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# GNU CC Command Options

When you invoke GNU CC, 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 GNU CC 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 multiletter names; therefore multiple single-letter options may not be grouped: ``-dr'` is very different from ``-d -r'`.

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

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

## 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.

      -c  -S  -E  -o file  -pipe  -v  -x language

C Language Options
  See section Options Controlling C Dialect.

      -ansi  -fallow-single-precision -fcond-mismatch  -fno-asm
      -fno-builtin  -fsigned-bitfields  -fsigned-char
      -funsigned-bitfields  -funsigned-char  -fwritable-strings
      -traditional  -traditional-cpp  -trigraphs

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

      -fall-virtual  -fdollars-in-identifiers  -felide-constructors
      -fenum-int-equiv -fexternal-templates  -ffor-scope -fno-for-scope
      -fhandle-signatures -fmemoize-lookups  -fno-default-inline -fno-gnu-keywords
      -fnonnull-objects  -foperator-names  -fstrict-prototype
      -fthis-is-variable -nostdinc++ -traditional  +en

Warning Options
  See section Options to Request or Suppress Warnings.

      -fsyntax-only  -pedantic  -pedantic-errors
      -w  -W  -Wall  -Waggregate-return  -Wbad-function-cast
      -Wcast-align -Wcast-qual  -Wchar-subscript  -Wcomment
      -Wconversion -Wenum-clash  -Werror  -Wformat
      -Wid-clash-len  -Wimplicit  -Wimport  -Winline
      -Wlarger-than-len  -Wmissing-declarations
      -Wmissing-prototypes  -Wnested-externs
      -Wno-import  -Woverloaded-virtual -Wparentheses
      -Wpointer-arith  -Wredundant-decls -Wreorder -Wreturn-type -Wshadow
      -Wstrict-prototypes  -Wswitch  -Wsynth  -Wtemplate-debugging
      -Wtraditional  -Wtrigraphs -Wuninitialized  -Wunused
      -Wwrite-strings

Debugging Options
  See section Options for Debugging Your Program or GNU CC.

      -a  -dletters  -fpretend-float
      -g  -glevel -gcoff  -gdwarf  -gdwarf+
      -ggdb  -gstabs  -gstabs+  -gxcoff  -gxcoff+
      -p  -pg  -print-file-name=library  -print-libgcc-file-name
      -print-prog-name=program  -print-search-dirs  -save-temps

Optimization Options
  See section Options That Control Optimization.

      -fcaller-saves  -fcse-follow-jumps  -fcse-skip-blocks
      -fdelayed-branch   -fexpensive-optimizations
      -ffast-math  -ffloat-store  -fforce-addr  -fforce-mem
      -finline-functions  -fkeep-inline-functions
      -fno-default-inline  -fno-defer-pop  -fno-function-cse
      -fno-inline  -fno-peephole  -fomit-frame-pointer
      -frerun-cse-after-loop  -fschedule-insns
      -fschedule-insns2  -fstrength-reduce  -fthread-jumps
      -funroll-all-loops  -funroll-loops
      -fmove-all-movables  -freduce-all-givs  -frerun-loop-opt
      -O  -O0  -O1  -O2  -O3

Preprocessor Options
  See section Options Controlling the Preprocessor.

      -Aquestion(answer)  -C  -dD  -dM  -dN
      -Dmacro[=defn]  -E  -H
      -idirafter dir
      -include file  -imacros file
      -iprefix file  -iwithprefix dir
      -iwithprefixbefore dir  -isystem dir
      -M  -MD  -MM  -MMD  -MG  -nostdinc  -P  -trigraphs
      -undef  -Umacro  -Wp,option

Assembler Option
  See section Passing Options to the Assembler.

      -Wa,option

Linker Options
  See section Options for Linking.

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

Directory Options
  See section Options for Directory Search.

      -Bprefix  -Idir  -I-  -Ldir

Target Options
  See section Specifying Target Machine and Compiler Version.

      -b machine  -V version

Machine Dependent Options
  See section Hardware Models and Configurations.

      M680x0 Options
      -m68000  -m68020  -m68020-40  -m68030  -m68040  -m68881
      -mbitfield  -mc68000  -mc68020  -mfpa  -mnobitfield
      -mrtd  -mshort  -msoft-float

      VAX Options
      -mg  -mgnu  -munix

      SPARC Options
      -mapp-regs  -mcypress  -mepilogue  -mflat  -mfpu  -mhard-float
      -mhard-quad-float  -mno-app-regs  -mno-flat  -mno-fpu
      -mno-epilogue  -mno-unaligned-doubles
      -msoft-float  -msoft-quad-float
      -msparclite  -msupersparc  -munaligned-doubles  -mv8

      SPARC V9 compilers support the following options
      in addition to the above:

      -mmedlow  -mmedany
      -mint32  -mint64  -mlong32  -mlong64
      -mno-stack-bias  -mstack-bias

      Convex Options
      -mc1  -mc2  -mc32  -mc34  -mc38
      -margcount  -mnoargcount
      -mlong32  -mlong64
      -mvolatile-cache  -mvolatile-nocache

      AMD29K Options
      -m29000  -m29050  -mbw  -mnbw  -mdw  -mndw
      -mlarge  -mnormal  -msmall
      -mkernel-registers  -mno-reuse-arg-regs
      -mno-stack-check  -mno-storem-bug
      -mreuse-arg-regs  -msoft-float  -mstack-check
      -mstorem-bug  -muser-registers

      ARM Options
      -mapcs -m2 -m3 -m6 -mbsd -mxopen -mno-symrename

      M88K Options
      -m88000  -m88100  -m88110  -mbig-pic
      -mcheck-zero-division  -mhandle-large-shift
      -midentify-revision  -mno-check-zero-division
      -mno-ocs-debug-info  -mno-ocs-frame-position
      -mno-optimize-arg-area  -mno-serialize-volatile
      -mno-underscores  -mocs-debug-info
      -mocs-frame-position  -moptimize-arg-area
      -mserialize-volatile  -mshort-data-num  -msvr3
      -msvr4  -mtrap-large-shift  -muse-div-instruction
      -mversion-03.00  -mwarn-passed-structs

      RS/6000 and PowerPC Options
      -mcpu=cpu  type
      -mpower  -mno-power  -mpower2  -mno-power2
      -mpowerpc  -mno-powerpc
      -mpowerpc-gpopt  -mno-powerpc-gpopt
      -mpowerpc-gfxopt  -mno-powerpc-gfxopt
      -mnew-mnemonics  -mno-new-mnemonics
      -mfull-toc   -mminimal-toc  -mno-fop-in-toc  -mno-sum-in-toc
      -msoft-float  -mhard-float -mmultiple -mno-multiple
      -mstring -mno-string -mbit-align -mno-bit-align
      -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
      -mtoc -mno-toc -mtraceback -mno-traceback
      -mlittle -mlittle-endian -mbig -mbig-endian
      -mcall-aix -mcall-sysv -mprototype

      RT Options
      -mcall-lib-mul  -mfp-arg-in-fpregs  -mfp-arg-in-gregs
      -mfull-fp-blocks  -mhc-struct-return  -min-line-mul
      -mminimum-fp-blocks  -mnohc-struct-return

      MIPS Options
      -mabicalls  -mcpu=cpu  type  -membedded-data
      -membedded-pic  -mfp32  -mfp64  -mgas  -mgp32  -mgp64
      -mgpopt  -mhalf-pic  -mhard-float  -mint64 -mips1
      -mips2 -mips3  -mlong64  -mlong-calls  -mmemcpy
      -mmips-as  -mmips-tfile  -mno-abicalls
      -mno-embedded-data  -mno-embedded-pic
      -mno-gpopt  -mno-long-calls
      -mno-memcpy  -mno-mips-tfile  -mno-rnames  -mno-stats
      -mrnames -msoft-float
      -m4650 -msingle-float -mmad
      -mstats  -EL  -EB  -G num  -nocpp

      i386 Options
      -m486  -m386 -mieee-fp  -mno-fancy-math-387
      -mno-fp-ret-in-387  -msoft-float  -msvr3-shlib
      -mno-wide-multiply -mrtd -malign-double
      -mreg-alloc=list -mregparm=num
      -malign-jumps=num -malign-loops=num
      -malign-functions=num

      HPPA Options
      -mdisable-fpregs  -mdisable-indexing  -mfast-indirect-calls
      -mgas  -mjump-in-delay -mlong-millicode-calls -mno-disable-fpregs
      -mno-disable-indexing -mno-fast-indirect-calls -mno-gas
      -mno-jump-in-delay -mno-millicode-long-calls
      -mno-portable-runtime -mno-soft-float -msoft-float
      -mpa-risc-1-0  -mpa-risc-1-1  -mportable-runtime -mschedule=list

      Intel 960 Options
      -mcpu type  -masm-compat  -mclean-linkage
      -mcode-align  -mcomplex-addr  -mleaf-procedures
      -mic-compat  -mic2.0-compat  -mic3.0-compat
      -mintel-asm  -mno-clean-linkage  -mno-code-align
      -mno-complex-addr  -mno-leaf-procedures
      -mno-old-align  -mno-strict-align  -mno-tail-call
      -mnumerics  -mold-align  -msoft-float  -mstrict-align
      -mtail-call

      DEC Alpha Options
      -mfp-regs  -mno-fp-regs  -mno-soft-float
      -msoft-float

      Clipper Options
      -mc300 -mc400

      H8/300 Options
      -mrelax  -mh

      System V Options
      -Qy  -Qn  -YP,paths  -Ym,dir

Code Generation Options
  See section Options for Code Generation Conventions.

      -fcall-saved-reg  -fcall-used-reg
      -ffixed-reg  -finhibit-size-directive
      -fno-common  -fno-ident  -fno-gnu-linker
      -fpcc-struct-return  -fpic  -fPIC
      -freg-struct-return  -fshared-data  -fshort-enums
      -fshort-double  -fvolatile  -fvolatile-global
      -fverbose-asm -fpack-struct +e0  +e1
      -fargument-alias  -fargument-noalias
      -fargument-noalias-global

## Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

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

`file.c`
  C source code which must be preprocessed.
`file.i`
  C source code which should not be preprocessed.
`file.ii`
  C++ source code which should not be preprocessed.
`file.m`
  Objective-C source code. Note that you must link with the library ``libobjc.a'` to make an Objective-C program work.
`file.h`
  C header file (not to be compiled or linked).
`file.cc`
`file.cxx`
`file.cpp`
`file.C`
  C++ source code which must be preprocessed. Note that in ``.cxx'`, the last two letters must both be literally ``x'`. Likewise, ``.C'` refers to a literal capital C.
`file.s`
  Assembler code.
`file.S`
  Assembler code which must be preprocessed.
`other`
  An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

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

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

      c  objective-c  c++
      c-header  cpp-output  c++-cpp-output
      assembler  assembler-with-cpp

`-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).

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

`-c`
  Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix ``.c'`, ``.i'`, ``.s'`, etc., with ``.o'`. Unrecognized input files, not requiring compilation or assembly, are ignored.
`-S`
  Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix ``.c'`, ``.i'`, etc., with ``.s'`. Input files that don't require compilation are ignored.
`-E`
  Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files which don't require preprocessing are ignored.
`-o file`
  Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. Since only one output file can be specified, it does not make sense to use ``-o'` when compiling more than one input file, unless you are producing an executable file as output. 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'`, and all preprocessed C source on standard output.
`-v`
  Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
`-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.

