RFR: 8275055: Improve HeapRegionRemSet::split_card() [v4]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this small change that improves HeapRegionRemSet::split_card() by reducing the number of (direct and indirect) memory accesses for it. It is hard to actually measure improvements because it's only called in concurrent code. So no particular improvements measured.
> 
> It also prepares that method for virtualizing the remembered set containers, allowing arbitrarily large heap region sizes [JDK-8275056](https://bugs.openjdk.java.net/browse/JDK-8275056). This change is required then because the splitting of a card index into "region" and "card within region" should not be dependent on *heap regions*.
> 
> Testing: gha, gc/g1 local testing.
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  kbarrett review

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5895/files
  - new: https://git.openjdk.java.net/jdk/pull/5895/files/615bf7c2..2825a499

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5895&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5895&range=02-03

  Stats: 32 lines in 6 files changed: 21 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5895.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5895/head:pull/5895

PR: https://git.openjdk.java.net/jdk/pull/5895