[Bug tree-optimization/104378] New: (N - x) ^ N should be optimized to x if x <= N (unsigned) and N is a pow2 - 1

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104378

            Bug ID: 104378
           Summary: (N - x) ^ N should be optimized to x if x <= N
                    (unsigned) and N is a pow2 - 1
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
#define n 8
#define N ((1u<<n)-1)

unsigned countLeadingZeros32(unsigned x)
{
   if (x > N) __builtin_unreachable();
   return (N - x) ^ N;
}

This should be optimized to just
return x;

Like it is done by LLVM.