[concurrent programming 049] talk about the classification of reordering?

Talk about the classification of reordering ?

When executing the program , To improve performance , compiler （jvm Inside ） And the processor （ Operating system level ） Instructions are often reordered . rearrangement Sequential Division 3 species type .

1） Compiler optimized reordering . Compiler without changing the semantics of single thread program , You can rearrange the execution order of statements .

2） Instruction level parallel reordering . Modern processor adopts instruction level parallel technology （Instruction-Level Parallelism, ILP） To put multiple instructions Overlapping execution . If there is no data dependency , The processor can change the execution order of machine instructions corresponding to statements .

3） Reordering of memory system . Because the processor uses cache and read / Write buffer , This makes loading and storage operations appear to be out of order .

Aforementioned 1 Belongs to compiler reorder , 2 and 3 It belongs to processor reordering . These reordering can lead to multithreaded programs Memory visibility problem . For compilers , JMM The compiler reordering rule of prevents a specific type of compiler reordering （ Not all compiler reordering is prohibited ） .

Reorder for processor , JMM The processor reordering rules for require Java When the compiler generates a sequence of instructions , Insert a specific type of memory barrier （Memory Barriers, Intel be called Memory Fence） Instructions , Disable specific types of processor reordering through memory barrier instructions .

In a single threaded program , Reorder operations that have data dependencies , Does not change the execution result （ This is also as-if-serial Semantics allows control over existence Reasons for reordering dependent operations ） ; But in multithreaded programs , Operations that depend on the existence of control Reorder , May change the execution of the program fruit , Therefore, it must be controlled by certain synchronization means .