GCC in-line assembly and the removal of -mcx16

[Alexander Monakov]

On Fri, 26 May 2017, Toebs Douglass wrote:
> > Just as a note to any future readers, although I may well be wrong, I
> > think this is only true for GCC 4.8.0 onwards, with the introduction of
> > the 128 bit types.

(4.6 actually)

> I *think* -mcx16 was introduced in 4.3.0.
> 
> Why would it *be* introduced here if GCC could not use it?
> 
> Was there then another way to indicate to the __sync functions they were
> being given a 128-bit type?

The intended way was via a typedef:

    typedef int my_int128 __attribute__((mode(TI)));

And it appears there's another, undocumented (and thus unsupported afaict) way,
via suffixing object size to the builtin name:  __sync_val_compare_and_swap_16.

Alexander