__HAVE_64B_ATOMICS and alignment

[Florian Weimer <fweimer at redhat dot com>]

I'm trying to figure out what __HAVE_64B_ATOMICS means.  Does it imply 
that atomics must be available for unaligned 64-bit objects  (such as a 
pair of longs), or only for 8-byte aligned values of type uint64_t? 
What happens if the architecture only mandates 4-byte alignment for 
uint64_t?

I'm describing the background for this question.  The opaque sem_t 
definition looks like this:

typedef union
{
  char __size[__SIZEOF_SEM_T];
  long int __align;
} sem_t;

But the __HAVE_64B_ATOMICS definition of the non-opaque version looks 
like this:

struct new_sem
{
  uint64_t data;
  int private;
  int pad;
};

This means that for an LP32 architecture such as i686 which could 
conceivable provide 64-bit atomics, we might try to perform an atomic 
operation on a potentially misaligned uint64_t value.

Could this be a problem on other architectures?  (IA-32 is generally 
fine with atomic operations on unaligned objects.)

Thanks,
Florian