[openjdk/jdk] 8338526: Don't store abstract and interface Klasses in class metaspace (PR #19157)

[Andrew Haley]

On 9/10/24 12:42, Coleen Phillimore wrote:
 > Thanks for reviewing Ioi and Thomas, and thank you Thomas for the suggested changes.

I'm a bit concerned about this one.

I'm working on megamorphic dispatch, and a uniform representation of
compressed class pointers allows me to squeeze klass+offset into a
single 64-bit word. This in turn allows fast and simple method
lookups. I need, at least, to be able to use compressed interface
pointers. If interfaces are "somewhere else", and thus incompressible,
I can't do what I need to do. If, however, klass and non-klass
metaspaces are contiguous I guess it'd be OK, if not ideal. I'd much
rather use compressed klass pointers without having to decode them.

All I need is a way to represent interface pointers in a compact way
in lookup tables, and to be able to get from compressed class pointers
to the address. As long as interface pointers are in a 32-bit range
and there's a fast way to get from compressed class to address that's
OK.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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