RFR: 8256298: Shenandoah: Enable concurrent stack processing [v4]

[Aleksey Shipilev]

On Mon, 25 Jan 2021 15:18:02 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Please review this patch that enables concurrent stack processing for Shenandoah GC.
>> 
>> After this patch, all root processing is done concurrently for concurrent GC.
>> 
>> Test:
>> - [x] hotspot_gc_shenandoah Linux x86_64 and x86_32
>> - [x] Nightly
>> - [x] tier1 with -XX:+UseShenandoahGC on Linux x86_32
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More of Roman's comments

Cursory review.

src/hotspot/share/gc/shenandoah/shenandoahClosures.inline.hpp line 79:

> 77: void ShenandoahKeepAliveClosure::do_oop(narrowOop* p) {
> 78:   do_oop_work(p);
> 79: }

These can move to declarations, right? See how the rest of `shenandoahOopClosures.hpp` does it.

src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp line 512:

> 510: //
> 511: public:
> 512:   bool uses_stack_watermark_barrier() const override { return true; }

I wonder that this single use of `override` caused CLang to complain about the rest of the file: https://github.com/zhengyu123/jdk/runs/1762873416

src/hotspot/share/gc/shenandoah/shenandoahStackWatermark.cpp line 59:

> 57: void ShenandoahStackWatermark::change_epoch_id() {
> 58:   shenandoah_assert_safepoint();
> 59:   _epoch_id ++;

Excess space before `++`.

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PR: https://git.openjdk.java.net/jdk/pull/2185