[Bug middle-end/101253] New: Optimize i % C1 == C0 || i % C1*C2 == C0 to i % C1 == C0

[antoshkka at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101253

            Bug ID: 101253
           Summary: Optimize i % C1 == C0 || i % C1*C2 == C0 to i % C1 ==
                    C0
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the following code

bool test_naive(short i) {
    return i % 100 == 0 || i % 400 == 0;
}

It could be optimized into

bool test_optim(short i) {
    return i % 100 == 0;
}


Godbolt playground: https://godbolt.org/z/zW49qcs7G

P.S.: Inspired by the manual optimizations in libstdc++
https://github.com/gcc-mirror/gcc/commit/b92d12d3fe3f1aa56d190d960e40c62869a6cfbb