RFR (round 2), JEP-318: Epsilon GC

[Aleksey Shipilev]

Hi,

This is round 2 code review for Epsilon GC changes.

JEP, targeted to 11:
  http://openjdk.java.net/jeps/318
  (you can find links to binary builds and sandbox locations there)

Webrev:
  http://cr.openjdk.java.net/~shade/epsilon/webrev.06/

Notes:

  *) See how the whole thing is almost completely isolated to hotspot/share/gc, without having
arch-specific mess -- thanks to GC interface work done over last years. The only leftover is the
change in graphKit.cpp that should go away after C2 barriers modularization [1] is complete;

  *) Recently added GC TLD facility and Elastic TLABs feature enables Epsilon to be more streamlined
in figuring out the TLAB sizes;

  *) Conditional GC compilation is supported. ISVs may choose not to build Epsilon, if they do not
want to extend whatever small notion of support that comes with experimental VM option [2].

  *) Half of the webrev are Epsilon-specific tests. They take ~30s in release and ~60s in fastdebug
on my desktop. They are not in tier1, so smoke testing time is not affected;


Builds:
    server X {x86_64, x86_32, aarch64, arm32, ppc64le, s390x}
      zero X {x86_64}
   minimal X {x86}

Testing: gc/epsilon on x86_64

Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8202377
[2] http://hg.openjdk.java.net/jdk/jdk/file/3661f31c6df4/src/hotspot/share/runtime/globals.hpp#l150

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