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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [AVX] bloated vector permute mask generation"
href="https://llvm.org/bugs/show_bug.cgi?id=23073">23073</a>
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<th>Summary</th>
<td>[AVX] bloated vector permute mask generation
</td>
</tr>
<tr>
<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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>$ cat badperm.ll
define <8 x float> @shuffle_v8f32_70000000(<8 x float> %a, <8 x float> %b) {
%shuffle = shufflevector <8 x float> %a, <8 x float> %b, <8 x i32> <i32 7,
i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0>
ret <8 x float> %shuffle
}
$ ./llc -mattr=avx2 badperm.ll -o -
...
movl $7, %eax
vmovd %eax, %xmm1
vpxor %ymm2, %ymm2, %ymm2
vpblendd $1, %ymm1, %ymm2, %ymm1 ## ymm1 = ymm1[0],ymm2[1,2,3,4,5,6,7]
vpermps %ymm0, %ymm1, %ymm0
retq
As noticed by Andrea in <a href="http://reviews.llvm.org/D8341">http://reviews.llvm.org/D8341</a>, this could've been a
vmovaps + vpermd (or vpermps depending on int/fp domain).
Or since we know that all of the high elements of the mask are zeros, the
permute mask could be shortened to a single 32-bit constant '7' load (vmovd
instead of vmovaps)?</pre>
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