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

[Aleksey Shipilev]

On Thu, 10 Apr 2025 18:26:39 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> 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

I think it would be "easier" to shift `@IntrinsicCandidate` to a private `Reference.getImpl` method, and intrinsify that instead. Pretty much like current `clear` and `refersTo` are doing it now.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/24315#discussion_r2039019037