This file documents the use of the GNU compilers.

Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License” and “Funding Free Software”, the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.

## Short Contents

- Introduction
- 1 Programming Languages Supported by GCC
- 2 Language Standards Supported by GCC
- 3 GCC Command Options
- 4 C Implementation-defined behavior
- 5 Extensions to the C Language Family
- 6 Extensions to the C++ Language
- 7 GNU Objective-C runtime features
- 8 Binary Compatibility
- 9 `gcov`—a Test Coverage Program
- 10 Known Causes of Trouble with GCC
- 11 Reporting Bugs
- 12 How To Get Help with GCC
- 13 Contributing to GCC Development
- Funding Free Software
- The GNU Project and GNU/Linux
- GNU GENERAL PUBLIC LICENSE
- GNU Free Documentation License
- Contributors to GCC
- Option Index
- Keyword Index

## Table of Contents

- Introduction
- 1 Programming Languages Supported by GCC
- 2 Language Standards Supported by GCC
- 3 GCC Command Options
  - 3.1 Option Summary
  - 3.2 Options Controlling the Kind of Output
  - 3.3 Compiling C++ Programs
  - 3.4 Options Controlling C Dialect
  - 3.5 Options Controlling C++ Dialect
  - 3.6 Options Controlling Objective-C and Objective-C++ Dialects
  - 3.7 Options to Control Diagnostic Messages Formatting
  - 3.8 Options to Request or Suppress Warnings
  - 3.9 Options for Debugging Your Program or GCC
  - 3.10 Options That Control Optimization
  - 3.11 Options Controlling the Preprocessor
  - 3.12 Passing Options to the Assembler
  - 3.13 Options for Linking
  - 3.14 Options for Directory Search
  - 3.15 Specifying subprocesses and the switches to pass to them
  - 3.16 Specifying Target Machine and Compiler Version
  - 3.17 Hardware Models and Configurations
    - 3.17.1 ARC Options
    - 3.17.2 ARM Options
    - 3.17.3 AVR Options
    - 3.17.4 Blackfin Options
    - 3.17.5 CRIS Options
    - 3.17.6 CRX Options
    - 3.17.7 Darwin Options
    - 3.17.8 DEC Alpha Options
    - 3.17.9 DEC Alpha/VMS Options
    - 3.17.10 FRV Options
    - 3.17.11 H8/300 Options
    - 3.17.12 HPPA Options
    - 3.17.13 Intel 386 and AMD x86-64 Options
    - 3.17.14 IA-64 Options
    - 3.17.15 M32C Options
    - 3.17.16 M32R/D Options
    - 3.17.17 M680x0 Options
    - 3.17.18 M68hc1x Options
    - 3.17.19 MCore Options
    - 3.17.20 MIPS Options
    - 3.17.21 MMIX Options
    - 3.17.22 MN10300 Options
    - 3.17.23 MT Options
    - 3.17.24 PDP-11 Options
    - 3.17.25 PowerPC Options
    - 3.17.26 IBM RS/6000 and PowerPC Options
    - 3.17.27 S/390 and zSeries Options
    - 3.17.28 SH Options
    - 3.17.29 SPARC Options
    - 3.17.30 Options for System V
    - 3.17.31 TMS320C3x/C4x Options
    - 3.17.32 V850 Options
    - 3.17.33 VAX Options
    - 3.17.34 x86-64 Options
    - 3.17.35 Xstormy16 Options
    - 3.17.36 Xtensa Options
    - 3.17.37 zSeries Options
  - 3.18 Options for Code Generation Conventions
  - 3.19 Environment Variables Affecting GCC
  - 3.20 Using Precompiled Headers
  - 3.21 Running Protoize
- 4 C Implementation-defined behavior
  - 4.1 Translation
  - 4.2 Environment
  - 4.3 Identifiers
  - 4.4 Characters
  - 4.5 Integers
  - 4.6 Floating point
  - 4.7 Arrays and pointers
  - 4.8 Hints
  - 4.9 Structures, unions, enumerations, and bit-fields
  - 4.10 Qualifiers
  - 4.11 Declarators
  - 4.12 Statements
  - 4.13 Preprocessing directives
  - 4.14 Library functions
  - 4.15 Architecture
  - 4.16 Locale-specific behavior
- 5 Extensions to the C Language Family
  - 5.1 Statements and Declarations in Expressions
  - 5.2 Locally Declared Labels
  - 5.3 Labels as Values
  - 5.4 Nested Functions
  - 5.5 Constructing Function Calls
  - 5.6 Referring to a Type with `typeof`
  - 5.7 Conditionals with Omitted Operands
  - 5.8 Double-Word Integers
  - 5.9 Complex Numbers
  - 5.10 Hex Floats
  - 5.11 Arrays of Length Zero
  - 5.12 Structures With No Members
  - 5.13 Arrays of Variable Length
  - 5.14 Macros with a Variable Number of Arguments.
  - 5.15 Slightly Looser Rules for Escaped Newlines
  - 5.16 Non-Lvalue Arrays May Have Subscripts
  - 5.17 Arithmetic on `void`- and Function-Pointers
  - 5.18 Non-Constant Initializers
  - 5.19 Compound Literals
  - 5.20 Designated Initializers
  - 5.21 Case Ranges
  - 5.22 Cast to a Union Type
  - 5.23 Mixed Declarations and Code
  - 5.24 Declaring Attributes of Functions
  - 5.25 Attribute Syntax
  - 5.26 Prototypes and Old-Style Function Definitions
  - 5.27 C++ Style Comments
  - 5.28 Dollar Signs in Identifier Names
  - 5.29 The Character <ESC> in Constants
  - 5.30 Inquiring on Alignment of Types or Variables
  - 5.31 Specifying Attributes of Variables
    - 5.31.1 M32R/D Variable Attributes
    - 5.31.2 i386 Variable Attributes
    - 5.31.3 Xstormy16 Variable Attributes
  - 5.32 Specifying Attributes of Types
    - 5.32.1 ARM Type Attributes
    - 5.32.2 i386 Type Attributes
  - 5.33 An Inline Function is As Fast As a Macro
  - 5.34 Assembler Instructions with C Expression Operands
    - 5.34.1 Size of an `asm`
    - 5.34.2 i386 floating point asm operands
  - 5.35 Constraints for `asm` Operands
    - 5.35.1 Simple Constraints
    - 5.35.2 Multiple Alternative Constraints
    - 5.35.3 Constraint Modifier Characters
    - 5.35.4 Constraints for Particular Machines
  - 5.36 Controlling Names Used in Assembler Code
  - 5.37 Variables in Specified Registers
    - 5.37.1 Defining Global Register Variables
    - 5.37.2 Specifying Registers for Local Variables
  - 5.38 Alternate Keywords
  - 5.39 Incomplete `enum` Types
  - 5.40 Function Names as Strings
  - 5.41 Getting the Return or Frame Address of a Function
  - 5.42 Using vector instructions through built-in functions
  - 5.43 Offsetof
  - 5.44 Built-in functions for atomic memory access
  - 5.45 Object Size Checking Builtins
  - 5.46 Other built-in functions provided by GCC
  - 5.47 Built-in Functions Specific to Particular Target Machines
    - 5.47.1 Alpha Built-in Functions
    - 5.47.2 ARM Built-in Functions
    - 5.47.3 Blackfin Built-in Functions
    - 5.47.4 FR-V Built-in Functions
      - 5.47.4.1 Argument Types
      - 5.47.4.2 Directly-mapped Integer Functions
      - 5.47.4.3 Directly-mapped Media Functions
      - 5.47.4.4 Raw read/write Functions
      - 5.47.4.5 Other Built-in Functions
    - 5.47.5 X86 Built-in Functions
    - 5.47.6 MIPS DSP Built-in Functions
    - 5.47.7 MIPS Paired-Single Support
      - 5.47.7.1 Paired-Single Arithmetic
      - 5.47.7.2 Paired-Single Built-in Functions
      - 5.47.7.3 MIPS-3D Built-in Functions
    - 5.47.8 PowerPC AltiVec Built-in Functions
    - 5.47.9 SPARC VIS Built-in Functions
  - 5.48 Format Checks Specific to Particular Target Machines
    - 5.48.1 Solaris Format Checks
  - 5.49 Pragmas Accepted by GCC
    - 5.49.1 ARM Pragmas
    - 5.49.2 M32C Pragmas
    - 5.49.3 RS/6000 and PowerPC Pragmas
    - 5.49.4 Darwin Pragmas
    - 5.49.5 Solaris Pragmas
    - 5.49.6 Symbol-Renaming Pragmas
    - 5.49.7 Structure-Packing Pragmas
    - 5.49.8 Weak Pragmas
  - 5.50 Unnamed struct/union fields within structs/unions
  - 5.51 Thread-Local Storage
    - 5.51.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage
    - 5.51.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage
- 6 Extensions to the C++ Language
  - 6.1 When is a Volatile Object Accessed?
  - 6.2 Restricting Pointer Aliasing
  - 6.3 Vague Linkage
  - 6.4 #pragma interface and implementation
  - 6.5 Where's the Template?
  - 6.6 Extracting the function pointer from a bound pointer to member function
  - 6.7 C++-Specific Variable, Function, and Type Attributes
  - 6.8 Strong Using
  - 6.9 Java Exceptions
  - 6.10 Deprecated Features
  - 6.11 Backwards Compatibility
- 7 GNU Objective-C runtime features
  - 7.1 `+load`: Executing code before main
    - 7.1.1 What you can and what you cannot do in `+load`
  - 7.2 Type encoding
  - 7.3 Garbage Collection
  - 7.4 Constant string objects
  - 7.5 compatibility_alias
- 8 Binary Compatibility
- 9 `gcov`—a Test Coverage Program
  - 9.1 Introduction to `gcov`
  - 9.2 Invoking gcov
  - 9.3 Using `gcov` with GCC Optimization
  - 9.4 Brief description of `gcov` data files
  - 9.5 Data file relocation to support cross-profiling
- 10 Known Causes of Trouble with GCC
  - 10.1 Actual Bugs We Haven't Fixed Yet
  - 10.2 Cross-Compiler Problems
  - 10.3 Interoperation
  - 10.4 Incompatibilities of GCC
  - 10.5 Fixed Header Files
  - 10.6 Standard Libraries
  - 10.7 Disappointments and Misunderstandings
  - 10.8 Common Misunderstandings with GNU C++
    - 10.8.1 Declare and Define Static Members
    - 10.8.2 Name lookup, templates, and accessing members of base classes
    - 10.8.3 Temporaries May Vanish Before You Expect
    - 10.8.4 Implicit Copy-Assignment for Virtual Bases
  - 10.9 Caveats of using `protoize`
  - 10.10 Certain Changes We Don't Want to Make
  - 10.11 Warning Messages and Error Messages
- 11 Reporting Bugs
  - 11.1 Have You Found a Bug?
  - 11.2 How and where to Report Bugs
- 12 How To Get Help with GCC
- 13 Contributing to GCC Development
- Funding Free Software
- The GNU Project and GNU/Linux
- GNU GENERAL PUBLIC LICENSE
  - Preamble
  - Appendix: How to Apply These Terms to Your New Programs
- GNU Free Documentation License
  - ADDENDUM: How to use this License for your documents
- Contributors to GCC
- Option Index
- Keyword Index

Next: G++ and GCC, Up: (DIR)

## Introduction

This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to GCC version 4.1.0. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See Introduction.

Next: Standards, Previous: Top, Up: Top

## 1 Programming Languages Supported by GCC

GCC stands for “GNU Compiler Collection”. GCC is an integrated distribution of compilers for several major programming languages. These languages currently include C, C++, Objective-C, Objective-C++, Java, Fortran, and Ada.

The abbreviation GCC has multiple meanings in common use. The current official meaning is “GNU Compiler Collection”, which refers generically to the complete suite of tools. The name historically stood for “GNU C Compiler”, and this usage is still common when the emphasis is on compiling C programs. Finally, the name is also used when speaking of the language-independent component of GCC: code shared among the compilers for all supported languages.

The language-independent component of GCC includes the majority of the optimizers, as well as the “back ends” that generate machine code for various processors.

The part of a compiler that is specific to a particular language is called the “front end”. In addition to the front ends that are integrated components of GCC, there are several other front ends that are maintained separately. These support languages such as Pascal, Mercury, and COBOL. To use these, they must be built together with GCC proper.

Most of the compilers for languages other than C have their own names. The C++ compiler is G++, the Ada compiler is GNAT, and so on. When we talk about compiling one of those languages, we might refer to that compiler by its own name, or as GCC. Either is correct.

Historically, compilers for many languages, including C++ and Fortran, have been implemented as “preprocessors” which emit another high level language such as C. None of the compilers included in GCC are implemented this way; they all generate machine code directly. This sort of preprocessor should not be confused with the C preprocessor, which is an integral feature of the C, C++, Objective-C and Objective-C++ languages.

Next: Invoking GCC, Previous: G++ and GCC, Up: Top

## 2 Language Standards Supported by GCC

For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.

GCC supports three versions of the C standard, although support for the most recent version is not yet complete.

The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options `-ansi`, `-std=c89` or `-std=iso9899:1990`; to obtain all the diagnostics required by the standard, you should also specify `-pedantic` (or `-pedantic-errors` if you want them to be errors rather than warnings). See Options Controlling C Dialect.

Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.

An amendment to the 1990 standard was published in 1995. This amendment added digraphs and `__STDC_VERSION__` to the language, but otherwise concerned the library. This amendment is commonly known as AMD1; the amended standard is sometimes known as C94 or C95. To select this standard in GCC, use the option `-std=iso9899:199409` (with, as for other standard versions, `-pedantic` to receive all required diagnostics).

A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-4.1/c99status.html for details. To select this standard, use `-std=c99` or `-std=iso9899:1999`. (While in development, drafts of this standard version were referred to as C9X.)

Errors in the 1999 ISO C standard were corrected in two Technical Corrigenda published in 2001 and 2004. GCC does not support the uncorrected version.

By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See Extensions to the C Language Family. Use of the `-std` options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with `-std=gnu89` (for C89 with GNU extensions) or `-std=gnu99` (for C99 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu89`; this will change to `-std=gnu99` in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.

The ISO C standard defines (in clause 4) two classes of conforming implementation. A conforming hosted implementation supports the whole standard including all the library facilities; a conforming freestanding implementation is only required to provide certain library facilities: those in `<float.h>`, `<limits.h>`, `<stdarg.h>`, and `<stddef.h>`; since AMD1, also those in `<iso646.h>`; and in C99, also those in `<stdbool.h>` and `<stdint.h>`. In addition, complex types, added in C99, are not required for freestanding implementations. The standard also defines two environments for programs, a freestanding environment, required of all implementations and which may not have library facilities beyond those required of freestanding implementations, where the handling of program startup and termination are implementation-defined, and a hosted environment, which is not required, in which all the library facilities are provided and startup is through a function `int main (void)` or `int main (int, char *[])`. An OS kernel would be a freestanding environment; a program using the facilities of an operating system would normally be in a hosted implementation.

GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for a conforming hosted implementation. By default, it will act as the compiler for a hosted implementation, defining `__STDC_HOSTED__` as `1` and presuming that when the names of ISO C functions are used, they have the semantics defined in the standard. To make it act as a conforming freestanding implementation for a freestanding environment, use the option `-ffreestanding`; it will then define `__STDC_HOSTED__` to `0` and not make assumptions about the meanings of function names from the standard library, with exceptions noted below. To build an OS kernel, you may well still need to make your own arrangements for linking and startup. See Options Controlling C Dialect.

GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See Standard Libraries.

Most of the compiler support routines used by GCC are present in `libgcc`, but there are a few exceptions. GCC requires the freestanding environment provide `memcpy`, `memmove`, `memset` and `memcmp`. Finally, if `__builtin_trap` is used, and the target does not implement the `trap` pattern, then GCC will emit a call to `abort`.

For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html

There is no formal written standard for Objective-C or Objective-C++. The most authoritative manual is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites:

- http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/ is a recent (and periodically updated) version;
- http://www.toodarkpark.org/computers/objc/ is an older example;
- http://www.gnustep.org and http://gcc.gnu.org/readings.html have additional useful information.

There is no standard for treelang, which is a sample language front end for GCC. Its only purpose is as a sample for people wishing to write a new language for GCC. The language is documented in `gcc/treelang/treelang.texi` which can be turned into info or HTML format.

See GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.

See Standards, for details of standards supported by `gfortran`.

See Compatibility with the Java Platform, for details of compatibility between `gcj` and the Java Platform.

Next: C Implementation, Previous: Standards, Up: Top

## 3 GCC Command Options

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The “overall options” allow you to stop this process at an intermediate stage. For example, the `-c` option says not to run the linker. Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.

See Compiling C++ Programs, for a summary of special options for compiling C++ programs.

The `gcc` program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: `-dr` is very different from ``-d -r`'.

You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify `-L` more than once, the directories are searched in the order specified.

Many options have long names starting with ``-f`' or with ``-W`'—for example, `-fstrength-reduce`, `-Wformat` and so on. Most of these have both positive and negative forms; the negative form of `-ffoo` would be `-fno-foo`. This manual documents only one of these two forms, whichever one is not the default.

See Option Index, for an index to GCC's options.

Next: Overall Options, Up: Invoking GCC

### 3.1 Option Summary

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

Overall Options
  See Options Controlling the Kind of Output.

                -c  -S  -E  -o file  -combine -pipe  -pass-exit-codes
                -x language  -v  -###  --help  --target-help  --version

C Language Options
  See Options Controlling C Dialect.

                -ansi  -std=standard  -aux-info filename
                -fno-asm  -fno-builtin  -fno-builtin-function
                -fhosted  -ffreestanding  -fms-extensions
                -trigraphs  -no-integrated-cpp  -traditional  -traditional-cpp
                -fallow-single-precision  -fcond-mismatch
                -fsigned-bitfields  -fsigned-char
                -funsigned-bitfields  -funsigned-char

C++ Language Options
  See Options Controlling C++ Dialect.

                -fabi-version=n  -fno-access-control  -fcheck-new
                -fconserve-space  -ffriend-injection  -fno-const-strings
                -fno-elide-constructors
                -fno-enforce-eh-specs
                -ffor-scope  -fno-for-scope  -fno-gnu-keywords
                -fno-implicit-templates
                -fno-implicit-inline-templates
                -fno-implement-inlines  -fms-extensions
                -fno-nonansi-builtins  -fno-operator-names
                -fno-optional-diags  -fpermissive
                -frepo  -fno-rtti  -fstats  -ftemplate-depth-n
                -fno-threadsafe-statics -fuse-cxa-atexit  -fno-weak  -nostdinc++
                -fno-default-inline  -fvisibility-inlines-hidden
                -Wabi  -Wctor-dtor-privacy
                -Wnon-virtual-dtor  -Wreorder
                -Weffc++  -Wno-deprecated  -Wstrict-null-sentinel
                -Wno-non-template-friend  -Wold-style-cast
                -Woverloaded-virtual  -Wno-pmf-conversions
                -Wsign-promo

Objective-C and Objective-C++ Language Options
  See Options Controlling Objective-C and Objective-C++ Dialects.

                -fconstant-string-class=class-name
                -fgnu-runtime  -fnext-runtime
                -fno-nil-receivers
                -fobjc-call-cxx-cdtors
                -fobjc-direct-dispatch
                -fobjc-exceptions
                -fobjc-gc
                -freplace-objc-classes
                -fzero-link
                -gen-decls
                -Wassign-intercept
                -Wno-protocol  -Wselector
                -Wstrict-selector-match
                -Wundeclared-selector

Language Independent Options
  See Options to Control Diagnostic Messages Formatting.

                -fmessage-length=n
                -fdiagnostics-show-location=[once|every-line]

  -fdiagnostics-show-options

Warning Options
  See Options to Request or Suppress Warnings.

                -fsyntax-only  -pedantic  -pedantic-errors
                -w  -Wextra  -Wall  -Waggregate-return -Wno-attributes
                -Wc++-compat -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment
                -Wconversion  -Wno-deprecated-declarations
                -Wdisabled-optimization  -Wno-div-by-zero  -Wno-endif-labels
                -Werror  -Werror-implicit-function-declaration
                -Wfatal-errors  -Wfloat-equal  -Wformat  -Wformat=2
                -Wno-format-extra-args -Wformat-nonliteral
                -Wformat-security  -Wformat-y2k
                -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int
                -Wimport  -Wno-import  -Winit-self  -Winline
                -Wno-int-to-pointer-cast
                -Wno-invalid-offsetof  -Winvalid-pch
                -Wlarger-than-len  -Wunsafe-loop-optimizations  -Wlong-long
                -Wmain  -Wmissing-braces  -Wmissing-field-initializers
                -Wmissing-format-attribute  -Wmissing-include-dirs
                -Wmissing-noreturn
                -Wno-multichar  -Wnonnull  -Wpacked  -Wpadded
                -Wparentheses  -Wpointer-arith  -Wno-pointer-to-int-cast
                -Wredundant-decls
                -Wreturn-type  -Wsequence-point  -Wshadow
                -Wsign-compare  -Wstack-protector
                -Wstrict-aliasing -Wstrict-aliasing=2
                -Wswitch  -Wswitch-default  -Wswitch-enum
                -Wsystem-headers  -Wtrigraphs  -Wundef  -Wuninitialized
                -Wunknown-pragmas  -Wno-pragmas -Wunreachable-code
                -Wunused  -Wunused-function  -Wunused-label  -Wunused-parameter
                -Wunused-value  -Wunused-variable  -Wvariadic-macros
                -Wvolatile-register-var  -Wwrite-strings

C-only Warning Options

                -Wbad-function-cast  -Wmissing-declarations
                -Wmissing-prototypes  -Wnested-externs  -Wold-style-definition
                -Wstrict-prototypes  -Wtraditional
                -Wdeclaration-after-statement -Wpointer-sign

Debugging Options
  See Options for Debugging Your Program or GCC.

                -dletters  -dumpspecs  -dumpmachine  -dumpversion
                -fdump-unnumbered  -fdump-translation-unit[-n]
                -fdump-class-hierarchy[-n]
                -fdump-ipa-all -fdump-ipa-cgraph
                -fdump-tree-all
                -fdump-tree-original[-n]
                -fdump-tree-optimized[-n]
                -fdump-tree-inlined[-n]
                -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias
                -fdump-tree-ch
                -fdump-tree-ssa[-n] -fdump-tree-pre[-n]
                -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
                -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n]
                -fdump-tree-dom[-n]
                -fdump-tree-dse[-n]
                -fdump-tree-phiopt[-n]
                -fdump-tree-forwprop[-n]
                -fdump-tree-copyrename[-n]
                -fdump-tree-nrv -fdump-tree-vect
                -fdump-tree-sink
                -fdump-tree-sra[-n]
                -fdump-tree-salias
                -fdump-tree-fre[-n]
                -fdump-tree-vrp[-n]
                -ftree-vectorizer-verbose=n
                -fdump-tree-storeccp[-n]
                -feliminate-dwarf2-dups -feliminate-unused-debug-types
                -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs
                -frandom-seed=string -fsched-verbose=n
                -ftest-coverage  -ftime-report -fvar-tracking
                -g  -glevel  -gcoff -gdwarf-2
                -ggdb  -gstabs  -gstabs+  -gvms  -gxcoff  -gxcoff+
                -p  -pg  -print-file-name=library  -print-libgcc-file-name
                -print-multi-directory  -print-multi-lib
                -print-prog-name=program  -print-search-dirs  -Q
                -save-temps  -time

Optimization Options
  See Options that Control Optimization.

                -falign-functions=n  -falign-jumps=n
                -falign-labels=n  -falign-loops=n
                -fbounds-check -fmudflap -fmudflapth -fmudflapir
                -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize
                -fbranch-target-load-optimize2 -fbtr-bb-exclusive
                -fcaller-saves  -fcprop-registers  -fcse-follow-jumps
                -fcse-skip-blocks  -fcx-limited-range  -fdata-sections
                -fdelayed-branch  -fdelete-null-pointer-checks -fearly-inlining
                -fexpensive-optimizations  -ffast-math  -ffloat-store
                -fforce-addr  -ffunction-sections
                -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las  -fgcse-after-reload
                -floop-optimize -fcrossjumping  -fif-conversion  -fif-conversion2
                -finline-functions  -finline-functions-called-once
                -finline-limit=n  -fkeep-inline-functions
                -fkeep-static-consts  -fmerge-constants  -fmerge-all-constants
                -fmodulo-sched -fno-branch-count-reg
                -fno-default-inline  -fno-defer-pop -floop-optimize2 -fmove-loop-invariants
                -fno-function-cse  -fno-guess-branch-probability
                -fno-inline  -fno-math-errno  -fno-peephole  -fno-peephole2
                -funsafe-math-optimizations  -funsafe-loop-optimizations  -ffinite-math-only
                -fno-trapping-math  -fno-zero-initialized-in-bss
                -fomit-frame-pointer  -foptimize-register-move
                -foptimize-sibling-calls  -fprefetch-loop-arrays
                -fprofile-generate -fprofile-use
                -fregmove  -frename-registers
                -freorder-blocks  -freorder-blocks-and-partition -freorder-functions
                -frerun-cse-after-loop  -frerun-loop-opt
                -frounding-math -fschedule-insns  -fschedule-insns2
                -fno-sched-interblock  -fno-sched-spec  -fsched-spec-load
                -fsched-spec-load-dangerous
                -fsched-stalled-insns=n -fsched-stalled-insns-dep=n
                -fsched2-use-superblocks
                -fsched2-use-traces -freschedule-modulo-scheduled-loops
                -fsignaling-nans -fsingle-precision-constant
                -fstack-protector  -fstack-protector-all
                -fstrength-reduce  -fstrict-aliasing  -ftracer  -fthread-jumps
                -funroll-all-loops  -funroll-loops  -fpeel-loops
                -fsplit-ivs-in-unroller -funswitch-loops
                -fvariable-expansion-in-unroller
                -ftree-pre  -ftree-ccp  -ftree-dce -ftree-loop-optimize
                -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts
                -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink
                -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize
                -ftree-vect-loop-version -ftree-salias -fweb
                -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fwhole-program
                --param name=value
                -O  -O0  -O1  -O2  -O3  -Os

Preprocessor Options
  See Options Controlling the Preprocessor.

                -Aquestion=answer
                -A-question[=answer]
                -C  -dD  -dI  -dM  -dN
                -Dmacro[=defn]  -E  -H
                -idirafter dir
                -include file  -imacros file
                -iprefix file  -iwithprefix dir
                -iwithprefixbefore dir  -isystem dir
                -isysroot dir
                -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc
                -P  -fworking-directory  -remap
                -trigraphs  -undef  -Umacro  -Wp,option
                -Xpreprocessor option

Assembler Option
  See Passing Options to the Assembler.

                -Wa,option  -Xassembler option

Linker Options
  See Options for Linking.

                object-file-name  -llibrary
                -nostartfiles  -nodefaultlibs  -nostdlib -pie -rdynamic
                -s  -static  -static-libgcc  -shared  -shared-libgcc  -symbolic
                -Wl,option  -Xlinker option
                -u symbol

Directory Options
  See Options for Directory Search.

                -Bprefix  -Idir  -iquotedir  -Ldir
                -specs=file  -I- --sysroot=dir

Target Options
  See Target Options.

                -V version  -b machine

Machine Dependent Options
  See Hardware Models and Configurations.

  ARC Options

                -EB  -EL
                -mmangle-cpu  -mcpu=cpu  -mtext=text-section
                -mdata=data-section  -mrodata=readonly-data-section

  ARM Options

                -mapcs-frame  -mno-apcs-frame
                -mabi=name
                -mapcs-stack-check  -mno-apcs-stack-check
                -mapcs-float  -mno-apcs-float
                -mapcs-reentrant  -mno-apcs-reentrant
                -msched-prolog  -mno-sched-prolog
                -mlittle-endian  -mbig-endian  -mwords-little-endian
                -mfloat-abi=name  -msoft-float  -mhard-float  -mfpe
                -mthumb-interwork  -mno-thumb-interwork
                -mcpu=name  -march=name  -mfpu=name
                -mstructure-size-boundary=n
                -mabort-on-noreturn
                -mlong-calls  -mno-long-calls
                -msingle-pic-base  -mno-single-pic-base
                -mpic-register=reg
                -mnop-fun-dllimport
                -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns
                -mpoke-function-name
                -mthumb  -marm
                -mtpcs-frame  -mtpcs-leaf-frame
                -mcaller-super-interworking  -mcallee-super-interworking
                -mtp=name

  AVR Options

                -mmcu=mcu  -msize  -minit-stack=n  -mno-interrupts
                -mcall-prologues  -mno-tablejump  -mtiny-stack  -mint8

  Blackfin Options

                -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
                -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly
                -mlow-64k -mno-low64k -mid-shared-library
                -mno-id-shared-library -mshared-library-id=n
                -mlong-calls  -mno-long-calls

  CRIS Options

                -mcpu=cpu  -march=cpu  -mtune=cpu
                -mmax-stack-frame=n  -melinux-stacksize=n
                -metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects
                -mstack-align  -mdata-align  -mconst-align
                -m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt
                -melf  -maout  -melinux  -mlinux  -sim  -sim2
                -mmul-bug-workaround  -mno-mul-bug-workaround

  CRX Options

                -mmac -mpush-args

  Darwin Options

                -all_load  -allowable_client  -arch  -arch_errors_fatal
                -arch_only  -bind_at_load  -bundle  -bundle_loader
                -client_name  -compatibility_version  -current_version
                -dead_strip
                -dependency-file  -dylib_file  -dylinker_install_name
                -dynamic  -dynamiclib  -exported_symbols_list
                -filelist  -flat_namespace  -force_cpusubtype_ALL
                -force_flat_namespace  -headerpad_max_install_names
                -image_base  -init  -install_name  -keep_private_externs
                -multi_module  -multiply_defined  -multiply_defined_unused
                -noall_load   -no_dead_strip_inits_and_terms
                -nofixprebinding -nomultidefs  -noprebind  -noseglinkedit
                -pagezero_size  -prebind  -prebind_all_twolevel_modules
                -private_bundle  -read_only_relocs  -sectalign
                -sectobjectsymbols  -whyload  -seg1addr
                -sectcreate  -sectobjectsymbols  -sectorder
                -segaddr -segs_read_only_addr -segs_read_write_addr
                -seg_addr_table  -seg_addr_table_filename  -seglinkedit
                -segprot  -segs_read_only_addr  -segs_read_write_addr
                -single_module  -static  -sub_library  -sub_umbrella
                -twolevel_namespace  -umbrella  -undefined
                -unexported_symbols_list  -weak_reference_mismatches
                -whatsloaded -F -gused -gfull -mmacosx-version-min=version
                -mone-byte-bool

  DEC Alpha Options

                -mno-fp-regs  -msoft-float  -malpha-as  -mgas
                -mieee  -mieee-with-inexact  -mieee-conformant
                -mfp-trap-mode=mode  -mfp-rounding-mode=mode
                -mtrap-precision=mode  -mbuild-constants
                -mcpu=cpu-type  -mtune=cpu-type
                -mbwx  -mmax  -mfix  -mcix
                -mfloat-vax  -mfloat-ieee
                -mexplicit-relocs  -msmall-data  -mlarge-data
                -msmall-text  -mlarge-text
                -mmemory-latency=time

  DEC Alpha/VMS Options

                -mvms-return-codes

  FRV Options

                -mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64
                -mhard-float  -msoft-float
                -malloc-cc  -mfixed-cc  -mdword  -mno-dword
                -mdouble  -mno-double
                -mmedia  -mno-media  -mmuladd  -mno-muladd
                -mfdpic  -minline-plt -mgprel-ro  -multilib-library-pic
                -mlinked-fp  -mlong-calls  -malign-labels
                -mlibrary-pic  -macc-4  -macc-8
                -mpack  -mno-pack  -mno-eflags  -mcond-move  -mno-cond-move
                -moptimize-membar -mno-optimize-membar
                -mscc  -mno-scc  -mcond-exec  -mno-cond-exec
                -mvliw-branch  -mno-vliw-branch
                -mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec
                -mno-nested-cond-exec  -mtomcat-stats
                -mTLS -mtls
                -mcpu=cpu

  H8/300 Options

                -mrelax  -mh  -ms  -mn  -mint32  -malign-300

  HPPA Options

                -march=architecture-type
                -mbig-switch  -mdisable-fpregs  -mdisable-indexing
                -mfast-indirect-calls  -mgas  -mgnu-ld   -mhp-ld
                -mfixed-range=register-range
                -mjump-in-delay -mlinker-opt -mlong-calls
                -mlong-load-store  -mno-big-switch  -mno-disable-fpregs
                -mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas
                -mno-jump-in-delay  -mno-long-load-store
                -mno-portable-runtime  -mno-soft-float
                -mno-space-regs  -msoft-float  -mpa-risc-1-0
                -mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime
                -mschedule=cpu-type  -mspace-regs  -msio  -mwsio
                -munix=unix-std  -nolibdld  -static  -threads

  i386 and x86-64 Options

                -mtune=cpu-type  -march=cpu-type
                -mfpmath=unit
                -masm=dialect  -mno-fancy-math-387
                -mno-fp-ret-in-387  -msoft-float  -msvr3-shlib
                -mno-wide-multiply  -mrtd  -malign-double
                -mpreferred-stack-boundary=num
                -mmmx  -msse  -msse2 -msse3 -m3dnow
                -mthreads  -mno-align-stringops  -minline-all-stringops
                -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double
                -m96bit-long-double  -mregparm=num  -msseregparm
                -momit-leaf-frame-pointer  -mno-red-zone -mno-tls-direct-seg-refs
                -mcmodel=code-model
                -m32  -m64 -mlarge-data-threshold=num

  IA-64 Options

                -mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic
                -mvolatile-asm-stop  -mregister-names  -mno-sdata
                -mconstant-gp  -mauto-pic  -minline-float-divide-min-latency
                -minline-float-divide-max-throughput
                -minline-int-divide-min-latency
                -minline-int-divide-max-throughput
                -minline-sqrt-min-latency -minline-sqrt-max-throughput
                -mno-dwarf2-asm -mearly-stop-bits
                -mfixed-range=register-range -mtls-size=tls-size
                -mtune=cpu-type -mt -pthread -milp32 -mlp64

  M32R/D Options

                -m32r2 -m32rx -m32r
                -mdebug
                -malign-loops -mno-align-loops
                -missue-rate=number
                -mbranch-cost=number
                -mmodel=code-size-model-type
                -msdata=sdata-type
                -mno-flush-func -mflush-func=name
                -mno-flush-trap -mflush-trap=number
                -G num

  M32C Options

                -mcpu=cpu -msim -memregs=number

  M680x0 Options

                -m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040
                -m68060  -mcpu32  -m5200  -m68881  -mbitfield  -mc68000  -mc68020
                -mnobitfield  -mrtd  -mshort  -msoft-float  -mpcrel
                -malign-int  -mstrict-align  -msep-data  -mno-sep-data
                -mshared-library-id=n  -mid-shared-library  -mno-id-shared-library

  M68hc1x Options

                -m6811  -m6812  -m68hc11  -m68hc12   -m68hcs12
                -mauto-incdec  -minmax  -mlong-calls  -mshort
                -msoft-reg-count=count

  MCore Options

                -mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates
                -mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields
                -m4byte-functions  -mno-4byte-functions  -mcallgraph-data
                -mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim
                -mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment

  MIPS Options

                -EL  -EB  -march=arch  -mtune=arch
                -mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2  -mips64
                -mips16  -mno-mips16  -mabi=abi  -mabicalls  -mno-abicalls
                -mxgot  -mno-xgot  -mgp32  -mgp64  -mfp32  -mfp64
                -mhard-float  -msoft-float  -msingle-float  -mdouble-float
                -mdsp  -mpaired-single  -mips3d
                -mlong64  -mlong32  -msym32  -mno-sym32
                -Gnum  -membedded-data  -mno-embedded-data
                -muninit-const-in-rodata  -mno-uninit-const-in-rodata
                -msplit-addresses  -mno-split-addresses
                -mexplicit-relocs  -mno-explicit-relocs
                -mcheck-zero-division  -mno-check-zero-division
                -mdivide-traps  -mdivide-breaks
                -mmemcpy  -mno-memcpy  -mlong-calls  -mno-long-calls
                -mmad  -mno-mad  -mfused-madd  -mno-fused-madd  -nocpp
                -mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400
                -mfix-vr4120  -mno-fix-vr4120  -mfix-vr4130
                -mfix-sb1  -mno-fix-sb1
                -mflush-func=func  -mno-flush-func
                -mbranch-likely  -mno-branch-likely
                -mfp-exceptions -mno-fp-exceptions
                -mvr4130-align -mno-vr4130-align

  MMIX Options

                -mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu
                -mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols
                -melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses
                -mno-base-addresses  -msingle-exit  -mno-single-exit

  MN10300 Options

                -mmult-bug  -mno-mult-bug
                -mam33  -mno-am33
                -mam33-2  -mno-am33-2
                -mreturn-pointer-on-d0
                -mno-crt0  -mrelax

  MT Options

                -mno-crt0 -mbacc -msim
                -march=cpu-type

  PDP-11 Options

                -mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10
                -mbcopy  -mbcopy-builtin  -mint32  -mno-int16
                -mint16  -mno-int32  -mfloat32  -mno-float64
                -mfloat64  -mno-float32  -mabshi  -mno-abshi
                -mbranch-expensive  -mbranch-cheap
                -msplit  -mno-split  -munix-asm  -mdec-asm

  PowerPC Options See RS/6000 and PowerPC Options.

  RS/6000 and PowerPC Options

                -mcpu=cpu-type
                -mtune=cpu-type
                -mpower  -mno-power  -mpower2  -mno-power2
                -mpowerpc  -mpowerpc64  -mno-powerpc
                -maltivec  -mno-altivec
                -mpowerpc-gpopt  -mno-powerpc-gpopt
                -mpowerpc-gfxopt  -mno-powerpc-gfxopt
                -mmfcrf  -mno-mfcrf  -mpopcntb  -mno-popcntb  -mfprnd  -mno-fprnd
                -mnew-mnemonics  -mold-mnemonics
                -mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc
                -m64  -m32  -mxl-compat  -mno-xl-compat  -mpe
                -malign-power  -malign-natural
                -msoft-float  -mhard-float  -mmultiple  -mno-multiple
                -mstring  -mno-string  -mupdate  -mno-update
                -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align
                -mstrict-align  -mno-strict-align  -mrelocatable
                -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib
                -mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian
                -mdynamic-no-pic  -maltivec  -mswdiv
                -mprioritize-restricted-insns=priority
                -msched-costly-dep=dependence_type
                -minsert-sched-nops=scheme
                -mcall-sysv  -mcall-netbsd
                -maix-struct-return  -msvr4-struct-return
                -mabi=abi-type -msecure-plt -mbss-plt
                -misel -mno-isel
                -misel=yes  -misel=no
                -mspe -mno-spe
                -mspe=yes  -mspe=no
                -mvrsave -mno-vrsave
                -mfloat-gprs=yes  -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double
                -mprototype  -mno-prototype
                -msim  -mmvme  -mads  -myellowknife  -memb  -msdata
                -msdata=opt  -mvxworks  -mwindiss  -G num  -pthread

  S/390 and zSeries Options

                -mtune=cpu-type  -march=cpu-type
                -mhard-float  -msoft-float -mlong-double-64 -mlong-double-128
                -mbackchain  -mno-backchain -mpacked-stack  -mno-packed-stack
                -msmall-exec  -mno-small-exec  -mmvcle -mno-mvcle
                -m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch
                -mtpf-trace -mno-tpf-trace  -mfused-madd  -mno-fused-madd
                -mwarn-framesize  -mwarn-dynamicstack  -mstack-size -mstack-guard

  SH Options

                -m1  -m2  -m2e  -m3  -m3e
                -m4-nofpu  -m4-single-only  -m4-single  -m4
                -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
                -m5-64media  -m5-64media-nofpu
                -m5-32media  -m5-32media-nofpu
                -m5-compact  -m5-compact-nofpu
                -mb  -ml  -mdalign  -mrelax
                -mbigtable  -mfmovd  -mhitachi -mrenesas -mno-renesas -mnomacsave
                -mieee  -misize  -mpadstruct  -mspace
                -mprefergot  -musermode -multcost=number -mdiv=strategy
                -mdivsi3_libfunc=name
                -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
                 -minvalid-symbols

  SPARC Options

                -mcpu=cpu-type
                -mtune=cpu-type
                -mcmodel=code-model
                -m32  -m64  -mapp-regs  -mno-app-regs
                -mfaster-structs  -mno-faster-structs
                -mfpu  -mno-fpu  -mhard-float  -msoft-float
                -mhard-quad-float  -msoft-quad-float
                -mimpure-text  -mno-impure-text  -mlittle-endian
                -mstack-bias  -mno-stack-bias
                -munaligned-doubles  -mno-unaligned-doubles
                -mv8plus  -mno-v8plus  -mvis  -mno-vis
                -threads -pthreads -pthread

  System V Options

                -Qy  -Qn  -YP,paths  -Ym,dir

  TMS320C3x/C4x Options

                -mcpu=cpu  -mbig  -msmall  -mregparm  -mmemparm
                -mfast-fix  -mmpyi  -mbk  -mti  -mdp-isr-reload
                -mrpts=count  -mrptb  -mdb  -mloop-unsigned
                -mparallel-insns  -mparallel-mpy  -mpreserve-float

  V850 Options

                -mlong-calls  -mno-long-calls  -mep  -mno-ep
                -mprolog-function  -mno-prolog-function  -mspace
                -mtda=n  -msda=n  -mzda=n
                -mapp-regs  -mno-app-regs
                -mdisable-callt  -mno-disable-callt
                -mv850e1
                -mv850e
                -mv850  -mbig-switch

  VAX Options

                -mg  -mgnu  -munix

  x86-64 Options See i386 and x86-64 Options.

  Xstormy16 Options

                -msim

  Xtensa Options

                -mconst16 -mno-const16
                -mfused-madd  -mno-fused-madd
                -mtext-section-literals  -mno-text-section-literals
                -mtarget-align  -mno-target-align
                -mlongcalls  -mno-longcalls

  zSeries Options See S/390 and zSeries Options.

Code Generation Options
  See Options for Code Generation Conventions.

                -fcall-saved-reg  -fcall-used-reg
                -ffixed-reg  -fexceptions
                -fnon-call-exceptions  -funwind-tables
                -fasynchronous-unwind-tables
                -finhibit-size-directive  -finstrument-functions
                -fno-common  -fno-ident
                -fpcc-struct-return  -fpic  -fPIC -fpie -fPIE
                -fno-jump-tables
                -freg-struct-return  -fshared-data  -fshort-enums
                -fshort-double  -fshort-wchar
                -fverbose-asm  -fpack-struct[=n]  -fstack-check
                -fstack-limit-register=reg  -fstack-limit-symbol=sym
                -fargument-alias  -fargument-noalias
                -fargument-noalias-global  -fleading-underscore
                -ftls-model=model
                -ftrapv  -fwrapv  -fbounds-check
                -fvisibility

Next: Invoking G++, Previous: Option Summary, Up: Invoking GCC

### 3.2 Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

file`.c`
  C source code which must be preprocessed.
file`.i`
  C source code which should not be preprocessed.
file`.ii`
  C++ source code which should not be preprocessed.
file`.m`
  Objective-C source code. Note that you must link with the `libobjc` library to make an Objective-C program work.
file`.mi`
  Objective-C source code which should not be preprocessed.
file`.mm`
file`.M`
  Objective-C++ source code. Note that you must link with the `libobjc` library to make an Objective-C++ program work. Note that ``.M`' refers to a literal capital M.
file`.mii`
  Objective-C++ source code which should not be preprocessed.
file`.h`
  C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file`.cc`
file`.cp`
file`.cxx`
file`.cpp`
file`.CPP`
file`.c++`
file`.C`
  C++ source code which must be preprocessed. Note that in ``.cxx`', the last two letters must both be literally ``x`'. Likewise, ``.C`' refers to a literal capital C.
file`.mm`
file`.M`
  Objective-C++ source code which must be preprocessed.
file`.mii`
  Objective-C++ source code which should not be preprocessed.
file`.hh`
file`.H`
  C++ header file to be turned into a precompiled header.
file`.f`
file`.for`
file`.FOR`
  Fixed form Fortran source code which should not be preprocessed.
file`.F`
file`.fpp`
file`.FPP`
  Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).
file`.f90`
file`.f95`
  Free form Fortran source code which should not be preprocessed.
file`.F90`
file`.F95`
  Free form Fortran source code which must be preprocessed (with the traditional preprocessor).
file`.ads`
  Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
file`.adb`
  Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
file`.s`
  Assembler code.
file`.S`
  Assembler code which must be preprocessed.
other
  An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the `-x` option:

`-x` language
  Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next `-x` option. Possible values for language are:

                c  c-header  c-cpp-output
                c++  c++-header  c++-cpp-output
                objective-c  objective-c-header  objective-c-cpp-output
                objective-c++ objective-c++-header objective-c++-cpp-output
                assembler  assembler-with-cpp
                ada
                f77  f77-cpp-input
                f95  f95-cpp-input
                java
                treelang

`-x none`
  Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if `-x` has not been used at all).
`-pass-exit-codes`
  Normally the `gcc` program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If you specify `-pass-exit-codes`, the `gcc` program will instead return with numerically highest error produced by any phase that returned an error indication.

If you only want some of the stages of compilation, you can use `-x` (or filename suffixes) to tell `gcc` where to start, and one of the options `-c`, `-S`, or `-E` to say where `gcc` is to stop. Note that some combinations (for example, ``-x cpp-output -E`') instruct `gcc` to do nothing at all.

`-c`
  Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

  By default, the object file name for a source file is made by replacing the suffix ``.c`', ``.i`', ``.s`', etc., with ``.o`'.

  Unrecognized input files, not requiring compilation or assembly, are ignored.

`-S`
  Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

  By default, the assembler file name for a source file is made by replacing the suffix ``.c`', ``.i`', etc., with ``.s`'.

  Input files that don't require compilation are ignored.

`-E`
  Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

  Input files which don't require preprocessing are ignored.

`-o` file
  Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

  If `-o` is not specified, the default is to put an executable file in `a.out`, the object file for `source.suffix` in `source.o`, its assembler file in `source.s`, a precompiled header file in `source.suffix.gch`, and all preprocessed C source on standard output.

`-v`
  Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
`-###`
  Like `-v` except the commands are not executed and all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.
`-pipe`
  Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
`-combine`
  If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with `-save-temps`, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) `.o` or `.s` file.
`--help`
  Print (on the standard output) a description of the command line options understood by `gcc`. If the `-v` option is also specified then `--help` will also be passed on to the various processes invoked by `gcc`, so that they can display the command line options they accept. If the `-Wextra` option is also specified then command line options which have no documentation associated with them will also be displayed.
`--target-help`
  Print (on the standard output) a description of target specific command line options for each tool.
`--version`
  Display the version number and copyrights of the invoked GCC.

Next: C Dialect Options, Previous: Overall Options, Up: Invoking GCC

### 3.3 Compiling C++ Programs

C++ source files conventionally use one of the suffixes ``.C`', ``.cc`', ``.cpp`', ``.CPP`', ``.c++`', ``.cp`', or ``.cxx`'; C++ header files often use ``.hh`' or ``.H`'; and preprocessed C++ files use the suffix ``.ii`'. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name `gcc`).

However, C++ programs often require class libraries as well as a compiler that understands the C++ language—and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C++ programs. You might also like to precompile a C header file with a ``.h`' extension to be used in C++ compilations. `g++` is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library. On many systems, `g++` is also installed with the name `c++`.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

Next: C++ Dialect Options, Previous: Invoking G++, Up: Invoking GCC

### 3.4 Options Controlling C Dialect

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

`-ansi`
  In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.

  This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the `asm` and `typeof` keywords, and predefined macros such as `unix` and `vax` that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style ``//`' comments as well as the `inline` keyword.

  The alternate keywords `__asm__`, `__extension__`, `__inline__` and `__typeof__` continue to work despite `-ansi`. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with `-ansi`. Alternate predefined macros such as `__unix__` and `__vax__` are also available, with or without `-ansi`.

  The `-ansi` option does not cause non-ISO programs to be rejected gratuitously. For that, `-pedantic` is required in addition to `-ansi`. See Warning Options.

  The macro `__STRICT_ANSI__` is predefined when the `-ansi` option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

  Functions which would normally be built in but do not have semantics defined by ISO C (such as `alloca` and `ffs`) are not built-in functions with `-ansi` is used. See Other built-in functions provided by GCC, for details of the functions affected.

`-std=`
  Determine the language standard. This option is currently only supported when compiling C or C++. A value for this option must be provided; possible values are
  ``c89`'
  ``iso9899:1990`'
    ISO C90 (same as `-ansi`).
  ``iso9899:199409`'
    ISO C90 as modified in amendment 1.
  ``c99`'
  ``c9x`'
  ``iso9899:1999`'
  ``iso9899:199x`'
    ISO C99. Note that this standard is not yet fully supported; see http://gcc.gnu.org/gcc-4.1/c99status.html for more information. The names ``c9x`' and ``iso9899:199x`' are deprecated.
  ``gnu89`'
    Default, ISO C90 plus GNU extensions (including some C99 features).
  ``gnu99`'
  ``gnu9x`'
    ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name ``gnu9x`' is deprecated.
  ``c++98`'
    The 1998 ISO C++ standard plus amendments.
  ``gnu++98`'
    The same as `-std=c++98` plus GNU extensions. This is the default for C++ code.

  Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use `__restrict__` even when `-std=c99` is not specified.

  The `-std` options specifying some version of ISO C have the same effects as `-ansi`, except that features that were not in ISO C90 but are in the specified version (for example, ``//`' comments and the `inline` keyword in ISO C99) are not disabled.

  See Language Standards Supported by GCC, for details of these standard versions.

`-aux-info` filename
  Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

  Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (``I`', ``N`' for new or ``O`' for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (``C`' or ``F`', respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

`-fno-asm`
  Do not recognize `asm`, `inline` or `typeof` as a keyword, so that code can use these words as identifiers. You can use the keywords `__asm__`, `__inline__` and `__typeof__` instead. `-ansi` implies `-fno-asm`.

  In C++, this switch only affects the `typeof` keyword, since `asm` and `inline` are standard keywords. You may want to use the `-fno-gnu-keywords` flag instead, which has the same effect. In C99 mode (`-std=c99` or `-std=gnu99`), this switch only affects the `asm` and `typeof` keywords, since `inline` is a standard keyword in ISO C99.

`-fno-builtin`
`-fno-builtin-`function
  Don't recognize built-in functions that do not begin with ``__builtin_`' as prefix. See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when `-ansi` or `-std` options for strict ISO C conformance are used because they do not have an ISO standard meaning.

  GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to `alloca` may become single instructions that adjust the stack directly, and calls to `memcpy` may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with `-Wformat` for bad calls to `printf`, when `printf` is built in, and `strlen` is known not to modify global memory.

  With the `-fno-builtin-function` option only the built-in function function is disabled. function must not begin with ``__builtin_`'. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding `-fbuiltin-function` option; if you wish to enable built-in functions selectively when using `-fno-builtin` or `-ffreestanding`, you may define macros such as:

                #define abs(n)          __builtin_abs ((n))
                #define strcpy(d, s)    __builtin_strcpy ((d), (s))

`-fhosted`
  Assert that compilation takes place in a hosted environment. This implies `-fbuiltin`. A hosted environment is one in which the entire standard library is available, and in which `main` has a return type of `int`. Examples are nearly everything except a kernel. This is equivalent to `-fno-freestanding`.
`-ffreestanding`
  Assert that compilation takes place in a freestanding environment. This implies `-fno-builtin`. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at `main`. The most obvious example is an OS kernel. This is equivalent to `-fno-hosted`.

  See Language Standards Supported by GCC, for details of freestanding and hosted environments.

`-fms-extensions`
  Accept some non-standard constructs used in Microsoft header files.

  Some cases of unnamed fields in structures and unions are only accepted with this option. See Unnamed struct/union fields within structs/unions, for details.

`-trigraphs`
  Support ISO C trigraphs. The `-ansi` option (and `-std` options for strict ISO C conformance) implies `-trigraphs`.
`-no-integrated-cpp`
  Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the `-B` option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

  The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

`-traditional`
`-traditional-cpp`
  Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the `-E` switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.
`-fcond-mismatch`
  Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.
`-funsigned-char`
  Let the type `char` be unsigned, like `unsigned char`.

  Each kind of machine has a default for what `char` should be. It is either like `unsigned char` by default or like `signed char` by default.

  Ideally, a portable program should always use `signed char` or `unsigned char` when it depends on the signedness of an object. But many programs have been written to use plain `char` and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.

  The type `char` is always a distinct type from each of `signed char` or `unsigned char`, even though its behavior is always just like one of those two.

`-fsigned-char`
  Let the type `char` be signed, like `signed char`.

  Note that this is equivalent to `-fno-unsigned-char`, which is the negative form of `-funsigned-char`. Likewise, the option `-fno-signed-char` is equivalent to `-funsigned-char`.

`-fsigned-bitfields`
`-funsigned-bitfields`
`-fno-signed-bitfields`
`-fno-unsigned-bitfields`
  These options control whether a bit-field is signed or unsigned, when the declaration does not use either `signed` or `unsigned`. By default, such a bit-field is signed, because this is consistent: the basic integer types such as `int` are signed types.

Next: Objective-C and Objective-C++ Dialect Options, Previous: C Dialect Options, Up: Invoking GCC

### 3.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file `firstClass.C` like this:

         g++ -g -frepo -O -c firstClass.C

In this example, only `-frepo` is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

`-fabi-version=`n
  Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

  The default is version 2.

`-fno-access-control`
  Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
`-fcheck-new`
  Check that the pointer returned by `operator new` is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that `operator new` will only return `0` if it is declared ``throw()`', in which case the compiler will always check the return value even without this option. In all other cases, when `operator new` has a non-empty exception specification, memory exhaustion is signalled by throwing `std::bad_alloc`. See also ``new (nothrow)`'.
`-fconserve-space`
  Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after `main()` has completed, you may have an object that is being destroyed twice because two definitions were merged.

  This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

`-ffriend-injection`
  Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function which is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they were in earlier releases.

  This option is for compatibility, and may be removed in a future release of G++.

`-fno-const-strings`
  Give string constants type `char *` instead of type `const char *`. By default, G++ uses type `const char *` as required by the standard. Even if you use `-fno-const-strings`, you cannot actually modify the value of a string constant.

  This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type `const char *`.

`-fno-elide-constructors`
  The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.
`-fno-enforce-eh-specs`
  Don't generate code to check for violation of exception specifications at runtime. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining ``NDEBUG`'. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler will still optimize based on the specifications, so throwing an unexpected exception will result in undefined behavior.
`-ffor-scope`
`-fno-for-scope`
  If `-ffor-scope` is specified, the scope of variables declared in a for-init-statement is limited to the ``for`' loop itself, as specified by the C++ standard. If `-fno-for-scope` is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.

  The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

`-fno-gnu-keywords`
  Do not recognize `typeof` as a keyword, so that code can use this word as an identifier. You can use the keyword `__typeof__` instead. `-ansi` implies `-fno-gnu-keywords`.
`-fno-implicit-templates`
  Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.
`-fno-implicit-inline-templates`
  Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.
`-fno-implement-inlines`
  To save space, do not emit out-of-line copies of inline functions controlled by ``#pragma implementation`'. This will cause linker errors if these functions are not inlined everywhere they are called.
`-fms-extensions`
  Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
`-fno-nonansi-builtins`
  Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include `ffs`, `alloca`, `_exit`, `index`, `bzero`, `conjf`, and other related functions.
`-fno-operator-names`
  Do not treat the operator name keywords `and`, `bitand`, `bitor`, `compl`, `not`, `or` and `xor` as synonyms as keywords.
`-fno-optional-diags`
  Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
`-fpermissive`
  Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using `-fpermissive` will allow some nonconforming code to compile.
`-frepo`
  Enable automatic template instantiation at link time. This option also implies `-fno-implicit-templates`. See Template Instantiation, for more information.
`-fno-rtti`
  Disable generation of information about every class with virtual functions for use by the C++ runtime type identification features (``dynamic_cast`' and ``typeid`'). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but it will generate it as needed.
`-fstats`
  Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.
`-ftemplate-depth-`n
  Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.
`-fno-threadsafe-statics`
  Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
`-fuse-cxa-atexit`
  Register destructors for objects with static storage duration with the `__cxa_atexit` function rather than the `atexit` function. This option is required for fully standards-compliant handling of static destructors, but will only work if your C library supports `__cxa_atexit`.
`-fvisibility-inlines-hidden`
  Causes all inlined methods to be marked with `__attribute__ ((visibility ("hidden")))` so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the `-fvisibility=hidden` switch.
`-fno-weak`
  Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++.
`-nostdinc++`
  Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

`-fno-default-inline`
  Do not assume ``inline`' for functions defined inside a class scope. See Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.
`-Wabi` (C++ only)
  Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible.

  You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

  The known incompatibilities at this point include:

  - Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:

                       struct A { virtual void f(); int f1 : 1; };
                       struct B : public A { int f2 : 1; };

    In this case, G++ will place `B::f2` into the same byte as`A::f1`; other compilers will not. You can avoid this problem by explicitly padding `A` so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout `B` identically.

  - Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

                       struct A { virtual void f(); char c1; };
                       struct B { B(); char c2; };
                       struct C : public A, public virtual B {};

    In this case, G++ will not place `B` into the tail-padding for `A`; other compilers will. You can avoid this problem by explicitly padding `A` so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout `C` identically.

  - Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

                       union U { int i : 4096; };

    Assuming that an `int` does not have 4096 bits, G++ will make the union too small by the number of bits in an `int`.

  - Empty classes can be placed at incorrect offsets. For example:

                       struct A {};

                       struct B {
                         A a;
                         virtual void f ();
                       };

                       struct C : public B, public A {};

    G++ will place the `A` base class of `C` at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the `A` data member of `B` is already at offset zero.

  - Names of template functions whose types involve `typename` or template template parameters can be mangled incorrectly.

                       template <typename Q>
                       void f(typename Q::X) {}

                       template <template <typename> class Q>
                       void f(typename Q<int>::X) {}

    Instantiations of these templates may be mangled incorrectly.

`-Wctor-dtor-privacy` (C++ only)
  Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.
`-Wnon-virtual-dtor` (C++ only)
  Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by `-Wall`.
`-Wreorder` (C++ only)
  Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

                struct A {
                  int i;
                  int j;
                  A(): j (0), i (1) { }
                };

  The compiler will rearrange the member initializers for ``i`' and ``j`' to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by `-Wall`.

The following `-W...` options are not affected by `-Wall`.

`-Weffc++` (C++ only)
  Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:
  - Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
  - Item 12: Prefer initialization to assignment in constructors.
  - Item 14: Make destructors virtual in base classes.
  - Item 15: Have `operator=` return a reference to `*this`.
  - Item 23: Don't try to return a reference when you must return an object.

  Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

  - Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
  - Item 7: Never overload `&&`, `||`, or `,`.

  When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ``grep -v`' to filter out those warnings.

`-Wno-deprecated` (C++ only)
  Do not warn about usage of deprecated features. See Deprecated Features.
`-Wstrict-null-sentinel` (C++ only)
  Warn also about the use of an uncasted `NULL` as sentinel. When compiling only with GCC this is a valid sentinel, as `NULL` is defined to `__null`. Although it is a null pointer constant not a null pointer, it is guaranteed to of the same size as a pointer. But this use is not portable across different compilers.
`-Wno-non-template-friend` (C++ only)
  Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., ``friend foo(int)`'), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, `-Wnon-template-friend` allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with `-Wno-non-template-friend` which keeps the conformant compiler code but disables the helpful warning.
`-Wold-style-cast` (C++ only)
  Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (``dynamic_cast`', ``static_cast`', ``reinterpret_cast`', and ``const_cast`') are less vulnerable to unintended effects and much easier to search for.
`-Woverloaded-virtual` (C++ only)
  Warn when a function declaration hides virtual functions from a base class. For example, in:

                struct A {
                  virtual void f();
                };

                struct B: public A {
                  void f(int);
                };

  the `A` class version of `f` is hidden in `B`, and code like:

                B* b;
                b->f();

  will fail to compile.

`-Wno-pmf-conversions` (C++ only)
  Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
`-Wsign-promo` (C++ only)
  Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ would try to preserve unsignedness, but the standard mandates the current behavior.

                struct A {
                  operator int ();
                  A& operator = (int);
                };

                main ()
                {
                  A a,b;
                  a = b;
                }

  In this example, G++ will synthesize a default ``A& operator = (const A&);`', while cfront will use the user-defined ``operator =`'.

Next: Language Independent Options, Previous: C++ Dialect Options, Up: Invoking GCC

### 3.6 Options Controlling Objective-C and Objective-C++ Dialects

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See See Language Standards Supported by GCC, for references.)

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but you can also use most of the language-independent GNU compiler options. For example, you might compile a file `some_class.m` like this:

         gcc -g -fgnu-runtime -O -c some_class.m

In this example, `-fgnu-runtime` is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., `-Wtraditional`). Similarly, Objective-C++ compilations may use C++-specific options (e.g., `-Wabi`).

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

`-fconstant-string-class=`class-name
  Use class-name as the name of the class to instantiate for each literal string specified with the syntax `@"..."`. The default class name is `NXConstantString` if the GNU runtime is being used, and `NSConstantString` if the NeXT runtime is being used (see below). The `-fconstant-cfstrings` option, if also present, will override the `-fconstant-string-class` setting and cause `@"..."` literals to be laid out as constant CoreFoundation strings.
`-fgnu-runtime`
  Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.
`-fnext-runtime`
  Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro `__NEXT_RUNTIME__` is predefined if (and only if) this option is used.
`-fno-nil-receivers`
  Assume that all Objective-C message dispatches (e.g., `[receiver message:arg]`) in this translation unit ensure that the receiver is not `nil`. This allows for more efficient entry points in the runtime to be used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
`-fobjc-call-cxx-cdtors`
  For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special `- (id) .cxx_construct` instance method that will run non-trivial default constructors on any such instance variables, in order, and then return `self`. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special `- (void) .cxx_destruct` method that will run all such default destructors, in reverse order.

  The `- (id) .cxx_construct` and/or `- (void) .cxx_destruct` methods thusly generated will only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The `- (id) .cxx_construct` methods will be invoked by the runtime immediately after a new object instance is allocated; the `- (void) .cxx_destruct` methods will be invoked immediately before the runtime deallocates an object instance.

  As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the `- (id) .cxx_construct` and `- (void) .cxx_destruct` methods.

`-fobjc-direct-dispatch`
  Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
`-fobjc-exceptions`
  Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

                  @try {
                    ...
                       @throw expr;
                    ...
                  }
                  @catch (AnObjCClass *exc) {
                    ...
                      @throw expr;
                    ...
                      @throw;
                    ...
                  }
                  @catch (AnotherClass *exc) {
                    ...
                  }
                  @catch (id allOthers) {
                    ...
                  }
                  @finally {
                    ...
                      @throw expr;
                    ...
                  }

  The `@throw` statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a `@catch` block, the `@throw` may appear without an argument (as shown above), in which case the object caught by the `@catch` will be rethrown.

  Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest `@catch` clause capable of handling objects of that type, analogously to how `catch` blocks work in C++ and Java. A `@catch(id ...)` clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous `@catch` clauses (if any).

  The `@finally` clause, if present, will be executed upon exit from the immediately preceding `@try ... @catch` section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the `@try ... @catch` section, analogously to the behavior of the `finally` clause in Java.

  There are several caveats to using the new exception mechanism:

  - Although currently designed to be binary compatible with `NS_HANDLER`-style idioms provided by the `NSException` class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.
  - As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time. This means you cannot `@throw` an exception from Objective-C and `catch` it in C++, or vice versa (i.e., `throw ... @catch`).

  The `-fobjc-exceptions` switch also enables the use of synchronization blocks for thread-safe execution:

                  @synchronized (ObjCClass *guard) {
                    ...
                  }

  Upon entering the `@synchronized` block, a thread of execution shall first check whether a lock has been placed on the corresponding `guard` object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once `guard` becomes available, the current thread will place its own lock on it, execute the code contained in the `@synchronized` block, and finally relinquish the lock (thereby making `guard` available to other threads).

  Unlike Java, Objective-C does not allow for entire methods to be marked `@synchronized`. Note that throwing exceptions out of `@synchronized` blocks is allowed, and will cause the guarding object to be unlocked properly.

`-fobjc-gc`
  Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
`-freplace-objc-classes`
  Emit a special marker instructing `ld(1)` not to statically link in the resulting object file, and allow `dyld(1)` to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
`-fzero-link`
  When compiling for the NeXT runtime, the compiler ordinarily replaces calls to `objc_getClass("...")` (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the `-fzero-link` flag suppresses this behavior and causes calls to `objc_getClass("...")` to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution.
`-gen-decls`
  Dump interface declarations for all classes seen in the source file to a file named `sourcename.decl`.
`-Wassign-intercept`
  Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
`-Wno-protocol`
  If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the `-Wno-protocol` option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
`-Wselector`
  Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a `@selector(...)` expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the `-fsyntax-only` option is being used.
`-Wstrict-selector-match`
  Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type `id` or `Class`. When this flag is off (which is the default behavior), the compiler will omit such warnings if any differences found are confined to types which share the same size and alignment.
`-Wundeclared-selector`
  Warn if a `@selector(...)` expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the `@selector(...)` expression, either explicitly in an `@interface` or `@protocol` declaration, or implicitly in an `@implementation` section. This option always performs its checks as soon as a `@selector(...)` expression is found, while `-Wselector` only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.
`-print-objc-runtime-info`
  Generate C header describing the largest structure that is passed by value, if any.

Next: Warning Options, Previous: Objective-C and Objective-C++ Dialect Options, Up: Invoking GCC

### 3.7 Options to Control Diagnostic Messages Formatting

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.

`-fmessage-length=`n
  Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for `g++` and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line.

`-fdiagnostics-show-location=once`
  Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.
`-fdiagnostics-show-location=every-line`
  Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.
`-fdiagnostics-show-options`
  This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option directly controls that diagnostic, when such an option is known to the diagnostic machinery.

Next: Debugging Options, Previous: Language Independent Options, Up: Invoking GCC

### 3.8 Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.

You can request many specific warnings with options beginning ``-W`', for example `-Wimplicit` to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning ``-Wno-`' to turn off warnings; for example, `-Wno-implicit`. This manual lists only one of the two forms, whichever is not the default.

The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

`-fsyntax-only`
  Check the code for syntax errors, but don't do anything beyond that.
`-pedantic`
  Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any `-std` option used.

  Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require `-ansi` or a `-std` option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

  `-pedantic` does not cause warning messages for use of the alternate keywords whose names begin and end with ``__`'. Pedantic warnings are also disabled in the expression that follows `__extension__`. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

  Some users try to use `-pedantic` to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

  A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic`. We don't have plans to support such a feature in the near future.

  Where the standard specified with `-std` represents a GNU extended dialect of C, such as ``gnu89`' or ``gnu99`', there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from `-pedantic` are given where they are required by the base standard. (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

`-pedantic-errors`
  Like `-pedantic`, except that errors are produced rather than warnings.
`-w`
  Inhibit all warning messages.
`-Wno-import`
  Inhibit warning messages about the use of ``#import`'.
`-Wchar-subscripts`
  Warn if an array subscript has type `char`. This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by `-Wall`.
`-Wcomment`
  Warn whenever a comment-start sequence ``/*`' appears in a ``/*`' comment, or whenever a Backslash-Newline appears in a ``//`' comment. This warning is enabled by `-Wall`.
`-Wfatal-errors`
  This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
`-Wformat`
  Check calls to `printf` and `scanf`, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes (see Function Attributes), in the `printf`, `scanf`, `strftime` and `strfmon` (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by `-ffreestanding` or `-fno-builtin`.

  The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if `-pedantic` is used with `-Wformat`, warnings will be given about format features not in the selected standard version (but not for `strfmon` formats, since those are not in any version of the C standard). See Options Controlling C Dialect.

  Since `-Wformat` also checks for null format arguments for several functions, `-Wformat` also implies `-Wnonnull`.

  `-Wformat` is included in `-Wall`. For more control over some aspects of format checking, the options `-Wformat-y2k`, `-Wno-format-extra-args`, `-Wno-format-zero-length`, `-Wformat-nonliteral`, `-Wformat-security`, and `-Wformat=2` are available, but are not included in `-Wall`.

`-Wformat-y2k`
  If `-Wformat` is specified, also warn about `strftime` formats which may yield only a two-digit year.
`-Wno-format-extra-args`
  If `-Wformat` is specified, do not warn about excess arguments to a `printf` or `scanf` format function. The C standard specifies that such arguments are ignored.

  Where the unused arguments lie between used arguments that are specified with ``$`' operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to `va_arg` to skip the unused arguments. However, in the case of `scanf` formats, this option will suppress the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

`-Wno-format-zero-length`
  If `-Wformat` is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.
`-Wformat-nonliteral`
  If `-Wformat` is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a `va_list`.
`-Wformat-security`
  If `-Wformat` is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to `printf` and `scanf` functions where the format string is not a string literal and there are no format arguments, as in `printf (foo);`. This may be a security hole if the format string came from untrusted input and contains ``%n`'. (This is currently a subset of what `-Wformat-nonliteral` warns about, but in future warnings may be added to `-Wformat-security` that are not included in `-Wformat-nonliteral`.)
`-Wformat=2`
  Enable `-Wformat` plus format checks not included in `-Wformat`. Currently equivalent to ``-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k`'.
`-Wnonnull`
  Warn about passing a null pointer for arguments marked as requiring a non-null value by the `nonnull` function attribute.

  `-Wnonnull` is included in `-Wall` and `-Wformat`. It can be disabled with the `-Wno-nonnull` option.

`-Winit-self` (C, C++, Objective-C and Objective-C++ only)
  Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the `-Wuninitialized` option, which in turn only works with `-O1` and above.

  For example, GCC will warn about `i` being uninitialized in the following snippet only when `-Winit-self` has been specified:

                int f()
                {
                  int i = i;
                  return i;
                }

`-Wimplicit-int`
  Warn when a declaration does not specify a type. This warning is enabled by `-Wall`.
`-Wimplicit-function-declaration`
`-Werror-implicit-function-declaration`
  Give a warning (or error) whenever a function is used before being declared. The form `-Wno-error-implicit-function-declaration` is not supported. This warning is enabled by `-Wall` (as a warning, not an error).
`-Wimplicit`
  Same as `-Wimplicit-int` and `-Wimplicit-function-declaration`. This warning is enabled by `-Wall`.
`-Wmain`
  Warn if the type of ``main`' is suspicious. ``main`' should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by `-Wall`.
`-Wmissing-braces`
  Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for ``a`' is not fully bracketed, but that for ``b`' is fully bracketed.

                int a[2][2] = { 0, 1, 2, 3 };
                int b[2][2] = { { 0, 1 }, { 2, 3 } };

  This warning is enabled by `-Wall`.

`-Wmissing-include-dirs` (C, C++, Objective-C and Objective-C++ only)
  Warn if a user-supplied include directory does not exist.
`-Wparentheses`
  Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Only the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are only supported when compiling C.

  Also warn if a comparison like ``x<=y<=z`' appears; this is equivalent to ``(x<=y ? 1 : 0) <= z`', which is a different interpretation from that of ordinary mathematical notation.

  Also warn about constructions where there may be confusion to which `if` statement an `else` branch belongs. Here is an example of such a case:

                {
                  if (a)
                    if (b)
                      foo ();
                  else
                    bar ();
                }

  In C, every `else` branch belongs to the innermost possible `if` statement, which in this example is `if (b)`. This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost `if` statement so there is no way the `else` could belong to the enclosing `if`. The resulting code would look like this:

                {
                  if (a)
                    {
                      if (b)
                        foo ();
                      else
                        bar ();
                    }
                }

  This warning is enabled by `-Wall`.

`-Wsequence-point`
  Warn about code that may have undefined semantics because of violations of sequence point rules in the C standard.

  The C standard defines the order in which expressions in a C program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a `&&`, `||`, `? :` or `,` (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

  It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C standard specifies that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

  Examples of code with undefined behavior are `a = a++;`, `a[n] = b[n++]` and `a[i++] = i;`. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

  The present implementation of this option only works for C programs. A future implementation may also work for C++ programs.

  The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

  This warning is enabled by `-Wall`.

`-Wreturn-type`
  Warn whenever a function is defined with a return-type that defaults to `int`. Also warn about any `return` statement with no return-value in a function whose return-type is not `void`.

  For C, also warn if the return type of a function has a type qualifier such as `const`. Such a type qualifier has no effect, since the value returned by a function is not an lvalue. ISO C prohibits qualified `void` return types on function definitions, so such return types always receive a warning even without this option.

  For C++, a function without return type always produces a diagnostic message, even when `-Wno-return-type` is specified. The only exceptions are ``main`' and functions defined in system headers.

  This warning is enabled by `-Wall`.

`-Wswitch`
  Warn whenever a `switch` statement has an index of enumerated type and lacks a `case` for one or more of the named codes of that enumeration. (The presence of a `default` label prevents this warning.) `case` labels outside the enumeration range also provoke warnings when this option is used. This warning is enabled by `-Wall`.
`-Wswitch-default`
  Warn whenever a `switch` statement does not have a `default` case.
`-Wswitch-enum`
  Warn whenever a `switch` statement has an index of enumerated type and lacks a `case` for one or more of the named codes of that enumeration. `case` labels outside the enumeration range also provoke warnings when this option is used.
`-Wtrigraphs`
  Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by `-Wall`.
`-Wunused-function`
  Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by `-Wall`.
`-Wunused-label`
  Warn whenever a label is declared but not used. This warning is enabled by `-Wall`.

  To suppress this warning use the ``unused`' attribute (see Variable Attributes).

`-Wunused-parameter`
  Warn whenever a function parameter is unused aside from its declaration.

  To suppress this warning use the ``unused`' attribute (see Variable Attributes).

`-Wunused-variable`
  Warn whenever a local variable or non-constant static variable is unused aside from its declaration This warning is enabled by `-Wall`.

  To suppress this warning use the ``unused`' attribute (see Variable Attributes).

`-Wunused-value`
  Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by `-Wall`.

  To suppress this warning cast the expression to ``void`'.

`-Wunused`
  All the above `-Wunused` options combined.

  In order to get a warning about an unused function parameter, you must either specify ``-Wextra -Wunused`' (note that ``-Wall`' implies ``-Wunused`'), or separately specify `-Wunused-parameter`.

`-Wuninitialized`
  Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a `setjmp` call.

  These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O`, you simply won't get these warnings.

  If you want to warn about code which uses the uninitialized value of the variable in its own initializer, use the `-Winit-self` option.

  These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables which are uninitialized or clobbered as a whole. They do not occur for variables or elements declared `volatile`. Because these warnings depend on optimization, the exact variables or elements for which there are warnings will depend on the precise optimization options and version of GCC used.

  Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

  These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

                {
                  int x;
                  switch (y)
                    {
                    case 1: x = 1;
                      break;
                    case 2: x = 4;
                      break;
                    case 3: x = 5;
                    }
                  foo (x);
                }

  If the value of `y` is always 1, 2 or 3, then `x` is always initialized, but GCC doesn't know this. Here is another common case:

                {
                  int save_y;
                  if (change_y) save_y = y, y = new_y;
                  ...
                  if (change_y) y = save_y;
                }

  This has no bug because `save_y` is used only if it is set.

  This option also warns when a non-volatile automatic variable might be changed by a call to `longjmp`. These warnings as well are possible only in optimizing compilation.

  The compiler sees only the calls to `setjmp`. It cannot know where `longjmp` will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because `longjmp` cannot in fact be called at the place which would cause a problem.

  Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn`. See Function Attributes.

  This warning is enabled by `-Wall`.

`-Wunknown-pragmas`
  Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were only enabled by the `-Wall` command line option.
`-Wno-pragmas`
  Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also ``-Wunknown-pragmas`'.
`-Wstrict-aliasing`
  This option is only active when `-fstrict-aliasing` is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in `-Wall`.
`-Wstrict-aliasing=2`
  This option is only active when `-fstrict-aliasing` is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization. This warning catches more cases than `-Wstrict-aliasing`, but it will also give a warning for some ambiguous cases that are safe.
`-Wall`
  All of the above ``-W`' options combined. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

The following `-W...` options are not implied by `-Wall`. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.

`-Wextra`
  (This option used to be called `-W`. The older name is still supported, but the newer name is more descriptive.) Print extra warning messages for these events:
  - A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning:

                       foo (a)
                       {
                         if (a > 0)
                           return a;
                       }

  - An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as ``x[i,j]`' will cause a warning, but ``x[(void)i,j]`' will not.
  - An unsigned value is compared against zero with ``<`' or ``>=`'.
  - Storage-class specifiers like `static` are not the first things in a declaration. According to the C Standard, this usage is obsolescent.
  - If `-Wall` or `-Wunused` is also specified, warn about unused arguments.
  - A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if `-Wno-sign-compare` is also specified.)
  - An aggregate has an initializer which does not initialize all members. This warning can be independently controlled by `-Wmissing-field-initializers`.
  - A function parameter is declared without a type specifier in K&R-style functions:

                       void foo(bar) { }

  - An empty body occurs in an ``if`' or ``else`' statement.
  - A pointer is compared against integer zero with ``<`', ``<=`', ``>`', or ``>=`'.
  - A variable might be changed by ``longjmp`' or ``vfork`'.
  - Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of precision, etc.
  - (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
  - (C++ only) A non-static reference or non-static ``const`' member appears in a class without constructors.
  - (C++ only) Ambiguous virtual bases.
  - (C++ only) Subscripting an array which has been declared ``register`'.
  - (C++ only) Taking the address of a variable which has been declared ``register`'.
  - (C++ only) A base class is not initialized in a derived class' copy constructor.
`-Wno-div-by-zero`
  Do not warn about compile-time integer division by zero. Floating point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.
`-Wsystem-headers`
  Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using `-Wall` in conjunction with this option will not warn about unknown pragmas in system headers—for that, `-Wunknown-pragmas` must also be used.
`-Wfloat-equal`
  Warn if floating point values are used in equality comparisons.

  The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

`-Wtraditional` (C only)
  Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs which should be avoided.
  - Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but does not in ISO C.
  - In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the ``#`' appeared in column 1 on the line. Therefore `-Wtraditional` warns about directives that traditional C understands but would ignore because the ``#`' does not appear as the first character on the line. It also suggests you hide directives like ``#pragma`' not understood by traditional C by indenting them. Some traditional implementations would not recognize ``#elif`', so it suggests avoiding it altogether.
  - A function-like macro that appears without arguments.
  - The unary plus operator.
  - The ``U`' integer constant suffix, or the ``F`' or ``L`' floating point constant suffixes. (Traditional C does support the ``L`' suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the ``_MIN`'/``_MAX`' macros in `<limits.h>`. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  - A function declared external in one block and then used after the end of the block.
  - A `switch` statement has an operand of type `long`.
  - A non-`static` function declaration follows a `static` one. This construct is not accepted by some traditional C compilers.
  - The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  - Usage of ISO string concatenation is detected.
  - Initialization of automatic aggregates.
  - Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  - Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. `__STDC__` to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  - Conversions by prototypes between fixed/floating point values and vice versa. The absence of these prototypes when compiling with traditional C would cause serious problems. This is a subset of the possible conversion warnings, for the full set use `-Wconversion`.
  - Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features will appear in your code when using libiberty's traditional C compatibility macros, `PARAMS` and `VPARAMS`. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
`-Wdeclaration-after-statement` (C only)
  Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by GCC versions before GCC 3.0. See Mixed Declarations.
`-Wundef`
  Warn if an undefined identifier is evaluated in an ``#if`' directive.
`-Wno-endif-labels`
  Do not warn whenever an ``#else`' or an ``#endif`' are followed by text.
`-Wshadow`
  Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.
`-Wlarger-than-`len
  Warn whenever an object of larger than len bytes is defined.
`-Wunsafe-loop-optimizations`
  Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices. With `-funsafe-loop-optimizations` warn if the compiler made such assumptions.
`-Wpointer-arith`
  Warn about anything that depends on the “size of” a function type or of `void`. GNU C assigns these types a size of 1, for convenience in calculations with `void *` pointers and pointers to functions.
`-Wbad-function-cast` (C only)
  Warn whenever a function call is cast to a non-matching type. For example, warn if `int malloc()` is cast to `anything *`.
`-Wc++-compat`
  Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from `void *` to a pointer to non-`void` type.
`-Wcast-qual`
  Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a `const char *` is cast to an ordinary `char *`.
`-Wcast-align`
  Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *` is cast to an `int *` on machines where integers can only be accessed at two- or four-byte boundaries.
`-Wwrite-strings`
  When compiling C, give string constants the type `const char[`length`]` so that copying the address of one into a non-`const` `char *` pointer will get a warning; when compiling C++, warn about the deprecated conversion from string constants to `char *`. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using `const` in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall` request these warnings.
`-Wconversion`
  Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion.

  Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment `x = -1` if `x` is unsigned. But do not warn about explicit casts like `(unsigned) -1`.

`-Wsign-compare`
  Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by `-Wextra`; to get the other warnings of `-Wextra` without this warning, use ``-Wextra -Wno-sign-compare`'.
`-Waggregate-return`
  Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
`-Wno-attributes`
  Do not warn if an unexpected `__attribute__` is used, such as unrecognized attributes, function attributes applied to variables, etc. This will not stop errors for incorrect use of supported attributes.
`-Wstrict-prototypes` (C only)
  Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.)
`-Wold-style-definition` (C only)
  Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.
`-Wmissing-prototypes` (C only)
  Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.
`-Wmissing-declarations` (C only)
  Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.
`-Wmissing-field-initializers`
  Warn if a structure's initializer has some fields missing. For example, the following code would cause such a warning, because `x.h` is implicitly zero:

                struct s { int f, g, h; };
                struct s x = { 3, 4 };

  This option does not warn about designated initializers, so the following modification would not trigger a warning:

                struct s { int f, g, h; };
                struct s x = { .f = 3, .g = 4 };

  This warning is included in `-Wextra`. To get other `-Wextra` warnings without this one, use ``-Wextra -Wno-missing-field-initializers`'.

`-Wmissing-noreturn`
  Warn about functions which might be candidates for attribute `noreturn`. Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the `noreturn` attribute, otherwise subtle code generation bugs could be introduced. You will not get a warning for `main` in hosted C environments.
`-Wmissing-format-attribute`
  Warn about function pointers which might be candidates for `format` attributes. Note these are only possible candidates, not absolute ones. GCC will guess that function pointers with `format` attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding `format` attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a `format` attribute to avoid the warning.

  GCC will also warn about function definitions which might be candidates for `format` attributes. Again, these are only possible candidates. GCC will guess that `format` attributes might be appropriate for any function that calls a function like `vprintf` or `vscanf`, but this might not always be the case, and some functions for which `format` attributes are appropriate may not be detected.

`-Wno-multichar`
  Do not warn if a multicharacter constant (``'FOOF'`') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.
`-Wnormalized=<none|id|nfc|nfkc>`
  In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers which have not been normalized; this option controls that warning.

  There are four levels of warning that GCC supports. The default is `-Wnormalized=nfc`, which warns about any identifier which is not in the ISO 10646 “C” normalized form, NFC. NFC is the recommended form for most uses.

  Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. `-Wnormalized=id` suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.

  You can switch the warning off for all characters by writing `-Wnormalized=none`. You would only want to do this if you were using some other normalization scheme (like “D”), because otherwise you can easily create bugs that are literally impossible to see.

  Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance `\u207F`, “SUPERSCRIPT LATIN SMALL LETTER N”, will display just like a regular `n` which has been placed in a superscript. ISO 10646 defines the NFKC normalisation scheme to convert all these into a standard form as well, and GCC will warn if your code is not in NFKC if you use `-Wnormalized=nfkc`. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment is unable to be fixed to display these characters distinctly.

`-Wno-deprecated-declarations`
  Do not warn about uses of functions, variables, and types marked as deprecated by using the `deprecated` attribute. (see Function Attributes, see Variable Attributes, see Type Attributes.)
`-Wpacked`
  Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable `f.x` in `struct bar` will be misaligned even though `struct bar` does not itself have the packed attribute:

                struct foo {
                  int x;
                  char a, b, c, d;
                } __attribute__((packed));
                struct bar {
                  char z;
                  struct foo f;
                };

`-Wpadded`
  Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.
`-Wredundant-decls`
  Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
`-Wnested-externs` (C only)
  Warn if an `extern` declaration is encountered within a function.
`-Wunreachable-code`
  Warn if the compiler detects that code will never be executed.

  This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.

  It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

  For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

  This option is not made part of `-Wall` because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time.

`-Winline`
  Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn about failures to inline functions declared in system headers.

  The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by `-Winline` to appear or disappear.

`-Wno-invalid-offsetof` (C++ only)
  Suppress warnings from applying the ``offsetof`' macro to a non-POD type. According to the 1998 ISO C++ standard, applying ``offsetof`' to a non-POD type is undefined. In existing C++ implementations, however, ``offsetof`' typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple ``struct`' that fails to be a POD type only by virtue of having a constructor.) This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

  The restrictions on ``offsetof`' may be relaxed in a future version of the C++ standard.

`-Wno-int-to-pointer-cast` (C only)
  Suppress warnings from casts to pointer type of an integer of a different size.
`-Wno-pointer-to-int-cast` (C only)
  Suppress warnings from casts from a pointer to an integer type of a different size.
`-Winvalid-pch`
  Warn if a precompiled header (see Precompiled Headers) is found in the search path but can't be used.
`-Wlong-long`
  Warn if ``long long`' type is used. This is default. To inhibit the warning messages, use `-Wno-long-long`. Flags `-Wlong-long` and `-Wno-long-long` are taken into account only when `-pedantic` flag is used.
`-Wvariadic-macros`
  Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode. This is default. To inhibit the warning messages, use `-Wno-variadic-macros`.
`-Wvolatile-register-var`
  Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables.
`-Wdisabled-optimization`
  Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers were unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of time.
`-Wpointer-sign`
  Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by `-Wall` and by `-pedantic`, which can be disabled with `-Wno-pointer-sign`.
`-Werror`
  Make all warnings into errors.
`-Wstack-protector`
  This option is only active when `-fstack-protector` is active. It warns about functions that will not be protected against stack smashing.

Next: Optimize Options, Previous: Warning Options, Up: Invoking GCC

### 3.9 Options for Debugging Your Program or GCC

GCC has various special options that are used for debugging either your program or GCC:

`-g`
  Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this debugging information.

  On most systems that use stabs format, `-g` enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+`, `-gstabs`, `-gxcoff+`, `-gxcoff`, or `-gvms` (see below).

  GCC allows you to use `-g` with `-O`. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

  Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

  The following options are useful when GCC is generated with the capability for more than one debugging format.

`-ggdb`
  Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
`-gstabs`
  Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.
`-feliminate-unused-debug-symbols`
  Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.
`-gstabs+`
  Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
`-gcoff`
  Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.
`-gxcoff`
  Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
`-gxcoff+`
  Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
`-gdwarf-2`
  Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6. With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is upward compatible with version 2, but may still cause problems for older debuggers.
`-gvms`
  Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems.
`-g` level
`-ggdb` level
`-gstabs` level
`-gcoff` level
`-gxcoff` level
`-gvms` level
  Request debugging information and also use level to specify how much information. The default level is 2.

  Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.

  Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3`.

  `-gdwarf-2` does not accept a concatenated debug level, because GCC used to support an option `-gdwarf` that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional `-glevel` option to change the debug level for DWARF2.

`-feliminate-dwarf2-dups`
  Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF2 debugging information with `-gdwarf-2`.

`-p`
  Generate extra code to write profile information suitable for the analysis program `prof`. You must use this option when compiling the source files you want data about, and you must also use it when linking.

`-pg`
  Generate extra code to write profile information suitable for the analysis program `gprof`. You must use this option when compiling the source files you want data about, and you must also use it when linking.
`-Q`
  Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
`-ftime-report`
  Makes the compiler print some statistics about the time consumed by each pass when it finishes.
`-fmem-report`
  Makes the compiler print some statistics about permanent memory allocation when it finishes.
`-fprofile-arcs`
  Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file called `auxname.gcda` for each source file. The data may be used for profile-directed optimizations (`-fbranch-probabilities`), or for test coverage analysis (`-ftest-coverage`). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. `foo.gcda` for input file `dir/foo.c`, or `dir/foo.gcda` for output file specified as `-o dir/foo.o`). See Cross-profiling.

`--coverage`
  This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for `-fprofile-arcs` `-ftest-coverage` (when compiling) and `-lgcov` (when linking). See the documentation for those options for more details.
  - Compile the source files with `-fprofile-arcs` plus optimization and code generation options. For test coverage analysis, use the additional `-ftest-coverage` option. You do not need to profile every source file in a program.
  - Link your object files with `-lgcov` or `-fprofile-arcs` (the latter implies the former).
  - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Also `fork` calls are detected and correctly handled (double counting will not happen).
  - For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus `-fbranch-probabilities` (see Options that Control Optimization).
  - For test coverage analysis, use `gcov` to produce human readable information from the `.gcno` and `.gcda` files. Refer to the `gcov` documentation for further information.

  With `-fprofile-arcs`, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

`-ftest-coverage`
  Produce a notes file that the `gcov` code-coverage utility (see `gcov`—a Test Coverage Program) can use to show program coverage. Each source file's note file is called `auxname.gcno`. Refer to the `-fprofile-arcs` option above for a description of auxname and instructions on how to generate test coverage data. Coverage data will match the source files more closely, if you do not optimize.
`-d` letters
`-fdump-rtl-`pass
  Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname. dumpname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file.

  Most debug dumps can be enabled either passing a letter to the `-d` option, or with a long `-fdump-rtl` switch; here are the possible letters for use in letters and pass, and their meanings:

  `-dA`
    Annotate the assembler output with miscellaneous debugging information.
  `-db`
  `-fdump-rtl-bp`
    Dump after computing branch probabilities, to `file.09.bp`.
  `-dB`
  `-fdump-rtl-bbro`
    Dump after block reordering, to `file.30.bbro`.
  `-dc`
  `-fdump-rtl-combine`
    Dump after instruction combination, to the file `file.17.combine`.
  `-dC`
  `-fdump-rtl-ce1`
  `-fdump-rtl-ce2`
    `-dC` and `-fdump-rtl-ce1` enable dumping after the first if conversion, to the file `file.11.ce1`. `-dC` and `-fdump-rtl-ce2` enable dumping after the second if conversion, to the file `file.18.ce2`.
  `-dd`
  `-fdump-rtl-btl`
  `-fdump-rtl-dbr`
    `-dd` and `-fdump-rtl-btl` enable dumping after branch target load optimization, to `file.31.btl`. `-dd` and `-fdump-rtl-dbr` enable dumping after delayed branch scheduling, to `file.36.dbr`.
  `-dD`
    Dump all macro definitions, at the end of preprocessing, in addition to normal output.
  `-dE`
  `-fdump-rtl-ce3`
    Dump after the third if conversion, to `file.28.ce3`.
  `-df`
  `-fdump-rtl-cfg`
  `-fdump-rtl-life`
    `-df` and `-fdump-rtl-cfg` enable dumping after control and data flow analysis, to `file.08.cfg`. `-df` and `-fdump-rtl-cfg` enable dumping dump after life analysis, to `file.16.life`.
  `-dg`
  `-fdump-rtl-greg`
    Dump after global register allocation, to `file.23.greg`.
  `-dG`
  `-fdump-rtl-gcse`
  `-fdump-rtl-bypass`
    `-dG` and `-fdump-rtl-gcse` enable dumping after GCSE, to `file.05.gcse`. `-dG` and `-fdump-rtl-bypass` enable dumping after jump bypassing and control flow optimizations, to `file.07.bypass`.
  `-dh`
  `-fdump-rtl-eh`
    Dump after finalization of EH handling code, to `file.02.eh`.
  `-di`
  `-fdump-rtl-sibling`
    Dump after sibling call optimizations, to `file.01.sibling`.
  `-dj`
  `-fdump-rtl-jump`
    Dump after the first jump optimization, to `file.03.jump`.
  `-dk`
  `-fdump-rtl-stack`
    Dump after conversion from registers to stack, to `file.33.stack`.
  `-dl`
  `-fdump-rtl-lreg`
    Dump after local register allocation, to `file.22.lreg`.
  `-dL`
  `-fdump-rtl-loop`
  `-fdump-rtl-loop2`
    `-dL` and `-fdump-rtl-loop` enable dumping after the first loop optimization pass, to `file.06.loop`. `-dL` and `-fdump-rtl-loop2` enable dumping after the second pass, to `file.13.loop2`.
  `-dm`
  `-fdump-rtl-sms`
    Dump after modulo scheduling, to `file.20.sms`.
  `-dM`
  `-fdump-rtl-mach`
    Dump after performing the machine dependent reorganization pass, to `file.35.mach`.
  `-dn`
  `-fdump-rtl-rnreg`
    Dump after register renumbering, to `file.29.rnreg`.
  `-dN`
  `-fdump-rtl-regmove`
    Dump after the register move pass, to `file.19.regmove`.
  `-do`
  `-fdump-rtl-postreload`
    Dump after post-reload optimizations, to `file.24.postreload`.
  `-dr`
  `-fdump-rtl-expand`
    Dump after RTL generation, to `file.00.expand`.
  `-dR`
  `-fdump-rtl-sched2`
    Dump after the second scheduling pass, to `file.32.sched2`.
  `-ds`
  `-fdump-rtl-cse`
    Dump after CSE (including the jump optimization that sometimes follows CSE), to `file.04.cse`.
  `-dS`
  `-fdump-rtl-sched`
    Dump after the first scheduling pass, to `file.21.sched`.
  `-dt`
  `-fdump-rtl-cse2`
    Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `file.15.cse2`.
  `-dT`
  `-fdump-rtl-tracer`
    Dump after running tracer, to `file.12.tracer`.
  `-dV`
  `-fdump-rtl-vpt`
  `-fdump-rtl-vartrack`
    `-dV` and `-fdump-rtl-vpt` enable dumping after the value profile transformations, to `file.10.vpt`. `-dV` and `-fdump-rtl-vartrack` enable dumping after variable tracking, to `file.34.vartrack`.
  `-dw`
  `-fdump-rtl-flow2`
    Dump after the second flow pass, to `file.26.flow2`.
  `-dz`
  `-fdump-rtl-peephole2`
    Dump after the peephole pass, to `file.27.peephole2`.
  `-dZ`
  `-fdump-rtl-web`
    Dump after live range splitting, to `file.14.web`.
  `-da`
  `-fdump-rtl-all`
    Produce all the dumps listed above.
  `-dH`
    Produce a core dump whenever an error occurs.
  `-dm`
    Print statistics on memory usage, at the end of the run, to standard error.
  `-dp`
    Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.
  `-dP`
    Dump the RTL in the assembler output as a comment before each instruction. Also turns on `-dp` annotation.
  `-dv`
    For each of the other indicated dump files (either with `-d` or `-fdump-rtl-pass`), dump a representation of the control flow graph suitable for viewing with VCG to `file.pass.vcg`.
  `-dx`
    Just generate RTL for a function instead of compiling it. Usually used with ``r`' (`-fdump-rtl-expand`).
  `-dy`
    Dump debugging information during parsing, to standard error.
`-fdump-unnumbered`
  When doing debugging dumps (see `-d` option above), suppress instruction numbers and line number note output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without `-g`.
`-fdump-translation-unit` (C++ only)
`-fdump-translation-unit-`options (C++ only)
  Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending `.tu` to the source file name. If the ``-options`' form is used, options controls the details of the dump as described for the `-fdump-tree` options.
`-fdump-class-hierarchy` (C++ only)
`-fdump-class-hierarchy-`options (C++ only)
  Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending `.class` to the source file name. If the ``-options`' form is used, options controls the details of the dump as described for the `-fdump-tree` options.
`-fdump-ipa-`switch
  Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name. The following dumps are possible:
  ``all`'
    Enables all inter-procedural analysis dumps; currently the only produced dump is the ``cgraph`' dump.
  ``cgraph`'
    Dumps information about call-graph optimization, unused function removal, and inlining decisions.
`-fdump-tree-`switch
`-fdump-tree-`switch`-`options
  Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch specific suffix to the source file name. If the ``-options`' form is used, options is a list of ``-`' separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be ignored. The following options are available
  ``address`'
    Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
  ``slim`'
    Inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path. When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
  ``raw`'
    Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
  ``details`'
    Enable more detailed dumps (not honored by every dump option).
  ``stats`'
    Enable dumping various statistics about the pass (not honored by every dump option).
  ``blocks`'
    Enable showing basic block boundaries (disabled in raw dumps).
  ``vops`'
    Enable showing virtual operands for every statement.
  ``lineno`'
    Enable showing line numbers for statements.
  ``uid`'
    Enable showing the unique ID (`DECL_UID`) for each variable.
  ``all`'
    Turn on all options, except `raw`, `slim` and `lineno`.

  The following tree dumps are possible:

  ``original`'
    Dump before any tree based optimization, to `file.original`.
  ``optimized`'
    Dump after all tree based optimization, to `file.optimized`.
  ``inlined`'
    Dump after function inlining, to `file.inlined`.
  ``gimple`'
    Dump each function before and after the gimplification pass to a file. The file name is made by appending `.gimple` to the source file name.
  ``cfg`'
    Dump the control flow graph of each function to a file. The file name is made by appending `.cfg` to the source file name.
  ``vcg`'
    Dump the control flow graph of each function to a file in VCG format. The file name is made by appending `.vcg` to the source file name. Note that if the file contains more than one function, the generated file cannot be used directly by VCG. You will need to cut and paste each function's graph into its own separate file first.
  ``ch`'
    Dump each function after copying loop headers. The file name is made by appending `.ch` to the source file name.
  ``ssa`'
    Dump SSA related information to a file. The file name is made by appending `.ssa` to the source file name.
  ``salias`'
    Dump structure aliasing variable information to a file. This file name is made by appending `.salias` to the source file name.
  ``alias`'
    Dump aliasing information for each function. The file name is made by appending `.alias` to the source file name.
  ``ccp`'
    Dump each function after CCP. The file name is made by appending `.ccp` to the source file name.
  ``storeccp`'
    Dump each function after STORE-CCP. The file name is made by appending `.storeccp` to the source file name.
  ``pre`'
    Dump trees after partial redundancy elimination. The file name is made by appending `.pre` to the source file name.
  ``fre`'
    Dump trees after full redundancy elimination. The file name is made by appending `.fre` to the source file name.
  ``copyprop`'
    Dump trees after copy propagation. The file name is made by appending `.copyprop` to the source file name.
  ``store_copyprop`'
    Dump trees after store copy-propagation. The file name is made by appending `.store_copyprop` to the source file name.
  ``dce`'
    Dump each function after dead code elimination. The file name is made by appending `.dce` to the source file name.
  ``mudflap`'
    Dump each function after adding mudflap instrumentation. The file name is made by appending `.mudflap` to the source file name.
  ``sra`'
    Dump each function after performing scalar replacement of aggregates. The file name is made by appending `.sra` to the source file name.
  ``sink`'
    Dump each function after performing code sinking. The file name is made by appending `.sink` to the source file name.
  ``dom`'
    Dump each function after applying dominator tree optimizations. The file name is made by appending `.dom` to the source file name.
  ``dse`'
    Dump each function after applying dead store elimination. The file name is made by appending `.dse` to the source file name.
  ``phiopt`'
    Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending `.phiopt` to the source file name.
  ``forwprop`'
    Dump each function after forward propagating single use variables. The file name is made by appending `.forwprop` to the source file name.
  ``copyrename`'
    Dump each function after applying the copy rename optimization. The file name is made by appending `.copyrename` to the source file name.
  ``nrv`'
    Dump each function after applying the named return value optimization on generic trees. The file name is made by appending `.nrv` to the source file name.
  ``vect`'
    Dump each function after applying vectorization of loops. The file name is made by appending `.vect` to the source file name.
  ``vrp`'
    Dump each function after Value Range Propagation (VRP). The file name is made by appending `.vrp` to the source file name.
  ``all`'
    Enable all the available tree dumps with the flags provided in this option.
`-ftree-vectorizer-verbose=`n
  This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless `-fdump-tree-all` or `-fdump-tree-vect` is specified, in which case it is output to the usual dump listing file, `.vect`.
`-frandom-seed=`string
  This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to generate certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the `-frandom-seed` option to produce reproducibly identical object files.