## Compiling C++ Programs

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

However, C++ programs often require class libraries as well as a compiler that understands the C++ language--and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. `g++` is a program that calls GNU CC with the default language set to C++, and automatically specifies linking against the GNU class library libg++. (1) On many systems, the script `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.

## Options Controlling C Dialect

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

`-ansi`
  Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the `asm`, `inline` 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 ANSI trigraph feature, disallows ``$'` as part of identifiers, and disables recognition of C++ style ``//'` comments. The alternate keywords `__asm__`, `__extension__`, `__inline__` and `__typeof__` continue to work despite ``-ansi'`. You would not want to use them in an ANSI 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-ANSI programs to be rejected gratuitously. For that, ``-pedantic'` is required in addition to ``-ansi'`. See section 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 ANSI standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. The functions `alloca`, `abort`, `exit`, and `_exit` are not builtin functions when ``-ansi'` is used.
`-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, as it also disables the other, C++-specific, extension keywords such as `headof`.
`-fno-builtin`
  Don't recognize builtin functions that do not begin with two leading underscores. Currently, the functions affected include `abort`, `abs`, `alloca`, `cos`, `exit`, `fabs`, `ffs`, `labs`, `memcmp`, `memcpy`, `sin`, `sqrt`, `strcmp`, `strcpy`, and `strlen`. GCC normally generates special code to handle certain builtin 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. The ``-ansi'` option prevents `alloca` and `ffs` from being builtin functions, since these functions do not have an ANSI standard meaning.
`-trigraphs`
  Support ANSI C trigraphs. You don't want to know about this brain-damage. The ``-ansi'` option implies ``-trigraphs'`.
`-traditional`
  Attempt to support some aspects of traditional C compilers. Specifically:
  - All `extern` declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions.
  - The newer keywords `typeof`, `inline`, `signed`, `const` and `volatile` are not recognized. (You can still use the alternative keywords such as `__typeof__`, `__inline__`, and so on.)
  - Comparisons between pointers and integers are always allowed.
  - Integer types `unsigned short` and `unsigned char` promote to `unsigned int`.
  - Out-of-range floating point literals are not an error.
  - Certain constructs which ANSI regards as a single invalid preprocessing number, such as ``0xe-0xd'`, are treated as expressions instead.
  - String "constants" are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of ``-fwritable-strings'`.)
  - All automatic variables not declared `register` are preserved by `longjmp`. Ordinarily, GNU C follows ANSI C: automatic variables not declared `volatile` may be clobbered.
  - The character escape sequences ``\x'` and ``\a'` evaluate as the literal characters ``x'` and ``a'` respectively. Without ``-traditional'`, ``\x'` is a prefix for the hexadecimal representation of a character, and ``\a'` produces a bell.
  - In C++ programs, assignment to `this` is permitted with ``-traditional'`. (The option ``-fthis-is-variable'` also has this effect.)
  You may wish to use ``-fno-builtin'` as well as ``-traditional'` if your program uses names that are normally GNU C builtin functions for other purposes of its own. You cannot use ``-traditional'` if you include any header files that rely on ANSI C features. Some vendors are starting to ship systems with ANSI C header files and you cannot use ``-traditional'` on such systems to compile files that include any system headers.
``
  In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.
``
  In preprocessing directive, the ``#'` symbol must appear as the first character of a line.
``
  In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline.
``
  The predefined macro `__STDC__` is not defined when you use ``-traditional'`, but `__GNUC__` is (since the GNU extensions which `__GNUC__` indicates are not affected by ``-traditional'`). If you need to write header files that work differently depending on whether ``-traditional'` is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers. The predefined macro `__STDC_VERSION__` is also not defined when you use ``-traditional'`. See section `Standard Predefined Macros' in The C Preprocessor, for more discussion of these and other predefined macros.
``
  The preprocessor considers a string constant to end at a newline (unless the newline is escaped with ``\'`). (Without ``-traditional'`, string constants can contain the newline character as typed.)
`-traditional-cpp`
  Attempt to support some aspects of traditional C preprocessors. This includes the last five items in the table immediately above, but none of the other effects of ``-traditional'`.
`-fcond-mismatch`
  Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.
`-funsigned-char`
  Let the type `char` be unsigned, like `unsigned char`. Each kind of machine has a default for what `char` should be. It is either like `unsigned char` by default or like `signed char` by default. Ideally, a portable program should always use `signed char` or `unsigned char` when it depends on the signedness of an object. But many programs have been written to use plain `char` and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type `char` is always a distinct type from each of `signed char` or `unsigned char`, even though its behavior is always just like one of those two.
`-fsigned-char`
  Let the type `char` be signed, like `signed char`. Note that this is equivalent to ``-fno-unsigned-char'`, which is the negative form of ``-funsigned-char'`. Likewise, the option ``-fno-signed-char'` is equivalent to ``-funsigned-char'`.
`-fsigned-bitfields`
`-funsigned-bitfields`
`-fno-signed-bitfields`
`-fno-unsigned-bitfields`
  These options control whether a bitfield is signed or unsigned, when the declaration does not use either `signed` or `unsigned`. By default, such a bitfield is signed, because this is consistent: the basic integer types such as `int` are signed types. However, when ``-traditional'` is used, bitfields are all unsigned no matter what.
`-fwritable-strings`
  Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. The option ``-traditional'` also has this effect. Writing into string constants is a very bad idea; "constants" should be constant.
`-fallow-single-precision`
  Do not promote single precision math operations to double precision, even when compiling with ``-traditional'`. Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use ``-traditional'`, but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default).

## 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 -felide-constructors -O -c firstClass.C

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

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

`-fno-access-control`
  Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
`-fall-virtual`
  Treat all possible member functions as virtual, implicitly. All member functions (except for constructor functions and `new` or `delete` member operators) are treated as virtual functions of the class where they appear. This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case.
`-fcheck-new`
  Check that the pointer returned by `operator new` is non-null before attempting to modify the storage allocated. The current Working Paper requires that `operator new` never return a null pointer, so this check is normally unnecessary.
`-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.
`-fdollars-in-identifiers`
  Accept ``$'` in identifiers. You can also explicitly prohibit use of ``$'` with the option ``-fno-dollars-in-identifiers'`. (GNU C++ allows ``$'` by default on some target systems but not others.) Traditional C allowed the character ``$'` to form part of identifiers. However, ANSI C and C++ forbid ``$'` in identifiers.
`-fenum-int-equiv`
  Anachronistically permit implicit conversion of `int` to enumeration types. Current C++ allows conversion of `enum` to `int`, but not the other way around.
`-fexternal-templates`
  Cause template instantiations to obey ``#pragma interface'` and ``implementation'`; template instances are emitted or not according to the location of the template definition. See section Where's the Template?, for more information.
`-falt-external-templates`
  Similar to -fexternal-templates, but template instances are emitted or not according to the place where they are first instantiated. See section Where's the Template?, for more information.
`-ffor-scope`
`-fno-for-scope`
  If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the ``for'` loop itself, as specified by the draft 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 gcc, and other (traditional) implementations of C++. The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.
`-fno-gnu-keywords`
  Do not recognize `classof`, `headof`, `signature`, `sigof` or `typeof` as a keyword, so that code can use these words as identifiers. You can use the keywords `__classof__`, `__headof__`, `__signature__`, `__sigof__`, and `__typeof__` instead. ``-ansi'` implies ``-fno-gnu-keywords'`.
`-fno-implicit-templates`
  Never emit code for templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See section Where's the Template?, for more information.
`-fhandle-signatures`
  Recognize the `signature` and `sigof` keywords for specifying abstract types. The default (``-fno-handle-signatures'`) is not to recognize them. See section Type Abstraction using Signatures.
`-fhuge-objects`
  Support virtual function calls for objects that exceed the size representable by a ``short int'`. Users should not use this flag by default; if you need to use it, the compiler will tell you so. If you compile any of your code with this flag, you must compile all of your code with this flag (including libg++, if you use it). This flag is not useful when compiling with -fvtable-thunks.
`-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.
`-fmemoize-lookups`
`-fsave-memoized`
  Use heuristics to compile faster. These heuristics are not enabled by default, since they are only effective for certain input files. Other input files compile more slowly. The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this:

      cout << "This " << p << " has " << n << " legs.\n";

  makes six passes through all three steps. By using a software cache, a "hit" significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. ``-fmemoize-lookups'` enables the software cache. Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the ``-fmemoize-lookups'` flag, the cache is flushed after every function that is compiled. The ``-fsave-memoized'` flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed. The code that implements these flags has rotted; you should probably avoid using them.
`-fstrict-prototype`
  Within an ``extern "C"'` linkage specification, treat a function declaration with no arguments, such as ``int foo ();'`, as declaring the function to take no arguments. Normally, such a declaration means that the function `foo` can take any combination of arguments, as in C. ``-pedantic'` implies ``-fstrict-prototype'` unless overridden with ``-fno-strict-prototype'`. This flag no longer affects declarations with C++ linkage.
`-fno-nonnull-objects`
  Don't assume that a reference is initialized to refer to a valid object. Although the current C++ Working Paper prohibits null references, some old code may rely on them, and you can use ``-fno-nonnull-objects'` to turn on checking. At the moment, the compiler only does this checking for conversions to virtual base classes.
`-foperator-names`
  Recognize the operator name keywords `and`, `bitand`, `bitor`, `compl`, `not`, `or` and `xor` as synonyms for the symbols they refer to. ``-ansi'` implies ``-foperator-names'`.
`-fthis-is-variable`
  Permit assignment to `this`. The incorporation of user-defined free store management into C++ has made assignment to ``this'` an anachronism. Therefore, by default it is invalid to assign to `this` within a class member function; that is, GNU C++ treats ``this'` in a member function of class `X` as a non-lvalue of type ``X *'`. However, for backwards compatibility, you can make it valid with ``-fthis-is-variable'`.
`-fvtable-thunks`
  Use ``thunks'` to implement the virtual function dispatch table (``vtable'`). The traditional (cfront-style) approach to implementing vtables was to store a pointer to the function and two offsets for adjusting the ``this'` pointer at the call site. Newer implementations store a single pointer to a ``thunk'` function which does any necessary adjustment and then calls the target function. This option also enables a heuristic for controlling emission of vtables; if a class has any non-inline virtual functions, the vtable will be emitted in the translation unit containing the first one of those.
`-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 libg++.)
`-traditional`
  For C++ programs (in addition to the effects that apply to both C and C++), this has the same effect as ``-fthis-is-variable'`. See section Options Controlling C Dialect.

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

`-fno-default-inline`
  Do not assume ``inline'` for functions defined inside a class scope. See section Options That Control Optimization.
`-Wenum-clash`
`-Woverloaded-virtual`
`-Wtemplate-debugging`
  Warnings that apply only to C++ programs. See section Options to Request or Suppress Warnings.
`+en`
  Control how virtual function definitions are used, in a fashion compatible with `cfront` 1.x. See section Options for Code Generation Conventions.

