[lworld] RFR: 8377451: [lworld] Add ValuePayload abstraction [v3]

[Frederic Parain]

On Tue, 10 Feb 2026 14:36:16 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)
>>   * 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 Like the `null_payload` which was superseded by the `Access::value_store_null`.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 incrementally with one additional commit since the last revision:
> 
>   Remove incorrect assert. JVM_CopyOfSpecialArray is currently broken

src/hotspot/share/oops/inlineKlassPayload.inline.hpp line 419:

> 417: }
> 418: 
> 419: inline void FlatValuePayload::copy_from_non_null(BufferedValuePayload& src) {

`copy_from_non_null` looks a bit weird here, because the method does something more precise: copying from a buffered value, which implies 1) the src is never null and 2) the method is allowed to update the null-marker.
`copy_from_buffered` looks like a better name to express what the method does.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/2068#discussion_r2788460110