[Bug target/95531] New: Failure to use TZCNT for __builtin_ffs

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95531

            Bug ID: 95531
           Summary: Failure to use TZCNT for __builtin_ffs
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int x)
{
    return __builtin_ffs(x);
}

With -O3 -mbmi, LLVM generates this :

f(int): # @f(int)
  tzcnt ecx, edi
  mov eax, -1
  cmovae eax, ecx
  add eax, 1
  ret

GCC outputs this :

f(int):
  bsf eax, edi
  mov edx, -1
  cmove eax, edx
  add eax, 1
  ret