[lworld] RFR: 8377451: [lworld] Add ValuePayload abstraction [v8]

[Stefan Karlsson]

On Thu, 12 Feb 2026 13:52:13 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 Like the `null_payload` which was superseded by the `Access::value_store_null`.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 incrementally with one additional commit since the last revision:
> 
>   Remove mutable

src/hotspot/share/oops/inlineKlassPayload.hpp line 81:

> 79:     inline address absolute_addr() const;
> 80: 
> 81:     InlineKlass* klass() const;

There are some inconsistency w.r.t. marking functions with `inline`:
Suggestion:

    inline InlineKlass* klass() const;

src/hotspot/share/oops/inlineKlassPayload.inline.hpp line 45:

> 43: 
> 44: template <typename OopOrHandle>
> 45: inline ValuePayload::StorageImpl<OopOrHandle>::StorageImpl()

There are some inconsistency w.r.t. marking the function definitions with `inline`. I'd propose that we put the `inline` declaration in the .hpp file and remove them from this file.

src/hotspot/share/oops/inlineKlassPayload.inline.hpp line 75:

> 73: inline ValuePayload::StorageImpl<OopOrHandle>::~StorageImpl() {
> 74: #ifdef ASSERT
> 75:   if (!_absolute_addr) {

Should this be:

Suggestion:

  if (!_use_absolute_addr) {

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PR Review Comment: https://git.openjdk.org/valhalla/pull/2068#discussion_r2803190722
PR Review Comment: https://git.openjdk.org/valhalla/pull/2068#discussion_r2803225329
PR Review Comment: https://git.openjdk.org/valhalla/pull/2068#discussion_r2803177797