When prefixing with a data, eagly get local name (#1614)

Fixes #1606

Previously, the Data itself would be put on the prefix stack and its
full name would be used as the prefix for subsequent names. This meant
that prefixes would grow quadratically as the prefix is present both on
the Data pushed to the stack, and on the stack itself. This is fixed by
just using the "local" name of the Data being pushed on the stack.

A related issue is deferring the name resolution. This led to unintuitive
behavior when the name of a Data used as a prefix is overridden later
(usually when the Data is a return value). The overriding name would
show up in prefixes twice instead of once. It is much more intuitive to
grab the name at the moment of the connection creating the prefix rather
than deferring to later.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

2 files changed, 44 insertions, 33 deletions

diff --git a/core/src/main/scala/chisel3/internal/Builder.scala b/core/src/main/scala/chisel3/internal/Builder.scala
index 3e0362e7..d6a44708 100644
--- a/core/src/main/scala/chisel3/internal/Builder.scala
+++ b/core/src/main/scala/chisel3/internal/Builder.scala
@@ -149,21 +149,6 @@ private[chisel3] trait HasId extends InstanceId {
     * @return the name, if it can be computed
     */
   def computeName(defaultPrefix: Option[String], defaultSeed: Option[String]): Option[String] = {
-    // Recursively builds a name if referenced fields of an aggregate type
-    def buildAggName(id: HasId): Option[String] = {
-      def recArg(node: Arg): Option[String] = node match {
-        case Slot(imm, name) => recArg(imm).map(_ + "_" + name)
-        case Index(imm, ILit(num)) => recArg(imm).map(_ + "_" + num)
-        case Index(imm, n: LitArg) => recArg(imm).map(_ + "_" + n.num)
-        case Index(imm, _: Node) => recArg(imm)
-        case Node(id) => recArg(id.getOptionRef.get)
-        case Ref(name) => Some(name)
-        case ModuleIO(mod, name) if _parent.contains(mod) => Some(name)
-        case ModuleIO(mod, name) => recArg(mod.getRef).map(_ + "_" + name)
-      }
-      id.getOptionRef.flatMap(recArg)
-    }
-
     /** Computes a name of this signal, given the seed and prefix
       * @param seed
       * @param prefix
@@ -173,18 +158,13 @@ private[chisel3] trait HasId extends InstanceId {
       val builder = new StringBuilder()
       prefix.foreach {
         case Left(s: String) => builder ++= s + "_"
-        case Right(d: HasId) =>
-          buildAggName(d) match {
-            case Some(n) => builder ++= n + "_"
-            case _ =>
-          }
-        case _ =>
+        case other           => Builder.exception(s"Only Strings should exist in Prefixes, got $other")
       }
       builder ++= seed
       builder.toString
     }
 
-    if(hasSeed) {
+    if (hasSeed) {
       Some(buildName(seedOpt.get, prefix_seed))
     } else {
       defaultSeed.map { default =>
@@ -379,19 +359,48 @@ private[chisel3] object Builder {
   def annotations: ArrayBuffer[ChiselAnnotation] = dynamicContext.annotations
   def namingStack: NamingStack = dynamicContext.namingStack
 
-  // Puts either a prefix string or hasId onto the prefix stack
-  def pushPrefix(d: Either[String, HasId]): Unit = {
-    chiselContext.get().prefixStack += d
-  }
-
   // Puts a prefix string onto the prefix stack
   def pushPrefix(d: String): Unit = {
     chiselContext.get().prefixStack += Left(d)
   }
 
-  // Puts a prefix data onto the prefix stack
-  def pushPrefix(d: HasId): Unit = {
-    chiselContext.get().prefixStack += Right(d)
+  /** Pushes the current name of a data onto the prefix stack
+    *
+    * @param d data to derive prefix from
+    * @return whether anything was pushed to the stack
+    */
+  def pushPrefix(d: HasId): Boolean = {
+    def buildAggName(id: HasId): Option[String] = {
+      // TODO This is slow, can we store this information upon binding?
+      def getSubName(field: Data, parent: Data): Option[String] = parent match {
+        case vec: Vec[_] =>
+          val idx = vec.indexOf(field)
+          if (idx >= 0) Some(idx.toString) else None
+        case rec: Record => rec.elements.collectFirst { case (name, d) if d == field => name }
+        case _ => Builder.exception(s"Shouldn't see non-Aggregate $parent as parent in ChildBinding!")
+      }
+      def map2[A, B](a: Option[A], b: Option[A])(f: (A, A) => B): Option[B] =
+        a.flatMap(ax => b.map(f(ax, _)))
+      // If the binding is None, this is an illegal connection and later logic will error
+      def recData(data: Data): Option[String] = data.binding.flatMap {
+        case (_: WireBinding | _: RegBinding | _: MemoryPortBinding | _: OpBinding) => data.seedOpt
+        case ChildBinding(parent) => recData(parent).map { p =>
+          // And name of the field if we have one, we don't for dynamic indexing of Vecs
+          getSubName(data, parent).map(p + "_" + _).getOrElse(p)
+        }
+        case SampleElementBinding(parent) => recData(parent)
+        case PortBinding(mod) if Builder.currentModule.contains(mod) => data.seedOpt
+        case PortBinding(mod) => map2(mod.seedOpt, data.seedOpt)(_ + "_" + _)
+        case (_: LitBinding | _: DontCareBinding) => None
+      }
+      id match {
+        case d: Data => recData(d)
+        case _ => None
+      }
+    }
+    buildAggName(d).map { name =>
+      chiselContext.get().prefixStack += Left(name)
+    }.isDefined
   }
 
   // Remove a prefix from top of the stack
@@ -598,7 +607,7 @@ private[chisel3] object Builder {
     * @param m exception message
     */
   @throws(classOf[ChiselException])
-  def exception(m: => String): Unit = {
+  def exception(m: => String): Nothing = {
     error(m)
     throwException(m)
   }
diff --git a/core/src/main/scala/chisel3/internal/prefix.scala b/core/src/main/scala/chisel3/internal/prefix.scala
index bbe31226..9dc14901 100644
--- a/core/src/main/scala/chisel3/internal/prefix.scala
+++ b/core/src/main/scala/chisel3/internal/prefix.scala
@@ -28,9 +28,11 @@ private[chisel3] object prefix { // scalastyle:ignore
     * @return The return value of the provided function
     */
   def apply[T](name: HasId)(f: => T): T = {
-    Builder.pushPrefix(name)
+    val pushed = Builder.pushPrefix(name)
     val ret = f
-    Builder.popPrefix()
+    if (pushed) {
+      Builder.popPrefix()
+    }
     ret
   }