Fixes #284 Add support for Thread.onSpinWait() while spinning

[franz1981]

I've opened this for discussion: I'll push different commits to propose places where onSpinWait could be welcome, if I'll solve first the compilation error:

InternalThreadHints.java:[52,41] method invoked with incorrect number of arguments; expected 0, found 1

[franz1981]

Probably @PolymorphicSignature is creating issues while compiling with 1.6, but with 1.7 is fine and, of course, if I use MethodHandle::invokeWithArguments, that doesn't use @PolymorphicSignature, is fine too

[franz1981]

Here LMAX-Exchange/disruptor#183 (comment) some more information on how we could bring this in with 1.6, if it worths doing it

[richardstartin]

If I understand correctly you want Thread.onSpinWait to be used in spin locks whenever it is available while maintaining bytecode compatibility with JDK6. You can achieve this with a shim class and a Multi-Release jar.

public class ThreadShim {

    public static void onSpinWait() {
        // do nothing
    }

}

In another directory e.g. src/main/java9 you add another implementation.

/** will load this one from META-INF/versions/9 in any JDK >= JDK9*/
public class ThreadShim {

    public static void onSpinWait() {
        Thread.onSpinWait();
    }

}

You now need to ensure that the JDK9+ implementation is in a special place in the jar (META-INF/versions/9, and the manifest has the property Multi-Release : true. JDK8 and less will never load the JDK+ shim.

There is an example of how to do this for a JDK9+ vectorised Arrays equality method in RoaringBitmap. An example implementation with Maven is here.

[nitsanw]

I used to do this too, but my current thinking is that it is pointless. The class is final, it has no fields, no methods that are not static. If some idiot wants to instantiate it, so what?

[nitsanw]

In its current state this will not fly with building on 8 with JDK6 compatibility.

[franz1981]

@nitsanw

In its current state this will not fly with building on 8 with JDK6 compatibility.

I know, I've yet to work on this to make it possible, but still not sure what to do: the idea of @richardstartin seems reasonable to me, but probably will need to support Multi-Release jar.

[franz1981]

@nitsanw Nitsan, bud, I cannot add any label to this PR, can you do it for me? is "DON'T MERGE IT YET" in red; I will change it accordly when I've a bit of time and will proceed on this 👍

[richardstartin]

There's a demo of how to do this with Maven here.