Using the GNU Compiler Collection (GCC)

This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to the compilers version 4.5.3. 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 section `Introduction' in GNU Compiler Collection (GCC) Internals.

1. Programming Languages Supported by GCC		You can compile C or C++ programs.
2. Language Standards Supported by GCC		Language standards supported by GCC.
3. GCC Command Options		Command options supported by `gcc'.
4. C Implementation-defined behavior		How GCC implements the ISO C specification.
5. C++ Implementation-defined behavior		How GCC implements the ISO C++ specification.
6. Extensions to the C Language Family		GNU extensions to the C language family.
7. Extensions to the C++ Language		GNU extensions to the C++ language.
8. GNU Objective-C runtime features
9. Binary Compatibility
10. gcov---a Test Coverage Program		gcov---a test coverage program.
11. Known Causes of Trouble with GCC		If you have trouble using GCC.
12. Reporting Bugs		How, why and where to report bugs.
13. How To Get Help with GCC		How to find suppliers of support for GCC.
14. Contributing to GCC Development		How to contribute to testing and developing GCC.

The GNU Project and GNU/Linux

Contributors to GCC

People who have contributed to GCC.

Option Index		Index to command line options.
Keyword Index		Index of concepts and symbol names.

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.

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.

2.1 C language

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=c90' 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 section 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-4.5/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 three Technical Corrigenda published in 2001, 2004 and 2007. 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 section 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=gnu90' (for C90 with GNU extensions) or `-std=gnu99' (for C99 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu90'; 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 C90 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 section 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 section 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

2.2 C++ language

GCC supports the ISO C++ standard (1998) and contains experimental support for the upcoming ISO C++ standard (200x).

The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998) and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These standards are referred to as C++98 and C++03, respectively. GCC implements the majority of C++98 (export is a notable exception) and most of the changes in C++03. To select this standard in GCC, use one of the options `-ansi' or `-std=c++98'; 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).

The ISO C++ committee is working on a new ISO C++ standard, dubbed C++0x, that is intended to be published by 2009. C++0x contains several changes to the C++ language, some of which have been implemented in an experimental C++0x mode in GCC. The C++0x mode in GCC tracks the draft working paper for the C++0x standard; the latest working paper is available on the ISO C++ committee's web site at http://www.open-std.org/jtc1/sc22/wg21/. For information regarding the C++0x features available in the experimental C++0x mode, see http://gcc.gnu.org/projects/cxx0x.html. To select this standard in GCC, use the option `-std=c++0x'; 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).

By default, GCC provides some extensions to the C++ language; See section Options Controlling C++ Dialect. Use of the `-std' option listed above will disable these extensions. You may also select an extended version of the C++ language explicitly with `-std=gnu++98' (for C++98 with GNU extensions) or `-std=gnu++0x' (for C++0x with GNU extensions). The default, if no C++ language dialect options are given, is `-std=gnu++98'.

2.3 Objective-C and Objective-C++ languages

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/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/ is a recent (and periodically updated) version;
• http://objc.toodarkpark.net is an older example;
• http://www.gnustep.org and http://gcc.gnu.org/readings.html have additional useful information.

See section `About This Guide' in GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.

See section `Standards' in The GNU Fortran Compiler, for details of standards supported by GNU Fortran.

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

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 section 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: `-dv' is very different from `-d -v'.

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. Also, the placement of the `-l' option is significant.

Many options have long names starting with `-f' or with `-W'---for example, `-fmove-loop-invariants', `-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 section Option Index, for an index to GCC's options.

3.1 Option Summary		Brief list of all options, without explanations.
3.2 Options Controlling the Kind of Output		Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
3.3 Compiling C++ Programs		Compiling C++ programs.
3.4 Options Controlling C Dialect		Controlling the variant of C language compiled.
3.5 Options Controlling C++ Dialect		Variations on C++.
3.6 Options Controlling Objective-C and Objective-C++ Dialects		Variations on Objective-C and Objective-C++.
3.7 Options to Control Diagnostic Messages Formatting		Controlling how diagnostics should be formatted.
3.8 Options to Request or Suppress Warnings		How picky should the compiler be?
3.9 Options for Debugging Your Program or GCC		Symbol tables, measurements, and debugging dumps.
3.10 Options That Control Optimization		How much optimization?
3.11 Options Controlling the Preprocessor		Controlling header files and macro definitions. Also, getting dependency information for Make.
3.12 Passing Options to the Assembler		Passing options to the assembler.
3.13 Options for Linking		Specifying libraries and so on.
3.14 Options for Directory Search		Where to find header files and libraries. Where to find the compiler executable files.
3.15 Specifying subprocesses and the switches to pass to them		How to pass switches to sub-processes.
3.16 Specifying Target Machine and Compiler Version		Running a cross-compiler, or an old version of GCC.
3.17 Hardware Models and Configurations		Specifying minor hardware or convention variations, such as 68010 vs 68020.
3.18 Options for Code Generation Conventions		Specifying conventions for function calls, data layout and register usage.
3.19 Environment Variables Affecting GCC		Env vars that affect GCC.
3.20 Using Precompiled Headers		Compiling a header once, and using it many times.

3.1 Option Summary

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

Overall Options

See section Options Controlling the Kind of Output. @gccoptlist{-c -S -E -o file -combine -no-canonical-prefixes @gol -pipe -pass-exit-codes @gol -x language -v -### --help[=class[,...]] --target-help @gol --version -wrapper@file -fplugin=file -fplugin-arg-name=arg}

C Language Options

See section Options Controlling C Dialect. @gccoptlist{-ansi -std=standard -fgnu89-inline @gol -aux-info filename @gol -fno-asm -fno-builtin -fno-builtin-function @gol -fhosted -ffreestanding -fopenmp -fms-extensions @gol -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol -fallow-single-precision -fcond-mismatch -flax-vector-conversions @gol -fsigned-bitfields -fsigned-char @gol -funsigned-bitfields -funsigned-char}

C++ Language Options

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

Objective-C and Objective-C++ Language Options

See section Options Controlling Objective-C and Objective-C++ Dialects. @gccoptlist{-fconstant-string-class=class-name @gol -fgnu-runtime -fnext-runtime @gol -fno-nil-receivers @gol -fobjc-call-cxx-cdtors @gol -fobjc-direct-dispatch @gol -fobjc-exceptions @gol -fobjc-gc @gol -freplace-objc-classes @gol -fzero-link @gol -gen-decls @gol -Wassign-intercept @gol -Wno-protocol -Wselector @gol -Wstrict-selector-match @gol -Wundeclared-selector}

Language Independent Options

See section Options to Control Diagnostic Messages Formatting. @gccoptlist{-fmessage-length=n @gol -fdiagnostics-show-location=[once|every-line] @gol -fdiagnostics-show-option}

Warning Options

See section Options to Request or Suppress Warnings. @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol -w -Wextra -Wall -Waddress -Waggregate-return -Warray-bounds @gol -Wno-attributes -Wno-builtin-macro-redefined @gol -Wc++-compat -Wc++0x-compat -Wcast-align -Wcast-qual @gol -Wchar-subscripts -Wclobbered -Wcomment @gol -Wconversion -Wcoverage-mismatch -Wno-deprecated @gol -Wno-deprecated-declarations -Wdisabled-optimization @gol -Wno-div-by-zero -Wempty-body -Wenum-compare -Wno-endif-labels @gol -Werror -Werror=* @gol -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral @gol -Wformat-security -Wformat-y2k @gol -Wframe-larger-than=len -Wjump-misses-init -Wignored-qualifiers @gol -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol -Winit-self -Winline @gol -Wno-int-to-pointer-cast -Wno-invalid-offsetof @gol -Winvalid-pch -Wlarger-than=len -Wunsafe-loop-optimizations @gol -Wlogical-op -Wlong-long @gol -Wmain -Wmissing-braces -Wmissing-field-initializers @gol -Wmissing-format-attribute -Wmissing-include-dirs @gol -Wmissing-noreturn -Wno-mudflap @gol -Wno-multichar -Wnonnull -Wno-overflow @gol -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded @gol -Wparentheses -Wpedantic-ms-format -Wno-pedantic-ms-format @gol -Wpointer-arith -Wno-pointer-to-int-cast @gol -Wredundant-decls @gol -Wreturn-type -Wsequence-point -Wshadow @gol -Wsign-compare -Wsign-conversion -Wstack-protector @gol -Wstrict-aliasing -Wstrict-aliasing=n @gol -Wstrict-overflow -Wstrict-overflow=n @gol -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand @gol -Wsystem-headers -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized @gol -Wunknown-pragmas -Wno-pragmas @gol -Wunsuffixed-float-constants -Wunused -Wunused-function @gol -Wunused-label -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable @gol -Wvariadic-macros -Wvla @gol -Wvolatile-register-var -Wwrite-strings}

C and Objective-C-only Warning Options

@gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs @gol -Wold-style-declaration -Wold-style-definition @gol -Wstrict-prototypes -Wtraditional -Wtraditional-conversion @gol -Wdeclaration-after-statement -Wpointer-sign}

Debugging Options

See section Options for Debugging Your Program or GCC. @gccoptlist{-dletters -dumpspecs -dumpmachine -dumpversion @gol -fdbg-cnt-list -fdbg-cnt=counter-value-list @gol -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links @gol -fdump-translation-unit[-n] @gol -fdump-class-hierarchy[-n] @gol -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline @gol -fdump-statistics @gol -fdump-tree-all @gol -fdump-tree-original[-n] @gol -fdump-tree-optimized[-n] @gol -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol -fdump-tree-ch @gol -fdump-tree-ssa[-n] -fdump-tree-pre[-n] @gol -fdump-tree-ccp[-n] -fdump-tree-dce[-n] @gol -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] @gol -fdump-tree-dom[-n] @gol -fdump-tree-dse[-n] @gol -fdump-tree-phiprop[-n] @gol -fdump-tree-phiopt[-n] @gol -fdump-tree-forwprop[-n] @gol -fdump-tree-copyrename[-n] @gol -fdump-tree-nrv -fdump-tree-vect @gol -fdump-tree-sink @gol -fdump-tree-sra[-n] @gol -fdump-tree-forwprop[-n] @gol -fdump-tree-fre[-n] @gol -fdump-tree-vrp[-n] @gol -ftree-vectorizer-verbose=n @gol -fdump-tree-storeccp[-n] @gol -fdump-final-insns=file @gol -fcompare-debug[=opts] -fcompare-debug-second @gol -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol -feliminate-unused-debug-symbols -femit-class-debug-always @gol -fenable-icf-debug @gol -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs @gol -frandom-seed=string -fsched-verbose=n @gol -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose @gol -ftest-coverage -ftime-report -fvar-tracking @gol -fvar-tracking-assignments -fvar-tracking-assignments-toggle @gol -g -glevel -gtoggle -gcoff -gdwarf-version @gol -ggdb -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf @gol -gvms -gxcoff -gxcoff+ @gol -fno-merge-debug-strings -fno-dwarf2-cfi-asm @gol -fdebug-prefix-map=old=new @gol -femit-struct-debug-baseonly -femit-struct-debug-reduced @gol -femit-struct-debug-detailed[=spec-list] @gol -p -pg -print-file-name=library -print-libgcc-file-name @gol -print-multi-directory -print-multi-lib -print-multi-os-directory @gol -print-prog-name=program -print-search-dirs -Q @gol -print-sysroot -print-sysroot-headers-suffix @gol -save-temps -save-temps=cwd -save-temps=obj -time[=file]}

Optimization Options

See section Options that Control Optimization. @gccoptlist{ -falign-functions[=n] -falign-jumps[=n] @gol -falign-labels[=n] -falign-loops[=n] -fassociative-math @gol -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize @gol -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves @gol -fcheck-data-deps -fconserve-stack -fcprop-registers -fcrossjumping @gol -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range @gol -fdata-sections -fdce -fdce @gol -fdelayed-branch -fdelete-null-pointer-checks -fdse -fdse @gol -fearly-inlining -fipa-sra -fexpensive-optimizations -ffast-math @gol -ffinite-math-only -ffloat-store -fexcess-precision=style @gol -fforward-propagate -ffunction-sections @gol -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm @gol -fgcse-sm -fif-conversion -fif-conversion2 -findirect-inlining @gol -finline-functions -finline-functions-called-once -fsort-data -finline-limit=n @gol -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-matrix-reorg -fipa-pta @gol -fipa-pure-const -fipa-reference -fipa-struct-reorg @gol -fipa-type-escape -fira-algorithm=algorithm @gol -fira-region=region -fira-coalesce @gol -fira-loop-pressure -fno-ira-share-save-slots @gol -fno-ira-share-spill-slots -fira-verbose=n @gol -fivopts -fkeep-inline-functions -fkeep-static-consts @gol -floop-block -floop-interchange -floop-strip-mine -fgraphite-identity @gol -floop-parallelize-all -flto -flto-compression-level -flto-report -fltrans @gol -fltrans-output-list -fmerge-all-constants -fmerge-constants -fmodulo-sched @gol -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmudflap @gol -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline @gol -fno-defer-pop -fno-function-cse -fno-guess-branch-probability @gol -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol -fno-sched-interblock -fno-sched-spec -fno-signed-zeros @gol -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss @gol -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls @gol -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays @gol -fprofile-correction -fprofile-dir=path -fprofile-generate @gol -fprofile-generate=path @gol -fprofile-use -fprofile-use=path -fprofile-values @gol -freciprocal-math -fregmove -frename-registers -freorder-blocks @gol -freorder-blocks-and-partition -freorder-functions @gol -frerun-cse-after-loop -freschedule-modulo-scheduled-loops @gol -frounding-math -fsched2-use-superblocks -fsched-pressure @gol -fsched-spec-load -fsched-spec-load-dangerous @gol -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] @gol -fsched-group-heuristic -fsched-critical-path-heuristic @gol -fsched-spec-insn-heuristic -fsched-rank-heuristic @gol -fsched-last-insn-heuristic -fsched-dep-count-heuristic @gol -fschedule-insns -fschedule-insns2 -fsection-anchors @gol -fselective-scheduling -fselective-scheduling2 @gol -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops @gol -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller @gol -fsplit-wide-types -fstack-protector -fstack-protector-all @gol -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer @gol -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copy-prop @gol -ftree-copyrename -ftree-dce @gol -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-im @gol -ftree-phiprop -ftree-loop-distribution @gol -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize @gol -ftree-parallelize-loops=n -ftree-pre -ftree-pta -ftree-reassoc @gol -ftree-sink -ftree-sra -ftree-switch-conversion @gol -ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp @gol -funit-at-a-time -funroll-all-loops -funroll-loops @gol -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops @gol -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb @gol -fwhole-program -fwhopr -fwpa -fuse-linker-plugin @gol --param name=value -O -O0 -O1 -O2 -O3 -Os}

