[lworld] RFR: 8376813: [lworld] Add print for dummy field reused as null-marker

[Frederic Parain]

On Fri, 30 Jan 2026 15:29:17 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

> Hello,
> 
> Right now it's not clear that the dummy field that is injected into empty inline klasses can be/is reused for the null-marker when looking at the printed layout from -XX:+PrintInlineLayout. I suggest we enhance this print to indicate if the dummy field has been reused for the null-marker.
> 
> I've tested that the added comment in the print is there when nullability is turned on (default), and not there when turned off (`-XX:-UseNullableValueFlattening -XX:-UseNullableNonAtomicValueFlattening`).
> 
> 
> Before:
> Instance fields:
>  @0 RESERVED 12/-
>  @12 REGULAR 1/1 ".empty" B
> 
> After:
> Instance fields:
>  @0 RESERVED 12/-
>  @12 REGULAR 1/1 ".empty" B (reused as null-marker)
> 
> 
> Testing:
> * GHA
> * Oracle's tier1

Looks good to me.
It's unfortunate that the null marker offset is stored in the FieldLayoutBuilder and not in FieldLayout, this would have prevented adding an argument to the print method.

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Marked as reviewed by fparain (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/2001#pullrequestreview-3729750699