<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63257>63257</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang: ABI incompatibility with MSVC on i686-windows for `__declspec(align(N))`, with N <= 4
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            ABI,
            platform:win32
      </td>
    </tr>

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

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

<pre>
    For the following three struct definitions
```c
struct NoRequestedAlign {
  uint64_t x;
  uint64_t y;
  uint64_t z;
};

__declspec(align(1))
struct RequestedAlign1 {
 uint64_t x;
  uint64_t y;
  uint64_t z;
};

__declspec(align(8))
struct RequestedAlign8 {
 uint64_t x;
  uint64_t y;
  uint64_t z;
};
```

when passed by value:

MSVC passes `NoRequestedAlign` and `RequestedAlign1` on the stack (like `byval`), and passes `RequestedAlign8` indirectly.
Clang passes `NoRequestedAlign` on the stack (with `byval`), and passes `RequestedAlign1` and `RequestedAlign8` indirectly.

It seems that Clang passes a struct indirectly if there is any `__declspec(align)` requested alignment, but MSVC only does so if the requested alignment is > 4.

Demo: https://clang.godbolt.org/z/fsn78PhxT
</pre>
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