[jdk16] RFR: 8258714: Shenandoah: Process references before evacuation during degen [v2]

[Roman Kennke]

> Currently, when doing degen-cycle, we process references right before immediate-garbage cleanup. It is imperative that we process references before any immediate garbage gets recycled, or else we may end up with bad references during reference-processing. However, the trouble is that immediate garbage can be recycled even before cleanup phase by recycle-assist. For this reason, we must process references before any evacuation during degen GC. It is also more natural: we process refs before weak roots and class-unloading during concurrent cycle, and should do the same during degen cycle.
> (Note that we already prevent recycle-assist in concurrent phase)
> 
> The change also adds STW timing for the weak-refs-processing, rather than polluting the conc-weak-refs timings.
> 
> Testing:
>  - [x] 30 good runs of hotspot_gc_shenandoah, which showed the crash fairly reliably before
> 
> Ok?

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Add new _weakrefs_process phases to ShPhaseTimings::is_worker_phase()

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

Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/55/files
  - new: https://git.openjdk.java.net/jdk16/pull/55/files/fbeb6f77..29b98531

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=55&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=55&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/55.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/55/head:pull/55

PR: https://git.openjdk.java.net/jdk16/pull/55