[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 #1 from Peter Dimov <pdimov at gmail dot com> ---
Reformulating the switch in terms of integral offsets

struct X2
{
    int x, y, z;

    int operator[]( std::size_t i ) const noexcept
    {
        std::ptrdiff_t k0 = &x - &x;
        std::ptrdiff_t k1 = &y - &x;
        std::ptrdiff_t k2 = &z - &x;

        std::ptrdiff_t k;

        switch( i )
        {
            case 0: k = k0; break;
            case 1: k = k1; break;
            case 2: k = k2; break;
            default: __builtin_unreachable();
        }

        return *( &x + k );
    }
};

results in the desired

f2(X2 const&, unsigned long):
        mov     eax, DWORD PTR [rdi+rsi*4]
        ret

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