<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - missed optimization: dead stores in after memcpy expansion"
href="https://bugs.llvm.org/show_bug.cgi?id=40550">40550</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>missed optimization: dead stores in after memcpy expansion
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>clement.courbet@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following code:
```
struct S {
char a[16];
};
void Swap(S& a, S& b) {
S tmp = a;
a = b;
b = tmp;
}
```
The generated code goes through the stack and generates 3
load/store pairs:
```
movups (%rdi), %xmm0
movaps %xmm0, -24(%rsp)
movups (%rsi), %xmm0
movups %xmm0, (%rdi)
movaps -24(%rsp), %xmm0
movups %xmm0, (%rsi)
```
With -combiner-global-alias-analysis, the combiner can avoid going through the
stack, but still generates a dead store:
```
movups (%rdi), %xmm0
movaps %xmm0, -24(%rsp)
movups (%rsi), %xmm1
movups %xmm1, (%rdi)
movups %xmm0, (%rsi)
```
The following code would be much better:
```
movups (%rdi), %xmm0
movups (%rsi), %xmm1
movups %xmm1, (%rdi)
movups %xmm0, (%rsi)
```</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>