Reducing class pointer size useful?

[John Rose]

On Sep 10, 2021, at 4:11 AM, Thomas Stüfe <thomas.stuefe at gmail.com<mailto:thomas.stuefe at gmail.com>> wrote:

Would it be of use for Lilliput to shrink the class pointer size beyond 32
bit? I did not closely follow the discussions. Therefore I am not sure
where the current thinking goes.

Such things are on the table I think.

There are two parameters here:  Number of
bits |ki| in the klass-ID, and size in bytes |ks|
(usually power of 2) of a klass struct.

Both |ki| and |ks| can be freely varied, I think,
as a design and optimization parameter.

1. *Not all* klasses need to be addressed using the
klass-ID of size |ki|; put another way, the first
2^|ki| glasses are privileged to have a compact
object header representation while other may
require more bits (an extension field in the
object layout).

2. *Not all* of the bytes of a klass need to be
represented in the |ks| bytes.  You can add
a level of indirection, and it won’t hurt much
as long as all the stuff JVM need fastest access
to is in the first |ks| bytes.

The second insight leads also to the concept
of “near klass” vs “far klass”, and opens the
conversation about having *several* near klasses
for one far klass.  In some designs, that allows
you to subdivide refine a system of classes
into a system of classes and “species”, where
several species can share on class.