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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] parity op gets transformed to popcount"
href="https://bugs.llvm.org/show_bug.cgi?id=52093">52093</a>
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<th>Summary</th>
<td>[x86] parity op gets transformed to popcount
</td>
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<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>
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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>
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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, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<p>
<div>
<pre>define i32 @xor4_popcount(i32 %x) {
%m = and i32 %x, 15
%p = tail call i32 @llvm.ctpop.i32(i32 %m)
%r = and i32 %p, 1
ret i32 %r
}
declare i32 @llvm.ctpop.i32(i32)
--------------------------------------------------------------------
% llc -o - pop.ll
xorl %eax, %eax
testb $15, %dil
setnp %al
% llc -o - pop.ll -mattr=popcnt
andl $15, %edi
popcntl %edi, %eax
andl $1, %eax
--------------------------------------------------------------------
Debug spew shows that we convert to a parity node either way, but then convert
back to ctpop for a target that supports that instruction.
Test and set likely has better latency/throughput than popcnt + 2 mask
instructions on all recent x86 CPUs.</pre>
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</p>
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