[lworld] RFR: 8349162: [lworld] VM Flags controlling flattening need an update [v3]

[Frederic Parain]

On Mon, 3 Feb 2025 17:09:42 GMT, Frederic Parain <fparain at openjdk.org> wrote:

>> 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 incrementally with one additional commit since the last revision:
> 
>   Fix identation

After a discussion with Tobias, we agreed on a more consistent scheme for flags names:
  - UseFlatArray -> UseArrayFlattening
  - UseFlatField -> UseFieldFlattening
  - NonAtomicValueFlattening -> UseNonAtomicValueFlattening
  - AtomicValueFlattening -> UseAtomicValueFlattening
  - NullableValueFlattening -> UseNullableValueFlattening

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PR Comment: https://git.openjdk.org/valhalla/pull/1345#issuecomment-2634822741