[Bug tree-optimization/123332] New: Missing optimization of loop to __builtin_popcount()

[antoshkka at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123332

            Bug ID: 123332
           Summary: Missing optimization of loop to __builtin_popcount()
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

One of the AI research papers https://arxiv.org/html/2505.11480v2 noted that
the following code:


int f(unsigned long x) {
    int res = 0;

    while (x > 0) {
        res += x & 1;
        x >>= 1;
    }

    return res;
}


can be optimized to `popcnt`, but is not optimized by compilers.

Please, add the missing optimization