[Bug target/80878] -mcx16 (enable 128 bit CAS) on x86_64 seems not to work on 7.1.0

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878

--- Comment #26 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Florian Weimer from comment #23)
> Ahh, I think this bug here is specific to __uint128 (with the C front end at
> least)
> 
> The C translation of the C++ reproducer from comment 20:
> 
> struct a
> {
>   long  _Alignas(16) x;
>   long y;
> };
> 
> _Bool
> cmpxchg (struct a *data, struct a expected, struct a newval)
> {
>   return __atomic_compare_exchange_n (&data, &expected, &newval, 1,
>                                       __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
> }
> 
> produces the atomic instruction.

Eh, no, that code does something else.