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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - alignas with placement new operator generate invalid instruction with sse"
href="https://llvm.org/bugs/show_bug.cgi?id=24957">24957</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>alignas with placement new operator generate invalid instruction with sse
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.6
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm@gyoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>When using alignas, clang will generate the mmx instruction if O1 optimization
or greater are used:
movaps %xmm0,(%rbx)
But if the memory has been allocated with a new placement operator on a non
16-bytes alignment, it will crash with a coredump
Here a test case:
// clang -std=c++11 -lstdc++ -O0 -g align.cc # ok
// clang -std=c++11 -lstdc++ -O1 -g align.cc # coredump because of
MOVAPS on non-aligned 128 bits
// clang -std=c++11 -lstdc++ -O1 -mno-sse -g align.cc # no more coredump
#include <iostream>
constexpr static int ALIGN_SIZE = 64;
class MyStruct
{
public:
MyStruct()
{
std::cout << "MyStruct: " << this << " " << sizeof(*this) << std::endl
<< "m_v1: " << &m_v1 << ", m_v2: " << &m_v2 << std::endl;
m_v1 = 0;
m_v2 = 0;
}
private:
alignas(ALIGN_SIZE) uint64_t m_v1;
uint64_t m_v2;
};
int main()
{
size_t offset = 1; // offset not a multiplier of 16 will crash
char* ptr = new char[sizeof(MyStruct) + offset];
MyStruct* myStruct = new (ptr+offset) MyStruct;
return 0;
}</pre>
</div>
</p>
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