RFR: 8352565: Add native method implementation of Reference.get() [v4]

[Vladimir Kozlov]

On Thu, 10 Apr 2025 08:08:32 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Kim Barrett has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - remove timeout by using waitForReferenceProcessing
>>  - make ill-timed gc in non-concurrent case less likely
>>  - fix test package use
>
> src/java.base/share/classes/java/lang/ref/Reference.java line 357:
> 
>> 355:     @IntrinsicCandidate
>> 356:     public T get() {
>> 357:         return get0();
> 
> I am looking at this now and wondering how current intrinsics matchers work in case of virtual calls. 
> 
> For example, when type information/profile tells us the receiver is generic `Reference`, but in reality it is a `PhantomReference` subclass, would the call to `Reference.get()` -- which is actually `PhantomReference.get()`! -- match accidentally to `Reference.get` intrinsic, and thus enter Access API with `ON_WEAK_REF`? Looks pre-existing, and I would have expected native code to assert, but I also think at least C2 intrinsics do not check the reference type.
> 
> It seems both `clear` and `refersTo` side-step all this by: a) not intrinsifying the virtual methods; b) doing `AS_NO_KEEPALIVE` -- so they are not as exposed. It might be another reason to do this change: to clearly separate `Reference.get` intrinsic and `PhantomReference.get` non-intrinsic...

You need to use intrinsic predicates to add runtime check for receiver. See:
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/library_call.cpp#L167

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PR Review Comment: https://git.openjdk.org/jdk/pull/24315#discussion_r2038013798