RFR: 8338526: Don't store abstract and interface Klasses in class metaspace [v2]

[Coleen Phillimore]

On Fri, 23 Aug 2024 20:37:55 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/oops/klass.hpp line 214:
>> 
>>> 212:   virtual bool is_klass() const { return true; }
>>> 213: 
>>> 214:   bool is_in_klass_space() const { return !is_interface() && !is_abstract(); }
>> 
>> This name is misleading. As a caller, I expect a function with this name to make a range check of Klass* to be inside class space range. This is more of a "should be".
>> 
>> (We also write class space with `c` throughout hotspot, its weird to have it with `k` now)
>> 
>> How about, instead: "needs_narrow_klass_id" or "must_be_narrow_encodable" or similar? That clearly says what you want, that this class needs to be encodable with a narrow id for whatever is your reason. This leaves room for future changes (e.g. a possible future where we need narrow klass ids for other reasons than to make heap objects smaller, or where there is no class space anymore).
>
> I renamed this is_in_class_space() with the lower case 'c'.  It's still directing metaspace or indicating where the object was allocated.  Your name is a little better but I think not enough until we want to expand the things we want allocated in the class space.  As we talked about, with Tiny Class Pointers, class space will have different things in it (not that these new things need a compressed pointers).  But I think we're better off having less things in the space where their pointers can be compressed since this space is constrained.

I have to think about this more.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1729492034