[libstdc++/65033] Give alignment info to libatomic

[Jonathan Wakely]

On 12/02/15 13:23 -0800, Richard Henderson wrote:
>When we fixed PR54005, making sure that atomic_is_lock_free returns the same
>value for all objects of a given type, we probably should have changed the
>interface so that we would pass size and alignment rather than size and object
>pointer.
>
>Instead, we decided that passing null for the object pointer would be
>sufficient.  But as this PR shows, we really do need to take alignment into
>account.
>
>The following patch constructs a fake object pointer that is maximally
>misaligned.  This allows the interface to both the builtin and to libatomic to
>remain unchanged.  Which probably makes this back-portable to maintenance
>releases as well.

Am I right in thinking that another option would be to ensure that
std::atomic<> objects are always suitably aligned? Would that make
std::atomic<> slightly more compatible with a C11 atomic_int, where
the _Atomic qualifier affects alignment?

https://gcc.gnu.org/PR62259 suggests we might need to enforce
alignment on std::atomic anyway, or am I barking up the wrong tree?