func (vm *VM) newThread() *thread {
	s := &stack{RWMutex: new(sync.RWMutex)}
	cfs := &callFrameStack{callFrames: []callFrame{}}
	t := &thread{stack: s, callFrameStack: cfs, sp: 0, cfp: 0}
	s.thread = t
	cfs.thread = t
	t.vm = vm
	return t
}