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

[Mandy Chung]

On Mon, 5 Oct 2020 10:51:02 GMT, Per Liden <pliden 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.
>
> src/hotspot/share/prims/jvm.cpp line 3451:
> 
>> 3449:   JVMWrapper("JVM_ReferenceRefersTo");
>> 3450:   return referenceRefersTo<ON_WEAK_OOP_REF>(ref, o);
>> 3451: JVM_END
> 
> ... and let this be something like:
> 
> JVM_ENTRY(jboolean, JVM_ReferenceRefersTo(JNIEnv* env, jobject ref, jobject o))
>   JVMWrapper("JVM_ReferenceRefersTo");
>   oop ref_oop = JNIHandles::resolve_non_null(ref);
>   oop cmp_oop = JNIHandles::resolve(o);
>   return java_lang_ref_Reference::refers_to(ref_oop, cmp_oop);
> JVM_END

I like Per's suggestion that makes it explicitly connect to `Reference::refers_to`.

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