RFR: Embed traversal_set and root_regions into ShenandoahTraversalGC to avoid derefs

[Aleksey Shipilev]

On 06/27/2018 05:34 PM, Roman Kennke wrote:
> I've noticed some unnecessary derefs in hot marking/traversal loops to
> get to stuff like cset, mark-bitmap and traversal set.
> 
> Here I'm proposing to embed the traversal_set and root_regions into
> ShenandoahTraversalGC. This helps to avoid some of those derefs. I
> changed all access to it to use the embedded object directly inside
> ShTraversalGC, and only hand it out as reference. Only exception is the
> ShHeapRegionSetIterator, which still converts it to pointer internally,
> because we can't reset a reference.
> 
> The patch also makes ShHeap and task-queues const in ShTraversalGC.
> 
> http://cr.openjdk.java.net/~rkennke/traversal-embed-stuff/webrev.00/

*) Why the changes to ShenandoahHeapRegionSetIterator needed? Can we keep ShHeapRegionSet* in its
interface, and only convert to pointer when needed? This would keep the SHRS* usages intact.

*) It also seems simpler to return the addresses right away here, and then just deal with SHRS*
everywhere, as usual. E.g. instead of:

  ShenandoahHeapRegionSet& traversal_set() { return _travers_set; }
  ShenandoahHeapRegionSet& root_regions()  { return _root_regions;}

Do:

  ShenandoahHeapRegionSet* traversal_set() { return &_traversal_set; }
  ShenandoahHeapRegionSet* root_regions()  { return &_root_regions;  }

This should trim down the patch significantly, and optimizing compilers should see through it:
  https://godbolt.org/g/HZ2hAv

Thanks,
-Aleksey