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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 vpermps instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=34370">34370</a>
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<th>Summary</th>
<td>[X86][AVX512] suboptimal shuffle sequence instead of one vpermps instruction
</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>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>
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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>ayman.musa@intel.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
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<pre>For the following IR:
define <8 x float> @test(<8 x float>* %vp, <8 x float> %vec2) {
%vec = load <8 x float>, <8 x float>* %vp
%shuf = shufflevector <8 x float> %vec, <8 x float> undef, <8 x i32> <i32 7,
i32 6, i32 3, i32 0, i32 7, i32 6, i32 3, i32 0>
%res = select <8 x i1> <i1 1, i1 1, i1 1, i1 0, i1 1, i1 1, i1 0, i1 0>, <8
x float> %shuf, <8 x float> %vec2
ret <8 x float> %res
}
<span class="quote">>> llc -mcpu=skx <file-name> -o out.s</span >
LLVM emits (showing 3.00 throughput on IACA tool):
vpermilps $59, (%rdi), %ymm1 # ymm1 = mem[3,2,3,0,7,6,7,4]
vpermpd $102, %ymm1, %ymm1 # ymm1 = ymm1[2,1,2,1]
movb $55, %al
kmovd %eax, %k1
vmovaps %ymm1, %ymm0 {%k1}
retq
While it can be replaced with (showing 2.86 throughput on IACA tool):
vmovaps .LCPI3327_0(%rip), %ymm1 # ymm1 = [7,6,3,0,7,6,3,0]
movb $55, %al
kmovd %eax, %k1
vpermps (%rdi), %ymm1, %ymm0 {%k1}
retq
** Throughput results from IACA tool => lower is better.</pre>
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