[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout

[Frederic Parain]

On Fri, 9 Jan 2026 20:13:29 GMT, Frederic Parain <fparain at openjdk.org> wrote:

> This would forbid the class which contains the `NULLABLE_NON_ATOMIC_FLAT` from being flattened into a class which contains it. I think we should only do so if the holder of the flattened strict final field should not be flattened if it is not itself a strict final field:
> 
>     * The holder is an identity class.
> 
>     * The holder is a large value class without `@LooselyConsistentValue`.
> 
> 
> This is equally valid to null-restricted field of non-`@LooselyConsistentValue`, too.

Yes, there are some restrictions where the NULLABLE_NON_ATOMIC_FLAT can be used, see the comments and the `can_use_atomic_flat` argument in the `field_layout_selection()` method.

> I assume this PR is for flattening of nullable fields at immutable memory (strict final fields) only. But I think the same flattening can be done for null-free field, right?

Strict final null-free fields could also be flattened without having to be concerned about atomicity, but null-freeness is not part of JEP 401. So this case is not supported in this PR. It can be added later when the null-restriction JEP has been finalized.

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PR Comment: https://git.openjdk.org/valhalla/pull/1407#issuecomment-3760127181