RFR: Single thread-local GC state flag for all barriers

[Aleksey Shipilev]

On 01/12/2018 12:11 PM, Aleksey Shipilev wrote:
> http://cr.openjdk.java.net/~shade/shenandoah/single-flag/webrev.01/
> 
> Please review this carefully. I did AArch64 change blindly, symmetric with x86. Taking care of SATB
> check requires some specialization in g1_wb_pre, and the relevant compiler matching changes. This
> would get convenient as we common the gc-state load between the safepoints, and that would not touch
> G1 SATB barriers then.
> 
> Example disassembly, (0x3d8(%r15) is our flag):
>   http://cr.openjdk.java.net/~shade/shenandoah/single-flag/single-flag.perfasm
> 
> Testing: hotspot_gc_shenandoah, eyeballing generated code

Renamed need_update_refs to is_unstable -- this captures the intent better, and updated comments a
little:
  http://cr.openjdk.java.net/~shade/shenandoah/single-flag/webrev.02/

Thanks,
-Aleksey