RFR (S): Specialize iteration over marked objects

[Aleksey Shipilev]

Hi,

Our current ShenandoahHeap::marked_object_iterate does the virtual call
to ObjectClosure::do_object for every marked object, for every task out
there. When the per-oop work is small, this is not good. Also limits
optimizeability (e.g. coalescing ShenandoahHeap::heap(), oopDesc::*, etc).

This is the template-based specialization:

http://cr.openjdk.java.net/~shade/shenandoah/specialize-mark-iterate/webrev.01/

After this change, our work is concentrated in specialized methods, e.g.:

   6.90%  ShenandoahMarkObjsClosure<ShenandoahMarkUpdateRefsClosure,
false>::do_object_or_array
   5.82%  ShenandoahCompactObjectsTask::work
   5.51%  ShenandoahPrepareForCompactionTask::work
   5.20%  ShenandoahAdjustPointersClosure::do_oop
   4.20%  ShenandoahAdjustPointersTask::work
   ...

(for ShenandoahAdjustPointersClosure::do_oop, inlining breaks elsewhere,
to be handled separately)

Testing: hotspot_gc_shenandoah, jcstress -m quick, some microbenchmarks

Thanks,
-Aleksey