  The string should be different for every file you compile.

`-fsched-verbose=`n
  On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless `-dS` or `-dR` is specified, in which case it is output to the usual dump listing file, `.sched` or `.sched2` respectively. However for n greater than nine, the output is always printed to standard error.

  For n greater than zero, `-fsched-verbose` outputs the same information as `-dRS`. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, `-fsched-verbose` also includes dependence info.

`-save-temps`
  Store the usual “temporary” intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c` with ``-c -save-temps`' would produce files `foo.i` and `foo.s`, as well as `foo.o`. This creates a preprocessed `foo.i` output file even though the compiler now normally uses an integrated preprocessor.

  When used in combination with the `-x` command line option, `-save-temps` is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using `-save-temps`.

`-time`
  Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). The output looks like this:

                # cc1 0.12 0.01
                # as 0.00 0.01

  The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

`-fvar-tracking`
  Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).

  It is enabled by default when compiling with optimization (`-Os`, `-O`, `-O2`, ...), debugging information (`-g`) and the debug info format supports it.

`-print-file-name=`library
  Print the full absolute name of the library file library that would be used when linking—and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
`-print-multi-directory`
  Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in `GCC_EXEC_PREFIX`.
`-print-multi-lib`
  Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ``;`', and each switch starts with an ``@`' instead of the ``-`', without spaces between multiple switches. This is supposed to ease shell-processing.
`-print-prog-name=`program
  Like `-print-file-name`, but searches for a program such as ``cpp`'.
`-print-libgcc-file-name`
  Same as `-print-file-name=libgcc.a`.

  This is useful when you use `-nostdlib` or `-nodefaultlibs` but you do want to link with `libgcc.a`. You can do

                gcc -nostdlib files... `gcc -print-libgcc-file-name`

`-print-search-dirs`
  Print the name of the configured installation directory and a list of program and library directories `gcc` will search—and don't do anything else.

  This is useful when `gcc` prints the error message ``installation problem, cannot exec cpp0: No such file or directory`'. To resolve this you either need to put `cpp0` and the other compiler components where `gcc` expects to find them, or you can set the environment variable `GCC_EXEC_PREFIX` to the directory where you installed them. Don't forget the trailing ``/`'. See Environment Variables.

`-dumpmachine`
  Print the compiler's target machine (for example, ``i686-pc-linux-gnu`')—and don't do anything else.
`-dumpversion`
  Print the compiler version (for example, ``3.0`')—and don't do anything else.
`-dumpspecs`
  Print the compiler's built-in specs—and don't do anything else. (This is used when GCC itself is being built.) See Spec Files.
`-feliminate-unused-debug-types`
  Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

Next: Preprocessor Options, Previous: Debugging Options, Up: Invoking GCC

### 3.10 Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program. Optimization levels `-O2` and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.

`-O`
`-O1`
  Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

  With `-O`, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

  `-O` turns on the following optimization flags:

                -fdefer-pop
                -fdelayed-branch
                -fguess-branch-probability
                -fcprop-registers
                -floop-optimize
                -fif-conversion
                -fif-conversion2
                -ftree-ccp
                -ftree-dce
                -ftree-dominator-opts
                -ftree-dse
                -ftree-ter
                -ftree-lrs
                -ftree-sra
                -ftree-copyrename
                -ftree-fre
                -ftree-ch
                -fmerge-constants

  `-O` also turns on `-fomit-frame-pointer` on machines where doing so does not interfere with debugging.

  `-O` doesn't turn on `-ftree-sra` for the Ada compiler. This option must be explicitly specified on the command line to be enabled for the Ada compiler.

`-O2`
  Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify `-O2`. As compared to `-O`, this option increases both compilation time and the performance of the generated code.

  `-O2` turns on all optimization flags specified by `-O`. It also turns on the following optimization flags:

                -fthread-jumps
                -fcrossjumping
                -foptimize-sibling-calls
                -fcse-follow-jumps  -fcse-skip-blocks
                -fgcse  -fgcse-lm
                -fexpensive-optimizations
                -fstrength-reduce
                -frerun-cse-after-loop  -frerun-loop-opt
                -fcaller-saves
                -fpeephole2
                -fschedule-insns  -fschedule-insns2
                -fsched-interblock  -fsched-spec
                -fregmove
                -fstrict-aliasing
                -fdelete-null-pointer-checks
                -freorder-blocks  -freorder-functions
                -funit-at-a-time
                -falign-functions  -falign-jumps
                -falign-loops  -falign-labels
                -ftree-vrp
                -ftree-pre

  Please note the warning under `-fgcse` about invoking `-O2` on programs that use computed gotos.

`-O3`
  Optimize yet more. `-O3` turns on all optimizations specified by `-O2` and also turns on the `-finline-functions`, `-funswitch-loops` and `-fgcse-after-reload` options.
`-O0`
  Do not optimize. This is the default.
`-Os`
  Optimize for size. `-Os` enables all `-O2` optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

  `-Os` disables the following optimization flags:

                -falign-functions  -falign-jumps  -falign-loops
                -falign-labels  -freorder-blocks  -freorder-blocks-and-partition
                -fprefetch-loop-arrays  -ftree-vect-loop-version

  If you use multiple `-O` options, with or without level numbers, the last such option is the one that is effective.

Options of the form `-fflag` specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo` would be `-fno-foo`. In the table below, only one of the forms is listed—the one you typically will use. You can figure out the other form by either removing ``no-`' or adding it.

The following options control specific optimizations. They are either activated by `-O` options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.

`-fno-default-inline`
  Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify `-O`, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add ``inline`' in front of the member function name.
`-fno-defer-pop`
  Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

  Disabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fforce-mem`
  Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. This option is now a nop and will be removed in 4.2.
`-fforce-addr`
  Force memory address constants to be copied into registers before doing arithmetic on them.
`-fomit-frame-pointer`
  Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.

  On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro `FRAME_POINTER_REQUIRED` controls whether a target machine supports this flag. See Register Usage.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-foptimize-sibling-calls`
  Optimize sibling and tail recursive calls.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fno-inline`
  Don't pay attention to the `inline` keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
`-finline-functions`
  Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

  If all calls to a given function are integrated, and the function is declared `static`, then the function is normally not output as assembler code in its own right.

  Enabled at level `-O3`.

`-finline-functions-called-once`
  Consider all `static` functions called once for inlining into their caller even if they are not marked `inline`. If a call to a given function is integrated, then the function is not output as assembler code in its own right.

  Enabled if `-funit-at-a-time` is enabled.

`-fearly-inlining`
  Inline functions marked by `always_inline` and functions whose body seems smaller than the function call overhead early before doing `-fprofile-generate` instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.

  Enabled by default.

`-finline-limit=`n
  By default, GCC limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 600. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with C++.

  Inlining is actually controlled by a number of parameters, which may be specified individually by using `--param name=value`. The `-finline-limit=n` option sets some of these parameters as follows:

  `max-inline-insns-single`
    is set to n/2.
  `max-inline-insns-auto`
    is set to n/2.
  `min-inline-insns`
    is set to 130 or n/4, whichever is smaller.
  `max-inline-insns-rtl`
    is set to n.

  See below for a documentation of the individual parameters controlling inlining.

  Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.

`-fkeep-inline-functions`
  In C, emit `static` functions that are declared `inline` into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the `extern inline` extension in GNU C. In C++, emit any and all inline functions into the object file.
`-fkeep-static-consts`
  Emit variables declared `static const` when optimization isn't turned on, even if the variables aren't referenced.

  GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts` option.

`-fmerge-constants`
  Attempt to merge identical constants (string constants and floating point constants) across compilation units.

  This option is the default for optimized compilation if the assembler and linker support it. Use `-fno-merge-constants` to inhibit this behavior.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fmerge-all-constants`
  Attempt to merge identical constants and identical variables.

  This option implies `-fmerge-constants`. In addition to `-fmerge-constants` this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types. Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior.

`-fmodulo-sched`
  Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
`-fno-branch-count-reg`
  Do not use “decrement and branch” instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

  The default is `-fbranch-count-reg`, enabled when `-fstrength-reduce` is enabled.

`-fno-function-cse`
  Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

  This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

  The default is `-ffunction-cse`

`-fno-zero-initialized-in-bss`
  If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.

  This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

  The default is `-fzero-initialized-in-bss`.

`-fbounds-check`
  For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran front-ends, where this option defaults to true and false respectively.
`-fmudflap -fmudflapth -fmudflapir`
  For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (`libmudflap`), which will be linked into a program if `-fmudflap` is given at link time. Run-time behavior of the instrumented program is controlled by the `MUDFLAP_OPTIONS` environment variable. See `env MUDFLAP_OPTIONS=-help a.out` for its options.

  Use `-fmudflapth` instead of `-fmudflap` to compile and to link if your program is multi-threaded. Use `-fmudflapir`, in addition to `-fmudflap` or `-fmudflapth`, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

`-fstrength-reduce`
  Perform the optimizations of loop strength reduction and elimination of iteration variables.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fthread-jumps`
  Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fcse-follow-jumps`
  In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an `if` statement with an `else` clause, CSE will follow the jump when the condition tested is false.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fcse-skip-blocks`
  This is similar to `-fcse-follow-jumps`, but causes CSE to follow jumps which conditionally skip over blocks. When CSE encounters a simple `if` statement with no else clause, `-fcse-skip-blocks` causes CSE to follow the jump around the body of the `if`.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-frerun-cse-after-loop`
  Re-run common subexpression elimination after loop optimizations has been performed.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-frerun-loop-opt`
  Run the loop optimizer twice.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fgcse`
  Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

  Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding `-fno-gcse` to the command line.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fgcse-lm`
  When `-fgcse-lm` is enabled, global common subexpression elimination will attempt to move loads which are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.

  Enabled by default when gcse is enabled.

`-fgcse-sm`
  When `-fgcse-sm` is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt to move stores out of loops. When used in conjunction with `-fgcse-lm`, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

  Not enabled at any optimization level.

`-fgcse-las`
  When `-fgcse-las` is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

  Not enabled at any optimization level.

`-fgcse-after-reload`
  When `-fgcse-after-reload` is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to cleanup redundant spilling.
`-floop-optimize`
  Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction as well.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-floop-optimize2`
  Perform loop optimizations using the new loop optimizer. The optimizations (loop unrolling, peeling and unswitching, loop invariant motion) are enabled by separate flags.
`-funsafe-loop-optimizations`
  If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using `-Wunsafe-loop-optimizations`, the compiler will warn you if it finds this kind of loop.
`-fcrossjumping`
  Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fif-conversion`
  Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by `if-conversion2`.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fif-conversion2`
  Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fdelete-null-pointer-checks`
  Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null.

  In some environments, this assumption is not true, and programs can safely dereference null pointers. Use `-fno-delete-null-pointer-checks` to disable this optimization for programs which depend on that behavior.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fexpensive-optimizations`
  Perform a number of minor optimizations that are relatively expensive.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-foptimize-register-move`
`-fregmove`
  Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.

  Note `-fregmove` and `-foptimize-register-move` are the same optimization.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fdelayed-branch`
  If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

  Enabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fschedule-insns`
  If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fschedule-insns2`
  Similar to `-fschedule-insns`, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fno-sched-interblock`
  Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.
`-fno-sched-spec`
  Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.
`-fsched-spec-load`
  Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.
`-fsched-spec-load-dangerous`
  Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.
`-fsched-stalled-insns=`n
  Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list, during the second scheduling pass.
`-fsched-stalled-insns-dep=`n
  Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature removal from the queue of stalled insns. Has an effect only during the second scheduling pass, and only if `-fsched-stalled-insns` is used and its value is not zero.
`-fsched2-use-superblocks`
  When scheduling after register allocation, do use superblock scheduling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on faster schedules. This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

  This only makes sense when scheduling after register allocation, i.e. with `-fschedule-insns2` or at `-O2` or higher.

`-fsched2-use-traces`
  Use `-fsched2-use-superblocks` algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass. See `-ftracer` for details on trace formation.

  This mode should produce faster but significantly longer programs. Also without `-fbranch-probabilities` the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with `-fschedule-insns2` or at `-O2` or higher.

`-freschedule-modulo-scheduled-loops`
  The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.
`-fcaller-saves`
  Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.

  This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-ftree-pre`
  Perform Partial Redundancy Elimination (PRE) on trees. This flag is enabled by default at `-O2` and `-O3`.
`-ftree-fre`
  Perform Full Redundancy Elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at `-O` and higher.
`-ftree-copy-prop`
  Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at `-O` and higher.
`-ftree-store-copy-prop`
  Perform copy propagation of memory loads and stores. This pass eliminates unnecessary copy operations in memory references (structures, global variables, arrays, etc). This flag is enabled by default at `-O2` and higher.
`-ftree-salias`
  Perform structural alias analysis on trees. This flag is enabled by default at `-O` and higher.
`-ftree-sink`
  Perform forward store motion on trees. This flag is enabled by default at `-O` and higher.
`-ftree-ccp`
  Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at `-O` and higher.
`-ftree-store-ccp`
  Perform sparse conditional constant propagation (CCP) on trees. This pass operates on both local scalar variables and memory stores and loads (global variables, structures, arrays, etc). This flag is enabled by default at `-O2` and higher.
`-ftree-dce`
  Perform dead code elimination (DCE) on trees. This flag is enabled by default at `-O` and higher.
`-ftree-dominator-opts`
  Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at `-O` and higher.
`-ftree-ch`
  Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at `-O` and higher. It is not enabled for `-Os`, since it usually increases code size.
`-ftree-loop-optimize`
  Perform loop optimizations on trees. This flag is enabled by default at `-O` and higher.
`-ftree-loop-linear`
  Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.
`-ftree-loop-im`
  Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With `-funswitch-loops` it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
`-ftree-loop-ivcanon`
  Create a canonical counter for number of iterations in the loop for that determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
`-fivopts`
  Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.
`-ftree-sra`
  Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at `-O` and higher.
`-ftree-copyrename`
  Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at `-O` and higher.
`-ftree-ter`
  Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at `-O` and higher.
`-ftree-lrs`
  Perform live range splitting during the SSA->normal phase. Distinct live ranges of a variable are split into unique variables, allowing for better optimization later. This is enabled by default at `-O` and higher.
`-ftree-vectorize`
  Perform loop vectorization on trees.
`-ftree-vect-loop-version`
  Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level `-Os` where it is disabled.
`-ftree-vrp`
  Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at `-O2` and higher. Null pointer check elimination is only done if `-fdelete-null-pointer-checks` is enabled.
`-ftracer`
  Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.
`-funroll-loops`
  Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. `-funroll-loops` implies both `-fstrength-reduce` and `-frerun-cse-after-loop`. This option makes code larger, and may or may not make it run faster.
`-funroll-all-loops`
  Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. `-funroll-all-loops` implies the same options as `-funroll-loops`,
`-fsplit-ivs-in-unroller`
  Enables expressing of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.

  Combination of `-fweb` and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.

  This optimization is enabled by default.

`-fvariable-expansion-in-unroller`
  With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.
`-fprefetch-loop-arrays`
  If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

  These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.

`-fno-peephole`
`-fno-peephole2`
  Disable any machine-specific peephole optimizations. The difference between `-fno-peephole` and `-fno-peephole2` is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

  `-fpeephole` is enabled by default. `-fpeephole2` enabled at levels `-O2`, `-O3`, `-Os`.

`-fno-guess-branch-probability`
  Do not guess branch probabilities using heuristics.

  GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (`-fprofile-arcs`). These heuristics are based on the control flow graph. If some branch probabilities are specified by ``__builtin_expect`', then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the ``__builtin_expect`' info into account. The interactions between the heuristics and ``__builtin_expect`' can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of ``__builtin_expect`' are easier to understand.

  The default is `-fguess-branch-probability` at levels `-O`, `-O2`, `-O3`, `-Os`.

`-freorder-blocks`
  Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

  Enabled at levels `-O2`, `-O3`.

`-freorder-blocks-and-partition`
  In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.

  This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

`-freorder-functions`
  Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections `.text.hot` for most frequently executed functions and `.text.unlikely` for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

  Also profile feedback must be available in to make this option effective. See `-fprofile-arcs` for details.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-fstrict-aliasing`
  Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an `unsigned int` can alias an `int`, but not a `void*` or a `double`. A character type may alias any other type.

  Pay special attention to code like this:

                union a_union {
                  int i;
                  double d;
                };

                int f() {
                  a_union t;
                  t.d = 3.0;
                  return t.i;
                }

  The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with `-fstrict-aliasing`, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:

                int f() {
                  a_union t;
                  int* ip;
                  t.d = 3.0;
                  ip = &t.i;
                  return *ip;
                }

  Every language that wishes to perform language-specific alias analysis should define a function that computes, given an `tree` node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function `c_get_alias_set`.

  Enabled at levels `-O2`, `-O3`, `-Os`.

`-falign-functions`
`-falign-functions=`n
  Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance, `-falign-functions=32` aligns functions to the next 32-byte boundary, but `-falign-functions=24` would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

  `-fno-align-functions` and `-falign-functions=1` are equivalent and mean that functions will not be aligned.

  Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.

  If n is not specified or is zero, use a machine-dependent default.

  Enabled at levels `-O2`, `-O3`.

`-falign-labels`
`-falign-labels=`n
  Align all branch targets to a power-of-two boundary, skipping up to n bytes like `-falign-functions`. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.

  `-fno-align-labels` and `-falign-labels=1` are equivalent and mean that labels will not be aligned.

  If `-falign-loops` or `-falign-jumps` are applicable and are greater than this value, then their values are used instead.

  If n is not specified or is zero, use a machine-dependent default which is very likely to be ``1`', meaning no alignment.

  Enabled at levels `-O2`, `-O3`.

`-falign-loops`
`-falign-loops=`n
  Align loops to a power-of-two boundary, skipping up to n bytes like `-falign-functions`. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

  `-fno-align-loops` and `-falign-loops=1` are equivalent and mean that loops will not be aligned.

  If n is not specified or is zero, use a machine-dependent default.

  Enabled at levels `-O2`, `-O3`.

`-falign-jumps`
`-falign-jumps=`n
  Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to n bytes like `-falign-functions`. In this case, no dummy operations need be executed.

  `-fno-align-jumps` and `-falign-jumps=1` are equivalent and mean that loops will not be aligned.

  If n is not specified or is zero, use a machine-dependent default.

  Enabled at levels `-O2`, `-O3`.

`-funit-at-a-time`
  Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory (in general). There are some compatibility issues with unit-at-at-time mode:
  - enabling unit-at-a-time mode may change the order in which functions, variables, and top-level `asm` statements are emitted, and will likely break code relying on some particular ordering. The majority of such top-level `asm` statements, though, can be replaced by `section` attributes.
  - unit-at-a-time mode removes unreferenced static variables and functions. This may result in undefined references when an `asm` statement refers directly to variables or functions that are otherwise unused. In that case either the variable/function shall be listed as an operand of the `asm` statement operand or, in the case of top-level `asm` statements the attribute `used` shall be used on the declaration.
  - Static functions now can use non-standard passing conventions that may break `asm` statements calling functions directly. Again, attribute `used` will prevent this behavior.

  As a temporary workaround, `-fno-unit-at-a-time` can be used, but this scheme may not be supported by future releases of GCC.

  Enabled at levels `-O2`, `-O3`.

`-fweb`
  Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables will no longer stay in a “home register”.

  Enabled by default with `-funroll-loops`.

`-fwhole-program`
  Assume that the current compilation unit represents whole program being compiled. All public functions and variables with the exception of `main` and those merged by attribute `externally_visible` become static functions and in a affect gets more aggressively optimized by interprocedural optimizers. While this option is equivalent to proper use of `static` keyword for programs consisting of single file, in combination with option `--combine` this flag can be used to compile most of smaller scale C programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.
`-fno-cprop-registers`
  After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

  Disabled at levels `-O`, `-O2`, `-O3`, `-Os`.

`-fprofile-generate`
  Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use `-fprofile-generate` both when compiling and when linking your program.

  The following options are enabled: `-fprofile-arcs`, `-fprofile-values`, `-fvpt`.

`-fprofile-use`
  Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

  The following options are enabled: `-fbranch-probabilities`, `-fvpt`, `-funroll-loops`, `-fpeel-loops`, `-ftracer`, `-fno-loop-optimize`.

The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.

`-ffloat-store`
  Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory.

  This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a `double` is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat-store` for such programs, after modifying them to store all pertinent intermediate computations into variables.

`-ffast-math`
  Sets `-fno-math-errno`, `-funsafe-math-optimizations`,
  `-fno-trapping-math`, `-ffinite-math-only`, `-fno-rounding-math`, `-fno-signaling-nans` and `fcx-limited-range`.

  This option causes the preprocessor macro `__FAST_MATH__` to be defined.

  This option should never be turned on by any `-O` option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

`-fno-math-errno`
  Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

  This option should never be turned on by any `-O` option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

  The default is `-fmath-errno`.

  On Darwin systems, the math library never sets `errno`. There is therefore no reason for the compiler to consider the possibility that it might, and `-fno-math-errno` is the default.

`-funsafe-math-optimizations`
  Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.

  This option should never be turned on by any `-O` option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

  The default is `-fno-unsafe-math-optimizations`.

`-ffinite-math-only`
  Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

  This option should never be turned on by any `-O` option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.

  The default is `-fno-finite-math-only`.

`-fno-trapping-math`
  Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option implies `-fno-signaling-nans`. Setting this option may allow faster code if one relies on “non-stop” IEEE arithmetic, for example.

  This option should never be turned on by any `-O` option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

  The default is `-ftrapping-math`.

`-frounding-math`
  Disable transformations and optimizations that assume default floating point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

  The default is `-fno-rounding-math`.

  This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's `FENV_ACCESS` pragma. This command line option will be used to specify the default state for `FENV_ACCESS`.

`-fsignaling-nans`
  Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies `-ftrapping-math`.

  This option causes the preprocessor macro `__SUPPORT_SNAN__` to be defined.

  The default is `-fno-signaling-nans`.

  This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

`-fsingle-precision-constant`
  Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.
`-fcx-limited-range`
`-fno-cx-limited-range`
  When enabled, this option states that a range reduction step is not needed when performing complex division. The default is `-fno-cx-limited-range`, but is enabled by `-ffast-math`.

  This option controls the default setting of the ISO C99 `CX_LIMITED_RANGE` pragma. Nevertheless, the option applies to all languages.

The following options control optimizations that may improve performance, but are not enabled by any `-O` options. This section includes experimental options that may produce broken code.

`-fbranch-probabilities`
  After running a program compiled with `-fprofile-arcs` (see Options for Debugging Your Program or `gcc`), you can compile it a second time using `-fbranch-probabilities`, to improve optimizations based on the number of times each branch was taken. When the program compiled with `-fprofile-arcs` exits it saves arc execution counts to a file called `sourcename.gcda` for each source file The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.

  With `-fbranch-probabilities`, GCC puts a ``REG_BR_PROB`' note on each ``JUMP_INSN`' and ``CALL_INSN`'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c`, instead of guessing which path a branch is mostly to take, the ``REG_BR_PROB`' values are used to exactly determine which path is taken more often.

`-fprofile-values`
  If combined with `-fprofile-arcs`, it adds code so that some data about values of expressions in the program is gathered.

  With `-fbranch-probabilities`, it reads back the data gathered from profiling values of expressions and adds ``REG_VALUE_PROFILE`' notes to instructions for their later usage in optimizations.

  Enabled with `-fprofile-generate` and `-fprofile-use`.

`-fvpt`
  If combined with `-fprofile-arcs`, it instructs the compiler to add a code to gather information about values of expressions.

  With `-fbranch-probabilities`, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.

`-frename-registers`
  Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization will most benefit processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer stay in a “home register”.

  Enabled by default with `-funroll-loops`.

`-ftracer`
  Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

  Enabled with `-fprofile-use`.

`-funroll-loops`
  Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. `-funroll-loops` implies `-frerun-cse-after-loop`, `-fweb` and `-frename-registers`. It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code larger, and may or may not make it run faster.

  Enabled with `-fprofile-use`.

`-funroll-all-loops`
  Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. `-funroll-all-loops` implies the same options as `-funroll-loops`.
`-fpeel-loops`
  Peels the loops for that there is enough information that they do not roll much (from profile feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

  Enabled with `-fprofile-use`.

`-fmove-loop-invariants`
  Enables the loop invariant motion pass in the new loop optimizer. Enabled at level `-O1`
`-funswitch-loops`
  Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
`-fprefetch-loop-arrays`
  If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

  Disabled at level `-Os`.

`-ffunction-sections`
`-fdata-sections`
  Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file.

  Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.

  Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use `gprof` on all systems if you specify this option and you may have problems with debugging if you specify both this option and `-g`.

`-fbranch-target-load-optimize`
  Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.
`-fbranch-target-load-optimize2`
  Perform branch target register load optimization after prologue / epilogue threading.
`-fbtr-bb-exclusive`
  When performing branch target register load optimization, don't reuse branch target registers in within any basic block.
`-fstack-protector`
  Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.
`-fstack-protector-all`
  Like `-fstack-protector` except that all functions are protected.
`--param` name`=`value
  In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not inline functions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the `--param` option.

  The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

  In each case, the value is an integer. The allowable choices for name are given in the following table:

  `salias-max-implicit-fields`
    The maximum number of fields in a variable without direct structure accesses for which structure aliasing will consider trying to track each field. The default is 5
  `sra-max-structure-size`
    The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization will perform block copies. The default value, 0, implies that GCC will select the most appropriate size itself.
  `sra-field-structure-ratio`
    The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used. The default is 75.
  `max-crossjump-edges`
    The maximum number of incoming edges to consider for crossjumping. The algorithm used by `-fcrossjumping` is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable size.
  `min-crossjump-insns`
    The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them. This value is ignored in the case where all instructions in the block being crossjumped from are matched. The default value is 5.
  `max-grow-copy-bb-insns`
    The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The default value is 8.
  `max-goto-duplication-insns`
    The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The default value is 8.
  `max-delay-slot-insn-search`
    The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run time.
  `max-delay-slot-live-search`
    When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
  `max-gcse-memory`
    The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done.
  `max-gcse-passes`
    The maximum number of passes of GCSE to run. The default is 1.
  `max-pending-list-length`
    The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
  `max-inline-insns-single`
    Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 450.
  `max-inline-insns-auto`
    When you use `-finline-functions` (included in `-O3`), a lot of functions that would otherwise not be considered for inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 90.
  `large-function-insns`
    The limit specifying really large functions. For functions larger than this limit after inlining inlining is constrained by `--param large-function-growth`. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the backend. This parameter is ignored when `-funit-at-a-time` is not used. The default value is 2700.
  `large-function-growth`
    Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored when `-funit-at-a-time` is not used. The default value is 100 which limits large function growth to 2.0 times the original size.
  `large-unit-insns`
    The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by `--param inline-unit-growth`. For small units this might be too tight (consider unit consisting of function A that is inline and B that just calls A three time. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlininable functions however the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to `--param large-unit-insns` before aplying `--param inline-unit-growth`. The default is 10000
  `inline-unit-growth`
    Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when `-funit-at-a-time` is not used. The default value is 50 which limits unit growth to 1.5 times the original size.
  `max-inline-insns-recursive`
  `max-inline-insns-recursive-auto`
    Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow into by performing recursive inlining.

    For functions declared inline `--param max-inline-insns-recursive` is taken into acount. For function not declared inline, recursive inlining happens only when `-finline-functions` (included in `-O3`) is enabled and `--param max-inline-insns-recursive-auto` is used. The default value is 450.

  `max-inline-recursive-depth`
  `max-inline-recursive-depth-auto`
    Specifies maximum recursion depth used by the recursive inlining.

    For functions declared inline `--param max-inline-recursive-depth` is taken into acount. For function not declared inline, recursive inlining happens only when `-finline-functions` (included in `-O3`) is enabled and `--param max-inline-recursive-depth-auto` is used. The default value is 450.

  `min-inline-recursive-probability`
    Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers.

    When profile feedback is available (see `-fprofile-generate`) the actual recursion depth can be guessed from probability that function will recurse via given call expression. This parameter limits inlining only to call expression whose probability exceeds given threshold (in percents). The default value is 10.

  `inline-call-cost`
    Specify cost of call instruction relative to simple arithmetics operations (having cost of 1). Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of leaf function that is believed to reduce function size by being inlined. In effect it increases amount of inlining for code having large abstraction penalty (many functions that just pass the arguments to other functions) and decrease inlining for code with low abstraction penalty. The default value is 16.
  `max-unrolled-insns`
    The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
  `max-average-unrolled-insns`
    The maximum number of instructions biased by probabilities of their execution that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
  `max-unroll-times`
    The maximum number of unrollings of a single loop.
  `max-peeled-insns`
    The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.
  `max-peel-times`
    The maximum number of peelings of a single loop.
  `max-completely-peeled-insns`
    The maximum number of insns of a completely peeled loop.
  `max-completely-peel-times`
    The maximum number of iterations of a loop to be suitable for complete peeling.
  `max-unswitch-insns`
    The maximum number of insns of an unswitched loop.
  `max-unswitch-level`
    The maximum number of branches unswitched in a single loop.
  `lim-expensive`
    The minimum cost of an expensive expression in the loop invariant motion.
  `iv-consider-all-candidates-bound`
    Bound on number of candidates for induction variables below that all candidates are considered for each use in induction variable optimizations. Only the most relevant candidates are considered if there are more candidates, to avoid quadratic time complexity.
  `iv-max-considered-uses`
    The induction variable optimizations give up on loops that contain more induction variable uses.
  `iv-always-prune-cand-set-bound`
    If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set.
  `scev-max-expr-size`
    Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
  `vect-max-version-checks`
    The maximum number of runtime checks that can be performed when doing loop versioning in the vectorizer. See option ftree-vect-loop-version for more information.
  `max-iterations-to-track`
    The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.
  `hot-bb-count-fraction`
    Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.
  `hot-bb-frequency-fraction`
    Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot
  `max-predicted-iterations`
    The maximum number of loop iterations we predict statically. This is useful in cases where function contain single loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds would appear artificially cold relative to the other one.
  `tracer-dynamic-coverage`
  `tracer-dynamic-coverage-feedback`
    This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.

    The `tracer-dynamic-coverage-feedback` is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.

  `tracer-max-code-growth`
    Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
  `tracer-min-branch-ratio`
    Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
  `tracer-min-branch-ratio`
  `tracer-min-branch-ratio-feedback`
    Stop forward growth if the best edge do have probability lower than this threshold.

    Similarly to `tracer-dynamic-coverage` two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

  `max-cse-path-length`
    Maximum number of basic blocks on path that cse considers. The default is 10.
  `max-cse-insns`
    The maximum instructions CSE process before flushing. The default is 1000.
  `global-var-threshold`
    Counts the number of function calls (n) and the number of call-clobbered variables (v). If nxv is larger than this limit, a single artificial variable will be created to represent all the call-clobbered variables at function call sites. This artificial variable will then be made to alias every call-clobbered variable. (done as `int * size_t` on the host machine; beware overflow).
  `max-aliased-vops`
    Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping heuristic. Alias grouping reduces compile times and memory consumption needed for aliasing at the expense of precision loss in alias information.
  `ggc-min-expand`
    GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.

    The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If `getrlimit` is available, the notion of "RAM" is the smallest of actual RAM and `RLIMIT_DATA` or `RLIMIT_AS`. If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and `ggc-min-heapsize` to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.

  `ggc-min-heapsize`
    Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by `ggc-min-expand`% beyond `ggc-min-heapsize`. Again, tuning this may improve compilation speed, and has no effect on code generation.

    The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and `ggc-min-expand` to zero causes a full collection to occur at every opportunity.

  `max-reload-search-insns`
    The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 100.
  `max-cselib-memory-location`
    The maximum number of memory locations cselib should take into acount. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500.
  `max-flow-memory-location`
    Similar as `max-cselib-memory-location` but for dataflow liveness. The default value is 100.
  `reorder-blocks-duplicate`
  `reorder-blocks-duplicate-feedback`
    Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.

    The `reorder-block-duplicate-feedback` is used only when profile feedback is available and may be set to higher values than `reorder-block-duplicate` since information about the hot spots is more accurate.

  `max-sched-region-blocks`
    The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.
  `max-sched-region-insns`
    The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.
  `min-sched-prob`
    The minimum probability of reaching a source block for interblock speculative scheduling. The default value is 40.
  `max-last-value-rtl`
    The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. The default is 10000.
  `integer-share-limit`
    Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant's. The default value is 256.
  `min-virtual-mappings`
    Specifies the minimum number of virtual mappings in the incremental SSA updater that should be registered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value is 100.
  `virtual-mappings-ratio`
    If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols to be updated, then the incremental SSA updater switches to a full update for those symbols. The default ratio is 3.
  `ssp-buffer-size`
    The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when `-fstack-protection` is used.
  `max-jump-thread-duplication-stmts`
    Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
  `max-fields-for-field-sensitive`
    Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analysis.

Next: Assembler Options, Previous: Optimize Options, Up: Invoking GCC

### 3.11 Options Controlling the Preprocessor

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the `-E` option, nothing is done except preprocessing. Some of these options make sense only together with `-E` because they cause the preprocessor output to be unsuitable for actual compilation.

You can use `-Wp,option` to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and `-Wp` forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using `-Wp` and let the driver handle the options instead.
`-Xpreprocessor` option
  Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options which GCC does not know how to recognize.

  If you want to pass an option that takes an argument, you must use `-Xpreprocessor` twice, once for the option and once for the argument.

`-D` name
  Predefine name as a macro, with definition `1`.
`-D` name`=`definition
  The contents of definition are tokenized and processed as if they appeared during translation phase three in a ``#define`' directive. In particular, the definition will be truncated by embedded newline characters.

  If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

  If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With `sh` and `csh`, `-D'name(args...)=definition'` works.

  `-D` and `-U` options are processed in the order they are given on the command line. All `-imacros file` and `-include file` options are processed after all `-D` and `-U` options.

`-U` name
  Cancel any previous definition of name, either built in or provided with a `-D` option.
`-undef`
  Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
`-I` dir
  Add the directory dir to the list of directories to be searched for header files. Directories named by `-I` are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated .
`-o` file
  Write output to file. This is the same as specifying file as the second non-option argument to `cpp`. `gcc` has a different interpretation of a second non-option argument, so you must use `-o` to specify the output file.
`-Wall`
  Turns on all optional warnings which are desirable for normal code. At present this is `-Wcomment`, `-Wtrigraphs`, `-Wmultichar` and a warning about integer promotion causing a change of sign in `#if` expressions. Note that many of the preprocessor's warnings are on by default and have no options to control them.
`-Wcomment`
`-Wcomments`
  Warn whenever a comment-start sequence ``/*`' appears in a ``/*`' comment, or whenever a backslash-newline appears in a ``//`' comment. (Both forms have the same effect.)
`-Wtrigraphs`
  Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form an escaped newline (``??/`' at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.

  This option is implied by `-Wall`. If `-Wall` is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other `-Wall` warnings, use ``-trigraphs -Wall -Wno-trigraphs`'.

`-Wtraditional`
  Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and problematic constructs which should be avoided.
`-Wimport`
  Warn the first time ``#import`' is used.
`-Wundef`
  Warn whenever an identifier which is not a macro is encountered in an ``#if`' directive, outside of ``defined`'. Such identifiers are replaced with zero.
`-Wunused-macros`
  Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor will also warn if the macro has not been used at the time it is redefined or undefined.

  Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

  Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

                #if defined the_macro_causing_the_warning
                #endif

`-Wendif-labels`
  Warn whenever an ``#else`' or an ``#endif`' are followed by text. This usually happens in code of the form

                #if FOO
                ...
                #else FOO
                ...
                #endif FOO

  The second and third `FOO` should be in comments, but often are not in older programs. This warning is on by default.

`-Werror`
  Make all warnings into hard errors. Source code which triggers warnings will be rejected.
`-Wsystem-headers`
  Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.
`-w`
  Suppress all warnings, including those which GNU CPP issues by default.
`-pedantic`
  Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.
`-pedantic-errors`
  Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without ``-pedantic`' but treats as warnings.
`-M`
  Instead of outputting the result of preprocessing, output a rule suitable for `make` describing the dependencies of the main source file. The preprocessor outputs one `make` rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from `-include` or `-imacros` command line options.

  Unless specified explicitly (with `-MT` or `-MQ`), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. If there are many included files then the rule is split into several lines using ``\`'-newline. The rule has no commands.

  This option does not suppress the preprocessor's debug output, such as `-dM`. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with `-MF`, or use an environment variable like `DEPENDENCIES_OUTPUT` (see Environment Variables). Debug output will still be sent to the regular output stream as normal.

  Passing `-M` to the driver implies `-E`, and suppresses warnings with an implicit `-w`.

`-MM`
  Like `-M` but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

  This implies that the choice of angle brackets or double quotes in an ``#include`' directive does not in itself determine whether that header will appear in `-MM` dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.

`-MF` file
  When used with `-M` or `-MM`, specifies a file to write the dependencies to. If no `-MF` switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output.

  When used with the driver options `-MD` or `-MMD`, `-MF` overrides the default dependency output file.

`-MG`
  In conjunction with an option such as `-M` requesting dependency generation, `-MG` assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the `#include` directive without prepending any path. `-MG` also suppresses preprocessed output, as a missing header file renders this useless.

  This feature is used in automatic updating of makefiles.

`-MP`
  This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors `make` gives if you remove header files without updating the `Makefile` to match.

  This is typical output:

                test.o: test.c test.h

                test.h:

`-MT` target
  Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, including any path, deletes any file suffix such as ``.c`', and appends the platform's usual object suffix. The result is the target.

  An `-MT` option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to `-MT`, or use multiple `-MT` options.

  For example, `-MT '$(objpfx)foo.o'` might give

                $(objpfx)foo.o: foo.c

`-MQ` target
  Same as `-MT`, but it quotes any characters which are special to Make. `-MQ '$(objpfx)foo.o'` gives

                $$(objpfx)foo.o: foo.c

  The default target is automatically quoted, as if it were given with `-MQ`.

`-MD`
  `-MD` is equivalent to `-M -MF file`, except that `-E` is not implied. The driver determines file based on whether an `-o` option is given. If it is, the driver uses its argument but with a suffix of `.d`, otherwise it take the basename of the input file and applies a `.d` suffix.

  If `-MD` is used in conjunction with `-E`, any `-o` switch is understood to specify the dependency output file (but see -MF), but if used without `-E`, each `-o` is understood to specify a target object file.

  Since `-E` is not implied, `-MD` can be used to generate a dependency output file as a side-effect of the compilation process.

`-MMD`
  Like `-MD` except mention only user header files, not system header files.
`-fpch-deps`
  When using precompiled headers (see Precompiled Headers), this flag will cause the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be listed and not the files that were used to create it because those files are not consulted when a precompiled header is used.
`-fpch-preprocess`
  This option allows use of a precompiled header (see Precompiled Headers) together with `-E`. It inserts a special `#pragma`, `#pragma GCC pch_preprocess "<filename>"` in the output to mark the place where the precompiled header was found, and its filename. When `-fpreprocessed` is in use, GCC recognizes this `#pragma` and loads the PCH.

  This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by `-save-temps`.

  You should not write this `#pragma` in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.

`-x c`
`-x c++`
`-x objective-c`
`-x assembler-with-cpp`
  Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: ``.c`', ``.cc`', ``.m`', or ``.S`'. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.

  Note: Previous versions of cpp accepted a `-lang` option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the `-l` option.

`-std=`standard
`-ansi`
  Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the future.

  standard may be one of:

  `iso9899:1990`
  `c89`
    The ISO C standard from 1990. ``c89`' is the customary shorthand for this version of the standard.

    The `-ansi` option is equivalent to `-std=c89`.

  `iso9899:199409`
    The 1990 C standard, as amended in 1994.
  `iso9899:1999`
  `c99`
  `iso9899:199x`
  `c9x`
    The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
  `gnu89`
    The 1990 C standard plus GNU extensions. This is the default.
  `gnu99`
  `gnu9x`
    The 1999 C standard plus GNU extensions.
  `c++98`
    The 1998 ISO C++ standard plus amendments.
  `gnu++98`
    The same as `-std=c++98` plus GNU extensions. This is the default for C++ code.
`-I-`
  Split the include path. Any directories specified with `-I` options before `-I-` are searched only for headers requested with `#include "`file`"`; they are not searched for `#include <`file`>`. If additional directories are specified with `-I` options after the `-I-`, those directories are searched for all ``#include`' directives.

  In addition, `-I-` inhibits the use of the directory of the current file directory as the first search directory for `#include "`file`"`. This option has been deprecated.

`-nostdinc`
  Do not search the standard system directories for header files. Only the directories you have specified with `-I` options (and the directory of the current file, if appropriate) are searched.
`-nostdinc++`
  Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
`-include` file
  Process file as if `#include "file"` appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the `#include "..."` search chain as normal.

  If multiple `-include` options are given, the files are included in the order they appear on the command line.

`-imacros` file
  Exactly like `-include`, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations.

  All files specified by `-imacros` are processed before all files specified by `-include`.

`-idirafter` dir
  Search dir for header files, but do it after all directories specified with `-I` and the standard system directories have been exhausted. dir is treated as a system include directory.
`-iprefix` prefix
  Specify prefix as the prefix for subsequent `-iwithprefix` options. If the prefix represents a directory, you should include the final ``/`'.
`-iwithprefix` dir
`-iwithprefixbefore` dir
  Append dir to the prefix specified previously with `-iprefix`, and add the resulting directory to the include search path. `-iwithprefixbefore` puts it in the same place `-I` would; `-iwithprefix` puts it where `-idirafter` would.
`-isysroot` dir
  This option is like the `--sysroot` option, but applies only to header files. See the `--sysroot` option for more information.
`-isystem` dir
  Search dir for header files, after all directories specified by `-I` but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.
`-iquote` dir
  Search dir only for header files requested with `#include "`file`"`; they are not searched for `#include <`file`>`, before all directories specified by `-I` and before the standard system directories.
`-fdollars-in-identifiers`
  Accept ``$`' in identifiers.
`-fextended-identifiers`
  Accept universal character names in identifiers. This option is experimental; in a future version of GCC, it will be enabled by default for C99 and C++.
`-fpreprocessed`
  Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with `-C` to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

  `-fpreprocessed` is implicit if the input file has one of the extensions ``.i`', ``.ii`' or ``.mi`'. These are the extensions that GCC uses for preprocessed files created by `-save-temps`.

`-ftabstop=`width
  Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
`-fexec-charset=`charset
  Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's `iconv` library routine.
`-fwide-exec-charset=`charset
  Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of `wchar_t`. As with `-fexec-charset`, charset can be any encoding supported by the system's `iconv` library routine; however, you will have problems with encodings that do not fit exactly in `wchar_t`.
`-finput-charset=`charset
  Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command line option. Currently the command line option takes precedence if there's a conflict. charset can be any encoding supported by the system's `iconv` library routine.
`-fworking-directory`
  Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form `-fno-working-directory`. If the `-P` flag is present in the command line, this option has no effect, since no `#line` directives are emitted whatsoever.
`-fno-show-column`
  Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as `dejagnu`.
`-A` predicate`=`answer
  Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form `-A predicate(answer)`, which is still supported, because it does not use shell special characters.
`-A -`predicate`=`answer
  Cancel an assertion with the predicate predicate and answer answer.
`-dCHARS`
  CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
  ``M`'
    Instead of the normal output, generate a list of ``#define`' directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file `foo.h`, the command

                       touch foo.h; cpp -dM foo.h

    will show all the predefined macros.

  ``D`'
    Like ``M`' except in two respects: it does not include the predefined macros, and it outputs both the ``#define`' directives and the result of preprocessing. Both kinds of output go to the standard output file.
  ``N`'
    Like ``D`', but emit only the macro names, not their expansions.
  ``I`'
    Output ``#include`' directives in addition to the result of preprocessing.
`-P`
  Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
`-C`
  Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

  You should be prepared for side effects when using `-C`; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a ``#`'.

`-CC`
  Do not discard comments, including during macro expansion. This is like `-C`, except that comments contained within macros are also passed through to the output file where the macro is expanded.

  In addition to the side-effects of the `-C` option, the `-CC` option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

  The `-CC` option is generally used to support lint comments.

`-traditional-cpp`
  Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
`-trigraphs`
  Process trigraph sequences. These are three-character sequences, all starting with ``??`', that are defined by ISO C to stand for single characters. For example, ``??/`' stands for ``\`', so ``'??/n'`' is a character constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the `-std` and `-ansi` options.

  The nine trigraphs and their replacements are

                Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
                Replacement:      [    ]    {    }    #    \    ^    |    ~

`-remap`
  Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
`--help`
`--target-help`
  Print text describing all the command line options instead of preprocessing anything.
`-v`
  Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.
`-H`
  Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the ``#include`' stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ``...x`' and a valid one with ``...!`' .
`-version`
`--version`
  Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.

Next: Link Options, Previous: Preprocessor Options, Up: Invoking GCC

### 3.12 Passing Options to the Assembler

You can pass options to the assembler.

`-Wa,`option
  Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
`-Xassembler` option
  Pass option as an option to the assembler. You can use this to supply system-specific assembler options which GCC does not know how to recognize.

  If you want to pass an option that takes an argument, you must use `-Xassembler` twice, once for the option and once for the argument.

Next: Directory Options, Previous: Assembler Options, Up: Invoking GCC

### 3.13 Options for Linking

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

object-file-name
  A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
`-c`
`-S`
`-E`
  If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See Overall Options.

`-l` library
`-l` library
  Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

  It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ``foo.o -lz bar.o`' searches library ``z`' after file `foo.o` but before `bar.o`. If `bar.o` refers to functions in ``z`', those functions may not be loaded.

  The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a`. The linker then uses this file as if it had been specified precisely by name.

  The directories searched include several standard system directories plus any that you specify with `-L`.

  Normally the files found this way are library files—archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an `-l` option and specifying a file name is that `-l` surrounds library with ``lib`' and ``.a`' and searches several directories.

`-lobjc`
  You need this special case of the `-l` option in order to link an Objective-C or Objective-C++ program.
`-nostartfiles`
  Do not use the standard system startup files when linking. The standard system libraries are used normally, unless `-nostdlib` or `-nodefaultlibs` is used.
`-nodefaultlibs`
  Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless `-nostartfiles` is used. The compiler may generate calls to `memcmp`, `memset`, `memcpy` and `memmove`. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
`-nostdlib`
  Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to `memcmp`, `memset`, `memcpy` and `memmove`. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

  One of the standard libraries bypassed by `-nostdlib` and `-nodefaultlibs` is `libgcc.a`, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See Interfacing to GCC Output, for more discussion of `libgcc.a`.) In most cases, you need `libgcc.a` even when you want to avoid other standard libraries. In other words, when you specify `-nostdlib` or `-nodefaultlibs` you should usually specify `-lgcc` as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example, ``__main`', used to ensure C++ constructors will be called; see `collect2`.)

`-pie`
  Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (`-fpie`, `-fPIE`, or model suboptions) when you specify this option.
`-rdynamic`
  Pass the flag `-export-dynamic` to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of `dlopen` or to allow obtaining backtraces from within a program.
`-s`
  Remove all symbol table and relocation information from the executable.
`-static`
  On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.
`-shared`
  Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options that were used to generate code (`-fpic`, `-fPIC`, or model suboptions) when you specify this option.1
`-shared-libgcc`
`-static-libgcc`
  On systems that provide `libgcc` as a shared library, these options force the use of either the shared or static version respectively. If no shared version of `libgcc` was built when the compiler was configured, these options have no effect.

  There are several situations in which an application should use the shared `libgcc` instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared `libgcc`.

  Therefore, the G++ and GCJ drivers automatically add `-shared-libgcc` whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

  If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared `libgcc`. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option `--eh-frame-hdr`, it will link the shared version of `libgcc` into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of `libgcc`, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

  However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option `-shared-libgcc`, such that it is linked with the shared `libgcc`.

`-symbolic`
  Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option ``-Xlinker -z -Xlinker defs`'). Only a few systems support this option.
`-Xlinker` option
  Pass option as an option to the linker. You can use this to supply system-specific linker options which GCC does not know how to recognize.

  If you want to pass an option that takes an argument, you must use `-Xlinker` twice, once for the option and once for the argument. For example, to pass `-assert definitions`, you must write ``-Xlinker -assert -Xlinker definitions`'. It does not work to write `-Xlinker "-assert definitions"`, because this passes the entire string as a single argument, which is not what the linker expects.

`-Wl,`option
  Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.
`-u` symbol
  Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use `-u` multiple times with different symbols to force loading of additional library modules.

Next: Spec Files, Previous: Link Options, Up: Invoking GCC

### 3.14 Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

`-I` dir
  Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use `-isystem` for that). If you use more than one `-I` option, the directories are scanned in left-to-right order; the standard system directories come after.

  If a standard system include directory, or a directory specified with `-isystem`, is also specified with `-I`, the `-I` option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the `-nostdinc` and/or `-isystem` options.

`-iquote` dir
  Add the directory dir to the head of the list of directories to be searched for header files only for the case of ``#include "file"`'; they are not searched for ``#include <file>`', otherwise just like `-I`.
`-L` dir
  Add directory dir to the list of directories to be searched for `-l`.
`-B` prefix
  This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

  The compiler driver program runs one or more of the subprograms `cpp`, `cc1`, `as` and `ld`. It tries prefix as a prefix for each program it tries to run, both with and without ``machine/version/`' (see Target Options).

  For each subprogram to be run, the compiler driver first tries the `-B` prefix, if any. If that name is not found, or if `-B` was not specified, the driver tries two standard prefixes, which are `/usr/lib/gcc/` and `/usr/local/lib/gcc/`. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH` environment variable.

  The compiler will check to see if the path provided by the `-B` refers to a directory, and if necessary it will add a directory separator character at the end of the path.

  `-B` prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L` options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into `-isystem` options for the preprocessor. In this case, the compiler appends ``include`' to the prefix.

  The run-time support file `libgcc.a` can also be searched for using the `-B` prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

  Another way to specify a prefix much like the `-B` prefix is to use the environment variable `GCC_EXEC_PREFIX`. See Environment Variables.

  As a special kludge, if the path provided by `-B` is `[dir/]stageN/`, where N is a number in the range 0 to 9, then it will be replaced by `[dir/]include`. This is to help with boot-strapping the compiler.

`-specs=`file
  Process file after the compiler reads in the standard `specs` file, in order to override the defaults that the `gcc` driver program uses when determining what switches to pass to `cc1`, `cc1plus`, `as`, `ld`, etc. More than one `-specs=file` can be specified on the command line, and they are processed in order, from left to right.
`--sysroot=`dir
  Use dir as the logical root directory for headers and libraries. For example, if the compiler would normally search for headers in `/usr/include` and libraries in `/usr/lib`, it will instead search `dir/usr/include` and `dir/usr/lib`.

  If you use both this option and the `-isysroot` option, then the `--sysroot` option will apply to libraries, but the `-isysroot` option will apply to header files.

  The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of `--sysroot` will still work, but the library aspect will not.

`-I-`
  This option has been deprecated. Please use `-iquote` instead for `-I` directories before the `-I-` and remove the `-I-`. Any directories you specify with `-I` options before the `-I-` option are searched only for the case of ``#include "file"`'; they are not searched for ``#include <file>`'.

  If additional directories are specified with `-I` options after the `-I-`, these directories are searched for all ``#include`' directives. (Ordinarily all `-I` directories are used this way.)

  In addition, the `-I-` option inhibits the use of the current directory (where the current input file came from) as the first search directory for ``#include "file"`'. There is no way to override this effect of `-I-`. With `-I.` you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

  `-I-` does not inhibit the use of the standard system directories for header files. Thus, `-I-` and `-nostdinc` are independent.

Next: Target Options, Previous: Directory Options, Up: Invoking GCC

### 3.15 Specifying subprocesses and the switches to pass to them

`gcc` is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings. In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into GCC can be overridden by using the `-specs=` command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:

`%`command
  Issues a command to the spec file processor. The commands that can appear here are:
  `%include <`file`>`
    Search for file and insert its text at the current point in the specs file.
  `%include_noerr <`file`>`
    Just like ``%include`', but do not generate an error message if the include file cannot be found.
  `%rename` old_name new_name
    Rename the spec string old_name to new_name.
`*[`spec_name`]:`
  This tells the compiler to create, override or delete the named spec string. All lines after this directive up to the next directive or blank line are considered to be the text for the spec string. If this results in an empty string then the spec will be deleted. (Or, if the spec did not exist, then nothing will happened.) Otherwise, if the spec does not currently exist a new spec will be created. If the spec does exist then its contents will be overridden by the text of this directive, unless the first character of that text is the ``+`' character, in which case the text will be appended to the spec.
`[`suffix`]:`
  Creates a new ``[suffix] spec`' pair. All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix. When the compiler encounters an input file with the named suffix, it will processes the spec string in order to work out how to compile that file. For example:

                .ZZ:
                z-compile -input %i

  This says that any input file whose name ends in ``.ZZ`' should be passed to the program ``z-compile`', which should be invoked with the command-line switch `-input` and with the result of performing the ``%i`' substitution. (See below.)

  As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:

  `@`language
    This says that the suffix is an alias for a known language. This is similar to using the `-x` command-line switch to GCC to specify a language explicitly. For example:

                       .ZZ:
                       @c++

    Says that .ZZ files are, in fact, C++ source files.

  `#`name
    This causes an error messages saying:

                       name compiler not installed on this system.

  GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.

         asm          Options to pass to the assembler
         asm_final    Options to pass to the assembler post-processor
         cpp          Options to pass to the C preprocessor
         cc1          Options to pass to the C compiler
         cc1plus      Options to pass to the C++ compiler
         endfile      Object files to include at the end of the link
         link         Options to pass to the linker
         lib          Libraries to include on the command line to the linker
         libgcc       Decides which GCC support library to pass to the linker
         linker       Sets the name of the linker
         predefines   Defines to be passed to the C preprocessor
         signed_char  Defines to pass to CPP to say whether char is signed
                      by default
         startfile    Object files to include at the start of the link

Here is a small example of a spec file:

         %rename lib                 old_lib

         *lib:
         --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called ``lib`' to ``old_lib`' and then overrides the previous definition of ``lib`' with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain ``%`'-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined ``%`'-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.

`%%`
  Substitute one ``%`' into the program name or argument.
`%i`
  Substitute the name of the input file being processed.
`%b`
  Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.
`%B`
  This is the same as ``%b`', but include the file suffix (text after the last period).
`%d`
  Marks the argument containing or following the ``%d`' as a temporary file name, so that that file will be deleted if GCC exits successfully. Unlike ``%g`', this contributes no text to the argument.
`%g` suffix
  Substitute a file name that has suffix suffix and is chosen once per compilation, and mark the argument in the same way as ``%d`'. To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known. For example, ``%g.s ... %g.o ... %g.s`' might turn into ``ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s`'. suffix matches the regexp ``[.A-Za-z]*`' or the special string ``%O`', which is treated exactly as if ``%O`' had been preprocessed. Previously, ``%g`' was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.
`%u` suffix
  Like ``%g`', but generates a new temporary file name even if ``%usuffix`' was already seen.
`%U` suffix
  Substitutes the last file name generated with ``%usuffix`', generating a new one if there is no such last file name. In the absence of any ``%usuffix`', this is just like ``%gsuffix`', except they don't share the same suffix space, so ``%g.s ... %U.s ... %g.s ... %U.s`' would involve the generation of two distinct file names, one for each ``%g.s`' and another for each ``%U.s`'. Previously, ``%U`' was simply substituted with a file name chosen for the previous ``%u`', without regard to any appended suffix.
`%j` suffix
  Substitutes the name of the `HOST_BIT_BUCKET`, if any, and if it is writable, and if save-temps is off; otherwise, substitute the name of a temporary file, just like ``%u`'. This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.
`%|`suffix
`%m` suffix
  Like ``%g`', except if `-pipe` is in effect. In that case ``%|`' substitutes a single dash and ``%m`' substitutes nothing at all. These are the two most common ways to instruct a program that it should read from standard input or write to standard output. If you need something more elaborate you can use an ``%{pipe:`X`}`' construct: see for example `f/lang-specs.h`.
`%.`SUFFIX
  Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with ``%*`'. SUFFIX is terminated by the next space or %.
`%w`
  Marks the argument containing or following the ``%w`' as the designated output file of this compilation. This puts the argument into the sequence of arguments that ``%o`' will substitute later.
`%o`
  Substitutes the names of all the output files, with spaces automatically placed around them. You should write spaces around the ``%o`' as well or the results are undefined. ``%o`' is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they will be linked.
`%O`
  Substitutes the suffix for object files. Note that this is handled specially when it immediately follows ``%g, %u, or %U`', because of the need for those to form complete file names. The handling is such that ``%O`' is treated exactly as if it had already been substituted, except that ``%g, %u, and %U`' do not currently support additional suffix characters following ``%O`' as they would following, for example, ``.o`'.
`%p`
  Substitutes the standard macro predefinitions for the current target machine. Use this when running `cpp`.
`%P`
  Like ``%p`', but puts ``__`' before and after the name of each predefined macro, except for macros that start with ``__`' or with ``_L`', where L is an uppercase letter. This is for ISO C.
`%I`
  Substitute any of `-iprefix` (made from `GCC_EXEC_PREFIX`), `-isysroot` (made from `TARGET_SYSTEM_ROOT`), and `-isystem` (made from `COMPILER_PATH` and `-B` options) as necessary.
`%s`
  Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.
`%e` str
  Print str as an error message. str is terminated by a newline. Use this when inconsistent options are detected.
`%(`name`)`
  Substitute the contents of spec string name at this point.
`%[`name`]`
  Like ``%(...)`' but put ``__`' around `-D` arguments.
`%x{`option`}`
  Accumulate an option for ``%X`'.
`%X`
  Output the accumulated linker options specified by `-Wl` or a ``%x`' spec string.
`%Y`
  Output the accumulated assembler options specified by `-Wa`.
`%Z`
  Output the accumulated preprocessor options specified by `-Wp`.
`%a`
  Process the `asm` spec. This is used to compute the switches to be passed to the assembler.
`%A`
  Process the `asm_final` spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed.
`%l`
  Process the `link` spec. This is the spec for computing the command line passed to the linker. Typically it will make use of the ``%L %G %S %D and %E`' sequences.
`%D`
  Dump out a `-L` option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory will be prepended to each of these paths.
`%L`
  Process the `lib` spec. This is a spec string for deciding which libraries should be included on the command line to the linker.
`%G`
  Process the `libgcc` spec. This is a spec string for deciding which GCC support library should be included on the command line to the linker.
`%S`
  Process the `startfile` spec. This is a spec for deciding which object files should be the first ones passed to the linker. Typically this might be a file named `crt0.o`.
`%E`
  Process the `endfile` spec. This is a spec string that specifies the last object files that will be passed to the linker.
`%C`
  Process the `cpp` spec. This is used to construct the arguments to be passed to the C preprocessor.
`%1`
  Process the `cc1` spec. This is used to construct the options to be passed to the actual C compiler (``cc1`').
`%2`
  Process the `cc1plus` spec. This is used to construct the options to be passed to the actual C++ compiler (``cc1plus`').
`%*`
  Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space.
`%<S`
  Remove all occurrences of `-S` from the command line. Note—this command is position dependent. ``%`' commands in the spec string before this one will see `-S`, ``%`' commands in the spec string after this one will not.
`%:`function`(`args`)`
  Call the named function function, passing it args. args is first processed as a nested spec string, then split into an argument vector in the usual fashion. The function returns a string which is processed as if it had appeared literally as part of the current spec.

  The following built-in spec functions are provided:

  `if-exists`
    The `if-exists` spec function takes one argument, an absolute pathname to a file. If the file exists, `if-exists` returns the pathname. Here is a small example of its usage:

                       *startfile:
                       crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s

  `if-exists-else`
    The `if-exists-else` spec function is similar to the `if-exists` spec function, except that it takes two arguments. The first argument is an absolute pathname to a file. If the file exists, `if-exists-else` returns the pathname. If it does not exist, it returns the second argument. This way, `if-exists-else` can be used to select one file or another, based on the existence of the first. Here is a small example of its usage:

                       *startfile:
                       crt0%O%s %:if-exists(crti%O%s) \
                       %:if-exists-else(crtbeginT%O%s crtbegin%O%s)

  `replace-outfile`
    The `replace-outfile` spec function takes two arguments. It looks for the first argument in the outfiles array and replaces it with the second argument. Here is a small example of its usage:

                       %{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}

`%{S}`
  Substitutes the `-S` switch, if that switch was given to GCC. If that switch was not specified, this substitutes nothing. Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed. Thus the spec string ``%{foo}`' would match the command-line option `-foo` and would output the command line option `-foo`.
`%W{S}`
  Like %{`S`} but mark last argument supplied within as a file to be deleted on failure.
`%{S*}`
  Substitutes all the switches specified to GCC whose names start with `-S`, but which also take an argument. This is used for switches like `-o`, `-D`, `-I`, etc. GCC considers `-o foo` as being one switch whose names starts with ``o`'. %{o*} would substitute this text, including the space. Thus two arguments would be generated.
`%{S*&T*}`
  Like %{`S`*}, but preserve order of `S` and `T` options (the order of `S` and `T` in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional. Useful for CPP as ``%{D*&U*&A*}`'.
`%{S:X}`
  Substitutes `X`, if the ``-S`' switch was given to GCC.
`%{!S:X}`
  Substitutes `X`, if the ``-S`' switch was not given to GCC.
`%{S*:X}`
  Substitutes `X` if one or more switches whose names start with `-S` are specified to GCC. Normally `X` is substituted only once, no matter how many such switches appeared. However, if `%*` appears somewhere in `X`, then `X` will be substituted once for each matching switch, with the `%*` replaced by the part of that switch that matched the `*`.
`%{.S:X}`
  Substitutes `X`, if processing a file with suffix `S`.
`%{!.S:X}`
  Substitutes `X`, if not processing a file with suffix `S`.
`%{S|P:X}`
  Substitutes `X` if either `-S` or `-P` was given to GCC. This may be combined with ``!`', ``.`', and `*` sequences as well, although they have a stronger binding than the ``|`'. If `%*` appears in `X`, all of the alternatives must be starred, and only the first matching alternative is substituted.

  For example, a spec string like this:

                %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}

  will output the following command-line options from the following input command-line options:

                fred.c        -foo -baz
                jim.d         -bar -boggle
                -d fred.c     -foo -baz -boggle
                -d jim.d      -bar -baz -boggle

`%{S:X; T:Y; :D}`
  If `S` was given to GCC, substitutes `X`; else if `T` was given to GCC, substitutes `Y`; else substitutes `D`. There can be as many clauses as you need. This may be combined with `.`, `!`, `|`, and `*` as needed.

The conditional text `X` in a %{`S`:`X`} or similar construct may contain other nested ``%`' constructs or spaces, or even newlines. They are processed as usual, as described above. Trailing white space in `X` is ignored. White space may also appear anywhere on the left side of the colon in these constructs, except between `.` or `*` and the corresponding word.

The `-O`, `-f`, `-m`, and `-W` switches are handled specifically in these constructs. If another value of `-O` or the negated form of a `-f`, `-m`, or `-W` switch is found later in the command line, the earlier switch value is ignored, except with {`S`*} where `S` is just one letter, which passes all matching options.

The character ``|`' at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if `-pipe` is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in `-l` are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.

Next: Submodel Options, Previous: Spec Files, Up: Invoking GCC

### 3.16 Specifying Target Machine and Compiler Version

The usual way to run GCC is to run the executable called `gcc`, or `<machine>-gcc` when cross-compiling, or `<machine>-gcc-<version>` to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.

`-b` machine
  The argument machine specifies the target machine for compilation.

  The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with ``configure arm-elf`', meaning to compile for an arm processor with elf binaries, then you would specify `-b arm-elf` to run that cross compiler. Because there are other options beginning with `-b`, the configuration must contain a hyphen.

`-V` version
  The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be ``4.0`', meaning to run GCC version 4.0.

The `-V` and `-b` options work by running the `<machine>-gcc-<version>` executable, so there's no real reason to use them if you can just run that directly.

Next: Code Gen Options, Previous: Target Options, Up: Invoking GCC

### 3.17 Hardware Models and Configurations

Earlier we discussed the standard option `-b` which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own special options, starting with ``-m`', to choose among various hardware models or configurations—for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.

Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.

Next: ARM Options, Up: Submodel Options

#### 3.17.1 ARC Options

These options are defined for ARC implementations:

`-EL`
  Compile code for little endian mode. This is the default.
`-EB`
  Compile code for big endian mode.
`-mmangle-cpu`
  Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are “almost identical”. This is an all or nothing option.
`-mcpu=`cpu
  Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support `-mcpu=base`, this is the default.
`-mtext=`text-section
`-mdata=`data-section
`-mrodata=`readonly-data-section
  Put functions, data, and readonly data in text-section, data-section, and readonly-data-section respectively by default. This can be overridden with the `section` attribute. See Variable Attributes.

Next: AVR Options, Previous: ARC Options, Up: Submodel Options

#### 3.17.2 ARM Options

These ``-m`' options are defined for Advanced RISC Machines (ARM) architectures:

`-mabi=`name
  Generate code for the specified ABI. Permissible values are: ``apcs-gnu`', ``atpcs`', ``aapcs`', ``aapcs-linux`' and ``iwmmxt`'.
`-mapcs-frame`
  Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying `-fomit-frame-pointer` with this option will cause the stack frames not to be generated for leaf functions. The default is `-mno-apcs-frame`.
`-mapcs`
  This is a synonym for `-mapcs-frame`.
`-mthumb-interwork`
  Generate code which supports calling between the ARM and Thumb instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is `-mno-thumb-interwork`, since slightly larger code is generated when `-mthumb-interwork` is specified.
`-mno-sched-prolog`
  Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code. The default is `-msched-prolog`.
`-mhard-float`
  Generate output containing floating point instructions. This is the default.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

  `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work.

`-mfloat-abi=`name
  Specifies which ABI to use for floating point values. Permissible values are: ``soft`', ``softfp`' and ``hard`'.

  ``soft`' and ``hard`' are equivalent to `-msoft-float` and `-mhard-float` respectively. ``softfp`' allows the generation of floating point instructions, but still uses the soft-float calling conventions.

`-mlittle-endian`
  Generate code for a processor running in little-endian mode. This is the default for all standard configurations.
`-mbig-endian`
  Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
`-mwords-little-endian`
  This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form ``32107654`'. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.
`-mcpu=`name
  This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: ``arm2`', ``arm250`', ``arm3`', ``arm6`', ``arm60`', ``arm600`', ``arm610`', ``arm620`', ``arm7`', ``arm7m`', ``arm7d`', ``arm7dm`', ``arm7di`', ``arm7dmi`', ``arm70`', ``arm700`', ``arm700i`', ``arm710`', ``arm710c`', ``arm7100`', ``arm7500`', ``arm7500fe`', ``arm7tdmi`', ``arm7tdmi-s`', ``arm8`', ``strongarm`', ``strongarm110`', ``strongarm1100`', ``arm8`', ``arm810`', ``arm9`', ``arm9e`', ``arm920`', ``arm920t`', ``arm922t`', ``arm946e-s`', ``arm966e-s`', ``arm968e-s`', ``arm926ej-s`', ``arm940t`', ``arm9tdmi`', ``arm10tdmi`', ``arm1020t`', ``arm1026ej-s`', ``arm10e`', ``arm1020e`', ``arm1022e`', ``arm1136j-s`', ``arm1136jf-s`', ``mpcore`', ``mpcorenovfp`', ``arm1176jz-s`', ``arm1176jzf-s`', ``xscale`', ``iwmmxt`', ``ep9312`'.
`-mtune=`name
  This option is very similar to the `-mcpu=` option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions that it will generate based on the cpu specified by a `-mcpu=` option. For some ARM implementations better performance can be obtained by using this option.
`-march=`name
  This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the `-mcpu=` option. Permissible names are: ``armv2`', ``armv2a`', ``armv3`', ``armv3m`', ``armv4`', ``armv4t`', ``armv5`', ``armv5t`', ``armv5te`', ``armv6`', ``armv6j`', ``iwmmxt`', ``ep9312`'.
`-mfpu=`name
`-mfpe=`number
`-mfp=`number
  This specifies what floating point hardware (or hardware emulation) is available on the target. Permissible names are: ``fpa`', ``fpe2`', ``fpe3`', ``maverick`', ``vfp`'. `-mfp` and `-mfpe` are synonyms for `-mfpu`=``fpe`'number, for compatibility with older versions of GCC.

  If `-msoft-float` is specified this specifies the format of floating point values.

`-mstructure-size-boundary=`n
  The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.

  Specifying the larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

`-mabort-on-noreturn`
  Generate a call to the function `abort` at the end of a `noreturn` function. It will be executed if the function tries to return.
`-mlong-calls`
`-mno-long-calls`
  Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function will lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.

  Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic is that static functions, functions which have the ``short-call`' attribute, functions that are inside the scope of a ``#pragma no_long_calls`' directive and functions whose definitions have already been compiled within the current compilation unit, will not be turned into long calls. The exception to this rule is that weak function definitions, functions with the ``long-call`' attribute or the ``section`' attribute, and functions that are within the scope of a ``#pragma long_calls`' directive, will always be turned into long calls.

  This feature is not enabled by default. Specifying `-mno-long-calls` will restore the default behavior, as will placing the function calls within the scope of a ``#pragma long_calls_off`' directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

`-mnop-fun-dllimport`
  Disable support for the `dllimport` attribute.
`-msingle-pic-base`
  Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The run-time system is responsible for initializing this register with an appropriate value before execution begins.
`-mpic-register=`reg
  Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is enabled, when R9 is used.
`-mcirrus-fix-invalid-insns`
  Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations. This option is only valid if the `-mcpu=ep9312` option has been used to enable generation of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by default, since the problem is only present in older Maverick implementations. The default can be re-enabled by use of the `-mno-cirrus-fix-invalid-insns` switch.
`-mpoke-function-name`
  Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

                     t0
                         .ascii "arm_poke_function_name", 0
                         .align
                     t1
                         .word 0xff000000 + (t1 - t0)
                     arm_poke_function_name
                         mov     ip, sp
                         stmfd   sp!, {fp, ip, lr, pc}
                         sub     fp, ip, #4

  When performing a stack backtrace, code can inspect the value of `pc` stored at `fp + 0`. If the trace function then looks at location `pc - 12` and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length `((pc[-3]) & 0xff000000)`.

`-mthumb`
  Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction set.
`-mtpcs-frame`
  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-tpcs-frame`.
`-mtpcs-leaf-frame`
  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-apcs-leaf-frame`.
`-mcallee-super-interworking`
  Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code.
`-mcaller-super-interworking`
  Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled.
`-mtp=`name
  Specify the access model for the thread local storage pointer. The valid models are `soft`, which generates calls to `__aeabi_read_tp`, `cp15`, which fetches the thread pointer from `cp15` directly (supported in the arm6k architecture), and `auto`, which uses the best available method for the selected processor. The default setting is `auto`.

Next: Blackfin Options, Previous: ARM Options, Up: Submodel Options

#### 3.17.3 AVR Options

These options are defined for AVR implementations:

`-mmcu=`mcu
  Specify ATMEL AVR instruction set or MCU type.

  Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).

  Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).

  Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).

  Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).

  Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).

`-msize`
  Output instruction sizes to the asm file.
`-minit-stack=`N
  Specify the initial stack address, which may be a symbol or numeric value, ``__stack`' is the default.
`-mno-interrupts`
  Generated code is not compatible with hardware interrupts. Code size will be smaller.
`-mcall-prologues`
  Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be smaller.
`-mno-tablejump`
  Do not generate tablejump insns which sometimes increase code size.
`-mtiny-stack`
  Change only the low 8 bits of the stack pointer.
`-mint8`
  Assume int to be 8 bit integer. This affects the sizes of all types: A char will be 1 byte, an int will be 1 byte, an long will be 2 bytes and long long will be 4 bytes. Please note that this option does not comply to the C standards, but it will provide you with smaller code size.

Next: CRIS Options, Previous: AVR Options, Up: Submodel Options

#### 3.17.4 Blackfin Options

`-momit-leaf-frame-pointer`
  Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option `-fomit-frame-pointer` removes the frame pointer for all functions which might make debugging harder.
`-mspecld-anomaly`
  When enabled, the compiler will ensure that the generated code does not contain speculative loads after jump instructions. This option is enabled by default.
`-mno-specld-anomaly`
  Don't generate extra code to prevent speculative loads from occurring.
`-mcsync-anomaly`
  When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. This option is enabled by default.
`-mno-csync-anomaly`
  Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
`-mlow-64k`
  When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.
`-mno-low-64k`
  Assume that the program is arbitrarily large. This is the default.
`-mid-shared-library`
  Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies `-fPIC`.
`-mno-id-shared-library`
  Generate code that doesn't assume ID based shared libraries are being used. This is the default.
`-mshared-library-id=n`
  Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time efficient than omitting this option.
`-mlong-calls`
`-mno-long-calls`
  Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function will lie outside of the 24 bit addressing range of the offset based version of subroutine call instruction.

  This feature is not enabled by default. Specifying `-mno-long-calls` will restore the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

Next: CRX Options, Previous: Blackfin Options, Up: Submodel Options

#### 3.17.5 CRIS Options

These options are defined specifically for the CRIS ports.

`-march=`architecture-type
`-mcpu=`architecture-type
  Generate code for the specified architecture. The choices for architecture-type are ``v3`', ``v8`' and ``v10`' for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is ``v0`' except for cris-axis-linux-gnu, where the default is ``v10`'.
`-mtune=`architecture-type
  Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for `-march=architecture-type`.
`-mmax-stack-frame=`n
  Warn when the stack frame of a function exceeds n bytes.
`-melinux-stacksize=`n
  Only available with the ``cris-axis-aout`' target. Arranges for indications in the program to the kernel loader that the stack of the program should be set to n bytes.
`-metrax4`
`-metrax100`
  The options `-metrax4` and `-metrax100` are synonyms for `-march=v3` and `-march=v8` respectively.
`-mmul-bug-workaround`
`-mno-mul-bug-workaround`
  Work around a bug in the `muls` and `mulu` instructions for CPU models where it applies. This option is active by default.
`-mpdebug`
  Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect to turn off the ``#NO_APP`' formatted-code indicator to the assembler at the beginning of the assembly file.
`-mcc-init`
  Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
`-mno-side-effects`
  Do not emit instructions with side-effects in addressing modes other than post-increment.
`-mstack-align`
`-mno-stack-align`
`-mdata-align`
`-mno-data-align`
`-mconst-align`
`-mno-const-align`
  These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
`-m32-bit`
`-m16-bit`
`-m8-bit`
  Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
`-mno-prologue-epilogue`
`-mprologue-epilogue`
  With `-mno-prologue-epilogue`, the normal function prologue and epilogue that sets up the stack-frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variable needs to be allocated.
`-mno-gotplt`
`-mgotplt`
  With `-fpic` and `-fPIC`, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is `-mgotplt`.
`-maout`
  Legacy no-op option only recognized with the cris-axis-aout target.
`-melf`
  Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
`-melinux`
  Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for `-march=v8`.
`-mlinux`
  Legacy no-op option only recognized with the cris-axis-linux-gnu target.
`-sim`
  This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
`-sim2`
  Like `-sim`, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

Next: Darwin Options, Previous: CRIS Options, Up: Submodel Options

#### 3.17.6 CRX Options

These options are defined specifically for the CRX ports.

`-mmac`
  Enable the use of multiply-accumulate instructions. Disabled by default.
`-mpush-args`
  Push instructions will be used to pass outgoing arguments when functions are called. Enabled by default.

Next: DEC Alpha Options, Previous: CRX Options, Up: Submodel Options

#### 3.17.7 Darwin Options

These options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create “fat” object files; it will create an object file for the single architecture that it was built to target. Apple's GCC on Darwin does create “fat” files if multiple `-arch` options are used; it does so by running the compiler or linker multiple times and joining the results together with `lipo`.

The subtype of the file created (like ``ppc7400`' or ``ppc970`' or ``i686`') is determined by the flags that specify the ISA that GCC is targetting, like `-mcpu` or `-march`. The `-force_cpusubtype_ALL` option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, `as`, will only permit instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in an ``ppc750`' object file. The linker for shared libraries, `/usr/bin/libtool`, will fail and print an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ``ppc970`' object file in a ``ppc7400`' library). The linker for executables, `ld`, will quietly give the executable the most restrictive subtype of any of its input files.

`-F` dir
  Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by `-I` options and are scanned in a left-to-right order.

  A framework directory is a directory with frameworks in it. A framework is a directory with a ``"Headers"`' and/or ``"PrivateHeaders"`' directory contained directly in it that ends in ``".framework"`'. The name of a framework is the name of this directory excluding the ``".framework"`'. Headers associated with the framework are found in one of those two directories, with ``"Headers"`' being searched first. A subframework is a framework directory that is in a framework's ``"Frameworks"`' directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework, a warning will be issued if this is violated. Currently a subframework cannot have subframeworks, in the future, the mechanism may be extended to support this. The standard frameworks can be found in ``"/System/Library/Frameworks"`' and ``"/Library/Frameworks"`'. An example include looks like `#include <Framework/header.h>`, where ``Framework`' denotes the name of the framework and header.h is found in the ``"PrivateHeaders"`' or ``"Headers"`' directory.

`-gused`
  Emit debugging information for symbols that are used. For STABS debugging format, this enables `-feliminate-unused-debug-symbols`. This is by default ON.
`-gfull`
  Emit debugging information for all symbols and types.
`-mmacosx-version-min=`version
  The earliest version of MacOS X that this executable will run on is version. Typical values of version include `10.1`, `10.2`, and `10.3.9`.

  The default for this option is to make choices that seem to be most useful.

`-mone-byte-bool`
  Override the defaults for ``bool`' so that ``sizeof(bool)==1`'. By default ``sizeof(bool)`' is ``4`' when compiling for Darwin/PowerPC and ``1`' when compiling for Darwin/x86, so this option has no effect on x86.

  Warning: The `-mone-byte-bool` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model.

`-mfix-and-continue`
`-ffix-and-continue`
`-findirect-data`
  Generate code suitable for fast turn around development. Needed to enable gdb to dynamically load `.o` files into already running programs. `-findirect-data` and `-ffix-and-continue` are provided for backwards compatibility.
`-all_load`
  Loads all members of static archive libraries. See man ld(1) for more information.
`-arch_errors_fatal`
  Cause the errors having to do with files that have the wrong architecture to be fatal.
`-bind_at_load`
  Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
`-bundle`
  Produce a Mach-o bundle format file. See man ld(1) for more information.
`-bundle_loader` executable
  This option specifies the executable that will be loading the build output file being linked. See man ld(1) for more information.
`-dynamiclib`
  When passed this option, GCC will produce a dynamic library instead of an executable when linking, using the Darwin `libtool` command.
`-force_cpusubtype_ALL`
  This causes GCC's output file to have the ALL subtype, instead of one controlled by the `-mcpu` or `-march` option.
`-allowable_client` client_name
`-client_name`
`-compatibility_version`
`-current_version`
`-dead_strip`
`-dependency-file`
`-dylib_file`
`-dylinker_install_name`
`-dynamic`
`-exported_symbols_list`
`-filelist`
`-flat_namespace`
`-force_flat_namespace`
`-headerpad_max_install_names`
`-image_base`
`-init`
`-install_name`
`-keep_private_externs`
`-multi_module`
`-multiply_defined`
`-multiply_defined_unused`
`-noall_load`
`-no_dead_strip_inits_and_terms`
`-nofixprebinding`
`-nomultidefs`
`-noprebind`
`-noseglinkedit`
`-pagezero_size`
`-prebind`
`-prebind_all_twolevel_modules`
`-private_bundle`
`-read_only_relocs`
`-sectalign`
`-sectobjectsymbols`
`-whyload`
`-seg1addr`
`-sectcreate`
`-sectobjectsymbols`
`-sectorder`
`-segaddr`
`-segs_read_only_addr`
`-segs_read_write_addr`
`-seg_addr_table`
`-seg_addr_table_filename`
`-seglinkedit`
`-segprot`
`-segs_read_only_addr`
`-segs_read_write_addr`
`-single_module`
`-static`
`-sub_library`
`-sub_umbrella`
`-twolevel_namespace`
`-umbrella`
`-undefined`
`-unexported_symbols_list`
`-weak_reference_mismatches`
`-whatsloaded`
  These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.

Next: DEC Alpha/VMS Options, Previous: Darwin Options, Up: Submodel Options

#### 3.17.8 DEC Alpha Options

These ``-m`' options are defined for the DEC Alpha implementations:

`-mno-soft-float`
`-msoft-float`
  Use (do not use) the hardware floating-point instructions for floating-point operations. When `-msoft-float` is specified, functions in `libgcc.a` will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

  Note that Alpha implementations without floating-point operations are required to have floating-point registers.

`-mfp-reg`
`-mno-fp-regs`
  Generate code that uses (does not use) the floating-point register set. `-mno-fp-regs` implies `-msoft-float`. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in `$0` instead of `$f0`. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with `-mno-fp-regs` must also be compiled with that option.

  A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

`-mieee`
  The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro `_IEEE_FP` is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option `-ieee_with_no_inexact`.
`-mieee-with-inexact`
  This is like `-mieee` except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to `_IEEE_FP`, `_IEEE_FP_EXACT` is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option `-ieee_with_inexact`.
`-mfp-trap-mode=`trap-mode
  This option controls what floating-point related traps are enabled. Other Alpha compilers call this option `-fptm trap-mode`. The trap mode can be set to one of four values:
  ``n`'
    This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  ``u`'
    In addition to the traps enabled by ``n`', underflow traps are enabled as well.
  ``su`'
    Like ``su`', but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  ``sui`'
    Like ``su`', but inexact traps are enabled as well.
`-mfp-rounding-mode=`rounding-mode
  Selects the IEEE rounding mode. Other Alpha compilers call this option `-fprm rounding-mode`. The rounding-mode can be one of:
  ``n`'
    Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
  ``m`'
    Round towards minus infinity.
  ``c`'
    Chopped rounding mode. Floating point numbers are rounded towards zero.
  ``d`'
    Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, ``d`' corresponds to round towards plus infinity.
`-mtrap-precision=`trap-precision
  In the Alpha architecture, floating point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating point trap. Depending on the requirements of an application, different levels of precisions can be selected:
  ``p`'
    Program precision. This option is the default and means a trap handler can only identify which program caused a floating point exception.
  ``f`'
    Function precision. The trap handler can determine the function that caused a floating point exception.
  ``i`'
    Instruction precision. The trap handler can determine the exact instruction that caused a floating point exception.

  Other Alpha compilers provide the equivalent options called `-scope_safe` and `-resumption_safe`.

`-mieee-conformant`
  This option marks the generated code as IEEE conformant. You must not use this option unless you also specify `-mtrap-precision=i` and either `-mfp-trap-mode=su` or `-mfp-trap-mode=sui`. Its only effect is to emit the line ``.eflag 48`' in the function prologue of the generated assembly file. Under DEC Unix, this has the effect that IEEE-conformant math library routines will be linked in.
`-mbuild-constants`
  Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime.

  Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

  You would typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

`-malpha-as`
`-mgas`
  Select whether to generate code to be assembled by the vendor-supplied assembler (`-malpha-as`) or by the GNU assembler `-mgas`.
`-mbwx`
`-mno-bwx`
`-mcix`
`-mno-cix`
`-mfix`
`-mno-fix`
`-mmax`
`-mno-max`
  Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via `-mcpu=` option or that of the CPU on which GCC was built if none was specified.
`-mfloat-vax`
`-mfloat-ieee`
  Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single and double precision.
`-mexplicit-relocs`
`-mno-explicit-relocs`
  Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
`-msmall-data`
`-mlarge-data`
  When `-mexplicit-relocs` is in effect, static data is accessed via gp-relative relocations. When `-msmall-data` is used, objects 8 bytes long or smaller are placed in a small data area (the `.sdata` and `.sbss` sections) and are accessed via 16-bit relocations off of the `$gp` register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

  The default is `-mlarge-data`. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use `malloc` or `mmap` to allocate the data in the heap instead of in the program's data segment.

  When generating code for shared libraries, `-fpic` implies `-msmall-data` and `-fPIC` implies `-mlarge-data`.

`-msmall-text`
`-mlarge-text`
  When `-msmall-text` is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When `-msmall-data` is used, the compiler can assume that all local symbols share the same `$gp` value, and thus reduce the number of instructions required for a function call from 4 to 1.

  The default is `-mlarge-text`.

`-mcpu=`cpu_type
  Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the ``EV`' style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and will choose the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC will default to the processor on which the compiler was built.

  Supported values for cpu_type are

  ``ev4`'
  ``ev45`'
  ``21064`'
    Schedules as an EV4 and has no instruction set extensions.
  ``ev5`'
  ``21164`'
    Schedules as an EV5 and has no instruction set extensions.
  ``ev56`'
  ``21164a`'
    Schedules as an EV5 and supports the BWX extension.
  ``pca56`'
  ``21164pc`'
  ``21164PC`'
    Schedules as an EV5 and supports the BWX and MAX extensions.
  ``ev6`'
  ``21264`'
    Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
  ``ev67`'
  ``21264a`'
    Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
`-mtune=`cpu_type
  Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.
`-mmemory-latency=`time
  Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

  Valid options for time are

  ``number`'
    A decimal number representing clock cycles.
  ``L1`'
  ``L2`'
  ``L3`'
  ``main`'
    The compiler contains estimates of the number of clock cycles for “typical” EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

Next: FRV Options, Previous: DEC Alpha Options, Up: Submodel Options

#### 3.17.9 DEC Alpha/VMS Options

These ``-m`' options are defined for the DEC Alpha/VMS implementations:

`-mvms-return-codes`
  Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error) codes.

Next: H8/300 Options, Previous: DEC Alpha/VMS Options, Up: Submodel Options

#### 3.17.10 FRV Options

`-mgpr-32`
  Only use the first 32 general purpose registers.
`-mgpr-64`
  Use all 64 general purpose registers.
`-mfpr-32`
  Use only the first 32 floating point registers.
`-mfpr-64`
  Use all 64 floating point registers
`-mhard-float`
  Use hardware instructions for floating point operations.
`-msoft-float`
  Use library routines for floating point operations.
`-malloc-cc`
  Dynamically allocate condition code registers.
`-mfixed-cc`
  Do not try to dynamically allocate condition code registers, only use `icc0` and `fcc0`.
`-mdword`
  Change ABI to use double word insns.
`-mno-dword`
  Do not use double word instructions.
`-mdouble`
  Use floating point double instructions.
`-mno-double`
  Do not use floating point double instructions.
`-mmedia`
  Use media instructions.
`-mno-media`
  Do not use media instructions.
`-mmuladd`
  Use multiply and add/subtract instructions.
`-mno-muladd`
  Do not use multiply and add/subtract instructions.
`-mfdpic`
  Select the FDPIC ABI, that uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies `-fPIE`. With `-fpic` or `-fpie`, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with `-fPIC` or `-fPIE`, GOT offsets are computed with 32 bits.
`-minline-plt`
  Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without `-mfdpic`. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., `-fPIC` or `-fpic`), or when an optimization option such as `-O3` or above is present in the command line.
`-mTLS`
  Assume a large TLS segment when generating thread-local code.
`-mtls`
  Do not assume a large TLS segment when generating thread-local code.
`-mgprel-ro`
  Enable the use of `GPREL` relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for `-fpic` or `-fpie`: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With `-fPIC` or `-fPIE`, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, `-mno-gprel-ro` can be used to disable it.
`-multilib-library-pic`
  Link with the (library, not FD) pic libraries. It's implied by `-mlibrary-pic`, as well as by `-fPIC` and `-fpic` without `-mfdpic`. You should never have to use it explicitly.
`-mlinked-fp`
  Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with `-mno-linked-fp`.
`-mlong-calls`
  Use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
`-malign-labels`
  Try to align labels to an 8-byte boundary by inserting nops into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds nops to existing ones.
`-mlibrary-pic`
  Generate position-independent EABI code.
`-macc-4`
  Use only the first four media accumulator registers.
`-macc-8`
  Use all eight media accumulator registers.
`-mpack`
  Pack VLIW instructions.
`-mno-pack`
  Do not pack VLIW instructions.
`-mno-eflags`
  Do not mark ABI switches in e_flags.
`-mcond-move`
  Enable the use of conditional-move instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-cond-move`
  Disable the use of conditional-move instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mscc`
  Enable the use of conditional set instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-scc`
  Disable the use of conditional set instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mcond-exec`
  Enable the use of conditional execution (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-cond-exec`
  Disable the use of conditional execution.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mvliw-branch`
  Run a pass to pack branches into VLIW instructions (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-vliw-branch`
  Do not run a pass to pack branches into VLIW instructions.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mmulti-cond-exec`
  Enable optimization of `&&` and `||` in conditional execution (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-multi-cond-exec`
  Disable optimization of `&&` and `||` in conditional execution.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mnested-cond-exec`
  Enable nested conditional execution optimizations (default).

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-mno-nested-cond-exec`
  Disable nested conditional execution optimizations.

  This switch is mainly for debugging the compiler and will likely be removed in a future version.

`-moptimize-membar`
  This switch removes redundant `membar` instructions from the compiler generated code. It is enabled by default.
`-mno-optimize-membar`
  This switch disables the automatic removal of redundant `membar` instructions from the generated code.
`-mtomcat-stats`
  Cause gas to print out tomcat statistics.
`-mcpu=`cpu
  Select the processor type for which to generate code. Possible values are ``frv`', ``fr550`', ``tomcat`', ``fr500`', ``fr450`', ``fr405`', ``fr400`', ``fr300`' and ``simple`'.

Next: HPPA Options, Previous: FRV Options, Up: Submodel Options

#### 3.17.11 H8/300 Options

These ``-m`' options are defined for the H8/300 implementations:

`-mrelax`
  Shorten some address references at link time, when possible; uses the linker option `-relax`. See `ld` and the H8/300, for a fuller description.
`-mh`
  Generate code for the H8/300H.
`-ms`
  Generate code for the H8S.
`-mn`
  Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with `-mh` or `-ms`.
`-ms2600`
  Generate code for the H8S/2600. This switch must be used with `-ms`.
`-mint32`
  Make `int` data 32 bits by default.
`-malign-300`
  On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4 byte boundaries. `-malign-300` causes them to be aligned on 2 byte boundaries. This option has no effect on the H8/300.

Next: i386 and x86-64 Options, Previous: H8/300 Options, Up: Submodel Options

#### 3.17.12 HPPA Options

These ``-m`' options are defined for the HPPA family of computers:

`-march=`architecture-type
  Generate code for the specified architecture. The choices for architecture-type are ``1.0`' for PA 1.0, ``1.1`' for PA 1.1, and ``2.0`' for PA 2.0 processors. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures will run on higher numbered architectures, but not the other way around.
`-mpa-risc-1-0`
`-mpa-risc-1-1`
`-mpa-risc-2-0`
  Synonyms for `-march=1.0`, `-march=1.1`, and `-march=2.0` respectively.
`-mbig-switch`
  Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.
`-mjump-in-delay`
  Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.
`-mdisable-fpregs`
  Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.
`-mdisable-indexing`
  Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
`-mno-space-regs`
  Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

  Such code is suitable for level 0 PA systems and kernels.

`-mfast-indirect-calls`
  Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs faster indirect calls.

  This option will not work in the presence of shared libraries or nested functions.

`-mfixed-range=`register-range
  Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
`-mlong-load-store`
  Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the ``+k`' option to the HP compilers.
`-mportable-runtime`
  Use the portable calling conventions proposed by HP for ELF systems.
`-mgas`
  Enable the use of assembler directives only GAS understands.
`-mschedule=`cpu-type
  Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are ``700`' ``7100`', ``7100LC`', ``7200`', ``7300`' and ``8000`'. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is ``8000`'.
`-mlinker-opt`
  Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded target ``hppa1.1-*-pro`' does provide software floating point support.

  `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work.

`-msio`
  Generate the predefine, `_SIO`, for server IO. The default is `-mwsio`. This generates the predefines, `__hp9000s700`, `__hp9000s700__` and `_WSIO`, for workstation IO. These options are available under HP-UX and HI-UX.
`-mgnu-ld`
  Use GNU ld specific options. This passes `-shared` to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using ``which `gcc -print-prog-name=ld``'. This option is only available on the 64 bit HP-UX GCC, i.e. configured with ``hppa*64*-*-hpux*`'.
`-mhp-ld`
  Use HP ld specific options. This passes `-b` to ld when building a shared library and passes `+Accept TypeMismatch` to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using ``which `gcc -print-prog-name=ld``'. This option is only available on the 64 bit HP-UX GCC, i.e. configured with ``hppa*64*-*-hpux*`'.
`-mlong-calls`
  Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

  Distances are measured from the beginning of functions when using the `-ffunction-sections` option, or when using the `-mgas` and `-mno-portable-runtime` options together under HP-UX with the SOM linker.

  It is normally not desirable to use this option as it will degrade performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

  The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

`-munix=`unix-std
  Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are ``93`', ``95`' and ``98`'. ``93`' is supported on all HP-UX versions. ``95`' is available on HP-UX 10.10 and later. ``98`' is available on HP-UX 11.11 and later. The default values are ``93`' for HP-UX 10.00, ``95`' for HP-UX 10.10 though to 11.00, and ``98`' for HP-UX 11.11 and later.

  `-munix=93` provides the same predefines as GCC 3.3 and 3.4. `-munix=95` provides additional predefines for `XOPEN_UNIX` and `_XOPEN_SOURCE_EXTENDED`, and the startfile `unix95.o`. `-munix=98` provides additional predefines for `_XOPEN_UNIX`, `_XOPEN_SOURCE_EXTENDED`, `_INCLUDE__STDC_A1_SOURCE` and `_INCLUDE_XOPEN_SOURCE_500`, and the startfile `unix98.o`.

  It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.

  Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability.

`-nolibdld`
  Suppress the generation of link options to search libdld.sl when the `-static` option is specified on HP-UX 10 and later.
`-static`
  The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Thus, when the `-static` option is specified, special link options are needed to resolve this dependency.

  On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the `-static` option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The `-nolibdld` option can be used to prevent the GCC driver from adding these link options.

`-threads`
  Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.

Next: IA-64 Options, Previous: HPPA Options, Up: Submodel Options

#### 3.17.13 Intel 386 and AMD x86-64 Options

These ``-m`' options are defined for the i386 and x86-64 family of computers:

`-mtune=`cpu-type
  Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for cpu-type are:
  i386
    Original Intel's i386 CPU.
  i486
    Intel's i486 CPU. (No scheduling is implemented for this chip.)
  i586, pentium
    Intel Pentium CPU with no MMX support.
  pentium-mmx
    Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
  i686, pentiumpro
    Intel PentiumPro CPU.
  pentium2
    Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
  pentium3, pentium3m
    Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.
  pentium-m
    Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
  pentium4, pentium4m
    Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
  prescott
    Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
  nocona
    Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
  k6
    AMD K6 CPU with MMX instruction set support.
  k6-2, k6-3
    Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
  athlon, athlon-tbird
    AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions support.
  athlon-4, athlon-xp, athlon-mp
    Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE instruction set support.
  k8, opteron, athlon64, athlon-fx
    AMD K8 core based CPUs with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
  winchip-c6
    IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
  winchip2
    IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW! instruction set support.
  c3
    Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is implemented for this chip.)
  c3-2
    Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

  While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the `-march=cpu-type` option being used.

`-march=`cpu-type
  Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for `-mtune`. Moreover, specifying `-march=cpu-type` implies `-mtune=cpu-type`.
`-mcpu=`cpu-type
  A deprecated synonym for `-mtune`.
`-m386`
`-m486`
`-mpentium`
`-mpentiumpro`
  These options are synonyms for `-mtune=i386`, `-mtune=i486`, `-mtune=pentium`, and `-mtune=pentiumpro` respectively. These synonyms are deprecated.
`-mfpmath=`unit
  Generate floating point arithmetics for selected unit unit. The choices for unit are:
  ``387`'
    Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise. Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precision instead of precision specified by the type resulting in slightly different results compared to most of other chips. See `-ffloat-store` for more detailed description.

    This is the default choice for i386 compiler.

  ``sse`'
    Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387. Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too.

    For the i386 compiler, you need to use `-march=cpu-type`, `-msse` or `-msse2` switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.

    The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit.

    This is the default choice for the x86-64 compiler.

  ``sse,387`'
    Attempt to utilize both instruction sets at once. This effectively double the amount of available registers and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well resulting in instable performance.
`-masm=`dialect
  Output asm instructions using selected dialect. Supported choices are ``intel`' or ``att`' (the default one). Darwin does not support ``intel`'.
`-mieee-fp`
`-mno-ieee-fp`
  Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

  On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if `-msoft-float` is used.

`-mno-fp-ret-in-387`
  Do not use the FPU registers for return values of functions.

  The usual calling convention has functions return values of types `float` and `double` in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

  The option `-mno-fp-ret-in-387` causes such values to be returned in ordinary CPU registers instead.

`-mno-fancy-math-387`
  Some 387 emulators do not support the `sin`, `cos` and `sqrt` instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden when `-march` indicates that the target cpu will always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, these instructions are not generated unless you also use the `-funsafe-math-optimizations` switch.
`-malign-double`
`-mno-align-double`
  Control whether GCC aligns `double`, `long double`, and `long long` variables on a two word boundary or a one word boundary. Aligning `double` variables on a two word boundary will produce code that runs somewhat faster on a ``Pentium`' at the expense of more memory.

  Warning: if you use the `-malign-double` switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386 and will not be binary compatible with structures in code compiled without that switch.

`-m96bit-long-double`
`-m128bit-long-double`
  These switches control the size of `long double` type. The i386 application binary interface specifies the size to be 96 bits, so `-m96bit-long-double` is the default in 32 bit mode.

  Modern architectures (Pentium and newer) would prefer `long double` to be aligned to an 8 or 16 byte boundary. In arrays or structures conforming to the ABI, this would not be possible. So specifying a `-m128bit-long-double` will align `long double` to a 16 byte boundary by padding the `long double` with an additional 32 bit zero.

  In the x86-64 compiler, `-m128bit-long-double` is the default choice as its ABI specifies that `long double` is to be aligned on 16 byte boundary.

  Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a `long double`.

  Warning: if you override the default value for your target ABI, the structures and arrays containing `long double` variables will change their size as well as function calling convention for function taking `long double` will be modified. Hence they will not be binary compatible with arrays or structures in code compiled without that switch.

`-mmlarge-data-threshold=`number
  When `-mcmodel=medium` is specified, the data greater than threshold are placed in large data section. This value must be the same across all object linked into the binary and defaults to 65535.
`-msvr3-shlib`
`-mno-svr3-shlib`
  Control whether GCC places uninitialized local variables into the `bss` or `data` segments. `-msvr3-shlib` places them into `bss`. These options are meaningful only on System V Release 3.
`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return with the `ret` num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

  You can specify that an individual function is called with this calling sequence with the function attribute ``stdcall`'. You can also override the `-mrtd` option by using the function attribute ``cdecl`'. See Function Attributes.

  Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

  Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise incorrect code will be generated for calls to those functions.

  In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

`-mregparm=`num
  Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute ``regparm`'. See Function Attributes.

  Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

`-msseregparm`
  Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute ``sseregparm`'. See Function Attributes.

  Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

`-mpreferred-stack-boundary=`num
  Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If `-mpreferred-stack-boundary` is not specified, the default is 4 (16 bytes or 128 bits), except when optimizing for code size (`-Os`), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64).

  On Pentium and PentiumPro, `double` and `long double` values should be aligned to an 8 byte boundary (see `-malign-double`) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type `__m128` suffers similar penalties if it is not 16 byte aligned.

  To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.

  This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to `-mpreferred-stack-boundary=2`.

