[Bug c/126338] New: Array of atomic structs has smaller alignment than its elements

[tellowkrinkle at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126338

            Bug ID: 126338
           Summary: Array of atomic structs has smaller alignment than its
                    elements
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tellowkrinkle at gmail dot com
  Target Milestone: ---

In the following code, on a 64-bit platform, `struct One` has 16-byte
alignment, while `struct Two` has 8-byte alignment.  Getting a pointer to
`Two::yay[0]` would therefore be misaligned for its `_Atomic struct Yay` type.

```
struct Yay {
    void *a;
    void *b;
};

struct One {
    _Atomic struct Yay yay;
};

struct Two {
    _Atomic struct Yay yay[1];
};

unsigned OneAlign() {
    return _Alignof(struct One);
}

unsigned TwoAlign() {
    return _Alignof(struct Two);
}
```

Godbolt link: https://gcc.godbolt.org/z/c3M8vvPhf