[Bug c/96753] New: Missed optimization [x86-64] on modulo when left side value is known to be greater than right side value

[goldstein.w.n at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96753

            Bug ID: 96753
           Summary: Missed optimization [x86-64] on modulo when left side
                    value is known to be greater than right side value
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goldstein.w.n at gmail dot com
  Target Milestone: ---

uint64_t
do_mod(uint64_t x, uint64_t y) {
    if(x >= y) {
        __builtin_unreachable();
    }
    return x % y;
}

With -O3 compiles to

do_mod(unsigned long, unsigned long):
        movq    %rdi, %rax
        xorl    %edx, %edx
        divq    %rsi
        movq    %rdx, %rax
        ret

given that x is known to be less than y this could be return x;

Godbolt link: https://gcc.godbolt.org/z/sfhr96