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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - wrong code generated for union with optimization"
href="http://llvm.org/bugs/show_bug.cgi?id=16236">16236</a>
</td>
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<th>Summary</th>
<td>wrong code generated for union with optimization
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dhazeghi@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following code produces 0 at -O0 and 1 at -O1 and above with clang trunk on
x86_64-linux. I believe the behavior is well-defined here, since the
assignment to the union member is through the union type. It is a change in
behavior from 2.8.
$ clang-trunk -v
clang version 3.4 (trunk 183303)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ clang-trunk -O0 wrong.c
$ ./a.out
0
$ clang-2.8 -O1 wrong.c
$ ./a.out
0
$ clang-trunk -O1 wrong.c
$ ./a.out
1
$
----------------------------------
int printf(const char *, ...);
union
{
short f0;
int f1;
} u, *up = &u;
int a;
int main ()
{
for (; a <= 0; a++)
if ((u.f0 = 1))
up->f1 = 0;
printf ("%d\n", u.f1);
return 0;
}</pre>
</div>
</p>
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