JVM究竟有没有对尾递归进行优化

JVM究竟有没有对尾递归进行优化？

看了好几篇博文，说没有，便在JDK8下做了个测试。

package fibonicc;

public class Test {
    //1 1 2 3 5 8
    private static int fibonicc(int n) {
        if(n<=1) {
            return 1;
        }
        return fibonicc(n-1)+fibonicc(n-2);
    }
    
    //n递减 代表一直运算到n=1为止
    private static int fibonicc2(int n,int last,int total) {
        if(n==1) {
            return total;
        }
        return fibonicc2(n-1,total,last+total);
    }
    //尾递归 还是没有 循环快
    private static int fibonicc3(int n) {
        if(n<=1) {
            return 1;
        }
        int last = 1;
        int current = 1;
        int i=2;
        int tmp = current;
        while(i++<=n) {
            tmp = current;
            current = last+current;
            last = tmp;
        }
        return current;
    }
    
    private static int fibonicc4(int n,int last,int total) {
        if(n==1) {
            return total;
        }
        int result = fibonicc4(n-1,total,last+total);
        return result;
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
//      System.out.println(fibonicc(50));
        long start = (System.nanoTime());
        System.out.println(fibonicc2(5000,1,1));
        System.out.println(System.nanoTime()-start);
        start = (System.nanoTime());
        System.out.println(fibonicc3(5000));
        System.out.println(System.nanoTime()-start);
        start = (System.nanoTime());
        System.out.println(fibonicc4(5000,1,1));
        System.out.println(System.nanoTime()-start);
    }

}

fibonicc是普通递归，很慢。

fibonicc2是尾递归实现，fibonicc2(5000,1,1)耗时1092785纳秒，比普通递归快。

fibonicc3是循环实现，fibonicc3(5000)耗时419347纳秒，比尾递归快。

fibonicc4是同fobonicc2的时间复杂度一致的非尾调，耗时1132819，看来和尾递归耗时差不多。