[master] RFR: 8353849: [Lilliput] Avoid race in compact identity hashcode

[William Kemper]

On Mon, 7 Apr 2025 12:48:02 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Multiple GC threads can concurrently update an object's mark-word (Parallel, G1 and Shenandoah). We need to be careful when doing object-size calculations and pass-through the pre-loaded, safe mark-word, instead of re-loading the mark-word from the object. In the latter case, we might read a forwarding pointer and interpret the bits as hash-bits which would lead to wrong size-calculation and crash.
> 
> Testing:
> - hotspot_gc (+UCOH)

LGTM

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Marked as reviewed by wkemper (Reviewer).

PR Review: https://git.openjdk.org/lilliput/pull/196#pullrequestreview-2747423621