The changes for:
6888336: G1: avoid explicitly marking and pushing objects in survivor spaces
leave around 1,500 lines of dead code (mainly: methods and classes that are not used any more) in the concurrent mark and other files. We decided not to remove the dead code as part of the 6888336 changes in order to keep those changes simpler and the code review quicker. But we should go ahead and remove the dead code, which was one of the major motivations for doing the 6888336 changes in the first place.