Forcefully inline reference types

[Julian Waters]

Going to leave a link in the meantime to the related Lilliput discussions:
https://github.com/openjdk/lilliput/pull/47
https://mail.openjdk.java.net/pipermail/lilliput-dev/2022-April/000406.html

best regards,
Julian

On Sun, Apr 24, 2022 at 10:19 PM Julian Waters <tanksherman27 at gmail.com>
wrote:

> Hi all,
>
> After some discussion with lilliput-dev about the possibility of removing
> the entire Mark Word, I'm wondering if being able to forcefully inline
> value and identity objects when they're instantiated could have any merit
> or benefit to be looked at in Valhalla. The idea came after the realization
> that to fully omit the Mark Word the GC bits in the header would also have
> to be dealt with, something which cannot be done by simply not having them
> in the header given that this may cause GC related performance issues. The
> discussed alternative was to not have GC support for certain objects at
> all, but that would require it to be able to take advantage of other memory
> freeing operations (Eg Popping off a stack frame, being inlined inside
> another object to be freed when the containing object is GC'd). To my
> knowledge regular objects can already be inlined by HotSpot via Escape
> Analysis, would it then be worth discussing to re-use the older "inline"
> keyword from Valhalla to allow control over this from the source level as
> well? (With the limitation that the object cannot escape its scope, and
> only copies of it may be made otherwise, something that may in turn require
> semantics for choosing to pass by value or reference)
>
> There are other issues with locking and identity hashes that relate to
> this, but that's probably better discussed back at lilliput-dev if this
> happens to be something we can look into
>
> best regards,
> Julian
>