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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Possible false positive: takes branches that contradict each other"
href="https://bugs.llvm.org/show_bug.cgi?id=36685">36685</a>
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<th>Summary</th>
<td>Possible false positive: takes branches that contradict each other
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>6.0
</td>
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<th>Hardware</th>
<td>Other
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Static Analyzer
</td>
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<th>Assignee</th>
<td>dcoughlin@apple.com
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<th>Reporter</th>
<td>manuelmohr@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=20048" name="attach_20048" title="Source code of median search and reduced caller.">attachment 20048</a> <a href="attachment.cgi?id=20048&action=edit" title="Source code of median search and reduced caller.">[details]</a></span>
Source code of median search and reduced caller.
The attached reduced source code implements a fast median computation according
to [1] and contains a placeholder caller.
If the caller passes its input array directly, clang does not report any
problems.
If the caller copies its input array to a local array and then runs the median
computation (compile with "-DCOPY"), clang finds a bug.
I therefore suspect a false positive.
The path it takes assumes the following conditions:
Lines 38--40:
array[1] <= array[2]
array[0] <= array[2]
array[1] <= array[0]
Line 43:
Swapping is a NOP due to middle == low + 1.
Lines 50:
array[0] > array[2]
This contradicts assumption array[0] <= array[2], hence the reported bug should
be impossible and thus a false positive.
[1] <a href="http://ndevilla.free.fr/median/median.pdf">http://ndevilla.free.fr/median/median.pdf</a></pre>
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