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

[Quan Anh Mai]

On Mon, 24 Mar 2025 20:49:25 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.

src/hotspot/share/classfile/classFileParser.hpp line 101:

> 99:                       _non_atomic_size_in_bytes(-1), _non_atomic_alignment(-1),
> 100:                       _atomic_layout_size_in_bytes(-1), _nullable_atomic_layout_size_in_bytes(-1),
> 101:                       _nullable_non_atomic_layout_size_in_bytes(-1),

Do we need `nullable_non_atomic_alignment`, similar to `non_atomic_alignment`. Besides, it may be better to rename the `null_free` fields, too.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1407#discussion_r2678654545