[lworld] RFR: 8349162: [lworld] VM Flags controlling flattening need an update [v6]

[Frederic Parain]

> This patch is mostly about VM flags update and internal renaming.
> 
> Renaming a few symbols to make them consistent with previous code cleanup:
>   - data_for_oop() -> payload_address()
>   - first_field_offset() -> payload_offset()
>   - LayoutKind::PAYLOAD -> LayoutKind::BUFFERED (it is only used to describe layout in buffered values, and it makes it an adjective like the other LayoutKind values)
>   
> VM flags controlling flattening:
>   - size based VM flags are removed:  FlatArrayElementMaxSize, InlineFieldMaxFlatSize. For now, FlatArrayElementMaxOops is still present because some compiler tests rely on it, but it will be removed eventually
>   - InlineArrayAtomicAccess has been removed, use AlwaysAtomicAccesses instead
>   - new VM flags to control the generation of the new layouts: NonAtomicValueFlattening, AtomicValueFlattening, NullableValueFlattening. Note that that those flags control the generation of the layouts, not the way they are used (see flags below)
>   - new VM flags to control use of flat layouts: UseFlatField, UseFlatArray
>   
> Example:
>   To enable flattening of nullable fields, the flag combination is -XX:+NullableValueFlattening -XX:+UseFlatField
> 
> The new VM flags controlling flattening are flags for testing or troubleshooting (they should probably be turned into DIAGNOSTIC flags before preview). They are not designed for performance tuning.
> 
> Feel free to propose new names for those flags if you think they are not explicit enough.
> 
> Thank you,
> 
> Fred

Frederic Parain has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge remote-tracking branch 'upstream/lworld' into renaming
 - Fix flag name typo
 - Change flags names for more consistency
 - Merge remote-tracking branch 'upstream/lworld' into renaming
 - Fix identation
 - Fix typo
 - Merge remote-tracking branch 'upstream/lworld' into renaming
 - Renaming and VM flags cleanup

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Changes: https://git.openjdk.org/valhalla/pull/1345/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1345&range=05
  Stats: 680 lines in 96 files changed: 324 ins; 20 del; 336 mod
  Patch: https://git.openjdk.org/valhalla/pull/1345.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1345/head:pull/1345

PR: https://git.openjdk.org/valhalla/pull/1345