在Java内存模型中发生并编程顺序(Happens before and program order in Java Memory Model)

 我有一些关于程序的顺序以及它如何影响JMM中的重新排序。 
 
 在Java内存模型中，程序顺序（ po ）被定义为程序中每个线程的总操作顺序。 根据JLS ，这导致了发生之前（ hb ）边缘： 
 
 
 
 如果x和y是同一个线程的动作，并且x按照程序顺序在y之前，那么hb（x，y） （即x发生在y之前）。 
 
 
 所以对于一个简单的程序P： 
 
  initially, x = y = 0
     T1     |     T2
 -----------|-----------
  1. r1 = x | 3. r2 = y  
  2. y = 1  | 4. x = r2
 
 我认为宝（1,2）和宝（3,4） 。 因此， hb（1,2）和hb（3,4） 。 
 
 现在假设我想对这些陈述重新排序，给我P'： 
 
  initially, x = y = 0
     T1     |     T2
 -----------|-----------
  2. y = 1  | 3. r2 = y  
  1. r1 = x | 4. x = r2
 
 根据本文 ，我们可以对任何两个相邻的语句（例如1和2）进行重新排序，前提是重新排序不会消除任何有效执行中的边缘之前发生的任何传递事件。 然而，由于hb是由po定义的（部分），而po是线程行为的全部顺序，所以在我看来，在不违反hb的情况下重新排序任何两个语句是不可能的，因此P'不是合法的转换。 
 
 我的问题是： 
 
 
 
 我对po和hb的理解是否正确，并且我是否正确地定义了po和hb关于上述程序P ？ 
 
 
 我对hb失败问题重新排序的理解在哪里？ 
 

I have some regarding program order and how it affects reorderings in the JMM.
 
In the Java Memory Model, program order (po) is defined as the total order of actions in each thread in a program. According to the JLS, this induces happens-before (hb) edges:
 
 
 
If x and y are actions of the same thread and x comes before y in program order, then hb(x, y) (i.e. x happens-before y).
 
 
So for a simple program P:
 
  initially, x = y = 0
     T1     |     T2
 -----------|-----------
  1. r1 = x | 3. r2 = y  
  2. y = 1  | 4. x = r2
 
I think po(1, 2) and po(3, 4). Thus, hb(1, 2) and hb(3, 4).
 
Now suppose I wanted to reorder some of these statements, giving me P':
 
  initially, x = y = 0
     T1     |     T2
 -----------|-----------
  2. y = 1  | 3. r2 = y  
  1. r1 = x | 4. x = r2
 
According to this paper, we can reorder any two adjacent statements (e.g. 1 and 2), provided that the reordering doesn't eliminate any transitive happens-before edges in any valid execution. However, since hb is defined (partially) by po, and po is a total order over a thread's actions, it seems to me that it would be impossible to reorder any two statements without violating hb, thus P' is not a legal transformation.
 
My questions are:
 
 
 
Is my understanding of po and hb correct, and have I correctly defined po and hb with respect to the above program P?
 
 
Where is my understanding about reordering with regards to hb failing?
 

原文：https://stackoverflow.com/questions/32492621