Make new Object() backward compatible

[forax at univ-mlv.fr]

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>, "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
> Envoyé: Dimanche 9 Mai 2021 17:04:31
> Objet: Re: Make new Object() backward compatible

>> The logical conclusion is that java.lang.Object is a parametric class with a
>> parameter saying if it should implement IdentityObject or not.
> 
> Magic hammer, meet nail :)

New tool, new tricks :)

Given that we want two classes for java.lang.Object, seeing them as one being a specialization of the other is not too outrageous.

> 
> To be explicit, what you're suggesting is something like
> 
>     class Object<ID : boolean> { ... }
> 
> where an identity class extends Object<true> and a primitive class
> extends Object<false> (and most interfaces remain parametric in ID.)

yes

> 
> Its conceptually elegant, and we've discussed it before; in theory
> parametricity could range over things other than types, such as
> constants, identity-ness, etc (e.g., the ref and val projections could
> be species).  But to apply it to a problem as prosaic as this seems like
> cracking a walnut with a two-ton bomb.  (I don't thing this is what you
> want to teach on day 1.)

It seems that you are thinking that i want to introduce a special syntax just for specializing java.lang.Object, I don't.

I'm not proposing to change to the source code of Object.java, only the bytecode.
The compiler will insert the SpecializationAnchor (and the attribute that adds the identity-ness) without needing any language support.

The VM can also dynamically insert the SpecializationAnchor, but modifying javac to get the bytecode we want seems an easier path.

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