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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [InstCombine] Replacing ((xor A, B) != 0) with (A != B) is not always good"
href="https://llvm.org/bugs/show_bug.cgi?id=26465">26465</a>
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<th>Summary</th>
<td>[InstCombine] Replacing ((xor A, B) != 0) with (A != B) is not always good
</td>
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<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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>twoh@fb.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>InstCombiner::visitICmpInstWithInstAndIntCst replaces ((xor A, B) != 0) with (A
!= B). However, this replacement generates suboptimal code if xor has uses
other than the compare.
*** Example code
...
char c = lhs ^ rhs;
if (k == 0) return true;
if (k != 32) return false;
...
*** What LLVM generates now
movzbl (%rbx),%eax
movzbl %r8b,%ecx
cmp %ecx,%eax
je 400c50
mov %r8b,%cl
xor %al,%cl
movzbl %cl,%ecx
cmp $0x20,%ecx
jne 400cb0
*** What LLVM generates if the replacement performed only when xor has one use
mov (%rbx),%al
mov %r13b,%cl
xor %al,%cl
je 400c48
movzbl %cl,%ecx
cmp $0x20,%ecx
jne 400ca0</pre>
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