[Issue 24594] New: ImportC: Packed struct has wrong layout

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24594

          Issue ID: 24594
           Summary: ImportC: Packed struct has wrong layout
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: tim.dlang at t-online.de

```
#include <stdio.h>
#include <stdint.h>

struct epoll_event
{
  uint32_t events;
  uint64_t data;
} __attribute__((packed));

int main()
{
    printf("size: %zd\n", sizeof(struct epoll_event));
    printf("data offset: %zd\n", &((struct epoll_event*)0)->data);
}
```

Output with gcc:

size: 12
data offset: 4

Output with dmd:

size: 16
data offset: 8

The type epoll_event is originally from Linux header /usr/include/sys/epoll.h,
but modified for this test.

The problem was found by comparing type sizes in druntime with type sizes found
with ImportC: https://github.com/dlang/dmd/pull/16571

--