[Bug middle-end/104971] New: Optimisation for __builtin_ia32_readeflags corrupts the stack

["andrew.cooper3 at citrix dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104971

            Bug ID: 104971
           Summary: Optimisation for __builtin_ia32_readeflags corrupts
                    the stack
           Product: gcc
           Version: 12.0
            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: ---

Full example: https://godbolt.org/z/xGq3c4Mnc

Given:

int broken(void)
{
    int fl = __builtin_ia32_readeflags_u64();
}

gcc -O2 generates:

broken:
        pushfq
        ret

Which is going explode very quickly.

Code generation appears to be safe without optimisation, but even -O alone is
enough to create problems.

At a guess, the optimiser has concluded that the result is unused, drops the
`pop %reg`, but fails to also drop the `pushf` too.

Looking through history on Godbolt, it appears that GCC 4.9 (which introduced
this builtin) has correct optimised code generation, and it regressed between
4.9 and 5.1.