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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/95867] New: Failure to optimize successive
 multiplications of ___uint128_t
Date: Wed, 24 Jun 2020 12:40:36 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95867

            Bug ID: 95867
           Summary: Failure to optimize successive multiplications of
                    ___uint128_t
           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: ---

__uint128_t f(__uint128_t n)
{
    return
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*=
n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n=
*n*n*n*n*n*n*n*n*n*n*n*n*n*n*n;
}

This can be optimized to=20

__uint128_t f2(__uint128_t n)
{
    __uint128_t tmp =3D n * n;
    tmp =3D (tmp * tmp) * n;
    tmp *=3D tmp;
    tmp *=3D tmp;
    tmp *=3D tmp;
    tmp =3D (tmp * tmp) * n;
    tmp =3D (tmp * tmp) * n;
    tmp *=3D tmp;
    return (tmp * n) * tmp;
}

This transformation is done by LLVM, but not by GCC.=