8341757: Field layout computation allowing atomic and nullable flattening

[fparain]

Initial implementation of fields layouts in order to support heap flattening with JEP 401 model.
The patch includes the generation of two new flat fields layouts: atomic and nullable.
The patch also includes the support in the interpreter (through the access API) to access the new layouts while respecting the new semantic (on x86_64 and aarch64).
Some areas do not support the new layouts yet, like JITs, Unsafe and arrays.

There's no automatic tests yet, but considering the impact those new layouts will have on the JVM, the priority was to make this code available to all developers as soon as possible. Automatic tests will be added in a following CR.

Fred

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Progress

• Change must not contain extraneous whitespace

Issue

• JDK-8341757: Field layout computation allowing atomic and nullable flattening (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/valhalla.git pull/1275/head:pull/1275
$ git checkout pull/1275

Update a local copy of the PR:
$ git checkout pull/1275
$ git pull https://git.openjdk.org/valhalla.git pull/1275/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 1275

View PR using the GUI difftool:
$ git pr show -t 1275

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/valhalla/pull/1275.diff

[bridgekeeper]

👋 Welcome back fparain! A progress list of the required criteria for merging this PR into lworld will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

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

[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 nullable_flat_2
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