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>

1 files changed, 6 insertions, 4 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 = {