## Options to Request or Suppress Warnings

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

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

These options control the amount and kinds of warnings produced by GNU CC:

`-fsyntax-only`
  Check the code for syntax errors, but don't do anything beyond that.
`-pedantic`
  Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require ``-ansi'`). However, without this option, certain GNU extensions and traditional 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 Alternate Keywords. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard. Some users try to use ``-pedantic'` to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all--only those for which ANSI C requires a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from ``-pedantic'`. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC.
`-pedantic-errors`
  Like ``-pedantic'`, except that errors are produced rather than warnings.
`-w`
  Inhibit all warning messages.
`-Wno-import`
  Inhibit warning messages about the use of ``#import'`.
`-Wchar-subscripts`
  Warn if an array subscript has type `char`. This is a common cause of error, as programmers often forget that this type is signed on some machines.
`-Wcomment`
  Warn whenever a comment-start sequence ``/*'` appears in a comment.
`-Wformat`
  Check calls to `printf` and `scanf`, etc., to make sure that the arguments supplied have types appropriate to the format string specified.
`-Wimplicit`
  Warn whenever a function or parameter is implicitly declared.
`-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.
`-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`.
`-Wswitch`
  Warn whenever a `switch` statement has an index of enumeral 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.
`-Wtrigraphs`
  Warn if any trigraphs are encountered (assuming they are enabled).
`-Wunused`
  Warn whenever a variable is unused aside from its declaration, whenever a function is declared static but never defined, whenever a label is declared but not used, and whenever a statement computes a result that is explicitly not used. To suppress this warning for an expression, simply cast it to void. For unused variables and parameters, use the ``unused'` attribute (see section Specifying Attributes of Variables).
`-Wuninitialized`
  An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify ``-O'`, you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared `volatile`, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. 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 GNU CC 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 GNU CC 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. Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn`. See section Declaring Attributes of Functions.
`-Wenum-clash`
  Warn about conversion between different enumeration types. (C++ only).
`-Wreorder (C++ only)`
  Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

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

  Here the compiler will warn that the member initializers for ``i'` and ``j'` will be rearranged to match the declaration order of the members.
`-Wtemplate-debugging`
  When using templates in a C++ program, warn if debugging is not yet fully available (C++ only).
`-Wall`
  All of the above ``-W'` options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros.

The remaining ``-W...'` options are not implied by ``-Wall'` because they warn about constructions that we consider reasonable to use, on occasion, in clean programs.

`-W`
  Print extra warning messages for these events:
  - A nonvolatile 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.
  - A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning:

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

  - An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as ``x[i,j]'` will cause a warning, but ``x[(void)i,j]'` will not.
  - An unsigned value is compared against zero with ``<'` or ``<='`.
  - 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.
  - Storage-class specifiers like `static` are not the first things in a declaration. According to the C Standard, this usage is obsolescent.
  - If ``-Wall'` or ``-Wunused'` is also specified, warn about unused arguments.
  - An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for `x.h`:

        struct s { int f, g; };
        struct t { struct s h; int i; };
        struct t x = { 1, 2, 3 };

`-Wtraditional`
  Warn about certain constructs that behave differently in traditional and ANSI C.
  - Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C.
  - 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`.
`-Wshadow`
  Warn whenever a local variable shadows another local variable.
`-Wid-clash-len`
  Warn whenever two distinct identifiers match in the first len characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers.
`-Wlarger-than-len`
  Warn whenever an object of larger than len bytes is defined.
`-Wpointer-arith`
  Warn about anything that depends on the "size of" a function type or of `void`. GNU C assigns these types a size of 1, for convenience in calculations with `void *` pointers and pointers to functions.
`-Wbad-function-cast`
  Warn whenever a function call is cast to a non-matching type. For example, warn if `int malloc()` is cast to `anything *`.
`-Wcast-qual`
  Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a `const char *` is cast to an ordinary `char *`.
`-Wcast-align`
  Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *` is cast to an `int *` on machines where integers can only be accessed at two- or four-byte boundaries.
`-Wwrite-strings`
  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.
`-Wconversion`
  Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment `x = -1` if `x` is unsigned. But do not warn about explicit casts like `(unsigned) -1`.
`-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.)
`-Wstrict-prototypes`
  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.)
`-Wmissing-prototypes`
  Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.
`-Wmissing-declarations`
  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.
`-Wredundant-decls`
  Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
`-Wnested-externs`
  Warn if an `extern` declaration is encountered within an function.
`-Winline`
  Warn if a function can not be inlined, and either it was declared as inline, or else the ``-finline-functions'` option was given.
`-Woverloaded-virtual`
  Warn when a derived class function declaration may be an error in defining a virtual function (C++ only). In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class.
`-Wsynth (C++ only)`
  Warn when g++'s synthesis behavior does not match that of cfront. For instance:

      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 ='`.
`-Werror`
  Make all warnings into errors.

## Options for Debugging Your Program or GNU CC

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

`-g`
  Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). 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'`, ``-gdwarf+'`, or ``-gdwarf'` (see below). Unlike most other C compilers, GNU CC 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 GNU CC is generated with the capability for more than one debugging format.
`-ggdb`
  Produce debugging information in the native format (if that is supported), including GDB extensions if at all possible.
`-gstabs`
  Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.
`-gstabs+`
  Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
`-gcoff`
  Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.
`-gxcoff`
  Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
`-gxcoff+`
  Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
`-gdwarf`
  Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on most System V Release 4 systems.
`-gdwarf+`
  Produce debugging information in DWARF 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.
`-glevel`
`-ggdblevel`
`-gstabslevel`
`-gcofflevel`
`-gxcofflevel`
`-gdwarflevel`
  Request debugging information and also use level to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use ``-g3'`.
`-p`
  Generate extra code to write profile information suitable for the analysis program `prof`. You must use this option when compiling the source files you want data about, and you must also use it when linking.
`-pg`
  Generate extra code to write profile information suitable for the analysis program `gprof`. You must use this option when compiling the source files you want data about, and you must also use it when linking.
`-a`
  Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If ``-g'` is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file ``bb.out'`. This data could be analyzed by a program like `tcov`. Note, however, that the format of the data is not what `tcov` expects. Eventually GNU `gprof` should be extended to process this data.
`-dletters`
  Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. ``foo.c.rtl'` or ``foo.c.jump'`). Here are the possible letters for use in letters, and their meanings:
  ``M'`
    Dump all macro definitions, at the end of preprocessing, and write no output.
  ``N'`
    Dump all macro names, at the end of preprocessing.
  ``D'`
    Dump all macro definitions, at the end of preprocessing, in addition to normal output.
  ``y'`
    Dump debugging information during parsing, to standard error.
  ``r'`
    Dump after RTL generation, to ``file.rtl'`.
  ``x'`
    Just generate RTL for a function instead of compiling it. Usually used with ``r'`.
  ``j'`
    Dump after first jump optimization, to ``file.jump'`.
  ``s'`
    Dump after CSE (including the jump optimization that sometimes follows CSE), to ``file.cse'`.
  ``L'`
    Dump after loop optimization, to ``file.loop'`.
  ``t'`
    Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to ``file.cse2'`.
  ``f'`
    Dump after flow analysis, to ``file.flow'`.
  ``c'`
    Dump after instruction combination, to the file ``file.combine'`.
  ``S'`
    Dump after the first instruction scheduling pass, to ``file.sched'`.
  ``l'`
    Dump after local register allocation, to ``file.lreg'`.
  ``g'`
    Dump after global register allocation, to ``file.greg'`.
  ``R'`
    Dump after the second instruction scheduling pass, to ``file.sched2'`.
  ``J'`
    Dump after last jump optimization, to ``file.jump2'`.
  ``d'`
    Dump after delayed branch scheduling, to ``file.dbr'`.
  ``k'`
    Dump after conversion from registers to stack, to ``file.stack'`.
  ``a'`
    Produce all the dumps listed above.
  ``m'`
    Print statistics on memory usage, at the end of the run, to standard error.
  ``p'`
    Annotate the assembler output with a comment indicating which pattern and alternative was used.
`-fpretend-float`
  When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine.
`-save-temps`
  Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling ``foo.c'` with ``-c -save-temps'` would produce files ``foo.i'` and ``foo.s'`, as well as ``foo.o'`.
`-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, GNU CC does not compile or link anything; it just prints the file name.
`-print-prog-name=program`
  Like ``-print-file-name'`, but searches for a program such as ``cpp'`.
`-print-libgcc-file-name`
  Same as ``-print-file-name=libgcc.a'`. This is useful when you use ``-nostdlib'` or ``-nodefaultlibs'` but you do want to link with ``libgcc.a'`. You can do

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

`-print-search-dirs`
  Print the name of the configured installation directory and a list of program and library directories gcc will search--and don't do anything else. This is useful when gcc prints the error message ``installation problem, cannot exec cpp: No such file or directory'`. To resolve this you either need to put ``cpp'` 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 Environment Variables Affecting GNU CC.

## Options That Control Optimization

These options control various sorts of optimizations:

`-O`
`-O1`
  Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without ``-O'`, 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. Without ``-O'`, the compiler only allocates variables declared `register` in registers. The resulting compiled code is a little worse than produced by PCC without ``-O'`. With ``-O'`, the compiler tries to reduce code size and execution time. When you specify ``-O'`, the compiler turns on ``-fthread-jumps'` and ``-fdefer-pop'` on all machines. The compiler turns on ``-fdelayed-branch'` on machines that have delay slots, and ``-fomit-frame-pointer'` on machines that can support debugging even without a frame pointer. On some machines the compiler also turns on other flags.
`-O2`
  Optimize even more. GNU CC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify ``-O2'`. As compared to ``-O'`, this option increases both compilation time and the performance of the generated code. ``-O2'` turns on all optional optimizations except for loop unrolling and function inlining. It also turns on the ``-fforce-mem'` option on all machines and frame pointer elimination on machines where doing so does not interfere with debugging.
`-O3`
  Optimize yet more. ``-O3'` turns on all optimizations specified by ``-O2'` and also turns on the ``inline-functions'` option.
`-O0`
  Do not optimize. 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 which is not the default. You can figure out the other form by either removing ``no-'` or adding it.

`-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. 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.
`-fno-default-inline`
  Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify ``-O'`, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add ``inline'` in front of the member function name.
`-fno-defer-pop`
  Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.
`-fforce-mem`
  Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. The ``-O2'` option turns on this option.
`-fforce-addr`
  Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as ``-fforce-mem'` may.
`-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.
`-fno-inline`
  Don't pay attention to the `inline` keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
`-finline-functions`
  Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared `static`, then the function is normally not output as assembler code in its own right.
`-fkeep-inline-functions`
  Even if all calls to a given function are integrated, and the function is declared `static`, nevertheless output a separate run-time callable version of the function.
`-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.
`-ffast-math`
  This option allows GCC to violate some ANSI or IEEE rules and/or specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the `sqrt` function are non-negative numbers and that no floating-point values are NaNs. 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 ANSI rules/specifications for math functions.

The following options control specific optimizations. The ``-O2'` option turns on all of these optimizations except ``-funroll-loops'` and ``-funroll-all-loops'`. On most machines, the ``-O'` option turns on the ``-fthread-jumps'` and ``-fdelayed-branch'` options, but specific machines may handle it differently.

You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.

