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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Use vpshuflb instead of vpshuflw/vpshufhw (AMD Zen)"
href="https://bugs.llvm.org/show_bug.cgi?id=44795">44795</a>
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<th>Summary</th>
<td>Use vpshuflb instead of vpshuflw/vpshufhw (AMD Zen)
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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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>david.bolvansky@gmail.com
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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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<pre>Orignally reported here:
<a href="https://www.realworldtech.com/forum/?threadid=176424&curpostid=176424&fbclid=IwAR0P-wuQYtb7X9C8K8cr-axqP4Gwh1ZRSz-yU69bm5-NTZHdud-Nvp5ypec">https://www.realworldtech.com/forum/?threadid=176424&curpostid=176424&fbclid=IwAR0P-wuQYtb7X9C8K8cr-axqP4Gwh1ZRSz-yU69bm5-NTZHdud-Nvp5ypec</a>
"In above example clang compiler replaced each of 4 vpshuflb instructions,
specified in the source code, by pair of instruction vpshuflw/vpshufhw. Since
the execution time is bottlenecked by 'Port 5' execution bandwidth, the
smart-assness of compiler leads to approximate doubling of execution time.
This particular Haswell/Broadwell/Skylake code generation bug is fixed in
clang6 (clang6 code looks horrible for other reason), but virtually identical
AMD Zen code generation bug is not fixed.
I have no Zen to measure, but according to Agner Fog's instruction tables 'Pipe
12 bottleneck' has approximately the same effect as 'Port 5 bottleneck' on
Intel parts."
<a href="https://godbolt.org/g/ZjjCmY">https://godbolt.org/g/ZjjCmY</a></pre>
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