runtime: rewrite channel implementation

This rewrite simplifies the channel implementation considerably, with
34% less LOC. Perhaps the most important change is the removal of the
channel state, which made sense when we had only send and receive
operations but only makes things more compliated when multiple select
operations can be pending on a single channel.

I did this rewrite originally to make it possible to make channels
parallelism-safe. The current implementation is not parallelism-safe,
but it will be easy to make it so (the main additions will be a channel
lock, a global select lock, and an atomic compare-and-swap in
chanQueue.pop).

compiler/channel.go

@@ -41,17 +41,17 @@ func (b *builder) createChanSend(instr *ssa.Send) {
 		b.CreateStore(chanValue, valueAlloca)
 	}
 
-	// Allocate blockedlist buffer.
-	channelBlockedList := b.getLLVMRuntimeType("channelBlockedList")
-	channelBlockedListAlloca, channelBlockedListAllocaSize := b.createTemporaryAlloca(channelBlockedList, "chan.blockedList")
+	// Allocate buffer for the channel operation.
+	channelOp := b.getLLVMRuntimeType("channelOp")
+	channelOpAlloca, channelOpAllocaSize := b.createTemporaryAlloca(channelOp, "chan.op")
 
 	// Do the send.
-	b.createRuntimeCall("chanSend", []llvm.Value{ch, valueAlloca, channelBlockedListAlloca}, "")
+	b.createRuntimeCall("chanSend", []llvm.Value{ch, valueAlloca, channelOpAlloca}, "")
 
 	// End the lifetime of the allocas.
 	// This also works around a bug in CoroSplit, at least in LLVM 8:
 	// https://bugs.llvm.org/show_bug.cgi?id=41742
-	b.emitLifetimeEnd(channelBlockedListAlloca, channelBlockedListAllocaSize)
+	b.emitLifetimeEnd(channelOpAlloca, channelOpAllocaSize)
 	if !isZeroSize {
 		b.emitLifetimeEnd(valueAlloca, valueAllocaSize)
 	}
@@ -72,20 +72,20 @@ func (b *builder) createChanRecv(unop *ssa.UnOp) llvm.Value {
 		valueAlloca, valueAllocaSize = b.createTemporaryAlloca(valueType, "chan.value")
 	}
 
-	// Allocate blockedlist buffer.
-	channelBlockedList := b.getLLVMRuntimeType("channelBlockedList")
-	channelBlockedListAlloca, channelBlockedListAllocaSize := b.createTemporaryAlloca(channelBlockedList, "chan.blockedList")
+	// Allocate buffer for the channel operation.
+	channelOp := b.getLLVMRuntimeType("channelOp")
+	channelOpAlloca, channelOpAllocaSize := b.createTemporaryAlloca(channelOp, "chan.op")
 
 	// Do the receive.
-	commaOk := b.createRuntimeCall("chanRecv", []llvm.Value{ch, valueAlloca, channelBlockedListAlloca}, "")
+	commaOk := b.createRuntimeCall("chanRecv", []llvm.Value{ch, valueAlloca, channelOpAlloca}, "")
 	var received llvm.Value
 	if isZeroSize {
 		received = llvm.ConstNull(valueType)
 	} else {
 		received = b.CreateLoad(valueType, valueAlloca, "chan.received")
 		b.emitLifetimeEnd(valueAlloca, valueAllocaSize)
 	}
-	b.emitLifetimeEnd(channelBlockedListAlloca, channelBlockedListAllocaSize)
+	b.emitLifetimeEnd(channelOpAlloca, channelOpAllocaSize)
 
 	if unop.CommaOk {
 		tuple := llvm.Undef(b.ctx.StructType([]llvm.Type{valueType, b.ctx.Int1Type()}, false))
@@ -198,26 +198,29 @@ func (b *builder) createSelect(expr *ssa.Select) llvm.Value {
 	if expr.Blocking {
 		// Stack-allocate operation structures.
 		// If these were simply created as a slice, they would heap-allocate.
-		chBlockAllocaType := llvm.ArrayType(b.getLLVMRuntimeType("channelBlockedList"), len(selectStates))
-		chBlockAlloca, chBlockSize := b.createTemporaryAlloca(chBlockAllocaType, "select.block.alloca")
-		chBlockLen := llvm.ConstInt(b.uintptrType, uint64(len(selectStates)), false)
-		chBlockPtr := b.CreateGEP(chBlockAllocaType, chBlockAlloca, []llvm.Value{
+		opsAllocaType := llvm.ArrayType(b.getLLVMRuntimeType("channelOp"), len(selectStates))
+		opsAlloca, opsSize := b.createTemporaryAlloca(opsAllocaType, "select.block.alloca")
+		opsLen := llvm.ConstInt(b.uintptrType, uint64(len(selectStates)), false)
+		opsPtr := b.CreateGEP(opsAllocaType, opsAlloca, []llvm.Value{
 			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
 			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
 		}, "select.block")
 
 		results = b.createRuntimeCall("chanSelect", []llvm.Value{
 			recvbuf,
 			statesPtr, statesLen, statesLen, // []chanSelectState
-			chBlockPtr, chBlockLen, chBlockLen, // []channelBlockList
+			opsPtr, opsLen, opsLen, // []channelOp
 		}, "select.result")
 
 		// Terminate the lifetime of the operation structures.
-		b.emitLifetimeEnd(chBlockAlloca, chBlockSize)
+		b.emitLifetimeEnd(opsAlloca, opsSize)
 	} else {
-		results = b.createRuntimeCall("tryChanSelect", []llvm.Value{
+		opsPtr := llvm.ConstNull(b.dataPtrType)
+		opsLen := llvm.ConstInt(b.uintptrType, 0, false)
+		results = b.createRuntimeCall("chanSelect", []llvm.Value{
 			recvbuf,
 			statesPtr, statesLen, statesLen, // []chanSelectState
+			opsPtr, opsLen, opsLen, // []channelOp (nil slice)
 		}, "select.result")
 	}