2 files changed, 47 insertions, 4 deletions
diff --git a/src/main/scala/firrtl/options/StageAnnotations.scala b/src/main/scala/firrtl/options/StageAnnotations.scala
index 2b12dd4e..584f2842 100644
--- a/src/main/scala/firrtl/options/StageAnnotations.scala
+++ b/src/main/scala/firrtl/options/StageAnnotations.scala
@@ -71,7 +71,33 @@ trait CustomFileEmission { this: Annotation =>
     val name = view[StageOptions](annotations).getBuildFileName(baseFileName(annotations), suffix)
     new File(name)
   }
+}
+
+/** A buffered version of [[CustomFileEmission]]
+  *
+  * This is especially useful for serializing large data structures. When emitting Strings, it makes
+  * it much easier to avoid materializing the entire serialized String in memory. It also helps
+  * avoid materializing intermediate datastructures in memory. Finally, it reduces iteration
+  * overhead and helps optimize I/O performance.
+  *
+  * It may seem strange to use `Array[Byte]` in an otherwise immutable API, but for maximal
+  * performance it is best to use the JVM primitive that file I/O uses. These Arrays should only
+  * used immutably even though the Java API technically does allow mutating them.
+  */
+trait BufferedCustomFileEmission extends CustomFileEmission { this: Annotation =>
+
+  /** A buffered version of [[getBytes]] for more efficient serialization
+    *
+    * If you only need to serialize an `Iterable[String]`, you can use the `String.getBytes` method.
+    * It's also helpful to create a `view` which will do the `.map` lazily instead of eagerly,
+    * improving GC performance.
+    * {{{
+    *  def getBytesBuffered: Iterable[Array[Byte]] = myListString.view.map(_.getBytes)
+    * }}}
+    */
+  def getBytesBuffered: Iterable[Array[Byte]]
 
+  final def getBytes: Iterable[Byte] = getBytesBuffered.flatten
 }
 
 /** Holds the name of the target directory
diff --git a/src/main/scala/firrtl/options/phases/WriteOutputAnnotations.scala b/src/main/scala/firrtl/options/phases/WriteOutputAnnotations.scala
index 0e26a5f7..2cf4c92f 100644
--- a/src/main/scala/firrtl/options/phases/WriteOutputAnnotations.scala
+++ b/src/main/scala/firrtl/options/phases/WriteOutputAnnotations.scala
@@ -4,7 +4,16 @@ package firrtl.options.phases
 
 import firrtl.AnnotationSeq
 import firrtl.annotations.{Annotation, DeletedAnnotation, JsonProtocol}
-import firrtl.options.{CustomFileEmission, Dependency, Phase, PhaseException, StageOptions, Unserializable, Viewer}
+import firrtl.options.{
+  BufferedCustomFileEmission,
+  CustomFileEmission,
+  Dependency,
+  Phase,
+  PhaseException,
+  StageOptions,
+  Unserializable,
+  Viewer
+}
 
 import java.io.{BufferedOutputStream, File, FileOutputStream, PrintWriter}
 
@@ -38,9 +47,17 @@ class WriteOutputAnnotations extends Phase {
         filesWritten.get(canonical) match {
           case None =>
             val w = new BufferedOutputStream(new FileOutputStream(filename))
-            a.getBytes match {
-              case arr: mutable.WrappedArray[Byte] => w.write(arr.array.asInstanceOf[Array[Byte]])
-              case other => other.foreach(w.write(_))
+            a match {
+              // Further optimized emission
+              case buf: BufferedCustomFileEmission =>
+                val it = buf.getBytesBuffered
+                it.foreach(bytearr => w.write(bytearr))
+              // Regular emission
+              case _ =>
+                a.getBytes match {
+                  case arr: mutable.WrappedArray[Byte] => w.write(arr.array.asInstanceOf[Array[Byte]])
+                  case other => other.foreach(w.write(_))
+                }
             }
             w.close()
             filesWritten(canonical) = a