[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout [v4]

[Coleen Phillimore]

On Wed, 21 Jan 2026 19:15:08 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 with a new target base due to a merge or a rebase. The pull request now contains 17 commits:
> 
>  - Fix to work with JDK-8374729
>  - Merge remote-tracking branch 'upstream/lworld' into flat_final
>  - Fix coding style
>  - More renaming
>  - Cleanups and fixes
>  - Fix Copyright years
>  - Fix compilation
>  - Merge remote-tracking branch 'upstream/lworld' into flat_final
>  - Improve tests
>  - Merge remote-tracking branch 'upstream/lworld' into flat_final
>  - ... and 7 more: https://git.openjdk.org/valhalla/compare/8f8398ed...eab559d6

Looks good!

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Marked as reviewed by coleenp (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1407#pullrequestreview-3688989799