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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - InstCombine: unsigned overflow subtraction miscompilation" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23639&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=wBBXKWLzGRuK1AAp8c5LP4ehWER5p5n_wCfbhYFyMFQ&s=BvFSqeBzehalMaToNJIx5VSBwqIY2FPptK912SoW0IQ&e=">23639</a>
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<th>Summary</th>
<td>InstCombine: unsigned overflow subtraction miscompilation
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>3.6
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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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>compnerd@compnerd.org
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=14374" name="attach_14374" title="reduced.c">attachment 14374</a> <a href="attachment.cgi?id=14374&action=edit" title="reduced.c">[details]</a></span>
reduced.c
Introduced in SVN r234388.
Basically, the X - undef -> undef => X - 0 -> {X, false} is mismatching the
__builtin_subcl with a carry in of 0, resulting in it dropping the arithmetic,
causing a miscomputation. The attached test case demonstrates the issue and
reverting the particular change does get it correct as does 3.6.</pre>
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