[Bug tree-optimization/90839] New: Detect lsb ones counting loop (final value replacement?)

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90839

            Bug ID: 90839
           Summary: Detect lsb ones counting loop (final value
                    replacement?)
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

The code:
int
foo (int a)
{
  int b = 0;
  while (a >>= 1)
    b++;

  return b;
}

Can calculates 31 - clz (a). A compiler could generate for x86:
        xorl      %eax, %eax
        bsr       %edi, %edx
        cmovne    %edx, %eax
        ret

But GCC generates the loop.
I believe we already have popcount-detecting code in final value replacement...