Auto merge of #10021 - pcwalton:skip-layout-traversals, r=mbrubeck

layout: Skip layout traversals that obviously won't do anything.

This reduces CPU usage when mousing over simple pages (example.com). More complex pages (Wikipedia) still reflow a lot due to other bugs.

Additionally, this change causes Servo to stop painting the results of hit test queries. This is also a win for CPU usage.

This significantly improves #9999, though there's more that can be done. I'll leave it open in case @paulrouget thinks this PR isn't enough.

r? @mbrubeck

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components/layout/incremental.rs

@@ -167,7 +167,8 @@ pub fn compute_damage(old: Option<&Arc<ComputedValues>>, new: &ComputedValues) -
                       [ REPAINT ], [
         get_color.color, get_background.background_color,
         get_border.border_top_color, get_border.border_right_color,
-        get_border.border_bottom_color, get_border.border_left_color
+        get_border.border_bottom_color, get_border.border_left_color,
+        get_effects.transform, get_box.z_index
     ]);
 
     add_if_not_equal!(old, new, damage,
@@ -252,7 +253,6 @@ impl<'a> LayoutDamageComputation for &'a mut Flow {
                 has_counter_affecting_children = has_counter_affecting_children ||
                     flow::base(kid).flags.intersects(AFFECTS_COUNTERS |
                                                      HAS_COUNTER_AFFECTING_CHILDREN);
-
             }
         }
 

components/layout/layout_thread.rs

@@ -30,7 +30,8 @@ use gfx::font_context;
 use gfx::paint_thread::LayoutToPaintMsg;
 use gfx_traits::{color, Epoch, LayerId, ScrollPolicy};
 use heapsize::HeapSizeOf;
-use incremental::{LayoutDamageComputation, REFLOW, REFLOW_ENTIRE_DOCUMENT, REPAINT};
+use incremental::{LayoutDamageComputation, REFLOW, REFLOW_ENTIRE_DOCUMENT, REFLOW_OUT_OF_FLOW};
+use incremental::{REPAINT};
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
 use layout_debug;
@@ -869,6 +870,8 @@ impl LayoutThread {
             flow::mut_base(flow_ref::deref_mut(layout_root)).clip =
                 ClippingRegion::from_rect(&data.page_clip_rect);
 
+            if flow::base(&**layout_root).restyle_damage.contains(REPAINT) ||
+                    rw_data.display_list.is_none() {
                 let mut root_stacking_context =
                     StackingContext::new(StackingContextId::new(0),
                                          StackingContextType::Real,
@@ -888,7 +891,6 @@ impl LayoutThread {
                                                                &mut root_stacking_context,
                                                                shared_layout_context);
 
-            if data.goal == ReflowGoal::ForDisplay {
                 debug!("Done building display list.");
 
                 let root_background_color = get_root_flow_background_color(
@@ -905,13 +907,20 @@ impl LayoutThread {
                 let origin = Rect::new(Point2D::new(Au(0), Au(0)), root_size);
                 root_stacking_context.bounds = origin;
                 root_stacking_context.overflow = origin;
-                root_stacking_context.layer_info = Some(LayerInfo::new(layout_root.layer_id(),
+                root_stacking_context.layer_info =
+                    Some(LayerInfo::new(layout_root.layer_id(),
                                         ScrollPolicy::Scrollable,
                                         None,
                                         root_background_color));
 
-                let display_list = DisplayList::new(root_stacking_context,
+                rw_data.display_list =
-                                                    &mut Some(display_list_entries));
+                    Some(Arc::new(DisplayList::new(root_stacking_context,
+                                                   &mut Some(display_list_entries))))
+            }
+
+            if data.goal == ReflowGoal::ForDisplay {
+                let display_list = (*rw_data.display_list.as_ref().unwrap()).clone();
+
                 if opts::get().dump_display_list {
                     display_list.print();
                 }
@@ -919,9 +928,6 @@ impl LayoutThread {
                     println!("{}", serde_json::to_string_pretty(&display_list).unwrap());
                 }
 
-                let display_list = Arc::new(display_list);
-                rw_data.display_list = Some(display_list.clone());
-
                 debug!("Layout done!");
 
                 self.epoch.next();
@@ -942,7 +948,10 @@ impl LayoutThread {
                         epoch,
                         Some(root_scroll_layer_id),
                         &mut auxiliary_lists_builder);
-                    let root_background_color = webrender_traits::ColorF::new(root_background_color.r,
+                    let root_background_color = get_root_flow_background_color(
+                        flow_ref::deref_mut(layout_root));
+                    let root_background_color =
+                        webrender_traits::ColorF::new(root_background_color.r,
                                                       root_background_color.g,
                                                       root_background_color.b,
                                                       root_background_color.a);
@@ -1323,6 +1332,7 @@ impl LayoutThread {
 
             // Perform the primary layout passes over the flow tree to compute the locations of all
             // the boxes.
+            if flow::base(&*root_flow).restyle_damage.intersects(REFLOW | REFLOW_OUT_OF_FLOW) {
                 profile(time::ProfilerCategory::LayoutMain,
                         self.profiler_metadata(),
                         self.time_profiler_chan.clone(),
@@ -1343,6 +1353,7 @@ impl LayoutThread {
                         }
                     }
                 });
+            }
 
             profile(time::ProfilerCategory::LayoutStoreOverflow,
                     self.profiler_metadata(),

components/layout/traversal.rs

@@ -234,6 +234,6 @@ impl<'a> BuildDisplayList<'a> {
 
     #[inline]
     fn should_process(&self) -> bool {
-        self.state.layout_context.shared_context().goal == ReflowGoal::ForDisplay
+        true
     }
 }

components/script/dom/window.rs

@@ -1021,7 +1021,11 @@ impl Window {
 
         debug!("script: layout joined");
 
+        // Pending reflows require display, so only reset the pending reflow count if this reflow
+        // was to be displayed.
+        if goal == ReflowGoal::ForDisplay {
             self.pending_reflow_count.set(0);
+        }
 
         if let Some(marker) = marker {
             self.emit_timeline_marker(marker.end());
@@ -1104,7 +1108,7 @@ impl Window {
 
     pub fn hit_test_query(&self, hit_test_request: Point2D<f32>, update_cursor: bool)
                           -> Option<UntrustedNodeAddress> {
-        self.reflow(ReflowGoal::ForDisplay,
+        self.reflow(ReflowGoal::ForScriptQuery,
                     ReflowQueryType::HitTestQuery(hit_test_request, update_cursor),
                     ReflowReason::Query);
         self.layout_rpc.hit_test().node_address