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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Some 2-vector vector select identity shuffles may be better represented as moves"
href="https://bugs.llvm.org/show_bug.cgi?id=41433">41433</a>
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<th>Summary</th>
<td>Some 2-vector vector select identity shuffles may be better represented as moves
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Split off from <a class="bz_bug_link
bz_status_NEW "
title="NEW - vector select optimization failures?"
href="show_bug.cgi?id=41429">https://bugs.llvm.org/show_bug.cgi?id=41429</a>
<a href="https://godbolt.org/z/_n1ggH">https://godbolt.org/z/_n1ggH</a>
void example(__m256i * __restrict__ dest, const __m256i * __restrict__ a) {
(*dest)[2] = (*a)[2];
(*dest)[3] = (*a)[3];
}
Here we do not ever touch the low half of dest, and replace the high part
of dest with high part of `a`.
The naive asm could be:
vmovaps ymm0, ymmword ptr [rdi]
vblendps ymm0, ymm0, ymmword ptr [rsi], 240 # ymm0 =
ymm0[0,1,2,3],mem[4,5,6,7]
vmovaps ymmword ptr [rdi], ymm0
vzeroupper
But we can also produce:
vmovaps xmm0, xmmword ptr [rsi + 16]
vmovaps xmmword ptr [rdi + 16], xmm0
I'm not quite sure what are the exact criteria when that is profitable to do
though.</pre>
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