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From: "krebbel at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/94613] New: combine: Wrong code due to
 splitting a simplified IF_THEN_ELSE
Date: Thu, 16 Apr 2020 07:01:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94613

            Bug ID: 94613
           Summary: combine: Wrong code due to splitting a simplified
                    IF_THEN_ELSE
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krebbel at gcc dot gnu.org
  Target Milestone: ---

Created attachment 48284
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48284&action=3Dedit
autoreduced testcase

When compiling the attached testcase for IBM Z GCC optimizes an OR operation
away during combine leading to wrong code being generated:

cc1plus -O3 -march=3Dz14 t.ii -quiet -o t.s

try_combine is invoked for the following 3 insns:

i1: 28: r111 =3D r110 | r158
i2: 31: r115 =3D r111 - r94
i3: 36: r122 =3D (r110 =3D=3D 0 ? r115 : r177)

i3 becomes the following after inserting i1 and i2

36: r122 =3D (r110 =3D=3D 0 ? r110 | r158 - r94 : r177)

simplification omits ORing r110 since it is 0 anyway:

36: r122 =3D (r110 =3D=3D 0 ? r158 - r94 : r177)

after not being able to recognize the insn it gets split into parts again:

28: deleted
31: r115 =3D r158 - r94
36: r122 =3D (r110 =3D=3D 0 ? r115 : r177)=