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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Overly optimized simultaneous copy of struct fields corrupts destination struct"
href="https://bugs.llvm.org/show_bug.cgi?id=50623">50623</a>
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<th>Summary</th>
<td>Overly optimized simultaneous copy of struct fields corrupts destination struct
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>12.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<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>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>samuel.sadok@bluewin.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Clang 12.0.0 modifies a field of a struct that should not be modified. This is
a regression from clang 11.0.1.
Input File:
```
struct S { char val1; char padding; short val2; };
void func(struct S src, struct S* dest) {
dest->val1 = src.val1;
dest->val2 = src.val2;
}
```
Output from clang 12.0.0 (clang -c test.c -o test.o -O1 && objdump -S test.o -M
intel):
```
0000000000000000 <func>:
0: 89 3e mov DWORD PTR [rsi],edi
2: c3 ret
```
Output from clang 11.0.1:
```
0000000000000000 <func>:
0: 40 88 3e mov BYTE PTR [rsi],dil
3: c1 ef 10 shr edi,0x10
6: 66 89 7e 02 mov WORD PTR [rsi+0x2],di
a: c3 ret
```
The output from clang 12.0.0 is incorrect because it modifies the field
`dest->padding`.
See also here: <a href="https://godbolt.org/z/dsr9soqbf">https://godbolt.org/z/dsr9soqbf</a>
"armv8-a clang (trunk)" exhibits the same problem.</pre>
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