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

[barry.revzin at gmail dot com]

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

            Bug ID: 92985
           Summary: missed optimization opportunity for switch linear
                    transformation
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

From Peter Dimov on Slack:

#include <cstddef>

struct X
{
    int x, y, z;

    int operator[]( std::size_t i ) const noexcept
    {
        switch( i )
        {
            case 0: return x;
            case 1: return y;
            case 2: return z;
            default: __builtin_unreachable();
        }
    }
};

int f( X const& x, std::size_t i )
{
    return x[ i ];
}

compiled with -O2 (or -O3) emits:

f(X const&, unsigned long):
        cmp     rsi, 1
        je      .L2
        cmp     rsi, 2
        jne     .L6
        mov     eax, DWORD PTR [rdi+8]
        ret
.L2:
        mov     eax, DWORD PTR [rdi+4]
        ret
.L6:
        mov     eax, DWORD PTR [rdi]
        ret

But it should be able to just emit:
        mov     eax, DWORD PTR [rdi+rsi*4]
        ret