Updated VM-bridges document

Remi Forax forax at univ-mlv.fr
Fri Apr 12 07:02:23 UTC 2019

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Karen Kinnear" <karen.kinnear at oracle.com>
> Cc: "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
> Envoyé: Vendredi 12 Avril 2019 01:04:15
> Objet: Re: Updated VM-bridges document

> On 4/11/2019 5:18 PM, Karen Kinnear wrote:
>>> OK, so at this point, the classfiles that have been loaded look like:
>>>     class D {
>>>         void m(LDT) { real method }
>>>         @Forwarding(m(LDT)) abstract void m(Date);
>>>     }
>>>     class E extends D {
>>>         @Override
>>>         m(Date) { impl }
>>>     }
>>> So D has members m(LTD) and m(Date), the latter is a forwarder.
>>> Therefore E has the same members (instance methods are inherited).
>> From a source perspective, E has the same names of members, although
>> it has overridden the contents of m(Date).
>>> Here's how I would imagine this turns into in the VM:
>> not important, but this was m(LDT) not m(LTD)
>>>     class D {
>>>         void m(LTD) { real method }
>>>         void m(Date d) { m(adapt(d)); }  // generated forwarder
>>>     }
>>>     class E extends D {
>>>         private void m$synthetic(Date d) { real method, body as
>>> present in classfile }
>> I would expect that the existing m(Date) with the real method would
>> stay unchanged - including
>> the name and the access controls - since there may be clients of
>> subclass E still trying to invoke it.
> I think this is our point of disconnect.
> The subclass has overridden a forwarder.  What we want to do is "heal
> the rift" by rewriting the subclass as if it had _only_ overridden the
> real method.  Hence, the "shunt it off to a synthetic" and create an
> overriding reverser that overrides the real method, adapting
> args/return, which delegates to the shuntee.
> If we left m(Date) in E, then this would be overriding the forwarder,
> effectively un-doing the effect of forwarding.
> Note that this is all "as if"; there are a hundred ways to _actually_ do
> it.

Another way to see this effect is to say that it actually override the forwarder but locally, just for that class, in subclasses, the forwarder is still present

This leads us to the next question, given that you can only override "locally" a forwarder, what if a forwarder overrides a forwarder ? You throw a LinkageError ?


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