[lworld] RFR: 8370450: [lworld] Alternate implementation of the substitutability test method [v6]

[Frederic Parain]

On Mon, 27 Oct 2025 18:57:50 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/classfile/fieldLayoutBuilder.cpp line 1301:
>> 
>>> 1299:   }
>>> 1300:   last_idx = last_idx == -1 ? 0 : last_idx;
>>> 1301:   int start = map->adr_at(last_idx)->first > offset ? 0 : last_idx;
>> 
>> last_idx is an optimization to try to prevent searching through the field map to find where to insert {offset,size} for the new entry.  When would it not be ascending?  super classes and inlined classes are inserted in order that they're found  in the LayoutBlocks so wouldn't the offsets be ascending?
>
> You answered this in slack for me.  Can you put this at the top of this function:
> 
> 
> The insert_segment creates an ordered list of maps {offset, size} from the LayoutRawBlocks.  LayoutRawBlocks are queued in ascending offset order, and are processed in this order.  This allows the last_idx optimization which would be critical for classes with a huge number of fields.
> But, this property is not required, and, if you look at the entire set of fields being processed, they might not all be processed by ascending offset order.
> Inherited fields are inserted first, by copying them from the super-class, and then local fields are processed. It is possible to have local fields interleaved with super-class fields. So all fields are not necessarily processed in ascending offset order. The algorithm tries to do it, and optimizes the insertion when it is done that way, but it also handles cases where this is not the case.

Comments have been added in `FieldLayoutBuilder::generate_acmp_maps()`.
Let me know if the explanation  is sufficient to clarify how the code works.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1695#discussion_r2466837183