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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Really bad codegen for libc++ vector"
href="https://bugs.llvm.org/show_bug.cgi?id=35637">35637</a>
</td>
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<th>Summary</th>
<td>Really bad codegen for libc++ vector
</td>
</tr>
<tr>
<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>Macintosh
</td>
</tr>
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<th>OS</th>
<td>All
</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>C++
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mclow.lists@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Given the following code:
#include <vector>
#include <memory>
auto f1()
{
return std::vector<unsigned char>( 1920*1080*4 );
}
auto f2()
{
return new std::vector<unsigned char>( 1920*1080*4 );
}
int main() {}
clang generates fine code for "f1", noticing that it has to zero 8+MB of data,
and calling memset to do so.
For "f2", it generates a loop. Clearing a byte? at a time, and recalculating
the pointer each time. Reproduced on godbolt with "-std=c++1z -Wall -W -O3
-stdlib=libc++" with both clang 5.0 and clang trunk.</pre>
</div>
</p>
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