Use ANTLR Listener to save memory during parsing

The ANTLR-generated concrete syntax tree (CST) takes up much more memory
than the parsed .fir file. By using a Listener, we can construct the
FIRRTL AST live with CST construction and null out the CST as we consume
pieces of it. Not only does this improve performance, it drastically
reduces max memory use for the parser.

1 files changed, 38 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/parser/Listener.scala b/src/main/scala/firrtl/parser/Listener.scala
new file mode 100644
index 00000000..ffaa22b2
--- /dev/null
+++ b/src/main/scala/firrtl/parser/Listener.scala
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package firrtl.parser
+
+import firrtl.antlr.{FIRRTLParser, _}
+import firrtl.Visitor
+import firrtl.Parser.InfoMode
+import firrtl.ir._
+
+import scala.collection.mutable
+import scala.concurrent.{Await, Future}
+import scala.concurrent.duration.Duration
+
+private[firrtl] class Listener(infoMode: InfoMode) extends FIRRTLBaseListener {
+  private var main: Option[String] = None
+  private var info: Option[Info] = None
+  private val modules = mutable.ArrayBuffer.empty[DefModule]
+
+  private val visitor = new Visitor(infoMode)
+
+  override def exitModule(ctx: FIRRTLParser.ModuleContext): Unit = {
+    val m = visitor.visitModule(ctx)
+    ctx.children = null // Null out to save memory
+    modules += m
+  }
+
+  override def exitCircuit(ctx: FIRRTLParser.CircuitContext): Unit = {
+    info = Some(visitor.visitInfo(Option(ctx.info), ctx))
+    main = Some(ctx.id.getText)
+    ctx.children = null // Null out to save memory
+  }
+
+  def getCircuit: Circuit = {
+    require(main.nonEmpty)
+    val mods = modules.toSeq
+    Circuit(info.get, mods, main.get)
+  }
+}