吊装和重新订购同样的东西？ [英] Are hoisting and reordering the same thing?

问题描述

我从 Effective Java 在没有同步的情况下，下面的序列A可以被虚拟机转换为序列B，这称为 hoisting 。我还在某处读到，如果变量未被声明为涉及变量的易失性指令，则可以重新排序。提升和重新排序是一样的吗？

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++;

推荐答案

它们略有不同。

提升表示您已将某些操作从循环中拉出，因为循环本身不会影响操作的结果。在您的情况下，您将从while循环中提升条件测试。

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.

重新排序意味着以某种方式更改指令序列这不会影响结果。通常，这将是相邻的指令，没有数据依赖性，例如，您执行以下两个语句的顺序无关紧要：

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;

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