RFR (M): 8201492: Properly implement non-contiguous generations for reference discovery

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adapts the reference
discovery for the non-contiguous generations (and non-contiguous
generations in general) of G1 to solve one day-one performance issue?

So reference discovery does not properly support non-contiguous
generations, resorting to an approximation of it. 

This in turn causes G1 to require the "preserve cm referents" phase
during GC during marking, which is very costly in some cases. 

The reason for the preserve cm referents phase is that References
(j.l.ref.References instances) that are discovered by concurrent
discovery, will currently prevent discovery and evacuation of referents
in the STW pauses, as G1 thinks that it already has discovered that
Reference and skips it. Still their referents can be in young gen,
while the Reference is in old gen (young gc may iterate over it during
card scanning), and this may cause crashes later. 

The preserve cm referents phase brute-force simply evacuates any
"leftover" referents and its followers. 

This is because the STW reference discovery currently does not treat
References in old gen as roots (i.e. to be scanned and referents
evacuated always), as the STW reference processor thinks that the g1
heap is a single generation spanning the whole heap. 

By giving the ref processor the correct idea of generations in G1, this
automatically works, and obsoletes the "preserve cm referents" phase.

To get an understanding how serious this issue may be, on theKitchensink reference stress test program, the "Preserve CM referents"
phase may take 105ms out of 115ms.
CR:
https://bugs.openjdk.java.net/browse/JDK-8201492
Webrev:
http://cr.openjdk.java.net/~tschatzl/8201492/webrev/
Testing:
hs-tier1-3, performance verification, lots of Kitchensink reference
stress testing runs

Thanks,
  Thomas