if n&(n-1) != 0 {
		throw("stack size not a power of 2")
	}
	if stackDebug >= 1 {
		print("stackalloc ", n, "\n")
	}

	if debug.efence != 0 || stackFromSystem != 0 {
		n = uint32(alignUp(uintptr(n), physPageSize))
		v := sysAlloc(uintptr(n), &memstats.stacks_sys)
		if v == nil {
			throw("out of memory (stackalloc)")
		}
		return stack{uintptr(v), uintptr(v) + uintptr(n)}
	}