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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/106884] New: ifcombine may move shift so it
 shifts more than bitwidth
Date: Thu, 08 Sep 2022 00:20:48 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: kristerw at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106884

            Bug ID: 106884
           Summary: ifcombine may move shift so it shifts more than
                    bitwidth
           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: ---

The function foo from gcc.dg/tree-ssa/ssa-ifcombine-1.c can be called
as foo(1, 1, 33) without invoking undefined behavior

int foo (int x, int a, int b)
{
  int c =3D 1 << a;
  if (x & c)
    if (x & (1 << b))
      return 2;
  return 0;
}

But ifcombine transforms this to

int foo (int x, int a, int b)
{
   int c;
   int _4;
   int _10;
   int _11;
   int _12;
   int _13;

   <bb 2>:
   _10 =3D 1 << b_8(D);
   _11 =3D 1 << a_5(D);
   _12 =3D _10 | _11;
   _13 =3D x_7(D) & _12;
   if (_12 =3D=3D _13)
     goto <bb 3>;
   else
     goto <bb 4>;

   <bb 3>:

   <bb 4>:
   # _4 =3D PHI <2(3), 0(2)>
   return _4;
}

and this will now calculate 1 << 33 unconditionally for _10.=