1. summary
When instructions refer to different memory locations, with regards to loads/reads from memory and stores/writes to memory there are 4 possible issues:
- writes can be reordered ahead of other reads
- writes can be reordered ahead of other writes
- reads can be reordered ahead of other reads
- reads can be reordered ahead of other writes
x86架构下只有最后一种情况会发生，我们称之为strong memory model。
举个例子，假设x和y初始值均为0，有两个线程分别执行下面每组指令：
// thread 1
mov [x],1 ; 向地址x写入1
mov eax,[y] ; 从地址y读取到eax
// thread 2
mov [y],1 ; 向地址y写入1
mov ebx,[x] ; 从地址x读取到ebx
在x86下eax和ebx可能的状态中有均为0的组合，按照常规逻辑由于读取x和y时至少有一组是赋值之后执行的，
所以必定发生了reorder。每组线程写和读的地址又不一样，所以cpu难以判定dependency关系而决定不重排。
而像ARM、PowerPC它们上述4种情况都可能会发生，这种称之为weak memory model。
2. TSO model
Another way to be able to predict the x86 processor behaviour is to have a model where each hardware thread has a FIFO buffer for writes, while reads are not immediate, not buffered. When reading a memory location, it is first looked up in the FIFO buffer for writes.
x86下每个hardware thread有一个用于store的FIFO buffer，但读不会有缓冲，读的时候会先查询store buffer。
这种称之为 total store ordering。
对于互斥的内存访问时需要设置一个全局锁global lock，比如lock指令flush write buffer到内存。
refer:
http://bajamircea.github.io/coding/cpp/2019/10/25/cpu-memory-model.html
1. summary
When instructions refer to different memory locations, with regards to loads/reads from memory and stores/writes to memory there are 4 possible issues:
x86架构下只有最后一种情况会发生，我们称之为strong memory model。
举个例子，假设x和y初始值均为0，有两个线程分别执行下面每组指令：
在x86下eax和ebx可能的状态中有均为0的组合，按照常规逻辑由于读取x和y时至少有一组是赋值之后执行的，
所以必定发生了reorder。每组线程写和读的地址又不一样，所以cpu难以判定dependency关系而决定不重排。
而像ARM、PowerPC它们上述4种情况都可能会发生，这种称之为weak memory model。
2. TSO model
Another way to be able to predict the x86 processor behaviour is to have a model where each hardware thread has a FIFO buffer for writes, while reads are not immediate, not buffered. When reading a memory location, it is first looked up in the FIFO buffer for writes.
x86下每个hardware thread有一个用于store的FIFO buffer，但读不会有缓冲，读的时候会先查询store buffer。
这种称之为 total store ordering。
对于互斥的内存访问时需要设置一个全局锁global lock，比如lock指令flush write buffer到内存。
refer:
http://bajamircea.github.io/coding/cpp/2019/10/25/cpu-memory-model.html