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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/108625] New: forwprop introduces new UB
Date: Wed, 01 Feb 2023 09:10:37 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108625

            Bug ID: 108625
           Summary: forwprop introduces new UB
           Product: gcc
           Version: 13.0
            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: ---

Consider the function

  unsigned char foo(int x)
  {
    int t =3D -x;
    unsigned char t1 =3D t;
    unsigned char t2 =3D t;
    return t1 + t2;
  }

This does not invoke undefined behavior when called as foo(0x40000001),
but forwprop1 optimizes this to

  unsigned char foo (int x)
  {
    int t;
    unsigned char _5;
    int _7;

    <bb 2> :
    t_2 =3D -x_1(D);
    _7 =3D t_2 - x_1(D);
    _5 =3D (unsigned char) _7;
    return _5;
  }

where _7 has signed overflow for x =3D 0x40000001.=