On 23 December 2014 at 00:27, Chaoran Yang <chaoran@xxxxxxxx> wrote:
>> 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.
Well then try testing it with something less pointless than "return 2"
N.B. that should be optimize("omit-frame-pointer"), you can grep in
the testsuite to find the correct syntax to use.
