runtime: remove Cond

I don't think this is used anywhere right now, and it would need to be
updated to work with multithreading. So instead of fixing it, I think we
can remove it.

My original intention was to have something like this that could be used
in the machine package, but since this is in the runtime package (and
the runtime package imports the machine package on baremetal) it can't
actually be used that way.

I checked the TinyGo repo and the drivers repo, and `runtime.Cond` isn't
used anywhere except in that one test.

src/runtime/cond.go

@@ -1,90 +0,0 @@
-//go:build !scheduler.none
-
-package runtime
-
-import (
-	"internal/task"
-	"sync/atomic"
-	"unsafe"
-)
-
-// notifiedPlaceholder is a placeholder task which is used to indicate that the condition variable has been notified.
-var notifiedPlaceholder task.Task
-
-// Cond is a simplified condition variable, useful for notifying goroutines of interrupts.
-type Cond struct {
-	t *task.Task
-}
-
-// Notify sends a notification.
-// If the condition variable already has a pending notification, this returns false.
-func (c *Cond) Notify() bool {
-	for {
-		t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
-		switch t {
-		case nil:
-			// Nothing is waiting yet.
-			// Apply the notification placeholder.
-			if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), unsafe.Pointer(&notifiedPlaceholder)) {
-				return true
-			}
-		case &notifiedPlaceholder:
-			// The condition variable has already been notified.
-			return false
-		default:
-			// Unblock the waiting task.
-			if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
-				scheduleTask(t)
-				return true
-			}
-		}
-	}
-}
-
-// Poll checks for a notification.
-// If a notification is found, it is cleared and this returns true.
-func (c *Cond) Poll() bool {
-	for {
-		t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
-		switch t {
-		case nil:
-			// No notifications are present.
-			return false
-		case &notifiedPlaceholder:
-			// A notification arrived and there is no waiting goroutine.
-			// Clear the notification and return.
-			if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
-				return true
-			}
-		default:
-			// A task is blocked on the condition variable, which means it has not been notified.
-			return false
-		}
-	}
-}
-
-// Wait for a notification.
-// If the condition variable was previously notified, this returns immediately.
-func (c *Cond) Wait() {
-	cur := task.Current()
-	for {
-		t := (*task.Task)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t))))
-		switch t {
-		case nil:
-			// Condition variable has not been notified.
-			// Block the current task on the condition variable.
-			if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), nil, unsafe.Pointer(cur)) {
-				task.Pause()
-				return
-			}
-		case &notifiedPlaceholder:
-			// A notification arrived and there is no waiting goroutine.
-			// Clear the notification and return.
-			if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.t)), unsafe.Pointer(t), nil) {
-				return
-			}
-		default:
-			panic("interrupt.Cond: condition variable in use by another goroutine")
-		}
-	}
-}

src/runtime/cond_nosched.go

@@ -1,38 +0,0 @@
-//go:build scheduler.none
-
-package runtime
-
-import "runtime/interrupt"
-
-// Cond is a simplified condition variable, useful for notifying goroutines of interrupts.
-type Cond struct {
-	notified bool
-}
-
-// Notify sends a notification.
-// If the condition variable already has a pending notification, this returns false.
-func (c *Cond) Notify() bool {
-	i := interrupt.Disable()
-	prev := c.notified
-	c.notified = true
-	interrupt.Restore(i)
-	return !prev
-}
-
-// Poll checks for a notification.
-// If a notification is found, it is cleared and this returns true.
-func (c *Cond) Poll() bool {
-	i := interrupt.Disable()
-	notified := c.notified
-	c.notified = false
-	interrupt.Restore(i)
-	return notified
-}
-
-// Wait for a notification.
-// If the condition variable was previously notified, this returns immediately.
-func (c *Cond) Wait() {
-	for !c.Poll() {
-		waitForEvents()
-	}
-}