On Mon, 9 Aug 2021 19:58:48 GMT, Vladimir Ivanov <vliva...@openjdk.org> wrote:
>> While fixing JDK-8270626 it was discovered that the C2 intrinsic for
>> `Reference.refersTo()` is not used as often as one would think. Instead C2
>> often prefers using the native implementation, which is much slower which
>> defeats the purpose of having an intrinsic in the first place.
>>
>> The problem arise from having `refersTo0()` be virtual, native and
>> @IntrinsicCandidate. This can be worked around by introducing an virtual
>> method layer and so that `refersTo0()` can be made `final`. That's what this
>> patch does, by adding a virtual `refersToImpl()` which in turn calls the now
>> final `refersTo0()`.
>>
>> It should be noted that `Object.clone()` and `Object.hashCode()` are also
>> virtual, native and @IntrinsicCandidate and these methods get special
>> treatment by C2 to get the intrinsic generated more optimally. We might want
>> to do a similar optimization for `Reference.refersTo0()`. In that case, it
>> is suggested that such an improvement could come later and be handled as a
>> separate enhancement, and @kimbarrett has already filed JDK-8272140 to track
>> that.
>>
>> Testing:
>> I found this hard to test without instrumenting Hotspot to tell me that the
>> intrinsic was called instead of the native method. So, for that reason
>> testing was ad-hoc, using an instrumented Hotspot in combination with the
>> test
>> (`vmTestbase/gc/gctests/PhantomReference/phantom002/TestDescription.java`)
>> that initially uncovered the problem. See JDK-8270626 for additional
>> information.
>
> src/java.base/share/classes/java/lang/ref/Reference.java line 374:
>
>> 372: * to call the native implementation over the intrinsic.
>> 373: */
>> 374: boolean refersToImpl(T obj) {
>
> I'm curious why can't you get rid of `refersToImpl` (virtual method) and just
> override `refersTo` on `PhantomReference`. Am I missing something important
> about keeping `refersTo` final?
We don't want user-defined subclass of `Reference` overriding the `refersTo`
implementation accidentally.
-------------
PR: https://git.openjdk.java.net/jdk/pull/5052
