The HeapRegion constructor takes an "is_zeroed" parameter that indicates whether the newly-allocated region is already zeroed. This is a hickup from the time when we had the concurrent zero fililng thread which has not been removed (6977804: G1: remove the zero-filling thread). It should have been removed as part of that CR but I just missed that extra parameter. It'd be good to get rid of it. Note that the is_zeroed parameter is propagated through several layers of constructors and initialization methods.