RFR (XS): Enable UseCountedLoopSafepoints with Shenandoah

[Lennart Börjeson]

> 20 dec. 2016 kl. 16:58 skrev Aleksey Shipilev <shade at redhat.com>:
> 
> On 12/20/2016 04:54 PM, Lennart Börjeson wrote:
>>> 20 dec. 2016 kl. 16:26 skrev Aleksey Shipilev <shade at redhat.com>:
>>> 
>>> I am a bit confused about the notion of "standard behavior". There is no 
>>> standard that mandates either putting safepoint checks into loops, or
>>> skipping them.
>>> 
>>> This Shenandoah change _inserts_ more safepoint checks, not eliminates
>>> them, so this seems like something you want?
>>> 
>> 
>> I was thinking about the flag UseCountedLoopSafepoints. The current default
>> is ”false”, and I gathered you were discussing to change this to ”true”?
> 
> Yes. "true" means Hotspot will emit safepoints checks in counted loops, thus
> improving time-to-safepoint, and therefore improving pause time. Isn't that the
> behavior you want for your product?
> 

Well, if there were to be a safe point check in every counted loop, I fear overall performance would suffer too much. But I would of course need to test that. 

Note that the problem we had with the other JVM was more that the definition of "counted loop" had changed, than a change of a default value for a flag.