RFR: 8215097: Do not create NonJavaThreads before BarrierSet

[David Holmes]

Hi Kim,

Didn't realize you were going to tackle this so soon. I was just ironing 
out the wrinkles in 8214097 before sending it for review later today. :)

On 9/12/2018 6:30 pm, Kim Barrett wrote:
> Please review this change to move the construction of some work gang
> threads by G1 and CMS to after they've created the barrier set.  This
> allows the removal of some bootstrapping code needed to support that
> construction order.  There isn't any requirement for the old order,
> it seems to just be a historical artifact.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8215097
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8215097/open.00/

Moving the GC thread creation out of the heap constructor into the heap 
initialize function seems quite reasonable. Does ZGC already defer the 
thread creation? Will this impact the merge of Shenandoah?

Everything else seems fine. (I'll be reworking the BarrierSet creation 
assertion as part of 8214097.)

Thanks,
David

> Testing:
> mach5 tier1-5
>