RFR [sh/8u]: Do not enable UseCountedLoopSafepoints in Shenandoah by default

[Aleksey Shipilev]

On 10/23/19 12:28 PM, Roman Kennke wrote:
> Enabling -XX:+UseCountedLoopSafepoints in 8u seems risky because it's
> known to trigger bugs. In particular, it shows up with the upcoming LRB
> backport and the TestMaybeNullUnsafeAccess.java testcase.
> 
> Let's not enable it by default, and print a warning when the user
> enables it. The downside of it is regressed TTSP for code-paths with
> long counted loops.
> 
> http://cr.openjdk.java.net/~rkennke/clsp/webrev.00/

Sounds fine.

*) Current checking code assumes the default is "false", maybe it should be more straight-forward
instead:

  if (UseCountedLoopSafepoints) {
    ...
  }

*) Let's not put the URLs in the logs? Not sure that would be the only bug, and it is dubious to
list just one. Put the reference to example bug in the code comment, if you want.

-- 
Thanks,
-Aleksey