Preprocessor Options

See section Options Controlling the Preprocessor. @gccoptlist{-Aquestion=answer @gol -A-question[=answer] @gol -C -dD -dI -dM -dN @gol -Dmacro[=defn] -E -H @gol -idirafter dir @gol -include file -imacros file @gol -iprefix file -iwithprefix dir @gol -iwithprefixbefore dir -isystem dir @gol -imultilib dir -isysroot dir @gol -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol -P -fworking-directory -remap @gol -trigraphs -undef -Umacro -Wp,option @gol -Xpreprocessor option}

Assembler Option

See section Passing Options to the Assembler. @gccoptlist{-Wa,option -Xassembler option}

Linker Options

See section Options for Linking. @gccoptlist{object-file-name -llibrary @gol -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic @gol -s -static -static-libgcc -static-libstdc++ -shared @gol -shared-libgcc -symbolic @gol -T script -Wl,option -Xlinker option @gol -u symbol}

Directory Options

See section Options for Directory Search. @gccoptlist{-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir}

Target Options

See section 3.16 Specifying Target Machine and Compiler Version. @gccoptlist{-V version -b machine}

Machine Dependent Options

See section Hardware Models and Configurations.

GNU/Linux Options @gccoptlist{-muclibc}

H8/300 Options @gccoptlist{-mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300}

M32C Options @gccoptlist{-mcpu=cpu -msim -memregs=number}

RX Options @gccoptlist{-m64bit-doubles -m32bit-doubles -fpu -nofpu@gol -mcpu= -patch=@gol -mbig-endian-data -mlittle-endian-data @gol -msmall-data @gol -msim -mno-sim@gol -mas100-syntax -mno-as100-syntax@gol -mrelax@gol -mmax-constant-size=@gol -mint-register=@gol -msave-acc-in-interrupts}

SH Options @gccoptlist{-m1 -m2 -m2e @gol -m2a-nofpu -m2a-single-only -m2a-single -m2a @gol -m3 -m3e @gol -m4-nofpu -m4-single-only -m4-single -m4 @gol -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol -m5-64media -m5-64media-nofpu @gol -m5-32media -m5-32media-nofpu @gol -m5-compact -m5-compact-nofpu @gol -mb -ml -mdalign -mrelax @gol -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol -mieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mspace @gol -mprefergot -musermode -multcost=number -mdiv=strategy @gol -mdivsi3_libfunc=name -mfixed-range=register-range @gol -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed @gol -minvalid-symbols}

V850 Options @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol -mprolog-function -mno-prolog-function -mspace @gol -mtda=n -msda=n -mzda=n @gol -mapp-regs -mno-app-regs @gol -mdisable-callt -mno-disable-callt @gol -mv850e2v3 @gol -mv850e2 @gol -mv850e1 -mv850es @gol -mv850e @gol -mv850 -mbig-switch}

VxWorks Options @gccoptlist{-mrtp -non-static -Bstatic -Bdynamic @gol -Xbind-lazy -Xbind-now}

Code Generation Options

See section Options for Code Generation Conventions. @gccoptlist{-fcall-saved-reg -fcall-used-reg @gol -ffixed-reg -fexceptions @gol -fnon-call-exceptions -funwind-tables @gol -fasynchronous-unwind-tables @gol -finhibit-size-directive -finstrument-functions @gol -finstrument-functions-exclude-function-list=sym,sym,... @gol -finstrument-functions-exclude-file-list=file,file,... @gol -fno-common -fno-ident @gol -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol -fno-jump-tables @gol -frecord-gcc-switches @gol -freg-struct-return -fshort-enums @gol -fshort-double -fshort-wchar @gol -fverbose-asm -fpack-struct[=n] -fstack-check @gol -fstack-limit-register=reg -fstack-limit-symbol=sym @gol -fno-stack-limit -fargument-alias -fargument-noalias @gol -fargument-noalias-global -fargument-noalias-anything @gol -fleading-underscore -ftls-model=model @gol -ftrapv -fwrapv -fbounds-check @gol -fvisibility}

3.2 Options Controlling the Kind of Output		Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
3.4 Options Controlling C Dialect		Controlling the variant of C language compiled.
3.5 Options Controlling C++ Dialect		Variations on C++.
3.6 Options Controlling Objective-C and Objective-C++ Dialects		Variations on Objective-C and Objective-C++.
3.7 Options to Control Diagnostic Messages Formatting		Controlling how diagnostics should be formatted.
3.8 Options to Request or Suppress Warnings		How picky should the compiler be?
3.9 Options for Debugging Your Program or GCC		Symbol tables, measurements, and debugging dumps.
3.10 Options That Control Optimization		How much optimization?
3.11 Options Controlling the Preprocessor		Controlling header files and macro definitions. Also, getting dependency information for Make.
3.12 Passing Options to the Assembler		Passing options to the assembler.
3.13 Options for Linking		Specifying libraries and so on.
3.14 Options for Directory Search		Where to find header files and libraries. Where to find the compiler executable files.
3.15 Specifying subprocesses and the switches to pass to them		How to pass switches to sub-processes.
3.16 Specifying Target Machine and Compiler Version		Running a cross-compiler, or an old version of 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:

@gcctabopt{file.c}

C source code which must be preprocessed.

@gcctabopt{file.i}

C source code which should not be preprocessed.

@gcctabopt{file.ii}

C++ source code which should not be preprocessed.

@gcctabopt{file.m}

Objective-C source code. Note that you must link with the `libobjc' library to make an Objective-C program work.

@gcctabopt{file.mi}

Objective-C source code which should not be preprocessed.

@gcctabopt{file.mm}

@gcctabopt{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.

@gcctabopt{file.mii}

Objective-C++ source code which should not be preprocessed.

@gcctabopt{file.h}

C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.

@gcctabopt{file.cc}

@gcctabopt{file.cp}

@gcctabopt{file.cxx}

@gcctabopt{file.cpp}

@gcctabopt{file.CPP}

@gcctabopt{file.c++}

@gcctabopt{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.

@gcctabopt{file.mm}

@gcctabopt{file.M}

Objective-C++ source code which must be preprocessed.

@gcctabopt{file.mii}

Objective-C++ source code which should not be preprocessed.

@gcctabopt{file.hh}

@gcctabopt{file.H}

@gcctabopt{file.hp}

@gcctabopt{file.hxx}

@gcctabopt{file.hpp}

@gcctabopt{file.HPP}

@gcctabopt{file.h++}

@gcctabopt{file.tcc}

C++ header file to be turned into a precompiled header.

@gcctabopt{file.f}

@gcctabopt{file.for}

@gcctabopt{file.ftn}

Fixed form Fortran source code which should not be preprocessed.

@gcctabopt{file.F}

@gcctabopt{file.FOR}

@gcctabopt{file.fpp}

@gcctabopt{file.FPP}

@gcctabopt{file.FTN}

Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).

@gcctabopt{file.f90}

@gcctabopt{file.f95}

@gcctabopt{file.f03}

@gcctabopt{file.f08}

Free form Fortran source code which should not be preprocessed.

@gcctabopt{file.F90}

@gcctabopt{file.F95}

@gcctabopt{file.F03}

@gcctabopt{file.F08}

Free form Fortran source code which must be preprocessed (with the traditional preprocessor).

@gcctabopt{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.

@gcctabopt{file.adb}

Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.

@gcctabopt{file.s}

Assembler code.

@gcctabopt{file.S}

@gcctabopt{file.sx}

Assembler code which must be preprocessed.

@gcctabopt{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:

@gcctabopt{-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

@gcctabopt{-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).

@gcctabopt{-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. The C, C++, and Fortran frontends return 4, if an internal compiler error is encountered.

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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-###}

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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{--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 has also been specified (prior to the `--help' option), then command line options which have no documentation associated with them will also be displayed.

@gcctabopt{--target-help}

Print (on the standard output) a description of target-specific command line options for each tool. For some targets extra target-specific information may also be printed.

@gcctabopt{--help={class|[^]qualifier}[,...]}

Print (on the standard output) a description of the command line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:

`optimizers'

This will display all of the optimization options supported by the compiler.

`warnings'

This will display all of the options controlling warning messages produced by the compiler.

`target'

This will display target-specific options. Unlike the `--target-help' option however, target-specific options of the linker and assembler will not be displayed. This is because those tools do not currently support the extended `--help=' syntax.

`params'

This will display the values recognized by the `--param' option.

language

This will display the options supported for language, where language is the name of one of the languages supported in this version of GCC.

`common'

This will display the options that are common to all languages.

These are the supported qualifiers:

`undocumented'

Display only those options which are undocumented.

`joined'

Display options which take an argument that appears after an equal sign in the same continuous piece of text, such as: `--help=target'.

`separate'

Display options which take an argument that appears as a separate word following the original option, such as: `-o output-file'.

Thus for example to display all the undocumented target-specific switches supported by the compiler the following can be used:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the `^' character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument), which have a description the following can be used:

--help=warnings,^joined,^undocumented

The argument to `--help=' should not consist solely of inverted qualifiers.

Combining several classes is possible, although this usually restricts the output by so much that there is nothing to display. One case where it does work however is when one of the classes is target. So for example to display all the target-specific optimization options the following can be used:

--help=target,optimizers

The `--help=' option can be repeated on the command line. Each successive use will display its requested class of options, skipping those that have already been displayed.

If the `-Q' option appears on the command line before the `--help=' option, then the descriptive text displayed by `--help=' is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the `--help=' option is used).

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c The following options are target specific: -mabi= 2 -mabort-on-noreturn [disabled] -mapcs [disabled]

The output is sensitive to the effects of previous command line options, so for example it is possible to find out which optimizations are enabled at `-O2' by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by `-O3' by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled

@gcctabopt{-no-canonical-prefixes}

Do not expand any symbolic links, resolve references to `/../' or `/./', or make the path absolute when generating a relative prefix.

@gcctabopt{--version}

Display the version number and copyrights of the invoked GCC.

@gcctabopt{-wrapper}

Invoke all subcommands under a wrapper program. It takes a single comma separated list as an argument, which will be used to invoke the wrapper:

gcc -c t.c -wrapper gdb,--args

This will invoke all subprograms of gcc under "gdb --args", thus cc1 invocation will be "gdb --args cc1 ...".

@gcctabopt{-fplugin=name.so}

Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See `-fplugin-arg-name-key=value' below). Each plugin should define the callback functions specified in the Plugins API.

@gcctabopt{-fplugin-arg-name-key=value}

Define an argument called key with a value of value for the plugin called name.

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', `.hpp', `.H', or (for shared template code) `.tcc'; 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, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats `.c', `.h' and `.i' files as C++ source files instead of C source files unless `-x' is used, and automatically specifies linking against the C++ library. This program is also useful when precompiling a C header file with a `.h' extension for use in C++ compilations. 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 section Options Controlling C Dialect, for explanations of options for languages related to C. See section Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

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:

@gcctabopt{-ansi}

In C mode, this is equivalent to `-std=c90'. In C++ mode, it is equivalent to `-std=c++98'.

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 section 3.8 Options to Request or Suppress Warnings.

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

@gcctabopt{-std=}

Determine the language standard. See section Language Standards Supported by GCC, for details of these standard versions. This option is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as `c90' or `c++98', and GNU dialects of those standards, such as `gnu90' or `gnu++98'. By specifying a base standard, the compiler will accept all programs following that standard and those using GNU extensions that do not contradict it. For example, `-std=c90' turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, by specifying a GNU dialect of a standard, all features the compiler support are enabled, even when those features change the meaning of the base standard and some strict-conforming programs may be rejected. The particular standard is used by `-pedantic' to identify which features are GNU extensions given that version of the standard. For example `-std=gnu90 -pedantic' would warn about C++ style `//' comments, while `-std=gnu99 -pedantic' would not.

A value for this option must be provided; possible values are

`c90'

`c89'

`iso9899:1990'

Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as `-ansi' for C code.

`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-4.5/c99status.html for more information. The names `c9x' and `iso9899:199x' are deprecated.

`gnu90'

`gnu89'

GNU dialect of ISO C90 (including some C99 features). This is the default for C code.

`gnu99'

`gnu9x'

GNU dialect of ISO C99. 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. Same as `-ansi' for C++ code.

`gnu++98'

GNU dialect of `-std=c++98'. This is the default for C++ code.

`c++0x'

The working draft of the upcoming ISO C++0x standard. This option enables experimental features that are likely to be included in C++0x. The working draft is constantly changing, and any feature that is enabled by this flag may be removed from future versions of GCC if it is not part of the C++0x standard.

