Using and Porting the GNU Compiler Collection (GCC): Invoking GCC

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# 2. GCC Command Options

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

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

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

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

The `gcc` program accepts options and file names as operands. Many options have 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.

> 2.1 Option Summary |  | Brief list of all options, without explanations.
> 2.2 Options Controlling the Kind of Output |  | Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
> 2.3 Compiling C++ Programs |  | Compiling C++ programs.
> 2.4 Options Controlling C Dialect |  | Controlling the variant of C language compiled.
> 2.5 Options Controlling C++ Dialect |  | Variations on C++.
> 2.6 Options to Request or Suppress Warnings |  | How picky should the compiler be?
> 2.7 Options for Debugging Your Program or GCC |  | Symbol tables, measurements, and debugging dumps.
> 2.8 Options That Control Optimization |  | How much optimization?
> 2.9 Options Controlling the Preprocessor |  | Controlling header files and macro definitions. Also, getting dependency information for Make.
> 2.10 Passing Options to the Assembler |  | Passing options to the assembler.
> 2.11 Options for Linking |  | Specifying libraries and so on.
> 2.12 Options for Directory Search |  | Where to find header files and libraries. Where to find the compiler executable files.
> 2.13 Specifying Target Machine and Compiler Version |  | Running a cross-compiler, or an old version of GCC.
> 2.14 Hardware Models and Configurations |  | Specifying minor hardware or convention variations, such as 68010 vs 68020.
> 2.15 Options for Code Generation Conventions |  | Specifying conventions for function calls, data layout and register usage.
> 2.16 Environment Variables Affecting GCC |  | Env vars that affect GCC.
> 2.17 Running Protoize |  | Automatically adding or removing function prototypes.

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## 2.1 Option Summary

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

Overall Options
  See section Options Controlling the Kind of Output.

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

C Language Options
  See section Options Controlling C Dialect.

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

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

      -fno-access-control  -fcheck-new  -fconserve-space  -fdollars-in-identifiers
      -fno-elide-constructors  -fexternal-templates  -ffor-scope
      -fno-for-scope  -fno-gnu-keywords  -fguiding-decls  -fhandle-signatures
      -fhonor-std -fhuge-objects  -fno-implicit-templates  -finit-priority
      -fno-implement-inlines -fname-mangling-version-n  -fno-default-inline
      -foperator-names  -fno-optional-diags  -fpermissive -frepo  -fstrict-prototype
      -fsquangle  -ftemplate-depth-n  -fthis-is-variable  -fvtable-thunks
      -nostdinc++  -Wctor-dtor-privacy -Wno-deprecated -Weffc++
      -Wno-non-template-friend
      -Wnon-virtual-dtor  -Wold-style-cast  -Woverloaded-virtual
      -Wno-pmf-conversions  -Wreorder  -Wsign-promo  -Wsynth

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-subscripts  -Wcomment
      -Wconversion  -Werror  -Wformat
      -Wid-clash-len  -Wimplicit -Wimplicit-int
      -Wimplicit-function-declaration  -Wimport
      -Werror-implicit-function-declaration  -Winline
      -Wlarger-than-len  -Wlong-long
      -Wmain  -Wmissing-declarations  -Wmissing-noreturn
      -Wmissing-prototypes  -Wmultichar  -Wnested-externs  -Wno-import
      -Wparentheses -Wpointer-arith  -Wredundant-decls
      -Wreturn-type -Wshadow  -Wsign-compare  -Wstrict-prototypes
      -Wswitch  -Wtraditional
      -Wtrigraphs -Wundef  -Wuninitialized  -Wunused  -Wwrite-strings
      -Wunknown-pragmas

Debugging Options
  See section Options for Debugging Your Program or GCC.

      -a  -ax  -dletters  -fdump-unnumbered -fpretend-float
      -fprofile-arcs  -ftest-coverage
      -g  -glevel  -gcoff  -gdwarf  -gdwarf-1  -gdwarf-1+  -gdwarf-2
      -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.

      -fbranch-probabilities  -foptimize-register-moves
      -fcaller-saves  -fcse-follow-jumps  -fcse-skip-blocks
      -fdelayed-branch   -fexpensive-optimizations
      -ffast-math  -ffloat-store  -fforce-addr  -fforce-mem
      -fdata-sections -ffunction-sections  -fgcse
      -finline-functions -finline-limit-n -fkeep-inline-functions
      -fno-default-inline -fno-defer-pop  -fno-function-cse
      -fno-inline  -fno-peephole  -fomit-frame-pointer -fregmove
      -frerun-cse-after-loop  -frerun-loop-opt -fschedule-insns
      -fschedule-insns2  -fstrength-reduce  -fthread-jumps
      -funroll-all-loops  -funroll-loops
      -fmove-all-movables  -freduce-all-givs -fstrict-aliasing
      -O  -O0  -O1  -O2  -O3 -Os

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 -isystem-c++ 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  -specs=file

Target Options
  See section 2.13 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  -m68020-60  -m68030  -m68040
      -m68060  -mcpu32 -m5200  -m68881  -mbitfield  -mc68000  -mc68020
      -mfpa -mnobitfield  -mrtd  -mshort  -msoft-float
      -malign-int

      VAX Options
      -mg  -mgnu  -munix

      SPARC Options
      -mcpu=cpu type
      -mtune=cpu type
      -mcmodel=code model
      -malign-jumps=num  -malign-loops=num
      -malign-functions=num
      -m32  -m64
      -mapp-regs  -mbroken-saverestore  -mcypress  -mepilogue
      -mflat  -mfpu  -mhard-float  -mhard-quad-float
      -mimpure-text  -mlive-g0  -mno-app-regs  -mno-epilogue
      -mno-flat  -mno-fpu  -mno-impure-text
      -mno-stack-bias  -mno-unaligned-doubles
      -msoft-float  -msoft-quad-float  -msparclite  -mstack-bias
      -msupersparc  -munaligned-doubles  -mv8

      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-frame -mno-apcs-frame
      -mapcs-26 -mapcs-32
      -mapcs-stack-check -mno-apcs-stack-check
      -mapcs-float -mno-apcs-float
      -mapcs-reentrant -mno-apcs-reentrant
      -msched-prolog -mno-sched-prolog
      -mlittle-endian -mbig-endian -mwords-little-endian
      -mshort-load-bytes -mno-short-load-bytes -mshort-load-words -mno-short-load-words
      -msoft-float -mhard-float -mfpe
      -mthumb-interwork -mno-thumb-interwork
      -mcpu= -march= -mfpe=
      -mstructure-size-boundary=
      -mbsd -mxopen -mno-symrename
      -mabort-on-noreturn
      -mno-sched-prolog

      Thumb Options
      -mtpcs-frame -mno-tpcs-frame
      -mtpcs-leaf-frame -mno-tpcs-leaf-frame
      -mlittle-endian  -mbig-endian
      -mthumb-interwork -mno-thumb-interwork
      -mstructure-size-boundary=

      MN10200 Options
      -mrelax

      MN10300 Options
      -mmult-bug
      -mno-mult-bug
      -mrelax

      M32R/D Options
      -mcode-model=model type  -msdata=sdata type
      -G num

      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
      -mtune=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
      -maix64  -maix32  -mxl-call  -mno-xl-call  -mthreads  -mpe
      -msoft-float  -mhard-float  -mmultiple  -mno-multiple
      -mstring  -mno-string  -mupdate  -mno-update
      -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align
      -mstrict-align  -mno-strict-align  -mrelocatable
      -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib
      -mtoc  -mno-toc -mlittle  -mlittle-endian  -mbig  -mbig-endian
      -mcall-aix  -mcall-sysv  -mprototype  -mno-prototype
      -msim  -mmvme  -mads  -myellowknife  -memb -msdata
      -msdata=opt  -G num

      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 -mips4 -mlong64  -mlong32 -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
      -mabi=32 -mabi=n32 -mabi=64 -mabi=eabi

      i386 Options
      -mcpu=cpu type
      -march=cpu type
      -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 -mpreferred-stack-boundary=num

      HPPA Options
      -march=architecture type
      -mbig-switch  -mdisable-fpregs  -mdisable-indexing
      -mfast-indirect-calls -mgas  -mjump-in-delay
      -mlong-load-store  -mno-big-switch  -mno-disable-fpregs
      -mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas
      -mno-jump-in-delay  -mno-long-load-store
      -mno-portable-runtime  -mno-soft-float  -mno-space
      -mno-space-regs  -msoft-float  -mpa-risc-1-0
      -mpa-risc-1-1  -mpa-risc-2-0 -mportable-runtime
      -mschedule=cpu type  -mspace  -mspace-regs

      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
      -malpha-as -mgas
      -mieee  -mieee-with-inexact  -mieee-conformant
      -mfp-trap-mode=mode  -mfp-rounding-mode=mode
      -mtrap-precision=mode  -mbuild-constants
      -mcpu=cpu type
      -mbwx -mno-bwx -mcix -mno-cix -mmax -mno-max
      -mmemory-latency=time

      Clipper Options
      -mc300  -mc400

      H8/300 Options
      -mrelax  -mh -ms -mint32  -malign-300

      SH Options
      -m1  -m2  -m3  -m3e  -mb  -ml  -mdalign -mrelax

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

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

      TMS320C3x/C4x Options
      -mcpu=cpu -mbig -msmall -mregparm -mmemparm
      -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload
      -mrpts=count  -mrptb -mdb -mloop-unsigned
      -mparallel-insns -mparallel-mpy -mpreserve-float

      V850 Options
      -mlong-calls -mno-long-calls -mep -mno-ep
      -mprolog-function -mno-prolog-function -mspace
      -mtda=n -msda=n -mzda=n
      -mv850 -mbig-switch

      NS32K Options
      -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add
      -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb
      -mbitfield -mnobitfield -mhimem -mnohimem

Code Generation Options
  See section Options for Code Generation Conventions.

