From the review discussion for JDK-8133456 the following suggestions for improving the interface for two HeapRegionManager methods came up:

I'd also prefer it slightly if HeapRegionManager::find_available_from_idx_reverse had the index as return value, with the count as the output parameter. Same for HeapRegionManager::find_unavailable_from_idx. Then they would be consistent with find_contiguous and find_highest_free, and there wouldn't be 2 index params to look odd.

Review thread starts at http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-August/014488.html for reference.