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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang-cl: SIMD vector elements are unaligned in Microsoft's std::array and std::tuple"
href="https://bugs.llvm.org/show_bug.cgi?id=41480">41480</a>
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<th>Summary</th>
<td>clang-cl: SIMD vector elements are unaligned in Microsoft's std::array and std::tuple
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</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
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>andreas.zapf@ableton.com
</td>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<pre>As a result, code using std::array's and std::tuple's with SIMD vector elements
crashes due to unaligned memory access.
It seems that the behavior of "#pragma pack" is the underlying issue.
Microsoft's std::array and std::tuple are declared with #pragma pack(8)
(exactly "#pragma pack(push,_CRT_PACKING)"), and it seems that in clang-cl
"#pragma pack" beats alignment of SIMD types while in cl it's the other way
around.
To reproduce, compile the following example:
clang-cl -c
#include <emmintrin.h>
#include <array>
#include <tuple>
std::array<__m128, 1> arr;
static_assert(alignof(decltype(arr)) >= alignof(__m128), "unaligned array");
std::tuple<__m128> tpl;
static_assert(alignof(decltype(tpl)) >= alignof(__m128), "unaligned tuple");</pre>
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