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

[Frederic Parain]

On Tue, 13 May 2025 13:35:19 GMT, Quan Anh Mai <qamai 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.

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PR Comment: https://git.openjdk.org/valhalla/pull/1407#issuecomment-3730431574