[jdk17u-dev] RFR: 8280784: VM_Cleanup unnecessarily processes all thread oops [v2]

[Aleksey Shipilev]

> Clean backport of performance enhancement for GCs that do concurrent thread roots (in JDK 17u those are Shenandoah and Z). On my machine, with Shenandoah running 20K threads, the safepoint cleanup time for "Cleanup" VM ops dropped from 2ms to nearly zero. A possible follow-up would be [JDK-8280817](https://bugs.openjdk.org/browse/JDK-8280817), which would extend this to other "empty" ops.
> 
> Additional testing:
>  - [x] Ad-hoc benchmarks
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2` 
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2`  with `-XX:+DeoptimizeALot`
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2`  with `-XX:+UseShenandoahGC`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8280784-cleanup-threadoops
 - Backport 8a3cca09ba427282f2712bec7298b85bbacf076b

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

Changes:
  - all: https://git.openjdk.org/jdk17u-dev/pull/889/files
  - new: https://git.openjdk.org/jdk17u-dev/pull/889/files/02fce546..8fd5b689

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=889&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=889&range=00-01

  Stats: 29596 lines in 427 files changed: 23888 ins; 2545 del; 3163 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/889.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/889/head:pull/889

PR: https://git.openjdk.org/jdk17u-dev/pull/889