[Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757

            Bug ID: 94757
           Summary: GCC does not optimise unsigned multiplication known
                    not to overflow
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.von.s at posteo dot de
  Target Milestone: ---

This code:

    #include <limits.h>

    unsigned f(unsigned x) {
        if (x > UINT_MAX / 3)
            __builtin_unreachable();
        return (x * 3) / 3;
    }

should be possible to optimise into the identity function, but (on x86_64) gcc
-O3 generates a multiplication (via LEA) then a division (via multiplication).
GCC knows that the multiplication cannot overflow, because replacing the
returned expression with __builtin_mul_overflow_p(x, 3, x) is makes it optimise
to returning constant 0.