Adding flattening of strict final fields

[fparain]

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.

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Progress

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Reviewing

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$ git checkout pull/1407

Update a local copy of the PR:
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$ git pull https://git.openjdk.org/valhalla.git pull/1407/head

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[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[merykitty]

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.

[bridgekeeper]

@fparain This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply issue a /touch or /keepalive command to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

[fparain]

/keepalive

[openjdk]

@fparain The pull request is being re-evaluated and the inactivity timeout has been reset.

[openjdk]

@fparain this pull request can not be integrated into lworld due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout flat_final
git fetch https://git.openjdk.org/valhalla.git lworld
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge lworld"
git push