[lworld] RFR: 8311219: [lworld] VM option "InlineFieldMaxFlatSize" cannot work well

[Jatin Bhateja]

On Fri, 21 Jul 2023 02:52:34 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> src/hotspot/share/classfile/fieldLayoutBuilder.cpp line 661:
>> 
>>> 659:         }
>>> 660:         if (!too_big_to_flatten &&
>>> 661:             (!(too_atomic_to_flatten | too_volatile_to_flatten) || fieldinfo.access_flags().is_final())) {
>> 
>> Hi @XiaohongGong , specifications (https://openjdk.org/projects/valhalla/design-notes/state-of-valhalla/03-vm-model#flattenable-contexts) also talks about flattening final L descriptor fields of value classes.
>
> Thanks for pointing out this! I got through of the specifications just now.
> 
> Final fields containing value objects, under both L and Q descriptors, can be routinely flattened. Mutable fields containing value objects under Q descriptors can be routinely flattened; mutable fields containing value objects under L descriptors must provide atomic loads and stores, which requires either atomic instructions or an indirection.
> 
> Per my understanding, this means:
> 1. The final fields can be flattened for all L/Q descriptors
> 2. The non-final fields can be flattened for Q descriptors
> 3. The non-final fields cannot be flattened for L descriptor value objects due to the atomic feature.
> 
> 
> For value objects with many fields, the JVM may decide to fall back to buffering its value in the heap, which is the traditional indirected representation.
> 
> And the above means size of the flattened fields should not exceed some value. Right? 
> 
> Please correct me if any misunderstanding! Thanks!

Yes, this is correct, but final fields can be flattened irrespective of sizing constraint.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/888#discussion_r1270179016