      -fcall-saved-reg  -fcall-used-reg
      -fexceptions -ffixed-reg  -finhibit-size-directive
      -fcheck-memory-usage  -fprefix-function-name
      -fno-common  -fno-ident  -fno-gnu-linker
      -fpcc-struct-return  -fpic  -fPIC
      -freg-struct-return  -fshared-data  -fshort-enums
      -fshort-double  -fvolatile  -fvolatile-global -fvolatile-static
      -fverbose-asm -fpack-struct  -fstack-check
      -fargument-alias  -fargument-noalias
      -fargument-noalias-global
      -fleading-underscore

> 2.2 Options Controlling the Kind of Output |  | Controlling the kind of output: an executable, object files, assembler files, or preprocessed source.
> 2.4 Options Controlling C Dialect |  | Controlling the variant of C language compiled.
> 2.5 Options Controlling C++ Dialect |  | Variations on C++.
> 2.6 Options to Request or Suppress Warnings |  | How picky should the compiler be?
> 2.7 Options for Debugging Your Program or GCC |  | Symbol tables, measurements, and debugging dumps.
> 2.8 Options That Control Optimization |  | How much optimization?
> 2.9 Options Controlling the Preprocessor |  | Controlling header files and macro definitions. Also, getting dependency information for Make.
> 2.10 Passing Options to the Assembler |  | Passing options to the assembler.
> 2.11 Options for Linking |  | Specifying libraries and so on.
> 2.12 Options for Directory Search |  | Where to find header files and libraries. Where to find the compiler executable files.
> 2.13 Specifying Target Machine and Compiler Version |  | Running a cross-compiler, or an old version of GCC.

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## 2.2 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.

`--help`
  Print (on the standard output) a description of the command line options understood by `gcc`. If the `-v` option is also specified then `--help` will also be passed on to the various processes invoked by `gcc`, so that they can display the command line options they accept. If the `-W` option is also specified then command line options which have no documentation associated with them will also be displayed.

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## 2.3 Compiling C++ Programs

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

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

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## 2.4 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`
  In C mode, support all ANSI standard C programs. In C++ mode, remove GNU extensions that conflict with ANSI C++.

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

  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 2.6 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.

`-fstd=`
  Determine the language standard. A value for this option must be provided; possible values are

  - iso9899:1990 Same as -ansi

  - iso9899:199409 ISO C as modified in amend. 1

  - iso9899:199x ISO C 9x

  - c89 same as -std=iso9899:1990

  - c9x same as -std=iso9899:199x

  - gnu89 default, iso9899:1990 + gnu extensions

  - gnu9x iso9899:199x + gnu extensions

  Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use `__restrict__` even when -fstd=c9x is not specified.

`-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 ``__builtin_'` as prefix. 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.

`-fhosted`

  Assert that compilation takes place in a hosted environment. This implies ``-fbuiltin'`. A hosted environment is one in which the entire standard library is available, and in which `main` has a return type of `int`. Examples are nearly everything except a kernel. This is equivalent to ``-fno-freestanding'`.

`-ffreestanding`

  Assert that compilation takes place in a freestanding environment. This implies ``-fno-builtin'`. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at `main`. The most obvious example is an OS kernel. This is equivalent to ``-fno-hosted'`.

`-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.

  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.

  The ``-traditional'` option also enables ``-traditional-cpp'`, which is described next.

`-traditional-cpp`
  Attempt to support some aspects of traditional C preprocessors. Specifically:

  - Comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.

  - In a preprocessing directive, the ``#'` symbol must appear as the first character of a line.

  - 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.)

`-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'`.

  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.

`-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).

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## 2.5 Options Controlling C++ Dialect

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

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

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

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

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

`-fcheck-new`
  Check that the pointer returned by `operator new` is non-null before attempting to modify the storage allocated. The current Working Paper requires that `operator new` never return a null pointer, so this check is normally unnecessary.

  An alternative to using this option is to specify that your `operator new` does not throw any exceptions; if you declare it ``throw()'`, g++ will check the return value. See also ``new (nothrow)'`.

`-fconserve-space`
  Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after `main()` has completed, you may have an object that is being destroyed twice because two definitions were merged.

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

`-fdollars-in-identifiers`
  Accept ``$'` in identifiers. You can also explicitly prohibit use of ``$'` with the option ``-fno-dollars-in-identifiers'`. (GNU C allows ``$'` by default on most target systems, but there are a few exceptions.) Traditional C allowed the character ``$'` to form part of identifiers. However, ANSI C and C++ forbid ``$'` in identifiers.

`-fno-elide-constructors`
  The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces g++ to call the copy constructor in all cases.

`-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 5.5 Where's the Template?, for more information.

  This option is deprecated.

`-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 5.5 Where's the Template?, for more information.

  This option is deprecated.

`-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'`.

`-fguiding-decls`
  Treat a function declaration with the same type as a potential function template instantiation as though it declares that instantiation, not a normal function. If a definition is given for the function later in the translation unit (or another translation unit if the target supports weak symbols), that definition will be used; otherwise the template will be instantiated. This behavior reflects the C++ language prior to September 1996, when guiding declarations were removed.

  This option implies ``-fname-mangling-version-0'`, and will not work with other name mangling versions. Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

`-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.

`-fhonor-std`
  Treat the `namespace std` as a namespace, instead of ignoring it. For compatibility with earlier versions of g++, the compiler will, by default, ignore `namespace-declarations`, `using-declarations`, `using-directives`, and `namespace-names`, if they involve `std`.

`-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.

  This flag is not useful when compiling with -fvtable-thunks.

  Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

`-fno-implicit-templates`
  Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See section 5.5 Where's the Template?, for more information.

`-fno-implicit-inline-templates`
  Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.

`-finit-priority`
  Support ``__attribute__ ((init_priority (n)))'` for controlling the order of initialization of file-scope objects. On ELF targets, this requires GNU ld 2.10 or later.

`-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.

`-fname-mangling-version-n`
  Control the way in which names are mangled. Version 0 is compatible with versions of g++ before 2.8. Version 1 is the default. Version 1 will allow correct mangling of function templates. For example, version 0 mangling does not mangle foo<int, double> and foo<int, char> given this declaration:

      template <class T, class U> void foo(T t);

  Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

`-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'`.

`-fno-optional-diags`
  Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by g++ is the one for a name having multiple meanings within a class.

`-fpermissive`
  Downgrade messages about nonconformant code from errors to warnings. By default, g++ effectively sets ``-pedantic-errors'` without ``-pedantic'`; this option reverses that. This behavior and this option are superceded by ``-pedantic'`, which works as it does for GNU C.

`-frepo`
  Enable automatic template instantiation. This option also implies ``-fno-implicit-templates'`. See section 5.5 Where's the Template?, for more information.

`-fno-rtti`
  Disable generation of the information used by C++ runtime type identification features (``dynamic_cast'` and ``typeid'`). If you don't use those parts of the language (or exception handling, which uses ``dynamic_cast'` internally), you can save some space by using this flag.

`-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'`.

  Specifying this option will also suppress implicit declarations of functions.

  This flag no longer affects declarations with C++ linkage.

`-fsquangle`
`-fno-squangle`
  ``-fsquangle'` will enable a compressed form of name mangling for identifiers. In particular, it helps to shorten very long names by recognizing types and class names which occur more than once, replacing them with special short ID codes. This option also requires any C++ libraries being used to be compiled with this option as well. The compiler has this disabled (the equivalent of ``-fno-squangle'`) by default.

  Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

`-ftemplate-depth-n`
  Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.

`-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=thunks-version`
  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.

  The original implementation of thunks (version 1) had a bug regarding virtual base classes; this bug is fixed with version 2 of the thunks implementation. With setting the version to 2, compatibility to the version 1 thunks is provided, at the cost of extra machine code. Version 3 does not include this compatibility.

  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.

  Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option. Since version 1 and version 2 are also incompatible (for classes with virtual bases defining virtual functions), all code must also be compiled with the same version.

  In this version of gcc, there are no targets for which version 2 thunks are the default. On all targets, not giving the option will use the traditional implementation, and -fvtable-thunks will produce version 2 thunks.

`-nostdinc++`
  Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

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

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

`-Wctor-dtor-privacy (C++ only)`
  Warn when a class seems unusable, because all the constructors or destructors in a class are private and the class has no friends or public static member functions.

`-Wnon-virtual-dtor (C++ only)`
  Warn when a class declares a non-virtual destructor that should probably be virtual, because it looks like the class will be used polymorphically.

`-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.

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

`-Weffc++ (C++ only)`
  Warn about violations of various style guidelines from Scott Meyers' Effective C++ books. If you use this option, you should be aware that the standard library headers do not obey all of these guidelines; you can use ``grep -v'` to filter out those warnings.

`-Wno-deprecated (C++ only)`
  Do not warn about usage of deprecated features. See section 4.40 Deprecated Features.

`-Wno-non-template-friend (C++ only)`
  Disable warnings when non-templatized friend functions are declared within a template. With the advent of explicit template specification support in g++, if the name of the friend is an unqualified-id (ie, ``friend foo(int)'`), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before g++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for g++, ``-Wnon-template-friend'` allows the compiler to check existing code for potential trouble spots, and is on by default. This new compiler behavior can also be turned off with the flag ``-fguiding-decls'`, which activates the older, non-specification compiler code, or with ``-Wno-non-template-friend'` which keeps the conformant compiler code but disables the helpful warning.

`-Wold-style-cast (C++ only)`
  Warn if an old-style (C-style) cast is used within a C++ program. The new-style casts (``static_cast'`, ``reinterpret_cast'`, and ``const_cast'`) are less vulnerable to unintended effects.

`-Woverloaded-virtual (C++ only)`
  Warn when a derived class function declaration may be an error in defining a virtual function. 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.

`-Wno-pmf-conversions (C++ only)`
  Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

`-Wsign-promo (C++ only)`
  Warn when overload resolution chooses a promotion from unsigned or enumeral type to a signed type over a conversion to an unsigned type of the same size. Previous versions of g++ would try to preserve unsignedness, but the standard mandates the current behavior.

`-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 ='`.

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## 2.6 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 GCC:

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

`-pedantic`
  Issue all the warnings demanded by strict ANSI C and ISO C++; reject all programs that use forbidden extensions.

  Valid ANSI C and ISO 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 and C++ features are supported as well. With this option, they are rejected.

  ``-pedantic'` does not cause warning messages for use of the alternate keywords whose names begin and end with ``__'`. Pedantic warnings are also disabled in the expression that follows `__extension__`. However, only system header files should use these escape routes; application programs should avoid them. See section 4.35 Alternate Keywords.

  This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GCC 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 don't have plans to support such a feature in the near future.

`-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, or whenever a Backslash-Newline 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-int`
  Warn when a declaration does not specify a type.

`-Wimplicit-function-declaration`
`-Werror-implicit-function-declaration`
  Give a warning (or error) whenever a function is used before being declared.

`-Wimplicit`
  Same as ``-Wimplicit-int'` and ``-Wimplicit-function-'`
  ``declaration'`.

`-Wmain`
  Warn if the type of ``main'` is suspicious. ``main'` should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types.

