JIT编译器和内部循环

问题描述

为什么JIT编译器不能使用以下代码?

Why is the JIT-compiler not working for the following code?

每个循环似乎都花费了很长时间

(请参阅我的其他有关执行时间的文章.简单循环的速度

(see my other post about executing time. Speed of simple loop

 public static void main (String[] args) {
    for (int j = 0; j < 10; j++) {
        float f;

        long start = System.nanoTime();

        for (int i = 0; i < 2000000000; i++) {
            f = i * 0.0001F;
        }
        long end = System.nanoTime();
        long timeToCallNanoTime = System.nanoTime() - end;
        long time = Math.max(0, end - start - timeToCallNanoTime);
        System.out.println("time: " + time + " ns.");
    }

}

结果:

 time: 6639317628 ns.
 time: 6630196045 ns.
 time: 6632583856 ns.
 time: 6617596798 ns.
 time: 6605243858 ns.
 time: 6609097755 ns.
 time: 6627151876 ns.
 time: 6623427381 ns.
 time: 6632506712 ns.
 time: 6615870257 ns.

推荐答案

它对我有效，正如您在上一个问题的答案中所见.

It works for me in as you can see in my answer to your previous question.

您很有可能正在使用32位Windows附带的客户端JVM.客户端JVM并未对代码进行尽可能多的优化以最小化启动时间.我建议您使用默认情况下使用-server JVM的64位JVM，它将更加积极地优化代码.

Most likely you are using the client JVM which comes with 32-bit Windows. The client JVM doesn't optimise the code as much to minimise startup time. I suggest you use the 64-bit JVM which uses the -server JVM by default and it will optimise the code more aggressively.

顺便说一句，我正在使用Java 7 update40.如果您的Java版本真的很旧，它可能无法优化循环.

BTW I was using Java 7 update 40. If you have a really old version of Java, it might not optimise the loop away.

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