From 63bc0bcbc6bdf41c06fbee3a8f300d01b835374e Mon Sep 17 00:00:00 2001
From: Paul Adenot <paul@paul.cx>
Date: Fri, 19 Jul 2013 16:40:56 +0200
Subject: [PATCH] Bug 882543 - Retain storage for media streams accross
 iterations instead of reallocating. r=jlebar,roc

---
 content/media/MediaStreamGraph.cpp   | 12 ++++++------
 content/media/MediaStreamGraphImpl.h |  5 +++++
 xpcom/glue/nsTArray.h                | 14 ++++++++++++++
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/content/media/MediaStreamGraph.cpp b/content/media/MediaStreamGraph.cpp
index 369d423cf192..f3facb4c4284 100644
--- a/content/media/MediaStreamGraph.cpp
+++ b/content/media/MediaStreamGraph.cpp
@@ -489,10 +489,10 @@ MediaStreamGraphImpl::UpdateStreamOrderForStream(nsTArray<MediaStream*>* aStack,
 void
 MediaStreamGraphImpl::UpdateStreamOrder()
 {
-  nsTArray<nsRefPtr<MediaStream> > oldStreams;
-  oldStreams.SwapElements(mStreams);
-  for (uint32_t i = 0; i < oldStreams.Length(); ++i) {
-    MediaStream* stream = oldStreams[i];
+  mOldStreams.SwapElements(mStreams);
+  mStreams.ClearAndRetainStorage();
+  for (uint32_t i = 0; i < mOldStreams.Length(); ++i) {
+    MediaStream* stream = mOldStreams[i];
     stream->mHasBeenOrdered = false;
     stream->mIsConsumed = false;
     stream->mIsOnOrderingStack = false;
@@ -504,8 +504,8 @@ MediaStreamGraphImpl::UpdateStreamOrder()
   }
 
   nsAutoTArray<MediaStream*,10> stack;
-  for (uint32_t i = 0; i < oldStreams.Length(); ++i) {
-    nsRefPtr<MediaStream>& s = oldStreams[i];
+  for (uint32_t i = 0; i < mOldStreams.Length(); ++i) {
+    nsRefPtr<MediaStream>& s = mOldStreams[i];
     if (!s->mAudioOutputs.IsEmpty() || !s->mVideoOutputs.IsEmpty()) {
       MarkConsumed(s);
     }
diff --git a/content/media/MediaStreamGraphImpl.h b/content/media/MediaStreamGraphImpl.h
index 628c1f5525a3..4a1f4c8ed475 100644
--- a/content/media/MediaStreamGraphImpl.h
+++ b/content/media/MediaStreamGraphImpl.h
@@ -371,6 +371,11 @@ public:
 
   nsTArray<nsRefPtr<MediaStream> > mStreams;
   /**
+   * mOldStreams is used as temporary storage for streams when computing the
+   * order in which we compute them.
+   */
+  nsTArray<nsRefPtr<MediaStream> > mOldStreams;
+  /**
    * The current graph time for the current iteration of the RunThread control
    * loop.
    */
diff --git a/xpcom/glue/nsTArray.h b/xpcom/glue/nsTArray.h
index 06a6ae64f3aa..3a386477005a 100644
--- a/xpcom/glue/nsTArray.h
+++ b/xpcom/glue/nsTArray.h
@@ -1042,6 +1042,20 @@ public:
   //
   // Mutation methods
   //
+  // This method call the destructor on each element of the array, empties it,
+  // but does not shrink the array's capacity.
+  //
+  // Make sure to call Compact() if needed to avoid keeping a huge array
+  // around.
+  void ClearAndRetainStorage() {
+    if (base_type::mHdr == EmptyHdr()) {
+      return;
+    }
+
+    DestructRange(0, Length());
+    base_type::mHdr->mLength = 0;
+  }
+
 
   // This method replaces a range of elements in this array.
   // @param start     The starting index of the elements to replace.
-- 
2.11.4.GIT