RFR: 8188055: (ref) Add Reference.refersTo predicate

[Gil Tene]

A very welcome change overall. However, I have concerns about
the semantic change to the PhantomReference specification. I propose
that PhantomReference semantics remain unchanged, and that
PhantomReference:RefersTo should return true only for null.

See more in comment at https://bugs.openjdk.java.net/browse/JDK-8188055?focusedCommentId=14329319&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14329319

> On Apr 7, 2020, at 5:25 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> [Note review on both core-libs and hotspot-gc-dev lists; try not to lose
> either when replying.]
> 
> Please review a new function: java.lang.ref.Reference.refersTo.
> 
> This function is needed to test the referent of a Reference object
> without artificially extending the lifetime of the referent object, as
> may happen when calling Reference.get.  Some garbage collectors
> require extending the lifetime of a weak referent when accessed, in
> order to maintain collector invariants.  Lifetime extension may occur
> with any collector when the Reference is a SoftReference, as calling
> get indicates recent access.  This new function also allows testing
> the referent of a PhantomReference, which can't be accessed by calling
> get.
> 
> The new function uses a native method whose implementation is in the
> VM so it can use the Access API.  It is the intent that this function
> will be intrinsified by optimizing compilers like C2 or graal, but
> that hasn't been implemented yet.  Bear that in mind before rushing
> off to change existing uses of Reference.get.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8188055
> https://bugs.openjdk.java.net/browse/JDK-8241029 (CSR)
> 
> Webrev:
> https://cr.openjdk.java.net/~kbarrett/8188055/open.04/
> 
> Testing:
> mach5 tier1
> 
> Locally (linux-x64) verified the new test passes with various garbage
> collectors.