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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - The performance of some vector operator is worse than GCC"
href="https://bugs.llvm.org/show_bug.cgi?id=49324">49324</a>
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<th>Summary</th>
<td>The performance of some vector operator is worse than GCC
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>499537630@qq.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=24563" name="attach_24563" title="testcase">attachment 24563</a> <a href="attachment.cgi?id=24563&action=edit" title="testcase">[details]</a></span>
testcase
clang++ -O2 test.cpp
./a,out
g++ -O2 test.cpp
./a.out
clang++ -v
clang version 8.0.1 (based on LLVM 8.0.1)
g++ -v
gcc version 7.3.0
result for example:
test clangxxO2/gxxO2
test_int8_t[data5(data.begin(), data.end())] 13.03
test_int8_t[data1.assign(100000000, 100)] 9.58
test_int8_t[data7.assign(data.begin(), data.end())] 18.81
We have other operator worse than GCC, like map and unordered_set.
For vector assign, the main function consumed is __construct_range_forward.
libcxx use the __construct_range_forward
Each element is constructed using the construct method, which is constructed
using the placement new method. GCC uses std::uninitialized_copy to copy the
memory. The performance is significantly better than that of libcxx.
see: <a class="bz_bug_link
bz_status_NEW "
title="NEW - The performance of vector::assign is 100 times worse than that of libstdc++."
href="show_bug.cgi?id=48893">https://bugs.llvm.org/show_bug.cgi?id=48893</a>#add_comment</pre>
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