RFR: JDK-8199620: Support for JNI object pinning

[Roman Kennke]

Currently, the Get/Release*Critical() family of functions use the
GCLocker protocol to ensure that no JNI critical arrays are in use when
a GC pause is entered.

Some GCs may instead want to use object pinning and guarantee that the
object does not move. For example, this is easy to do with region-based
GCs (G1, Shenandoah, ZGC) by simply not including regions with pinned
objects in the collection set.

The implementation/API that I'm proposing is fairly simple: add two
methods oop pin_object(oop) and void unpin_object(oop) to CollectedHeap,
and call them from JNI's Get/Release*Critical methods.

This approach has been working perfectly fine since a long time in
Shenandoah.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8199620
Webrev:
http://cr.openjdk.java.net/~rkennke/8199620/webrev.00/


What do you think?

Thanks, Roman

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