Arrays now implement "Arrayish"

Rémi Forax forax at
Thu Apr 28 13:02:52 UTC 2016

Hi David,
did you check that Arrayish can contain default methods ?


Le 27 avril 2016 14:52:41 CEST, David Simms <david.simms at> a écrit :
>I've just pushed a set of changes to enable all arrays to implement the
>"java.lang.Arrayish" interface:
>    public interface Arrayish<any T> extends Cloneable,
> {
>         int arraySize();
>         T arrayGet(int index);
>         void arraySet(int index, T element);
>    }
>Thanks to John Rose for the original patch, which I have adapted
>How does it work ?
>  * "Klass->extra_super()" contains an optional "InstanceKlass*"
>    pointer, which all array klasses have set to their appropriate
>    Arrayish, i.e.:
>      o int[] extra_super() = "Arrayish<int>"
>      o Object[] extra_super() = "Arrayish"
> * Classes used as "extra_super" must currently be interfaces, and have
>    their own "self itable" generated at load time.
>  * The extra_super mechanism allows the "Arrayish" interface to be
>    injected during JVM start-up, but after the basic array types need
>    to be initialized
>  * "extra_super()" is used when, checkcast/instanceof and looking up
>    interface methods
>  * See jtreg test: "hotspot/runtime/valhalla/arrays/"
>This is a prototype, there are caveats (and dragons):
>  * The current "Model 3" implementation uses a specializer written in
>    Java, for bootstrapping the Arrayish specializations, these are
>    currently built at JDK compile time (placed
>    "$JAVA_HOME/valhalla-prespecialized")
>      o There is no support yet for double slot types (double and long,
>        and hence their arrays)
>      o These classes are specific to the JVM runtime, and must not be
>        seen by "javac" ( "VALHALLA_PRESPECIALIZED_DIR" is known to the
>        classloader implementation, but not the classpath).
>  * x86 32 & 64 bit architectures only at this point
>   o arrays on other architectures will simply not implement "Arrayish"
>  * Nothing about this implementation is set in stone, prototype code
>    and quality (piggy-backing on these changes, then ymmv)
>  * The type hierarchy for arrays will no doubt continue to be in flux
>/David Simms

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