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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][AVX512] suboptimal shuffle sequence instead of one vpermw instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=34369">34369</a>
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<th>Summary</th>
<td>[X86][AVX512] suboptimal shuffle sequence instead of one vpermw instruction
</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>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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
</td>
</tr>
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<th>Reporter</th>
<td>ayman.musa@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>For the following IR:
define <16 x i16> @test(<16 x i16> %vec) {
%shuf = shufflevector <16 x i16> %vec, <16 x i16> undef, <16 x i32> <i32 3,
i32 0, i32 0, i32 13, i32 5, i32 2, i32 2, i32 10, i32 15, i32 8, i32 14, i32
8, i32 9, i32 10, i32 12, i32 12>
%res = select <16 x i1> <i1 1, i1 1, i1 1, i1 0, i1 1, i1 0, i1 0, i1 1, i1
1, i1 1, i1 0, i1 1, i1 1, i1 1, i1 1, i1 1>, <16 x i16> %shuf, <16 x i16>
zeroinitializer
ret <16 x i16> %res
}
<span class="quote">>> llc -mcpu=skx <file-name> -o out.s</span >
LLVM emits (showing 9.52 throughput on IACA tool):
vextracti128 $1, %ymm0, %xmm1
vpshufb .LCPI41_0(%rip), %xmm1, %xmm2 # xmm2 =
xmm1[8,9,10,11,4,5,10,11,8,9,10,11,4,5,4,5]
vpshufb .LCPI41_1(%rip), %xmm0, %xmm0 # xmm0 =
xmm0[6,7,0,1,0,1,6,7,10,11,4,5,4,5,6,7]
vpblendw $136, %xmm2, %xmm0, %xmm0 # xmm0 =
xmm0[0,1,2],xmm2[3],xmm0[4,5,6],xmm2[7]
vpshufb .LCPI41_2(%rip), %xmm1, %xmm1 # xmm1 =
xmm1[14,15,0,1,12,13,0,1,2,3,4,5,8,9,8,9]
vinserti128 $1, %xmm1, %ymm0, %ymm0
movw $-1129, %ax # imm = 0xFB97
kmovd %eax, %k1
vmovdqu16 %ymm0, %ymm0 {%k1} {z}
retq
While it can be replaced with (showing 5.76 throughput on IACA tool):
vmovdqu .LCPI43_0(%rip), %ymm1 # ymm1 =
[3,0,0,13,5,2,2,10,15,8,14,8,9,10,12,12]
movw $-1129, %ax
kmovd %eax, %k1
vpermw %ymm0, %ymm1, %ymm0 {%k1} {z}
retq
** Throughput results from IACA tool => lower is better.</pre>
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