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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use of the GNU compilers.

   Published by the Free Software Foundation
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   A GNU Manual

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   You have freedom to copy and modify this GNU Manual, like GNU
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File: gcc.info,  Node: Debugging Options,  Next: Optimize Options,  Prev: Warning Options,  Up: Invoking GCC

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+',
     `-gdwarf-1', or `-gvms' (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.

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

`-gLEVEL'
`-ggdbLEVEL'
`-gstabsLEVEL'
`-gcoffLEVEL'
`-gxcoffLEVEL'
`-gvmsLEVEL'
     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'.

     Note that in order to avoid confusion between DWARF1 debug level 2,
     and DWARF2, neither `-gdwarf' nor `-gdwarf-2' accept a
     concatenated debug level.  Instead use an additional `-gLEVEL'
     option to change the debug level for DWARF1 or DWARF2.

`-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.

`-ftime-report'
     Makes the compiler print some statistics about the time consumed
     by each pass when it finishes.

`-fmem-report'
     Makes the compiler print some statistics about permanent memory
     allocation when it finishes.

`-fprofile-arcs'
     Instrument "arcs" during compilation to generate coverage data or
     for profile-directed block ordering.  During execution the program
     records how many times each branch is executed and how many times
     it is taken.  When the compiled program exits it saves this data
     to a file called `SOURCENAME.da' for each source file.

     For profile-directed block ordering, compile the program with
     `-fprofile-arcs' plus optimization and code generation options,
     generate the arc profile information by running the program on a
     selected workload, and then compile the program again with the same
     optimization and code generation options plus
     `-fbranch-probabilities' (*note Options that Control Optimization:
     Optimize Options.).

     The other use of `-fprofile-arcs' is for use with `gcov', when it
     is used with the `-ftest-coverage' option.

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

`-ftest-coverage'
     Create data files for the `gcov' code-coverage utility (*note
     `gcov': a GCC Test Coverage Program: Gcov.).  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.

     Use `-ftest-coverage' with `-fprofile-arcs'; the latter option
     adds instrumentation to the program, which then writes execution
     counts to another data file:

    `SOURCENAME.da'
          Runtime arc execution counts, used in conjunction with the arc
          information in the file `SOURCENAME.bbg'.

     Coverage data will map better to the source files if
     `-ftest-coverage' is used without optimization.

`-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 pass number
     and a word to the source file name (e.g.  `foo.c.00.rtl' or
     `foo.c.01.sibling').  Here are the possible letters for use in
     LETTERS, and their meanings:

    `A'
          Annotate the assembler output with miscellaneous debugging
          information.

    `b'
          Dump after computing branch probabilities, to `FILE.14.bp'.

    `B'
          Dump after block reordering, to `FILE.29.bbro'.

    `c'
          Dump after instruction combination, to the file
          `FILE.16.combine'.

    `C'
          Dump after the first if conversion, to the file `FILE.17.ce'.

    `d'
          Dump after delayed branch scheduling, to `FILE.31.dbr'.

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

    `e'
          Dump after SSA optimizations, to `FILE.04.ssa' and
          `FILE.07.ussa'.

    `E'
          Dump after the second if conversion, to `FILE.26.ce2'.

    `f'
          Dump after life analysis, to `FILE.15.life'.

    `F'
          Dump after purging `ADDRESSOF' codes, to `FILE.09.addressof'.

    `g'
          Dump after global register allocation, to `FILE.21.greg'.

    `h'
          Dump after finalization of EH handling code, to `FILE.02.eh'.

    `k'
          Dump after reg-to-stack conversion, to `FILE.28.stack'.

    `o'
          Dump after post-reload optimizations, to `FILE.22.postreload'.

    `G'
          Dump after GCSE, to `FILE.10.gcse'.

    `i'
          Dump after sibling call optimizations, to `FILE.01.sibling'.

    `j'
          Dump after the first jump optimization, to `FILE.03.jump'.

    `k'
          Dump after conversion from registers to stack, to
          `FILE.32.stack'.

    `l'
          Dump after local register allocation, to `FILE.20.lreg'.

    `L'
          Dump after loop optimization, to `FILE.11.loop'.

    `M'
          Dump after performing the machine dependent reorganisation
          pass, to `FILE.30.mach'.

    `n'
          Dump after register renumbering, to `FILE.25.rnreg'.

    `N'
          Dump after the register move pass, to `FILE.18.regmove'.

    `r'
          Dump after RTL generation, to `FILE.00.rtl'.

    `R'
          Dump after the second scheduling pass, to `FILE.27.sched2'.

    `s'
          Dump after CSE (including the jump optimization that
          sometimes follows CSE), to `FILE.08.cse'.

    `S'
          Dump after the first scheduling pass, to `FILE.19.sched'.

    `t'
          Dump after the second CSE pass (including the jump
          optimization that sometimes follows CSE), to `FILE.12.cse2'.

    `w'
          Dump after the second flow pass, to `FILE.23.flow2'.

    `X'
          Dump after SSA dead code elimination, to `FILE.06.ssadce'.

    `z'
          Dump after the peephole pass, to `FILE.24.peephole2'.

