RFR: 8260574: Remove parallel constructs in GenCollectedHeap::process_roots [v4]

Sat Jan 30 00:59:54 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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  remove surrounding braces

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2280/files
  - new: https://git.openjdk.java.net/jdk/pull/2280/files/9585038c..f81abed6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=02-03

  Stats: 30 lines in 1 file changed: 4 ins; 12 del; 14 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