[Bug middle-end/92237] New: [x86] Missed optimisation opportunity with bit tests

[andrew.cooper3 at citrix dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92237

            Bug ID: 92237
           Summary: [x86] Missed optimisation opportunity with bit tests
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrew.cooper3 at citrix dot com
  Target Milestone: ---

See https://godbolt.org/z/mP-8Y7

An expression such as:

bool foo(uint64_t val)
{
    return (val & 0x120) == 0x20;
}

gets assembled to:
<foo>:
   0:   81 e7 20 01 00 00       and    $0x120,%edi
   6:   48 83 ff 20             cmp    $0x20,%rdi
   a:   0f 94 c0                sete   %al
   d:   c3                      retq


Some part of optimisation has noticed that, due to the 32bit constant, the AND
can be performed on %edi, but hasn't spotted that the same is true for the
following CMP.

In this example, the CMP could use %edi as well, and save emitting the REX
prefix into the instruction stream.