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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [Clang-optimization] wrong code for optimization the union assignment in a called function"
href="https://bugs.llvm.org/show_bug.cgi?id=35571">35571</a>
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<th>Summary</th>
<td>[Clang-optimization] wrong code for optimization the union assignment in a called function
</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>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>yangyibiao@nju.edu.cn
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>$ clang -v
clang version 6.0.0-svn320088-1~exp1 (trunk)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.2.0
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
$ cat small.c
int printf(const char *, ...);
union {
int a;
short b;
} c, *d = &c, e = {1};
void f() {
c.a = 0;
*d = e;
c.b = 0;
}
int main() {
f();
printf("%d\n", c.b);
}
$ clang -O0 small.c; ./a.out
0
$ clang -Ofast small.c; ./a.out
1
When removing function f and put the three lines of code in the main function,
the output is the same.</pre>
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