[Bug target/65146] alignment of _Atomic structure member is not correct

["peter at cordes dot ca" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65146

--- Comment #8 from Peter Cordes <peter at cordes dot ca> ---
BTW, all of my proposals are really ABI changes, even if struct layout stays
the same.

All code has to agree on which objects are lock-free or not, and whether they
need to check alignment before using an SSE load instead of lock cmpxchg8b or
something.

It won't be safe to link with binaries compiled with old gcc that assumes a
simple SSE load/store is atomic on an 8B atomic_llong*, if the new design can
still pass it underaligned pointers.  The existing ABI is broken.

Some code may happen to not be affected, especially when running on Intel
hardware (where only 64B boundaries matter, not 8B boundaries for x86 in
general).  Or because they only depend on atomic RMW being atomic, not pure
load or pure store, so they just take the ~100x performance hit without losing
correctness in cases where a boundary is crossed.