| Issue |
134694
|
| Summary |
Misaligned member variables of templated SIMD types (like __m128) when using /Zp flag with clang-cl
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
januszn
|
Given the following source:
```
#include <cstddef>
#include <type_traits>
#include <xmmintrin.h>
// __m128 is defined as the following in clang's xmmintrin.h:
// typedef float __m128 __attribute__((__vector_size__(16), __aligned__(16)));
template<typename T>
using MyVectorRegisterType = std::conditional_t<std::is_same_v<T, float>, __m128, void >;
struct STemplated
{
int I;
MyVectorRegisterType<float> V;
size_t M()
{
static_assert(offsetof(STemplated, V) % alignof(decltype(V)) == 0, "STemplated::V is misaligned");
return offsetof(STemplated, V);
}
};
struct SDirect
{
int I;
__m128 V;
size_t M()
{
static_assert(offsetof(SDirect, V) % alignof(decltype(V)) == 0, "SDirect::V is misaligned");
return offsetof(SDirect, V);
}
};
int main()
{
STemplated ST;
SDirect SD;
return (int)ST.M() + (int)SD.M();
}
```
...clang-cl misaligns the `STemplated::V` member variable when using `/Zp8`:
```
D:\work>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.43.34809 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
usage: cl [ option... ] filename... [ /link linkoption... ]
D:\work>clang-cl --version
clang version 19.1.1
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\Llvm\x64\bin
D:\work>cl test.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.43.34809 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
test.cpp
Microsoft (R) Incremental Linker Version 14.43.34809.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:test.exe
test.obj
D:\work>clang-cl test.cpp
D:\work>cl /Zp8 test.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.43.34809 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
test.cpp
Microsoft (R) Incremental Linker Version 14.43.34809.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:test.exe
test.obj
D:\work>clang-cl /Zp8 test.cpp
test.cpp(19,17): error: static assertion failed due to requirement '__builtin_offsetof(STemplated, V) % alignof(__attribute__((__vector_size__(4 * sizeof(float)))) float) == 0': STemplated::V is misaligned
19 | static_assert(offsetof(STemplated, V) % alignof(decltype(V)) == 0, "STemplated::V is misaligned");
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\Llvm\x64\lib\clang\19\include\__stddef_offsetof.h(16,24): note: expanded from macro 'offsetof'
16 | #define offsetof(t, d) __builtin_offsetof(t, d)
| ^
test.cpp(19,64): note: _expression_ evaluates to '8 == 0'
19 | static_assert(offsetof(STemplated, V) % alignof(decltype(V)) == 0, "STemplated::V is misaligned");
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
1 error generated.
```
A similar issue was previously reported in https://github.com/llvm/llvm-project/issues/44765 , with a fix for the `SDirect` case above committed, but it seems the fix does not work if the type with alignment requirements is wrapped in a template. I tested the issue on Clang versions 16, 18, 19 and 20. The minimal example is adapted from code in Unreal Engine (`HierarchicalSpatialHash.h`, `TSpatialHashGridPoints::CellSize` member variable, `TSpatialHashGridPoints::InitializePoints` fails at runtime due to misaligned access of `CellSize`).
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