`gnu++0x'

GNU dialect of `-std=c++0x'. This option enables experimental features that may be removed in future versions of GCC.

@gcctabopt{-fgnu89-inline}

The option `-fgnu89-inline' tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See section An Inline Function is As Fast As a Macro. This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions (see section 6.29 Declaring Attributes of Functions).

The option `-fno-gnu89-inline' explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option was first supported in GCC 4.3. This option is not supported in `-std=c90' or `-std=gnu90' mode.

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. See section `Common Predefined Macros' in The C Preprocessor.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fno-builtin}

@gcctabopt{-fno-builtin-function}

Don't recognize built-in functions that do not begin with `__builtin_' as prefix. See section 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 that 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))

@gcctabopt{-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'.

@gcctabopt{-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 section Language Standards Supported by GCC, for details of freestanding and hosted environments.

@gcctabopt{-fopenmp}

Enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran. When `-fopenmp' is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v3.0 http://www.openmp.org/. This option implies `-pthread', and thus is only supported on targets that have support for `-pthread'.

@gcctabopt{-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 section Unnamed struct/union fields within structs/unions, for details.

@gcctabopt{-trigraphs}

Support ISO C trigraphs. The `-ansi' option (and `-std' options for strict ISO C conformance) implies `-trigraphs'.

@gcctabopt{-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.

@gcctabopt{-traditional}

@gcctabopt{-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.

@gcctabopt{-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++.

@gcctabopt{-flax-vector-conversions}

Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-fsigned-bitfields}

@gcctabopt{-funsigned-bitfields}

@gcctabopt{-fno-signed-bitfields}

@gcctabopt{-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.

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:

@gcctabopt{-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.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4 implements a standard mangling for vector types.

See also `-Wabi'.

@gcctabopt{-fno-access-control}

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

@gcctabopt{-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)'.

@gcctabopt{-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.

@gcctabopt{-fno-deduce-init-list}

Disable deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e.

template <class T> auto forward(T t) -> decltype (realfn (t)) { return realfn (t); } void f() { forward({1,2}); // call forward<std::initializer_list<int>> }

This option is present because this deduction is an extension to the current specification in the C++0x working draft, and there was some concern about potential overload resolution problems.

@gcctabopt{-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++.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-ffor-scope}

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-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 section 7.5 Where's the Template?, for more information.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fno-operator-names}

Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.

@gcctabopt{-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.

@gcctabopt{-fpermissive}

Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using `-fpermissive' will allow some nonconforming code to compile.

@gcctabopt{-fno-pretty-templates}

When an error message refers to a specialization of a function template, the compiler will normally print the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler will omit any template arguments which match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, using `-fno-pretty-templates' will disable them.

@gcctabopt{-frepo}

Enable automatic template instantiation at link time. This option also implies `-fno-implicit-templates'. See section 7.5 Where's the Template?, for more information.

@gcctabopt{-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. The `dynamic_cast' operator can still be used for casts that do not require runtime type information, i.e. casts to void * or to unambiguous base classes.

@gcctabopt{-fstats}

Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

@gcctabopt{-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 (changed to 1024 in C++0x).

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fno-use-cxa-get-exception-ptr}

Don't use the __cxa_get_exception_ptr runtime routine. This will cause std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available.

@gcctabopt{-fvisibility-inlines-hidden}

This switch declares that the user does not attempt to compare pointers to inline methods where the addresses of the two functions were taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods 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.

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility will have no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See section 7.5 Where's the Template?.

@gcctabopt{-fvisibility-ms-compat}

This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC's linkage model:

1. It sets the default visibility to hidden, like `-fvisibility=hidden'.

2. Types, but not their members, are not hidden by default.

3. The One Definition Rule is relaxed for types without explicit visibility specifications which are defined in more than one different shared object: those declarations are permitted if they would have been permitted when this option was not used.

In new code it is better to use `-fvisibility=hidden' and export those classes which are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects will be different, so changing one will not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

@gcctabopt{-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++.

@gcctabopt{-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:

@gcctabopt{-fno-default-inline}

Do not assume `inline' for functions defined inside a class scope. See section Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.

@gcctabopt{-Wabi (C, Objective-C, C++ and Objective-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 in `-fabi-version=2' (the default) include:

• A template with a non-type template parameter of reference type is mangled incorrectly:

  extern int N; template <int &> struct S {}; void n (S<N>) {2}

  This is fixed in `-fabi-version=3'.

• SIMD vector types declared using __attribute ((vector_size)) are mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

  The mangling is changed in `-fabi-version=4'.

The known incompatibilities in `-fabi-version=1' 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 asA::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.

It also warns psABI related changes. The known psABI changes at this point include:

• For SYSV/x86-64, when passing union with long double, it is changed to pass in memory as specified in psABI. For example:

  union U { long double ld; int i; };

  union U will always be passed in memory.

@gcctabopt{-Wctor-dtor-privacy (C++ and Objective-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.

@gcctabopt{-Wnon-virtual-dtor (C++ and Objective-C++ only)}

Warn when a class has virtual functions and accessible non-virtual destructor, in which case it would be possible but unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if -Weffc++ is specified.

@gcctabopt{-Wreorder (C++ and Objective-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'.

@gcctabopt{-Weffc++ (C++ and Objective-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.

@gcctabopt{-Wstrict-null-sentinel (C++ and Objective-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 be of the same size as a pointer. But this use is not portable across different compilers.

@gcctabopt{-Wno-non-template-friend (C++ and Objective-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.

@gcctabopt{-Wold-style-cast (C++ and Objective-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.

@gcctabopt{-Woverloaded-virtual (C++ and Objective-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.

@gcctabopt{-Wno-pmf-conversions (C++ and Objective-C++ only)}

Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

@gcctabopt{-Wsign-promo (C++ and Objective-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 ='.

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 section 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:

@gcctabopt{-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.

@gcctabopt{-fgnu-runtime}

Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fobjc-direct-dispatch}

Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.

@gcctabopt{-fobjc-exceptions}

Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. This option is unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier.

@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.

@gcctabopt{-fobjc-gc}

Enable garbage collection (GC) in Objective-C and Objective-C++ programs.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-gen-decls}

Dump interface declarations for all classes seen in the source file to a file named `sourcename.decl'.

@gcctabopt{-Wassign-intercept (Objective-C and Objective-C++ only)}

Warn whenever an Objective-C assignment is being intercepted by the garbage collector.

@gcctabopt{-Wno-protocol (Objective-C and Objective-C++ only)}

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.

@gcctabopt{-Wselector (Objective-C and Objective-C++ only)}

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.

@gcctabopt{-Wstrict-selector-match (Objective-C and Objective-C++ only)}

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.

@gcctabopt{-Wundeclared-selector (Objective-C and Objective-C++ only)}

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.

@gcctabopt{-print-objc-runtime-info}

Generate C header describing the largest structure that is passed by value, if any.

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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fdiagnostics-show-option}

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.

@gcctabopt{-Wcoverage-mismatch}

Warn if feedback profiles do not match when using the `-fprofile-use' option. If a source file was changed between `-fprofile-gen' and `-fprofile-use', the files with the profile feedback can fail to match the source file and GCC can not use the profile feedback information. By default, GCC emits an error message in this case. The option `-Wcoverage-mismatch' emits a warning instead of an error. GCC does not use appropriate feedback profiles, so using this option can result in poorly optimized code. This option is useful only in the case of very minor changes such as bug fixes to an existing code-base.

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.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

@gcctabopt{-fsyntax-only}

Check the code for syntax errors, but don't do anything beyond that.

@gcctabopt{-w}

Inhibit all warning messages.

@gcctabopt{-Werror}

Make all warnings into errors.

@gcctabopt{-Werror=}

Make the specified warning into an error. The specifier for a warning is appended, for example `-Werror=switch' turns the warnings controlled by `-Wswitch' into errors. This switch takes a negative form, to be used to negate `-Werror' for specific warnings, for example `-Wno-error=switch' makes `-Wswitch' warnings not be errors, even when `-Werror' is in effect. You can use the `-fdiagnostics-show-option' option to have each controllable warning amended with the option which controls it, to determine what to use with this option.

Note that specifying `-Werror='foo automatically implies `-W'foo. However, `-Wno-error='foo does not imply anything.

@gcctabopt{-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.

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. For further, language-specific options also refer to 3.5 Options Controlling C++ Dialect and 3.6 Options Controlling Objective-C and Objective-C++ Dialects.

@gcctabopt{-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 section 6.43 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 `gnu90' 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.)

@gcctabopt{-pedantic-errors}

Like `-pedantic', except that errors are produced rather than warnings.

@gcctabopt{-Wall}

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 3.5 Options Controlling C++ Dialect and 3.6 Options Controlling Objective-C and Objective-C++ Dialects.

`-Wall' turns on the following warning flags:

@gccoptlist{-Waddress @gol -Warray-bounds (only with `-O2') @gol -Wc++0x-compat @gol -Wchar-subscripts @gol -Wenum-compare (in C/Objc; this is on by default in C++) @gol -Wimplicit-int @gol -Wimplicit-function-declaration @gol -Wcomment @gol -Wformat @gol -Wmain (only for C/ObjC and unless `-ffreestanding') @gol -Wmissing-braces @gol -Wnonnull @gol -Wparentheses @gol -Wpointer-sign @gol -Wreorder @gol -Wreturn-type @gol -Wsequence-point @gol -Wsign-compare (only in C++) @gol -Wstrict-aliasing @gol -Wstrict-overflow=1 @gol -Wswitch @gol -Wtrigraphs @gol -Wuninitialized @gol -Wunknown-pragmas @gol -Wunused-function @gol -Wunused-label @gol -Wunused-value @gol -Wunused-variable @gol -Wvolatile-register-var @gol }

Note that some warning flags 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. Some of them are enabled by `-Wextra' but many of them must be enabled individually.

@gcctabopt{-Wextra}

This enables some extra warning flags that are not enabled by `-Wall'. (This option used to be called `-W'. The older name is still supported, but the newer name is more descriptive.)

@gccoptlist{-Wclobbered @gol -Wempty-body @gol -Wignored-qualifiers @gol -Wmissing-field-initializers @gol -Wmissing-parameter-type (C only) @gol -Wold-style-declaration (C only) @gol -Woverride-init @gol -Wsign-compare @gol -Wtype-limits @gol -Wuninitialized @gol -Wunused-parameter (only with `-Wunused' or `-Wall') @gol }

The option `-Wextra' also prints warning messages for the following cases:

• A pointer is compared against integer zero with `<', `<=', `>', or `>='.

• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.

• (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.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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 section 6.29 Declaring Attributes of Functions), 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 section 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'.

@gcctabopt{-Wformat-y2k}

If `-Wformat' is specified, also warn about strftime formats which may yield only a two-digit year.

@gcctabopt{-Wno-format-contains-nul}

If `-Wformat' is specified, do not warn about format strings that contain NUL bytes.

@gcctabopt{-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.

@gcctabopt{-Wno-format-zero-length (C and Objective-C only)}

If `-Wformat' is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

@gcctabopt{-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.

@gcctabopt{-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'.)

@gcctabopt{-Wformat=2}

Enable `-Wformat' plus format checks not included in `-Wformat'. Currently equivalent to `-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k'.

@gcctabopt{-Wnonnull (C and Objective-C only)}

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.

@gcctabopt{-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.

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; }

@gcctabopt{-Wimplicit-int (C and Objective-C only)}

Warn when a declaration does not specify a type. This warning is enabled by `-Wall'.

@gcctabopt{-Wimplicit-function-declaration (C and Objective-C only)}

Give a warning whenever a function is used before being declared. In C99 mode (`-std=c99' or `-std=gnu99'), this warning is enabled by default and it is made into an error by `-pedantic-errors'. This warning is also enabled by `-Wall'.

@gcctabopt{-Wimplicit}

Same as `-Wimplicit-int' and `-Wimplicit-function-declaration'. This warning is enabled by `-Wall'.

@gcctabopt{-Wignored-qualifiers (C and C++ only)}

Warn if the return type of a function has a type qualifier such as const. For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.

This warning is also enabled by `-Wextra'.

@gcctabopt{-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 default in C++ and is enabled by either `-Wall' or `-pedantic'.

@gcctabopt{-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'.

@gcctabopt{-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)}

Warn if a user-supplied include directory does not exist.

@gcctabopt{-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.

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/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'.

@gcctabopt{-Wsequence-point}

Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

The C and C++ standards defines the order in which expressions in a C/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 and C++ standards specify 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 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' for C and C++.

@gcctabopt{-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 (falling off the end of the function body is considered returning without a value), and about a return statement with an expression in a function whose return-type is void.

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'.

@gcctabopt{-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 (even if there is a default label). This warning is enabled by `-Wall'.

@gcctabopt{-Wswitch-default}

Warn whenever a switch statement does not have a default case.

@gcctabopt{-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. The only difference between `-Wswitch' and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.

@gcctabopt{-Wsync-nand (C and C++ only)}

Warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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 section 6.35 Specifying Attributes of Variables).

@gcctabopt{-Wunused-parameter}

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

To suppress this warning use the `unused' attribute (see section 6.35 Specifying Attributes of Variables).

@gcctabopt{-Wno-unused-result}

Do not warn if a caller of a function marked with attribute warn_unused_result (see section 6.35 Specifying Attributes of Variables) does not use its return value. The default is `-Wunused-result'.

@gcctabopt{-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 section 6.35 Specifying Attributes of Variables).

@gcctabopt{-Wunused-value}

Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to `void'. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as `x[i,j]' will cause a warning, while `x[(void)i,j]' will not.

This warning is enabled by `-Wall'.

@gcctabopt{-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'.

@gcctabopt{-Wuninitialized}

Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call. In C++, warn if a non-static reference or non-static `const' member appears in a class without constructors.

