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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect result at self-comparison of uninitialized value"
href="https://bugs.llvm.org/show_bug.cgi?id=37774">37774</a>
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<th>Summary</th>
<td>Incorrect result at self-comparison of uninitialized value
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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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>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>m-takahiro@jp.fujitsu.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>I have tested a program using self-comparison of uninitialized value as below.
--- test.c ---
#include <stdio.h>
int main() {
int a;
if (a == a) puts("equal");
else puts("not equal");
return 0;
}
--------------
A correct result is output at -O0, but an incorrect result is output at -O1.
$ clang --version
clang version 7.0.0 (trunk 334448)
$ clang -O0 test.c
test.c:4:9: warning: self-comparison always evaluates to true
[-Wtautological-compare]
if (a == a) puts("equal");
^
1 warning generated.
$ ./a.out
equal
$ clang -O1 test.c
test.c:4:9: warning: self-comparison always evaluates to true
[-Wtautological-compare]
if (a == a) puts("equal");
^
1 warning generated.
$ ./a.out
not equal
This is a rare case and I think no one writes a program like this, but I think
self-comparison program should always pass true route even if an uninitialized
value is used.</pre>
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