`-fstrength-reduce`
  Perform the optimizations of loop strength reduction and elimination of iteration variables.
`-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.
`-fcse-follow-jumps`
  In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an `if` statement with an `else` clause, CSE will follow the jump when the condition tested is false.
`-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`.
`-frerun-cse-after-loop`
  Re-run common subexpression elimination after loop optimizations has been performed.
`-fexpensive-optimizations`
  Perform a number of minor optimizations that are relatively expensive.
`-fdelayed-branch`
  If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.
`-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.
`-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.
`-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 enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
`-funroll-loops`
  Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. ``-funroll-loop'` implies both ``-fstrength-reduce'` and ``-frerun-cse-after-loop'`.
`-funroll-all-loops`
  Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. ``-funroll-all-loops'` implies ``-fstrength-reduce'` as well as ``-frerun-cse-after-loop'`.
`-fmove-all-movables`
  Forces all invariant computations in loops to be moved outside the loop. This option is provided primarily to improve performance for some Fortran code, though it might improve code written in other languages. Note: When compiling programs written in Fortran, this option is enabled by default. Analysis of Fortran code optimization and the resulting optimizations triggered by this option, and the ``-freduce-all-givs'` and ``-frerun-loop-opt'` options as well, were contributed by Toon Moene (`toon@moene.indiv.nluug.nl`). These three options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving the performance of Fortran code. Please let us (`fortran@gnu.ai.mit.edu`) know how use of these options affects the performance of your production code. We're very interested in code that runs slower when these options are enabled.
`-freduce-all-givs`
  Forces all general-induction variables in loops to be strength-reduced. This option is provided primarily to improve performance for some Fortran code, though it might improve code written in other languages. Note: When compiling programs written in Fortran, this option is enabled by default.
`-frerun-loop-opt`
  Runs loop optimizations a second time. This option is provided primarily to improve performance for some Fortran code, though it might improve code written in other languages. Note: When compiling programs written in Fortran, this option is enabled by default.
`-fno-peephole`
  Disable any machine-specific peephole optimizations.

## 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.

`-include file`
  Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any ``-D'` and ``-U'` options on the command line are always processed before ``-include file'`, regardless of the order in which they are written. All the ``-include'` and ``-imacros'` options are processed in the order in which they are written.
`-imacros file`
  Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of ``-imacros file'` is to make the macros defined in file available for use in the main input. Any ``-D'` and ``-U'` options on the command line are always processed before ``-imacros file'`, regardless of the order in which they are written. All the ``-include'` and ``-imacros'` options are processed in the order in which they are written.
`-idirafter dir`
  Add the directory dir to the second include path. The directories on the second include path are searched when a header file is not found in any of the directories in the main include path (the one that ``-I'` adds to).
`-iprefix prefix`
  Specify prefix as the prefix for subsequent ``-iwithprefix'` options.
`-iwithprefix dir`
  Add a directory to the second include path. The directory's name is made by concatenating prefix and dir, where prefix was specified previously with ``-iprefix'`. If you have not specified a prefix yet, the directory containing the installed passes of the compiler is used as the default.
`-iwithprefixbefore dir`
  Add a directory to the main include path. The directory's name is made by concatenating prefix and dir, as in the case of ``-iwithprefix'`.
`-isystem dir`
  Add a directory to the beginning of the second include path, marking it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.
`-nostdinc`
  Do not search the standard system directories for header files. Only the directories you have specified with ``-I'` options (and the current directory, if appropriate) are searched. See section Options for Directory Search, for information on ``-I'`. By using both ``-nostdinc'` and ``-I-'`, you can limit the include-file search path to only those directories you specify explicitly.
`-undef`
  Do not predefine any nonstandard macros. (Including architecture flags).
`-E`
  Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file.
`-C`
  Tell the preprocessor not to discard comments. Used with the ``-E'` option.
`-P`
  Tell the preprocessor not to generate ``#line'` directives. Used with the ``-E'` option.
`-M`
  Tell the preprocessor to output a rule suitable for `make` describing the dependencies of each object file. For each source file, the preprocessor outputs one `make`-rule whose target is the object file name for that source file and whose dependencies are all the `#include` header files it uses. This rule may be a single line or may be continued with ``\'`-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program. ``-M'` implies ``-E'`. Another way to specify output of a `make` rule is by setting the environment variable `DEPENDENCIES_OUTPUT` (see section Environment Variables Affecting GNU CC).
`-MM`
  Like ``-M'` but the output mentions only the user header files included with ``#include "file"'`. System header files included with ``#include <file>'` are omitted.
`-MD`
  Like ``-M'` but the dependency information is written to a file made by replacing ".c" with ".d" at the end of the input file names. This is in addition to compiling the file as specified---``-MD'` does not inhibit ordinary compilation the way ``-M'` does. In Mach, you can use the utility `md` to merge multiple dependency files into a single dependency file suitable for using with the ``make'` command.
`-MMD`
  Like ``-MD'` except mention only user header files, not system header files.
`-MG`
  Treat missing header files as generated files and assume they live in the same directory as the source file. If you specify ``-MG'`, you must also specify either ``-M'` or ``-MM'`. ``-MG'` is not supported with ``-MD'` or ``-MMD'`.
`-H`
  Print the name of each header file used, in addition to other normal activities.
`-Aquestion(answer)`
  Assert the answer answer for question, in case it is tested with a preprocessing conditional such as ``#if #question(answer)'`. ``-A-'` disables the standard assertions that normally describe the target machine.
`-Dmacro`
  Define macro macro with the string ``1'` as its definition.
`-Dmacro=defn`
  Define macro macro as defn. All instances of ``-D'` on the command line are processed before any ``-U'` options.
`-Umacro`
  Undefine macro macro. ``-U'` options are evaluated after all ``-D'` options, but before any ``-include'` and ``-imacros'` options.
`-dM`
  Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the ``-E'` option.
`-dD`
  Tell the preprocessing to pass all macro definitions into the output, in their proper sequence in the rest of the output.
`-dN`
  Like ``-dD'` except that the macro arguments and contents are omitted. Only ``#define name'` is included in the output.
`-trigraphs`
  Support ANSI C trigraphs. The ``-ansi'` option also has this effect.
`-Wp,option`
  Pass option as an option to the preprocessor. If option contains commas, it is split into multiple options at the commas.

## Passing Options to the Assembler

You can pass options to the assembler.

`-Wa,option`
  Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.

## Options for Linking

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

`object-file-name`
  A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
`-c`
`-S`
`-E`
  If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See section Options Controlling the Kind of Output.
`-llibrary`
  Search the library named library when linking. It makes a difference where in the command you write this option; the linker searches processes libraries and object files in the order they are specified. Thus, ``foo.o -lz bar.o'` searches library ``z'` after file ``foo.o'` but before ``bar.o'`. If ``bar.o'` refers to functions in ``z'`, those functions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named ``liblibrary.a'`. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with ``-L'`. Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an ``-l'` option and specifying a file name is that ``-l'` surrounds library with ``lib'` and ``.a'` and searches several directories.
`-lobjc`
  You need this special case of the ``-l'` option in order to link an Objective C program.
`-nostartfiles`
  Do not use the standard system startup files when linking. The standard system libraries are used normally, unless `-nostdlib` or `-nodefaultlibs` is used.
`-nodefaultlibs`
  Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless `-nostartfiles` is used.
`-nostdlib`
  Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. One of the standard libraries bypassed by ``-nostdlib'` and ``-nodefaultlibs'` is ``libgcc.a'`, a library of internal subroutines that GNU CC uses to overcome shortcomings of particular machines, or special needs for some languages. (See section Interfacing to GNU CC Output, for more discussion of ``libgcc.a'`.) In most cases, you need ``libgcc.a'` even when you want to avoid other standard libraries. In other words, when you specify ``-nostdlib'` or ``-nodefaultlibs'` you should usually specify ``-lgcc'` as well. This ensures that you have no unresolved references to internal GNU CC library subroutines. (For example, ``__main'`, used to ensure C++ constructors will be called; see section `collect2`.)
`-s`
  Remove all symbol table and relocation information from the executable.
`-static`
  On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.
`-shared`
  Produce a shared object which can then be linked with other objects to form an executable. Only a few systems support this option.
`-symbolic`
  Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option ``-Xlinker -z -Xlinker defs'`). Only a few systems support this option.
`-Xlinker option`
  Pass option as an option to the linker. You can use this to supply system-specific linker options which GNU CC does not know how to recognize. If you want to pass an option that takes an argument, you must use ``-Xlinker'` twice, once for the option and once for the argument. For example, to pass ``-assert definitions'`, you must write ``-Xlinker -assert -Xlinker definitions'`. It does not work to write ``-Xlinker "-assert definitions"'`, because this passes the entire string as a single argument, which is not what the linker expects.
`-Wl,option`
  Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.
`-u symbol`
  Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use ``-u'` multiple times with different symbols to force loading of additional library modules.

## Options for Directory Search

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

`-Idir`
  Add the directory directory to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. If you use more than one ``-I'` option, the directories are scanned in left-to-right order; the standard system directories come after.
`-I-`
  Any directories you specify with ``-I'` options before the ``-I-'` option are searched only for the case of ``#include "file"'`; they are not searched for ``#include <file>'`. If additional directories are specified with ``-I'` options after the ``-I-'`, these directories are searched for all ``#include'` directives. (Ordinarily all ``-I'` directories are used this way.) In addition, the ``-I-'` option inhibits the use of the current directory (where the current input file came from) as the first search directory for ``#include "file"'`. There is no way to override this effect of ``-I-'`. With ``-I.'` you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory. ``-I-'` does not inhibit the use of the standard system directories for header files. Thus, ``-I-'` and ``-nostdinc'` are independent.
`-Ldir`
  Add directory dir to the list of directories to be searched for ``-l'`.
`-Bprefix`
  This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. The compiler driver program runs one or more of the subprograms ``cpp'`, ``cc1'`, ``as'` and ``ld'`. It tries prefix as a prefix for each program it tries to run, both with and without ``machine/version/'` (see section Specifying Target Machine and Compiler Version). For each subprogram to be run, the compiler driver first tries the ``-B'` prefix, if any. If that name is not found, or if ``-B'` was not specified, the driver tries two standard prefixes, which are ``/usr/lib/gcc/'` and ``/usr/local/lib/gcc-lib/'`. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your ``PATH'` environment variable. ``-B'` prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into ``-L'` options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into ``-isystem'` options for the preprocessor. In this case, the compiler appends ``include'` to the prefix. The run-time support file ``libgcc.a'` can also be searched for using the ``-B'` prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the ``-B'` prefix is to use the environment variable `GCC_EXEC_PREFIX`. See section Environment Variables Affecting GNU CC.

## Specifying Target Machine and Compiler Version

By default, GNU CC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to use with the ``-b'` option.

In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.

`-b machine`
  The argument machine specifies the target machine for compilation. This is useful when you have installed GNU CC as a cross-compiler. The value to use for machine is the same as was specified as the machine type when configuring GNU CC as a cross-compiler. For example, if a cross-compiler was configured with ``configure i386v'`, meaning to compile for an 80386 running System V, then you would specify ``-b i386v'` to run that cross compiler. When you do not specify ``-b'`, it normally means to compile for the same type of machine that you are using.
`-V version`
  The argument version specifies which version of GNU CC to run. This is useful when multiple versions are installed. For example, version might be ``2.0'`, meaning to run GNU CC version 2.0. The default version, when you do not specify ``-V'`, is the last version of GNU CC that you installed.

