Bug 1484780 - Cache gfx::Path to avoid building and flattening path at each restyle cycle. r=heycam

In most cases, we run an animation on an object by changing its
offset-distance/offset-rotate, but keep its offset-path the same.
Building and flattening the path is sometime expensive, especially for
large path, so caching it makes sense to us and have a significant
performance improvement. This is for the main thread motion path
animations.

Note: Even though we support compositor animations for motion path,
nsIFrame::GetTransformMatrix() is still called during the animations for
other usages, so we may still build the gfx::Path on the main thread
without this patch, so this improvement becomes necessary for most cases.

Differential Revision: https://phabricator.services.mozilla.com/D46667

--HG--
extra : moz-landing-system : lando

layout/base/MotionPathUtils.cpp

@@ -439,17 +439,10 @@ static OffsetPathData GenerateOffsetPathData(const nsIFrame* aFrame) {
   const StyleOffsetPath& path = aFrame->StyleDisplay()->mOffsetPath;
   switch (path.tag) {
     case StyleOffsetPath::Tag::Path: {
-      // Here we only need to build a valid path for motion path, so
-      // using the default values of stroke-width, stoke-linecap, and fill-rule
-      // is fine for now because what we want is get the point and its normal
-      // vector along the path, instead of rendering it.
-      // FIXME: Use cache for main-thread in Bug 1484780.
-      RefPtr<gfx::PathBuilder> builder =
-          gfxPlatform::GetPlatform()
-              ->ScreenReferenceDrawTarget()
-              ->CreatePathBuilder(gfx::FillRule::FILL_WINDING);
       RefPtr<gfx::Path> gfxPath =
-          MotionPathUtils::BuildPath(path.AsPath(), builder);
+          aFrame->GetProperty(nsIFrame::OffsetPathCache());
+      MOZ_ASSERT(gfxPath,
+                 "Should have a valid cached gfx::Path for offset-path");
       return OffsetPathData::Path(path.AsPath(), gfxPath.forget());
     }
     case StyleOffsetPath::Tag::Ray:

layout/generic/nsFrame.cpp

@@ -112,6 +112,7 @@
 #include "mozilla/ServoStyleSet.h"
 #include "mozilla/ServoStyleSetInlines.h"
 #include "mozilla/css/ImageLoader.h"
+#include "mozilla/dom/SVGPathData.h"
 #include "mozilla/dom/TouchEvent.h"
 #include "mozilla/gfx/Tools.h"
 #include "mozilla/layers/WebRenderUserData.h"
@@ -1335,6 +1336,30 @@ void nsFrame::DidSetComputedStyle(ComputedStyle* aOldComputedStyle) {
     PresContext()->SetBidiEnabled();
   }
 
+  // The following part is for caching offset-path:path(). We cache the
+  // flatten gfx path, so we don't have to rebuild and re-flattern it at
+  // each cycle if we have animations on offset-* with a fixed offset-path.
+  const StyleOffsetPath* oldPath =
+      aOldComputedStyle ? &aOldComputedStyle->StyleDisplay()->mOffsetPath
+                        : nullptr;
+  const StyleOffsetPath& newPath = StyleDisplay()->mOffsetPath;
+  if (!oldPath || *oldPath != newPath) {
+    if (newPath.IsPath()) {
+      // Here we only need to build a valid path for motion path, so
+      // using the default values of stroke-width, stoke-linecap, and fill-rule
+      // is fine for now because what we want is to get the point and its normal
+      // vector along the path, instead of rendering it.
+      RefPtr<gfx::PathBuilder> builder =
+          gfxPlatform::GetPlatform()
+              ->ScreenReferenceDrawTarget()
+              ->CreatePathBuilder(gfx::FillRule::FILL_WINDING);
+      SetProperty(nsIFrame::OffsetPathCache(),
+                  MotionPathUtils::BuildPath(newPath.AsPath(), builder).take());
+    } else if (oldPath) {
+      DeleteProperty(nsIFrame::OffsetPathCache());
+    }
+  }
+
   RemoveStateBits(NS_FRAME_SIMPLE_EVENT_REGIONS | NS_FRAME_SIMPLE_DISPLAYLIST);
 
   mMayHaveRoundedCorners = true;

layout/generic/nsIFrame.h

@@ -1257,6 +1257,8 @@ class nsIFrame : public nsQueryFrame {
   NS_DECLARE_FRAME_PROPERTY_WITHOUT_DTOR(PlaceholderFrameProperty,
                                          nsPlaceholderFrame)
 
+  NS_DECLARE_FRAME_PROPERTY_RELEASABLE(OffsetPathCache, mozilla::gfx::Path)
+
   mozilla::FrameBidiData GetBidiData() const {
     bool exists;
     mozilla::FrameBidiData bidiData = GetProperty(BidiDataProperty(), &exists);