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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM generates SIMD instructions on copying a large object which is slow."
href="https://llvm.org/bugs/show_bug.cgi?id=27630">27630</a>
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<th>Summary</th>
<td>LLVM generates SIMD instructions on copying a large object which is slow.
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>congh@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LLVM generates SSE instructions (on x86 each movaps copies 16 bytes) on copying
a large object for the following code, which underperforms GCC's generated code
in which each mov instruction copies 8 bytes:
struct A {
int a[16];
};
constexpr int N = 10000;
A a[N], b[N];
int main() {
for (int j = 0; j < 1000; ++j) {
for (int i = 0; i < N; ++i) {
a[i] = b[i];
}
}
}
Running time (on a Haswell machine):
LLVM: 0.071s
GCC: 0.039s
The performance becomes worse if we don't unroll the loop.</pre>
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