The ``-b'` and ``-V'` options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GNU CC, for a given target machine, is normally kept in the directory ``/usr/local/lib/gcc-lib/machine/version'`.

Thus, sites can customize the effect of ``-b'` or ``-V'` either by changing the names of these directories or adding alternate names (or symbolic links). If in directory ``/usr/local/lib/gcc-lib/'` the file ``80386'` is a link to the file ``i386v'`, then ``-b 80386'` becomes an alias for ``-b i386v'`.

In one respect, the ``-b'` or ``-V'` do not completely change to a different compiler: the top-level driver program `gcc` that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target and version.

The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version.

The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as `ogcc` and that for version 2.1 is installed as `gcc`, then the command `gcc` will use version 2.1 by default, while `ogcc` will use 2.0 by default. However, you can choose either version with either command with the ``-V'` option.

## Hardware Models and Configurations

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

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

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

These options are defined by the macro `TARGET_SWITCHES` in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.

### M680x0 Options

These are the ``-m'` options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

`-m68000`
`-mc68000`
  Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
`-m68020`
`-mc68020`
  Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.
`-m68881`
  Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless ``-nfp'` was specified when the compiler was configured.
`-m68030`
  Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
`-m68040`
  Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. If your 68040 does not have code to emulate those instructions, use ``-m68040'`.
`-m68020-40`
  Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
`-mfpa`
  Generate output containing Sun FPA instructions for floating point.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets ``m68k-*-aout'` and ``m68k-*-coff'` do provide software floating point support.
`-mshort`
  Consider type `int` to be 16 bits wide, like `short int`.
`-mnobitfield`
  Do not use the bit-field instructions. The ``-m68000'` option implies ``-mnobitfield'`.
`-mbitfield`
  Do use the bit-field instructions. The ``-m68020'` option implies ``-mbitfield'`. This is the default if you use a configuration designed for a 68020.
`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return with the `rtd` instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) The `rtd` instruction is supported by the 68010 and 68020 processors, but not by the 68000.

### VAX Options

These ``-m'` options are defined for the Vax:

`-munix`
  Do not output certain jump instructions (`aobleq` and so on) that the Unix assembler for the Vax cannot handle across long ranges.
`-mgnu`
  Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
`-mg`
  Output code for g-format floating point numbers instead of d-format.

### SPARC Options

These ``-m'` switches are supported on the SPARC:

`-mno-app-regs`
`-mapp-regs`
  Specify ``-mapp-regs'` to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default. To be fully SVR4 ABI compliant at the cost of some performance loss, specify ``-mno-app-regs'`. You should compile libraries and system software with this option.
`-mfpu`
`-mhard-float`
  Generate output containing floating point instructions. This is the default.
`-mno-fpu`
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets ``sparc-*-aout'` and ``sparclite-*-*'` do provide software floating point support. ``-msoft-float'` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile ``libgcc.a'`, the library that comes with GNU CC, with ``-msoft-float'` in order for this to work.
`-mhard-quad-float`
  Generate output containing quad-word (long double) floating point instructions.
`-msoft-quad-float`
  Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default. As of this writing, there are no sparc implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the ``-msoft-quad-float'` option is the default.
`-mno-epilogue`
`-mepilogue`
  With ``-mepilogue'` (the default), the compiler always emits code for function exit at the end of each function. Any function exit in the middle of the function (such as a return statement in C) will generate a jump to the exit code at the end of the function. With ``-mno-epilogue'`, the compiler tries to emit exit code inline at every function exit.
`-mno-flat`
`-mflat`
  With ``-mflat'`, the compiler does not generate save/restore instructions and will use a "flat" or single register window calling convention. This model uses %i7 as the frame pointer and is compatible with the normal register window model. Code from either may be intermixed although debugger support is still incomplete. The local registers and the input registers (0-5) are still treated as "call saved" registers and will be saved on the stack as necessary. With ``-mno-flat'` (the default), the compiler emits save/restore instructions (except for leaf functions) and is the normal mode of operation.
`-mno-unaligned-doubles`
`-munaligned-doubles`
  Assume that doubles have 8 byte alignment. This is the default. With ``-munaligned-doubles'`, GNU CC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.
`-mv8`
`-msparclite`
  These two options select variations on the SPARC architecture. By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture. ``-mv8'` will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC v7. ``-msparclite'` will give you SPARClite code. This adds the integer multiply, integer divide step and scan (`ffs`) instructions which exist in SPARClite but not in SPARC v7.
`-mcypress`
`-msupersparc`
  These two options select the processor for which the code is optimised. With ``-mcypress'` (the default), the compiler optimizes code for the Cypress CY7C602 chip, as used in the SparcStation/SparcServer 3xx series. This is also appropriate for the older SparcStation 1, 2, IPX etc. With ``-msupersparc'` the compiler optimizes code for the SuperSparc cpu, as used in the SparcStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set.

In a future version of GCC, these options will very likely be renamed to ``-mcpu=cypress'` and ``-mcpu=supersparc'`.

These ``-m'` switches are supported in addition to the above on SPARC V9 processors:

`-mmedlow`
  Generate code for the Medium/Low code model: assume a 32 bit address space. Programs are statically linked, PIC is not supported. Pointers are still 64 bits. It is very likely that a future version of GCC will rename this option.
`-mmedany`
  Generate code for the Medium/Anywhere code model: assume a 32 bit text segment starting at offset 0, and a 32 bit data segment starting anywhere (determined at link time). Programs are statically linked, PIC is not supported. Pointers are still 64 bits. It is very likely that a future version of GCC will rename this option.
`-mint64`
  Types long and int are 64 bits.
`-mlong32`
  Types long and int are 32 bits.
`-mlong64`
`-mint32`
  Type long is 64 bits, and type int is 32 bits.
`-mstack-bias`
`-mno-stack-bias`
  With ``-mstack-bias'`, GNU CC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. Otherwise, assume no such offset is present.

### Convex Options

These ``-m'` options are defined for Convex:

`-mc1`
  Generate output for C1. The code will run on any Convex machine. The preprocessor symbol `__convex__c1__` is defined.
`-mc2`
  Generate output for C2. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C2. The preprocessor symbol `__convex_c2__` is defined.
`-mc32`
  Generate output for C32xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C32. The preprocessor symbol `__convex_c32__` is defined.
`-mc34`
  Generate output for C34xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C34. The preprocessor symbol `__convex_c34__` is defined.
`-mc38`
  Generate output for C38xx. Uses instructions not available on C1. Scheduling and other optimizations are chosen for max performance on C38. The preprocessor symbol `__convex_c38__` is defined.
`-margcount`
  Generate code which puts an argument count in the word preceding each argument list. This is compatible with regular CC, and a few programs may need the argument count word. GDB and other source-level debuggers do not need it; this info is in the symbol table.
`-mnoargcount`
  Omit the argument count word. This is the default.
`-mvolatile-cache`
  Allow volatile references to be cached. This is the default.
`-mvolatile-nocache`
  Volatile references bypass the data cache, going all the way to memory. This is only needed for multi-processor code that does not use standard synchronization instructions. Making non-volatile references to volatile locations will not necessarily work.
`-mlong32`
  Type long is 32 bits, the same as type int. This is the default.
`-mlong64`
  Type long is 64 bits, the same as type long long. This option is useless, because no library support exists for it.

### AMD29K Options

These ``-m'` options are defined for the AMD Am29000:

`-mdw`
  Generate code that assumes the `DW` bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default.
`-mndw`
  Generate code that assumes the `DW` bit is not set.
`-mbw`
  Generate code that assumes the system supports byte and halfword write operations. This is the default.
`-mnbw`
  Generate code that assumes the systems does not support byte and halfword write operations. ``-mnbw'` implies ``-mndw'`.
`-msmall`
  Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256k. This allows the `call` instruction to be used instead of a `const`, `consth`, `calli` sequence.
`-mnormal`
  Use the normal memory model: Generate `call` instructions only when calling functions in the same file and `calli` instructions otherwise. This works if each file occupies less than 256 KB but allows the entire executable to be larger than 256 KB. This is the default.
`-mlarge`
  Always use `calli` instructions. Specify this option if you expect a single file to compile into more than 256 KB of code.
`-m29050`
  Generate code for the Am29050.
`-m29000`
  Generate code for the Am29000. This is the default.
`-mkernel-registers`
  Generate references to registers `gr64-gr95` instead of to registers `gr96-gr127`. This option can be used when compiling kernel code that wants a set of global registers disjoint from that used by user-mode code. Note that when this option is used, register names in ``-f'` flags must use the normal, user-mode, names.
`-muser-registers`
  Use the normal set of global registers, `gr96-gr127`. This is the default.
`-mstack-check`
`-mno-stack-check`
  Insert (or do not insert) a call to `__msp_check` after each stack adjustment. This is often used for kernel code.
`-mstorem-bug`
`-mno-storem-bug`
  ``-mstorem-bug'` handles 29k processors which cannot handle the separation of a mtsrim insn and a storem instruction (most 29000 chips to date, but not the 29050).
`-mno-reuse-arg-regs`
`-mreuse-arg-regs`
  ``-mno-reuse-arg-regs'` tells the compiler to only use incoming argument registers for copying out arguments. This helps detect calling a function with fewer arguments than it was declared with.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

### ARM Options

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

`-m2`
`-m3`
  These options are identical. Generate code for the ARM2 and ARM3 processors. This option is the default. You should also use this option to generate code for ARM6 processors that are running with a 26-bit program counter.
`-m6`
  Generate code for the ARM6 processor when running with a 32-bit program counter.
`-mapcs`
  Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code.
`-mbsd`
  This option only applies to RISC iX. Emulate the native BSD-mode compiler. This is the default if ``-ansi'` is not specified.
`-mxopen`
  This option only applies to RISC iX. Emulate the native X/Open-mode compiler.
`-mno-symrename`
  This option only applies to RISC iX. Do not run the assembler post-processor, ``symrename'`, after code has been assembled. Normally it is necessary to modify some of the standard symbols in preparation for linking with the RISC iX C library; this option suppresses this pass. The post-processor is never run when the compiler is built for cross-compilation.

