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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Performance disparty between g++ and clang++ for recursive template execution"
href="https://llvm.org/bugs/show_bug.cgi?id=26866">26866</a>
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<th>Summary</th>
<td>Performance disparty between g++ and clang++ for recursive template execution
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.5
</td>
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<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>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++11
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>wolfgang.brehm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15998" name="attach_15998" title="cpp source code to calculate floor of log2 using tail call recursion">attachment 15998</a> <a href="attachment.cgi?id=15998&action=edit" title="cpp source code to calculate floor of log2 using tail call recursion">[details]</a></span>
cpp source code to calculate floor of log2 using tail call recursion
This probably has to do with non guaranteed tail call recursion optimization.
I think my recursieve templated floor_log2 implementation could be a good test
case to optimize clang++ as the g++ compiled executable is about 3 times
faster.
The relevant code:
[...]
constexpr floor_log2(const T& x,const T& lower,const T& upper){
return (upper-lower==T(1))?lower:(x&(T(0)-T(1)<<((upper+lower)/2))?
floor_log2(x,(upper+lower)/2,upper):
floor_log2(x,lower,(upper+lower)/2));
}
[...]
see attachment for minimal working example.</pre>
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