__attribute__((optimize("-O2"))) doesn't work

[Jonathan Wakely]

On 22 December 2014 at 22:14, Chaoran Yang 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.

> Anyway, I just want to say that the __attribute__((optimize(“xx”))) doesn’t seem to work.

It does work, if you use it right, and you put it on functions where
optimisation makes a difference.

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. And for
a function as trivial as "return 2" you're not going to notice any
difference between -O1 and -O3.

> I tried to use __attribute__((optimize(0))) to turn off optimization on certain functions, but also failed.

That should be __attribute__((optimize("O0"))) although I don't know
if -O0 can be used with the attribute.