`-Wmultichar`
  Warn if a multicharacter constant (``'FOOF''`) is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.
`-Wparentheses`
  Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

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

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

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

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

`-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.

  In order to get a warning about an unused function parameter, you must specify both ``-W'` and ``-Wunused'`.

  To suppress this warning for an expression, simply cast it to void. For unused variables, parameters and labels, use the ``unused'` attribute (see section 4.29 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 GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

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

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

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

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

  Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn`. See section 4.23 Declaring Attributes of Functions.

`-Wunknown-pragmas`
  Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were only enabled by the ``-Wall'` command line option.

`-Wall`
  All of the above ``-W'` options combined. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros.

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

`-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.

  - A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if ``-Wno-sign-compare'` is also specified.)

  - An aggregate has 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 };

  - An aggregate has an initializer which does not initialize all members. For example, the following code would cause such a warning, because `x.h` would be implicitly initialized to zero:

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

`-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`.

  - A non-`static` function declaration follows a `static` one. This construct is not accepted by some traditional C compilers.

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

`-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`.

`-Wsign-compare`
  Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by ``-W'`; to get the other warnings of ``-W'` without this warning, use ``-W -Wno-sign-compare'`.

`-Waggregate-return`
  Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)

`-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.

`-Wmissing-noreturn`
  Warn about functions which might be candidates for attribute `noreturn`. Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the `noreturn` attribute, otherwise subtle code generation bugs could be introduced.

`-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.

`-Wlong-long`
  Warn if ``long long'` type is used. This is default. To inhibit the warning messages, use ``-Wno-long-long'`. Flags ``-Wlong-long'` and ``-Wno-long-long'` are taken into account only when ``-pedantic'` flag is used.

`-Werror`
  Make all warnings into errors.

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## 2.7 Options for Debugging Your Program or GCC

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

`-g`
  Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). 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-1+'`, or ``-gdwarf-1'` (see below).

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

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

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

`-ggdb`
  Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

`-gstabs`
  Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

`-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 version 1 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 version 1 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.

`-gdwarf-2`
  Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6.

`-glevel`
`-ggdblevel`
`-gstabslevel`
`-gcofflevel`
`-gxcofflevel`
`-gdwarflevel`
`-gdwarf-2level`
  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.

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

`-ax`
  Generate extra code to profile basic blocks. Your executable will produce output that is a superset of that produced when ``-a'` is used. Additional output is the source and target address of the basic blocks where a jump takes place, the number of times a jump is executed, and (optionally) the complete sequence of basic blocks being executed. The output is appended to file ``bb.out'`.

  You can examine different profiling aspects without recompilation. Your executable will read a list of function names from file ``bb.in'`. Profiling starts when a function on the list is entered and stops when that invocation is exited. To exclude a function from profiling, prefix its name with `-'. If a function name is not unique, you can disambiguate it by writing it in the form ``/path/filename.d:functionname'`. Your executable will write the available paths and filenames in file ``bb.out'`.

  Several function names have a special meaning:

  `__bb_jumps__`
    Write source, target and frequency of jumps to file ``bb.out'`.
  `__bb_hidecall__`
    Exclude function calls from frequency count.
  `__bb_showret__`
    Include function returns in frequency count.
  `__bb_trace__`
    Write the sequence of basic blocks executed to file ``bbtrace.gz'`. The file will be compressed using the program ``gzip'`, which must exist in your `PATH`. On systems without the ``popen'` function, the file will be named ``bbtrace'` and will not be compressed. Profiling for even a few seconds on these systems will produce a very large file. Note: `__bb_hidecall__` and `__bb_showret__` will not affect the sequence written to ``bbtrace.gz'`.

  Here's a short example using different profiling parameters in file ``bb.in'`. Assume function `foo` consists of basic blocks 1 and 2 and is called twice from block 3 of function `main`. After the calls, block 3 transfers control to block 4 of `main`.

  With `__bb_trace__` and `main` contained in file ``bb.in'`, the following sequence of blocks is written to file ``bbtrace.gz'`: 0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because the return is to a point inside the block and not to the top. The block address 0 always indicates, that control is transferred to the trace from somewhere outside the observed functions. With ``-foo'` added to ``bb.in'`, the blocks of function `foo` are removed from the trace, so only 0 3 4 remains.

  With `__bb_jumps__` and `main` contained in file ``bb.in'`, jump frequencies will be written to file ``bb.out'`. The frequencies are obtained by constructing a trace of blocks and incrementing a counter for every neighbouring pair of blocks in the trace. The trace 0 3 1 2 1 2 4 displays the following frequencies:

      Jump from block 0x0 to block 0x3 executed 1 time(s)
      Jump from block 0x3 to block 0x1 executed 1 time(s)
      Jump from block 0x1 to block 0x2 executed 2 time(s)
      Jump from block 0x2 to block 0x1 executed 1 time(s)
      Jump from block 0x2 to block 0x4 executed 1 time(s)

  With `__bb_hidecall__`, control transfer due to call instructions is removed from the trace, that is the trace is cut into three parts: 0 3 4, 0 1 2 and 0 1 2. With `__bb_showret__`, control transfer due to return instructions is added to the trace. The trace becomes: 0 3 1 2 3 1 2 3 4. Note, that this trace is not the same, as the sequence written to ``bbtrace.gz'`. It is solely used for counting jump frequencies.

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

  Since not every arc in the program must be instrumented, programs compiled with this option run faster than programs compiled with ``-a'`, which adds instrumentation code to every basic block in the program. The tradeoff: since `gcov` does not have execution counts for all branches, it must start with the execution counts for the instrumented branches, and then iterate over the program flow graph until the entire graph has been solved. Hence, `gcov` runs a little more slowly than a program which uses information from ``-a'`.

  ``-fprofile-arcs'` also makes it possible to estimate branch probabilities, and to calculate basic block execution counts. In general, basic block execution counts do not give enough information to estimate all branch probabilities. When the compiled program exits, it saves the arc execution counts to a file called ``sourcename.da'`. Use the compiler option ``-fbranch-probabilities'` (see section Options that Control Optimization) when recompiling, to optimize using estimated branch probabilities.

`-ftest-coverage`
  Create data files for the `gcov` code-coverage utility (see section `gcov`: a GCC Test Coverage Program). The data file names begin with the name of your source file:

  `sourcename.bb`
    A mapping from basic blocks to line numbers, which `gcov` uses to associate basic block execution counts with line numbers.

  `sourcename.bbg`
    A list of all arcs in the program flow graph. This allows `gcov` to reconstruct the program flow graph, so that it can compute all basic block and arc execution counts from the information in the `sourcename.da` file (this last file is the output from ``-fprofile-arcs'`).

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

`-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:

  ``b'`
    Dump after computing branch probabilities, to ``file.bp'`.
  ``c'`
    Dump after instruction combination, to the file ``file.combine'`.
  ``d'`
    Dump after delayed branch scheduling, to ``file.dbr'`.
  ``D'`
    Dump all macro definitions, at the end of preprocessing, in addition to normal output.
  ``r'`
    Dump after RTL generation, to ``file.rtl'`.
  ``j'`
    Dump after first jump optimization, to ``file.jump'`.
  ``F'`
    Dump after purging ADDRESSOF, to ``file.addressof'`.
  ``f'`
    Dump after flow analysis, to ``file.flow'`.
  ``g'`
    Dump after global register allocation, to ``file.greg'`.
  ``G'`
    Dump after GCSE, to ``file.gcse'`.
  ``j'`
    Dump after first jump optimization, to ``file.jump'`.
  ``J'`
    Dump after last jump optimization, to ``file.jump2'`.
  ``k'`
    Dump after conversion from registers to stack, to ``file.stack'`.
  ``l'`
    Dump after local register allocation, to ``file.lreg'`.
  ``L'`
    Dump after loop optimization, to ``file.loop'`.
  ``M'`
    Dump after performing the machine dependent reorganisation pass, to ``file.mach'`.
  ``N'`
    Dump after the register move pass, to ``file.regmove'`.
  ``r'`
    Dump after RTL generation, to ``file.rtl'`.
  ``R'`
    Dump after the second instruction scheduling pass, to ``file.sched2'`.
  ``s'`
    Dump after CSE (including the jump optimization that sometimes follows CSE), to ``file.cse'`.
  ``S'`
    Dump after the first instruction scheduling pass, to ``file.sched'`.
  ``t'`
    Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to ``file.cse2'`.
  ``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. The length of each instruction is also printed.
  ``x'`
    Just generate RTL for a function instead of compiling it. Usually used with ``r'`.
  ``y'`
    Dump debugging information during parsing, to standard error.
  ``A'`
    Annotate the assembler output with miscellaneous debugging information.

`-fdump-unnumbered`
  When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output. This makes it more feasible to use diff on debugging dumps for compiler invokations with different options, in particular with and without -g.

`-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 GCC 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, GCC 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 2.16 Environment Variables Affecting GCC.

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

  ``-O2'` turns on all optional optimizations except for loop unrolling, function inlining, and strict aliasing optimizations. 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.

`-Os`
  Optimize for size. ``-Os'` enables all ``-O2'` optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

  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. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use ``-ffloat-store'` for such programs, after modifying them to store all pertinent intermediate computations into variables.

`-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 17.5 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.

`-finline-limit-n`
  By default, gcc limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (ie marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 10000. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with c++.

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

`-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. This switch does not affect `extern inline` functions.

`-fkeep-static-consts`
  Emit variables declared `static const` when optimization isn't turned on, even if the variables aren't referenced.

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

`-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'` ``-funroll-all-loops'`, and ``-fstrict-aliasing'`. 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.

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

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

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

`-foptimize-register-moves`
`-fregmove`
  Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. GCC enables this optimization by default with ``-O2'` or higher.

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

`-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.

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

  Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. HPPA processors running HP-UX and Sparc processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future.

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

`-fcaller-saves`
  Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.

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

  For all machines, optimization level 2 and higher enables this flag by default.

`-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-loops'` 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.

`-freduce-all-givs`
  Forces all general-induction variables in loops to be strength-reduced.

  Note: When compiling programs written in Fortran, ``-fmove-all-movables'` and ``-freduce-all-givs'` are enabled by default when you use the optimizer.

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

  These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations.

  Please let us (`gcc@gcc.gnu.org` and `fortran@gnu.org`) 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.

`-fno-peephole`
  Disable any machine-specific peephole optimizations.

`-fbranch-probabilities`
  After running a program compiled with ``-fprofile-arcs'` (see section Options for Debugging Your Program or `gcc`), you can compile it a second time using ``-fbranch-probabilities'`, to improve optimizations based on guessing the path a branch might take.

