[Bug c/88959] New: Unnecessary xor before bsf/tzcnt

[bugzilla@poradnik-webmastera.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88959

            Bug ID: 88959
           Summary: Unnecessary xor before bsf/tzcnt
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla@poradnik-webmastera.com
  Target Milestone: ---

[code]
int test(int x)
{
    return __builtin_ctz(x);
}
[/code]

gcc 4.9.1 with -O3 produces this:

[asm]
test(int):
  rep bsf eax, edi
  ret
[/asm]

And this with -O3 -mbmi:

[asm]
test(int):
  tzcnt eax, edi
  ret
[/asm]

gcc 4.9.2 and newer (including gcc 9) produces this for both cases:

[asm]
test(int):
  xor eax, eax
  rep bsf eax, edi
  ret
[/asm]

[asm]
test(int):
  xor eax, eax
  tzcnt eax, edi
  ret
[/asm]

This extra xor instruction is not needed here.