RFR: Enable C2 loop strip mining by default

[Aleksey Shipilev]

On 12/15/2017 01:38 PM, Per Liden wrote:
> Patch to enable loop strip mining by default when using ZGC. I also noticed that the file had an
> incorrect header, so I fixed that too.
> 
> http://cr.openjdk.java.net/~pliden/zgc/c2_loop_strip_mining_by_default/webrev.0/

Yup. It worked very well for Shenandoah.

But, the relevant code block from Shenandoah code is:

#ifdef COMPILER2
  // Shenandoah cares more about pause times, rather than raw throughput.
  if (FLAG_IS_DEFAULT(UseCountedLoopSafepoints)) {
    FLAG_SET_DEFAULT(UseCountedLoopSafepoints, true);
  }
  if (UseCountedLoopSafepoints && FLAG_IS_DEFAULT(LoopStripMiningIter)) {
    FLAG_SET_DEFAULT(LoopStripMiningIter, 1000);
  }
#ifdef ASSERT

...which is slightly different from what you are suggesting for ZGC. Don't you want to enable
LoopStripMiningIter when user explicitly sets -XX:+UseCountedLoopSafepoints (which, I guess, are
most users concerned with TTSP-related latency)?

Thanks,
-Aleksey