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

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

  Pay special attention to code like this:

      union a_union {
        int i;
        double d;
      };

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

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

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

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

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## 2.9 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 2.12 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 2.16 Environment Variables Affecting GCC).

`-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.

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## 2.10 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.

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## 2.11 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 2.2 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. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ANSI C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

`-nostdlib`
  Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ANSI C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

  One of the standard libraries bypassed by ``-nostdlib'` and ``-nodefaultlibs'` is ``libgcc.a'`, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See section Interfacing to GCC Output, for more discussion of ``libgcc.a'`.) In most cases, you need ``libgcc.a'` even when you want to avoid other standard libraries. In other words, when you specify ``-nostdlib'` or ``-nodefaultlibs'` you should usually specify ``-lgcc'` as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example, ``__main'`, used to ensure C++ constructors will be called; see 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. Not all systems support this option. You must also specify ``-fpic'` or ``-fPIC'` on some systems when you specify 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 GCC does not know how to recognize.

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

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

`-u symbol`
  Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use ``-u'` multiple times with different symbols to force loading of additional library modules.

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## 2.12 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 dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. 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 2.13 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 2.16 Environment Variables Affecting GCC.

`-specs=file`
  Process file after the compiler reads in the standard ``specs'` file, in order to override the defaults that the ``gcc'` driver program uses when determining what switches to pass to ``cc1'`, ``cc1plus'`, ``as'`, ``ld'`, etc. More than one ``-specs='`file can be specified on the command line, and they are processed in order, from left to right.

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## 2.13 Specifying Target Machine and Compiler Version

By default, GCC 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 GCC, 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 GCC 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 GCC as a cross-compiler.

  The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with ``configure 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 GCC to run. This is useful when multiple versions are installed. For example, version might be ``2.0'`, meaning to run GCC version 2.0.

  The default version, when you do not specify ``-V'`, is the last version of GCC 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 GCC, 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.

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## 2.14 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.

> 2.14.1 M680x0 Options |  |
> 2.14.2 VAX Options |  |
> 2.14.3 SPARC Options |  |
> 2.14.4 Convex Options |  |
> 2.14.5 AMD29K Options |  |
> 2.14.6 ARM Options |  |
> 2.14.7 Thumb Options |  |
> 2.14.8 MN10200 Options |  |
> 2.14.9 MN10300 Options |  |
> 2.14.10 M32R/D Options |  |
> 2.14.11 M88K Options |  |
> 2.14.12 IBM RS/6000 and PowerPC Options |  |
> 2.14.13 IBM RT Options |  |
> 2.14.14 MIPS Options |  |
> 2.14.15 Intel 386 Options |  |
> 2.14.16 HPPA Options |  |
> 2.14.17 Intel 960 Options |  |
> 2.14.18 DEC Alpha Options |  |
> 2.14.19 Clipper Options |  |
> 2.14.20 H8/300 Options |  |
> 2.14.21 SH Options |  |
> 2.14.22 Options for System V |  |
> 2.14.23 TMS320C3x/C4x Options |  |
> 2.14.24 V850 Options |  |
> 2.14.25 ARC Options |  |
> 2.14.26 NS32K Options |  |

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### 2.14.1 M680x0 Options

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

`-m68000`
`-mc68000`
  Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.

  Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

`-m68020`
`-mc68020`
  Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.

`-m68881`
  Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless ``-nfp'` was specified when the compiler was configured.

`-m68030`
  Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.

`-m68040`
  Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.

  This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.

`-m68060`
  Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.

  This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.

`-mcpu32`
  Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.

  Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

`-m5200`
  Generate output for a 520X "coldfire" family cpu. This is the default when the compiler is configured for 520X-based systems.

  Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

`-m68020-40`
  Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

`-m68020-60`
  Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

`-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'`, ``-mcpu32'` and ``-m5200'` options imply ``-mnobitfield'`.

`-mbitfield`
  Do use the bit-field instructions. The ``-m68020'` option implies ``-mbitfield'`. This is the default if you use a configuration designed for a 68020.

`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return with the `rtd` instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

  This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

  Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise incorrect code will be generated for calls to those functions.

  In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

  The `rtd` instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

`-malign-int`
`-mno-align-int`
  Control whether GCC aligns `int`, `long`, `long long`, `float`, `double`, and `long double` variables on a 32-bit boundary (``-malign-int'`) or a 16-bit boundary (``-mno-align-int'`). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

  Warning: if you use the ``-malign-int'` switch, GCC will align structures containing the above types differently than most published application binary interface specifications for the m68k.

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### 2.14.2 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.

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### 2.14.3 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 GCC, with ``-msoft-float'` in order for this to work.

`-mhard-quad-float`
  Generate output containing quad-word (long double) floating point instructions.

`-msoft-quad-float`
  Generate output containing library calls for quad-word (long double) floating point instructions. The functions called are those specified in the SPARC ABI. This is the default.

  As of this writing, there are no sparc implementations that have hardware support for the quad-word floating point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the ``-msoft-quad-float'` option is the default.

`-mno-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. 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'`, GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating point code.

`-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.

  These options are deprecated and will be deleted in a future GCC release. They have been replaced with ``-mcpu=xxx'`.

`-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.

  These options are deprecated and will be deleted in a future GCC release. They have been replaced with ``-mcpu=xxx'`.

`-mcpu=cpu_type`
  Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``v7'`, ``cypress'`, ``v8'`, ``supersparc'`, ``sparclite'`, ``hypersparc'`, ``sparclite86x'`, ``f930'`, ``f934'`, ``sparclet'`, ``tsc701'`, ``v9'`, and ``ultrasparc'`.

  Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are ``v7'`, ``v8'`, ``sparclite'`, ``sparclet'`, ``v9'`.

  Here is a list of each supported architecture and their supported implementations.

          v7:             cypress
          v8:             supersparc, hypersparc
          sparclite:      f930, f934, sparclite86x
          sparclet:       tsc701
          v9:             ultrasparc

`-mtune=cpu_type`
  Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option ``-mcpu='`cpu_type would.

  The same values for ``-mcpu='`cpu_type are used for ``-mtune='`
  cpu_type, though the only useful values are those that select a particular cpu implementation: ``cypress'`, ``supersparc'`, ``hypersparc'`, ``f930'`, ``f934'`, ``sparclite86x'`, ``tsc701'`, ``ultrasparc'`.

`-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.

`-malign-functions=num`
  Align the start of functions to a 2 raised to num byte boundary. If ``-malign-functions'` is not specified, the default is 2 if compiling for 32 bit sparc, and 5 if compiling for 64 bit sparc.

These ``-m'` switches are supported in addition to the above on the SPARCLET processor.

`-mlittle-endian`
  Generate code for a processor running in little-endian mode.

`-mlive-g0`
  Treat register `%g0` as a normal register. GCC will continue to clobber it as necessary but will not assume it always reads as 0.

`-mbroken-saverestore`
  Generate code that does not use non-trivial forms of the `save` and `restore` instructions. Early versions of the SPARCLET processor do not correctly handle `save` and `restore` instructions used with arguments. They correctly handle them used without arguments. A `save` instruction used without arguments increments the current window pointer but does not allocate a new stack frame. It is assumed that the window overflow trap handler will properly handle this case as will interrupt handlers.

These ``-m'` switches are supported in addition to the above on SPARC V9 processors in 64 bit environments.

`-mlittle-endian`
  Generate code for a processor running in little-endian mode.

`-m32`
`-m64`
  Generate code for a 32 bit or 64 bit environment. The 32 bit environment sets int, long and pointer to 32 bits. The 64 bit environment sets int to 32 bits and long and pointer to 64 bits.

`-mcmodel=medlow`
  Generate code for the Medium/Low code model: the program must be linked in the low 32 bits of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked.

`-mcmodel=medmid`
  Generate code for the Medium/Middle code model: the program must be linked in the low 44 bits of the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

`-mcmodel=medany`
  Generate code for the Medium/Anywhere code model: the program may be linked anywhere in the address space, the text segment must be less than 2G bytes, and data segment must be within 2G of the text segment. Pointers are 64 bits.

`-mcmodel=embmedany`
  Generate code for the Medium/Anywhere code model for embedded systems: assume a 32 bit text and a 32 bit data segment, both starting anywhere (determined at link time). Register %g4 points to the base of the data segment. Pointers still 64 bits. Programs are statically linked, PIC is not supported.

`-mstack-bias`
`-mno-stack-bias`
  With ``-mstack-bias'`, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. Otherwise, assume no such offset is present.

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### 2.14.4 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.

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### 2.14.5 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.

`-mno-impure-text`
`-mimpure-text`
  ``-mimpure-text'`, used in addition to ``-shared'`, tells the compiler to not pass ``-assert pure-text'` to the linker when linking a shared object.

`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

`-mno-multm`
  Do not generate multm or multmu instructions. This is useful for some embedded systems which do not have trap handlers for these instructions.

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### 2.14.6 ARM Options

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

`-mapcs-frame`
  Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying ``-fomit-frame-pointer'` with this option will cause the stack frames not to be generated for leaf functions. The default is ``-mno-apcs-frame'`.

`-mapcs`
  This is a synonym for ``-mapcs-frame'`.

`-mapcs-26`
  Generate code for a processor running with a 26-bit program counter, and conforming to the function calling standards for the APCS 26-bit option. This option replaces the ``-m2'` and ``-m3'` options of previous releases of the compiler.

`-mapcs-32`
  Generate code for a processor running with a 32-bit program counter, and conforming to the function calling standards for the APCS 32-bit option. This option replaces the ``-m6'` option of previous releases of the compiler.

`-mapcs-stack-check`
  Generate code to check the amount of stack space available upon entry to every function (that actually uses some stack space). If there is insufficient space available then either the function ``__rt_stkovf_split_small'` or ``__rt_stkovf_split_big'` will be called, depending upon the amount of stack space required. The run time system is required to provide these functions. The default is ``-mno-apcs-stack-check'`, since this produces smaller code.

`-mapcs-float`
  Pass floating point arguments using the float point registers. This is one of the variants of the APCS. This option is reccommended if the target hardware has a floating point unit or if a lot of floating point arithmetic is going to be performed by the code. The default is ``-mno-apcs-float'`, since integer only code is slightly increased in size if ``-mapcs-float'` is used.

`-mapcs-reentrant`
  Generate reentrant, position independent code. This is the equivalent to specifying the ``-fpic'` option. The default is ``-mno-apcs-reentrant'`.

