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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed optimization by O2? LTO generates faster code than O2 with all code visible"
href="http://llvm.org/bugs/show_bug.cgi?id=21201">21201</a>
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<th>Summary</th>
<td>Missed optimization by O2? LTO generates faster code than O2 with all code visible
</td>
</tr>
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<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</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>
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<th>Component</th>
<td>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>vlovich@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13169" name="attach_13169" title="Example project that benchmarks tagged">attachment 13169</a> <a href="attachment.cgi?id=13169&action=edit" title="Example project that benchmarks tagged">[details]</a></span>
Example project that benchmarks tagged
I know this sounds like expected behaviour, but is it possible there's some
optimization opportunity that clang is missing @ -02/-O3 whereas it's caught by
LTO?
The benchmark is I build a vector of a type that wraps a boost::variant of 2
types. I then iterate invoking a function on each element of the vector & time
the result.
-O2 -flto: vector with types in random-order:
TaggedDispatch.TaggedDispatch (457.022675 ms)
-O2 -flto: vector with types in-order:
TaggedDispatch.TaggedDispatch (100.611670 ms)
-O2: vector with types in random order:
TaggedDispatch.TaggedDispatch (652.344259 ms)
-O2: vector in-order:
TaggedDispatch.TaggedDispatch (326.946609 ms)
-O3: vector with types in random order:
TaggedDispatch.TaggedDispatch (647.796656 ms)
-O3: vector in-order:
TaggedDispatch.TaggedDispatch (330.778245 ms)
We can see that somehow -flto manages to squeeze out quite a bit of performance
which is unexpected. There's no difference between O2 & O3.
Numbers generated with Xcode's clang on OSX 10.10. The version of boost
appeared to not matter (numbers consistent between boost 1.51 & boost 1.55).
You may have to edit the makefile to point to your boost if it's not in your
system path.</pre>
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