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From: "kurkindmit at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96480] New: missed optimisation: unnecessary compare in
 standard algorithms
Date: Wed, 05 Aug 2020 08:53:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96480

            Bug ID: 96480
           Summary: missed optimisation: unnecessary compare in standard
                    algorithms
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kurkindmit at gmail dot com
  Target Milestone: ---

Consider the C++ code:

#include <algorithm>
#include <array>

enum ENUM { A, B, C, D, E, F, G, };

const std::array<ENUM, 4> foo{A, B, C, D};

bool is_foo(ENUM e) {
    return std::any_of(foo.begin(), foo.end(),
    [e] (auto ee) { return e =3D=3D ee; });
}

GCC 7.4 optimizes if_foo to a single compare operation e <=3D 3,
but newer GCC versions do two comare operations e <=3D 2 || e =3D=3D 3.

see https://godbolt.org/z/5a6aPa=