[Bug middle-end/82479] New: missing popcount builtin detection

[kugan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82479

            Bug ID: 82479
           Summary: missing popcount builtin detection
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kugan at gcc dot gnu.org
  Target Milestone: ---

gcc does not have support to detect builtin pop count. As a results, gcc
generates bad code for

int PopCount (long b) {
    int c = 0;

    while (b) {
        b &= b - 1;
        c++;
    }
    return c;
}

clang seems to do that and generates (for aarch64):

_Z8PopCounty:
                fmov     d0, x0
                cnt          v0.8b, v0.8b
                uaddlv  h0, v0.8b
                fmov     w0, s0
                ret