Yes, the JIT has intrinsic knowledge of some JDK classes and their methods,
and emits optimized code - Integer has several intrinsics. Look at
src/share/vm/opto/library_call.cpp
Cheers
Sent from my phone
On Sep 28, 2012 10:51 AM, "Raffaello Giulietti" <
raffaello.giulie...@gmail.com> wrote:
> 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);
> }
>
> }
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>
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