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From: "klaus.doldinger64 at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107622] Missing optimization of switch-statement
Date: Fri, 11 Nov 2022 05:01:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107622

--- Comment #4 from Wilhelm M <klaus.doldinger64 at googlemail dot com> ---
In the following class the static data member is accessible via explicit
template instantiation from the outside. So the compiler cannot reason that=
 the
value is in [0,2]. But this does not hold for the function g(): here mState=
2 is
definitely not accessible from outside. But here the std::unreachable() is
still neccessary. here the optimization is missing.=20

struct FSM {
    enum class State : uint8_t {A, B, C};
    static void f() {
        switch(mState) {
        case State::A:
            o =3D 10;
            break;
        case State::B:
            o =3D 11;
            break;
        case State::C:
            o =3D 12;
            break;
        default:
    //        std::unreachable();
            break;
        }
    }
    static void g() {
        static State mState2{State::A}; // not accessible from outside
        switch(mState2) {
        case State::A:
            o =3D 10;
            break;
        case State::B:
            o =3D 11;
            break;
        case State::C:
            o =3D 12;
            break;
        default:
    //        std::unreachable();
            break;
        }
    }
private:
    inline static State mState{State::A}; // still modifyable via explicit
template instantiation
};=