`-mmmx`
`-mno-mmx`
`-msse`
`-mno-sse`
`-msse2`
`-mno-sse2`
`-msse3`
`-mno-sse3`
`-m3dnow`
`-mno-3dnow`
  These switches enable or disable the use of instructions in the MMX, SSE, SSE2 or 3DNow! extended instruction sets. These extensions are also available as built-in functions: see X86 Built-in Functions, for details of the functions enabled and disabled by these switches.

  To have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instructions), see `-mfpmath=sse`.

  These options will enable GCC to use these extended instructions in generated code, even without `-mfpmath=sse`. Applications which perform runtime CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.

`-mpush-args`
`-mno-push-args`
  Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.
`-maccumulate-outgoing-args`
  If enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies `-mno-push-args`.
`-mthreads`
  Support thread-safe exception handling on ``Mingw32`'. Code that relies on thread-safe exception handling must compile and link all code with the `-mthreads` option. When compiling, `-mthreads` defines `-D_MT`; when linking, it links in a special thread helper library `-lmingwthrd` which cleans up per thread exception handling data.
`-mno-align-stringops`
  Do not align destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it.
`-minline-all-stringops`
  By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte boundary. This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths.
`-momit-leaf-frame-pointer`
  Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option `-fomit-frame-pointer` removes the frame pointer for all functions which might make debugging harder.
`-mtls-direct-seg-refs`
`-mno-tls-direct-seg-refs`
  Controls whether TLS variables may be accessed with offsets from the TLS segment register (`%gs` for 32-bit, `%fs` for 64-bit), or whether the thread base pointer must be added. Whether or not this is legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.

  For systems that use GNU libc, the default is on.

These ``-m`' switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.

`-m32`
`-m64`
  Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.
`-mno-red-zone`
  Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag `-mno-red-zone` disables this red zone.
`-mcmodel=small`
  Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
`-mcmodel=kernel`
  Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.
`-mcmodel=medium`
  Generate code for the medium model: The program is linked in the lower 2 GB of the address space but symbols can be located anywhere in the address space. Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model.
`-mcmodel=large`
  Generate code for the large model: This model makes no assumptions about addresses and sizes of sections. Currently GCC does not implement this model.

Next: M32C Options, Previous: i386 and x86-64 Options, Up: Submodel Options

#### 3.17.14 IA-64 Options

These are the ``-m`' options defined for the Intel IA-64 architecture.

`-mbig-endian`
  Generate code for a big endian target. This is the default for HP-UX.
`-mlittle-endian`
  Generate code for a little endian target. This is the default for AIX5 and GNU/Linux.
`-mgnu-as`
`-mno-gnu-as`
  Generate (or don't) code for the GNU assembler. This is the default.
`-mgnu-ld`
`-mno-gnu-ld`
  Generate (or don't) code for the GNU linker. This is the default.
`-mno-pic`
  Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.
`-mvolatile-asm-stop`
`-mno-volatile-asm-stop`
  Generate (or don't) a stop bit immediately before and after volatile asm statements.
`-mregister-names`
`-mno-register-names`
  Generate (or don't) ``in`', ``loc`', and ``out`' register names for the stacked registers. This may make assembler output more readable.
`-mno-sdata`
`-msdata`
  Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
`-mconstant-gp`
  Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
`-mauto-pic`
  Generate code that is self-relocatable. This implies `-mconstant-gp`. This is useful when compiling firmware code.
`-minline-float-divide-min-latency`
  Generate code for inline divides of floating point values using the minimum latency algorithm.
`-minline-float-divide-max-throughput`
  Generate code for inline divides of floating point values using the maximum throughput algorithm.
`-minline-int-divide-min-latency`
  Generate code for inline divides of integer values using the minimum latency algorithm.
`-minline-int-divide-max-throughput`
  Generate code for inline divides of integer values using the maximum throughput algorithm.
`-minline-sqrt-min-latency`
  Generate code for inline square roots using the minimum latency algorithm.
`-minline-sqrt-max-throughput`
  Generate code for inline square roots using the maximum throughput algorithm.
`-mno-dwarf2-asm`
`-mdwarf2-asm`
  Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful when not using the GNU assembler.
`-mearly-stop-bits`
`-mno-early-stop-bits`
  Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.
`-mfixed-range=`register-range
  Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
`-mtls-size=`tls-size
  Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
`-mtune=`cpu-type
  Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.
`-mt`
`-pthread`
  Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. It does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it. These are HP-UX specific flags.
`-milp32`
`-mlp64`
  Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.

Next: M32R/D Options, Previous: IA-64 Options, Up: Submodel Options

#### 3.17.15 M32C Options

`-mcpu=`name
  Select the CPU for which code is generated. name may be one of ``r8c`' for the R8C/Tiny series, ``m16c`' for the M16C (up to /60) series, ``m32cm`' for the M16C/80 series, or ``m32c`' for the M32C/80 series.
`-msim`
  Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
`-memregs=`number
  Specifies the number of memory-based pseudo-registers GCC will use during code generation. These pseudo-registers will be used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with the default runtime libraries gcc builds.

Next: M680x0 Options, Previous: M32C Options, Up: Submodel Options

#### 3.17.16 M32R/D Options

These `-m` options are defined for Renesas M32R/D architectures:

`-m32r2`
  Generate code for the M32R/2.
`-m32rx`
  Generate code for the M32R/X.
`-m32r`
  Generate code for the M32R. This is the default.
`-mmodel=small`
  Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24` instruction), and assume all subroutines are reachable with the `bl` instruction. This is the default.

  The addressability of a particular object can be set with the `model` attribute.

`-mmodel=medium`
  Assume objects may be anywhere in the 32-bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and assume all subroutines are reachable with the `bl` instruction.
`-mmodel=large`
  Assume objects may be anywhere in the 32-bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and assume subroutines may not be reachable with the `bl` instruction (the compiler will generate the much slower `seth/add3/jl` instruction sequence).
`-msdata=none`
  Disable use of the small data area. Variables will be put into one of ``.data`', ``bss`', or ``.rodata`' (unless the `section` attribute has been specified). This is the default.

  The small data area consists of sections ``.sdata`' and ``.sbss`'. Objects may be explicitly put in the small data area with the `section` attribute using one of these sections.

`-msdata=sdata`
  Put small global and static data in the small data area, but do not generate special code to reference them.
`-msdata=use`
  Put small global and static data in the small data area, and generate special instructions to reference them.
`-G` num
  Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal data or bss sections. The default value of num is 8. The `-msdata` option must be set to one of ``sdata`' or ``use`' for this option to have any effect.

  All modules should be compiled with the same `-G num` value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message—incorrect code will not be generated.

`-mdebug`
  Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.
`-malign-loops`
  Align all loops to a 32-byte boundary.
`-mno-align-loops`
  Do not enforce a 32-byte alignment for loops. This is the default.
`-missue-rate=`number
  Issue number instructions per cycle. number can only be 1 or 2.
`-mbranch-cost=`number
  number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it is 2, then the opposite will apply.
`-mflush-trap=`number
  Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
`-mno-flush-trap`
  Specifies that the cache cannot be flushed by using a trap.
`-mflush-func=`name
  Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call will only be used if a trap is not available.
`-mno-flush-func`
  Indicates that there is no OS function for flushing the cache.

Next: M68hc1x Options, Previous: M32R/D Options, Up: Submodel Options

#### 3.17.17 M680x0 Options

These are the ``-m`' options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

`-m68000`
`-mc68000`
  Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.

  Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

`-m68020`
`-mc68020`
  Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.
`-m68881`
  Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless `--nfp` was specified when the compiler was configured.
`-m68030`
  Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
`-m68040`
  Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.

  This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.

`-m68060`
  Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.

  This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

`-mcpu32`
  Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.

  Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

`-m5200`
  Generate output for a 520X “coldfire” family cpu. This is the default when the compiler is configured for 520X-based systems.

  Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

`-m68020-40`
  Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
`-m68020-60`
  Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets ``m68k-*-aout`' and ``m68k-*-coff`' do provide software floating point support.
`-mshort`
  Consider type `int` to be 16 bits wide, like `short int`. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
`-mnobitfield`
  Do not use the bit-field instructions. The `-m68000`, `-mcpu32` and `-m5200` options imply `-mnobitfield`.
`-mbitfield`
  Do use the bit-field instructions. The `-m68020` option implies `-mbitfield`. This is the default if you use a configuration designed for a 68020.
`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return with the `rtd` instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

  This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

  Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise incorrect code will be generated for calls to those functions.

  In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

  The `rtd` instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

`-malign-int`
`-mno-align-int`
  Control whether GCC aligns `int`, `long`, `long long`, `float`, `double`, and `long double` variables on a 32-bit boundary (`-malign-int`) or a 16-bit boundary (`-mno-align-int`). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

  Warning: if you use the `-malign-int` switch, GCC will align structures containing the above types differently than most published application binary interface specifications for the m68k.

`-mpcrel`
  Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies `-fpic`, allowing at most a 16-bit offset for pc-relative addressing. `-fPIC` is not presently supported with `-mpcrel`, though this could be supported for 68020 and higher processors.
`-mno-strict-align`
`-mstrict-align`
  Do not (do) assume that unaligned memory references will be handled by the system.
`-msep-data`
  Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management. This option implies `-fPIC`.
`-mno-sep-data`
  Generate code that assumes that the data segment follows the text segment. This is the default.
`-mid-shared-library`
  Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies `-fPIC`.
`-mno-id-shared-library`
  Generate code that doesn't assume ID based shared libraries are being used. This is the default.
`-mshared-library-id=n`
  Specified the identification number of the ID based shared library being compiled. Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time efficient than omitting this option.

Next: MCore Options, Previous: M680x0 Options, Up: Submodel Options

#### 3.17.18 M68hc1x Options

These are the ``-m`' options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.

`-m6811`
`-m68hc11`
  Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based systems.
`-m6812`
`-m68hc12`
  Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based systems.
`-m68S12`
`-m68hcs12`
  Generate output for a 68HCS12.
`-mauto-incdec`
  Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.
`-minmax`
`-nominmax`
  Enable the use of 68HC12 min and max instructions.
`-mlong-calls`
`-mno-long-calls`
  Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use the `call` instruction to call a function and the `rtc` instruction for returning.
`-mshort`
  Consider type `int` to be 16 bits wide, like `short int`.
`-msoft-reg-count=`count
  Specify the number of pseudo-soft registers which are used for the code generation. The maximum number is 32. Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.

Next: MIPS Options, Previous: M68hc1x Options, Up: Submodel Options

#### 3.17.19 MCore Options

These are the ``-m`' options defined for the Motorola M*Core processors.

`-mhardlit`
`-mno-hardlit`
  Inline constants into the code stream if it can be done in two instructions or less.
`-mdiv`
`-mno-div`
  Use the divide instruction. (Enabled by default).
`-mrelax-immediate`
`-mno-relax-immediate`
  Allow arbitrary sized immediates in bit operations.
`-mwide-bitfields`
`-mno-wide-bitfields`
  Always treat bit-fields as int-sized.
`-m4byte-functions`
`-mno-4byte-functions`
  Force all functions to be aligned to a four byte boundary.
`-mcallgraph-data`
`-mno-callgraph-data`
  Emit callgraph information.
`-mslow-bytes`
`-mno-slow-bytes`
  Prefer word access when reading byte quantities.
`-mlittle-endian`
`-mbig-endian`
  Generate code for a little endian target.
`-m210`
`-m340`
  Generate code for the 210 processor.

Next: MMIX Options, Previous: MCore Options, Up: Submodel Options

#### 3.17.20 MIPS Options

`-EB`
  Generate big-endian code.
`-EL`
  Generate little-endian code. This is the default for ``mips*el-*-*`' configurations.
`-march=`arch
  Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: ``mips1`', ``mips2`', ``mips3`', ``mips4`', ``mips32`', ``mips32r2`', and ``mips64`'. The processor names are: ``4kc`', ``4km`', ``4kp`', ``5kc`', ``5kf`', ``20kc`', ``24k`', ``24kc`', ``24kf`', ``24kx`', ``m4k`', ``orion`', ``r2000`', ``r3000`', ``r3900`', ``r4000`', ``r4400`', ``r4600`', ``r4650`', ``r6000`', ``r8000`', ``rm7000`', ``rm9000`', ``sb1`', ``sr71000`', ``vr4100`', ``vr4111`', ``vr4120`', ``vr4130`', ``vr4300`', ``vr5000`', ``vr5400`' and ``vr5500`'. The special value ``from-abi`' selects the most compatible architecture for the selected ABI (that is, ``mips1`' for 32-bit ABIs and ``mips3`' for 64-bit ABIs).

  In processor names, a final ``000`' can be abbreviated as ``k`' (for example, ``-march=r2k`'). Prefixes are optional, and ``vr`' may be written ``r`'.

  GCC defines two macros based on the value of this option. The first is ``_MIPS_ARCH`', which gives the name of target architecture, as a string. The second has the form ``_MIPS_ARCH_foo`', where foo is the capitalized value of ``_MIPS_ARCH`'. For example, ``-march=r2000`' will set ``_MIPS_ARCH`' to ``"r2000"`' and define the macro ``_MIPS_ARCH_R2000`'.

  Note that the ``_MIPS_ARCH`' macro uses the processor names given above. In other words, it will have the full prefix and will not abbreviate ``000`' as ``k`'. In the case of ``from-abi`', the macro names the resolved architecture (either ``"mips1"`' or ``"mips3"`'). It names the default architecture when no `-march` option is given.

`-mtune=`arch
  Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for `-march`.

  When this option is not used, GCC will optimize for the processor specified by `-march`. By using `-march` and `-mtune` together, it is possible to generate code that will run on a family of processors, but optimize the code for one particular member of that family.

  ``-mtune`' defines the macros ``_MIPS_TUNE`' and ``_MIPS_TUNE_foo`', which work in the same way as the ``-march`' ones described above.

`-mips1`
  Equivalent to ``-march=mips1`'.
`-mips2`
  Equivalent to ``-march=mips2`'.
`-mips3`
  Equivalent to ``-march=mips3`'.
`-mips4`
  Equivalent to ``-march=mips4`'.
`-mips32`
  Equivalent to ``-march=mips32`'.
`-mips32r2`
  Equivalent to ``-march=mips32r2`'.
`-mips64`
  Equivalent to ``-march=mips64`'.
`-mips16`
`-mno-mips16`
  Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE.
`-mabi=32`
`-mabi=o64`
`-mabi=n32`
`-mabi=64`
`-mabi=eabi`
  Generate code for the given ABI.

  Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use `-mgp32` to get 32-bit code instead.

  For information about the O64 ABI, see http://gcc.gnu.org/projects/mipso64-abi.html.

`-mabicalls`
`-mno-abicalls`
  Generate (do not generate) SVR4-style position-independent code. `-mabicalls` is the default for SVR4-based systems.
`-mxgot`
`-mno-xgot`
  Lift (do not lift) the usual restrictions on the size of the global offset table.

  GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it will only work if the GOT is smaller than about 64k. Anything larger will cause the linker to report an error such as:

                relocation truncated to fit: R_MIPS_GOT16 foobar

  If this happens, you should recompile your code with `-mxgot`. It should then work with very large GOTs, although it will also be less efficient, since it will take three instructions to fetch the value of a global symbol.

  Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use `-mxgot` when a single object file accesses more than 64k's worth of GOT entries. Very few do.

  These options have no effect unless GCC is generating position independent code.

`-mgp32`
  Assume that general-purpose registers are 32 bits wide.
`-mgp64`
  Assume that general-purpose registers are 64 bits wide.
`-mfp32`
  Assume that floating-point registers are 32 bits wide.
`-mfp64`
  Assume that floating-point registers are 64 bits wide.
`-mhard-float`
  Use floating-point coprocessor instructions.
`-msoft-float`
  Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
`-msingle-float`
  Assume that the floating-point coprocessor only supports single-precision operations.
`-mdouble-float`
  Assume that the floating-point coprocessor supports double-precision operations. This is the default.
`-mdsp`
`-mno-dsp`
  Use (do not use) the MIPS DSP ASE. See MIPS DSP Built-in Functions.
`-mpaired-single`
`-mno-paired-single`
  Use (do not use) paired-single floating-point instructions. See MIPS Paired-Single Support. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
`-mips3d`
`-mno-mips3d`
  Use (do not use) the MIPS-3D ASE. See MIPS-3D Built-in Functions. The option `-mips3d` implies `-mpaired-single`.
`-mlong64`
  Force `long` types to be 64 bits wide. See `-mlong32` for an explanation of the default and the way that the pointer size is determined.
`-mlong32`
  Force `long`, `int`, and pointer types to be 32 bits wide.

  The default size of `int`s, `long`s and pointers depends on the ABI. All the supported ABIs use 32-bit `int`s. The n64 ABI uses 64-bit `long`s, as does the 64-bit EABI; the others use 32-bit `long`s. Pointers are the same size as `long`s, or the same size as integer registers, whichever is smaller.

`-msym32`
`-mno-sym32`
  Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with `-mabi=64` and `-mno-abicalls` because it allows GCC to generate shorter and faster references to symbolic addresses.
`-G` num
  Put global and static items less than or equal to num bytes into the small data or bss section instead of the normal data or bss section. This allows the data to be accessed using a single instruction.

  All modules should be compiled with the same `-G num` value.

`-membedded-data`
`-mno-embedded-data`
  Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.
`-muninit-const-in-rodata`
`-mno-uninit-const-in-rodata`
  Put uninitialized `const` variables in the read-only data section. This option is only meaningful in conjunction with `-membedded-data`.
`-msplit-addresses`
`-mno-split-addresses`
  Enable (disable) use of the `%hi()` and `%lo()` assembler relocation operators. This option has been superseded by `-mexplicit-relocs` but is retained for backwards compatibility.
`-mexplicit-relocs`
`-mno-explicit-relocs`
  Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by `-mno-explicit-relocs`, is to use assembler macros instead.

  `-mexplicit-relocs` is the default if GCC was configured to use an assembler that supports relocation operators.

`-mcheck-zero-division`
`-mno-check-zero-division`
  Trap (do not trap) on integer division by zero. The default is `-mcheck-zero-division`.
`-mdivide-traps`
`-mdivide-breaks`
  MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (`SIGFPE`). Use `-mdivide-traps` to allow conditional traps on architectures that support them and `-mdivide-breaks` to force the use of breaks.

  The default is usually `-mdivide-traps`, but this can be overridden at configure time using `--with-divide=breaks`. Divide-by-zero checks can be completely disabled using `-mno-check-zero-division`.

`-mmemcpy`
`-mno-memcpy`
  Force (do not force) the use of `memcpy()` for non-trivial block moves. The default is `-mno-memcpy`, which allows GCC to inline most constant-sized copies.
`-mlong-calls`
`-mno-long-calls`
  Disable (do not disable) use of the `jal` instruction. Calling functions using `jal` is more efficient but requires the caller and callee to be in the same 256 megabyte segment.

  This option has no effect on abicalls code. The default is `-mno-long-calls`.

`-mmad`
`-mno-mad`
  Enable (disable) use of the `mad`, `madu` and `mul` instructions, as provided by the R4650 ISA.
`-mfused-madd`
`-mno-fused-madd`
  Enable (disable) use of the floating point multiply-accumulate instructions, when they are available. The default is `-mfused-madd`.

  When multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances.

`-nocpp`
  Tell the MIPS assembler to not run its preprocessor over user assembler files (with a ``.s`' suffix) when assembling them.
`-mfix-r4000`
`-mno-fix-r4000`
  Work around certain R4000 CPU errata:
  - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
  - A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
  - An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
`-mfix-r4400`
`-mno-fix-r4400`
  Work around certain R4400 CPU errata:
  - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
`-mfix-vr4120`
`-mno-fix-vr4120`
  Work around certain VR4120 errata:
  - `dmultu` does not always produce the correct result.
  - `div` and `ddiv` do not always produce the correct result if one of the operands is negative.
  The workarounds for the division errata rely on special functions in `libgcc.a`. At present, these functions are only provided by the `mips64vr*-elf` configurations.

  Other VR4120 errata require a nop to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

`-mfix-vr4130`
  Work around the VR4130 `mflo`/`mfhi` errata. The workarounds are implemented by the assembler rather than by GCC, although GCC will avoid using `mflo` and `mfhi` if the VR4130 `macc`, `macchi`, `dmacc` and `dmacchi` instructions are available instead.
`-mfix-sb1`
`-mno-fix-sb1`
  Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 “F1” and “F2” floating point errata.)
`-mflush-func=`func
`-mno-flush-func`
  Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common `_flush_func()`, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either ``_flush_func`' or ``__cpu_flush`'.
`-mbranch-likely`
`-mno-branch-likely`
  Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors which implement those architectures; for those, Branch Likely instructions will not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.
`-mfp-exceptions`
`-mno-fp-exceptions`
  Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions for some processors. The default is that FP exceptions are enabled.

  For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe.

`-mvr4130-align`
`-mno-vr4130-align`
  The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC will align pairs of instructions that it thinks should execute in parallel.

  This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level `-O3`.

Next: MN10300 Options, Previous: MIPS Options, Up: Submodel Options

#### 3.17.21 MMIX Options

These options are defined for the MMIX:

`-mlibfuncs`
`-mno-libfuncs`
  Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
`-mepsilon`
`-mno-epsilon`
  Generate floating-point comparison instructions that compare with respect to the `rE` epsilon register.
`-mabi=mmixware`
`-mabi=gnu`
  Generate code that passes function parameters and return values that (in the called function) are seen as registers `$0` and up, as opposed to the GNU ABI which uses global registers `$231` and up.
`-mzero-extend`
`-mno-zero-extend`
  When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
`-mknuthdiv`
`-mno-knuthdiv`
  Make the result of a division yielding a remainder have the same sign as the divisor. With the default, `-mno-knuthdiv`, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used.
`-mtoplevel-symbols`
`-mno-toplevel-symbols`
  Prepend (do not prepend) a ``:`' to all global symbols, so the assembly code can be used with the `PREFIX` assembly directive.
`-melf`
  Generate an executable in the ELF format, rather than the default ``mmo`' format used by the `mmix` simulator.
`-mbranch-predict`
`-mno-branch-predict`
  Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
`-mbase-addresses`
`-mno-base-addresses`
  Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require `-mno-base-addresses`.
`-msingle-exit`
`-mno-single-exit`
  Force (do not force) generated code to have a single exit point in each function.

Next: MT Options, Previous: MMIX Options, Up: Submodel Options

#### 3.17.22 MN10300 Options

These `-m` options are defined for Matsushita MN10300 architectures:

`-mmult-bug`
  Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
`-mno-mult-bug`
  Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
`-mam33`
  Generate code which uses features specific to the AM33 processor.
`-mno-am33`
  Do not generate code which uses features specific to the AM33 processor. This is the default.
`-mreturn-pointer-on-d0`
  When generating a function which returns a pointer, return the pointer in both `a0` and `d0`. Otherwise, the pointer is returned only in a0, and attempts to call such functions without a prototype would result in errors. Note that this option is on by default; use `-mno-return-pointer-on-d0` to disable it.
`-mno-crt0`
  Do not link in the C run-time initialization object file.
`-mrelax`
  Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.

  This option makes symbolic debugging impossible.

Next: PDP-11 Options, Previous: MN10300 Options, Up: Submodel Options

#### 3.17.23 MT Options

These `-m` options are defined for Morpho MT architectures:

`-march=`cpu-type
  Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are ``ms1-64-001`', ``ms1-16-002`', ``ms1-16-003`' and ``ms2`'.

  When this option is not used, the default is `-march=ms1-16-002`.

`-mbacc`
  Use byte loads and stores when generating code.
`-mno-bacc`
  Do not use byte loads and stores when generating code.
`-msim`
  Use simulator runtime
`-mno-crt0`
  Do not link in the C run-time initialization object file `crti.o`. Other run-time initialization and termination files such as `startup.o` and `exit.o` are still included on the linker command line.

Next: PowerPC Options, Previous: MT Options, Up: Submodel Options

#### 3.17.24 PDP-11 Options

These options are defined for the PDP-11:

`-mfpu`
  Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.)
`-msoft-float`
  Do not use hardware floating point.
`-mac0`
  Return floating-point results in ac0 (fr0 in Unix assembler syntax).
`-mno-ac0`
  Return floating-point results in memory. This is the default.
`-m40`
  Generate code for a PDP-11/40.
`-m45`
  Generate code for a PDP-11/45. This is the default.
`-m10`
  Generate code for a PDP-11/10.
`-mbcopy-builtin`
  Use inline `movmemhi` patterns for copying memory. This is the default.
`-mbcopy`
  Do not use inline `movmemhi` patterns for copying memory.
`-mint16`
`-mno-int32`
  Use 16-bit `int`. This is the default.
`-mint32`
`-mno-int16`
  Use 32-bit `int`.
`-mfloat64`
`-mno-float32`
  Use 64-bit `float`. This is the default.
`-mfloat32`
`-mno-float64`
  Use 32-bit `float`.
`-mabshi`
  Use `abshi2` pattern. This is the default.
`-mno-abshi`
  Do not use `abshi2` pattern.
`-mbranch-expensive`
  Pretend that branches are expensive. This is for experimenting with code generation only.
`-mbranch-cheap`
  Do not pretend that branches are expensive. This is the default.
`-msplit`
  Generate code for a system with split I&D.
`-mno-split`
  Generate code for a system without split I&D. This is the default.
`-munix-asm`
  Use Unix assembler syntax. This is the default when configured for ``pdp11-*-bsd`'.
`-mdec-asm`
  Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than ``pdp11-*-bsd`'.

Next: RS/6000 and PowerPC Options, Previous: PDP-11 Options, Up: Submodel Options

#### 3.17.25 PowerPC Options

These are listed under See RS/6000 and PowerPC Options.

Next: S/390 and zSeries Options, Previous: PowerPC Options, Up: Submodel Options

#### 3.17.26 IBM RS/6000 and PowerPC Options

These ``-m`' options are defined for the IBM RS/6000 and PowerPC:

`-mpower`
`-mno-power`
`-mpower2`
`-mno-power2`
`-mpowerpc`
`-mno-powerpc`
`-mpowerpc-gpopt`
`-mno-powerpc-gpopt`
`-mpowerpc-gfxopt`
`-mno-powerpc-gfxopt`
`-mpowerpc64`
`-mno-powerpc64`
`-mmfcrf`
`-mno-mfcrf`
`-mpopcntb`
`-mno-popcntb`
`-mfprnd`
`-mno-fprnd`
  GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the ``rios`' chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx, 6xx, and follow-on microprocessors.

  Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.

  You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the `-mcpu=cpu_type` overrides the specification of these options. We recommend you use the `-mcpu=cpu_type` option rather than the options listed above.

  The `-mpower` option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying `-mpower2` implies `-power` and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.

  The `-mpowerpc` option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying `-mpowerpc-gpopt` implies `-mpowerpc` and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt` implies `-mpowerpc` and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

  The `-mmfcrf` option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The `-mpopcntb` option allows GCC to generate the popcount and double precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The `-mfprnd` option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture.

  The `-mpowerpc64` option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to `-mno-powerpc64`.

  If you specify both `-mno-power` and `-mno-powerpc`, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both `-mpower` and `-mpowerpc` permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

`-mnew-mnemonics`
`-mold-mnemonics`
  Select which mnemonics to use in the generated assembler code. With `-mnew-mnemonics`, GCC uses the assembler mnemonics defined for the PowerPC architecture. With `-mold-mnemonics` it uses the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified.

  GCC defaults to the mnemonics appropriate for the architecture in use. Specifying `-mcpu=cpu_type` sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either `-mnew-mnemonics` or `-mold-mnemonics`, but should instead accept the default.

`-mcpu=`cpu_type
  Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``401`', ``403`', ``405`', ``405fp`', ``440`', ``440fp`', ``505`', ``601`', ``602`', ``603`', ``603e`', ``604`', ``604e`', ``620`', ``630`', ``740`', ``7400`', ``7450`', ``750`', ``801`', ``821`', ``823`', ``860`', ``970`', ``8540`', ``ec603e`', ``G3`', ``G4`', ``G5`', ``power`', ``power2`', ``power3`', ``power4`', ``power5`', ``power5+`', ``common`', ``powerpc`', ``powerpc64`', ``rios`', ``rios1`', ``rios2`', ``rsc`', and ``rs64`'.

  `-mcpu=common` selects a completely generic processor. Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes.

  `-mcpu=power`, `-mcpu=power2`, `-mcpu=powerpc`, and `-mcpu=powerpc64` specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

  The other options specify a specific processor. Code generated under those options will run best on that processor, and may not run at all on others.

  The `-mcpu` options automatically enable or disable the following options: `-maltivec`, `-mfprnd`, `-mhard-float`, `-mmfcrf`, `-mmultiple`, `-mnew-mnemonics`, `-mpopcntb`, `-mpower`, `-mpower2`, `-mpowerpc64`, `-mpowerpc-gpopt`, `-mpowerpc-gfxopt`, `-mstring`. The particular options set for any particular CPU will vary between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the `-mcpu` option, like ``-mcpu=970 -mno-altivec`'.

  On AIX, the `-maltivec` and `-mpowerpc64` options are not enabled or disabled by the `-mcpu` option at present because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment.

`-mtune=`cpu_type
  Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, or choice of mnemonics, as `-mcpu=cpu_type` would. The same values for cpu_type are used for `-mtune` as for `-mcpu`. If both are specified, the code generated will use the architecture, registers, and mnemonics set by `-mcpu`, but the scheduling parameters set by `-mtune`.
`-mswdiv`
`-mno-swdiv`
  Generate code to compute division as reciprocal estimate and iterative refinement, creating opportunities for increased throughput. This feature requires: optional PowerPC Graphics instruction set for single precision and FRE instruction for double precision, assuming divides cannot generate user-visible traps, and the domain values not include Infinities, denormals or zero denominator.
`-maltivec`
`-mno-altivec`
  Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set `-mabi=altivec` to adjust the current ABI with AltiVec ABI enhancements.
`-mvrsave`
`-mno-vrsave`
  Generate VRSAVE instructions when generating AltiVec code.
`-msecure-plt`
  Generate code that allows ld and ld.so to build executables and shared libraries with non-exec .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option.
`-mbss-plt`
  Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
`-misel`
`-mno-isel`
  This switch enables or disables the generation of ISEL instructions.
`-misel=`yes/no
  This switch has been deprecated. Use `-misel` and `-mno-isel` instead.
`-mspe`
`-mno-isel`
  This switch enables or disables the generation of SPE simd instructions.
`-mspe=`yes/no
  This option has been deprecated. Use `-mspe` and `-mno-spe` instead.
`-mfloat-gprs=`yes/single/double/no
`-mfloat-gprs`
  This switch enables or disables the generation of floating point operations on the general purpose registers for architectures that support it.

  The argument yes or single enables the use of single-precision floating point operations.

  The argument double enables the use of single and double-precision floating point operations.

  The argument no disables floating point operations on the general purpose registers.

  This option is currently only available on the MPC854x.

`-m32`
`-m64`
  Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for `-mpowerpc64`.
`-mfull-toc`
`-mno-fp-in-toc`
`-mno-sum-in-toc`
`-mminimal-toc`
  Modify generation of the TOC (Table Of Contents), which is created for every executable file. The `-mfull-toc` option is selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.

  If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the `-mno-fp-in-toc` and `-mno-sum-in-toc` options. `-mno-fp-in-toc` prevents GCC from putting floating-point constants in the TOC and `-mno-sum-in-toc` forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

  If you still run out of space in the TOC even when you specify both of these options, specify `-mminimal-toc` instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

`-maix64`
`-maix32`
  Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit `long` type, and the infrastructure needed to support them. Specifying `-maix64` implies `-mpowerpc64` and `-mpowerpc`, while `-maix32` disables the 64-bit ABI and implies `-mno-powerpc64`. GCC defaults to `-maix32`.
`-mxl-compat`
`-mno-xl-compat`
  Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines.

  The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization.

`-mpe`
  Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (`/usr/lpp/ppe.poe/`), or the `specs` file must be overridden with the `-specs=` option to specify the appropriate directory location. The Parallel Environment does not support threads, so the `-mpe` option and the `-pthread` option are incompatible.
`-malign-natural`
`-malign-power`
  On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option `-malign-natural` overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option `-malign-power` instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

  On 64-bit Darwin, natural alignment is the default, and `-malign-power` is not supported.

`-msoft-float`
`-mhard-float`
  Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the `-msoft-float` option, and pass the option to GCC when linking.
`-mmultiple`
`-mno-multiple`
  Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mmultiple` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.
`-mstring`
`-mno-string`
  Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mstring` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.
`-mupdate`
`-mno-update`
  Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use `-mno-update`, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.
`-mfused-madd`
`-mno-fused-madd`
  Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used.
`-mno-bit-align`
`-mbit-align`
  On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

  For example, by default a structure containing nothing but 8 `unsigned` bit-fields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using `-mno-bit-align`, the structure would be aligned to a 1 byte boundary and be one byte in size.

`-mno-strict-align`
`-mstrict-align`
  On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.
`-mrelocatable`
`-mno-relocatable`
  On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. If you use `-mrelocatable` on any module, all objects linked together must be compiled with `-mrelocatable` or `-mrelocatable-lib`.
`-mrelocatable-lib`
`-mno-relocatable-lib`
  On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. Modules compiled with `-mrelocatable-lib` can be linked with either modules compiled without `-mrelocatable` and `-mrelocatable-lib` or with modules compiled with the `-mrelocatable` options.
`-mno-toc`
`-mtoc`
  On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
`-mlittle`
`-mlittle-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The `-mlittle-endian` option is the same as `-mlittle`.
`-mbig`
`-mbig-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The `-mbig-endian` option is the same as `-mbig`.
`-mdynamic-no-pic`
  On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries.
`-mprioritize-restricted-insns=`priority
  This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0/1/2 to assign no/highest/second-highest priority to dispatch slot restricted instructions.
`-msched-costly-dep=`dependence_type
  This option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values: no: no dependence is costly, all: all dependences are costly, true_store_to_load: a true dependence from store to load is costly, store_to_load: any dependence from store to load is costly, number: any dependence which latency >= number is costly.
`-minsert-sched-nops=`scheme
  This option controls which nop insertion scheme will be used during the second scheduling pass. The argument scheme takes one of the following values: no: Don't insert nops. pad: Pad with nops any dispatch group which has vacant issue slots, according to the scheduler's grouping. regroup_exact: Insert nops to force costly dependent insns into separate groups. Insert exactly as many nops as needed to force an insn to a new group, according to the estimated processor grouping. number: Insert nops to force costly dependent insns into separate groups. Insert number nops to force an insn to a new group.
`-mcall-sysv`
  On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using ``powerpc-*-eabiaix`'.
`-mcall-sysv-eabi`
  Specify both `-mcall-sysv` and `-meabi` options.
`-mcall-sysv-noeabi`
  Specify both `-mcall-sysv` and `-mno-eabi` options.
`-mcall-solaris`
  On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.
`-mcall-linux`
  On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
`-mcall-gnu`
  On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.
`-mcall-netbsd`
  On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
`-maix-struct-return`
  Return all structures in memory (as specified by the AIX ABI).
`-msvr4-struct-return`
  Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
`-mabi=`abi-type
  Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec, no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble.
`-mabi=spe`
  Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI.
`-mabi=no-spe`
  Disable Booke SPE ABI extensions for the current ABI.
`-mabi=ibmlongdouble`
  Change the current ABI to use IBM extended precision long double. This is a PowerPC 32-bit SYSV ABI option.
`-mabi=ieeelongdouble`
  Change the current ABI to use IEEE extended precision long double. This is a PowerPC 32-bit Linux ABI option.
`-mprototype`
`-mno-prototype`
  On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With `-mprototype`, only calls to prototyped variable argument functions will set or clear the bit.
`-msim`
  On embedded PowerPC systems, assume that the startup module is called `sim-crt0.o` and that the standard C libraries are `libsim.a` and `libc.a`. This is the default for ``powerpc-*-eabisim`'. configurations.
`-mmvme`
  On embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libmvme.a` and `libc.a`.
`-mads`
  On embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libads.a` and `libc.a`.
`-myellowknife`
  On embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libyk.a` and `libc.a`.
`-mvxworks`
  On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
`-mwindiss`
  Specify that you are compiling for the WindISS simulation environment.
`-memb`
  On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that ``eabi`' extended relocations are used.
`-meabi`
`-mno-eabi`
  On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting `-meabi` means that the stack is aligned to an 8 byte boundary, a function `__eabi` is called to from `main` to set up the eabi environment, and the `-msdata` option can use both `r2` and `r13` to point to two separate small data areas. Selecting `-mno-eabi` means that the stack is aligned to a 16 byte boundary, do not call an initialization function from `main`, and the `-msdata` option will only use `r13` to point to a single small data area. The `-meabi` option is on by default if you configured GCC using one of the ``powerpc*-*-eabi*`' options.
`-msdata=eabi`
  On System V.4 and embedded PowerPC systems, put small initialized `const` global and static data in the ``.sdata2`' section, which is pointed to by register `r2`. Put small initialized non-`const` global and static data in the ``.sdata`' section, which is pointed to by register `r13`. Put small uninitialized global and static data in the ``.sbss`' section, which is adjacent to the ``.sdata`' section. The `-msdata=eabi` option is incompatible with the `-mrelocatable` option. The `-msdata=eabi` option also sets the `-memb` option.
`-msdata=sysv`
  On System V.4 and embedded PowerPC systems, put small global and static data in the ``.sdata`' section, which is pointed to by register `r13`. Put small uninitialized global and static data in the ``.sbss`' section, which is adjacent to the ``.sdata`' section. The `-msdata=sysv` option is incompatible with the `-mrelocatable` option.
`-msdata=default`
`-msdata`
  On System V.4 and embedded PowerPC systems, if `-meabi` is used, compile code the same as `-msdata=eabi`, otherwise compile code the same as `-msdata=sysv`.
`-msdata-data`
  On System V.4 and embedded PowerPC systems, put small global and static data in the ``.sdata`' section. Put small uninitialized global and static data in the ``.sbss`' section. Do not use register `r13` to address small data however. This is the default behavior unless other `-msdata` options are used.
`-msdata=none`
`-mno-sdata`
  On embedded PowerPC systems, put all initialized global and static data in the ``.data`' section, and all uninitialized data in the ``.bss`' section.
`-G` num
  On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The `-G num` switch is also passed to the linker. All modules should be compiled with the same `-G num` value.
`-mregnames`
`-mno-regnames`
  On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.
`-mlongcall`
`-mno-longcall`
  Default to making all function calls indirectly, using a register, so that functions which reside further than 32 megabytes (33,554,432 bytes) from the current location can be called. This setting can be overridden by the `shortcall` function attribute, or by `#pragma longcall(0)`.

  Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

  On Darwin/PPC systems, `#pragma longcall` will generate “jbsr callee, L42”, plus a “branch island” (glue code). The two target addresses represent the callee and the “branch island”. The Darwin/PPC linker will prefer the first address and generate a “bl callee” if the PPC “bl” instruction will reach the callee directly; otherwise, the linker will generate “bl L42” to call the “branch island”. The “branch island” is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

  On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

  In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.

`-pthread`
  Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

Next: SH Options, Previous: RS/6000 and PowerPC Options, Up: Submodel Options

#### 3.17.27 S/390 and zSeries Options

These are the ``-m`' options defined for the S/390 and zSeries architecture.

`-mhard-float`
`-msoft-float`
  Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When `-msoft-float` is specified, functions in `libgcc.a` will be used to perform floating-point operations. When `-mhard-float` is specified, the compiler generates IEEE floating-point instructions. This is the default.
`-mlong-double-64`
`-mlong-double-128`
  These switches control the size of `long double` type. A size of 64bit makes the `long double` type equivalent to the `double` type. This is the default.
`-mbackchain`
`-mno-backchain`
  Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call frame information. When `-mno-packed-stack` is in effect, the backchain pointer is stored at the bottom of the stack frame; when `-mpacked-stack` is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

  In general, code compiled with `-mbackchain` is call-compatible with code compiled with `-mmo-backchain`; however, use of the backchain for debugging purposes usually requires that the whole binary is built with `-mbackchain`. Note that the combination of `-mbackchain`, `-mpacked-stack` and `-mhard-float` is not supported. In order to build a linux kernel use `-msoft-float`.

  The default is to not maintain the backchain.

`-mpacked-stack`
`-mno-packed-stack`
  Use (do not use) the packed stack layout. When `-mno-packed-stack` is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When `-mpacked-stack` is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when `-mbackchain` is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

  As long as the stack frame backchain is not used, code generated with `-mpacked-stack` is call-compatible with code generated with `-mno-packed-stack`. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with `-mpacked-stack`. Also, note that the combination of `-mbackchain`, `-mpacked-stack` and `-mhard-float` is not supported. In order to build a linux kernel use `-msoft-float`.

  The default is to not use the packed stack layout.

`-msmall-exec`
`-mno-small-exec`
  Generate (or do not generate) code using the `bras` instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the `basr` instruction instead, which does not have this limitation.
`-m64`
`-m31`
  When `-m31` is specified, generate code compliant to the GNU/Linux for S/390 ABI. When `-m64` is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the ``s390`' targets, the default is `-m31`, while the ``s390x`' targets default to `-m64`.
`-mzarch`
`-mesa`
  When `-mzarch` is specified, generate code using the instructions available on z/Architecture. When `-mesa` is specified, generate code using the instructions available on ESA/390. Note that `-mesa` is not possible with `-m64`. When generating code compliant to the GNU/Linux for S/390 ABI, the default is `-mesa`. When generating code compliant to the GNU/Linux for zSeries ABI, the default is `-mzarch`.
`-mmvcle`
`-mno-mvcle`
  Generate (or do not generate) code using the `mvcle` instruction to perform block moves. When `-mno-mvcle` is specified, use a `mvc` loop instead. This is the default unless optimizing for size.
`-mdebug`
`-mno-debug`
  Print (or do not print) additional debug information when compiling. The default is to not print debug information.
`-march=`cpu-type
  Generate code that will run on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are ``g5`', ``g6`', ``z900`', and ``z990`'. When generating code using the instructions available on z/Architecture, the default is `-march=z900`. Otherwise, the default is `-march=g5`.
`-mtune=`cpu-type
  Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for `-march`. The default is the value used for `-march`.
`-mtpf-trace`
`-mno-tpf-trace`
  Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.
`-mfused-madd`
`-mno-fused-madd`
  Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
`-mwarn-framesize=`framesize
  Emit a warning if the current function exceeds the given frame size. Because this is a compile time check it doesn't need to be a real problem when the program runs. It is intended to identify functions which most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
`-mwarn-dynamicstack`
  Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a bad idea with a limited stack size.
`-mstack-guard=`stack-guard
`-mstack-size=`stack-size
  These arguments always have to be used in conjunction. If they are present the s390 back end emits additional instructions in the function prologue which trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on s390 grows downward). These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size.

Next: SPARC Options, Previous: S/390 and zSeries Options, Up: Submodel Options

#### 3.17.28 SH Options

These ``-m`' options are defined for the SH implementations:

`-m1`
  Generate code for the SH1.
`-m2`
  Generate code for the SH2.
`-m2e`
  Generate code for the SH2e.
`-m3`
  Generate code for the SH3.
`-m3e`
  Generate code for the SH3e.
`-m4-nofpu`
  Generate code for the SH4 without a floating-point unit.
`-m4-single-only`
  Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
`-m4-single`
  Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
`-m4`
  Generate code for the SH4.
`-m4a-nofpu`
  Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
`-m4a-single-only`
  Generate code for the SH4a, in such a way that no double-precision floating point operations are used.
`-m4a-single`
  Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
`-m4a`
  Generate code for the SH4a.
`-m4al`
  Same as `-m4a-nofpu`, except that it implicitly passes `-dsp` to the assembler. GCC doesn't generate any DSP instructions at the moment.
`-mb`
  Compile code for the processor in big endian mode.
`-ml`
  Compile code for the processor in little endian mode.
`-mdalign`
  Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with `-mdalign`.
`-mrelax`
  Shorten some address references at link time, when possible; uses the linker option `-relax`.
`-mbigtable`
  Use 32-bit offsets in `switch` tables. The default is to use 16-bit offsets.
`-mfmovd`
  Enable the use of the instruction `fmovd`.
`-mhitachi`
  Comply with the calling conventions defined by Renesas.
`-mrenesas`
  Comply with the calling conventions defined by Renesas.
`-mno-renesas`
  Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain except for ``sh-symbianelf`'.
`-mnomacsave`
  Mark the `MAC` register as call-clobbered, even if `-mhitachi` is given.
`-mieee`
  Increase IEEE-compliance of floating-point code. At the moment, this is equivalent to `-fno-finite-math-only`. When generating 16 bit SH opcodes, getting IEEE-conforming results for comparisons of NANs / infinities incurs extra overhead in every floating point comparison, therefore the default is set to `-ffinite-math-only`.
`-misize`
  Dump instruction size and location in the assembly code.
`-mpadstruct`
  This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
`-mspace`
  Optimize for space instead of speed. Implied by `-Os`.
`-mprefergot`
  When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.
`-musermode`
  Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline. This library function call doesn't assume it can write to the whole memory address space. This is the default when the target is `sh-*-linux*`.
`-multcost=`number
  Set the cost to assume for a multiply insn.
`-mdiv=`strategy
  Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating point. This has a very high latency, but needs only a few instructions, so it might be a good choice if your code has enough easily exploitable ILP to allow the compiler to schedule the floating point instructions together with other instructions. Division by zero causes a floating point exception. "inv" uses integer operations to calculate the inverse of the divisor, and then multiplies the dividend with the inverse. This strategy allows cse and hoisting of the inverse calculation. Division by zero calculates an unspecified result, but does not trap. "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities have been found, or if the entire operation has been hoisted to the same place, the last stages of the inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense of using a few more instructions, and thus offering fewer scheduling opportunities with other code. "call" calls a library function that usually implements the inv:minlat strategy. This gives high code density for m5-*media-nofpu compilations. "call2" uses a different entry point of the same library function, where it assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to cse / code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial code generation, but if the code stays unoptimized, revert to the "call", "call2", or "fp" strategies, respectively. Note that the potentially-trapping side effect of division by zero is carried by a separate instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the side effect stays where it is. A recombination to fp operations or a call is not possible in that case. "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case that the inverse calculation was nor separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable), by inserting a test to skip a number of operations in this case; this test slows down the case of larger dividends. inv20u assumes the case of a such a small dividend to be unlikely, and inv20l assumes it to be likely.
`-mdivsi3_libfunc=`name
  Set the name of the library function used for 32 bit signed division to name. This only affect the name used in the call and inv:call division strategies, and the compiler will still expect the same sets of input/output/clobbered registers as if this option was not present.
`-madjust-unroll`
  Throttle unrolling to avoid thrashing target registers. This option only has an effect if the gcc code base supports the TARGET_ADJUST_UNROLL_MAX target hook.
`-mindexed-addressing`
  Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware and/or OS implement 32 bit wrap-around semantics for the indexed addressing mode. The architecture allows the implementation of processors with 64 bit MMU, which the OS could use to get 32 bit addressing, but since no current hardware implementation supports this or any other way to make the indexed addressing mode safe to use in the 32 bit ABI, the default is -mno-indexed-addressing.
`-mgettrcost=`number
  Set the cost assumed for the gettr instruction to number. The default is 2 if `-mpt-fixed` is in effect, 100 otherwise.
`-mpt-fixed`
  Assume pt* instructions won't trap. This will generally generate better scheduled code, but is unsafe on current hardware. The current architecture definition says that ptabs and ptrel trap when the target anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule ptabs / ptrel before a branch, or hoist it out of a loop. For example, __do_global_ctors, a part of libgcc that runs constructors at program startup, calls functions in a list which is delimited by -1. With the -mpt-fixed option, the ptabs will be done before testing against -1. That means that all the constructors will be run a bit quicker, but when the loop comes to the end of the list, the program crashes because ptabs loads -1 into a target register. Since this option is unsafe for any hardware implementing the current architecture specification, the default is -mno-pt-fixed. Unless the user specifies a specific cost with `-mgettrcost`, -mno-pt-fixed also implies `-mgettrcost=100`; this deters register allocation using target registers for storing ordinary integers.
`-minvalid-symbols`
  Assume symbols might be invalid. Ordinary function symbols generated by the compiler will always be valid to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible to generate symbols that will cause ptabs / ptrel to trap. This option is only meaningful when `-mno-pt-fixed` is in effect. It will then prevent cross-basic-block cse, hoisting and most scheduling of symbol loads. The default is `-mno-invalid-symbols`.

Next: System V Options, Previous: SH Options, Up: Submodel Options

#### 3.17.29 SPARC Options

These ``-m`' options are supported on the SPARC:

`-mno-app-regs`
`-mapp-regs`
  Specify `-mapp-regs` to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default.

  To be fully SVR4 ABI compliant at the cost of some performance loss, specify `-mno-app-regs`. You should compile libraries and system software with this option.

`-mfpu`
`-mhard-float`
  Generate output containing floating point instructions. This is the default.
`-mno-fpu`
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets ``sparc-*-aout`' and ``sparclite-*-*`' do provide software floating point support.

  `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work.

`-mhard-quad-float`
  Generate output containing quad-word (long double) floating point instructions.
`-msoft-quad-float`
  Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default.

  As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the `-msoft-quad-float` option is the default.

`-mno-unaligned-doubles`
`-munaligned-doubles`
  Assume that doubles have 8 byte alignment. This is the default.

  With `-munaligned-doubles`, GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.

`-mno-faster-structs`
`-mfaster-structs`
  With `-mfaster-structs`, the compiler assumes that structures should have 8 byte alignment. This enables the use of pairs of `ldd` and `std` instructions for copies in structure assignment, in place of twice as many `ld` and `st` pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.
`-mimpure-text`
  `-mimpure-text`, used in addition to `-shared`, tells the compiler to not pass `-z text` to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

  `-mimpure-text` suppresses the “relocations remain against allocatable but non-writable sections” linker error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using `-mimpure-text`, you should compile all source code with `-fpic` or `-fPIC`.

  This option is only available on SunOS and Solaris.

`-mcpu=`cpu_type
  Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``v7`', ``cypress`', ``v8`', ``supersparc`', ``sparclite`', ``f930`', ``f934`', ``hypersparc`', ``sparclite86x`', ``sparclet`', ``tsc701`', ``v9`', ``ultrasparc`', and ``ultrasparc3`'.

  Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are ``v7`', ``v8`', ``sparclite`', ``sparclet`', ``v9`'.

  Here is a list of each supported architecture and their supported implementations.

                    v7:             cypress
                    v8:             supersparc, hypersparc
                    sparclite:      f930, f934, sparclite86x
                    sparclet:       tsc701
                    v9:             ultrasparc, ultrasparc3

  By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With `-mcpu=cypress`, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

  With `-mcpu=v8`, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With `-mcpu=supersparc`, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

  With `-mcpu=sparclite`, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan (`ffs`) instructions which exist in SPARClite but not in SPARC-V7. With `-mcpu=f930`, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With `-mcpu=f934`, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

  With `-mcpu=sparclet`, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan (`ffs`) instructions which exist in SPARClet but not in SPARC-V7. With `-mcpu=tsc701`, the compiler additionally optimizes it for the TEMIC SPARClet chip.

  With `-mcpu=v9`, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With `-mcpu=ultrasparc`, the compiler additionally optimizes it for the Sun UltraSPARC I/II chips. With `-mcpu=ultrasparc3`, the compiler additionally optimizes it for the Sun UltraSPARC III chip.

`-mtune=`cpu_type
  Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option `-mcpu=cpu_type` would.

  The same values for `-mcpu=cpu_type` can be used for `-mtune=cpu_type`, but the only useful values are those that select a particular cpu implementation. Those are ``cypress`', ``supersparc`', ``hypersparc`', ``f930`', ``f934`', ``sparclite86x`', ``tsc701`', ``ultrasparc`', and ``ultrasparc3`'.

`-mv8plus`
`-mno-v8plus`
  With `-mv8plus`, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
`-mvis`
`-mno-vis`
  With `-mvis`, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is `-mno-vis`.

These ``-m`' options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:

`-mlittle-endian`
  Generate code for a processor running in little-endian mode. It is only available for a few configurations and most notably not on Solaris and Linux.
`-m32`
`-m64`
  Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
`-mcmodel=medlow`
  Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
`-mcmodel=medmid`
  Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
`-mcmodel=medany`
  Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
`-mcmodel=embmedany`
  Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported.
`-mstack-bias`
`-mno-stack-bias`
  With `-mstack-bias`, GCC assumes that the stack pointer, and frame pointer if present, are offset by −2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

These switches are supported in addition to the above on Solaris:

`-threads`
  Add support for multithreading using the Solaris threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.
`-pthreads`
  Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.
`-pthread`
  This is a synonym for `-pthreads`.

Next: TMS320C3x/C4x Options, Previous: SPARC Options, Up: Submodel Options

#### 3.17.30 Options for System V

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

`-G`
  Create a shared object. It is recommended that `-symbolic` or `-shared` be used instead.
`-Qy`
  Identify the versions of each tool used by the compiler, in a `.ident` assembler directive in the output.
`-Qn`
  Refrain from adding `.ident` directives to the output file (this is the default).
`-YP,`dirs
  Search the directories dirs, and no others, for libraries specified with `-l`.
`-Ym,`dir
  Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

Next: V850 Options, Previous: System V Options, Up: Submodel Options

#### 3.17.31 TMS320C3x/C4x Options

These ``-m`' options are defined for TMS320C3x/C4x implementations:

`-mcpu=`cpu_type
  Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``c30`', ``c31`', ``c32`', ``c40`', and ``c44`'. The default is ``c40`' to generate code for the TMS320C40.
`-mbig-memory`
`-mbig`
`-msmall-memory`
`-msmall`
  Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access.
`-mbk`
`-mno-bk`
  Allow (disallow) allocation of general integer operands into the block count register BK.
`-mdb`
`-mno-db`
  Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^23 + 1 (but who iterates loops more than 2^23 times on the C3x?). Note that GCC will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized.
`-mdp-isr-reload`
`-mparanoid`
  Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library.
`-mmpyi`
`-mno-mpyi`
  For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the `-mmpyi` option is not specified for the C3x, then squaring operations are performed inline instead of a library call.
`-mfast-fix`
`-mno-fast-fix`
  The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of the additional code required to correct the result.
`-mrptb`
`-mno-rptb`
  Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with `-O2`.
`-mrpts=`count
`-mno-rpts`
  Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands. However, since interrupts are blocked by this instruction, it is disabled by default.
`-mloop-unsigned`
`-mno-loop-unsigned`
  The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^31 + 1 since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^31 + 1 maximum iteration count may be exceeded. This switch allows an unsigned iteration count.
`-mti`
  Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers.
`-mregparm`
`-mmemparm`
  Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack.
`-mparallel-insns`
`-mno-parallel-insns`
  Allow the generation of parallel instructions. This is enabled by default with `-O2`.
`-mparallel-mpy`
`-mno-parallel-mpy`
  Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided `-mparallel-insns` is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.

Next: VAX Options, Previous: TMS320C3x/C4x Options, Up: Submodel Options

#### 3.17.32 V850 Options

These ``-m`' options are defined for V850 implementations:

`-mlong-calls`
`-mno-long-calls`
  Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer.
`-mno-ep`
`-mep`
  Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the `ep` register, and use the shorter `sld` and `sst` instructions. The `-mep` option is on by default if you optimize.
`-mno-prolog-function`
`-mprolog-function`
  Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The `-mprolog-function` option is on by default if you optimize.
`-mspace`
  Try to make the code as small as possible. At present, this just turns on the `-mep` and `-mprolog-function` options.
`-mtda=`n
  Put static or global variables whose size is n bytes or less into the tiny data area that register `ep` points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
`-msda=`n
  Put static or global variables whose size is n bytes or less into the small data area that register `gp` points to. The small data area can hold up to 64 kilobytes.
`-mzda=`n
  Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
`-mv850`
  Specify that the target processor is the V850.
`-mbig-switch`
  Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.
`-mapp-regs`
  This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is the default.
`-mno-app-regs`
  This option will cause r2 and r5 to be treated as fixed registers.
`-mv850e1`
  Specify that the target processor is the V850E1. The preprocessor constants ``__v850e1__`' and ``__v850e__`' will be defined if this option is used.
`-mv850e`
  Specify that the target processor is the V850E. The preprocessor constant ``__v850e__`' will be defined if this option is used.

  If neither `-mv850` nor `-mv850e` nor `-mv850e1` are defined then a default target processor will be chosen and the relevant ``__v850*__`' preprocessor constant will be defined.

  The preprocessor constants ``__v850`' and ``__v851__`' are always defined, regardless of which processor variant is the target.

`-mdisable-callt`
  This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850 architecture. The default is `-mno-disable-callt` which allows the CALLT instruction to be used.

Next: x86-64 Options, Previous: V850 Options, Up: Submodel Options

#### 3.17.33 VAX Options

These ``-m`' options are defined for the VAX:

`-munix`
  Do not output certain jump instructions (`aobleq` and so on) that the Unix assembler for the VAX cannot handle across long ranges.
`-mgnu`
  Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
`-mg`
  Output code for g-format floating point numbers instead of d-format.

Next: Xstormy16 Options, Previous: VAX Options, Up: Submodel Options

#### 3.17.34 x86-64 Options

These are listed under See i386 and x86-64 Options.

Next: Xtensa Options, Previous: x86-64 Options, Up: Submodel Options

#### 3.17.35 Xstormy16 Options

These options are defined for Xstormy16:

`-msim`
  Choose startup files and linker script suitable for the simulator.

Next: zSeries Options, Previous: Xstormy16 Options, Up: Submodel Options

#### 3.17.36 Xtensa Options

These options are supported for Xtensa targets:

`-mconst16`
`-mno-const16`
  Enable or disable use of `CONST16` instructions for loading constant values. The `CONST16` instruction is currently not a standard option from Tensilica. When enabled, `CONST16` instructions are always used in place of the standard `L32R` instructions. The use of `CONST16` is enabled by default only if the `L32R` instruction is not available.
`-mfused-madd`
`-mno-fused-madd`
  Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations.
`-mtext-section-literals`
`-mno-text-section-literals`
  Control the treatment of literal pools. The default is `-mno-text-section-literals`, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With `-mtext-section-literals`, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files.
`-mtarget-align`
`-mno-target-align`
  When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening will be performed. The default is `-mtarget-align`. These options do not affect the treatment of auto-aligned instructions like `LOOP`, which the assembler will always align, either by widening density instructions or by inserting no-op instructions.
`-mlongcalls`
`-mno-longcalls`
  When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct `CALL` instruction into an `L32R` followed by a `CALLX` instruction. The default is `-mno-longcalls`. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC will still show direct call instructions—look at the disassembled object code to see the actual instructions. Note that the assembler will use an indirect call for every cross-file call, not just those that really will be out of range.

Previous: Xtensa Options, Up: Submodel Options

#### 3.17.37 zSeries Options

These are listed under See S/390 and zSeries Options.

Next: Environment Variables, Previous: Submodel Options, Up: Invoking GCC

### 3.18 Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of `-ffoo` would be `-fno-foo`. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing ``no-`' or adding it.

`-fbounds-check`
  For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran 77 front-ends, where this option defaults to true and false respectively.
`-ftrapv`
  This option generates traps for signed overflow on addition, subtraction, multiplication operations.
`-fwrapv`
  This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. This option is enabled by default for the Java front-end, as required by the Java language specification.
`-fexceptions`
  Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC will generate frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC will enable it by default for languages like C++ which normally require exception handling, and disable it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.
`-fnon-call-exceptions`
  Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as `SIGALRM`.
`-funwind-tables`
  Similar to `-fexceptions`, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf.
`-fasynchronous-unwind-tables`
  Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
`-fpcc-struct-return`
  Return “short” `struct` and `union` values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

  The precise convention for returning structures in memory depends on the target configuration macros.

  Short structures and unions are those whose size and alignment match that of some integer type.

  Warning: code compiled with the `-fpcc-struct-return` switch is not binary compatible with code compiled with the `-freg-struct-return` switch. Use it to conform to a non-default application binary interface.

`-freg-struct-return`
  Return `struct` and `union` values in registers when possible. This is more efficient for small structures than `-fpcc-struct-return`.

  If you specify neither `-fpcc-struct-return` nor `-freg-struct-return`, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to `-fpcc-struct-return`, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

  Warning: code compiled with the `-freg-struct-return` switch is not binary compatible with code compiled with the `-fpcc-struct-return` switch. Use it to conform to a non-default application binary interface.

`-fshort-enums`
  Allocate to an `enum` type only as many bytes as it needs for the declared range of possible values. Specifically, the `enum` type will be equivalent to the smallest integer type which has enough room.

  Warning: the `-fshort-enums` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fshort-double`
  Use the same size for `double` as for `float`.

  Warning: the `-fshort-double` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fshort-wchar`
  Override the underlying type for ``wchar_t`' to be ``short unsigned int`' instead of the default for the target. This option is useful for building programs to run under WINE.

  Warning: the `-fshort-wchar` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fshared-data`
  Requests that the data and non-`const` variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.
`-fno-common`
  In C, allocate even uninitialized global variables in the data section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without `extern`) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.
`-fno-ident`
  Ignore the ``#ident`' directive.
`-finhibit-size-directive`
  Don't output a `.size` assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c`; you should not need to use it for anything else.
`-fverbose-asm`
  Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

  `-fno-verbose-asm`, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

`-fpic`
  Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that `-fpic` does not work; in that case, recompile with `-fPIC` instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no such limit.)

  Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

`-fPIC`
  If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, PowerPC and SPARC.

  Position-independent code requires special support, and therefore works only on certain machines.

`-fpie`
`-fPIE`
  These options are similar to `-fpic` and `-fPIC`, but generated position independent code can be only linked into executables. Usually these options are used when `-pie` GCC option will be used during linking.
`-fno-jump-tables`
  Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with `-fpic` or `-fPIC` for building code which forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.
`-ffixed-`reg
  Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

  reg must be the name of a register. The register names accepted are machine-specific and are defined in the `REGISTER_NAMES` macro in the machine description macro file.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fcall-used-`reg
  Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg.

  It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fcall-saved-`reg
  Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

  It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

  A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fpack-struct[=`n`]`
  Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this will be output potentially unaligned at the next fitting location.

  Warning: the `-fpack-struct` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

`-finstrument-functions`
  Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site. (On some platforms, `__builtin_return_address` does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

                void __cyg_profile_func_enter (void *this_fn,
                                               void *call_site);
                void __cyg_profile_func_exit  (void *this_fn,
                                               void *call_site);

  The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

  This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use ``extern inline`' in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

  A function may be given the attribute `no_instrument_function`, in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

`-fstack-check`
  Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

  Note that this switch does not actually cause checking to be done; the operating system must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended.

`-fstack-limit-register=`reg
`-fstack-limit-symbol=`sym
`-fno-stack-limit`
  Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If the stack would grow beyond the value, a signal is raised. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

  For instance, if the stack starts at absolute address ``0x80000000`' and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit` and `-Wl,--defsym,__stack_limit=0x7ffe0000` to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

`-fargument-alias`
`-fargument-noalias`
`-fargument-noalias-global`
  Specify the possible relationships among parameters and between parameters and global data.

  `-fargument-alias` specifies that arguments (parameters) may alias each other and may alias global storage.
  `-fargument-noalias` specifies that arguments do not alias each other, but may alias global storage.
  `-fargument-noalias-global` specifies that arguments do not alias each other and do not alias global storage.

  Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.

`-fleading-underscore`
  This option and its counterpart, `-fno-leading-underscore`, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

  Warning: the `-fleading-underscore` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

`-ftls-model=`model
  Alter the thread-local storage model to be used (see Thread-Local). The model argument should be one of `global-dynamic`, `local-dynamic`, `initial-exec` or `local-exec`.

  The default without `-fpic` is `initial-exec`; with `-fpic` the default is `global-dynamic`.

`-fvisibility=`default|internal|hidden|protected
  Set the default ELF image symbol visibility to the specified option—all symbols will be marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

  Despite the nomenclature, `default` always means public ie; available to be linked against from outside the shared object. `protected` and `internal` are pretty useless in real-world usage so the only other commonly used option will be `hidden`. The default if `-fvisibility` isn't specified is `default`, i.e., make every symbol public—this causes the same behavior as previous versions of GCC.

  A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by “How To Write Shared Libraries” by Ulrich Drepper (which can be found at http://people.redhat.com/~drepper/)—however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLL's on Windows and with `-fvisibility=hidden` and `__attribute__ ((visibility("default")))` instead of `__declspec(dllexport)` you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.

  For those adding visibility support to existing code, you may find ``#pragma GCC visibility`' of use. This works by you enclosing the declarations you wish to set visibility for with (for example) ``#pragma GCC visibility push(hidden)`' and ``#pragma GCC visibility pop`'. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads—making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

  An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/wiki/Visibility.

Next: Precompiled Headers, Previous: Code Gen Options, Up: Invoking GCC

### 3.19 Environment Variables Affecting GCC

This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as `-B`, `-I` and `-L` (see Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Controlling the Compilation Driver `gcc`.

`LANG`
`LC_CTYPE`
`LC_MESSAGES`
`LC_ALL`
  These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions. GCC inspects the locale categories `LC_CTYPE` and `LC_MESSAGES` if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is ``en_GB.UTF-8`' for English in the United Kingdom encoded in UTF-8.

  The `LC_CTYPE` environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.

  The `LC_MESSAGES` environment variable specifies the language to use in diagnostic messages.

  If the `LC_ALL` environment variable is set, it overrides the value of `LC_CTYPE` and `LC_MESSAGES`; otherwise, `LC_CTYPE` and `LC_MESSAGES` default to the value of the `LANG` environment variable. If none of these variables are set, GCC defaults to traditional C English behavior.

`TMPDIR`
  If `TMPDIR` is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.
`GCC_EXEC_PREFIX`
  If `GCC_EXEC_PREFIX` is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

  If `GCC_EXEC_PREFIX` is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.

  If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

  The default value of `GCC_EXEC_PREFIX` is `prefix/lib/gcc/` where prefix is the value of `prefix` when you ran the `configure` script.

  Other prefixes specified with `-B` take precedence over this prefix.

  This prefix is also used for finding files such as `crt0.o` that are used for linking.

  In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with ``/usr/local/lib/gcc`' (more precisely, with the value of `GCC_INCLUDE_DIR`), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with `-Bfoo/`, GCC will search `foo/bar` where it would normally search `/usr/local/lib/bar`. These alternate directories are searched first; the standard directories come next.

`COMPILER_PATH`
  The value of `COMPILER_PATH` is a colon-separated list of directories, much like `PATH`. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using `GCC_EXEC_PREFIX`.
`LIBRARY_PATH`
  The value of `LIBRARY_PATH` is a colon-separated list of directories, much like `PATH`. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using `GCC_EXEC_PREFIX`. Linking using GCC also uses these directories when searching for ordinary libraries for the `-l` option (but directories specified with `-L` come first).
`LANG`
  This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for `LANG` are recognized:
  ``C-JIS`'
    Recognize JIS characters.
  ``C-SJIS`'
    Recognize SJIS characters.
  ``C-EUCJP`'
    Recognize EUCJP characters.

  If `LANG` is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

`CPATH`
`C_INCLUDE_PATH`
`CPLUS_INCLUDE_PATH`
`OBJC_INCLUDE_PATH`
  Each variable's value is a list of directories separated by a special character, much like `PATH`, in which to look for header files. The special character, `PATH_SEPARATOR`, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

  `CPATH` specifies a list of directories to be searched as if specified with `-I`, but after any paths given with `-I` options on the command line. This environment variable is used regardless of which language is being preprocessed.

  The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with `-isystem`, but after any paths given with `-isystem` options on the command line.

  In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of `CPATH` is `:/special/include`, that has the same effect as ``-I. -I/special/include`'.

`DEPENDENCIES_OUTPUT`
  If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

  The value of `DEPENDENCIES_OUTPUT` can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form ``file target`', in which case the rules are written to file file using target as the target name.

  In other words, this environment variable is equivalent to combining the options `-MM` and `-MF` (see Preprocessor Options), with an optional `-MT` switch too.

`SUNPRO_DEPENDENCIES`
  This variable is the same as `DEPENDENCIES_OUTPUT` (see above), except that system header files are not ignored, so it implies `-M` rather than `-MM`. However, the dependence on the main input file is omitted. See Preprocessor Options.

Next: Running Protoize, Previous: Environment Variables, Up: Invoking GCC

### 3.20 Using Precompiled Headers

Often large projects have many header files that are included in every source file. The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project. To make builds faster, GCC allows users to `precompile' a header file; then, if builds can use the precompiled header file they will be much faster.

To create a precompiled header file, simply compile it as you would any other file, if necessary using the `-x` option to make the driver treat it as a C or C++ header file. You will probably want to use a tool like `make` to keep the precompiled header up-to-date when the headers it contains change.

A precompiled header file will be searched for when `#include` is seen in the compilation. As it searches for the included file (see Search Path) the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory. The name searched for is the name specified in the `#include` with ``.gch`' appended. If the precompiled header file can't be used, it is ignored.

For instance, if you have `#include "all.h"`, and you have `all.h.gch` in the same directory as `all.h`, then the precompiled header file will be used if possible, and the original header will be used otherwise.

Alternatively, you might decide to put the precompiled header file in a directory and use `-I` to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an `#error` command.

This also works with `-include`. So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and `-include` the precompiled header. If the header files have guards against multiple inclusion, they will be skipped because they've already been included (in the precompiled header).

If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like `all.h.gch`, and put each precompiled header in the directory, perhaps using `-o`. It doesn't matter what you call the files in the directory, every precompiled header in the directory will be considered. The first precompiled header encountered in the directory that is valid for this compilation will be used; they're searched in no particular order.

There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.

A precompiled header file can be used only when these conditions apply:

- Only one precompiled header can be used in a particular compilation.
- A precompiled header can't be used once the first C token is seen. You can have preprocessor directives before a precompiled header; you can even include a precompiled header from inside another header, so long as there are no C tokens before the `#include`.
- The precompiled header file must be produced for the same language as the current compilation. You can't use a C precompiled header for a C++ compilation.
- The precompiled header file must have been produced by the same compiler binary as the current compilation is using.
- Any macros defined before the precompiled header is included must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that they don't appear in the precompiled header at all.

  The `-D` option is one way to define a macro before a precompiled header is included; using a `#define` can also do it. There are also some options that define macros implicitly, like `-O` and `-Wdeprecated`; the same rule applies to macros defined this way.

- If debugging information is output when using the precompiled header, using `-g` or similar, the same kind of debugging information must have been output when building the precompiled header. However, a precompiled header built using `-g` can be used in a compilation when no debugging information is being output.
- The same `-m` options must generally be used when building and using the precompiled header. See Submodel Options, for any cases where this rule is relaxed.
- Each of the following options must be the same when building and using the precompiled header:

                -fexceptions -funit-at-a-time

- Some other command-line options starting with `-f`, `-p`, or `-O` must be defined in the same way as when the precompiled header was generated. At present, it's not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header. The following are known to be safe:

                -fmessage-length= -fpreprocessed
                -fsched-interblock -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
                -fsched-verbose=<number> -fschedule-insns -fvisibility=
                -pedantic-errors

For all of these except the last, the compiler will automatically ignore the precompiled header if the conditions aren't met. If you find an option combination that doesn't work and doesn't cause the precompiled header to be ignored, please consider filing a bug report, see Bugs.

If you do use differing options when generating and using the precompiled header, the actual behavior will be a mixture of the behavior for the options. For instance, if you use `-g` to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.

Previous: Precompiled Headers, Up: Invoking GCC

### 3.21 Running Protoize

The program `protoize` is an optional part of GCC. You can use it to add prototypes to a program, thus converting the program to ISO C in one respect. The companion program `unprotoize` does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named `foo.X`.

After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted. By default, `protoize` and `unprotoize` convert only source and header files in the current directory. You can specify additional directories whose files should be converted with the `-d directory` option. You can also specify particular files to exclude with the `-x file` option. A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with `protoize` consists of rewriting most function definitions and function declarations to specify the types of the arguments. The only ones not rewritten are those for varargs functions.

`protoize` optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function's definition. Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with `unprotoize` consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ISO form.

Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with `-q`.

The output from `protoize` or `unprotoize` replaces the original source file. The original file is renamed to a name ending with ``.save`' (for DOS, the saved filename ends in ``.sav`' without the original ``.c`' suffix). If the ``.save`' (``.sav`' for DOS) file already exists, then the source file is simply discarded.

`protoize` and `unprotoize` both depend on GCC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GCC is installed.

Here is a table of the options you can use with `protoize` and `unprotoize`. Each option works with both programs unless otherwise stated.

`-B` directory
  Look for the file `SYSCALLS.c.X` in directory, instead of the usual directory (normally `/usr/local/lib`). This file contains prototype information about standard system functions. This option applies only to `protoize`.
`-c` compilation-options
  Use compilation-options as the options when running `gcc` to produce the ``.X`' files. The special option `-aux-info` is always passed in addition, to tell `gcc` to write a ``.X`' file.

  Note that the compilation options must be given as a single argument to `protoize` or `unprotoize`. If you want to specify several `gcc` options, you must quote the entire set of compilation options to make them a single word in the shell.

  There are certain `gcc` arguments that you cannot use, because they would produce the wrong kind of output. These include `-g`, `-O`, `-c`, `-S`, and `-o` If you include these in the compilation-options, they are ignored.

`-C`
  Rename files to end in ``.C`' (``.cc`' for DOS-based file systems) instead of ``.c`'. This is convenient if you are converting a C program to C++. This option applies only to `protoize`.
`-g`
  Add explicit global declarations. This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared. These declarations precede the first function definition that contains a call to an undeclared function. This option applies only to `protoize`.
`-i` string
  Indent old-style parameter declarations with the string string. This option applies only to `protoize`.

  `unprotoize` converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial ``{`'. By default, `unprotoize` uses five spaces as the indentation. If you want to indent with just one space instead, use `-i " "`.

`-k`
  Keep the ``.X`' files. Normally, they are deleted after conversion is finished.
`-l`
  Add explicit local declarations. `protoize` with `-l` inserts a prototype declaration for each function in each block which calls the function without any declaration. This option applies only to `protoize`.
`-n`
  Make no real changes. This mode just prints information about the conversions that would have been done without `-n`.
`-N`
  Make no ``.save`' files. The original files are simply deleted. Use this option with caution.
`-p` program
  Use the program program as the compiler. Normally, the name `gcc` is used.
`-q`
  Work quietly. Most warnings are suppressed.
`-v`
  Print the version number, just like `-v` for `gcc`.

If you need special compiler options to compile one of your program's source files, then you should generate that file's ``.X`' file specially, by running `gcc` on that source file with the appropriate options and the option `-aux-info`. Then run `protoize` on the entire set of files. `protoize` will use the existing ``.X`' file because it is newer than the source file. For example:

         gcc -Dfoo=bar file1.c -aux-info file1.X
         protoize *.c

You need to include the special files along with the rest in the `protoize` command, even though their ``.X`' files already exist, because otherwise they won't get converted.

See Protoize Caveats, for more information on how to use `protoize` successfully.

Next: C Extensions, Previous: Invoking GCC, Up: Top

## 4 C Implementation-defined behavior

A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated “implementation defined”. The following lists all such areas, along with the section numbers from the ISO/IEC 9899:1990 and ISO/IEC 9899:1999 standards. Some areas are only implementation-defined in one version of the standard.

Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as “determined by ABI” below. See Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See Implementation-defined behavior. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.

Next: Environment implementation, Up: C Implementation

### 4.1 Translation

- How a diagnostic is identified (C90 3.7, C99 3.10, C90 and C99 5.1.1.3).

  Diagnostics consist of all the output sent to stderr by GCC.

- Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character in translation phase 3 (C90 and C99 5.1.1.2).

  See Implementation-defined behavior.

Next: Identifiers implementation, Previous: Translation implementation, Up: C Implementation

### 4.2 Environment

The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

- The mapping between physical source file multibyte characters and the source character set in translation phase 1 (C90 and C99 5.1.1.2).

  See Implementation-defined behavior.

Next: Characters implementation, Previous: Environment implementation, Up: C Implementation

### 4.3 Identifiers

- Which additional multibyte characters may appear in identifiers and their correspondence to universal character names (C99 6.4.2).

  See Implementation-defined behavior.

- The number of significant initial characters in an identifier (C90 6.1.2, C90 and C99 5.2.4.1, C99 6.4.2).

  For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.

- Whether case distinctions are significant in an identifier with external linkage (C90 6.1.2).

  This is a property of the linker. C99 requires that case distinctions are always significant in identifiers with external linkage and systems without this property are not supported by GCC.

Next: Integers implementation, Previous: Identifiers implementation, Up: C Implementation

### 4.4 Characters

- The number of bits in a byte (C90 3.4, C99 3.6).

  Determined by ABI.

- The values of the members of the execution character set (C90 and C99 5.2.1).

  Determined by ABI.

- The unique value of the member of the execution character set produced for each of the standard alphabetic escape sequences (C90 and C99 5.2.2).

  Determined by ABI.

- The value of a `char` object into which has been stored any character other than a member of the basic execution character set (C90 6.1.2.5, C99 6.2.5).

  Determined by ABI.

- Which of `signed char` or `unsigned char` has the same range, representation, and behavior as “plain” `char` (C90 6.1.2.5, C90 6.2.1.1, C99 6.2.5, C99 6.3.1.1).

  Determined by ABI. The options `-funsigned-char` and `-fsigned-char` change the default. See Options Controlling C Dialect.

- The mapping of members of the source character set (in character constants and string literals) to members of the execution character set (C90 6.1.3.4, C99 6.4.4.4, C90 and C99 5.1.1.2).

  Determined by ABI.

- The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character (C90 6.1.3.4, C99 6.4.4.4).

  See Implementation-defined behavior.

- The value of a wide character constant containing more than one multibyte character, or containing a multibyte character or escape sequence not represented in the extended execution character set (C90 6.1.3.4, C99 6.4.4.4).

  See Implementation-defined behavior.

- The current locale used to convert a wide character constant consisting of a single multibyte character that maps to a member of the extended execution character set into a corresponding wide character code (C90 6.1.3.4, C99 6.4.4.4).

  See Implementation-defined behavior.

- The current locale used to convert a wide string literal into corresponding wide character codes (C90 6.1.4, C99 6.4.5).

  See Implementation-defined behavior.

- The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set (C90 6.1.4, C99 6.4.5).

  See Implementation-defined behavior.

Next: Floating point implementation, Previous: Characters implementation, Up: C Implementation

### 4.5 Integers

- Any extended integer types that exist in the implementation (C99 6.2.5).

  GCC does not support any extended integer types.

- Whether signed integer types are represented using sign and magnitude, two's complement, or one's complement, and whether the extraordinary value is a trap representation or an ordinary value (C99 6.2.6.2).

  GCC supports only two's complement integer types, and all bit patterns are ordinary values.

- The rank of any extended integer type relative to another extended integer type with the same precision (C99 6.3.1.1).

  GCC does not support any extended integer types.

- The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (C90 6.2.1.2, C99 6.3.1.3).

  For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.

- The results of some bitwise operations on signed integers (C90 6.3, C99 6.5).

  Bitwise operators act on the representation of the value including both the sign and value bits, where the sign bit is considered immediately above the highest-value value bit. Signed ``>>`' acts on negative numbers by sign extension.

  GCC does not use the latitude given in C99 only to treat certain aspects of signed ``<<`' as undefined, but this is subject to change.

- The sign of the remainder on integer division (C90 6.3.5).

  GCC always follows the C99 requirement that the result of division is truncated towards zero.

Next: Arrays and pointers implementation, Previous: Integers implementation, Up: C Implementation

### 4.6 Floating point

- The accuracy of the floating-point operations and of the library functions in `<math.h>` and `<complex.h>` that return floating-point results (C90 and C99 5.2.4.2.2).

  The accuracy is unknown.

- The rounding behaviors characterized by non-standard values of `FLT_ROUNDS` (C90 and C99 5.2.4.2.2).

  GCC does not use such values.

- The evaluation methods characterized by non-standard negative values of `FLT_EVAL_METHOD` (C99 5.2.4.2.2).

  GCC does not use such values.

- The direction of rounding when an integer is converted to a floating-point number that cannot exactly represent the original value (C90 6.2.1.3, C99 6.3.1.4).

  C99 Annex F is followed.

- The direction of rounding when a floating-point number is converted to a narrower floating-point number (C90 6.2.1.4, C99 6.3.1.5).

  C99 Annex F is followed.

- How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (C90 6.1.3.1, C99 6.4.4.2).

  C99 Annex F is followed.

- Whether and how floating expressions are contracted when not disallowed by the `FP_CONTRACT` pragma (C99 6.5).

  Expressions are currently only contracted if `-funsafe-math-optimizations` or `-ffast-math` are used. This is subject to change.

- The default state for the `FENV_ACCESS` pragma (C99 7.6.1).

  This pragma is not implemented, but the default is to “off” unless `-frounding-math` is used in which case it is “on”.

- Additional floating-point exceptions, rounding modes, environments, and classifications, and their macro names (C99 7.6, C99 7.12).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

- The default state for the `FP_CONTRACT` pragma (C99 7.12.2).

  This pragma is not implemented. Expressions are currently only contracted if `-funsafe-math-optimizations` or `-ffast-math` are used. This is subject to change.

- Whether the “inexact” floating-point exception can be raised when the rounded result actually does equal the mathematical result in an IEC 60559 conformant implementation (C99 F.9).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

- Whether the “underflow” (and “inexact”) floating-point exception can be raised when a result is tiny but not inexact in an IEC 60559 conformant implementation (C99 F.9).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

Next: Hints implementation, Previous: Floating point implementation, Up: C Implementation

### 4.7 Arrays and pointers

- The result of converting a pointer to an integer or vice versa (C90 6.3.4, C99 6.3.2.3).

  A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends2 if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.

  A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.

  When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 6.5.6/8.

- The size of the result of subtracting two pointers to elements of the same array (C90 6.3.6, C99 6.5.6).

  The value is as specified in the standard and the type is determined by the ABI.

Next: Structures unions enumerations and bit-fields implementation, Previous: Arrays and pointers implementation, Up: C Implementation

### 4.8 Hints

- The extent to which suggestions made by using the `register` storage-class specifier are effective (C90 6.5.1, C99 6.7.1).

  The `register` specifier affects code generation only in these ways:

  - When used as part of the register variable extension, see Explicit Reg Vars.
  - When `-O0` is in use, the compiler allocates distinct stack memory for all variables that do not have the `register` storage-class specifier; if `register` is specified, the variable may have a shorter lifespan than the code would indicate and may never be placed in memory.
  - On some rare x86 targets, `setjmp` doesn't save the registers in all circumstances. In those cases, GCC doesn't allocate any variables in registers unless they are marked `register`.
- The extent to which suggestions made by using the inline function specifier are effective (C99 6.7.4).

  GCC will not inline any functions if the `-fno-inline` option is used or if `-O0` is used. Otherwise, GCC may still be unable to inline a function for many reasons; the `-Winline` option may be used to determine if a function has not been inlined and why not.

Next: Qualifiers implementation, Previous: Hints implementation, Up: C Implementation

### 4.9 Structures, unions, enumerations, and bit-fields

- A member of a union object is accessed using a member of a different type (C90 6.3.2.3).

  The relevant bytes of the representation of the object are treated as an object of the type used for the access. This may be a trap representation.

- Whether a “plain” `int` bit-field is treated as a `signed int` bit-field or as an `unsigned int` bit-field (C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99 6.7.2.1).

  By default it is treated as `signed int` but this may be changed by the `-funsigned-bitfields` option.

- Allowable bit-field types other than `_Bool`, `signed int`, and `unsigned int` (C99 6.7.2.1).

  No other types are permitted in strictly conforming mode.

- Whether a bit-field can straddle a storage-unit boundary (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The order of allocation of bit-fields within a unit (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The alignment of non-bit-field members of structures (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The integer type compatible with each enumerated type (C90 6.5.2.2, C99 6.7.2.2).

  Normally, the type is `unsigned int` if there are no negative values in the enumeration, otherwise `int`. If `-fshort-enums` is specified, then if there are negative values it is the first of `signed char`, `short` and `int` that can represent all the values, otherwise it is the first of `unsigned char`, `unsigned short` and `unsigned int` that can represent all the values.

  On some targets, `-fshort-enums` is the default; this is determined by the ABI.

Next: Declarators implementation, Previous: Structures unions enumerations and bit-fields implementation, Up: C Implementation

### 4.10 Qualifiers

- What constitutes an access to an object that has volatile-qualified type (C90 6.5.3, C99 6.7.3).

  See When is a Volatile Object Accessed?.

Next: Statements implementation, Previous: Qualifiers implementation, Up: C Implementation

### 4.11 Declarators

- The maximum number of declarators that may modify an arithmetic, structure or union type (C90 6.5.4).

  GCC is only limited by available memory.

Next: Preprocessing directives implementation, Previous: Declarators implementation, Up: C Implementation

### 4.12 Statements

- The maximum number of `case` values in a `switch` statement (C90 6.6.4.2).

  GCC is only limited by available memory.

Next: Library functions implementation, Previous: Statements implementation, Up: C Implementation

### 4.13 Preprocessing directives

See Implementation-defined behavior, for details of these aspects of implementation-defined behavior.

- How sequences in both forms of header names are mapped to headers or external source file names (C90 6.1.7, C99 6.4.7).
- Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (C90 6.8.1, C99 6.10.1).
- Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (C90 6.8.1, C99 6.10.1).
- The places that are searched for an included ```<>' delimited header, and how the places are specified or the header is identified (C90 6.8.2, C99 6.10.2).
- How the named source file is searched for in an included ```""' delimited header (C90 6.8.2, C99 6.10.2).
- The method by which preprocessing tokens (possibly resulting from macro expansion) in a `#include` directive are combined into a header name (C90 6.8.2, C99 6.10.2).
- The nesting limit for `#include` processing (C90 6.8.2, C99 6.10.2).
- Whether the ```#' operator inserts a ```\' character before the ```\' character that begins a universal character name in a character constant or string literal (C99 6.10.3.2).
- The behavior on each recognized non-`STDC #pragma` directive (C90 6.8.6, C99 6.10.6).

  See Pragmas, for details of pragmas accepted by GCC on all targets. See Pragmas Accepted by GCC, for details of target-specific pragmas.

- The definitions for `__DATE__` and `__TIME__` when respectively, the date and time of translation are not available (C90 6.8.8, C99 6.10.8).

Next: Architecture implementation, Previous: Preprocessing directives implementation, Up: C Implementation

### 4.14 Library functions

The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

- The null pointer constant to which the macro `NULL` expands (C90 7.1.6, C99 7.17).

  In `<stddef.h>`, `NULL` expands to `((void *)0)`. GCC does not provide the other headers which define `NULL` and some library implementations may use other definitions in those headers.

Next: Locale-specific behavior implementation, Previous: Library functions implementation, Up: C Implementation

### 4.15 Architecture

- The values or expressions assigned to the macros specified in the headers `<float.h>`, `<limits.h>`, and `<stdint.h>` (C90 and C99 5.2.4.2, C99 7.18.2, C99 7.18.3).

  Determined by ABI.

- The number, order, and encoding of bytes in any object (when not explicitly specified in this International Standard) (C99 6.2.6.1).

  Determined by ABI.

- The value of the result of the `sizeof` operator (C90 6.3.3.4, C99 6.5.3.4).

  Determined by ABI.

Previous: Architecture implementation, Up: C Implementation

### 4.16 Locale-specific behavior

The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

Next: C++ Extensions, Previous: C Implementation, Up: Top

## 5 Extensions to the C Language Family

GNU C provides several language features not found in ISO standard C. (The `-pedantic` option directs GCC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro `__GNUC__`, which is always defined under GCC.

These extensions are available in C and Objective-C. Most of them are also available in C++. See Extensions to the C++ Language, for extensions that apply only to C++.

Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.

Next: Local Labels, Up: C Extensions

### 5.1 Statements and Declarations in Expressions

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:

         ({ int y = foo (); int z;
            if (y > 0) z = y;
            else z = - y;
            z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of `foo ()`.

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type `void`, and thus effectively no value.)

This feature is especially useful in making macro definitions “safe” (so that they evaluate each operand exactly once). For example, the “maximum” function is commonly defined as a macro in standard C as follows:

         #define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here taken as `int`), you can define the macro safely as follows:

         #define maxint(a,b) \
           ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you must use `typeof` (see Typeof).

In G++, the result value of a statement expression undergoes array and function pointer decay, and is returned by value to the enclosing expression. For instance, if `A` is a class, then

                 A a;

                 ({a;}).Foo ()

will construct a temporary `A` object to hold the result of the statement expression, and that will be used to invoke `Foo`. Therefore the `this` pointer observed by `Foo` will not be the address of `a`.

Any temporaries created within a statement within a statement expression will be destroyed at the statement's end. This makes statement expressions inside macros slightly different from function calls. In the latter case temporaries introduced during argument evaluation will be destroyed at the end of the statement that includes the function call. In the statement expression case they will be destroyed during the statement expression. For instance,

         #define macro(a)  ({__typeof__(a) b = (a); b + 3; })
         template<typename T> T function(T a) { T b = a; return b + 3; }

         void foo ()
         {
           macro (X ());
           function (X ());
         }

will have different places where temporaries are destroyed. For the `macro` case, the temporary `X` will be destroyed just after the initialization of `b`. In the `function` case that temporary will be destroyed when the function returns.

These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)

Jumping into a statement expression with `goto` or using a `switch` statement outside the statement expression with a `case` or `default` label inside the statement expression is not permitted. Jumping into a statement expression with a computed `goto` (see Labels as Values) yields undefined behavior. Jumping out of a statement expression is permitted, but if the statement expression is part of a larger expression then it is unspecified which other subexpressions of that expression have been evaluated except where the language definition requires certain subexpressions to be evaluated before or after the statement expression. In any case, as with a function call the evaluation of a statement expression is not interleaved with the evaluation of other parts of the containing expression. For example,

           foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();

will call `foo` and `bar1` and will not call `baz` but may or may not call `bar2`. If `bar2` is called, it will be called after `foo` and before `bar1`

Next: Labels as Values, Previous: Statement Exprs, Up: C Extensions

### 5.2 Locally Declared Labels

GCC allows you to declare local labels in any nested block scope. A local label is just like an ordinary label, but you can only reference it (with a `goto` statement, or by taking its address) within the block in which it was declared.

A local label declaration looks like this:

         __label__ label;

or

         __label__ label1, label2, /* ... */;

Local label declarations must come at the beginning of the block, before any ordinary declarations or statements.

The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label`:`, within the statements of the statement expression.

The local label feature is useful for complex macros. If a macro contains nested loops, a `goto` can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example:

         #define SEARCH(value, array, target)              \
         do {                                              \
           __label__ found;                                \
           typeof (target) _SEARCH_target = (target);      \
           typeof (*(array)) *_SEARCH_array = (array);     \
           int i, j;                                       \
           int value;                                      \
           for (i = 0; i < max; i++)                       \
             for (j = 0; j < max; j++)                     \
               if (_SEARCH_array[i][j] == _SEARCH_target)  \
                 { (value) = i; goto found; }              \
           (value) = -1;                                   \
          found:;                                          \
         } while (0)

This could also be written using a statement-expression:

         #define SEARCH(array, target)                     \
         ({                                                \
           __label__ found;                                \
           typeof (target) _SEARCH_target = (target);      \
           typeof (*(array)) *_SEARCH_array = (array);     \
           int i, j;                                       \
           int value;                                      \
           for (i = 0; i < max; i++)                       \
             for (j = 0; j < max; j++)                     \
               if (_SEARCH_array[i][j] == _SEARCH_target)  \
                 { value = i; goto found; }                \
           value = -1;                                     \
          found:                                           \
           value;                                          \
         })

Local label declarations also make the labels they declare visible to nested functions, if there are any. See Nested Functions, for details.

Next: Nested Functions, Previous: Local Labels, Up: C Extensions

### 5.3 Labels as Values

You can get the address of a label defined in the current function (or a containing function) with the unary operator ``&&`'. The value has type `void *`. This value is a constant and can be used wherever a constant of that type is valid. For example:

         void *ptr;
         /* ... */
         ptr = &&foo;

To use these values, you need to be able to jump to one. This is done with the computed goto statement3, `goto *`exp`;`. For example,

         goto *ptr;

Any expression of type `void *` is allowed.

One way of using these constants is in initializing a static array that will serve as a jump table:

         static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

         goto *array[i];

Note that this does not check whether the subscript is in bounds—array indexing in C never does that.

Such an array of label values serves a purpose much like that of the `switch` statement. The `switch` statement is cleaner, so use that rather than an array unless the problem does not fit a `switch` statement very well.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.

You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.

An alternate way to write the above example is

         static const int array[] = { &&foo - &&foo, &&bar - &&foo,
                                      &&hack - &&foo };
         goto *(&&foo + array[i]);

This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.

Next: Constructing Calls, Previous: Labels as Values, Up: C Extensions

### 5.4 Nested Functions

A nested function is a function defined inside another function. (Nested functions are not supported for GNU C++.) The nested function's name is local to the block where it is defined. For example, here we define a nested function named `square`, and call it twice:

         foo (double a, double b)
         {
           double square (double z) { return z * z; }

           return square (a) + square (b);
         }

The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited variable named `offset`:

         bar (int *array, int offset, int size)
         {
           int access (int *array, int index)
             { return array[index + offset]; }
           int i;
           /* ... */
           for (i = 0; i < size; i++)
             /* ... */ access (array, i) /* ... */
         }

Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, mixed with the other declarations and statements in the block.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:

         hack (int *array, int size)
         {
           void store (int index, int value)
             { array[index] = value; }

           intermediate (store, size);
         }

Here, the function `intermediate` receives the address of `store` as an argument. If `intermediate` calls `store`, the arguments given to `store` are used to store into `array`. But this technique works only so long as the containing function (`hack`, in this example) does not exit.

If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.

GCC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as

http://people.debian.org/~aaronl/Usenix88-lexic.pdf.

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see Local Labels). Such a jump returns instantly to the containing function, exiting the nested function which did the `goto` and any intermediate functions as well. Here is an example:

         bar (int *array, int offset, int size)
         {
           __label__ failure;
           int access (int *array, int index)
             {
               if (index > size)
                 goto failure;
               return array[index + offset];
             }
           int i;
           /* ... */
           for (i = 0; i < size; i++)
             /* ... */ access (array, i) /* ... */
           /* ... */
           return 0;

          /* Control comes here from access
             if it detects an error.  */
          failure:
           return -1;
         }

A nested function always has no linkage. Declaring one with `extern` or `static` is erroneous. If you need to declare the nested function before its definition, use `auto` (which is otherwise meaningless for function declarations).

         bar (int *array, int offset, int size)
         {
           __label__ failure;
           auto int access (int *, int);
           /* ... */
           int access (int *array, int index)
             {
               if (index > size)
                 goto failure;
               return array[index + offset];
             }
           /* ... */
         }

Next: Typeof, Previous: Nested Functions, Up: C Extensions

### 5.5 Constructing Function Calls

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

However, these built-in functions may interact badly with some sophisticated features or other extensions of the language. It is, therefore, not recommended to use them outside very simple functions acting as mere forwarders for their arguments.

— Built-in Function: void * __builtin_apply_args ()

> This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.

> The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.

— Built-in Function: void * __builtin_apply (void (*function)(), void *arguments, size_t size)

> This built-in function invokes function with a copy of the parameters described by arguments and size.

> The value of arguments should be the value returned by `__builtin_apply_args`. The argument size specifies the size of the stack argument data, in bytes.

> This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.

> It is not always simple to compute the proper value for size. The value is used by `__builtin_apply` to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

— Built-in Function: void __builtin_return (void *result)

> This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by `__builtin_apply`.

Next: Conditionals, Previous: Constructing Calls, Up: C Extensions

### 5.6 Referring to a Type with `typeof`

Another way to refer to the type of an expression is with `typeof`. The syntax of using of this keyword looks like `sizeof`, but the construct acts semantically like a type name defined with `typedef`.

There are two ways of writing the argument to `typeof`: with an expression or with a type. Here is an example with an expression:

         typeof (x[0](1))

This assumes that `x` is an array of pointers to functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

         typeof (int *)

Here the type described is that of pointers to `int`.

If you are writing a header file that must work when included in ISO C programs, write `__typeof__` instead of `typeof`. See Alternate Keywords.

A `typeof`-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of `sizeof` or `typeof`.

`typeof` is often useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe “maximum” macro that operates on any arithmetic type and evaluates each of its arguments exactly once:

         #define max(a,b) \
           ({ typeof (a) _a = (a); \
               typeof (b) _b = (b); \
             _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for `a` and `b`. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

Some more examples of the use of `typeof`:

- This declares `y` with the type of what `x` points to.

                typeof (*x) y;

- This declares `y` as an array of such values.

                typeof (*x) y[4];

- This declares `y` as an array of pointers to characters:

                typeof (typeof (char *)[4]) y;

  It is equivalent to the following traditional C declaration:

                char *y[4];

  To see the meaning of the declaration using `typeof`, and why it might be a useful way to write, rewrite it with these macros:

                #define pointer(T)  typeof(T *)
                #define array(T, N) typeof(T [N])

  Now the declaration can be rewritten this way:

                array (pointer (char), 4) y;

  Thus, `array (pointer (char), 4)` is the type of arrays of 4 pointers to `char`.

Compatibility Note: In addition to `typeof`, GCC 2 supported a more limited extension which permitted one to write

         typedef T = expr;

with the effect of declaring T to have the type of the expression expr. This extension does not work with GCC 3 (versions between 3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which relies on it should be rewritten to use `typeof`:

         typedef typeof(expr) T;

This will work with all versions of GCC.

Next: Long Long, Previous: Typeof, Up: C Extensions

### 5.7 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

         x ? : y

has the value of `x` if that is nonzero; otherwise, the value of `y`.

This example is perfectly equivalent to

         x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.

Next: Complex, Previous: Conditionals, Up: C Extensions

### 5.8 Double-Word Integers

ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C89 mode and in C++. Simply write `long long int` for a signed integer, or `unsigned long long int` for an unsigned integer. To make an integer constant of type `long long int`, add the suffix ``LL`' to the integer. To make an integer constant of type `unsigned long long int`, add the suffix ``ULL`' to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.

There may be pitfalls when you use `long long` types for function arguments, unless you declare function prototypes. If a function expects type `int` for its argument, and you pass a value of type `long long int`, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects `long long int` and you pass `int`. The best way to avoid such problems is to use prototypes.

Next: Hex Floats, Previous: Long Long, Up: C Extensions

### 5.9 Complex Numbers

ISO C99 supports complex floating data types, and as an extension GCC supports them in C89 mode and in C++, and supports complex integer data types which are not part of ISO C99. You can declare complex types using the keyword `_Complex`. As an extension, the older GNU keyword `__complex__` is also supported.

For example, ``_Complex double x;`' declares `x` as a variable whose real part and imaginary part are both of type `double`. ``_Complex short int y;`' declares `y` to have real and imaginary parts of type `short int`; this is not likely to be useful, but it shows that the set of complex types is complete.

To write a constant with a complex data type, use the suffix ``i`' or ``j`' (either one; they are equivalent). For example, `2.5fi` has type `_Complex float` and `3i` has type `_Complex int`. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant. This is a GNU extension; if you have an ISO C99 conforming C library (such as GNU libc), and want to construct complex constants of floating type, you should include `<complex.h>` and use the macros `I` or `_Complex_I` instead.

To extract the real part of a complex-valued expression exp, write `__real__` exp. Likewise, use `__imag__` to extract the imaginary part. This is a GNU extension; for values of floating type, you should use the ISO C99 functions `crealf`, `creal`, `creall`, `cimagf`, `cimag` and `cimagl`, declared in `<complex.h>` and also provided as built-in functions by GCC.

The operator ``~`' performs complex conjugation when used on a value with a complex type. This is a GNU extension; for values of floating type, you should use the ISO C99 functions `conjf`, `conj` and `conjl`, declared in `<complex.h>` and also provided as built-in functions by GCC.

