RFR: 8343218: Disable allocating interface and abstract classes in non-class metaspace

[David Holmes]

On Tue, 29 Oct 2024 17:15:44 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> Change the code to allocate all Klasses to class-metaspace until c2 dependencies are understood and fixed, and more urgent need arises when compressing klass pointers to smaller numbers of bits.
> Tested with tier1-4.

So this is just disabling [JDK-8338526](https://bugs.openjdk.org/browse/JDK-8338526) until other C2 changes are in place. Are they coming soon? If not shouldn't [JDK-8338526](https://bugs.openjdk.org/browse/JDK-8338526) be backed out?

src/hotspot/share/oops/instanceKlass.cpp line 459:

> 457:   // Only Klasses that aren't interface or abstract classes are found in oop headers so they
> 458:   // are the only ones that require compression and allocation in the limited class metaspace
> 459:   // region.  But ... there are C2 dependencies that all klasses are allocated to the class metaspace.

Suggestion:

  // region.  But ... there are C2 dependencies that all klasses are allocated to the class metaspace, so
  // always use the class metaspace until JDK-NNNNNNN is resolved.

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Changes requested by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21769#pullrequestreview-2403674772
PR Review Comment: https://git.openjdk.org/jdk/pull/21769#discussion_r1821915266