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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - cloop overflow test is broken in recent clang"
href="https://bugs.webkit.org/show_bug.cgi?id=143345">143345</a>
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<th>Summary</th>
<td>cloop overflow test is broken in recent clang
</td>
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<th>Classification</th>
<td>Unclassified
</td>
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<th>Product</th>
<td>WebKit
</td>
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<th>Version</th>
<td>528+ (Nightly build)
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<th>Hardware</th>
<td>Unspecified
</td>
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<th>OS</th>
<td>Unspecified
</td>
</tr>
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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>P2
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<th>Component</th>
<td>JavaScriptCore
</td>
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<th>Assignee</th>
<td>webkit-unassigned@lists.webkit.org
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</tr>
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<th>Reporter</th>
<td>rjogrady@google.com
</td>
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<p>
<div>
<pre>cloop.rb's cloopAddOverflowTest() uses this check: "((#{signBit}(b) != #{signBit}(a)) && (#{signBit}(b-a) == #{signBit}(a)))" to detect overflow, and similarly for subtraction.
In clang r221045, an optimization was introduced which breaks this for the case where we are incrementing a value by 1.
Repro on Linux (with clang r221045 or above):
$ CC=clang
$ CXX=clang
$ build-jsc --cloop --release --gtk
Then execute this in jsc:
for (var i = 0x7fffffff; i < 0x80000000; i++) {print(i);}
Notice that i wraps around to become negative.
Using safeAdd() and safeSub() from CheckedArithmetic.h avoids the problem.</pre>
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