ARM/POWER Relaxed Memory Model

ARM和POWER系统的概念模型是，每个处理器从其自己的完整内存副本中读取和向其写入，每个写入独立地传播到其他处理器，随着写入的传播，允许重新排序。

在这个宽松的(relaxed)模型中，我们迄今为止所看到的每一个litmus test的答案都是“yes，这真的可能发生。”

Litmus Test: Message Passing
Can this program see r1 = 1, r2 = 0?
// Thread 1           // Thread 2
x = 1                 r1 = y
y = 1                 r2 = x
On sequentially consistent hardware: no.
On x86 (or other TSO): no.
On ARM/POWER: yes!

Litmus Test: Store Buffering
Can this program see r1 = 0, r2 = 0?
// Thread 1           // Thread 2
x = 1                 y = 1
r1 = y                r2 = x
On sequentially consistent hardware: no.
On x86 (or other TSO): yes!
On ARM/POWER: yes!

Litmus Test: Independent Reads of Independent Writes (IRIW)
Can this program see r1 = 1, r2 = 0, r3 = 1, r4 = 0?
(Can Threads 3 and 4 see x and y change in different orders?)
// Thread 1    // Thread 2    // Thread 3    // Thread 4
x = 1          y = 1          r1 = x         r3 = y
							  r2 = y         r4 = x
On sequentially consistent hardware: no.
On x86 (or other TSO): no.
On ARM/POWER: yes!

Litmus Test: Load Buffering
Can this program see r1 = 1, r2 = 1?
(Can each thread's read happen after the other thread's write?)
// Thread 1    // Thread 2
r1 = x         r2 = y
y = 1          x = 1
On sequentially consistent hardware: no.
On x86 (or other TSO): no.
On ARM/POWER: yes!