RFR: 8254075: Shenandoah: Remove ShenandoahCodeRootsStyle diagnostic flag and related test

[Aleksey Shipilev]

On Tue, 6 Oct 2020 13:40:02 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Shenandoah introduced fast code roots iteration (ShenandoahCodeRootsStyle = 2), also kept 2 additional iteration
> mechanism (ShenandoahCodeRootsStyle = 0 and 1), mainly for verification purpose.
> Today, Shenandoah normally iterates code roots concurrently using style 2, while style 0 and 1 require safepoints, that
> makes style 0 and 1 verification less useful. Besides, Shenandoah has much improved verifier, more suitable for the
> purpose.  Let's remove this diagnostic flag and related test.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah

Looks fine to me, except `ClassUnloading` handling in one place.

src/hotspot/share/gc/shenandoah/shenandoahConcurrentRoots.cpp line 40:

> 38:
> 39: bool ShenandoahConcurrentRoots::can_do_concurrent_class_unloading() {
> 40:   return true;

Hold on, this used to check `ClassUnloading`, should it continue to check it?

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Changes requested by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/524