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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/94800] New: Failure to optimize yet another
 popcount idiom
Date: Mon, 27 Apr 2020 14:42:31 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94800

            Bug ID: 94800
           Summary: Failure to optimize yet another popcount idiom
           Product: gcc
           Version: 10.0
            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 populationCount(uint32_t x)
{
    x =3D x - ((x >> 1) & 0x55555555);
    x =3D (x & 0x33333333) + ((x >> 2) & 0x33333333);
    x =3D (x + (x >> 4)) & 0x0F0F0F0F;
    x +=3D (x << 8);
    return ((x + (x << 16)) >> 24);
}

This can be optimized to `__builtin_popcount(x)` (when `sizeof(int) =3D=3D
sizeof(uint32_t)`). This transformation is done by LLVM, but not by GCC

Comparison here : https://godbolt.org/z/iz9qJf=