Merge pull request #3638 from cgaebel/parallel-dom-traversal

Factors out DOM traversal, keeping the code in `parallel` free of traversal-specific logic.

5 files changed, 501 insertions, 403 deletions

diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index b2653496a6b..d252b856f06 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -16,6 +16,7 @@ use flow_ref::FlowRef;
 use layout_debug;
 use parallel::UnsafeFlow;
 use parallel;
+use traversal;
 use util::{LayoutDataAccess, LayoutDataWrapper, OpaqueNodeMethods, ToGfxColor};
 use wrapper::{LayoutNode, TLayoutNode, ThreadSafeLayoutNode};
 
@@ -146,108 +147,6 @@ pub struct LayoutTask {
     pub rw_data: Arc<Mutex<LayoutTaskData>>,
 }
 
-/// The flow tree verification traversal. This is only on in debug builds.
-#[cfg(debug)]
-struct FlowTreeVerificationTraversal;
-
-#[cfg(debug)]
-impl PreorderFlowTraversal for FlowTreeVerificationTraversal {
-    #[inline]
-    fn process(&mut self, flow: &mut Flow) -> bool {
-        let base = flow::base(flow);
-        if !base.flags.is_leaf() && !base.flags.is_nonleaf() {
-            println("flow tree verification failed: flow wasn't a leaf or a nonleaf!");
-            flow.dump();
-            fail!("flow tree verification failed")
-        }
-        true
-    }
-}
-
-/// The bubble-inline-sizes traversal, the first part of layout computation. This computes preferred
-/// and intrinsic inline-sizes and bubbles them up the tree.
-pub struct BubbleISizesTraversal<'a> {
-    pub layout_context: &'a LayoutContext<'a>,
-}
-
-impl<'a> PostorderFlowTraversal for BubbleISizesTraversal<'a> {
-    #[inline]
-    fn process(&mut self, flow: &mut Flow) -> bool {
-        flow.bubble_inline_sizes(self.layout_context);
-        true
-    }
-
-    // FIXME: We can't prune until we start reusing flows
-    /*
-    #[inline]
-    fn should_prune(&mut self, flow: &mut Flow) -> bool {
-        flow::mut_base(flow).restyle_damage.lacks(BubbleISizes)
-    }
-    */
-}
-
-/// The assign-inline-sizes traversal. In Gecko this corresponds to `Reflow`.
-pub struct AssignISizesTraversal<'a> {
-    pub layout_context: &'a LayoutContext<'a>,
-}
-
-impl<'a> PreorderFlowTraversal for AssignISizesTraversal<'a> {
-    #[inline]
-    fn process(&mut self, flow: &mut Flow) -> bool {
-        flow.assign_inline_sizes(self.layout_context);
-        true
-    }
-}
-
-/// The assign-block-sizes-and-store-overflow traversal, the last (and most expensive) part of layout
-/// computation. Determines the final block-sizes for all layout objects, computes positions, and
-/// computes overflow regions. In Gecko this corresponds to `FinishAndStoreOverflow`.
-pub struct AssignBSizesAndStoreOverflowTraversal<'a> {
-    pub layout_context: &'a LayoutContext<'a>,
-}
-
-impl<'a> PostorderFlowTraversal for AssignBSizesAndStoreOverflowTraversal<'a> {
-    #[inline]
-    fn process(&mut self, flow: &mut Flow) -> bool {
-        flow.assign_block_size(self.layout_context);
-        // Skip store-overflow for absolutely positioned flows. That will be
-        // done in a separate traversal.
-        if !flow.is_store_overflow_delayed() {
-            flow.store_overflow(self.layout_context);
-        }
-        true
-    }
-
-    #[inline]
-    fn should_process(&mut self, flow: &mut Flow) -> bool {
-        !flow::base(flow).flags.impacted_by_floats()
-    }
-}
-
-/// The display list construction traversal.
-pub struct BuildDisplayListTraversal<'a> {
-    layout_context: &'a LayoutContext<'a>,
-}
-
-impl<'a> BuildDisplayListTraversal<'a> {
-    #[inline]
-    fn process(&mut self, flow: &mut Flow) {
-        flow.compute_absolute_position();
-
-        for kid in flow::mut_base(flow).child_iter() {
-            if !kid.is_absolutely_positioned() {
-                self.process(kid)
-            }
-        }
-
-        for absolute_descendant_link in flow::mut_base(flow).abs_descendants.iter() {
-            self.process(absolute_descendant_link)
-        }
-
-        flow.build_display_list(self.layout_context)
-    }
-}
-
 struct LayoutImageResponder {
     id: PipelineId,
     script_chan: ScriptControlChan,
@@ -617,7 +516,7 @@ impl LayoutTask {
         let _scope = layout_debug_scope!("solve_constraints");
 
         if layout_context.shared.opts.bubble_inline_sizes_separately {
-            let mut traversal = BubbleISizesTraversal {
+            let mut traversal = traversal::BubbleISizes {
                 layout_context: layout_context,
             };
             layout_root.traverse_postorder(&mut traversal);
@@ -625,14 +524,14 @@ impl LayoutTask {
 
