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

[Zhengyu Gu]

On Tue, 6 Oct 2020 13:52:23 GMT, Aleksey Shipilev <shade 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
>
> 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?

No. I actually puzzled why checked it here, our concurrent roots scanner can handle both scenarios.
Maybe a leftover from concurrent roots to concurrent class unloading transition?

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