[Bug middle-end/87655] New: `i = i % constant` for static local `i` is not optimized

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87655

            Bug ID: 87655
           Summary: `i = i % constant` for static local `i` is not
                    optimized
           Product: gcc
           Version: 9.0
            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 example:

unsigned next_trivial() {
    static int i = 0;

    auto ret = i;
    ++i;
    i = i % 10;
    return ret;
}

For that example a very suboptimal assembly with multiplication, many registers
usage and multiple instructions is generated.

However, the above example could be rewritten in the following way:
unsigned next_trivial_optim() {
    static int i = 0;

    auto ret = i;
    ++i;
    if (i == 10) { i = 0; }
    return ret;
}

For the above code snippet a very short and clear assembly is produced, without
any multiplications and unnecessary instructions:
  mov eax, DWORD PTR next_trivial_optim()::i[rip]
  mov ecx, 0
  lea edx, [rax+1]
  cmp eax, 9
  cmove edx, ecx
  mov DWORD PTR next_trivial_optim()::i[rip], edx
  ret

Please, add an optimization to do that transformation.