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

[Stefan Karlsson]

On Wed, 17 Dec 2025 11:18:51 GMT, Paul Hübner <phubner 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?
>
> 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?

Of course! Thanks. This is left-overs from an earlier prototype, where I only saved the offset to the Members. That removes the need for CDS to patch this address, but it adds extra instructions to the compiler, so I skipped that.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1812#discussion_r2626951939