RFR: 8304412: Serial: Refactor old generation cards update after Full GC

Fri Mar 17 14:00:12 UTC 2023

Simple refactoring to use more informative names/comments.

Test: tier1-6

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Commit messages:
 - s1-card-table-refactor

Changes: https://git.openjdk.org/jdk/pull/13076/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13076&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304412
  Stats: 46 lines in 3 files changed: 0 ins; 27 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/13076.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13076/head:pull/13076

PR: https://git.openjdk.org/jdk/pull/13076