RFR: 8188055: (ref) Add Reference::refersTo predicate [v7]

[Kim Barrett]

> Finally returning to this review that was started in April 2020.  I've
> recast it as a github PR.  I think the security concern raised by Gil
> has been adequately answered.
> https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-April/029203.html
> https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-July/030401.html
> https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-August/030677.html
> https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-September/030793.html
> 
> 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 native methods whose implementations are in the VM so
> they can use the Access API.  It is the intent that these methods 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.
> 
> There are two native methods involved, one in Reference and an override in
> PhantomReference, both package private in java.lang.ref. The reason for this
> split is to simplify the intrinsification. This is a change from the version
> from April 2020; that version had a single native method in Reference,
> implemented using the ON_UNKNOWN_OOP_REF Access reference strength category.
> However, adding support for that category in the compilers adds significant
> implementation effort and complexity. Splitting avoids that complexity.
> 
> Testing:
> mach5 tier1
> Locally (linux-x64) verified the new test passes with various garbage collectors.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 13 additional commits since the last revision:

 - Merge branch 'master' into refersto
 - improve wording in refersTo javadoc
 - Merge branch 'master' into refersto
 - More explicit refersTo0 comment.
 - simplify test
 - cleanup nits from Mandy
 - use Object instead of TestObject
 - improve refersTo0 descriptions
 - basic functional test
 - change referent access
 - ... and 3 more: https://git.openjdk.java.net/jdk/compare/f06d7348...79277ff3

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/498/files
  - new: https://git.openjdk.java.net/jdk/pull/498/files/3a15b6a9..79277ff3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=498&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=498&range=05-06

  Stats: 90837 lines in 1555 files changed: 63919 ins; 19502 del; 7416 mod
  Patch: https://git.openjdk.java.net/jdk/pull/498.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/498/head:pull/498

PR: https://git.openjdk.java.net/jdk/pull/498