how to understand instruction reordering has no effect on single thread, it will not affect the running result of the program, but it will affect multi-thread.
for example:
int i = 0; //1
i = i + 1; //2
System.out.println(i); //3if reordering is performed and the order of execution of 2 and 3 is changed, it will definitely affect the result of the run.
first of all, you need to understand the meaning of instruction reordering: the processor is as efficient as possible without affecting the final calculation result.
how to make sure that the final calculation result is not affected? When calculating, there is a data dependency, such as the printf, here, which depends on the data I and does not output until I completes the final calculation. Or the calculation of the variable value I must be happens-before printf.
JMM (java's memory model) is special, the happens-before relationship is very important, you can learn about it.
instruction rearrangement is not a disorder, 2 3 dependency is not reversed.
single-threaded instructions can be considered to be executed serially, and only
a = 1; // 1
b = 2; // 2
System.out.println(a); // 3
System.out.println(b); // 4 like 1 2 can be reversed without affecting the result. 1 3 , 2 4 and 3 4 will not.
you will know that
instruction rearrangement has an important premise in a single-threaded environment, instruction rearrangement cannot affect the final result , so you can't rearrange it here.
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