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 section 6.29 Declaring Attributes of Functions.

This warning is enabled by `-Wall' or `-Wextra'.

@gcctabopt{-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.

@gcctabopt{-Wno-pragmas}

Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also `-Wunknown-pragmas'.

@gcctabopt{-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'. It is equivalent to `-Wstrict-aliasing=3'

@gcctabopt{-Wstrict-aliasing=n}

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. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. `-Wstrict-aliasing' is equivalent to `-Wstrict-aliasing=n', with n=3.

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the frontend only.

Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the frontend only.

Level 3 (default for `-Wstrict-aliasing'): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the frontend: *(int*)&some_float. If optimization is enabled, it also runs in the backend, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

@gcctabopt{-Wstrict-overflow}

@gcctabopt{-Wstrict-overflow=n}

This option is only active when `-fstrict-overflow' is active. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.

An optimization which assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code which is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop will require, in particular when determining whether a loop will be executed at all.

@gcctabopt{-Wstrict-overflow=1}

Warn about cases which are both questionable and easy to avoid. For example: x + 1 > x; with `-fstrict-overflow', the compiler will simplify this to 1. This level of `-Wstrict-overflow' is enabled by `-Wall'; higher levels are not, and must be explicitly requested.

@gcctabopt{-Wstrict-overflow=2}

Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when `-fstrict-overflow' is in effect, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. `-Wstrict-overflow' (with no level) is the same as `-Wstrict-overflow=2'.

@gcctabopt{-Wstrict-overflow=3}

Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 will be simplified to x > 0.

@gcctabopt{-Wstrict-overflow=4}

Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 will be simplified to x * 2.

@gcctabopt{-Wstrict-overflow=5}

Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y will be simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level will give a very large number of false positives.

@gcctabopt{-Warray-bounds}

This option is only active when `-ftree-vrp' is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by `-Wall'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-Wtraditional (C and Objective-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 `-Wtraditional-conversion'.

• 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.

@gcctabopt{-Wtraditional-conversion (C and Objective-C only)}

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.

@gcctabopt{-Wdeclaration-after-statement (C and Objective-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 section 6.28 Mixed Declarations and Code.

@gcctabopt{-Wundef}

Warn if an undefined identifier is evaluated in an `#if' directive.

@gcctabopt{-Wno-endif-labels}

Do not warn whenever an `#else' or an `#endif' are followed by text.

@gcctabopt{-Wshadow}

Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.

@gcctabopt{-Wlarger-than=len}

Warn whenever an object of larger than len bytes is defined.

@gcctabopt{-Wframe-larger-than=len}

Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a warning. In addition, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.

@gcctabopt{-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.

@gcctabopt{-Wno-pedantic-ms-format (MinGW targets only)}

Disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets depending on the MS runtime, when you are using the options `-Wformat' and `-pedantic' without gnu-extensions.

@gcctabopt{-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. In C++, warn also when an arithmetic operation involves NULL. This warning is also enabled by `-pedantic'.

@gcctabopt{-Wtype-limits}

Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with `<' or `>='. This warning is also enabled by `-Wextra'.

@gcctabopt{-Wbad-function-cast (C and Objective-C only)}

Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.

@gcctabopt{-Wc++-compat (C and Objective-C only)}

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.

@gcctabopt{-Wc++0x-compat (C++ and Objective-C++ only)}

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 200x, e.g., identifiers in ISO C++ 1998 that will become keywords in ISO C++ 200x. This warning is enabled by `-Wall'.

@gcctabopt{-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 *.

Also warn when making a cast which introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:

/* p is char ** value. */ const char **q = (const char **) p; /* Assignment of readonly string to const char * is OK. */ *q = "string"; /* Now char** pointer points to read-only memory. */ **p = 'b';

@gcctabopt{-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.

@gcctabopt{-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. 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.

When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.

@gcctabopt{-Wclobbered}

Warn for variables that might be changed by `longjmp' or `vfork'. This warning is also enabled by `-Wextra'.

@gcctabopt{-Wconversion}

Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using `-Wno-sign-conversion'.

For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that will never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless `-Wsign-conversion' is explicitly enabled.

@gcctabopt{-Wno-conversion-null (C++ and Objective-C++ only)}

Do not warn for conversions between NULL and non-pointer types. `-Wconversion-null' is enabled by default.

@gcctabopt{-Wempty-body}

Warn if an empty body occurs in an `if', `else' or `do while' statement. This warning is also enabled by `-Wextra'.

@gcctabopt{-Wenum-compare}

Warn about a comparison between values of different enum types. In C++ this warning is enabled by default. In C this warning is enabled by `-Wall'.

@gcctabopt{-Wjump-misses-init (C, Objective-C only)}

Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables which are initialized when they are declared. This warning is only supported for C and Objective C; in C++ this sort of branch is an error in any case.

`-Wjump-misses-init' is included in `-Wc++-compat'. It can be disabled with the `-Wno-jump-misses-init' option.

@gcctabopt{-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'.

@gcctabopt{-Wsign-conversion}

Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by `-Wconversion'.

@gcctabopt{-Waddress}

Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp. This warning is enabled by `-Wall'.

@gcctabopt{-Wlogical-op}

Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected.

@gcctabopt{-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.)

@gcctabopt{-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.

@gcctabopt{-Wno-builtin-macro-redefined}

Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.

@gcctabopt{-Wstrict-prototypes (C and Objective-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.)

@gcctabopt{-Wold-style-declaration (C and Objective-C only)}

Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by `-Wextra'.

@gcctabopt{-Wold-style-definition (C and Objective-C only)}

Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.

@gcctabopt{-Wmissing-parameter-type (C and Objective-C only)}

A function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by `-Wextra'.

@gcctabopt{-Wmissing-prototypes (C and Objective-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.

@gcctabopt{-Wmissing-declarations}

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. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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 normalization 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.

@gcctabopt{-Wno-deprecated}

Do not warn about usage of deprecated features. See section 7.11 Deprecated Features.

@gcctabopt{-Wno-deprecated-declarations}

Do not warn about uses of functions (see section 6.29 Declaring Attributes of Functions), variables (see section 6.35 Specifying Attributes of Variables), and types (see section 6.36 Specifying Attributes of Types) marked as deprecated by using the deprecated attribute.

@gcctabopt{-Wno-overflow}

Do not warn about compile-time overflow in constant expressions.

@gcctabopt{-Woverride-init (C and Objective-C only)}

Warn if an initialized field without side effects is overridden when using designated initializers (see section Designated Initializers).

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

@gcctabopt{-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; };

@gcctabopt{-Wpacked-bitfield-compat}

The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure:

struct foo { char a:4; char b:8; } __attribute__ ((packed));

This warning is enabled by default. Use `-Wno-packed-bitfield-compat' to disable this warning.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-Wnested-externs (C and Objective-C only)}

Warn if an extern declaration is encountered within a function.

@gcctabopt{-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.

@gcctabopt{-Wno-invalid-offsetof (C++ and Objective-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.

@gcctabopt{-Wno-int-to-pointer-cast (C and Objective-C only)}

Suppress warnings from casts to pointer type of an integer of a different size.

@gcctabopt{-Wno-pointer-to-int-cast (C and Objective-C only)}

Suppress warnings from casts from a pointer to an integer type of a different size.

@gcctabopt{-Winvalid-pch}

Warn if a precompiled header (see section 3.20 Using Precompiled Headers) is found in the search path but can't be used.

@gcctabopt{-Wlong-long}

Warn if `long long' type is used. This is enabled by either `-pedantic' or `-Wtraditional' in ISO C90 and C++98 modes. To inhibit the warning messages, use `-Wno-long-long'.

@gcctabopt{-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'.

@gcctabopt{-Wvla}

Warn if variable length array is used in the code. `-Wno-vla' will prevent the `-pedantic' warning of the variable length array.

@gcctabopt{-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. This warning is enabled by `-Wall'.

@gcctabopt{-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.

@gcctabopt{-Wpointer-sign (C and Objective-C only)}

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'.

@gcctabopt{-Wstack-protector}

This option is only active when `-fstack-protector' is active. It warns about functions that will not be protected against stack smashing.

@gcctabopt{-Wno-mudflap}

Suppress warnings about constructs that cannot be instrumented by `-fmudflap'.

@gcctabopt{-Woverlength-strings}

Warn about string constants which are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants which are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by `-pedantic', and can be disabled with `-Wno-overlength-strings'.

@gcctabopt{-Wunsuffixed-float-constants (C and Objective-C only)}

GCC will issue a warning for any floating constant that does not have a suffix. When used together with `-Wsystem-headers' it will warn about such constants in system header files. This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.

3.9 Options for Debugging Your Program or GCC

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

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-feliminate-unused-debug-symbols}

Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.

@gcctabopt{-femit-class-debug-always}

Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option will increase the size of debugging information by as much as a factor of two.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-gdwarf-version}

Produce debugging information in DWARF format (if that is supported). This is the format used by DBX on IRIX 6. The value of version may be either 2, 3 or 4; the default version is 2.

Note that with DWARF version 2 some ports require, and will always use, some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and `-fvar-tracking-assignments' for maximum benefit.

@gcctabopt{-gstrict-dwarf}

Disallow using extensions of later DWARF standard version than selected with `-gdwarf-version'. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.

@gcctabopt{-gno-strict-dwarf}

Allow using extensions of later DWARF standard version than selected with `-gdwarf-version'.

@gcctabopt{-gvms}

Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems.

@gcctabopt{-glevel}

@gcctabopt{-ggdblevel}

@gcctabopt{-gstabslevel}

@gcctabopt{-gcofflevel}

@gcctabopt{-gxcofflevel}

@gcctabopt{-gvmslevel}

Request debugging information and also use level to specify how much information. The default level is 2.

Level 0 produces no debug information at all. Thus, `-g0' negates `-g'.

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 DWARF.

@gcctabopt{-gtoggle}

Turn off generation of debug info, if leaving out this option would have generated it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter, it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with `-fcompare-debug'.

@gcctabopt{-fdump-final-insns[=file]}

Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file will be determined by appending .gkd to the compilation output file name.

@gcctabopt{-fcompare-debug[=opts]}

If no error occurs during compilation, run the compiler a second time, adding opts and `-fcompare-debug-second' to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.

If the equal sign is omitted, the default `-gtoggle' is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables `-fcompare-debug'. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default `-gtoggle' is used.

`-fcompare-debug=', with the equal sign but without opts, is equivalent to `-fno-compare-debug', which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during `-fcompare-debug' testing, set GCC_COMPARE_DEBUG to say `-fcompare-debug-not-overridden', which GCC will reject as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to `-w%n-fcompare-debug not overridden' will do.

@gcctabopt{-fcompare-debug-second}

This option is implicitly passed to the compiler for the second compilation requested by `-fcompare-debug', along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first.

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

@gcctabopt{-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'.

@gcctabopt{-femit-struct-debug-baseonly}

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct was defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See `-femit-struct-debug-reduced' for a less aggressive option. See `-femit-struct-debug-detailed' for more detailed control.

This option works only with DWARF 2.

@gcctabopt{-femit-struct-debug-reduced}

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type was defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See `-femit-struct-debug-baseonly' for a more aggressive option. See `-femit-struct-debug-detailed' for more detailed control.

This option works only with DWARF 2.

@gcctabopt{-femit-struct-debug-detailed[=spec-list]}

Specify the struct-like types for which the compiler will generate debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

This option is a detailed version of `-femit-struct-debug-reduced' and `-femit-struct-debug-baseonly', which will serve for most needs.

A specification has the syntax [`dir:'|`ind:'][`ord:'|`gen:'](`any'|`sys'|`base'|`none')

The optional first word limits the specification to structs that are used directly (`dir:') or used indirectly (`ind:'). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct would be legal, the use is indirect. An example is `struct one direct; struct two * indirect;'.

The optional second word limits the specification to ordinary structs (`ord:') or generic structs (`gen:'). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but `-femit-struct-debug-detailed' does not yet implement them.

The third word specifies the source files for those structs for which the compiler will emit debug information. The values `none' and `any' have the normal meaning. The value `base' means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that types declared in `foo.c' and `foo.h' will have debug information, but types declared in other header will not. The value `sys' means those types satisfying `base' or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is `-femit-struct-debug-detailed=all'.

This option works only with DWARF 2.

@gcctabopt{-fenable-icf-debug}

Generate additional debug information to support identical code folding (ICF). This option only works with DWARF version 2 or higher.

@gcctabopt{-fno-merge-debug-strings}

Direct the linker to not merge together strings in the debugging information which are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.

@gcctabopt{-fdebug-prefix-map=old=new}

When compiling files in directory `old', record debugging information describing them as in `new' instead.

@gcctabopt{-fno-dwarf2-cfi-asm}

Emit DWARF 2 unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-Q}

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

@gcctabopt{-ftime-report}

Makes the compiler print some statistics about the time consumed by each pass when it finishes.

@gcctabopt{-fmem-report}

Makes the compiler print some statistics about permanent memory allocation when it finishes.

@gcctabopt{-fpre-ipa-mem-report}

@gcctabopt{-fpost-ipa-mem-report}

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

@gcctabopt{-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 section 10.5 Data file relocation to support cross-profiling.

@gcctabopt{--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 section 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.

@gcctabopt{-ftest-coverage}

Produce a notes file that the gcov code-coverage utility (see section 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.

@gcctabopt{-fdbg-cnt-list}

Print the name and the counter upperbound for all debug counters.

@gcctabopt{-fdbg-cnt=counter-value-list}

Set the internal debug counter upperbound. counter-value-list is a comma-separated list of name:value pairs which sets the upperbound of each debug counter name to value. All debug counters have the initial upperbound of UINT_MAX, thus dbg_cnt() returns true always unless the upperbound is set by this option. e.g. With -fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce) will return true only for first 10 invocations and dbg_cnt(tail_call) will return false always.

