__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 not defined on aarch64

[Alexander Monakov]

On Fri, 30 Jun 2017, Andrew Haley wrote:
> Perhaps so.  However, it would make more sense and be more maintainable
> if, rather than use a command-line option, a variable attribute was used
> to tell the compiler what this program is supposed to mean.  IMO.

Correction: this can't be a variable attribute, because when you take a pointer
to such object, the property remains relevant to the result of dereferencing
that pointer.  Therefore, it would need to be a type attribute as Richard said.

I'd caution against adding attributes due to source code uglification and
compiler compatibility concerns.

An option is also simple; adding the attribute would be repeating history with
refusing native 128-bit CAS-based atomics due to r/o issue: creating extra work
for compiler writers and frustration for users, only to handle an edge case they
don't care about.

Alexander