<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/146047>146047</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang][CUDA][HIP] __declspec(empty_bases) leads to inconsistent struct layout between host and device
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    Since #87651, the struct layout when compiling CUDA/HIP code for Windows is consistent between host and device. However, there is the `__declspec(empty_bases)` attribute that can modify the struct layout. Right now this attribute is respected on the host side but ignored on the device side.

Consider this example:
```c++
struct A {};
struct B {};
struct __declspec(empty_bases) C : A, B {
    int i;
};

__attribute__((global)) void C_kernel(C c)
{
  c.i = 1;
}
```
The device-side compilation reports `warning: __declspec attribute 'empty_bases' is not supported [-Wignored-attributes]`. With `-fdump-record-layouts`, you can see that the device-side layout is
```
         0 | struct C
         0 |   struct A (base) (empty)
         1 |   struct B (base) (empty)
         4 |   int i
           | [sizeof=8, align=4,
           |  nvsize=8, nvalign=4]
```
while the host-side layout is
```
         0 | struct C
 0 |   struct A (base) (empty)
         0 |   struct B (base) (empty)
         0 |   int i
           | [sizeof=4, align=4,
           | nvsize=4, nvalign=4]
```
Godbolt: https://cuda.godbolt.org/z/ccrs4Ecf6

Since #87651 you no longer need the `empty_bases` attribute to manually make struct layouts consistent between host and device. The attribute is used e.g. in Nvidia's CCCL library for nvcc compatibility and since https://github.com/NVIDIA/cccl/pull/3155/commits/fc84efda5b15a6eee0224a4685a4270653bfbe16 it is causing a layout consistency with Clang on Windows.

To solve this issue, Clang should respect `__declspec(empty_bases)` on the device side too when targeting Windows.
</pre>
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