RFR: Implement heuristics to switch between merged and separate update-refs phase

[Roman Kennke]

Am 08.05.2017 um 18:23 schrieb Aleksey Shipilev:
> On 05/08/2017 05:12 PM, Roman Kennke wrote:
>> This also means that, at least when running with adaptive heuristics
>> (the default), the argument -XX:+/-ShenandoahUpdateRefsEarly is unused.
> Um. I wonder if there is an option to turn off UR unconditionally. Should we
> rewire that flag have three values: always-on, always-off, adaptive?

Like this ?

http://cr.openjdk.java.net/~rkennke/merge-uprefs/webrev.02/
<http://cr.openjdk.java.net/%7Erkennke/merge-uprefs/webrev.02/>

it takes 'true' / 'on', 'false', 'off' and 'adaptive'. Under adaptive
heuristics, it defaults to adaptive, otherwise to true. It prints a
warning when attempting 'adaptive' on non-adaptive heuristics (mostly
because we need to call the same constructor from Adaptive), fails on
other values.

Roman