RFR: 8188055: (ref) Add Reference::refersTo predicate [v5]

[David Holmes]

On Thu, 15 Oct 2020 17:04:37 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> 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 11 additional commits since
> the last revision:
>  - 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
>  - expand test
>  - remove CMS comment
>  - ... and 1 more: https://git.openjdk.java.net/jdk/compare/844db4bb...ab4e519b

src/java.base/share/classes/java/lang/ref/PhantomReference.java line 63:

> 61:     }
> 62:
> 63:     /* Override the implementation of Reference.refersTo.

s/refersTo/refersTo0/

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

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