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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	[thread overview]
Message-ID: <bug-94757-4@http.gcc.gnu.org/bugzilla/> (raw)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757

            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 multiplication).
GCC knows that the multiplication cannot overflow, because replacing the
returned expression with __builtin_mul_overflow_p(x, 3, x) is makes it optimise
to returning constant 0.

             reply	other threads:[~2020-04-25  8:55 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-25  8:55 felix.von.s at posteo dot de [this message]
2020-04-27  6:35 ` [Bug tree-optimization/94757] " rguenth at gcc dot gnu.org
2020-06-19  8:09 ` qianjh at cn dot fujitsu.com
2021-12-15 23:56 ` pinskia at gcc dot gnu.org

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