[Bug tree-optimization/106727] New: Missed fold / canonicalization for checking if a number is a power of 2

[llvm at rifkin dot dev via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

            Bug ID: 106727
           Summary: Missed fold / canonicalization for checking if a
                    number is a power of 2
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

These two are equivalent but generate different code:

bool foo(unsigned n) {
    return std::popcount(n) <= 1;
}
bool bar(unsigned n) {
    return (n & (n - 1)) == 0;
}

https://godbolt.org/z/crrxnfWo7

For systems that don't have popcount instructions it would be very beneficial
to transform foo -> bar. For systems that do have popcount instructions it
still would be beneficial to canonicalize the two.