GCC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). Only the DWARF2 debug info format can represent this, so use of DWARF2 is recommended. If you are using the stabs debug info format, GCC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is `foo`, the two fictitious variables are named `foo$real` and `foo$imag`. You can examine and set these two fictitious variables with your debugger.

Next: Zero Length, Previous: Complex, Up: C Extensions

### 5.10 Hex Floats

ISO C99 supports floating-point numbers written not only in the usual decimal notation, such as `1.55e1`, but also numbers such as `0x1.fp3` written in hexadecimal format. As a GNU extension, GCC supports this in C89 mode (except in some cases when strictly conforming) and in C++. In that format the ``0x`' hex introducer and the ``p`' or ``P`' exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significant part will be multiplied. Thus ``0x1.f`' is 1 15/16, ``p3`' multiplies it by 8, and the value of `0x1.fp3` is the same as `1.55e1`.

Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., `0x1.f`. This could mean `1.0f` or `1.9375` since ``f`' is also the extension for floating-point constants of type `float`.

Next: Variable Length, Previous: Hex Floats, Up: C Extensions

### 5.11 Arrays of Length Zero

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:

         struct line {
           int length;
           char contents[0];
         };

         struct line *thisline = (struct line *)
           malloc (sizeof (struct line) + this_length);
         thisline->length = this_length;

In ISO C90, you would have to give `contents` a length of 1, which means either you waste space or complicate the argument to `malloc`.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

- Flexible array members are written as `contents[]` without the `0`.
- Flexible array members have incomplete type, and so the `sizeof` operator may not be applied. As a quirk of the original implementation of zero-length arrays, `sizeof` evaluates to zero.
- Flexible array members may only appear as the last member of a `struct` that is otherwise non-empty.
- A structure containing a flexible array member, or a union containing such a structure (possibly recursively), may not be a member of a structure or an element of an array. (However, these uses are permitted by GCC as extensions.)

GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.

Instead GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the original structure followed by an array of sufficient size to contain the data. I.e. in the following, `f1` is constructed as if it were declared like `f2`.

         struct f1 {
           int x; int y[];
         } f1 = { 1, { 2, 3, 4 } };

         struct f2 {
           struct f1 f1; int data[3];
         } f2 = { { 1 }, { 2, 3, 4 } };

The convenience of this extension is that `f1` has the desired type, eliminating the need to consistently refer to `f2.f1`.

This has symmetry with normal static arrays, in that an array of unknown size is also written with `[]`.

Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:

         struct foo { int x; int y[]; };
         struct bar { struct foo z; };

         struct foo a = { 1, { 2, 3, 4 } };        // Valid.
         struct bar b = { { 1, { 2, 3, 4 } } };    // Invalid.
         struct bar c = { { 1, { } } };            // Valid.
         struct foo d[1] = { { 1 { 2, 3, 4 } } };  // Invalid.

Next: Variadic Macros, Previous: Variable Length, Up: C Extensions

### 5.12 Structures With No Members

GCC permits a C structure to have no members:

         struct empty {
         };

The structure will have size zero. In C++, empty structures are part of the language. G++ treats empty structures as if they had a single member of type `char`.

Next: Empty Structures, Previous: Zero Length, Up: C Extensions

### 5.13 Arrays of Variable Length

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:

         FILE *
         concat_fopen (char *s1, char *s2, char *mode)
         {
           char str[strlen (s1) + strlen (s2) + 1];
           strcpy (str, s1);
           strcat (str, s2);
           return fopen (str, mode);
         }

Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.

You can use the function `alloca` to get an effect much like variable-length arrays. The function `alloca` is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant.

There are other differences between these two methods. Space allocated with `alloca` exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and `alloca` in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with `alloca`.)

You can also use variable-length arrays as arguments to functions:

         struct entry
         tester (int len, char data[len][len])
         {
           /* ... */
         }

The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with `sizeof`.

If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list—another GNU extension.

         struct entry
         tester (int len; char data[len][len], int len)
         {
           /* ... */
         }

The ``int len`' before the semicolon is a parameter forward declaration, and it serves the purpose of making the name `len` known when the declaration of `data` is parsed.

You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the “real” parameter declarations. Each forward declaration must match a “real” declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.

Next: Escaped Newlines, Previous: Empty Structures, Up: C Extensions

### 5.14 Macros with a Variable Number of Arguments.

In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:

         #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

Here ``...`' is a variable argument. In the invocation of such a macro, it represents the zero or more tokens until the closing parenthesis that ends the invocation, including any commas. This set of tokens replaces the identifier `__VA_ARGS__` in the macro body wherever it appears. See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:

         #define debug(format, args...) fprintf (stderr, format, args)

This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:

         debug ("A message")

GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.

To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, ``##`'. If instead you write

         #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

and if the variable arguments are omitted or empty, the ``##`' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.

Next: Subscripting, Previous: Variadic Macros, Up: C Extensions

### 5.15 Slightly Looser Rules for Escaped Newlines

Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.

Next: Pointer Arith, Previous: Escaped Newlines, Up: C Extensions

### 5.16 Non-Lvalue Arrays May Have Subscripts

In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary ``&`' operator may not be applied to them. As an extension, GCC allows such arrays to be subscripted in C89 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C89:

         struct foo {int a[4];};

         struct foo f();

         bar (int index)
         {
           return f().a[index];
         }

Next: Initializers, Previous: Subscripting, Up: C Extensions

### 5.17 Arithmetic on `void`- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to `void` and on pointers to functions. This is done by treating the size of a `void` or of a function as 1.

A consequence of this is that `sizeof` is also allowed on `void` and on function types, and returns 1.

The option `-Wpointer-arith` requests a warning if these extensions are used.

Next: Compound Literals, Previous: Pointer Arith, Up: C Extensions

### 5.18 Non-Constant Initializers

As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:

         foo (float f, float g)
         {
           float beat_freqs[2] = { f-g, f+g };
           /* ... */
         }

Next: Designated Inits, Previous: Initializers, Up: C Extensions

### 5.19 Compound Literals

ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C89 mode and in C++.

Usually, the specified type is a structure. Assume that `struct foo` and `structure` are declared as shown:

         struct foo {int a; char b[2];} structure;

Here is an example of constructing a `struct foo` with a compound literal:

         structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

         {
           struct foo temp = {x + y, 'a', 0};
           structure = temp;
         }

You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:

         char **foo = (char *[]) { "x", "y", "z" };

Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.

As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if compound literal's and object types match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.

         static struct foo x = (struct foo) {1, 'a', 'b'};
         static int y[] = (int []) {1, 2, 3};
         static int z[] = (int [3]) {1};

The above lines are equivalent to the following:

         static struct foo x = {1, 'a', 'b'};
         static int y[] = {1, 2, 3};
         static int z[] = {1, 0, 0};

Next: Cast to Union, Previous: Compound Literals, Up: C Extensions

### 5.20 Designated Initializers

Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well. This extension is not implemented in GNU C++.

To specify an array index, write ``[index] =`' before the element value. For example,

         int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

         int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write ``[index]`' before the element value, with no ``=`'.

To initialize a range of elements to the same value, write ``[first ... last] = value`'. This is a GNU extension. For example,

         int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with ``.fieldname =`' before the element value. For example, given the following structure,

         struct point { int x, y; };

the following initialization

         struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

         struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is ``fieldname:`', as shown here:

         struct point p = { y: yvalue, x: xvalue };

The ``[index]`' or ``.fieldname`' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,

         union foo { int i; double d; };

         union foo f = { .d = 4 };

will convert 4 to a `double` to store it in the union using the second element. By contrast, casting 4 to type `union foo` would store it into the union as the integer `i`, since it is an integer. (See Cast to Union.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,

         int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

         int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an `enum` type. For example:

         int whitespace[256]
           = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
               ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

You can also write a series of ``.fieldname`' and ``[index]`' designators before an ``=`' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the ``struct point`' declaration above:

         struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, GCC will discard them and issue a warning.

Next: Mixed Declarations, Previous: Cast to Union, Up: C Extensions

### 5.21 Case Ranges

You can specify a range of consecutive values in a single `case` label, like this:

         case low ... high:

This has the same effect as the proper number of individual `case` labels, one for each integer value from low to high, inclusive.

This feature is especially useful for ranges of ASCII character codes:

         case 'A' ... 'Z':

Be careful: Write spaces around the `...`, for otherwise it may be parsed wrong when you use it with integer values. For example, write this:

         case 1 ... 5:

rather than this:

         case 1...5:

Next: Case Ranges, Previous: Designated Inits, Up: C Extensions

### 5.22 Cast to a Union Type

A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with `union` tag or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (See Compound Literals.)

The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:

         union foo { int i; double d; };
         int x;
         double y;

both `x` and `y` can be cast to type `union foo`.

Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:

         union foo u;
         /* ... */
         u = (union foo) x  ==  u.i = x
         u = (union foo) y  ==  u.d = y

You can also use the union cast as a function argument:

         void hack (union foo);
         /* ... */
         hack ((union foo) x);

Next: Function Attributes, Previous: Case Ranges, Up: C Extensions

### 5.23 Mixed Declarations and Code

ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C89 mode. For example, you could do:

         int i;
         /* ... */
         i++;
         int j = i + 2;

Each identifier is visible from where it is declared until the end of the enclosing block.

Next: Attribute Syntax, Previous: Mixed Declarations, Up: C Extensions

### 5.24 Declaring Attributes of Functions

In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.

The keyword `__attribute__` allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. The following attributes are currently defined for functions on all targets: `noreturn`, `returns_twice`, `noinline`, `always_inline`, `flatten`, `pure`, `const`, `nothrow`, `sentinel`, `format`, `format_arg`, `no_instrument_function`, `section`, `constructor`, `destructor`, `used`, `unused`, `deprecated`, `weak`, `malloc`, `alias`, `warn_unused_result`, `nonnull` and `externally_visible`. Several other attributes are defined for functions on particular target systems. Other attributes, including `section` are supported for variables declarations (see Variable Attributes) and for types (see Type Attributes).

You may also specify attributes with ``__`' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use `__noreturn__` instead of `noreturn`.

See Attribute Syntax, for details of the exact syntax for using attributes.

`alias ("`target`")`
  The `alias` attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

                void __f () { /* Do something. */; }
                void f () __attribute__ ((weak, alias ("__f")));

  declares ``f`' to be a weak alias for ``__f`'. In C++, the mangled name for the target must be used. It is an error if ``__f`' is not defined in the same translation unit.

  Not all target machines support this attribute.

`always_inline`
  Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.

`flatten`
  Generally, inlining into a function is limited. For a function marked with this attribute, every call inside this function will be inlined, if possible. Whether the function itself is considered for inlining depends on its size and the current inlining parameters. The `flatten` attribute only works reliably in unit-at-a-time mode.
`cdecl`
  On the Intel 386, the `cdecl` attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd` switch.
`const`
  Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the `pure` attribute below, since function is not allowed to read global memory.

  Note that a function that has pointer arguments and examines the data pointed to must not be declared `const`. Likewise, a function that calls a non-`const` function usually must not be `const`. It does not make sense for a `const` function to return `void`.

  The attribute `const` is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows:

                typedef int intfn ();

                extern const intfn square;

  This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the ``const`' must be attached to the return value.

`constructor`
`destructor`
  The `constructor` attribute causes the function to be called automatically before execution enters `main ()`. Similarly, the `destructor` attribute causes the function to be called automatically after `main ()` has completed or `exit ()` has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.

  These attributes are not currently implemented for Objective-C.

`deprecated`
  The `deprecated` attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses:

                int old_fn () __attribute__ ((deprecated));
                int old_fn ();
                int (*fn_ptr)() = old_fn;

  results in a warning on line 3 but not line 2.

  The `deprecated` attribute can also be used for variables and types (see Variable Attributes, see Type Attributes.)

`dllexport`
  On Microsoft Windows targets and Symbian OS targets the `dllexport` attribute causes the compiler to provide a global pointer to a pointer in a DLL, so that it can be referenced with the `dllimport` attribute. On Microsoft Windows targets, the pointer name is formed by combining `_imp__` and the function or variable name.

  You can use `__declspec(dllexport)` as a synonym for `__attribute__ ((dllexport))` for compatibility with other compilers.

  On systems that support the `visibility` attribute, this attribute also implies “default” visibility, unless a `visibility` attribute is explicitly specified. You should avoid the use of `dllexport` with “hidden” or “internal” visibility; in the future GCC may issue an error for those cases.

  Currently, the `dllexport` attribute is ignored for inlined functions, unless the `-fkeep-inline-functions` flag has been used. The attribute is also ignored for undefined symbols.

  When applied to C++ classes, the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.

  For Microsoft Windows targets there are alternative methods for including the symbol in the DLL's export table such as using a `.def` file with an `EXPORTS` section or, with GNU ld, using the `--export-all` linker flag.

`dllimport`
  On Microsoft Windows and Symbian OS targets, the `dllimport` attribute causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the DLL exporting the symbol. The attribute implies `extern` storage. On Microsoft Windows targets, the pointer name is formed by combining `_imp__` and the function or variable name.

  You can use `__declspec(dllimport)` as a synonym for `__attribute__ ((dllimport))` for compatibility with other compilers.

  Currently, the attribute is ignored for inlined functions. If the attribute is applied to a symbol definition, an error is reported. If a symbol previously declared `dllimport` is later defined, the attribute is ignored in subsequent references, and a warning is emitted. The attribute is also overridden by a subsequent declaration as `dllexport`.

  When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.

  On the SH Symbian OS target the `dllimport` attribute also has another affect—it can cause the vtable and run-time type information for a class to be exported. This happens when the class has a dllimport'ed constructor or a non-inline, non-pure virtual function and, for either of those two conditions, the class also has a inline constructor or destructor and has a key function that is defined in the current translation unit.

  For Microsoft Windows based targets the use of the `dllimport` attribute on functions is not necessary, but provides a small performance benefit by eliminating a thunk in the DLL. The use of the `dllimport` attribute on imported variables was required on older versions of the GNU linker, but can now be avoided by passing the `--enable-auto-import` switch to the GNU linker. As with functions, using the attribute for a variable eliminates a thunk in the DLL.

  One drawback to using this attribute is that a pointer to a function or variable marked as `dllimport` cannot be used as a constant address. On Microsoft Windows targets, the attribute can be disabled for functions by setting the `-mnop-fun-dllimport` flag.

`eightbit_data`
  Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data.

  You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

`exception_handler`
  Use this attribute on the Blackfin to indicate that the specified function is an exception handler. The compiler will generate function entry and exit sequences suitable for use in an exception handler when this attribute is present.
`far`
  On 68HC11 and 68HC12 the `far` attribute causes the compiler to use a calling convention that takes care of switching memory banks when entering and leaving a function. This calling convention is also the default when using the `-mlong-calls` option.

  On 68HC12 the compiler will use the `call` and `rtc` instructions to call and return from a function.

  On 68HC11 the compiler will generate a sequence of instructions to invoke a board-specific routine to switch the memory bank and call the real function. The board-specific routine simulates a `call`. At the end of a function, it will jump to a board-specific routine instead of using `rts`. The board-specific return routine simulates the `rtc`.

`fastcall`
  On the Intel 386, the `fastcall` attribute causes the compiler to pass the first argument (if of integral type) in the register ECX and the second argument (if of integral type) in the register EDX. Subsequent and other typed arguments are passed on the stack. The called function will pop the arguments off the stack. If the number of arguments is variable all arguments are pushed on the stack.
`format (`archetype`,` string-index`,` first-to-check`)`
  The `format` attribute specifies that a function takes `printf`, `scanf`, `strftime` or `strfmon` style arguments which should be type-checked against a format string. For example, the declaration:

                extern int
                my_printf (void *my_object, const char *my_format, ...)
                      __attribute__ ((format (printf, 2, 3)));

  causes the compiler to check the arguments in calls to `my_printf` for consistency with the `printf` style format string argument `my_format`.

  The parameter archetype determines how the format string is interpreted, and should be `printf`, `scanf`, `strftime` or `strfmon`. (You can also use `__printf__`, `__scanf__`, `__strftime__` or `__strfmon__`.) The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as `vprintf`), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For `strftime` formats, the third parameter is required to be zero. Since non-static C++ methods have an implicit `this` argument, the arguments of such methods should be counted from two, not one, when giving values for string-index and first-to-check.

  In the example above, the format string (`my_format`) is the second argument of the function `my_print`, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.

  The `format` attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless `-ffreestanding` or `-fno-builtin` is used) checks formats for the standard library functions `printf`, `fprintf`, `sprintf`, `scanf`, `fscanf`, `sscanf`, `strftime`, `vprintf`, `vfprintf` and `vsprintf` whenever such warnings are requested (using `-Wformat`), so there is no need to modify the header file `stdio.h`. In C99 mode, the functions `snprintf`, `vsnprintf`, `vscanf`, `vfscanf` and `vsscanf` are also checked. Except in strictly conforming C standard modes, the X/Open function `strfmon` is also checked as are `printf_unlocked` and `fprintf_unlocked`. See Options Controlling C Dialect.

  The target may provide additional types of format checks. See Format Checks Specific to Particular Target Machines.

`format_arg (`string-index`)`
  The `format_arg` attribute specifies that a function takes a format string for a `printf`, `scanf`, `strftime` or `strfmon` style function and modifies it (for example, to translate it into another language), so the result can be passed to a `printf`, `scanf`, `strftime` or `strfmon` style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). For example, the declaration:

                extern char *
                my_dgettext (char *my_domain, const char *my_format)
                      __attribute__ ((format_arg (2)));

  causes the compiler to check the arguments in calls to a `printf`, `scanf`, `strftime` or `strfmon` type function, whose format string argument is a call to the `my_dgettext` function, for consistency with the format string argument `my_format`. If the `format_arg` attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when `-Wformat-nonliteral` is used, but the calls could not be checked without the attribute.

  The parameter string-index specifies which argument is the format string argument (starting from one). Since non-static C++ methods have an implicit `this` argument, the arguments of such methods should be counted from two.

  The `format-arg` attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to `printf`, `scanf`, `strftime` or `strfmon` type function whose operands are a call to one of your own function. The compiler always treats `gettext`, `dgettext`, and `dcgettext` in this manner except when strict ISO C support is requested by `-ansi` or an appropriate `-std` option, or `-ffreestanding` or `-fno-builtin` is used. See Options Controlling C Dialect.

`function_vector`
  Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H and H8S) and shares space with the interrupt vector.

  You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

`interrupt`
  Use this attribute on the ARM, AVR, C4x, CRX, M32C, M32R/D, MS1, and Xstormy16 ports to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

  Note, interrupt handlers for the Blackfin, m68k, H8/300, H8/300H, H8S, and SH processors can be specified via the `interrupt_handler` attribute.

  Note, on the AVR, interrupts will be enabled inside the function.

  Note, for the ARM, you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:

                void f () __attribute__ ((interrupt ("IRQ")));

  Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.

`interrupt_handler`
  Use this attribute on the Blackfin, m68k, H8/300, H8/300H, H8S, and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.
`kspisusp`
  When used together with `interrupt_handler`, `exception_handler` or `nmi_handler`, code will be generated to load the stack pointer from the USP register in the function prologue.
`long_call/short_call`
  This attribute specifies how a particular function is called on ARM. Both attributes override the `-mlong-calls` (see ARM Options) command line switch and `#pragma long_calls` settings. The `long_call` attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The `short_call` attribute always places the offset to the function from the call site into the ``BL`' instruction directly.
`longcall/shortcall`
  On the Blackfin, RS/6000 and PowerPC, the `longcall` attribute causes the compiler to always call this function via a pointer, just as it would if the `-mlongcall` option had been specified. The `shortcall` attribute causes the compiler not to do this. These attributes override both the `-mlongcall` switch and, on the RS/6000 and PowerPC, the `#pragma longcall` setting.

  See RS/6000 and PowerPC Options, for more information on whether long calls are necessary.

`long_call`
  This attribute specifies how a particular function is called on MIPS. The attribute overrides the `-mlong-calls` (see MIPS Options) command line switch. This attribute causes the compiler to always call the function by first loading its address into a register, and then using the contents of that register.
`malloc`
  The `malloc` attribute is used to tell the compiler that a function may be treated as if any non-`NULL` pointer it returns cannot alias any other pointer valid when the function returns. This will often improve optimization. Standard functions with this property include `malloc` and `calloc`. `realloc`-like functions have this property as long as the old pointer is never referred to (including comparing it to the new pointer) after the function returns a non-`NULL` value.
`model (`model-name`)`
  On the M32R/D, use this attribute to set the addressability of an object, and of the code generated for a function. The identifier model-name is one of `small`, `medium`, or `large`, representing each of the code models.

  Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24` instruction), and are callable with the `bl` instruction.

  Medium model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and are callable with the `bl` instruction.

  Large model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and may not be reachable with the `bl` instruction (the compiler will generate the much slower `seth/add3/jl` instruction sequence).

  On IA-64, use this attribute to set the addressability of an object. At present, the only supported identifier for model-name is `small`, indicating addressability via “small” (22-bit) addresses (so that their addresses can be loaded with the `addl` instruction). Caveat: such addressing is by definition not position independent and hence this attribute must not be used for objects defined by shared libraries.

`naked`
  Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the specified function does not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences.
`near`
  On 68HC11 and 68HC12 the `near` attribute causes the compiler to use the normal calling convention based on `jsr` and `rts`. This attribute can be used to cancel the effect of the `-mlong-calls` option.
`nesting`
  Use this attribute together with `interrupt_handler`, `exception_handler` or `nmi_handler` to indicate that the function entry code should enable nested interrupts or exceptions.
`nmi_handler`
  Use this attribute on the Blackfin to indicate that the specified function is an NMI handler. The compiler will generate function entry and exit sequences suitable for use in an NMI handler when this attribute is present.
`no_instrument_function`
  If `-finstrument-functions` is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.
`noinline`
  This function attribute prevents a function from being considered for inlining.
`nonnull (`arg-index`, ...)`
  The `nonnull` attribute specifies that some function parameters should be non-null pointers. For instance, the declaration:

                extern void *
                my_memcpy (void *dest, const void *src, size_t len)
                	__attribute__((nonnull (1, 2)));

  causes the compiler to check that, in calls to `my_memcpy`, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the `-Wnonnull` option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null.

  If no argument index list is given to the `nonnull` attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example:

                extern void *
                my_memcpy (void *dest, const void *src, size_t len)
                	__attribute__((nonnull));

`noreturn`
  A few standard library functions, such as `abort` and `exit`, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them `noreturn` to tell the compiler this fact. For example,

                void fatal () __attribute__ ((noreturn));

                void
                fatal (/* ... */)
                {
                  /* ... */ /* Print error message. */ /* ... */
                  exit (1);
                }

  The `noreturn` keyword tells the compiler to assume that `fatal` cannot return. It can then optimize without regard to what would happen if `fatal` ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables.

  The `noreturn` keyword does not affect the exceptional path when that applies: a `noreturn`-marked function may still return to the caller by throwing an exception or calling `longjmp`.

  Do not assume that registers saved by the calling function are restored before calling the `noreturn` function.

  It does not make sense for a `noreturn` function to have a return type other than `void`.

  The attribute `noreturn` is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

                typedef void voidfn ();

                volatile voidfn fatal;

  This approach does not work in GNU C++.

`nothrow`
  The `nothrow` attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions of `qsort` and `bsearch` that take function pointer arguments. The `nothrow` attribute is not implemented in GCC versions earlier than 3.3.
`pure`
  Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute `pure`. For example,

                int square (int) __attribute__ ((pure));

  says that the hypothetical function `square` is safe to call fewer times than the program says.

  Some of common examples of pure functions are `strlen` or `memcmp`. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as `feof` in a multithreading environment).

  The attribute `pure` is not implemented in GCC versions earlier than 2.96.

`regparm (`number`)`
  On the Intel 386, the `regparm` attribute causes the compiler to pass arguments number one to number if they are of integral type in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.

  Beware that on some ELF systems this attribute is unsuitable for global functions in shared libraries with lazy binding (which is the default). Lazy binding will send the first call via resolving code in the loader, which might assume EAX, EDX and ECX can be clobbered, as per the standard calling conventions. Solaris 8 is affected by this. GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be safe since the loaders there save all registers. (Lazy binding can be disabled with the linker or the loader if desired, to avoid the problem.)

`sseregparm`
  On the Intel 386 with SSE support, the `sseregparm` attribute causes the compiler to pass up to 8 floating point arguments in SSE registers instead of on the stack. Functions that take a variable number of arguments will continue to pass all of their floating point arguments on the stack.
`returns_twice`
  The `returns_twice` attribute tells the compiler that a function may return more than one time. The compiler will ensure that all registers are dead before calling such a function and will emit a warning about the variables that may be clobbered after the second return from the function. Examples of such functions are `setjmp` and `vfork`. The `longjmp`-like counterpart of such function, if any, might need to be marked with the `noreturn` attribute.
`saveall`
  Use this attribute on the Blackfin, H8/300, H8/300H, and H8S to indicate that all registers except the stack pointer should be saved in the prologue regardless of whether they are used or not.
`section ("`section-name`")`
  Normally, the compiler places the code it generates in the `text` section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The `section` attribute specifies that a function lives in a particular section. For example, the declaration:

                extern void foobar (void) __attribute__ ((section ("bar")));

  puts the function `foobar` in the `bar` section.

  Some file formats do not support arbitrary sections so the `section` attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

`sentinel`
  This function attribute ensures that a parameter in a function call is an explicit `NULL`. The attribute is only valid on variadic functions. By default, the sentinel is located at position zero, the last parameter of the function call. If an optional integer position argument P is supplied to the attribute, the sentinel must be located at position P counting backwards from the end of the argument list.

                __attribute__ ((sentinel))
                is equivalent to
                __attribute__ ((sentinel(0)))

  The attribute is automatically set with a position of 0 for the built-in functions `execl` and `execlp`. The built-in function `execle` has the attribute set with a position of 1.

  A valid `NULL` in this context is defined as zero with any pointer type. If your system defines the `NULL` macro with an integer type then you need to add an explicit cast. GCC replaces `stddef.h` with a copy that redefines NULL appropriately.

  The warnings for missing or incorrect sentinels are enabled with `-Wformat`.

`short_call`
  See long_call/short_call.
`shortcall`
  See longcall/shortcall.
`signal`
  Use this attribute on the AVR to indicate that the specified function is a signal handler. The compiler will generate function entry and exit sequences suitable for use in a signal handler when this attribute is present. Interrupts will be disabled inside the function.
`sp_switch`
  Use this attribute on the SH to indicate an `interrupt_handler` function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack.

                void *alt_stack;
                void f () __attribute__ ((interrupt_handler,
                                          sp_switch ("alt_stack")));

`stdcall`
  On the Intel 386, the `stdcall` attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments.
`tiny_data`
  Use this attribute on the H8/300H and H8S to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.
`trap_exit`
  Use this attribute on the SH for an `interrupt_handler` to return using `trapa` instead of `rte`. This attribute expects an integer argument specifying the trap number to be used.
`unused`
  This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function.
`used`
  This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.
`visibility ("`visibility_type`")`
  The `visibility` attribute on ELF targets causes the declaration to be emitted with default, hidden, protected or internal visibility.

                void __attribute__ ((visibility ("protected")))
                f () { /* Do something. */; }
                int i __attribute__ ((visibility ("hidden")));

  See the ELF gABI for complete details, but the short story is:

  default
    Default visibility is the normal case for ELF. This value is available for the visibility attribute to override other options that may change the assumed visibility of symbols.
  hidden
    Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.
  internal
    Internal visibility is like hidden visibility, but with additional processor specific semantics. Unless otherwise specified by the psABI, GCC defines internal visibility to mean that the function is never called from another module. Note that hidden symbols, while they cannot be referenced directly by other modules, can be referenced indirectly via function pointers. By indicating that a symbol cannot be called from outside the module, GCC may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value.
  protected
    Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol. That is, the symbol cannot be overridden by another module.

  Not all ELF targets support this attribute.

`warn_unused_result`
  The `warn_unused_result` attribute causes a warning to be emitted if a caller of the function with this attribute does not use its return value. This is useful for functions where not checking the result is either a security problem or always a bug, such as `realloc`.

                int fn () __attribute__ ((warn_unused_result));
                int foo ()
                {
                  if (fn () < 0) return -1;
                  fn ();
                  return 0;
                }

  results in warning on line 5.

`weak`
  The `weak` attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.
`weakref`
`weakref ("`target`")`
  The `weakref` attribute marks a declaration as a weak reference. Without arguments, it should be accompanied by an `alias` attribute naming the target symbol. Optionally, the target may be given as an argument to `weakref` itself. In either case, `weakref` implicitly marks the declaration as `weak`. Without a target, given as an argument to `weakref` or to `alias`, `weakref` is equivalent to `weak`.

                extern int x() __attribute__ ((weakref ("y")));
                /* is equivalent to... */
                extern int x() __attribute__ ((weak, weakref, alias ("y")));
                /* and to... */
                extern int x() __attribute__ ((weakref));
                extern int x() __attribute__ ((alias ("y")));

  A weak reference is an alias that does not by itself require a definition to be given for the target symbol. If the target symbol is only referenced through weak references, then the becomes a `weak` undefined symbol. If it is directly referenced, however, then such strong references prevail, and a definition will be required for the symbol, not necessarily in the same translation unit.

  The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a reloadable link on them.

`externally_visible`
  This attribute, attached to a global variable or function nullify effect of `-fwhole-program` command line option, so the object remain visible outside the current compilation unit

You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.

Some people object to the `__attribute__` feature, suggesting that ISO C's `#pragma` should be used instead. At the time `__attribute__` was designed, there were two reasons for not doing this.

1. It is impossible to generate `#pragma` commands from a macro.
2. There is no telling what the same `#pragma` might mean in another compiler.

These two reasons applied to almost any application that might have been proposed for `#pragma`. It was basically a mistake to use `#pragma` for anything.

The ISO C99 standard includes `_Pragma`, which now allows pragmas to be generated from macros. In addition, a `#pragma GCC` namespace is now in use for GCC-specific pragmas. However, it has been found convenient to use `__attribute__` to achieve a natural attachment of attributes to their corresponding declarations, whereas `#pragma GCC` is of use for constructs that do not naturally form part of the grammar. See Miscellaneous Preprocessing Directives.

Next: Function Prototypes, Previous: Function Attributes, Up: C Extensions

### 5.25 Attribute Syntax

This section describes the syntax with which `__attribute__` may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective-C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.

There are some problems with the semantics of attributes in C++. For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading. Similarly, `typeid` does not distinguish between types with different attributes. Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators.

See Function Attributes, for details of the semantics of attributes applying to functions. See Variable Attributes, for details of the semantics of attributes applying to variables. See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types.

An attribute specifier is of the form `__attribute__ ((`attribute-list`))`. An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:

- Empty. Empty attributes are ignored.
- A word (which may be an identifier such as `unused`, or a reserved word such as `const`).
- A word, followed by, in parentheses, parameters for the attribute. These parameters take one of the following forms:
  - An identifier. For example, `mode` attributes use this form.
  - An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, `format` attributes use this form.
  - A possibly empty comma-separated list of expressions. For example, `format_arg` attributes use this form with the list being a single integer constant expression, and `alias` attributes use this form with the list being a single string constant.

An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.

In GNU C, an attribute specifier list may appear after the colon following a label, other than a `case` or `default` label. The only attribute it makes sense to use after a label is `unused`. This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with `-Wall`. It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an `#ifdef` conditional. GNU C++ does not permit such placement of attribute lists, as it is permissible for a declaration, which could begin with an attribute list, to be labelled in C++. Declarations cannot be labelled in C90 or C99, so the ambiguity does not arise there.

An attribute specifier list may appear as part of a `struct`, `union` or `enum` specifier. It may go either immediately after the `struct`, `union` or `enum` keyword, or after the closing brace. It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used—that is, in usages such as `struct __attribute__((foo)) bar` with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.

Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.

Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, `section`.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of `int` is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.

At present, the first parameter in a function prototype must have some type specifier which is not an attribute specifier; this resolves an ambiguity in the interpretation of `void f(int (__attribute__((foo)) x))`, but is subject to change. At present, if the parentheses of a function declarator contain only attributes then those attributes are ignored, rather than yielding an error or warning or implying a single parameter of type int, but this is subject to change.

An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in

         __attribute__((noreturn)) void d0 (void),
             __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
              d2 (void)

the `noreturn` attribute applies to all the functions declared; the `format` attribute only applies to `d1`.

An attribute specifier list may appear immediately before the comma, `=` or semicolon terminating the declaration of an identifier other than a function definition. At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator. In simple cases there is no difference, but, for example, in

         void (****f)(void) __attribute__((noreturn));

at present the `noreturn` attribute applies to `f`, which causes a warning since `f` is not a function, but in future it may apply to the function `****f`. The precise semantics of what attributes in such cases will apply to are not yet specified. Where an assembler name for an object or function is specified (see Asm Labels), at present the attribute must follow the `asm` specification; in future, attributes before the `asm` specification may apply to the adjacent declarator, and those after it to the declared object or function.

An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).

Attribute specifiers may be mixed with type qualifiers appearing inside the `[]` of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted. Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored.

An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the `*` of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax. It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration `T D1`, where `T` contains declaration specifiers that specify a type Type (such as `int`) and `D1` is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.

If `D1` has the form `(` attribute-specifier-list `D )`, and the declaration `T D` specifies the type “derived-declarator-type-list Type” for ident, then `T D1` specifies the type “derived-declarator-type-list attribute-specifier-list Type” for ident.

If `D1` has the form `*` type-qualifier-and-attribute-specifier-list `D`, and the declaration `T D` specifies the type “derived-declarator-type-list Type” for ident, then `T D1` specifies the type “derived-declarator-type-list type-qualifier-and-attribute-specifier-list Type” for ident.

For example,

         void (__attribute__((noreturn)) ****f) (void);

specifies the type “pointer to pointer to pointer to pointer to non-returning function returning `void`”. As another example,

         char *__attribute__((aligned(8))) *f;

specifies the type “pointer to 8-byte-aligned pointer to `char`”. Note again that this does not work with most attributes; for example, the usage of ``aligned`' and ``noreturn`' attributes given above is not yet supported.

For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.

Next: C++ Comments, Previous: Attribute Syntax, Up: C Extensions

### 5.26 Prototypes and Old-Style Function Definitions

GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:

         /* Use prototypes unless the compiler is old-fashioned.  */
         #ifdef __STDC__
         #define P(x) x
         #else
         #define P(x) ()
         #endif

         /* Prototype function declaration.  */
         int isroot P((uid_t));

         /* Old-style function definition.  */
         int
         isroot (x)   /* ??? lossage here ??? */
              uid_t x;
         {
           return x == 0;
         }

Suppose the type `uid_t` happens to be `short`. ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an `int`, which does not match the prototype argument type of `short`.

This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the `uid_t` type is `short`, `int`, or `long`. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following:

         int isroot (uid_t);

         int
         isroot (uid_t x)
         {
           return x == 0;
         }

GNU C++ does not support old-style function definitions, so this extension is irrelevant.

Next: Dollar Signs, Previous: Function Prototypes, Up: C Extensions

### 5.27 C++ Style Comments

In GNU C, you may use C++ style comments, which start with ``//`' and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an `-std` option specifying a version of ISO C before C99, or `-ansi` (equivalent to `-std=c89`).

Next: Character Escapes, Previous: C++ Comments, Up: C Extensions

### 5.28 Dollar Signs in Identifier Names

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.

Next: Variable Attributes, Previous: Dollar Signs, Up: C Extensions

### 5.29 The Character <ESC> in Constants

You can use the sequence ``\e`' in a string or character constant to stand for the ASCII character <ESC>.

Next: Inline, Previous: Type Attributes, Up: C Extensions

### 5.30 Inquiring on Alignment of Types or Variables

The keyword `__alignof__` allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like `sizeof`.

For example, if the target machine requires a `double` value to be aligned on an 8-byte boundary, then `__alignof__ (double)` is 8. This is true on many RISC machines. On more traditional machine designs, `__alignof__ (double)` is 4 or even 2.

Some machines never actually require alignment; they allow reference to any data type even at an odd address. For these machines, `__alignof__` reports the recommended alignment of a type.

If the operand of `__alignof__` is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's `__attribute__` extension (see Variable Attributes). For example, after this declaration:

         struct foo { int x; char y; } foo1;

the value of `__alignof__ (foo1.y)` is 1, even though its actual alignment is probably 2 or 4, the same as `__alignof__ (int)`.

It is an error to ask for the alignment of an incomplete type.

Next: Type Attributes, Previous: Character Escapes, Up: C Extensions

### 5.31 Specifying Attributes of Variables

The keyword `__attribute__` allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Some attributes are currently defined generically for variables. Other attributes are defined for variables on particular target systems. Other attributes are available for functions (see Function Attributes) and for types (see Type Attributes). Other front ends might define more attributes (see Extensions to the C++ Language).

You may also specify attributes with ``__`' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use `__aligned__` instead of `aligned`.

See Attribute Syntax, for details of the exact syntax for using attributes.

`aligned (`alignment`)`
  This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration:

                int x __attribute__ ((aligned (16))) = 0;

  causes the compiler to allocate the global variable `x` on a 16-byte boundary. On a 68040, this could be used in conjunction with an `asm` expression to access the `move16` instruction which requires 16-byte aligned operands.

  You can also specify the alignment of structure fields. For example, to create a double-word aligned `int` pair, you could write:

                struct foo { int x[2] __attribute__ ((aligned (8))); };

  This is an alternative to creating a union with a `double` member that forces the union to be double-word aligned.

  As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

                short array[3] __attribute__ ((aligned));

  Whenever you leave out the alignment factor in an `aligned` attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way.

  The `aligned` attribute can only increase the alignment; but you can decrease it by specifying `packed` as well. See below.

  Note that the effectiveness of `aligned` attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)` in an `__attribute__` will still only provide you with 8 byte alignment. See your linker documentation for further information.

`cleanup (`cleanup_function`)`
  The `cleanup` attribute runs a function when the variable goes out of scope. This attribute can only be applied to auto function scope variables; it may not be applied to parameters or variables with static storage duration. The function must take one parameter, a pointer to a type compatible with the variable. The return value of the function (if any) is ignored.

  If `-fexceptions` is enabled, then cleanup_function will be run during the stack unwinding that happens during the processing of the exception. Note that the `cleanup` attribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally.

`common`
`nocommon`
  The `common` attribute requests GCC to place a variable in “common” storage. The `nocommon` attribute requests the opposite—to allocate space for it directly.

  These attributes override the default chosen by the `-fno-common` and `-fcommon` flags respectively.

`deprecated`
  The `deprecated` attribute results in a warning if the variable is used anywhere in the source file. This is useful when identifying variables that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated variable, to enable users to easily find further information about why the variable is deprecated, or what they should do instead. Note that the warning only occurs for uses:

                extern int old_var __attribute__ ((deprecated));
                extern int old_var;
                int new_fn () { return old_var; }

  results in a warning on line 3 but not line 2.

  The `deprecated` attribute can also be used for functions and types (see Function Attributes, see Type Attributes.)

`mode (`mode`)`
  This attribute specifies the data type for the declaration—whichever type corresponds to the mode mode. This in effect lets you request an integer or floating point type according to its width.

  You may also specify a mode of ``byte`' or ``__byte__`' to indicate the mode corresponding to a one-byte integer, ``word`' or ``__word__`' for the mode of a one-word integer, and ``pointer`' or ``__pointer__`' for the mode used to represent pointers.

`packed`
  The `packed` attribute specifies that a variable or structure field should have the smallest possible alignment—one byte for a variable, and one bit for a field, unless you specify a larger value with the `aligned` attribute.

  Here is a structure in which the field `x` is packed, so that it immediately follows `a`:

                struct foo
                {
                  char a;
                  int x[2] __attribute__ ((packed));
                };

`section ("`section-name`")`
  Normally, the compiler places the objects it generates in sections like `data` and `bss`. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The `section` attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names:

                struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
                struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
                char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
                int init_data __attribute__ ((section ("INITDATA"))) = 0;

                main()
                {
                  /* Initialize stack pointer */
                  init_sp (stack + sizeof (stack));

                  /* Initialize initialized data */
                  memcpy (&init_data, &data, &edata - &data);

                  /* Turn on the serial ports */
                  init_duart (&a);
                  init_duart (&b);
                }

  Use the `section` attribute with an initialized definition of a global variable, as shown in the example. GCC issues a warning and otherwise ignores the `section` attribute in uninitialized variable declarations.

  You may only use the `section` attribute with a fully initialized global definition because of the way linkers work. The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the `common` (or `bss`) section and can be multiply “defined”. You can force a variable to be initialized with the `-fno-common` flag or the `nocommon` attribute.

  Some file formats do not support arbitrary sections so the `section` attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

`shared`
  On Microsoft Windows, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL. For example, this small program defines shared data by putting it in a named section `shared` and marking the section shareable:

                int foo __attribute__((section ("shared"), shared)) = 0;

                int
                main()
                {
                  /* Read and write foo.  All running
                     copies see the same value.  */
                  return 0;
                }

  You may only use the `shared` attribute along with `section` attribute with a fully initialized global definition because of the way linkers work. See `section` attribute for more information.

  The `shared` attribute is only available on Microsoft Windows.

`tls_model ("`tls_model`")`
  The `tls_model` attribute sets thread-local storage model (see Thread-Local) of a particular `__thread` variable, overriding `-ftls-model=` command line switch on a per-variable basis. The tls_model argument should be one of `global-dynamic`, `local-dynamic`, `initial-exec` or `local-exec`.

  Not all targets support this attribute.

`unused`
  This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC will not produce a warning for this variable.
`vector_size (`bytes`)`
  This attribute specifies the vector size for the variable, measured in bytes. For example, the declaration:

                int foo __attribute__ ((vector_size (16)));

  causes the compiler to set the mode for `foo`, to be 16 bytes, divided into `int` sized units. Assuming a 32-bit int (a vector of 4 units of 4 bytes), the corresponding mode of `foo` will be V4SI.

  This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.

  Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:

                struct S { int a; };
                struct S  __attribute__ ((vector_size (16))) foo;

  is invalid even if the size of the structure is the same as the size of the `int`.

`selectany`
  The `selectany` attribute causes an initialized global variable to have link-once semantics. When multiple definitions of the variable are encountered by the linker, the first is selected and the remainder are discarded. Following usage by the Microsoft compiler, the linker is told not to warn about size or content differences of the multiple definitions.

  Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.

  The `selectany` attribute is only available on Microsoft Windows targets. You can use `__declspec (selectany)` as a synonym for `__attribute__ ((selectany))` for compatibility with other compilers.

`weak`
  The `weak` attribute is described in See Function Attributes.
`dllimport`
  The `dllimport` attribute is described in See Function Attributes.
`dlexport`
  The `dllexport` attribute is described in See Function Attributes.

#### 5.31.1 M32R/D Variable Attributes

One attribute is currently defined for the M32R/D.

`model (`model-name`)`
  Use this attribute on the M32R/D to set the addressability of an object. The identifier model-name is one of `small`, `medium`, or `large`, representing each of the code models.

  Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24` instruction).

  Medium and large model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3` instructions to load their addresses).

#### 5.31.2 i386 Variable Attributes

Two attributes are currently defined for i386 configurations: `ms_struct` and `gcc_struct`

`ms_struct`
`gcc_struct`
  If `packed` is used on a structure, or if bit-fields are used it may be that the Microsoft ABI packs them differently than GCC would normally pack them. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.

  Currently `-m[no-]ms-bitfields` is provided for the Microsoft Windows X86 compilers to match the native Microsoft compiler.

#### 5.31.3 Xstormy16 Variable Attributes

One attribute is currently defined for xstormy16 configurations: `below100`

`below100`
  If a variable has the `below100` attribute (`BELOW100` is allowed also), GCC will place the variable in the first 0x100 bytes of memory and use special opcodes to access it. Such variables will be placed in either the `.bss_below100` section or the `.data_below100` section.

Next: Alignment, Previous: Variable Attributes, Up: C Extensions

### 5.32 Specifying Attributes of Types

The keyword `__attribute__` allows you to specify special attributes of `struct` and `union` types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Six attributes are currently defined for types: `aligned`, `packed`, `transparent_union`, `unused`, `deprecated` and `may_alias`. Other attributes are defined for functions (see Function Attributes) and for variables (see Variable Attributes).

You may also specify any one of these attributes with ``__`' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use `__aligned__` instead of `aligned`.

You may specify the `aligned` and `transparent_union` attributes either in a `typedef` declaration or just past the closing curly brace of a complete enum, struct or union type definition and the `packed` attribute only past the closing brace of a definition.

You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.

See Attribute Syntax, for details of the exact syntax for using attributes.

`aligned (`alignment`)`
  This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations:

                struct S { short f[3]; } __attribute__ ((aligned (8)));
                typedef int more_aligned_int __attribute__ ((aligned (8)));

  force the compiler to insure (as far as it can) that each variable whose type is `struct S` or `more_aligned_int` will be allocated and aligned at least on a 8-byte boundary. On a SPARC, having all variables of type `struct S` aligned to 8-byte boundaries allows the compiler to use the `ldd` and `std` (doubleword load and store) instructions when copying one variable of type `struct S` to another, thus improving run-time efficiency.

  Note that the alignment of any given `struct` or `union` type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the `struct` or `union` in question. This means that you can effectively adjust the alignment of a `struct` or `union` type by attaching an `aligned` attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire `struct` or `union` type.

  As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given `struct` or `union` type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

                struct S { short f[3]; } __attribute__ ((aligned));

  Whenever you leave out the alignment factor in an `aligned` attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way.

  In the example above, if the size of each `short` is 2 bytes, then the size of the entire `struct S` type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire `struct S` type to 8 bytes.

  Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.

  The `aligned` attribute can only increase the alignment; but you can decrease it by specifying `packed` as well. See below.

  Note that the effectiveness of `aligned` attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)` in an `__attribute__` will still only provide you with 8 byte alignment. See your linker documentation for further information.

`packed`
  This attribute, attached to `struct` or `union` type definition, specifies that each member (other than zero-width bitfields) of the structure or union is placed to minimize the memory required. When attached to an `enum` definition, it indicates that the smallest integral type should be used.

  Specifying this attribute for `struct` and `union` types is equivalent to specifying the `packed` attribute on each of the structure or union members. Specifying the `-fshort-enums` flag on the line is equivalent to specifying the `packed` attribute on all `enum` definitions.

  In the following example `struct my_packed_struct`'s members are packed closely together, but the internal layout of its `s` member is not packed—to do that, `struct my_unpacked_struct` would need to be packed too.

                struct my_unpacked_struct
                 {
                    char c;
                    int i;
                 };

                struct __attribute__ ((__packed__)) my_packed_struct
                  {
                     char c;
                     int  i;
                     struct my_unpacked_struct s;
                  };

  You may only specify this attribute on the definition of a `enum`, `struct` or `union`, not on a `typedef` which does not also define the enumerated type, structure or union.

`transparent_union`
  This attribute, attached to a `union` type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way.

  First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required. Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like `const` on the referenced type must be respected, just as with normal pointer conversions.

  Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.

  Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons. For example, suppose the `wait` function must accept either a value of type `int *` to comply with Posix, or a value of type `union wait *` to comply with the 4.1BSD interface. If `wait`'s parameter were `void *`, `wait` would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful. Instead, `<sys/wait.h>` might define the interface as follows:

                typedef union
                  {
                    int *__ip;
                    union wait *__up;
                  } wait_status_ptr_t __attribute__ ((__transparent_union__));

                pid_t wait (wait_status_ptr_t);

  This interface allows either `int *` or `union wait *` arguments to be passed, using the `int *` calling convention. The program can call `wait` with arguments of either type:

                int w1 () { int w; return wait (&w); }
                int w2 () { union wait w; return wait (&w); }

  With this interface, `wait`'s implementation might look like this:

                pid_t wait (wait_status_ptr_t p)
                {
                  return waitpid (-1, p.__ip, 0);
                }

`unused`
  When attached to a type (including a `union` or a `struct`), this attribute means that variables of that type are meant to appear possibly unused. GCC will not produce a warning for any variables of that type, even if the variable appears to do nothing. This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions.
`deprecated`
  The `deprecated` attribute results in a warning if the type is used anywhere in the source file. This is useful when identifying types that are expected to be removed in a future version of a program. If possible, the warning also includes the location of the declaration of the deprecated type, to enable users to easily find further information about why the type is deprecated, or what they should do instead. Note that the warnings only occur for uses and then only if the type is being applied to an identifier that itself is not being declared as deprecated.

                typedef int T1 __attribute__ ((deprecated));
                T1 x;
                typedef T1 T2;
                T2 y;
                typedef T1 T3 __attribute__ ((deprecated));
                T3 z __attribute__ ((deprecated));

  results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6.

  The `deprecated` attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)

`may_alias`
  Accesses to objects with types with this attribute are not subjected to type-based alias analysis, but are instead assumed to be able to alias any other type of objects, just like the `char` type. See `-fstrict-aliasing` for more information on aliasing issues.

  Example of use:

                typedef short __attribute__((__may_alias__)) short_a;

                int
                main (void)
                {
                  int a = 0x12345678;
                  short_a *b = (short_a *) &a;

                  b[1] = 0;

                  if (a == 0x12345678)
                    abort();

                  exit(0);
                }

  If you replaced `short_a` with `short` in the variable declaration, the above program would abort when compiled with `-fstrict-aliasing`, which is on by default at `-O2` or above in recent GCC versions.

  #### 5.32.1 ARM Type Attributes

  On those ARM targets that support `dllimport` (such as Symbian OS), you can use the `notshared` attribute to indicate that the virtual table and other similar data for a class should not be exported from a DLL. For example:

                class __declspec(notshared) C {
                public:
                  __declspec(dllimport) C();
                  virtual void f();
                }

                __declspec(dllexport)
                C::C() {}

  In this code, `C::C` is exported from the current DLL, but the virtual table for `C` is not exported. (You can use `__attribute__` instead of `__declspec` if you prefer, but most Symbian OS code uses `__declspec`.)

  #### 5.32.2 i386 Type Attributes

  Two attributes are currently defined for i386 configurations: `ms_struct` and `gcc_struct`

`ms_struct`
`gcc_struct`
  If `packed` is used on a structure, or if bit-fields are used it may be that the Microsoft ABI packs them differently than GCC would normally pack them. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.

  Currently `-m[no-]ms-bitfields` is provided for the Microsoft Windows X86 compilers to match the native Microsoft compiler.

To specify multiple attributes, separate them by commas within the double parentheses: for example, ``__attribute__ ((aligned (16), packed))`'.

Next: Extended Asm, Previous: Alignment, Up: C Extensions

### 5.33 An Inline Function is As Fast As a Macro

By declaring a function `inline`, you can direct GCC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really “works” only in optimizing compilation. If you don't use `-O`, no function is really inline.

Inline functions are included in the ISO C99 standard, but there are currently substantial differences between what GCC implements and what the ISO C99 standard requires.

To declare a function inline, use the `inline` keyword in its declaration, like this:

         inline int
         inc (int *a)
         {
           (*a)++;
         }

(If you are writing a header file to be included in ISO C programs, write `__inline__` instead of `inline`. See Alternate Keywords.) You can also make all “simple enough” functions inline with the option `-finline-functions`.

Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: use of varargs, use of alloca, use of variable sized data types (see Variable Length), use of computed goto (see Labels as Values), use of nonlocal goto, and nested functions (see Nested Functions). Using `-Winline` will warn when a function marked `inline` could not be substituted, and will give the reason for the failure.

Note that in C and Objective-C, unlike C++, the `inline` keyword does not affect the linkage of the function.

GCC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared `inline`. (You can override this with `-fno-default-inline`; see Options Controlling C++ Dialect.)

When a function is both inline and `static`, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions`. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined.

When an inline function is not `static`, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-`static` inline function is always compiled on its own in the usual fashion.

If you specify both `inline` and `extern` in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of `inline` and `extern` has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking `inline` and `extern`) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library.

Since GCC eventually will implement ISO C99 semantics for inline functions, it is best to use `static inline` only to guarantee compatibility. (The existing semantics will remain available when `-std=gnu89` is specified, but eventually the default will be `-std=gnu99` and that will implement the C99 semantics, though it does not do so yet.)

GCC does not inline any functions when not optimizing unless you specify the ``always_inline`' attribute for the function, like this:

         /* Prototype.  */
         inline void foo (const char) __attribute__((always_inline));

Next: Constraints, Previous: Inline, Up: C Extensions

### 5.34 Assembler Instructions with C Expression Operands

In an assembler instruction using `asm`, you can specify the operands of the instruction using C expressions. This means you need not guess which registers or memory locations will contain the data you want to use.

You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.

For example, here is how to use the 68881's `fsinx` instruction:

         asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here `angle` is the C expression for the input operand while `result` is that of the output operand. Each has ``"f"`' as its operand constraint, saying that a floating point register is required. The ``=`' in ``=f`' indicates that the operand is an output; all output operands' constraints must use ``=`'. The constraints use the same language used in the machine description (see Constraints).

Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.

If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.

As of GCC version 3.1, it is also possible to specify input and output operands using symbolic names which can be referenced within the assembler code. These names are specified inside square brackets preceding the constraint string, and can be referenced inside the assembler code using `%[`name`]` instead of a percentage sign followed by the operand number. Using named operands the above example could look like:

         asm ("fsinx %[angle],%[output]"
              : [output] "=f" (result)
              : [angle] "f" (angle));

Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but you must ensure that no two operands within the same assembler construct use the same symbolic name.

Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input. The extended `asm` feature is most often used for machine instructions the compiler itself does not know exist. If the output expression cannot be directly addressed (for example, it is a bit-field), your constraint must allow a register. In that case, GCC will use the register as the output of the `asm`, and then store that register into the output.

The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character ``+`' to indicate such an operand and list it with the output operands. You should only use read-write operands when the constraints for the operand (or the operand in which only some of the bits are to be changed) allow a register.

You may, as an alternative, logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) ``combine`' instruction with `bar` as its read-only source operand and `foo` as its read-write destination:

         asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint ``"0"`' for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.

Only a number in the constraint can guarantee that one operand will be in the same place as another. The mere fact that `foo` is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work reliably:

         asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GCC knows no reason not to do so. For example, the compiler might find a copy of the value of `foo` in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to `foo`'s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GCC can't tell that.

As of GCC version 3.1, one may write `[`name`]` instead of the operand number for a matching constraint. For example:

         asm ("cmoveq %1,%2,%[result]"
              : [result] "=r"(result)
              : "r" (test), "r"(new), "[result]"(old));

Sometimes you need to make an `asm` operand be a specific register, but there's no matching constraint letter for that register by itself. To force the operand into that register, use a local variable for the operand and specify the register in the variable declaration. See Explicit Reg Vars. Then for the `asm` operand, use any register constraint letter that matches the register:

         register int *p1 asm ("r0") = ...;
         register int *p2 asm ("r1") = ...;
         register int *result asm ("r0");
         asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));

In the above example, beware that a register that is call-clobbered by the target ABI will be overwritten by any function call in the assignment, including library calls for arithmetic operators. Assuming it is a call-clobbered register, this may happen to `r0` above by the assignment to `p2`. If you have to use such a register, use temporary variables for expressions between the register assignment and use:

         int t1 = ...;
         register int *p1 asm ("r0") = ...;
         register int *p2 asm ("r1") = t1;
         register int *result asm ("r0");
         asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));

Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:

         asm volatile ("movc3 %0,%1,%2"
                       : /* no outputs */
                       : "g" (from), "g" (to), "g" (count)
                       : "r0", "r1", "r2", "r3", "r4", "r5");

You may not write a clobber description in a way that overlaps with an input or output operand. For example, you may not have an operand describing a register class with one member if you mention that register in the clobber list. Variables declared to live in specific registers (see Explicit Reg Vars), and used as asm input or output operands must have no part mentioned in the clobber description. There is no way for you to specify that an input operand is modified without also specifying it as an output operand. Note that if all the output operands you specify are for this purpose (and hence unused), you will then also need to specify `volatile` for the `asm` construct, as described below, to prevent GCC from deleting the `asm` statement as unused.

If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with ``%`'; to produce one ``%`' in the assembler code, you must write ``%%`' in the input.

If your assembler instruction can alter the condition code register, add ``cc`' to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; ``cc`' serves to name this register. On other machines, the condition code is handled differently, and specifying ``cc`' has no effect. But it is valid no matter what the machine.

If your assembler instructions access memory in an unpredictable fashion, add ``memory`' to the list of clobbered registers. This will cause GCC to not keep memory values cached in registers across the assembler instruction and not optimize stores or loads to that memory. You will also want to add the `volatile` keyword if the memory affected is not listed in the inputs or outputs of the `asm`, as the ``memory`' clobber does not count as a side-effect of the `asm`. If you know how large the accessed memory is, you can add it as input or output but if this is not known, you should add ``memory`'. As an example, if you access ten bytes of a string, you can use a memory input like:

         {"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.

Note that in the following example the memory input is necessary, otherwise GCC might optimize the store to `x` away:

         int foo ()
         {
           int x = 42;
           int *y = &x;
           int result;
           asm ("magic stuff accessing an 'int' pointed to by '%1'"
                 "=&d" (r) : "a" (y), "m" (*y));
           return result;
         }

You can put multiple assembler instructions together in a single `asm` template, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as ``\n\t`'). Sometimes semicolons can be used, if the assembler allows semicolons as a line-breaking character. Note that some assembler dialects use semicolons to start a comment. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes the subroutine `_foo` accepts arguments in registers 9 and 10:

         asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
              : /* no outputs */
              : "g" (from), "g" (to)
              : "r9", "r10");

Unless an output operand has the ``&`' constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use ``&`' for each output operand that may not overlap an input. See Modifiers.

If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the `asm` construct, as follows:

         asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
              : "g" (result)
              : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.

Speaking of labels, jumps from one `asm` to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize.

Usually the most convenient way to use these `asm` instructions is to encapsulate them in macros that look like functions. For example,

         #define sin(x)       \
         ({ double __value, __arg = (x);   \
            asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
            __value; })

Here the variable `__arg` is used to make sure that the instruction operates on a proper `double` value, and to accept only those arguments `x` which can convert automatically to a `double`.

Another way to make sure the instruction operates on the correct data type is to use a cast in the `asm`. This is different from using a variable `__arg` in that it converts more different types. For example, if the desired type were `int`, casting the argument to `int` would accept a pointer with no complaint, while assigning the argument to an `int` variable named `__arg` would warn about using a pointer unless the caller explicitly casts it.

If an `asm` has output operands, GCC assumes for optimization purposes the instruction has no side effects except to change the output operands. This does not mean instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register.

You can prevent an `asm` instruction from being deleted by writing the keyword `volatile` after the `asm`. For example:

         #define get_and_set_priority(new)              \
         ({ int __old;                                  \
            asm volatile ("get_and_set_priority %0, %1" \
                          : "=g" (__old) : "g" (new));  \
            __old; })

The `volatile` keyword indicates that the instruction has important side-effects. GCC will not delete a volatile `asm` if it is reachable. (The instruction can still be deleted if GCC can prove that control-flow will never reach the location of the instruction.) Note that even a volatile `asm` instruction can be moved relative to other code, including across jump instructions. For example, on many targets there is a system register which can be set to control the rounding mode of floating point operations. You might try setting it with a volatile `asm`, like this PowerPC example:

                asm volatile("mtfsf 255,%0" : : "f" (fpenv));
                sum = x + y;

This will not work reliably, as the compiler may move the addition back before the volatile `asm`. To make it work you need to add an artificial dependency to the `asm` referencing a variable in the code you don't want moved, for example:

             asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
             sum = x + y;

Similarly, you can't expect a sequence of volatile `asm` instructions to remain perfectly consecutive. If you want consecutive output, use a single `asm`. Also, GCC will perform some optimizations across a volatile `asm` instruction; GCC does not “forget everything” when it encounters a volatile `asm` instruction the way some other compilers do.

An `asm` instruction without any output operands will be treated identically to a volatile `asm` instruction.

It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following “store” instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary “test” and “compare” instructions because they don't have any output operands.

For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.

If you are writing a header file that should be includable in ISO C programs, write `__asm__` instead of `asm`. See Alternate Keywords.

#### 5.34.1 Size of an `asm`

Some targets require that GCC track the size of each instruction used in order to generate correct code. Because the final length of an `asm` is only known by the assembler, GCC must make an estimate as to how big it will be. The estimate is formed by counting the number of statements in the pattern of the `asm` and multiplying that by the length of the longest instruction on that processor. Statements in the `asm` are identified by newline characters and whatever statement separator characters are supported by the assembler; on most processors this is the ``;`' character.

Normally, GCC's estimate is perfectly adequate to ensure that correct code is generated, but it is possible to confuse the compiler if you use pseudo instructions or assembler macros that expand into multiple real instructions or if you use assembler directives that expand to more space in the object file than would be needed for a single instruction. If this happens then the assembler will produce a diagnostic saying that a label is unreachable.

#### 5.34.2 i386 floating point asm operands

There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:

1. Given a set of input regs that die in an asm_operands, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by gcc.

   An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.

2. For any input reg that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped reg, it would not be possible to know what the stack looked like—it's not clear how the rest of the stack “slides up”.

   All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.

   It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example:

                 asm ("foo" : "=t" (a) : "f" (b));

   This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.

   If any input operand uses the `f` constraint, all output reg constraints must use the `&` earlyclobber.

   The asm above would be written as

                 asm ("foo" : "=&t" (a) : "f" (b));

3. Some operands need to be in particular places on the stack. All output operands fall in this category—there is no other way to know which regs the outputs appear in unless the user indicates this in the constraints.

   Output operands must specifically indicate which reg an output appears in after an asm. `=f` is not allowed: the operand constraints must select a class with a single reg.

4. Output operands may not be “inserted” between existing stack regs. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm_operands, and are pushed by the asm_operands. It makes no sense to push anywhere but the top of the reg-stack.

   Output operands must start at the top of the reg-stack: output operands may not “skip” a reg.

5. Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs.

Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.

         asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

This asm takes two inputs, which are popped by the `fyl2xp1` opcode, and replaces them with one output. The user must code the `st(1)` clobber for reg-stack.c to know that `fyl2xp1` pops both inputs.

         asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");

Next: Asm Labels, Previous: Extended Asm, Up: C Extensions

### 5.35 Constraints for `asm` Operands

Here are specific details on what constraint letters you can use with `asm` operands. Constraints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match.

Next: Multi-Alternative, Up: Constraints

#### 5.35.1 Simple Constraints

The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:

whitespace
  Whitespace characters are ignored and can be inserted at any position except the first. This enables each alternative for different operands to be visually aligned in the machine description even if they have different number of constraints and modifiers.

``m`'
  A memory operand is allowed, with any kind of address that the machine supports in general.

``o`'
  A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address.

  For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.

  Note that in an output operand which can be matched by another operand, the constraint letter ``o`' is valid only when accompanied by both ``<`' (if the target machine has predecrement addressing) and ``>`' (if the target machine has preincrement addressing).

``V`'
  A memory operand that is not offsettable. In other words, anything that would fit the ``m`' constraint but not the ``o`' constraint.

``<`'
  A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed.

``>`'
  A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed.

``r`'
  A register operand is allowed provided that it is in a general register.

``i`'
  An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time or later.

``n`'
  An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use ``n`' rather than ``i`'.

``I`', ``J`', ``K`', ... ``P`'
  Other letters in the range ``I`' through ``P`' may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, ``I`' is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions.

``E`'
  An immediate floating operand (expression code `const_double`) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running).

``F`'
  An immediate floating operand (expression code `const_double` or `const_vector`) is allowed.

``G`', ``H`'
  ``G`' and ``H`' may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values.

``s`'
  An immediate integer operand whose value is not an explicit integer is allowed.

  This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use ``s`' instead of ``i`'? Sometimes it allows better code to be generated.

  For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between −128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a ``moveq`' instruction. We arrange for this to happen by defining the letter ``K`' to mean “any integer outside the range −128 to 127”, and then specifying ``Ks`' in the operand constraints.

``g`'
  Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.

``X`'
  Any operand whatsoever is allowed.

``0`', ``1`', ``2`', ... ``9`'
  An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last.

  This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that ``10`' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.

  This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles which `asm` distinguishes. For example, an add instruction uses two input operands and an output operand, but on most CISC machines an add instruction really has only two operands, one of them an input-output operand:

                addl #35,r12

  Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.

``p`'
  An operand that is a valid memory address is allowed. This is for “load address” and “push address” instructions.

  ``p`' in the constraint must be accompanied by `address_operand` as the predicate in the `match_operand`. This predicate interprets the mode specified in the `match_operand` as the mode of the memory reference for which the address would be valid.

other-letters
  Other letters can be defined in machine-dependent fashion to stand for particular classes of registers or other arbitrary operand types. ``d`', ``a`' and ``f`' are defined on the 68000/68020 to stand for data, address and floating point registers.

Next: Modifiers, Previous: Simple Constraints, Up: Constraints

#### 5.35.2 Multiple Alternative Constraints

Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.

These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.

If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the ``?`' and ``!`' characters:

`?`
  Disparage slightly the alternative that the ``?`' appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each ``?`' that appears in it.

`!`
  Disparage severely the alternative that the ``!`' appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used.

Next: Machine Constraints, Previous: Multi-Alternative, Up: Constraints

#### 5.35.3 Constraint Modifier Characters

Here are constraint modifier characters.

``=`'
  Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.

``+`'
  Means that this operand is both read and written by the instruction.

  When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. ``=`' identifies an output; ``+`' identifies an operand that is both input and output; all other operands are assumed to be input only.

  If you specify ``=`' or ``+`' in a constraint, you put it in the first character of the constraint string.

``&`'
  Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address.

  ``&`' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires ``&`' while others do not. See, for example, the ``movdf`' insn of the 68000.

  An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the ``mulsi3`' insn of the ARM.

  ``&`' does not obviate the need to write ``=`'.

``%`'
  Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. GCC can only handle one commutative pair in an asm; if you use more, the compiler may fail. Note that you need not use the modifier if the two alternatives are strictly identical; this would only waste time in the reload pass. The modifier is not operational after register allocation, so the result of `define_peephole2` and `define_split`s performed after reload cannot rely on ``%`' to make the intended insn match.

``#`'
  Says that all following characters, up to the next comma, are to be ignored as a constraint. They are significant only for choosing register preferences.

``*`'
  Says that the following character should be ignored when choosing register preferences. ``*`' has no effect on the meaning of the constraint as a constraint, and no effect on reloading.

Previous: Modifiers, Up: Constraints

#### 5.35.4 Constraints for Particular Machines

Whenever possible, you should use the general-purpose constraint letters in `asm` arguments, since they will convey meaning more readily to people reading your code. Failing that, use the constraint letters that usually have very similar meanings across architectures. The most commonly used constraints are ``m`' and ``r`' (for memory and general-purpose registers respectively; see Simple Constraints), and ``I`', usually the letter indicating the most common immediate-constant format.

For each machine architecture, the `config/machine/machine.h` file defines additional constraints. These constraints are used by the compiler itself for instruction generation, as well as for `asm` statements; therefore, some of the constraints are not particularly interesting for `asm`. The constraints are defined through these macros:

`REG_CLASS_FROM_LETTER`
  Register class constraints (usually lowercase).
`CONST_OK_FOR_LETTER_P`
  Immediate constant constraints, for non-floating point constants of word size or smaller precision (usually uppercase).
`CONST_DOUBLE_OK_FOR_LETTER_P`
  Immediate constant constraints, for all floating point constants and for constants of greater than word size precision (usually uppercase).
`EXTRA_CONSTRAINT`
  Special cases of registers or memory. This macro is not required, and is only defined for some machines.

Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.

ARM family—``arm.h
  `f`
    Floating-point register
  `w`
    VFP floating-point register
  `F`
    One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 or 10.0
  `G`
    Floating-point constant that would satisfy the constraint ``F`' if it were negated
  `I`
    Integer that is valid as an immediate operand in a data processing instruction. That is, an integer in the range 0 to 255 rotated by a multiple of 2
  `J`
    Integer in the range −4095 to 4095
  `K`
    Integer that satisfies constraint ``I`' when inverted (ones complement)
  `L`
    Integer that satisfies constraint ``I`' when negated (twos complement)
  `M`
    Integer in the range 0 to 32
  `Q`
    A memory reference where the exact address is in a single register (```m`'' is preferable for `asm` statements)
  `R`
    An item in the constant pool
  `S`
    A symbol in the text segment of the current file
  `Uv`
    A memory reference suitable for VFP load/store insns (reg+constant offset)
  `Uy`
    A memory reference suitable for iWMMXt load/store instructions.
  `Uq`
    A memory reference suitable for the ARMv4 ldrsb instruction.
AVR family—``avr.h
  `l`
    Registers from r0 to r15
  `a`
    Registers from r16 to r23
  `d`
    Registers from r16 to r31
  `w`
    Registers from r24 to r31. These registers can be used in ``adiw`' command
  `e`
    Pointer register (r26–r31)
  `b`
    Base pointer register (r28–r31)
  `q`
    Stack pointer register (SPH:SPL)
  `t`
    Temporary register r0
  `x`
    Register pair X (r27:r26)
  `y`
    Register pair Y (r29:r28)
  `z`
    Register pair Z (r31:r30)
  `I`
    Constant greater than −1, less than 64
  `J`
    Constant greater than −64, less than 1
  `K`
    Constant integer 2
  `L`
    Constant integer 0
  `M`
    Constant that fits in 8 bits
  `N`
    Constant integer −1
  `O`
    Constant integer 8, 16, or 24
  `P`
    Constant integer 1
  `G`
    A floating point constant 0.0
CRX Architecture—``crx.h
  `b`
    Registers from r0 to r14 (registers without stack pointer)
  `l`
    Register r16 (64-bit accumulator lo register)
  `h`
    Register r17 (64-bit accumulator hi register)
  `k`
    Register pair r16-r17. (64-bit accumulator lo-hi pair)
  `I`
    Constant that fits in 3 bits
  `J`
    Constant that fits in 4 bits
  `K`
    Constant that fits in 5 bits
  `L`
    Constant that is one of -1, 4, -4, 7, 8, 12, 16, 20, 32, 48
  `G`
    Floating point constant that is legal for store immediate
PowerPC and IBM RS6000—``rs6000.h
  `b`
    Address base register
  `f`
    Floating point register
  `v`
    Vector register
  `h`
    ``MQ`', ``CTR`', or ``LINK`' register
  `q`
    ``MQ`' register
  `c`
    ``CTR`' register
  `l`
    ``LINK`' register
  `x`
    ``CR`' register (condition register) number 0
  `y`
    ``CR`' register (condition register)
  `z`
    ``FPMEM`' stack memory for FPR-GPR transfers
  `I`
    Signed 16-bit constant
  `J`
    Unsigned 16-bit constant shifted left 16 bits (use ``L`' instead for `SImode` constants)
  `K`
    Unsigned 16-bit constant
  `L`
    Signed 16-bit constant shifted left 16 bits
  `M`
    Constant larger than 31
  `N`
    Exact power of 2
  `O`
    Zero
  `P`
    Constant whose negation is a signed 16-bit constant
  `G`
    Floating point constant that can be loaded into a register with one instruction per word
  `Q`
    Memory operand that is an offset from a register (``m`' is preferable for `asm` statements)
  `R`
    AIX TOC entry
  `S`
    Constant suitable as a 64-bit mask operand
  `T`
    Constant suitable as a 32-bit mask operand
  `U`
    System V Release 4 small data area reference
MorphoTech family—``mt.h
  `I`
    Constant for an arithmetic insn (16-bit signed integer).
  `J`
    The constant 0.
  `K`
    Constant for a logical insn (16-bit zero-extended integer).
  `L`
    A constant that can be loaded with `lui` (i.e. the bottom 16 bits are zero).
  `M`
    A constant that takes two words to load (i.e. not matched by `I`, `K`, or `L`).
  `N`
    Negative 16-bit constants other than -65536.
  `O`
    A 15-bit signed integer constant.
  `P`
    A positive 16-bit constant.
Intel 386—``i386.h
  `q`
    ``a`', `b`, `c`, or `d` register for the i386. For x86-64 it is equivalent to ``r`' class (for 8-bit instructions that do not use upper halves).
  `Q`
    ``a`', `b`, `c`, or `d` register (for 8-bit instructions, that do use upper halves).
  `R`
    Legacy register—equivalent to `r` class in i386 mode. (for non-8-bit registers used together with 8-bit upper halves in a single instruction)
  `A`
    Specifies the ``a`' or ``d`' registers. This is primarily useful for 64-bit integer values (when in 32-bit mode) intended to be returned with the ``d`' register holding the most significant bits and the ``a`' register holding the least significant bits.
  `f`
    Floating point register
  `t`
    First (top of stack) floating point register
  `u`
    Second floating point register
  `a`
    ``a`' register
  `b`
    ``b`' register
  `c`
    ``c`' register
  `C`
    Specifies constant that can be easily constructed in SSE register without loading it from memory.
  `d`
    ``d`' register
  `D`
    ``di`' register
  `S`
    ``si`' register
  `x`
    ``xmm`' SSE register
  `y`
    MMX register
  `I`
    Constant in range 0 to 31 (for 32-bit shifts)
  `J`
    Constant in range 0 to 63 (for 64-bit shifts)
  `K`
    ``0xff`'
  `L`
    ``0xffff`'
  `M`
    0, 1, 2, or 3 (shifts for `lea` instruction)
  `N`
    Constant in range 0 to 255 (for `out` instruction)
  `Z`
    Constant in range 0 to `0xffffffff` or symbolic reference known to fit specified range. (for using immediates in zero extending 32-bit to 64-bit x86-64 instructions)
  `e`
    Constant in range −2147483648 to 2147483647 or symbolic reference known to fit specified range. (for using immediates in 64-bit x86-64 instructions)
  `G`
    Standard 80387 floating point constant
Intel IA-64—``ia64.h
  `a`
    General register `r0` to `r3` for `addl` instruction
  `b`
    Branch register
  `c`
    Predicate register (``c`' as in “conditional”)
  `d`
    Application register residing in M-unit
  `e`
    Application register residing in I-unit
  `f`
    Floating-point register
  `m`
    Memory operand. Remember that ``m`' allows postincrement and postdecrement which require printing with ``%Pn`' on IA-64. Use ``S`' to disallow postincrement and postdecrement.
  `G`
    Floating-point constant 0.0 or 1.0
  `I`
    14-bit signed integer constant
  `J`
    22-bit signed integer constant
  `K`
    8-bit signed integer constant for logical instructions
  `L`
    8-bit adjusted signed integer constant for compare pseudo-ops
  `M`
    6-bit unsigned integer constant for shift counts
  `N`
    9-bit signed integer constant for load and store postincrements
  `O`
    The constant zero
  `P`
    0 or −1 for `dep` instruction
  `Q`
    Non-volatile memory for floating-point loads and stores
  `R`
    Integer constant in the range 1 to 4 for `shladd` instruction
  `S`
    Memory operand except postincrement and postdecrement
FRV—``frv.h
  `a`
    Register in the class `ACC_REGS` (`acc0` to `acc7`).
  `b`
    Register in the class `EVEN_ACC_REGS` (`acc0` to `acc7`).
  `c`
    Register in the class `CC_REGS` (`fcc0` to `fcc3` and `icc0` to `icc3`).
  `d`
    Register in the class `GPR_REGS` (`gr0` to `gr63`).
  `e`
    Register in the class `EVEN_REGS` (`gr0` to `gr63`). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.
  `f`
    Register in the class `FPR_REGS` (`fr0` to `fr63`).
  `h`
    Register in the class `FEVEN_REGS` (`fr0` to `fr63`). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.
  `l`
    Register in the class `LR_REG` (the `lr` register).
  `q`
    Register in the class `QUAD_REGS` (`gr2` to `gr63`). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.
  `t`
    Register in the class `ICC_REGS` (`icc0` to `icc3`).
  `u`
    Register in the class `FCC_REGS` (`fcc0` to `fcc3`).
  `v`
    Register in the class `ICR_REGS` (`cc4` to `cc7`).
  `w`
    Register in the class `FCR_REGS` (`cc0` to `cc3`).
  `x`
    Register in the class `QUAD_FPR_REGS` (`fr0` to `fr63`). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.
  `z`
    Register in the class `SPR_REGS` (`lcr` and `lr`).
  `A`
    Register in the class `QUAD_ACC_REGS` (`acc0` to `acc7`).
  `B`
    Register in the class `ACCG_REGS` (`accg0` to `accg7`).
  `C`
    Register in the class `CR_REGS` (`cc0` to `cc7`).
  `G`
    Floating point constant zero
  `I`
    6-bit signed integer constant
  `J`
    10-bit signed integer constant
  `L`
    16-bit signed integer constant
  `M`
    16-bit unsigned integer constant
  `N`
    12-bit signed integer constant that is negative—i.e. in the range of −2048 to −1
  `O`
    Constant zero
  `P`
    12-bit signed integer constant that is greater than zero—i.e. in the range of 1 to 2047.
Blackfin family—``bfin.h
  `a`
    P register
  `d`
    D register
  `z`
    A call clobbered P register.
  `D`
    Even-numbered D register
  `W`
    Odd-numbered D register
  `e`
    Accumulator register.
  `A`
    Even-numbered accumulator register.
  `B`
    Odd-numbered accumulator register.
  `b`
    I register
  `B`
    B register
  `f`
    M register
  `c`
    Registers used for circular buffering, i.e. I, B, or L registers.
  `C`
    The CC register.
  `x`
    Any D, P, B, M, I or L register.
  `y`
    Additional registers typically used only in prologues and epilogues: RETS, RETN, RETI, RETX, RETE, ASTAT, SEQSTAT and USP.
  `w`
    Any register except accumulators or CC.
  `Ksh`
    Signed 16 bit integer (in the range -32768 to 32767)
  `Kuh`
    Unsigned 16 bit integer (in the range 0 to 65535)
  `Ks7`
    Signed 7 bit integer (in the range -64 to 63)
  `Ku7`
    Unsigned 7 bit integer (in the range 0 to 127)
  `Ku5`
    Unsigned 5 bit integer (in the range 0 to 31)
  `Ks4`
    Signed 4 bit integer (in the range -8 to 7)
  `Ks3`
    Signed 3 bit integer (in the range -3 to 4)
  `Ku3`
    Unsigned 3 bit integer (in the range 0 to 7)
  `P` n
    Constant n, where n is a single-digit constant in the range 0 to 4.
  `M1`
    Constant 255.
  `M2`
    Constant 65535.
  `J`
    An integer constant with exactly a single bit set.
  `L`
    An integer constant with all bits set except exactly one.
  `H`
  `Q`
    Any SYMBOL_REF.
M32C—``m32c.c
Rsp
Rfb
Rsb
  ``$sp`', ``$fb`', ``$sb`'.
Rcr
  Any control register, when they're 16 bits wide (nothing if control registers are 24 bits wide)
Rcl
  Any control register, when they're 24 bits wide.
R0w
R1w
R2w
R3w
  $r0, $r1, $r2, $r3.
R02
  $r0 or $r2, or $r2r0 for 32 bit values.
R13
  $r1 or $r3, or $r3r1 for 32 bit values.
Rdi
  A register that can hold a 64 bit value.
Rhl
  $r0 or $r1 (registers with addressable high/low bytes)
R23
  $r2 or $r3
Raa
  Address registers
Raw
  Address registers when they're 16 bits wide.
Ral
  Address registers when they're 24 bits wide.
Rqi
  Registers that can hold QI values.
Rad
  Registers that can be used with displacements ($a0, $a1, $sb).
Rsi
  Registers that can hold 32 bit values.
Rhi
  Registers that can hold 16 bit values.
Rhc
  Registers chat can hold 16 bit values, including all control registers.
Rra
  $r0 through R1, plus $a0 and $a1.
Rfl
  The flags register.
Rmm
  The memory-based pseudo-registers $mem0 through $mem15.
Rpi
  Registers that can hold pointers (16 bit registers for r8c, m16c; 24 bit registers for m32cm, m32c).
Rpa
  Matches multiple registers in a PARALLEL to form a larger register. Used to match function return values.
Is3
  -8 ... 7
IS1
  -128 ... 127
IS2
  -32768 ... 32767
IU2
  0 ... 65535
In4
  -8 ... -1 or 1 ... 8
In5
  -16 ... -1 or 1 ... 16
In4
  -8 ... -1 or 1 ... 8
IM2
  -65536 ... -1
Ilb
  An 8 bit value with exactly one bit set.
Ilw
  A 16 bit value with exactly one bit set.
Sd
  The common src/dest memory addressing modes.
Sa
  Memory addressed using $a0 or $a1.
Si
  Memory addressed with immediate addresses.
Ss
  Memory addressed using the stack pointer ($sp).
Sf
  Memory addressed using the frame base register ($fb).
Ss
  Memory addressed using the small base register ($sb).
S1
  $r1h
MIPS—``mips.h
  `d`
    General-purpose integer register
  `f`
    Floating-point register (if available)
  `h`
    ``Hi`' register
  `l`
    ``Lo`' register
  `x`
    ``Hi`' or ``Lo`' register
  `y`
    General-purpose integer register
  `z`
    Floating-point status register
  `I`
    Signed 16-bit constant (for arithmetic instructions)
  `J`
    Zero
  `K`
    Zero-extended 16-bit constant (for logic instructions)
  `L`
    Constant with low 16 bits zero (can be loaded with `lui`)
  `M`
    32-bit constant which requires two instructions to load (a constant which is not ``I`', ``K`', or ``L`')
  `N`
    Negative 16-bit constant
  `O`
    Exact power of two
  `P`
    Positive 16-bit constant
  `G`
    Floating point zero
  `Q`
    Memory reference that can be loaded with more than one instruction (``m`' is preferable for `asm` statements)
  `R`
    Memory reference that can be loaded with one instruction (``m`' is preferable for `asm` statements)
  `S`
    Memory reference in external OSF/rose PIC format (``m`' is preferable for `asm` statements)
Motorola 680x0—``m68k.h
  `a`
    Address register
  `d`
    Data register
  `f`
    68881 floating-point register, if available
  `I`
    Integer in the range 1 to 8
  `J`
    16-bit signed number
  `K`
    Signed number whose magnitude is greater than 0x80
  `L`
    Integer in the range −8 to −1
  `M`
    Signed number whose magnitude is greater than 0x100
  `G`
    Floating point constant that is not a 68881 constant
Motorola 68HC11 & 68HC12 families—``m68hc11.h
  `a`
    Register `a'
  `b`
    Register `b'
  `d`
    Register `d'
  `q`
    An 8-bit register
  `t`
    Temporary soft register _.tmp
  `u`
    A soft register _.d1 to _.d31
  `w`
    Stack pointer register
  `x`
    Register `x'
  `y`
    Register `y'
  `z`
    Pseudo register `z' (replaced by `x' or `y' at the end)
  `A`
    An address register: x, y or z
  `B`
    An address register: x or y
  `D`
    Register pair (x:d) to form a 32-bit value
  `L`
    Constants in the range −65536 to 65535
  `M`
    Constants whose 16-bit low part is zero
  `N`
    Constant integer 1 or −1
  `O`
    Constant integer 16
  `P`
    Constants in the range −8 to 2
SPARC—``sparc.h
  `f`
    Floating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture.
  `e`
    Floating-point register. It is equivalent to ``f`' on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture.
  `c`
    Floating-point condition code register.
  `d`
    Lower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
  `b`
    Floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
  `h`
    64-bit global or out register for the SPARC-V8+ architecture.
  `I`
    Signed 13-bit constant
  `J`
    Zero
  `K`
    32-bit constant with the low 12 bits clear (a constant that can be loaded with the `sethi` instruction)
  `L`
    A constant in the range supported by `movcc` instructions
  `M`
    A constant in the range supported by `movrcc` instructions
  `N`
    Same as ``K`', except that it verifies that bits that are not in the lower 32-bit range are all zero. Must be used instead of ``K`' for modes wider than `SImode`
  `O`
    The constant 4096
  `G`
    Floating-point zero
  `H`
    Signed 13-bit constant, sign-extended to 32 or 64 bits
  `Q`
    Floating-point constant whose integral representation can be moved into an integer register using a single sethi instruction
  `R`
    Floating-point constant whose integral representation can be moved into an integer register using a single mov instruction
  `S`
    Floating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence
  `T`
    Memory address aligned to an 8-byte boundary
  `U`
    Even register
  `W`
    Memory address for ``e`' constraint registers
  `Y`
    Vector zero
TMS320C3x/C4x—``c4x.h
  `a`
    Auxiliary (address) register (ar0-ar7)
  `b`
    Stack pointer register (sp)
  `c`
    Standard (32-bit) precision integer register
  `f`
    Extended (40-bit) precision register (r0-r11)
  `k`
    Block count register (bk)
  `q`
    Extended (40-bit) precision low register (r0-r7)
  `t`
    Extended (40-bit) precision register (r0-r1)
  `u`
    Extended (40-bit) precision register (r2-r3)
  `v`
    Repeat count register (rc)
  `x`
    Index register (ir0-ir1)
  `y`
    Status (condition code) register (st)
  `z`
    Data page register (dp)
  `G`
    Floating-point zero
  `H`
    Immediate 16-bit floating-point constant
  `I`
    Signed 16-bit constant
  `J`
    Signed 8-bit constant
  `K`
    Signed 5-bit constant
  `L`
    Unsigned 16-bit constant
  `M`
    Unsigned 8-bit constant
  `N`
    Ones complement of unsigned 16-bit constant
  `O`
    High 16-bit constant (32-bit constant with 16 LSBs zero)
  `Q`
    Indirect memory reference with signed 8-bit or index register displacement
  `R`
    Indirect memory reference with unsigned 5-bit displacement
  `S`
    Indirect memory reference with 1 bit or index register displacement
  `T`
    Direct memory reference
  `U`
    Symbolic address
S/390 and zSeries—``s390.h
  `a`
    Address register (general purpose register except r0)
  `c`
    Condition code register
  `d`
    Data register (arbitrary general purpose register)
  `f`
    Floating-point register
  `I`
    Unsigned 8-bit constant (0–255)
  `J`
    Unsigned 12-bit constant (0–4095)
  `K`
    Signed 16-bit constant (−32768–32767)
  `L`
    Value appropriate as displacement.
    `(0..4095)`
      for short displacement
    `(-524288..524287)`
      for long displacement
  `M`
    Constant integer with a value of 0x7fffffff.
  `N`
    Multiple letter constraint followed by 4 parameter letters.
    `0..9:`
      number of the part counting from most to least significant
    `H,Q:`
      mode of the part
    `D,S,H:`
      mode of the containing operand
    `0,F:`
      value of the other parts (F—all bits set)
    The constraint matches if the specified part of a constant has a value different from it's other parts.
  `Q`
    Memory reference without index register and with short displacement.
  `R`
    Memory reference with index register and short displacement.
  `S`
    Memory reference without index register but with long displacement.
  `T`
    Memory reference with index register and long displacement.
  `U`
    Pointer with short displacement.
  `W`
    Pointer with long displacement.
  `Y`
    Shift count operand.
Xstormy16—``stormy16.h
  `a`
    Register r0.
  `b`
    Register r1.
  `c`
    Register r2.
  `d`
    Register r8.
  `e`
    Registers r0 through r7.
  `t`
    Registers r0 and r1.
  `y`
    The carry register.
  `z`
    Registers r8 and r9.
  `I`
    A constant between 0 and 3 inclusive.
  `J`
    A constant that has exactly one bit set.
  `K`
    A constant that has exactly one bit clear.
  `L`
    A constant between 0 and 255 inclusive.
  `M`
    A constant between −255 and 0 inclusive.
  `N`
    A constant between −3 and 0 inclusive.
  `O`
    A constant between 1 and 4 inclusive.
  `P`
    A constant between −4 and −1 inclusive.
  `Q`
    A memory reference that is a stack push.
  `R`
    A memory reference that is a stack pop.
  `S`
    A memory reference that refers to a constant address of known value.
  `T`
    The register indicated by Rx (not implemented yet).
  `U`
    A constant that is not between 2 and 15 inclusive.
  `Z`
    The constant 0.
Xtensa—``xtensa.h
  `a`
    General-purpose 32-bit register
  `b`
    One-bit boolean register
  `A`
    MAC16 40-bit accumulator register
  `I`
    Signed 12-bit integer constant, for use in MOVI instructions
  `J`
    Signed 8-bit integer constant, for use in ADDI instructions
  `K`
    Integer constant valid for BccI instructions
  `L`
    Unsigned constant valid for BccUI instructions

Next: Explicit Reg Vars, Previous: Constraints, Up: C Extensions

### 5.36 Controlling Names Used in Assembler Code

You can specify the name to be used in the assembler code for a C function or variable by writing the `asm` (or `__asm__`) keyword after the declarator as follows:

         int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable `foo` in the assembler code should be ``myfoo`' rather than the usual ``_foo`'.

On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.

It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see Explicit Reg Vars. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.

You cannot use `asm` in this way in a function definition; but you can get the same effect by writing a declaration for the function before its definition and putting `asm` there, like this:

         extern func () asm ("FUNC");

         func (x, y)
              int x, y;
         /* ... */

It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.

Next: Alternate Keywords, Previous: Asm Labels, Up: C Extensions

### 5.37 Variables in Specified Registers

GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.

- Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often.
- Local register variables in specific registers do not reserve the registers, except at the point where they are used as input or output operands in an `asm` statement and the `asm` statement itself is not deleted. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.

  These local variables are sometimes convenient for use with the extended `asm` feature (see Extended Asm), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the `asm`.)

Next: Local Reg Vars, Up: Explicit Reg Vars

#### 5.37.1 Defining Global Register Variables

You can define a global register variable in GNU C like this:

         register int *foo asm ("a5");

Here `a5` is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register `a5` would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a “global” register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5`.

Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.

Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.

It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).

It is not safe for one function that uses a global register variable to call another such function `foo` by way of a third function `lose` that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because `lose` might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to `qsort`, since `qsort` might have put something else in that register. (If you are prepared to recompile `qsort` with the same global register variable, you can solve this problem.)

If you want to recompile `qsort` or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option `-ffixed-reg`. You need not actually add a global register declaration to their source code.

A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.

On most machines, `longjmp` will restore to each global register variable the value it had at the time of the `setjmp`. On some machines, however, `longjmp` will not change the value of global register variables. To be portable, the function that called `setjmp` should make other arrangements to save the values of the global register variables, and to restore them in a `longjmp`. This way, the same thing will happen regardless of what `longjmp` does.

All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.

On the SPARC, there are reports that g3 ... g7 are suitable registers, but certain library functions, such as `getwd`, as well as the subroutines for division and remainder, modify g3 and g4. g1 and g2 are local temporaries.

On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.

Previous: Global Reg Vars, Up: Explicit Reg Vars

#### 5.37.2 Specifying Registers for Local Variables

You can define a local register variable with a specified register like this:

         register int *foo asm ("a5");

Here `a5` is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see Extended Asm). Both of these things generally require that you conditionalize your program according to cpu type.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5`.

Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live.

This option does not guarantee that GCC will generate code that has this variable in the register you specify at all times. You may not code an explicit reference to this register in the assembler instruction template part of an `asm` statement and assume it will always refer to this variable. However, using the variable as an `asm` operand guarantees that the specified register is used for the operand.

Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.

As for global register variables, it's recommended that you choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it. A common pitfall is to initialize multiple call-clobbered registers with arbitrary expressions, where a function call or library call for an arithmetic operator will overwrite a register value from a previous assignment, for example `r0` below:

         register int *p1 asm ("r0") = ...;
         register int *p2 asm ("r1") = ...;

In those cases, a solution is to use a temporary variable for each arbitrary expression. See Example of asm with clobbered asm reg.

Next: Incomplete Enums, Previous: Explicit Reg Vars, Up: C Extensions

### 5.38 Alternate Keywords

`-ansi` and the various `-std` options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords `asm`, `typeof` and `inline` are not available in programs compiled with `-ansi` or `-std` (although `inline` can be used in a program compiled with `-std=c99`). The ISO C99 keyword `restrict` is only available when `-std=gnu99` (which will eventually be the default) or `-std=c99` (or the equivalent `-std=iso9899:1999`) is used.

The way to solve these problems is to put ``__`' at the beginning and end of each problematical keyword. For example, use `__asm__` instead of `asm`, and `__inline__` instead of `inline`.

Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:

         #ifndef __GNUC__
         #define __asm__ asm
         #endif

`-pedantic` and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing `__extension__` before the expression. `__extension__` has no effect aside from this.

Next: Function Names, Previous: Alternate Keywords, Up: C Extensions

### 5.39 Incomplete `enum` Types

You can define an `enum` tag without specifying its possible values. This results in an incomplete type, much like what you get if you write `struct foo` without describing the elements. A later declaration which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of `enum` more consistent with the way `struct` and `union` are handled.

This extension is not supported by GNU C++.

Next: Return Address, Previous: Incomplete Enums, Up: C Extensions

### 5.40 Function Names as Strings

GCC provides three magic variables which hold the name of the current function, as a string. The first of these is `__func__`, which is part of the C99 standard:

         The identifier __func__ is implicitly declared by the translator
         as if, immediately following the opening brace of each function
         definition, the declaration
                   static const char __func__[] = "function-name";

         appeared, where function-name is the name of the lexically-enclosing
         function.  This name is the unadorned name of the function.

`__FUNCTION__` is another name for `__func__`. Older versions of GCC recognize only this name. However, it is not standardized. For maximum portability, we recommend you use `__func__`, but provide a fallback definition with the preprocessor:

         #if __STDC_VERSION__ < 199901L
         # if __GNUC__ >= 2
         #  define __func__ __FUNCTION__
         # else
         #  define __func__ "<unknown>"
         # endif
         #endif

In C, `__PRETTY_FUNCTION__` is yet another name for `__func__`. However, in C++, `__PRETTY_FUNCTION__` contains the type signature of the function as well as its bare name. For example, this program:

         extern "C" {
         extern int printf (char *, ...);
         }

         class a {
          public:
           void sub (int i)
             {
               printf ("__FUNCTION__ = %s\n", __FUNCTION__);
               printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
             }
         };

         int
         main (void)
         {
           a ax;
           ax.sub (0);
           return 0;
         }

gives this output:

         __FUNCTION__ = sub
         __PRETTY_FUNCTION__ = void a::sub(int)

These identifiers are not preprocessor macros. In GCC 3.3 and earlier, in C only, `__FUNCTION__` and `__PRETTY_FUNCTION__` were treated as string literals; they could be used to initialize `char` arrays, and they could be concatenated with other string literals. GCC 3.4 and later treat them as variables, like `__func__`. In C++, `__FUNCTION__` and `__PRETTY_FUNCTION__` have always been variables.

Next: Vector Extensions, Previous: Function Names, Up: C Extensions

### 5.41 Getting the Return or Frame Address of a Function

These functions may be used to get information about the callers of a function.

— Built-in Function: void * __builtin_return_address (unsigned int level)

> This function returns the return address of the current function, or of one of its callers. The level argument is number of frames to scan up the call stack. A value of `0` yields the return address of the current function, a value of `1` yields the return address of the caller of the current function, and so forth. When inlining the expected behavior is that the function will return the address of the function that will be returned to. To work around this behavior use the `noinline` function attribute.

> The level argument must be a constant integer.

> On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return `0` or a random value. In addition, `__builtin_frame_address` may be used to determine if the top of the stack has been reached.

> This function should only be used with a nonzero argument for debugging purposes.

— Built-in Function: void * __builtin_frame_address (unsigned int level)

> This function is similar to `__builtin_return_address`, but it returns the address of the function frame rather than the return address of the function. Calling `__builtin_frame_address` with a value of `0` yields the frame address of the current function, a value of `1` yields the frame address of the caller of the current function, and so forth.

> The frame is the area on the stack which holds local variables and saved registers. The frame address is normally the address of the first word pushed on to the stack by the function. However, the exact definition depends upon the processor and the calling convention. If the processor has a dedicated frame pointer register, and the function has a frame, then `__builtin_frame_address` will return the value of the frame pointer register.

> On some machines it may be impossible to determine the frame address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return `0` if the first frame pointer is properly initialized by the startup code.

> This function should only be used with a nonzero argument for debugging purposes.

Next: Offsetof, Previous: Return Address, Up: C Extensions

### 5.42 Using vector instructions through built-in functions

On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used this way.

The first step in using these extensions is to provide the necessary data types. This should be done using an appropriate `typedef`:

         typedef int v4si __attribute__ ((vector_size (16)));

The `int` type specifies the base type, while the attribute specifies the vector size for the variable, measured in bytes. For example, the declaration above causes the compiler to set the mode for the `v4si` type to be 16 bytes wide and divided into `int` sized units. For a 32-bit `int` this means a vector of 4 units of 4 bytes, and the corresponding mode of `foo` will be V4SI.

The `vector_size` attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.

All the basic integer types can be used as base types, both as signed and as unsigned: `char`, `short`, `int`, `long`, `long long`. In addition, `float` and `double` can be used to build floating-point vector types.

Specifying a combination that is not valid for the current architecture will cause GCC to synthesize the instructions using a narrower mode. For example, if you specify a variable of type `V4SI` and your architecture does not allow for this specific SIMD type, GCC will produce code that uses 4 `SIs`.

The types defined in this manner can be used with a subset of normal C operations. Currently, GCC will allow using the following operators on these types: `+, -, *, /, unary minus, ^, |, &, ~`.

The operations behave like C++ `valarrays`. Addition is defined as the addition of the corresponding elements of the operands. For example, in the code below, each of the 4 elements in a will be added to the corresponding 4 elements in b and the resulting vector will be stored in c.

         typedef int v4si __attribute__ ((vector_size (16)));

         v4si a, b, c;

         c = a + b;

Subtraction, multiplication, division, and the logical operations operate in a similar manner. Likewise, the result of using the unary minus or complement operators on a vector type is a vector whose elements are the negative or complemented values of the corresponding elements in the operand.

You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).

You cannot operate between vectors of different lengths or different signedness without a cast.

A port that supports hardware vector operations, usually provides a set of built-in functions that can be used to operate on vectors. For example, a function to add two vectors and multiply the result by a third could look like this:

         v4si f (v4si a, v4si b, v4si c)
         {
           v4si tmp = __builtin_addv4si (a, b);
           return __builtin_mulv4si (tmp, c);
         }

Next: Atomic Builtins, Previous: Vector Extensions, Up: C Extensions

### 5.43 Offsetof

GCC implements for both C and C++ a syntactic extension to implement the `offsetof` macro.

         primary:
         	"__builtin_offsetof" "(" typename "," offsetof_member_designator ")"

         offsetof_member_designator:
         	  identifier
         	| offsetof_member_designator "." identifier
         	| offsetof_member_designator "[" expr "]"

This extension is sufficient such that

         #define offsetof(type, member)  __builtin_offsetof (type, member)

is a suitable definition of the `offsetof` macro. In C++, type may be dependent. In either case, member may consist of a single identifier, or a sequence of member accesses and array references.

Next: Object Size Checking, Previous: Offsetof, Up: C Extensions

### 5.44 Built-in functions for atomic memory access

The following builtins are intended to be compatible with those described in the Intel Itanium Processor-specific Application Binary Interface, section 7.4. As such, they depart from the normal GCC practice of using the “__builtin_” prefix, and further that they are overloaded such that they work on multiple types.

The definition given in the Intel documentation allows only for the use of the types `int`, `long`, `long long` as well as their unsigned counterparts. GCC will allow any integral scalar or pointer type that is 1, 2, 4 or 8 bytes in length.

Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning will be generated and a call an external function will be generated. The external function will carry the same name as the builtin, with an additional suffix ``_n`' where n is the size of the data type.

In most cases, these builtins are considered a full barrier. That is, no memory operand will be moved across the operation, either forward or backward. Further, instructions will be issued as necessary to prevent the processor from speculating loads across the operation and from queuing stores after the operation.

