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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - range-v3 tensor indices range vs manual loop nesting, 100-400x slowdown"
href="https://llvm.org/bugs/show_bug.cgi?id=26867">26867</a>
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<th>Summary</th>
<td>range-v3 tensor indices range vs manual loop nesting, 100-400x slowdown
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gonzalobg88@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre><a href="https://github.com/ericniebler/range-v3/issues/281">https://github.com/ericniebler/range-v3/issues/281</a>
Basically there are two range implementations of iteration over
multi-dimensional tensors:
- one using range comprehensions (using view::for_each, 100-400x slowdown)
- one using a view written by hand (10x slowdown)
The slowdowns are measured with respect to a manually written triple nested
loop.
That is, when using range views instead of raw loops, clang produces slower
code. (GCC does as well, but the slowdown is much lower: 1-2x slowdown).
I can provide preprocessed sources that trigger this issue, but whoever wants
to tackle this should probably get range-v3 from source anyways (I will be glad
to test and benchmark in my machines: from SSE to AVX2).</pre>
</div>
</p>
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