[lworld] RFR: 8377451: [lworld] Add ValuePayload abstraction [v16]

[Frederic Parain]

On Mon, 23 Feb 2026 14:08:00 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Create a wrapper class which represents the payload of an InlineKlass object.
>> 
>> The current convention is to use a `void*` representing where the payload starts, the `InlineKlass*` (as we do not always have a header when flattened) and a `LayoutKind` (describing the payloads layout). 
>> 
>> I suggest we introduce something like a `ValuePayload` which encapsulate these properties. As well as a hierarchy built upon these, with the proper interfaces implemented.
>> * ValuePayload (Any payload)
>>   * ~RawValuePayload (Payload with holder erased)~ (Removed in favour of using a static function)
>>   * BufferedValuePayload (Payload of normal heap object)
>>   * FlatValuePayload (Payload of flattened value)
>>     * FlatFieldPayload (Payload of flattened field)
>>     * FlatArrayPayload (Payload of flattened array element)
>>    
>> The goal is to both make interfaces clearer, and easier to understand. As well as consolidating the implementation in one place rather than spread across different subsystems. 
>> 
>> Each type (except RawValuePayload) also allows for the creation of a Handle, (thread local, or in an OopStorage) for keeping the payload as a thread or global root.
>> 
>> The ValuePayload class is also the interface for interacting with the Access API for InlineKlass objects.
>> 
>> * Testing
>>   * Running tier 1-4 with preview enabled
>>   * Running app tests with preview enabled
>>   * Running normal tier 1-5 
>> 
>> #### _Extra Notes:_
>>  * The `OopHandle` type is there so that we can migrate the JVMTI payload abstraction implementation to using this instead. (Future RFE)
>>  * Some interfaces got cleaned up. Some are unused. Like the `null_payload` which was superseded by the `Access::value_store_null`. C1 still uses the `.null_reset` but if that dependency is removed we should be able to remove that weird object all together.
>>  * Simply adding the Java to VM transition deep inside the payload code created a circular include dependency here. So rather than fixing that, I implemented the relevant bytecodes in the BytecodeInterpreter.
>
> Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 74 commits:
> 
>  - Merge remote-tracking branch 'upstream_valhalla/lworld' into lworld-payload-rebase
>  - Merge remote-tracking branch 'upstream_valhalla/lworld' into lworld-payload-rebase
>  - Merge remote-tracking branch 'upstream_valhalla/lworld' into lworld-payload-rebase
>  - Destructor is a CHECK_UNHANDLED_OOPS property
>  - @shipilev formatted table
>  - Revert "Fix BytecodeInterpreter dispatch table"
>    
>    This reverts commit 59c1fc2e7dedd079a61dced4401267dace7c642d.
>  - Revert "Avoid having to align label entires"
>    
>    This reverts commit 342cd358eec59e1798ef62d03bbbd3a0e0f79d9d.
>  - Missed header guard rename
>  - Rename inlineKlassPayload files to valuePayload
>  - Add missing inline keywords
>  - ... and 64 more: https://git.openjdk.org/valhalla/compare/7b3ac63d...02fada7d

Thank you for this very clean refactoring and encapsulation of the value payload copying code. Copying a value's content is so complex and so specific that isolating this feature in its own files makes perfect sense.

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Marked as reviewed by fparain (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/2068#pullrequestreview-3843211667