All of the routines are are described in the Intel documentation to take “an optional list of variables protected by the memory barrier”. It's not clear what is meant by that; it could mean that only the following variables are protected, or it could mean that these variables should in addition be protected. At present GCC ignores this list and protects all variables which are globally accessible. If in the future we make some use of this list, an empty list will continue to mean all globally accessible variables.

type`__sync_fetch_and_add (`type`*ptr,` type `value, ...)`
type`__sync_fetch_and_sub (`type`*ptr,` type `value, ...)`
type`__sync_fetch_and_or (`type`*ptr,` type `value, ...)`
type`__sync_fetch_and_and (`type`*ptr,` type `value, ...)`
type`__sync_fetch_and_xor (`type`*ptr,` type `value, ...)`
type`__sync_fetch_and_nand (`type`*ptr,` type `value, ...)`
  These builtins perform the operation suggested by the name, and returns the value that had previously been in memory. That is,

                { tmp = *ptr; *ptr op= value; return tmp; }
                { tmp = *ptr; *ptr = ~tmp & value; return tmp; }   // nand

type`__sync_add_and_fetch (`type`*ptr,` type `value, ...)`
type`__sync_sub_and_fetch (`type`*ptr,` type `value, ...)`
type`__sync_or_and_fetch (`type`*ptr,` type `value, ...)`
type`__sync_and_and_fetch (`type`*ptr,` type `value, ...)`
type`__sync_xor_and_fetch (`type`*ptr,` type `value, ...)`
type`__sync_nand_and_fetch (`type`*ptr,` type `value, ...)`
  These builtins perform the operation suggested by the name, and return the new value. That is,

                { *ptr op= value; return *ptr; }
                { *ptr = ~*ptr & value; return *ptr; }   // nand

`bool __sync_bool_compare_and_swap (`type`*ptr,` type `oldval` type `newval, ...)`
type`__sync_val_compare_and_swap (`type`*ptr,` type `oldval` type `newval, ...)`
  These builtins perform an atomic compare and swap. That is, if the current value of `*`ptr is oldval, then write newval into `*`ptr.

  The “bool” version returns true if the comparison is successful and newval was written. The “val” version returns the contents of `*`ptr before the operation.

`__sync_synchronize (...)`
  This builtin issues a full memory barrier.
type`__sync_lock_test_and_set (`type`*ptr,` type `value, ...)`
  This builtin, as described by Intel, is not a traditional test-and-set operation, but rather an atomic exchange operation. It writes value into `*`ptr, and returns the previous contents of `*`ptr.

  Many targets have only minimal support for such locks, and do not support a full exchange operation. In this case, a target may support reduced functionality here by which the only valid value to store is the immediate constant 1. The exact value actually stored in `*`ptr is implementation defined.

  This builtin is not a full barrier, but rather an acquire barrier. This means that references after the builtin cannot move to (or be speculated to) before the builtin, but previous memory stores may not be globally visible yet, and previous memory loads may not yet be satisfied.

`void __sync_lock_release (`type`*ptr, ...)`
  This builtin releases the lock acquired by `__sync_lock_test_and_set`. Normally this means writing the constant 0 to `*`ptr.

  This builtin is not a full barrier, but rather a release barrier. This means that all previous memory stores are globally visible, and all previous memory loads have been satisfied, but following memory reads are not prevented from being speculated to before the barrier.

Next: Other Builtins, Previous: Atomic Builtins, Up: C Extensions

### 5.45 Object Size Checking Builtins

GCC implements a limited buffer overflow protection mechanism that can prevent some buffer overflow attacks.

— Built-in Function: size_t __builtin_object_size (void * ptr, int type)

> is a built-in construct that returns a constant number of bytes from ptr to the end of the object ptr pointer points to (if known at compile time). `__builtin_object_size` never evaluates its arguments for side-effects. If there are any side-effects in them, it returns `(size_t) -1` for type 0 or 1 and `(size_t) 0` for type 2 or 3. If there are multiple objects ptr can point to and all of them are known at compile time, the returned number is the maximum of remaining byte counts in those objects if type & 2 is 0 and minimum if nonzero. If it is not possible to determine which objects ptr points to at compile time, `__builtin_object_size` should return `(size_t) -1` for type 0 or 1 and `(size_t) 0` for type 2 or 3.

> type is an integer constant from 0 to 3. If the least significant bit is clear, objects are whole variables, if it is set, a closest surrounding subobject is considered the object a pointer points to. The second bit determines if maximum or minimum of remaining bytes is computed.

>               struct V { char buf1[10]; int b; char buf2[10]; } var;
>               char *p = &var.buf1[1], *q = &var.b;

>               /* Here the object p points to is var.  */
>               assert (__builtin_object_size (p, 0) == sizeof (var) - 1);
>               /* The subobject p points to is var.buf1.  */
>               assert (__builtin_object_size (p, 1) == sizeof (var.buf1) - 1);
>               /* The object q points to is var.  */
>               assert (__builtin_object_size (q, 0)
>               	== (char *) (&var + 1) - (char *) &var.b);
>               /* The subobject q points to is var.b.  */
>               assert (__builtin_object_size (q, 1) == sizeof (var.b));

There are built-in functions added for many common string operation functions, e.g. for `memcpy` `__builtin___memcpy_chk` built-in is provided. This built-in has an additional last argument, which is the number of bytes remaining in object the dest argument points to or `(size_t) -1` if the size is not known.

The built-in functions are optimized into the normal string functions like `memcpy` if the last argument is `(size_t) -1` or if it is known at compile time that the destination object will not be overflown. If the compiler can determine at compile time the object will be always overflown, it issues a warning.

The intended use can be e.g.

         #undef memcpy
         #define bos0(dest) __builtin_object_size (dest, 0)
         #define memcpy(dest, src, n) \
           __builtin___memcpy_chk (dest, src, n, bos0 (dest))

         char *volatile p;
         char buf[10];
         /* It is unknown what object p points to, so this is optimized
            into plain memcpy - no checking is possible.  */
         memcpy (p, "abcde", n);
         /* Destination is known and length too.  It is known at compile
            time there will be no overflow.  */
         memcpy (&buf[5], "abcde", 5);
         /* Destination is known, but the length is not known at compile time.
            This will result in __memcpy_chk call that can check for overflow
            at runtime.  */
         memcpy (&buf[5], "abcde", n);
         /* Destination is known and it is known at compile time there will
            be overflow.  There will be a warning and __memcpy_chk call that
            will abort the program at runtime.  */
         memcpy (&buf[6], "abcde", 5);

Such built-in functions are provided for `memcpy`, `mempcpy`, `memmove`, `memset`, `strcpy`, `stpcpy`, `strncpy`, `strcat` and `strncat`.

There are also checking built-in functions for formatted output functions.

         int __builtin___sprintf_chk (char *s, int flag, size_t os, const char *fmt, ...);
         int __builtin___snprintf_chk (char *s, size_t maxlen, int flag, size_t os,
         			      const char *fmt, ...);
         int __builtin___vsprintf_chk (char *s, int flag, size_t os, const char *fmt,
         			      va_list ap);
         int __builtin___vsnprintf_chk (char *s, size_t maxlen, int flag, size_t os,
         			       const char *fmt, va_list ap);

The added flag argument is passed unchanged to `__sprintf_chk` etc. functions and can contain implementation specific flags on what additional security measures the checking function might take, such as handling `%n` differently.

The os argument is the object size s points to, like in the other built-in functions. There is a small difference in the behavior though, if os is `(size_t) -1`, the built-in functions are optimized into the non-checking functions only if flag is 0, otherwise the checking function is called with os argument set to `(size_t) -1`.

In addition to this, there are checking built-in functions `__builtin___printf_chk`, `__builtin___vprintf_chk`, `__builtin___fprintf_chk` and `__builtin___vfprintf_chk`. These have just one additional argument, flag, right before format string fmt. If the compiler is able to optimize them to `fputc` etc. functions, it will, otherwise the checking function should be called and the flag argument passed to it.

Next: Target Builtins, Previous: Object Size Checking, Up: C Extensions

### 5.46 Other built-in functions provided by GCC

GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.

The remaining functions are provided for optimization purposes.

GCC includes built-in versions of many of the functions in the standard C library. The versions prefixed with `__builtin_` will always be treated as having the same meaning as the C library function even if you specify the `-fno-builtin` option. (see C Dialect Options) Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function will be emitted.

Outside strict ISO C mode (`-ansi`, `-std=c89` or `-std=c99`), the functions `_exit`, `alloca`, `bcmp`, `bzero`, `dcgettext`, `dgettext`, `dremf`, `dreml`, `drem`, `exp10f`, `exp10l`, `exp10`, `ffsll`, `ffsl`, `ffs`, `fprintf_unlocked`, `fputs_unlocked`, `gammaf`, `gammal`, `gamma`, `gettext`, `index`, `isascii`, `j0f`, `j0l`, `j0`, `j1f`, `j1l`, `j1`, `jnf`, `jnl`, `jn`, `mempcpy`, `pow10f`, `pow10l`, `pow10`, `printf_unlocked`, `rindex`, `scalbf`, `scalbl`, `scalb`, `signbit`, `signbitf`, `signbitl`, `significandf`, `significandl`, `significand`, `sincosf`, `sincosl`, `sincos`, `stpcpy`, `stpncpy`, `strcasecmp`, `strdup`, `strfmon`, `strncasecmp`, `strndup`, `toascii`, `y0f`, `y0l`, `y0`, `y1f`, `y1l`, `y1`, `ynf`, `ynl` and `yn` may be handled as built-in functions. All these functions have corresponding versions prefixed with `__builtin_`, which may be used even in strict C89 mode.

The ISO C99 functions `_Exit`, `acoshf`, `acoshl`, `acosh`, `asinhf`, `asinhl`, `asinh`, `atanhf`, `atanhl`, `atanh`, `cabsf`, `cabsl`, `cabs`, `cacosf`, `cacoshf`, `cacoshl`, `cacosh`, `cacosl`, `cacos`, `cargf`, `cargl`, `carg`, `casinf`, `casinhf`, `casinhl`, `casinh`, `casinl`, `casin`, `catanf`, `catanhf`, `catanhl`, `catanh`, `catanl`, `catan`, `cbrtf`, `cbrtl`, `cbrt`, `ccosf`, `ccoshf`, `ccoshl`, `ccosh`, `ccosl`, `ccos`, `cexpf`, `cexpl`, `cexp`, `cimagf`, `cimagl`, `cimag`, `clogf`, `clogl`, `clog`, `conjf`, `conjl`, `conj`, `copysignf`, `copysignl`, `copysign`, `cpowf`, `cpowl`, `cpow`, `cprojf`, `cprojl`, `cproj`, `crealf`, `creall`, `creal`, `csinf`, `csinhf`, `csinhl`, `csinh`, `csinl`, `csin`, `csqrtf`, `csqrtl`, `csqrt`, `ctanf`, `ctanhf`, `ctanhl`, `ctanh`, `ctanl`, `ctan`, `erfcf`, `erfcl`, `erfc`, `erff`, `erfl`, `erf`, `exp2f`, `exp2l`, `exp2`, `expm1f`, `expm1l`, `expm1`, `fdimf`, `fdiml`, `fdim`, `fmaf`, `fmal`, `fmaxf`, `fmaxl`, `fmax`, `fma`, `fminf`, `fminl`, `fmin`, `hypotf`, `hypotl`, `hypot`, `ilogbf`, `ilogbl`, `ilogb`, `imaxabs`, `isblank`, `iswblank`, `lgammaf`, `lgammal`, `lgamma`, `llabs`, `llrintf`, `llrintl`, `llrint`, `llroundf`, `llroundl`, `llround`, `log1pf`, `log1pl`, `log1p`, `log2f`, `log2l`, `log2`, `logbf`, `logbl`, `logb`, `lrintf`, `lrintl`, `lrint`, `lroundf`, `lroundl`, `lround`, `nearbyintf`, `nearbyintl`, `nearbyint`, `nextafterf`, `nextafterl`, `nextafter`, `nexttowardf`, `nexttowardl`, `nexttoward`, `remainderf`, `remainderl`, `remainder`, `remquof`, `remquol`, `remquo`, `rintf`, `rintl`, `rint`, `roundf`, `roundl`, `round`, `scalblnf`, `scalblnl`, `scalbln`, `scalbnf`, `scalbnl`, `scalbn`, `snprintf`, `tgammaf`, `tgammal`, `tgamma`, `truncf`, `truncl`, `trunc`, `vfscanf`, `vscanf`, `vsnprintf` and `vsscanf` are handled as built-in functions except in strict ISO C90 mode (`-ansi` or `-std=c89`).

There are also built-in versions of the ISO C99 functions `acosf`, `acosl`, `asinf`, `asinl`, `atan2f`, `atan2l`, `atanf`, `atanl`, `ceilf`, `ceill`, `cosf`, `coshf`, `coshl`, `cosl`, `expf`, `expl`, `fabsf`, `fabsl`, `floorf`, `floorl`, `fmodf`, `fmodl`, `frexpf`, `frexpl`, `ldexpf`, `ldexpl`, `log10f`, `log10l`, `logf`, `logl`, `modfl`, `modf`, `powf`, `powl`, `sinf`, `sinhf`, `sinhl`, `sinl`, `sqrtf`, `sqrtl`, `tanf`, `tanhf`, `tanhl` and `tanl` that are recognized in any mode since ISO C90 reserves these names for the purpose to which ISO C99 puts them. All these functions have corresponding versions prefixed with `__builtin_`.

The ISO C94 functions `iswalnum`, `iswalpha`, `iswcntrl`, `iswdigit`, `iswgraph`, `iswlower`, `iswprint`, `iswpunct`, `iswspace`, `iswupper`, `iswxdigit`, `towlower` and `towupper` are handled as built-in functions except in strict ISO C90 mode (`-ansi` or `-std=c89`).

The ISO C90 functions `abort`, `abs`, `acos`, `asin`, `atan2`, `atan`, `calloc`, `ceil`, `cosh`, `cos`, `exit`, `exp`, `fabs`, `floor`, `fmod`, `fprintf`, `fputs`, `frexp`, `fscanf`, `isalnum`, `isalpha`, `iscntrl`, `isdigit`, `isgraph`, `islower`, `isprint`, `ispunct`, `isspace`, `isupper`, `isxdigit`, `tolower`, `toupper`, `labs`, `ldexp`, `log10`, `log`, `malloc`, `memcmp`, `memcpy`, `memset`, `modf`, `pow`, `printf`, `putchar`, `puts`, `scanf`, `sinh`, `sin`, `snprintf`, `sprintf`, `sqrt`, `sscanf`, `strcat`, `strchr`, `strcmp`, `strcpy`, `strcspn`, `strlen`, `strncat`, `strncmp`, `strncpy`, `strpbrk`, `strrchr`, `strspn`, `strstr`, `tanh`, `tan`, `vfprintf`, `vprintf` and `vsprintf` are all recognized as built-in functions unless `-fno-builtin` is specified (or `-fno-builtin-function` is specified for an individual function). All of these functions have corresponding versions prefixed with `__builtin_`.

GCC provides built-in versions of the ISO C99 floating point comparison macros that avoid raising exceptions for unordered operands. They have the same names as the standard macros ( `isgreater`, `isgreaterequal`, `isless`, `islessequal`, `islessgreater`, and `isunordered`) , with `__builtin_` prefixed. We intend for a library implementor to be able to simply `#define` each standard macro to its built-in equivalent.

— Built-in Function: int __builtin_types_compatible_p (type1, type2)

> You can use the built-in function `__builtin_types_compatible_p` to determine whether two types are the same.

> This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.

> This built-in function ignores top level qualifiers (e.g., `const`, `volatile`). For example, `int` is equivalent to `const int`.

> The type `int[]` and `int[5]` are compatible. On the other hand, `int` and `char *` are not compatible, even if the size of their types, on the particular architecture are the same. Also, the amount of pointer indirection is taken into account when determining similarity. Consequently, `short *` is not similar to `short **`. Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible.

> An `enum` type is not considered to be compatible with another `enum` type even if both are compatible with the same integer type; this is what the C standard specifies. For example, `enum {foo, bar}` is not similar to `enum {hot, dog}`.

> You would typically use this function in code whose execution varies depending on the arguments' types. For example:

>               #define foo(x)                                                  \
>                 ({                                                           \
>                   typeof (x) tmp;                                             \
>                   if (__builtin_types_compatible_p (typeof (x), long double)) \
>                     tmp = foo_long_double (tmp);                              \
>                   else if (__builtin_types_compatible_p (typeof (x), double)) \
>                     tmp = foo_double (tmp);                                   \
>                   else if (__builtin_types_compatible_p (typeof (x), float))  \
>                     tmp = foo_float (tmp);                                    \
>                   else                                                        \
>                     abort ();                                                 \
>                   tmp;                                                        \
>                 })

> Note: This construct is only available for C.

— Built-in Function: type __builtin_choose_expr (const_exp, exp1, exp2)

> You can use the built-in function `__builtin_choose_expr` to evaluate code depending on the value of a constant expression. This built-in function returns exp1 if const_exp, which is a constant expression that must be able to be determined at compile time, is nonzero. Otherwise it returns 0.

> This built-in function is analogous to the ``? :`' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.

> This built-in function can return an lvalue if the chosen argument is an lvalue.

> If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.

> Example:

>               #define foo(x)                                                    \
>                 __builtin_choose_expr (                                         \
>                   __builtin_types_compatible_p (typeof (x), double),            \
>                   foo_double (x),                                               \
>                   __builtin_choose_expr (                                       \
>                     __builtin_types_compatible_p (typeof (x), float),           \
>                     foo_float (x),                                              \
>                     /* The void expression results in a compile-time error  \
>                        when assigning the result to something.  */          \
>                     (void)0))

> Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions.

— Built-in Function: int __builtin_constant_p (exp)

> You can use the built-in function `__builtin_constant_p` to determine if a value is known to be constant at compile-time and hence that GCC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the `-O` option.

> You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:

>               #define Scale_Value(X)      \
>                 (__builtin_constant_p (X) \
>                 ? ((X) * SCALE + OFFSET) : Scale (X))

> You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O` option.

> You may also use `__builtin_constant_p` in initializers for static data. For instance, you can write

>               static const int table[] = {
>                  __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
>                  /* ... */
>               };

> This is an acceptable initializer even if EXPRESSION is not a constant expression. GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.

> Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.

— Built-in Function: long __builtin_expect (long exp, long c)

> You may use `__builtin_expect` to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (`-fprofile-arcs`), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.

> The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c. For example:

>               if (__builtin_expect (x, 0))
>                 foo ();

> would indicate that we do not expect to call `foo`, since we expect `x` to be zero. Since you are limited to integral expressions for exp, you should use constructions such as

>               if (__builtin_expect (ptr != NULL, 1))
>                 error ();

> when testing pointer or floating-point values.

— Built-in Function: void __builtin_prefetch (const void *addr, ...)

> This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to `__builtin_prefetch` into code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions will be generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed.

> The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.

>               for (i = 0; i < n; i++)
>                 {
>                   a[i] = a[i] + b[i];
>                   __builtin_prefetch (&a[i+j], 1, 1);
>                   __builtin_prefetch (&b[i+j], 0, 1);
>                   /* ... */
>                 }

> Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of `p->next` will not fault if `p->next` is not a valid address, but evaluation will fault if `p` is not a valid address.

> If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.

— Built-in Function: double __builtin_huge_val (void)

> Returns a positive infinity, if supported by the floating-point format, else `DBL_MAX`. This function is suitable for implementing the ISO C macro `HUGE_VAL`.

— Built-in Function: float __builtin_huge_valf (void)

> Similar to `__builtin_huge_val`, except the return type is `float`.

— Built-in Function: long double __builtin_huge_vall (void)

> Similar to `__builtin_huge_val`, except the return type is `long double`.

— Built-in Function: double __builtin_inf (void)

> Similar to `__builtin_huge_val`, except a warning is generated if the target floating-point format does not support infinities.

— Built-in Function: float __builtin_inff (void)

> Similar to `__builtin_inf`, except the return type is `float`. This function is suitable for implementing the ISO C99 macro `INFINITY`.

— Built-in Function: long double __builtin_infl (void)

> Similar to `__builtin_inf`, except the return type is `long double`.

— Built-in Function: double __builtin_nan (const char *str)

> This is an implementation of the ISO C99 function `nan`.

> Since ISO C99 defines this function in terms of `strtod`, which we do not implement, a description of the parsing is in order. The string is parsed as by `strtol`; that is, the base is recognized by leading ``0`' or ``0x`' prefixes. The number parsed is placed in the significand such that the least significant bit of the number is at the least significant bit of the significand. The number is truncated to fit the significand field provided. The significand is forced to be a quiet NaN.

> This function, if given a string literal, is evaluated early enough that it is considered a compile-time constant.

— Built-in Function: float __builtin_nanf (const char *str)

> Similar to `__builtin_nan`, except the return type is `float`.

— Built-in Function: long double __builtin_nanl (const char *str)

> Similar to `__builtin_nan`, except the return type is `long double`.

— Built-in Function: double __builtin_nans (const char *str)

> Similar to `__builtin_nan`, except the significand is forced to be a signaling NaN. The `nans` function is proposed by WG14 N965.

— Built-in Function: float __builtin_nansf (const char *str)

> Similar to `__builtin_nans`, except the return type is `float`.

— Built-in Function: long double __builtin_nansl (const char *str)

> Similar to `__builtin_nans`, except the return type is `long double`.

— Built-in Function: int __builtin_ffs (unsigned int x)

> Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.

— Built-in Function: int __builtin_clz (unsigned int x)

> Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

— Built-in Function: int __builtin_ctz (unsigned int x)

> Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.

— Built-in Function: int __builtin_popcount (unsigned int x)

> Returns the number of 1-bits in x.

— Built-in Function: int __builtin_parity (unsigned int x)

> Returns the parity of x, i.e. the number of 1-bits in x modulo 2.

— Built-in Function: int __builtin_ffsl (unsigned long)

> Similar to `__builtin_ffs`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_clzl (unsigned long)

> Similar to `__builtin_clz`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_ctzl (unsigned long)

> Similar to `__builtin_ctz`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_popcountl (unsigned long)

> Similar to `__builtin_popcount`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_parityl (unsigned long)

> Similar to `__builtin_parity`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_ffsll (unsigned long long)

> Similar to `__builtin_ffs`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_clzll (unsigned long long)

> Similar to `__builtin_clz`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_ctzll (unsigned long long)

> Similar to `__builtin_ctz`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_popcountll (unsigned long long)

> Similar to `__builtin_popcount`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_parityll (unsigned long long)

> Similar to `__builtin_parity`, except the argument type is `unsigned long long`.

— Built-in Function: double __builtin_powi (double, int)

> Returns the first argument raised to the power of the second. Unlike the `pow` function no guarantees about precision and rounding are made.

— Built-in Function: float __builtin_powif (float, int)

> Similar to `__builtin_powi`, except the argument and return types are `float`.

— Built-in Function: long double __builtin_powil (long double, int)

> Similar to `__builtin_powi`, except the argument and return types are `long double`.

Next: Target Format Checks, Previous: Other Builtins, Up: C Extensions

### 5.47 Built-in Functions Specific to Particular Target Machines

On some target machines, GCC supports many built-in functions specific to those machines. Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls.

Next: ARM Built-in Functions, Up: Target Builtins

#### 5.47.1 Alpha Built-in Functions

These built-in functions are available for the Alpha family of processors, depending on the command-line switches used.

The following built-in functions are always available. They all generate the machine instruction that is part of the name.

         long __builtin_alpha_implver (void)
         long __builtin_alpha_rpcc (void)
         long __builtin_alpha_amask (long)
         long __builtin_alpha_cmpbge (long, long)
         long __builtin_alpha_extbl (long, long)
         long __builtin_alpha_extwl (long, long)
         long __builtin_alpha_extll (long, long)
         long __builtin_alpha_extql (long, long)
         long __builtin_alpha_extwh (long, long)
         long __builtin_alpha_extlh (long, long)
         long __builtin_alpha_extqh (long, long)
         long __builtin_alpha_insbl (long, long)
         long __builtin_alpha_inswl (long, long)
         long __builtin_alpha_insll (long, long)
         long __builtin_alpha_insql (long, long)
         long __builtin_alpha_inswh (long, long)
         long __builtin_alpha_inslh (long, long)
         long __builtin_alpha_insqh (long, long)
         long __builtin_alpha_mskbl (long, long)
         long __builtin_alpha_mskwl (long, long)
         long __builtin_alpha_mskll (long, long)
         long __builtin_alpha_mskql (long, long)
         long __builtin_alpha_mskwh (long, long)
         long __builtin_alpha_msklh (long, long)
         long __builtin_alpha_mskqh (long, long)
         long __builtin_alpha_umulh (long, long)
         long __builtin_alpha_zap (long, long)
         long __builtin_alpha_zapnot (long, long)

The following built-in functions are always with `-mmax` or `-mcpu=cpu` where cpu is `pca56` or later. They all generate the machine instruction that is part of the name.

         long __builtin_alpha_pklb (long)
         long __builtin_alpha_pkwb (long)
         long __builtin_alpha_unpkbl (long)
         long __builtin_alpha_unpkbw (long)
         long __builtin_alpha_minub8 (long, long)
         long __builtin_alpha_minsb8 (long, long)
         long __builtin_alpha_minuw4 (long, long)
         long __builtin_alpha_minsw4 (long, long)
         long __builtin_alpha_maxub8 (long, long)
         long __builtin_alpha_maxsb8 (long, long)
         long __builtin_alpha_maxuw4 (long, long)
         long __builtin_alpha_maxsw4 (long, long)
         long __builtin_alpha_perr (long, long)

The following built-in functions are always with `-mcix` or `-mcpu=cpu` where cpu is `ev67` or later. They all generate the machine instruction that is part of the name.

         long __builtin_alpha_cttz (long)
         long __builtin_alpha_ctlz (long)
         long __builtin_alpha_ctpop (long)

The following builtins are available on systems that use the OSF/1 PALcode. Normally they invoke the `rduniq` and `wruniq` PAL calls, but when invoked with `-mtls-kernel`, they invoke `rdval` and `wrval`.

         void *__builtin_thread_pointer (void)
         void __builtin_set_thread_pointer (void *)

Next: Blackfin Built-in Functions, Previous: Alpha Built-in Functions, Up: Target Builtins

#### 5.47.2 ARM Built-in Functions

These built-in functions are available for the ARM family of processors, when the `-mcpu=iwmmxt` switch is used:

         typedef int v2si __attribute__ ((vector_size (8)));
         typedef short v4hi __attribute__ ((vector_size (8)));
         typedef char v8qi __attribute__ ((vector_size (8)));

         int __builtin_arm_getwcx (int)
         void __builtin_arm_setwcx (int, int)
         int __builtin_arm_textrmsb (v8qi, int)
         int __builtin_arm_textrmsh (v4hi, int)
         int __builtin_arm_textrmsw (v2si, int)
         int __builtin_arm_textrmub (v8qi, int)
         int __builtin_arm_textrmuh (v4hi, int)
         int __builtin_arm_textrmuw (v2si, int)
         v8qi __builtin_arm_tinsrb (v8qi, int)
         v4hi __builtin_arm_tinsrh (v4hi, int)
         v2si __builtin_arm_tinsrw (v2si, int)
         long long __builtin_arm_tmia (long long, int, int)
         long long __builtin_arm_tmiabb (long long, int, int)
         long long __builtin_arm_tmiabt (long long, int, int)
         long long __builtin_arm_tmiaph (long long, int, int)
         long long __builtin_arm_tmiatb (long long, int, int)
         long long __builtin_arm_tmiatt (long long, int, int)
         int __builtin_arm_tmovmskb (v8qi)
         int __builtin_arm_tmovmskh (v4hi)
         int __builtin_arm_tmovmskw (v2si)
         long long __builtin_arm_waccb (v8qi)
         long long __builtin_arm_wacch (v4hi)
         long long __builtin_arm_waccw (v2si)
         v8qi __builtin_arm_waddb (v8qi, v8qi)
         v8qi __builtin_arm_waddbss (v8qi, v8qi)
         v8qi __builtin_arm_waddbus (v8qi, v8qi)
         v4hi __builtin_arm_waddh (v4hi, v4hi)
         v4hi __builtin_arm_waddhss (v4hi, v4hi)
         v4hi __builtin_arm_waddhus (v4hi, v4hi)
         v2si __builtin_arm_waddw (v2si, v2si)
         v2si __builtin_arm_waddwss (v2si, v2si)
         v2si __builtin_arm_waddwus (v2si, v2si)
         v8qi __builtin_arm_walign (v8qi, v8qi, int)
         long long __builtin_arm_wand(long long, long long)
         long long __builtin_arm_wandn (long long, long long)
         v8qi __builtin_arm_wavg2b (v8qi, v8qi)
         v8qi __builtin_arm_wavg2br (v8qi, v8qi)
         v4hi __builtin_arm_wavg2h (v4hi, v4hi)
         v4hi __builtin_arm_wavg2hr (v4hi, v4hi)
         v8qi __builtin_arm_wcmpeqb (v8qi, v8qi)
         v4hi __builtin_arm_wcmpeqh (v4hi, v4hi)
         v2si __builtin_arm_wcmpeqw (v2si, v2si)
         v8qi __builtin_arm_wcmpgtsb (v8qi, v8qi)
         v4hi __builtin_arm_wcmpgtsh (v4hi, v4hi)
         v2si __builtin_arm_wcmpgtsw (v2si, v2si)
         v8qi __builtin_arm_wcmpgtub (v8qi, v8qi)
         v4hi __builtin_arm_wcmpgtuh (v4hi, v4hi)
         v2si __builtin_arm_wcmpgtuw (v2si, v2si)
         long long __builtin_arm_wmacs (long long, v4hi, v4hi)
         long long __builtin_arm_wmacsz (v4hi, v4hi)
         long long __builtin_arm_wmacu (long long, v4hi, v4hi)
         long long __builtin_arm_wmacuz (v4hi, v4hi)
         v4hi __builtin_arm_wmadds (v4hi, v4hi)
         v4hi __builtin_arm_wmaddu (v4hi, v4hi)
         v8qi __builtin_arm_wmaxsb (v8qi, v8qi)
         v4hi __builtin_arm_wmaxsh (v4hi, v4hi)
         v2si __builtin_arm_wmaxsw (v2si, v2si)
         v8qi __builtin_arm_wmaxub (v8qi, v8qi)
         v4hi __builtin_arm_wmaxuh (v4hi, v4hi)
         v2si __builtin_arm_wmaxuw (v2si, v2si)
         v8qi __builtin_arm_wminsb (v8qi, v8qi)
         v4hi __builtin_arm_wminsh (v4hi, v4hi)
         v2si __builtin_arm_wminsw (v2si, v2si)
         v8qi __builtin_arm_wminub (v8qi, v8qi)
         v4hi __builtin_arm_wminuh (v4hi, v4hi)
         v2si __builtin_arm_wminuw (v2si, v2si)
         v4hi __builtin_arm_wmulsm (v4hi, v4hi)
         v4hi __builtin_arm_wmulul (v4hi, v4hi)
         v4hi __builtin_arm_wmulum (v4hi, v4hi)
         long long __builtin_arm_wor (long long, long long)
         v2si __builtin_arm_wpackdss (long long, long long)
         v2si __builtin_arm_wpackdus (long long, long long)
         v8qi __builtin_arm_wpackhss (v4hi, v4hi)
         v8qi __builtin_arm_wpackhus (v4hi, v4hi)
         v4hi __builtin_arm_wpackwss (v2si, v2si)
         v4hi __builtin_arm_wpackwus (v2si, v2si)
         long long __builtin_arm_wrord (long long, long long)
         long long __builtin_arm_wrordi (long long, int)
         v4hi __builtin_arm_wrorh (v4hi, long long)
         v4hi __builtin_arm_wrorhi (v4hi, int)
         v2si __builtin_arm_wrorw (v2si, long long)
         v2si __builtin_arm_wrorwi (v2si, int)
         v2si __builtin_arm_wsadb (v8qi, v8qi)
         v2si __builtin_arm_wsadbz (v8qi, v8qi)
         v2si __builtin_arm_wsadh (v4hi, v4hi)
         v2si __builtin_arm_wsadhz (v4hi, v4hi)
         v4hi __builtin_arm_wshufh (v4hi, int)
         long long __builtin_arm_wslld (long long, long long)
         long long __builtin_arm_wslldi (long long, int)
         v4hi __builtin_arm_wsllh (v4hi, long long)
         v4hi __builtin_arm_wsllhi (v4hi, int)
         v2si __builtin_arm_wsllw (v2si, long long)
         v2si __builtin_arm_wsllwi (v2si, int)
         long long __builtin_arm_wsrad (long long, long long)
         long long __builtin_arm_wsradi (long long, int)
         v4hi __builtin_arm_wsrah (v4hi, long long)
         v4hi __builtin_arm_wsrahi (v4hi, int)
         v2si __builtin_arm_wsraw (v2si, long long)
         v2si __builtin_arm_wsrawi (v2si, int)
         long long __builtin_arm_wsrld (long long, long long)
         long long __builtin_arm_wsrldi (long long, int)
         v4hi __builtin_arm_wsrlh (v4hi, long long)
         v4hi __builtin_arm_wsrlhi (v4hi, int)
         v2si __builtin_arm_wsrlw (v2si, long long)
         v2si __builtin_arm_wsrlwi (v2si, int)
         v8qi __builtin_arm_wsubb (v8qi, v8qi)
         v8qi __builtin_arm_wsubbss (v8qi, v8qi)
         v8qi __builtin_arm_wsubbus (v8qi, v8qi)
         v4hi __builtin_arm_wsubh (v4hi, v4hi)
         v4hi __builtin_arm_wsubhss (v4hi, v4hi)
         v4hi __builtin_arm_wsubhus (v4hi, v4hi)
         v2si __builtin_arm_wsubw (v2si, v2si)
         v2si __builtin_arm_wsubwss (v2si, v2si)
         v2si __builtin_arm_wsubwus (v2si, v2si)
         v4hi __builtin_arm_wunpckehsb (v8qi)
         v2si __builtin_arm_wunpckehsh (v4hi)
         long long __builtin_arm_wunpckehsw (v2si)
         v4hi __builtin_arm_wunpckehub (v8qi)
         v2si __builtin_arm_wunpckehuh (v4hi)
         long long __builtin_arm_wunpckehuw (v2si)
         v4hi __builtin_arm_wunpckelsb (v8qi)
         v2si __builtin_arm_wunpckelsh (v4hi)
         long long __builtin_arm_wunpckelsw (v2si)
         v4hi __builtin_arm_wunpckelub (v8qi)
         v2si __builtin_arm_wunpckeluh (v4hi)
         long long __builtin_arm_wunpckeluw (v2si)
         v8qi __builtin_arm_wunpckihb (v8qi, v8qi)
         v4hi __builtin_arm_wunpckihh (v4hi, v4hi)
         v2si __builtin_arm_wunpckihw (v2si, v2si)
         v8qi __builtin_arm_wunpckilb (v8qi, v8qi)
         v4hi __builtin_arm_wunpckilh (v4hi, v4hi)
         v2si __builtin_arm_wunpckilw (v2si, v2si)
         long long __builtin_arm_wxor (long long, long long)
         long long __builtin_arm_wzero ()

Next: FR-V Built-in Functions, Previous: ARM Built-in Functions, Up: Target Builtins

#### 5.47.3 Blackfin Built-in Functions

Currently, there are two Blackfin-specific built-in functions. These are used for generating `CSYNC` and `SSYNC` machine insns without using inline assembly; by using these built-in functions the compiler can automatically add workarounds for hardware errata involving these instructions. These functions are named as follows:

         void __builtin_bfin_csync (void)
         void __builtin_bfin_ssync (void)

Next: X86 Built-in Functions, Previous: Blackfin Built-in Functions, Up: Target Builtins

#### 5.47.4 FR-V Built-in Functions

GCC provides many FR-V-specific built-in functions. In general, these functions are intended to be compatible with those described by FR-V Family, Softune C/C++ Compiler Manual (V6), Fujitsu Semiconductor. The two exceptions are `__MDUNPACKH` and `__MBTOHE`, the gcc forms of which pass 128-bit values by pointer rather than by value.

Most of the functions are named after specific FR-V instructions. Such functions are said to be “directly mapped” and are summarized here in tabular form.

Next: Directly-mapped Integer Functions, Up: FR-V Built-in Functions

##### 5.47.4.1 Argument Types

The arguments to the built-in functions can be divided into three groups: register numbers, compile-time constants and run-time values. In order to make this classification clear at a glance, the arguments and return values are given the following pseudo types:

Pseudo type | Real C type | Constant? | Description
`uh` | `unsigned short` | No | an unsigned halfword
`uw1` | `unsigned int` | No | an unsigned word
`sw1` | `int` | No | a signed word
`uw2` | `unsigned long long` | No | an unsigned doubleword
`sw2` | `long long` | No | a signed doubleword
`const` | `int` | Yes | an integer constant
`acc` | `int` | Yes | an ACC register number
`iacc` | `int` | Yes | an IACC register number

These pseudo types are not defined by GCC, they are simply a notational convenience used in this manual.

Arguments of type `uh`, `uw1`, `sw1`, `uw2` and `sw2` are evaluated at run time. They correspond to register operands in the underlying FR-V instructions.

`const` arguments represent immediate operands in the underlying FR-V instructions. They must be compile-time constants.

`acc` arguments are evaluated at compile time and specify the number of an accumulator register. For example, an `acc` argument of 2 will select the ACC2 register.

`iacc` arguments are similar to `acc` arguments but specify the number of an IACC register. See see Other Built-in Functions for more details.

Next: Directly-mapped Media Functions, Previous: Argument Types, Up: FR-V Built-in Functions

##### 5.47.4.2 Directly-mapped Integer Functions

The functions listed below map directly to FR-V I-type instructions.

Function prototype | Example usage | Assembly output
`sw1 __ADDSS (sw1, sw1)` | c`= __ADDSS (`a`,` b`)` | `ADDSS` a`,`b`,`c
`sw1 __SCAN (sw1, sw1)` | c`= __SCAN (`a`,` b`)` | `SCAN` a`,`b`,`c
`sw1 __SCUTSS (sw1)` | b`= __SCUTSS (`a`)` | `SCUTSS` a`,`b
`sw1 __SLASS (sw1, sw1)` | c`= __SLASS (`a`,` b`)` | `SLASS` a`,`b`,`c
`void __SMASS (sw1, sw1)` | `__SMASS (`a`,` b`)` | `SMASS` a`,`b
`void __SMSSS (sw1, sw1)` | `__SMSSS (`a`,` b`)` | `SMSSS` a`,`b
`void __SMU (sw1, sw1)` | `__SMU (`a`,` b`)` | `SMU` a`,`b
`sw2 __SMUL (sw1, sw1)` | c`= __SMUL (`a`,` b`)` | `SMUL` a`,`b`,`c
`sw1 __SUBSS (sw1, sw1)` | c`= __SUBSS (`a`,` b`)` | `SUBSS` a`,`b`,`c
`uw2 __UMUL (uw1, uw1)` | c`= __UMUL (`a`,` b`)` | `UMUL` a`,`b`,`c

Next: Raw read/write Functions, Previous: Directly-mapped Integer Functions, Up: FR-V Built-in Functions

##### 5.47.4.3 Directly-mapped Media Functions

The functions listed below map directly to FR-V M-type instructions.

Function prototype | Example usage | Assembly output
`uw1 __MABSHS (sw1)` | b`= __MABSHS (`a`)` | `MABSHS` a`,`b
`void __MADDACCS (acc, acc)` | `__MADDACCS (`b`,` a`)` | `MADDACCS` a`,`b
`sw1 __MADDHSS (sw1, sw1)` | c`= __MADDHSS (`a`,` b`)` | `MADDHSS` a`,`b`,`c
`uw1 __MADDHUS (uw1, uw1)` | c`= __MADDHUS (`a`,` b`)` | `MADDHUS` a`,`b`,`c
`uw1 __MAND (uw1, uw1)` | c`= __MAND (`a`,` b`)` | `MAND` a`,`b`,`c
`void __MASACCS (acc, acc)` | `__MASACCS (`b`,` a`)` | `MASACCS` a`,`b
`uw1 __MAVEH (uw1, uw1)` | c`= __MAVEH (`a`,` b`)` | `MAVEH` a`,`b`,`c
`uw2 __MBTOH (uw1)` | b`= __MBTOH (`a`)` | `MBTOH` a`,`b
`void __MBTOHE (uw1 *, uw1)` | `__MBTOHE (&`b`,` a`)` | `MBTOHE` a`,`b
`void __MCLRACC (acc)` | `__MCLRACC (`a`)` | `MCLRACC` a
`void __MCLRACCA (void)` | `__MCLRACCA ()` | `MCLRACCA`
`uw1 __Mcop1 (uw1, uw1)` | c`= __Mcop1 (`a`,` b`)` | `Mcop1` a`,`b`,`c
`uw1 __Mcop2 (uw1, uw1)` | c`= __Mcop2 (`a`,` b`)` | `Mcop2` a`,`b`,`c
`uw1 __MCPLHI (uw2, const)` | c`= __MCPLHI (`a`,` b`)` | `MCPLHI` a`,#`b`,`c
`uw1 __MCPLI (uw2, const)` | c`= __MCPLI (`a`,` b`)` | `MCPLI` a`,#`b`,`c
`void __MCPXIS (acc, sw1, sw1)` | `__MCPXIS (`c`,` a`,` b`)` | `MCPXIS` a`,`b`,`c
`void __MCPXIU (acc, uw1, uw1)` | `__MCPXIU (`c`,` a`,` b`)` | `MCPXIU` a`,`b`,`c
`void __MCPXRS (acc, sw1, sw1)` | `__MCPXRS (`c`,` a`,` b`)` | `MCPXRS` a`,`b`,`c
`void __MCPXRU (acc, uw1, uw1)` | `__MCPXRU (`c`,` a`,` b`)` | `MCPXRU` a`,`b`,`c
`uw1 __MCUT (acc, uw1)` | c`= __MCUT (`a`,` b`)` | `MCUT` a`,`b`,`c
`uw1 __MCUTSS (acc, sw1)` | c`= __MCUTSS (`a`,` b`)` | `MCUTSS` a`,`b`,`c
`void __MDADDACCS (acc, acc)` | `__MDADDACCS (`b`,` a`)` | `MDADDACCS` a`,`b
`void __MDASACCS (acc, acc)` | `__MDASACCS (`b`,` a`)` | `MDASACCS` a`,`b
`uw2 __MDCUTSSI (acc, const)` | c`= __MDCUTSSI (`a`,` b`)` | `MDCUTSSI` a`,#`b`,`c
`uw2 __MDPACKH (uw2, uw2)` | c`= __MDPACKH (`a`,` b`)` | `MDPACKH` a`,`b`,`c
`uw2 __MDROTLI (uw2, const)` | c`= __MDROTLI (`a`,` b`)` | `MDROTLI` a`,#`b`,`c
`void __MDSUBACCS (acc, acc)` | `__MDSUBACCS (`b`,` a`)` | `MDSUBACCS` a`,`b
`void __MDUNPACKH (uw1 *, uw2)` | `__MDUNPACKH (&`b`,` a`)` | `MDUNPACKH` a`,`b
`uw2 __MEXPDHD (uw1, const)` | c`= __MEXPDHD (`a`,` b`)` | `MEXPDHD` a`,#`b`,`c
`uw1 __MEXPDHW (uw1, const)` | c`= __MEXPDHW (`a`,` b`)` | `MEXPDHW` a`,#`b`,`c
`uw1 __MHDSETH (uw1, const)` | c`= __MHDSETH (`a`,` b`)` | `MHDSETH` a`,#`b`,`c
`sw1 __MHDSETS (const)` | b`= __MHDSETS (`a`)` | `MHDSETS #`a`,`b
`uw1 __MHSETHIH (uw1, const)` | b`= __MHSETHIH (`b`,` a`)` | `MHSETHIH #`a`,`b
`sw1 __MHSETHIS (sw1, const)` | b`= __MHSETHIS (`b`,` a`)` | `MHSETHIS #`a`,`b
`uw1 __MHSETLOH (uw1, const)` | b`= __MHSETLOH (`b`,` a`)` | `MHSETLOH #`a`,`b
`sw1 __MHSETLOS (sw1, const)` | b`= __MHSETLOS (`b`,` a`)` | `MHSETLOS #`a`,`b
`uw1 __MHTOB (uw2)` | b`= __MHTOB (`a`)` | `MHTOB` a`,`b
`void __MMACHS (acc, sw1, sw1)` | `__MMACHS (`c`,` a`,` b`)` | `MMACHS` a`,`b`,`c
`void __MMACHU (acc, uw1, uw1)` | `__MMACHU (`c`,` a`,` b`)` | `MMACHU` a`,`b`,`c
`void __MMRDHS (acc, sw1, sw1)` | `__MMRDHS (`c`,` a`,` b`)` | `MMRDHS` a`,`b`,`c
`void __MMRDHU (acc, uw1, uw1)` | `__MMRDHU (`c`,` a`,` b`)` | `MMRDHU` a`,`b`,`c
`void __MMULHS (acc, sw1, sw1)` | `__MMULHS (`c`,` a`,` b`)` | `MMULHS` a`,`b`,`c
`void __MMULHU (acc, uw1, uw1)` | `__MMULHU (`c`,` a`,` b`)` | `MMULHU` a`,`b`,`c
`void __MMULXHS (acc, sw1, sw1)` | `__MMULXHS (`c`,` a`,` b`)` | `MMULXHS` a`,`b`,`c
`void __MMULXHU (acc, uw1, uw1)` | `__MMULXHU (`c`,` a`,` b`)` | `MMULXHU` a`,`b`,`c
`uw1 __MNOT (uw1)` | b`= __MNOT (`a`)` | `MNOT` a`,`b
`uw1 __MOR (uw1, uw1)` | c`= __MOR (`a`,` b`)` | `MOR` a`,`b`,`c
`uw1 __MPACKH (uh, uh)` | c`= __MPACKH (`a`,` b`)` | `MPACKH` a`,`b`,`c
`sw2 __MQADDHSS (sw2, sw2)` | c`= __MQADDHSS (`a`,` b`)` | `MQADDHSS` a`,`b`,`c
`uw2 __MQADDHUS (uw2, uw2)` | c`= __MQADDHUS (`a`,` b`)` | `MQADDHUS` a`,`b`,`c
`void __MQCPXIS (acc, sw2, sw2)` | `__MQCPXIS (`c`,` a`,` b`)` | `MQCPXIS` a`,`b`,`c
`void __MQCPXIU (acc, uw2, uw2)` | `__MQCPXIU (`c`,` a`,` b`)` | `MQCPXIU` a`,`b`,`c
`void __MQCPXRS (acc, sw2, sw2)` | `__MQCPXRS (`c`,` a`,` b`)` | `MQCPXRS` a`,`b`,`c
`void __MQCPXRU (acc, uw2, uw2)` | `__MQCPXRU (`c`,` a`,` b`)` | `MQCPXRU` a`,`b`,`c
`sw2 __MQLCLRHS (sw2, sw2)` | c`= __MQLCLRHS (`a`,` b`)` | `MQLCLRHS` a`,`b`,`c
`sw2 __MQLMTHS (sw2, sw2)` | c`= __MQLMTHS (`a`,` b`)` | `MQLMTHS` a`,`b`,`c
`void __MQMACHS (acc, sw2, sw2)` | `__MQMACHS (`c`,` a`,` b`)` | `MQMACHS` a`,`b`,`c
`void __MQMACHU (acc, uw2, uw2)` | `__MQMACHU (`c`,` a`,` b`)` | `MQMACHU` a`,`b`,`c
`void __MQMACXHS (acc, sw2, sw2)` | `__MQMACXHS (`c`,` a`,` b`)` | `MQMACXHS` a`,`b`,`c
`void __MQMULHS (acc, sw2, sw2)` | `__MQMULHS (`c`,` a`,` b`)` | `MQMULHS` a`,`b`,`c
`void __MQMULHU (acc, uw2, uw2)` | `__MQMULHU (`c`,` a`,` b`)` | `MQMULHU` a`,`b`,`c
`void __MQMULXHS (acc, sw2, sw2)` | `__MQMULXHS (`c`,` a`,` b`)` | `MQMULXHS` a`,`b`,`c
`void __MQMULXHU (acc, uw2, uw2)` | `__MQMULXHU (`c`,` a`,` b`)` | `MQMULXHU` a`,`b`,`c
`sw2 __MQSATHS (sw2, sw2)` | c`= __MQSATHS (`a`,` b`)` | `MQSATHS` a`,`b`,`c
`uw2 __MQSLLHI (uw2, int)` | c`= __MQSLLHI (`a`,` b`)` | `MQSLLHI` a`,`b`,`c
`sw2 __MQSRAHI (sw2, int)` | c`= __MQSRAHI (`a`,` b`)` | `MQSRAHI` a`,`b`,`c
`sw2 __MQSUBHSS (sw2, sw2)` | c`= __MQSUBHSS (`a`,` b`)` | `MQSUBHSS` a`,`b`,`c
`uw2 __MQSUBHUS (uw2, uw2)` | c`= __MQSUBHUS (`a`,` b`)` | `MQSUBHUS` a`,`b`,`c
`void __MQXMACHS (acc, sw2, sw2)` | `__MQXMACHS (`c`,` a`,` b`)` | `MQXMACHS` a`,`b`,`c
`void __MQXMACXHS (acc, sw2, sw2)` | `__MQXMACXHS (`c`,` a`,` b`)` | `MQXMACXHS` a`,`b`,`c
`uw1 __MRDACC (acc)` | b`= __MRDACC (`a`)` | `MRDACC` a`,`b
`uw1 __MRDACCG (acc)` | b`= __MRDACCG (`a`)` | `MRDACCG` a`,`b
`uw1 __MROTLI (uw1, const)` | c`= __MROTLI (`a`,` b`)` | `MROTLI` a`,#`b`,`c
`uw1 __MROTRI (uw1, const)` | c`= __MROTRI (`a`,` b`)` | `MROTRI` a`,#`b`,`c
`sw1 __MSATHS (sw1, sw1)` | c`= __MSATHS (`a`,` b`)` | `MSATHS` a`,`b`,`c
`uw1 __MSATHU (uw1, uw1)` | c`= __MSATHU (`a`,` b`)` | `MSATHU` a`,`b`,`c
`uw1 __MSLLHI (uw1, const)` | c`= __MSLLHI (`a`,` b`)` | `MSLLHI` a`,#`b`,`c
`sw1 __MSRAHI (sw1, const)` | c`= __MSRAHI (`a`,` b`)` | `MSRAHI` a`,#`b`,`c
`uw1 __MSRLHI (uw1, const)` | c`= __MSRLHI (`a`,` b`)` | `MSRLHI` a`,#`b`,`c
`void __MSUBACCS (acc, acc)` | `__MSUBACCS (`b`,` a`)` | `MSUBACCS` a`,`b
`sw1 __MSUBHSS (sw1, sw1)` | c`= __MSUBHSS (`a`,` b`)` | `MSUBHSS` a`,`b`,`c
`uw1 __MSUBHUS (uw1, uw1)` | c`= __MSUBHUS (`a`,` b`)` | `MSUBHUS` a`,`b`,`c
`void __MTRAP (void)` | `__MTRAP ()` | `MTRAP`
`uw2 __MUNPACKH (uw1)` | b`= __MUNPACKH (`a`)` | `MUNPACKH` a`,`b
`uw1 __MWCUT (uw2, uw1)` | c`= __MWCUT (`a`,` b`)` | `MWCUT` a`,`b`,`c
`void __MWTACC (acc, uw1)` | `__MWTACC (`b`,` a`)` | `MWTACC` a`,`b
`void __MWTACCG (acc, uw1)` | `__MWTACCG (`b`,` a`)` | `MWTACCG` a`,`b
`uw1 __MXOR (uw1, uw1)` | c`= __MXOR (`a`,` b`)` | `MXOR` a`,`b`,`c

Next: Other Built-in Functions, Previous: Directly-mapped Media Functions, Up: FR-V Built-in Functions

##### 5.47.4.4 Raw read/write Functions

This sections describes built-in functions related to read and write instructions to access memory. These functions generate `membar` instructions to flush the I/O load and stores where appropriate, as described in Fujitsu's manual described above.

`unsigned char __builtin_read8 (void *`data`)`
`unsigned short __builtin_read16 (void *`data`)`
`unsigned long __builtin_read32 (void *`data`)`
`unsigned long long __builtin_read64 (void *`data`)`
`void __builtin_write8 (void *`data`, unsigned char` datum`)`
`void __builtin_write16 (void *`data`, unsigned short` datum`)`
`void __builtin_write32 (void *`data`, unsigned long` datum`)`
`void __builtin_write64 (void *`data`, unsigned long long` datum`)`

Previous: Raw read/write Functions, Up: FR-V Built-in Functions

##### 5.47.4.5 Other Built-in Functions

This section describes built-in functions that are not named after a specific FR-V instruction.

`sw2 __IACCreadll (iacc` reg`)`
  Return the full 64-bit value of IACC0. The reg argument is reserved for future expansion and must be 0.
`sw1 __IACCreadl (iacc` reg`)`
  Return the value of IACC0H if reg is 0 and IACC0L if reg is 1. Other values of reg are rejected as invalid.
`void __IACCsetll (iacc` reg`, sw2` x`)`
  Set the full 64-bit value of IACC0 to x. The reg argument is reserved for future expansion and must be 0.
`void __IACCsetl (iacc` reg`, sw1` x`)`
  Set IACC0H to x if reg is 0 and IACC0L to x if reg is 1. Other values of reg are rejected as invalid.
`void __data_prefetch0 (const void *`x`)`
  Use the `dcpl` instruction to load the contents of address x into the data cache.
`void __data_prefetch (const void *`x`)`
  Use the `nldub` instruction to load the contents of address x into the data cache. The instruction will be issued in slot I1.

Next: MIPS DSP Built-in Functions, Previous: FR-V Built-in Functions, Up: Target Builtins

#### 5.47.5 X86 Built-in Functions

These built-in functions are available for the i386 and x86-64 family of computers, depending on the command-line switches used.

Note that, if you specify command-line switches such as `-msse`, the compiler could use the extended instruction sets even if the built-ins are not used explicitly in the program. For this reason, applications which perform runtime CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.

The following machine modes are available for use with MMX built-in functions (see Vector Extensions): `V2SI` for a vector of two 32-bit integers, `V4HI` for a vector of four 16-bit integers, and `V8QI` for a vector of eight 8-bit integers. Some of the built-in functions operate on MMX registers as a whole 64-bit entity, these use `DI` as their mode.

If 3Dnow extensions are enabled, `V2SF` is used as a mode for a vector of two 32-bit floating point values.

If SSE extensions are enabled, `V4SF` is used for a vector of four 32-bit floating point values. Some instructions use a vector of four 32-bit integers, these use `V4SI`. Finally, some instructions operate on an entire vector register, interpreting it as a 128-bit integer, these use mode `TI`.

The following built-in functions are made available by `-mmmx`. All of them generate the machine instruction that is part of the name.

         v8qi __builtin_ia32_paddb (v8qi, v8qi)
         v4hi __builtin_ia32_paddw (v4hi, v4hi)
         v2si __builtin_ia32_paddd (v2si, v2si)
         v8qi __builtin_ia32_psubb (v8qi, v8qi)
         v4hi __builtin_ia32_psubw (v4hi, v4hi)
         v2si __builtin_ia32_psubd (v2si, v2si)
         v8qi __builtin_ia32_paddsb (v8qi, v8qi)
         v4hi __builtin_ia32_paddsw (v4hi, v4hi)
         v8qi __builtin_ia32_psubsb (v8qi, v8qi)
         v4hi __builtin_ia32_psubsw (v4hi, v4hi)
         v8qi __builtin_ia32_paddusb (v8qi, v8qi)
         v4hi __builtin_ia32_paddusw (v4hi, v4hi)
         v8qi __builtin_ia32_psubusb (v8qi, v8qi)
         v4hi __builtin_ia32_psubusw (v4hi, v4hi)
         v4hi __builtin_ia32_pmullw (v4hi, v4hi)
         v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
         di __builtin_ia32_pand (di, di)
         di __builtin_ia32_pandn (di,di)
         di __builtin_ia32_por (di, di)
         di __builtin_ia32_pxor (di, di)
         v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
         v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
         v2si __builtin_ia32_pcmpeqd (v2si, v2si)
         v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
         v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
         v2si __builtin_ia32_pcmpgtd (v2si, v2si)
         v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
         v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
         v2si __builtin_ia32_punpckhdq (v2si, v2si)
         v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
         v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
         v2si __builtin_ia32_punpckldq (v2si, v2si)
         v8qi __builtin_ia32_packsswb (v4hi, v4hi)
         v4hi __builtin_ia32_packssdw (v2si, v2si)
         v8qi __builtin_ia32_packuswb (v4hi, v4hi)

The following built-in functions are made available either with `-msse`, or with a combination of `-m3dnow` and `-march=athlon`. All of them generate the machine instruction that is part of the name.

         v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
         v8qi __builtin_ia32_pavgb (v8qi, v8qi)
         v4hi __builtin_ia32_pavgw (v4hi, v4hi)
         v4hi __builtin_ia32_psadbw (v8qi, v8qi)
         v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
         v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
         v8qi __builtin_ia32_pminub (v8qi, v8qi)
         v4hi __builtin_ia32_pminsw (v4hi, v4hi)
         int __builtin_ia32_pextrw (v4hi, int)
         v4hi __builtin_ia32_pinsrw (v4hi, int, int)
         int __builtin_ia32_pmovmskb (v8qi)
         void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
         void __builtin_ia32_movntq (di *, di)
         void __builtin_ia32_sfence (void)

The following built-in functions are available when `-msse` is used. All of them generate the machine instruction that is part of the name.

         int __builtin_ia32_comieq (v4sf, v4sf)
         int __builtin_ia32_comineq (v4sf, v4sf)
         int __builtin_ia32_comilt (v4sf, v4sf)
         int __builtin_ia32_comile (v4sf, v4sf)
         int __builtin_ia32_comigt (v4sf, v4sf)
         int __builtin_ia32_comige (v4sf, v4sf)
         int __builtin_ia32_ucomieq (v4sf, v4sf)
         int __builtin_ia32_ucomineq (v4sf, v4sf)
         int __builtin_ia32_ucomilt (v4sf, v4sf)
         int __builtin_ia32_ucomile (v4sf, v4sf)
         int __builtin_ia32_ucomigt (v4sf, v4sf)
         int __builtin_ia32_ucomige (v4sf, v4sf)
         v4sf __builtin_ia32_addps (v4sf, v4sf)
         v4sf __builtin_ia32_subps (v4sf, v4sf)
         v4sf __builtin_ia32_mulps (v4sf, v4sf)
         v4sf __builtin_ia32_divps (v4sf, v4sf)
         v4sf __builtin_ia32_addss (v4sf, v4sf)
         v4sf __builtin_ia32_subss (v4sf, v4sf)
         v4sf __builtin_ia32_mulss (v4sf, v4sf)
         v4sf __builtin_ia32_divss (v4sf, v4sf)
         v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
         v4si __builtin_ia32_cmpltps (v4sf, v4sf)
         v4si __builtin_ia32_cmpleps (v4sf, v4sf)
         v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
         v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
         v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
         v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
         v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
         v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
         v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
         v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
         v4si __builtin_ia32_cmpordps (v4sf, v4sf)
         v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
         v4si __builtin_ia32_cmpltss (v4sf, v4sf)
         v4si __builtin_ia32_cmpless (v4sf, v4sf)
         v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
         v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
         v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
         v4si __builtin_ia32_cmpnless (v4sf, v4sf)
         v4si __builtin_ia32_cmpordss (v4sf, v4sf)
         v4sf __builtin_ia32_maxps (v4sf, v4sf)
         v4sf __builtin_ia32_maxss (v4sf, v4sf)
         v4sf __builtin_ia32_minps (v4sf, v4sf)
         v4sf __builtin_ia32_minss (v4sf, v4sf)
         v4sf __builtin_ia32_andps (v4sf, v4sf)
         v4sf __builtin_ia32_andnps (v4sf, v4sf)
         v4sf __builtin_ia32_orps (v4sf, v4sf)
         v4sf __builtin_ia32_xorps (v4sf, v4sf)
         v4sf __builtin_ia32_movss (v4sf, v4sf)
         v4sf __builtin_ia32_movhlps (v4sf, v4sf)
         v4sf __builtin_ia32_movlhps (v4sf, v4sf)
         v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
         v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
         v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
         v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
         v2si __builtin_ia32_cvtps2pi (v4sf)
         int __builtin_ia32_cvtss2si (v4sf)
         v2si __builtin_ia32_cvttps2pi (v4sf)
         int __builtin_ia32_cvttss2si (v4sf)
         v4sf __builtin_ia32_rcpps (v4sf)
         v4sf __builtin_ia32_rsqrtps (v4sf)
         v4sf __builtin_ia32_sqrtps (v4sf)
         v4sf __builtin_ia32_rcpss (v4sf)
         v4sf __builtin_ia32_rsqrtss (v4sf)
         v4sf __builtin_ia32_sqrtss (v4sf)
         v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
         void __builtin_ia32_movntps (float *, v4sf)
         int __builtin_ia32_movmskps (v4sf)

The following built-in functions are available when `-msse` is used.

`v4sf __builtin_ia32_loadaps (float *)`
  Generates the `movaps` machine instruction as a load from memory.
`void __builtin_ia32_storeaps (float *, v4sf)`
  Generates the `movaps` machine instruction as a store to memory.
`v4sf __builtin_ia32_loadups (float *)`
  Generates the `movups` machine instruction as a load from memory.
`void __builtin_ia32_storeups (float *, v4sf)`
  Generates the `movups` machine instruction as a store to memory.
`v4sf __builtin_ia32_loadsss (float *)`
  Generates the `movss` machine instruction as a load from memory.
`void __builtin_ia32_storess (float *, v4sf)`
  Generates the `movss` machine instruction as a store to memory.
`v4sf __builtin_ia32_loadhps (v4sf, v2si *)`
  Generates the `movhps` machine instruction as a load from memory.
`v4sf __builtin_ia32_loadlps (v4sf, v2si *)`
  Generates the `movlps` machine instruction as a load from memory
`void __builtin_ia32_storehps (v4sf, v2si *)`
  Generates the `movhps` machine instruction as a store to memory.
`void __builtin_ia32_storelps (v4sf, v2si *)`
  Generates the `movlps` machine instruction as a store to memory.

The following built-in functions are available when `-msse3` is used. All of them generate the machine instruction that is part of the name.

         v2df __builtin_ia32_addsubpd (v2df, v2df)
         v2df __builtin_ia32_addsubps (v2df, v2df)
         v2df __builtin_ia32_haddpd (v2df, v2df)
         v2df __builtin_ia32_haddps (v2df, v2df)
         v2df __builtin_ia32_hsubpd (v2df, v2df)
         v2df __builtin_ia32_hsubps (v2df, v2df)
         v16qi __builtin_ia32_lddqu (char const *)
         void __builtin_ia32_monitor (void *, unsigned int, unsigned int)
         v2df __builtin_ia32_movddup (v2df)
         v4sf __builtin_ia32_movshdup (v4sf)
         v4sf __builtin_ia32_movsldup (v4sf)
         void __builtin_ia32_mwait (unsigned int, unsigned int)

The following built-in functions are available when `-msse3` is used.

`v2df __builtin_ia32_loadddup (double const *)`
  Generates the `movddup` machine instruction as a load from memory.

The following built-in functions are available when `-m3dnow` is used. All of them generate the machine instruction that is part of the name.

         void __builtin_ia32_femms (void)
         v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
         v2si __builtin_ia32_pf2id (v2sf)
         v2sf __builtin_ia32_pfacc (v2sf, v2sf)
         v2sf __builtin_ia32_pfadd (v2sf, v2sf)
         v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
         v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
         v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
         v2sf __builtin_ia32_pfmax (v2sf, v2sf)
         v2sf __builtin_ia32_pfmin (v2sf, v2sf)
         v2sf __builtin_ia32_pfmul (v2sf, v2sf)
         v2sf __builtin_ia32_pfrcp (v2sf)
         v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
         v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
         v2sf __builtin_ia32_pfrsqrt (v2sf)
         v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
         v2sf __builtin_ia32_pfsub (v2sf, v2sf)
         v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
         v2sf __builtin_ia32_pi2fd (v2si)
         v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)

The following built-in functions are available when both `-m3dnow` and `-march=athlon` are used. All of them generate the machine instruction that is part of the name.

         v2si __builtin_ia32_pf2iw (v2sf)
         v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
         v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
         v2sf __builtin_ia32_pi2fw (v2si)
         v2sf __builtin_ia32_pswapdsf (v2sf)
         v2si __builtin_ia32_pswapdsi (v2si)

Next: MIPS Paired-Single Support, Previous: X86 Built-in Functions, Up: Target Builtins

#### 5.47.6 MIPS DSP Built-in Functions

The MIPS DSP Application-Specific Extension (ASE) includes new instructions that are designed to improve the performance of DSP and media applications. It provides instructions that operate on packed 8-bit integer data, Q15 fractional data and Q31 fractional data.

GCC supports MIPS DSP operations using both the generic vector extensions (see Vector Extensions) and a collection of MIPS-specific built-in functions. Both kinds of support are enabled by the `-mdsp` command-line option.

At present, GCC only provides support for operations on 32-bit vectors. The vector type associated with 8-bit integer data is usually called `v4i8` and the vector type associated with Q15 is usually called `v2q15`. They can be defined in C as follows:

         typedef char v4i8 __attribute__ ((vector_size(4)));
         typedef short v2q15 __attribute__ ((vector_size(4)));

`v4i8` and `v2q15` values are initialized in the same way as aggregates. For example:

         v4i8 a = {1, 2, 3, 4};
         v4i8 b;
         b = (v4i8) {5, 6, 7, 8};

         v2q15 c = {0x0fcb, 0x3a75};
         v2q15 d;
         d = (v2q15) {0.1234 * 0x1.0p15, 0.4567 * 0x1.0p15};

Note: The CPU's endianness determines the order in which values are packed. On little-endian targets, the first value is the least significant and the last value is the most significant. The opposite order applies to big-endian targets. For example, the code above will set the lowest byte of `a` to `1` on little-endian targets and `4` on big-endian targets.

Note: Q15 and Q31 values must be initialized with their integer representation. As shown in this example, the integer representation of a Q15 value can be obtained by multiplying the fractional value by `0x1.0p15`. The equivalent for Q31 values is to multiply by `0x1.0p31`.

The table below lists the `v4i8` and `v2q15` operations for which hardware support exists. `a` and `b` are `v4i8` values, and `c` and `d` are `v2q15` values.

C code | MIPS instruction
`a + b` | `addu.qb`
`c + d` | `addq.ph`
`a - b` | `subu.qb`
`c - d` | `subq.ph`

It is easier to describe the DSP built-in functions if we first define the following types:

         typedef int q31;
         typedef int i32;
         typedef long long a64;

`q31` and `i32` are actually the same as `int`, but we use `q31` to indicate a Q31 fractional value and `i32` to indicate a 32-bit integer value. Similarly, `a64` is the same as `long long`, but we use `a64` to indicate values that will be placed in one of the four DSP accumulators (`$ac0`, `$ac1`, `$ac2` or `$ac3`).

Also, some built-in functions prefer or require immediate numbers as parameters, because the corresponding DSP instructions accept both immediate numbers and register operands, or accept immediate numbers only. The immediate parameters are listed as follows.

         imm0_7: 0 to 7.
         imm0_15: 0 to 15.
         imm0_31: 0 to 31.
         imm0_63: 0 to 63.
         imm0_255: 0 to 255.
         imm_n32_31: -32 to 31.
         imm_n512_511: -512 to 511.

The following built-in functions map directly to a particular MIPS DSP instruction. Please refer to the architecture specification for details on what each instruction does.

         v2q15 __builtin_mips_addq_ph (v2q15, v2q15)
         v2q15 __builtin_mips_addq_s_ph (v2q15, v2q15)
         q31 __builtin_mips_addq_s_w (q31, q31)
         v4i8 __builtin_mips_addu_qb (v4i8, v4i8)
         v4i8 __builtin_mips_addu_s_qb (v4i8, v4i8)
         v2q15 __builtin_mips_subq_ph (v2q15, v2q15)
         v2q15 __builtin_mips_subq_s_ph (v2q15, v2q15)
         q31 __builtin_mips_subq_s_w (q31, q31)
         v4i8 __builtin_mips_subu_qb (v4i8, v4i8)
         v4i8 __builtin_mips_subu_s_qb (v4i8, v4i8)
         i32 __builtin_mips_addsc (i32, i32)
         i32 __builtin_mips_addwc (i32, i32)
         i32 __builtin_mips_modsub (i32, i32)
         i32 __builtin_mips_raddu_w_qb (v4i8)
         v2q15 __builtin_mips_absq_s_ph (v2q15)
         q31 __builtin_mips_absq_s_w (q31)
         v4i8 __builtin_mips_precrq_qb_ph (v2q15, v2q15)
         v2q15 __builtin_mips_precrq_ph_w (q31, q31)
         v2q15 __builtin_mips_precrq_rs_ph_w (q31, q31)
         v4i8 __builtin_mips_precrqu_s_qb_ph (v2q15, v2q15)
         q31 __builtin_mips_preceq_w_phl (v2q15)
         q31 __builtin_mips_preceq_w_phr (v2q15)
         v2q15 __builtin_mips_precequ_ph_qbl (v4i8)
         v2q15 __builtin_mips_precequ_ph_qbr (v4i8)
         v2q15 __builtin_mips_precequ_ph_qbla (v4i8)
         v2q15 __builtin_mips_precequ_ph_qbra (v4i8)
         v2q15 __builtin_mips_preceu_ph_qbl (v4i8)
         v2q15 __builtin_mips_preceu_ph_qbr (v4i8)
         v2q15 __builtin_mips_preceu_ph_qbla (v4i8)
         v2q15 __builtin_mips_preceu_ph_qbra (v4i8)
         v4i8 __builtin_mips_shll_qb (v4i8, imm0_7)
         v4i8 __builtin_mips_shll_qb (v4i8, i32)
         v2q15 __builtin_mips_shll_ph (v2q15, imm0_15)
         v2q15 __builtin_mips_shll_ph (v2q15, i32)
         v2q15 __builtin_mips_shll_s_ph (v2q15, imm0_15)
         v2q15 __builtin_mips_shll_s_ph (v2q15, i32)
         q31 __builtin_mips_shll_s_w (q31, imm0_31)
         q31 __builtin_mips_shll_s_w (q31, i32)
         v4i8 __builtin_mips_shrl_qb (v4i8, imm0_7)
         v4i8 __builtin_mips_shrl_qb (v4i8, i32)
         v2q15 __builtin_mips_shra_ph (v2q15, imm0_15)
         v2q15 __builtin_mips_shra_ph (v2q15, i32)
         v2q15 __builtin_mips_shra_r_ph (v2q15, imm0_15)
         v2q15 __builtin_mips_shra_r_ph (v2q15, i32)
         q31 __builtin_mips_shra_r_w (q31, imm0_31)
         q31 __builtin_mips_shra_r_w (q31, i32)
         v2q15 __builtin_mips_muleu_s_ph_qbl (v4i8, v2q15)
         v2q15 __builtin_mips_muleu_s_ph_qbr (v4i8, v2q15)
         v2q15 __builtin_mips_mulq_rs_ph (v2q15, v2q15)
         q31 __builtin_mips_muleq_s_w_phl (v2q15, v2q15)
         q31 __builtin_mips_muleq_s_w_phr (v2q15, v2q15)
         a64 __builtin_mips_dpau_h_qbl (a64, v4i8, v4i8)
         a64 __builtin_mips_dpau_h_qbr (a64, v4i8, v4i8)
         a64 __builtin_mips_dpsu_h_qbl (a64, v4i8, v4i8)
         a64 __builtin_mips_dpsu_h_qbr (a64, v4i8, v4i8)
         a64 __builtin_mips_dpaq_s_w_ph (a64, v2q15, v2q15)
         a64 __builtin_mips_dpaq_sa_l_w (a64, q31, q31)
         a64 __builtin_mips_dpsq_s_w_ph (a64, v2q15, v2q15)
         a64 __builtin_mips_dpsq_sa_l_w (a64, q31, q31)
         a64 __builtin_mips_mulsaq_s_w_ph (a64, v2q15, v2q15)
         a64 __builtin_mips_maq_s_w_phl (a64, v2q15, v2q15)
         a64 __builtin_mips_maq_s_w_phr (a64, v2q15, v2q15)
         a64 __builtin_mips_maq_sa_w_phl (a64, v2q15, v2q15)
         a64 __builtin_mips_maq_sa_w_phr (a64, v2q15, v2q15)
         i32 __builtin_mips_bitrev (i32)
         i32 __builtin_mips_insv (i32, i32)
         v4i8 __builtin_mips_repl_qb (imm0_255)
         v4i8 __builtin_mips_repl_qb (i32)
         v2q15 __builtin_mips_repl_ph (imm_n512_511)
         v2q15 __builtin_mips_repl_ph (i32)
         void __builtin_mips_cmpu_eq_qb (v4i8, v4i8)
         void __builtin_mips_cmpu_lt_qb (v4i8, v4i8)
         void __builtin_mips_cmpu_le_qb (v4i8, v4i8)
         i32 __builtin_mips_cmpgu_eq_qb (v4i8, v4i8)
         i32 __builtin_mips_cmpgu_lt_qb (v4i8, v4i8)
         i32 __builtin_mips_cmpgu_le_qb (v4i8, v4i8)
         void __builtin_mips_cmp_eq_ph (v2q15, v2q15)
         void __builtin_mips_cmp_lt_ph (v2q15, v2q15)
         void __builtin_mips_cmp_le_ph (v2q15, v2q15)
         v4i8 __builtin_mips_pick_qb (v4i8, v4i8)
         v2q15 __builtin_mips_pick_ph (v2q15, v2q15)
         v2q15 __builtin_mips_packrl_ph (v2q15, v2q15)
         i32 __builtin_mips_extr_w (a64, imm0_31)
         i32 __builtin_mips_extr_w (a64, i32)
         i32 __builtin_mips_extr_r_w (a64, imm0_31)
         i32 __builtin_mips_extr_s_h (a64, i32)
         i32 __builtin_mips_extr_rs_w (a64, imm0_31)
         i32 __builtin_mips_extr_rs_w (a64, i32)
         i32 __builtin_mips_extr_s_h (a64, imm0_31)
         i32 __builtin_mips_extr_r_w (a64, i32)
         i32 __builtin_mips_extp (a64, imm0_31)
         i32 __builtin_mips_extp (a64, i32)
         i32 __builtin_mips_extpdp (a64, imm0_31)
         i32 __builtin_mips_extpdp (a64, i32)
         a64 __builtin_mips_shilo (a64, imm_n32_31)
         a64 __builtin_mips_shilo (a64, i32)
         a64 __builtin_mips_mthlip (a64, i32)
         void __builtin_mips_wrdsp (i32, imm0_63)
         i32 __builtin_mips_rddsp (imm0_63)
         i32 __builtin_mips_lbux (void *, i32)
         i32 __builtin_mips_lhx (void *, i32)
         i32 __builtin_mips_lwx (void *, i32)
         i32 __builtin_mips_bposge32 (void)

Next: PowerPC AltiVec Built-in Functions, Previous: MIPS DSP Built-in Functions, Up: Target Builtins

#### 5.47.7 MIPS Paired-Single Support

The MIPS64 architecture includes a number of instructions that operate on pairs of single-precision floating-point values. Each pair is packed into a 64-bit floating-point register, with one element being designated the “upper half” and the other being designated the “lower half”.

GCC supports paired-single operations using both the generic vector extensions (see Vector Extensions) and a collection of MIPS-specific built-in functions. Both kinds of support are enabled by the `-mpaired-single` command-line option.

The vector type associated with paired-single values is usually called `v2sf`. It can be defined in C as follows:

         typedef float v2sf __attribute__ ((vector_size (8)));

`v2sf` values are initialized in the same way as aggregates. For example:

         v2sf a = {1.5, 9.1};
         v2sf b;
         float e, f;
         b = (v2sf) {e, f};

Note: The CPU's endianness determines which value is stored in the upper half of a register and which value is stored in the lower half. On little-endian targets, the first value is the lower one and the second value is the upper one. The opposite order applies to big-endian targets. For example, the code above will set the lower half of `a` to `1.5` on little-endian targets and `9.1` on big-endian targets.

Next: Paired-Single Built-in Functions, Up: MIPS Paired-Single Support

##### 5.47.7.1 Paired-Single Arithmetic

The table below lists the `v2sf` operations for which hardware support exists. `a`, `b` and `c` are `v2sf` values and `x` is an integral value.

C code | MIPS instruction
`a + b` | `add.ps`
`a - b` | `sub.ps`
`-a` | `neg.ps`
`a * b` | `mul.ps`
`a * b + c` | `madd.ps`
`a * b - c` | `msub.ps`
`-(a * b + c)` | `nmadd.ps`
`-(a * b - c)` | `nmsub.ps`
`x ? a : b` | `movn.ps`/`movz.ps`

Note that the multiply-accumulate instructions can be disabled using the command-line option `-mno-fused-madd`.

Next: MIPS-3D Built-in Functions, Previous: Paired-Single Arithmetic, Up: MIPS Paired-Single Support

##### 5.47.7.2 Paired-Single Built-in Functions

The following paired-single functions map directly to a particular MIPS instruction. Please refer to the architecture specification for details on what each instruction does.

`v2sf __builtin_mips_pll_ps (v2sf, v2sf)`
  Pair lower lower (`pll.ps`).
`v2sf __builtin_mips_pul_ps (v2sf, v2sf)`
  Pair upper lower (`pul.ps`).
`v2sf __builtin_mips_plu_ps (v2sf, v2sf)`
  Pair lower upper (`plu.ps`).
`v2sf __builtin_mips_puu_ps (v2sf, v2sf)`
  Pair upper upper (`puu.ps`).
`v2sf __builtin_mips_cvt_ps_s (float, float)`
  Convert pair to paired single (`cvt.ps.s`).
`float __builtin_mips_cvt_s_pl (v2sf)`
  Convert pair lower to single (`cvt.s.pl`).
`float __builtin_mips_cvt_s_pu (v2sf)`
  Convert pair upper to single (`cvt.s.pu`).
`v2sf __builtin_mips_abs_ps (v2sf)`
  Absolute value (`abs.ps`).
`v2sf __builtin_mips_alnv_ps (v2sf, v2sf, int)`
  Align variable (`alnv.ps`).

  Note: The value of the third parameter must be 0 or 4 modulo 8, otherwise the result will be unpredictable. Please read the instruction description for details.

The following multi-instruction functions are also available. In each case, cond can be any of the 16 floating-point conditions: `f`, `un`, `eq`, `ueq`, `olt`, `ult`, `ole`, `ule`, `sf`, `ngle`, `seq`, `ngl`, `lt`, `nge`, `le` or `ngt`.

`v2sf __builtin_mips_movt_c_`cond`_ps (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
`v2sf __builtin_mips_movf_c_`cond`_ps (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
  Conditional move based on floating point comparison (`c.`cond`.ps`, `movt.ps`/`movf.ps`).

  The `movt` functions return the value x computed by:

                c.cond.ps cc,a,b
                mov.ps x,c
                movt.ps x,d,cc

  The `movf` functions are similar but use `movf.ps` instead of `movt.ps`.

`int __builtin_mips_upper_c_`cond`_ps (v2sf` a`, v2sf` b`)`
`int __builtin_mips_lower_c_`cond`_ps (v2sf` a`, v2sf` b`)`
  Comparison of two paired-single values (`c.`cond`.ps`, `bc1t`/`bc1f`).

  These functions compare a and b using `c.`cond`.ps` and return either the upper or lower half of the result. For example:

                v2sf a, b;
                if (__builtin_mips_upper_c_eq_ps (a, b))
                  upper_halves_are_equal ();
                else
                  upper_halves_are_unequal ();

                if (__builtin_mips_lower_c_eq_ps (a, b))
                  lower_halves_are_equal ();
                else
                  lower_halves_are_unequal ();

Previous: Paired-Single Built-in Functions, Up: MIPS Paired-Single Support

##### 5.47.7.3 MIPS-3D Built-in Functions

The MIPS-3D Application-Specific Extension (ASE) includes additional paired-single instructions that are designed to improve the performance of 3D graphics operations. Support for these instructions is controlled by the `-mips3d` command-line option.

The functions listed below map directly to a particular MIPS-3D instruction. Please refer to the architecture specification for more details on what each instruction does.

`v2sf __builtin_mips_addr_ps (v2sf, v2sf)`
  Reduction add (`addr.ps`).
`v2sf __builtin_mips_mulr_ps (v2sf, v2sf)`
  Reduction multiply (`mulr.ps`).
`v2sf __builtin_mips_cvt_pw_ps (v2sf)`
  Convert paired single to paired word (`cvt.pw.ps`).
`v2sf __builtin_mips_cvt_ps_pw (v2sf)`
  Convert paired word to paired single (`cvt.ps.pw`).
`float __builtin_mips_recip1_s (float)`
`double __builtin_mips_recip1_d (double)`
`v2sf __builtin_mips_recip1_ps (v2sf)`
  Reduced precision reciprocal (sequence step 1) (`recip1.`fmt).
`float __builtin_mips_recip2_s (float, float)`
`double __builtin_mips_recip2_d (double, double)`
`v2sf __builtin_mips_recip2_ps (v2sf, v2sf)`
  Reduced precision reciprocal (sequence step 2) (`recip2.`fmt).
`float __builtin_mips_rsqrt1_s (float)`
`double __builtin_mips_rsqrt1_d (double)`
`v2sf __builtin_mips_rsqrt1_ps (v2sf)`
  Reduced precision reciprocal square root (sequence step 1) (`rsqrt1.`fmt).
`float __builtin_mips_rsqrt2_s (float, float)`
`double __builtin_mips_rsqrt2_d (double, double)`
`v2sf __builtin_mips_rsqrt2_ps (v2sf, v2sf)`
  Reduced precision reciprocal square root (sequence step 2) (`rsqrt2.`fmt).

The following multi-instruction functions are also available. In each case, cond can be any of the 16 floating-point conditions: `f`, `un`, `eq`, `ueq`, `olt`, `ult`, `ole`, `ule`, `sf`, `ngle`, `seq`, `ngl`, `lt`, `nge`, `le` or `ngt`.

`int __builtin_mips_cabs_`cond`_s (float` a`, float` b`)`
`int __builtin_mips_cabs_`cond`_d (double` a`, double` b`)`
  Absolute comparison of two scalar values (`cabs.`cond`.`fmt, `bc1t`/`bc1f`).

  These functions compare a and b using `cabs.`cond`.s` or `cabs.`cond`.d` and return the result as a boolean value. For example:

                float a, b;
                if (__builtin_mips_cabs_eq_s (a, b))
                  true ();
                else
                  false ();

`int __builtin_mips_upper_cabs_`cond`_ps (v2sf` a`, v2sf` b`)`
`int __builtin_mips_lower_cabs_`cond`_ps (v2sf` a`, v2sf` b`)`
  Absolute comparison of two paired-single values (`cabs.`cond`.ps`, `bc1t`/`bc1f`).

  These functions compare a and b using `cabs.`cond`.ps` and return either the upper or lower half of the result. For example:

                v2sf a, b;
                if (__builtin_mips_upper_cabs_eq_ps (a, b))
                  upper_halves_are_equal ();
                else
                  upper_halves_are_unequal ();

                if (__builtin_mips_lower_cabs_eq_ps (a, b))
                  lower_halves_are_equal ();
                else
                  lower_halves_are_unequal ();

`v2sf __builtin_mips_movt_cabs_`cond`_ps (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
`v2sf __builtin_mips_movf_cabs_`cond`_ps (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
  Conditional move based on absolute comparison (`cabs.`cond`.ps`, `movt.ps`/`movf.ps`).

  The `movt` functions return the value x computed by:

                cabs.cond.ps cc,a,b
                mov.ps x,c
                movt.ps x,d,cc

  The `movf` functions are similar but use `movf.ps` instead of `movt.ps`.

`int __builtin_mips_any_c_`cond`_ps (v2sf` a`, v2sf` b`)`
`int __builtin_mips_all_c_`cond`_ps (v2sf` a`, v2sf` b`)`
`int __builtin_mips_any_cabs_`cond`_ps (v2sf` a`, v2sf` b`)`
`int __builtin_mips_all_cabs_`cond`_ps (v2sf` a`, v2sf` b`)`
  Comparison of two paired-single values (`c.`cond`.ps`/`cabs.`cond`.ps`, `bc1any2t`/`bc1any2f`).

  These functions compare a and b using `c.`cond`.ps` or `cabs.`cond`.ps`. The `any` forms return true if either result is true and the `all` forms return true if both results are true. For example:

                v2sf a, b;
                if (__builtin_mips_any_c_eq_ps (a, b))
                  one_is_true ();
                else
                  both_are_false ();

                if (__builtin_mips_all_c_eq_ps (a, b))
                  both_are_true ();
                else
                  one_is_false ();

`int __builtin_mips_any_c_`cond`_4s (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
`int __builtin_mips_all_c_`cond`_4s (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
`int __builtin_mips_any_cabs_`cond`_4s (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
`int __builtin_mips_all_cabs_`cond`_4s (v2sf` a`, v2sf` b`, v2sf` c`, v2sf` d`)`
  Comparison of four paired-single values (`c.`cond`.ps`/`cabs.`cond`.ps`, `bc1any4t`/`bc1any4f`).

  These functions use `c.`cond`.ps` or `cabs.`cond`.ps` to compare a with b and to compare c with d. The `any` forms return true if any of the four results are true and the `all` forms return true if all four results are true. For example:

                v2sf a, b, c, d;
                if (__builtin_mips_any_c_eq_4s (a, b, c, d))
                  some_are_true ();
                else
                  all_are_false ();

                if (__builtin_mips_all_c_eq_4s (a, b, c, d))
                  all_are_true ();
                else
                  some_are_false ();

Next: SPARC VIS Built-in Functions, Previous: MIPS Paired-Single Support, Up: Target Builtins

#### 5.47.8 PowerPC AltiVec Built-in Functions

GCC provides an interface for the PowerPC family of processors to access the AltiVec operations described in Motorola's AltiVec Programming Interface Manual. The interface is made available by including `<altivec.h>` and using `-maltivec` and `-mabi=altivec`. The interface supports the following vector types.

         vector unsigned char
         vector signed char
         vector bool char

         vector unsigned short
         vector signed short
         vector bool short
         vector pixel

         vector unsigned int
         vector signed int
         vector bool int
         vector float

GCC's implementation of the high-level language interface available from C and C++ code differs from Motorola's documentation in several ways.

- A vector constant is a list of constant expressions within curly braces.
- A vector initializer requires no cast if the vector constant is of the same type as the variable it is initializing.
- If `signed` or `unsigned` is omitted, the signedness of the vector type is the default signedness of the base type. The default varies depending on the operating system, so a portable program should always specify the signedness.
- Compiling with `-maltivec` adds keywords `__vector`, `__pixel`, and `__bool`. Macros `vector`, `pixel`, and `bool` are defined in `<altivec.h>` and can be undefined.
- GCC allows using a `typedef` name as the type specifier for a vector type.
- For C, overloaded functions are implemented with macros so the following does not work:

                  vec_add ((vector signed int){1, 2, 3, 4}, foo);

  Since `vec_add` is a macro, the vector constant in the example is treated as four separate arguments. Wrap the entire argument in parentheses for this to work.

Note: Only the `<altivec.h>` interface is supported. Internally, GCC uses built-in functions to achieve the functionality in the aforementioned header file, but they are not supported and are subject to change without notice.

The following interfaces are supported for the generic and specific AltiVec operations and the AltiVec predicates. In cases where there is a direct mapping between generic and specific operations, only the generic names are shown here, although the specific operations can also be used.

