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From: "felix.von.s at posteo dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94757] New: GCC does not optimise unsigned
 multiplication known not to overflow
Date: Sat, 25 Apr 2020 08:55:08 +0000
X-Bugzilla-Reason: CC
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: felix.von.s at posteo dot de
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94757

            Bug ID: 94757
           Summary: GCC does not optimise unsigned multiplication known
                    not to overflow
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.von.s at posteo dot de
  Target Milestone: ---

This code:

    #include <limits.h>

    unsigned f(unsigned x) {
        if (x > UINT_MAX / 3)
            __builtin_unreachable();
        return (x * 3) / 3;
    }

should be possible to optimise into the identity function, but (on x86_64) =
gcc
-O3 generates a multiplication (via LEA) then a division (via multiplicatio=
n).
GCC knows that the multiplication cannot overflow, because replacing the
returned expression with __builtin_mul_overflow_p(x, 3, x) is makes it opti=
mise
to returning constant 0.=