[Bug c++/107622] New: Missing optimization of switch-statement

["klaus.doldinger64 at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107622

            Bug ID: 107622
           Summary: Missing optimization of switch-statement
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: klaus.doldinger64 at googlemail dot com
  Target Milestone: ---

In the following example the default-case could not be reached. Therefore
introducing std::unreachable() should be useless. But the compiler produces
slightly better code with std::unreachable() as it removes one (unneccessary)
comparison (tested for x86 and avr targets).

volatile uint8_t o;

enum class State : uint8_t {A, B, C};

void g(const State s) {
    switch(s) {
    case State::A:
        o = 10;
        break;
    case State::B:
        o = 11;
        break;
    case State::C:
        o = 12;
        break;
    default:
//        std::unreachable(); // __builtin_unreachable();
        break;
    }
}