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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - libc++'s string::assign isn't O(1) when assigning the string to itself if the system's memmove() doesn't check for that"
href="https://llvm.org/bugs/show_bug.cgi?id=25413">25413</a>
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<th>Summary</th>
<td>libc++'s string::assign isn't O(1) when assigning the string to itself if the system's memmove() doesn't check for that
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>nicolasweber@gmx.de
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>Consider this program:
int main() {
string s(8 * 1024 * 1024, ' ');
string d; d.reserve(s.size());
for (int i = 0; i < s.size(); i += 4096) {
d.append(&s[i], 4096);
const char* p = d.data();
const char* end = p + d.size();
d.assign(p, end - p);
}
}
It might look silly, but it's reduced from real-world code that reads chunks of
a pipe, processes completely read packets, and uses assign() to move chunks of
incomplete packets to the front of d. If a packet spans several chunks, that's
the code that effectively runs (from <a href="http://crbug.com/547387">http://crbug.com/547387</a>).
In all standard library implementations that aren't libc++, this runs quickly,
because they all check if p overlaps with the string stored in d and do
something intelligent in that case. libc++'s generic char_traits::move() does
that too from what I can tell, but the char specialization of char_traits just
calls memmove, and then we're at the mercy of the system C library. On most
systems, memmove seems to do something smart, but not on all of them: On OS X
versions 10.8 or older, this program takes a long time to run. With libstdc++
it's fast there too.</pre>
</div>
</p>
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