Speed up updating the hit-test tree

webrender/src/clip_scroll_tree.rs

@@ -249,10 +249,14 @@ impl ClipScrollTree {
         &mut self,
         pan: WorldPoint,
         scene_properties: &SceneProperties,
-    ) -> TransformPalette {
-        let mut transform_palette = TransformPalette::new(self.spatial_nodes.len());
+        mut transform_palette: Option<&mut TransformPalette>,
+    ) {
         if self.spatial_nodes.is_empty() {
-            return transform_palette;
+            return;
+        }
+
+        if let Some(ref mut palette) = transform_palette {
+            palette.reserve(self.spatial_nodes.len());
         }
 
         self.coord_systems.clear();
@@ -275,36 +279,38 @@ impl ClipScrollTree {
             &mut transform_palette,
             scene_properties,
         );
-
-        transform_palette
     }
 
     fn update_node(
         &mut self,
         node_index: SpatialNodeIndex,
         state: &mut TransformUpdateState,
-        transform_palette: &mut TransformPalette,
+        transform_palette: &mut Option<&mut TransformPalette>,
         scene_properties: &SceneProperties,
     ) {
         // TODO(gw): This is an ugly borrow check workaround to clone these.
         //           Restructure this to avoid the clones!
         let mut state = state.clone();
-        let node_children = {
+        let mut node_children: SmallVec<[SpatialNodeIndex; 4]> = SmallVec::new();
+
+        {
             let node = match self.spatial_nodes.get_mut(node_index.0) {
                 Some(node) => node,
                 None => return,
             };
 
             node.update(&mut state, &mut self.coord_systems, scene_properties);
-            node.push_gpu_data(transform_palette, node_index);
+            if let Some(palette) = transform_palette {
+                node.push_gpu_data(palette, node_index);
+            }
 
             if node.children.is_empty() {
                 return;
             }
 
             node.prepare_state_for_children(&mut state);
-            node.children.clone()
-        };
+            node_children.extend_from_slice(&node.children);
+        }
 
         for child_node_index in node_children {
             self.update_node(
@@ -525,7 +531,7 @@ fn test_cst_simple_translation() {
         LayoutVector2D::zero(),
     );
 
-    cst.update_tree(WorldPoint::zero(), &SceneProperties::new());
+    cst.update_tree(WorldPoint::zero(), &SceneProperties::new(), None);
 
     test_pt(100.0, 100.0, &cst, child1, root, 200.0, 100.0);
     test_pt(100.0, 100.0, &cst, root, child1, 0.0, 100.0);
@@ -570,7 +576,7 @@ fn test_cst_simple_scale() {
         LayoutVector2D::zero(),
     );
 
-    cst.update_tree(WorldPoint::zero(), &SceneProperties::new());
+    cst.update_tree(WorldPoint::zero(), &SceneProperties::new(), None);
 
     test_pt(100.0, 100.0, &cst, child1, root, 400.0, 100.0);
     test_pt(100.0, 100.0, &cst, root, child1, 25.0, 100.0);
@@ -624,7 +630,7 @@ fn test_cst_scale_translation() {
         LayoutVector2D::zero(),
     );
 
-    cst.update_tree(WorldPoint::zero(), &SceneProperties::new());
+    cst.update_tree(WorldPoint::zero(), &SceneProperties::new(), None);
 
     test_pt(100.0, 100.0, &cst, child1, root, 200.0, 150.0);
     test_pt(100.0, 100.0, &cst, child2, root, 300.0, 450.0);
@@ -663,7 +669,7 @@ fn test_cst_translation_rotate() {
         LayoutVector2D::zero(),
     );
 
-    cst.update_tree(WorldPoint::zero(), &SceneProperties::new());
+    cst.update_tree(WorldPoint::zero(), &SceneProperties::new(), None);
 
     test_pt(100.0, 0.0, &cst, child1, root, 0.0, -100.0);
 }

webrender/src/frame_builder.rs

@@ -340,9 +340,11 @@ impl FrameBuilder {
         resource_cache.begin_frame(frame_id);
         gpu_cache.begin_frame();
 
-        let mut transform_palette = clip_scroll_tree.update_tree(
+        let mut transform_palette = TransformPalette::new();
+        clip_scroll_tree.update_tree(
             pan,
             scene_properties,
+            Some(&mut transform_palette),
         );
 
         self.update_scroll_bars(clip_scroll_tree, gpu_cache);

webrender/src/gpu_types.rs

@@ -415,14 +415,19 @@ pub struct TransformPalette {
 }
 
 impl TransformPalette {
-    pub fn new(spatial_node_count: usize) -> Self {
+    pub fn new() -> Self {
         TransformPalette {
-            transforms: vec![TransformData::invalid(); spatial_node_count],
-            metadata: vec![TransformMetadata::invalid(); spatial_node_count],
+            transforms: Vec::new(),
+            metadata: Vec::new(),
             map: FastHashMap::default(),
         }
     }
 
