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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Inefficient loop unrolling on Silvermont (20% performance penalty vs gcc trunk)"
href="https://llvm.org/bugs/show_bug.cgi?id=24936">24936</a>
</td>
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<th>Summary</th>
<td>Inefficient loop unrolling on Silvermont (20% performance penalty vs gcc trunk)
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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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>
<tr>
<th>Reporter</th>
<td>egor.kochetov@intel.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>Created <span class=""><a href="attachment.cgi?id=14933" name="attach_14933" title="c++ reproducer and Makefile">attachment 14933</a> <a href="attachment.cgi?id=14933&action=edit" title="c++ reproducer and Makefile">[details]</a></span>
c++ reproducer and Makefile
While compiling one of the programs I've encountered the source that works
faster by 20% if compiled by gcc than if compiled by clang. The target for
compilation is Linux, Intel Silvermont CPU. Affected optsets are (-m32, -m64) ×
(-Ofast, -O2).
The reason for the problem turned out to be the fact that gcc unwinds
constant-sized loop (be it #define in C or constexpr in C++, it does not
matter), while clang does not.
Attached is the source that adds and multiplies floating-point zeros and
reproduces this performance difference. Makefile for this source is also
attached, fix compiler paths in it and run the sample like that:
make clean run; make clean run COMPILER=gcc
Make will produce the binary main.g++ or main.clang++ and the corresponding .s
file with disassembly of the slow_function.
Compilers under consideration were gcc 6.0 trunk and clang 3.8 trunk.</pre>
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