Are java.lang classes better served by the JVM?

[Raffaello Giulietti]

Hello,

are java.lang classes better served by the JVM than other classes?

Here's a small experiment.

I created a MyInteger class that exposes the very same implementation
of Integer.numberOfTrailingZeros(int), copied verbatim.

And here is a test that, on my JVM, shows that the implementation in
Integer is about 5 times faster. I tried several JVM flags, e.g.,
-server, -XX:+AggressiveOpts, -Xshare:off, unsatisfactorily. Similar
results with factor of about 3-5 are observed on other platforms.

Why the big performance difference? I know, micro-benchmarks are evil,
etc, etc, ... But this is hard to understand, except if Integer were
already super-optimized "a priori", intrinsically, while building the
JVM. Is this the case?



Greetings
Raffaello



---------------

public class Trailing {

    private static int COUNT = 1 << 30;

    public static void main(String[] args) {
        warmup();
        my();
        their();
    }

    private static void warmup() {
        int t = 0;
        for (int i = 0; i < COUNT; ++i) {
            t += MyInteger.numberOfTrailingZeros(i);
        }
        System.out.println("warmup, t=" + t);
    }

    private static void their() {
        int t = 0;
        long begin = System.nanoTime();
        for (int i = 0; i < COUNT; ++i) {
            t += Integer.numberOfTrailingZeros(i);
        }
        System.out.println((System.nanoTime() - begin) / 1000000 +
"ms, t=" + t);
    }

    private static void my() {
        int t = 0;
        long begin = System.nanoTime();
        for (int i = 0; i < COUNT; ++i) {
            t += MyInteger.numberOfTrailingZeros(i);
        }
        System.out.println((System.nanoTime() - begin) / 1000000 +
"ms, t=" + t);
    }

}

------------------

public class MyInteger {

    public static int numberOfTrailingZeros(int i) {
        // HD, Figure 5-14
        int y;
        if (i == 0) return 32;
        int n = 31;
        y = i <<16; if (y != 0) { n = n -16; i = y; }
        y = i << 8; if (y != 0) { n = n - 8; i = y; }
        y = i << 4; if (y != 0) { n = n - 4; i = y; }
        y = i << 2; if (y != 0) { n = n - 2; i = y; }
        return n - ((i << 1) >>> 31);
    }

}