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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - A move to a specific target register is often not optimized away"
href="http://llvm.org/bugs/show_bug.cgi?id=18602">18602</a>
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<tr>
<th>Summary</th>
<td>A move to a specific target register is often not optimized away
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jn@sirrida.de
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>A move to a specific target register is often not optimized away.
This unnecessarily costs 2 bytes code:
int test(int x) {
return ((x>>2) & 15) ^ ((x>>3) & 31);
}
=> clang -O3
movl %edi, %eax
shrl $2, %eax
andl $15, %eax
shrl $3, %edi
andl $31, %edi
xorl %eax, %edi //
movl %edi, %eax // could become xorl %edi, %eax
retq // we need %eax here
int test(int x, int y) {
return (((x>>2) & 15) ^ ((x<<3) & ~31)) / y;
}
=> clang -O3
movl %edi, %eax
shrl $2, %eax
andl $15, %eax
shll $3, %edi
andl $-32, %edi
orl %eax, %edi //
movl %edi, %eax // could become orl %edi, %eax
cltd // we need %eax here
idivl %esi
retq</pre>
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</p>
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