RFR: 8293207: Add assert to JVM_ReferenceRefersTo to clarify its API

[Kim Barrett]

On Thu, 1 Sep 2022 10:47:50 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> JVM_ReferenceRefersTo should only be used for soft/weak references and not phantom references, which has its own implementation. This is not clear from the name. `java_lang_ref_Reference::weak_referent_no_keepalive(ref_oop)` is only a valid call if `!java_lang_ref_Reference::is_phantom(ref_oop)`.
> 
> Adds an assert and comment to make this clear.
> 
> Testing: Oracle platforms tier 1-3

src/hotspot/share/prims/jvm.cpp line 3286:

> 3284:   oop ref_oop = JNIHandles::resolve_non_null(ref);
> 3285:   // PhantomReference has it's own implementation of refersTo().
> 3286:   assert(!java_lang_ref_Reference::is_phantom(ref_oop), "precondition");

Maybe instead assertions should be added to the java_lang_ref_Reference functions for accessing the referent?  (weak_referent_no_keepalive, weak_referent, phantom_referent_no_keepalive)

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PR: https://git.openjdk.org/jdk/pull/10117