Atom in C++11

Memory ordering

1. Sequentially consistent ordering :
   memory_order_seq_cst
2. Acquire-release ordering :

• memory_order_acquire
• memory_order_release
• memory_order_consume
• memory_order_acq_rel

3. Relaxed ordering :

• memory_order_relaxed

memory_order_relaxed 没有fencing作用
memory_order_consume 后面依赖此原子变量的访存指令勿重排至此条指令之前
memory_order_acquire 后面访存指令勿重排至此条指令之前
memory_order_release 前面访存指令勿重排至此条指令之后。当此条指令的结果对其他线程可见后，之前的所有指令都可见
memory_order_acq_rel acquire + release语意
memory_order_seq_cst acq_rel语意外加所有使用seq_cst的指令有严格地全序关系

fence

• atomic_thread_fence