[lworld] RFR: 8373494: [lworld] markWord preservation for ParallelGC [v2]

[Joel Sikström]

On Thu, 11 Dec 2025 12:55:41 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/psParallelCompact.cpp line 1481:
>> 
>>> 1479:           const bool full_header_in_current_region = cur_addr + oopDesc::header_size() <= end;
>>> 1480: 
>>> 1481:           if (EnableValhalla && !full_header_in_current_region) {
>> 
>> We could make this criteria even more specific by checking something like the following, but I'm not sure it's worth it since not copying the full header in the same region is pretty rare.
>> 
>> 
>> static bool requires_valhalla_preservation(markWord mark) {
>>   return EnableValhalla && (mark.is_larval_state() || mark.is_null_free_array() || mark.is_flat_array() || mark.is_inline_type());
>> }
>
> If I understand it correctly, the version you have right now is less conservative than `requires_valhalla_preservation` which may include false positives. I think I'm inclined to checking region containment, it also makes maintainability easier down the line (if we end up changing the markWord bits).

Yes, the version I have is less conservative than it could be, and I agree that this makes the code easier to maintain.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1785#discussion_r2613389856