@gcctabopt{-dletters}

@gcctabopt{-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, and the files are created in the directory of the output file. 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. These switches may have different effects when `-E' is used for preprocessing.

Debug dumps can be enabled with a `-fdump-rtl' switch or some `-d' option letters. Here are the possible letters for use in pass and letters, and their meanings:

@gcctabopt{-fdump-rtl-alignments}

Dump after branch alignments have been computed.

@gcctabopt{-fdump-rtl-asmcons}

Dump after fixing rtl statements that have unsatisfied in/out constraints.

@gcctabopt{-fdump-rtl-auto_inc_dec}

Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.

@gcctabopt{-fdump-rtl-barriers}

Dump after cleaning up the barrier instructions.

@gcctabopt{-fdump-rtl-bbpart}

Dump after partitioning hot and cold basic blocks.

@gcctabopt{-fdump-rtl-bbro}

Dump after block reordering.

@gcctabopt{-fdump-rtl-btl1}

@gcctabopt{-fdump-rtl-btl2}

`-fdump-rtl-btl1' and `-fdump-rtl-btl2' enable dumping after the two branch target load optimization passes.

@gcctabopt{-fdump-rtl-bypass}

Dump after jump bypassing and control flow optimizations.

@gcctabopt{-fdump-rtl-combine}

Dump after the RTL instruction combination pass.

@gcctabopt{-fdump-rtl-compgotos}

Dump after duplicating the computed gotos.

@gcctabopt{-fdump-rtl-ce1}

@gcctabopt{-fdump-rtl-ce2}

@gcctabopt{-fdump-rtl-ce3}

`-fdump-rtl-ce1', `-fdump-rtl-ce2', and `-fdump-rtl-ce3' enable dumping after the three if conversion passes.

@gcctabopt{-fdump-rtl-cprop_hardreg}

Dump after hard register copy propagation.

@gcctabopt{-fdump-rtl-csa}

Dump after combining stack adjustments.

@gcctabopt{-fdump-rtl-cse1}

@gcctabopt{-fdump-rtl-cse2}

`-fdump-rtl-cse1' and `-fdump-rtl-cse2' enable dumping after the two common sub-expression elimination passes.

@gcctabopt{-fdump-rtl-dce}

Dump after the standalone dead code elimination passes.

@gcctabopt{-fdump-rtl-dbr}

Dump after delayed branch scheduling.

@gcctabopt{-fdump-rtl-dce1}

@gcctabopt{-fdump-rtl-dce2}

`-fdump-rtl-dce1' and `-fdump-rtl-dce2' enable dumping after the two dead store elimination passes.

@gcctabopt{-fdump-rtl-eh}

Dump after finalization of EH handling code.

@gcctabopt{-fdump-rtl-eh_ranges}

Dump after conversion of EH handling range regions.

@gcctabopt{-fdump-rtl-expand}

Dump after RTL generation.

@gcctabopt{-fdump-rtl-fwprop1}

@gcctabopt{-fdump-rtl-fwprop2}

`-fdump-rtl-fwprop1' and `-fdump-rtl-fwprop2' enable dumping after the two forward propagation passes.

@gcctabopt{-fdump-rtl-gcse1}

@gcctabopt{-fdump-rtl-gcse2}

`-fdump-rtl-gcse1' and `-fdump-rtl-gcse2' enable dumping after global common subexpression elimination.

@gcctabopt{-fdump-rtl-init-regs}

Dump after the initialization of the registers.

@gcctabopt{-fdump-rtl-initvals}

Dump after the computation of the initial value sets.

@gcctabopt{-fdump-rtl-into_cfglayout}

Dump after converting to cfglayout mode.

@gcctabopt{-fdump-rtl-ira}

Dump after iterated register allocation.

@gcctabopt{-fdump-rtl-jump}

Dump after the second jump optimization.

@gcctabopt{-fdump-rtl-loop2}

`-fdump-rtl-loop2' enables dumping after the rtl loop optimization passes.

@gcctabopt{-fdump-rtl-mach}

Dump after performing the machine dependent reorganization pass, if that pass exists.

@gcctabopt{-fdump-rtl-mode_sw}

Dump after removing redundant mode switches.

@gcctabopt{-fdump-rtl-rnreg}

Dump after register renumbering.

@gcctabopt{-fdump-rtl-outof_cfglayout}

Dump after converting from cfglayout mode.

@gcctabopt{-fdump-rtl-peephole2}

Dump after the peephole pass.

@gcctabopt{-fdump-rtl-postreload}

Dump after post-reload optimizations.

@gcctabopt{-fdump-rtl-pro_and_epilogue}

Dump after generating the function pro and epilogues.

@gcctabopt{-fdump-rtl-regmove}

Dump after the register move pass.

@gcctabopt{-fdump-rtl-sched1}

@gcctabopt{-fdump-rtl-sched2}

`-fdump-rtl-sched1' and `-fdump-rtl-sched2' enable dumping after the basic block scheduling passes.

@gcctabopt{-fdump-rtl-see}

Dump after sign extension elimination.

@gcctabopt{-fdump-rtl-seqabstr}

Dump after common sequence discovery.

@gcctabopt{-fdump-rtl-shorten}

Dump after shortening branches.

@gcctabopt{-fdump-rtl-sibling}

Dump after sibling call optimizations.

@gcctabopt{-fdump-rtl-split1}

@gcctabopt{-fdump-rtl-split2}

@gcctabopt{-fdump-rtl-split3}

@gcctabopt{-fdump-rtl-split4}

@gcctabopt{-fdump-rtl-split5}

`-fdump-rtl-split1', `-fdump-rtl-split2', `-fdump-rtl-split3', `-fdump-rtl-split4' and `-fdump-rtl-split5' enable dumping after five rounds of instruction splitting.

@gcctabopt{-fdump-rtl-sms}

Dump after modulo scheduling. This pass is only run on some architectures.

@gcctabopt{-fdump-rtl-stack}

Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.

@gcctabopt{-fdump-rtl-subreg1}

@gcctabopt{-fdump-rtl-subreg2}

`-fdump-rtl-subreg1' and `-fdump-rtl-subreg2' enable dumping after the two subreg expansion passes.

@gcctabopt{-fdump-rtl-unshare}

Dump after all rtl has been unshared.

@gcctabopt{-fdump-rtl-vartrack}

Dump after variable tracking.

@gcctabopt{-fdump-rtl-vregs}

Dump after converting virtual registers to hard registers.

@gcctabopt{-fdump-rtl-web}

Dump after live range splitting.

@gcctabopt{-fdump-rtl-regclass}

@gcctabopt{-fdump-rtl-subregs_of_mode_init}

@gcctabopt{-fdump-rtl-subregs_of_mode_finish}

@gcctabopt{-fdump-rtl-dfinit}

@gcctabopt{-fdump-rtl-dfinish}

These dumps are defined but always produce empty files.

@gcctabopt{-fdump-rtl-all}

Produce all the dumps listed above.

@gcctabopt{-dA}

Annotate the assembler output with miscellaneous debugging information.

@gcctabopt{-dD}

Dump all macro definitions, at the end of preprocessing, in addition to normal output.

@gcctabopt{-dH}

Produce a core dump whenever an error occurs.

@gcctabopt{-dm}

Print statistics on memory usage, at the end of the run, to standard error.

@gcctabopt{-dp}

Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.

@gcctabopt{-dP}

Dump the RTL in the assembler output as a comment before each instruction. Also turns on `-dp' annotation.

@gcctabopt{-dv}

For each of the other indicated dump files (`-fdump-rtl-pass'), dump a representation of the control flow graph suitable for viewing with VCG to `file.pass.vcg'.

@gcctabopt{-dx}

Just generate RTL for a function instead of compiling it. Usually used with `-fdump-rtl-expand'.

@gcctabopt{-dy}

Dump debugging information during parsing, to standard error.

@gcctabopt{-fdump-noaddr}

When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.

@gcctabopt{-fdump-unnumbered}

When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without `-g'.

@gcctabopt{-fdump-unnumbered-links}

When doing debugging dumps (see `-d' option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

@gcctabopt{-fdump-translation-unit (C++ only)}

@gcctabopt{-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, and the file is created in the same directory as the output file. If the `-options' form is used, options controls the details of the dump as described for the `-fdump-tree' options.

@gcctabopt{-fdump-class-hierarchy (C++ only)}

@gcctabopt{-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, and the file is created in the same directory as the output file. If the `-options' form is used, options controls the details of the dump as described for the `-fdump-tree' options.

@gcctabopt{-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, and the file is created in the same directory as the output file. The following dumps are possible:

`all'

Enables all inter-procedural analysis dumps.

`cgraph'

Dumps information about call-graph optimization, unused function removal, and inlining decisions.

`inline'

Dump after function inlining.

@gcctabopt{-fdump-statistics-option}

Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in `.statistics' to the source file name, and the file is created in the same directory as the output file. If the `-option' form is used, `-stats' will cause counters to be summed over the whole compilation unit while `-details' will dump every event as the passes generate them. The default with no option is to sum counters for each function compiled.

@gcctabopt{-fdump-tree-switch}

@gcctabopt{-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, and the file is created in the same directory as the output file. 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.

`asmname'

If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. Its primary use is ease of use working backward from mangled names in the assembly file.

`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.

`verbose'

Enable showing the tree dump for each statement.

`eh'

Enable showing the EH region number holding each statement.

`all'

Turn on all options, except `raw', `slim', `verbose' 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'.

`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.

`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.

`slp'

Dump each function after applying vectorization of basic blocks. The file name is made by appending `.slp' 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.

@gcctabopt{-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'. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops. This is the same verbosity level that `-fdump-tree-vect-stats' uses. Higher verbosity levels mean either more information dumped for each reported loop, or same amount of information reported for more loops: if n=3, vectorizer cost model information is reported. If n=4, alignment related information is added to the reports. If n=5, data-references related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=6, the vectorizer reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be countable, or may have complicated control-flow). If n=7, the vectorizer reports also non-vectorized nested loops. If n=8, SLP related information is added to the reports. For n=9, all the information the vectorizer generates during its analysis and transformation is reported. This is the same verbosity level that `-fdump-tree-vect-details' uses.

@gcctabopt{-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.

@gcctabopt{-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 `-fdump-rtl-sched1' or `-fdump-rtl-sched2' is specified, in which case it is output to the usual dump listing file, `.sched1' 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 `-fdump-rtl-sched1' and `-fdump-rtl-sched2'. 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.

@gcctabopt{-save-temps}

@gcctabopt{-save-temps=cwd}

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'.

If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:

gcc -save-temps -o outdir1/foo.o indir1/foo.c& gcc -save-temps -o outdir2/foo.o indir2/foo.c&

may result in `foo.i' and `foo.o' being written to simultaneously by both compilers.

@gcctabopt{-save-temps=obj}

Store the usual "temporary" intermediate files permanently. If the `-o' option is used, the temporary files are based on the object file. If the `-o' option is not used, the `-save-temps=obj' switch behaves like `-save-temps'.

For example:

gcc -save-temps=obj -c foo.c gcc -save-temps=obj -c bar.c -o dir/xbar.o gcc -save-temps=obj foobar.c -o dir2/yfoobar

would create `foo.i', `foo.s', `dir/xbar.i', `dir/xbar.s', `dir2/yfoobar.i', `dir2/yfoobar.s', and `dir2/yfoobar.o'.

@gcctabopt{-time[=file]}

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).

Without the specification of an output file, 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.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 options 0.00 0.01 as options

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.

@gcctabopt{-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.

@gcctabopt{-fvar-tracking-assignments}

Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of `-gdwarf-4' is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations will be created and maintained, but discarded at the end.

@gcctabopt{-fvar-tracking-assignments-toggle}

Toggle `-fvar-tracking-assignments', in the same way that `-gtoggle' toggles `-g'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-print-multi-os-directory}

Print the path to OS libraries for the selected multilib, relative to some `lib' subdirectory. If OS libraries are present in the `lib' subdirectory and no multilibs are used, this is usually just `.', if OS libraries are present in `libsuffix' sibling directories this prints e.g. `../lib64', `../lib' or `../lib32', or if OS libraries are present in `lib/subdir' subdirectories it prints e.g. `amd64', `sparcv9' or `ev6'.

@gcctabopt{-print-prog-name=program}

Like `-print-file-name', but searches for a program such as `cpp'.

@gcctabopt{-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`

@gcctabopt{-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 section 3.19 Environment Variables Affecting GCC.

@gcctabopt{-print-sysroot}

Print the target sysroot directory that will be used during compilation. This is the target sysroot specified either at configure time or using the `--sysroot' option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.

@gcctabopt{-print-sysroot-headers-suffix}

Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix--and don't do anything else.

@gcctabopt{-dumpmachine}

Print the compiler's target machine (for example, `i686-pc-linux-gnu')---and don't do anything else.

@gcctabopt{-dumpversion}

Print the compiler version (for example, `3.0')---and don't do anything else.

@gcctabopt{-dumpspecs}

Print the compiler's built-in specs--and don't do anything else. (This is used when GCC itself is being built.) See section 3.15 Specifying subprocesses and the switches to pass to them.

@gcctabopt{-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.

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. Compiling multiple files at once to a single output file 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 in this section.

Most optimizations are only enabled if an `-O' level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each `-O' level than those listed here. You can invoke GCC with `-Q --help=optimizers' to find out the exact set of optimizations that are enabled at each level. See section 3.2 Options Controlling the Kind of Output, for examples.