         // FIXME(pcwalton): Prune these two passes.
         {
-            let mut traversal = AssignISizesTraversal {
+            let mut traversal = traversal::AssignISizes {
                 layout_context: layout_context,
             };
             layout_root.traverse_preorder(&mut traversal);
         }
 
         {
-            let mut traversal = AssignBSizesAndStoreOverflowTraversal {
+            let mut traversal = traversal::AssignBSizesAndStoreOverflow {
                 layout_context: layout_context,
             };
             layout_root.traverse_postorder(&mut traversal);
@@ -650,7 +549,7 @@ impl LayoutTask {
                                   layout_root: &mut FlowRef,
                                   shared_layout_context: &SharedLayoutContext) {
         if shared_layout_context.opts.bubble_inline_sizes_separately {
-            let mut traversal = BubbleISizesTraversal {
+            let mut traversal = traversal::BubbleISizes {
                 layout_context: &LayoutContext::new(shared_layout_context),
             };
             layout_root.get_mut().traverse_postorder(&mut traversal);
@@ -677,7 +576,7 @@ impl LayoutTask {
     #[inline(never)]
     #[cfg(debug)]
     fn verify_flow_tree(&self, layout_root: &mut FlowRef) {
-        let mut traversal = FlowTreeVerificationTraversal;
+        let mut traversal = traversal::FlowTreeVerification;
         layout_root.traverse_preorder(&mut traversal);
     }
 
@@ -757,7 +656,7 @@ impl LayoutTask {
                                                    None)
                 }
                 Some(ref mut traversal) => {
-                    parallel::recalc_style_for_subtree(node, &mut shared_layout_ctx, traversal)
+                    parallel::traverse_dom_preorder(*node, &mut shared_layout_ctx, traversal)
                 }
             }
 
@@ -807,7 +706,7 @@ impl LayoutTask {
                 match rw_data.parallel_traversal {
                     None => {
                         let layout_ctx = LayoutContext::new(&shared_layout_ctx);
-                        let mut traversal = BuildDisplayListTraversal {
+                        let mut traversal = traversal::BuildDisplayList {
                             layout_context: &layout_ctx,
                         };
                         traversal.process(layout_root.get_mut());
diff --git a/components/layout/lib.rs b/components/layout/lib.rs
index c15b2658ab4..13e11e06c54 100644
--- a/components/layout/lib.rs
+++ b/components/layout/lib.rs
@@ -62,6 +62,7 @@ pub mod table_rowgroup;
 pub mod table_row;
 pub mod table_cell;
 pub mod text;
+pub mod traversal;
 pub mod util;
 pub mod incremental;
 pub mod wrapper;
diff --git a/components/layout/parallel.rs b/components/layout/parallel.rs
index 87478be9320..3a85ee9a629 100644
--- a/components/layout/parallel.rs
+++ b/components/layout/parallel.rs
@@ -6,32 +6,24 @@
 //!
 //! This code is highly unsafe. Keep this file small and easy to audit.
 
-use css::node_style::StyledNode;
-use css::matching::{ApplicableDeclarations, CannotShare, MatchMethods, StyleWasShared};
-use construct::FlowConstructor;
 use context::{LayoutContext, SharedLayoutContext};
 use flow::{Flow, MutableFlowUtils, PreorderFlowTraversal, PostorderFlowTraversal};
 use flow;
 use flow_ref::FlowRef;
-use incremental::RestyleDamage;
-use layout_task::{AssignBSizesAndStoreOverflowTraversal, AssignISizesTraversal};
-use layout_task::{BubbleISizesTraversal};
+use traversal::{RecalcStyleForNode, ConstructFlows};
+use traversal::{AssignBSizesAndStoreOverflow, AssignISizes, BubbleISizes};
 use url::Url;
-use util::{LayoutDataAccess, LayoutDataWrapper, OpaqueNodeMethods};
+use util::{LayoutDataAccess, LayoutDataWrapper};
 use wrapper::{layout_node_to_unsafe_layout_node, layout_node_from_unsafe_layout_node, LayoutNode};
-use wrapper::{PostorderNodeMutTraversal, ThreadSafeLayoutNode, UnsafeLayoutNode};
+use wrapper::{PostorderNodeMutTraversal, UnsafeLayoutNode};
+use wrapper::{PreorderDOMTraversal, PostorderDOMTraversal};
 