`-mthumb-interwork`
  Generate code which supports calling between the ARM and THUMB instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is ``-mno-thumb-interwork'`, since slightly larger code is generated when ``-mthumb-interwork'` is specified.

`-mno-sched-prolog`
  Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will start with a recognisable set of instructions (or in fact one of a chioce from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code. The default is ``-msched-prolog'`.

`-mhard-float`
  Generate output containing floating point instructions. This is the default.

`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

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

`-mlittle-endian`
  Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

`-mbig-endian`
  Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

`-mwords-little-endian`
  This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form ``32107654'`. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

`-mshort-load-bytes`
  Do not try to load half-words (eg ``short'`s) by loading a word from an unaligned address. For some targets the MMU is configured to trap unaligned loads; use this option to generate code that is safe in these environments.

`-mno-short-load-bytes`
  Use unaligned word loads to load half-words (eg ``short'`s). This option produces more efficient code, but the MMU is sometimes configured to trap these instructions.

`-mshort-load-words`
  This is a synonym for the ``-mno-short-load-bytes'`.

`-mno-short-load-words`
  This is a synonym for the ``-mshort-load-bytes'`.

`-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.

`-mcpu=<name>`
`-mtune=<name>`
  This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can use when generating assembly code. Permissable names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9tdmi. ``-mtune='` is a synonym for ``-mcpue='` to support older versions of GCC.

`-march=<name>`
  This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can use when generating assembly code. This option can be used in conjunction with or instead of the ``-mcpu='` option. Permissable names are: armv2, armv2a, armv3, armv3m, armv4, armv4t

`-mfpe=<number>`
`-mfp=<number>`
  This specifes the version of the floating point emulation available on the target. Permissable values are 2 and 3. ``-mfp='` is a synonym for ``-mfpe='` to support older versions of GCC.

`-mstructure-size-boundary=<n>`
  The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissable values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produced faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions. Programmers are encouraged to use the 32 value as future versions of the toolchain may default to this value.

`-mabort-on-noreturn`
  Generate a call to the function abort at the end of a noreturn function. It will be executed if the function tries to return.

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### 2.14.7 Thumb Options

`-mthumb-interwork`
  Generate code which supports calling between the THUMB and ARM instruction sets. Without this option the two instruction sets cannot be reliably used inside one program. The default is ``-mno-thumb-interwork'`, since slightly smaller code is generated with this option.

`-mtpcs-frame`
  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions). The default is ``-mno-apcs-frame'`.

`-mtpcs-leaf-frame`
  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions). The default is ``-mno-apcs-leaf-frame'`.

`-mlittle-endian`
  Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

`-mbig-endian`
  Generate code for a processor running in big-endian mode.

`-mstructure-size-boundary=<n>`
  The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option. Permissable values are 8 and 32. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. Specifying the larger number can produced faster, more efficient code, but can also increase the size of the program. The two values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with the other value, if they exchange information using structures or unions. Programmers are encouraged to use the 32 value as future versions of the toolchain may default to this value.

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### 2.14.8 MN10200 Options

These ``-m'` options are defined for Matsushita MN10200 architectures:

`-mrelax`
  Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.

  This option makes symbolic debugging impossible.

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### 2.14.9 MN10300 Options

These ``-m'` options are defined for Matsushita MN10300 architectures:

`-mmult-bug`
  Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.

`-mno-mult-bug`
  Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.

`-mrelax`
  Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.

  This option makes symbolic debugging impossible.

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### 2.14.10 M32R/D Options

These ``-m'` options are defined for Mitsubishi M32R/D architectures:

`-mcode-model=small`
  Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24` instruction), and assume all subroutines are reachable with the `bl` instruction. This is the default.

  The addressability of a particular object can be set with the `model` attribute.

`-mcode-model=medium`
  Assume objects may be anywhere in the 32 bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and assume all subroutines are reachable with the `bl` instruction.

`-mcode-model=large`
  Assume objects may be anywhere in the 32 bit address space (the compiler will generate `seth/add3` instructions to load their addresses), and assume subroutines may not be reachable with the `bl` instruction (the compiler will generate the much slower `seth/add3/jl` instruction sequence).

`-msdata=none`
  Disable use of the small data area. Variables will be put into one of ``.data'`, ``bss'`, or ``.rodata'` (unless the `section` attribute has been specified). This is the default.

  The small data area consists of sections ``.sdata'` and ``.sbss'`. Objects may be explicitly put in the small data area with the `section` attribute using one of these sections.

`-msdata=sdata`
  Put small global and static data in the small data area, but do not generate special code to reference them.

`-msdata=use`
  Put small global and static data in the small data area, and generate special instructions to reference them.

`-G num`
  Put global and static objects less than or equal to num bytes into the small data or bss sections instead of the normal data or bss sections. The default value of num is 8. The ``-msdata'` option must be set to one of ``sdata'` or ``use'` for this option to have any effect.

  All modules should be compiled with the same ``-G num'` value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message - incorrect code will not be generated.

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### 2.14.11 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 GCC 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 GCC 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 GCC 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, GCC issues no such warning.

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### 2.14.12 IBM RS/6000 and PowerPC Options

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

`-mpower`
`-mno-power`
`-mpower2`
`-mno-power2`
`-mpowerpc`
`-mno-powerpc`
`-mpowerpc-gpopt`
`-mno-powerpc-gpopt`
`-mpowerpc-gfxopt`
`-mno-powerpc-gfxopt`
`-mpowerpc64`
`-mno-powerpc64`
  GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the ``rios'` chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.

  Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.

  You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the ``-mcpu=cpu_type'` overrides the specification of these options. We recommend you use the ``-mcpu=cpu_type'` option rather than the options listed above.

  The ``-mpower'` option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying ``-mpower2'` implies ``-power'` and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.

  The ``-mpowerpc'` option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying ``-mpowerpc-gpopt'` implies ``-mpowerpc'` and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying ``-mpowerpc-gfxopt'` implies ``-mpowerpc'` and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

  The ``-mpowerpc64'` option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to ``-mno-powerpc64'`.

  If you specify both ``-mno-power'` and ``-mno-powerpc'`, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both ``-mpower'` and ``-mpowerpc'` permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

`-mnew-mnemonics`
`-mold-mnemonics`
  Select which mnemonics to use in the generated assembler code. ``-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; GCC uses that mnemonic irrespective of which of these options is specified.

  GCC defaults to the mnemonics appropriate for the architecture in use. Specifying ``-mcpu=cpu_type'` sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either ``-mnew-mnemonics'` or ``-mold-mnemonics'`, but should instead accept the default.

`-mcpu=cpu_type`
  Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``rs6000'`, ``rios1'`, ``rios2'`, ``rsc'`, ``601'`, ``602'`, ``603'`, ``603e'`, ``604'`, ``604e'`, ``620'`, ``740'`, ``750'`, ``power'`, ``power2'`, ``powerpc'`, ``403'`, ``505'`, ``801'`, ``821'`, ``823'`, and ``860'` and ``common'`. ``-mcpu=power'`, ``-mcpu=power2'`, and ``-mcpu=powerpc'` specify generic POWER, POWER2 and pure PowerPC (i.e., not MPC601) architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

  Specifying any of the following options: ``-mcpu=rios1'`, ``-mcpu=rios2'`, ``-mcpu=rsc'`, ``-mcpu=power'`, or ``-mcpu=power2'` enables the ``-mpower'` option and disables the ``-mpowerpc'` option; ``-mcpu=601'` enables both the ``-mpower'` and ``-mpowerpc'` options. All of ``-mcpu=602'`, ``-mcpu=603'`, ``-mcpu=603e'`, ``-mcpu=604'`, ``-mcpu=620'`, enable the ``-mpowerpc'` option and disable the ``-mpower'` option. Exactly similarly, all of ``-mcpu=403'`, ``-mcpu=505'`, ``-mcpu=821'`, ``-mcpu=860'` and ``-mcpu=powerpc'` enable the ``-mpowerpc'` option and disable the ``-mpower'` option. ``-mcpu=common'` disables both the ``-mpower'` and ``-mpowerpc'` options.

  AIX versions 4 or greater selects ``-mcpu=common'` by default, so that code will operate on all members of the RS/6000 and PowerPC families. In that case, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes.

  Specifying any of the options ``-mcpu=rios1'`, ``-mcpu=rios2'`, ``-mcpu=rsc'`, ``-mcpu=power'`, or ``-mcpu=power2'` also disables the ``new-mnemonics'` option. Specifying ``-mcpu=601'`, ``-mcpu=602'`, ``-mcpu=603'`, ``-mcpu=603e'`, ``-mcpu=604'`, ``620'`, ``403'`, or ``-mcpu=powerpc'` also enables the ``new-mnemonics'` option.

  Specifying ``-mcpu=403'`, ``-mcpu=821'`, or ``-mcpu=860'` also enables the ``-msoft-float'` option.

`-mtune=cpu_type`
  Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, choice of mnemonics like ``-mcpu='`cpu_type would. The same values for cpu_type are used for ``-mtune='`cpu_type as for ``-mcpu='`cpu_type. The ``-mtune='`cpu_type option overrides the ``-mcpu='`cpu_type option in terms of instruction scheduling parameters.

`-mfull-toc`
`-mno-fp-in-toc`
`-mno-sum-in-toc`
`-mminimal-toc`
  Modify generation of the TOC (Table Of Contents), which is created for every executable file. The ``-mfull-toc'` option is selected by default. In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GCC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.

  If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the ``-mno-fp-in-toc'` and ``-mno-sum-in-toc'` options. ``-mno-fp-in-toc'` prevents GCC from putting floating-point constants in the TOC and ``-mno-sum-in-toc'` forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

  If you still run out of space in the TOC even when you specify both of these options, specify ``-mminimal-toc'` instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

`-maix64`
`-maix32`
  Enable AIX 64-bit ABI and calling convention: 64-bit pointers, 64-bit `long` type, and the infrastructure needed to support them. Specifying ``-maix64'` implies ``-mpowerpc64'` and ``-mpowerpc'`, while ``-maix32'` disables the 64-bit ABI and implies ``-mno-powerpc64'`. GCC defaults to ``-maix32'`.

`-mxl-call`
`-mno-xl-call`
  On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.

`-mthreads`
  Support AIX Threads. Link an application written to use pthreads with special libraries and startup code to enable the application to run.

`-mpe`
  Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (``/usr/lpp/ppe.poe/'`), or the ``specs'` file must be overridden with the ``-specs='` option to specify the appropriate directory location. The Parallel Environment does not support threads, so the ``-mpe'` option and the ``-mthreads'` option are incompatible.