@gcctabopt{-O}

@gcctabopt{-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: @gccoptlist{ -fauto-inc-dec @gol -fcprop-registers @gol -fdce @gol -fdefer-pop @gol -fdelayed-branch @gol -fdse @gol -fguess-branch-probability @gol -fif-conversion2 @gol -fif-conversion @gol -fipa-pure-const @gol -fipa-reference @gol -fmerge-constants -fsplit-wide-types @gol -ftree-builtin-call-dce @gol -ftree-ccp @gol -ftree-ch @gol -ftree-copyrename @gol -ftree-dce @gol -ftree-dominator-opts @gol -ftree-dse @gol -ftree-forwprop @gol -ftree-fre @gol -ftree-phiprop @gol -ftree-sra @gol -ftree-pta @gol -ftree-ter @gol -funit-at-a-time}

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

@gcctabopt{-O2}

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. 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: @gccoptlist{-fthread-jumps @gol -falign-functions -falign-jumps @gol -falign-loops -falign-labels @gol -fcaller-saves @gol -fcrossjumping @gol -fcse-follow-jumps -fcse-skip-blocks @gol -fdelete-null-pointer-checks @gol -fexpensive-optimizations @gol -fgcse -fgcse-lm @gol -finline-small-functions @gol -findirect-inlining @gol -fipa-sra @gol -foptimize-sibling-calls @gol -fpeephole2 @gol -fregmove @gol -freorder-blocks -freorder-functions @gol -frerun-cse-after-loop @gol -fsched-interblock -fsched-spec @gol -fschedule-insns -fschedule-insns2 @gol -fstrict-aliasing -fstrict-overflow @gol -ftree-switch-conversion @gol -ftree-pre @gol -ftree-vrp}

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

@gcctabopt{-O3}

Optimize yet more. `-O3' turns on all optimizations specified by `-O2' and also turns on the `-finline-functions', `-funswitch-loops', `-fpredictive-commoning', `-fgcse-after-reload', `-ftree-vectorize' and `-fipa-cp-clone' options.

@gcctabopt{-O0}

Reduce compilation time and make debugging produce the expected results. This is the default.

@gcctabopt{-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. It enables option `-fsort-data'.

`-Os' disables the following optimization flags: @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol -falign-labels -freorder-blocks -freorder-blocks-and-partition @gol -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.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-fforward-propagate}

Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels `-O', `-O2', `-O3', `-Os'.

@gcctabopt{-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 section `Register Usage' in GNU Compiler Collection (GCC) Internals.

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

@gcctabopt{-foptimize-sibling-calls}

Optimize sibling and tail recursive calls.

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

@gcctabopt{-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.

@gcctabopt{-finline-small-functions}

Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

Enabled at level `-O2'.

@gcctabopt{-findirect-inlining}

Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the `-finline-functions' or `-finline-small-functions' options.

Enabled at level `-O2'.

@gcctabopt{-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'.

@gcctabopt{-fsort-data}

Sorts initialized data variables and constant data based on their alignment and place in seperate sections. The N(1/2/4) byte aligned data will be placed either in section ".data.alignN" or in ".rodata.alignN" depending on readonly attribute. The COFF tool chain ignores this option. This option will be ignored if used along with option `-fdata-sections'.

Enabled at level `-Os'.

@gcctabopt{-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 at levels `-O1', `-O2', `-O3' and `-Os'.

@gcctabopt{-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.

@gcctabopt{-fipa-sra}

Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.

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

@gcctabopt{-finline-limit=n}

By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.

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:

@gcctabopt{max-inline-insns-single}

is set to n/2.

@gcctabopt{max-inline-insns-auto}

is set to n/2.

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

Note: there may be no value to `-finline-limit' that results in default behavior.

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.

@gcctabopt{-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 C90. In C++, emit any and all inline functions into the object file.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-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 variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option will result in non-conforming behavior.

@gcctabopt{-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.

@gcctabopt{-fmodulo-sched-allow-regmoves}

Perform more aggressive SMS based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This option is effective only with `-fmodulo-sched' enabled.

@gcctabopt{-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'.

@gcctabopt{-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'

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-fsplit-wide-types}

When using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult.

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

@gcctabopt{-fcse-follow-jumps}

In common subexpression elimination (CSE), 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'.

@gcctabopt{-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'.

@gcctabopt{-frerun-cse-after-loop}

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

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

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-fauto-inc-dec}

Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at `-O' and higher on architectures that support this.

@gcctabopt{-fdce}

Perform dead code elimination (DCE) on RTL. Enabled by default at `-O' and higher.

@gcctabopt{-fdse}

Perform dead store elimination (DSE) on RTL. Enabled by default at `-O' and higher.

@gcctabopt{-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'.

@gcctabopt{-fif-conversion2}

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

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

@gcctabopt{-fdelete-null-pointer-checks}

Assume that programs cannot safely dereference null pointers, and that no code or data element resides there. This enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that in some environments this assumption is not true. Use `-fno-delete-null-pointer-checks' to disable this optimization for programs which depend on that behavior.

Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled at all levels: `-O0', `-O1', `-O2', `-O3', `-Os'. Passes that use the information are enabled independently at different optimization levels.

@gcctabopt{-fexpensive-optimizations}

Perform a number of minor optimizations that are relatively expensive.

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

@gcctabopt{-foptimize-register-move}

@gcctabopt{-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'.

@gcctabopt{-fira-algorithm=algorithm}

Use specified coloring algorithm for the integrated register allocator. The algorithm argument should be priority or CB. The first algorithm specifies Chow's priority coloring, the second one specifies Chaitin-Briggs coloring. The second algorithm can be unimplemented for some architectures. If it is implemented, it is the default because Chaitin-Briggs coloring as a rule generates a better code.

@gcctabopt{-fira-region=region}

Use specified regions for the integrated register allocator. The region argument should be one of all, mixed, or one. The first value means using all loops as register allocation regions, the second value which is the default means using all loops except for loops with small register pressure as the regions, and third one means using all function as a single region. The first value can give best result for machines with small size and irregular register set, the third one results in faster and generates decent code and the smallest size code, and the default value usually give the best results in most cases and for most architectures.

@gcctabopt{-fira-coalesce}

Do optimistic register coalescing. This option might be profitable for architectures with big regular register files.

@gcctabopt{-fira-loop-pressure}

Use IRA to evaluate register pressure in loops for decision to move loop invariants. Usage of this option usually results in generation of faster and smaller code on machines with big register files (>= 32 registers) but it can slow compiler down.

This option is enabled at level `-O3' for some targets.

@gcctabopt{-fno-ira-share-save-slots}

Switch off sharing stack slots used for saving call used hard registers living through a call. Each hard register will get a separate stack slot and as a result function stack frame will be bigger.

@gcctabopt{-fno-ira-share-spill-slots}

Switch off sharing stack slots allocated for pseudo-registers. Each pseudo-register which did not get a hard register will get a separate stack slot and as a result function stack frame will be bigger.

@gcctabopt{-fira-verbose=n}

Set up how verbose dump file for the integrated register allocator will be. Default value is 5. If the value is greater or equal to 10, the dump file will be stderr as if the value were n minus 10.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fsched-pressure}

Enable register pressure sensitive insn scheduling before the register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with `-fschedule-insns' or at `-O2' or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and as a consequence register spills in the register allocation.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fsched-stalled-insns}

@gcctabopt{-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. `-fno-sched-stalled-insns' means that no insns will be moved prematurely, `-fsched-stalled-insns=0' means there is no limit on how many queued insns can be moved prematurely. `-fsched-stalled-insns' without a value is equivalent to `-fsched-stalled-insns=1'.

@gcctabopt{-fsched-stalled-insns-dep}

@gcctabopt{-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. This has an effect only during the second scheduling pass, and only if `-fsched-stalled-insns' is used. `-fno-sched-stalled-insns-dep' is equivalent to `-fsched-stalled-insns-dep=0'. `-fsched-stalled-insns-dep' without a value is equivalent to `-fsched-stalled-insns-dep=1'.

@gcctabopt{-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.

@gcctabopt{-fsched-group-heuristic}

Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-fsched-critical-path-heuristic}

Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-fsched-spec-insn-heuristic}

Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-fsched-rank-heuristic}

Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-fsched-last-insn-heuristic}

Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-fsched-dep-count-heuristic}

Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with `-fschedule-insns' or `-fschedule-insns2' or at `-O2' or higher.

@gcctabopt{-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.

@gcctabopt{-fselective-scheduling}

Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.

@gcctabopt{-fselective-scheduling2}

Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.

@gcctabopt{-fsel-sched-pipelining}

Enable software pipelining of innermost loops during selective scheduling. This option has no effect until one of `-fselective-scheduling' or `-fselective-scheduling2' is turned on.

@gcctabopt{-fsel-sched-pipelining-outer-loops}

When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect until `-fsel-sched-pipelining' is turned on.

@gcctabopt{-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'.

@gcctabopt{-fconserve-stack}

Attempt to minimize stack usage. The compiler will attempt to use less stack space, even if that makes the program slower. This option implies setting the `large-stack-frame' parameter to 100 and the `large-stack-frame-growth' parameter to 400.

@gcctabopt{-ftree-reassoc}

Perform reassociation on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-ftree-pre}

Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at `-O2' and `-O3'.

@gcctabopt{-ftree-forwprop}

Perform forward propagation on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-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 is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at `-O' and higher.

@gcctabopt{-ftree-phiprop}

Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at `-O' and higher.

@gcctabopt{-ftree-copy-prop}

Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at `-O' and higher.

@gcctabopt{-fipa-pure-const}

Discover which functions are pure or constant. Enabled by default at `-O' and higher.

@gcctabopt{-fipa-reference}

Discover which static variables do not escape cannot escape the compilation unit. Enabled by default at `-O' and higher.

@gcctabopt{-fipa-struct-reorg}

Perform structure reorganization optimization, that change C-like structures layout in order to better utilize spatial locality. This transformation is effective for programs containing arrays of structures. Available in two compilation modes: profile-based (enabled with `-fprofile-generate') or static (which uses built-in heuristics). Require `-fipa-type-escape' to provide the safety of this transformation. It works only in whole program mode, so it requires `-fwhole-program' and `-combine' to be enabled. Structures considered `cold' by this transformation are not affected (see `--param struct-reorg-cold-struct-ratio=value').

With this flag, the program debug info reflects a new structure layout.

@gcctabopt{-fipa-pta}

Perform interprocedural pointer analysis. This option is experimental and does not affect generated code.

@gcctabopt{-fipa-cp}

Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at `-O2', `-Os' and `-O3'.

@gcctabopt{-fipa-cp-clone}

Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation will perform function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see `--param ipcp-unit-growth=value'). This flag is enabled by default at `-O3'.

@gcctabopt{-fipa-matrix-reorg}

Perform matrix flattening and transposing. Matrix flattening tries to replace an m-dimensional matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of the matrix. The second optimization is matrix transposing that attempts to change the order of the matrix's dimensions in order to improve cache locality. Both optimizations need the `-fwhole-program' flag. Transposing is enabled only if profiling information is available.

@gcctabopt{-ftree-sink}

Perform forward store motion on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-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.

@gcctabopt{-ftree-switch-conversion}

Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at `-O2' and higher.

@gcctabopt{-ftree-dce}

Perform dead code elimination (DCE) on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-ftree-builtin-call-dce}

Perform conditional dead code elimination (DCE) for calls to builtin functions that may set errno but are otherwise side-effect free. This flag is enabled by default at `-O2' and higher if `-Os' is not also specified.

@gcctabopt{-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.

@gcctabopt{-ftree-dse}

Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location which will later be overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at `-O' and higher.

@gcctabopt{-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.

@gcctabopt{-ftree-loop-optimize}

Perform loop optimizations on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-ftree-loop-linear}

Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.

@gcctabopt{-floop-interchange}

Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and outer loops. For example, given a loop like:

DO J = 1, M DO I = 1, N A(J, I) = A(J, I) * C ENDDO ENDDO

loop interchange will transform the loop as if the user had written:

DO I = 1, N DO J = 1, M A(J, I) = A(J, I) * C ENDDO ENDDO

which can be beneficial when N is larger than the caches, because in Fortran, the elements of an array are stored in memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache miss. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with `--with-ppl' and `--with-cloog' to enable the Graphite loop transformation infrastructure.

@gcctabopt{-floop-strip-mine}

Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops. The outer loop has strides equal to the strip size and the inner loop has strides of the original loop within a strip. The strip length can be changed using the `loop-block-tile-size' parameter. For example, given a loop like:

DO I = 1, N A(I) = A(I) + C ENDDO

loop strip mining will transform the loop as if the user had written:

DO II = 1, N, 51 DO I = II, min (II + 50, N) A(I) = A(I) + C ENDDO ENDDO

This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with `--with-ppl' and `--with-cloog' to enable the Graphite loop transformation infrastructure.

@gcctabopt{-floop-block}

Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such that the memory accesses of the element loops fit inside caches. The strip length can be changed using the `loop-block-tile-size' parameter. For example, given a loop like:

DO I = 1, N DO J = 1, M A(J, I) = B(I) + C(J) ENDDO ENDDO

loop blocking will transform the loop as if the user had written:

DO II = 1, N, 51 DO JJ = 1, M, 51 DO I = II, min (II + 50, N) DO J = JJ, min (JJ + 50, M) A(J, I) = B(I) + C(J) ENDDO ENDDO ENDDO ENDDO

which can be beneficial when M is larger than the caches, because the innermost loop will iterate over a smaller amount of data that can be kept in the caches. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with `--with-ppl' and `--with-cloog' to enable the Graphite loop transformation infrastructure.

@gcctabopt{-fgraphite-identity}

Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using `-fgraphite-identity' we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator CLooG, like index splitting and dead code elimination in loops.

@gcctabopt{-floop-parallelize-all}

Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.

@gcctabopt{-fcheck-data-deps}

Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.

