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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - GVN's propagateEquality propagates wrong equality"
href="https://llvm.org/bugs/show_bug.cgi?id=27506">27506</a>
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<th>Summary</th>
<td>GVN's propagateEquality propagates wrong equality
</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>PC
</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>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>kyoonss@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</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=16264" name="attach_16264" title="llvm code that exposes error">attachment 16264</a> <a href="attachment.cgi?id=16264&action=edit" title="llvm code that exposes error">[details]</a></span>
llvm code that exposes error
GVN's `propagateEquality` propagates wrong equality related to icmp
instructions.
When it propagates such as "(icmp eq A B) == true", it also propagates "A ==
B". However, this may cause an incorrect behavior if either A or B is an
undefined value.
When the source code like below is given, GVN transforms the "b1" block as
shown. Suppose %x has a normal value and %u has the undef value. In the source,
"%aa" is always defined. However, in the target, "%aa" is replaced with "%u",
which is undef.
We attach an input llvm file of "opt -gvn" and a C example code that exposes
the same problem when compiled using "clang -O2". We tested this in both clang
3.7.1 and the trunk.
b0:
%a = add i32 %x, 1
%c = icmp eq i32 %a, %u
br i1 %c, label %b1, label %b2
b1:
%aa = add i32 %x, 1
%d = call i32 @bar(i32 %aa)
->
b1:
%d = call i32 @bar(i32 %u)</pre>
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