Replace some options with nullable vars (backport #2658) (#2659)

* Replace some options with nullable vars (#2658)

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit ac460bfeb16c8e7d0dc00975bb03f73c0fea2103)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Fix backport conflicts (#2661)

Co-authored-by: Zachary Yedidia <zyedidia@gmail.com>

2 files changed, 27 insertions, 13 deletions

diff --git a/core/src/main/scala/chisel3/Data.scala b/core/src/main/scala/chisel3/Data.scala
index 592ebe25..956c7996 100644
--- a/core/src/main/scala/chisel3/Data.scala
+++ b/core/src/main/scala/chisel3/Data.scala
@@ -501,14 +501,15 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
   // Binding stores information about this node's position in the hardware graph.
   // This information is supplemental (more than is necessary to generate FIRRTL) and is used to
   // perform checks in Chisel, where more informative error messages are possible.
-  private var _binding: Option[Binding] = None
+  private var _bindingVar: Binding = null // using nullable var for better memory usage
+  private def _binding:    Option[Binding] = Option(_bindingVar)
   // Only valid after node is bound (synthesizable), crashes otherwise
   protected[chisel3] def binding: Option[Binding] = _binding
   protected def binding_=(target: Binding) {
     if (_binding.isDefined) {
       throw RebindingException(s"Attempted reassignment of binding to $this, from: ${target}")
     }
-    _binding = Some(target)
+    _bindingVar = target
   }
 
   // Similar to topBindingOpt except it explicitly excludes SampleElements which are bound but not
@@ -540,14 +541,15 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
   // Both are only valid after binding is set.
 
   // Direction of this node, accounting for parents (force Input / Output) and children.
-  private var _direction: Option[ActualDirection] = None
+  private var _directionVar: ActualDirection = null // using nullable var for better memory usage
+  private def _direction:    Option[ActualDirection] = Option(_directionVar)
 
   private[chisel3] def direction: ActualDirection = _direction.get
   private[chisel3] def direction_=(actualDirection: ActualDirection) {
     if (_direction.isDefined) {
       throw RebindingException(s"Attempted reassignment of resolved direction to $this")
     }
-    _direction = Some(actualDirection)
+    _directionVar = actualDirection
   }
 
   private[chisel3] def stringAccessor(chiselType: String): String = {
diff --git a/core/src/main/scala/chisel3/internal/Builder.scala b/core/src/main/scala/chisel3/internal/Builder.scala
index 61f94f8f..25f4c44c 100644
--- a/core/src/main/scala/chisel3/internal/Builder.scala
+++ b/core/src/main/scala/chisel3/internal/Builder.scala
@@ -88,10 +88,19 @@ trait InstanceId {
 
 private[chisel3] trait HasId extends InstanceId {
   private[chisel3] def _onModuleClose: Unit = {}
-  private[chisel3] var _parent: Option[BaseModule] = Builder.currentModule
+  // using nullable var for better memory usage
+  private var _parentVar:       BaseModule = Builder.currentModule.getOrElse(null)
+  private[chisel3] def _parent: Option[BaseModule] = Option(_parentVar)
+  private[chisel3] def _parent_=(target: Option[BaseModule]): Unit = {
+    _parentVar = target.getOrElse(null)
+  }
 
   // Set if the returned top-level module of a nested call to the Chisel Builder, see Definition.apply
-  private[chisel3] var _circuit: Option[BaseModule] = None
+  private var _circuitVar:       BaseModule = null // using nullable var for better memory usage
+  private[chisel3] def _circuit: Option[BaseModule] = Option(_circuitVar)
+  private[chisel3] def _circuit_=(target: Option[BaseModule]): Unit = {
+    _circuitVar = target.getOrElse(null)
+  }
 
   private[chisel3] val _id: Long = Builder.idGen.next
 
@@ -100,10 +109,12 @@ private[chisel3] trait HasId extends InstanceId {
   override def equals(that: Any): Boolean = super.equals(that)
 
   // Contains suggested seed (user-decided seed)
-  private var suggested_seed: Option[String] = None
+  private var suggested_seedVar: String = null // using nullable var for better memory usage
+  private def suggested_seed:    Option[String] = Option(suggested_seedVar)
 
   // Contains the seed computed automatically by the compiler plugin
-  private var auto_seed: Option[String] = None
+  private var auto_seedVar: String = null // using nullable var for better memory usage
+  private def auto_seed:    Option[String] = Option(auto_seedVar)
 
   // Prefix for use in naming
   // - Defaults to prefix at time when object is created
@@ -131,7 +142,7 @@ private[chisel3] trait HasId extends InstanceId {
   private[chisel3] def autoSeed(seed: String): this.type = forceAutoSeed(seed)
   // Bypass the overridden behavior of autoSeed in [[Data]], apply autoSeed even to ports
   private[chisel3] def forceAutoSeed(seed: String): this.type = {
-    auto_seed = Some(seed)
+    auto_seedVar = seed
     for (hook <- auto_postseed_hooks.reverse) { hook(seed) }
     naming_prefix = Builder.getPrefix
     this
@@ -159,7 +170,7 @@ private[chisel3] trait HasId extends InstanceId {
     * @return this object
     */
   def suggestName(seed: => String): this.type = {
-    if (suggested_seed.isEmpty) suggested_seed = Some(seed)
+    if (suggested_seed.isEmpty) suggested_seedVar = seed
     naming_prefix = Builder.getPrefix
     for (hook <- suggest_postseed_hooks.reverse) { hook(seed) }
     this
@@ -171,7 +182,7 @@ private[chisel3] trait HasId extends InstanceId {
   private[chisel3] def forceFinalName(seed: String): this.type = {
     // This could be called with user prefixes, ignore them
     noPrefix {
-      suggested_seed = Some(seed)
+      suggested_seedVar = seed
       this.suggestName(seed)
     }
   }
@@ -212,11 +223,12 @@ private[chisel3] trait HasId extends InstanceId {
       naming_prefix = Nil
     }
 
-  private var _ref: Option[Arg] = None
+  private var _refVar: Arg = null // using nullable var for better memory usage
+  private def _ref:    Option[Arg] = Option(_refVar)
   private[chisel3] def setRef(imm: Arg): Unit = setRef(imm, false)
   private[chisel3] def setRef(imm: Arg, force: Boolean): Unit = {
     if (_ref.isEmpty || force) {
-      _ref = Some(imm)
+      _refVar = imm
     }
   }
   private[chisel3] def setRef(parent: HasId, name:  String): Unit = setRef(Slot(Node(parent), name))