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.
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
-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
-O3 turns on all optimizations specified by
-O2 and also turns on the -finline-functions and
-frename-registers options.
-O0
-Os
-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
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
-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
-fforce-mem
-O2 option turns on this option.
-fforce-addr
-fforce-mem may.
-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 Register Usage.
-foptimize-sibling-calls
-ftrapv
-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=nNote: 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.
-fmerge-constants
This option is default for optimized compilation if assembler and linker
support it. Use -fno-merge-constants to inhibit this behavior.
-fmerge-all-constants
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
-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
-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
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
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
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
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
-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
-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
-frerun-loop-opt
-fgcse
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
-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
-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
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
-foptimize-register-move
-fregmove
-O2
or higher.
Note -fregmove and -foptimize-register-move are the same
optimization.
-fdelayed-branch
-fschedule-insns
-fschedule-insns2
-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
-fschedule-insns or at -O2 or higher.
-fno-sched-spec
-fschedule-insns or at -O2 or higher.
-fsched-spec-load
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
-fschedule-insns or at -O2 or higher.
-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-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
-funroll-all-loops implies the same options as
-funroll-loops,
-fprefetch-loop-arrays
-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 ([email protected] and [email protected])
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
-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
-fprofile-arcs
(see Options for Debugging Your Program or gcc), 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 .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
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
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-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-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-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-falign-functions. In this case, no dummy operations
need be executed.
If n is not specified, use a machine-dependent default.
-fssa
-fssa-ccp
-fssa. Like -fssa, this is an experimental feature.
-fssa-dce
-fssa.
Like -fssa, this is an experimental feature.
-fsingle-precision-constant
-frename-registers
-fno-cprop-registers
--param name=value--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
max-delay-slot-live-search
max-gcse-memory
max-gcse-passes
max-pending-list-length
max-inline-insns
-finline-limit.