RFR: 8260574: Remove parallel constructs in GenCollectedHeap::process_roots

Thu Jan 28 09:15:52 UTC 2021

This PR is broken into two commits for easier reviewing, one for removing the usage of `SubTasksDone` in `GenCollectedHeap`, and the other for removing `StrongRootsScope` in the call chain of `GenCollectedHeap::process_roots`.

Tested: hotspot_gc

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Commit messages:
 - StrongRootsScope sequential support
 - remove SubTasksDone in serial gc

Changes: https://git.openjdk.java.net/jdk/pull/2280/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260574
  Stats: 57 lines in 7 files changed: 5 ins; 34 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2280.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2280/head:pull/2280

PR: https://git.openjdk.java.net/jdk/pull/2280