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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Slow 'for' loop unrolling"
href="https://bugs.llvm.org/show_bug.cgi?id=44359">44359</a>
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<th>Summary</th>
<td>Slow 'for' loop unrolling
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>9.0
</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>
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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>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>hehaochen13@nudt.edu.cn
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=22959" name="attach_22959" title="Adobe_C++Benchmarks">attachment 22959</a> <a href="attachment.cgi?id=22959&action=edit" title="Adobe_C++Benchmarks">[details]</a></span>
Adobe_C++Benchmarks
Compile and run 'loop unroll test' in Adobe C++ benchmark
(<a href="https://stlab.adobe.com/performance/">https://stlab.adobe.com/performance/</a>). We find slowdown in clang-7,8,9(docker)
when adding '-O3':
╔══════════════════════════════════════╦═════╦═════╦═════╦══════════╗
║ Total absolute execution time (sec.) ║ O1 ║ O2 ║ O3 ║ comments ║
╠════════════════════════════╦═════════╬═════╬═════╬═════╬══════════╣
║ ║ gcc-8 ║ 259 ║ 167 ║ 149 ║ ok ║
║ ╠═════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-6 ║ 326 ║ 206 ║ 181 ║ ok? ║
║ ╠═════════╬═════╬═════╬═════╬══════════╣
║ int32_t 'for' ║ clang-7 ║ 249 ║ 171 ║ 182 ║ SLOW ║
║ loop unrolling ╠═════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-8 ║ 247 ║ 170 ║ 183 ║ SLOW ║
║ ╠═════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-9 ║ 248 ║ 170 ║ 183 ║ SLOW ║
╚════════════════════════════╩═════════╩═════╩═════╩═════╩══════════╝
╔══════════════════════════════════════╦═════╦═════╦═════╦══════════╗
║ Total absolute execution time (sec.) ║ O1 ║ O2 ║ O3 ║ comments ║
╠═══════════════════════╦══════════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-6 ║ 347 ║ 206 ║ 181 ║ ok ║
║ ╠══════════════╬═════╬═════╬═════╬══════════╣
║ int32_t 'while' ║ clang-7 ║ 248 ║ 167 ║ 164 ║ ok ║
║ loop unrolling ╠══════════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-8 ║ 249 ║ 167 ║ 165 ║ ok ║
║ ╠══════════════╬═════╬═════╬═════╬══════════╣
║ ║ clang-9 ║ 248 ║ 167 ║ 165 ║ ok ║
╚═══════════════════════╩══════════════╩═════╩═════╩═════╩══════════╝
The this performance issue do not occur in 'while' loop unrolling test or gcc
either.
Thank you.</pre>
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