I read from Effective Java that In the absence of synchronization the following sequence A below can be converted into sequence B by the virtual machine and this is called hoisting. I also read somewhere that if variables are not declared as volatile instructions involving the variables can be reordered . Are hoisting and reordering the same thing?
while (!done) sequence A
i++;
if (!done)
while (true) sequence B
i++;
They are slightly different.
Hoisting means that you have pulled some operation out of a loop because the loop itself does not affect the result of the operation. In your case, you are hoisting the conditional test out of the while loop.
Re-ordering means changing the sequence of instructions in a way that does not affect the result. Typically this would be adjacent instructions with no data dependencies, e.g. it does not matter which order you perform the following two statements:
int a = x;
int b = y;
The term "reordering" as it's used in the Java Memory Model refers to all possible optimizations that may affect correctness of improperly synchronized programs.
So, reordering in this sense is a generic term that includes optimizations such as hoisting, effects of out-of-order execution, inconsistencies caused by memory hierarchy (i.e. caches), and so on.
Reordering relates to performance of execution where as reordering doesn't impact performance.
Hoisting is done to reordering of Memory barrier instructions.
