[lworld] RFR: 8369134: [lworld] We should preserve the markWord for all GCs [v2]

[duke]

On Mon, 20 Oct 2025 09:53:50 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> Hi all,
>> 
>> This PR follows up on changes made in [JDK-8368875](https://bugs.openjdk.org/browse/JDK-8368875). I updated the documentation to reflect the new information gained.
>> 
>> Furthermore, the investigation described in the issue yielded that we _must_ preserve the larval bit explicitly. I introduce a regression test. If we change the mark word preservation condition to `return (!is_unlocked() || !has_no_hash())` (i.e. what we had before), I have confirmed that we will crash a debug VM via failed assertion. Note that ZGC does not require mark word preservation. I've nonetheless decided to include it in case this behaviour is introduced later.
>> 
>> Testing: tiers 1-3.
>
> Paul Hübner has updated the pull request incrementally with one additional commit since the last revision:
> 
>   WhiteBox API.

@Arraying 
Your change (at version a75ca117bc4e686d863b468158bd58eba424fcb6) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/valhalla/pull/1666#issuecomment-3425092016