[master] RFR: JDK-8324523: Lilliput: if +UseCOH, always use the archive's encoding base and shift

[Roman Kennke]

On Tue, 23 Jan 2024 13:21:11 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> We have two ways to initialize narrow Klass encoding: either we let the JVM choose base and shift freely, or we dictate base and shift. The former gives the JVM more leeway, e.g. to go with unscaled encoding. The latter, however, is required if we load a CDS archive and that archive contains precomputed narrow Klass IDs.
> 
> In the Legacy VM, this can only happen if the archive contains heap objects. In Lilliput, the markword carries the nKlass, and therefore the prototype baked into archived Klass structures carries it also. Therefore, we must always choose the strict initialization when +UseCOH.

Change looks ok.

Questions, though: what is the impact of this? Is it a bug? Does it improve or regress performance? Should it be backported?

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Marked as reviewed by rkennke (Lead).

PR Review: https://git.openjdk.org/lilliput/pull/124#pullrequestreview-1839123144