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From: "andrew.cooper3 at citrix dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/104971] New: Optimisation for
 __builtin_ia32_readeflags corrupts the stack
Date: Thu, 17 Mar 2022 18:49:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104971

            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 =3D __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 introduc=
ed
this builtin) has correct optimised code generation, and it regressed betwe=
en
4.9 and 5.1.=