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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - {32, 64} - __builtin_clz{,l}(...) does not get optimized correctly on x86."
href="http://llvm.org/bugs/show_bug.cgi?id=16115">16115</a>
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<th>Summary</th>
<td>{32, 64} - __builtin_clz{,l}(...) does not get optimized correctly on x86.
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>bigcheesegs@gmail.com
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</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>unsigned foo(unsigned i) {
return 32 - __builtin_clz(i);
}
Is compiled to:
bsrl %edi, %eax
xorl $-32, %eax
addl $33, %eax
ret
I expect:
bsrl %edi, %eax
ret
Which is what this produces:
unsigned foo(unsigned i) {
return __builtin_clz(i) ^ 31;
}
I believe the issue is that the optimizer doesn't know that bsrl will never
result in a value greater than 31 and thus there's no need to worry about wrap
around.</pre>
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