compiler: add support for channel operations

Support for channels is not complete. The following pieces are missing:

  * Channels with values bigger than int. An int in TinyGo can always
    contain at least a pointer, so pointers are okay to send.
  * Buffered channels.
  * The select statement.

src/runtime/chan.go

@@ -2,7 +2,152 @@ package runtime
 
 // This file implements the 'chan' type and send/receive/select operations.
 
-// dummy
+// A channel can be in one of the following states:
+//     empty:
+//       No goroutine is waiting on a send or receive operation. The 'blocked'
+//       member is nil.
+//     recv:
+//       A goroutine tries to receive from the channel. This goroutine is stored
+//       in the 'blocked' member.
+//     send:
+//       The reverse of send. A goroutine tries to send to the channel. This
+//       goroutine is stored in the 'blocked' member.
+//     closed:
+//       The channel is closed. Sends will panic, receives will get a zero value
+//       plus optionally the indication that the channel is zero (with the
+//       commao-ok value in the coroutine).
+//
+// A send/recv transmission is completed by copying from the data element of the
+// sending coroutine to the data element of the receiving coroutine, and setting
+// the 'comma-ok' value to true.
+// A receive operation on a closed channel is completed by zeroing the data
+// element of the receiving coroutine and setting the 'comma-ok' value to false.
+
+import (
+	"unsafe"
+)
 
 type channel struct {
+	state   uint8
+	blocked *coroutine
+}
+
+const (
+	chanStateEmpty = iota
+	chanStateRecv
+	chanStateSend
+	chanStateClosed
+)
+
+func chanSendStub(caller *coroutine, ch *channel, _ unsafe.Pointer, size uintptr)
+func chanRecvStub(caller *coroutine, ch *channel, _ unsafe.Pointer, _ *bool, size uintptr)
+
+// chanSend sends a single value over the channel. If this operation can
+// complete immediately (there is a goroutine waiting for a value), it sends the
+// value and re-activates both goroutines. If not, it sets itself as waiting on
+// a value.
+//
+// The unsafe.Pointer value is used during lowering. During IR generation, it
+// points to the to-be-received value. During coroutine lowering, this value is
+// replaced with a read from the coroutine promise.
+func chanSend(sender *coroutine, ch *channel, _ unsafe.Pointer, size uintptr) {
+	if ch == nil {
+		// A nil channel blocks forever. Do not scheduler this goroutine again.
+		return
+	}
+	switch ch.state {
+	case chanStateEmpty:
+		ch.state = chanStateSend
+		ch.blocked = sender
+	case chanStateRecv:
+		receiver := ch.blocked
+		receiverPromise := receiver.promise()
+		senderPromise := sender.promise()
+		memcpy(unsafe.Pointer(&receiverPromise.data), unsafe.Pointer(&senderPromise.data), size)
+		receiverPromise.commaOk = true
+		ch.blocked = receiverPromise.next
+		receiverPromise.next = nil
+		activateTask(receiver)
+		activateTask(sender)
+		if ch.blocked == nil {
+			ch.state = chanStateEmpty
+		}
+	case chanStateClosed:
+		runtimePanic("send on closed channel")
+	case chanStateSend:
+		sender.promise().next = ch.blocked
+		ch.blocked = sender
+	}
+}
+
+// chanRecv receives a single value over a channel. If there is an available
+// sender, it receives the value immediately and re-activates both coroutines.
+// If not, it sets itself as available for receiving. If the channel is closed,
+// it immediately activates itself with a zero value as the result.
+//
+// The two unnamed values exist to help during lowering. The unsafe.Pointer
+// points to the value, and the *bool points to the comma-ok value. Both are
+// replaced by reads from the coroutine promise.
+func chanRecv(receiver *coroutine, ch *channel, _ unsafe.Pointer, _ *bool, size uintptr) {
+	if ch == nil {
+		// A nil channel blocks forever. Do not scheduler this goroutine again.
+		return
+	}
+	switch ch.state {
+	case chanStateSend:
+		sender := ch.blocked
+		receiverPromise := receiver.promise()
+		senderPromise := sender.promise()
+		memcpy(unsafe.Pointer(&receiverPromise.data), unsafe.Pointer(&senderPromise.data), size)
+		receiverPromise.commaOk = true
+		ch.blocked = senderPromise.next
+		senderPromise.next = nil
+		activateTask(receiver)
+		activateTask(sender)
+		if ch.blocked == nil {
+			ch.state = chanStateEmpty
+		}
+	case chanStateEmpty:
+		ch.state = chanStateRecv
+		ch.blocked = receiver
+	case chanStateClosed:
+		receiverPromise := receiver.promise()
+		memzero(unsafe.Pointer(&receiverPromise.data), size)
+		receiverPromise.commaOk = false
+		activateTask(receiver)
+	case chanStateRecv:
+		receiver.promise().next = ch.blocked
+		ch.blocked = receiver
+	}
+}
+
+// chanClose closes the given channel. If this channel has a receiver or is
+// empty, it closes the channel. Else, it panics.
+func chanClose(ch *channel, size uintptr) {
+	if ch == nil {
+		// Not allowed by the language spec.
+		runtimePanic("close of nil channel")
+	}
+	switch ch.state {
+	case chanStateClosed:
+		// Not allowed by the language spec.
+		runtimePanic("close of closed channel")
+	case chanStateSend:
+		// This panic should ideally on the sending side, not in this goroutine.
+		// But when a goroutine tries to send while the channel is being closed,
+		// that is clearly invalid: the send should have been completed already
+		// before the close.
+		runtimePanic("close channel during send")
+	case chanStateRecv:
+		// The receiver must be re-activated with a zero value.
+		receiverPromise := ch.blocked.promise()
+		memzero(unsafe.Pointer(&receiverPromise.data), size)
+		receiverPromise.commaOk = false
+		activateTask(ch.blocked)
+		ch.state = chanStateClosed
+		ch.blocked = nil
+	case chanStateEmpty:
+		// Easy case. No available sender or receiver.
+		ch.state = chanStateClosed
+	}
 }

src/runtime/scheduler.go

@@ -51,10 +51,11 @@ func makeGoroutine(*uint8) *uint8
 
 // State/promise of a task. Internally represented as:
 //
-//     {i8* next, i32/i64 data}
+//     {i8* next, i1 commaOk, i32/i64 data}
 type taskState struct {
-	next *coroutine
-	data uint
+	next    *coroutine
+	commaOk bool // 'comma-ok' flag for channel receive operation
+	data    uint
 }
 
 // Queues used by the scheduler.