<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/58323>58323</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Suboptimal lowering of `vget_high_XXX` on AArch64
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    Clang/LLVM lowers `a = vget_high_XXX(b)` NEON intrinsics to `EXT Va.16B, Vb.16B, Vb.16B, #8` instead of the `DUP Va.1D, Vb.D[1]` suggested by [ARM NEON intrinsics guide](https://developer.arm.com/architectures/instruction-sets/intrinsics/). This lowering has two drawbacks:
- On AArch64 cores which split NEON instructions into 64-bit microoperations, `EXT Va.16B, Vb.16B, Vb.16B, #8` generates two 64-bit microoperations instead of one.
- On all AArch64 cores `EXT Va.16B, Vb.16B, Vb.16B, #8` leaves the upper part of the destination register initialized (but unused), thereby preventing its power-gating.
</pre>
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