diplomacy: eliminate some wasted IdentityNodes using cross-module refs

src/main/scala/tilelink/AsyncCrossing.scala

@@ -87,16 +87,11 @@ object TLAsyncCrossingSink
 
 class TLAsyncCrossing(depth: Int = 8, sync: Int = 3)(implicit p: Parameters) extends LazyModule
 {
-  val nodeIn = TLIdentityNode()
-  val nodeOut = TLIdentityNode()
-  val node = NodeHandle(nodeIn, nodeOut)
-
   val source = LazyModule(new TLAsyncCrossingSource(sync))
   val sink = LazyModule(new TLAsyncCrossingSink(depth, sync))
+  val node = NodeHandle(source.node, sink.node)
 
   sink.node := source.node
-  source.node := nodeIn
-  nodeOut := sink.node
 
   lazy val module = new LazyModuleImp(this) {
     val io = IO(new Bundle {

src/main/scala/tilelink/Buffer.scala

@@ -74,31 +74,25 @@ object TLBuffer
 }
 
 class TLBufferChain(depth: Int)(implicit p: Parameters) extends LazyModule {
-
-  val nodeIn = TLIdentityNode()
-  val nodeOut = TLIdentityNode()
-  val node = NodeHandle(nodeIn, nodeOut)
-
-  val buf_chain = if (depth > 0) {
-    val chain = List.fill(depth)(LazyModule(new TLBuffer(BufferParams.default)))
-
-    (chain.init zip chain.tail) foreach { case(prev, next) => next.node :=? prev.node }
-    chain
+  val buf_chain = List.fill(depth)(LazyModule(new TLBuffer(BufferParams.default)))
+  val node = if (depth > 0) {
+    (buf_chain.init zip buf_chain.tail) foreach { case (prev, next) => next.node :=? prev.node }
+    NodeHandle(buf_chain.head.node, buf_chain.last.node)
   } else {
-    List(LazyModule(new TLBuffer(BufferParams.none)))
+    TLIdentityNode()
   }
-
-  buf_chain.head.node :=? nodeIn
-  nodeOut :=? buf_chain.last.node
-
   lazy val module = new LazyModuleImp(this) { }
 }
 
 object TLBufferChain
 {
   def apply(depth: Int)(x: TLOutwardNode)(implicit p: Parameters, sourceInfo: SourceInfo): TLOutwardNode = {
-    val buffer = LazyModule(new TLBufferChain(depth))
-    buffer.node :=? x
-    buffer.node
+    if (depth > 0) {
+      val buffer = LazyModule(new TLBufferChain(depth))
+      buffer.node :=? x
+      buffer.node
+    } else {
+      x
+    }
   }
 }

src/main/scala/tilelink/Bus.scala

@@ -71,7 +71,7 @@ abstract class TLBusWrapper(params: TLBusParams, val busName: String)(implicit p
     SourceCardinality { implicit p =>
       val chain = LazyModule(new TLBufferChain(depth))
       name.foreach { n => chain.suggestName(s"${busName}_${n}_TLBufferChain")}
-      (chain.nodeIn, chain.nodeOut)
+      (chain.node, chain.node)
     }
   }
 

src/main/scala/tilelink/RationalCrossing.scala

@@ -99,16 +99,11 @@ object TLRationalCrossingSink
 
 class TLRationalCrossing(direction: RationalDirection = Symmetric)(implicit p: Parameters) extends LazyModule
 {
-  val nodeIn = TLIdentityNode()
-  val nodeOut = TLIdentityNode()
-  val node = NodeHandle(nodeIn, nodeOut)
-
   val source = LazyModule(new TLRationalCrossingSource)
   val sink = LazyModule(new TLRationalCrossingSink(direction))
+  val node = NodeHandle(source.node, sink.node)
 
   sink.node := source.node
-  source.node := nodeIn
-  nodeOut := sink.node
 
   lazy val module = new LazyModuleImp(this) {
     val io = IO(new Bundle {