so para acresentar....
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, 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.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and
possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only opti-
mizations that have a flag are listed.
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of com-
-O turns on the following optimization flags: -fdefer-pop
-fmerge-constants -fthread-jumps -floop-optimize -fcrossjumping
-fif-conversion -fif-conversion2 -fdelayed-branch
-O also turns on -fomit-frame-pointer on machines where doing so
does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported optimiza-
tions that do not involve a space-speed tradeoff. The compiler
does not perform loop unrolling or function inlining when you spec-
ify -O2. As compared to -O, this option increases both compilation
time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns
on the following optimization flags: -fforce-mem -foptimize-sib-
ling-calls -fstrength-reduce -fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm
-fgcse-sm -fdelete-null-pointer-checks -fexpensive-optimizations
-fregmove -fschedule-insns -fschedule-insns2 -fsched-interblock
-fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks -fre-
order-functions -fstrict-aliasing -falign-functions -falign-jumps
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
-O0 Do not optimize. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not
typically increase code size. It also performs further optimiza-
tions designed to reduce code size.
-Os disables the following optimization flags: -falign-functions
-falign-jumps -falign-loops -falign-labels -freorder-blocks
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of comput-
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for cpu-type are i386, i486, i586, i686, pentium, pentium-
mmx, pentiumpro, pentium2, pentium3, pentium4, prescott, nocona,
k6, k6-2, k6-3, athlon, athlon-tbird, athlon-4, athlon-xp, athlon-
mp, winchip-c6, winchip2 and c3.
While picking a specific cpu-type will schedule things appropri-
ately 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 and athlon are the AMD chips as
opposed to the Intel ones.
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.
These options are synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pen-
tium, and -mcpu=pentiumpro respectively. These synonyms are depre-
slkcflags se orientam pelo gcc entao.... sejam felizes..
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