[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout [v3]

[Coleen Phillimore]

On Fri, 16 Jan 2026 14:20:32 GMT, Frederic Parain <fparain at openjdk.org> wrote:

>> Strict final instance fields are not subject to concurrent writes during a read access, so they can be flattened even if they are nullable and bigger than 64 bits. The NULLABLE_NON_ATOMIC_FLAT layout is added for this particular case.
>> This new layout can also be used in the special case of nullable empty value classes, because their payload contains a single entry, the null-marker, which is naturally atomic.
>
> Frederic Parain has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix coding style

Looks good!

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Marked as reviewed by coleenp (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1407#pullrequestreview-3672298838