[Bug middle-end/98817] New: Optimize if (a != b) a = b;

[Bug middle-end/98817] New: Optimize if (a != b) a = b;

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

            Bug ID: 98817
           Summary: Optimize if (a != b) a = b;
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:


void arithmetic(int& result, int value) {
    if (result != value) {
        result = value;
    }
}


GCC generates the following assembly:


arithmetic(int&, int):
  cmp DWORD PTR [rdi], esi
  je .L1
  mov DWORD PTR [rdi], esi
.L1:
  ret


The assembly seems suboptimal, because
1) cmov could be used
2) conditional jump could be totally removed, reducing the binary size and
leaving only one mov instruction:

arithmetic(int&, int):
  mov DWORD PTR [rdi], esi
  ret



Godbolt playground https://godbolt.org/z/Pdz7eP with above sample and
std::vector::clear() sample that would also benefit from the above
optimization.

[Bug middle-end/98817] Optimize if (a != b) a = b;

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'm not sure about this.  Turning it into an unconditional store would mean
that the memory the reference points to must be writable, that might not be
always the case.
E.g. if it refers to a .rodata object, or an object within something mapped
PROT_READ only etc.

[Bug middle-end/98817] Optimize if (a != b) a = b;

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

--- Comment #2 from Antony Polukhin <antoshkka at gmail dot com> ---
(In reply to Jakub Jelinek from comment #1)
> I'm not sure about this.  Turning it into an unconditional store would mean
> that the memory the reference points to must be writable, that might not be
> always the case.

Fair pint.

How about emitting cmov instead of cmp+je?

[Bug middle-end/98817] Optimize if (a != b) a = b;

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This cannot be done due to race conditions too:
https://gcc.gnu.org/wiki/Atomic/GCCMM/DataRaces

[Bug middle-end/98817] Optimize if (a != b) a = b;

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
cmov has the exact same problem as the unconditional store.  At least on x86 a
cmov for one doesn't have a MEM destination and so the conditionally assigned
register then needs to be unconditionally stored.
You'd need an instruction that would store only if the condition is true
(something like SSE/AVX masked stores, but for scalars).

[Bug middle-end/98817] Optimize if (a != b) a = b;

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

--- Comment #5 from Antony Polukhin <antoshkka at gmail dot com> ---
Please, close as invalid

[Bug middle-end/98817] Optimize if (a != b) a = b;

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98817

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
.