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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] unnecessary clearing of bits with 16-bit value"
href="https://bugs.llvm.org/show_bug.cgi?id=39840">39840</a>
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<th>Summary</th>
<td>[x86] unnecessary clearing of bits with 16-bit value
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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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>spatel+llvm@rotateright.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>As discussed here:
<a href="https://reviews.llvm.org/D54640">https://reviews.llvm.org/D54640</a>
We have this regression test in:
test/CodeGen/X86/clear-lowbits.ll
define i16 @clear_lowbits16_ic0(i16 %val, i16 %numlowbits) nounwind {
%numhighbits = sub i16 16, %numlowbits
%mask = shl i16 -1, %numhighbits
%masked = and i16 %mask, %val
ret i16 %masked
}
$ llc -o - lowbits.ll
movzwl %di, %eax <--- don't need this
movl $16, %ecx
subl %esi, %ecx
shrl %cl, %eax
shll %cl, %eax
retq
(Note that D54640 will modify that output slightly, but not the leading
'movzwl'.)</pre>
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