    `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.

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

    `v'
          For each of the other indicated dump files (except for
          `FILE.00.rtl'), dump a representation of the control flow
          graph suitable for viewing with VCG to `FILE.PASS.vcg'.

    `x'
          Just generate RTL for a function instead of compiling it.
          Usually used with `r'.

    `y'
          Dump debugging information during parsing, to standard error.

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

`-fdump-translation-unit (C and C++ only)'
`-fdump-translation-unit-OPTIONS (C and C++ only)'
     Dump a representation of the tree structure for the entire
     translation unit to a file.  The file name is made by appending
     `.tu' to the source file name.  If the `-OPTIONS' form is used,
     OPTIONS controls the details of the dump as described for the
     `-fdump-tree' options.

`-fdump-class-hierarchy (C++ only)'
`-fdump-class-hierarchy-OPTIONS (C++ only)'
     Dump a representation of each class's hierarchy and virtual
     function table layout to a file.  The file name is made by
     appending `.class' to the source file name.  If the `-OPTIONS'
     form is used, OPTIONS controls the details of the dump as
     described for the `-fdump-tree' options.

`-fdump-tree-SWITCH (C++ only)'
`-fdump-tree-SWITCH-OPTIONS (C++ only)'
     Control the dumping at various stages of processing the
     intermediate language tree to a file.  The file name is generated
     by appending a switch specific suffix to the source file name.  If
     the `-OPTIONS' form is used, OPTIONS is a list of `-' separated
     options that control the details of the dump. Not all options are
     applicable to all dumps, those which are not meaningful will be
     ignored. The following options are available

    `address'
          Print the address of each node.  Usually this is not
          meaningful as it changes according to the environment and
          source file. Its primary use is for tying up a dump file with
          a debug environment.

    `slim'
          Inhibit dumping of members of a scope or body of a function
          merely because that scope has been reached. Only dump such
          items when they are directly reachable by some other path.

    `all'
          Turn on all options.

     The following tree dumps are possible:
    `original'
          Dump before any tree based optimization, to `FILE.original'.

    `optimized'
          Dump after all tree based optimization, to `FILE.optimized'.

    `inlined'
          Dump after function inlining, to `FILE.inlined'.

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

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

`-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'.  This creates a
     preprocessed `foo.i' output file even though the compiler now
     normally uses an integrated preprocessor.

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

          # cc1 0.12 0.01
          # as 0.00 0.01

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

`-print-file-name=LIBRARY'
     Print the full absolute name of the library file LIBRARY that
     would be used when linking--and don't do anything else.  With this
     option, GCC does not compile or link anything; it just prints the
     file name.

`-print-multi-directory'
     Print the directory name corresponding to the multilib selected by
     any other switches present in the command line.  This directory is
     supposed to exist in `GCC_EXEC_PREFIX'.

`-print-multi-lib'
     Print the mapping from multilib directory names to compiler
     switches that enable them.  The directory name is separated from
     the switches by `;', and each switch starts with an `@' instead of
     the `-', without spaces between multiple switches.  This is
     supposed to ease shell-processing.

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

`-print-libgcc-file-name'
     Same as `-print-file-name=libgcc.a'.

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

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

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

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

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

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

`-dumpspecs'
     Print the compiler's built-in specs--and don't do anything else.
     (This is used when GCC itself is being built.)  *Note Spec Files::.


File: gcc.info,  Node: Optimize Options,  Next: Preprocessor Options,  Prev: Debugging Options,  Up: Invoking GCC

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.

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

`-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 register renaming.  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.

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

`-O3'
     Optimize yet more.  `-O3' turns on all optimizations specified by
     `-O2' and also turns on the `-finline-functions' and
     `-frename-registers' options.

`-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.  *Note Register
     Usage: (gccint)Registers.

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

`-ftrapv'
     This option generates traps for signed overflow on addition,
     subtraction, multiplication operations.

`-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 600.
     Increasing this value can result in more inlined code at the cost
     of compilation time and memory consumption.  Decreasing usually
     makes the compilation faster and less code will be inlined (which
     presumably means slower programs).  This option is particularly
     useful for programs that use inlining heavily such as those based
     on recursive templates with C++.

     _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.

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

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

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

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

`-fno-branch-count-reg'
     Do not use "decrement and branch" instructions on a count register,
     but instead generate a sequence of instructions that decrement a
     register, compare it against zero, then branch based upon the
     result.  This option is only meaningful on architectures that
     support such instructions, which include x86, PowerPC, IA-64 and
     S/390.

`-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'
     Sets `-fno-math-errno', `-funsafe-math-optimizations', and
     `-fno-trapping-math'.

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

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

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

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

     The default is `-fmath-errno'.

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

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

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

`-fno-trapping-math'
     Compile code assuming that floating-point operations cannot
     generate user-visible traps.  Setting this option may allow faster
     code if one relies on "non-stop" IEEE arithmetic, for example.

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

     The default is `-ftrapping-math'.

`-fbounds-check'
     For front-ends that support it, generate additional code to check
     that indices used to access arrays are within the declared range.
     This is currenly only supported by the Java and Fortran 77
     front-ends, where this option defaults to true and false
     respectively.