@gcctabopt{-ftree-loop-distribution}

Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop

DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO

is transformed to

DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fivopts}

Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.

@gcctabopt{-ftree-parallelize-loops=n}

Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies `-pthread', and thus is only supported on targets that have support for `-pthread'.

@gcctabopt{-ftree-pta}

Perform function-local points-to analysis on trees. This flag is enabled by default at `-O' and higher.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-ftree-vectorize}

Perform loop vectorization on trees. This flag is enabled by default at `-O3'.

@gcctabopt{-ftree-slp-vectorize}

Perform basic block vectorization on trees. This flag is enabled by default at `-O3' and when `-ftree-vectorize' is enabled.

@gcctabopt{-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.

@gcctabopt{-fvect-cost-model}

Enable cost model for vectorization.

@gcctabopt{-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.

@gcctabopt{-ftracer}

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

@gcctabopt{-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'. This option makes code larger, and may or may not make it run faster.

@gcctabopt{-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',

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fpredictive-commoning}

Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops.

This option is enabled at level `-O3'.

@gcctabopt{-fprefetch-loop-arrays}

If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

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

Disabled at level `-Os'.

@gcctabopt{-fno-peephole}

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-fstrict-aliasing}

Allow 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() { union 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. See section 4.9 Structures, unions, enumerations, and bit-fields. However, this code might not:

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

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() { double d = 3.0; return ((union a_union *) &d)->i; }

The `-fstrict-aliasing' option is enabled at levels `-O2', `-O3', `-Os'.

@gcctabopt{-fstrict-overflow}

Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it will not happen. This permits various optimizations. For example, the compiler will assume that an expression like i + 10 > i will always be true for signed i. This assumption is only valid if signed overflow is undefined, as the expression is false if i + 10 overflows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually involve overflow.

This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the compiler to conclude that p + u > p is always true for a pointer p and unsigned integer u. This assumption is only valid because pointer wraparound is undefined, as the expression is false if p + u overflows using twos complement arithmetic.

See also the `-fwrapv' option. Using `-fwrapv' means that integer signed overflow is fully defined: it wraps. When `-fwrapv' is used, there is no difference between `-fstrict-overflow' and `-fno-strict-overflow' for integers. With `-fwrapv' certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with `-fwrapv', but not otherwise.

The `-fstrict-overflow' option is enabled at levels `-O2', `-O3', `-Os'.

@gcctabopt{-falign-functions}

@gcctabopt{-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'.

@gcctabopt{-falign-labels}

@gcctabopt{-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'.

@gcctabopt{-falign-loops}

@gcctabopt{-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'.

@gcctabopt{-falign-jumps}

@gcctabopt{-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'.

@gcctabopt{-funit-at-a-time}

This option is left for compatibility reasons. `-funit-at-a-time' has no effect, while `-fno-unit-at-a-time' implies `-fno-toplevel-reorder' and `-fno-section-anchors'.

Enabled by default.

@gcctabopt{-fno-toplevel-reorder}

Do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables will not be removed. This option is intended to support existing code which relies on a particular ordering. For new code, it is better to use attributes.

Enabled at level `-O0'. When disabled explicitly, it also imply `-fno-section-anchors' that is otherwise enabled at `-O0' on some targets.

@gcctabopt{-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'.

@gcctabopt{-fwhole-program}

Assume that the current compilation unit represents the 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 effect are optimized more aggressively by interprocedural optimizers. While this option is equivalent to proper use of the static keyword for programs consisting of a single file, in combination with option `-combine', `-flto' or `-fwhopr' this flag can be used to compile many smaller scale programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.

This option implies `-fwhole-file' for Fortran programs.

@gcctabopt{-flto}

This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.

To use the link-timer optimizer, `-flto' needs to be specified at compile time and during the final link. For example,

gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC will save a bytecode representation of GIMPLE into special ELF sections inside `foo.o' and `bar.o'. The final invocation will read the GIMPLE bytecode from `foo.o' and `bar.o', merge the two files into a single internal image, and compile the result as usual. Since both `foo.o' and `bar.o' are merged into a single image, this causes all the inter-procedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner will be able to inline functions in `bar.o' into functions in `foo.o' and vice-versa.

Another (simpler) way to enable link-time optimization is,

gcc -o myprog -flto -O2 foo.c bar.c

The above will generate bytecode for `foo.c' and `bar.c', merge them together into a single GIMPLE representation and optimize them as usual to produce `myprog'.

The only important thing to keep in mind is that to enable link-time optimizations the `-flto' flag needs to be passed to both the compile and the link commands.

Note that when a file is compiled with `-flto', the generated object file will be larger than a regular object file because it will contain GIMPLE bytecodes and the usual final code. This means that object files with LTO information can be linked as a normal object file. So, in the previous example, if the final link is done with

gcc -o myprog foo.o bar.o

The only difference will be that no inter-procedural optimizations will be applied to produce `myprog'. The two object files `foo.o' and `bar.o' will be simply sent to the regular linker.

Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link-time. For instance,

gcc -c -O0 -flto foo.c gcc -c -O0 -flto bar.c gcc -o myprog -flto -O3 foo.o bar.o

This will produce individual object files with unoptimized assembler code, but the resulting binary `myprog' will be optimized at `-O3'. Now, if the final binary is generated without `-flto', then `myprog' will not be optimized.

When producing the final binary with `-flto', GCC will only apply link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC will automatically select which files to optimize in LTO mode and which files to link without further processing.

There are some code generation flags that GCC will preserve when generating bytecodes, as they need to be used during the final link stage. Currently, the following options are saved into the GIMPLE bytecode files: `-fPIC', `-fcommon' and all the `-m' target flags.

At link time, these options are read-in and reapplied. Note that the current implementation makes no attempt at recognizing conflicting values for these options. If two or more files have a conflicting value (e.g., one file is compiled with `-fPIC' and another isn't), the compiler will simply use the last value read from the bytecode files. It is recommended, then, that all the files participating in the same link be compiled with the same options.

Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages. This requires some support in the language front end. Currently, the C, C++ and Fortran front ends are capable of emitting GIMPLE bytecodes, so something like this should work

gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries and `-lgfortran' is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command used when mixing languages in a regular (non-LTO) compilation. This means that if your build process was mixing languages before, all you need to add is `-flto' to all the compile and link commands.

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at runtime.

If object files containing GIMPLE bytecode are stored in a library archive, say `libfoo.a', it is possible to extract and use them in an LTO link if you are using gold as the linker (which, in turn requires GCC to be configured with `--enable-gold'). To enable this feature, use the flag `-fuse-linker-plugin' at link-time:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, gold will extract the needed GIMPLE files from `libfoo.a' and pass them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.

If you are not using gold and/or do not specify `-fuse-linker-plugin' then the objects inside `libfoo.a' will be extracted and linked as usual, but they will not participate in the LTO optimization process.

Link time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine `-flto' and `-fwhopr' with `-fwhole-program' to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities.

Regarding portability: the current implementation of LTO makes no attempt at generating bytecode that can be ported between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC will not work with an older/newer version of GCC.

Link time optimization does not play well with generating debugging information. Combining `-flto' or `-fwhopr' with `-g' is experimental.

This option is disabled by default.

@gcctabopt{-fwhopr}

This option is identical in functionality to `-flto' but it differs in how the final link stage is executed. Instead of loading all the function bodies in memory, the callgraph is analyzed and optimization decisions are made (whole program analysis or WPA). Once optimization decisions are made, the callgraph is partitioned and the different sections are compiled separately (local transformations or LTRANS). This process allows optimizations on very large programs that otherwise would not fit in memory. This option enables `-fwpa' and `-fltrans' automatically.

Disabled by default.

This option is experimental.

@gcctabopt{-fwpa}

This is an internal option used by GCC when compiling with `-fwhopr'. You should never need to use it.

This option runs the link-time optimizer in the whole-program-analysis (WPA) mode, which reads in summary information from all inputs and performs a whole-program analysis based on summary information only. It generates object files for subsequent runs of the link-time optimizer where individual object files are optimized using both summary information from the WPA mode and the actual function bodies. It then drives the LTRANS phase.

Disabled by default.

@gcctabopt{-fltrans}

This is an internal option used by GCC when compiling with `-fwhopr'. You should never need to use it.

This option runs the link-time optimizer in the local-transformation (LTRANS) mode, which reads in output from a previous run of the LTO in WPA mode. In the LTRANS mode, LTO optimizes an object and produces the final assembly.

Disabled by default.

@gcctabopt{-fltrans-output-list=file}

This is an internal option used by GCC when compiling with `-fwhopr'. You should never need to use it.

This option specifies a file to which the names of LTRANS output files are written. This option is only meaningful in conjunction with `-fwpa'.

Disabled by default.

@gcctabopt{-flto-compression-level=n}

This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (`-fwhopr', `-flto'). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is used.

@gcctabopt{-flto-report}

Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version, it is meant to be useful to GCC developers when processing object files in LTO mode (via `-fwhopr' or `-flto').

Disabled by default.

@gcctabopt{-fuse-linker-plugin}

Enables the extraction of objects with GIMPLE bytecode information from library archives. This option relies on features available only in gold, so to use this you must configure GCC with `--enable-gold'. See `-flto' for a description on the effect of this flag and how to use it.

Disabled by default.

@gcctabopt{-fcprop-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.

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

@gcctabopt{-fprofile-correction}

Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC will use heuristics to correct or smooth out such inconsistencies. By default, GCC will emit an error message when an inconsistent profile is detected.

@gcctabopt{-fprofile-dir=path}

Set the directory to search the profile data files in to path. This option affects only the profile data generated by `-fprofile-generate', `-ftest-coverage', `-fprofile-arcs' and used by `-fprofile-use' and `-fbranch-probabilities' and its related options. By default, GCC will use the current directory as path thus the profile data file will appear in the same directory as the object file.

@gcctabopt{-fprofile-generate}

@gcctabopt{-fprofile-generate=path}

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.

If path is specified, GCC will look at the path to find the profile feedback data files. See `-fprofile-dir'.

@gcctabopt{-fprofile-use}

@gcctabopt{-fprofile-use=path}

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

By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using `-Wcoverage-mismatch'. Note this may result in poorly optimized code.

If path is specified, GCC will look at the path to find the profile feedback data files. See `-fprofile-dir'.

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

@gcctabopt{-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.

@gcctabopt{-fexcess-precision=style}

This option allows further control over excess precision on machines where floating-point registers have more precision than the IEEE float and double types and the processor does not support operations rounding to those types. By default, `-fexcess-precision=fast' is in effect; this means that operations are carried out in the precision of the registers and that it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if `-fexcess-precision=standard' is specified then excess precision will follow the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas `-ffloat-store' only affects assignments). This option is enabled by default for C if a strict conformance option such as `-std=c99' is used.

`-fexcess-precision=standard' is not implemented for languages other than C, and has no effect if `-funsafe-math-optimizations' or `-ffast-math' is specified. On the x86, it also has no effect if `-mfpmath=sse' or `-mfpmath=sse+387' is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.

@gcctabopt{-ffast-math}

Sets `-fno-math-errno', `-funsafe-math-optimizations', `-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 is not 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. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

@gcctabopt{-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 is not 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. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

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.

@gcctabopt{-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 is not 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. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables `-fno-signed-zeros', `-fno-trapping-math', `-fassociative-math' and `-freciprocal-math'.

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

@gcctabopt{-fassociative-math}

Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on a code which relies on rounding behavior like (x + 2**52) - 2**52). May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both `-fno-signed-zeros' and `-fno-trapping-math' be in effect. Moreover, it doesn't make much sense with `-frounding-math'. For Fortran the option is automatically enabled when both `-fno-signed-zeros' and `-fno-trapping-math' are in effect.

The default is `-fno-associative-math'.

@gcctabopt{-freciprocal-math}

Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y) which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value.

The default is `-fno-reciprocal-math'.

@gcctabopt{-ffinite-math-only}

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

This option is not 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. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

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

@gcctabopt{-fno-signed-zeros}

Allow optimizations for floating point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with `-ffinite-math-only'). This option implies that the sign of a zero result isn't significant.

The default is `-fsigned-zeros'.

@gcctabopt{-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 requires that `-fno-signaling-nans' be in effect. 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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-fsingle-precision-constant}

Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

@gcctabopt{-fcx-limited-range}

When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. 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.

@gcctabopt{-fcx-fortran-rules}

Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.

The default is `-fno-cx-fortran-rules'.

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.

@gcctabopt{-fbranch-probabilities}

After running a program compiled with `-fprofile-arcs' (see section 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.

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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' and `-fpeel-loops'.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-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'.

@gcctabopt{-fmove-loop-invariants}

Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level `-O1'

@gcctabopt{-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).

@gcctabopt{-ffunction-sections}

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-fbranch-target-load-optimize2}

Perform branch target register load optimization after prologue / epilogue threading.

@gcctabopt{-fbtr-bb-exclusive}

When performing branch target register load optimization, don't reuse branch target registers in within any basic block.

@gcctabopt{-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.

@gcctabopt{-fstack-protector-all}

Like `-fstack-protector' except that all functions are protected.

@gcctabopt{-fsection-anchors}

Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.

For example, the implementation of the following function foo:

static int a, b, c; int foo (void) { return a + b + c; }

would usually calculate the addresses of all three variables, but if you compile it with `-fsection-anchors', it will access the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):

int foo (void) { register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x]; }

Not all targets support this option.

@gcctabopt{--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:

@gcctabopt{struct-reorg-cold-struct-ratio}

The threshold ratio (as a percentage) between a structure frequency and the frequency of the hottest structure in the program. This parameter is used by struct-reorg optimization enabled by `-fipa-struct-reorg'. We say that if the ratio of a structure frequency, calculated by profiling, to the hottest structure frequency in the program is less than this parameter, then structure reorganization is not applied to this structure. The default is 10.

