2 files changed, 34 insertions, 2 deletions
diff --git a/core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala b/core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala
index 59b4c692..a8cfac2f 100644
--- a/core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala
+++ b/core/src/main/scala/chisel3/experimental/hierarchy/Definition.scala
@@ -11,6 +11,7 @@ import chisel3.internal.sourceinfo.{DefinitionTransform, DefinitionWrapTransform
 import chisel3.experimental.BaseModule
 import chisel3.internal.BaseModule.IsClone
 import firrtl.annotations.{IsModule, ModuleTarget}
+import firrtl.annotations.{IsModule, ModuleTarget, NoTargetAnnotation}
 
 /** User-facing Definition type.
   * Represents a definition of an object of type [[A]] which are marked as @instantiable
@@ -112,3 +113,9 @@ object Definition extends SourceInfoDoc {
   }
 
 }
+
+/** Stores a [[Definition]] that is imported so that its Instances can be
+  * compiled separately.
+  */
+case class ImportDefinitionAnnotation[T <: BaseModule with IsInstantiable](definition: Definition[T])
+    extends NoTargetAnnotation
diff --git a/core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala b/core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala
index cc926771..9f96dff1 100644
--- a/core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala
+++ b/core/src/main/scala/chisel3/experimental/hierarchy/Instance.scala
@@ -2,12 +2,13 @@
 
 package chisel3.experimental.hierarchy
 
-import scala.collection.mutable.{ArrayBuffer, HashMap}
 import scala.language.experimental.macros
 import chisel3._
 import chisel3.internal.BaseModule.{InstantiableClone, IsClone, ModuleClone}
+import chisel3.internal.Builder
 import chisel3.internal.sourceinfo.{InstanceTransform, SourceInfo}
-import chisel3.experimental.BaseModule
+import chisel3.experimental.{BaseModule, ExtModule}
+import chisel3.internal.firrtl.{Component, DefBlackBox, DefModule, Port}
 import firrtl.annotations.IsModule
 
 /** User-facing Instance type.
@@ -107,9 +108,33 @@ object Instance extends SourceInfoDoc {
     implicit sourceInfo: SourceInfo,
     compileOptions:      CompileOptions
   ): Instance[T] = {
+    // Check to see if the module is already defined internally or externally
+    val existingMod = Builder.components.map {
+      case c: DefModule if c.id == definition.proto          => Some(c)
+      case c: DefBlackBox if c.name == definition.proto.name => Some(c)
+      case _ => None
+    }.flatten
+
+    if (existingMod.isEmpty) {
+      // Add a Definition that will get emitted as an ExtModule so that FIRRTL
+      // does not complain about a missing element
+      class EmptyExtModule extends ExtModule {
+        override def desiredName: String = definition.proto.name
+        override def generateComponent(): Option[Component] = {
+          require(!_closed, s"Can't generate $desiredName module more than once")
+          _closed = true
+          val firrtlPorts = definition.proto.getModulePorts.map { port => Port(port, port.specifiedDirection) }
+          val component = DefBlackBox(this, definition.proto.name, firrtlPorts, SpecifiedDirection.Unspecified, params)
+          Some(component)
+        }
+      }
+      Definition(new EmptyExtModule() {})
+    }
+
     val ports = experimental.CloneModuleAsRecord(definition.proto)
     val clone = ports._parent.get.asInstanceOf[ModuleClone[T]]
     clone._madeFromDefinition = true
+
     new Instance(Clone(clone))
   }