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From: "llvm at rifkin dot dev" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106727] New: Missed fold / canonicalization
 for checking if a number is a power of 2
Date: Wed, 24 Aug 2022 02:51:21 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.1.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106727

            Bug ID: 106727
           Summary: Missed fold / canonicalization for checking if a
                    number is a power of 2
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

These two are equivalent but generate different code:

bool foo(unsigned n) {
    return std::popcount(n) <=3D 1;
}
bool bar(unsigned n) {
    return (n & (n - 1)) =3D=3D 0;
}

https://godbolt.org/z/crrxnfWo7

For systems that don't have popcount instructions it would be very benefici=
al
to transform foo -> bar. For systems that do have popcount instructions it
still would be beneficial to canonicalize the two.=