### M88K Options

These ``-m'` options are defined for Motorola 88k architectures:

`-m88000`
  Generate code that works well on both the m88100 and the m88110.
`-m88100`
  Generate code that works best for the m88100, but that also runs on the m88110.
`-m88110`
  Generate code that works best for the m88110, and may not run on the m88100.
`-mbig-pic`
  Obsolete option to be removed from the next revision. Use ``-fPIC'`.
`-midentify-revision`
  Include an `ident` directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used.
`-mno-underscores`
  In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name.
`-mocs-debug-info`
`-mno-ocs-debug-info`
  Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Standard, "OCS". This extra information allows debugging of code that has had the frame pointer eliminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default.
`-mocs-frame-position`
  When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the canonical frame address, which is the stack pointer (register 31) on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use ``-mocs-frame-position'`; other 88k configurations have the default ``-mno-ocs-frame-position'`.
`-mno-ocs-frame-position`
  When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the frame pointer register (register 30). When this option is in effect, the frame pointer is not eliminated when debugging information is selected by the -g switch.
`-moptimize-arg-area`
`-mno-optimize-arg-area`
  Control how function arguments are stored in stack frames. ``-moptimize-arg-area'` saves space by optimizing them, but this conflicts with the 88open specifications. The opposite alternative, ``-mno-optimize-arg-area'`, agrees with 88open standards. By default GNU CC does not optimize the argument area.
`-mshort-data-num`
  Generate smaller data references by making them relative to `r0`, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify ``-mshort-data-512'`, then the data references affected are those involving displacements of less than 512 bytes. ``-mshort-data-num'` is not effective for num greater than 64k.
`-mserialize-volatile`
`-mno-serialize-volatile`
  Do, or don't, generate code to guarantee sequential consistency of volatile memory references. By default, consistency is guaranteed. The order of memory references made by the MC88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When consistency must be guaranteed, GNU C generates special instructions, as needed, to force execution in the proper order. The MC88100 processor does not reorder memory references and so always provides sequential consistency. However, by default, GNU C generates the special instructions to guarantee consistency even when you use ``-m88100'`, so that the code may be run on an MC88110 processor. If you intend to run your code only on the MC88100 processor, you may use ``-mno-serialize-volatile'`. The extra code generated to guarantee consistency may affect the performance of your application. If you know that you can safely forgo this guarantee, you may use ``-mno-serialize-volatile'`.
`-msvr4`
`-msvr3`
  Turn on (``-msvr4'`) or off (``-msvr3'`) compiler extensions related to System V release 4 (SVr4). This controls the following:
  1. Which variant of the assembler syntax to emit.
  2. ``-msvr4'` makes the C preprocessor recognize ``#pragma weak'` that is used on System V release 4.
  3. ``-msvr4'` makes GNU CC issue additional declaration directives used in SVr4.
  ``-msvr4'` is the default for the m88k-motorola-sysv4 and m88k-dg-dgux m88k configurations. ``-msvr3'` is the default for all other m88k configurations.
`-mversion-03.00`
  This option is obsolete, and is ignored.
`-mno-check-zero-division`
`-mcheck-zero-division`
  Do, or don't, generate code to guarantee that integer division by zero will be detected. By default, detection is guaranteed. Some models of the MC88100 processor fail to trap upon integer division by zero under certain conditions. By default, when compiling code that might be run on such a processor, GNU C generates code that explicitly checks for zero-valued divisors and traps with exception number 503 when one is detected. Use of mno-check-zero-division suppresses such checking for code generated to run on an MC88100 processor. GNU C assumes that the MC88110 processor correctly detects all instances of integer division by zero. When ``-m88110'` is specified, both ``-mcheck-zero-division'` and ``-mno-check-zero-division'` are ignored, and no explicit checks for zero-valued divisors are generated.
`-muse-div-instruction`
  Use the div instruction for signed integer division on the MC88100 processor. By default, the div instruction is not used. On the MC88100 processor the signed integer division instruction div) traps to the operating system on a negative operand. The operating system transparently completes the operation, but at a large cost in execution time. By default, when compiling code that might be run on an MC88100 processor, GNU C emulates signed integer division using the unsigned integer division instruction divu), thereby avoiding the large penalty of a trap to the operating system. Such emulation has its own, smaller, execution cost in both time and space. To the extent that your code's important signed integer division operations are performed on two nonnegative operands, it may be desirable to use the div instruction directly. On the MC88110 processor the div instruction (also known as the divs instruction) processes negative operands without trapping to the operating system. When ``-m88110'` is specified, ``-muse-div-instruction'` is ignored, and the div instruction is used for signed integer division. Note that the result of dividing INT_MIN by -1 is undefined. In particular, the behavior of such a division with and without ``-muse-div-instruction'` may differ.
`-mtrap-large-shift`
`-mhandle-large-shift`
  Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GNU CC makes no special provision for large bit shifts.
`-mwarn-passed-structs`
  Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GNU CC issues no such warning.

### IBM RS/6000 and PowerPC Options

These ``-m'` options are defined for the IBM RS/6000 and PowerPC:

`-mpower`
`-mno-power`
`-mpower2`
`-mno-power2`
`-mpowerpc`
`-mno-powerpc`
`-mpowerpc-gpopt`
`-mno-powerpc-gpopt`
`-mpowerpc-gfxopt`
`-mno-powerpc-gfxopt`
  GNU CC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the ``rios'` chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC6xx microprocessors. The PowerPC architecture defines 64-bit instructions, but they are not supported by any current processors. Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture. You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GNU CC. Specifying the ``-mcpu=cpu_type'` overrides the specification of these options. We recommend you use that option rather than these. The ``-mpower'` option allows GNU CC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying ``-mpower2'` implies ``-power'` and also allows GNU CC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture. The ``-mpowerpc'` option allows GNU CC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying ``-mpowerpc-gpopt'` implies ``-mpowerpc'` and also allows GNU CC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying ``-mpowerpc-gfxopt'` implies ``-mpowerpc'` and also allows GNU CC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. If you specify both ``-mno-power'` and ``-mno-powerpc'`, GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both ``-mpower'` and ``-mpowerpc'` permits GNU CC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.
`-mnew-mnemonics`
`-mold-mnemonics`
  Select which mnemonics to use in the generated assembler code. ``-mnew-mnemonics'` requests output that uses the assembler mnemonics defined for the PowerPC architecture, while ``-mold-mnemonics'` requests the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GNU CC uses that mnemonic irrespective of which of these options is specified. PowerPC assemblers support both the old and new mnemonics, as will later POWER assemblers. Current POWER assemblers only support the old mnemonics. Specify ``-mnew-mnemonics'` if you have an assembler that supports them, otherwise specify ``-mold-mnemonics'`. The default value of these options depends on how GNU CC was configured. Specifying ``-mcpu=cpu_type'` sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either ``-mnew-mnemonics'` or ``-mold-mnemonics'`, but should instead accept the default.
`-mcpu=cpu_type`
  Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. By default, cpu_type is the target system defined when GNU CC was configured. Supported values for cpu_type are ``rios1'`, ``rios2'`, ``rsc'`, ``601'`, ``603'`, ``604'`, ``power'`, ``powerpc'`, ``403'`, and ``common'`. ``-mcpu=power'` and ``-mcpu=powerpc'` specify generic POWER and pure PowerPC (i.e., not MPC601) architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes. Specifying ``-mcpu=rios1'`, ``-mcpu=rios2'`, ``-mcpu=rsc'`, or ``-mcpu=power'` enables the ``-mpower'` option and disables the ``-mpowerpc'` option; ``-mcpu=601'` enables both the ``-mpower'` and ``-mpowerpc'` options; ``-mcpu=603'`, ``-mcpu=604'`, ``-mcpu=403'`, and ``-mcpu=powerpc'` enable the ``-mpowerpc'` option and disable the ``-mpower'` option; ``-mcpu=common'` disables both the ``-mpower'` and ``-mpowerpc'` options. To generate code that will operate on all members of the RS/6000 and PowerPC families, specify ``-mcpu=common'`. In that case, GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. GNU CC assumes a generic processor model for scheduling purposes. Specifying ``-mcpu=rios1'`, ``-mcpu=rios2'`, ``-mcpu=rsc'`, or ``-mcpu=power'` also disables the ``new-mnemonics'` option. Specifying ``-mcpu=601'`, ``-mcpu=603'`, ``-mcpu=604'`, ``403'`, or ``-mcpu=powerpc'` also enables the ``new-mnemonics'` option.
`-mfull-toc`
`-mno-fp-in-toc`
`-mno-sum-in-toc`
`-mminimal-toc`
  Modify generation of the TOC (Table Of Contents), which is created for every executable file. The ``-mfull-toc'` option is selected by default. In that case, GNU CC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GNU CC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the ``-mno-fp-in-toc'` and ``-mno-sum-in-toc'` options. ``-mno-fp-in-toc'` prevents GNU CC from putting floating-point constants in the TOC and ``-mno-sum-in-toc'` forces GNU CC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GNU CC to produce very slightly slower and larger code at the expense of conserving TOC space. If you still run out of space in the TOC even when you specify both of these options, specify ``-mminimal-toc'` instead. This option causes GNU CC to make only one TOC entry for every file. When you specify this option, GNU CC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.
`-msoft-float`
`-mhard-float`
  Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the ``-msoft-float'` option, and pass the option to GNU CC when linking.
`-mmultiple`
`-mno-multiple`
  Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use ``-mmultiple'` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.
`-mstring`
`-mno-string`
  Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, anod not generated on PowerPC systems. Do not use ``-mstring'` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.
`-mno-bit-align`
`-mbit-align`
  On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit fields to be aligned to the base type of the bit field. For example, by default a structure containing nothing but 8 `unsigned` bitfields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using ``-mno-bit-align'`, the structure would be aligned to a 1 byte boundary and be one byte in size.
`-mno-strict-align`
`-mstrict-align`
  On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.
`-mrelocatable`
`-mno-relocatable`
  On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.
`-mno-toc`
`-mtoc`
  On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
`-mno-traceback`
`-mtraceback`
  On embedded PowerPC systems do not (do) generate a traceback tag before the start of the function. This tag can be used by the debugger to identify where the start of a function is.
`-mlittle`
`-mlittle-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The ``-mlittle-endian'` option is the same as ``-mlittle'`.
`-mbig`
`-mbig-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The ``-mbig-endian'` option is the same as ``-mbig'`.
`-mcall-sysv`
  On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using ``powerpc-*-eabiaix'`.
`-mcall-aix`
  On System V.4 and embedded PowerPC systems compile code using calling conventions that are similar to those used on AIX. This is the default if you configured GCC using ``powerpc-*-eabiaix'`.
`-mprototype`
`-mno-prototype`
  On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With ``-mprototype'`, only calls to prototyped variable argument functions will set or clear the bit.

### IBM RT Options

These ``-m'` options are defined for the IBM RT PC:

`-min-line-mul`
  Use an in-line code sequence for integer multiplies. This is the default.
`-mcall-lib-mul`
  Call `lmul$$` for integer multiples.
`-mfull-fp-blocks`
  Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default.
`-mminimum-fp-blocks`
  Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically.
`-mfp-arg-in-fpregs`
  Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that `varargs.h` and `stdargs.h` will not work with floating point operands if this option is specified.
`-mfp-arg-in-gregs`
  Use the normal calling convention for floating point arguments. This is the default.
`-mhc-struct-return`
  Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use the option ``-fpcc-struct-return'` for compatibility with the Portable C Compiler (pcc).
`-mnohc-struct-return`
  Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use the option ``-fpcc-struct-return'` or the option ``-mhc-struct-return'`.

