> On Dec 22, 2014, at 5:59 PM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
>
>> Shouldn’t it be optimized to:
>>
>> 0x0000000000400530 <+0>: mov $0x2,%eax
>> 0x0000000000400535 <+5>: retq
>>
>> just like I compiled it with "gcc -O2 t.c”?
>
> When you compile the whole file with -O2 you also compile main() with
> -O2, which is not the same as just optimising one function.
Even if I take out the main function, using __attribute__((optimize(“O2”))) is still not the same as -O2. See the following output.
$ cat t.c
int foo()
{
return 2;
}
$ gcc -c -O2 t.c
$ gdb t.o
(gdb) disas foo
Dump of assembler code for function foo:
0x0000000000000000 <+0>: mov $0x2,%eax <==== this is compiled with -O2
0x0000000000000005 <+5>: retq
End of assembler dump.
$ gcc -c t.c
$ gdb t.o
(gdb) disas foo
Dump of assembler code for function foo:
0x0000000000000000 <+0>: push %rbp <==== this is compiled with no optimization
0x0000000000000001 <+1>: mov %rsp,%rbp
0x0000000000000004 <+4>: mov $0x2,%eax
0x0000000000000009 <+9>: pop %rbp
0x000000000000000a <+10>: retq
End of assembler dump.
$ cat t.c
int foo() __attribute__((optimize("O2"))); <==== Adding the __attribute__
int foo()
{
return 2;
}
$ gcc -c t.c
$ gdb t.o
(gdb) disas foo
Dump of assembler code for function foo:
0x0000000000000000 <+0>: push %rbp <==== output is the same as compiled with no optimization
0x0000000000000001 <+1>: mov $0x2,%eax
0x0000000000000006 <+6>: mov %rsp,%rbp
0x0000000000000009 <+9>: pop %rbp
0x000000000000000a <+10>: retq
End of assembler dump.
> But I believe unless you use some -Ox option on the command line the
> optimize attribute has no effect (which is true for optimization
> options like -finline-functions). You have to enable optimization in
> the first place before you can control the optimization level.
I don’t this this is true, at least not true for -fomit-frame-pointer. I've tested __attribute__((optimize(“-fomit-frame-pointer”))), which works regardless optimization is turned on or not.
Thanks,
Chaoran