Arguments that are documented as `const int` require literal integral values within the range required for that operation.

         vector signed char vec_abs (vector signed char);
         vector signed short vec_abs (vector signed short);
         vector signed int vec_abs (vector signed int);
         vector float vec_abs (vector float);

         vector signed char vec_abss (vector signed char);
         vector signed short vec_abss (vector signed short);
         vector signed int vec_abss (vector signed int);

         vector signed char vec_add (vector bool char, vector signed char);
         vector signed char vec_add (vector signed char, vector bool char);
         vector signed char vec_add (vector signed char, vector signed char);
         vector unsigned char vec_add (vector bool char, vector unsigned char);
         vector unsigned char vec_add (vector unsigned char, vector bool char);
         vector unsigned char vec_add (vector unsigned char,
                                       vector unsigned char);
         vector signed short vec_add (vector bool short, vector signed short);
         vector signed short vec_add (vector signed short, vector bool short);
         vector signed short vec_add (vector signed short, vector signed short);
         vector unsigned short vec_add (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_add (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_add (vector unsigned short,
                                        vector unsigned short);
         vector signed int vec_add (vector bool int, vector signed int);
         vector signed int vec_add (vector signed int, vector bool int);
         vector signed int vec_add (vector signed int, vector signed int);
         vector unsigned int vec_add (vector bool int, vector unsigned int);
         vector unsigned int vec_add (vector unsigned int, vector bool int);
         vector unsigned int vec_add (vector unsigned int, vector unsigned int);
         vector float vec_add (vector float, vector float);

         vector float vec_vaddfp (vector float, vector float);

         vector signed int vec_vadduwm (vector bool int, vector signed int);
         vector signed int vec_vadduwm (vector signed int, vector bool int);
         vector signed int vec_vadduwm (vector signed int, vector signed int);
         vector unsigned int vec_vadduwm (vector bool int, vector unsigned int);
         vector unsigned int vec_vadduwm (vector unsigned int, vector bool int);
         vector unsigned int vec_vadduwm (vector unsigned int,
                                          vector unsigned int);

         vector signed short vec_vadduhm (vector bool short,
                                          vector signed short);
         vector signed short vec_vadduhm (vector signed short,
                                          vector bool short);
         vector signed short vec_vadduhm (vector signed short,
                                          vector signed short);
         vector unsigned short vec_vadduhm (vector bool short,
                                            vector unsigned short);
         vector unsigned short vec_vadduhm (vector unsigned short,
                                            vector bool short);
         vector unsigned short vec_vadduhm (vector unsigned short,
                                            vector unsigned short);

         vector signed char vec_vaddubm (vector bool char, vector signed char);
         vector signed char vec_vaddubm (vector signed char, vector bool char);
         vector signed char vec_vaddubm (vector signed char, vector signed char);
         vector unsigned char vec_vaddubm (vector bool char,
                                           vector unsigned char);
         vector unsigned char vec_vaddubm (vector unsigned char,
                                           vector bool char);
         vector unsigned char vec_vaddubm (vector unsigned char,
                                           vector unsigned char);

         vector unsigned int vec_addc (vector unsigned int, vector unsigned int);

         vector unsigned char vec_adds (vector bool char, vector unsigned char);
         vector unsigned char vec_adds (vector unsigned char, vector bool char);
         vector unsigned char vec_adds (vector unsigned char,
                                        vector unsigned char);
         vector signed char vec_adds (vector bool char, vector signed char);
         vector signed char vec_adds (vector signed char, vector bool char);
         vector signed char vec_adds (vector signed char, vector signed char);
         vector unsigned short vec_adds (vector bool short,
                                         vector unsigned short);
         vector unsigned short vec_adds (vector unsigned short,
                                         vector bool short);
         vector unsigned short vec_adds (vector unsigned short,
                                         vector unsigned short);
         vector signed short vec_adds (vector bool short, vector signed short);
         vector signed short vec_adds (vector signed short, vector bool short);
         vector signed short vec_adds (vector signed short, vector signed short);
         vector unsigned int vec_adds (vector bool int, vector unsigned int);
         vector unsigned int vec_adds (vector unsigned int, vector bool int);
         vector unsigned int vec_adds (vector unsigned int, vector unsigned int);
         vector signed int vec_adds (vector bool int, vector signed int);
         vector signed int vec_adds (vector signed int, vector bool int);
         vector signed int vec_adds (vector signed int, vector signed int);

         vector signed int vec_vaddsws (vector bool int, vector signed int);
         vector signed int vec_vaddsws (vector signed int, vector bool int);
         vector signed int vec_vaddsws (vector signed int, vector signed int);

         vector unsigned int vec_vadduws (vector bool int, vector unsigned int);
         vector unsigned int vec_vadduws (vector unsigned int, vector bool int);
         vector unsigned int vec_vadduws (vector unsigned int,
                                          vector unsigned int);

         vector signed short vec_vaddshs (vector bool short,
                                          vector signed short);
         vector signed short vec_vaddshs (vector signed short,
                                          vector bool short);
         vector signed short vec_vaddshs (vector signed short,
                                          vector signed short);

         vector unsigned short vec_vadduhs (vector bool short,
                                            vector unsigned short);
         vector unsigned short vec_vadduhs (vector unsigned short,
                                            vector bool short);
         vector unsigned short vec_vadduhs (vector unsigned short,
                                            vector unsigned short);

         vector signed char vec_vaddsbs (vector bool char, vector signed char);
         vector signed char vec_vaddsbs (vector signed char, vector bool char);
         vector signed char vec_vaddsbs (vector signed char, vector signed char);

         vector unsigned char vec_vaddubs (vector bool char,
                                           vector unsigned char);
         vector unsigned char vec_vaddubs (vector unsigned char,
                                           vector bool char);
         vector unsigned char vec_vaddubs (vector unsigned char,
                                           vector unsigned char);

         vector float vec_and (vector float, vector float);
         vector float vec_and (vector float, vector bool int);
         vector float vec_and (vector bool int, vector float);
         vector bool int vec_and (vector bool int, vector bool int);
         vector signed int vec_and (vector bool int, vector signed int);
         vector signed int vec_and (vector signed int, vector bool int);
         vector signed int vec_and (vector signed int, vector signed int);
         vector unsigned int vec_and (vector bool int, vector unsigned int);
         vector unsigned int vec_and (vector unsigned int, vector bool int);
         vector unsigned int vec_and (vector unsigned int, vector unsigned int);
         vector bool short vec_and (vector bool short, vector bool short);
         vector signed short vec_and (vector bool short, vector signed short);
         vector signed short vec_and (vector signed short, vector bool short);
         vector signed short vec_and (vector signed short, vector signed short);
         vector unsigned short vec_and (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_and (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_and (vector unsigned short,
                                        vector unsigned short);
         vector signed char vec_and (vector bool char, vector signed char);
         vector bool char vec_and (vector bool char, vector bool char);
         vector signed char vec_and (vector signed char, vector bool char);
         vector signed char vec_and (vector signed char, vector signed char);
         vector unsigned char vec_and (vector bool char, vector unsigned char);
         vector unsigned char vec_and (vector unsigned char, vector bool char);
         vector unsigned char vec_and (vector unsigned char,
                                       vector unsigned char);

         vector float vec_andc (vector float, vector float);
         vector float vec_andc (vector float, vector bool int);
         vector float vec_andc (vector bool int, vector float);
         vector bool int vec_andc (vector bool int, vector bool int);
         vector signed int vec_andc (vector bool int, vector signed int);
         vector signed int vec_andc (vector signed int, vector bool int);
         vector signed int vec_andc (vector signed int, vector signed int);
         vector unsigned int vec_andc (vector bool int, vector unsigned int);
         vector unsigned int vec_andc (vector unsigned int, vector bool int);
         vector unsigned int vec_andc (vector unsigned int, vector unsigned int);
         vector bool short vec_andc (vector bool short, vector bool short);
         vector signed short vec_andc (vector bool short, vector signed short);
         vector signed short vec_andc (vector signed short, vector bool short);
         vector signed short vec_andc (vector signed short, vector signed short);
         vector unsigned short vec_andc (vector bool short,
                                         vector unsigned short);
         vector unsigned short vec_andc (vector unsigned short,
                                         vector bool short);
         vector unsigned short vec_andc (vector unsigned short,
                                         vector unsigned short);
         vector signed char vec_andc (vector bool char, vector signed char);
         vector bool char vec_andc (vector bool char, vector bool char);
         vector signed char vec_andc (vector signed char, vector bool char);
         vector signed char vec_andc (vector signed char, vector signed char);
         vector unsigned char vec_andc (vector bool char, vector unsigned char);
         vector unsigned char vec_andc (vector unsigned char, vector bool char);
         vector unsigned char vec_andc (vector unsigned char,
                                        vector unsigned char);

         vector unsigned char vec_avg (vector unsigned char,
                                       vector unsigned char);
         vector signed char vec_avg (vector signed char, vector signed char);
         vector unsigned short vec_avg (vector unsigned short,
                                        vector unsigned short);
         vector signed short vec_avg (vector signed short, vector signed short);
         vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
         vector signed int vec_avg (vector signed int, vector signed int);

         vector signed int vec_vavgsw (vector signed int, vector signed int);

         vector unsigned int vec_vavguw (vector unsigned int,
                                         vector unsigned int);

         vector signed short vec_vavgsh (vector signed short,
                                         vector signed short);

         vector unsigned short vec_vavguh (vector unsigned short,
                                           vector unsigned short);

         vector signed char vec_vavgsb (vector signed char, vector signed char);

         vector unsigned char vec_vavgub (vector unsigned char,
                                          vector unsigned char);

         vector float vec_ceil (vector float);

         vector signed int vec_cmpb (vector float, vector float);

         vector bool char vec_cmpeq (vector signed char, vector signed char);
         vector bool char vec_cmpeq (vector unsigned char, vector unsigned char);
         vector bool short vec_cmpeq (vector signed short, vector signed short);
         vector bool short vec_cmpeq (vector unsigned short,
                                      vector unsigned short);
         vector bool int vec_cmpeq (vector signed int, vector signed int);
         vector bool int vec_cmpeq (vector unsigned int, vector unsigned int);
         vector bool int vec_cmpeq (vector float, vector float);

         vector bool int vec_vcmpeqfp (vector float, vector float);

         vector bool int vec_vcmpequw (vector signed int, vector signed int);
         vector bool int vec_vcmpequw (vector unsigned int, vector unsigned int);

         vector bool short vec_vcmpequh (vector signed short,
                                         vector signed short);
         vector bool short vec_vcmpequh (vector unsigned short,
                                         vector unsigned short);

         vector bool char vec_vcmpequb (vector signed char, vector signed char);
         vector bool char vec_vcmpequb (vector unsigned char,
                                        vector unsigned char);

         vector bool int vec_cmpge (vector float, vector float);

         vector bool char vec_cmpgt (vector unsigned char, vector unsigned char);
         vector bool char vec_cmpgt (vector signed char, vector signed char);
         vector bool short vec_cmpgt (vector unsigned short,
                                      vector unsigned short);
         vector bool short vec_cmpgt (vector signed short, vector signed short);
         vector bool int vec_cmpgt (vector unsigned int, vector unsigned int);
         vector bool int vec_cmpgt (vector signed int, vector signed int);
         vector bool int vec_cmpgt (vector float, vector float);

         vector bool int vec_vcmpgtfp (vector float, vector float);

         vector bool int vec_vcmpgtsw (vector signed int, vector signed int);

         vector bool int vec_vcmpgtuw (vector unsigned int, vector unsigned int);

         vector bool short vec_vcmpgtsh (vector signed short,
                                         vector signed short);

         vector bool short vec_vcmpgtuh (vector unsigned short,
                                         vector unsigned short);

         vector bool char vec_vcmpgtsb (vector signed char, vector signed char);

         vector bool char vec_vcmpgtub (vector unsigned char,
                                        vector unsigned char);

         vector bool int vec_cmple (vector float, vector float);

         vector bool char vec_cmplt (vector unsigned char, vector unsigned char);
         vector bool char vec_cmplt (vector signed char, vector signed char);
         vector bool short vec_cmplt (vector unsigned short,
                                      vector unsigned short);
         vector bool short vec_cmplt (vector signed short, vector signed short);
         vector bool int vec_cmplt (vector unsigned int, vector unsigned int);
         vector bool int vec_cmplt (vector signed int, vector signed int);
         vector bool int vec_cmplt (vector float, vector float);

         vector float vec_ctf (vector unsigned int, const int);
         vector float vec_ctf (vector signed int, const int);

         vector float vec_vcfsx (vector signed int, const int);

         vector float vec_vcfux (vector unsigned int, const int);

         vector signed int vec_cts (vector float, const int);

         vector unsigned int vec_ctu (vector float, const int);

         void vec_dss (const int);

         void vec_dssall (void);

         void vec_dst (const vector unsigned char *, int, const int);
         void vec_dst (const vector signed char *, int, const int);
         void vec_dst (const vector bool char *, int, const int);
         void vec_dst (const vector unsigned short *, int, const int);
         void vec_dst (const vector signed short *, int, const int);
         void vec_dst (const vector bool short *, int, const int);
         void vec_dst (const vector pixel *, int, const int);
         void vec_dst (const vector unsigned int *, int, const int);
         void vec_dst (const vector signed int *, int, const int);
         void vec_dst (const vector bool int *, int, const int);
         void vec_dst (const vector float *, int, const int);
         void vec_dst (const unsigned char *, int, const int);
         void vec_dst (const signed char *, int, const int);
         void vec_dst (const unsigned short *, int, const int);
         void vec_dst (const short *, int, const int);
         void vec_dst (const unsigned int *, int, const int);
         void vec_dst (const int *, int, const int);
         void vec_dst (const unsigned long *, int, const int);
         void vec_dst (const long *, int, const int);
         void vec_dst (const float *, int, const int);

         void vec_dstst (const vector unsigned char *, int, const int);
         void vec_dstst (const vector signed char *, int, const int);
         void vec_dstst (const vector bool char *, int, const int);
         void vec_dstst (const vector unsigned short *, int, const int);
         void vec_dstst (const vector signed short *, int, const int);
         void vec_dstst (const vector bool short *, int, const int);
         void vec_dstst (const vector pixel *, int, const int);
         void vec_dstst (const vector unsigned int *, int, const int);
         void vec_dstst (const vector signed int *, int, const int);
         void vec_dstst (const vector bool int *, int, const int);
         void vec_dstst (const vector float *, int, const int);
         void vec_dstst (const unsigned char *, int, const int);
         void vec_dstst (const signed char *, int, const int);
         void vec_dstst (const unsigned short *, int, const int);
         void vec_dstst (const short *, int, const int);
         void vec_dstst (const unsigned int *, int, const int);
         void vec_dstst (const int *, int, const int);
         void vec_dstst (const unsigned long *, int, const int);
         void vec_dstst (const long *, int, const int);
         void vec_dstst (const float *, int, const int);

         void vec_dststt (const vector unsigned char *, int, const int);
         void vec_dststt (const vector signed char *, int, const int);
         void vec_dststt (const vector bool char *, int, const int);
         void vec_dststt (const vector unsigned short *, int, const int);
         void vec_dststt (const vector signed short *, int, const int);
         void vec_dststt (const vector bool short *, int, const int);
         void vec_dststt (const vector pixel *, int, const int);
         void vec_dststt (const vector unsigned int *, int, const int);
         void vec_dststt (const vector signed int *, int, const int);
         void vec_dststt (const vector bool int *, int, const int);
         void vec_dststt (const vector float *, int, const int);
         void vec_dststt (const unsigned char *, int, const int);
         void vec_dststt (const signed char *, int, const int);
         void vec_dststt (const unsigned short *, int, const int);
         void vec_dststt (const short *, int, const int);
         void vec_dststt (const unsigned int *, int, const int);
         void vec_dststt (const int *, int, const int);
         void vec_dststt (const unsigned long *, int, const int);
         void vec_dststt (const long *, int, const int);
         void vec_dststt (const float *, int, const int);

         void vec_dstt (const vector unsigned char *, int, const int);
         void vec_dstt (const vector signed char *, int, const int);
         void vec_dstt (const vector bool char *, int, const int);
         void vec_dstt (const vector unsigned short *, int, const int);
         void vec_dstt (const vector signed short *, int, const int);
         void vec_dstt (const vector bool short *, int, const int);
         void vec_dstt (const vector pixel *, int, const int);
         void vec_dstt (const vector unsigned int *, int, const int);
         void vec_dstt (const vector signed int *, int, const int);
         void vec_dstt (const vector bool int *, int, const int);
         void vec_dstt (const vector float *, int, const int);
         void vec_dstt (const unsigned char *, int, const int);
         void vec_dstt (const signed char *, int, const int);
         void vec_dstt (const unsigned short *, int, const int);
         void vec_dstt (const short *, int, const int);
         void vec_dstt (const unsigned int *, int, const int);
         void vec_dstt (const int *, int, const int);
         void vec_dstt (const unsigned long *, int, const int);
         void vec_dstt (const long *, int, const int);
         void vec_dstt (const float *, int, const int);

         vector float vec_expte (vector float);

         vector float vec_floor (vector float);

         vector float vec_ld (int, const vector float *);
         vector float vec_ld (int, const float *);
         vector bool int vec_ld (int, const vector bool int *);
         vector signed int vec_ld (int, const vector signed int *);
         vector signed int vec_ld (int, const int *);
         vector signed int vec_ld (int, const long *);
         vector unsigned int vec_ld (int, const vector unsigned int *);
         vector unsigned int vec_ld (int, const unsigned int *);
         vector unsigned int vec_ld (int, const unsigned long *);
         vector bool short vec_ld (int, const vector bool short *);
         vector pixel vec_ld (int, const vector pixel *);
         vector signed short vec_ld (int, const vector signed short *);
         vector signed short vec_ld (int, const short *);
         vector unsigned short vec_ld (int, const vector unsigned short *);
         vector unsigned short vec_ld (int, const unsigned short *);
         vector bool char vec_ld (int, const vector bool char *);
         vector signed char vec_ld (int, const vector signed char *);
         vector signed char vec_ld (int, const signed char *);
         vector unsigned char vec_ld (int, const vector unsigned char *);
         vector unsigned char vec_ld (int, const unsigned char *);

         vector signed char vec_lde (int, const signed char *);
         vector unsigned char vec_lde (int, const unsigned char *);
         vector signed short vec_lde (int, const short *);
         vector unsigned short vec_lde (int, const unsigned short *);
         vector float vec_lde (int, const float *);
         vector signed int vec_lde (int, const int *);
         vector unsigned int vec_lde (int, const unsigned int *);
         vector signed int vec_lde (int, const long *);
         vector unsigned int vec_lde (int, const unsigned long *);

         vector float vec_lvewx (int, float *);
         vector signed int vec_lvewx (int, int *);
         vector unsigned int vec_lvewx (int, unsigned int *);
         vector signed int vec_lvewx (int, long *);
         vector unsigned int vec_lvewx (int, unsigned long *);

         vector signed short vec_lvehx (int, short *);
         vector unsigned short vec_lvehx (int, unsigned short *);

         vector signed char vec_lvebx (int, char *);
         vector unsigned char vec_lvebx (int, unsigned char *);

         vector float vec_ldl (int, const vector float *);
         vector float vec_ldl (int, const float *);
         vector bool int vec_ldl (int, const vector bool int *);
         vector signed int vec_ldl (int, const vector signed int *);
         vector signed int vec_ldl (int, const int *);
         vector signed int vec_ldl (int, const long *);
         vector unsigned int vec_ldl (int, const vector unsigned int *);
         vector unsigned int vec_ldl (int, const unsigned int *);
         vector unsigned int vec_ldl (int, const unsigned long *);
         vector bool short vec_ldl (int, const vector bool short *);
         vector pixel vec_ldl (int, const vector pixel *);
         vector signed short vec_ldl (int, const vector signed short *);
         vector signed short vec_ldl (int, const short *);
         vector unsigned short vec_ldl (int, const vector unsigned short *);
         vector unsigned short vec_ldl (int, const unsigned short *);
         vector bool char vec_ldl (int, const vector bool char *);
         vector signed char vec_ldl (int, const vector signed char *);
         vector signed char vec_ldl (int, const signed char *);
         vector unsigned char vec_ldl (int, const vector unsigned char *);
         vector unsigned char vec_ldl (int, const unsigned char *);

         vector float vec_loge (vector float);

         vector unsigned char vec_lvsl (int, const volatile unsigned char *);
         vector unsigned char vec_lvsl (int, const volatile signed char *);
         vector unsigned char vec_lvsl (int, const volatile unsigned short *);
         vector unsigned char vec_lvsl (int, const volatile short *);
         vector unsigned char vec_lvsl (int, const volatile unsigned int *);
         vector unsigned char vec_lvsl (int, const volatile int *);
         vector unsigned char vec_lvsl (int, const volatile unsigned long *);
         vector unsigned char vec_lvsl (int, const volatile long *);
         vector unsigned char vec_lvsl (int, const volatile float *);

         vector unsigned char vec_lvsr (int, const volatile unsigned char *);
         vector unsigned char vec_lvsr (int, const volatile signed char *);
         vector unsigned char vec_lvsr (int, const volatile unsigned short *);
         vector unsigned char vec_lvsr (int, const volatile short *);
         vector unsigned char vec_lvsr (int, const volatile unsigned int *);
         vector unsigned char vec_lvsr (int, const volatile int *);
         vector unsigned char vec_lvsr (int, const volatile unsigned long *);
         vector unsigned char vec_lvsr (int, const volatile long *);
         vector unsigned char vec_lvsr (int, const volatile float *);

         vector float vec_madd (vector float, vector float, vector float);

         vector signed short vec_madds (vector signed short,
                                        vector signed short,
                                        vector signed short);

         vector unsigned char vec_max (vector bool char, vector unsigned char);
         vector unsigned char vec_max (vector unsigned char, vector bool char);
         vector unsigned char vec_max (vector unsigned char,
                                       vector unsigned char);
         vector signed char vec_max (vector bool char, vector signed char);
         vector signed char vec_max (vector signed char, vector bool char);
         vector signed char vec_max (vector signed char, vector signed char);
         vector unsigned short vec_max (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_max (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_max (vector unsigned short,
                                        vector unsigned short);
         vector signed short vec_max (vector bool short, vector signed short);
         vector signed short vec_max (vector signed short, vector bool short);
         vector signed short vec_max (vector signed short, vector signed short);
         vector unsigned int vec_max (vector bool int, vector unsigned int);
         vector unsigned int vec_max (vector unsigned int, vector bool int);
         vector unsigned int vec_max (vector unsigned int, vector unsigned int);
         vector signed int vec_max (vector bool int, vector signed int);
         vector signed int vec_max (vector signed int, vector bool int);
         vector signed int vec_max (vector signed int, vector signed int);
         vector float vec_max (vector float, vector float);

         vector float vec_vmaxfp (vector float, vector float);

         vector signed int vec_vmaxsw (vector bool int, vector signed int);
         vector signed int vec_vmaxsw (vector signed int, vector bool int);
         vector signed int vec_vmaxsw (vector signed int, vector signed int);

         vector unsigned int vec_vmaxuw (vector bool int, vector unsigned int);
         vector unsigned int vec_vmaxuw (vector unsigned int, vector bool int);
         vector unsigned int vec_vmaxuw (vector unsigned int,
                                         vector unsigned int);

         vector signed short vec_vmaxsh (vector bool short, vector signed short);
         vector signed short vec_vmaxsh (vector signed short, vector bool short);
         vector signed short vec_vmaxsh (vector signed short,
                                         vector signed short);

         vector unsigned short vec_vmaxuh (vector bool short,
                                           vector unsigned short);
         vector unsigned short vec_vmaxuh (vector unsigned short,
                                           vector bool short);
         vector unsigned short vec_vmaxuh (vector unsigned short,
                                           vector unsigned short);

         vector signed char vec_vmaxsb (vector bool char, vector signed char);
         vector signed char vec_vmaxsb (vector signed char, vector bool char);
         vector signed char vec_vmaxsb (vector signed char, vector signed char);

         vector unsigned char vec_vmaxub (vector bool char,
                                          vector unsigned char);
         vector unsigned char vec_vmaxub (vector unsigned char,
                                          vector bool char);
         vector unsigned char vec_vmaxub (vector unsigned char,
                                          vector unsigned char);

         vector bool char vec_mergeh (vector bool char, vector bool char);
         vector signed char vec_mergeh (vector signed char, vector signed char);
         vector unsigned char vec_mergeh (vector unsigned char,
                                          vector unsigned char);
         vector bool short vec_mergeh (vector bool short, vector bool short);
         vector pixel vec_mergeh (vector pixel, vector pixel);
         vector signed short vec_mergeh (vector signed short,
                                         vector signed short);
         vector unsigned short vec_mergeh (vector unsigned short,
                                           vector unsigned short);
         vector float vec_mergeh (vector float, vector float);
         vector bool int vec_mergeh (vector bool int, vector bool int);
         vector signed int vec_mergeh (vector signed int, vector signed int);
         vector unsigned int vec_mergeh (vector unsigned int,
                                         vector unsigned int);

         vector float vec_vmrghw (vector float, vector float);
         vector bool int vec_vmrghw (vector bool int, vector bool int);
         vector signed int vec_vmrghw (vector signed int, vector signed int);
         vector unsigned int vec_vmrghw (vector unsigned int,
                                         vector unsigned int);

         vector bool short vec_vmrghh (vector bool short, vector bool short);
         vector signed short vec_vmrghh (vector signed short,
                                         vector signed short);
         vector unsigned short vec_vmrghh (vector unsigned short,
                                           vector unsigned short);
         vector pixel vec_vmrghh (vector pixel, vector pixel);

         vector bool char vec_vmrghb (vector bool char, vector bool char);
         vector signed char vec_vmrghb (vector signed char, vector signed char);
         vector unsigned char vec_vmrghb (vector unsigned char,
                                          vector unsigned char);

         vector bool char vec_mergel (vector bool char, vector bool char);
         vector signed char vec_mergel (vector signed char, vector signed char);
         vector unsigned char vec_mergel (vector unsigned char,
                                          vector unsigned char);
         vector bool short vec_mergel (vector bool short, vector bool short);
         vector pixel vec_mergel (vector pixel, vector pixel);
         vector signed short vec_mergel (vector signed short,
                                         vector signed short);
         vector unsigned short vec_mergel (vector unsigned short,
                                           vector unsigned short);
         vector float vec_mergel (vector float, vector float);
         vector bool int vec_mergel (vector bool int, vector bool int);
         vector signed int vec_mergel (vector signed int, vector signed int);
         vector unsigned int vec_mergel (vector unsigned int,
                                         vector unsigned int);

         vector float vec_vmrglw (vector float, vector float);
         vector signed int vec_vmrglw (vector signed int, vector signed int);
         vector unsigned int vec_vmrglw (vector unsigned int,
                                         vector unsigned int);
         vector bool int vec_vmrglw (vector bool int, vector bool int);

         vector bool short vec_vmrglh (vector bool short, vector bool short);
         vector signed short vec_vmrglh (vector signed short,
                                         vector signed short);
         vector unsigned short vec_vmrglh (vector unsigned short,
                                           vector unsigned short);
         vector pixel vec_vmrglh (vector pixel, vector pixel);

         vector bool char vec_vmrglb (vector bool char, vector bool char);
         vector signed char vec_vmrglb (vector signed char, vector signed char);
         vector unsigned char vec_vmrglb (vector unsigned char,
                                          vector unsigned char);

         vector unsigned short vec_mfvscr (void);

         vector unsigned char vec_min (vector bool char, vector unsigned char);
         vector unsigned char vec_min (vector unsigned char, vector bool char);
         vector unsigned char vec_min (vector unsigned char,
                                       vector unsigned char);
         vector signed char vec_min (vector bool char, vector signed char);
         vector signed char vec_min (vector signed char, vector bool char);
         vector signed char vec_min (vector signed char, vector signed char);
         vector unsigned short vec_min (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_min (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_min (vector unsigned short,
                                        vector unsigned short);
         vector signed short vec_min (vector bool short, vector signed short);
         vector signed short vec_min (vector signed short, vector bool short);
         vector signed short vec_min (vector signed short, vector signed short);
         vector unsigned int vec_min (vector bool int, vector unsigned int);
         vector unsigned int vec_min (vector unsigned int, vector bool int);
         vector unsigned int vec_min (vector unsigned int, vector unsigned int);
         vector signed int vec_min (vector bool int, vector signed int);
         vector signed int vec_min (vector signed int, vector bool int);
         vector signed int vec_min (vector signed int, vector signed int);
         vector float vec_min (vector float, vector float);

         vector float vec_vminfp (vector float, vector float);

         vector signed int vec_vminsw (vector bool int, vector signed int);
         vector signed int vec_vminsw (vector signed int, vector bool int);
         vector signed int vec_vminsw (vector signed int, vector signed int);

         vector unsigned int vec_vminuw (vector bool int, vector unsigned int);
         vector unsigned int vec_vminuw (vector unsigned int, vector bool int);
         vector unsigned int vec_vminuw (vector unsigned int,
                                         vector unsigned int);

         vector signed short vec_vminsh (vector bool short, vector signed short);
         vector signed short vec_vminsh (vector signed short, vector bool short);
         vector signed short vec_vminsh (vector signed short,
                                         vector signed short);

         vector unsigned short vec_vminuh (vector bool short,
                                           vector unsigned short);
         vector unsigned short vec_vminuh (vector unsigned short,
                                           vector bool short);
         vector unsigned short vec_vminuh (vector unsigned short,
                                           vector unsigned short);

         vector signed char vec_vminsb (vector bool char, vector signed char);
         vector signed char vec_vminsb (vector signed char, vector bool char);
         vector signed char vec_vminsb (vector signed char, vector signed char);

         vector unsigned char vec_vminub (vector bool char,
                                          vector unsigned char);
         vector unsigned char vec_vminub (vector unsigned char,
                                          vector bool char);
         vector unsigned char vec_vminub (vector unsigned char,
                                          vector unsigned char);

         vector signed short vec_mladd (vector signed short,
                                        vector signed short,
                                        vector signed short);
         vector signed short vec_mladd (vector signed short,
                                        vector unsigned short,
                                        vector unsigned short);
         vector signed short vec_mladd (vector unsigned short,
                                        vector signed short,
                                        vector signed short);
         vector unsigned short vec_mladd (vector unsigned short,
                                          vector unsigned short,
                                          vector unsigned short);

         vector signed short vec_mradds (vector signed short,
                                         vector signed short,
                                         vector signed short);

         vector unsigned int vec_msum (vector unsigned char,
                                       vector unsigned char,
                                       vector unsigned int);
         vector signed int vec_msum (vector signed char,
                                     vector unsigned char,
                                     vector signed int);
         vector unsigned int vec_msum (vector unsigned short,
                                       vector unsigned short,
                                       vector unsigned int);
         vector signed int vec_msum (vector signed short,
                                     vector signed short,
                                     vector signed int);

         vector signed int vec_vmsumshm (vector signed short,
                                         vector signed short,
                                         vector signed int);

         vector unsigned int vec_vmsumuhm (vector unsigned short,
                                           vector unsigned short,
                                           vector unsigned int);

         vector signed int vec_vmsummbm (vector signed char,
                                         vector unsigned char,
                                         vector signed int);

         vector unsigned int vec_vmsumubm (vector unsigned char,
                                           vector unsigned char,
                                           vector unsigned int);

         vector unsigned int vec_msums (vector unsigned short,
                                        vector unsigned short,
                                        vector unsigned int);
         vector signed int vec_msums (vector signed short,
                                      vector signed short,
                                      vector signed int);

         vector signed int vec_vmsumshs (vector signed short,
                                         vector signed short,
                                         vector signed int);

         vector unsigned int vec_vmsumuhs (vector unsigned short,
                                           vector unsigned short,
                                           vector unsigned int);

         void vec_mtvscr (vector signed int);
         void vec_mtvscr (vector unsigned int);
         void vec_mtvscr (vector bool int);
         void vec_mtvscr (vector signed short);
         void vec_mtvscr (vector unsigned short);
         void vec_mtvscr (vector bool short);
         void vec_mtvscr (vector pixel);
         void vec_mtvscr (vector signed char);
         void vec_mtvscr (vector unsigned char);
         void vec_mtvscr (vector bool char);

         vector unsigned short vec_mule (vector unsigned char,
                                         vector unsigned char);
         vector signed short vec_mule (vector signed char,
                                       vector signed char);
         vector unsigned int vec_mule (vector unsigned short,
                                       vector unsigned short);
         vector signed int vec_mule (vector signed short, vector signed short);

         vector signed int vec_vmulesh (vector signed short,
                                        vector signed short);

         vector unsigned int vec_vmuleuh (vector unsigned short,
                                          vector unsigned short);

         vector signed short vec_vmulesb (vector signed char,
                                          vector signed char);

         vector unsigned short vec_vmuleub (vector unsigned char,
                                           vector unsigned char);

         vector unsigned short vec_mulo (vector unsigned char,
                                         vector unsigned char);
         vector signed short vec_mulo (vector signed char, vector signed char);
         vector unsigned int vec_mulo (vector unsigned short,
                                       vector unsigned short);
         vector signed int vec_mulo (vector signed short, vector signed short);

         vector signed int vec_vmulosh (vector signed short,
                                        vector signed short);

         vector unsigned int vec_vmulouh (vector unsigned short,
                                          vector unsigned short);

         vector signed short vec_vmulosb (vector signed char,
                                          vector signed char);

         vector unsigned short vec_vmuloub (vector unsigned char,
                                            vector unsigned char);

         vector float vec_nmsub (vector float, vector float, vector float);

         vector float vec_nor (vector float, vector float);
         vector signed int vec_nor (vector signed int, vector signed int);
         vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
         vector bool int vec_nor (vector bool int, vector bool int);
         vector signed short vec_nor (vector signed short, vector signed short);
         vector unsigned short vec_nor (vector unsigned short,
                                        vector unsigned short);
         vector bool short vec_nor (vector bool short, vector bool short);
         vector signed char vec_nor (vector signed char, vector signed char);
         vector unsigned char vec_nor (vector unsigned char,
                                       vector unsigned char);
         vector bool char vec_nor (vector bool char, vector bool char);

         vector float vec_or (vector float, vector float);
         vector float vec_or (vector float, vector bool int);
         vector float vec_or (vector bool int, vector float);
         vector bool int vec_or (vector bool int, vector bool int);
         vector signed int vec_or (vector bool int, vector signed int);
         vector signed int vec_or (vector signed int, vector bool int);
         vector signed int vec_or (vector signed int, vector signed int);
         vector unsigned int vec_or (vector bool int, vector unsigned int);
         vector unsigned int vec_or (vector unsigned int, vector bool int);
         vector unsigned int vec_or (vector unsigned int, vector unsigned int);
         vector bool short vec_or (vector bool short, vector bool short);
         vector signed short vec_or (vector bool short, vector signed short);
         vector signed short vec_or (vector signed short, vector bool short);
         vector signed short vec_or (vector signed short, vector signed short);
         vector unsigned short vec_or (vector bool short, vector unsigned short);
         vector unsigned short vec_or (vector unsigned short, vector bool short);
         vector unsigned short vec_or (vector unsigned short,
                                       vector unsigned short);
         vector signed char vec_or (vector bool char, vector signed char);
         vector bool char vec_or (vector bool char, vector bool char);
         vector signed char vec_or (vector signed char, vector bool char);
         vector signed char vec_or (vector signed char, vector signed char);
         vector unsigned char vec_or (vector bool char, vector unsigned char);
         vector unsigned char vec_or (vector unsigned char, vector bool char);
         vector unsigned char vec_or (vector unsigned char,
                                      vector unsigned char);

         vector signed char vec_pack (vector signed short, vector signed short);
         vector unsigned char vec_pack (vector unsigned short,
                                        vector unsigned short);
         vector bool char vec_pack (vector bool short, vector bool short);
         vector signed short vec_pack (vector signed int, vector signed int);
         vector unsigned short vec_pack (vector unsigned int,
                                         vector unsigned int);
         vector bool short vec_pack (vector bool int, vector bool int);

         vector bool short vec_vpkuwum (vector bool int, vector bool int);
         vector signed short vec_vpkuwum (vector signed int, vector signed int);
         vector unsigned short vec_vpkuwum (vector unsigned int,
                                            vector unsigned int);

         vector bool char vec_vpkuhum (vector bool short, vector bool short);
         vector signed char vec_vpkuhum (vector signed short,
                                         vector signed short);
         vector unsigned char vec_vpkuhum (vector unsigned short,
                                           vector unsigned short);

         vector pixel vec_packpx (vector unsigned int, vector unsigned int);

         vector unsigned char vec_packs (vector unsigned short,
                                         vector unsigned short);
         vector signed char vec_packs (vector signed short, vector signed short);
         vector unsigned short vec_packs (vector unsigned int,
                                          vector unsigned int);
         vector signed short vec_packs (vector signed int, vector signed int);

         vector signed short vec_vpkswss (vector signed int, vector signed int);

         vector unsigned short vec_vpkuwus (vector unsigned int,
                                            vector unsigned int);

         vector signed char vec_vpkshss (vector signed short,
                                         vector signed short);

         vector unsigned char vec_vpkuhus (vector unsigned short,
                                           vector unsigned short);

         vector unsigned char vec_packsu (vector unsigned short,
                                          vector unsigned short);
         vector unsigned char vec_packsu (vector signed short,
                                          vector signed short);
         vector unsigned short vec_packsu (vector unsigned int,
                                           vector unsigned int);
         vector unsigned short vec_packsu (vector signed int, vector signed int);

         vector unsigned short vec_vpkswus (vector signed int,
                                            vector signed int);

         vector unsigned char vec_vpkshus (vector signed short,
                                           vector signed short);

         vector float vec_perm (vector float,
                                vector float,
                                vector unsigned char);
         vector signed int vec_perm (vector signed int,
                                     vector signed int,
                                     vector unsigned char);
         vector unsigned int vec_perm (vector unsigned int,
                                       vector unsigned int,
                                       vector unsigned char);
         vector bool int vec_perm (vector bool int,
                                   vector bool int,
                                   vector unsigned char);
         vector signed short vec_perm (vector signed short,
                                       vector signed short,
                                       vector unsigned char);
         vector unsigned short vec_perm (vector unsigned short,
                                         vector unsigned short,
                                         vector unsigned char);
         vector bool short vec_perm (vector bool short,
                                     vector bool short,
                                     vector unsigned char);
         vector pixel vec_perm (vector pixel,
                                vector pixel,
                                vector unsigned char);
         vector signed char vec_perm (vector signed char,
                                      vector signed char,
                                      vector unsigned char);
         vector unsigned char vec_perm (vector unsigned char,
                                        vector unsigned char,
                                        vector unsigned char);
         vector bool char vec_perm (vector bool char,
                                    vector bool char,
                                    vector unsigned char);

         vector float vec_re (vector float);

         vector signed char vec_rl (vector signed char,
                                    vector unsigned char);
         vector unsigned char vec_rl (vector unsigned char,
                                      vector unsigned char);
         vector signed short vec_rl (vector signed short, vector unsigned short);
         vector unsigned short vec_rl (vector unsigned short,
                                       vector unsigned short);
         vector signed int vec_rl (vector signed int, vector unsigned int);
         vector unsigned int vec_rl (vector unsigned int, vector unsigned int);

         vector signed int vec_vrlw (vector signed int, vector unsigned int);
         vector unsigned int vec_vrlw (vector unsigned int, vector unsigned int);

         vector signed short vec_vrlh (vector signed short,
                                       vector unsigned short);
         vector unsigned short vec_vrlh (vector unsigned short,
                                         vector unsigned short);

         vector signed char vec_vrlb (vector signed char, vector unsigned char);
         vector unsigned char vec_vrlb (vector unsigned char,
                                        vector unsigned char);

         vector float vec_round (vector float);

         vector float vec_rsqrte (vector float);

         vector float vec_sel (vector float, vector float, vector bool int);
         vector float vec_sel (vector float, vector float, vector unsigned int);
         vector signed int vec_sel (vector signed int,
                                    vector signed int,
                                    vector bool int);
         vector signed int vec_sel (vector signed int,
                                    vector signed int,
                                    vector unsigned int);
         vector unsigned int vec_sel (vector unsigned int,
                                      vector unsigned int,
                                      vector bool int);
         vector unsigned int vec_sel (vector unsigned int,
                                      vector unsigned int,
                                      vector unsigned int);
         vector bool int vec_sel (vector bool int,
                                  vector bool int,
                                  vector bool int);
         vector bool int vec_sel (vector bool int,
                                  vector bool int,
                                  vector unsigned int);
         vector signed short vec_sel (vector signed short,
                                      vector signed short,
                                      vector bool short);
         vector signed short vec_sel (vector signed short,
                                      vector signed short,
                                      vector unsigned short);
         vector unsigned short vec_sel (vector unsigned short,
                                        vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_sel (vector unsigned short,
                                        vector unsigned short,
                                        vector unsigned short);
         vector bool short vec_sel (vector bool short,
                                    vector bool short,
                                    vector bool short);
         vector bool short vec_sel (vector bool short,
                                    vector bool short,
                                    vector unsigned short);
         vector signed char vec_sel (vector signed char,
                                     vector signed char,
                                     vector bool char);
         vector signed char vec_sel (vector signed char,
                                     vector signed char,
                                     vector unsigned char);
         vector unsigned char vec_sel (vector unsigned char,
                                       vector unsigned char,
                                       vector bool char);
         vector unsigned char vec_sel (vector unsigned char,
                                       vector unsigned char,
                                       vector unsigned char);
         vector bool char vec_sel (vector bool char,
                                   vector bool char,
                                   vector bool char);
         vector bool char vec_sel (vector bool char,
                                   vector bool char,
                                   vector unsigned char);

         vector signed char vec_sl (vector signed char,
                                    vector unsigned char);
         vector unsigned char vec_sl (vector unsigned char,
                                      vector unsigned char);
         vector signed short vec_sl (vector signed short, vector unsigned short);
         vector unsigned short vec_sl (vector unsigned short,
                                       vector unsigned short);
         vector signed int vec_sl (vector signed int, vector unsigned int);
         vector unsigned int vec_sl (vector unsigned int, vector unsigned int);

         vector signed int vec_vslw (vector signed int, vector unsigned int);
         vector unsigned int vec_vslw (vector unsigned int, vector unsigned int);

         vector signed short vec_vslh (vector signed short,
                                       vector unsigned short);
         vector unsigned short vec_vslh (vector unsigned short,
                                         vector unsigned short);

         vector signed char vec_vslb (vector signed char, vector unsigned char);
         vector unsigned char vec_vslb (vector unsigned char,
                                        vector unsigned char);

         vector float vec_sld (vector float, vector float, const int);
         vector signed int vec_sld (vector signed int,
                                    vector signed int,
                                    const int);
         vector unsigned int vec_sld (vector unsigned int,
                                      vector unsigned int,
                                      const int);
         vector bool int vec_sld (vector bool int,
                                  vector bool int,
                                  const int);
         vector signed short vec_sld (vector signed short,
                                      vector signed short,
                                      const int);
         vector unsigned short vec_sld (vector unsigned short,
                                        vector unsigned short,
                                        const int);
         vector bool short vec_sld (vector bool short,
                                    vector bool short,
                                    const int);
         vector pixel vec_sld (vector pixel,
                               vector pixel,
                               const int);
         vector signed char vec_sld (vector signed char,
                                     vector signed char,
                                     const int);
         vector unsigned char vec_sld (vector unsigned char,
                                       vector unsigned char,
                                       const int);
         vector bool char vec_sld (vector bool char,
                                   vector bool char,
                                   const int);

         vector signed int vec_sll (vector signed int,
                                    vector unsigned int);
         vector signed int vec_sll (vector signed int,
                                    vector unsigned short);
         vector signed int vec_sll (vector signed int,
                                    vector unsigned char);
         vector unsigned int vec_sll (vector unsigned int,
                                      vector unsigned int);
         vector unsigned int vec_sll (vector unsigned int,
                                      vector unsigned short);
         vector unsigned int vec_sll (vector unsigned int,
                                      vector unsigned char);
         vector bool int vec_sll (vector bool int,
                                  vector unsigned int);
         vector bool int vec_sll (vector bool int,
                                  vector unsigned short);
         vector bool int vec_sll (vector bool int,
                                  vector unsigned char);
         vector signed short vec_sll (vector signed short,
                                      vector unsigned int);
         vector signed short vec_sll (vector signed short,
                                      vector unsigned short);
         vector signed short vec_sll (vector signed short,
                                      vector unsigned char);
         vector unsigned short vec_sll (vector unsigned short,
                                        vector unsigned int);
         vector unsigned short vec_sll (vector unsigned short,
                                        vector unsigned short);
         vector unsigned short vec_sll (vector unsigned short,
                                        vector unsigned char);
         vector bool short vec_sll (vector bool short, vector unsigned int);
         vector bool short vec_sll (vector bool short, vector unsigned short);
         vector bool short vec_sll (vector bool short, vector unsigned char);
         vector pixel vec_sll (vector pixel, vector unsigned int);
         vector pixel vec_sll (vector pixel, vector unsigned short);
         vector pixel vec_sll (vector pixel, vector unsigned char);
         vector signed char vec_sll (vector signed char, vector unsigned int);
         vector signed char vec_sll (vector signed char, vector unsigned short);
         vector signed char vec_sll (vector signed char, vector unsigned char);
         vector unsigned char vec_sll (vector unsigned char,
                                       vector unsigned int);
         vector unsigned char vec_sll (vector unsigned char,
                                       vector unsigned short);
         vector unsigned char vec_sll (vector unsigned char,
                                       vector unsigned char);
         vector bool char vec_sll (vector bool char, vector unsigned int);
         vector bool char vec_sll (vector bool char, vector unsigned short);
         vector bool char vec_sll (vector bool char, vector unsigned char);

         vector float vec_slo (vector float, vector signed char);
         vector float vec_slo (vector float, vector unsigned char);
         vector signed int vec_slo (vector signed int, vector signed char);
         vector signed int vec_slo (vector signed int, vector unsigned char);
         vector unsigned int vec_slo (vector unsigned int, vector signed char);
         vector unsigned int vec_slo (vector unsigned int, vector unsigned char);
         vector signed short vec_slo (vector signed short, vector signed char);
         vector signed short vec_slo (vector signed short, vector unsigned char);
         vector unsigned short vec_slo (vector unsigned short,
                                        vector signed char);
         vector unsigned short vec_slo (vector unsigned short,
                                        vector unsigned char);
         vector pixel vec_slo (vector pixel, vector signed char);
         vector pixel vec_slo (vector pixel, vector unsigned char);
         vector signed char vec_slo (vector signed char, vector signed char);
         vector signed char vec_slo (vector signed char, vector unsigned char);
         vector unsigned char vec_slo (vector unsigned char, vector signed char);
         vector unsigned char vec_slo (vector unsigned char,
                                       vector unsigned char);

         vector signed char vec_splat (vector signed char, const int);
         vector unsigned char vec_splat (vector unsigned char, const int);
         vector bool char vec_splat (vector bool char, const int);
         vector signed short vec_splat (vector signed short, const int);
         vector unsigned short vec_splat (vector unsigned short, const int);
         vector bool short vec_splat (vector bool short, const int);
         vector pixel vec_splat (vector pixel, const int);
         vector float vec_splat (vector float, const int);
         vector signed int vec_splat (vector signed int, const int);
         vector unsigned int vec_splat (vector unsigned int, const int);
         vector bool int vec_splat (vector bool int, const int);

         vector float vec_vspltw (vector float, const int);
         vector signed int vec_vspltw (vector signed int, const int);
         vector unsigned int vec_vspltw (vector unsigned int, const int);
         vector bool int vec_vspltw (vector bool int, const int);

         vector bool short vec_vsplth (vector bool short, const int);
         vector signed short vec_vsplth (vector signed short, const int);
         vector unsigned short vec_vsplth (vector unsigned short, const int);
         vector pixel vec_vsplth (vector pixel, const int);

         vector signed char vec_vspltb (vector signed char, const int);
         vector unsigned char vec_vspltb (vector unsigned char, const int);
         vector bool char vec_vspltb (vector bool char, const int);

         vector signed char vec_splat_s8 (const int);

         vector signed short vec_splat_s16 (const int);

         vector signed int vec_splat_s32 (const int);

         vector unsigned char vec_splat_u8 (const int);

         vector unsigned short vec_splat_u16 (const int);

         vector unsigned int vec_splat_u32 (const int);

         vector signed char vec_sr (vector signed char, vector unsigned char);
         vector unsigned char vec_sr (vector unsigned char,
                                      vector unsigned char);
         vector signed short vec_sr (vector signed short,
                                     vector unsigned short);
         vector unsigned short vec_sr (vector unsigned short,
                                       vector unsigned short);
         vector signed int vec_sr (vector signed int, vector unsigned int);
         vector unsigned int vec_sr (vector unsigned int, vector unsigned int);

         vector signed int vec_vsrw (vector signed int, vector unsigned int);
         vector unsigned int vec_vsrw (vector unsigned int, vector unsigned int);

         vector signed short vec_vsrh (vector signed short,
                                       vector unsigned short);
         vector unsigned short vec_vsrh (vector unsigned short,
                                         vector unsigned short);

         vector signed char vec_vsrb (vector signed char, vector unsigned char);
         vector unsigned char vec_vsrb (vector unsigned char,
                                        vector unsigned char);

         vector signed char vec_sra (vector signed char, vector unsigned char);
         vector unsigned char vec_sra (vector unsigned char,
                                       vector unsigned char);
         vector signed short vec_sra (vector signed short,
                                      vector unsigned short);
         vector unsigned short vec_sra (vector unsigned short,
                                        vector unsigned short);
         vector signed int vec_sra (vector signed int, vector unsigned int);
         vector unsigned int vec_sra (vector unsigned int, vector unsigned int);

         vector signed int vec_vsraw (vector signed int, vector unsigned int);
         vector unsigned int vec_vsraw (vector unsigned int,
                                        vector unsigned int);

         vector signed short vec_vsrah (vector signed short,
                                        vector unsigned short);
         vector unsigned short vec_vsrah (vector unsigned short,
                                          vector unsigned short);

         vector signed char vec_vsrab (vector signed char, vector unsigned char);
         vector unsigned char vec_vsrab (vector unsigned char,
                                         vector unsigned char);

         vector signed int vec_srl (vector signed int, vector unsigned int);
         vector signed int vec_srl (vector signed int, vector unsigned short);
         vector signed int vec_srl (vector signed int, vector unsigned char);
         vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
         vector unsigned int vec_srl (vector unsigned int,
                                      vector unsigned short);
         vector unsigned int vec_srl (vector unsigned int, vector unsigned char);
         vector bool int vec_srl (vector bool int, vector unsigned int);
         vector bool int vec_srl (vector bool int, vector unsigned short);
         vector bool int vec_srl (vector bool int, vector unsigned char);
         vector signed short vec_srl (vector signed short, vector unsigned int);
         vector signed short vec_srl (vector signed short,
                                      vector unsigned short);
         vector signed short vec_srl (vector signed short, vector unsigned char);
         vector unsigned short vec_srl (vector unsigned short,
                                        vector unsigned int);
         vector unsigned short vec_srl (vector unsigned short,
                                        vector unsigned short);
         vector unsigned short vec_srl (vector unsigned short,
                                        vector unsigned char);
         vector bool short vec_srl (vector bool short, vector unsigned int);
         vector bool short vec_srl (vector bool short, vector unsigned short);
         vector bool short vec_srl (vector bool short, vector unsigned char);
         vector pixel vec_srl (vector pixel, vector unsigned int);
         vector pixel vec_srl (vector pixel, vector unsigned short);
         vector pixel vec_srl (vector pixel, vector unsigned char);
         vector signed char vec_srl (vector signed char, vector unsigned int);
         vector signed char vec_srl (vector signed char, vector unsigned short);
         vector signed char vec_srl (vector signed char, vector unsigned char);
         vector unsigned char vec_srl (vector unsigned char,
                                       vector unsigned int);
         vector unsigned char vec_srl (vector unsigned char,
                                       vector unsigned short);
         vector unsigned char vec_srl (vector unsigned char,
                                       vector unsigned char);
         vector bool char vec_srl (vector bool char, vector unsigned int);
         vector bool char vec_srl (vector bool char, vector unsigned short);
         vector bool char vec_srl (vector bool char, vector unsigned char);

         vector float vec_sro (vector float, vector signed char);
         vector float vec_sro (vector float, vector unsigned char);
         vector signed int vec_sro (vector signed int, vector signed char);
         vector signed int vec_sro (vector signed int, vector unsigned char);
         vector unsigned int vec_sro (vector unsigned int, vector signed char);
         vector unsigned int vec_sro (vector unsigned int, vector unsigned char);
         vector signed short vec_sro (vector signed short, vector signed char);
         vector signed short vec_sro (vector signed short, vector unsigned char);
         vector unsigned short vec_sro (vector unsigned short,
                                        vector signed char);
         vector unsigned short vec_sro (vector unsigned short,
                                        vector unsigned char);
         vector pixel vec_sro (vector pixel, vector signed char);
         vector pixel vec_sro (vector pixel, vector unsigned char);
         vector signed char vec_sro (vector signed char, vector signed char);
         vector signed char vec_sro (vector signed char, vector unsigned char);
         vector unsigned char vec_sro (vector unsigned char, vector signed char);
         vector unsigned char vec_sro (vector unsigned char,
                                       vector unsigned char);

         void vec_st (vector float, int, vector float *);
         void vec_st (vector float, int, float *);
         void vec_st (vector signed int, int, vector signed int *);
         void vec_st (vector signed int, int, int *);
         void vec_st (vector unsigned int, int, vector unsigned int *);
         void vec_st (vector unsigned int, int, unsigned int *);
         void vec_st (vector bool int, int, vector bool int *);
         void vec_st (vector bool int, int, unsigned int *);
         void vec_st (vector bool int, int, int *);
         void vec_st (vector signed short, int, vector signed short *);
         void vec_st (vector signed short, int, short *);
         void vec_st (vector unsigned short, int, vector unsigned short *);
         void vec_st (vector unsigned short, int, unsigned short *);
         void vec_st (vector bool short, int, vector bool short *);
         void vec_st (vector bool short, int, unsigned short *);
         void vec_st (vector pixel, int, vector pixel *);
         void vec_st (vector pixel, int, unsigned short *);
         void vec_st (vector pixel, int, short *);
         void vec_st (vector bool short, int, short *);
         void vec_st (vector signed char, int, vector signed char *);
         void vec_st (vector signed char, int, signed char *);
         void vec_st (vector unsigned char, int, vector unsigned char *);
         void vec_st (vector unsigned char, int, unsigned char *);
         void vec_st (vector bool char, int, vector bool char *);
         void vec_st (vector bool char, int, unsigned char *);
         void vec_st (vector bool char, int, signed char *);

         void vec_ste (vector signed char, int, signed char *);
         void vec_ste (vector unsigned char, int, unsigned char *);
         void vec_ste (vector bool char, int, signed char *);
         void vec_ste (vector bool char, int, unsigned char *);
         void vec_ste (vector signed short, int, short *);
         void vec_ste (vector unsigned short, int, unsigned short *);
         void vec_ste (vector bool short, int, short *);
         void vec_ste (vector bool short, int, unsigned short *);
         void vec_ste (vector pixel, int, short *);
         void vec_ste (vector pixel, int, unsigned short *);
         void vec_ste (vector float, int, float *);
         void vec_ste (vector signed int, int, int *);
         void vec_ste (vector unsigned int, int, unsigned int *);
         void vec_ste (vector bool int, int, int *);
         void vec_ste (vector bool int, int, unsigned int *);

         void vec_stvewx (vector float, int, float *);
         void vec_stvewx (vector signed int, int, int *);
         void vec_stvewx (vector unsigned int, int, unsigned int *);
         void vec_stvewx (vector bool int, int, int *);
         void vec_stvewx (vector bool int, int, unsigned int *);

         void vec_stvehx (vector signed short, int, short *);
         void vec_stvehx (vector unsigned short, int, unsigned short *);
         void vec_stvehx (vector bool short, int, short *);
         void vec_stvehx (vector bool short, int, unsigned short *);
         void vec_stvehx (vector pixel, int, short *);
         void vec_stvehx (vector pixel, int, unsigned short *);

         void vec_stvebx (vector signed char, int, signed char *);
         void vec_stvebx (vector unsigned char, int, unsigned char *);
         void vec_stvebx (vector bool char, int, signed char *);
         void vec_stvebx (vector bool char, int, unsigned char *);

         void vec_stl (vector float, int, vector float *);
         void vec_stl (vector float, int, float *);
         void vec_stl (vector signed int, int, vector signed int *);
         void vec_stl (vector signed int, int, int *);
         void vec_stl (vector unsigned int, int, vector unsigned int *);
         void vec_stl (vector unsigned int, int, unsigned int *);
         void vec_stl (vector bool int, int, vector bool int *);
         void vec_stl (vector bool int, int, unsigned int *);
         void vec_stl (vector bool int, int, int *);
         void vec_stl (vector signed short, int, vector signed short *);
         void vec_stl (vector signed short, int, short *);
         void vec_stl (vector unsigned short, int, vector unsigned short *);
         void vec_stl (vector unsigned short, int, unsigned short *);
         void vec_stl (vector bool short, int, vector bool short *);
         void vec_stl (vector bool short, int, unsigned short *);
         void vec_stl (vector bool short, int, short *);
         void vec_stl (vector pixel, int, vector pixel *);
         void vec_stl (vector pixel, int, unsigned short *);
         void vec_stl (vector pixel, int, short *);
         void vec_stl (vector signed char, int, vector signed char *);
         void vec_stl (vector signed char, int, signed char *);
         void vec_stl (vector unsigned char, int, vector unsigned char *);
         void vec_stl (vector unsigned char, int, unsigned char *);
         void vec_stl (vector bool char, int, vector bool char *);
         void vec_stl (vector bool char, int, unsigned char *);
         void vec_stl (vector bool char, int, signed char *);

         vector signed char vec_sub (vector bool char, vector signed char);
         vector signed char vec_sub (vector signed char, vector bool char);
         vector signed char vec_sub (vector signed char, vector signed char);
         vector unsigned char vec_sub (vector bool char, vector unsigned char);
         vector unsigned char vec_sub (vector unsigned char, vector bool char);
         vector unsigned char vec_sub (vector unsigned char,
                                       vector unsigned char);
         vector signed short vec_sub (vector bool short, vector signed short);
         vector signed short vec_sub (vector signed short, vector bool short);
         vector signed short vec_sub (vector signed short, vector signed short);
         vector unsigned short vec_sub (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_sub (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_sub (vector unsigned short,
                                        vector unsigned short);
         vector signed int vec_sub (vector bool int, vector signed int);
         vector signed int vec_sub (vector signed int, vector bool int);
         vector signed int vec_sub (vector signed int, vector signed int);
         vector unsigned int vec_sub (vector bool int, vector unsigned int);
         vector unsigned int vec_sub (vector unsigned int, vector bool int);
         vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
         vector float vec_sub (vector float, vector float);

         vector float vec_vsubfp (vector float, vector float);

         vector signed int vec_vsubuwm (vector bool int, vector signed int);
         vector signed int vec_vsubuwm (vector signed int, vector bool int);
         vector signed int vec_vsubuwm (vector signed int, vector signed int);
         vector unsigned int vec_vsubuwm (vector bool int, vector unsigned int);
         vector unsigned int vec_vsubuwm (vector unsigned int, vector bool int);
         vector unsigned int vec_vsubuwm (vector unsigned int,
                                          vector unsigned int);

         vector signed short vec_vsubuhm (vector bool short,
                                          vector signed short);
         vector signed short vec_vsubuhm (vector signed short,
                                          vector bool short);
         vector signed short vec_vsubuhm (vector signed short,
                                          vector signed short);
         vector unsigned short vec_vsubuhm (vector bool short,
                                            vector unsigned short);
         vector unsigned short vec_vsubuhm (vector unsigned short,
                                            vector bool short);
         vector unsigned short vec_vsubuhm (vector unsigned short,
                                            vector unsigned short);

         vector signed char vec_vsububm (vector bool char, vector signed char);
         vector signed char vec_vsububm (vector signed char, vector bool char);
         vector signed char vec_vsububm (vector signed char, vector signed char);
         vector unsigned char vec_vsububm (vector bool char,
                                           vector unsigned char);
         vector unsigned char vec_vsububm (vector unsigned char,
                                           vector bool char);
         vector unsigned char vec_vsububm (vector unsigned char,
                                           vector unsigned char);

         vector unsigned int vec_subc (vector unsigned int, vector unsigned int);

         vector unsigned char vec_subs (vector bool char, vector unsigned char);
         vector unsigned char vec_subs (vector unsigned char, vector bool char);
         vector unsigned char vec_subs (vector unsigned char,
                                        vector unsigned char);
         vector signed char vec_subs (vector bool char, vector signed char);
         vector signed char vec_subs (vector signed char, vector bool char);
         vector signed char vec_subs (vector signed char, vector signed char);
         vector unsigned short vec_subs (vector bool short,
                                         vector unsigned short);
         vector unsigned short vec_subs (vector unsigned short,
                                         vector bool short);
         vector unsigned short vec_subs (vector unsigned short,
                                         vector unsigned short);
         vector signed short vec_subs (vector bool short, vector signed short);
         vector signed short vec_subs (vector signed short, vector bool short);
         vector signed short vec_subs (vector signed short, vector signed short);
         vector unsigned int vec_subs (vector bool int, vector unsigned int);
         vector unsigned int vec_subs (vector unsigned int, vector bool int);
         vector unsigned int vec_subs (vector unsigned int, vector unsigned int);
         vector signed int vec_subs (vector bool int, vector signed int);
         vector signed int vec_subs (vector signed int, vector bool int);
         vector signed int vec_subs (vector signed int, vector signed int);

         vector signed int vec_vsubsws (vector bool int, vector signed int);
         vector signed int vec_vsubsws (vector signed int, vector bool int);
         vector signed int vec_vsubsws (vector signed int, vector signed int);

         vector unsigned int vec_vsubuws (vector bool int, vector unsigned int);
         vector unsigned int vec_vsubuws (vector unsigned int, vector bool int);
         vector unsigned int vec_vsubuws (vector unsigned int,
                                          vector unsigned int);

         vector signed short vec_vsubshs (vector bool short,
                                          vector signed short);
         vector signed short vec_vsubshs (vector signed short,
                                          vector bool short);
         vector signed short vec_vsubshs (vector signed short,
                                          vector signed short);

         vector unsigned short vec_vsubuhs (vector bool short,
                                            vector unsigned short);
         vector unsigned short vec_vsubuhs (vector unsigned short,
                                            vector bool short);
         vector unsigned short vec_vsubuhs (vector unsigned short,
                                            vector unsigned short);

         vector signed char vec_vsubsbs (vector bool char, vector signed char);
         vector signed char vec_vsubsbs (vector signed char, vector bool char);
         vector signed char vec_vsubsbs (vector signed char, vector signed char);

         vector unsigned char vec_vsububs (vector bool char,
                                           vector unsigned char);
         vector unsigned char vec_vsububs (vector unsigned char,
                                           vector bool char);
         vector unsigned char vec_vsububs (vector unsigned char,
                                           vector unsigned char);

         vector unsigned int vec_sum4s (vector unsigned char,
                                        vector unsigned int);
         vector signed int vec_sum4s (vector signed char, vector signed int);
         vector signed int vec_sum4s (vector signed short, vector signed int);

         vector signed int vec_vsum4shs (vector signed short, vector signed int);

         vector signed int vec_vsum4sbs (vector signed char, vector signed int);

         vector unsigned int vec_vsum4ubs (vector unsigned char,
                                           vector unsigned int);

         vector signed int vec_sum2s (vector signed int, vector signed int);

         vector signed int vec_sums (vector signed int, vector signed int);

         vector float vec_trunc (vector float);

         vector signed short vec_unpackh (vector signed char);
         vector bool short vec_unpackh (vector bool char);
         vector signed int vec_unpackh (vector signed short);
         vector bool int vec_unpackh (vector bool short);
         vector unsigned int vec_unpackh (vector pixel);

         vector bool int vec_vupkhsh (vector bool short);
         vector signed int vec_vupkhsh (vector signed short);

         vector unsigned int vec_vupkhpx (vector pixel);

         vector bool short vec_vupkhsb (vector bool char);
         vector signed short vec_vupkhsb (vector signed char);

         vector signed short vec_unpackl (vector signed char);
         vector bool short vec_unpackl (vector bool char);
         vector unsigned int vec_unpackl (vector pixel);
         vector signed int vec_unpackl (vector signed short);
         vector bool int vec_unpackl (vector bool short);

         vector unsigned int vec_vupklpx (vector pixel);

         vector bool int vec_vupklsh (vector bool short);
         vector signed int vec_vupklsh (vector signed short);

         vector bool short vec_vupklsb (vector bool char);
         vector signed short vec_vupklsb (vector signed char);

         vector float vec_xor (vector float, vector float);
         vector float vec_xor (vector float, vector bool int);
         vector float vec_xor (vector bool int, vector float);
         vector bool int vec_xor (vector bool int, vector bool int);
         vector signed int vec_xor (vector bool int, vector signed int);
         vector signed int vec_xor (vector signed int, vector bool int);
         vector signed int vec_xor (vector signed int, vector signed int);
         vector unsigned int vec_xor (vector bool int, vector unsigned int);
         vector unsigned int vec_xor (vector unsigned int, vector bool int);
         vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
         vector bool short vec_xor (vector bool short, vector bool short);
         vector signed short vec_xor (vector bool short, vector signed short);
         vector signed short vec_xor (vector signed short, vector bool short);
         vector signed short vec_xor (vector signed short, vector signed short);
         vector unsigned short vec_xor (vector bool short,
                                        vector unsigned short);
         vector unsigned short vec_xor (vector unsigned short,
                                        vector bool short);
         vector unsigned short vec_xor (vector unsigned short,
                                        vector unsigned short);
         vector signed char vec_xor (vector bool char, vector signed char);
         vector bool char vec_xor (vector bool char, vector bool char);
         vector signed char vec_xor (vector signed char, vector bool char);
         vector signed char vec_xor (vector signed char, vector signed char);
         vector unsigned char vec_xor (vector bool char, vector unsigned char);
         vector unsigned char vec_xor (vector unsigned char, vector bool char);
         vector unsigned char vec_xor (vector unsigned char,
                                       vector unsigned char);

         int vec_all_eq (vector signed char, vector bool char);
         int vec_all_eq (vector signed char, vector signed char);
         int vec_all_eq (vector unsigned char, vector bool char);
         int vec_all_eq (vector unsigned char, vector unsigned char);
         int vec_all_eq (vector bool char, vector bool char);
         int vec_all_eq (vector bool char, vector unsigned char);
         int vec_all_eq (vector bool char, vector signed char);
         int vec_all_eq (vector signed short, vector bool short);
         int vec_all_eq (vector signed short, vector signed short);
         int vec_all_eq (vector unsigned short, vector bool short);
         int vec_all_eq (vector unsigned short, vector unsigned short);
         int vec_all_eq (vector bool short, vector bool short);
         int vec_all_eq (vector bool short, vector unsigned short);
         int vec_all_eq (vector bool short, vector signed short);
         int vec_all_eq (vector pixel, vector pixel);
         int vec_all_eq (vector signed int, vector bool int);
         int vec_all_eq (vector signed int, vector signed int);
         int vec_all_eq (vector unsigned int, vector bool int);
         int vec_all_eq (vector unsigned int, vector unsigned int);
         int vec_all_eq (vector bool int, vector bool int);
         int vec_all_eq (vector bool int, vector unsigned int);
         int vec_all_eq (vector bool int, vector signed int);
         int vec_all_eq (vector float, vector float);

         int vec_all_ge (vector bool char, vector unsigned char);
         int vec_all_ge (vector unsigned char, vector bool char);
         int vec_all_ge (vector unsigned char, vector unsigned char);
         int vec_all_ge (vector bool char, vector signed char);
         int vec_all_ge (vector signed char, vector bool char);
         int vec_all_ge (vector signed char, vector signed char);
         int vec_all_ge (vector bool short, vector unsigned short);
         int vec_all_ge (vector unsigned short, vector bool short);
         int vec_all_ge (vector unsigned short, vector unsigned short);
         int vec_all_ge (vector signed short, vector signed short);
         int vec_all_ge (vector bool short, vector signed short);
         int vec_all_ge (vector signed short, vector bool short);
         int vec_all_ge (vector bool int, vector unsigned int);
         int vec_all_ge (vector unsigned int, vector bool int);
         int vec_all_ge (vector unsigned int, vector unsigned int);
         int vec_all_ge (vector bool int, vector signed int);
         int vec_all_ge (vector signed int, vector bool int);
         int vec_all_ge (vector signed int, vector signed int);
         int vec_all_ge (vector float, vector float);

         int vec_all_gt (vector bool char, vector unsigned char);
         int vec_all_gt (vector unsigned char, vector bool char);
         int vec_all_gt (vector unsigned char, vector unsigned char);
         int vec_all_gt (vector bool char, vector signed char);
         int vec_all_gt (vector signed char, vector bool char);
         int vec_all_gt (vector signed char, vector signed char);
         int vec_all_gt (vector bool short, vector unsigned short);
         int vec_all_gt (vector unsigned short, vector bool short);
         int vec_all_gt (vector unsigned short, vector unsigned short);
         int vec_all_gt (vector bool short, vector signed short);
         int vec_all_gt (vector signed short, vector bool short);
         int vec_all_gt (vector signed short, vector signed short);
         int vec_all_gt (vector bool int, vector unsigned int);
         int vec_all_gt (vector unsigned int, vector bool int);
         int vec_all_gt (vector unsigned int, vector unsigned int);
         int vec_all_gt (vector bool int, vector signed int);
         int vec_all_gt (vector signed int, vector bool int);
         int vec_all_gt (vector signed int, vector signed int);
         int vec_all_gt (vector float, vector float);

         int vec_all_in (vector float, vector float);

         int vec_all_le (vector bool char, vector unsigned char);
         int vec_all_le (vector unsigned char, vector bool char);
         int vec_all_le (vector unsigned char, vector unsigned char);
         int vec_all_le (vector bool char, vector signed char);
         int vec_all_le (vector signed char, vector bool char);
         int vec_all_le (vector signed char, vector signed char);
         int vec_all_le (vector bool short, vector unsigned short);
         int vec_all_le (vector unsigned short, vector bool short);
         int vec_all_le (vector unsigned short, vector unsigned short);
         int vec_all_le (vector bool short, vector signed short);
         int vec_all_le (vector signed short, vector bool short);
         int vec_all_le (vector signed short, vector signed short);
         int vec_all_le (vector bool int, vector unsigned int);
         int vec_all_le (vector unsigned int, vector bool int);
         int vec_all_le (vector unsigned int, vector unsigned int);
         int vec_all_le (vector bool int, vector signed int);
         int vec_all_le (vector signed int, vector bool int);
         int vec_all_le (vector signed int, vector signed int);
         int vec_all_le (vector float, vector float);

         int vec_all_lt (vector bool char, vector unsigned char);
         int vec_all_lt (vector unsigned char, vector bool char);
         int vec_all_lt (vector unsigned char, vector unsigned char);
         int vec_all_lt (vector bool char, vector signed char);
         int vec_all_lt (vector signed char, vector bool char);
         int vec_all_lt (vector signed char, vector signed char);
         int vec_all_lt (vector bool short, vector unsigned short);
         int vec_all_lt (vector unsigned short, vector bool short);
         int vec_all_lt (vector unsigned short, vector unsigned short);
         int vec_all_lt (vector bool short, vector signed short);
         int vec_all_lt (vector signed short, vector bool short);
         int vec_all_lt (vector signed short, vector signed short);
         int vec_all_lt (vector bool int, vector unsigned int);
         int vec_all_lt (vector unsigned int, vector bool int);
         int vec_all_lt (vector unsigned int, vector unsigned int);
         int vec_all_lt (vector bool int, vector signed int);
         int vec_all_lt (vector signed int, vector bool int);
         int vec_all_lt (vector signed int, vector signed int);
         int vec_all_lt (vector float, vector float);

         int vec_all_nan (vector float);

         int vec_all_ne (vector signed char, vector bool char);
         int vec_all_ne (vector signed char, vector signed char);
         int vec_all_ne (vector unsigned char, vector bool char);
         int vec_all_ne (vector unsigned char, vector unsigned char);
         int vec_all_ne (vector bool char, vector bool char);
         int vec_all_ne (vector bool char, vector unsigned char);
         int vec_all_ne (vector bool char, vector signed char);
         int vec_all_ne (vector signed short, vector bool short);
         int vec_all_ne (vector signed short, vector signed short);
         int vec_all_ne (vector unsigned short, vector bool short);
         int vec_all_ne (vector unsigned short, vector unsigned short);
         int vec_all_ne (vector bool short, vector bool short);
         int vec_all_ne (vector bool short, vector unsigned short);
         int vec_all_ne (vector bool short, vector signed short);
         int vec_all_ne (vector pixel, vector pixel);
         int vec_all_ne (vector signed int, vector bool int);
         int vec_all_ne (vector signed int, vector signed int);
         int vec_all_ne (vector unsigned int, vector bool int);
         int vec_all_ne (vector unsigned int, vector unsigned int);
         int vec_all_ne (vector bool int, vector bool int);
         int vec_all_ne (vector bool int, vector unsigned int);
         int vec_all_ne (vector bool int, vector signed int);
         int vec_all_ne (vector float, vector float);

         int vec_all_nge (vector float, vector float);

         int vec_all_ngt (vector float, vector float);

         int vec_all_nle (vector float, vector float);

         int vec_all_nlt (vector float, vector float);

         int vec_all_numeric (vector float);

         int vec_any_eq (vector signed char, vector bool char);
         int vec_any_eq (vector signed char, vector signed char);
         int vec_any_eq (vector unsigned char, vector bool char);
         int vec_any_eq (vector unsigned char, vector unsigned char);
         int vec_any_eq (vector bool char, vector bool char);
         int vec_any_eq (vector bool char, vector unsigned char);
         int vec_any_eq (vector bool char, vector signed char);
         int vec_any_eq (vector signed short, vector bool short);
         int vec_any_eq (vector signed short, vector signed short);
         int vec_any_eq (vector unsigned short, vector bool short);
         int vec_any_eq (vector unsigned short, vector unsigned short);
         int vec_any_eq (vector bool short, vector bool short);
         int vec_any_eq (vector bool short, vector unsigned short);
         int vec_any_eq (vector bool short, vector signed short);
         int vec_any_eq (vector pixel, vector pixel);
         int vec_any_eq (vector signed int, vector bool int);
         int vec_any_eq (vector signed int, vector signed int);
         int vec_any_eq (vector unsigned int, vector bool int);
         int vec_any_eq (vector unsigned int, vector unsigned int);
         int vec_any_eq (vector bool int, vector bool int);
         int vec_any_eq (vector bool int, vector unsigned int);
         int vec_any_eq (vector bool int, vector signed int);
         int vec_any_eq (vector float, vector float);

         int vec_any_ge (vector signed char, vector bool char);
         int vec_any_ge (vector unsigned char, vector bool char);
         int vec_any_ge (vector unsigned char, vector unsigned char);
         int vec_any_ge (vector signed char, vector signed char);
         int vec_any_ge (vector bool char, vector unsigned char);
         int vec_any_ge (vector bool char, vector signed char);
         int vec_any_ge (vector unsigned short, vector bool short);
         int vec_any_ge (vector unsigned short, vector unsigned short);
         int vec_any_ge (vector signed short, vector signed short);
         int vec_any_ge (vector signed short, vector bool short);
         int vec_any_ge (vector bool short, vector unsigned short);
         int vec_any_ge (vector bool short, vector signed short);
         int vec_any_ge (vector signed int, vector bool int);
         int vec_any_ge (vector unsigned int, vector bool int);
         int vec_any_ge (vector unsigned int, vector unsigned int);
         int vec_any_ge (vector signed int, vector signed int);
         int vec_any_ge (vector bool int, vector unsigned int);
         int vec_any_ge (vector bool int, vector signed int);
         int vec_any_ge (vector float, vector float);

         int vec_any_gt (vector bool char, vector unsigned char);
         int vec_any_gt (vector unsigned char, vector bool char);
         int vec_any_gt (vector unsigned char, vector unsigned char);
         int vec_any_gt (vector bool char, vector signed char);
         int vec_any_gt (vector signed char, vector bool char);
         int vec_any_gt (vector signed char, vector signed char);
         int vec_any_gt (vector bool short, vector unsigned short);
         int vec_any_gt (vector unsigned short, vector bool short);
         int vec_any_gt (vector unsigned short, vector unsigned short);
         int vec_any_gt (vector bool short, vector signed short);
         int vec_any_gt (vector signed short, vector bool short);
         int vec_any_gt (vector signed short, vector signed short);
         int vec_any_gt (vector bool int, vector unsigned int);
         int vec_any_gt (vector unsigned int, vector bool int);
         int vec_any_gt (vector unsigned int, vector unsigned int);
         int vec_any_gt (vector bool int, vector signed int);
         int vec_any_gt (vector signed int, vector bool int);
         int vec_any_gt (vector signed int, vector signed int);
         int vec_any_gt (vector float, vector float);

         int vec_any_le (vector bool char, vector unsigned char);
         int vec_any_le (vector unsigned char, vector bool char);
         int vec_any_le (vector unsigned char, vector unsigned char);
         int vec_any_le (vector bool char, vector signed char);
         int vec_any_le (vector signed char, vector bool char);
         int vec_any_le (vector signed char, vector signed char);
         int vec_any_le (vector bool short, vector unsigned short);
         int vec_any_le (vector unsigned short, vector bool short);
         int vec_any_le (vector unsigned short, vector unsigned short);
         int vec_any_le (vector bool short, vector signed short);
         int vec_any_le (vector signed short, vector bool short);
         int vec_any_le (vector signed short, vector signed short);
         int vec_any_le (vector bool int, vector unsigned int);
         int vec_any_le (vector unsigned int, vector bool int);
         int vec_any_le (vector unsigned int, vector unsigned int);
         int vec_any_le (vector bool int, vector signed int);
         int vec_any_le (vector signed int, vector bool int);
         int vec_any_le (vector signed int, vector signed int);
         int vec_any_le (vector float, vector float);

         int vec_any_lt (vector bool char, vector unsigned char);
         int vec_any_lt (vector unsigned char, vector bool char);
         int vec_any_lt (vector unsigned char, vector unsigned char);
         int vec_any_lt (vector bool char, vector signed char);
         int vec_any_lt (vector signed char, vector bool char);
         int vec_any_lt (vector signed char, vector signed char);
         int vec_any_lt (vector bool short, vector unsigned short);
         int vec_any_lt (vector unsigned short, vector bool short);
         int vec_any_lt (vector unsigned short, vector unsigned short);
         int vec_any_lt (vector bool short, vector signed short);
         int vec_any_lt (vector signed short, vector bool short);
         int vec_any_lt (vector signed short, vector signed short);
         int vec_any_lt (vector bool int, vector unsigned int);
         int vec_any_lt (vector unsigned int, vector bool int);
         int vec_any_lt (vector unsigned int, vector unsigned int);
         int vec_any_lt (vector bool int, vector signed int);
         int vec_any_lt (vector signed int, vector bool int);
         int vec_any_lt (vector signed int, vector signed int);
         int vec_any_lt (vector float, vector float);

         int vec_any_nan (vector float);

         int vec_any_ne (vector signed char, vector bool char);
         int vec_any_ne (vector signed char, vector signed char);
         int vec_any_ne (vector unsigned char, vector bool char);
         int vec_any_ne (vector unsigned char, vector unsigned char);
         int vec_any_ne (vector bool char, vector bool char);
         int vec_any_ne (vector bool char, vector unsigned char);
         int vec_any_ne (vector bool char, vector signed char);
         int vec_any_ne (vector signed short, vector bool short);
         int vec_any_ne (vector signed short, vector signed short);
         int vec_any_ne (vector unsigned short, vector bool short);
         int vec_any_ne (vector unsigned short, vector unsigned short);
         int vec_any_ne (vector bool short, vector bool short);
         int vec_any_ne (vector bool short, vector unsigned short);
         int vec_any_ne (vector bool short, vector signed short);
         int vec_any_ne (vector pixel, vector pixel);
         int vec_any_ne (vector signed int, vector bool int);
         int vec_any_ne (vector signed int, vector signed int);
         int vec_any_ne (vector unsigned int, vector bool int);
         int vec_any_ne (vector unsigned int, vector unsigned int);
         int vec_any_ne (vector bool int, vector bool int);
         int vec_any_ne (vector bool int, vector unsigned int);
         int vec_any_ne (vector bool int, vector signed int);
         int vec_any_ne (vector float, vector float);

         int vec_any_nge (vector float, vector float);

         int vec_any_ngt (vector float, vector float);

         int vec_any_nle (vector float, vector float);

         int vec_any_nlt (vector float, vector float);

         int vec_any_numeric (vector float);

         int vec_any_out (vector float, vector float);

Previous: PowerPC AltiVec Built-in Functions, Up: Target Builtins

#### 5.47.9 SPARC VIS Built-in Functions

GCC supports SIMD operations on the SPARC using both the generic vector extensions (see Vector Extensions) as well as built-in functions for the SPARC Visual Instruction Set (VIS). When you use the `-mvis` switch, the VIS extension is exposed as the following built-in functions:

         typedef int v2si __attribute__ ((vector_size (8)));
         typedef short v4hi __attribute__ ((vector_size (8)));
         typedef short v2hi __attribute__ ((vector_size (4)));
         typedef char v8qi __attribute__ ((vector_size (8)));
         typedef char v4qi __attribute__ ((vector_size (4)));

         void * __builtin_vis_alignaddr (void *, long);
         int64_t __builtin_vis_faligndatadi (int64_t, int64_t);
         v2si __builtin_vis_faligndatav2si (v2si, v2si);
         v4hi __builtin_vis_faligndatav4hi (v4si, v4si);
         v8qi __builtin_vis_faligndatav8qi (v8qi, v8qi);

         v4hi __builtin_vis_fexpand (v4qi);

         v4hi __builtin_vis_fmul8x16 (v4qi, v4hi);
         v4hi __builtin_vis_fmul8x16au (v4qi, v4hi);
         v4hi __builtin_vis_fmul8x16al (v4qi, v4hi);
         v4hi __builtin_vis_fmul8sux16 (v8qi, v4hi);
         v4hi __builtin_vis_fmul8ulx16 (v8qi, v4hi);
         v2si __builtin_vis_fmuld8sux16 (v4qi, v2hi);
         v2si __builtin_vis_fmuld8ulx16 (v4qi, v2hi);

         v4qi __builtin_vis_fpack16 (v4hi);
         v8qi __builtin_vis_fpack32 (v2si, v2si);
         v2hi __builtin_vis_fpackfix (v2si);
         v8qi __builtin_vis_fpmerge (v4qi, v4qi);

         int64_t __builtin_vis_pdist (v8qi, v8qi, int64_t);

Next: Pragmas, Previous: Target Builtins, Up: C Extensions

### 5.48 Format Checks Specific to Particular Target Machines

For some target machines, GCC supports additional options to the format attribute (see Declaring Attributes of Functions).

Up: Target Format Checks

#### 5.48.1 Solaris Format Checks

Solaris targets support the `cmn_err` (or `__cmn_err__`) format check. `cmn_err` accepts a subset of the standard `printf` conversions, and the two-argument `%b` conversion for displaying bit-fields. See the Solaris man page for `cmn_err` for more information.

Next: Unnamed Fields, Previous: Target Format Checks, Up: C Extensions

### 5.49 Pragmas Accepted by GCC

GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; See Function Attributes, for further explanation.

Next: M32C Pragmas, Up: Pragmas

#### 5.49.1 ARM Pragmas

The ARM target defines pragmas for controlling the default addition of `long_call` and `short_call` attributes to functions. See Function Attributes, for information about the effects of these attributes.

`long_calls`
  Set all subsequent functions to have the `long_call` attribute.
`no_long_calls`
  Set all subsequent functions to have the `short_call` attribute.
`long_calls_off`
  Do not affect the `long_call` or `short_call` attributes of subsequent functions.

Next: RS/6000 and PowerPC Pragmas, Previous: ARM Pragmas, Up: Pragmas

#### 5.49.2 M32C Pragmas

`memregs` number
  Overrides the command line option `-memregs=` for the current file. Use with care! This pragma must be before any function in the file, and mixing different memregs values in different objects may make them incompatible. This pragma is useful when a performance-critical function uses a memreg for temporary values, as it may allow you to reduce the number of memregs used.

Next: Darwin Pragmas, Previous: M32C Pragmas, Up: Pragmas

#### 5.49.3 RS/6000 and PowerPC Pragmas

The RS/6000 and PowerPC targets define one pragma for controlling whether or not the `longcall` attribute is added to function declarations by default. This pragma overrides the `-mlongcall` option, but not the `longcall` and `shortcall` attributes. See RS/6000 and PowerPC Options, for more information about when long calls are and are not necessary.

`longcall (1)`
  Apply the `longcall` attribute to all subsequent function declarations.
`longcall (0)`
  Do not apply the `longcall` attribute to subsequent function declarations.

Next: Solaris Pragmas, Previous: RS/6000 and PowerPC Pragmas, Up: Pragmas

#### 5.49.4 Darwin Pragmas

The following pragmas are available for all architectures running the Darwin operating system. These are useful for compatibility with other Mac OS compilers.

`mark` tokens`...`
  This pragma is accepted, but has no effect.
`options align=`alignment
  This pragma sets the alignment of fields in structures. The values of alignment may be `mac68k`, to emulate m68k alignment, or `power`, to emulate PowerPC alignment. Uses of this pragma nest properly; to restore the previous setting, use `reset` for the alignment.
`segment` tokens`...`
  This pragma is accepted, but has no effect.
`unused (`var`[,` var`]...)`
  This pragma declares variables to be possibly unused. GCC will not produce warnings for the listed variables. The effect is similar to that of the `unused` attribute, except that this pragma may appear anywhere within the variables' scopes.

Next: Symbol-Renaming Pragmas, Previous: Darwin Pragmas, Up: Pragmas

#### 5.49.5 Solaris Pragmas

The Solaris target supports `#pragma redefine_extname` (see Symbol-Renaming Pragmas). It also supports additional `#pragma` directives for compatibility with the system compiler.

`align` alignment`(`variable`[,` variable`]...)`
  Increase the minimum alignment of each variable to alignment. This is the same as GCC's `aligned` attribute see Variable Attributes). Macro expansion occurs on the arguments to this pragma when compiling C and Objective-C. It does not currently occur when compiling C++, but this is a bug which may be fixed in a future release.
`fini (`function`[,` function`]...)`
  This pragma causes each listed function to be called after main, or during shared module unloading, by adding a call to the `.fini` section.
`init (`function`[,` function`]...)`
  This pragma causes each listed function to be called during initialization (before `main`) or during shared module loading, by adding a call to the `.init` section.

Next: Structure-Packing Pragmas, Previous: Solaris Pragmas, Up: Pragmas

#### 5.49.6 Symbol-Renaming Pragmas

For compatibility with the Solaris and Tru64 UNIX system headers, GCC supports two `#pragma` directives which change the name used in assembly for a given declaration. These pragmas are only available on platforms whose system headers need them. To get this effect on all platforms supported by GCC, use the asm labels extension (see Asm Labels).

`redefine_extname` oldname newname
  This pragma gives the C function oldname the assembly symbol newname. The preprocessor macro `__PRAGMA_REDEFINE_EXTNAME` will be defined if this pragma is available (currently only on Solaris).
`extern_prefix` string
  This pragma causes all subsequent external function and variable declarations to have string prepended to their assembly symbols. This effect may be terminated with another `extern_prefix` pragma whose argument is an empty string. The preprocessor macro `__PRAGMA_EXTERN_PREFIX` will be defined if this pragma is available (currently only on Tru64 UNIX).

These pragmas and the asm labels extension interact in a complicated manner. Here are some corner cases you may want to be aware of.

1. Both pragmas silently apply only to declarations with external linkage. Asm labels do not have this restriction.
2. In C++, both pragmas silently apply only to declarations with “C” linkage. Again, asm labels do not have this restriction.
3. If any of the three ways of changing the assembly name of a declaration is applied to a declaration whose assembly name has already been determined (either by a previous use of one of these features, or because the compiler needed the assembly name in order to generate code), and the new name is different, a warning issues and the name does not change.
4. The oldname used by `#pragma redefine_extname` is always the C-language name.
5. If `#pragma extern_prefix` is in effect, and a declaration occurs with an asm label attached, the prefix is silently ignored for that declaration.
6. If `#pragma extern_prefix` and `#pragma redefine_extname` apply to the same declaration, whichever triggered first wins, and a warning issues if they contradict each other. (We would like to have `#pragma redefine_extname` always win, for consistency with asm labels, but if `#pragma extern_prefix` triggers first we have no way of knowing that that happened.)

Next: Weak Pragmas, Previous: Symbol-Renaming Pragmas, Up: Pragmas

#### 5.49.7 Structure-Packing Pragmas

For compatibility with Win32, GCC supports a set of `#pragma` directives which change the maximum alignment of members of structures (other than zero-width bitfields), unions, and classes subsequently defined. The n value below always is required to be a small power of two and specifies the new alignment in bytes.

1. `#pragma pack(`n`)` simply sets the new alignment.
2. `#pragma pack()` sets the alignment to the one that was in effect when compilation started (see also command line option `-fpack-struct[=<n>]` see Code Gen Options).
3. `#pragma pack(push[,`n`])` pushes the current alignment setting on an internal stack and then optionally sets the new alignment.
4. `#pragma pack(pop)` restores the alignment setting to the one saved at the top of the internal stack (and removes that stack entry). Note that `#pragma pack([`n`])` does not influence this internal stack; thus it is possible to have `#pragma pack(push)` followed by multiple `#pragma pack(`n`)` instances and finalized by a single `#pragma pack(pop)`.

Previous: Structure-Packing Pragmas, Up: Pragmas

#### 5.49.8 Weak Pragmas

For compatibility with SVR4, GCC supports a set of `#pragma` directives for declaring symbols to be weak, and defining weak aliases.

`#pragma weak` symbol
  This pragma declares symbol to be weak, as if the declaration had the attribute of the same name. The pragma may appear before or after the declaration of symbol, but must appear before either its first use or its definition. It is not an error for symbol to never be defined at all.
`#pragma weak` symbol1`=` symbol2
  This pragma declares symbol1 to be a weak alias of symbol2. It is an error if symbol2 is not defined in the current translation unit.

Next: Thread-Local, Previous: Pragmas, Up: C Extensions

### 5.50 Unnamed struct/union fields within structs/unions

For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example:

         struct {
           int a;
           union {
             int b;
             float c;
           };
           int d;
         } foo;

In this example, the user would be able to access members of the unnamed union with code like ``foo.b`'. Note that only unnamed structs and unions are allowed, you may not have, for example, an unnamed `int`.

You must never create such structures that cause ambiguous field definitions. For example, this structure:

         struct {
           int a;
           struct {
             int a;
           };
         } foo;

It is ambiguous which `a` is being referred to with ``foo.a`'. Such constructs are not supported and must be avoided. In the future, such constructs may be detected and treated as compilation errors.

Unless `-fms-extensions` is used, the unnamed field must be a structure or union definition without a tag (for example, ``struct { int a; };`'). If `-fms-extensions` is used, the field may also be a definition with a tag such as ``struct foo { int a; };`', a reference to a previously defined structure or union such as ``struct foo;`', or a reference to a `typedef` name for a previously defined structure or union type.

Previous: Unnamed Fields, Up: C Extensions

### 5.51 Thread-Local Storage

Thread-local storage (TLS) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The run-time model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (`ld`), dynamic linker (`ld.so`), and system libraries (`libc.so` and `libpthread.so`), so it is not available everywhere.

At the user level, the extension is visible with a new storage class keyword: `__thread`. For example:

         __thread int i;
         extern __thread struct state s;
         static __thread char *p;

The `__thread` specifier may be used alone, with the `extern` or `static` specifiers, but with no other storage class specifier. When used with `extern` or `static`, `__thread` must appear immediately after the other storage class specifier.

The `__thread` specifier may be applied to any global, file-scoped static, function-scoped static, or static data member of a class. It may not be applied to block-scoped automatic or non-static data member.

When the address-of operator is applied to a thread-local variable, it is evaluated at run-time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid.

No static initialization may refer to the address of a thread-local variable.

In C++, if an initializer is present for a thread-local variable, it must be a constant-expression, as defined in 5.19.2 of the ANSI/ISO C++ standard.

See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the run-time is expected to function.

Next: C++98 Thread-Local Edits, Up: Thread-Local

#### 5.51.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage

The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension.

- 5.1.2 Execution environments

  Add new text after paragraph 1

  > Within either execution environment, a thread is a flow of control within a program. It is implementation defined whether or not there may be more than one thread associated with a program. It is implementation defined how threads beyond the first are created, the name and type of the function called at thread startup, and how threads may be terminated. However, objects with thread storage duration shall be initialized before thread startup.

- 6.2.4 Storage durations of objects

  Add new text before paragraph 3

  > An object whose identifier is declared with the storage-class specifier `__thread` has thread storage duration. Its lifetime is the entire execution of the thread, and its stored value is initialized only once, prior to thread startup.

- 6.4.1 Keywords

  Add `__thread`.

- 6.7.1 Storage-class specifiers

  Add `__thread` to the list of storage class specifiers in paragraph 1.

  Change paragraph 2 to

  > With the exception of `__thread`, at most one storage-class specifier may be given [...]. The `__thread` specifier may be used alone, or immediately following `extern` or `static`.

  Add new text after paragraph 6

  > The declaration of an identifier for a variable that has block scope that specifies `__thread` shall also specify either `extern` or `static`.

  > The `__thread` specifier shall be used only with variables.

Previous: C99 Thread-Local Edits, Up: Thread-Local

#### 5.51.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage

The following are a set of changes to ISO/IEC 14882:1998 (aka C++98) that document the exact semantics of the language extension.

- [intro.execution]

  New text after paragraph 4

  > A thread is a flow of control within the abstract machine. It is implementation defined whether or not there may be more than one thread.

  New text after paragraph 7

  > It is unspecified whether additional action must be taken to ensure when and whether side effects are visible to other threads.

- [lex.key]

  Add `__thread`.

- [basic.start.main]

  Add after paragraph 5

  > The thread that begins execution at the `main` function is called the main thread. It is implementation defined how functions beginning threads other than the main thread are designated or typed. A function so designated, as well as the `main` function, is called a thread startup function. It is implementation defined what happens if a thread startup function returns. It is implementation defined what happens to other threads when any thread calls `exit`.

- [basic.start.init]

  Add after paragraph 4

  > The storage for an object of thread storage duration shall be statically initialized before the first statement of the thread startup function. An object of thread storage duration shall not require dynamic initialization.

- [basic.start.term]

  Add after paragraph 3

  > The type of an object with thread storage duration shall not have a non-trivial destructor, nor shall it be an array type whose elements (directly or indirectly) have non-trivial destructors.

- [basic.stc]

  Add “thread storage duration” to the list in paragraph 1.

  Change paragraph 2

  > Thread, static, and automatic storage durations are associated with objects introduced by declarations [...].

  Add `__thread` to the list of specifiers in paragraph 3.

- [basic.stc.thread]

  New section before [basic.stc.static]

  > The keyword `__thread` applied to a non-local object gives the object thread storage duration.

  > A local variable or class data member declared both `static` and `__thread` gives the variable or member thread storage duration.

- [basic.stc.static]

  Change paragraph 1

  > All objects which have neither thread storage duration, dynamic storage duration nor are local [...].

- [dcl.stc]

  Add `__thread` to the list in paragraph 1.

  Change paragraph 1

  > With the exception of `__thread`, at most one storage-class-specifier shall appear in a given decl-specifier-seq. The `__thread` specifier may be used alone, or immediately following the `extern` or `static` specifiers. [...]

  Add after paragraph 5

  > The `__thread` specifier can be applied only to the names of objects and to anonymous unions.

- [class.mem]

  Add after paragraph 6

  > Non-`static` members shall not be `__thread`.

Next: Objective-C, Previous: C Extensions, Up: Top

## 6 Extensions to the C++ Language

The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro `__GNUC__`. You can also use `__GNUG__` to test specifically for GNU C++ (see Predefined Macros).

Next: Restricted Pointers, Up: C++ Extensions

### 6.1 When is a Volatile Object Accessed?

Both the C and C++ standard have the concept of volatile objects. These are normally accessed by pointers and used for accessing hardware. The standards encourage compilers to refrain from optimizations concerning accesses to volatile objects that it might perform on non-volatile objects. The C standard leaves it implementation defined as to what constitutes a volatile access. The C++ standard omits to specify this, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible. The minimum either standard specifies is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Thus an implementation is free to reorder and combine volatile accesses which occur between sequence points, but cannot do so for accesses across a sequence point. The use of volatiles does not allow you to violate the restriction on updating objects multiple times within a sequence point.

In most expressions, it is intuitively obvious what is a read and what is a write. For instance

         volatile int *dst = somevalue;
         volatile int *src = someothervalue;
         *dst = *src;

will cause a read of the volatile object pointed to by src and stores the value into the volatile object pointed to by dst. There is no guarantee that these reads and writes are atomic, especially for objects larger than `int`.

Less obvious expressions are where something which looks like an access is used in a void context. An example would be,

         volatile int *src = somevalue;
         *src;

With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to. The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is this lvalue to rvalue conversion which is responsible for causing an access. However, there is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type in a void context as a read of the object. When the object has incomplete type, G++ issues a warning.

         struct S;
         struct T {int m;};
         volatile S *ptr1 = somevalue;
         volatile T *ptr2 = somevalue;
         *ptr1;
         *ptr2;

In this example, a warning is issued for `*ptr1`, and `*ptr2` causes a read of the object pointed to. If you wish to force an error on the first case, you must force a conversion to rvalue with, for instance a static cast, `static_cast<S>(*ptr1)`.

When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue.

Next: Vague Linkage, Previous: Volatiles, Up: C++ Extensions

### 6.2 Restricting Pointer Aliasing

As with the C front end, G++ understands the C99 feature of restricted pointers, specified with the `__restrict__`, or `__restrict` type qualifier. Because you cannot compile C++ by specifying the `-std=c99` language flag, `restrict` is not a keyword in C++.

In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context.

         void fn (int *__restrict__ rptr, int &__restrict__ rref)
         {
           /* ... */
         }

In the body of `fn`, rptr points to an unaliased integer and rref refers to a (different) unaliased integer.

You may also specify whether a member function's this pointer is unaliased by using `__restrict__` as a member function qualifier.

         void T::fn () __restrict__
         {
           /* ... */
         }

Within the body of `T::fn`, this will have the effective definition `T *__restrict__ const this`. Notice that the interpretation of a `__restrict__` member function qualifier is different to that of `const` or `volatile` qualifier, in that it is applied to the pointer rather than the object. This is consistent with other compilers which implement restricted pointers.

As with all outermost parameter qualifiers, `__restrict__` is ignored in function definition matching. This means you only need to specify `__restrict__` in a function definition, rather than in a function prototype as well.

Next: C++ Interface, Previous: Restricted Pointers, Up: C++ Extensions

### 6.3 Vague Linkage

There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have “vague linkage”. Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.

Inline Functions
  Inline functions are typically defined in a header file which can be included in many different compilations. Hopefully they can usually be inlined, but sometimes an out-of-line copy is necessary, if the address of the function is taken or if inlining fails. In general, we emit an out-of-line copy in all translation units where one is needed. As an exception, we only emit inline virtual functions with the vtable, since it will always require a copy.

  Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function.

VTables
  C++ virtual functions are implemented in most compilers using a lookup table, known as a vtable. The vtable contains pointers to the virtual functions provided by a class, and each object of the class contains a pointer to its vtable (or vtables, in some multiple-inheritance situations). If the class declares any non-inline, non-pure virtual functions, the first one is chosen as the “key method” for the class, and the vtable is only emitted in the translation unit where the key method is defined.

  Note: If the chosen key method is later defined as inline, the vtable will still be emitted in every translation unit which defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there.

type_info objects
  C++ requires information about types to be written out in order to implement ``dynamic_cast`', ``typeid`' and exception handling. For polymorphic classes (classes with virtual functions), the type_info object is written out along with the vtable so that ``dynamic_cast`' can determine the dynamic type of a class object at runtime. For all other types, we write out the type_info object when it is used: when applying ``typeid`' to an expression, throwing an object, or referring to a type in a catch clause or exception specification.
Template Instantiations
  Most everything in this section also applies to template instantiations, but there are other options as well. See Where's the Template?.

When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support.

On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable.

For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.

See Declarations and Definitions in One Header, for another way to control placement of these constructs.

Next: Template Instantiation, Previous: Vague Linkage, Up: C++ Extensions

### 6.4 #pragma interface and implementation

`#pragma interface` and `#pragma implementation` provide the user with a way of explicitly directing the compiler to emit entities with vague linkage (and debugging information) in a particular translation unit.

Note: As of GCC 2.7.2, these `#pragma`s are not useful in most cases, because of COMDAT support and the “key method” heuristic mentioned in Vague Linkage. Using them can actually cause your program to grow due to unnecessary out-of-line copies of inline functions. Currently (3.4) the only benefit of these `#pragma`s is reduced duplication of debugging information, and that should be addressed soon on DWARF 2 targets with the use of COMDAT groups.

`#pragma interface`
`#pragma interface "`subdir`/`objects`.h"`
  Use this directive in header files that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing ``#pragma interface`' is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses ``#pragma implementation`'). Instead, the object files will contain references to be resolved at link time.

  The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to ``#pragma implementation`'.

`#pragma implementation`
`#pragma implementation "`objects`.h"`
  Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use ``#pragma interface`'. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files.

  If you use ``#pragma implementation`' with no argument, it applies to an include file with the same basename4 as your source file. For example, in `allclass.cc`, giving just ``#pragma implementation`' by itself is equivalent to ``#pragma implementation "allclass.h"`'.

  In versions of GNU C++ prior to 2.6.0 `allclass.h` was treated as an implementation file whenever you would include it from `allclass.cc` even if you never specified ``#pragma implementation`'. This was deemed to be more trouble than it was worth, however, and disabled.

  Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use ``#include`' to include the header file; ``#pragma implementation`' only specifies how to use the file—it doesn't actually include it.)

  There is no way to split up the contents of a single header file into multiple implementation files.

``#pragma implementation`' and ``#pragma interface`' also have an effect on function inlining.

If you define a class in a header file marked with ``#pragma interface`', the effect on an inline function defined in that class is similar to an explicit `extern` declaration—the compiler emits no code at all to define an independent version of the function. Its definition is used only for inlining with its callers.

Conversely, when you include the same header file in a main source file that declares it as ``#pragma implementation`', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with `-fno-implement-inlines`. If any calls were not inlined, you will get linker errors.

Next: Bound member functions, Previous: C++ Interface, Up: C++ Extensions

### 6.5 Where's the Template?

C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which are referred to as the Borland model and the Cfront model.

Borland model
  Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them, and the linker collapses them together. The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about. This disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file, since they must be seen to be instantiated.
Cfront model
  The AT&T C++ translator, Cfront, solved the template instantiation problem by creating the notion of a template repository, an automatically maintained place where template instances are stored. A more modern version of the repository works as follows: As individual object files are built, the compiler places any template definitions and instantiations encountered in the repository. At link time, the link wrapper adds in the objects in the repository and compiles any needed instances that were not previously emitted. The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker. The disadvantages are vastly increased complexity, and thus potential for error; for some code this can be just as transparent, but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories. Code written for this model tends to separate definitions of non-inline member templates into a separate file, which should be compiled separately.

When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, G++ supports the Borland model. On other systems, G++ implements neither automatic model.

A future version of G++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations.

In the mean time, you have the following options for dealing with template instantiations:

1. Compile your template-using code with `-frepo`. The compiler will generate files with the extension ``.rpo`' listing all of the template instantiations used in the corresponding object files which could be instantiated there; the link wrapper, ``collect2`', will then update the ``.rpo`' files to tell the compiler where to place those instantiations and rebuild any affected object files. The link-time overhead is negligible after the first pass, as the compiler will continue to place the instantiations in the same files.

   This is your best option for application code written for the Borland model, as it will just work. Code written for the Cfront model will need to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding `#include <tmethods.cc>` to the end of each template header.

   For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.

2. Compile your code with `-fno-implicit-templates` to disable the implicit generation of template instances, and explicitly instantiate all the ones you use. This approach requires more knowledge of exactly which instances you need than do the others, but it's less mysterious and allows greater control. You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like

                 #include "Foo.h"
                 #include "Foo.cc"

                 template class Foo<int>;
                 template ostream& operator <<
                                 (ostream&, const Foo<int>&);

   for each of the instances you need, and create a template instantiation library from those.

   If you are using Cfront-model code, you can probably get away with not using `-fno-implicit-templates` when compiling files that don't ``#include`' the member template definitions.

   If you use one big file to do the instantiations, you may want to compile it without `-fno-implicit-templates` so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.

   G++ has extended the template instantiation syntax given in the ISO standard to allow forward declaration of explicit instantiations (with `extern`), instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with `inline`), and instantiation of only the static data members of a template class, without the support data or member functions (with (`static`):

                 extern template int max (int, int);
                 inline template class Foo<int>;
                 static template class Foo<int>;

3. Do nothing. Pretend G++ does implement automatic instantiation management. Code written for the Borland model will work fine, but each translation unit will contain instances of each of the templates it uses. In a large program, this can lead to an unacceptable amount of code duplication.

Next: C++ Attributes, Previous: Template Instantiation, Up: C++ Extensions

### 6.6 Extracting the function pointer from a bound pointer to member function

In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the ``this`' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.

Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls.

The syntax for this extension is

         extern A a;
         extern int (A::*fp)();
         typedef int (*fptr)(A *);

         fptr p = (fptr)(a.*fp);

For PMF constants (i.e. expressions of the form ``&Klasse::Member`'), no object is needed to obtain the address of the function. They can be converted to function pointers directly:

         fptr p1 = (fptr)(&A::foo);

You must specify `-Wno-pmf-conversions` to use this extension.

Next: Strong Using, Previous: Bound member functions, Up: C++ Extensions

### 6.7 C++-Specific Variable, Function, and Type Attributes

Some attributes only make sense for C++ programs.

`init_priority (`priority`)`

  In Standard C++, objects defined at namespace scope are guaranteed to be initialized in an order in strict accordance with that of their definitions in a given translation unit. No guarantee is made for initializations across translation units. However, GNU C++ allows users to control the order of initialization of objects defined at namespace scope with the `init_priority` attribute by specifying a relative priority, a constant integral expression currently bounded between 101 and 65535 inclusive. Lower numbers indicate a higher priority.

  In the following example, `A` would normally be created before `B`, but the `init_priority` attribute has reversed that order:

                Some_Class  A  __attribute__ ((init_priority (2000)));
                Some_Class  B  __attribute__ ((init_priority (543)));

  Note that the particular values of priority do not matter; only their relative ordering.

`java_interface`
  This type attribute informs C++ that the class is a Java interface. It may only be applied to classes declared within an `extern "Java"` block. Calls to methods declared in this interface will be dispatched using GCJ's interface table mechanism, instead of regular virtual table dispatch.

See also See Strong Using.

Next: Java Exceptions, Previous: C++ Attributes, Up: C++ Extensions

### 6.8 Strong Using

Caution: The semantics of this extension are not fully defined. Users should refrain from using this extension as its semantics may change subtly over time. It is possible that this extension wil be removed in future versions of G++.

A using-directive with `__attribute ((strong))` is stronger than a normal using-directive in two ways:

- Templates from the used namespace can be specialized as though they were members of the using namespace.
- The using namespace is considered an associated namespace of all templates in the used namespace for purposes of argument-dependent name lookup.

This is useful for composing a namespace transparently from implementation namespaces. For example:

         namespace std {
           namespace debug {
             template <class T> struct A { };
           }
           using namespace debug __attribute ((__strong__));
           template <> struct A<int> { };   // ok to specialize

           template <class T> void f (A<T>);
         }

         int main()
         {
           f (std::A<float>());             // lookup finds std::f
           f (std::A<int>());
         }

Next: Deprecated Features, Previous: Strong Using, Up: C++ Extensions

### 6.9 Java Exceptions

The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:

           struct S { ~S(); };
           extern void bar();    // is written in Java, and may throw exceptions
           void foo()
           {
             S s;
             bar();
           }

The usual effect of an incorrect guess is a link failure, complaining of a missing routine called ``__gxx_personality_v0`'.

You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing ``#pragma GCC java_exceptions`' at the head of the file. This ``#pragma`' must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.

You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.

Next: Backwards Compatibility, Previous: Java Exceptions, Up: C++ Extensions

### 6.10 Deprecated Features

In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.

While the list below is not exhaustive, it documents some of the options that are now deprecated:

`-fexternal-templates`
`-falt-external-templates`
  These are two of the many ways for G++ to implement template instantiation. See Template Instantiation. The C++ standard clearly defines how template definitions have to be organized across implementation units. G++ has an implicit instantiation mechanism that should work just fine for standard-conforming code.
`-fstrict-prototype`
`-fno-strict-prototype`
  Previously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands. This feature has been removed, except where it is required for backwards compatibility See Backwards Compatibility.

G++ allows a virtual function returning ``void *`' to be overridden by one returning a different pointer type. This extension to the covariant return type rules is now deprecated and will be removed from a future version.

The G++ minimum and maximum operators (``<?`' and ``>?`') and their compound forms (``<?=`') and ``>?=`') have been deprecated and will be removed in a future version. Code using these operators should be modified to use `std::min` and `std::max` instead.

The named return value extension has been deprecated, and is now removed from G++.

The use of initializer lists with new expressions has been deprecated, and is now removed from G++.

Floating and complex non-type template parameters have been deprecated, and are now removed from G++.

The implicit typename extension has been deprecated and is now removed from G++.

The use of default arguments in function pointers, function typedefs and and other places where they are not permitted by the standard is deprecated and will be removed from a future version of G++.

G++ allows floating-point literals to appear in integral constant expressions, e.g. ``enum E { e = int(2.2 * 3.7) }` ' This extension is deprecated and will be removed from a future version.

G++ allows static data members of const floating-point type to be declared with an initializer in a class definition. The standard only allows initializers for static members of const integral types and const enumeration types so this extension has been deprecated and will be removed from a future version.

Previous: Deprecated Features, Up: C++ Extensions

### 6.11 Backwards Compatibility

Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated See Deprecated Features.

`For scope`
  If a variable is declared at for scope, it used to remain in scope until the end of the scope which contained the for statement (rather than just within the for scope). G++ retains this, but issues a warning, if such a variable is accessed outside the for scope.
`Implicit C language`
  Old C system header files did not contain an `extern "C" {...}` scope to set the language. On such systems, all header files are implicitly scoped inside a C language scope. Also, an empty prototype `()` will be treated as an unspecified number of arguments, rather than no arguments, as C++ demands.

Next: Compatibility, Previous: C++ Extensions, Up: Top

## 7 GNU Objective-C runtime features

This document is meant to describe some of the GNU Objective-C runtime features. It is not intended to teach you Objective-C, there are several resources on the Internet that present the language. Questions and comments about this document to Ovidiu Predescu ovidiu@cup.hp.com.

Next: Type encoding, Previous: Objective-C, Up: Objective-C

### 7.1 `+load`: Executing code before main

The GNU Objective-C runtime provides a way that allows you to execute code before the execution of the program enters the `main` function. The code is executed on a per-class and a per-category basis, through a special class method `+load`.

This facility is very useful if you want to initialize global variables which can be accessed by the program directly, without sending a message to the class first. The usual way to initialize global variables, in the `+initialize` method, might not be useful because `+initialize` is only called when the first message is sent to a class object, which in some cases could be too late.

