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.
