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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 - Miscompile of zext(cttz())"
href="https://bugs.llvm.org/show_bug.cgi?id=45762">45762</a>
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<th>Summary</th>
<td>Miscompile of zext(cttz())
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>meheff@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23430" name="attach_23430" title="Repro .ll file">attachment 23430</a> <a href="attachment.cgi?id=23430&action=edit" title="Repro .ll file">[details]</a></span>
Repro .ll file
It looks like cttz(X, false) is unsafely relaxed to cttz(X, true) if the only
use of the cttz is a zext/trunc. Transformation happens here:
<a href="https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L889">https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L889</a>
It checks to see if the cttz has a single use (unsafely assuming it is the
select), but earlier the analysis can traverse through zext/trunc so the
assumption that the cttz's single use is the select is wrong:
<a href="https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L864">https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L864</a>
If the zext/trunc has other uses then the relaxed cttz(X, true) incorrectly
leaks out around the guarding select.
Repro attached.
$ lli bad.ll
$ echo $?
8
$ opt bad.ll --O2 | lli
$ echo $?
1</pre>
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