Suppose for example you have a `FileStream` class that declares `Stdin`, `Stdout` and `Stderr` as global variables, like below:

         FileStream *Stdin = nil;
         FileStream *Stdout = nil;
         FileStream *Stderr = nil;

         @implementation FileStream

         + (void)initialize
         {
             Stdin = [[FileStream new] initWithFd:0];
             Stdout = [[FileStream new] initWithFd:1];
             Stderr = [[FileStream new] initWithFd:2];
         }

         /* Other methods here */
         @end

In this example, the initialization of `Stdin`, `Stdout` and `Stderr` in `+initialize` occurs too late. The programmer can send a message to one of these objects before the variables are actually initialized, thus sending messages to the `nil` object. The `+initialize` method which actually initializes the global variables is not invoked until the first message is sent to the class object. The solution would require these variables to be initialized just before entering `main`.

The correct solution of the above problem is to use the `+load` method instead of `+initialize`:

         @implementation FileStream

         + (void)load
         {
             Stdin = [[FileStream new] initWithFd:0];
             Stdout = [[FileStream new] initWithFd:1];
             Stderr = [[FileStream new] initWithFd:2];
         }

         /* Other methods here */
         @end

The `+load` is a method that is not overridden by categories. If a class and a category of it both implement `+load`, both methods are invoked. This allows some additional initializations to be performed in a category.

This mechanism is not intended to be a replacement for `+initialize`. You should be aware of its limitations when you decide to use it instead of `+initialize`.

Previous: Executing code before main, Up: Executing code before main

#### 7.1.1 What you can and what you cannot do in `+load`

The `+load` implementation in the GNU runtime guarantees you the following things:

- you can write whatever C code you like;
- you can send messages to Objective-C constant strings (`@"this is a constant string"`);
- you can allocate and send messages to objects whose class is implemented in the same file;
- the `+load` implementation of all super classes of a class are executed before the `+load` of that class is executed;
- the `+load` implementation of a class is executed before the `+load` implementation of any category.

In particular, the following things, even if they can work in a particular case, are not guaranteed:

- allocation of or sending messages to arbitrary objects;
- allocation of or sending messages to objects whose classes have a category implemented in the same file;

You should make no assumptions about receiving `+load` in sibling classes when you write `+load` of a class. The order in which sibling classes receive `+load` is not guaranteed.

The order in which `+load` and `+initialize` are called could be problematic if this matters. If you don't allocate objects inside `+load`, it is guaranteed that `+load` is called before `+initialize`. If you create an object inside `+load` the `+initialize` method of object's class is invoked even if `+load` was not invoked. Note if you explicitly call `+load` on a class, `+initialize` will be called first. To avoid possible problems try to implement only one of these methods.

The `+load` method is also invoked when a bundle is dynamically loaded into your running program. This happens automatically without any intervening operation from you. When you write bundles and you need to write `+load` you can safely create and send messages to objects whose classes already exist in the running program. The same restrictions as above apply to classes defined in bundle.

Next: Garbage Collection, Previous: Executing code before main, Up: Objective-C

### 7.2 Type encoding

The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.

The types are encoded in the following way:

`char` | `c`
`unsigned char` | `C`
`short` | `s`
`unsigned short` | `S`
`int` | `i`
`unsigned int` | `I`
`long` | `l`
`unsigned long` | `L`
`long long` | `q`
`unsigned long long` | `Q`
`float` | `f`
`double` | `d`
`void` | `v`
`id` | `@`
`Class` | `#`
`SEL` | `:`
`char*` | `*`
unknown type | `?`
bit-fields | `b` followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)

The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.

The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.

The non-atomic types are encoded as follows:

pointers | ``^`' followed by the pointed type.
arrays | ``[`' followed by the number of elements in the array followed by the type of the elements followed by ``]`'
structures | ``{`' followed by the name of the structure (or ``?`' if the structure is unnamed), the ``=`' sign, the type of the members and by ``}`'
unions | ``(`' followed by the name of the structure (or ``?`' if the union is unnamed), the ``=`' sign, the type of the members followed by ``)`'

Here are some types and their encodings, as they are generated by the compiler on an i386 machine:

Objective-C type | Compiler encoding
         int a[10];
| `[10i]`
         struct {
           int i;
           float f[3];
           int a:3;
           int b:2;
           char c;
         }
| `{?=i[3f]b128i3b131i2c}`

In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:

Specifier | Encoding
`const` | `r`
`in` | `n`
`inout` | `N`
`out` | `o`
`bycopy` | `O`
`oneway` | `V`

The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.

Next: Constant string objects, Previous: Type encoding, Up: Objective-C

### 7.3 Garbage Collection

Support for a new memory management policy has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector. It is available from http://www.hpl.hp.com/personal/Hans_Boehm/gc/.

To enable the support for it you have to configure the compiler using an additional argument, `--enable-objc-gc`. You need to have garbage collector installed before building the compiler. This will build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, `libobjc_gc.a` to not conflict with the non-garbage-collected library.

When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized.

There is a new runtime function `class_ivar_set_gcinvisible()` which can be used to declare a so-called weak pointer reference. Such a pointer is basically hidden for the garbage collector; this can be useful in certain situations, especially when you want to keep track of the allocated objects, yet allow them to be collected. This kind of pointers can only be members of objects, you cannot declare a global pointer as a weak reference. Every type which is a pointer type can be declared a weak pointer, including `id`, `Class` and `SEL`.

Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this:

         @interface WeakPointer : Object
         {
             const void* weakPointer;
         }

         - initWithPointer:(const void*)p;
         - (const void*)weakPointer;
         @end

         @implementation WeakPointer

         + (void)initialize
         {
           class_ivar_set_gcinvisible (self, "weakPointer", YES);
         }

         - initWithPointer:(const void*)p
         {
           weakPointer = p;
           return self;
         }

         - (const void*)weakPointer
         {
           return weakPointer;
         }

         @end

Weak pointers are supported through a new type character specifier represented by the ``!`' character. The `class_ivar_set_gcinvisible()` function adds or removes this specifier to the string type description of the instance variable named as argument.

Next: compatibility_alias, Previous: Garbage Collection, Up: Objective-C

### 7.4 Constant string objects

GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character ``@`':

           id myString = @"this is a constant string object";

The constant string objects are by default instances of the `NXConstantString` class which is provided by the GNU Objective-C runtime. To get the definition of this class you must include the `objc/NXConstStr.h` header file.

User defined libraries may want to implement their own constant string class. To be able to support them, the GNU Objective-C compiler provides a new command line options `-fconstant-string-class=class-name`. The provided class should adhere to a strict structure, the same as `NXConstantString`'s structure:

         @interface MyConstantStringClass
         {
           Class isa;
           char *c_string;
           unsigned int len;
         }
         @end

`NXConstantString` inherits from `Object`; user class libraries may choose to inherit the customized constant string class from a different class than `Object`. There is no requirement in the methods the constant string class has to implement, but the final ivar layout of the class must be the compatible with the given structure.

When the compiler creates the statically allocated constant string object, the `c_string` field will be filled by the compiler with the string; the `length` field will be filled by the compiler with the string length; the `isa` pointer will be filled with `NULL` by the compiler, and it will later be fixed up automatically at runtime by the GNU Objective-C runtime library to point to the class which was set by the `-fconstant-string-class` option when the object file is loaded (if you wonder how it works behind the scenes, the name of the class to use, and the list of static objects to fixup, are stored by the compiler in the object file in a place where the GNU runtime library will find them at runtime).

As a result, when a file is compiled with the `-fconstant-string-class` option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this.

Previous: Constant string objects, Up: Objective-C

### 7.5 compatibility_alias

This is a feature of the Objective-C compiler rather than of the runtime, anyway since it is documented nowhere and its existence was forgotten, we are documenting it here.

The keyword `@compatibility_alias` allows you to define a class name as equivalent to another class name. For example:

         @compatibility_alias WOApplication GSWApplication;

tells the compiler that each time it encounters `WOApplication` as a class name, it should replace it with `GSWApplication` (that is, `WOApplication` is just an alias for `GSWApplication`).

There are some constraints on how this can be used—

- `WOApplication` (the alias) must not be an existing class;
- `GSWApplication` (the real class) must be an existing class.

Next: Gcov, Previous: Objective-C, Up: Top

## 8 Binary Compatibility

Binary compatibility encompasses several related concepts:

application binary interface (ABI)
  The set of runtime conventions followed by all of the tools that deal with binary representations of a program, including compilers, assemblers, linkers, and language runtime support. Some ABIs are formal with a written specification, possibly designed by multiple interested parties. Others are simply the way things are actually done by a particular set of tools.
ABI conformance
  A compiler conforms to an ABI if it generates code that follows all of the specifications enumerated by that ABI. A library conforms to an ABI if it is implemented according to that ABI. An application conforms to an ABI if it is built using tools that conform to that ABI and does not contain source code that specifically changes behavior specified by the ABI.
calling conventions
  Calling conventions are a subset of an ABI that specify of how arguments are passed and function results are returned.
interoperability
  Different sets of tools are interoperable if they generate files that can be used in the same program. The set of tools includes compilers, assemblers, linkers, libraries, header files, startup files, and debuggers. Binaries produced by different sets of tools are not interoperable unless they implement the same ABI. This applies to different versions of the same tools as well as tools from different vendors.
intercallability
  Whether a function in a binary built by one set of tools can call a function in a binary built by a different set of tools is a subset of interoperability.
implementation-defined features
  Language standards include lists of implementation-defined features whose behavior can vary from one implementation to another. Some of these features are normally covered by a platform's ABI and others are not. The features that are not covered by an ABI generally affect how a program behaves, but not intercallability.
compatibility
  Conformance to the same ABI and the same behavior of implementation-defined features are both relevant for compatibility.

The application binary interface implemented by a C or C++ compiler affects code generation and runtime support for:

- size and alignment of data types
- layout of structured types
- calling conventions
- register usage conventions
- interfaces for runtime arithmetic support
- object file formats

In addition, the application binary interface implemented by a C++ compiler affects code generation and runtime support for:

- name mangling
- exception handling
- invoking constructors and destructors
- layout, alignment, and padding of classes
- layout and alignment of virtual tables

Some GCC compilation options cause the compiler to generate code that does not conform to the platform's default ABI. Other options cause different program behavior for implementation-defined features that are not covered by an ABI. These options are provided for consistency with other compilers that do not follow the platform's default ABI or the usual behavior of implementation-defined features for the platform. Be very careful about using such options.

Most platforms have a well-defined ABI that covers C code, but ABIs that cover C++ functionality are not yet common.

Starting with GCC 3.2, GCC binary conventions for C++ are based on a written, vendor-neutral C++ ABI that was designed to be specific to 64-bit Itanium but also includes generic specifications that apply to any platform. This C++ ABI is also implemented by other compiler vendors on some platforms, notably GNU/Linux and BSD systems. We have tried hard to provide a stable ABI that will be compatible with future GCC releases, but it is possible that we will encounter problems that make this difficult. Such problems could include different interpretations of the C++ ABI by different vendors, bugs in the ABI, or bugs in the implementation of the ABI in different compilers. GCC's `-Wabi` switch warns when G++ generates code that is probably not compatible with the C++ ABI.

The C++ library used with a C++ compiler includes the Standard C++ Library, with functionality defined in the C++ Standard, plus language runtime support. The runtime support is included in a C++ ABI, but there is no formal ABI for the Standard C++ Library. Two implementations of that library are interoperable if one follows the de-facto ABI of the other and if they are both built with the same compiler, or with compilers that conform to the same ABI for C++ compiler and runtime support.

When G++ and another C++ compiler conform to the same C++ ABI, but the implementations of the Standard C++ Library that they normally use do not follow the same ABI for the Standard C++ Library, object files built with those compilers can be used in the same program only if they use the same C++ library. This requires specifying the location of the C++ library header files when invoking the compiler whose usual library is not being used. The location of GCC's C++ header files depends on how the GCC build was configured, but can be seen by using the G++ `-v` option. With default configuration options for G++ 3.3 the compile line for a different C++ compiler needs to include

             -Igcc_install_directory/include/c++/3.3

Similarly, compiling code with G++ that must use a C++ library other than the GNU C++ library requires specifying the location of the header files for that other library.

The most straightforward way to link a program to use a particular C++ library is to use a C++ driver that specifies that C++ library by default. The `g++` driver, for example, tells the linker where to find GCC's C++ library (`libstdc++`) plus the other libraries and startup files it needs, in the proper order.

If a program must use a different C++ library and it's not possible to do the final link using a C++ driver that uses that library by default, it is necessary to tell `g++` the location and name of that library. It might also be necessary to specify different startup files and other runtime support libraries, and to suppress the use of GCC's support libraries with one or more of the options `-nostdlib`, `-nostartfiles`, and `-nodefaultlibs`.

Next: Trouble, Previous: Compatibility, Up: Top

## 9 `gcov`—a Test Coverage Program

`gcov` is a tool you can use in conjunction with GCC to test code coverage in your programs.

Next: Invoking Gcov, Up: Gcov

### 9.1 Introduction to `gcov`

`gcov` is a test coverage program. Use it in concert with GCC to analyze your programs to help create more efficient, faster running code and to discover untested parts of your program. You can use `gcov` as a profiling tool to help discover where your optimization efforts will best affect your code. You can also use `gcov` along with the other profiling tool, `gprof`, to assess which parts of your code use the greatest amount of computing time.

Profiling tools help you analyze your code's performance. Using a profiler such as `gcov` or `gprof`, you can find out some basic performance statistics, such as:

- how often each line of code executes
- what lines of code are actually executed
- how much computing time each section of code uses

Once you know these things about how your code works when compiled, you can look at each module to see which modules should be optimized. `gcov` helps you determine where to work on optimization.

Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.

You should compile your code without optimization if you plan to use `gcov` because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look for `hot spots' where the code is using a great deal of computer time. Likewise, because `gcov` accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less helpful—they only report on the line where the macro call appears. If your complex macros behave like functions, you can replace them with inline functions to solve this problem.

`gcov` creates a logfile called `sourcefile.gcov` which indicates how many times each line of a source file `sourcefile.c` has executed. You can use these logfiles along with `gprof` to aid in fine-tuning the performance of your programs. `gprof` gives timing information you can use along with the information you get from `gcov`.

`gcov` works only on code compiled with GCC. It is not compatible with any other profiling or test coverage mechanism.

Next: Gcov and Optimization, Previous: Gcov Intro, Up: Gcov

### 9.2 Invoking gcov

         gcov [options] sourcefile

`gcov` accepts the following options:

`-h`
`--help`
  Display help about using `gcov` (on the standard output), and exit without doing any further processing.
`-v`
`--version`
  Display the `gcov` version number (on the standard output), and exit without doing any further processing.
`-a`
`--all-blocks`
  Write individual execution counts for every basic block. Normally gcov outputs execution counts only for the main blocks of a line. With this option you can determine if blocks within a single line are not being executed.
`-b`
`--branch-probabilities`
  Write branch frequencies to the output file, and write branch summary info to the standard output. This option allows you to see how often each branch in your program was taken. Unconditional branches will not be shown, unless the `-u` option is given.
`-c`
`--branch-counts`
  Write branch frequencies as the number of branches taken, rather than the percentage of branches taken.
`-n`
`--no-output`
  Do not create the `gcov` output file.
`-l`
`--long-file-names`
  Create long file names for included source files. For example, if the header file `x.h` contains code, and was included in the file `a.c`, then running `gcov` on the file `a.c` will produce an output file called `a.c##x.h.gcov` instead of `x.h.gcov`. This can be useful if `x.h` is included in multiple source files. If you use the ``-p`' option, both the including and included file names will be complete path names.
`-p`
`--preserve-paths`
  Preserve complete path information in the names of generated `.gcov` files. Without this option, just the filename component is used. With this option, all directories are used, with ``/`' characters translated to ``#`' characters, `.` directory components removed and `..` components renamed to ``^`'. This is useful if sourcefiles are in several different directories. It also affects the ``-l`' option.
`-f`
`--function-summaries`
  Output summaries for each function in addition to the file level summary.
`-o` directory|file
`--object-directory` directory
`--object-file` file
  Specify either the directory containing the gcov data files, or the object path name. The `.gcno`, and `.gcda` data files are searched for using this option. If a directory is specified, the data files are in that directory and named after the source file name, without its extension. If a file is specified here, the data files are named after that file, without its extension. If this option is not supplied, it defaults to the current directory.
`-u`
`--unconditional-branches`
  When branch probabilities are given, include those of unconditional branches. Unconditional branches are normally not interesting.

`gcov` should be run with the current directory the same as that when you invoked the compiler. Otherwise it will not be able to locate the source files. `gcov` produces files called `mangledname.gcov` in the current directory. These contain the coverage information of the source file they correspond to. One `.gcov` file is produced for each source file containing code, which was compiled to produce the data files. The mangledname part of the output file name is usually simply the source file name, but can be something more complicated if the ``-l`' or ``-p`' options are given. Refer to those options for details.

The `.gcov` files contain the ``:`' separated fields along with program source code. The format is

         execution_count:line_number:source line text

Additional block information may succeed each line, when requested by command line option. The execution_count is ``-`' for lines containing no code and ``#####`' for lines which were never executed. Some lines of information at the start have line_number of zero.

The preamble lines are of the form

         -:0:tag:value

The ordering and number of these preamble lines will be augmented as `gcov` development progresses — do not rely on them remaining unchanged. Use tag to locate a particular preamble line.

The additional block information is of the form

         tag information

The information is human readable, but designed to be simple enough for machine parsing too.

When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-boundary value.

When using `gcov`, you must first compile your program with two special GCC options: ``-fprofile-arcs -ftest-coverage`'. This tells the compiler to generate additional information needed by gcov (basically a flow graph of the program) and also includes additional code in the object files for generating the extra profiling information needed by gcov. These additional files are placed in the directory where the object file is located.

Running the program will cause profile output to be generated. For each source file compiled with `-fprofile-arcs`, an accompanying `.gcda` file will be placed in the object file directory.

Running `gcov` with your program's source file names as arguments will now produce a listing of the code along with frequency of execution for each line. For example, if your program is called `tmp.c`, this is what you see when you use the basic `gcov` facility:

         $ gcc -fprofile-arcs -ftest-coverage tmp.c
         $ a.out
         $ gcov tmp.c
         90.00% of 10 source lines executed in file tmp.c
         Creating tmp.c.gcov.

The file `tmp.c.gcov` contains output from `gcov`. Here is a sample:

                 -:    0:Source:tmp.c
                 -:    0:Graph:tmp.gcno
                 -:    0:Data:tmp.gcda
                 -:    0:Runs:1
                 -:    0:Programs:1
                 -:    1:#include <stdio.h>
                 -:    2:
                 -:    3:int main (void)
                 1:    4:{
                 1:    5:  int i, total;
                 -:    6:
                 1:    7:  total = 0;
                 -:    8:
                11:    9:  for (i = 0; i < 10; i++)
                10:   10:    total += i;
                 -:   11:
                 1:   12:  if (total != 45)
             #####:   13:    printf ("Failure\n");
                 -:   14:  else
                 1:   15:    printf ("Success\n");
                 1:   16:  return 0;
                 -:   17:}

When you use the `-a` option, you will get individual block counts, and the output looks like this:

                 -:    0:Source:tmp.c
                 -:    0:Graph:tmp.gcno
                 -:    0:Data:tmp.gcda
                 -:    0:Runs:1
                 -:    0:Programs:1
                 -:    1:#include <stdio.h>
                 -:    2:
                 -:    3:int main (void)
                 1:    4:{
                 1:    4-block  0
                 1:    5:  int i, total;
                 -:    6:
                 1:    7:  total = 0;
                 -:    8:
                11:    9:  for (i = 0; i < 10; i++)
                11:    9-block  0
                10:   10:    total += i;
                10:   10-block  0
                 -:   11:
                 1:   12:  if (total != 45)
                 1:   12-block  0
             #####:   13:    printf ("Failure\n");
             $$$$$:   13-block  0
                 -:   14:  else
                 1:   15:    printf ("Success\n");
                 1:   15-block  0
                 1:   16:  return 0;
                 1:   16-block  0
                 -:   17:}

In this mode, each basic block is only shown on one line – the last line of the block. A multi-line block will only contribute to the execution count of that last line, and other lines will not be shown to contain code, unless previous blocks end on those lines. The total execution count of a line is shown and subsequent lines show the execution counts for individual blocks that end on that line. After each block, the branch and call counts of the block will be shown, if the `-b` option is given.

Because of the way GCC instruments calls, a call count can be shown after a line with no individual blocks. As you can see, line 13 contains a basic block that was not executed.

When you use the `-b` option, your output looks like this:

         $ gcov -b tmp.c
         90.00% of 10 source lines executed in file tmp.c
         80.00% of 5 branches executed in file tmp.c
         80.00% of 5 branches taken at least once in file tmp.c
         50.00% of 2 calls executed in file tmp.c
         Creating tmp.c.gcov.

Here is a sample of a resulting `tmp.c.gcov` file:

                 -:    0:Source:tmp.c
                 -:    0:Graph:tmp.gcno
                 -:    0:Data:tmp.gcda
                 -:    0:Runs:1
                 -:    0:Programs:1
                 -:    1:#include <stdio.h>
                 -:    2:
                 -:    3:int main (void)
         function main called 1 returned 1 blocks executed 75%
                 1:    4:{
                 1:    5:  int i, total;
                 -:    6:
                 1:    7:  total = 0;
                 -:    8:
                11:    9:  for (i = 0; i < 10; i++)
         branch  0 taken 91% (fallthrough)
         branch  1 taken 9%
                10:   10:    total += i;
                 -:   11:
                 1:   12:  if (total != 45)
         branch  0 taken 0% (fallthrough)
         branch  1 taken 100%
             #####:   13:    printf ("Failure\n");
         call    0 never executed
                 -:   14:  else
                 1:   15:    printf ("Success\n");
         call    0 called 1 returned 100%
                 1:   16:  return 0;
                 -:   17:}

For each function, a line is printed showing how many times the function is called, how many times it returns and what percentage of the function's blocks were executed.

For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.

For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message “never executed” is printed.

For a call, if it was executed at least once, then a percentage indicating the number of times the call returned divided by the number of times the call was executed will be printed. This will usually be 100%, but may be less for functions call `exit` or `longjmp`, and thus may not return every time they are called.

The execution counts are cumulative. If the example program were executed again without removing the `.gcda` file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs.

The data in the `.gcda` files is saved immediately before the program exits. For each source file compiled with `-fprofile-arcs`, the profiling code first attempts to read in an existing `.gcda` file; if the file doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file.

Next: Gcov Data Files, Previous: Invoking Gcov, Up: Gcov

### 9.3 Using `gcov` with GCC Optimization

If you plan to use `gcov` to help optimize your code, you must first compile your program with two special GCC options: ``-fprofile-arcs -ftest-coverage`'. Aside from that, you can use any other GCC options; but if you want to prove that every single line in your program was executed, you should not compile with optimization at the same time. On some machines the optimizer can eliminate some simple code lines by combining them with other lines. For example, code like this:

         if (a != b)
           c = 1;
         else
           c = 0;

can be compiled into one instruction on some machines. In this case, there is no way for `gcov` to calculate separate execution counts for each line because there isn't separate code for each line. Hence the `gcov` output looks like this if you compiled the program with optimization:

               100:   12:if (a != b)
               100:   13:  c = 1;
               100:   14:else
               100:   15:  c = 0;

The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.

Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all.

If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If `fileA.o` and `fileB.o` both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When `fileA.o` and `fileB.o` are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts.

If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent.

Next: Cross-profiling, Previous: Gcov and Optimization, Up: Gcov

### 9.4 Brief description of `gcov` data files

`gcov` uses two files for profiling. The names of these files are derived from the original object file by substituting the file suffix with either `.gcno`, or `.gcda`. All of these files are placed in the same directory as the object file, and contain data stored in a platform-independent format.

The `.gcno` file is generated when the source file is compiled with the GCC `-ftest-coverage` option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks.

The `.gcda` file is generated when a program containing object files built with the GCC `-fprofile-arcs` option is executed. A separate `.gcda` file is created for each object file compiled with this option. It contains arc transition counts, and some summary information.

The full details of the file format is specified in `gcov-io.h`, and functions provided in that header file should be used to access the coverage files.

Previous: Gcov Data Files, Up: Gcov

### 9.5 Data file relocation to support cross-profiling

Running the program will cause profile output to be generated. For each source file compiled with `-fprofile-arcs`, an accompanying `.gcda` file will be placed in the object file directory. That implicitly requires running the program on the same system as it was built or having the same absolute directory structure on the target system. The program will try to create the needed directory structure, if it is not already present.

To support cross-profiling, a program compiled with `-fprofile-arcs` can relocate the data files based on two environment variables:

- GCOV_PREFIX contains the prefix to add to the absolute paths in the object file. Prefix must be absolute as well, otherwise its value is ignored. The default is no prefix.
- GCOV_PREFIX_STRIP indicates the how many initial directory names to strip off the hardwired absolute paths. Default value is 0.

  Note: GCOV_PREFIX_STRIP has no effect if GCOV_PREFIX is undefined, empty or non-absolute.

For example, if the object file `/user/build/foo.o` was built with `-fprofile-arcs`, the final executable will try to create the data file `/user/build/foo.gcda` when running on the target system. This will fail if the corresponding directory does not exist and it is unable to create it. This can be overcome by, for example, setting the environment as ``GCOV_PREFIX=/target/run`' and ``GCOV_PREFIX_STRIP=1`'. Such a setting will name the data file `/target/run/build/foo.gcda`.

You must move the data files to the expected directory tree in order to use them for profile directed optimizations (`--use-profile`), or to use the `gcov` tool.

Next: Bugs, Previous: Gcov, Up: Top

## 10 Known Causes of Trouble with GCC

This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se—if they were, we would fix them. But the result for a user may be like the result of a bug.

Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.

Next: Cross-Compiler Problems, Up: Trouble

### 10.1 Actual Bugs We Haven't Fixed Yet

- The `fixincludes` script interacts badly with automounters; if the directory of system header files is automounted, it tends to be unmounted while `fixincludes` is running. This would seem to be a bug in the automounter. We don't know any good way to work around it.
- The `fixproto` script will sometimes add prototypes for the `sigsetjmp` and `siglongjmp` functions that reference the `jmp_buf` type before that type is defined. To work around this, edit the offending file and place the typedef in front of the prototypes.

Next: Interoperation, Previous: Actual Bugs, Up: Trouble

### 10.2 Cross-Compiler Problems

You may run into problems with cross compilation on certain machines, for several reasons.

- At present, the program `mips-tfile` which adds debug support to object files on MIPS systems does not work in a cross compile environment.

Next: Incompatibilities, Previous: Cross-Compiler Problems, Up: Trouble

### 10.3 Interoperation

This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.

- On many platforms, GCC supports a different ABI for C++ than do other compilers, so the object files compiled by GCC cannot be used with object files generated by another C++ compiler.

  An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers—but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time.

- On some BSD systems, including some versions of Ultrix, use of profiling causes static variable destructors (currently used only in C++) not to be run.
- On some SGI systems, when you use `-lgl_s` as an option, it gets translated magically to ``-lgl_s -lX11_s -lc_s`'. Naturally, this does not happen when you use GCC. You must specify all three options explicitly.
- On a SPARC, GCC aligns all values of type `double` on an 8-byte boundary, and it expects every `double` to be so aligned. The Sun compiler usually gives `double` values 8-byte alignment, with one exception: function arguments of type `double` may not be aligned.

  As a result, if a function compiled with Sun CC takes the address of an argument of type `double` and passes this pointer of type `double *` to a function compiled with GCC, dereferencing the pointer may cause a fatal signal.

  One way to solve this problem is to compile your entire program with GCC. Another solution is to modify the function that is compiled with Sun CC to copy the argument into a local variable; local variables are always properly aligned. A third solution is to modify the function that uses the pointer to dereference it via the following function `access_double` instead of directly with ``*`':

                inline double
                access_double (double *unaligned_ptr)
                {
                  union d2i { double d; int i[2]; };

                  union d2i *p = (union d2i *) unaligned_ptr;
                  union d2i u;

                  u.i[0] = p->i[0];
                  u.i[1] = p->i[1];

                  return u.d;
                }

  Storing into the pointer can be done likewise with the same union.

- On Solaris, the `malloc` function in the `libmalloc.a` library may allocate memory that is only 4 byte aligned. Since GCC on the SPARC assumes that doubles are 8 byte aligned, this may result in a fatal signal if doubles are stored in memory allocated by the `libmalloc.a` library.

  The solution is to not use the `libmalloc.a` library. Use instead `malloc` and related functions from `libc.a`; they do not have this problem.

- On the HP PA machine, ADB sometimes fails to work on functions compiled with GCC. Specifically, it fails to work on functions that use `alloca` or variable-size arrays. This is because GCC doesn't generate HP-UX unwind descriptors for such functions. It may even be impossible to generate them.
- Debugging (`-g`) is not supported on the HP PA machine, unless you use the preliminary GNU tools.
- Taking the address of a label may generate errors from the HP-UX PA assembler. GAS for the PA does not have this problem.
- Using floating point parameters for indirect calls to static functions will not work when using the HP assembler. There simply is no way for GCC to specify what registers hold arguments for static functions when using the HP assembler. GAS for the PA does not have this problem.
- In extremely rare cases involving some very large functions you may receive errors from the HP linker complaining about an out of bounds unconditional branch offset. This used to occur more often in previous versions of GCC, but is now exceptionally rare. If you should run into it, you can work around by making your function smaller.
- GCC compiled code sometimes emits warnings from the HP-UX assembler of the form:

                (warning) Use of GR3 when
                  frame >= 8192 may cause conflict.

  These warnings are harmless and can be safely ignored.

- In extremely rare cases involving some very large functions you may receive errors from the AIX Assembler complaining about a displacement that is too large. If you should run into it, you can work around by making your function smaller.
- The `libstdc++.a` library in GCC relies on the SVR4 dynamic linker semantics which merges global symbols between libraries and applications, especially necessary for C++ streams functionality. This is not the default behavior of AIX shared libraries and dynamic linking. `libstdc++.a` is built on AIX with “runtime-linking” enabled so that symbol merging can occur. To utilize this feature, the application linked with `libstdc++.a` must include the `-Wl,-brtl` flag on the link line. G++ cannot impose this because this option may interfere with the semantics of the user program and users may not always use ``g++`' to link his or her application. Applications are not required to use the `-Wl,-brtl` flag on the link line—the rest of the `libstdc++.a` library which is not dependent on the symbol merging semantics will continue to function correctly.
- An application can interpose its own definition of functions for functions invoked by `libstdc++.a` with “runtime-linking” enabled on AIX. To accomplish this the application must be linked with “runtime-linking” option and the functions explicitly must be exported by the application (`-Wl,-brtl,-bE:exportfile`).
- AIX on the RS/6000 provides support (NLS) for environments outside of the United States. Compilers and assemblers use NLS to support locale-specific representations of various objects including floating-point numbers (``.`' vs ``,`' for separating decimal fractions). There have been problems reported where the library linked with GCC does not produce the same floating-point formats that the assembler accepts. If you have this problem, set the `LANG` environment variable to ``C`' or ``En_US`'.
- Even if you specify `-fdollars-in-identifiers`, you cannot successfully use ``$`' in identifiers on the RS/6000 due to a restriction in the IBM assembler. GAS supports these identifiers.

- On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved by function calls. However, the C compiler uses conventions compatible with BSD Unix: registers 2 through 5 may be clobbered by function calls.

  GCC uses the same convention as the Ultrix C compiler. You can use these options to produce code compatible with the Fortran compiler:

                -fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5

Next: Fixed Headers, Previous: Interoperation, Up: Trouble

### 10.4 Incompatibilities of GCC

There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C.

- GCC normally makes string constants read-only. If several identical-looking string constants are used, GCC stores only one copy of the string.

  One consequence is that you cannot call `mktemp` with a string constant argument. The function `mktemp` always alters the string its argument points to.

  Another consequence is that `sscanf` does not work on some very old systems when passed a string constant as its format control string or input. This is because `sscanf` incorrectly tries to write into the string constant. Likewise `fscanf` and `scanf`.

  The solution to these problems is to change the program to use `char`-array variables with initialization strings for these purposes instead of string constants.

- `-2147483648` is positive.

  This is because 2147483648 cannot fit in the type `int`, so (following the ISO C rules) its data type is `unsigned long int`. Negating this value yields 2147483648 again.

- GCC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GCC

                #define foo(a) "a"

  will produce output `"a"` regardless of what the argument a is.

- When you use `setjmp` and `longjmp`, the only automatic variables guaranteed to remain valid are those declared `volatile`. This is a consequence of automatic register allocation. Consider this function:

                jmp_buf j;

                foo ()
                {
                  int a, b;

                  a = fun1 ();
                  if (setjmp (j))
                    return a;

                  a = fun2 ();
                  /* longjmp (j) may occur in fun3. */
                  return a + fun3 ();
                }

  Here `a` may or may not be restored to its first value when the `longjmp` occurs. If `a` is allocated in a register, then its first value is restored; otherwise, it keeps the last value stored in it.

  If you use the `-W` option with the `-O` option, you will get a warning when GCC thinks such a problem might be possible.

- Programs that use preprocessing directives in the middle of macro arguments do not work with GCC. For example, a program like this will not work:

                foobar (
                #define luser
                        hack)

  ISO C does not permit such a construct.

- K&R compilers allow comments to cross over an inclusion boundary (i.e. started in an include file and ended in the including file).

- Declarations of external variables and functions within a block apply only to the block containing the declaration. In other words, they have the same scope as any other declaration in the same place.

  In some other C compilers, a `extern` declaration affects all the rest of the file even if it happens within a block.

- In traditional C, you can combine `long`, etc., with a typedef name, as shown here:

                typedef int foo;
                typedef long foo bar;

  In ISO C, this is not allowed: `long` and other type modifiers require an explicit `int`.

- PCC allows typedef names to be used as function parameters.
- Traditional C allows the following erroneous pair of declarations to appear together in a given scope:

                typedef int foo;
                typedef foo foo;

- GCC treats all characters of identifiers as significant. According to K&R-1 (2.2), “No more than the first eight characters are significant, although more may be used.”. Also according to K&R-1 (2.2), “An identifier is a sequence of letters and digits; the first character must be a letter. The underscore _ counts as a letter.”, but GCC also allows dollar signs in identifiers.

- PCC allows whitespace in the middle of compound assignment operators such as ``+=`'. GCC, following the ISO standard, does not allow this.

- GCC complains about unterminated character constants inside of preprocessing conditionals that fail. Some programs have English comments enclosed in conditionals that are guaranteed to fail; if these comments contain apostrophes, GCC will probably report an error. For example, this code would produce an error:

                #if 0
                You can't expect this to work.
                #endif

  The best solution to such a problem is to put the text into an actual C comment delimited by ``/*...*/`'.

- Many user programs contain the declaration ``long time ();`'. In the past, the system header files on many systems did not actually declare `time`, so it did not matter what type your program declared it to return. But in systems with ISO C headers, `time` is declared to return `time_t`, and if that is not the same as `long`, then ``long time ();`' is erroneous.

  The solution is to change your program to use appropriate system headers (`<time.h>` on systems with ISO C headers) and not to declare `time` if the system header files declare it, or failing that to use `time_t` as the return type of `time`.

- When compiling functions that return `float`, PCC converts it to a double. GCC actually returns a `float`. If you are concerned with PCC compatibility, you should declare your functions to return `double`; you might as well say what you mean.

- When compiling functions that return structures or unions, GCC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GCC cannot call a structure-returning function compiled with PCC, and vice versa.

  The method used by GCC is as follows: a structure or union which is 1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union with any other size is stored into an address supplied by the caller (usually in a special, fixed register, but on some machines it is passed on the stack). The target hook `TARGET_STRUCT_VALUE_RTX` tells GCC where to pass this address.

  By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant.

  On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers.

  You can tell GCC to use a compatible convention for all structure and union returning with the option `-fpcc-struct-return`.

- GCC complains about program fragments such as ``0x74ae-0x4000`' which appear to be two hexadecimal constants separated by the minus operator. Actually, this string is a single preprocessing token. Each such token must correspond to one token in C. Since this does not, GCC prints an error message. Although it may appear obvious that what is meant is an operator and two values, the ISO C standard specifically requires that this be treated as erroneous.

  A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and ``e+`', ``e-`', ``E+`', ``E-`', ``p+`', ``p-`', ``P+`', or ``P-`' character sequences. (In strict C89 mode, the sequences ``p+`', ``p-`', ``P+`' and ``P-`' cannot appear in preprocessing numbers.)

  To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number.

Next: Standard Libraries, Previous: Incompatibilities, Up: Trouble

### 10.5 Fixed Header Files

GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.

Installing GCC automatically creates and installs the fixed header files, by running a program called `fixincludes`. Normally, you don't need to pay attention to this. But there are cases where it doesn't do the right thing automatically.

- If you update the system's header files, such as by installing a new system version, the fixed header files of GCC are not automatically updated. They can be updated using the `mkheaders` script installed in `libexecdir/gcc/target/version/install-tools/`.
- On some systems, header file directories contain machine-specific symbolic links in certain places. This makes it possible to share most of the header files among hosts running the same version of the system on different machine models.

  The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.

  It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand.

Next: Disappointments, Previous: Fixed Headers, Up: Trouble

### 10.6 Standard Libraries

GCC by itself attempts to be a conforming freestanding implementation. See Language Standards Supported by GCC, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using `-Wall`) that you don't expect.

For example, the `sprintf` function on SunOS 4.1.3 returns `char *` while the C standard says that `sprintf` returns an `int`. The `fixincludes` program could make the prototype for this function match the Standard, but that would be wrong, since the function will still return `char *`.

If you need a Standard compliant library, then you need to find one, as GCC does not provide one. The GNU C library (called `glibc`) provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for GNU/Linux and HURD-based GNU systems; no recent version of it supports other systems, though some very old versions did. Version 2.2 of the GNU C library includes nearly complete C99 support. You could also ask your operating system vendor if newer libraries are available.

Next: C++ Misunderstandings, Previous: Standard Libraries, Up: Trouble

### 10.7 Disappointments and Misunderstandings

These problems are perhaps regrettable, but we don't know any practical way around them.

- Certain local variables aren't recognized by debuggers when you compile with optimization.

  This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable “would have had”, and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information.

  You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.

- Users often think it is a bug when GCC reports an error for code like this:

                int foo (struct mumble *);

                struct mumble { ... };

                int foo (struct mumble *x)
                { ... }

  This code really is erroneous, because the scope of `struct mumble` in the prototype is limited to the argument list containing it. It does not refer to the `struct mumble` defined with file scope immediately below—they are two unrelated types with similar names in different scopes.

  But in the definition of `foo`, the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error.

  This behavior may seem silly, but it's what the ISO standard specifies. It is easy enough for you to make your code work by moving the definition of `struct mumble` above the prototype. It's not worth being incompatible with ISO C just to avoid an error for the example shown above.

- Accesses to bit-fields even in volatile objects works by accessing larger objects, such as a byte or a word. You cannot rely on what size of object is accessed in order to read or write the bit-field; it may even vary for a given bit-field according to the precise usage.

  If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields.

- GCC comes with shell scripts to fix certain known problems in system header files. They install corrected copies of various header files in a special directory where only GCC will normally look for them. The scripts adapt to various systems by searching all the system header files for the problem cases that we know about.

  If new system header files are installed, nothing automatically arranges to update the corrected header files. They can be updated using the `mkheaders` script installed in `libexecdir/gcc/target/version/install-tools/`.

- On 68000 and x86 systems, for instance, you can get paradoxical results if you test the precise values of floating point numbers. For example, you can find that a floating point value which is not a NaN is not equal to itself. This results from the fact that the floating point registers hold a few more bits of precision than fit in a `double` in memory. Compiled code moves values between memory and floating point registers at its convenience, and moving them into memory truncates them.

  You can partially avoid this problem by using the `-ffloat-store` option (see Optimize Options).

- On AIX and other platforms without weak symbol support, templates need to be instantiated explicitly and symbols for static members of templates will not be generated.
- On AIX, GCC scans object files and library archives for static constructors and destructors when linking an application before the linker prunes unreferenced symbols. This is necessary to prevent the AIX linker from mistakenly assuming that static constructor or destructor are unused and removing them before the scanning can occur. All static constructors and destructors found will be referenced even though the modules in which they occur may not be used by the program. This may lead to both increased executable size and unexpected symbol references.

Next: Protoize Caveats, Previous: Disappointments, Up: Trouble

### 10.8 Common Misunderstandings with GNU C++

C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.

Next: Name lookup, Up: C++ Misunderstandings

#### 10.8.1 Declare and Define Static Members

When a class has static data members, it is not enough to declare the static member; you must also define it. For example:

         class Foo
         {
           ...
           void method();
           static int bar;
         };

This declaration only establishes that the class `Foo` has an `int` named `Foo::bar`, and a member function named `Foo::method`. But you still need to define both `method` and `bar` elsewhere. According to the ISO standard, you must supply an initializer in one (and only one) source file, such as:

         int Foo::bar = 0;

Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to `g++` from one of these compilers, you may discover that a program that appeared to work correctly in fact does not conform to the standard: `g++` reports as undefined symbols any static data members that lack definitions.

Next: Temporaries, Previous: Static Definitions, Up: C++ Misunderstandings

#### 10.8.2 Name lookup, templates, and accessing members of base classes

The C++ standard prescribes that all names that are not dependent on template parameters are bound to their present definitions when parsing a template function or class.5 Only names that are dependent are looked up at the point of instantiation. For example, consider

           void foo(double);

           struct A {
             template <typename T>
             void f () {
               foo (1);        // 1
               int i = N;      // 2
               T t;
               t.bar();        // 3
               foo (t);        // 4
             }

             static const int N;
           };

Here, the names `foo` and `N` appear in a context that does not depend on the type of `T`. The compiler will thus require that they are defined in the context of use in the template, not only before the point of instantiation, and will here use `::foo(double)` and `A::N`, respectively. In particular, it will convert the integer value to a `double` when passing it to `::foo(double)`.

Conversely, `bar` and the call to `foo` in the fourth marked line are used in contexts that do depend on the type of `T`, so they are only looked up at the point of instantiation, and you can provide declarations for them after declaring the template, but before instantiating it. In particular, if you instantiate `A::f<int>`, the last line will call an overloaded `::foo(int)` if one was provided, even if after the declaration of `struct A`.

This distinction between lookup of dependent and non-dependent names is called two-stage (or dependent) name lookup. G++ implements it since version 3.4.

Two-stage name lookup sometimes leads to situations with behavior different from non-template codes. The most common is probably this:

           template <typename T> struct Base {
             int i;
           };

           template <typename T> struct Derived : public Base<T> {
             int get_i() { return i; }
           };

In `get_i()`, `i` is not used in a dependent context, so the compiler will look for a name declared at the enclosing namespace scope (which is the global scope here). It will not look into the base class, since that is dependent and you may declare specializations of `Base` even after declaring `Derived`, so the compiler can't really know what `i` would refer to. If there is no global variable `i`, then you will get an error message.

In order to make it clear that you want the member of the base class, you need to defer lookup until instantiation time, at which the base class is known. For this, you need to access `i` in a dependent context, by either using `this->i` (remember that `this` is of type `Derived<T>*`, so is obviously dependent), or using `Base<T>::i`. Alternatively, `Base<T>::i` might be brought into scope by a `using`-declaration.

Another, similar example involves calling member functions of a base class:

           template <typename T> struct Base {
               int f();
           };

           template <typename T> struct Derived : Base<T> {
               int g() { return f(); };
           };

Again, the call to `f()` is not dependent on template arguments (there are no arguments that depend on the type `T`, and it is also not otherwise specified that the call should be in a dependent context). Thus a global declaration of such a function must be available, since the one in the base class is not visible until instantiation time. The compiler will consequently produce the following error message:

           x.cc: In member function `int Derived<T>::g()':
           x.cc:6: error: there are no arguments to `f' that depend on a template
              parameter, so a declaration of `f' must be available
           x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but
              allowing the use of an undeclared name is deprecated)

To make the code valid either use `this->f()`, or `Base<T>::f()`. Using the `-fpermissive` flag will also let the compiler accept the code, by marking all function calls for which no declaration is visible at the time of definition of the template for later lookup at instantiation time, as if it were a dependent call. We do not recommend using `-fpermissive` to work around invalid code, and it will also only catch cases where functions in base classes are called, not where variables in base classes are used (as in the example above).

Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above code without an error. Those compilers do not implement two-stage name lookup correctly.

Next: Copy Assignment, Previous: Name lookup, Up: C++ Misunderstandings

#### 10.8.3 Temporaries May Vanish Before You Expect

It is dangerous to use pointers or references to portions of a temporary object. The compiler may very well delete the object before you expect it to, leaving a pointer to garbage. The most common place where this problem crops up is in classes like string classes, especially ones that define a conversion function to type `char *` or `const char *`—which is one reason why the standard `string` class requires you to call the `c_str` member function. However, any class that returns a pointer to some internal structure is potentially subject to this problem.

For example, a program may use a function `strfunc` that returns `string` objects, and another function `charfunc` that operates on pointers to `char`:

         string strfunc ();
         void charfunc (const char *);

         void
         f ()
         {
           const char *p = strfunc().c_str();
           ...
           charfunc (p);
           ...
           charfunc (p);
         }

In this situation, it may seem reasonable to save a pointer to the C string returned by the `c_str` member function and use that rather than call `c_str` repeatedly. However, the temporary string created by the call to `strfunc` is destroyed after `p` is initialized, at which point `p` is left pointing to freed memory.

Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.

The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:

         const string& tmp = strfunc ();
         charfunc (tmp.c_str ());

Previous: Temporaries, Up: C++ Misunderstandings

#### 10.8.4 Implicit Copy-Assignment for Virtual Bases

When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:

         struct Base{
           char *name;
           Base(char *n) : name(strdup(n)){}
           Base& operator= (const Base& other){
            free (name);
            name = strdup (other.name);
           }
         };

         struct A:virtual Base{
           int val;
           A():Base("A"){}
         };

         struct B:virtual Base{
           int bval;
           B():Base("B"){}
         };

         struct Derived:public A, public B{
           Derived():Base("Derived"){}
         };

         void func(Derived &d1, Derived &d2)
         {
           d1 = d2;
         }

The C++ standard specifies that ``Base::Base`' is only called once when constructing or copy-constructing a Derived object. It is unspecified whether ``Base::operator=`' is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside ``func`' in the example).

G++ implements the “intuitive” algorithm for copy-assignment: assign all direct bases, then assign all members. In that algorithm, the virtual base subobject can be encountered more than once. In the example, copying proceeds in the following order: ``val`', ``name`' (via `strdup`), ``bval`', and ``name`' again.

If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.

Next: Non-bugs, Previous: C++ Misunderstandings, Up: Trouble

### 10.9 Caveats of using `protoize`

The conversion programs `protoize` and `unprotoize` can sometimes change a source file in a way that won't work unless you rearrange it.

- `protoize` can insert references to a type name or type tag before the definition, or in a file where they are not defined.

  If this happens, compiler error messages should show you where the new references are, so fixing the file by hand is straightforward.

- There are some C constructs which `protoize` cannot figure out. For example, it can't determine argument types for declaring a pointer-to-function variable; this you must do by hand. `protoize` inserts a comment containing ``???`' each time it finds such a variable; so you can find all such variables by searching for this string. ISO C does not require declaring the argument types of pointer-to-function types.
- Using `unprotoize` can easily introduce bugs. If the program relied on prototypes to bring about conversion of arguments, these conversions will not take place in the program without prototypes. One case in which you can be sure `unprotoize` is safe is when you are removing prototypes that were made with `protoize`; if the program worked before without any prototypes, it will work again without them.

  You can find all the places where this problem might occur by compiling the program with the `-Wconversion` option. It prints a warning whenever an argument is converted.

- Both conversion programs can be confused if there are macro calls in and around the text to be converted. In other words, the standard syntax for a declaration or definition must not result from expanding a macro. This problem is inherent in the design of C and cannot be fixed. If only a few functions have confusing macro calls, you can easily convert them manually.
- `protoize` cannot get the argument types for a function whose definition was not actually compiled due to preprocessing conditionals. When this happens, `protoize` changes nothing in regard to such a function. `protoize` tries to detect such instances and warn about them.

  You can generally work around this problem by using `protoize` step by step, each time specifying a different set of `-D` options for compilation, until all of the functions have been converted. There is no automatic way to verify that you have got them all, however.

- Confusion may result if there is an occasion to convert a function declaration or definition in a region of source code where there is more than one formal parameter list present. Thus, attempts to convert code containing multiple (conditionally compiled) versions of a single function header (in the same vicinity) may not produce the desired (or expected) results.

  If you plan on converting source files which contain such code, it is recommended that you first make sure that each conditionally compiled region of source code which contains an alternative function header also contains at least one additional follower token (past the final right parenthesis of the function header). This should circumvent the problem.

- `unprotoize` can become confused when trying to convert a function definition or declaration which contains a declaration for a pointer-to-function formal argument which has the same name as the function being defined or declared. We recommend you avoid such choices of formal parameter names.
- You might also want to correct some of the indentation by hand and break long lines. (The conversion programs don't write lines longer than eighty characters in any case.)

Next: Warnings and Errors, Previous: Protoize Caveats, Up: Trouble

### 10.10 Certain Changes We Don't Want to Make

This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.

- Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype.

  Such a feature would work only occasionally—only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.

- Warning about using an expression whose type is signed as a shift count.

  Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.

- Warning about assigning a signed value to an unsigned variable.

  Such assignments must be very common; warning about them would cause more annoyance than good.

- Warning when a non-void function value is ignored.

  C contains many standard functions that return a value that most programs choose to ignore. One obvious example is `printf`. Warning about this practice only leads the defensive programmer to clutter programs with dozens of casts to `void`. Such casts are required so frequently that they become visual noise. Writing those casts becomes so automatic that they no longer convey useful information about the intentions of the programmer. For functions where the return value should never be ignored, use the `warn_unused_result` function attribute (see Function Attributes).

- Making `-fshort-enums` the default.

  This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.

- Making bit-fields unsigned by default on particular machines where “the ABI standard” says to do so.

  The ISO C standard leaves it up to the implementation whether a bit-field declared plain `int` is signed or not. This in effect creates two alternative dialects of C.

  The GNU C compiler supports both dialects; you can specify the signed dialect with `-fsigned-bitfields` and the unsigned dialect with `-funsigned-bitfields`. However, this leaves open the question of which dialect to use by default.

  Currently, the preferred dialect makes plain bit-fields signed, because this is simplest. Since `int` is the same as `signed int` in every other context, it is cleanest for them to be the same in bit-fields as well.

  Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.

  A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect.

  Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.

  Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.

  This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).

  There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future.

  (Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)

- Undefining `__STDC__` when `-ansi` is not used.

  Currently, GCC defines `__STDC__` unconditionally. This provides good results in practice.

  Programmers normally use conditionals on `__STDC__` to ask whether it is safe to use certain features of ISO C, such as function prototypes or ISO token concatenation. Since plain `gcc` supports all the features of ISO C, the correct answer to these questions is “yes”.

  Some users try to use `__STDC__` to check for the availability of certain library facilities. This is actually incorrect usage in an ISO C program, because the ISO C standard says that a conforming freestanding implementation should define `__STDC__` even though it does not have the library facilities. ``gcc -ansi -pedantic`' is a conforming freestanding implementation, and it is therefore required to define `__STDC__`, even though it does not come with an ISO C library.

  Sometimes people say that defining `__STDC__` in a compiler that does not completely conform to the ISO C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ISO C, such as ``gcc -ansi`'—not for other compilers such as plain `gcc`. Whatever the ISO C standard says is relevant to the design of plain `gcc` without `-ansi` only for pragmatic reasons, not as a requirement.

  GCC normally defines `__STDC__` to be 1, and in addition defines `__STRICT_ANSI__` if you specify the `-ansi` option, or a `-std` option for strict conformance to some version of ISO C. On some hosts, system include files use a different convention, where `__STDC__` is normally 0, but is 1 if the user specifies strict conformance to the C Standard. GCC follows the host convention when processing system include files, but when processing user files it follows the usual GNU C convention.

- Undefining `__STDC__` in C++.

  Programs written to compile with C++-to-C translators get the value of `__STDC__` that goes with the C compiler that is subsequently used. These programs must test `__STDC__` to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ISO C fashion or in the traditional fashion.

  These programs work properly with GNU C++ if `__STDC__` is defined. They would not work otherwise.

  In addition, many header files are written to provide prototypes in ISO C but not in traditional C. Many of these header files can work without change in C++ provided `__STDC__` is defined. If `__STDC__` is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well.

- Deleting “empty” loops.

  Historically, GCC has not deleted “empty” loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.

  However, the rationale here is that optimization of a nonempty loop cannot produce an empty one. This held for carefully written C compiled with less powerful optimizers but is not always the case for carefully written C++ or with more powerful optimizers. Thus GCC will remove operations from loops whenever it can determine those operations are not externally visible (apart from the time taken to execute them, of course). In case the loop can be proved to be finite, GCC will also remove the loop itself.

  Be aware of this when performing timing tests, for instance the following loop can be completely removed, provided `some_expression` can provably not change any global state.

                {
                   int sum = 0;
                   int ix;

                   for (ix = 0; ix != 10000; ix++)
                      sum += some_expression;
                }

  Even though `sum` is accumulated in the loop, no use is made of that summation, so the accumulation can be removed.

- Making side effects happen in the same order as in some other compiler.

  It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another:

                void func (int, int);

                int i = 2;
                func (i++, i++);

  There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. `func` might get the arguments ``2, 3`', or it might get ``3, 2`', or even ``2, 2`'.

- Making certain warnings into errors by default.

  Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.

  ISO C requires a “diagnostic” message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If testsuites call this “failure”, they should be run with the GCC option `-pedantic-errors`, which will turn these warnings into errors.

Previous: Non-bugs, Up: Trouble

### 10.11 Warning Messages and Error Messages

The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:

- Errors report problems that make it impossible to compile your program. GCC reports errors with the source file name and line number where the problem is apparent.
- Warnings report other unusual conditions in your code that may indicate a problem, although compilation can (and does) proceed. Warning messages also report the source file name and line number, but include the text ``warning:`' to distinguish them from error messages.

Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the `-W` options (for instance, `-Wall` requests a variety of useful warnings).

GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The `-pedantic` option tells GCC to issue warnings in such cases; `-pedantic-errors` says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors.

See Options to Request or Suppress Warnings, for more detail on these and related command-line options.

Next: Service, Previous: Trouble, Up: Top

## 11 Reporting Bugs

Your bug reports play an essential role in making GCC reliable.

When you encounter a problem, the first thing to do is to see if it is already known. See Trouble. If it isn't known, then you should report the problem.

Next: Bug Reporting, Up: Bugs

### 11.1 Have You Found a Bug?

If you are not sure whether you have found a bug, here are some guidelines:

- If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash.

- If the compiler produces invalid assembly code, for any input whatever (except an `asm` statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run.

- If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug.

  However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.

  For example, in many nonoptimizing compilers, you can write ``x;`' at the end of a function instead of ``return x;`', with the same results. But the value of the function is undefined if `return` is omitted; it is not a bug when GCC produces different results.

  Problems often result from expressions with two increment operators, as in `f (*p++, *p++)`. Your previous compiler might have interpreted that expression the way you intended; GCC might interpret it another way. Neither compiler is wrong. The bug is in your code.

  After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.

- If the compiler produces an error message for valid input, that is a compiler bug.

- If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of “invalid input” might be someone else's idea of “an extension” or “support for traditional practice”.
- If you are an experienced user of one of the languages GCC supports, your suggestions for improvement of GCC are welcome in any case.

Previous: Bug Criteria, Up: Bugs

### 11.2 How and where to Report Bugs

Bugs should be reported to the GCC bug database. Please refer to http://gcc.gnu.org/bugs.html for up-to-date instructions how to submit bug reports. Copies of this file in HTML (`bugs.html`) and plain text (`BUGS`) are also part of GCC releases.

Next: Contributing, Previous: Bugs, Up: Top

## 12 How To Get Help with GCC

If you need help installing, using or changing GCC, there are two ways to find it:

- Send a message to a suitable network mailing list. First try gcc-help@gcc.gnu.org (for help installing or using GCC), and if that brings no response, try gcc@gcc.gnu.org. For help changing GCC, ask gcc@gcc.gnu.org. If you think you have found a bug in GCC, please report it following the instructions at see Bug Reporting.
- Look in the service directory for someone who might help you for a fee. The service directory is found at http://www.gnu.org/prep/service.html.

For further information, see http://gcc.gnu.org/faq.html#support.

Next: Funding, Previous: Service, Up: Top

## 13 Contributing to GCC Development

If you would like to help pretest GCC releases to assure they work well, current development sources are available by CVS (see http://gcc.gnu.org/cvs.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html.

If you would like to work on improvements to GCC, please read the advice at these URLs:

         http://gcc.gnu.org/contribute.html
         http://gcc.gnu.org/contributewhy.html

for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/.

Next: GNU Project, Previous: Contributing, Up: Top

## Funding Free Software

If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.

Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.

The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.

To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don't be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn't give a basis for comparison.

Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.

Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.

By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.

         Copyright © 1994 Free Software Foundation, Inc.
         Verbatim copying and redistribution of this section is permitted
         without royalty; alteration is not permitted.

Next: Copying, Previous: Funding, Up: Top

## The GNU Project and GNU/Linux

The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for “GNU's Not Unix”; it is pronounced “guh-NEW”.) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as “Linux”, they are more accurately called GNU/Linux systems.

For more information, see:

         http://www.gnu.org/
         http://www.gnu.org/gnu/linux-and-gnu.html

Next: GNU Free Documentation License, Previous: GNU Project, Up: Top

## GNU GENERAL PUBLIC LICENSE

Version 2, June 1991

         Copyright © 1989, 1991 Free Software Foundation, Inc.
         51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

         Everyone is permitted to copy and distribute verbatim copies
         of this license document, but changing it is not allowed.

### Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.

Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and modification follow.

TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

0. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The “Program”, below, refers to any such program or work, and a “work based on the Program” means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term “modification”.) Each licensee is addressed as “you”.

   Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.

1. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.

   You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.

2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
   1. You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
   2. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
   3. If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)

   These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.

   Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.

   In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.

3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
   1. Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
   2. Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
   3. Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)

   The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

   If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.

4. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
5. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.
6. Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License.
7. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program.

   If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.

   It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.

   This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

8. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
9. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

   Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and “any later version”, you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.

10. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.
    NO WARRANTY
11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

END OF TERMS AND CONDITIONS

### Appendix: How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

         one line to give the program's name and a brief idea of what it does.
         Copyright (C) year  name of author

         This program is free software; you can redistribute it and/or modify
         it under the terms of the GNU General Public License as published by
         the Free Software Foundation; either version 2 of the License, or
         (at your option) any later version.

         This program is distributed in the hope that it will be useful,
         but WITHOUT ANY WARRANTY; without even the implied warranty of
         MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
         GNU General Public License for more details.

         You should have received a copy of the GNU General Public License
         along with this program; if not, write to the Free Software
         Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode:

         Gnomovision version 69, Copyright (C) year name of author
         Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
         type `show w'.
         This is free software, and you are welcome to redistribute it
         under certain conditions; type `show c' for details.

The hypothetical commands ``show w`' and ``show c`' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than ``show w`' and ``show c`'; they could even be mouse-clicks or menu items—whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:

         Yoyodyne, Inc., hereby disclaims all copyright interest in the program
         `Gnomovision' (which makes passes at compilers) written by James Hacker.

         signature of Ty Coon, 1 April 1989
         Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.

Next: Contributors, Previous: Copying, Up: Top

## GNU Free Documentation License

Version 1.2, November 2002

         Copyright © 2000,2001,2002 Free Software Foundation, Inc.
         51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

         Everyone is permitted to copy and distribute verbatim copies
         of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

   The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

   A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.

   Examples of suitable formats for Transparent copies include plain ascii without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

   The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

   A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.

   The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

### ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

           Copyright (C)  year  your name.
           Permission is granted to copy, distribute and/or modify this document
           under the terms of the GNU Free Documentation License, Version 1.2
           or any later version published by the Free Software Foundation;
           with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
           Texts.  A copy of the license is included in the section entitled ``GNU
           Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

             with the Invariant Sections being list their titles, with
             the Front-Cover Texts being list, and with the Back-Cover Texts
             being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

Next: Option Index, Previous: GNU Free Documentation License, Up: Top

## Contributors to GCC

The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order.

- Analog Devices helped implement the support for complex data types and iterators.
- John David Anglin for threading-related fixes and improvements to libstdc++-v3, and the HP-UX port.
- James van Artsdalen wrote the code that makes efficient use of the Intel 80387 register stack.
- Abramo and Roberto Bagnara for the SysV68 Motorola 3300 Delta Series port.
- Alasdair Baird for various bug fixes.
- Giovanni Bajo for analyzing lots of complicated C++ problem reports.
- Peter Barada for his work to improve code generation for new ColdFire cores.
- Gerald Baumgartner added the signature extension to the C++ front end.
- Godmar Back for his Java improvements and encouragement.
- Scott Bambrough for help porting the Java compiler.
- Wolfgang Bangerth for processing tons of bug reports.
- Jon Beniston for his Microsoft Windows port of Java.
- Daniel Berlin for better DWARF2 support, faster/better optimizations, improved alias analysis, plus migrating GCC to Bugzilla.
- Geoff Berry for his Java object serialization work and various patches.
- Eric Blake for helping to make GCJ and libgcj conform to the specifications.
- Janne Blomqvist for contributions to gfortran.
- Segher Boessenkool for various fixes.
- Hans-J. Boehm for his garbage collector, IA-64 libffi port, and other Java work.
- Neil Booth for work on cpplib, lang hooks, debug hooks and other miscellaneous clean-ups.
- Steven Bosscher for integrating the gfortran front end into GCC and for contributing to the tree-ssa branch.
- Eric Botcazou for fixing middle- and backend bugs left and right.
- Per Bothner for his direction via the steering committee and various improvements to the infrastructure for supporting new languages. Chill front end implementation. Initial implementations of cpplib, fix-header, config.guess, libio, and past C++ library (libg++) maintainer. Dreaming up, designing and implementing much of GCJ.
- Devon Bowen helped port GCC to the Tahoe.
- Don Bowman for mips-vxworks contributions.
- Dave Brolley for work on cpplib and Chill.
- Paul Brook for work on the ARM architecture and maintaining gfortran.
- Robert Brown implemented the support for Encore 32000 systems.
- Christian Bruel for improvements to local store elimination.
- Herman A.J. ten Brugge for various fixes.
- Joerg Brunsmann for Java compiler hacking and help with the GCJ FAQ.
- Joe Buck for his direction via the steering committee.
- Craig Burley for leadership of the G77 Fortran effort.
- Stephan Buys for contributing Doxygen notes for libstdc++.
- Paolo Carlini for libstdc++ work: lots of efficiency improvements to the C++ strings, streambufs and formatted I/O, hard detective work on the frustrating localization issues, and keeping up with the problem reports.
- John Carr for his alias work, SPARC hacking, infrastructure improvements, previous contributions to the steering committee, loop optimizations, etc.
- Stephane Carrez for 68HC11 and 68HC12 ports.
- Steve Chamberlain for support for the Renesas SH and H8 processors and the PicoJava processor, and for GCJ config fixes.
- Glenn Chambers for help with the GCJ FAQ.
- John-Marc Chandonia for various libgcj patches.
- Scott Christley for his Objective-C contributions.
- Eric Christopher for his Java porting help and clean-ups.
- Branko Cibej for more warning contributions.
- The GNU Classpath project for all of their merged runtime code.
- Nick Clifton for arm, mcore, fr30, v850, m32r work, `--help`, and other random hacking.
- Michael Cook for libstdc++ cleanup patches to reduce warnings.
- R. Kelley Cook for making GCC buildable from a read-only directory as well as other miscellaneous build process and documentation clean-ups.
- Ralf Corsepius for SH testing and minor bugfixing.
- Stan Cox for care and feeding of the x86 port and lots of behind the scenes hacking.
- Alex Crain provided changes for the 3b1.
- Ian Dall for major improvements to the NS32k port.
- Paul Dale for his work to add uClinux platform support to the m68k backend.
- Dario Dariol contributed the four varieties of sample programs that print a copy of their source.
- Russell Davidson for fstream and stringstream fixes in libstdc++.
- Bud Davis for work on the G77 and gfortran compilers.
- Mo DeJong for GCJ and libgcj bug fixes.
- DJ Delorie for the DJGPP port, build and libiberty maintenance, various bug fixes, and the M32C port.
- Arnaud Desitter for helping to debug gfortran.
- Gabriel Dos Reis for contributions to G++, contributions and maintenance of GCC diagnostics infrastructure, libstdc++-v3, including `valarray<>`, `complex<>`, maintaining the numerics library (including that pesky `<limits>` :-) and keeping up-to-date anything to do with numbers.
- Ulrich Drepper for his work on glibc, testing of GCC using glibc, ISO C99 support, CFG dumping support, etc., plus support of the C++ runtime libraries including for all kinds of C interface issues, contributing and maintaining `complex<>`, sanity checking and disbursement, configuration architecture, libio maintenance, and early math work.
- Zdenek Dvorak for a new loop unroller and various fixes.
- Richard Earnshaw for his ongoing work with the ARM.
- David Edelsohn for his direction via the steering committee, ongoing work with the RS6000/PowerPC port, help cleaning up Haifa loop changes, doing the entire AIX port of libstdc++ with his bare hands, and for ensuring GCC properly keeps working on AIX.
- Kevin Ediger for the floating point formatting of num_put::do_put in libstdc++.
- Phil Edwards for libstdc++ work including configuration hackery, documentation maintainer, chief breaker of the web pages, the occasional iostream bug fix, and work on shared library symbol versioning.
- Paul Eggert for random hacking all over GCC.
- Mark Elbrecht for various DJGPP improvements, and for libstdc++ configuration support for locales and fstream-related fixes.
- Vadim Egorov for libstdc++ fixes in strings, streambufs, and iostreams.
- Christian Ehrhardt for dealing with bug reports.
- Ben Elliston for his work to move the Objective-C runtime into its own subdirectory and for his work on autoconf.
- Marc Espie for OpenBSD support.
- Doug Evans for much of the global optimization framework, arc, m32r, and SPARC work.
- Christopher Faylor for his work on the Cygwin port and for caring and feeding the gcc.gnu.org box and saving its users tons of spam.
- Fred Fish for BeOS support and Ada fixes.
- Ivan Fontes Garcia for the Portuguese translation of the GCJ FAQ.
- Peter Gerwinski for various bug fixes and the Pascal front end.
- Kaveh R. Ghazi for his direction via the steering committee, amazing work to make ``-W -Wall -W* -Werror`' useful, and continuously testing GCC on a plethora of platforms. Kaveh extends his gratitude to the CAIP Center at Rutgers University for providing him with computing resources to work on Free Software since the late 1980s.
- John Gilmore for a donation to the FSF earmarked improving GNU Java.
- Judy Goldberg for c++ contributions.
- Torbjorn Granlund for various fixes and the c-torture testsuite, multiply- and divide-by-constant optimization, improved long long support, improved leaf function register allocation, and his direction via the steering committee.
- Anthony Green for his `-Os` contributions and Java front end work.
- Stu Grossman for gdb hacking, allowing GCJ developers to debug Java code.
- Michael K. Gschwind contributed the port to the PDP-11.
- Ron Guilmette implemented the `protoize` and `unprotoize` tools, the support for Dwarf symbolic debugging information, and much of the support for System V Release 4. He has also worked heavily on the Intel 386 and 860 support.
- Mostafa Hagog for Swing Modulo Scheduling (SMS) and post reload GCSE.
- Bruno Haible for improvements in the runtime overhead for EH, new warnings and assorted bug fixes.
- Andrew Haley for his amazing Java compiler and library efforts.
- Chris Hanson assisted in making GCC work on HP-UX for the 9000 series 300.
- Michael Hayes for various thankless work he's done trying to get the c30/c40 ports functional. Lots of loop and unroll improvements and fixes.
- Dara Hazeghi for wading through myriads of target-specific bug reports.
- Kate Hedstrom for staking the G77 folks with an initial testsuite.
- Richard Henderson for his ongoing SPARC, alpha, ia32, and ia64 work, loop opts, and generally fixing lots of old problems we've ignored for years, flow rewrite and lots of further stuff, including reviewing tons of patches.
- Aldy Hernandez for working on the PowerPC port, SIMD support, and various fixes.
- Nobuyuki Hikichi of Software Research Associates, Tokyo, contributed the support for the Sony NEWS machine.
- Kazu Hirata for caring and feeding the Renesas H8/300 port and various fixes.
- Katherine Holcomb for work on gfortran.
- Manfred Hollstein for his ongoing work to keep the m88k alive, lots of testing and bug fixing, particularly of GCC configury code.
- Steve Holmgren for MachTen patches.
- Jan Hubicka for his x86 port improvements.
- Falk Hueffner for working on C and optimization bug reports.
- Bernardo Innocenti for his m68k work, including merging of ColdFire improvements and uClinux support.
- Christian Iseli for various bug fixes.
- Kamil Iskra for general m68k hacking.
- Lee Iverson for random fixes and MIPS testing.
- Andreas Jaeger for testing and benchmarking of GCC and various bug fixes.
- Jakub Jelinek for his SPARC work and sibling call optimizations as well as lots of bug fixes and test cases, and for improving the Java build system.
- Janis Johnson for ia64 testing and fixes, her quality improvement sidetracks, and web page maintenance.
- Kean Johnston for SCO OpenServer support and various fixes.
- Tim Josling for the sample language treelang based originally on Richard Kenner's “toy” language.
- Nicolai Josuttis for additional libstdc++ documentation.
- Klaus Kaempf for his ongoing work to make alpha-vms a viable target.
- Steven G. Kargl for work on gfortran.
- David Kashtan of SRI adapted GCC to VMS.
- Ryszard Kabatek for many, many libstdc++ bug fixes and optimizations of strings, especially member functions, and for auto_ptr fixes.
- Geoffrey Keating for his ongoing work to make the PPC work for GNU/Linux and his automatic regression tester.
- Brendan Kehoe for his ongoing work with G++ and for a lot of early work in just about every part of libstdc++.
- Oliver M. Kellogg of Deutsche Aerospace contributed the port to the MIL-STD-1750A.
- Richard Kenner of the New York University Ultracomputer Research Laboratory wrote the machine descriptions for the AMD 29000, the DEC Alpha, the IBM RT PC, and the IBM RS/6000 as well as the support for instruction attributes. He also made changes to better support RISC processors including changes to common subexpression elimination, strength reduction, function calling sequence handling, and condition code support, in addition to generalizing the code for frame pointer elimination and delay slot scheduling. Richard Kenner was also the head maintainer of GCC for several years.
- Mumit Khan for various contributions to the Cygwin and Mingw32 ports and maintaining binary releases for Microsoft Windows hosts, and for massive libstdc++ porting work to Cygwin/Mingw32.
- Robin Kirkham for cpu32 support.
- Mark Klein for PA improvements.
- Thomas Koenig for various bug fixes.
- Bruce Korb for the new and improved fixincludes code.
- Benjamin Kosnik for his G++ work and for leading the libstdc++-v3 effort.
- Charles LaBrec contributed the support for the Integrated Solutions 68020 system.
- Asher Langton and Mike Kumbera for contributing Cray pointer support to gfortran, and for other gfortran improvements.
- Jeff Law for his direction via the steering committee, coordinating the entire egcs project and GCC 2.95, rolling out snapshots and releases, handling merges from GCC2, reviewing tons of patches that might have fallen through the cracks else, and random but extensive hacking.
- Marc Lehmann for his direction via the steering committee and helping with analysis and improvements of x86 performance.
- Victor Leikehman for work on gfortran.
- Ted Lemon wrote parts of the RTL reader and printer.
- Kriang Lerdsuwanakij for C++ improvements including template as template parameter support, and many C++ fixes.
- Warren Levy for tremendous work on libgcj (Java Runtime Library) and random work on the Java front end.
- Alain Lichnewsky ported GCC to the MIPS CPU.
- Oskar Liljeblad for hacking on AWT and his many Java bug reports and patches.
- Robert Lipe for OpenServer support, new testsuites, testing, etc.
- Weiwen Liu for testing and various bug fixes.
- Dave Love for his ongoing work with the Fortran front end and runtime libraries.
- Martin von Löwis for internal consistency checking infrastructure, various C++ improvements including namespace support, and tons of assistance with libstdc++/compiler merges.
- H.J. Lu for his previous contributions to the steering committee, many x86 bug reports, prototype patches, and keeping the GNU/Linux ports working.
- Greg McGary for random fixes and (someday) bounded pointers.
- Andrew MacLeod for his ongoing work in building a real EH system, various code generation improvements, work on the global optimizer, etc.
- Vladimir Makarov for hacking some ugly i960 problems, PowerPC hacking improvements to compile-time performance, overall knowledge and direction in the area of instruction scheduling, and design and implementation of the automaton based instruction scheduler.
- Bob Manson for his behind the scenes work on dejagnu.
- Philip Martin for lots of libstdc++ string and vector iterator fixes and improvements, and string clean up and testsuites.
- All of the Mauve project contributors, for Java test code.
- Bryce McKinlay for numerous GCJ and libgcj fixes and improvements.
- Adam Megacz for his work on the Microsoft Windows port of GCJ.
- Michael Meissner for LRS framework, ia32, m32r, v850, m88k, MIPS, powerpc, haifa, ECOFF debug support, and other assorted hacking.
- Jason Merrill for his direction via the steering committee and leading the G++ effort.
- David Miller for his direction via the steering committee, lots of SPARC work, improvements in jump.c and interfacing with the Linux kernel developers.
- Gary Miller ported GCC to Charles River Data Systems machines.
- Alfred Minarik for libstdc++ string and ios bug fixes, and turning the entire libstdc++ testsuite namespace-compatible.
- Mark Mitchell for his direction via the steering committee, mountains of C++ work, load/store hoisting out of loops, alias analysis improvements, ISO C `restrict` support, and serving as release manager for GCC 3.x.
- Alan Modra for various GNU/Linux bits and testing.
- Toon Moene for his direction via the steering committee, Fortran maintenance, and his ongoing work to make us make Fortran run fast.
- Jason Molenda for major help in the care and feeding of all the services on the gcc.gnu.org (formerly egcs.cygnus.com) machine—mail, web services, ftp services, etc etc. Doing all this work on scrap paper and the backs of envelopes would have been... difficult.
- Catherine Moore for fixing various ugly problems we have sent her way, including the haifa bug which was killing the Alpha & PowerPC Linux kernels.
- Mike Moreton for his various Java patches.
- David Mosberger-Tang for various Alpha improvements, and for the initial IA-64 port.
- Stephen Moshier contributed the floating point emulator that assists in cross-compilation and permits support for floating point numbers wider than 64 bits and for ISO C99 support.
- Bill Moyer for his behind the scenes work on various issues.
- Philippe De Muyter for his work on the m68k port.
- Joseph S. Myers for his work on the PDP-11 port, format checking and ISO C99 support, and continuous emphasis on (and contributions to) documentation.
- Nathan Myers for his work on libstdc++-v3: architecture and authorship through the first three snapshots, including implementation of locale infrastructure, string, shadow C headers, and the initial project documentation (DESIGN, CHECKLIST, and so forth). Later, more work on MT-safe string and shadow headers.
- Felix Natter for documentation on porting libstdc++.
- Nathanael Nerode for cleaning up the configuration/build process.
- NeXT, Inc. donated the front end that supports the Objective-C language.
- Hans-Peter Nilsson for the CRIS and MMIX ports, improvements to the search engine setup, various documentation fixes and other small fixes.
- Geoff Noer for his work on getting cygwin native builds working.
- Diego Novillo for his SPEC performance tracking web pages and assorted fixes in the middle end and various back ends.
- David O'Brien for the FreeBSD/alpha, FreeBSD/AMD x86-64, FreeBSD/ARM, FreeBSD/PowerPC, and FreeBSD/SPARC64 ports and related infrastructure improvements.
- Alexandre Oliva for various build infrastructure improvements, scripts and amazing testing work, including keeping libtool issues sane and happy.
- Stefan Olsson for work on mt_alloc.
- Melissa O'Neill for various NeXT fixes.
- Rainer Orth for random MIPS work, including improvements to GCC's o32 ABI support, improvements to dejagnu's MIPS support, Java configuration clean-ups and porting work, etc.
- Hartmut Penner for work on the s390 port.
- Paul Petersen wrote the machine description for the Alliant FX/8.
- Alexandre Petit-Bianco for implementing much of the Java compiler and continued Java maintainership.
- Matthias Pfaller for major improvements to the NS32k port.
- Gerald Pfeifer for his direction via the steering committee, pointing out lots of problems we need to solve, maintenance of the web pages, and taking care of documentation maintenance in general.
- Andrew Pinski for processing bug reports by the dozen.
- Ovidiu Predescu for his work on the Objective-C front end and runtime libraries.
- Jerry Quinn for major performance improvements in C++ formatted I/O.
- Ken Raeburn for various improvements to checker, MIPS ports and various cleanups in the compiler.
- Rolf W. Rasmussen for hacking on AWT.
- David Reese of Sun Microsystems contributed to the Solaris on PowerPC port.
- Volker Reichelt for keeping up with the problem reports.
- Joern Rennecke for maintaining the sh port, loop, regmove & reload hacking.
- Loren J. Rittle for improvements to libstdc++-v3 including the FreeBSD port, threading fixes, thread-related configury changes, critical threading documentation, and solutions to really tricky I/O problems, as well as keeping GCC properly working on FreeBSD and continuous testing.
- Craig Rodrigues for processing tons of bug reports.
- Ola Rönnerup for work on mt_alloc.
- Gavin Romig-Koch for lots of behind the scenes MIPS work.
- David Ronis inspired and encouraged Craig to rewrite the G77 documentation in texinfo format by contributing a first pass at a translation of the old `g77-0.5.16/f/DOC` file.
- Ken Rose for fixes to GCC's delay slot filling code.
- Paul Rubin wrote most of the preprocessor.
- Pétur Runólfsson for major performance improvements in C++ formatted I/O and large file support in C++ filebuf.
- Chip Salzenberg for libstdc++ patches and improvements to locales, traits, Makefiles, libio, libtool hackery, and “long long” support.
- Juha Sarlin for improvements to the H8 code generator.
- Greg Satz assisted in making GCC work on HP-UX for the 9000 series 300.
- Roger Sayle for improvements to constant folding and GCC's RTL optimizers as well as for fixing numerous bugs.
- Bradley Schatz for his work on the GCJ FAQ.
- Peter Schauer wrote the code to allow debugging to work on the Alpha.
- William Schelter did most of the work on the Intel 80386 support.
- Tobias Schlüter for work on gfortran.
- Bernd Schmidt for various code generation improvements and major work in the reload pass as well a serving as release manager for GCC 2.95.3.
- Peter Schmid for constant testing of libstdc++—especially application testing, going above and beyond what was requested for the release criteria—and libstdc++ header file tweaks.
- Jason Schroeder for jcf-dump patches.
- Andreas Schwab for his work on the m68k port.
- Lars Segerlund for work on gfortran.
- Joel Sherrill for his direction via the steering committee, RTEMS contributions and RTEMS testing.
- Nathan Sidwell for many C++ fixes/improvements.
- Jeffrey Siegal for helping RMS with the original design of GCC, some code which handles the parse tree and RTL data structures, constant folding and help with the original VAX & m68k ports.
- Kenny Simpson for prompting libstdc++ fixes due to defect reports from the LWG (thereby keeping GCC in line with updates from the ISO).
- Franz Sirl for his ongoing work with making the PPC port stable for GNU/Linux.
- Andrey Slepuhin for assorted AIX hacking.
- Christopher Smith did the port for Convex machines.
- Danny Smith for his major efforts on the Mingw (and Cygwin) ports.
- Randy Smith finished the Sun FPA support.
- Scott Snyder for queue, iterator, istream, and string fixes and libstdc++ testsuite entries. Also for providing the patch to G77 to add rudimentary support for `INTEGER*1`, `INTEGER*2`, and `LOGICAL*1`.
- Brad Spencer for contributions to the GLIBCPP_FORCE_NEW technique.
- Richard Stallman, for writing the original GCC and launching the GNU project.
- Jan Stein of the Chalmers Computer Society provided support for Genix, as well as part of the 32000 machine description.
- Nigel Stephens for various mips16 related fixes/improvements.
- Jonathan Stone wrote the machine description for the Pyramid computer.
- Graham Stott for various infrastructure improvements.
- John Stracke for his Java HTTP protocol fixes.
- Mike Stump for his Elxsi port, G++ contributions over the years and more recently his vxworks contributions
- Jeff Sturm for Java porting help, bug fixes, and encouragement.
- Shigeya Suzuki for this fixes for the bsdi platforms.
- Ian Lance Taylor for his mips16 work, general configury hacking, fixincludes, etc.
- Holger Teutsch provided the support for the Clipper CPU.
- Gary Thomas for his ongoing work to make the PPC work for GNU/Linux.
- Philipp Thomas for random bug fixes throughout the compiler
- Jason Thorpe for thread support in libstdc++ on NetBSD.
- Kresten Krab Thorup wrote the run time support for the Objective-C language and the fantastic Java bytecode interpreter.
- Michael Tiemann for random bug fixes, the first instruction scheduler, initial C++ support, function integration, NS32k, SPARC and M88k machine description work, delay slot scheduling.
- Andreas Tobler for his work porting libgcj to Darwin.
- Teemu Torma for thread safe exception handling support.
- Leonard Tower wrote parts of the parser, RTL generator, and RTL definitions, and of the VAX machine description.
- Tom Tromey for internationalization support and for his many Java contributions and libgcj maintainership.
- Lassi Tuura for improvements to config.guess to determine HP processor types.
- Petter Urkedal for libstdc++ CXXFLAGS, math, and algorithms fixes.
- Andy Vaught for the design and initial implementation of the gfortran front end.
- Brent Verner for work with the libstdc++ cshadow files and their associated configure steps.
- Todd Vierling for contributions for NetBSD ports.
- Jonathan Wakely for contributing libstdc++ Doxygen notes and XHTML guidance.
- Dean Wakerley for converting the install documentation from HTML to texinfo in time for GCC 3.0.
- Krister Walfridsson for random bug fixes.
- Feng Wang for contributions to gfortran.
- Stephen M. Webb for time and effort on making libstdc++ shadow files work with the tricky Solaris 8+ headers, and for pushing the build-time header tree.
- John Wehle for various improvements for the x86 code generator, related infrastructure improvements to help x86 code generation, value range propagation and other work, WE32k port.
- Ulrich Weigand for work on the s390 port.
- Zack Weinberg for major work on cpplib and various other bug fixes.
- Matt Welsh for help with Linux Threads support in GCJ.
- Urban Widmark for help fixing java.io.
- Mark Wielaard for new Java library code and his work integrating with Classpath.
- Dale Wiles helped port GCC to the Tahoe.
- Bob Wilson from Tensilica, Inc. for the Xtensa port.
- Jim Wilson for his direction via the steering committee, tackling hard problems in various places that nobody else wanted to work on, strength reduction and other loop optimizations.
- Paul Woegerer and Tal Agmon for the CRX port.
- Carlo Wood for various fixes.
- Tom Wood for work on the m88k port.
- Canqun Yang for work on gfortran.
- Masanobu Yuhara of Fujitsu Laboratories implemented the machine description for the Tron architecture (specifically, the Gmicro).
- Kevin Zachmann helped port GCC to the Tahoe.
- Ayal Zaks for Swing Modulo Scheduling (SMS).
- Xiaoqiang Zhang for work on gfortran.
- Gilles Zunino for help porting Java to Irix.

The following people are recognized for their contributions to GNAT, the Ada front end of GCC:

- Bernard Banner
- Romain Berrendonner
- Geert Bosch
- Emmanuel Briot
- Joel Brobecker
- Ben Brosgol
- Vincent Celier
- Arnaud Charlet
- Chien Chieng
- Cyrille Comar
- Cyrille Crozes
- Robert Dewar
- Gary Dismukes
- Robert Duff
- Ed Falis
- Ramon Fernandez
- Sam Figueroa
- Vasiliy Fofanov
- Michael Friess
- Franco Gasperoni
- Ted Giering
- Matthew Gingell
- Laurent Guerby
- Jerome Guitton
- Olivier Hainque
- Jerome Hugues
- Hristian Kirtchev
- Jerome Lambourg
- Bruno Leclerc
- Albert Lee
- Sean McNeil
- Javier Miranda
- Laurent Nana
- Pascal Obry
- Dong-Ik Oh
- Laurent Pautet
- Brett Porter
- Thomas Quinot
- Nicolas Roche
- Pat Rogers
- Jose Ruiz
- Douglas Rupp
- Sergey Rybin
- Gail Schenker
- Ed Schonberg
- Nicolas Setton
- Samuel Tardieu

The following people are recognized for their contributions of new features, bug reports, testing and integration of classpath/libgcj for GCC version 4.1:

- Lillian Angel for `JTree` implementation and lots Free Swing additions and bugfixes.
- Wolfgang Baer for `GapContent` bugfixes.
- Anthony Balkissoon for `JList`, Free Swing 1.5 updates and mouse event fixes, lots of Free Swing work including `JTable` editing.
- Stuart Ballard for RMI constant fixes.
- Goffredo Baroncelli for `HTTPURLConnection` fixes.
- Gary Benson for `MessageFormat` fixes.
- Daniel Bonniot for `Serialization` fixes.
- Chris Burdess for lots of gnu.xml and http protocol fixes, `StAX` and `DOM xml:id` support.
- Ka-Hing Cheung for `TreePath` and `TreeSelection` fixes.
- Archie Cobbs for build fixes, VM interface updates, `URLClassLoader` updates.
- Kelley Cook for build fixes.
- Martin Cordova for Suggestions for better `SocketTimeoutException`.
- David Daney for `BitSet` bugfixes, `HttpURLConnection` rewrite and improvements.
- Thomas Fitzsimmons for lots of upgrades to the gtk+ AWT and Cairo 2D support. Lots of imageio framework additions, lots of AWT and Free Swing bugfixes.
- Jeroen Frijters for `ClassLoader` and nio cleanups, serialization fixes, better `Proxy` support, bugfixes and IKVM integration.
- Santiago Gala for `AccessControlContext` fixes.
- Nicolas Geoffray for `VMClassLoader` and `AccessController` improvements.
- David Gilbert for `basic` and `metal` icon and plaf support and lots of documenting, Lots of Free Swing and metal theme additions. `MetalIconFactory` implementation.
- Anthony Green for `MIDI` framework, `ALSA` and `DSSI` providers.
- Andrew Haley for `Serialization` and `URLClassLoader` fixes, gcj build speedups.
- Kim Ho for `JFileChooser` implementation.
- Andrew John Hughes for `Locale` and net fixes, URI RFC2986 updates, `Serialization` fixes, `Properties` XML support and generic branch work, VMIntegration guide update.
- Bastiaan Huisman for `TimeZone` bugfixing.
- Andreas Jaeger for mprec updates.
- Paul Jenner for better `-Werror` support.
- Ito Kazumitsu for `NetworkInterface` implementation and updates.
- Roman Kennke for `BoxLayout`, `GrayFilter` and `SplitPane`, plus bugfixes all over. Lots of Free Swing work including styled text.
- Simon Kitching for `String` cleanups and optimization suggestions.
- Michael Koch for configuration fixes, `Locale` updates, bug and build fixes.
- Guilhem Lavaux for configuration, thread and channel fixes and Kaffe integration. JCL native `Pointer` updates. Logger bugfixes.
- David Lichteblau for JCL support library global/local reference cleanups.
- Aaron Luchko for JDWP updates and documentation fixes.
- Ziga Mahkovec for `Graphics2D` upgraded to Cairo 0.5 and new regex features.
- Sven de Marothy for BMP imageio support, CSS and `TextLayout` fixes. `GtkImage` rewrite, 2D, awt, free swing and date/time fixes and implementing the Qt4 peers.
- Casey Marshall for crypto algorithm fixes, `FileChannel` lock, `SystemLogger` and `FileHandler` rotate implementations, NIO `FileChannel.map` support, security and policy updates.
- Bryce McKinlay for RMI work.
- Audrius Meskauskas for lots of Free Corba, RMI and HTML work plus testing and documenting.
- Kalle Olavi Niemitalo for build fixes.
- Rainer Orth for build fixes.
- Andrew Overholt for `File` locking fixes.
- Ingo Proetel for `Image`, `Logger` and `URLClassLoader` updates.
- Olga Rodimina for `MenuSelectionManager` implemenation.
- Jan Roehrich for `BasicTreeUI` and `JTree` fixes.
- Julian Scheid for documentation updates and gjdoc support.
- Christian Schlichtherle for zip fixes and cleanups.
- Robert Schuster for documentation updates and beans fixes, `TreeNode` enumerations and `ActionCommand` and various fixes, XML and URL, AWT and Free Swing bugfixes.
- Keith Seitz for lots of JDWP work.
- Christian Thalinger for 64-bit cleanups, Configuration and VM interface fixes and `CACAO` integration, `fdlibm` updates.
- Gael Thomas for `VMClassLoader` boot packages support sugestions.
- Andreas Tobler for Darwin and Solaris testing and fixing, `Qt4` support for Darwin/OS X, `Graphics2D` support, `gtk+` updates.
- Dalibor Topic for better `DEBUG` support, build cleanups and Kaffe integration. `Qt4` build infrastructure, `SHA1PRNG` and `GdkPixbugDecoder` updates.
- Tom Tromey for Eclipse integration, generics work, lots of bugfixes and gcj integration including coordinating The Big Merge.
- Mark Wielaard for bugfixes, packaging and release management, `Clipboard` implementation, system call interrupts and network timeouts and `GdkPixpufDecoder` fixes.

In addition to the above, all of which also contributed time and energy in testing GCC, we would like to thank the following for their contributions to testing:

- Michael Abd-El-Malek
- Thomas Arend
- Bonzo Armstrong
- Steven Ashe
- Chris Baldwin
- David Billinghurst
- Jim Blandy
- Stephane Bortzmeyer
- Horst von Brand
- Frank Braun
- Rodney Brown
- Sidney Cadot
- Bradford Castalia
- Jonathan Corbet
- Ralph Doncaster
- Richard Emberson
- Levente Farkas
- Graham Fawcett
- Mark Fernyhough
- Robert A. French
- Jörgen Freyh
- Mark K. Gardner
- Charles-Antoine Gauthier
- Yung Shing Gene
- David Gilbert
- Simon Gornall
- Fred Gray
- John Griffin
- Patrik Hagglund
- Phil Hargett
- Amancio Hasty
- Takafumi Hayashi
- Bryan W. Headley
- Kevin B. Hendricks
- Joep Jansen
- Christian Joensson
- Michel Kern
- David Kidd
- Tobias Kuipers
- Anand Krishnaswamy
- A. O. V. Le Blanc
- llewelly
- Damon Love
- Brad Lucier
- Matthias Klose
- Martin Knoblauch
- Rick Lutowski
- Jesse Macnish
- Stefan Morrell
- Anon A. Mous
- Matthias Mueller
- Pekka Nikander
- Rick Niles
- Jon Olson
- Magnus Persson
- Chris Pollard
- Richard Polton
- Derk Reefman
- David Rees
- Paul Reilly
- Tom Reilly
- Torsten Rueger
- Danny Sadinoff
- Marc Schifer
- Erik Schnetter
- Wayne K. Schroll
- David Schuler
- Vin Shelton
- Tim Souder
- Adam Sulmicki
- Bill Thorson
- George Talbot
- Pedro A. M. Vazquez
- Gregory Warnes
- Ian Watson
- David E. Young
- And many others

And finally we'd like to thank everyone who uses the compiler, submits bug reports and generally reminds us why we're doing this work in the first place.

Next: Keyword Index, Previous: Contributors, Up: Top

## Option Index

GCC's command line options are indexed here without any initial ``-`' or ``--`'. Where an option has both positive and negative forms (such as `-foption` and `-fno-option`), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.

