RFR: Clean up CondCardMark code: remove excess barriers, enable it by default [v2]

[Zhengyu Gu]

On Wed, 31 Mar 2021 14:24:02 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> The `StoreLoad`, `StoreStore` barriers in that code are only needed for CMS, and does not relate to Shenandoah. Also, `UseCondCardMark` is a recommended option for most machines today, and therefore we better just enable it ergonomically.
>> 
>> Additional testing:
>>  - [x] hotspot_gc_shenandoah, x86_64, {fastdebug,release}
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More cleanups of scanned_concurrently

src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp line 393:

> 391:   if (UseCondCardMark) {
> 392:     Label L_already_dirty;
> 393:     __ membar(Assembler::StoreLoad);

I believe this StoreStore barrier is for preventing race against concurrent refinement. Question is that, generational Shenandoah going to implement concurrent refinement in the future?

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PR: https://git.openjdk.java.net/shenandoah/pull/26