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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86][AVX] suboptimal splats of high elements of 256-bit vectors"
href="https://llvm.org/bugs/show_bug.cgi?id=23027">23027</a>
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<th>Summary</th>
<td>[X86][AVX] suboptimal splats of high elements of 256-bit vectors
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>AVX shuffling of 256-bit vectors isn't supported very well in hardware. It
looks like real instructions for this don't show up until AVX2.
But I think we can do better than this by using vpermild + vperm2f128:
define <4 x double> @shuffle_splat_v4f64_3(<4 x double> %v) {
%z = shufflevector <4 x double> %v, <4 x double> undef, <4 x i32> <i32 3, i32
3, i32 3, i32 3>
ret <4 x double> %z
}
$ ./llc -o - -mattr=avx splat256.ll
...
vextractf128 $1, %ymm0, %xmm0
vmovhlps %xmm0, %xmm0, %xmm0 ## xmm0 = xmm0[1,1]
vinsertf128 $1, %xmm0, %ymm0, %ymm0
retq</pre>
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</p>
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