Add goroutines and function pointers

src/runtime/scheduler.go

@@ -0,0 +1,249 @@
+
+package runtime
+
+// This file implements the Go scheduler using coroutines.
+// A goroutine contains a whole stack. A coroutine is just a single function.
+// How do we use coroutines for goroutines, then?
+//   * Every function that contains a blocking call (like sleep) is marked
+//     blocking, and all it's parents (callers) are marked blocking as well
+//     transitively until the root (main.main or a go statement).
+//   * A blocking function that calls a non-blocking function is called as
+//     usual.
+//   * A blocking function that calls a blocking function passes its own
+//     coroutine handle as a parameter to the subroutine and will make sure it's
+//     own coroutine is removed from the scheduler. When the subroutine returns,
+//     it will re-insert the parent into the scheduler.
+// Note that a goroutine is generally called a 'task' for brevity and because
+// that's the more common term among RTOSes. But a goroutine and a task are
+// basically the same thing. Although, the code often uses the word 'task' to
+// refer to both a coroutine and a goroutine, as most of the scheduler isn't
+// aware of the difference.
+//
+// For more background on coroutines in LLVM:
+// https://llvm.org/docs/Coroutines.html
+
+import (
+	"unsafe"
+)
+
+// State/promise of a task. Internally represented as:
+//
+//     {i8 state, i32 data, i8* next}
+type taskState struct {
+	state uint8
+	data  uint32
+	next  taskInstance
+}
+
+// Pointer to a task. Wrap unsafe.Pointer to provide some sort of type safety.
+type taskInstance unsafe.Pointer
+
+// Various states a task can be in. Not always updated (especially
+// TASK_STATE_RUNNABLE).
+const (
+	TASK_STATE_RUNNABLE = iota
+	TASK_STATE_SLEEP
+	TASK_STATE_CALL // waiting for a sub-coroutine
+)
+
+// Queues used by the scheduler.
+//
+// TODO: runqueueFront can be removed by making the run queue a circular linked
+// list. The runqueueBack will simply refer to the front in the 'next' pointer.
+var (
+	runqueueFront      taskInstance
+	runqueueBack       taskInstance
+	sleepQueue         taskInstance
+	sleepQueueBaseTime uint64
+)
+
+// Translated to void @llvm.coro.resume(i8*).
+func _llvm_coro_resume(taskInstance)
+
+// Translated to void @llvm.coro.destroy(i8*).
+func _llvm_coro_destroy(taskInstance)
+
+// Translated to i1 @llvm.coro.done(i8*).
+func _llvm_coro_done(taskInstance) bool
+
+// Translated to i8* @llvm.coro.promise(i8*, i32, i1).
+func _llvm_coro_promise(taskInstance, int32, bool) unsafe.Pointer
+
+// Get the promise belonging to a task.
+func taskPromise(t taskInstance) *taskState {
+	return (*taskState)(_llvm_coro_promise(t, 4, false))
+}
+
+// Simple logging, for debugging.
+func scheduleLog(msg string) {
+	//println(msg)
+}
+
+// Simple logging with a task pointer, for debugging.
+func scheduleLogTask(msg string, t taskInstance) {
+	//println(msg, t)
+}
+
+// Set the task state to sleep for a given time.
+//
+// This is a compiler intrinsic.
+func sleepTask(caller taskInstance, duration Duration) {
+	promise := taskPromise(caller)
+	promise.state = TASK_STATE_SLEEP
+	promise.data = uint32(duration) // TODO: longer durations
+}
+
+// Wait for the result of an async call. This means that the parent goroutine
+// will be removed from the runqueue and be rescheduled by the callee.
+//
+// This is a compiler intrinsic.
+func waitForAsyncCall(caller taskInstance) {
+	promise := taskPromise(caller)
+	promise.state = TASK_STATE_CALL
+}
+
+// Add a task to the runnable or sleep queue, depending on the state.
+//
+// This is a compiler intrinsic.
+func scheduleTask(t taskInstance) {
+	if t == nil {
+		return
+	}
+	scheduleLogTask("  schedule task:", t)
+	// See what we should do with this task: try to execute it directly
+	// again or let it sleep for a bit.
