From 0b55cc54f32b9d4b450ff1eac9d5cb45d5cc9c6d Mon Sep 17 00:00:00 2001
From: Kartikaya Gupta <kgupta@mozilla.com>
Date: Thu, 9 Nov 2017 23:18:39 -0500
Subject: [PATCH] Bug 1416073 - Avoid calling the expensive
 ComputeScrollMetadata function if we already computed that metadata before.
 r=jrmuizel

MozReview-Commit-ID: 5SwAEA4Wpqw

--HG--
extra : rebase_source : 1c717c3f0cfb1c5a94970d5d7cc267ee9570c0ad
---
 gfx/layers/wr/WebRenderCommandBuilder.cpp |  2 +-
 gfx/layers/wr/WebRenderScrollData.cpp     | 21 ++++++++++++++-------
 gfx/layers/wr/WebRenderScrollData.h       |  2 +-
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/gfx/layers/wr/WebRenderCommandBuilder.cpp b/gfx/layers/wr/WebRenderCommandBuilder.cpp
index d0b477ccf166..ffda9ed4289d 100644
--- a/gfx/layers/wr/WebRenderCommandBuilder.cpp
+++ b/gfx/layers/wr/WebRenderCommandBuilder.cpp
@@ -81,7 +81,7 @@ WebRenderCommandBuilder::BuildWebRenderCommands(wr::DisplayListBuilder& aBuilder
     mLayerScrollData.emplace_back();
     mLayerScrollData.back().InitializeRoot(mLayerScrollData.size() - 1);
     auto callback = [&aScrollData](FrameMetrics::ViewID aScrollId) -> bool {
-      return aScrollData.HasMetadataFor(aScrollId);
+      return aScrollData.HasMetadataFor(aScrollId).isSome();
     };
     if (Maybe<ScrollMetadata> rootMetadata = nsLayoutUtils::GetRootMetadata(
           aDisplayListBuilder, mManager, ContainerLayerParameters(), callback)) {
diff --git a/gfx/layers/wr/WebRenderScrollData.cpp b/gfx/layers/wr/WebRenderScrollData.cpp
index 97ce9a5b1de0..b7a8d539e9b4 100644
--- a/gfx/layers/wr/WebRenderScrollData.cpp
+++ b/gfx/layers/wr/WebRenderScrollData.cpp
@@ -54,11 +54,17 @@ WebRenderLayerScrollData::Initialize(WebRenderScrollData& aOwner,
        asr && asr != aStopAtAsr;
        asr = asr->mParent) {
     MOZ_ASSERT(aOwner.GetManager());
-    Maybe<ScrollMetadata> metadata = asr->mScrollableFrame->ComputeScrollMetadata(
-        nullptr, aOwner.GetManager(), aItem->ReferenceFrame(),
-        ContainerLayerParameters(), nullptr);
-    MOZ_ASSERT(metadata);
-    mScrollIds.AppendElement(aOwner.AddMetadata(metadata.ref()));
+    FrameMetrics::ViewID scrollId = nsLayoutUtils::ViewIDForASR(asr);
+    if (Maybe<size_t> index = aOwner.HasMetadataFor(scrollId)) {
+      mScrollIds.AppendElement(index.ref());
+    } else {
+      Maybe<ScrollMetadata> metadata = asr->mScrollableFrame->ComputeScrollMetadata(
+          nullptr, aOwner.GetManager(), aItem->ReferenceFrame(),
+          ContainerLayerParameters(), nullptr);
+      MOZ_ASSERT(metadata);
+      MOZ_ASSERT(metadata->GetMetrics().GetScrollId() == scrollId);
+      mScrollIds.AppendElement(aOwner.AddMetadata(metadata.ref()));
+    }
   }
 }
 
@@ -200,10 +206,11 @@ WebRenderScrollData::GetScrollMetadata(size_t aIndex) const
   return mScrollMetadatas[aIndex];
 }
 
-bool
+Maybe<size_t>
 WebRenderScrollData::HasMetadataFor(const FrameMetrics::ViewID& aScrollId) const
 {
-  return mScrollIdMap.find(aScrollId) != mScrollIdMap.end();
+  auto it = mScrollIdMap.find(aScrollId);
+  return (it == mScrollIdMap.end() ? Nothing() : Some(it->second));
 }
 
 void
diff --git a/gfx/layers/wr/WebRenderScrollData.h b/gfx/layers/wr/WebRenderScrollData.h
index 1e016331a604..0e270d14a8a5 100644
--- a/gfx/layers/wr/WebRenderScrollData.h
+++ b/gfx/layers/wr/WebRenderScrollData.h
@@ -151,7 +151,7 @@ public:
   const WebRenderLayerScrollData* GetLayerData(size_t aIndex) const;
 
   const ScrollMetadata& GetScrollMetadata(size_t aIndex) const;
-  bool HasMetadataFor(const FrameMetrics::ViewID& aScrollId) const;
+  Maybe<size_t> HasMetadataFor(const FrameMetrics::ViewID& aScrollId) const;
 
   const FocusTarget& GetFocusTarget() const { return mFocusTarget; }
   void SetFocusTarget(const FocusTarget& aFocusTarget);