[Bug c++/92985] missed optimization opportunity for switch linear transformation

[pdimov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92985

--- Comment #2 from Peter Dimov <pdimov at gmail dot com> ---
Two more reformulations that activate the linear transformation are:

    int operator[]( std::size_t i ) const noexcept
    {
        std::ptrdiff_t offset;

        switch( i )
        {
            case 0: offset = offsetof(X, x); break;
            case 1: offset = offsetof(X, y); break;
            case 2: offset = offsetof(X, z); break;
            default: __builtin_unreachable();
        }

        return *(int const*)((char const*)this + offset);
    }

(https://godbolt.org/z/cJDB_m)

and

    int operator[]( std::size_t i ) const noexcept
    {
        int X::* p;

        switch( i )
        {
            case 0: p = &X::x; break;
            case 1: p = &X::y; break;
            case 2: p = &X::z; break;
            default: __builtin_unreachable();
        }

        return this->*p;
    }

(https://godbolt.org/z/xfsKh5)