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

[Paul Hübner]

On Tue, 7 Oct 2025 13:02:10 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.

This pull request has now been integrated.

Changeset: b5e30d53
Author:    Paul Hübner <phubner at openjdk.org>
Committer: David Simms <dsimms at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/b5e30d53fbe964c48c89ce8fcd30e8abe0b3fac5
Stats:     130 lines in 2 files changed: 127 ins; 0 del; 3 mod

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

Reviewed-by: coleenp

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PR: https://git.openjdk.org/valhalla/pull/1666