The happened-before relationship between memory operations

It all comes down to how memory operations are ordered at runtime, and how the runtime guarantees when the effects of those memory operations are observable. To explain the Go memory model, we need to define three relationships that define different orderings of memory operations.

In any goroutine, the ordering of memory operations must correspond to the correct sequential execution of that goroutine as determined by the control flow statements and expression evaluation order. This ordering is the sequenced-before relationship. This, however, does not mean that the compiler has to execute a program in the order it is written. The compiler can rearrange the execution order of statements as long as a memory read operation of a variable reads the last value written to that variable.

Let’s refer to the following program:

1: x=1
2: y=2
3: z=x
4: y++
5: w=y

The z variable will always be set to 1, and the w variable...