<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/124538>124538</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            GlobalISel BUILD_VECTOR hurts performance with more masks
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:AMDGPU,
            llvm:globalisel
      </td>
    </tr>

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

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

<pre>
    I'm not sure about the tradeoffs of BUILD_VECTOR, so I'm not sure how to find and avoid cases where this does benefit codegen.

https://godbolt.org/z/369z3hqv9

# To Reproduce

A command like `llc -march=amdgcn -mtriple=amdgcn-amd-amdhsa -mcpu=gfx942 -global-isel` can be used on the IR

# Symptom and Cause
Various commands like `v_and_b32_e32 v3, 0xffff, v7` appear when constructing the BUILD_VECTOR result.

If `CombinerHelper::matchCombineInsertVecElts` is disabled, these masks go away and I've seen noticeable performance improvements in the full program that I'm running.

https://github.com/llvm/llvm-project/blob/6087c3049656bbaef51fffb48e2404e86f7e0d3f/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp#L3015

</pre>
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