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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Reading from stdin is 10x slower than libstdc++"
href="http://llvm.org/bugs/show_bug.cgi?id=21192">21192</a>
</td>
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<tr>
<th>Summary</th>
<td>Reading from stdin is 10x slower than libstdc++
</td>
</tr>
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<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>Macintosh
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>All Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>donovanhide@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>If I have a large file and want to read the contents line by line using
iostream, the performance of libc++ is around 10x worse than that of libstdc++.
Below is a comparison of a simple line counter for a binary compiled with g++
4.9, clang 3.5 and the standard coreutils wc tool. More discussion here:
<a href="http://info.prelert.com/blog/stdgetline-is-the-poor-relation">http://info.prelert.com/blog/stdgetline-is-the-poor-relation</a>
Profiling seems to confirm that with libc++ a mutex is held for each single
character read from stdin.
$cat wc.cc
#include <iostream>
#include <string>
int main() {
std::ios_base::sync_with_stdio(false);
std::size_t i = 0;
for (std::string line; std::getline(std::cin, line);) i++;
std::cout << i << std::endl;
}
$ g++-4.9 --version
g++-4.9 (Homebrew gcc 4.9.1) 4.9.1
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ g++-4.9 -O3 wc.cc -o wc
$ time gzip -cd large_file.gz|head -n 100000 | ./wc
100000
real 0m2.811s
user 0m2.953s
sys 0m0.408s
$ c++ --version
Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
Thread model: posix
$ c++ -O3 wc.cc -o wc
$ time gzip -cd large_file.gz|head -n 100000 | ./wc
100000
real 0m24.420s
user 0m27.114s
sys 0m0.322s
$ time gzip -cd large_file.gz|head -n 100000 | wc -l
100000
real 0m2.888s
user 0m3.229s
sys 0m0.304s</pre>
</div>
</p>
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