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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 - swap goes through memory when it could stay in registers"
href="https://bugs.llvm.org/show_bug.cgi?id=47438">47438</a>
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</tr>
<tr>
<th>Summary</th>
<td>swap goes through memory when it could stay in registers
</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>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>benny.kra@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Godbolt: <a href="https://godbolt.org/z/Yez1Mc">https://godbolt.org/z/Yez1Mc</a>
$ cat t.cc
#include <utility>
struct foo {
float x, y, z, w;
};
void bar(foo& a, foo& b) { std::swap(a, b); }
$ gcc-11 -O3 -S -o - t.cc
bar(foo&, foo&):
movups (%rdi), %xmm0
movdqu (%rsi), %xmm1
movups %xmm1, (%rdi)
movups %xmm0, (%rsi)
ret
$ clang-11 -O3 -S -o - t.cc
bar(foo&, foo&): # @bar(foo&, foo&)
movups (%rdi), %xmm0
movaps %xmm0, -24(%rsp)
movups (%rsi), %xmm0
movups %xmm0, (%rdi)
movaps -24(%rsp), %xmm0
movups %xmm0, (%rsi)
retq
The std::swap expands into an alloca and 3 memcpy's.</pre>
</div>
</p>
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