<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 - Clang-cl inefficient assembly, 'inc' not generated"
   href="https://bugs.llvm.org/show_bug.cgi?id=37912">37912</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Clang-cl inefficient assembly, 'inc' not generated
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>6.0
          </td>
        </tr>

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

        <tr>
          <th>OS</th>
          <td>Windows NT
          </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>-New Bugs
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>jvapen@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org
          </td>
        </tr></table>
      <p>
        <div>
        <pre>Unfortunately, I am unable to share all details for this.
However in investigating why the generated executable by clang-cl is 10% slower
than the one of MSVC, I did notice the following already when comparing the
generated assembly:

MSVC uses specialized inc/dec instructions while clang uses add instructions:
        MSVC: incq   0x20(%rbx)
        Clang: addq   $0x1,(%rcx)
        MSVC: inc    %rcx
        Clang: add    $0x1,%rdx
        MSVC: dec    %eax
        Clang: add    $0xffffffff,%edi

File was compiled with:
clang-cl.exe -fms-compatibility-version=19.11 /DBOOST_USE_WINDOWS_H
-Weverything -Wno-unused-command-line-argument /nologo /c /W4 /Wall /wd4710
/wd4711 /GR /EHsc /favor:INTEL64 /fp:precise /FS /std:c++17  /Ox /Z7 /MD
/bigobj  t.cpp 

Note that /favor:INTEL64 ain't recognized  by clang, which might be the reason
the inc instruction doesn't get selected.

At the same time, it is remarkable that for most functions, more non-volotile
registers get pushed/popped by clang than by MSVC. In the hot path, I've
noticed 8 pop instructions for clang and 3 for MSVC.</pre>
        </div>
      </p>


      <hr>
      <span>You are receiving this mail because:</span>

      <ul>
          <li>You are on the CC list for the bug.</li>
      </ul>
    </body>
</html>