-use gfx::display_list::OpaqueNode;
-use servo_util::bloom::BloomFilter;
-use servo_util::tid::tid;
 use servo_util::time::{TimeProfilerChan, profile};
 use servo_util::time;
 use servo_util::workqueue::{WorkQueue, WorkUnit, WorkerProxy};
 use std::mem;
 use std::ptr;
 use std::sync::atomics::{AtomicInt, Relaxed, SeqCst};
-use style;
-use style::TNode;
 
 #[allow(dead_code)]
 fn static_assertion(node: UnsafeLayoutNode) {
@@ -85,6 +77,113 @@ impl DomParallelInfo {
     }
 }
 
+/// A parallel top-down DOM traversal.
+pub trait ParallelPreorderDOMTraversal : PreorderDOMTraversal {
+    fn run_parallel(&mut self,
+                    node: UnsafeLayoutNode,
+                    proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeLayoutNode>);
+
+    #[inline(always)]
+    fn run_parallel_helper(&mut self,
+                           unsafe_node: UnsafeLayoutNode,
+                           proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeLayoutNode>,
+                           top_down_func: extern "Rust" fn(UnsafeFlow,
+                                                           &mut WorkerProxy<*const SharedLayoutContext,
+                                                                            UnsafeLayoutNode>),
+                           bottom_up_func: extern "Rust" fn(UnsafeFlow,
+                                                            &mut WorkerProxy<*const SharedLayoutContext,
+                                                                             UnsafeFlow>)) {
+        // Get a real layout node.
+        let node: LayoutNode = unsafe {
+            layout_node_from_unsafe_layout_node(&unsafe_node)
+        };
+
+        // Perform the appropriate traversal.
+        self.process(node);
+
+        // NB: O(n).
+        let child_count = node.children().count();
+
+        // Reset the count of children.
+        {
+            let mut layout_data_ref = node.mutate_layout_data();
+            let layout_data = layout_data_ref.as_mut().expect("no layout data");
+            layout_data.data.parallel.children_count.store(child_count as int, Relaxed);
+        }
+
+        // Possibly enqueue the children.
+        if child_count != 0 {
+            for kid in node.children() {
+                proxy.push(WorkUnit {
+                    fun:  top_down_func,
+                    data: layout_node_to_unsafe_layout_node(&kid),
+                });
+            }
+        } else {
+            // If there were no more children, start walking back up.
+            bottom_up_func(unsafe_node, proxy)
+        }
+    }
+}
+
+/// A parallel bottom-up DOM traversal.
+trait ParallelPostorderDOMTraversal : PostorderDOMTraversal {
+    /// Process current node and potentially traverse its ancestors.
+    ///
+    /// If we are the last child that finished processing, recursively process
+    /// our parent. Else, stop. Also, stop at the root.
+    ///
+    /// Thus, if we start with all the leaves of a tree, we end up traversing
+    /// the whole tree bottom-up because each parent will be processed exactly
+    /// once (by the last child that finishes processing).
+    ///
+    /// The only communication between siblings is that they both
+    /// fetch-and-subtract the parent's children count.
+    fn run_parallel(&mut self,
+                    mut unsafe_node: UnsafeLayoutNode,
+                    proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeLayoutNode>) {
+        loop {
+            // Get a real layout node.
+            let node: LayoutNode = unsafe {
+                layout_node_from_unsafe_layout_node(&unsafe_node)
+            };
+
+            // Perform the appropriate traversal.
+            self.process(node);
+
+            let shared_layout_context = unsafe { &**proxy.user_data() };
+            let layout_context = LayoutContext::new(shared_layout_context);
+
+            let parent =
+                match node.layout_parent_node(layout_context.shared) {
+                    None         => break,
+                    Some(parent) => parent,
+                };
+
+            unsafe {
+                let parent_layout_data =
+                    (*parent.borrow_layout_data_unchecked())
+                    .as_ref()
+                    .expect("no layout data");
+
+                unsafe_node = layout_node_to_unsafe_layout_node(&parent);
+
+                let parent_layout_data: &mut LayoutDataWrapper = mem::transmute(parent_layout_data);
+                if parent_layout_data
+                    .data
+                    .parallel
+                    .children_count
+                    .fetch_sub(1, SeqCst) == 1 {
+                    // We were the last child of our parent. Construct flows for our parent.
+                } else {
+                    // Get out of here and find another node to work on.
+                    break
+                }
+            }
+        }
+    }
+}
+
 /// Information that we need stored in each flow.
 pub struct FlowParallelInfo {
     /// The number of children that still need work done.
@@ -131,6 +230,7 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
                     self.process(flow.get_mut());
                 }
 
