On Fri, Sep 28, 2012 at 10:21 AM, Raffaello Giulietti
<raffaello.giulie...@gmail.com> wrote:
> I'm not sure that we are speaking about the same thing.
>
> The Java source code of numberOfTrailingZeros() is exactly the same in
> Integer as it is in MyInteger. But, as far as I understand, what
> really runs on the metal upon invocation of the Integer method is not
> JITted code but something else that probably makes use of CPU specific
> instructions. This code is built directly into the JVM and need not
> bear any resemblance with the code that would have been produced by
> JITting the bytecode.
Regardless of whether the method is implemented in Java or not, the
JVM "knows" native/intrinsic/optimized versions of many java.lang core
methods. numberOfTrailingZeros is one such method.
Here, the JVM is using its intrinsified version rather than the JITed
version, presumably because the intrinsified version is pre-optimized
and faster than what the JVM JIT can do for the JVM bytecode version.
system ~/projects/jruby-ruby $ java -XX:+PrintCompilation
-XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining Blah
65 1 java.lang.String::hashCode (55 bytes)
78 2 Blah::doIt (5 bytes)
78 3 java.lang.Integer::numberOfTrailingZeros (79 bytes)
@ 1
java.lang.Integer::numberOfTrailingZeros (79 bytes) (intrinsic)
79 1 % Blah::main @ 2 (29 bytes)
@ 9 Blah::doIt (5 bytes) inline (hot)
@ 1
java.lang.Integer::numberOfTrailingZeros (79 bytes) (intrinsic)
@ 15 Blah::doIt (5 bytes) inline (hot)
@ 1
java.lang.Integer::numberOfTrailingZeros (79 bytes) (intrinsic)
system ~/projects/jruby-ruby $ cat Blah.java
public class Blah {
public static int value = 0;
public static void main(String[] args) {
for (int i = 0; i < 10_000_000; i++) {
value = doIt(i) + doIt(i * 2);
}
}
public static int doIt(int i) {
return Integer.numberOfTrailingZeros(i);
}
}
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