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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 - [InstCombine] Dropping pointless masking before shift"
href="https://bugs.llvm.org/show_bug.cgi?id=42456">42456</a>
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<tr>
<th>Summary</th>
<td>[InstCombine] Dropping pointless masking before shift
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre><a href="https://godbolt.org/z/cuuf0P">https://godbolt.org/z/cuuf0P</a>
<a href="https://rise4fun.com/Alive/PcZk">https://rise4fun.com/Alive/PcZk</a>
int bad(unsigned input, unsigned len) {
unsigned shamt = (32 - len);
unsigned diff = input >> shamt;
return (int(diff) << shamt) >> shamt;
}
int good(unsigned input, unsigned len) {
return int(input) >> (32 - len);
}
Name:
%sub = sub i32 32, %len
%zz = shl i32 -1, %sub
%shl = and i32 %zz, %input
%r = ashr i32 %shl, %sub
=>
%r = ashr i32 %input, %sub</pre>
</div>
</p>
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