RFR: 8258714: Shenandoah: Process references before evacuation during degen

[Aleksey Shipilev]

On Mon, 21 Dec 2020 11:37:31 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> 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: 30 good runs of hotspot_gc_shenandoah, which showed the crash fairly reliably before
> 
> Ok?

This looks good, but I think this PR should be against `openjdk/jdk16` to get it fixed in JDK 16 (where I think the bug is).

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1859