### MIPS Options

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

`-mcpu=cpu type`
  Assume the defaults for the machine type cpu type when scheduling instructions. The choices for cpu type are ``r2000'`, ``r3000'`, ``r4000'`, ``r4400'`, ``r4600'`, and ``r6000'`. While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without the ``-mips2'` or ``-mips3'` switches being used.
`-mips1`
  Issue instructions from level 1 of the MIPS ISA. This is the default. ``r3000'` is the default cpu type at this ISA level.
`-mips2`
  Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). ``r6000'` is the default cpu type at this ISA level.
`-mips3`
  Issue instructions from level 3 of the MIPS ISA (64 bit instructions). ``r4000'` is the default cpu type at this ISA level. This option does not change the sizes of any of the C data types.
`-mfp32`
  Assume that 32 32-bit floating point registers are available. This is the default.
`-mfp64`
  Assume that 32 64-bit floating point registers are available. This is the default when the ``-mips3'` option is used.
`-mgp32`
  Assume that 32 32-bit general purpose registers are available. This is the default.
`-mgp64`
  Assume that 32 64-bit general purpose registers are available. This is the default when the ``-mips3'` option is used.
`-mint64`
  Types long, int, and pointer are 64 bits. This works only if ``-mips3'` is also specified.
`-mlong64`
  Types long and pointer are 64 bits, and type int is 32 bits. This works only if ``-mips3'` is also specified.
`-mmips-as`
  Generate code for the MIPS assembler, and invoke ``mips-tfile'` to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If the either of the ``-gstabs'` or ``-gstabs+'` switches are used, the ``mips-tfile'` program will encapsulate the stabs within MIPS ECOFF.
`-mgas`
  Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format.
`-mrnames`
`-mno-rnames`
  The ``-mrnames'` switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The only known assembler that supports this option is the Algorithmics assembler.
`-mgpopt`
`-mno-gpopt`
  The ``-mgpopt'` switch says to write all of the data declarations before the instructions in the text section, this allows the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected.
`-mstats`
`-mno-stats`
  For each non-inline function processed, the ``-mstats'` switch causes the compiler to emit one line to the standard error file to print statistics about the program (number of registers saved, stack size, etc.).
`-mmemcpy`
`-mno-memcpy`
  The ``-mmemcpy'` switch makes all block moves call the appropriate string function (``memcpy'` or ``bcopy'`) instead of possibly generating inline code.
`-mmips-tfile`
`-mno-mips-tfile`
  The ``-mno-mips-tfile'` switch causes the compiler not postprocess the object file with the ``mips-tfile'` program, after the MIPS assembler has generated it to add debug support. If ``mips-tfile'` is not run, then no local variables will be available to the debugger. In addition, ``stage2'` and ``stage3'` objects will have the temporary file names passed to the assembler embedded in the object file, which means the objects will not compare the same. The ``-mno-mips-tfile'` switch should only be used when there are bugs in the ``mips-tfile'` program that prevents compilation.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
`-mhard-float`
  Generate output containing floating point instructions. This is the default if you use the unmodified sources.
`-mabicalls`
`-mno-abicalls`
  Emit (or do not emit) the pseudo operations ``.abicalls'`, ``.cpload'`, and ``.cprestore'` that some System V.4 ports use for position independent code.
`-mlong-calls`
`-mno-long-calls`
  Do all calls with the ``JALR'` instruction, which requires loading up a function's address into a register before the call. You need to use this switch, if you call outside of the current 512 megabyte segment to functions that are not through pointers.
`-mhalf-pic`
`-mno-half-pic`
  Put pointers to extern references into the data section and load them up, rather than put the references in the text section.
`-membedded-pic`
`-mno-embedded-pic`
  Generate PIC code suitable for some embedded systems. All calls are made using PC relative address, and all data is addressed using the $gp register. This requires GNU as and GNU ld which do most of the work.
`-membedded-data`
`-mno-embedded-data`
  Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.
`-msingle-float`
`-mdouble-float`
  The ``-msingle-float'` switch tells gcc to assume that the floating point coprocessor only supports single precision operations, as on the ``r4650'` chip. The ``-mdouble-float'` switch permits gcc to use double precision operations. This is the default.
`-mmad`
`-mno-mad`
  Permit use of the ``mad'`, ``madu'` and ``mul'` instructions, as on the ``r4650'` chip.
`-m4650`
  Turns on ``-msingle-float'`, ``-mmad'`, and, at least for now, ``-mcpu=r4650'`.
`-EL`
  Compile code for the processor in little endian mode. The requisite libraries are assumed to exist.
`-EB`
  Compile code for the processor in big endian mode. The requisite libraries are assumed to exist.
`-G num`
  Put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. This allows the assembler to emit one word memory reference instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The ``-G num'` switch is also passed to the assembler and linker. All modules should be compiled with the same ``-G num'` value.
`-nocpp`
  Tell the MIPS assembler to not run it's preprocessor over user assembler files (with a ``.s'` suffix) when assembling them.

These options are defined by the macro `TARGET_SWITCHES` in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.

### Intel 386 Options

These ``-m'` options are defined for the i386 family of computers:

`-m486`
`-m386`
  Control whether or not code is optimized for a 486 instead of an 386. Code generated for an 486 will run on a 386 and vice versa.
`-mieee-fp`
`-mno-ieee-fp`
  Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if ``-msoft-float'` is used.
`-mno-fp-ret-in-387`
  Do not use the FPU registers for return values of functions. The usual calling convention has functions return values of types `float` and `double` in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. The option ``-mno-fp-ret-in-387'` causes such values to be returned in ordinary CPU registers instead.
`-mno-fancy-math-387`
  Some 387 emulators do not support the `sin`, `cos` and `sqrt` instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD. As of revision 2.6.1, these instructions are not generated unless you also use the ``-ffast-math'` switch.
`-malign-double`
`-mno-align-double`
  Control whether GNU CC aligns `double`, `long double`, and `long long` variables on a two word boundary or a one word boundary. Aligning `double` variables on a two word boundary will produce code that runs somewhat faster on a ``Pentium'` at the expense of more memory. Warning: if you use the ``-malign-double'` switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386.
`-msvr3-shlib`
`-mno-svr3-shlib`
  Control whether GNU CC places uninitialized locals into `bss` or `data`. ``-msvr3-shlib'` places these locals into `bss`. These options are meaningful only on System V Release 3.
`-mno-wide-multiply`
`-mwide-multiply`
  Control whether GNU CC uses the `mul` and `imul` that produce 64 bit results in `eax:edx` from 32 bit operands to do `long long` multiplies and 32-bit division by constants.
`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return with the `ret` num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute ``stdcall'`. You can also override the ``-mrtd'` option by using the function attribute ``cdecl'`. See section Declaring Attributes of Functions Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
`-mreg-alloc=regs`
  Control the default allocation order of integer registers. The string regs is a series of letters specifying a register. The supported letters are: `a` allocate EAX; `b` allocate EBX; `c` allocate ECX; `d` allocate EDX; `S` allocate ESI; `D` allocate EDI; `B` allocate EBP.
`-mregparm=num`
  Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute ``regparm'`. See section Declaring Attributes of Functions Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
`-malign-loops=num`
  Align loops to a 2 raised to a num byte boundary. If ``-malign-loops'` is not specified, the default is 2.
`-malign-jumps=num`
  Align instructions that are only jumped to to a 2 raised to a num byte boundary. If ``-malign-jumps'` is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486.
`-malign-functions=num`
  Align the start of functions to a 2 raised to num byte boundary. If ``-malign-jumps'` is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486.

### HPPA Options

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

`-mpa-risc-1-0`
  Generate code for a PA 1.0 processor.
`-mpa-risc-1-1`
  Generate code for a PA 1.1 processor.
`-mjump-in-delay`
  Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.
`-mmillicode-long-calls`
  Generate code which assumes millicode routines can not be reached by the standard millicode call sequence, linker-generated long-calls, or linker-modified millicode calls. In practice this should only be needed for dynamicly linked executables with extremely large SHLIB_INFO sections.
`-mdisable-fpregs`
  Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.
`-mdisable-indexing`
  Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
`-mfast-indirect-calls`
  Generate code which performs faster indirect calls. Such code is suitable for kernels and for static linking. The fast indirect call code will fail miserably if it's part of a dynamically linked executable and in the presense of nested functions.
`-mportable-runtime`
  Use the portable calling conventions proposed by HP for ELF systems.
`-mgas`
  Enable the use of assembler directives only GAS understands.
`-mschedule=cpu type`
  Schedule code according to the constraints for the machine type cpu type. The choices for cpu type are ``700'` for 7n0 machines, ``7100'` for 7n5 machines, and ``7100'` for 7n2 machines. ``700'` is the default for cpu type. Note the ``7100LC'` scheduling information is incomplete and using ``7100LC'` often leads to bad schedules. For now it's probably best to use ``7100'` instead of ``7100LC'` for the 7n2 machines.
`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded target ``hppa1.1-*-pro'` does provide software floating point support. ``-msoft-float'` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile ``libgcc.a'`, the library that comes with GNU CC, with ``-msoft-float'` in order for this to work.

### Intel 960 Options

These ``-m'` options are defined for the Intel 960 implementations:

`-mcpu type`
  Assume the defaults for the machine type cpu type for some of the other options, including instruction scheduling, floating point support, and addressing modes. The choices for cpu type are ``ka'`, ``kb'`, ``mc'`, ``ca'`, ``cf'`, ``sa'`, and ``sb'`. The default is ``kb'`.
`-mnumerics`
`-msoft-float`
  The ``-mnumerics'` option indicates that the processor does support floating-point instructions. The ``-msoft-float'` option indicates that floating-point support should not be assumed.
`-mleaf-procedures`
`-mno-leaf-procedures`
  Do (or do not) attempt to alter leaf procedures to be callable with the `bal` instruction as well as `call`. This will result in more efficient code for explicit calls when the `bal` instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn't support this optimization.
`-mtail-call`
`-mno-tail-call`
  Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is ``-mno-tail-call'`.
`-mcomplex-addr`
`-mno-complex-addr`
  Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently ``-mcomplex-addr'` for all processors except the CB and CC.
`-mcode-align`
`-mno-code-align`
  Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by default for C-series implementations only.
`-mic-compat`
`-mic2.0-compat`
`-mic3.0-compat`
  Enable compatibility with iC960 v2.0 or v3.0.
`-masm-compat`
`-mintel-asm`
  Enable compatibility with the iC960 assembler.
`-mstrict-align`
`-mno-strict-align`
  Do not permit (do permit) unaligned accesses.
`-mold-align`
  Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37). Currently this is buggy in that ``#pragma align 1'` is always assumed as well, and cannot be turned off.

### DEC Alpha Options

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

`-mno-soft-float`
`-msoft-float`
  Use (do not use) the hardware floating-point instructions for floating-point operations. When `-msoft-float` is specified, functions in ``libgcc1.c'` will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers.
`-mfp-reg`
`-mno-fp-regs`
  Generate code that uses (does not use) the floating-point register set. `-mno-fp-regs` implies `-msoft-float`. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with `-mno-fp-regs` must also be compiled with that option. A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

### Clipper Options

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

`-mc300`
  Produce code for a C300 Clipper processor. This is the default.
`-mc400`
  Produce code for a C400 Clipper processor i.e. use floating point registers f8..f15.

### H8/300 Options

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

`-mrelax`
  Shorten some address references at link time, when possible; uses the linker option ``-relax'`. See section ``ld` and the H8/300' in Using ld, for a fuller description.
`-mh`
  Generate code for the H8/300H.

### Options for System V

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

`-Qy`
  Identify the versions of each tool used by the compiler, in a `.ident` assembler directive in the output.
`-Qn`
  Refrain from adding `.ident` directives to the output file (this is the default).
`-YP,dirs`
  Search the directories dirs, and no others, for libraries specified with ``-l'`.
`-Ym,dir`
  Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

## Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of ``-ffoo'` would be ``-fno-foo'`. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing ``no-'` or adding it.

