refactor coroutine lowering and tasks

src/internal/task/queue.go

@@ -0,0 +1,98 @@
+package task
+
+const asserts = false
+
+// Queue is a FIFO container of tasks.
+// The zero value is an empty queue.
+type Queue struct {
+	head, tail *Task
+}
+
+// Push a task onto the queue.
+func (q *Queue) Push(t *Task) {
+	if asserts && t.Next != nil {
+		panic("runtime: pushing a task to a queue with a non-nil Next pointer")
+	}
+	if q.tail != nil {
+		q.tail.Next = t
+	}
+	q.tail = t
+	t.Next = nil
+	if q.head == nil {
+		q.head = t
+	}
+}
+
+// Pop a task off of the queue.
+func (q *Queue) Pop() *Task {
+	t := q.head
+	if t == nil {
+		return nil
+	}
+	q.head = t.Next
+	if q.tail == t {
+		q.tail = nil
+	}
+	t.Next = nil
+	return t
+}
+
+// Append pops the contents of another queue and pushes them onto the end of this queue.
+func (q *Queue) Append(other *Queue) {
+	if q.head == nil {
+		q.head = other.head
+	} else {
+		q.tail.Next = other.head
+	}
+	q.tail = other.tail
+	other.head, other.tail = nil, nil
+}
+
+// Stack is a LIFO container of tasks.
+// The zero value is an empty stack.
+// This is slightly cheaper than a queue, so it can be preferable when strict ordering is not necessary.
+type Stack struct {
+	top *Task
+}
+
+// Push a task onto the stack.
+func (s *Stack) Push(t *Task) {
+	if asserts && t.Next != nil {
+		panic("runtime: pushing a task to a stack with a non-nil Next pointer")
+	}
+	s.top, t.Next = t, s.top
+}
+
+// Pop a task off of the stack.
+func (s *Stack) Pop() *Task {
+	t := s.top
+	if t != nil {
+		s.top = t.Next
+	}
+	t.Next = nil
+	return t
+}
+
+// tail follows the chain of tasks.
+// If t is nil, returns nil.
+// Otherwise, returns the task in the chain where the Next field is nil.
+func (t *Task) tail() *Task {
+	if t == nil {
+		return nil
+	}
+	for t.Next != nil {
+		t = t.Next
+	}
+	return t
+}
+
+// Queue moves the contents of the stack into a queue.
+// Elements can be popped from the queue in the same order that they would be popped from the stack.
+func (s *Stack) Queue() Queue {
+	head := s.top
+	s.top = nil
+	return Queue{
+		head: head,
+		tail: head.tail(),
+	}
+}