layout: Perform cascading in parallel.

Speeds up the cascade by 3x.

src/components/main/css/matching.rs

@@ -17,6 +17,7 @@ pub trait MatchMethods {
     fn match_node(&self, stylist: &Stylist);
     fn match_subtree(&self, stylist: &Stylist);
 
+    unsafe fn cascade_node(&self, parent: Option<LayoutNode>);
     fn cascade_subtree(&self, parent: Option<LayoutNode>);
 }
 
@@ -51,18 +52,34 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
         }
     }
 
-    fn cascade_subtree(&self, parent: Option<LayoutNode>) {
+    unsafe fn cascade_node(&self, parent: Option<LayoutNode>) {
         macro_rules! cascade_node(
             ($applicable_declarations: ident, $style: ident) => {{
+                // Get our parent's style. This must be unsafe so that we don't touch the parent's
+                // borrow flags.
+                //
+                // FIXME(pcwalton): Isolate this unsafety into the `wrapper` module to allow
+                // enforced safe, race-free access to the parent style.
                 let parent_style = match parent {
-                    Some(ref parent) => Some(parent.style()),
-                    None => None
+                    None => None,
+                    Some(parent_node) => {
+                        let parent_layout_data = parent_node.borrow_layout_data_unchecked();
+                        match *parent_layout_data {
+                            None => fail!("no parent data?!"),
+                            Some(ref parent_layout_data) => {
+                                match parent_layout_data.data.style {
+                                    None => fail!("parent hasn't been styled yet?!"),
+                                    Some(ref style) => Some(style.get()),
+                                }
+                            }
+                        }
+                    }
                 };
 
                 let computed_values = {
                     let layout_data_ref = self.borrow_layout_data();
                     let layout_data = layout_data_ref.get().as_ref().unwrap();
-                    Arc::new(cascade(layout_data.data.$applicable_declarations, parent_style.map(|parent_style| parent_style.get())))
+                    Arc::new(cascade(layout_data.data.$applicable_declarations, parent_style))
                 };
 
                 let mut layout_data_ref = self.mutate_layout_data();
@@ -102,6 +119,12 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
                 cascade_node!(after_applicable_declarations, after_style);
             }
         }
+    }
+
+    fn cascade_subtree(&self, parent: Option<LayoutNode>) {
+        unsafe {
+            self.cascade_node(parent);
+        }
 
         for kid in self.children() {
             if kid.is_element() {

src/components/main/layout/context.rs

@@ -7,6 +7,7 @@
 use extra::arc::MutexArc;
 use green::task::GreenTask;
 use layout::flow::LeafSet;
+use layout::util::OpaqueNode;
 use std::cast;
 use std::ptr;
 use std::rt::Runtime;
@@ -45,6 +46,9 @@ pub struct LayoutContext {
     ///
     /// FIXME(pcwalton): Make this no longer an unsafe pointer once we have fast `RWArc`s.
     stylist: *Stylist,
+
+    /// The root node at which we're starting the layout.
+    reflow_root: OpaqueNode,
 }
 
 impl LayoutContext {

src/components/main/layout/layout_task.rs

@@ -289,7 +289,7 @@ impl LayoutTask {
     }
 
     // Create a layout context for use in building display lists, hit testing, &c.
-    fn build_layout_context(&self) -> LayoutContext {
+    fn build_layout_context(&self, reflow_root: &LayoutNode) -> LayoutContext {
         let font_context_info = FontContextInfo {
             backend: self.opts.render_backend,
             needs_font_list: true,
@@ -303,6 +303,7 @@ impl LayoutTask {
             leaf_set: self.leaf_set.clone(),
             font_context_info: font_context_info,
             stylist: &*self.stylist,
+            reflow_root: OpaqueNode::from_layout_node(reflow_root),
         }
     }
 
@@ -505,7 +506,7 @@ impl LayoutTask {
         self.screen_size = current_screen_size;
 
         // Create a layout context for use throughout the following passes.
-        let mut layout_ctx = self.build_layout_context();
+        let mut layout_ctx = self.build_layout_context(node);
 
         // Initialize layout data for each node.
         //
@@ -522,15 +523,21 @@ impl LayoutTask {
                     match self.parallel_traversal {
                         None => node.match_subtree(self.stylist),
                         Some(ref mut traversal) => {
-                            parallel::match_subtree(node, &mut layout_ctx, traversal)
+                            parallel::match_and_cascade_subtree(node, &mut layout_ctx, traversal)
                         }
                     }
                 });
+
+                // If we're doing layout sequentially, do the cascade separately.
+                //
+                // TODO(pcwalton): Integrate this into `match_subtree`.
+                if self.parallel_traversal.is_none() {
                     profile(time::LayoutSelectorCascadeCategory, self.profiler_chan.clone(), || {
                         node.cascade_subtree(None);
                     });
                 }
             }
+        }
 
