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From: "ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/115667] New: Improve expansion for popcountti2
Date: Wed, 26 Jun 2024 14:36:35 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 13.3.1
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ktkachov at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115667

            Bug ID: 115667
           Summary: Improve expansion for popcountti2
           Product: gcc
           Version: 13.3.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

Maybe this is aarch64-specific but for the testcase:
int
cnt (unsigned __int128 a)
{
  return __builtin_popcountg (a);
}

GCC for aarch64 will generate:
cnt:
        fmov    d30, x0
        fmov    d31, x1
        cnt     v30.8b, v30.8b
        cnt     v31.8b, v31.8b
        addv    b30, v30.8b
        addv    b31, v31.8b
        fmov    x1, d30
        fmov    x0, d31
        add     w0, w1, w0
        ret

Effectively doing two DImode popcount expansions and adding the results.
Clang does the more effective:
cnt:                                    // @cnt
        fmov    d0, x0
        mov     v0.d[1], x1
        cnt     v0.16b, v0.16b
        uaddlv  h0, v0.16b
        fmov    w0, s0
        ret=