+    pub fn reserve(&mut self, count: usize) {
+        self.transforms = vec![TransformData::invalid(); count];
+        self.metadata = vec![TransformMetadata::invalid(); count];
+    }
+
     pub fn set_world_transform(
         &mut self,
         index: SpatialNodeIndex,

webrender/src/render_backend.rs

@@ -279,6 +279,21 @@ impl Document {
         }
     }
 
+    fn rebuild_hit_tester(&mut self) {
+        if let Some(ref mut frame_builder) = self.frame_builder {
+            let accumulated_scale_factor = self.view.accumulated_scale_factor();
+            let pan = self.view.pan.to_f32() / accumulated_scale_factor;
+
+            self.clip_scroll_tree.update_tree(
+                pan,
+                &self.dynamic_properties,
+                None,
+            );
+
+            self.hit_tester = Some(frame_builder.create_hit_tester(&self.clip_scroll_tree));
+        }
+    }
+
     pub fn updated_pipeline_info(&mut self) -> PipelineInfo {
         let removed_pipelines = replace(&mut self.removed_pipelines, Vec::new());
         PipelineInfo {
@@ -333,14 +348,12 @@ impl Document {
 
 struct DocumentOps {
     scroll: bool,
-    build_frame: bool,
 }
 
 impl DocumentOps {
     fn nop() -> Self {
         DocumentOps {
             scroll: false,
-            build_frame: false,
         }
     }
 }
@@ -597,7 +610,6 @@ impl RenderBackend {
                             replace(&mut txn.resource_updates, Vec::new()),
                             replace(&mut txn.frame_ops, Vec::new()),
                             replace(&mut txn.notifications, Vec::new()),
-                            txn.build_frame,
                             txn.render_frame,
                             &mut frame_counter,
                             &mut profile_counters,
@@ -850,7 +862,6 @@ impl RenderBackend {
             rasterized_blobs: Vec::new(),
             notifications: transaction_msg.notifications,
             set_root_pipeline: None,
-            build_frame: transaction_msg.generate_frame,
             render_frame: transaction_msg.generate_frame,
         });
 
@@ -885,7 +896,6 @@ impl RenderBackend {
                 replace(&mut txn.resource_updates, Vec::new()),
                 replace(&mut txn.frame_ops, Vec::new()),
                 replace(&mut txn.notifications, Vec::new()),
-                txn.build_frame,
                 txn.render_frame,
                 frame_counter,
                 profile_counters,
@@ -922,7 +932,6 @@ impl RenderBackend {
         resource_updates: Vec<ResourceUpdate>,
         mut frame_ops: Vec<FrameMsg>,
         mut notifications: Vec<NotificationRequest>,
-        mut build_frame: bool,
         mut render_frame: bool,
         frame_counter: &mut u32,
         profile_counters: &mut BackendProfileCounters,
@@ -935,7 +944,7 @@ impl RenderBackend {
         // fiddle with things after a potentially long scene build, but just
         // before rendering. This is useful for rendering with the latest
         // async transforms.
-        if build_frame {
+        if requested_frame {
             if let Some(ref sampler) = self.sampler {
                 frame_ops.append(&mut sampler.sample());
             }
@@ -949,7 +958,6 @@ impl RenderBackend {
         for frame_msg in frame_ops {
             let _timer = profile_counters.total_time.timer();
             let op = doc.process_frame_msg(frame_msg);
-            build_frame |= op.build_frame;
             scroll |= op.scroll;
         }
 
@@ -964,30 +972,23 @@ impl RenderBackend {
             &mut profile_counters.resources,
         );
 
-        // After applying the new scene we need to
-        // rebuild the hit-tester, so we trigger a frame generation
-        // step.
-        //
-        // TODO: We could avoid some the cost of building the frame by only
-        // building the information required for hit-testing (See #2807).
-        build_frame |= has_built_scene;
-
         if doc.dynamic_properties.flush_pending_updates() {
             doc.frame_is_valid = false;
             doc.hit_tester_is_valid = false;
-            build_frame = true;
         }
 
         if !doc.can_render() {
             // TODO: this happens if we are building the first scene asynchronously and
             // scroll at the same time. we should keep track of the fact that we skipped
             // composition here and do it as soon as we receive the scene.
-            build_frame = false;
             render_frame = false;
         }
 
-        if doc.frame_is_valid {
-            build_frame = false;
+        // Avoid re-building the frame if the current built frame is still valid.
+        let build_frame = render_frame && !doc.frame_is_valid;
+
+        if !doc.hit_tester_is_valid && !build_frame {
+            doc.rebuild_hit_tester();
         }
 
         let mut frame_build_time = None;

webrender/src/scene_builder.rs

@@ -33,7 +33,6 @@ pub struct Transaction {
     pub frame_ops: Vec<FrameMsg>,
     pub notifications: Vec<NotificationRequest>,
     pub set_root_pipeline: Option<PipelineId>,
-    pub build_frame: bool,
     pub render_frame: bool,
 }
 
@@ -66,7 +65,6 @@ pub struct BuiltTransaction {
     pub notifications: Vec<NotificationRequest>,
     pub scene_build_start_time: u64,
     pub scene_build_end_time: u64,
-    pub build_frame: bool,
     pub render_frame: bool,
 }
 
@@ -246,7 +244,6 @@ impl SceneBuilder {
 
             let txn = Box::new(BuiltTransaction {
                 document_id: item.document_id,
-                build_frame: true,
                 render_frame: item.build_frame,
                 built_scene,
                 resource_updates: Vec::new(),
@@ -334,7 +331,6 @@ impl SceneBuilder {
 
         Box::new(BuiltTransaction {
             document_id: txn.document_id,
-            build_frame: txn.build_frame || built_scene.is_some(),
             render_frame: txn.render_frame,
             built_scene,
             rasterized_blobs,