Will reordering instructions under a single thread in Java lead to changes in the output results?

Java single-threaded instruction reordering: Will the output result be changed?

The Java compiler and processor reorder instructions for performance optimization. This reordering can cause problems in multithreaded environments, but generally does not affect the correctness of the program in a single threaded environment.

Let's look at an example:

 System.out.println("1");
System.out.println("2");

Will these two simple print statements in a single thread result in the output of "21" due to instruction reordering in a single thread? At first glance, due to the lack of data dependencies, it seems possible.

The answer is no. If reordering can lead to such a result, it is not just a simple reordering, but a complete out-of-order execution. Although the Java memory model allows instructions to be reordered, it must abide by certain rules to ensure that the execution results of single-threaded programs remain unchanged. In this example, even without data dependencies, the Java memory model ensures that the execution order under a single thread is not disrupted. Therefore, the output is always "12", not "21".

It should be noted that instruction reordering mainly has an impact in a multi-threaded environment. In a multithreaded environment, visibility and order issues between threads may lead to unpredictable results due to instruction reordering. However, in a single-threaded environment, the Java memory model ensures predictability of program execution results, and instruction reordering does not change the final output.

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