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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine: shift amount reassociation"
href="https://bugs.llvm.org/show_bug.cgi?id=42391">42391</a>
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</tr>
<tr>
<th>Summary</th>
<td>InstCombine: shift amount reassociation
</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>
<tr>
<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/S9Zn-y">https://godbolt.org/z/S9Zn-y</a>
<a href="https://rise4fun.com/Alive/plLm">https://rise4fun.com/Alive/plLm</a>
int bad2(unsigned diff, unsigned len1, unsigned len2) {
return ((diff >> (32-len1)) >> (len1 - 1));
}
int good2(unsigned diff, unsigned len1, unsigned len2) {
return diff >> ((32-len1)+(len1-1));
}
Name: dd
Pre: C > 0 ; signed positive
%4 = sub i32 32, %1
%5 = lshr i32 %0, %4
%6 = sub i32 %1, C
%r = lshr i32 %5, %6
=>
%z4 = sub i32 32, C
%r = lshr i32 %0, %z4</pre>
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</p>
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