----- Original Message -----
> From: "Brian Goetz" <brian.go...@oracle.com>
> To: "Remi Forax" <fo...@univ-mlv.fr>
> Cc: "daniel smith" <daniel.sm...@oracle.com>, "valhalla-spec-experts"
> <valhalla-spec-experts@openjdk.java.net>
> Sent: Tuesday, April 26, 2022 4:12:43 PM
> Subject: Re: B3 ref model
>> so if we have
>>
>> primitive class Prim {
>> long value;
>> }
>>
>> class Container {
>> LPrim; prim;
>> }
>>
>> and Prim has been loaded before Container is seen by the VM, the VM can not
>> decide to flatten LPrim; to a long + a bit for nullability because the VM has
>> to ensure atomicity even if the user has declared Prim as a primitive class.
>
> Of course, we don’t put L or Q in the source code, but we put .val or .ref
> which
> translates appropriately. If you have a field of type X.ref, the compiler
> should put X in the preload attribute. If this doesn’t happen, then yes,
> there
> will be no flattening. If it does happen, there may be flattening.
but the VM has already loaded Prim, it knows that its a B3.
The preload attribute is to pre-load if the VM does not know if it's a value
type or not.
It seems you want to restrict the VM to take local decisions independently of
the classes already loaded ?
For me, L-type means: if you do not already know, you will discover later if
it's a B1/B2/B3 when the class will be loaded.
The preload attribute means: if you do not already know, you should load the
class now (at least when you want to take a decison based on the class being a
B1/B2/B3 or not).
A L-type does not mean, it's a pointer and it's always be a pointer, because if
a user has chosen a class to be a B3, the VM should do whatever is possible to
flatten it, even if the declared type is a L-type.
By example, if you have a code like this
record Holder(Object o) { }
primitive record Complex(double re, double im) { }
Holder holder = new Holder(new Complex(2., 3.));
... // more codes
The VM may found that "holder" escapes but at the same time that Complex is a
B3, in that case the instance of Holder is allocated on the heap but the
instance of Complex can be flattened inside the heap representation of Holder.
In this example, there is no QComplex; in the bytecode of Holder, there is no
Preload attribute too, the VM knowning that a Complex is a B3 is enough.
And i'm not suggesting that it's what a particular VM should do that, just that
a VM should be able to do that.
Rémi
