__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 not defined on aarch64

[Toebs Douglass]

On 29/06/17 11:28, Andrew Haley wrote:
> I think I now properly understand Richard Earnshaw's point: that we
> *do* support a full set of atomic primitives for 16-byte types via
> libatomic, but for them to work as a sequentially-consistent set we
> must use the same locking scheme for all of them.  It's ugly, and
> horrible for anyone who simply wants a double-word CAS, but it is what
> it is.  We can't use LDXP because it isn't atomic on its own, and the
> ARM manual is quite explicit about this.  Anyone who wants to use
> a real compare-and-swap-16 is on their own.

I may be utterly, utterly wrong, but I think what's happened makes
perfect sense at each step of the way but has led to an outcome which is
crazy.  I publish a lock-free data structure library and I must
specifically avoid libatomic.