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

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

   Not all of the optimizations performed by GCC have `-f' options to
control them.

`-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.

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

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

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

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

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

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

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

     Note `-fregmove' and `-foptimize-register-move' 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.

`-fno-sched-interblock'
     Don't schedule instructions across basic blocks.  This is normally
     enabled by default when scheduling before register allocation, i.e.
     with `-fschedule-insns' or at `-O2' or higher.

`-fno-sched-spec'
     Don't allow speculative motion of non-load instructions.  This is
     normally enabled by default when scheduling before register
     allocation, i.e.  with `-fschedule-insns' or at `-O2' or higher.

`-fsched-spec-load'
     Allow speculative motion of some load instructions.  This only
     makes sense when scheduling before register allocation, i.e. with
     `-fschedule-insns' or at `-O2' or higher.

`-fsched-spec-load-dangerous'
     Allow speculative motion of more load instructions.  This only
     makes sense when scheduling before register allocation, i.e. with
     `-fschedule-insns' or at `-O2' or higher.

`-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'
     Unroll loops whose number of iterations can be determined at
     compile time or upon entry to the loop.  `-funroll-loops' implies
     both `-fstrength-reduce' and `-frerun-cse-after-loop'.  This
     option makes code larger, and may or may not make it run faster.

`-funroll-all-loops'
     Unroll all loops, even if their number of iterations is uncertain
     when the loop is entered.  This usually makes programs run more
     slowly.  `-funroll-all-loops' implies the same options as
     `-funroll-loops',

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

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

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

     With `-fbranch-probabilities', GCC puts a `REG_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.

`-fno-guess-branch-probability'
     Do not guess branch probabilities using a randomized model.

     Sometimes gcc will opt to use a randomized model to guess branch
     probabilities, when none are available from either profiling
     feedback (`-fprofile-arcs') or `__builtin_expect'.  This means that
     different runs of the compiler on the same program may produce
     different object code.

     In a hard real-time system, people don't want different runs of the
     compiler to produce code that has different behavior; minimizing
     non-determinism is of paramount import.  This switch allows users
     to reduce non-determinism, possibly at the expense of inferior
     optimization.

`-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'.

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

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

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

     If N is not specified, use a machine-dependent default.

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

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

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

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

     If N is not specified, use a machine-dependent default.

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

     If N is not specified, use a machine-dependent default.

`-fssa'
     Perform optimizations in static single assignment form.  Each
     function's flow graph is translated into SSA form, optimizations
     are performed, and the flow graph is translated back from SSA
     form.  Users should not specify this option, since it is not yet
     ready for production use.

`-fssa-ccp'
     Perform Sparse Conditional Constant Propagation in SSA form.
     Requires `-fssa'.  Like `-fssa', this is an experimental feature.

`-fssa-dce'
     Perform aggressive dead-code elimination in SSA form.  Requires
     `-fssa'.  Like `-fssa', this is an experimental feature.

`-fsingle-precision-constant'
     Treat floating point constant as single precision constant instead
     of implicitly converting it to double precision constant.

`-frename-registers'
     Attempt to avoid false dependencies in scheduled code by making use
     of registers left over after register allocation.  This
     optimization will most benefit processors with lots of registers.
     It can, however, make debugging impossible, since variables will
     no longer stay in a "home register".

`-fno-cprop-registers'
     After register allocation and post-register allocation instruction
     splitting, we perform a copy-propagation pass to try to reduce
     scheduling dependencies and occasionally eliminate the copy.

`--param NAME=VALUE'
     In some places, GCC uses various constants to control the amount of
     optimization that is done.  For example, GCC will not inline
     functions that contain more that a certain number of instructions.
     You can control some of these constants on the command-line using
     the `--param' option.

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

    `max-delay-slot-insn-search'
          The maximum number of instructions to consider when looking
          for an instruction to fill a delay slot.  If more than this
          arbitrary number of instructions is searched, the time
          savings from filling the delay slot will be minimal so stop
          searching.  Increasing values mean more aggressive
          optimization, making the compile time increase with probably
          small improvement in executable run time.

    `max-delay-slot-live-search'
          When trying to fill delay slots, the maximum number of
          instructions to consider when searching for a block with
          valid live register information.  Increasing this arbitrarily
          chosen value means more aggressive optimization, increasing
          the compile time.  This parameter should be removed when the
          delay slot code is rewritten to maintain the control-flow
          graph.

    `max-gcse-memory'
          The approximate maximum amount of memory that will be
          allocated in order to perform the global common subexpression
          elimination optimization.  If more memory than specified is
          required, the optimization will not be done.

    `max-gcse-passes'
          The maximum number of passes of GCSE to run.

    `max-pending-list-length'
          The maximum number of pending dependencies scheduling will
          allow before flushing the current state and starting over.
          Large functions with few branches or calls can create
          excessively large lists which needlessly consume memory and
          resources.

    `max-inline-insns'
          If an function contains more than this many instructions, it
          will not be inlined.  This option is precisely equivalent to
          `-finline-limit'.

