[Bug c/97307] New: Optimization for pure vs. const function

["benjamin.meier70 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97307

            Bug ID: 97307
           Summary: Optimization for pure vs. const function
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: benjamin.meier70 at gmail dot com
  Target Milestone: ---

This bug report is based on this stackoverflow post:
https://stackoverflow.com/q/64034889/916672

The source code I use in this post, is also available here:
https://gcc.godbolt.org/z/dGvxnv

Given this C source code:

> int pure_f(int a, int b) __attribute__((pure));
> 
> int const_f(int a, int b) __attribute__((const));
> 
> int my_f(int a, int b) {
>     int x = pure_f(a, b);
>     if (a > 0) {
>         return x;
>     }
>     return a;
> }

If this is compiled with gcc with -O3, I would expect that the evaluation of
pure_f(a, b) is moved into the if. But it is not done:

> my_f(int, int):
>         push    r12
>         mov     r12d, edi
>         call    pure_f(int, int)
>         test    r12d, r12d
>         cmovg   r12d, eax
>         mov     eax, r12d
>         pop     r12
>         ret

On the other side, if const_f is called instead of pure_f, it is moved into the
if:


> my_f(int, int):
>         test    edi, edi
>         jg      .L4
>         mov     eax, edi
>         ret
> .L4:
>         jmp     const_f(int, int)


Why isn't this optimization applied for a pure function? From my understanding,
this should also be possible and it seems to be beneficial.