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

[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

-------------

Commit messages:
 - Merge remote-tracking branch 'upstream/lworld' into renaming
 - Renaming and VM flags cleanup

Changes: https://git.openjdk.org/valhalla/pull/1345/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1345&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349162
  Stats: 644 lines in 89 files changed: 324 ins; 20 del; 300 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