Understand the classification of reordering

1. Compiler reordering: If the statements have no sequential dependencies, the compiler can re-adjust the execution order of the statements in order to optimize performance.
2. CPU instruction reordering: At the instruction level, multiple instructions without dependencies are executed in parallel.
3. CPU memory reordering: The CPU has its own cache, and the order of execution of instructions and the order of writing to main memory are not completely consistent.