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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [InstCombine] Failure to recognize sign bit test - shift amount reassociation"
href="https://bugs.llvm.org/show_bug.cgi?id=43595">43595</a>
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<th>Summary</th>
<td>[InstCombine] Failure to recognize sign bit test - shift amount reassociation
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<th>Product</th>
<td>libraries
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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
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<pre>This is motivated by <a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine: lshr + "sext" => ashr canonicalization"
href="show_bug.cgi?id=42389">https://bugs.llvm.org/show_bug.cgi?id=42389</a>
While we succeed to produce sign bit test if we use unsigned:
<a href="https://godbolt.org/z/iSlCpW">https://godbolt.org/z/iSlCpW</a>
We get stuck if we use signed:
<a href="https://godbolt.org/z/VO1-0F">https://godbolt.org/z/VO1-0F</a>
Boo. <a href="https://rise4fun.com/Alive/PRU">https://rise4fun.com/Alive/PRU</a>
I'm not sure how to approach this yet.
We can't treat this just as shift amount reassociation,
maybe icmp can be treated as `trunc to i1`.</pre>
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