refactor coroutine lowering and tasks

src/internal/task/task_stack.go

@@ -0,0 +1,74 @@
+// +build scheduler.tasks
+
+package task
+
+import "unsafe"
+
+//go:linkname runtimePanic runtime.runtimePanic
+func runtimePanic(str string)
+
+// Stack canary, to detect a stack overflow. The number is a random number
+// generated by random.org. The bit fiddling dance is necessary because
+// otherwise Go wouldn't allow the cast to a smaller integer size.
+const stackCanary = uintptr(uint64(0x670c1333b83bf575) & uint64(^uintptr(0)))
+
+// state is a structure which holds a reference to the state of the task.
+// When the task is suspended, the registers are stored onto the stack and the stack pointer is stored into sp.
+type state struct {
+	// sp is the stack pointer of the saved state.
+	// When the task is inactive, the saved registers are stored at the top of the stack.
+	sp uintptr
+
+	// canaryPtr points to the top word of the stack (the lowest address).
+	// This is used to detect stack overflows.
+	// When initializing the goroutine, the stackCanary constant is stored there.
+	// If the stack overflowed, the word will likely no longer equal stackCanary.
+	canaryPtr *uintptr
+}
+
+// currentTask is the current running task, or nil if currently in the scheduler.
+var currentTask *Task
+
+// Current returns the current active task.
+func Current() *Task {
+	return currentTask
+}
+
+// Pause suspends the current task and returns to the scheduler.
+// This function may only be called when running on a goroutine stack, not when running on the system stack or in an interrupt.
+func Pause() {
+	// Check whether the canary (the lowest address of the stack) is still
+	// valid. If it is not, a stack overflow has occured.
+	if *currentTask.state.canaryPtr != stackCanary {
+		runtimePanic("goroutine stack overflow")
+	}
+	currentTask.state.pause()
+}
+
+// Resume the task until it pauses or completes.
+// This may only be called from the scheduler.
+func (t *Task) Resume() {
+	currentTask = t
+	t.state.resume()
+	currentTask = nil
+}
+
+// initialize the state and prepare to call the specified function with the specified argument bundle.
+func (s *state) initialize(fn uintptr, args unsafe.Pointer) {
+	// Create a stack.
+	stack := make([]uintptr, stackSize/unsafe.Sizeof(uintptr(0)))
+
+	// Invoke architecture-specific initialization.
+	s.archInit(stack, fn, args)
+}
+
+//go:linkname runqueuePushBack runtime.runqueuePushBack
+func runqueuePushBack(*Task)
+
+// start creates and starts a new goroutine with the given function and arguments.
+// The new goroutine is scheduled to run later.
+func start(fn uintptr, args unsafe.Pointer) {
+	t := &Task{}
+	t.state.initialize(fn, args)
+	runqueuePushBack(t)
+}