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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - copy constructor memcpy optimization tramples virtual bases in tail padding"
href="https://bugs.llvm.org/show_bug.cgi?id=36992">36992</a>
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<th>Summary</th>
<td>copy constructor memcpy optimization tramples virtual bases in tail padding
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</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
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
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<pre>Testcase:
struct A { char c; A(const A&) : c(1) {} };
struct B { int n; char c[3]; ~B(); };
struct C : B, virtual A {}; C f(C c) { return c; }
The move constructor we emit for C performs an 8-byte memcpy to construct its B
base class, which tramples over the initialization we've already performed for
the A virtual base class.
Similar issues will arise when memcpyizing non-static data members due to P0840
-- <a href="http://wg21.link/p0840r2">http://wg21.link/p0840r2</a>, or <a href="http://wg21.link/p0840r1">http://wg21.link/p0840r1</a> for the latest
publicly-visible version:
struct A { char c; A(const A&) : c(1) {} };
struct B { int n; char c[3]; ~B(); };
struct C : virtual A { [[no_unique_address]] B b; }; C f(C c) { return c; }</pre>
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