'[llvm-bugs] [Bug 43513] New: Missing 32 - __builtin_popcount(x | -x) -> __builtin_ctz(x)'

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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 43513] New: Missing 32 - __builtin_popcount(x | -x) -> __builtin_ctz(x)
From:       via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2019-09-30 20:07:06
Message-ID: bug-43513-206 () http ! bugs ! llvm ! org/
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Date: Mon, 30 Sep 2019 13:07:06 -0700
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https://bugs.llvm.org/show_bug.cgi?id=43513

            Bug ID: 43513
           Summary: Missing 32 - __builtin_popcount(x | -x) ->
                    __builtin_ctz(x)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs@nondot.org
          Reporter: david.bolvansky@gmail.com
                CC: llvm-bugs@lists.llvm.org

-O3 -march=haswell

int foo(int i) {
    return i == 0 ? 32 : __builtin_ctz(i);
    // return __builtin_ctz(i);
}

int food(int i) {
    return 32 - __builtin_popcount(i | -i);
}

int foodd(int i) {
    return __builtin_popcount(~i & (i-1));
}

foo(int):                                # @foo(int)
        tzcnt   eax, edi
        ret
food(int):                               # @food(int)
        mov     eax, edi
        neg     eax
        or      eax, edi
        popcnt  ecx, eax
        mov     eax, 32
        sub     eax, ecx
        ret

foodd(int):                              # @foodd(int)
        lea     eax, [rdi - 1]
        andn    eax, edi, eax
        popcnt  eax, eax
        ret

https://godbolt.org/z/iNeY43

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Date: Mon, 30 Sep 2019 13:07:06 -0700
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<html>
    <head>
      <base href="https://bugs.llvm.org/">
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Missing 32 - __builtin_popcount(x | -x) -&gt; __builtin_ctz(x)"
   href="https://bugs.llvm.org/show_bug.cgi?id=43513">43513</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Missing 32 - __builtin_popcount(x | -x) -&gt; __builtin_ctz(x)
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libraries
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>trunk
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>Transformation Utilities
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedbugs&#64;nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>david.bolvansky&#64;gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs&#64;lists.llvm.org
          </td>
        </tr></table>
      <p>
        <div>
        <pre>-O3 -march=haswell

int foo(int i) {
    return i == 0 ? 32 : __builtin_ctz(i);
    // return __builtin_ctz(i);
}

int food(int i) {
    return 32 - __builtin_popcount(i | -i);
}

int foodd(int i) {
    return __builtin_popcount(~i &amp; (i-1));
}

foo(int):                                # &#64;foo(int)
        tzcnt   eax, edi
        ret
food(int):                               # &#64;food(int)
        mov     eax, edi
        neg     eax
        or      eax, edi
        popcnt  ecx, eax
        mov     eax, 32
        sub     eax, ecx
        ret

foodd(int):                              # &#64;foodd(int)
        lea     eax, [rdi - 1]
        andn    eax, edi, eax
        popcnt  eax, eax
        ret

<a href="https://godbolt.org/z/iNeY43">https://godbolt.org/z/iNeY43</a></pre>
        </div>
      </p>


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