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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - wrong code at -O2 and above on x86_64-linux-gnu (affecting clang-6.0 and above versions)"
href="https://bugs.llvm.org/show_bug.cgi?id=52495">52495</a>
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<th>Summary</th>
<td>wrong code at -O2 and above on x86_64-linux-gnu (affecting clang-6.0 and above versions)
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>
</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>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>haoxintu@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Hi all.
I guess the following program is well-defined but makes clang produce wrong
code across many versions.
$cat small.c
#include <stdio.h>
#include <stdint.h>
union a {
int16_t b;
uint64_t c;
} d;
uint64_t *e = &d.c;
static uint16_t f(int16_t a, int32_t b, uint64_t c);
static int64_t g(int32_t aa, uint8_t h, union a bb) {
int16_t *i = &d.b;
f(0, h, 0);
*i = h;
return 0;
}
uint16_t f(int16_t a, int32_t b, uint64_t c) {
for (d.c = 0; 0;)
;
*e = 0;
return 0;
}
int main() {
uint32_t j = 8;
g(1, j, d);
printf("%d\n", d.b);
return 0;
}
$clang -w -O1 small.c ; ./a.out
8
$clang -w -O2 small.c ; ./a.out
0
$clang -w -O3 small.c ; ./a.out
0
$clang-5.0 -w -O small.c ; ./a.out
8
Please check more in Godbolt: <a href="https://godbolt.org/z/c3YzvYe7a">https://godbolt.org/z/c3YzvYe7a</a>
Thanks,
Haoxin</pre>
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