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

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #34 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Niall Douglas from comment #33)
> Re: your original point, I cannot say anything about _Atomic. However, for
> std::atomic<__int128>, as __int128 is not an integral type it seems

That depends whether you use -std=c++NN or -std=gnu++NN

> 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.

std::atomic<T> requires T to have a trivial destructor, so the destructor is
always trivial.

> 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?

Isn't that covered by PR 94649?

std::atomic just calls the relevant __atomic built-in for all operations. What
the built-in does is not up to libstdc++.