On Mon, 9 Aug 2021 13:13:33 GMT, Per Liden <[email protected]> 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?
src/java.base/share/classes/java/lang/ref/Reference.java line 379:
> 377:
> 378: @IntrinsicCandidate
> 379: private native final boolean refersTo0(Object o);
No need to have it both `private` and `final`. Just `private` should be enough.
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PR: https://git.openjdk.java.net/jdk/pull/5052
