[Issue 21230] New: Incorrect stack alignment of 16byte aligned aggregates on linux

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21230

          Issue ID: 21230
           Summary: Incorrect stack alignment of 16byte aligned aggregates
                    on linux
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: mrsmith33 at yandex.ru

When struct is passed on the stack and alignment of struct is 16 bytes compiler
does not align the struct properly. Here is a comparison between ldc, dmd and
gcc. Both ldc and dmd miss the alignment.

https://godbolt.org/z/xKc8WK

D code:
```
align(16)
struct s { long a; }
extern(C) void foo(int, int, int, int, int, int, s, s);
void bar() {
    // push    10
    // push     7
    // call    foo at PLT
    foo(1,2,3,4,5,6, s(7),s(10));
}
```

C code:
```
struct __attribute__((aligned(16))) s { long a; };
void foo(int, int, int, int, int, int, s, s);
void bar() {
    // push     0
    // push    10
    // push     0
    // push     7
    foo(1,2,3,4,5,6, s{7},s{10});
}
```

However alignment starts to work if you replace 16 with 32.

https://godbolt.org/z/6r7GbW

Issue for ldc: https://github.com/ldc-developers/ldc/issues/3557

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