[lworld] RFR: 8348547: [lworld] Investigate false adapter sharing due to inherited fields in the calling convention

[Quan Anh Mai]

On Wed, 15 Oct 2025 10:28:36 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> Hi,
>> 
>> Currently, the class `AdapterFingerPrint` only tracks the types of the arguments in the calling convention. This does not work well with the inclusion of value classes which need to be packed and unpacked when passed between different tiers. This is because 2 types can have the same sequence of field types, yet the fields are placed at different offsets. In those cases, the code to pack those types cannot be the same, hence the adapter code cannot be shared. Currently, a hack is implemented to cover the cases of inherited fields. For example:
>> 
>>     value class MyAbstract {
>>         int x;
>>     }
>> 
>>     value class MyValue1 extends MyAbstract {}
>> 
>>     value class MyValue2 {
>>         int x;
>>     }
>> 
>> In this case, since `MyAbstract` is an abstract class, we do not know the complete field sequence of every class that has the field `MyAbstract.x`. Moreover, we need to ensure that the payload of a value class is stable when it is flattened, and it can be accessed atomically if atomicity is required. As a result, `MyAbstract.x` must have the highest alignment, which is currently 8 because the largest atomic flat access is 8-byte. However, `MyValue2.x` does not need such a large alignment.
>> 
>> The current hack put a pair of `T_METADATA`, `T_VOID` before an inherited field. This is fine for the case above, but it clashes with another case:
>> 
>>     value class MyValue3 {
>>         @NullRestricted
>>         Empty e;
>>         int x;
>>     }
>> 
>> The solution I propose is to add field offsets into `AdapterFingerPrint`, as well as removing the current hack. Please take a look and leave your reviews, thanks a lot.
>
> Thanks for working on this! The fix looks good to me.

@TobiHartmann Thanks a lot for your review

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PR Comment: https://git.openjdk.org/valhalla/pull/1679#issuecomment-3407277997