Change RawModule._commands to a VectorBuilder (backport #2839) (#2841)

* Change RawModule._commands to a VectorBuilder (#2839)

* Change RawModule._commands to a VectorBuilder

Use the resulting Vector to build the underlying Component's commands
and then use those instead of copying the original ArrayBuffer when
iterating on commands. Previously, the Component was using a List to
hold the commands which is particularly memory inefficient, especially
for large modules.

* Optimize Converter's handling of Seq[Command]

It previously converted the Commands to a List (which, while not
captured in the type system, they were already a List) and then used
head and tail iteration. This is less efficient with the new underlying
Vector implementation.

(cherry picked from commit 48a1ef0a3872c6b68d46145764d977926923a270)

* Waive false binary compatibility failures

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

1 files changed, 9 insertions, 5 deletions

diff --git a/core/src/main/scala/chisel3/RawModule.scala b/core/src/main/scala/chisel3/RawModule.scala
index f2ce4c70..9668313a 100644
--- a/core/src/main/scala/chisel3/RawModule.scala
+++ b/core/src/main/scala/chisel3/RawModule.scala
@@ -2,7 +2,6 @@
 
 package chisel3
 
-import scala.collection.mutable.{ArrayBuffer, HashMap}
 import scala.util.Try
 import scala.language.experimental.macros
 import scala.annotation.nowarn
@@ -13,6 +12,7 @@ import chisel3.internal.Builder._
 import chisel3.internal.firrtl._
 import chisel3.internal.sourceinfo.UnlocatableSourceInfo
 import _root_.firrtl.annotations.{IsModule, ModuleTarget}
+import scala.collection.immutable.VectorBuilder
 
 /** Abstract base class for Modules that contain Chisel RTL.
   * This abstract base class is a user-defined module which does not include implicit clock and reset and supports
@@ -23,14 +23,18 @@ abstract class RawModule(implicit moduleCompileOptions: CompileOptions) extends
   //
   // RTL construction internals
   //
-  private val _commands = ArrayBuffer[Command]()
+  // Perhaps this should be an ArrayBuffer (or ArrayBuilder), but DefModule is public and has Seq[Command]
+  // so our best option is to share a single Seq datastructure with that
+  private val _commands = new VectorBuilder[Command]()
   private[chisel3] def addCommand(c: Command) {
     require(!_closed, "Can't write to module after module close")
     _commands += c
   }
-  protected def getCommands = {
+  protected def getCommands: Seq[Command] = {
     require(_closed, "Can't get commands before module close")
-    _commands.toSeq
+    // Unsafe cast but we know that any RawModule uses a DefModule
+    // _component is defined as a var on BaseModule and we cannot override mutable vars
+    _component.get.asInstanceOf[DefModule].commands
   }
 
   //
@@ -153,7 +157,7 @@ abstract class RawModule(implicit moduleCompileOptions: CompileOptions) extends
         Seq()
       }
     }
-    val component = DefModule(this, name, firrtlPorts, invalidateCommands ++ getCommands)
+    val component = DefModule(this, name, firrtlPorts, invalidateCommands ++: _commands.result())
     _component = Some(component)
     _component
   }