On Mon, 27 Oct 2025 18:57:50 GMT, Coleen Phillimore <[email protected]> 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.
-------------
PR Review Comment:
https://git.openjdk.org/valhalla/pull/1695#discussion_r2466837183
