在Java中编译循环 [英] Compiling loops in Java

问题描述

我可以从 JVM中看到规范这段代码：

void spin() {
    int i;
    for (i = 0; i < 100; i++) {
        ;    // Loop body is empty
    }
}

应编译成：

0   iconst_0
1   istore_1
2   goto 8
5   iinc 1 1
8   iload_1
9   bipush 100
11  if_icmplt 5
14  return  

其中条件检查 if_icmplt 是循环体之后，但当我自己编译和查看与javap，我看到：

where condition check if_icmplt is after the loop body, but when I compile it myself and view with javap, I see:

0:   iconst_0
1:   istore_1
2:   iload_1
3:   bipush  100
5:   if_icmpge       14
8:   iinc    1, 1
11:  goto    2
14:  return

循环条件在循环体之前。为什么会发生这种情况？

and loop condition is before the cycle body. Why does this happen?

在身体之后放置条件阻止我们在每个循环后执行goto，并且对我看起来符合逻辑。那么为什么OracleJDK做另一种方式？

Placing condition after body prevents us from doing goto after every cycle and looks logical to me. So why is OracleJDK doing another way?

推荐答案

这不是为了更好的JIT优化 - 对于JIT，这些代码片段是等价的。这是因为没有意义在javac中进行优化，因为JIT优化无论如何都是更强大的。

It is not for better JIT optimizations - for JIT, these code snippets are equivalent. It is because there is no sense to make optimizations in javac, since JIT optimizations are more powerful anyway.

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