<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/134694>134694</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Misaligned member variables of templated SIMD types (like __m128) when using /Zp flag with clang-cl
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          januszn
      </td>
    </tr>
</table>

<pre>
    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`).
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzcWEtv4zgS_jX0pRBBpiTbOvjgyMlMgAl2MPHmsBeBlkp2dVOklqScx2F_-4KU_Eg63TPduws0VhC6lSJZ_OqrB4sW1tJOIS5Zds2y9UT0bq_N8pNQvX1Vk62uX5a_0AEVuD1Co6XUT6R2YHVvKmTJisXhncXjG68YT0hVsq8RWFJU1tU1Niy5-WLIvXRYOiPI2Y-Gn9uWlDOkov047GfcMn4LZdlO-QLIQo0NKaxB2HcASUElhdoxPrfwRtWFHo-gxgYaqYU7ai1L4Zyhbe-wLBlfML4oywNWTpvS0usgnM4Yzxkv_GzpGawvxeFNrgfMDttOCoejxUq0CJvBot56qPcvj0H7H7gj69BsXjpPwBqsqz3cZFVpVZMjrYQsHUuK0wDZ0ooWywNLio2HEyzxygM0b4__OmiqwUuPmKwzfeXgYTNiq7147gcBAEg5uBvmfgiOJcVxH3g86fQrAz8O7gNt-SgcFVsnHFWlsBaNY3yhm8ai0w3jiwscvIBHxnNgPIPAbJhQYyU9eYwvHgd6PUGeo9ivYJxfqAjMPProaMmO3mGcn3wC42PQ9UbBN3GMK9h8HRhaf8HgmgxW7qv0ARzD6s-IOq8P498ga9zyh5ka1_8HNL1F8FWOPBGtIHUy8mzjmWl42Jz3GzXDw_osG_dnfEHKMZ4_bKKRNmD8-kK-PspHAAOei8oURVGoCVeVPNk81A02i78IIDaLocV2iwYOwpDYSoSnPSoYstar5Lf_6Bb-I3lfBNdelBVP2nxmyU0lWby6p8poqxvnIf_h4ReM3xaMX3sz_tY5aunVay5025FEA49oLGkF0zxKkyhJF3EOjTbwPEtZvCp092Jotw_6Cq_vvEOhTaeN8CUjAlhJCWGmBYMWzQHraPBQb8XOV3GoJLDsGnQXlkQRsGwNDclQroa_PdW3ktRn8P-8mTkoe2_zyPTV1WGwg8WrIITD2a5pNGXxaiPMDn1EwvNiVs7Sq666eiJV6yd71dpD5afsDYoaWl2j9BM7bemZxas7ZZ2QEus1GS8vBhC_G70zooVbkmhZVpypeSTbCwkPrq9Js6zgMefDggatDTWWZcVjwbJio7X0i3-Th5Zlhac9K7akPjYXHFoXVV33k_r6m_DuVGWwReWEhN9Ifb5ElJ4QRfF_CQvjt7r3Dg-g8BmP-PT20zeD6cKIj1wwZORP4Yn_c0d8yfTpky-mvjWazkMpXgEao0NuDocaDIeat6gRJLGGukdwGgz-s6fBeGB8XpbbnqQjVX5Pp_CXurcUGF-FMzisGZqZU-PGczhKLs7Oubfgz7qMcGJNc2Dz4mfrdzwklt3867sfH2n_66IqacuyYmjYs2Kas6wYbwIsK8pyuEOc4iDaj612wdMxxpR24RzD506oGmtojG6hFZXRPpbO3M9HH80GQngy3CAu25vQ2NSe8w9D8DT8htqvPCy7-SA1Zh_ANgNRgAche-HQ-pRgfL54E4M_eYB9b2iFeGTxajoUCdihQuP3jN71Uz5bW5LCAFnbIzwJC53BA-neyhcw2Gnjm0lSsHeusx5zuOPtyO37bVTp1ncvPuaG_646oz-FJvY2aLSM36bpfJaBt_-J3B4ENPQcSv-xRRz73lkMlbAIYqsPCJVuW3Ij1dveATmwiO14I6VnqDVa72rwRRSoCQOe_XEfz2ioehcV0Hq6n4zousEqAcd7ZAR3oexiHfQMfGgFxWVvZSHkB0zD_W-ag1A18DiCzR6hJUWtkIDPou2kVwGiFp07pk2la_R7_l0ZFBJu1M5niC-ms_hXQiNMtadKyIdOOBLyV2H30T70w4WnaXMh_8VQ_bsm5ewQRQVK-UCv-EFz_RdW3ynyQ_SKo3QWhzPEgnBgeuWoxeNpcg5VEFWF1oJuvP4LBIzn0aReJnWe5GKCy-k8TeM0m_Nksl_mVRonCWZpzXmdzRaIcZ0nfDtP8kW6recTWvKYZ3Eaz6fzeJEm0WzWJIjJXCymeYMJsjTGVpCMfLxF2uwmwVfLaZLO8nQixRalDT-8cK7wafCkz6tsPTHLEKTbfmdZGkuyzp7VOHISl_dnA98xGUx15wvW3f06hJv1PpT0GU8_DeRvbjT-SIdGit0Ql8ezftIbufzxrBrNPSz5vwMAAP__Q6jEdg">