+	promise := taskPromise(t)
+	if promise.state == TASK_STATE_CALL {
+		return // calling an async task, the subroutine will re-active the parent
+	} else if promise.state == TASK_STATE_SLEEP && promise.data != 0 {
+		addSleepTask(t)
+	} else {
+		pushTask(t)
+	}
+}
+
+// Add this task to the end of the run queue. May also destroy the task if it's
+// done.
+func pushTask(t taskInstance) {
+	if _llvm_coro_done(t) {
+		scheduleLogTask("  destroy task:", t)
+		_llvm_coro_destroy(t)
+		return
+	}
+	if runqueueBack == nil { // empty runqueue
+		runqueueBack = t
+		runqueueFront = t
+	} else {
+		lastTaskPromise := taskPromise(runqueueBack)
+		lastTaskPromise.next = t
+		runqueueBack = t
+	}
+}
+
+// Get a task from the front of the run queue. May return nil if there is none.
+func popTask() taskInstance {
+	t := runqueueFront
+	if t == nil {
+		return nil
+	}
+	scheduleLogTask("    popTask:", t)
+	promise := taskPromise(t)
+	runqueueFront = promise.next
+	if runqueueFront == nil {
+		runqueueBack = nil
+	}
+	promise.next = nil
+	return t
+}
+
+// Add this task to the sleep queue, assuming its state is set to sleeping.
+func addSleepTask(t taskInstance) {
+	now := monotime()
+	if sleepQueue == nil {
+		scheduleLog("  -> sleep new queue")
+		// Create new linked list for the sleep queue.
+		sleepQueue = t
+		sleepQueueBaseTime = now
+		return
+	}
+
+	// Make sure promise.data is relative to the queue time base.
+	promise := taskPromise(t)
+
+	// Insert at front of sleep queue.
+	if promise.data < taskPromise(sleepQueue).data {
+		scheduleLog("  -> sleep at start")
+		taskPromise(sleepQueue).data -= promise.data
+		promise.next = sleepQueue
+		sleepQueue = t
+		return
+	}
+
+	// Add to sleep queue (in the middle or at the end).
+	queueIndex := sleepQueue
+	for {
+		promise.data -= taskPromise(queueIndex).data
+		if taskPromise(queueIndex).next == nil || taskPromise(queueIndex).data > promise.data {
+			if taskPromise(queueIndex).next == nil {
+				scheduleLog("  -> sleep at end")
+				promise.next = nil
+			} else {
+				scheduleLog("  -> sleep in middle")
+				promise.next = taskPromise(queueIndex).next
+				taskPromise(promise.next).data -= promise.data
+			}
+			taskPromise(queueIndex).next = t
+			break
+		}
+		queueIndex = taskPromise(queueIndex).next
+	}
+}
+
+// Run the scheduler until all tasks have finished.
+// It takes an initial task (main.main) to bootstrap.
+func scheduler(main taskInstance) {
+	// Initial task.
+	scheduleTask(main)
+
+	// Main scheduler loop.
+	for {
+		scheduleLog("\n  schedule")
+		now := monotime()
+
+		// Add tasks that are done sleeping to the end of the runqueue so they
+		// will be executed soon.
+		if sleepQueue != nil && now - sleepQueueBaseTime >= uint64(taskPromise(sleepQueue).data) {
+			scheduleLog("  run <- sleep")
+			t := sleepQueue
+			promise := taskPromise(t)
+			sleepQueueBaseTime += uint64(promise.data)
+			sleepQueue = promise.next
+			promise.next = nil
+			pushTask(t)
+		}
+
+		scheduleLog("  <- popTask")
+		t := popTask()
+		if t == nil {
+			if sleepQueue == nil {
+				// No more tasks to execute.
+				// It would be nice if we could detect deadlocks here, because
+				// there might still be functions waiting on each other in a
+				// deadlock.
+				scheduleLog("  no tasks left!")
+				return
+			}
+			scheduleLog("  sleeping...")
+			timeLeft := uint64(taskPromise(sleepQueue).data) - (now - sleepQueueBaseTime)
+			sleep(Duration(timeLeft))
+			continue
+		}
+
+		// Run the given task.
+		scheduleLogTask("  run:", t)
+		_llvm_coro_resume(t)
+
+		// Add the just resumed task to the run queue or the sleep queue.
+		scheduleTask(t)
+	}
+}