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From: "kristerw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110434] New: tree-nrv introduces incorrect
 CLOBBER(eol)
Date: Tue, 27 Jun 2023 10:48:55 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: kristerw at gcc dot gnu.org
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110434

            Bug ID: 110434
           Summary: tree-nrv introduces incorrect CLOBBER(eol)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
  Target Milestone: ---

The tree-nrv pass may introduce incorrect CLOBBER(eol) of the form
  <retval> =3D{v} {CLOBBER(eol)};
  return <retval>;

One example of this can be seen by compiling gcc.c-torture/execute/921204-1=
.c
for x86 using the flags "-O -m32", where it changes the IR

  union bu o;
  ...
  o =3D i;
  MEM[(union  *)&o].b18 =3D _11;
  MEM[(union  *)&o].b20 =3D _11;
  <retval> =3D o;
  o =3D{v} {CLOBBER(eol)};
  return <retval>;

to just use <retval> instead of o

  union bu o [value-expr: <retval>];
  ...
  <retval> =3D i;
  MEM[(union  *)&<retval>].b18 =3D _11;
  MEM[(union  *)&<retval>].b20 =3D _11;
  <retval> =3D{v} {CLOBBER(eol)};
  return <retval>;

so the CLOBBER(eol) now refers to <retval>.=