[Bug libstdc++/71660] [5/6/7 regression] alignment of std::atomic<8 byte primitive type> (long long, double) is wrong on x86

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71660

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-06-27
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
However alignof(std::atomic<long long>) != alignof(_Atomic long long) for
x86-32, which is wrong, since that was the point of PR65147.

#ifdef __cplusplus
#include <atomic>
using std::atomic_llong;
#else
#include <stdalign.h>
#include <stdatomic.h>
#endif

#include <stdio.h>

struct InStruct { long long x; };
struct AtomicStruct { int i; atomic_llong x; };

int main()
{
  printf("%zu %zu %zu %zu\n",
      alignof(struct InStruct),
      alignof(atomic_llong),
      sizeof(struct AtomicStruct),
      alignof(struct AtomicStruct));
}

With GCC this prints 4 4 12 4 but with G++ it prints 4 8 16 8

We might need a front-end attribute so std::atomic can emulate what the C
front-end does with _Atomic.