Slight inconsistency between state of valhalla and the value objects draft JEP

[Remi Forax]

----- Original Message -----
> From: "Stig Rohde Døssing" <stigdoessing at gmail.com>
> To: "valhalla-dev" <valhalla-dev at openjdk.java.net>
> Sent: Thursday, December 23, 2021 11:13:03 PM
> Subject: Slight inconsistency between state of valhalla and the value objects draft JEP

> Hi,
> 
> The updated state of valhalla at
> https://openjdk.java.net/projects/valhalla/design-notes/state-of-valhalla/03-vm-model#flattening-and-representation
> says that final fields containing value objects can regularly be flattened,
> even when using the L descriptor. It also says that value objects might not
> be flattenable if referenced through a non-sealed interface, which I
> understand to mean that if it is referenced through a sealed interface, we
> might not have to pay the polymorphism tax, and might still be able to
> flatten?
> 
> The draft JEP at https://openjdk.java.net/jeps/8277163 is more
> conservative, and says that value objects in fields and value objects
> viewed as an instance of a supertype will be heap allocated.
> 
> Is the JEP just being a bit too conservative? It sounds great if flattening
> is still possible as long as the fields are final or the superinterface is
> sealed. I think this would make a big difference for Scala's Option type,
> which is a sealed interface.

To be able to flatten on heap, you need
- the type to be loaded eagerly (a Q-type or a L-type in the Preload table)
  - I see no issue to store a sealed interface in the Preload table
- if it's a primitive type, it should not be too big
- if it's a value type, it should be <= 64 bits knowing that you need a bit for nullability
  (in practice a byte)
- if it's a sealed interface, you need several bits to store the index of the permitted subtypes
  (in practice a byte but you may reuse the same byte as above (255 or 128 permitted subtypes seems a lot))

So for Option, if the VM is using compressed oops (so no ZGC !), it should be okay.

Now, does this heroic bits packing stuff will be implemented one day, maybe, the VM already does bits packing in the object header after all,
but don't expect this to be implemented when Valhalla will be first released. 

Rémi