Fresh Fish 4/FreshFish_May-June1994.bin/gnu/info/gcc.info-3

GNU Info File | 1994-02-20 | 46KB | 877 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input
file gcc.texi.
   This file documents the use and the internals of the GNU compiler.
   Published by the Free Software Foundation 675 Massachusetts Avenue
Cambridge, MA 02139 USA
   Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc.
   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License" and "Protect
Your Freedom--Fight `Look And Feel'" are included exactly as in the
original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License" and "Protect Your Freedom--Fight `Look And Feel'", and this
permission notice, may be included in translations approved by the Free
Software Foundation instead of in the original English.
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:
`-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', only variables declared `register' are allocated 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 `-O' is specified, the two options `-fthread-jumps' and
     `-fdelayed-branch' are turned on.  On some machines other flags may
     also be turned on.
`-O2'
     Optimize even more.  Nearly all supported optimizations that do not
     involve a space-speed tradeoff are performed.  As compared to `-O',
     this option increases both compilation time and the performance of
     the generated code.
     `-O2' turns on all optional optimizations except for loop unrolling
     and frame pointer elimination.
`-O0'
     Do not optimize.
     If you use multiple `-O' options, with or without level numbers,
     the last such option is the one that is effective.
   Options of the form `-fFLAG' specify machine-independent flags.
Most flags have both positive and negative forms; the negative form of
`-ffoo' would be `-fno-foo'.  In the table below, only one of the forms
is listed--the one which is not the default.  You can figure out the
other form by either removing `no-' or adding it.
`-ffloat-store'
     Do not store floating point variables in registers, and inhibit
     other options that might change whether a floating point value is
     taken from a register or memory.
     This option prevents undesirable excess precision on machines such
     as the 68000 where the floating registers (of the 68881) keep more
     precision than a `double' is supposed to have.  For most programs,
     the excess precision does only good, but a few programs rely on the
     precise definition of IEEE floating point.  Use `-ffloat-store' for
     such programs.
`-fno-default-inline'
     Do not make member functions inline by default merely because they
     are defined inside the class scope (C++ only).  Otherwise, when
     you specify `-O', member functions defined inside class scope are
     compiled inline by default; i.e., you don't need to add `inline'
     in front of the member function name.
`-fno-defer-pop'
     Always pop the arguments to each function call as soon as that
     function returns.  For machines which must pop arguments after a
     function call, the compiler normally lets arguments accumulate on
     the stack for several function calls and pops them all at once.
`-fforce-mem'
     Force memory operands to be copied into registers before doing
     arithmetic on them.  This may produce better code by making all
     memory references potential common subexpressions.  When they are
     not common subexpressions, instruction combination should
     eliminate the separate register-load.  I am interested in hearing
     about the difference this makes.
`-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.  I am interested in hearing about the
     difference this makes.
`-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 Registers::.
`-fno-inline'
     Don't pay attention to the `inline' keyword.  Normally this option
     is used to keep the compiler from expanding any functions inline.
     Note that if you are not optimizing, no functions can be expanded
     inline.
`-finline-functions'
     Integrate all simple functions into their callers.  The compiler
     heuristically decides which functions are simple enough to be worth
     integrating in this way.
     If all calls to a given function are integrated, and the function
     is declared `static', then the function is normally not output as
     assembler code in its own right.
`-fkeep-inline-functions'
     Even if all calls to a given function are integrated, and the
     function is declared `static', nevertheless output a separate
     run-time callable version of the function.
`-fno-function-cse'
     Do not put function addresses in registers; make each instruction
     that calls a constant function contain the function's address
     explicitly.
     This option results in less efficient code, but some strange hacks
     that alter the assembler output may be confused by the
     optimizations performed when this option is not used.
`-ffast-math'
     This option allows GCC to violate some ANSI or IEEE rules and/or
     specifications in the interest of optimizing code for speed.  For
     example, it allows the compiler to assume arguments to the `sqrt'
     function are non-negative numbers.
     This option should never be turned on by any `-O' option since it
     can result in incorrect output for programs which depend on an
     exact implementation of IEEE or ANSI rules/specifications for math
     functions.
   The following options control specific optimizations.  The `-O2'
option turns on all of these optimizations except `-funroll-loops' and
`-funroll-all-loops'.  On most machines, the `-O' option turns on the
`-fthread-jumps' and `-fdelayed-branch' options, but specific machines
may handle it differently.
   You can use the following flags in the rare cases when "fine-tuning"
of optimizations to be performed is desired.
`-fstrength-reduce'
     Perform the optimizations of loop strength reduction and
     elimination of iteration variables.
`-fthread-jumps'
     Perform optimizations where we check to see if a jump branches to a
     locati