Add alternative graph IR (#671)

* add graph node classes

* add graph representation usage pass

* remove pass using graph nodes so that firrtl can compile

* move google graph ir nodes to altIR package

src/main/scala/firrtl/altIR/FirrtlGraphNode.scala

@@ -0,0 +1,150 @@
+package firrtl.altIR
+import firrtl.ir._
+import scala.collection.mutable
+
+/**
+ * Base class for the graph nodes in the graph representation of firrtl AST tree.
+ */
+abstract class FirrtlGraphNode {
+  def neighbors: Seq[FirrtlGraphNode]
+}
+
+/**
+ * Graph node equivalent of Statement AST tree nodes in IR.scala.
+ */
+abstract class StatementGraphNode extends FirrtlGraphNode
+
+/**
+ * Graph node representation of nodes that can be refered to by name in the tree form of the IR
+ */
+abstract class NamedGraphNode extends StatementGraphNode {
+  val name: String
+  val references = mutable.ArrayBuffer.empty[ReferenceGraphNode]
+  def addReference(ref: ReferenceGraphNode): Unit
+}
+
+/**
+ * Graph node representation of nodes that can be assigned to by connect statements in the tree form
+ * of firrtl.
+ */
+abstract class AssignableGraphNode extends NamedGraphNode {
+  def neighbors = references
+  def addReference(ref: ReferenceGraphNode): Unit = {
+    references += ref
+    ref.namedNode = Some(this)
+  }
+}
+
+/**
+ * Graph node equivalent to Expression nodes in the tree form of the IR.
+ */
+abstract class ExpressionGraphNode extends FirrtlGraphNode {
+  val tpe: Type
+  var parent: Option[FirrtlGraphNode]
+  def addParent(node: ExpressionGraphNode): Unit = {
+    parent = Some(node)
+    node match {
+      case subField: SubFieldGraphNode =>
+        subField.expr = Some(this)
+      case subIndex: SubIndexGraphNode =>
+        subIndex.expr = Some(this)
+    }
+  }
+}
+
+/**
+ * Graph node equivalent to Reference nodes in the tree form of the IR
+ */
+class ReferenceGraphNode(
+  val name: String, val tpe: Type
+) extends ExpressionGraphNode {
+  var namedNode: Option[NamedGraphNode] = None
+  var parent: Option[FirrtlGraphNode] = None
+  def neighbors = namedNode.toList ++ parent.toList
+}
+
+/**
+ * Graph node equivalent to SubField nodes in the tree form of the IR
+ */
+class SubFieldGraphNode(
+  val name: String, val tpe: Type
+) extends ExpressionGraphNode {
+  var expr: Option[ExpressionGraphNode] = None
+  var parent: Option[FirrtlGraphNode] = None
+  def neighbors = expr.toList ++ parent.toList
+}
+
+/**
+ * Graph node equivalent to SubIndex nodes in the tree form of the IR
+ */
+class SubIndexGraphNode(
+  val value: Int, val tpe: Type
+) extends ExpressionGraphNode {
+  var expr: Option[ExpressionGraphNode] = None
+  var parent: Option[FirrtlGraphNode] = None
+  def neighbors = expr.toList ++ parent.toList
+}
+
+/**
+ * Graph node equivalent to DefWire nodes in the tree form of the IR
+ */
+class DefWireGraphNode(
+  val info: Info,
+  val name: String,
+  val tpe: Type
+) extends AssignableGraphNode
+
+/**
+ * Graph node equivalent to DefInstance nodes in the tree form of the IR
+ */
+class DefInstanceGraphNode(
+  val info: Info,
+  val name: String,
+  val module: String
+) extends AssignableGraphNode
+
+/**
+ * Graph nodes that represent the IO ports of a module not present in the tree form of the IR.
+ */
+class PortGraphNode(
+  val info: Info,
+  val name: String,
+  val direction: Direction,
+  val tpe: Type
+) extends AssignableGraphNode
+
+/**
+ * Graph node equivalent to Connect nodes in the tree form of the IR
+ */
+class ConnectGraphNode(val info: Info) extends StatementGraphNode {
+  var loc: Option[ExpressionGraphNode] = None
+  var expr: Option[ExpressionGraphNode] = None
+  def neighbors = loc.toList ++ expr.toList
+  def addLoc(node: ExpressionGraphNode): Unit = {
+    loc = Some(node)
+    node.parent = Some(this)
+  }
+  def addExpr(node: ExpressionGraphNode): Unit = {
+    expr = Some(node)
+    node.parent = Some(this)
+  }
+}
+
+/**
+ * Graph node equivalent to IsInvalid nodes in the tree form of the IR
+ */
+class IsInvalidGraphNode(val info: Info) extends StatementGraphNode {
+  var expr: Option[ExpressionGraphNode] = None
+  def neighbors = expr.toList
+}
+
+object getGraphNode {
+  def apply[T](pointer: Option[T]): T = {
+    pointer.getOrElse(
+      throw new Exception(
+        s"InsertWrapperModules pass encountered unexpectedly unconnected "
+        + s"graph node pointer"
+      )
+    )
+  }
+}