         // Construct the flow tree.
         let mut layout_root = profile(time::LayoutTreeBuilderCategory,

src/components/main/layout/parallel.rs

@@ -11,6 +11,7 @@ use layout::context::LayoutContext;
 use layout::flow::{Flow, LeafSet, PostorderFlowTraversal};
 use layout::flow;
 use layout::layout_task::{AssignHeightsAndStoreOverflowTraversal, BubbleWidthsTraversal};
+use layout::util::OpaqueNode;
 use layout::wrapper::LayoutNode;
 
 use extra::arc::MutexArc;
@@ -115,7 +116,7 @@ impl<'a> ParallelPostorderFlowTraversal for BubbleWidthsTraversal<'a> {}
 
 impl<'a> ParallelPostorderFlowTraversal for AssignHeightsAndStoreOverflowTraversal<'a> {}
 
-fn match_node(unsafe_layout_node: UnsafeLayoutNode,
+fn match_and_cascade_node(unsafe_layout_node: UnsafeLayoutNode,
                           proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeLayoutNode>) {
     unsafe {
         let layout_context: &mut LayoutContext = cast::transmute(*proxy.user_data());
@@ -123,19 +124,27 @@ fn match_node(unsafe_layout_node: UnsafeLayoutNode,
         // Get a real layout node.
         let node: LayoutNode = cast::transmute(unsafe_layout_node);
 
+        // Perform the CSS selector matching.
+        let stylist: &Stylist = cast::transmute(layout_context.stylist);
+        node.match_node(stylist);
+
+        // Perform the CSS cascade.
+        let parent_opt = if OpaqueNode::from_layout_node(&node) == layout_context.reflow_root {
+            None
+        } else {
+            node.parent_node()
+        };
+        node.cascade_node(parent_opt);
+
         // Enqueue kids.
         for kid in node.children() {
             if kid.is_element() {
                 proxy.push(WorkUnit {
-                    fun: match_node,
+                    fun: match_and_cascade_node,
                     data: layout_node_to_unsafe_layout_node(&kid),
                 });
             }
         }
-
-        // Perform the CSS selector matching.
-        let stylist: &Stylist = cast::transmute(layout_context.stylist);
-        node.match_node(stylist);
     }
 }
 
@@ -160,7 +169,7 @@ fn assign_heights_and_store_overflow(unsafe_flow: UnsafeFlow,
     assign_heights_traversal.run_parallel(unsafe_flow)
 }
 
-pub fn match_subtree(root_node: &LayoutNode,
+pub fn match_and_cascade_subtree(root_node: &LayoutNode,
                                  layout_context: &mut LayoutContext,
                                  queue: &mut WorkQueue<*mut LayoutContext,UnsafeLayoutNode>) {
     unsafe {
@@ -169,7 +178,7 @@ pub fn match_subtree(root_node: &LayoutNode,
 
     // Enqueue the root node.
     queue.push(WorkUnit {
-        fun: match_node,
+        fun: match_and_cascade_node,
         data: layout_node_to_unsafe_layout_node(root_node),
     });
 

src/components/main/layout/util.rs

@@ -174,9 +174,7 @@ pub struct LayoutDataWrapper {
 /// A trait that allows access to the layout data of a DOM node.
 pub trait LayoutDataAccess {
     /// Borrows the layout data without checks.
-    ///
-    /// FIXME(pcwalton): Make safe.
-    // unsafe fn borrow_layout_data_unchecked<'a>(&'a self) -> &'a Option<~LayoutData>;
+    unsafe fn borrow_layout_data_unchecked(&self) -> *Option<LayoutDataWrapper>;
     /// Borrows the layout data immutably. Fails on a conflicting borrow.
     fn borrow_layout_data<'a>(&'a self) -> Ref<'a,Option<LayoutDataWrapper>>;
     /// Borrows the layout data mutably. Fails on a conflicting borrow.
@@ -184,6 +182,11 @@ pub trait LayoutDataAccess {
 }
 
 impl<'ln> LayoutDataAccess for LayoutNode<'ln> {
+    #[inline(always)]
+    unsafe fn borrow_layout_data_unchecked(&self) -> *Option<LayoutDataWrapper> {
+        cast::transmute(self.get().layout_data.borrow_unchecked())
+    }
+
     #[inline(always)]
     fn borrow_layout_data<'a>(&'a self) -> Ref<'a,Option<LayoutDataWrapper>> {
         unsafe {

src/components/script/dom/node.rs

@@ -154,13 +154,10 @@ impl LayoutDataRef {
     /// Borrows the layout data immutably, *asserting that there are no mutators*. Bad things will
     /// happen if you try to mutate the layout data while this is held. This is the only thread-
     /// safe layout data accessor.
-    ///
-    /// FIXME(pcwalton): Enforce this invariant via the type system. Will require traversal
-    /// functions to be trusted, but c'est la vie.
-    // #[inline]
-    // pub unsafe fn borrow_unchecked<'a>(&'a self) -> &'a () {
-    //     self.data.borrow_unchecked()
-    // }
+    #[inline]
+    pub unsafe fn borrow_unchecked(&self) -> *Option<LayoutData> {
+        cast::transmute(&self.data_cell)
+    }
 
     /// Borrows the layout data immutably. This function is *not* thread-safe.
     #[inline]