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From: "andres at anarazel dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/106590] New: x86-64 miscompilation starting with "i386:
 Improve ix86_expand_int_movcc" w/ mtune=skylake
Date: Fri, 12 Aug 2022 04:23:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106590

            Bug ID: 106590
           Summary: x86-64 miscompilation starting with "i386: Improve
                    ix86_expand_int_movcc" w/ mtune=3Dskylake
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andres at anarazel dot de
  Target Milestone: ---

Hi,

The attached reproducer shows a miscompilation I found building postgres. I=
've
bisected postgres' failure to 1ceddd7497e, but it's of course possible it's
just surfacing a prior issue.

In my reproducer, and in postgres, the problem only occurs with -mtune=3Dsk=
ylake
or higher, but I'm not sure how related that actually is.

$ /home/andres/build/gcc-master/install/bin/gcc --version
gcc (GCC) 12.0.1 20220423 (experimental)
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc-11 --version
gcc-11 (Debian 11.3.0-5) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ /home/andres/build/gcc-master/install/bin/gcc -Wall -Wextra -O1
-mtune=3Dskylake /tmp/test.i -o /tmp/test
$ /tmp/test
wrong results: procform->prorettype: 23, restype: 20

$ /home/andres/build/gcc-master/install/bin/gcc -Wall -Wextra -O1
-mtune=3Dbroadwell /tmp/test.i -o /tmp/test
$ /tmp/test
everything ok: procform->prorettype: 23, restype: 23

$ gcc-11 -Wall -Wextra -O1 -mtune=3Dskylake /tmp/test.i -o /tmp/test
$ /tmp/test
everything ok: procform->prorettype: 23, restype: 23

I think it's pretty obvious that the code should never be able to result in
restype =3D=3D 20.

Regards,

Andres=