RFR: JDK-8305334: GenShen: reference processing needs a card-marking barrier

[William Kemper]

On Thu, 30 Mar 2023 20:30:26 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> Global collections can create new cross-generational pointers during j.l.r.Reference processing which should be added to the card marking remembered set. The issue was found with dacapo during heap verification and happens somewhat rarely. I added the requisite barrier and have provided a comment describing the sole situation in reference processing that should need the barrier. Assertions check this condition, but the card is also redundantly dirtied for young collections too where it's strictly not needed.
> 
> I have tested with and without heap verification using product and fastdebug builds using dacapo and specjbb. Will run it through our internal tests pipeline too before pushing the fix.
> 
> I have not yet run any performance comparisons to assess the impact of the checks for applications that traffic heavily in j.l.r.Refs, but am open to any suggestions.
> 
> Thank you for your reviews.

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/238#pullrequestreview-1367481200