RFR: 8047818: G1 HeapRegions can no longer be ContiguousSpaces

[Thomas Schatzl]

Hi,

On Mon, 2014-06-23 at 16:26 +0200, Mikael Gerdin wrote:
> Hi!
> 
> When G1 is modified to unload classes without doing full collections the old 
> HeapRegions can contain unparseable objects. This makes ContiguousSpace 
> unsuitable as a base class for HeapRegion since it assumes that all objects 
> below _top are parseable.
> 
> Modify G1OffsetTableContigSpace to implement allocation with a separate _top 
> and reimplement some Space pure virtuals to make object iteration work as 
> expected.
> 
> This change is the last part of a set of 4 changes: 8047818, 8047819, 8047820, 
> 8047821 which are needed to refactor the HeapRegion class and its superclasses 
> in order to simplify the G1 class unloading change which is coming.
> This change depends on the 19, 20 and 21 changes.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8047818
> Webrev:
> http://cr.openjdk.java.net/~mgerdin/8047818/webrev/
> 
> Notes:
> The moving of set_offset_range is due to an introduced circular dependency 
> between g1BlockOffsetTable.inline.hpp and heapRegion.inline.hpp

  a few minor nits:

 - in G1OffsetTableContigSpace::cas_allocate_inner(), the method should
access _top directly per coding guidelines
 - just a note: _top should be declared volatile as it is used in the
CAS, although the code is correct. However there is already an issue for
that https://bugs.openjdk.java.net/browse/JDK-8033552, so I suggest
postponing this.
 - extra newline after G1OffsetTableContigSpace::allocate_inner()
 - extra newline after G1BlockOffsetSharedArray::set_offset_array()

Thanks,
  Thomas