[lworld] RFR: 8373864: [lworld] Hide and rename InstanceKlassFixedBlock

[Paul Hübner]

On Wed, 17 Dec 2025 10:21:24 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Sub-classes of `InstanceKlass` can't have C++ fields because they would end up overlayed on top of the vtable and other dynamically sized sections of the `InstanceKlass` object.
> 
> To handle that the `InlineKlass` has a companion class named `InlineKlassFixedBlock`, which lists all the member fields that belongs to the InlineKlass, and an instance of that gets stamped into the `InlineKlass` object after the parts that are provided by the `InstanceKlass`.
> 
> I propose a few changes:
> 
> 1) Move `InlineKlassFixedBlock` away from instanceKlass.hpp and place it inside inlineKlass.hpp instead.
> 
> 2) Nest `InlineKlassFixedBlock` it inside `InlineKlass`. It's only `InlineKlass `(and the compilers) that touch these fields, so it doesn't have to be a public, top-level class.
> 
> 3) Rename it from `InlineKlassFixedBlock` to `InlineKlass::Members`. I think that "fixed block" term is unclear and doesn't help the reader understand its role in the `InlineKlass`. Hopefully, the name `Members` is a clearer.
> 
> WDYT?

Looks good overall, just minor comments.

src/hotspot/share/ci/ciMethod.hpp line 44:

> 42: class BCEscapeAnalyzer;
> 43: class InlineTree;
> 44: class SigEntry;

Why do we need this declaration? AFAICT `ciMethod`'s method should be unaffected by this change?

src/hotspot/share/oops/inlineKlass.hpp line 90:

> 88: 
> 89:   Members& members() {
> 90:     assert(_adr_inline_klass_members != 0, "Should have been initialized");

Nitpick: `nullptr` for clarity as that's what we initialize it as?

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Marked as reviewed by phubner (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1812#pullrequestreview-3587240158
PR Review Comment: https://git.openjdk.org/valhalla/pull/1812#discussion_r2626646022
PR Review Comment: https://git.openjdk.org/valhalla/pull/1812#discussion_r2626660311