[Bug c++/84436] New: Missed optimization with switch on enum constants returning the same value

[mferoldif at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84436

            Bug ID: 84436
           Summary: Missed optimization with switch on enum constants
                    returning the same value
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mferoldif at gmail dot com
  Target Milestone: ---

The following snippet:

    enum class E
    {
        A, B, C,
    };

    int foo(E e)
    {
        switch (e)
        {
            case E::A: return 0;
            case E::B: return 1;
            case E::C: return 2;
        }
    }

Produces the following ineffective code:

    foo(E):
      cmp edi, 1
      mov eax, 1
      je .L1
      cmp edi, 2
      mov eax, 2
      je .L1
      xor eax, eax
    .L1:
      rep ret

There is no reason why it should not produce a single `mov eax, edi`
and ret instruction.