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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/98334] New: Failure to optimally optimize add
 loop to mul
Date: Wed, 16 Dec 2020 18:48:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98334

            Bug ID: 98334
           Summary: Failure to optimally optimize add loop to mul
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int i, unsigned int n) {
    int result =3D 0;
    while (n > 0) {
        result +=3D i;
        n -=3D 1;
    }
    return result;
}

GCC optimizes this function to code that effectively does `return (n =3D=3D=
 0) ? 0
: (i - 1) * n + n;`. It could instead emit the more optimal `return (n =3D=
=3D 0) ?
0 : i * n;`, or just `return i * n;` on platforms where multiplication is f=
ast.
This transformation is done by LLVM, but not by GCC.=