`-msoft-float`
`-mhard-float`
  Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the ``-msoft-float'` option, and pass the option to GCC when linking.

`-mmultiple`
`-mno-multiple`
  Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use ``-mmultiple'` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

`-mstring`
`-mno-string`
  Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use ``-mstring'` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

`-mupdate`
`-mno-update`
  Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use ``-mno-update'`, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

`-mfused-madd`
`-mno-fused-madd`
  Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used.

`-mno-bit-align`
`-mbit-align`
  On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit fields to be aligned to the base type of the bit field.

  For example, by default a structure containing nothing but 8 `unsigned` 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. If you use ``-mrelocatable'` on any module, all objects linked together must be compiled with ``-mrelocatable'` or ``-mrelocatable-lib'`.

`-mrelocatable-lib`
`-mno-relocatable-lib`
  On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. Modules compiled with ``-mrelocatable-lib'` can be linked with either modules compiled without ``-mrelocatable'` and ``-mrelocatable-lib'` or with modules compiled with the ``-mrelocatable'` options.

`-mno-toc`
`-mtoc`
  On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

`-mlittle`
`-mlittle-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The ``-mlittle-endian'` option is the same as ``-mlittle'`.

`-mbig`
`-mbig-endian`
  On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The ``-mbig-endian'` option is the same as ``-mbig'`.

`-mcall-sysv`
  On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using ``powerpc-*-eabiaix'`.

`-mcall-sysv-eabi`
  Specify both ``-mcall-sysv'` and ``-meabi'` options.

`-mcall-sysv-noeabi`
  Specify both ``-mcall-sysv'` and ``-mno-eabi'` options.

`-mcall-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'`.

`-mcall-solaris`
  On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.

`-mcall-linux`
  On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

`-mprototype`
`-mno-prototype`
  On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments. With ``-mprototype'`, only calls to prototyped variable argument functions will set or clear the bit.

`-msim`
  On embedded PowerPC systems, assume that the startup module is called ``sim-crt0.o'` and that the standard C libraries are ``libsim.a'` and ``libc.a'`. This is the default for ``powerpc-*-eabisim'`. configurations.

`-mmvme`
  On embedded PowerPC systems, assume that the startup module is called ``crt0.o'` and the standard C libraries are ``libmvme.a'` and ``libc.a'`.

`-mads`
  On embedded PowerPC systems, assume that the startup module is called ``crt0.o'` and the standard C libraries are ``libads.a'` and ``libc.a'`.

`-myellowknife`
  On embedded PowerPC systems, assume that the startup module is called ``crt0.o'` and the standard C libraries are ``libyk.a'` and ``libc.a'`.

`-memb`
  On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that ``eabi'` extended relocations are used.

`-meabi`
`-mno-eabi`
  On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting `-meabi` means that the stack is aligned to an 8 byte boundary, a function `__eabi` is called to from `main` to set up the eabi environment, and the ``-msdata'` option can use both `r2` and `r13` to point to two separate small data areas. Selecting `-mno-eabi` means that the stack is aligned to a 16 byte boundary, do not call an initialization function from `main`, and the ``-msdata'` option will only use `r13` to point to a single small data area. The ``-meabi'` option is on by default if you configured GCC using one of the ``powerpc*-*-eabi*'` options.

`-msdata=eabi`
  On System V.4 and embedded PowerPC systems, put small initialized `const` global and static data in the ``.sdata2'` section, which is pointed to by register `r2`. Put small initialized non-`const` global and static data in the ``.sdata'` section, which is pointed to by register `r13`. Put small uninitialized global and static data in the ``.sbss'` section, which is adjacent to the ``.sdata'` section. The ``-msdata=eabi'` option is incompatible with the ``-mrelocatable'` option. The ``-msdata=eabi'` option also sets the ``-memb'` option.

`-msdata=sysv`
  On System V.4 and embedded PowerPC systems, put small global and static data in the ``.sdata'` section, which is pointed to by register `r13`. Put small uninitialized global and static data in the ``.sbss'` section, which is adjacent to the ``.sdata'` section. The ``-msdata=sysv'` option is incompatible with the ``-mrelocatable'` option.

`-msdata=default`
`-msdata`
  On System V.4 and embedded PowerPC systems, if ``-meabi'` is used, compile code the same as ``-msdata=eabi'`, otherwise compile code the same as ``-msdata=sysv'`.

`-msdata-data`
  On System V.4 and embedded PowerPC systems, put small global and static data in the ``.sdata'` section. Put small uninitialized global and static data in the ``.sbss'` section. Do not use register `r13` to address small data however. This is the default behavior unless other ``-msdata'` options are used.

`-msdata=none`
`-mno-sdata`
  On embedded PowerPC systems, put all initialized global and static data in the ``.data'` section, and all uninitialized data in the ``.bss'` section.

`-G num`
  On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The ``-G num'` switch is also passed to the linker. All modules should be compiled with the same ``-G num'` value.

`-mregnames`
`-mno-regnames`
  On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.

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### 2.14.13 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'`.

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### 2.14.14 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'`, ``r3900'`, ``r4000'`, ``r4100'`, ``r4300'`, ``r4400'`, ``r4600'`, ``r4650'`, ``r5000'`, ``r6000'`, ``r8000'`, and ``orion'`. Additionally, the ``r2000'`, ``r3000'`, ``r4000'`, ``r5000'`, and ``r6000'` can be abbreviated as ``r2k'` (or ``r2K'`), ``r3k'`, etc. 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 a ``-mipsX'` or ``-mabi'` switch 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.

`-mips4`
  Issue instructions from level 4 of the MIPS ISA (conditional move, prefetch, enhanced FPU instructions). ``r8000'` is the default cpu type at this ISA level.

`-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`
  Force int and long types to be 64 bits wide. See ``-mlong32'` for an explanation of the default, and the width of pointers.

`-mlong64`
  Force long types to be 64 bits wide. See ``-mlong32'` for an explanation of the default, and the width of pointers.

`-mlong32`
  Force long, int, and pointer types to be 32 bits wide.

  If none of ``-mlong32'`, ``-mlong64'`, or ``-mint64'` are set, the size of ints, longs, and pointers depends on the ABI and ISA choosen. For ``-mabi=32'`, and ``-mabi=n32'`, ints and longs are 32 bits wide. For ``-mabi=64'`, ints are 32 bits, and longs are 64 bits wide. For ``-mabi=eabi'` and either ``-mips1'` or ``-mips2'`, ints and longs are 32 bits wide. For ``-mabi=eabi'` and higher ISAs, ints are 32 bits, and longs are 64 bits wide. The width of pointer types is the smaller of the width of longs or the width of general purpose registers (which in turn depends on the ISA).

`-mabi=32`
`-mabi=o64`
`-mabi=n32`
`-mabi=64`
`-mabi=eabi`
  Generate code for the indicated ABI. The default instruction level is ``-mips1'` for ``32'`, ``-mips3'` for ``n32'`, and ``-mips4'` otherwise. Conversely, with ``-mips1'` or ``-mips2'`, the default ABI is ``32'`; otherwise, the default ABI is ``64'`.

`-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. Also, this is the default if the configure option ``--with-gnu-as'` is used.

`-msplit-addresses`
`-mno-split-addresses`
  Generate code to load the high and low parts of address constants separately. This allows `gcc` to optimize away redundant loads of the high order bits of addresses. This optimization requires GNU as and GNU ld. This optimization is enabled by default for some embedded targets where GNU as and GNU ld are standard.

`-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 GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

`-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. No more than 65536 bytes of global data may be used. This requires GNU as and GNU ld which do most of the work. This currently only works on targets which use ECOFF; it does not work with ELF.

`-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'`.

`-mips16`
`-mno-mips16`
  Enable 16-bit instructions.

`-mentry`
  Use the entry and exit pseudo ops. This option can only be used with ``-mips16'`.

`-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 its 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.

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### 2.14.15 Intel 386 Options

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

`-mcpu=cpu type`
  Assume the defaults for the machine type cpu type when scheduling instructions. The choices for cpu type are:

  ``i386'` | ``i486'` | ``i586'` | ``i686'`
  ``pentium'` | ``pentiumpro'` | ``k6'`

  While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the ``-march=cpu type'` option being used. ``i586'` is equivalent to ``pentium'` and ``i686'` is equivalent to ``pentiumpro'`. ``k6'` is the AMD chip as opposed to the Intel ones.

`-march=cpu type`
  Generate instructions for the machine type cpu type. The choices for cpu type are the same as for ``-mcpu'`. Moreover, specifying ``-march=cpu type'` implies ``-mcpu=cpu type'`.

`-m386`
`-m486`
`-mpentium`
`-mpentiumpro`
  Synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=pentiumpro respectively. These synonyms are deprecated.

`-mieee-fp`
`-mno-ieee-fp`
  Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the case where the result of a comparison is unordered.

`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

  On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if ``-msoft-float'` is used.

`-mno-fp-ret-in-387`
  Do not use the FPU registers for return values of functions.

  The usual calling convention has functions return values of types `float` and `double` in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

  The option ``-mno-fp-ret-in-387'` causes such values to be returned in ordinary CPU registers instead.

`-mno-fancy-math-387`
  Some 387 emulators do not support the `sin`, `cos` and `sqrt` instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD. 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 GCC aligns `double`, `long double`, and `long long` variables on a two word boundary or a one word boundary. Aligning `double` variables on a two word boundary will produce code that runs somewhat faster on a ``Pentium'` at the expense of more memory.

  Warning: if you use the ``-malign-double'` switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386.

`-msvr3-shlib`
`-mno-svr3-shlib`
  Control whether GCC 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 GCC 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 4.23 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 4.23 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 unless gas 2.8 (or later) is being used in which case the default is to align the loop on a 16 byte boundary if it is less than 8 bytes away.

`-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 unless gas 2.8 (or later) is being used in which case the default is to align the instruction on a 16 byte boundary if it is less than 8 bytes away.

`-malign-functions=num`
  Align the start of functions to a 2 raised to num byte boundary. If ``-malign-functions'` is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486.

`-mpreferred-stack-boundary=num`
  Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If ``-mpreferred-stack-boundary'` is not specified, the default is 4 (16 bytes or 128 bits).

  The stack is required to be aligned on a 4 byte boundary. On Pentium and PentiumPro, `double` and `long double` values should be aligned to an 8 byte boundary (see ``-malign-double'`) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extention (SSE) data type `__m128` suffers similar penalties if it is not 16 byte aligned.

  To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.

