[Bug tree-optimization/95862] New: Failure to optimize usage of __builtin_mul_overflow to small __int128-based check

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95862

            Bug ID: 95862
           Summary: Failure to optimize usage of __builtin_mul_overflow to
                    small __int128-based check
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(int32_t a, int32_t b)
{
    uint64_t r;
    return __builtin_mul_overflow (a, b, &r);
}

This can be optimized to `return ((__uint128_t)(a * (__int128_t)b) >> 64) & 1;`
(idk if this might invoke UB, but rn at least it generates much better code on
x86 than what GCC currently generates for the example I gave). This
transformation is done by LLVM, but not by GCC.