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From: "manolis.tsamis at vrull dot eu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114326] New: Missed optimization for A || B
 when !B implies A.
Date: Wed, 13 Mar 2024 14:19:19 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114326

            Bug ID: 114326
           Summary: Missed optimization for A || B when !B implies A.
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manolis.tsamis at vrull dot eu
  Target Milestone: ---

The function below doesn't fold to return 0;

int cmp1(uint64_t d1, uint64_t d2) {
  if (((d1 ^ d2) & 0xabcd) =3D=3D 0 || d1 !=3D d2)
    return 0;
  return foo();
}

while the following function does:=20

int cmp2(uint64_t d1, uint64_t d2) {
  if (d1 !=3D d2 || ((d1 ^ d2) & 0xabcd) =3D=3D 0)
    return 0;
  return foo();
}

The functions are equivalent since the lhs and rhs of || don't have side
effects.

In general, there pattern here is a side-effect free expression a || b wher=
e !b
implies a should be optimized to true. As in the testcase above, a doesn't
necessarily imply !b. Something similar could be stated for && expressions.

Complementary godbolt link: https://godbolt.org/z/qK5bYf36T=