On Wed, 9 Oct 2024 06:07:13 GMT, John R Rose <jr...@openjdk.org> wrote:
> If the interfaces had a compact numbering, and there were a side table > mapping the compact numbers to interface Klass* pointers, then I think > Andrew's code would still work (with natural adjustments). Probably, yes. I can pack klass ID+ [iv]table offset into a word to identify a method, or use a compressed metadata pointer. But I have a data dependency on a table load to get from a compressed Klass ID of some sort to a `Klass*`, that's a 3-to-5-cycle stall. > But managing such a side table is at least as complicated as just doing the > pointer compression stuff we do. (Hence my comment that we haven't had to > invent side tables yet.) > > Because of CDS I don't think we can treat concretes and abstracts (or even > just classes and interfaces) as disjoint metadata types with separate > independent compression tactics or representations. I think we need a > subtype/supertype relation between the "narrowest" and merely "narrower" > klass IDs. I think that's right. Notwithstanding any side tables etc., it would be nice to make sure that all metadata is reachable with a 4Gbyte offset from some base. It's not so hard to make sure that concrete and abstract `Klass*` (and all `Method*`s) can be in that range, simply(?) by allocating everything contiguously. ------------- PR Comment: https://git.openjdk.org/jdk/pull/19157#issuecomment-2402742912
