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

["s_gccbugzilla at nedprod dot com" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #33 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
(In reply to Andrew Pinski from comment #31)
> 
> Again the problem is stuff like:
> static const _Atomic __int128_t t = 2000;
> 
> __int128_t g(void)
> {
>   return t;
> }
> 
> DOES NOT WORK if you use CAS (or ldaxp/stlxp).

I think we are talking about different things here. You are talking about
calling convention. I'm talking about
std::atomic<__int128>::compare_exchange_weak() i.e. that the specific member
function compare_exchange_weak() is not generating cmpxchg16b if compiled with
GCC, but does with clang.

Re: your original point, I cannot say anything about _Atomic. However, for
std::atomic<__int128>, as __int128 is not an integral type it seems reasonable
to me that its specialisation tell the compiler to not store it in read only
memory. Mark it with attribute section, give it a non-trivial destructor, or
whatever it needs.

Perhaps I ought to open a separate issue here, as my specific problem is that
std::atomic<__int128>::compare_exchange_weak() is not using cmpxchg16b? Mr.
Wakely your thoughts?