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From: "chfast at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113764] New: [X86] Generates lzcnt when bsr is
 sufficient
Date: Mon, 05 Feb 2024 11:38:07 +0000
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X-Bugzilla-Version: 13.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113764

            Bug ID: 113764
           Summary: [X86] Generates lzcnt when bsr is sufficient
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chfast at gmail dot com
  Target Milestone: ---

When lzcnt instructions is enabled (-mlzcnt) the compiler generates lzcnt f=
or
__builtin_clz() in the context where the bsr instruction is sufficient and
better.

unsigned bsr(unsigned x)
{
    return __builtin_clz(x) ^ 31;
}

bsr:
  xor eax, eax
  lzcnt eax, edi
  xor eax, 31
  ret


Without -mlzcnt the generated code is optimal.

bsr:
  bsr eax, edi
  ret


https://godbolt.org/z/5qcTq18nr=