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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/115555] New: [Ranger] deduce 'a >= 0' from  'b << a'
Date: Thu, 20 Jun 2024 07:50:39 +0000
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X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 15.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115555

            Bug ID: 115555
           Summary: [Ranger] deduce 'a >=3D 0' from  'b << a'
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: aldyh at gcc dot gnu.org
  Target Milestone: ---

Stumbled over this when looking at PR 115551.

C has for the bitwise shift operators:

"If the value of the right operand is negative or is greater than or equal =
to
the
width of the promoted left operand, the behavior is undefined."

Thus, it seems as if the ranger should be able to deduce 'a >=3D 0' in the
following, such that both conditions can be optimized away:

int f (int a, float *b) {
   *b =3D a >=3D 0 ? 1.0 : 2.0;
   int mask1 =3D 1U << a;  // 'a' must be >=3D 0 =E2=86=92 '*b=3D1.0' above=
 and 'return -1'
below
   if (a >=3D 0)
     return -1;
   return mask1;
}=