Instruction rearrangement, happens before, as if serial

There are certain restrictions on instruction rearrangement , There are two rules ：happens-before、as-if-serial To restrain .

Catalog

One . happens-before

1 Definition

2. understand

3. Specific rules

Two .as-if-serial The rules

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One . happens-before

1 Definition

• If operation A happens-before B, that A The execution result of will be right B so , and A The execution order of must be in B Before .
• There is... Between the two operations happens-before Relationship , It doesn't mean that Java The specific implementation of the platform must be in accordance with happens-before Execute in the order specified by the relationship ⾏. If the execution after reordering ⾏ result , And press happens-before Relationship to hold ⾏ Result ⼀ Cause , Then the instructions can be reordered .

2. understand

From the perspective of programmers ： It provides a memory structure similar to strong memory for programmers , Easy to program .

Stand in the editor and processor manufacturers ： Without affecting the correct results , You can let editors and processor manufacturers optimize .

3. Specific rules

• Rules of procedure sequence ： Every operation of a thread ,happens-before Any subsequent operation in the thread .
• Monitor lock rules ： Unlocking a lock ,happens-before Then lock the lock .
• volatile Variable rule ： To a volatile Domain writing ,happens-before In any follow-up to this volatile Domain reading .
• Transitivity ： If A happens-before B, And B happens-before C, that A happens-before C.
• start() The rules ： If the thread A Perform the operation ThreadB.start()( Start thread B), So thread A Of ThreadB.start() operation happens-before Thread B Any operation in .
• join() The rules ： If the thread A perform ThreadB.join() And return successfully , be B Any operation in should happens-before Thread A from ThreadB.join() Operation successful return .

Two .as-if-serial The rules

as-if-serial Semantic meaning means ： No matter how reorder （ Compiler and processor to improve parallelism ）,（ Single thread ） The execution result of the program cannot be changed . compiler ,runtime And the processor must comply as-if-serial semantics .

In order to observe as-if-serial semantics , Compilers and processors Do not reorder operations that have data dependencies , Because this reordering will change the execution result . however , If there are no data dependencies between operations , These operations can be reordered by the compiler and processor .