@gcctabopt{predictable-branch-outcome}

When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. The default is 10.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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 300.

@gcctabopt{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 50.

@gcctabopt{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. The default value is 2700.

@gcctabopt{large-function-growth}

Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits large function growth to 2.0 times the original size.

@gcctabopt{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 inlineable 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 applying `--param inline-unit-growth'. The default is 10000

@gcctabopt{inline-unit-growth}

Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 30 which limits unit growth to 1.3 times the original size.

@gcctabopt{ipcp-unit-growth}

Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. The default value is 10 which limits unit growth to 1.1 times the original size.

@gcctabopt{large-stack-frame}

The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. Default value is 256 bytes.

@gcctabopt{large-stack-frame-growth}

Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which limits large stack frame growth to 11 times the original size.

@gcctabopt{max-inline-insns-recursive}

@gcctabopt{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 account. 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.

@gcctabopt{max-inline-recursive-depth}

@gcctabopt{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 account. 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 8.

@gcctabopt{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.

@gcctabopt{early-inlining-insns}

Specify growth that early inliner can make. In effect it increases amount of inlining for code having large abstraction penalty. The default value is 8.

@gcctabopt{max-early-inliner-iterations}

@gcctabopt{max-early-inliner-iterations}

Limit of iterations of early inliner. This basically bounds number of nested indirect calls early inliner can resolve. Deeper chains are still handled by late inlining.

@gcctabopt{min-vect-loop-bound}

The minimum number of iterations under which a loop will not get vectorized when `-ftree-vectorize' is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. The default value is 0.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{max-unroll-times}

The maximum number of unrollings of a single loop.

@gcctabopt{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.

@gcctabopt{max-peel-times}

The maximum number of peelings of a single loop.

@gcctabopt{max-completely-peeled-insns}

The maximum number of insns of a completely peeled loop.

@gcctabopt{max-completely-peel-times}

The maximum number of iterations of a loop to be suitable for complete peeling.

@gcctabopt{max-completely-peel-loop-nest-depth}

The maximum depth of a loop nest suitable for complete peeling.

@gcctabopt{max-unswitch-insns}

The maximum number of insns of an unswitched loop.

@gcctabopt{max-unswitch-level}

The maximum number of branches unswitched in a single loop.

@gcctabopt{lim-expensive}

The minimum cost of an expensive expression in the loop invariant motion.

@gcctabopt{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.

@gcctabopt{iv-max-considered-uses}

The induction variable optimizations give up on loops that contain more induction variable uses.

@gcctabopt{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.

@gcctabopt{scev-max-expr-size}

Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.

@gcctabopt{omega-max-vars}

The maximum number of variables in an Omega constraint system. The default value is 128.

@gcctabopt{omega-max-geqs}

The maximum number of inequalities in an Omega constraint system. The default value is 256.

@gcctabopt{omega-max-eqs}

The maximum number of equalities in an Omega constraint system. The default value is 128.

@gcctabopt{omega-max-wild-cards}

The maximum number of wildcard variables that the Omega solver will be able to insert. The default value is 18.

@gcctabopt{omega-hash-table-size}

The size of the hash table in the Omega solver. The default value is 550.

@gcctabopt{omega-max-keys}

The maximal number of keys used by the Omega solver. The default value is 500.

@gcctabopt{omega-eliminate-redundant-constraints}

When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.

@gcctabopt{vect-max-version-for-alignment-checks}

The maximum number of runtime checks that can be performed when doing loop versioning for alignment in the vectorizer. See option ftree-vect-loop-version for more information.

@gcctabopt{vect-max-version-for-alias-checks}

The maximum number of runtime checks that can be performed when doing loop versioning for alias in the vectorizer. See option ftree-vect-loop-version for more information.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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

@gcctabopt{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.

@gcctabopt{align-threshold}

Select fraction of the maximal frequency of executions of basic block in function given basic block will get aligned.

@gcctabopt{align-loop-iterations}

A loop expected to iterate at lest the selected number of iterations will get aligned.

@gcctabopt{tracer-dynamic-coverage}

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{tracer-min-branch-ratio}

Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).

@gcctabopt{tracer-min-branch-ratio}

@gcctabopt{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.

@gcctabopt{max-cse-path-length}

Maximum number of basic blocks on path that cse considers. The default is 10.

@gcctabopt{max-cse-insns}

The maximum instructions CSE process before flushing. The default is 1000.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{max-cselib-memory-locations}

The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500.

@gcctabopt{reorder-blocks-duplicate}

@gcctabopt{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.

@gcctabopt{max-sched-ready-insns}

The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. The default value is 100.

@gcctabopt{max-sched-region-blocks}

The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.

@gcctabopt{max-pipeline-region-blocks}

The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The default value is 15.

@gcctabopt{max-sched-region-insns}

The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.

@gcctabopt{max-pipeline-region-insns}

The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The default value is 200.

@gcctabopt{min-spec-prob}

The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default value is 40.

@gcctabopt{max-sched-extend-regions-iters}

The maximum number of iterations through CFG to extend regions. 0 - disable region extension, N - do at most N iterations. The default value is 0.

@gcctabopt{max-sched-insn-conflict-delay}

The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.

@gcctabopt{sched-spec-prob-cutoff}

The minimal probability of speculation success (in percents), so that speculative insn will be scheduled. The default value is 40.

@gcctabopt{sched-mem-true-dep-cost}

Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default value is 1.

@gcctabopt{selsched-max-lookahead}

The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. The default value is 50.

@gcctabopt{selsched-max-sched-times}

The maximum number of times that an instruction will be scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. The default value is 2.

@gcctabopt{selsched-max-insns-to-rename}

The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. The default value is 2.

@gcctabopt{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.

@gcctabopt{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. The default value is 256.

@gcctabopt{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.

@gcctabopt{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.

@gcctabopt{ssp-buffer-size}

The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when `-fstack-protection' is used.

@gcctabopt{max-jump-thread-duplication-stmts}

Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.

@gcctabopt{max-fields-for-field-sensitive}

Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analysis. The default is zero for -O0, and -O1 and 100 for -Os, -O2, and -O3.

@gcctabopt{prefetch-latency}

Estimate on average number of instructions that are executed before prefetch finishes. The distance we prefetch ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see `simultaneous-prefetches').

@gcctabopt{simultaneous-prefetches}

Maximum number of prefetches that can run at the same time.

@gcctabopt{l1-cache-line-size}

The size of cache line in L1 cache, in bytes.

@gcctabopt{l1-cache-size}

The size of L1 cache, in kilobytes.

@gcctabopt{l2-cache-size}

The size of L2 cache, in kilobytes.

@gcctabopt{min-insn-to-prefetch-ratio}

The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop with an unknown trip count.

@gcctabopt{prefetch-min-insn-to-mem-ratio}

The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.

@gcctabopt{use-canonical-types}

Whether the compiler should use the "canonical" type system. By default, this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.

@gcctabopt{switch-conversion-max-branch-ratio}

Switch initialization conversion will refuse to create arrays that are bigger than `switch-conversion-max-branch-ratio' times the number of branches in the switch.

@gcctabopt{max-partial-antic-length}

Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (`-ftree-pre') when optimizing at `-O3' and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter will allow an unlimited set length.

@gcctabopt{sccvn-max-scc-size}

Maximum size of a strongly connected component (SCC) during SCCVN processing. If this limit is hit, SCCVN processing for the whole function will not be done and optimizations depending on it will be disabled. The default maximum SCC size is 10000.

@gcctabopt{ira-max-loops-num}

IRA uses a regional register allocation by default. If a function contains loops more than number given by the parameter, only at most given number of the most frequently executed loops will form regions for the regional register allocation. The default value of the parameter is 100.

@gcctabopt{ira-max-conflict-table-size}

Although IRA uses a sophisticated algorithm of compression conflict table, the table can be still big for huge functions. If the conflict table for a function could be more than size in MB given by the parameter, the conflict table is not built and faster, simpler, and lower quality register allocation algorithm will be used. The algorithm do not use pseudo-register conflicts. The default value of the parameter is 2000.

@gcctabopt{ira-loop-reserved-regs}

IRA can be used to evaluate more accurate register pressure in loops for decision to move loop invariants (see `-O3'). The number of available registers reserved for some other purposes is described by this parameter. The default value of the parameter is 2 which is minimal number of registers needed for execution of typical instruction. This value is the best found from numerous experiments.

@gcctabopt{loop-invariant-max-bbs-in-loop}

Loop invariant motion can be very expensive, both in compile time and in amount of needed compile time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.

@gcctabopt{max-vartrack-size}

Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.

@gcctabopt{min-nondebug-insn-uid}

Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by `-fvar-tracking-assignments', but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.

@gcctabopt{ipa-sra-ptr-growth-factor}

IPA-SRA will replace a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to `ipa-sra-ptr-growth-factor' times the size of the original pointer parameter.

@gcctabopt{graphite-max-nb-scop-params}

To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. The default value is 10 parameters. A variable whose value is unknown at compile time and defined outside a SCoP is a parameter of the SCoP.

@gcctabopt{graphite-max-bbs-per-function}

To avoid exponential effects in the detection of SCoPs, the size of the functions analyzed by Graphite is bounded. The default value is 100 basic blocks.

@gcctabopt{loop-block-tile-size}

Loop blocking or strip mining transforms, enabled with `-floop-block' or `-floop-strip-mine', strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the `loop-block-tile-size' parameter. The default value is 51 iterations.

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.

@gcctabopt{-Wp,option}

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.

@gcctabopt{-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.

@gcctabopt{-D name}

Predefine name as a macro, with definition 1.

@gcctabopt{-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.

@gcctabopt{-U name}

Cancel any previous definition of name, either built in or provided with a `-D' option.

@gcctabopt{-undef}

Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

@gcctabopt{-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 . If dir begins with =, then the = will be replaced by the sysroot prefix; see `--sysroot' and `-isysroot'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-Wcomment}

@gcctabopt{-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.)

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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

@gcctabopt{-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.

@gcctabopt{-Werror}

Make all warnings into hard errors. Source code which triggers warnings will be rejected.

@gcctabopt{-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.

@gcctabopt{-w}

Suppress all warnings, including those which GNU CPP issues by default.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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 name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. 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 section 3.19 Environment Variables Affecting GCC). 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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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:

@gcctabopt{-MT target}

Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and 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

@gcctabopt{-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'.

@gcctabopt{-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 takes the name of the input file, removes any directory components and suffix, and applies a `.d' suffix.

If `-MD' is used in conjunction with `-E', any `-o' switch is understood to specify the dependency output file (see section -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.

@gcctabopt{-MMD}

Like `-MD' except mention only user header files, not system header files.

@gcctabopt{-fpch-deps}

When using precompiled headers (see section 3.20 Using 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.

@gcctabopt{-fpch-preprocess}

This option allows use of a precompiled header (see section 3.20 Using 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.

@gcctabopt{-x c}

@gcctabopt{-x c++}

@gcctabopt{-x objective-c}

@gcctabopt{-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.

@gcctabopt{-std=standard}

@gcctabopt{-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:

c90

c89

iso9899:1990

The ISO C standard from 1990. `c90' is the customary shorthand for this version of the standard.

The `-ansi' option is equivalent to `-std=c90'.

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.

gnu90

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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.)

@gcctabopt{-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.

@gcctabopt{-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'.

@gcctabopt{-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. If dir begins with =, then the = will be replaced by the sysroot prefix; see `--sysroot' and `-isysroot'.

@gcctabopt{-iprefix prefix}

Specify prefix as the prefix for subsequent `-iwithprefix' options. If the prefix represents a directory, you should include the final `/'.

@gcctabopt{-iwithprefix dir}

@gcctabopt{-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.

@gcctabopt{-isysroot dir}

This option is like the `--sysroot' option, but applies only to header files. See the `--sysroot' option for more information.

@gcctabopt{-imultilib dir}

Use dir as a subdirectory of the directory containing target-specific C++ headers.

@gcctabopt{-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. If dir begins with =, then the = will be replaced by the sysroot prefix; see `--sysroot' and `-isysroot'.

@gcctabopt{-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. If dir begins with =, then the = will be replaced by the sysroot prefix; see `--sysroot' and `-isysroot'.

@gcctabopt{-fdirectives-only}

When preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the `-E' and `-fpreprocessed' options.

With `-E', preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the `-dD' option is implicitly enabled.

With `-fpreprocessed', predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.

With both `-E' and `-fpreprocessed', the rules for `-fpreprocessed' take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.

@gcctabopt{-fdollars-in-identifiers}

Accept `$' in identifiers.

@gcctabopt{-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++.

@gcctabopt{-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'.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-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.

@gcctabopt{-A -predicate=answer}

Cancel an assertion with the predicate predicate and answer answer.

@gcctabopt{-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.

If you use `-dM' without the `-E' option, `-dM' is interpreted as a synonym for `-fdump-rtl-mach'. See section 3.9 Options for Debugging Your Program or GCC.

`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.

`U'

Like `D' except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and `#undef' directives are also output for macros tested but undefined at the time.

@gcctabopt{-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.

@gcctabopt{-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 `#'.

@gcctabopt{-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.

@gcctabopt{-traditional-cpp}

Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.

@gcctabopt{-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: [ ] { } # \ ^ | ~

@gcctabopt{-remap}

Enable special code to work around file systems which only permit very short file names, such as MS-DOS.

@gcctabopt{--help}

@gcctabopt{--target-help}

Print text describing all the command line options instead of preprocessing anything.

@gcctabopt{-v}

Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.

@gcctabopt{-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 `...!' .

@gcctabopt{-version}

@gcctabopt{--version}

Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.