[Bug middle-end/66713] atomic compare_exchange_strong creates spurious store for x86-64 at -O3

[tkoeppe at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66713

--- Comment #3 from Thomas Köppe <tkoeppe at google dot com> ---
Note: The code in question, and the hand-written assembly, are taken from the
ZMQ library:

https://github.com/zeromq/libzmq/blob/master/src/atomic_ptr.hpp

I added the C++11 atomic support recenlty.