RFR (8u-backport): Implement early update references phase

[Roman Kennke]

Am 09.05.2017 um 19:24 schrieb Aleksey Shipilev:
> On 05/09/2017 07:14 PM, Roman Kennke wrote:
>> This is a straight backport of the early update references phase and all
>> related code from JDK9.
>>
>> http://cr.openjdk.java.net/~rkennke/backport-uprefs/webrev.00/
>> <http://cr.openjdk.java.net/%7Erkennke/backport-uprefs/webrev.00/>
>>
>> Testing: hotspot_gc_shenandoah, specjvm compiler
>>
>> Ok?
> Nice and tidy, thanks.
>
> Comparing with 9, these are the interesting differences, which might be good to
> fix in this iteration:
>
>  *) ShenandoahHeuristics::record_gc_start/end() seem to be missing?
This one seems to be used only for partial GC. However, it looks more
consistent in this iteration. Added.

>  *) ShenandoahHeap::update_refs_in_progress_addr() seems to be missing?
Similar: this is only used for the improved store checks, but looks more
consistent here. Added too.

I also added heap occupancy logging for the update refs pause, as
present in jdk9.

http://cr.openjdk.java.net/~rkennke/backport-uprefs/webrev.01/
<http://cr.openjdk.java.net/%7Erkennke/backport-uprefs/webrev.01/>

Ok?

Roman