`-fpcc-struct-return`
  Return "short" `struct` and `union` values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GNU CC-compiled files and files compiled with other compilers. The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type.
`-freg-struct-return`
  Use the convention that `struct` and `union` values are returned in registers when possible. This is more efficient for small structures than ``-fpcc-struct-return'`. If you specify neither ``-fpcc-struct-return'` nor its contrary ``-freg-struct-return'`, GNU CC defaults to whichever convention is standard for the target. If there is no standard convention, GNU CC defaults to ``-fpcc-struct-return'`, except on targets where GNU CC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
`-fshort-enums`
  Allocate to an `enum` type only as many bytes as it needs for the declared range of possible values. Specifically, the `enum` type will be equivalent to the smallest integer type which has enough room.
`-fshort-double`
  Use the same size for `double` as for `float`.
`-fshared-data`
  Requests that the data and non-`const` variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.
`-fno-common`
  Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without `extern`) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.
`-fno-ident`
  Ignore the ``#ident'` directive.
`-fno-gnu-linker`
  Do not output global initializations (such as C++ constructors and destructors) in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a non-GNU linker, which also requires using the `collect2` program to make sure the system linker includes constructors and destructors. (`collect2` is included in the GNU CC distribution.) For systems which must use `collect2`, the compiler driver `gcc` is configured to do this automatically.
`-finhibit-size-directive`
  Don't output a `.size` assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling ``crtstuff.c'`; you should not need to use it for anything else.
`-fverbose-asm`
  Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).
`-fvolatile`
  Consider all memory references through pointers to be volatile.
`-fvolatile-global`
  Consider all memory references to extern and global data items to be volatile.
`-fpic`
  Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that ``-fpic'` does not work; in that case, recompile with ``-fPIC'` instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the 386, GNU CC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. The GNU assembler does not fully support PIC. Currently, you must use some other assembler in order for PIC to work. We would welcome volunteers to upgrade GAS to handle this; the first part of the job is to figure out what the assembler must do differently.
`-fPIC`
  If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, m88k and the Sparc. Position-independent code requires special support, and therefore works only on certain machines.
`-ffixed-reg`
  Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the `REGISTER_NAMES` macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice.
`-fcall-used-reg`
  Treat the register named reg as an allocatable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. This flag does not have a negative form, because it specifies a three-way choice.
`-fcall-saved-reg`
  Treat the register named reg as an allocatable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice.
`-fpack-struct`
  Pack all structure members together without holes. Usually you would not want to use this option, since it makes the code suboptimal, and the offsets of structure members won't agree with system libraries.
`+e0`
`+e1`
  Control whether virtual function definitions in classes are used to generate code, or only to define interfaces for their callers. (C++ only). These options are provided for compatibility with `cfront` 1.x usage; the recommended alternative GNU C++ usage is in flux. See section Declarations and Definitions in One Header. With ``+e0'`, virtual function definitions in classes are declared `extern`; the declaration is used only as an interface specification, not to generate code for the virtual functions (in this compilation). With ``+e1'`, G++ actually generates the code implementing virtual functions defined in the code, and makes them publicly visible.
`-fargument-alias`
`-fargument-noalias`
`-fargument-noalias-global`
  Specify the possible relationships among parameters and between parameters and global data. ``-fargument-alias'` specifies that arguments (parameters) may alias each other and may alias global storage. ``-fargument-noalias'` specifies that arguments do not alias each other, but may alias global storage. ``-fargument-noalias-global'` specifies that arguments do not alias each other and do not alias global storage. For code written in C, C++, and Objective-C, ``-fargument-alias'` is the default. For code written in Fortran, ``-fargument-noalias-global'` is the default, though this is pertinent only on systems where `g77` is installed. (See the documentation for other compilers for information on the defaults for their respective languages.) Normally, `gcc` assumes that a write through a pointer passed as a parameter to the current function might modify a value pointed to by another pointer passed as a parameter, or in global storage. For example, consider this code:

      void x(int *i, int *j)
      {
        extern int k;

        ++*i;
        ++*j;
        ++k;
      }

  When compiling the above function, `gcc` assumes that ``i'` might be a pointer to the same variable as ``j'`, and that either ``i'`, ``j'`, or both might be a pointer to ``k'`. Therefore, `gcc` does not assume it can generate code to read ``*i'`, ``*j'`, and ``k'` into separate registers, increment each register, then write the incremented values back out. Instead, `gcc` must generate code that reads ``*i'`, increments it, and writes it back before reading ``*j'`, in case ``i'` and ``j'` are aliased, and, similarly, that writes ``*j'` before reading ``k'`. The result is code that, on many systems, takes longer to execute, due to the way many processors schedule instruction execution. Compiling the above code with the ``-fargument-noalias'` option allows `gcc` to assume that ``i'` and ``j'` do not alias each other, but either might alias ``k'`. Compiling the above code with the ``-fargument-noalias-global'` option allows `gcc` to assume that no combination of ``i'`, ``j'`, and ``k'` are aliases for each other. Note: Use the ``-fargument-noalias'` and ``-fargument-noalias-global'` options with care. While they can result in faster executables, they can also result in executables with subtle bugs, bugs that show up only when compiled for specific target systems, or bugs that show up only when compiled by specific versions of `g77`.

## Environment Variables Affecting GNU CC

This section describes several environment variables that affect how GNU CC operates. They work by specifying directories or prefixes to use when searching for various kinds of files.

Note that you can also specify places to search using options such as ``-B'`, ``-I'` and ``-L'` (see section Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC. See section Controlling the Compilation Driver, ``gcc'`.

`TMPDIR`
  If `TMPDIR` is set, it specifies the directory to use for temporary files. GNU CC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.
`GCC_EXEC_PREFIX`
  If `GCC_EXEC_PREFIX` is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. If GNU CC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. The default value of `GCC_EXEC_PREFIX` is ``prefix/lib/gcc-lib/'` where prefix is the value of `prefix` when you ran the ``configure'` script. Other prefixes specified with ``-B'` take precedence over this prefix. This prefix is also used for finding files such as ``crt0.o'` that are used for linking. In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with ``/usr/local/lib/gcc-lib'` (more precisely, with the value of `GCC_INCLUDE_DIR`), GNU CC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with ``-Bfoo/'`, GNU CC will search ``foo/bar'` where it would normally search ``/usr/local/lib/bar'`. These alternate directories are searched first; the standard directories come next.
`COMPILER_PATH`
  The value of `COMPILER_PATH` is a colon-separated list of directories, much like `PATH`. GNU CC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using `GCC_EXEC_PREFIX`.
`LIBRARY_PATH`
  The value of `LIBRARY_PATH` is a colon-separated list of directories, much like `PATH`. When configured as a native compiler, GNU CC tries the directories thus specified when searching for special linker files, if it can't find them using `GCC_EXEC_PREFIX`. Linking using GNU CC also uses these directories when searching for ordinary libraries for the ``-l'` option (but directories specified with ``-L'` come first).
`C_INCLUDE_PATH`
`CPLUS_INCLUDE_PATH`
`OBJC_INCLUDE_PATH`
  These environment variables pertain to particular languages. Each variable's value is a colon-separated list of directories, much like `PATH`. When GNU CC searches for header files, it tries the directories listed in the variable for the language you are using, after the directories specified with ``-I'` but before the standard header file directories.
`DEPENDENCIES_OUTPUT`
  If this variable is set, its value specifies how to output dependencies for Make based on the header files processed by the compiler. This output looks much like the output from the ``-M'` option (see section Options Controlling the Preprocessor), but it goes to a separate file, and is in addition to the usual results of compilation. The value of `DEPENDENCIES_OUTPUT` can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form ``file target'`, in which case the rules are written to file file using target as the target name.

## Running Protoize

The program `protoize` is an optional part of GNU C. You can use it to add prototypes to a program, thus converting the program to ANSI C in one respect. The companion program `unprotoize` does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named ``foo.X'`.

After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted. By default, `protoize` and `unprotoize` convert only source and header files in the current directory. You can specify additional directories whose files should be converted with the ``-d directory'` option. You can also specify particular files to exclude with the ``-x file'` option. A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with `protoize` consists of rewriting most function definitions and function declarations to specify the types of the arguments. The only ones not rewritten are those for varargs functions.

`protoize` optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function's definition. Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with `unprotoize` consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ANSI form.

Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with ``-q'`.

The output from `protoize` or `unprotoize` replaces the original source file. The original file is renamed to a name ending with ``.save'`. If the ``.save'` file already exists, then the source file is simply discarded.

`protoize` and `unprotoize` both depend on GNU CC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GNU CC is installed.

Here is a table of the options you can use with `protoize` and `unprotoize`. Each option works with both programs unless otherwise stated.

`-B directory`
  Look for the file ``SYSCALLS.c.X'` in directory, instead of the usual directory (normally ``/usr/local/lib'`). This file contains prototype information about standard system functions. This option applies only to `protoize`.
`-c compilation-options`
  Use compilation-options as the options when running `gcc` to produce the ``.X'` files. The special option ``-aux-info'` is always passed in addition, to tell `gcc` to write a ``.X'` file. Note that the compilation options must be given as a single argument to `protoize` or `unprotoize`. If you want to specify several `gcc` options, you must quote the entire set of compilation options to make them a single word in the shell. There are certain `gcc` arguments that you cannot use, because they would produce the wrong kind of output. These include ``-g'`, ``-O'`, ``-c'`, ``-S'`, and ``-o'` If you include these in the compilation-options, they are ignored.
`-C`
  Rename files to end in ``.C'` instead of ``.c'`. This is convenient if you are converting a C program to C++. This option applies only to `protoize`.
`-g`
  Add explicit global declarations. This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared. These declarations precede the first function definition that contains a call to an undeclared function. This option applies only to `protoize`.
`-i string`
  Indent old-style parameter declarations with the string string. This option applies only to `protoize`. `unprotoize` converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial ``{'`. By default, `unprotoize` uses five spaces as the indentation. If you want to indent with just one space instead, use ``-i " "'`.
`-k`
  Keep the ``.X'` files. Normally, they are deleted after conversion is finished.
`-l`
  Add explicit local declarations. `protoize` with ``-l'` inserts a prototype declaration for each function in each block which calls the function without any declaration. This option applies only to `protoize`.
`-n`
  Make no real changes. This mode just prints information about the conversions that would have been done without ``-n'`.
`-N`
  Make no ``.save'` files. The original files are simply deleted. Use this option with caution.
`-p program`
  Use the program program as the compiler. Normally, the name ``gcc'` is used.
`-q`
  Work quietly. Most warnings are suppressed.
`-v`
  Print the version number, just like ``-v'` for `gcc`.

If you need special compiler options to compile one of your program's source files, then you should generate that file's ``.X'` file specially, by running `gcc` on that source file with the appropriate options and the option ``-aux-info'`. Then run `protoize` on the entire set of files. `protoize` will use the existing ``.X'` file because it is newer than the source file. For example:

    gcc -Dfoo=bar file1.c -aux-info
    protoize *.c

You need to include the special files along with the rest in the `protoize` command, even though their ``.X'` files already exist, because otherwise they won't get converted.

See section Caveats of using `protoize`, for more information on how to use `protoize` successfully.

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