RFR: 8255847: Shenandoah: Shenandoah should not mark through weak roots

[Roman Kennke]

On Wed, 4 Nov 2020 00:09:13 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> After moving weak root processing into concurrent phase, Shenandoah should no longer marks through weak roots, even when class unloading is disabled, given weak root processing no longer contributes to latency.
> 
> There are a couple of bugs:
> 1) ShenandoahRootVerifier was not updated to reflect the change.
> The problem did not show up due to SH::parallel_cleaning() uses wrong flag to determine if it should cleanup weak roots, and it will be addressed in separate CR.
> 
> 2) Concurrent roots scanner should not mark through string dedup roots.
> 
> Test:
> - [x]  hotspot_gc_shenandoah
> - [x]  hotspot_gc_shenandoah with -XX:+ShenandoahVerify -XX:-ClassUnloading
> - [x]  hotspot_gc_shenandoah with -XX:+ShenandoahVerify -XX:+UseStringDeduplication

Looks good. Make sure to merge latest tip to get full pre-submit testing in GitHub actions.

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1050