  This extra alignment does consume extra stack space. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to ``-mpreferred-stack-boundary=2'`.

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### 2.14.16 HPPA Options

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

`-march=architecture type`
  Generate code for the specified architecture. The choices for architecture type are ``1.0'` for PA 1.0, ``1.1'` for PA 1.1, and ``2.0'` for PA 2.0 processors. Refer to ``/usr/lib/sched.models'` on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures will run on higher numbered architectures, but not the other way around.

  PA 2.0 support currently requires gas snapshot 19990413 or later. The next release of binutils (current is 2.9.1) will probably contain PA 2.0 support.

`-mpa-risc-1-0`
`-mpa-risc-1-1`
`-mpa-risc-2-0`
  Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

`-mbig-switch`
  Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

`-mjump-in-delay`
  Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.

`-mdisable-fpregs`
  Prevent floating point registers from being used in any manner. This is necessary for compiling kernels which perform lazy context switching of floating point registers. If you use this option and attempt to perform floating point operations, the compiler will abort.

`-mdisable-indexing`
  Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.

`-mno-space-regs`
  Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

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

`-mfast-indirect-calls`
  Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which performs faster indirect calls.

  This option will not work in the presense of shared libraries or nested functions.

`-mspace`
  Optimize for space rather than execution time. Currently this only enables out of line function prologues and epilogues. This option is incompatible with PIC code generation and profiling.

`-mlong-load-store`
  Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the ``+k'` option to the HP compilers.

`-mportable-runtime`
  Use the portable calling conventions proposed by HP for ELF systems.

`-mgas`
  Enable the use of assembler directives only GAS understands.

`-mschedule=cpu type`
  Schedule code according to the constraints for the machine type cpu type. The choices for cpu type are ``700'` ``7100'`, ``7100LC'`, ``7200'`, and ``8000'`. Refer to ``/usr/lib/sched.models'` on an HP-UX system to determine the proper scheduling option for your machine.

`-mlinker-opt`
  Enable the optimization pass in the HPUX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HPUX 8 and HPUX 9 linkers in which they give bogus error messages when linking some programs.

`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded target ``hppa1.1-*-pro'` does provide software floating point support.

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

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### 2.14.17 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). This option implies ``-mstrict-align'`.

`-mlong-double-64`
  Implement type ``long double'` as 64-bit floating point numbers. Without the option ``long double'` is implemented by 80-bit floating point numbers. The only reason we have it because there is no 128-bit ``long double'` support in ``fp-bit.c'` yet. So it is only useful for people using soft-float targets. Otherwise, we should recommend against use of it.

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### 2.14.18 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.

`-mieee`
  The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE compliant code except that the inexact flag is not maintained (see below). If this option is turned on, the CPP macro `_IEEE_FP` is defined during compilation. The option is a shorthand for: ``-D_IEEE_FP -mfp-trap-mode=su -mtrap-precision=i -mieee-conformant'`. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option `-ieee_with_no_inexact`.

`-mieee-with-inexact`
  This is like ``-mieee'` except the generated code also maintains the IEEE inexact flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. The option is a shorthand for ``-D_IEEE_FP -D_IEEE_FP_INEXACT'` plus the three following: ``-mieee-conformant'`, ``-mfp-trap-mode=sui'`, and ``-mtrap-precision=i'`. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact flag, you should normally not specify this option. Other Alpha compilers call this option ``-ieee_with_inexact'`.

`-mfp-trap-mode=trap mode`
  This option controls what floating-point related traps are enabled. Other Alpha compilers call this option ``-fptm '`trap mode. The trap mode can be set to one of four values:

  ``n'`
    This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).

  ``u'`
    In addition to the traps enabled by ``n'`, underflow traps are enabled as well.

  ``su'`
    Like ``su'`, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).

  ``sui'`
    Like ``su'`, but inexact traps are enabled as well.

`-mfp-rounding-mode=rounding mode`
  Selects the IEEE rounding mode. Other Alpha compilers call this option ``-fprm '`rounding mode. The rounding mode can be one of:

  ``n'`
    Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.

  ``m'`
    Round towards minus infinity.

  ``c'`
    Chopped rounding mode. Floating point numbers are rounded towards zero.

  ``d'`
    Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, ``d'` corresponds to round towards plus infinity.

`-mtrap-precision=trap precision`
  In the Alpha architecture, floating point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating point trap. Depending on the requirements of an application, different levels of precisions can be selected:

  ``p'`
    Program precision. This option is the default and means a trap handler can only identify which program caused a floating point exception.

  ``f'`
    Function precision. The trap handler can determine the function that caused a floating point exception.

  ``i'`
    Instruction precision. The trap handler can determine the exact instruction that caused a floating point exception.

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

`-mieee-conformant`
  This option marks the generated code as IEEE conformant. You must not use this option unless you also specify ``-mtrap-precision=i'` and either ``-mfp-trap-mode=su'` or ``-mfp-trap-mode=sui'`. Its only effect is to emit the line ``.eflag 48'` in the function prologue of the generated assembly file. Under DEC Unix, this has the effect that IEEE-conformant math library routines will be linked in.

`-mbuild-constants`
  Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime.

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

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

`-malpha-as`
`-mgas`
  Select whether to generate code to be assembled by the vendor-supplied assembler (``-malpha-as'`) or by the GNU assembler ``-mgas'`.

`-mbwx`
`-mno-bwx`
`-mcix`
`-mno-cix`
`-mmax`
`-mno-max`
  Indicate whether GCC should generate code to use the optional BWX, CIX, and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via ``-mcpu='` option or that of the CPU on which GCC was built if none was specified.

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

  Supported values for cpu_type are

  ``ev4'`
  ``21064'`
    Schedules as an EV4 and has no instruction set extensions.

  ``ev5'`
  ``21164'`
    Schedules as an EV5 and has no instruction set extensions.

  ``ev56'`
  ``21164a'`
    Schedules as an EV5 and supports the BWX extension.

  ``pca56'`
  ``21164pc'`
  ``21164PC'`
    Schedules as an EV5 and supports the BWX and MAX extensions.

  ``ev6'`
  ``21264'`
    Schedules as an EV5 (until Digital releases the scheduling parameters for the EV6) and supports the BWX, CIX, and MAX extensions.

`-mmemory-latency=time`
  Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependant on the memory access patterns used by the application and the size of the external cache on the machine.

  Valid options for time are

  ``number'`
    A decimal number representing clock cycles.

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

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### 2.14.19 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.

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### 2.14.20 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.

`-ms`
  Generate code for the H8/S.

`-mint32`
  Make `int` data 32 bits by default.

`-malign-300`
  On the h8/300h, use the same alignment rules as for the h8/300. The default for the h8/300h is to align longs and floats on 4 byte boundaries. ``-malign-300'` causes them to be aligned on 2 byte boundaries. This option has no effect on the h8/300.

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### 2.14.21 SH Options

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

`-m1`
  Generate code for the SH1.

`-m2`
  Generate code for the SH2.

`-m3`
  Generate code for the SH3.

`-m3e`
  Generate code for the SH3e.

`-mb`
  Compile code for the processor in big endian mode.

`-ml`
  Compile code for the processor in little endian mode.

`-mdalign`
  Align doubles at 64 bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with -mdalign.

`-mrelax`
  Shorten some address references at link time, when possible; uses the linker option ``-relax'`.

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### 2.14.22 Options for System V

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

`-G`
  Create a shared object. It is recommended that ``-symbolic'` or ``-shared'` be used instead.

`-Qy`
  Identify the versions of each tool used by the compiler, in a `.ident` assembler directive in the output.

`-Qn`
  Refrain from adding `.ident` directives to the output file (this is the default).

`-YP,dirs`
  Search the directories dirs, and no others, for libraries specified with ``-l'`.

`-Ym,dir`
  Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

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### 2.14.23 TMS320C3x/C4x Options

These ``-m'` options are defined for TMS320C3x/C4x implementations:

`-mcpu=cpu_type`
  Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are ``c30'`, ``c31'`, ``c32'`, ``c40'`, and ``c44'`. The default is ``c40'` to generate code for the TMS320C40.

`-mbig-memory`
`-mbig`
`-msmall-memory`
`-msmall`
  Generates code for the big or small memory model. The small memory model assumed that all data fits into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections. The big memory model is the default and requires reloading of the DP register for every direct memory access.

`-mbk`
`-mno-bk`
  Allow (disallow) allocation of general integer operands into the block count register BK.

`-mdb`
`-mno-db`
  Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^23 + 1 (but who iterates loops more than 2^23 times on the C3x?). Note that GCC will try to reverse a loop so that it can utilise the decrement and branch instruction, but will give up if there is more than one memory reference in the loop. Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilised.

`-mdp-isr-reload`
`-mparanoid`
  Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR. This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library.

`-mmpyi`
`-mno-mpyi`
  For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are performed inline instead of a library call.

`-mfast-fix`
`-mno-fast-fix`
  The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer. Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case. This option can be used to disable generation of the additional code required to correct the result.

`-mrptb`
`-mno-rptb`
  Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping. The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.

`-mrpts=count`
`-mno-rpts`
  Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value count, GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time. Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for oeprands. However, since interrupts are blocked by this instruction, it is disabled by default.

`-mloop-unsigned`
`-mno-loop-unsigned`
  The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^31 + 1 since these instructions test if the iteration count is negative to terminate the loop. If the iteration count is unsigned there is a possibility than the 2^31 + 1 maximum iteration count may be exceeded. This switch allows an unsigned iteration count.

`-mti`
  Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed as structures rather than in floating point registers.

`-mregparm`
`-mmemparm`
  Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack.

`-mparallel-insns`
`-mno-parallel-insns`
  Allow the generation of parallel instructions. This is enabled by default with -O2.

`-mparallel-mpy`
`-mno-parallel-mpy`
  Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified. These instructions have tight register constraints which can pessimize the code generation of large functions.

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### 2.14.24 V850 Options

These ``-m'` options are defined for V850 implementations:

`-mlong-calls`
`-mno-long-calls`
  Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer.

`-mno-ep`
`-mep`
  Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the `ep` register, and use the shorter `sld` and `sst` instructions. The ``-mep'` option is on by default if you optimize.

`-mno-prolog-function`
`-mprolog-function`
  Do not use (do use) external functions to save and restore registers at the prolog and epilog of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The ``-mprolog-function'` option is on by default if you optimize.

`-mspace`
  Try to make the code as small as possible. At present, this just turns on the ``-mep'` and ``-mprolog-function'` options.

`-mtda=n`
  Put static or global variables whose size is n bytes or less into the tiny data area that register `ep` points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).

