```
inline size_t
ParallelCompactData::region_offset(const HeapWord* addr) const
{
  assert(addr >= _region_start, "bad addr");
  assert(addr <= _region_end, "bad addr");
  return (size_t(addr) & RegionAddrOffsetMask) >> LogHeapWordSize;
}
```
`region_offset` calculates the distance (in words) from `_region_start` to `addr`. For `addr == _region_end`, it would mistakenly return 0. It's better to disallow such value as the argument, like `assert(addr < _region_end, "bad addr");`.