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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - padding in nested std::pair"
href="https://bugs.llvm.org/show_bug.cgi?id=40230">40230</a>
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<th>Summary</th>
<td>padding in nested std::pair
</td>
</tr>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>7.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>FreeBSD
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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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
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<th>Reporter</th>
<td>maxime.arthaud@nasa.gov
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
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<pre>Hi everyone,
I am getting a weird padding at the beginning of a std::pair.
See the following example:
#include <cstdio>
#include <utility>
int main() {
std::pair< std::pair< char, char >, char > p;
printf("%lu %p %p\n", sizeof(p), &p, &p.first);
return 0;
}
Output:
4 0x7fffffffe738 0x7fffffffe739
There is an additional byte at the beginning of the pair, for no reason. This
is not really a bug, but is worth improving.
It causes crashes in boost::container::flat_map, see
<a href="https://github.com/boostorg/container/issues/97">https://github.com/boostorg/container/issues/97</a>
Please note that this only happens on FreeBSD.
System: FreeBSD 12.0
Compiler: Clang 6.0.1 (default compiler on FreeBSD)
libc++: master branch on Github
I'm pretty sure this is related to <a href="https://reviews.llvm.org/D25389">https://reviews.llvm.org/D25389</a>
The base class __non_trivially_copyable_base introduces the padding. I don't
understand why Empty Base Optimization doesn't work in that case.
To reproduce this, I used the following virtual machine:
<a href="https://download.freebsd.org/ftp/releases/VM-IMAGES/12.0-RELEASE/amd64/Latest/">https://download.freebsd.org/ftp/releases/VM-IMAGES/12.0-RELEASE/amd64/Latest/</a>
Upstream issue in IKOS: <a href="https://github.com/NASA-SW-VnV/ikos/issues/22">https://github.com/NASA-SW-VnV/ikos/issues/22</a></pre>
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