没有JIT优化 [英] No JIT Optimization

问题描述

查看这个问题：

代码：

class test
{
    public static void main(String abc[])
    {
        for( int k=1; k<=3; k++)
        {
            for( int N=1; N<=1_000_000_000; N=N*10)
            {
                long t1 = System.nanoTime();

                int j=1;
                for(int i=0; i<=N; i++)
                    j=j*i;

                long t2 = System.nanoTime() - t1;
                System.out.println("Time taken for "+ N + " : "+ t2);
            }
        }
    }
}

以上代码的输出：

Time taken for 1 : 2160
Time taken for 10 : 1142
Time taken for 100 : 2651
Time taken for 1000 : 19453
Time taken for 10000 : 407754
Time taken for 100000 : 4648124
Time taken for 1000000 : 12859417
Time taken for 10000000 : 13706643
Time taken for 100000000 : 136928177
Time taken for 1000000000 : 1368847843
Time taken for 1 : 264
Time taken for 10 : 233
Time taken for 100 : 332
Time taken for 1000 : 1562
Time taken for 10000 : 17341
Time taken for 100000 : 136869
Time taken for 1000000 : 1366934
Time taken for 10000000 : 13689017
Time taken for 100000000 : 136887869
Time taken for 1000000000 : 1368178175
Time taken for 1 : 231
Time taken for 10 : 242
Time taken for 100 : 328
Time taken for 1000 : 1551
Time taken for 10000 : 13854
Time taken for 100000 : 136850
Time taken for 1000000 : 1366919
Time taken for 10000000 : 13692465
Time taken for 100000000 : 136833634
Time taken for 1000000000 : 1368862705

在循环中，即使i的值从0开始，表示产品为零，没有JIT优化。为什么不 ？
在上面提供的链接中，我之前将for循环放在JIT正在优化的方法调用中。是否将语句放在促进优化过程的方法中？

In the loop, even though the value of i starts from 0, indicating the product to be zero, there is no JIT Optimization. Why not ? In the link provided above, I had previously put the for loop in a method call, which the JIT was optimizing. Is putting the statements in a method facilitating in the optimization process ?

推荐答案

1. 在你的上一个问题JIT优化了方法的完整代码 start ，而没有分析方法返回时变量中出现的数字。这是因为你选择使你的方法 void ，给JIT一个简单的线索，任何计算出的值都将被丢弃。

1. In your previous question the JIT optimized away the complete code of the method start without any analysis as to what number happened to be present in the variables upon method return. This is because you chose to make your method void, giving the JIT a dead-easy clue that any values calculated will be discarded.

将当前示例与上一个问题中的示例进行对比，没有 void 方法被调用，因此自然不会发生优化。为什么没有一些其他优化可以帮助这个完全不同的案例是一个无法回答的问题。在specefic JVM实现和特定的JVM调用中没有这样的优化，您已经使用它来测试代码。

Contrasting your current example with the one from your previous question, there are no void methods called so naturally the optimization does not occur. Why there is not some other optimization which would help this completely different case is an unanswerable question. There is just no such optimization in the specefic JVM implementation, and the specific JVM invocation, with which you have tested your code.

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