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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Instcombine: transform shl(inexact shr(x)) with different constant shifts."
href="https://bugs.llvm.org/show_bug.cgi?id=37872">37872</a>
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<th>Summary</th>
<td>Instcombine: transform shl(inexact shr(x)) with different constant shifts.
</td>
</tr>
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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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>IIn <a href="https://reviews.llvm.org/D46760#1123713">https://reviews.llvm.org/D46760#1123713</a>, Sanjay Patel suggests transforming
lshr+shl into mask(and)+shl, which is not only a canonicalization, but
would also sidestep the problem which D46760 is trying to fix.
This canonicalization is already being done for the non-constants,
or equal constants, but not unequal constants.
The commit that introduced that restriction: <a href="https://reviews.llvm.org/rL155136">https://reviews.llvm.org/rL155136</a>
suggests that said transform negatively impacts backend (rotate instruction),
and SCEV.
The backend part will likely be resolved in <a href="https://reviews.llvm.org/D47681">https://reviews.llvm.org/D47681</a> or
followups.</pre>
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