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

[toby at winterflaw dot net]

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

--- Comment #13 from toby at winterflaw dot net ---
On 29 Mar 2018, andysem at mail dot ru <gcc-bugzilla@gcc.gnu.org> wrote:
>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
>
>--- Comment #12 from andysem at mail dot ru ---
>Is read-only memory a valid use case for __atomic intrinsics anyway?
>These
>intrinsics are primarily targeted to implement std::atomic, but does
>the
>standard guarantee these operations (primarily, std::atomic::load()) do
>not
>issue writes to the memory?

On Intel, all CAS operations always write, even if thr compare failed.