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These options control various sorts of optimizations:
-O
-O1
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
`-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
-O0
-Os
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
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
-fno-defer-pop
-fforce-mem
-fforce-addr
-fomit-frame-pointer
On some machines, such as the Vax, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED
controls
whether a target machine supports this flag. See section `Register Usage' in Using and Porting GCC.
-fno-inline
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
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
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
static
, nevertheless output a separate run-time
callable version of the function. This switch does not affect
extern inline
functions.
-fkeep-static-consts
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
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
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
-fthread-jumps
-fcse-follow-jumps
if
statement with an
else
clause, CSE will follow the jump when the condition
tested is false.
-fcse-skip-blocks
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
-frerun-loop-opt
-fgcse
-fexpensive-optimizations
-foptimize-register-moves
-fregmove
Note -fregmove
and -foptimize-register-moves
are the same
optimization.
-fdelayed-branch
-fschedule-insns
-fschedule-insns2
-ffunction-sections
-fdata-sections
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
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
-funroll-all-loops
-fmove-all-movables
-freduce-all-givs
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
-fbranch-probabilities
gcc
), you can compile it a second time using
`-fbranch-probabilities', to improve optimizations based on
guessing the path a branch might take.
-fstrict-aliasing
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; } |
int f() { a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; } |
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