- `###`: Overall Options
- `-dynamiclib`: Darwin Options
- `-force_cpusubtype_ALL`: Darwin Options
- `-fsplit-ivs-in-unroller`: Optimize Options
- `-fvariable-expansion-in-unroller`: Optimize Options
- `-gfull`: Darwin Options
- `-gused`: Darwin Options
- `-mone-byte-bool`: Darwin Options
- `A`: Preprocessor Options
- `all_load`: Darwin Options
- `allowable_client`: Darwin Options
- `ansi`: Non-bugs
- `ansi`: Other Builtins
- `ansi`: Preprocessor Options
- `ansi`: C Dialect Options
- `ansi`: Standards
- `arch_errors_fatal`: Darwin Options
- `aux-info`: C Dialect Options
- `b`: Target Options
- `B`: Directory Options
- `bcopy-builtin`: PDP-11 Options
- `bind_at_load`: Darwin Options
- `bundle`: Darwin Options
- `bundle_loader`: Darwin Options
- `c`: Link Options
- `C`: Preprocessor Options
- `c`: Overall Options
- `client_name`: Darwin Options
- `combine`: Overall Options
- `compatibility_version`: Darwin Options
- `coverage`: Debugging Options
- `crossjumping`: Optimize Options
- `current_version`: Darwin Options
- `D`: Preprocessor Options
- `d`: Debugging Options
- `da`: Debugging Options
- `dA`: Debugging Options
- `dB`: Debugging Options
- `db`: Debugging Options
- `dC`: Debugging Options
- `dc`: Debugging Options
- `dD`: Preprocessor Options
- `dD`: Debugging Options
- `dd`: Debugging Options
- `dE`: Debugging Options
- `dead_strip`: Darwin Options
- `dependency-file`: Darwin Options
- `df`: Debugging Options
- `dG`: Debugging Options
- `dg`: Debugging Options
- `dH`: Debugging Options
- `dh`: Debugging Options
- `dI`: Preprocessor Options
- `di`: Debugging Options
- `dj`: Debugging Options
- `dk`: Debugging Options
- `dL`: Debugging Options
- `dl`: Debugging Options
- `dM`: Preprocessor Options
- `dm`: Debugging Options
- `dM`: Debugging Options
- `dm`: Debugging Options
- `dN`: Preprocessor Options
- `dN`: Debugging Options
- `dn`: Debugging Options
- `do`: Debugging Options
- `dP`: Debugging Options
- `dp`: Debugging Options
- `dR`: Debugging Options
- `dr`: Debugging Options
- `dS`: Debugging Options
- `ds`: Debugging Options
- `dT`: Debugging Options
- `dt`: Debugging Options
- `dumpmachine`: Debugging Options
- `dumpspecs`: Debugging Options
- `dumpversion`: Debugging Options
- `dv`: Debugging Options
- `dV`: Debugging Options
- `dw`: Debugging Options
- `dx`: Debugging Options
- `dy`: Debugging Options
- `dylib_file`: Darwin Options
- `dylinker_install_name`: Darwin Options
- `dynamic`: Darwin Options
- `dZ`: Debugging Options
- `dz`: Debugging Options
- `E`: Link Options
- `E`: Overall Options
- `EB`: MIPS Options
- `EB`: ARC Options
- `EL`: MIPS Options
- `EL`: ARC Options
- `exported_symbols_list`: Darwin Options
- `F`: Darwin Options
- `fabi-version`: C++ Dialect Options
- `falign-functions`: Optimize Options
- `falign-jumps`: Optimize Options
- `falign-labels`: Optimize Options
- `falign-loops`: Optimize Options
- `fargument-alias`: Code Gen Options
- `fargument-noalias`: Code Gen Options
- `fargument-noalias-global`: Code Gen Options
- `fasynchronous-unwind-tables`: Code Gen Options
- `fbounds-check`: Code Gen Options
- `fbounds-check`: Optimize Options
- `fbranch-probabilities`: Optimize Options
- `fbranch-target-load-optimize`: Optimize Options
- `fbranch-target-load-optimize2`: Optimize Options
- `fbtr-bb-exclusive`: Optimize Options
- `fcall-saved`: Interoperation
- `fcall-saved`: Code Gen Options
- `fcall-used`: Code Gen Options
- `fcaller-saves`: Optimize Options
- `fcheck-new`: C++ Dialect Options
- `fcommon`: Variable Attributes
- `fcond-mismatch`: C Dialect Options
- `fconserve-space`: C++ Dialect Options
- `fconstant-string-class`: Objective-C and Objective-C++ Dialect Options
- `fcse-follow-jumps`: Optimize Options
- `fcse-skip-blocks`: Optimize Options
- `fcx-limited-range`: Optimize Options
- `fdata-sections`: Optimize Options
- `fdelayed-branch`: Optimize Options
- `fdelete-null-pointer-checks`: Optimize Options
- `fdiagnostics-show-location`: Language Independent Options
- `fdiagnostics-show-options`: Language Independent Options
- `fdollars-in-identifiers`: Interoperation
- `fdollars-in-identifiers`: Preprocessor Options
- `fdump-class-hierarchy`: Debugging Options
- `fdump-ipa`: Debugging Options
- `fdump-rtl-all`: Debugging Options
- `fdump-rtl-bbro`: Debugging Options
- `fdump-rtl-bp`: Debugging Options
- `fdump-rtl-btl`: Debugging Options
- `fdump-rtl-bypass`: Debugging Options
- `fdump-rtl-ce1`: Debugging Options
- `fdump-rtl-ce2`: Debugging Options
- `fdump-rtl-ce3`: Debugging Options
- `fdump-rtl-cfg`: Debugging Options
- `fdump-rtl-combine`: Debugging Options
- `fdump-rtl-cse`: Debugging Options
- `fdump-rtl-cse2`: Debugging Options
- `fdump-rtl-dbr`: Debugging Options
- `fdump-rtl-eh`: Debugging Options
- `fdump-rtl-expand`: Debugging Options
- `fdump-rtl-flow2`: Debugging Options
- `fdump-rtl-gcse`: Debugging Options
- `fdump-rtl-greg`: Debugging Options
- `fdump-rtl-jump`: Debugging Options
- `fdump-rtl-life`: Debugging Options
- `fdump-rtl-loop`: Debugging Options
- `fdump-rtl-loop2`: Debugging Options
- `fdump-rtl-lreg`: Debugging Options
- `fdump-rtl-mach`: Debugging Options
- `fdump-rtl-peephole2`: Debugging Options
- `fdump-rtl-postreload`: Debugging Options
- `fdump-rtl-regmove`: Debugging Options
- `fdump-rtl-rnreg`: Debugging Options
- `fdump-rtl-sched`: Debugging Options
- `fdump-rtl-sched2`: Debugging Options
- `fdump-rtl-sibling`: Debugging Options
- `fdump-rtl-sms`: Debugging Options
- `fdump-rtl-stack`: Debugging Options
- `fdump-rtl-tracer`: Debugging Options
- `fdump-rtl-vartrack`: Debugging Options
- `fdump-rtl-vpt`: Debugging Options
- `fdump-rtl-web`: Debugging Options
- `fdump-translation-unit`: Debugging Options
- `fdump-tree`: Debugging Options
- `fdump-tree-alias`: Debugging Options
- `fdump-tree-all`: Debugging Options
- `fdump-tree-ccp`: Debugging Options
- `fdump-tree-cfg`: Debugging Options
- `fdump-tree-ch`: Debugging Options
- `fdump-tree-copyprop`: Debugging Options
- `fdump-tree-copyrename`: Debugging Options
- `fdump-tree-dce`: Debugging Options
- `fdump-tree-dom`: Debugging Options
- `fdump-tree-dse`: Debugging Options
- `fdump-tree-forwprop`: Debugging Options
- `fdump-tree-fre`: Debugging Options
- `fdump-tree-gimple`: Debugging Options
- `fdump-tree-mudflap`: Debugging Options
- `fdump-tree-nrv`: Debugging Options
- `fdump-tree-phiopt`: Debugging Options
- `fdump-tree-pre`: Debugging Options
- `fdump-tree-salias`: Debugging Options
- `fdump-tree-sink`: Debugging Options
- `fdump-tree-sra`: Debugging Options
- `fdump-tree-ssa`: Debugging Options
- `fdump-tree-store_copyprop`: Debugging Options
- `fdump-tree-storeccp`: Debugging Options
- `fdump-tree-vcg`: Debugging Options
- `fdump-tree-vect`: Debugging Options
- `fdump-tree-vrp`: Debugging Options
- `fdump-unnumbered`: Debugging Options
- `fearly-inlining`: Optimize Options
- `feliminate-dwarf2-dups`: Debugging Options
- `feliminate-unused-debug-symbols`: Debugging Options
- `feliminate-unused-debug-types`: Debugging Options
- `fexceptions`: Code Gen Options
- `fexec-charset`: Preprocessor Options
- `fexpensive-optimizations`: Optimize Options
- `fextended-identifiers`: Preprocessor Options
- `ffast-math`: Optimize Options
- `ffinite-math-only`: Optimize Options
- `ffix-and-continue`: Darwin Options
- `ffixed`: Code Gen Options
- `ffloat-store`: Disappointments
- `ffloat-store`: Optimize Options
- `ffor-scope`: C++ Dialect Options
- `fforce-addr`: Optimize Options
- `fforce-mem`: Optimize Options
- `ffreestanding`: Function Attributes
- `ffreestanding`: Warning Options
- `ffreestanding`: C Dialect Options
- `ffreestanding`: Standards
- `ffriend-injection`: C++ Dialect Options
- `ffunction-sections`: Optimize Options
- `fgcse`: Optimize Options
- `fgcse-after-reload`: Optimize Options
- `fgcse-las`: Optimize Options
- `fgcse-lm`: Optimize Options
- `fgcse-sm`: Optimize Options
- `fgnu-runtime`: Objective-C and Objective-C++ Dialect Options
- `fhosted`: C Dialect Options
- `filelist`: Darwin Options
- `findirect-data`: Darwin Options
- `finhibit-size-directive`: Code Gen Options
- `finline-functions`: Optimize Options
- `finline-functions-called-once`: Optimize Options
- `finline-limit`: Optimize Options
- `finput-charset`: Preprocessor Options
- `finstrument-functions`: Function Attributes
- `finstrument-functions`: Code Gen Options
- `fkeep-inline-functions`: Inline
- `fkeep-inline-functions`: Optimize Options
- `fkeep-static-consts`: Optimize Options
- `flat_namespace`: Darwin Options
- `fleading-underscore`: Code Gen Options
- `floop-optimize`: Optimize Options
- `floop-optimize2`: Optimize Options
- `fmem-report`: Debugging Options
- `fmessage-length`: Language Independent Options
- `fmodulo-sched`: Optimize Options
- `fmove-loop-invariants`: Optimize Options
- `fms-extensions`: Unnamed Fields
- `fms-extensions`: C++ Dialect Options
- `fms-extensions`: C Dialect Options
- `fmudflap`: Optimize Options
- `fmudflapir`: Optimize Options
- `fmudflapth`: Optimize Options
- `fnext-runtime`: Objective-C and Objective-C++ Dialect Options
- `fno-access-control`: C++ Dialect Options
- `fno-asm`: C Dialect Options
- `fno-branch-count-reg`: Optimize Options
- `fno-builtin`: Other Builtins
- `fno-builtin`: Function Attributes
- `fno-builtin`: Warning Options
- `fno-builtin`: C Dialect Options
- `fno-common`: Variable Attributes
- `fno-common`: Code Gen Options
- `fno-const-strings`: C++ Dialect Options
- `fno-cprop-registers`: Optimize Options
- `fno-cx-limited-range`: Optimize Options
- `fno-default-inline`: Inline
- `fno-default-inline`: Optimize Options
- `fno-default-inline`: C++ Dialect Options
- `fno-defer-pop`: Optimize Options
- `fno-elide-constructors`: C++ Dialect Options
- `fno-enforce-eh-specs`: C++ Dialect Options
- `fno-for-scope`: C++ Dialect Options
- `fno-function-cse`: Optimize Options
- `fno-gnu-keywords`: C++ Dialect Options
- `fno-guess-branch-probability`: Optimize Options
- `fno-ident`: Code Gen Options
- `fno-implement-inlines`: C++ Interface
- `fno-implement-inlines`: C++ Dialect Options
- `fno-implicit-inline-templates`: C++ Dialect Options
- `fno-implicit-templates`: Template Instantiation
- `fno-implicit-templates`: C++ Dialect Options
- `fno-inline`: Optimize Options
- `fno-jump-tables`: Code Gen Options
- `fno-math-errno`: Optimize Options
- `fno-nil-receivers`: Objective-C and Objective-C++ Dialect Options
- `fno-nonansi-builtins`: C++ Dialect Options
- `fno-operator-names`: C++ Dialect Options
- `fno-optional-diags`: C++ Dialect Options
- `fno-peephole`: Optimize Options
- `fno-peephole2`: Optimize Options
- `fno-rtti`: C++ Dialect Options
- `fno-sched-interblock`: Optimize Options
- `fno-sched-spec`: Optimize Options
- `fno-show-column`: Preprocessor Options
- `fno-signed-bitfields`: C Dialect Options
- `fno-stack-limit`: Code Gen Options
- `fno-threadsafe-statics`: C++ Dialect Options
- `fno-trapping-math`: Optimize Options
- `fno-unsigned-bitfields`: C Dialect Options
- `fno-weak`: C++ Dialect Options
- `fno-working-directory`: Preprocessor Options
- `fno-zero-initialized-in-bss`: Optimize Options
- `fnon-call-exceptions`: Code Gen Options
- `fobjc-call-cxx-cdtors`: Objective-C and Objective-C++ Dialect Options
- `fobjc-direct-dispatch`: Objective-C and Objective-C++ Dialect Options
- `fobjc-exceptions`: Objective-C and Objective-C++ Dialect Options
- `fobjc-gc`: Objective-C and Objective-C++ Dialect Options
- `fomit-frame-pointer`: Optimize Options
- `foptimize-register-move`: Optimize Options
- `foptimize-sibling-calls`: Optimize Options
- `force_flat_namespace`: Darwin Options
- `fpack-struct`: Code Gen Options
- `fpcc-struct-return`: Incompatibilities
- `fpcc-struct-return`: Code Gen Options
- `fpch-deps`: Preprocessor Options
- `fpch-preprocess`: Preprocessor Options
- `fpeel-loops`: Optimize Options
- `fpermissive`: C++ Dialect Options
- `fPIC`: Code Gen Options
- `fpic`: Code Gen Options
- `fPIE`: Code Gen Options
- `fpie`: Code Gen Options
- `fprefetch-loop-arrays`: Optimize Options
- `fpreprocessed`: Preprocessor Options
- `fprofile-arcs`: Other Builtins
- `fprofile-arcs`: Debugging Options
- `fprofile-generate`: Optimize Options
- `fprofile-use`: Optimize Options
- `fprofile-values`: Optimize Options
- `frandom-string`: Debugging Options
- `freg-struct-return`: Code Gen Options
- `fregmove`: Optimize Options
- `frename-registers`: Optimize Options
- `freorder-blocks`: Optimize Options
- `freorder-blocks-and-partition`: Optimize Options
- `freorder-functions`: Optimize Options
- `freplace-objc-classes`: Objective-C and Objective-C++ Dialect Options
- `frepo`: Template Instantiation
- `frepo`: C++ Dialect Options
- `frerun-cse-after-loop`: Optimize Options
- `frerun-loop-opt`: Optimize Options
- `frounding-math`: Optimize Options
- `fsched-spec-load`: Optimize Options
- `fsched-spec-load-dangerous`: Optimize Options
- `fsched-stalled-insns`: Optimize Options
- `fsched-stalled-insns-dep`: Optimize Options
- `fsched-verbose`: Debugging Options
- `fsched2-use-superblocks`: Optimize Options
- `fsched2-use-traces`: Optimize Options
- `fschedule-insns`: Optimize Options
- `fschedule-insns2`: Optimize Options
- `fscheduling-in-modulo-scheduled-loops`: Optimize Options
- `fshared-data`: Code Gen Options
- `fshort-double`: Code Gen Options
- `fshort-enums`: Non-bugs
- `fshort-enums`: Type Attributes
- `fshort-enums`: Structures unions enumerations and bit-fields implementation
- `fshort-enums`: Code Gen Options
- `fshort-wchar`: Code Gen Options
- `fsignaling-nans`: Optimize Options
- `fsigned-bitfields`: Non-bugs
- `fsigned-bitfields`: C Dialect Options
- `fsigned-char`: Characters implementation
- `fsigned-char`: C Dialect Options
- `fsingle-precision-constant`: Optimize Options
- `fstack-check`: Code Gen Options
- `fstack-limit-register`: Code Gen Options
- `fstack-limit-symbol`: Code Gen Options
- `fstats`: C++ Dialect Options
- `fstrength-reduce`: Optimize Options
- `fstrict-aliasing`: Optimize Options
- `fsyntax-only`: Warning Options
- `ftabstop`: Preprocessor Options
- `ftemplate-depth`: C++ Dialect Options
- `ftest-coverage`: Debugging Options
- `fthread-jumps`: Optimize Options
- `ftime-report`: Debugging Options
- `ftracer`: Optimize Options
- `ftrapv`: Code Gen Options
- `ftree-vect-loop-version`: Optimize Options
- `ftree-vectorizer-verbose`: Debugging Options
- `funit-at-a-time`: Optimize Options
- `funroll-all-loops`: Optimize Options
- `funroll-loops`: Optimize Options
- `funsafe-loop-optimizations`: Optimize Options
- `funsafe-math-optimizations`: Optimize Options
- `funsigned-bitfields`: Non-bugs
- `funsigned-bitfields`: Structures unions enumerations and bit-fields implementation
- `funsigned-bitfields`: C Dialect Options
- `funsigned-char`: Characters implementation
- `funsigned-char`: C Dialect Options
- `funswitch-loops`: Optimize Options
- `funwind-tables`: Code Gen Options
- `fuse-cxa-atexit`: C++ Dialect Options
- `fvar-tracking`: Debugging Options
- `fverbose-asm`: Code Gen Options
- `fvisibility`: Code Gen Options
- `fvisibility-inlines-hidden`: C++ Dialect Options
- `fvpt`: Optimize Options
- `fweb`: Optimize Options
- `fwhole-program`: Optimize Options
- `fwide-exec-charset`: Preprocessor Options
- `fworking-directory`: Preprocessor Options
- `fwrapv`: Code Gen Options
- `fzero-link`: Objective-C and Objective-C++ Dialect Options
- `G`: System V Options
- `G`: RS/6000 and PowerPC Options
- `G`: MIPS Options
- `G`: M32R/D Options
- `g`: Debugging Options
- `gcoff`: Debugging Options
- `gdwarf-2`: Debugging Options
- `gen-decls`: Objective-C and Objective-C++ Dialect Options
- `ggdb`: Debugging Options
- `gnu-ld`: HPPA Options
- `gstabs`: Debugging Options
- `gstabs+`: Debugging Options
- `gvms`: Debugging Options
- `gxcoff`: Debugging Options
- `gxcoff+`: Debugging Options
- `H`: Preprocessor Options
- `headerpad_max_install_names`: Darwin Options
- `help`: Preprocessor Options
- `help`: Overall Options
- `hp-ld`: HPPA Options
- `I`: Directory Options
- `I`: Preprocessor Options
- `I-`: Directory Options
- `I-`: Preprocessor Options
- `idirafter`: Preprocessor Options
- `if-conversion`: Optimize Options
- `if-conversion2`: Optimize Options
- `imacros`: Preprocessor Options
- `image_base`: Darwin Options
- `include`: Preprocessor Options
- `init`: Darwin Options
- `install_name`: Darwin Options
- `iprefix`: Preprocessor Options
- `iquote`: Directory Options
- `iquote`: Preprocessor Options
- `isysroot`: Preprocessor Options
- `isystem`: Preprocessor Options
- `iwithprefix`: Preprocessor Options
- `iwithprefixbefore`: Preprocessor Options
- `keep_private_externs`: Darwin Options
- `L`: Directory Options
- `l`: Link Options
- `lobjc`: Link Options
- `M`: Preprocessor Options
- `m1`: SH Options
- `m10`: PDP-11 Options
- `m128bit-long-double`: i386 and x86-64 Options
- `m16-bit`: CRIS Options
- `m2`: SH Options
- `m210`: MCore Options
- `m3`: SH Options
- `m31`: S/390 and zSeries Options
- `m32`: SPARC Options
- `m32`: RS/6000 and PowerPC Options
- `m32`: i386 and x86-64 Options
- `m32-bit`: CRIS Options
- `m32r`: M32R/D Options
- `m32r2`: M32R/D Options
- `m32rx`: M32R/D Options
- `m340`: MCore Options
- `m386`: i386 and x86-64 Options
- `m3dnow`: i386 and x86-64 Options
- `m3e`: SH Options
- `m4`: SH Options
- `m4-nofpu`: SH Options
- `m4-single`: SH Options
- `m4-single-only`: SH Options
- `m40`: PDP-11 Options
- `m45`: PDP-11 Options
- `m486`: i386 and x86-64 Options
- `m4a`: SH Options
- `m4a-nofpu`: SH Options
- `m4a-single`: SH Options
- `m4a-single-only`: SH Options
- `m4al`: SH Options
- `m4byte-functions`: MCore Options
- `m5200`: M680x0 Options
- `m64`: SPARC Options
- `m64`: S/390 and zSeries Options
- `m64`: RS/6000 and PowerPC Options
- `m64`: i386 and x86-64 Options
- `m68000`: M680x0 Options
- `m68020`: M680x0 Options
- `m68020-40`: M680x0 Options
- `m68020-60`: M680x0 Options
- `m68030`: M680x0 Options
- `m68040`: M680x0 Options
- `m68060`: M680x0 Options
- `m6811`: M68hc1x Options
- `m6812`: M68hc1x Options
- `m68881`: M680x0 Options
- `m68hc11`: M68hc1x Options
- `m68hc12`: M68hc1x Options
- `m68hcs12`: M68hc1x Options
- `m68S12`: M68hc1x Options
- `m8-bit`: CRIS Options
- `m96bit-long-double`: i386 and x86-64 Options
- `mabi`: RS/6000 and PowerPC Options
- `mabi`: ARM Options
- `mabi-mmixware`: MMIX Options
- `mabi=32`: MIPS Options
- `mabi=64`: MIPS Options
- `mabi=eabi`: MIPS Options
- `mabi=gnu`: MMIX Options
- `mabi=ibmlongdouble`: RS/6000 and PowerPC Options
- `mabi=ieeelongdouble`: RS/6000 and PowerPC Options
- `mabi=n32`: MIPS Options
- `mabi=no-spe`: RS/6000 and PowerPC Options
- `mabi=o64`: MIPS Options
- `mabi=spe`: RS/6000 and PowerPC Options
- `mabicalls`: MIPS Options
- `mabort-on-noreturn`: ARM Options
- `mabshi`: PDP-11 Options
- `mac0`: PDP-11 Options
- `macc-4`: FRV Options
- `macc-8`: FRV Options
- `maccumulate-outgoing-args`: i386 and x86-64 Options
- `madjust-unroll`: SH Options
- `mads`: RS/6000 and PowerPC Options
- `maix-struct-return`: RS/6000 and PowerPC Options
- `maix32`: RS/6000 and PowerPC Options
- `maix64`: RS/6000 and PowerPC Options
- `malign-300`: H8/300 Options
- `malign-double`: i386 and x86-64 Options
- `malign-int`: M680x0 Options
- `malign-labels`: FRV Options
- `malign-loops`: M32R/D Options
- `malign-natural`: RS/6000 and PowerPC Options
- `malign-power`: RS/6000 and PowerPC Options
- `malloc-cc`: FRV Options
- `malpha-as`: DEC Alpha Options
- `maltivec`: RS/6000 and PowerPC Options
- `mam33`: MN10300 Options
- `maout`: CRIS Options
- `mapcs`: ARM Options
- `mapcs-frame`: ARM Options
- `mapp-regs`: V850 Options
- `mapp-regs`: SPARC Options
- `march`: S/390 and zSeries Options
- `march`: MT Options
- `march`: MIPS Options
- `march`: i386 and x86-64 Options
- `march`: HPPA Options
- `march`: CRIS Options
- `march`: ARM Options
- `masm=`dialect: i386 and x86-64 Options
- `mauto-incdec`: M68hc1x Options
- `mauto-pic`: IA-64 Options
- `mb`: SH Options
- `mbacc`: MT Options
- `mbackchain`: S/390 and zSeries Options
- `mbase-addresses`: MMIX Options
- `mbcopy`: PDP-11 Options
- `mbig`: TMS320C3x/C4x Options
- `mbig`: RS/6000 and PowerPC Options
- `mbig-endian`: RS/6000 and PowerPC Options
- `mbig-endian`: MCore Options
- `mbig-endian`: IA-64 Options
- `mbig-endian`: ARM Options
- `mbig-memory`: TMS320C3x/C4x Options
- `mbig-switch`: V850 Options
- `mbig-switch`: HPPA Options
- `mbigtable`: SH Options
- `mbit-align`: RS/6000 and PowerPC Options
- `mbitfield`: M680x0 Options
- `mbk`: TMS320C3x/C4x Options
- `mbranch-cheap`: PDP-11 Options
- `mbranch-cost=`number: M32R/D Options
- `mbranch-expensive`: PDP-11 Options
- `mbranch-likely`: MIPS Options
- `mbranch-predict`: MMIX Options
- `mbss-plt`: RS/6000 and PowerPC Options
- `mbuild-constants`: DEC Alpha Options
- `mbwx`: DEC Alpha Options
- `mc68000`: M680x0 Options
- `mc68020`: M680x0 Options
- `mcall-gnu`: RS/6000 and PowerPC Options
- `mcall-linux`: RS/6000 and PowerPC Options
- `mcall-netbsd`: RS/6000 and PowerPC Options
- `mcall-prologues`: AVR Options
- `mcall-solaris`: RS/6000 and PowerPC Options
- `mcall-sysv`: RS/6000 and PowerPC Options
- `mcall-sysv-eabi`: RS/6000 and PowerPC Options
- `mcall-sysv-noeabi`: RS/6000 and PowerPC Options
- `mcallee-super-interworking`: ARM Options
- `mcaller-super-interworking`: ARM Options
- `mcallgraph-data`: MCore Options
- `mcc-init`: CRIS Options
- `mcheck-zero-division`: MIPS Options
- `mcirrus-fix-invalid-insns`: ARM Options
- `mcix`: DEC Alpha Options
- `mcmodel=embmedany`: SPARC Options
- `mcmodel=kernel`: i386 and x86-64 Options
- `mcmodel=large`: i386 and x86-64 Options
- `mcmodel=medany`: SPARC Options
- `mcmodel=medium`: i386 and x86-64 Options
- `mcmodel=medlow`: SPARC Options
- `mcmodel=medmid`: SPARC Options
- `mcmodel=small`: i386 and x86-64 Options
- `mcond-exec`: FRV Options
- `mcond-move`: FRV Options
- `mconst-align`: CRIS Options
- `mconst16`: Xtensa Options
- `mconstant-gp`: IA-64 Options
- `mcpu`: TMS320C3x/C4x Options
- `mcpu`: SPARC Options
- `mcpu`: RS/6000 and PowerPC Options
- `mcpu`: i386 and x86-64 Options
- `mcpu`: FRV Options
- `mcpu`: DEC Alpha Options
- `mcpu`: CRIS Options
- `mcpu`: ARM Options
- `mcpu`: ARC Options
- `mcpu32`: M680x0 Options
- `mcpu=`: M32C Options
- `mcsync-anomaly`: Blackfin Options
- `MD`: Preprocessor Options
- `mdalign`: SH Options
- `mdata`: ARC Options
- `mdata-align`: CRIS Options
- `mdb`: TMS320C3x/C4x Options
- `mdebug`: S/390 and zSeries Options
- `mdebug`: M32R/D Options
- `mdec-asm`: PDP-11 Options
- `mdisable-callt`: V850 Options
- `mdisable-fpregs`: HPPA Options
- `mdisable-indexing`: HPPA Options
- `mdiv`: MCore Options
- `mdiv=`strategy: SH Options
- `mdivide-breaks`: MIPS Options
- `mdivide-traps`: MIPS Options
- `mdivsi3_libfunc=`name: SH Options
- `mdouble`: FRV Options
- `mdouble-float`: MIPS Options
- `mdp-isr-reload`: TMS320C3x/C4x Options
- `mdsp`: MIPS Options
- `mdwarf2-asm`: IA-64 Options
- `mdword`: FRV Options
- `mdynamic-no-pic`: RS/6000 and PowerPC Options
- `meabi`: RS/6000 and PowerPC Options
- `mearly-stop-bits`: IA-64 Options
- `melf`: MMIX Options
- `melf`: CRIS Options
- `melinux`: CRIS Options
- `melinux-stacksize`: CRIS Options
- `memb`: RS/6000 and PowerPC Options
- `membedded-data`: MIPS Options
- `memregs=`: M32C Options
- `mep`: V850 Options
- `mepsilon`: MMIX Options
- `mesa`: S/390 and zSeries Options
- `metrax100`: CRIS Options
- `metrax4`: CRIS Options
- `mexplicit-relocs`: MIPS Options
- `mexplicit-relocs`: DEC Alpha Options
- `MF`: Preprocessor Options
- `mfast-fix`: TMS320C3x/C4x Options
- `mfast-indirect-calls`: HPPA Options
- `mfaster-structs`: SPARC Options
- `mfdpic`: FRV Options
- `mfix`: DEC Alpha Options
- `mfix-and-continue`: Darwin Options
- `mfix-r4000`: MIPS Options
- `mfix-r4400`: MIPS Options
- `mfix-sb1`: MIPS Options
- `mfix-vr4120`: MIPS Options
- `mfix-vr4130`: MIPS Options
- `mfixed-cc`: FRV Options
- `mfixed-range`: IA-64 Options
- `mfixed-range`: HPPA Options
- `mfloat-abi`: ARM Options
- `mfloat-gprs`: RS/6000 and PowerPC Options
- `mfloat-ieee`: DEC Alpha Options
- `mfloat-vax`: DEC Alpha Options
- `mfloat32`: PDP-11 Options
- `mfloat64`: PDP-11 Options
- `mflush-func`: MIPS Options
- `mflush-func=`name: M32R/D Options
- `mflush-trap=`number: M32R/D Options
- `mfmovd`: SH Options
- `mfp`: ARM Options
- `mfp-exceptions`: MIPS Options
- `mfp-reg`: DEC Alpha Options
- `mfp-rounding-mode`: DEC Alpha Options
- `mfp-trap-mode`: DEC Alpha Options
- `mfp32`: MIPS Options
- `mfp64`: MIPS Options
- `mfpe`: ARM Options
- `mfpr-32`: FRV Options
- `mfpr-64`: FRV Options
- `mfprnd`: RS/6000 and PowerPC Options
- `mfpu`: SPARC Options
- `mfpu`: PDP-11 Options
- `mfpu`: ARM Options
- `mfull-toc`: RS/6000 and PowerPC Options
- `mfused-madd`: Xtensa Options
- `mfused-madd`: S/390 and zSeries Options
- `mfused-madd`: RS/6000 and PowerPC Options
- `mfused-madd`: MIPS Options
- `mg`: VAX Options
- `MG`: Preprocessor Options
- `mgas`: HPPA Options
- `mgas`: DEC Alpha Options
- `mgettrcost=`number: SH Options
- `mgnu`: VAX Options
- `mgnu-as`: IA-64 Options
- `mgnu-ld`: IA-64 Options
- `mgotplt`: CRIS Options
- `mgp32`: MIPS Options
- `mgp64`: MIPS Options
- `mgpr-32`: FRV Options
- `mgpr-64`: FRV Options
- `mgprel-ro`: FRV Options
- `mh`: H8/300 Options
- `mhard-float`: SPARC Options
- `mhard-float`: S/390 and zSeries Options
- `mhard-float`: RS/6000 and PowerPC Options
- `mhard-float`: MIPS Options
- `mhard-float`: FRV Options
- `mhard-float`: ARM Options
- `mhard-quad-float`: SPARC Options
- `mhardlit`: MCore Options
- `mhitachi`: SH Options
- `mid-shared-library`: Blackfin Options
- `mieee`: SH Options
- `mieee`: DEC Alpha Options
- `mieee-conformant`: DEC Alpha Options
- `mieee-fp`: i386 and x86-64 Options
- `mieee-with-inexact`: DEC Alpha Options
- `milp32`: IA-64 Options
- `mimpure-text`: SPARC Options
- `mindexed-addressing`: SH Options
- `minit-stack`: AVR Options
- `minline-all-stringops`: i386 and x86-64 Options
- `minline-float-divide-max-throughput`: IA-64 Options
- `minline-float-divide-min-latency`: IA-64 Options
- `minline-int-divide-max-throughput`: IA-64 Options
- `minline-int-divide-min-latency`: IA-64 Options
- `minline-plt`: FRV Options
- `minline-sqrt-max-throughput`: IA-64 Options
- `minline-sqrt-min-latency`: IA-64 Options
- `minmax`: M68hc1x Options
- `minsert-sched-nops`: RS/6000 and PowerPC Options
- `mint16`: PDP-11 Options
- `mint32`: PDP-11 Options
- `mint32`: H8/300 Options
- `mint8`: AVR Options
- `minvalid-symbols`: SH Options
- `mips1`: MIPS Options
- `mips16`: MIPS Options
- `mips2`: MIPS Options
- `mips3`: MIPS Options
- `mips32`: MIPS Options
- `mips32r2`: MIPS Options
- `mips3d`: MIPS Options
- `mips4`: MIPS Options
- `mips64`: MIPS Options
- `misel`: RS/6000 and PowerPC Options
- `misize`: SH Options
- `missue-rate=`number: M32R/D Options
- `mjump-in-delay`: HPPA Options
- `mknuthdiv`: MMIX Options
- `ml`: SH Options
- `mlarge-data`: DEC Alpha Options
- `mlarge-data-threshold=`number: i386 and x86-64 Options
- `mlarge-text`: DEC Alpha Options
- `mlibfuncs`: MMIX Options
- `mlibrary-pic`: FRV Options
- `mlinked-fp`: FRV Options
- `mlinker-opt`: HPPA Options
- `mlinux`: CRIS Options
- `mlittle`: RS/6000 and PowerPC Options
- `mlittle-endian`: SPARC Options
- `mlittle-endian`: RS/6000 and PowerPC Options
- `mlittle-endian`: MCore Options
- `mlittle-endian`: IA-64 Options
- `mlittle-endian`: ARM Options
- `mlong-calls`: V850 Options
- `mlong-calls`: MIPS Options
- `mlong-calls`: M68hc1x Options
- `mlong-calls`: FRV Options
- `mlong-calls`: Blackfin Options
- `mlong-calls`: ARM Options
- `mlong-double-128`: S/390 and zSeries Options
- `mlong-double-64`: S/390 and zSeries Options
- `mlong-load-store`: HPPA Options
- `mlong32`: MIPS Options
- `mlong64`: MIPS Options
- `mlongcall`: RS/6000 and PowerPC Options
- `mlongcalls`: Xtensa Options
- `mloop-unsigned`: TMS320C3x/C4x Options
- `mlow-64k`: Blackfin Options
- `mlp64`: IA-64 Options
- `MM`: Preprocessor Options
- `mmac`: CRX Options
- `mmad`: MIPS Options
- `mmangle-cpu`: ARC Options
- `mmax`: DEC Alpha Options
- `mmax-stack-frame`: CRIS Options
- `mmcu`: AVR Options
- `MMD`: Preprocessor Options
- `mmedia`: FRV Options
- `mmemcpy`: MIPS Options
- `mmemory-latency`: DEC Alpha Options
- `mmemparm`: TMS320C3x/C4x Options
- `mmfcrf`: RS/6000 and PowerPC Options
- `mminimal-toc`: RS/6000 and PowerPC Options
- `mmmx`: i386 and x86-64 Options
- `mmodel=large`: M32R/D Options
- `mmodel=medium`: M32R/D Options
- `mmodel=small`: M32R/D Options
- `mmpyi`: TMS320C3x/C4x Options
- `mmul-bug-workaround`: CRIS Options
- `mmuladd`: FRV Options
- `mmult-bug`: MN10300 Options
- `mmulti-cond-exec`: FRV Options
- `mmultiple`: RS/6000 and PowerPC Options
- `mmvcle`: S/390 and zSeries Options
- `mmvme`: RS/6000 and PowerPC Options
- `mn`: H8/300 Options
- `mnested-cond-exec`: FRV Options
- `mnew-mnemonics`: RS/6000 and PowerPC Options
- `mno-3dnow`: i386 and x86-64 Options
- `mno-4byte-functions`: MCore Options
- `mno-abicalls`: MIPS Options
- `mno-abshi`: PDP-11 Options
- `mno-ac0`: PDP-11 Options
- `mno-align-double`: i386 and x86-64 Options
- `mno-align-int`: M680x0 Options
- `mno-align-loops`: M32R/D Options
- `mno-align-stringops`: i386 and x86-64 Options
- `mno-altivec`: RS/6000 and PowerPC Options
- `mno-am33`: MN10300 Options
- `mno-app-regs`: V850 Options
- `mno-app-regs`: SPARC Options
- `mno-bacc`: MT Options
- `mno-backchain`: S/390 and zSeries Options
- `mno-base-addresses`: MMIX Options
- `mno-bit-align`: RS/6000 and PowerPC Options
- `mno-bk`: TMS320C3x/C4x Options
- `mno-branch-likely`: MIPS Options
- `mno-branch-predict`: MMIX Options
- `mno-bwx`: DEC Alpha Options
- `mno-callgraph-data`: MCore Options
- `mno-check-zero-division`: MIPS Options
- `mno-cirrus-fix-invalid-insns`: ARM Options
- `mno-cix`: DEC Alpha Options
- `mno-cond-exec`: FRV Options
- `mno-cond-move`: FRV Options
- `mno-const-align`: CRIS Options
- `mno-const16`: Xtensa Options
- `mno-crt0`: MT Options
- `mno-crt0`: MN10300 Options
- `mno-csync-anomaly`: Blackfin Options
- `mno-data-align`: CRIS Options
- `mno-db`: TMS320C3x/C4x Options
- `mno-debug`: S/390 and zSeries Options
- `mno-div`: MCore Options
- `mno-double`: FRV Options
- `mno-dsp`: MIPS Options
- `mno-dwarf2-asm`: IA-64 Options
- `mno-dword`: FRV Options
- `mno-eabi`: RS/6000 and PowerPC Options
- `mno-early-stop-bits`: IA-64 Options
- `mno-eflags`: FRV Options
- `mno-embedded-data`: MIPS Options
- `mno-ep`: V850 Options
- `mno-epsilon`: MMIX Options
- `mno-explicit-relocs`: MIPS Options
- `mno-explicit-relocs`: DEC Alpha Options
- `mno-fancy-math-387`: i386 and x86-64 Options
- `mno-fast-fix`: TMS320C3x/C4x Options
- `mno-faster-structs`: SPARC Options
- `mno-fix`: DEC Alpha Options
- `mno-fix-r4000`: MIPS Options
- `mno-fix-r4400`: MIPS Options
- `mno-float32`: PDP-11 Options
- `mno-float64`: PDP-11 Options
- `mno-flush-func`: M32R/D Options
- `mno-flush-trap`: M32R/D Options
- `mno-fp-in-toc`: RS/6000 and PowerPC Options
- `mno-fp-regs`: DEC Alpha Options
- `mno-fp-ret-in-387`: i386 and x86-64 Options
- `mno-fprnd`: RS/6000 and PowerPC Options
- `mno-fpu`: SPARC Options
- `mno-fused-madd`: Xtensa Options
- `mno-fused-madd`: S/390 and zSeries Options
- `mno-fused-madd`: RS/6000 and PowerPC Options
- `mno-fused-madd`: MIPS Options
- `mno-gnu-as`: IA-64 Options
- `mno-gnu-ld`: IA-64 Options
- `mno-gotplt`: CRIS Options
- `mno-hardlit`: MCore Options
- `mno-id-shared-library`: Blackfin Options
- `mno-ieee-fp`: i386 and x86-64 Options
- `mno-int16`: PDP-11 Options
- `mno-int32`: PDP-11 Options
- `mno-interrupts`: AVR Options
- `mno-isel`: RS/6000 and PowerPC Options
- `mno-knuthdiv`: MMIX Options
- `mno-libfuncs`: MMIX Options
- `mno-long-calls`: V850 Options
- `mno-long-calls`: MIPS Options
- `mno-long-calls`: M68hc1x Options
- `mno-long-calls`: HPPA Options
- `mno-long-calls`: Blackfin Options
- `mno-long-calls`: ARM Options
- `mno-longcall`: RS/6000 and PowerPC Options
- `mno-longcalls`: Xtensa Options
- `mno-loop-unsigned`: TMS320C3x/C4x Options
- `mno-low-64k`: Blackfin Options
- `mno-mad`: MIPS Options
- `mno-max`: DEC Alpha Options
- `mno-media`: FRV Options
- `mno-memcpy`: MIPS Options
- `mno-mfcrf`: RS/6000 and PowerPC Options
- `mno-mips16`: MIPS Options
- `mno-mips3d`: MIPS Options
- `mno-mmx`: i386 and x86-64 Options
- `mno-mpyi`: TMS320C3x/C4x Options
- `mno-mul-bug-workaround`: CRIS Options
- `mno-muladd`: FRV Options
- `mno-mult-bug`: MN10300 Options
- `mno-multi-cond-exec`: FRV Options
- `mno-multiple`: RS/6000 and PowerPC Options
- `mno-mvcle`: S/390 and zSeries Options
- `mno-nested-cond-exec`: FRV Options
- `mno-optimize-membar`: FRV Options
- `mno-pack`: FRV Options
- `mno-packed-stack`: S/390 and zSeries Options
- `mno-paired-single`: MIPS Options
- `mno-parallel-insns`: TMS320C3x/C4x Options
- `mno-parallel-mpy`: TMS320C3x/C4x Options
- `mno-pic`: IA-64 Options
- `mno-popcntb`: RS/6000 and PowerPC Options
- `mno-power`: RS/6000 and PowerPC Options
- `mno-power2`: RS/6000 and PowerPC Options
- `mno-powerpc`: RS/6000 and PowerPC Options
- `mno-powerpc-gfxopt`: RS/6000 and PowerPC Options
- `mno-powerpc-gpopt`: RS/6000 and PowerPC Options
- `mno-powerpc64`: RS/6000 and PowerPC Options
- `mno-prolog-function`: V850 Options
- `mno-prologue-epilogue`: CRIS Options
- `mno-prototype`: RS/6000 and PowerPC Options
- `mno-push-args`: i386 and x86-64 Options
- `mno-register-names`: IA-64 Options
- `mno-regnames`: RS/6000 and PowerPC Options
- `mno-relax-immediate`: MCore Options
- `mno-relocatable`: RS/6000 and PowerPC Options
- `mno-relocatable-lib`: RS/6000 and PowerPC Options
- `mno-rptb`: TMS320C3x/C4x Options
- `mno-rpts`: TMS320C3x/C4x Options
- `mno-scc`: FRV Options
- `mno-sched-prolog`: ARM Options
- `mno-sdata`: RS/6000 and PowerPC Options
- `mno-sdata`: IA-64 Options
- `mno-side-effects`: CRIS Options
- `mno-single-exit`: MMIX Options
- `mno-slow-bytes`: MCore Options
- `mno-small-exec`: S/390 and zSeries Options
- `mno-soft-float`: DEC Alpha Options
- `mno-space-regs`: HPPA Options
- `mno-spe`: RS/6000 and PowerPC Options
- `mno-specld-anomaly`: Blackfin Options
- `mno-split`: PDP-11 Options
- `mno-split-addresses`: MIPS Options
- `mno-sse`: i386 and x86-64 Options
- `mno-stack-align`: CRIS Options
- `mno-stack-bias`: SPARC Options
- `mno-strict-align`: RS/6000 and PowerPC Options
- `mno-strict-align`: M680x0 Options
- `mno-string`: RS/6000 and PowerPC Options
- `mno-sum-in-toc`: RS/6000 and PowerPC Options
- `mno-svr3-shlib`: i386 and x86-64 Options
- `mno-swdiv`: RS/6000 and PowerPC Options
- `mno-sym32`: MIPS Options
- `mno-tablejump`: AVR Options
- `mno-target-align`: Xtensa Options
- `mno-text-section-literals`: Xtensa Options
- `mno-toc`: RS/6000 and PowerPC Options
- `mno-toplevel-symbols`: MMIX Options
- `mno-tpf-trace`: S/390 and zSeries Options
- `mno-unaligned-doubles`: SPARC Options
- `mno-uninit-const-in-rodata`: MIPS Options
- `mno-update`: RS/6000 and PowerPC Options
- `mno-v8plus`: SPARC Options
- `mno-vis`: SPARC Options
- `mno-vliw-branch`: FRV Options
- `mno-volatile-asm-stop`: IA-64 Options
- `mno-vrsave`: RS/6000 and PowerPC Options
- `mno-wide-bitfields`: MCore Options
- `mno-xgot`: MIPS Options
- `mno-xl-compat`: RS/6000 and PowerPC Options
- `mno-zero-extend`: MMIX Options
- `mnobitfield`: M680x0 Options
- `mnomacsave`: SH Options
- `mnominmax`: M68hc1x Options
- `mnop-fun-dllimport`: ARM Options
- `mold-mnemonics`: RS/6000 and PowerPC Options
- `momit-leaf-frame-pointer`: i386 and x86-64 Options
- `momit-leaf-frame-pointer`: Blackfin Options
- `moptimize-membar`: FRV Options
- `MP`: Preprocessor Options
- `mpa-risc-1-0`: HPPA Options
- `mpa-risc-1-1`: HPPA Options
- `mpa-risc-2-0`: HPPA Options
- `mpack`: FRV Options
- `mpacked-stack`: S/390 and zSeries Options
- `mpadstruct`: SH Options
- `mpaired-single`: MIPS Options
- `mparallel-insns`: TMS320C3x/C4x Options
- `mparallel-mpy`: TMS320C3x/C4x Options
- `mparanoid`: TMS320C3x/C4x Options
- `mpcrel`: M680x0 Options
- `mpdebug`: CRIS Options
- `mpe`: RS/6000 and PowerPC Options
- `mpentium`: i386 and x86-64 Options
- `mpentiumpro`: i386 and x86-64 Options
- `mpic-register`: ARM Options
- `mpoke-function-name`: ARM Options
- `mpopcntb`: RS/6000 and PowerPC Options
- `mportable-runtime`: HPPA Options
- `mpower`: RS/6000 and PowerPC Options
- `mpower2`: RS/6000 and PowerPC Options
- `mpowerpc`: RS/6000 and PowerPC Options
- `mpowerpc-gfxopt`: RS/6000 and PowerPC Options
- `mpowerpc-gpopt`: RS/6000 and PowerPC Options
- `mpowerpc64`: RS/6000 and PowerPC Options
- `mprefergot`: SH Options
- `mpreferred-stack-boundary`: i386 and x86-64 Options
- `mprioritize-restricted-insns`: RS/6000 and PowerPC Options
- `mprolog-function`: V850 Options
- `mprologue-epilogue`: CRIS Options
- `mprototype`: RS/6000 and PowerPC Options
- `mpt-fixed`: SH Options
- `mpush-args`: i386 and x86-64 Options
- `mpush-args`: CRX Options
- `MQ`: Preprocessor Options
- `mregister-names`: IA-64 Options
- `mregnames`: RS/6000 and PowerPC Options
- `mregparm`: TMS320C3x/C4x Options
- `mregparm`: i386 and x86-64 Options
- `mrelax`: SH Options
- `mrelax`: MN10300 Options
- `mrelax`: H8/300 Options
- `mrelax-immediate`: MCore Options
- `mrelocatable`: RS/6000 and PowerPC Options
- `mrelocatable-lib`: RS/6000 and PowerPC Options
- `mreturn-pointer-on-d0`: MN10300 Options
- `mrodata`: ARC Options
- `mrptb`: TMS320C3x/C4x Options
- `mrpts`: TMS320C3x/C4x Options
- `mrtd`: Function Attributes
- `mrtd`: M680x0 Options
- `mrtd`: i386 and x86-64 Options
- `ms`: H8/300 Options
- `ms2600`: H8/300 Options
- `mscc`: FRV Options
- `msched-costly-dep`: RS/6000 and PowerPC Options
- `mschedule`: HPPA Options
- `msda`: V850 Options
- `msdata`: RS/6000 and PowerPC Options
- `msdata`: IA-64 Options
- `msdata-data`: RS/6000 and PowerPC Options
- `msdata=default`: RS/6000 and PowerPC Options
- `msdata=eabi`: RS/6000 and PowerPC Options
- `msdata=none`: RS/6000 and PowerPC Options
- `msdata=none`: M32R/D Options
- `msdata=sdata`: M32R/D Options
- `msdata=sysv`: RS/6000 and PowerPC Options
- `msdata=use`: M32R/D Options
- `msecure-plt`: RS/6000 and PowerPC Options
- `mshared-library-id`: Blackfin Options
- `mshort`: M68hc1x Options
- `mshort`: M680x0 Options
- `msim`: Xstormy16 Options
- `msim`: RS/6000 and PowerPC Options
- `msim`: MT Options
- `msim`: M32C Options
- `msingle-exit`: MMIX Options
- `msingle-float`: MIPS Options
- `msingle-pic-base`: ARM Options
- `msio`: HPPA Options
- `msize`: AVR Options
- `mslow-bytes`: MCore Options
- `msmall`: TMS320C3x/C4x Options
- `msmall-data`: DEC Alpha Options
- `msmall-exec`: S/390 and zSeries Options
- `msmall-memory`: TMS320C3x/C4x Options
- `msmall-text`: DEC Alpha Options
- `msoft-float`: SPARC Options
- `msoft-float`: S/390 and zSeries Options
- `msoft-float`: RS/6000 and PowerPC Options
- `msoft-float`: PDP-11 Options
- `msoft-float`: MIPS Options
- `msoft-float`: M680x0 Options
- `msoft-float`: i386 and x86-64 Options
- `msoft-float`: HPPA Options
- `msoft-float`: FRV Options
- `msoft-float`: DEC Alpha Options
- `msoft-float`: ARM Options
- `msoft-quad-float`: SPARC Options
- `msoft-reg-count`: M68hc1x Options
- `mspace`: V850 Options
- `mspace`: SH Options
- `mspe`: RS/6000 and PowerPC Options
- `mspecld-anomaly`: Blackfin Options
- `msplit`: PDP-11 Options
- `msplit-addresses`: MIPS Options
- `msse`: i386 and x86-64 Options
- `msseregparm`: i386 and x86-64 Options
- `mstack-align`: CRIS Options
- `mstack-bias`: SPARC Options
- `mstack-guard`: S/390 and zSeries Options
- `mstack-size`: S/390 and zSeries Options
- `mstrict-align`: RS/6000 and PowerPC Options
- `mstrict-align`: M680x0 Options
- `mstring`: RS/6000 and PowerPC Options
- `mstructure-size-boundary`: ARM Options
- `msvr3-shlib`: i386 and x86-64 Options
- `msvr4-struct-return`: RS/6000 and PowerPC Options
- `mswdiv`: RS/6000 and PowerPC Options
- `msym32`: MIPS Options
- `mt`: IA-64 Options
- `MT`: Preprocessor Options
- `mtarget-align`: Xtensa Options
- `mtda`: V850 Options
- `mtext`: ARC Options
- `mtext-section-literals`: Xtensa Options
- `mthreads`: i386 and x86-64 Options
- `mthumb`: ARM Options
- `mthumb-interwork`: ARM Options
- `mti`: TMS320C3x/C4x Options
- `mtiny-stack`: AVR Options
- `mtls-direct-seg-refs`: i386 and x86-64 Options
- `mtls-size`: IA-64 Options
- `mtoc`: RS/6000 and PowerPC Options
- `mtomcat-stats`: FRV Options
- `mtoplevel-symbols`: MMIX Options
- `mtp`: ARM Options
- `mtpcs-frame`: ARM Options
- `mtpcs-leaf-frame`: ARM Options
- `mtpf-trace`: S/390 and zSeries Options
- `mtrap-precision`: DEC Alpha Options
- `mtune`: SPARC Options
- `mtune`: S/390 and zSeries Options
- `mtune`: RS/6000 and PowerPC Options
- `mtune`: MIPS Options
- `mtune`: IA-64 Options
- `mtune`: i386 and x86-64 Options
- `mtune`: DEC Alpha Options
- `mtune`: CRIS Options
- `mtune`: ARM Options
- `multcost=`number: SH Options
- `multi_module`: Darwin Options
- `multilib-library-pic`: FRV Options
- `multiply_defined`: Darwin Options
- `multiply_defined_unused`: Darwin Options
- `munaligned-doubles`: SPARC Options
- `muninit-const-in-rodata`: MIPS Options
- `munix`: VAX Options
- `munix-asm`: PDP-11 Options
- `mupdate`: RS/6000 and PowerPC Options
- `musermode`: SH Options
- `mv850`: V850 Options
- `mv850e`: V850 Options
- `mv850e1`: V850 Options
- `mv8plus`: SPARC Options
- `mvis`: SPARC Options
- `mvliw-branch`: FRV Options
- `mvms-return-codes`: DEC Alpha/VMS Options
- `mvolatile-asm-stop`: IA-64 Options
- `mvr4130-align`: MIPS Options
- `mvrsave`: RS/6000 and PowerPC Options
- `mvxworks`: RS/6000 and PowerPC Options
- `mwarn-dynamicstack`: S/390 and zSeries Options
- `mwarn-framesize`: S/390 and zSeries Options
- `mwide-bitfields`: MCore Options
- `mwindiss`: RS/6000 and PowerPC Options
- `mwords-little-endian`: ARM Options
- `mxgot`: MIPS Options
- `mxl-compat`: RS/6000 and PowerPC Options
- `myellowknife`: RS/6000 and PowerPC Options
- `mzarch`: S/390 and zSeries Options
- `mzda`: V850 Options
- `mzero-extend`: MMIX Options
- `no-integrated-cpp`: C Dialect Options
- `no-red-zone`: i386 and x86-64 Options
- `no_dead_strip_inits_and_terms`: Darwin Options
- `noall_load`: Darwin Options
- `nocpp`: MIPS Options
- `nodefaultlibs`: Link Options
- `nofixprebinding`: Darwin Options
- `nolibdld`: HPPA Options
- `nomultidefs`: Darwin Options
- `noprebind`: Darwin Options
- `noseglinkedit`: Darwin Options
- `nostartfiles`: Link Options
- `nostdinc`: Preprocessor Options
- `nostdinc++`: Preprocessor Options
- `nostdinc++`: C++ Dialect Options
- `nostdlib`: Link Options
- `o`: Preprocessor Options
- `O`: Optimize Options
- `o`: Overall Options
- `O0`: Optimize Options
- `O1`: Optimize Options
- `O2`: Optimize Options
- `O3`: Optimize Options
- `Os`: Optimize Options
- `P`: Preprocessor Options
- `p`: Debugging Options
- `pagezero_size`: Darwin Options
- `param`: Optimize Options
- `pass-exit-codes`: Overall Options
- `pedantic`: Warnings and Errors
- `pedantic`: Alternate Keywords
- `pedantic`: C Extensions
- `pedantic`: Preprocessor Options
- `pedantic`: Warning Options
- `pedantic`: Standards
- `pedantic-errors`: Warnings and Errors
- `pedantic-errors`: Non-bugs
- `pedantic-errors`: Preprocessor Options
- `pedantic-errors`: Warning Options
- `pedantic-errors`: Standards
- `pg`: Debugging Options
- `pie`: Link Options
- `pipe`: Overall Options
- `prebind`: Darwin Options
- `prebind_all_twolevel_modules`: Darwin Options
- `preprocessor`: Preprocessor Options
- `print-file-name`: Debugging Options
- `print-libgcc-file-name`: Debugging Options
- `print-multi-directory`: Debugging Options
- `print-multi-lib`: Debugging Options
- `print-objc-runtime-info`: Objective-C and Objective-C++ Dialect Options
- `print-prog-name`: Debugging Options
- `print-search-dirs`: Debugging Options
- `private_bundle`: Darwin Options
- `pthread`: SPARC Options
- `pthread`: RS/6000 and PowerPC Options
- `pthread`: IA-64 Options
- `pthreads`: SPARC Options
- `Q`: Debugging Options
- `Qn`: System V Options
- `Qy`: System V Options
- `rdynamic`: Link Options
- `read_only_relocs`: Darwin Options
- `remap`: Preprocessor Options
- `s`: Link Options
- `S`: Link Options
- `S`: Overall Options
- `save-temps`: Debugging Options
- `sectalign`: Darwin Options
- `sectcreate`: Darwin Options
- `sectobjectsymbols`: Darwin Options
- `sectorder`: Darwin Options
- `seg1addr`: Darwin Options
- `seg_addr_table`: Darwin Options
- `seg_addr_table_filename`: Darwin Options
- `segaddr`: Darwin Options
- `seglinkedit`: Darwin Options
- `segprot`: Darwin Options
- `segs_read_only_addr`: Darwin Options
- `segs_read_write_addr`: Darwin Options
- `shared`: Link Options
- `shared-libgcc`: Link Options
- `sim`: CRIS Options
- `sim2`: CRIS Options
- `single_module`: Darwin Options
- `specs`: Directory Options
- `static`: HPPA Options
- `static`: Darwin Options
- `static`: Link Options
- `static-libgcc`: Link Options
- `std`: Non-bugs
- `std`: Other Builtins
- `std`: C Dialect Options
- `std`: Standards
- `std=`: Preprocessor Options
- `sub_library`: Darwin Options
- `sub_umbrella`: Darwin Options
- `symbolic`: Link Options
- `sysroot`: Directory Options
- `target-help`: Preprocessor Options
- `target-help`: Overall Options
- `threads`: SPARC Options
- `threads`: HPPA Options
- `time`: Debugging Options
- `tls`: FRV Options
- `TLS`: FRV Options
- `traditional`: Incompatibilities
- `traditional`: C Dialect Options
- `traditional-cpp`: Preprocessor Options
- `traditional-cpp`: C Dialect Options
- `trigraphs`: Preprocessor Options
- `trigraphs`: C Dialect Options
- `twolevel_namespace`: Darwin Options
- `u`: Link Options
- `U`: Preprocessor Options
- `umbrella`: Darwin Options
- `undef`: Preprocessor Options
- `undefined`: Darwin Options
- `unexported_symbols_list`: Darwin Options
- `V`: Target Options
- `v`: Preprocessor Options
- `v`: Overall Options
- `version`: Preprocessor Options
- `version`: Overall Options
- `W`: Incompatibilities
- `w`: Preprocessor Options
- `W`: Warning Options
- `w`: Warning Options
- `Wa`: Assembler Options
- `Wabi`: C++ Dialect Options
- `Waggregate-return`: Warning Options
- `Wall`: Standard Libraries
- `Wall`: Preprocessor Options
- `Wall`: Warning Options
- `Wassign-intercept`: Objective-C and Objective-C++ Dialect Options
- `Wattributes`: Warning Options
- `Wbad-function-cast`: Warning Options
- `Wcast-align`: Warning Options
- `Wcast-qual`: Warning Options
- `Wchar-subscripts`: Warning Options
- `Wcomment`: Preprocessor Options
- `Wcomment`: Warning Options
- `Wcomments`: Preprocessor Options
- `Wconversion`: Protoize Caveats
- `Wconversion`: Warning Options
- `Wctor-dtor-privacy`: C++ Dialect Options
- `Wdeclaration-after-statement`: Warning Options
- `Wdisabled-optimization`: Warning Options
- `Wdiv-by-zero`: Warning Options
- `weak_reference_mismatches`: Darwin Options
- `Weffc++`: C++ Dialect Options
- `Wendif-labels`: Preprocessor Options
- `Wendif-labels`: Warning Options
- `Werror`: Preprocessor Options
- `Werror`: Warning Options
- `Werror-implicit-function-declaration`: Warning Options
- `Wextra`: Warning Options
- `Wfatal-errors`: Warning Options
- `Wfloat-equal`: Warning Options
- `Wformat`: Function Attributes
- `Wformat`: Warning Options
- `Wformat-nonliteral`: Function Attributes
- `Wformat-nonliteral`: Warning Options
- `Wformat-security`: Warning Options
- `Wformat-y2k`: Warning Options
- `Wformat=2`: Warning Options
- `whatsloaded`: Darwin Options
- `whyload`: Darwin Options
- `Wimplicit`: Warning Options
- `Wimplicit-function-declaration`: Warning Options
- `Wimplicit-int`: Warning Options
- `Wimport`: Preprocessor Options
- `Winit-self`: Warning Options
- `Winline`: Inline
- `Winline`: Warning Options
- `Winvalid-pch`: Warning Options
- `Wl`: Link Options
- `Wlarger-than`: Warning Options
- `Wlong-long`: Warning Options
- `Wmain`: Warning Options
- `Wmissing-braces`: Warning Options
- `Wmissing-declarations`: Warning Options
- `Wmissing-field-initializers`: Warning Options
- `Wmissing-format-attribute`: Warning Options
- `Wmissing-include-dirs`: Warning Options
- `Wmissing-noreturn`: Warning Options
- `Wmissing-prototypes`: Warning Options
- `Wmultichar`: Warning Options
- `Wnested-externs`: Warning Options
- `Wno-attributes`: Warning Options
- `Wno-deprecated`: C++ Dialect Options
- `Wno-deprecated-declarations`: Warning Options
- `Wno-div-by-zero`: Warning Options
- `Wno-endif-labels`: Warning Options
- `Wno-format-extra-args`: Warning Options
- `Wno-format-zero-length`: Warning Options
- `Wno-import`: Warning Options
- `Wno-int-to-pointer-cast`: Warning Options
- `Wno-invalid-offsetof`: Warning Options
- `Wno-long-long`: Warning Options
- `Wno-multichar`: Warning Options
- `Wno-non-template-friend`: C++ Dialect Options
- `Wno-pmf-conversions`: Bound member functions
- `Wno-pmf-conversions`: C++ Dialect Options
- `Wno-pointer-sign`: Warning Options
- `Wno-pointer-to-int-cast`: Warning Options
- `Wno-pragmas`: Warning Options
- `Wno-protocol`: Objective-C and Objective-C++ Dialect Options
- `Wno-variadic-macros`: Warning Options
- `Wno-volatile-register-var`: Warning Options
- `Wnon-virtual-dtor`: C++ Dialect Options
- `Wnonnull`: Warning Options
- `Wnormalized`: Warning Options
- `Wold-style-cast`: C++ Dialect Options
- `Wold-style-definition`: Warning Options
- `Woverloaded-virtual`: C++ Dialect Options
- `Wp`: Preprocessor Options
- `Wpacked`: Warning Options
- `Wpadded`: Warning Options
- `Wparentheses`: Warning Options
- `Wpointer-arith`: Pointer Arith
- `Wpointer-arith`: Warning Options
- `Wpointer-sign`: Warning Options
- `Wpragmas`: Warning Options
- `Wredundant-decls`: Warning Options
- `Wreorder`: C++ Dialect Options
- `Wreturn-type`: Warning Options
- `Wselector`: Objective-C and Objective-C++ Dialect Options
- `Wsequence-point`: Warning Options
- `Wshadow`: Warning Options
- `Wsign-compare`: Warning Options
- `Wsign-promo`: C++ Dialect Options
- `Wstrict-aliasing`: Warning Options
- `Wstrict-aliasing=2`: Warning Options
- `Wstrict-null-sentinel`: C++ Dialect Options
- `Wstrict-prototypes`: Warning Options
- `Wstrict-selector-match`: Objective-C and Objective-C++ Dialect Options
- `Wswitch`: Warning Options
- `Wswitch-enum`: Warning Options
- `Wswitch-switch`: Warning Options
- `Wsystem-headers`: Preprocessor Options
- `Wsystem-headers`: Warning Options
- `Wtraditional`: Preprocessor Options
- `Wtraditional`: Warning Options
- `Wtrigraphs`: Preprocessor Options
- `Wtrigraphs`: Warning Options
- `Wundeclared-selector`: Objective-C and Objective-C++ Dialect Options
- `Wundef`: Preprocessor Options
- `Wundef`: Warning Options
- `Wuninitialized`: Warning Options
- `Wunknown-pragmas`: Warning Options
- `Wunreachable-code`: Warning Options
- `Wunsafe-loop-optimizations`: Warning Options
- `Wunused`: Warning Options
- `Wunused-function`: Warning Options
- `Wunused-label`: Warning Options
- `Wunused-macros`: Preprocessor Options
- `Wunused-parameter`: Warning Options
- `Wunused-value`: Warning Options
- `Wunused-variable`: Warning Options
- `Wvariadic-macros`: Warning Options
- `Wvolatile-register-var`: Warning Options
- `Wwrite-strings`: Warning Options
- `x`: Preprocessor Options
- `x`: Overall Options
- `Xassembler`: Assembler Options
- `Xlinker`: Link Options
- `Ym`: System V Options
- `YP`: System V Options

Previous: Option Index, Up: Top

## Keyword Index

- ``!`' in constraint: Multi-Alternative
- ``#`' in constraint: Modifiers
- #pragma: Pragmas
- `#pragma implementation`: C++ Interface
- `#pragma implementation`, implied: C++ Interface
- `#pragma interface`: C++ Interface
- `#pragma`, reason for not using: Function Attributes
- $: Dollar Signs
- ``%`' in constraint: Modifiers
- %include: Spec Files
- %include_noerr: Spec Files
- %rename: Spec Files
- ``&`' in constraint: Modifiers
- ': Incompatibilities
- ``*`' in constraint: Modifiers
- ``+`' in constraint: Modifiers
- `-lgcc`, use with `-nodefaultlibs`: Link Options
- `-lgcc`, use with `-nostdlib`: Link Options
- `-nodefaultlibs` and unresolved references: Link Options
- `-nostdlib` and unresolved references: Link Options
- .sdata/.sdata2 references (PowerPC): RS/6000 and PowerPC Options
- //: C++ Comments
- ``0`' in constraint: Simple Constraints
- ``<`' in constraint: Simple Constraints
- ``=`' in constraint: Modifiers
- ``>`' in constraint: Simple Constraints
- ``?`' in constraint: Multi-Alternative
- `?:` extensions: Conditionals
- ?: side effect: Conditionals
- ``_`' in variables in macros: Typeof
- `__builtin___fprintf_chk`: Object Size Checking
- `__builtin___memcpy_chk`: Object Size Checking
- `__builtin___memmove_chk`: Object Size Checking
- `__builtin___mempcpy_chk`: Object Size Checking
- `__builtin___memset_chk`: Object Size Checking
- `__builtin___printf_chk`: Object Size Checking
- `__builtin___snprintf_chk`: Object Size Checking
- `__builtin___sprintf_chk`: Object Size Checking
- `__builtin___stpcpy_chk`: Object Size Checking
- `__builtin___strcat_chk`: Object Size Checking
- `__builtin___strcpy_chk`: Object Size Checking
- `__builtin___strncat_chk`: Object Size Checking
- `__builtin___strncpy_chk`: Object Size Checking
- `__builtin___vfprintf_chk`: Object Size Checking
- `__builtin___vprintf_chk`: Object Size Checking
- `__builtin___vsnprintf_chk`: Object Size Checking
- `__builtin___vsprintf_chk`: Object Size Checking
- `__builtin_apply`: Constructing Calls
- `__builtin_apply_args`: Constructing Calls
- `__builtin_choose_expr`: Other Builtins
- `__builtin_clz`: Other Builtins
- `__builtin_clzl`: Other Builtins
- `__builtin_clzll`: Other Builtins
- `__builtin_constant_p`: Other Builtins
- `__builtin_ctz`: Other Builtins
- `__builtin_ctzl`: Other Builtins
- `__builtin_ctzll`: Other Builtins
- `__builtin_expect`: Other Builtins
- `__builtin_ffs`: Other Builtins
- `__builtin_ffsl`: Other Builtins
- `__builtin_ffsll`: Other Builtins
- `__builtin_frame_address`: Return Address
- `__builtin_huge_val`: Other Builtins
- `__builtin_huge_valf`: Other Builtins
- `__builtin_huge_vall`: Other Builtins
- `__builtin_inf`: Other Builtins
- `__builtin_inff`: Other Builtins
- `__builtin_infl`: Other Builtins
- `__builtin_isgreater`: Other Builtins
- `__builtin_isgreaterequal`: Other Builtins
- `__builtin_isless`: Other Builtins
- `__builtin_islessequal`: Other Builtins
- `__builtin_islessgreater`: Other Builtins
- `__builtin_isunordered`: Other Builtins
- `__builtin_nan`: Other Builtins
- `__builtin_nanf`: Other Builtins
- `__builtin_nanl`: Other Builtins
- `__builtin_nans`: Other Builtins
- `__builtin_nansf`: Other Builtins
- `__builtin_nansl`: Other Builtins
- `__builtin_object_size`: Object Size Checking
- `__builtin_offsetof`: Offsetof
- `__builtin_parity`: Other Builtins
- `__builtin_parityl`: Other Builtins
- `__builtin_parityll`: Other Builtins
- `__builtin_popcount`: Other Builtins
- `__builtin_popcountl`: Other Builtins
- `__builtin_popcountll`: Other Builtins
- `__builtin_powi`: Other Builtins
- `__builtin_powif`: Other Builtins
- `__builtin_powil`: Other Builtins
- `__builtin_prefetch`: Other Builtins
- `__builtin_return`: Constructing Calls
- `__builtin_return_address`: Return Address
- `__builtin_types_compatible_p`: Other Builtins
- `__complex__` keyword: Complex
- `__declspec(dllexport)`: Function Attributes
- `__declspec(dllimport)`: Function Attributes
- `__extension__`: Alternate Keywords
- `__func__` identifier: Function Names
- `__FUNCTION__` identifier: Function Names
- `__imag__` keyword: Complex
- `__PRETTY_FUNCTION__` identifier: Function Names
- `__real__` keyword: Complex
- `__STDC_HOSTED__`: Standards
- `__sync_add_and_fetch`: Atomic Builtins
- `__sync_and_and_fetch`: Atomic Builtins
- `__sync_bool_compare_and_swap`: Atomic Builtins
- `__sync_fetch_and_add`: Atomic Builtins
- `__sync_fetch_and_and`: Atomic Builtins
- `__sync_fetch_and_nand`: Atomic Builtins
- `__sync_fetch_and_or`: Atomic Builtins
- `__sync_fetch_and_sub`: Atomic Builtins
- `__sync_fetch_and_xor`: Atomic Builtins
- `__sync_lock_release`: Atomic Builtins
- `__sync_lock_test_and_set`: Atomic Builtins
- `__sync_nand_and_fetch`: Atomic Builtins
- `__sync_or_and_fetch`: Atomic Builtins
- `__sync_sub_and_fetch`: Atomic Builtins
- `__sync_synchronize`: Atomic Builtins
- `__sync_val_compare_and_swap`: Atomic Builtins
- `__sync_xor_and_fetch`: Atomic Builtins
- __thread: Thread-Local
- `_Complex` keyword: Complex
- `_exit`: Other Builtins
- `_Exit`: Other Builtins
- ABI: Compatibility
- `abort`: Other Builtins
- `abs`: Other Builtins
- accessing volatiles: Volatiles
- `acos`: Other Builtins
- `acosf`: Other Builtins
- `acosh`: Other Builtins
- `acoshf`: Other Builtins
- `acoshl`: Other Builtins
- `acosl`: Other Builtins
- Ada: G++ and GCC
- address constraints: Simple Constraints
- address of a label: Labels as Values
- `address_operand`: Simple Constraints
- `alias` attribute: Function Attributes
- aliasing of parameters: Code Gen Options
- `aligned` attribute: Type Attributes
- `aligned` attribute: Variable Attributes
- alignment: Alignment
- `alloca`: Other Builtins
- `alloca` vs variable-length arrays: Variable Length
- Allow nesting in an interrupt handler on the Blackfin processor.: Function Attributes
- alternate keywords: Alternate Keywords
- `always_inline` function attribute: Function Attributes
- AMD x86-64 Options: i386 and x86-64 Options
- AMD1: Standards
- ANSI C: Standards
- ANSI C standard: Standards
- ANSI C89: Standards
- ANSI support: C Dialect Options
- ANSI X3.159-1989: Standards
- apostrophes: Incompatibilities
- application binary interface: Compatibility
- ARC Options: ARC Options
- ARM [Annotated C++ Reference Manual]: Backwards Compatibility
- ARM options: ARM Options
- arrays of length zero: Zero Length
- arrays of variable length: Variable Length
- arrays, non-lvalue: Subscripting
- `asin`: Other Builtins
- `asinf`: Other Builtins
- `asinh`: Other Builtins
- `asinhf`: Other Builtins
- `asinhl`: Other Builtins
- `asinl`: Other Builtins
- `asm` constraints: Constraints
- `asm` expressions: Extended Asm
- assembler instructions: Extended Asm
- assembler names for identifiers: Asm Labels
- assembly code, invalid: Bug Criteria
- `atan`: Other Builtins
- `atan2`: Other Builtins
- `atan2f`: Other Builtins
- `atan2l`: Other Builtins
- `atanf`: Other Builtins
- `atanh`: Other Builtins
- `atanhf`: Other Builtins
- `atanhl`: Other Builtins
- `atanl`: Other Builtins
- attribute of types: Type Attributes
- attribute of variables: Variable Attributes
- attribute syntax: Attribute Syntax
- autoincrement/decrement addressing: Simple Constraints
- automatic `inline` for C++ member fns: Inline
- AVR Options: AVR Options
- Backwards Compatibility: Backwards Compatibility
- base class members: Name lookup
- `bcmp`: Other Builtins
- `below100` attribute: Variable Attributes
- binary compatibility: Compatibility
- Blackfin Options: Blackfin Options
- bound pointer to member function: Bound member functions
- bounds checking: Optimize Options
- bug criteria: Bug Criteria
- bugs: Bugs
- bugs, known: Trouble
- built-in functions: Other Builtins
- built-in functions: C Dialect Options
- `bzero`: Other Builtins
- C compilation options: Invoking GCC
- C intermediate output, nonexistent: G++ and GCC
- C language extensions: C Extensions
- C language, traditional: C Dialect Options
- C standard: Standards
- C standards: Standards
- `c++`: Invoking G++
- C++: G++ and GCC
- C++ comments: C++ Comments
- C++ compilation options: Invoking GCC
- C++ interface and implementation headers: C++ Interface
- C++ language extensions: C++ Extensions
- C++ member fns, automatically `inline`: Inline
- C++ misunderstandings: C++ Misunderstandings
- C++ options, command line: C++ Dialect Options
- C++ pragmas, effect on inlining: C++ Interface
- C++ source file suffixes: Invoking G++
- C++ static data, declaring and defining: Static Definitions
- C89: Standards
- C90: Standards
- C94: Standards
- C95: Standards
- C99: Standards
- C9X: Standards
- `C_INCLUDE_PATH`: Environment Variables
- `cabs`: Other Builtins
- `cabsf`: Other Builtins
- `cabsl`: Other Builtins
- `cacos`: Other Builtins
- `cacosf`: Other Builtins
- `cacosh`: Other Builtins
- `cacoshf`: Other Builtins
- `cacoshl`: Other Builtins
- `cacosl`: Other Builtins
- calling functions through the function vector on the H8/300 processors: Function Attributes
- `calloc`: Other Builtins
- `carg`: Other Builtins
- `cargf`: Other Builtins
- `cargl`: Other Builtins
- case labels in initializers: Designated Inits
- case ranges: Case Ranges
- `casin`: Other Builtins
- `casinf`: Other Builtins
- `casinh`: Other Builtins
- `casinhf`: Other Builtins
- `casinhl`: Other Builtins
- `casinl`: Other Builtins
- cast to a union: Cast to Union
- `catan`: Other Builtins
- `catanf`: Other Builtins
- `catanh`: Other Builtins
- `catanhf`: Other Builtins
- `catanhl`: Other Builtins
- `catanl`: Other Builtins
- `cbrt`: Other Builtins
- `cbrtf`: Other Builtins
- `cbrtl`: Other Builtins
- `ccos`: Other Builtins
- `ccosf`: Other Builtins
- `ccosh`: Other Builtins
- `ccoshf`: Other Builtins
- `ccoshl`: Other Builtins
- `ccosl`: Other Builtins
- `ceil`: Other Builtins
- `ceilf`: Other Builtins
- `ceill`: Other Builtins
- `cexp`: Other Builtins
- `cexpf`: Other Builtins
- `cexpl`: Other Builtins
- character set, execution: Preprocessor Options
- character set, input: Preprocessor Options
- character set, input normalization: Warning Options
- character set, wide execution: Preprocessor Options
- `cimag`: Other Builtins
- `cimagf`: Other Builtins
- `cimagl`: Other Builtins
- `cleanup` attribute: Variable Attributes
- `clog`: Other Builtins
- `clogf`: Other Builtins
- `clogl`: Other Builtins
- COBOL: G++ and GCC
- code generation conventions: Code Gen Options
- code, mixed with declarations: Mixed Declarations
- command options: Invoking GCC
- comments, C++ style: C++ Comments
- `common` attribute: Variable Attributes
- comparison of signed and unsigned values, warning: Warning Options
- compiler bugs, reporting: Bug Reporting
- compiler compared to C++ preprocessor: G++ and GCC
- compiler options, C++: C++ Dialect Options
- compiler options, Objective-C and Objective-C++: Objective-C and Objective-C++ Dialect Options
- compiler version, specifying: Target Options
- `COMPILER_PATH`: Environment Variables
- complex conjugation: Complex
- complex numbers: Complex
- compound literals: Compound Literals
- computed gotos: Labels as Values
- conditional expressions, extensions: Conditionals
- conflicting types: Disappointments
- `conj`: Other Builtins
- `conjf`: Other Builtins
- `conjl`: Other Builtins
- `const` applied to function: Function Attributes
- `const` function attribute: Function Attributes
- constants in constraints: Simple Constraints
- constraint modifier characters: Modifiers
- constraint, matching: Simple Constraints
- constraints, `asm`: Constraints
- constraints, machine specific: Machine Constraints
- constructing calls: Constructing Calls
- constructor expressions: Compound Literals
- `constructor` function attribute: Function Attributes
- contributors: Contributors
- `copysign`: Other Builtins
- `copysignf`: Other Builtins
- `copysignl`: Other Builtins
- core dump: Bug Criteria
- `cos`: Other Builtins
- `cosf`: Other Builtins
- `cosh`: Other Builtins
- `coshf`: Other Builtins
- `coshl`: Other Builtins
- `cosl`: Other Builtins
- `CPATH`: Environment Variables
- `CPLUS_INCLUDE_PATH`: Environment Variables
- `cpow`: Other Builtins
- `cpowf`: Other Builtins
- `cpowl`: Other Builtins
- `cproj`: Other Builtins
- `cprojf`: Other Builtins
- `cprojl`: Other Builtins
- `creal`: Other Builtins
- `crealf`: Other Builtins
- `creall`: Other Builtins
- CRIS Options: CRIS Options
- cross compiling: Target Options
- CRX Options: CRX Options
- `csin`: Other Builtins
- `csinf`: Other Builtins
- `csinh`: Other Builtins
- `csinhf`: Other Builtins
- `csinhl`: Other Builtins
- `csinl`: Other Builtins
- `csqrt`: Other Builtins
- `csqrtf`: Other Builtins
- `csqrtl`: Other Builtins
- `ctan`: Other Builtins
- `ctanf`: Other Builtins
- `ctanh`: Other Builtins
- `ctanhf`: Other Builtins
- `ctanhl`: Other Builtins
- `ctanl`: Other Builtins
- Darwin options: Darwin Options
- `dcgettext`: Other Builtins
- deallocating variable length arrays: Variable Length
- debugging information options: Debugging Options
- declaration scope: Incompatibilities
- declarations inside expressions: Statement Exprs
- declarations, mixed with code: Mixed Declarations
- declaring attributes of functions: Function Attributes
- declaring static data in C++: Static Definitions
- defining static data in C++: Static Definitions
- dependencies for make as output: Environment Variables
- dependencies, make: Preprocessor Options
- `DEPENDENCIES_OUTPUT`: Environment Variables
- dependent name lookup: Name lookup
- `deprecated` attribute: Variable Attributes
- `deprecated` attribute.: Function Attributes
- designated initializers: Designated Inits
- designator lists: Designated Inits
- designators: Designated Inits
- `destructor` function attribute: Function Attributes
- `dgettext`: Other Builtins
- diagnostic messages: Language Independent Options
- dialect options: C Dialect Options
- digits in constraint: Simple Constraints
- directory options: Directory Options
- dollar signs in identifier names: Dollar Signs
- double-word arithmetic: Long Long
- downward funargs: Nested Functions
- `drem`: Other Builtins
- `dremf`: Other Builtins
- `dreml`: Other Builtins
- ``E`' in constraint: Simple Constraints
- earlyclobber operand: Modifiers
- eight bit data on the H8/300, H8/300H, and H8S: Function Attributes
- empty structures: Empty Structures
- environment variables: Environment Variables
- `erf`: Other Builtins
- `erfc`: Other Builtins
- `erfcf`: Other Builtins
- `erfcl`: Other Builtins
- `erff`: Other Builtins
- `erfl`: Other Builtins
- error messages: Warnings and Errors
- escaped newlines: Escaped Newlines
- exception handler functions on the Blackfin processor: Function Attributes
- exclamation point: Multi-Alternative
- `exit`: Other Builtins
- `exp`: Other Builtins
- `exp10`: Other Builtins
- `exp10f`: Other Builtins
- `exp10l`: Other Builtins
- `exp2`: Other Builtins
- `exp2f`: Other Builtins
- `exp2l`: Other Builtins
- `expf`: Other Builtins
- `expl`: Other Builtins
- explicit register variables: Explicit Reg Vars
- `expm1`: Other Builtins
- `expm1f`: Other Builtins
- `expm1l`: Other Builtins
- expressions containing statements: Statement Exprs
- expressions, constructor: Compound Literals
- extended `asm`: Extended Asm
- extensible constraints: Simple Constraints
- extensions, `?:`: Conditionals
- extensions, C language: C Extensions
- extensions, C++ language: C++ Extensions
- external declaration scope: Incompatibilities
- `externally_visible` attribute.: Function Attributes
- ``F`' in constraint: Simple Constraints
- `fabs`: Other Builtins
- `fabsf`: Other Builtins
- `fabsl`: Other Builtins
- fatal signal: Bug Criteria
- `fdim`: Other Builtins
- `fdimf`: Other Builtins
- `fdiml`: Other Builtins
- FDL, GNU Free Documentation License: GNU Free Documentation License
- `ffs`: Other Builtins
- file name suffix: Overall Options
- file names: Link Options
- `flatten` function attribute: Function Attributes
- flexible array members: Zero Length
- `float` as function value type: Incompatibilities
- floating point precision: Disappointments
- floating point precision: Optimize Options
- `floor`: Other Builtins
- `floorf`: Other Builtins
- `floorl`: Other Builtins
- `fma`: Other Builtins
- `fmaf`: Other Builtins
- `fmal`: Other Builtins
- `fmax`: Other Builtins
- `fmaxf`: Other Builtins
- `fmaxl`: Other Builtins
- `fmin`: Other Builtins
- `fminf`: Other Builtins
- `fminl`: Other Builtins
- `fmod`: Other Builtins
- `fmodf`: Other Builtins
- `fmodl`: Other Builtins
- `format` function attribute: Function Attributes
- `format_arg` function attribute: Function Attributes
- Fortran: G++ and GCC
- forwarding calls: Constructing Calls
- `fprintf`: Other Builtins
- `fprintf_unlocked`: Other Builtins
- `fputs`: Other Builtins
- `fputs_unlocked`: Other Builtins
- freestanding environment: Standards
- freestanding implementation: Standards
- `frexp`: Other Builtins
- `frexpf`: Other Builtins
- `frexpl`: Other Builtins
- FRV Options: FRV Options
- `fscanf`: Other Builtins
- `fscanf`, and constant strings: Incompatibilities
- function addressability on the M32R/D: Function Attributes
- function attributes: Function Attributes
- function pointers, arithmetic: Pointer Arith
- function prototype declarations: Function Prototypes
- function without a prologue/epilogue code: Function Attributes
- function, size of pointer to: Pointer Arith
- functions called via pointer on the RS/6000 and PowerPC: Function Attributes
- functions in arbitrary sections: Function Attributes
- functions that are passed arguments in registers on the 386: Function Attributes
- functions that behave like malloc: Function Attributes
- functions that do not pop the argument stack on the 386: Function Attributes
- functions that do pop the argument stack on the 386: Function Attributes
- functions that have no side effects: Function Attributes
- functions that never return: Function Attributes
- functions that pop the argument stack on the 386: Function Attributes
- functions that return more than once: Function Attributes
- functions which do not handle memory bank switching on 68HC11/68HC12: Function Attributes
- functions which handle memory bank switching: Function Attributes
- functions with non-null pointer arguments: Function Attributes
- functions with `printf`, `scanf`, `strftime` or `strfmon` style arguments: Function Attributes
- ``g`' in constraint: Simple Constraints
- ``G`' in constraint: Simple Constraints
- `g++`: Invoking G++
- G++: G++ and GCC
- `gamma`: Other Builtins
- `gammaf`: Other Builtins
- `gammal`: Other Builtins
- GCC: G++ and GCC
- GCC command options: Invoking GCC
- `GCC_EXEC_PREFIX`: Environment Variables
- `gcc_struct`: Type Attributes
- `gcc_struct` attribute: Variable Attributes
- `gcov`: Debugging Options
- `gettext`: Other Builtins
- global offset table: Code Gen Options
- global register after `longjmp`: Global Reg Vars
- global register variables: Global Reg Vars
- GNAT: G++ and GCC
- GNU C Compiler: G++ and GCC
- GNU Compiler Collection: G++ and GCC
- goto with computed label: Labels as Values
- gp-relative references (MIPS): MIPS Options
- `gprof`: Debugging Options
- grouping options: Invoking GCC
- ``H`' in constraint: Simple Constraints
- hardware models and configurations, specifying: Submodel Options
- hex floats: Hex Floats
- hosted environment: C Dialect Options
- hosted environment: Standards
- hosted implementation: Standards
- HPPA Options: HPPA Options
- `hypot`: Other Builtins
- `hypotf`: Other Builtins
- `hypotl`: Other Builtins
- ``I`' in constraint: Simple Constraints
- ``i`' in constraint: Simple Constraints
- i386 Options: i386 and x86-64 Options
- IA-64 Options: IA-64 Options
- IBM RS/6000 and PowerPC Options: RS/6000 and PowerPC Options
- identifier names, dollar signs in: Dollar Signs
- identifiers, names in assembler code: Asm Labels
- `ilogb`: Other Builtins
- `ilogbf`: Other Builtins
- `ilogbl`: Other Builtins
- `imaxabs`: Other Builtins
- implementation-defined behavior, C language: C Implementation
- implied `#pragma implementation`: C++ Interface
- incompatibilities of GCC: Incompatibilities
- increment operators: Bug Criteria
- `index`: Other Builtins
- indirect calls on ARM: Function Attributes
- indirect calls on MIPS: Function Attributes
- init_priority attribute: C++ Attributes
- initializations in expressions: Compound Literals
- initializers with labeled elements: Designated Inits
- initializers, non-constant: Initializers
- `inline` automatic for C++ member fns: Inline
- inline functions: Inline
- inline functions, omission of: Inline
- inlining and C++ pragmas: C++ Interface
- installation trouble: Trouble
- integrating function code: Inline
- Intel 386 Options: i386 and x86-64 Options
- interface and implementation headers, C++: C++ Interface
- intermediate C version, nonexistent: G++ and GCC
- interrupt handler functions: Function Attributes
- interrupt handler functions on the Blackfin, m68k, H8/300 and SH processors: Function Attributes
- introduction: Top
- invalid assembly code: Bug Criteria
- invalid input: Bug Criteria
- invoking `g++`: Invoking G++
- `isalnum`: Other Builtins
- `isalpha`: Other Builtins
- `isascii`: Other Builtins
- `isblank`: Other Builtins
- `iscntrl`: Other Builtins
- `isdigit`: Other Builtins
- `isgraph`: Other Builtins
- `islower`: Other Builtins
- ISO 9899: Standards
- ISO C: Standards
- ISO C standard: Standards
- ISO C90: Standards
- ISO C94: Standards
- ISO C95: Standards
- ISO C99: Standards
- ISO C9X: Standards
- ISO support: C Dialect Options
- ISO/IEC 9899: Standards
- `isprint`: Other Builtins
- `ispunct`: Other Builtins
- `isspace`: Other Builtins
- `isupper`: Other Builtins
- `iswalnum`: Other Builtins
- `iswalpha`: Other Builtins
- `iswblank`: Other Builtins
- `iswcntrl`: Other Builtins
- `iswdigit`: Other Builtins
- `iswgraph`: Other Builtins
- `iswlower`: Other Builtins
- `iswprint`: Other Builtins
- `iswpunct`: Other Builtins
- `iswspace`: Other Builtins
- `iswupper`: Other Builtins
- `iswxdigit`: Other Builtins
- `isxdigit`: Other Builtins
- `j0`: Other Builtins
- `j0f`: Other Builtins
- `j0l`: Other Builtins
- `j1`: Other Builtins
- `j1f`: Other Builtins
- `j1l`: Other Builtins
- Java: G++ and GCC
- java_interface attribute: C++ Attributes
- `jn`: Other Builtins
- `jnf`: Other Builtins
- `jnl`: Other Builtins
- keywords, alternate: Alternate Keywords
- known causes of trouble: Trouble
- labeled elements in initializers: Designated Inits
- labels as values: Labels as Values
- `labs`: Other Builtins
- `LANG`: Environment Variables
- language dialect options: C Dialect Options
- `LC_ALL`: Environment Variables
- `LC_CTYPE`: Environment Variables
- `LC_MESSAGES`: Environment Variables
- `ldexp`: Other Builtins
- `ldexpf`: Other Builtins
- `ldexpl`: Other Builtins
- length-zero arrays: Zero Length
- `lgamma`: Other Builtins
- `lgammaf`: Other Builtins
- `lgammal`: Other Builtins
- Libraries: Link Options
- `LIBRARY_PATH`: Environment Variables
- link options: Link Options
- `LL` integer suffix: Long Long
- `llabs`: Other Builtins
- `llrint`: Other Builtins
- `llrintf`: Other Builtins
- `llrintl`: Other Builtins
- `llround`: Other Builtins
- `llroundf`: Other Builtins
- `llroundl`: Other Builtins
- load address instruction: Simple Constraints
- local labels: Local Labels
- local variables in macros: Typeof
- local variables, specifying registers: Local Reg Vars
- locale: Environment Variables
- locale definition: Environment Variables
- `log`: Other Builtins
- `log10`: Other Builtins
- `log10f`: Other Builtins
- `log10l`: Other Builtins
- `log1p`: Other Builtins
- `log1pf`: Other Builtins
- `log1pl`: Other Builtins
- `log2`: Other Builtins
- `log2f`: Other Builtins
- `log2l`: Other Builtins
- `logb`: Other Builtins
- `logbf`: Other Builtins
- `logbl`: Other Builtins
- `logf`: Other Builtins
- `logl`: Other Builtins
- `long long` data types: Long Long
- `longjmp`: Global Reg Vars
- `longjmp` incompatibilities: Incompatibilities
- `longjmp` warnings: Warning Options
- `lrint`: Other Builtins
- `lrintf`: Other Builtins
- `lrintl`: Other Builtins
- `lround`: Other Builtins
- `lroundf`: Other Builtins
- `lroundl`: Other Builtins
- ``m`' in constraint: Simple Constraints
- M32C options: M32C Options
- M32R/D options: M32R/D Options
- M680x0 options: M680x0 Options
- M68hc1x options: M68hc1x Options
- machine dependent options: Submodel Options
- machine specific constraints: Machine Constraints
- macro with variable arguments: Variadic Macros
- macros containing `asm`: Extended Asm
- macros, inline alternative: Inline
- macros, local labels: Local Labels
- macros, local variables in: Typeof
- macros, statements in expressions: Statement Exprs
- macros, types of arguments: Typeof
- make: Preprocessor Options
- `malloc`: Other Builtins
- `malloc` attribute: Function Attributes
- matching constraint: Simple Constraints
- MCore options: MCore Options
- member fns, automatically `inline`: Inline
- `memcmp`: Other Builtins
- `memcpy`: Other Builtins
- memory references in constraints: Simple Constraints
- `mempcpy`: Other Builtins
- `memset`: Other Builtins
- Mercury: G++ and GCC
- message formatting: Language Independent Options
- messages, warning: Warning Options
- messages, warning and error: Warnings and Errors
- middle-operands, omitted: Conditionals
- MIPS options: MIPS Options
- misunderstandings in C++: C++ Misunderstandings
- mixed declarations and code: Mixed Declarations
- `mktemp`, and constant strings: Incompatibilities
- MMIX Options: MMIX Options
- MN10300 options: MN10300 Options
- `mode` attribute: Variable Attributes
- `modf`: Other Builtins
- `modff`: Other Builtins
- `modfl`: Other Builtins
- modifiers in constraints: Modifiers
- `ms_struct`: Type Attributes
- `ms_struct` attribute: Variable Attributes
- MT options: MT Options
- mudflap: Optimize Options
- multiple alternative constraints: Multi-Alternative
- multiprecision arithmetic: Long Long
- ``n`' in constraint: Simple Constraints
- names used in assembler code: Asm Labels
- naming convention, implementation headers: C++ Interface
- `nearbyint`: Other Builtins
- `nearbyintf`: Other Builtins
- `nearbyintl`: Other Builtins
- nested functions: Nested Functions
- newlines (escaped): Escaped Newlines
- `nextafter`: Other Builtins
- `nextafterf`: Other Builtins
- `nextafterl`: Other Builtins
- `nexttoward`: Other Builtins
- `nexttowardf`: Other Builtins
- `nexttowardl`: Other Builtins
- NFC: Warning Options
- NFKC: Warning Options
- NMI handler functions on the Blackfin processor: Function Attributes
- `no_instrument_function` function attribute: Function Attributes
- `nocommon` attribute: Variable Attributes
- `noinline` function attribute: Function Attributes
- non-constant initializers: Initializers
- non-static inline function: Inline
- `nonnull` function attribute: Function Attributes
- `noreturn` function attribute: Function Attributes
- `nothrow` function attribute: Function Attributes
- ``o`' in constraint: Simple Constraints
- `OBJC_INCLUDE_PATH`: Environment Variables
- Objective-C: Standards
- Objective-C: G++ and GCC
- Objective-C and Objective-C++ options, command line: Objective-C and Objective-C++ Dialect Options
- Objective-C++: Standards
- Objective-C++: G++ and GCC
- offsettable address: Simple Constraints
- old-style function definitions: Function Prototypes
- omitted middle-operands: Conditionals
- open coding: Inline
- operand constraints, `asm`: Constraints
- optimize options: Optimize Options
- options to control diagnostics formatting: Language Independent Options
- options to control warnings: Warning Options
- options, C++: C++ Dialect Options
- options, code generation: Code Gen Options
- options, debugging: Debugging Options
- options, dialect: C Dialect Options
- options, directory search: Directory Options
- options, GCC command: Invoking GCC
- options, grouping: Invoking GCC
- options, linking: Link Options
- options, Objective-C and Objective-C++: Objective-C and Objective-C++ Dialect Options
- options, optimization: Optimize Options
- options, order: Invoking GCC
- options, preprocessor: Preprocessor Options
- order of evaluation, side effects: Non-bugs
- order of options: Invoking GCC
- other register constraints: Simple Constraints
- output file option: Overall Options
- overloaded virtual fn, warning: C++ Dialect Options
- ``p`' in constraint: Simple Constraints
- `packed` attribute: Variable Attributes
- parameter forward declaration: Variable Length
- parameters, aliased: Code Gen Options
- Pascal: G++ and GCC
- PDP-11 Options: PDP-11 Options
- PIC: Code Gen Options
- pmf: Bound member functions
- pointer arguments: Function Attributes
- pointer to member function: Bound member functions
- portions of temporary objects, pointers to: Temporaries
- `pow`: Other Builtins
- `pow10`: Other Builtins
- `pow10f`: Other Builtins
- `pow10l`: Other Builtins
- PowerPC options: PowerPC Options
- `powf`: Other Builtins
- `powl`: Other Builtins
- pragma, align: Solaris Pragmas
- pragma, extern_prefix: Symbol-Renaming Pragmas
- pragma, fini: Solaris Pragmas
- pragma, init: Solaris Pragmas
- pragma, long_calls: ARM Pragmas
- pragma, long_calls_off: ARM Pragmas
- pragma, longcall: RS/6000 and PowerPC Pragmas
- pragma, mark: Darwin Pragmas
- pragma, memregs: M32C Pragmas
- pragma, no_long_calls: ARM Pragmas
- pragma, options align: Darwin Pragmas
- pragma, reason for not using: Function Attributes
- pragma, redefine_extname: Symbol-Renaming Pragmas
- pragma, segment: Darwin Pragmas
- pragma, unused: Darwin Pragmas
- pragma, weak: Weak Pragmas
- pragmas: Pragmas
- pragmas in C++, effect on inlining: C++ Interface
- pragmas, interface and implementation: C++ Interface
- pragmas, warning of unknown: Warning Options
- precompiled headers: Precompiled Headers
- preprocessing numbers: Incompatibilities
- preprocessing tokens: Incompatibilities
- preprocessor options: Preprocessor Options
- `printf`: Other Builtins
- `printf_unlocked`: Other Builtins
- `prof`: Debugging Options
- promotion of formal parameters: Function Prototypes
- `pure` function attribute: Function Attributes
- push address instruction: Simple Constraints
- `putchar`: Other Builtins
- `puts`: Other Builtins
- `qsort`, and global register variables: Global Reg Vars
- question mark: Multi-Alternative
- ``r`' in constraint: Simple Constraints
- ranges in case statements: Case Ranges
- read-only strings: Incompatibilities
- register variable after `longjmp`: Global Reg Vars
- registers: Extended Asm
- registers for local variables: Local Reg Vars
- registers in constraints: Simple Constraints
- registers, global allocation: Explicit Reg Vars
- registers, global variables in: Global Reg Vars
- `regparm` attribute: Function Attributes
- relocation truncated to fit (MIPS): MIPS Options
- `remainder`: Other Builtins
- `remainderf`: Other Builtins
- `remainderl`: Other Builtins
- `remquo`: Other Builtins
- `remquof`: Other Builtins
- `remquol`: Other Builtins
- reordering, warning: C++ Dialect Options
- reporting bugs: Bugs
- rest argument (in macro): Variadic Macros
- restricted pointers: Restricted Pointers
- restricted references: Restricted Pointers
- restricted this pointer: Restricted Pointers
- `returns_twice` attribute: Function Attributes
- `rindex`: Other Builtins
- `rint`: Other Builtins
- `rintf`: Other Builtins
- `rintl`: Other Builtins
- `round`: Other Builtins
- `roundf`: Other Builtins
- `roundl`: Other Builtins
- RS/6000 and PowerPC Options: RS/6000 and PowerPC Options
- RTTI: Vague Linkage
- run-time options: Code Gen Options
- ``s`' in constraint: Simple Constraints
- S/390 and zSeries Options: S/390 and zSeries Options
- save all registers on the Blackfin, H8/300, H8/300H, and H8S: Function Attributes
- `scalb`: Other Builtins
- `scalbf`: Other Builtins
- `scalbl`: Other Builtins
- `scalbln`: Other Builtins
- `scalblnf`: Other Builtins
- `scalbn`: Other Builtins
- `scalbnf`: Other Builtins
- `scanf`, and constant strings: Incompatibilities
- `scanfnl`: Other Builtins
- scope of a variable length array: Variable Length
- scope of declaration: Disappointments
- scope of external declarations: Incompatibilities
- search path: Directory Options
- `section` function attribute: Function Attributes
- `section` variable attribute: Variable Attributes
- `sentinel` function attribute: Function Attributes
- `setjmp`: Global Reg Vars
- `setjmp` incompatibilities: Incompatibilities
- shared strings: Incompatibilities
- `shared` variable attribute: Variable Attributes
- side effect in ?:: Conditionals
- side effects, macro argument: Statement Exprs
- side effects, order of evaluation: Non-bugs
- signal handler functions on the AVR processors: Function Attributes
- `signbit`: Other Builtins
- `signbitf`: Other Builtins
- `signbitl`: Other Builtins
- signed and unsigned values, comparison warning: Warning Options
- `significand`: Other Builtins
- `significandf`: Other Builtins
- `significandl`: Other Builtins
- simple constraints: Simple Constraints
- `sin`: Other Builtins
- `sincos`: Other Builtins
- `sincosf`: Other Builtins
- `sincosl`: Other Builtins
- `sinf`: Other Builtins
- `sinh`: Other Builtins
- `sinhf`: Other Builtins
- `sinhl`: Other Builtins
- `sinl`: Other Builtins
- `sizeof`: Typeof
- smaller data references: M32R/D Options
- smaller data references (MIPS): MIPS Options
- smaller data references (PowerPC): RS/6000 and PowerPC Options
- `snprintf`: Other Builtins
- SPARC options: SPARC Options
- Spec Files: Spec Files
- specified registers: Explicit Reg Vars
- specifying compiler version and target machine: Target Options
- specifying hardware config: Submodel Options
- specifying machine version: Target Options
- specifying registers for local variables: Local Reg Vars
- speed of compilation: Precompiled Headers
- `sprintf`: Other Builtins
- `sqrt`: Other Builtins
- `sqrtf`: Other Builtins
- `sqrtl`: Other Builtins
- `sscanf`: Other Builtins
- `sscanf`, and constant strings: Incompatibilities
- `sseregparm` attribute: Function Attributes
- statements inside expressions: Statement Exprs
- static data in C++, declaring and defining: Static Definitions
- `stpcpy`: Other Builtins
- `stpncpy`: Other Builtins
- `strcasecmp`: Other Builtins
- `strcat`: Other Builtins
- `strchr`: Other Builtins
- `strcmp`: Other Builtins
- `strcpy`: Other Builtins
- `strcspn`: Other Builtins
- `strdup`: Other Builtins
- `strfmon`: Other Builtins
- `strftime`: Other Builtins
- string constants: Incompatibilities
- `strlen`: Other Builtins
- `strncasecmp`: Other Builtins
- `strncat`: Other Builtins
- `strncmp`: Other Builtins
- `strncpy`: Other Builtins
- `strndup`: Other Builtins
- `strpbrk`: Other Builtins
- `strrchr`: Other Builtins
- `strspn`: Other Builtins
- `strstr`: Other Builtins
- struct: Unnamed Fields
- structures: Incompatibilities
- structures, constructor expression: Compound Literals
- submodel options: Submodel Options
- subscripting: Subscripting
- subscripting and function values: Subscripting
- suffixes for C++ source: Invoking G++
- `SUNPRO_DEPENDENCIES`: Environment Variables
- suppressing warnings: Warning Options
- surprises in C++: C++ Misunderstandings
- syntax checking: Warning Options
- system headers, warnings from: Warning Options
- `tan`: Other Builtins
- `tanf`: Other Builtins
- `tanh`: Other Builtins
- `tanhf`: Other Builtins
- `tanhl`: Other Builtins
- `tanl`: Other Builtins
- target machine, specifying: Target Options
- target options: Target Options
- TC1: Standards
- TC2: Standards
- Technical Corrigenda: Standards
- Technical Corrigendum 1: Standards
- Technical Corrigendum 2: Standards
- template instantiation: Template Instantiation
- temporaries, lifetime of: Temporaries
- `tgamma`: Other Builtins
- `tgammaf`: Other Builtins
- `tgammal`: Other Builtins
- Thread-Local Storage: Thread-Local
- thunks: Nested Functions
- tiny data section on the H8/300H and H8S: Function Attributes
- TLS: Thread-Local
- `tls_model` attribute: Variable Attributes
- `TMPDIR`: Environment Variables
- TMS320C3x/C4x Options: TMS320C3x/C4x Options
- `toascii`: Other Builtins
- `tolower`: Other Builtins
- `toupper`: Other Builtins
- `towlower`: Other Builtins
- `towupper`: Other Builtins
- traditional C language: C Dialect Options
- treelang: Standards
- treelang: G++ and GCC
- `trunc`: Other Builtins
- `truncf`: Other Builtins
- `truncl`: Other Builtins
- two-stage name lookup: Name lookup
- type alignment: Alignment
- type attributes: Type Attributes
- type_info: Vague Linkage
- typedef names as function parameters: Incompatibilities
- `typeof`: Typeof
- `ULL` integer suffix: Long Long
- Ultrix calling convention: Interoperation
- undefined behavior: Bug Criteria
- undefined function value: Bug Criteria
- underscores in variables in macros: Typeof
- union: Unnamed Fields
- union, casting to a: Cast to Union
- unions: Incompatibilities
- unknown pragmas, warning: Warning Options
- unresolved references and `-nodefaultlibs`: Link Options
- unresolved references and `-nostdlib`: Link Options
- `unused` attribute.: Function Attributes
- `used` attribute.: Function Attributes
- User stack pointer in interrupts on the Blackfin: Function Attributes
- ``V`' in constraint: Simple Constraints
- V850 Options: V850 Options
- vague linkage: Vague Linkage
- value after `longjmp`: Global Reg Vars
- variable addressability on the IA-64: Function Attributes
- variable addressability on the M32R/D: Variable Attributes
- variable alignment: Alignment
- variable attributes: Variable Attributes
- variable number of arguments: Variadic Macros
- variable-length array scope: Variable Length
- variable-length arrays: Variable Length
- variables in specified registers: Explicit Reg Vars
- variables, local, in macros: Typeof
- variadic macros: Variadic Macros
- VAX calling convention: Interoperation
- VAX options: VAX Options
- `vfprintf`: Other Builtins
- `vfscanf`: Other Builtins
- `visibility` attribute: Function Attributes
- VLAs: Variable Length
- void pointers, arithmetic: Pointer Arith
- void, size of pointer to: Pointer Arith
- volatile access: Volatiles
- `volatile` applied to function: Function Attributes
- volatile read: Volatiles
- volatile write: Volatiles
- `vprintf`: Other Builtins
- `vscanf`: Other Builtins
- `vsnprintf`: Other Builtins
- `vsprintf`: Other Builtins
- `vsscanf`: Other Builtins
- vtable: Vague Linkage
- `warn_unused_result` attribute: Function Attributes
- warning for comparison of signed and unsigned values: Warning Options
- warning for overloaded virtual fn: C++ Dialect Options
- warning for reordering of member initializers: C++ Dialect Options
- warning for unknown pragmas: Warning Options
- warning messages: Warning Options
- warnings from system headers: Warning Options
- warnings vs errors: Warnings and Errors
- `weak` attribute: Function Attributes
- `weakref` attribute: Function Attributes
- whitespace: Incompatibilities
- ``X`' in constraint: Simple Constraints
- X3.159-1989: Standards
- x86-64 options: x86-64 Options
- x86-64 Options: i386 and x86-64 Options
- Xstormy16 Options: Xstormy16 Options
- Xtensa Options: Xtensa Options
- `y0`: Other Builtins
- `y0f`: Other Builtins
- `y0l`: Other Builtins
- `y1`: Other Builtins
- `y1f`: Other Builtins
- `y1l`: Other Builtins
- `yn`: Other Builtins
- `ynf`: Other Builtins
- `ynl`: Other Builtins
- zero-length arrays: Zero Length
- zero-size structures: Empty Structures
- zSeries options: zSeries Options

#### Footnotes

[1] On some systems, ``gcc -shared`' needs to build supplementary stub code for constructors to work. On multi-libbed systems, ``gcc -shared`' must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.

[2] Future versions of GCC may zero-extend, or use a target-defined `ptr_extend` pattern. Do not rely on sign extension.

[3] The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.

[4] A file's basename was the name stripped of all leading path information and of trailing suffixes, such as ``.h`' or ``.C`' or ``.cc`'.

[5] The C++ standard just uses the term “dependent” for names that depend on the type or value of template parameters. This shorter term will also be used in the rest of this section.