[rvm-research] [GC]About MarkCompact and ComplexPhase

Hello All，
After reading MarkCompact GCPlan, I  have several questions:

1.MarkCompactCollector.ToCursor
   void copy(ObjectReference from, ObjectReference to) {   
     cursor = VM.objectModel.copyTo(from, to, cursor);

      MarkCompactSpace.setForwardingPointer(to, ObjectReference.nullReference());    
    }
the "cursor" is:
@param region The start of the region that was reserved for this object
@param region The start (or an address less than) the region that was reserved for this object.
but "cursor" may change during a collection, not always the start of the region.

2.MarkCompactCollector
public void calculateForwardingPointers() {
  /* Loop through the objects in the current 'from' region */
      while (fromCursor.hasMoreObjects()) {
       ......

        if (MarkCompactSpace.toBeCompacted(current)) {  
        .......
         toCursor.incTo(Allocator.alignAllocationNoFill(toCursor.get(), align, offset));//【1】
        .......
          if (!toCursor.sameRegion(fromCursor) && !toCursor.isAvailable(size)) {
            toCursor.advanceToNextRegion();
            toCursor.incTo(Allocator.alignAllocationNoFill(toCursor.get(), align, offset));//【2】
          }

          ObjectReference target = VM.objectModel.getReferenceWhenCopiedTo(current, toCursor.get());
          if (toCursor.sameRegion(fromCursor) && target.toAddress().GE(current.toAddress())) {
            // Don't move the object.//【3】
            MarkCompactSpace.setForwardingPointer(current, current);
            toCursor.incTo(VM.objectModel.getObjectEndAddress(current));
          } else {
            MarkCompactSpace.setForwardingPointer(current, target);
            toCursor.inc(size);
          }
        }
      }

}
Why do "toCursor.incTo(Allocator.alignAllocationNoFill(toCursor.get(), align, offset));"  twice? See【1】【2】
What does "if (toCursor.sameRegion(fromCursor) && target.toAddress().GE(current.toAddress())) "mean? See【3】
In which condition the collector will not move the object?


3.MarkCompactSpace
Some comments are confusing，just Crtl C+Crtl V?
traceObject()/traceMarkObject()/traceForwardObject()
testAndMark()/testAndClearMark()/clearMark()

traceObject() is just a stub(Maybe I can reuse to perform Copying)
In traceMarkObject() method, why return "object" not "getForwardingPointer(object)"? See 【4】
public ObjectReference traceMarkObject(TraceLocal trace, ObjectReference object) {    
    if (testAndMark(object)) {
      trace.processNode(object);
    } else if (!getForwardingPointer(object).isNull()) {
      return getForwardingPointer(object);
    }
    }
    return object;【4】
  }


4.MC & ComplexPhase
In org.mmtk.plan.markcompact.MC.java, ComplexPhase"mcColletion" is created and it replaces "colletion" in Simple.
When a GC is triggered, perform Phase.beginNewPhaseStack(Phase.scheduleComplex(global().collection))

=> pushScheduledPhase() & processPhaseStack()
There is no switch casestatementfor "SCHEDULE_COMPLEX" in processPhaseStack(), how to process ComplexPhase in Phase? 

Simple defines lots of Phases, which is necessary for a Plan? And when should I define my own phase?
I'm still not so clear about how to write collctionPhase().


Thanks for any help.
Best Wishes.                                                                                    Chen Hao

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