`-msda=n`
  Put static or global variables whose size is n bytes or less into the small data area that register `gp` points to. The small data area can hold up to 64 kilobytes.

`-mzda=n`
  Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
`-mv850`
  Specify that the target processor is the V850.

`-mbig-switch`
  Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.

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### 2.14.25 ARC Options

These options are defined for ARC implementations:

`-EL`
  Compile code for little endian mode. This is the default.

`-EB`
  Compile code for big endian mode.

`-mmangle-cpu`
  Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics. This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are "almost identical". This is an all or nothing option.

`-mcpu=cpu`
  Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All variants support ``-mcpu=base'`, this is the default.

`-mtext=text section`
`-mdata=data section`
`-mrodata=readonly data section`
  Put functions, data, and readonly data in text section, data section, and readonly data section respectively by default. This can be overridden with the `section` attribute. See section 4.29 Specifying Attributes of Variables.

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### 2.14.26 NS32K Options

These are the ``-m'` options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.

`-m32032`
`-m32032`
  Generate output for a 32032. This is the default when the compiler is configured for 32032 and 32016 based systems.

`-m32332`
`-m32332`
  Generate output for a 32332. This is the default when the compiler is configured for 32332-based systems.

`-m32532`
`-m32532`
  Generate output for a 32532. This is the default when the compiler is configured for 32532-based systems.

`-m32081`
  Generate output containing 32081 instructions for floating point. This is the default for all systems.

`-m32381`
  Generate output containing 32381 instructions for floating point. This also implies ``-m32081'`. The 32381 is only compatible with the 32332 and 32532 cpus. This is the default for the pc532-netbsd configuration.

`-mmulti-add`
  Try and generate multiply-add floating point instructions `polyF` and `dotF`. This option is only available if the ``-m32381'` option is in effect. Using these instructions requires changes to to register allocation which generally has a negative impact on performance. This option should only be enabled when compiling code particularly likely to make heavy use of multiply-add instructions.

`-mnomulti-add`
  Do not try and generate multiply-add floating point instructions `polyF` and `dotF`. This is the default on all platforms.

`-msoft-float`
  Generate output containing library calls for floating point. Warning: the requisite libraries may not be available.

`-mnobitfield`
  Do not use the bit-field instructions. On some machines it is faster to use shifting and masking operations. This is the default for the pc532.

`-mbitfield`
  Do use the bit-field instructions. This is the default for all platforms except the pc532.

`-mrtd`
  Use a different function-calling convention, in which functions that take a fixed number of arguments return pop their arguments on return with the `ret` instruction.

  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.)

  This option takes its name from the 680x0 `rtd` instruction.

`-mregparam`
  Use a different function-calling convention where the first two arguments are passed in registers.

  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.

`-mnoregparam`
  Do not pass any arguments in registers. This is the default for all targets.

`-msb`
  It is OK to use the sb as an index register which is always loaded with zero. This is the default for the pc532-netbsd target.

`-mnosb`
  The sb register is not available for use or has not been initialized to zero by the run time system. This is the default for all targets except the pc532-netbsd. It is also implied whenever ``-mhimem'` or ``-fpic'` is set.

`-mhimem`
  Many ns32000 series addressing modes use displacements of up to 512MB. If an address is above 512MB then displacements from zero can not be used. This option causes code to be generated which can be loaded above 512MB. This may be useful for operating systems or ROM code.

`-mnohimem`
  Assume code will be loaded in the first 512MB of virtual address space. This is the default for all platforms.

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## 2.15 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.

`-fexceptions`
  Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies generation of frame unwind information for all functions. This can produce significant data size overhead, although it does not affect execution. If you do not specify this option, it is enabled by default for languages like C++ which normally require exception handling, and disabled for languages like C that do not normally require it. However, when compiling C code that needs to interoperate properly with exception handlers written in C++, you may need to enable this option. You may also wish to disable this option is you are compiling older C++ programs that don't use exception handling.

`-fpcc-struct-return`
  Return "short" `struct` and `union` values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers.

  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'`, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to ``-fpcc-struct-return'`, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

`-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 GCC 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).

  ``-fno-verbose-asm'`, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

`-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. GCC does not consider static data items to be volatile because of this switch.

`-fvolatile-static`
  Consider all memory references to static data 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). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that ``-fpic'` does not work; in that case, recompile with ``-fPIC'` instead. (These maximums are 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, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

`-fPIC`
  If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, 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 allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg.

  It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fcall-saved-reg`
  Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

  It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

  A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fpack-struct`
  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.

`-fcheck-memory-usage`
  Generate extra code to check each memory access. GCC will generate code that is suitable for a detector of bad memory accesses such as ``Checker'`.

  Normally, you should compile all, or none, of your code with this option.

  If you do mix code compiled with and without this option, you must ensure that all code that has side effects and that is called by code compiled with this option is, itself, compiled with this option. If you do not, you might get erroneous messages from the detector.

  If you use functions from a library that have side-effects (such as `read`), you might not be able to recompile the library and specify this option. In that case, you can enable the ``-fprefix-function-name'` option, which requests GCC to encapsulate your code and make other functions look as if they were compiled with ``-fcheck-memory-usage'`. This is done by calling "stubs", which are provided by the detector. If you cannot find or build stubs for every function you call, you might have to specify ``-fcheck-memory-usage'` without ``-fprefix-function-name'`.

  If you specify this option, you can not use the `asm` or `__asm__` keywords in functions with memory checking enabled. The compiler cannot understand what the `asm` statement will do, and therefore cannot generate the appropriate code, so it is rejected. However, the function attribute `no_check_memory_usage` will disable memory checking within a function, and `asm` statements can be put inside such functions. Inline expansion of a non-checked function within a checked function is permitted; the inline function's memory accesses won't be checked, but the rest will.

  If you move your `asm` statements to non-checked inline functions, but they do access memory, you can add calls to the support code in your inline function, to indicate any reads, writes, or copies being done. These calls would be similar to those done in the stubs described above.

`-fprefix-function-name`
  Request GCC to add a prefix to the symbols generated for function names. GCC adds a prefix to the names of functions defined as well as functions called. Code compiled with this option and code compiled without the option can't be linked together, unless stubs are used.

  If you compile the following code with ``-fprefix-function-name'`

      extern void bar (int);
      void
      foo (int a)
      {
        return bar (a + 5);
      }

  GCC will compile the code as if it was written:

      extern void prefix_bar (int);
      void
      prefix_foo (int a)
      {
        return prefix_bar (a + 5);
      }

  This option is designed to be used with ``-fcheck-memory-usage'`.

`-finstrument-functions`
  Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site. (On some platforms, `__builtin_return_address` does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

      void __cyg_profile_func_enter (void *this_fn, void *call_site);
      void __cyg_profile_func_exit  (void *this_fn, void *call_site);

  The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

  This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use ``extern inline'` in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

  A function may be given the attribute `no_instrument_function`, in which case this instrumentation will not be done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

`-fstack-check`
  Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

`-fargument-alias`
`-fargument-noalias`
`-fargument-noalias-global`
  Specify the possible relationships among parameters and between parameters and global data.

  ``-fargument-alias'` specifies that arguments (parameters) may alias each other and may alias global storage. ``-fargument-noalias'` specifies that arguments do not alias each other, but may alias global storage. ``-fargument-noalias-global'` specifies that arguments do not alias each other and do not alias global storage.

  Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.

`-fleading-underscore`
  This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

  Be warned that you should know what you are doing when invoking this option, and that not all targets provide complete support for it.

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## 2.16 Environment Variables Affecting GCC

This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as ``-B'`, ``-I'` and ``-L'` (see section 2.12 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 GCC. See section 17.1 Controlling the Compilation Driver, ``gcc'`.

`LANG`
`LC_CTYPE`
`LC_MESSAGES`
`LC_ALL`
  These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions. GCC inspects the locale categories `LC_CTYPE` and `LC_MESSAGES` if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is ``en_UK'` for English in the United Kingdom.

  The `LC_CTYPE` environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.

  The `LC_MESSAGES` environment variable specifies the language to use in diagnostic messages.

  If the `LC_ALL` environment variable is set, it overrides the value of `LC_CTYPE` and `LC_MESSAGES`; otherwise, `LC_CTYPE` and `LC_MESSAGES` default to the value of the `LANG` environment variable. If none of these variables are set, GCC defaults to traditional C English behavior.

`TMPDIR`
  If `TMPDIR` is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

`GCC_EXEC_PREFIX`
  If `GCC_EXEC_PREFIX` is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

  If GCC 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`), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with ``-Bfoo/'`, GCC will search ``foo/bar'` where it would normally search ``/usr/local/lib/bar'`. These alternate directories are searched first; the standard directories come next.

`COMPILER_PATH`
  The value of `COMPILER_PATH` is a colon-separated list of directories, much like `PATH`. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using `GCC_EXEC_PREFIX`.

`LIBRARY_PATH`
  The value of `LIBRARY_PATH` is a colon-separated list of directories, much like `PATH`. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using `GCC_EXEC_PREFIX`. Linking using GCC also uses these directories when searching for ordinary libraries for the ``-l'` option (but directories specified with ``-L'` come first).

`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 GCC 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 2.9 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.

`LANG`
  This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for `LANG` are recognized:

  `C-JIS`
    Recognize JIS characters.
  `C-SJIS`
    Recognize SJIS characters.
  `C-EUCJP`
    Recognize EUCJP characters.

  If `LANG` is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

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## 2.17 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 GCC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GCC is installed.

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

`-B directory`
  Look for the file ``SYSCALLS.c.X'` in directory, instead of the usual directory (normally ``/usr/local/lib'`). This file contains prototype information about standard system functions. This option applies only to `protoize`.

`-c compilation-options`
  Use compilation-options as the options when running `gcc` to produce the ``.X'` files. The special option ``-aux-info'` is always passed in addition, to tell `gcc` to write a ``.X'` file.

  Note that the compilation options must be given as a single argument to `protoize` or `unprotoize`. If you want to specify several `gcc` options, you must quote the entire set of compilation options to make them a single word in the shell.

  There are certain `gcc` arguments that you cannot use, because they would produce the wrong kind of output. These include ``-g'`, ``-O'`, ``-c'`, ``-S'`, and ``-o'` If you include these in the compilation-options, they are ignored.

`-C`
  Rename files to end in ``.C'` 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 7.11 Caveats of using `protoize`, for more information on how to use `protoize` successfully.

Note most of this information is out of date and superceded by the EGCS install procedures. It is provided for historical reference only.

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