<html>
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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - Clang miscompiles assignments to packed struct members" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23599&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=ykVRlO6FZN7RHNvt7BLOkMImfmePqh5Z9_ATIldYRQE&s=q5opSN9AfdtOOUcW2jrah2gKja-cAsvsx07iEfDUb_c&e=">23599</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Clang miscompiles assignments to packed struct members
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>vonosmas@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, nlewycky@google.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>$ cat tmp/a.cc
#include <stdio.h>
#include <stdint.h>
#define ETH_ALEN 6
struct ether_addr
{
uint8_t ether_addr_octet[ETH_ALEN];
} __attribute__ ((__packed__));
struct ether_header
{
uint8_t ether_dhost[ETH_ALEN];
uint8_t ether_shost[ETH_ALEN];
} __attribute__ ((__packed__));
void Print(const ether_header ð) {
for (int i = 0; i < 6; i++)
printf("%d ", eth.ether_dhost[i]);
printf("\n");
for (int i = 0; i < 6; i++)
printf("%d ", eth.ether_shost[i]);
printf("\n");
}
void FooBarBaz(const ether_addr& ether_src, const ether_addr& ether_dst) {
ether_header eth{};
*((ether_addr*)eth.ether_shost) = ether_src;
*((ether_addr*)eth.ether_dhost) = ether_dst;
Print(eth);
}
int main() {
FooBarBaz({{1,2,3,4,5,6}}, {{7,8,9,10,11,12}});
return 0;
}
$ ./bin/clang++ -O2 tmp/a.cc -std=c++11 && ./a.out
7 8 9 10 11 12
0 0 0 0 0 0
$ ./bin/clang++ -O0 tmp/a.cc -std=c++11 && ./a.out
7 8 9 10 11 12
1 2 3 4 5 6
I haven't yet figured out which optimization pass is responsible for that. Note
that if I swap the assignments of ether_src and ether_dst, the program works as
expected.</pre>
</div>
</p>
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