runtime: add fast path for non-blocking single-state select

src/runtime/chan.go

@@ -432,11 +432,52 @@ func unlockAllStates(states []chanSelectState) {
 func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelOp) (uint32, bool) {
 	mask := interrupt.Disable()
 
+	const selectNoIndex = ^uint32(0)
+
+	// Fast path for non-blocking single-state selects, for example:
+	//
+	//	select {
+	//	case ch <- value:
+	//	default:
+	//	}
+	//
+	// Such selects can only lock at most one channel, so they don't need the
+	// global select lock. Also avoid lockAllStates/unlockAllStates here: they use
+	// channel.selectLocked, which is shared state protected by chanSelectLock.
+	if len(states) == 1 && len(ops) == 0 {
+		state := states[0]
+		selectIndex := selectNoIndex
+		selectOk := true
+
+		if state.ch != nil {
+			if hasParallelism {
+				state.ch.lock.Lock()
+			}
+
+			if state.value == nil { // chan receive
+				if received, ok := state.ch.tryRecv(recvbuf); received {
+					selectIndex = 0
+					selectOk = ok
+				}
+			} else { // chan send
+				if state.ch.trySend(state.value) {
+					selectIndex = 0
+				}
+			}
+
+			if hasParallelism {
+				state.ch.lock.Unlock()
+			}
+		}
+
+		interrupt.Restore(mask)
+		return selectIndex, selectOk
+	}
+
 	// Lock everything.
 	chanSelectLock.Lock()
 	lockAllStates(states)
 
-	const selectNoIndex = ^uint32(0)
 	selectIndex := selectNoIndex
 	selectOk := true