<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/66751>66751</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Large compile time regression with new MatchTable combiner
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            llvm:globalisel
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          aemerson
      </td>
    </tr>
</table>

<pre>
    It seems we forgot to measure the compile time impact of the new MT combiner. When it was enabled for AArch64 in July it resulted in a large -Os 9% regression on CTMark/sqlite3 (and others too but I'm just using sqlite3 as a test case). Some time soon afterwards that regression went down to 3-4% but I haven't identified the commit responsible for that improvement.

However, even 3% is a very large CT regression.

The measurements were taken using a release + noasserts build of clang, without LTO/PGO on trunk.

Unfortunately some quick profiling doesn't show much except that `ExecuteMatchTable()` is taking most of the time in the combiner, which isn't surprising.
</pre>
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