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<base href="https://bugs.llvm.org/">
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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 - Multiple moves compiled inefficiently"
href="https://bugs.llvm.org/show_bug.cgi?id=32086">32086</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Multiple moves compiled inefficiently
</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>enhancement
</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>drraph@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider:
double a[4], b[4], c[4];
void foo ()
{
a[0] = c[0];
a[1] = c[1];
a[2] = c[0];
a[3] = c[1];
b[0] = c[2];
b[1] = c[3];
b[2] = c[2];
b[3] = c[3];
}
clang compiles this with -O3 to:
foo: # @foo
mov rax, qword ptr [rip + c]
mov qword ptr [rip + a], rax
mov rcx, qword ptr [rip + c+8]
mov qword ptr [rip + a+8], rcx
mov qword ptr [rip + a+16], rax
mov qword ptr [rip + a+24], rcx
mov rax, qword ptr [rip + c+16]
mov qword ptr [rip + b], rax
mov rcx, qword ptr [rip + c+24]
mov qword ptr [rip + b+8], rcx
mov qword ptr [rip + b+16], rax
mov qword ptr [rip + b+24], rcx
ret
gcc on the other hand gives the more efficient:
foo:
movapd xmm1, XMMWORD PTR c[rip]
movapd xmm0, XMMWORD PTR c[rip+16]
movaps XMMWORD PTR a[rip], xmm1
movaps XMMWORD PTR a[rip+16], xmm1
movaps XMMWORD PTR b[rip], xmm0
movaps XMMWORD PTR b[rip+16], xmm0
ret</pre>
</div>
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