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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] shuffle lowering should try harder to match unpack"
href="https://bugs.llvm.org/show_bug.cgi?id=40434">40434</a>
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<th>Summary</th>
<td>[x86] shuffle lowering should try harder to match unpack
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>enhancement
</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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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>define <8 x i32> @shuf_unpack(<8 x i32> %x, <8 x i32> %y) {
%unpckh = shufflevector <8 x i32> %x, <8 x i32> %y, <8 x i32> <i32 2, i32 14,
i32 3, i32 15, i32 undef, i32 undef, i32 undef, i32 undef>
ret <8 x i32> %unpckh
}
This translates to what I think is the optimal extract+unpckhps with AVX1:
$ llc -o - unpckh.ll -mattr=avx
vextractf128 $1, %ymm1, %xmm1
vunpckhps %xmm1, %xmm0, %xmm0 ## xmm0 =
xmm0[2],xmm1[2],xmm0[3],xmm1[3]
retq
But with AVX2, the availability of more shuffle instructions leads to a worse
outcome:
$ llc -o - unpckh.ll -mattr=avx2
vblendps $192, %ymm1, %ymm0, %ymm0 ## ymm0 =
ymm0[0,1,2,3,4,5],ymm1[6,7]
vmovaps LCPI0_0(%rip), %ymm1 ## ymm1 = <2,6,3,7,u,u,u,u>
vpermps %ymm0, %ymm1, %ymm0
retq</pre>
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