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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to recognize table-based ctz implementation"
href="https://bugs.llvm.org/show_bug.cgi?id=46434">46434</a>
</td>
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<tr>
<th>Summary</th>
<td>Failure to recognize table-based ctz implementation
</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>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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int f(unsigned x)
{
static const char table[32] = {0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15,
25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9};
return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
}
This can be optimized to `__builtin_ctz(x);`. This transformation is done by
GCC (or at least <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838</a> claims to
do so, though I can't get it to trigger), but not by LLVM.</pre>
</div>
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