func offAddrToLevelIndex(level int, addr offAddr) int {
	return int((addr.a - arenaBaseOffset) >> levelShift[level])
}

// levelIndexToOffAddr converts an index into summary[level] into
// the corresponding address in the offset address space.
func levelIndexToOffAddr(level, idx int) offAddr {
	return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset}
}

// addrsToSummaryRange converts base and limit pointers into a range