func (b *atomicOffAddr) StoreUnmark(markedAddr, newAddr uintptr) {
	b.a.CompareAndSwap(-int64(markedAddr-arenaBaseOffset), int64(newAddr-arenaBaseOffset))
}

// StoreMarked stores addr but first converted to the offset address
// space and then negated.
func (b *atomicOffAddr) StoreMarked(addr uintptr) {
	b.a.Store(-int64(addr - arenaBaseOffset))
}

// Load returns the address in the box as a virtual address. It also