+
                 let base = flow::mut_base(flow.get_mut());
 
                 // Reset the count of children for the next layout traversal.
@@ -204,9 +304,9 @@ trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
     }
 }
 
-impl<'a> ParallelPostorderFlowTraversal for BubbleISizesTraversal<'a> {}
+impl<'a> ParallelPostorderFlowTraversal for BubbleISizes<'a> {}
 
-impl<'a> ParallelPreorderFlowTraversal for AssignISizesTraversal<'a> {
+impl<'a> ParallelPreorderFlowTraversal for AssignISizes<'a> {
     fn run_parallel(&mut self,
                     unsafe_flow: UnsafeFlow,
                     proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
@@ -217,291 +317,46 @@ impl<'a> ParallelPreorderFlowTraversal for AssignISizesTraversal<'a> {
     }
 }
 
-impl<'a> ParallelPostorderFlowTraversal for AssignBSizesAndStoreOverflowTraversal<'a> {}
-
-/// Every time we do another layout, the old bloom filters are invalid. This is
-/// detected by ticking a generation number every layout.
-type Generation = uint;
-
-/// A pair of the bloom filter used for css selector matching, and the node to
-/// which it applies. This is used to efficiently do `Descendant` selector
-/// matches. Thanks to the bloom filter, we can avoid walking up the tree
-/// looking for ancestors that aren't there in the majority of cases.
-///
-/// As we walk down the DOM tree a task-local bloom filter is built of all the
-/// CSS `SimpleSelector`s which are part of a `Descendant` compound selector
-/// (i.e. paired with a `Descendant` combinator, in the `next` field of a
-/// `CompoundSelector`.
-///
-/// Before a `Descendant` selector match is tried, it's compared against the
-/// bloom filter. If the bloom filter can exclude it, the selector is quickly
-/// rejected.
-///
-/// When done styling a node, all selectors previously inserted into the filter
-/// are removed.
-///
-/// Since a work-stealing queue is used for styling, sometimes, the bloom filter
-/// will no longer be the for the parent of the node we're currently on. When
-/// this happens, the task local bloom filter will be thrown away and rebuilt.
-local_data_key!(style_bloom: (BloomFilter, UnsafeLayoutNode, Generation))
-
-/// Returns the task local bloom filter.
-///
-/// If one does not exist, a new one will be made for you. If it is out of date,
-/// it will be thrown out and a new one will be made for you.
-fn take_task_local_bloom_filter(
-  parent_node: Option<LayoutNode>,
-  layout_context: &LayoutContext)
-      -> BloomFilter {
-
-    let new_bloom =
-        |p: Option<LayoutNode>| -> BloomFilter {
-            let mut bf = BloomFilter::new(style::RECOMMENDED_SELECTOR_BLOOM_FILTER_SIZE);
-            p.map(|p| insert_ancestors_into_bloom_filter(&mut bf, p, layout_context));
-            if p.is_none() {
-                debug!("[{}] No parent, but new bloom filter!", tid());
-            }
-            bf
-        };
-
-    match (parent_node, style_bloom.replace(None)) {
-        // Root node. Needs new bloom filter.
-        (None,     _  ) => new_bloom(None),
-        // No bloom filter for this thread yet.
-        (Some(p), None) => new_bloom(Some(p)),
-        // Found cached bloom filter.
-        (Some(p), Some((bf, old_node, old_generation))) => {
-            // Hey, the cached parent is our parent! We can reuse the bloom filter.
-            if old_node == layout_node_to_unsafe_layout_node(&p) &&
-                old_generation == layout_context.shared.generation {
-                debug!("[{}] Parent matches (={}). Reusing bloom filter.", tid(), old_node.val0());
-                bf
-            // Oh no. the cached parent is stale. I guess we need a new one...
-            } else {
-                new_bloom(Some(p))
-            }
-        },
-    }
-}
+impl<'a> ParallelPostorderFlowTraversal for AssignBSizesAndStoreOverflow<'a> {}
 
-fn put_task_local_bloom_filter(bf: BloomFilter, unsafe_node: &UnsafeLayoutNode, layout_context: &LayoutContext) {
-    match style_bloom.replace(Some((bf, *unsafe_node, layout_context.shared.generation))) {
-        None => {},
-        Some(_) => fail!("Putting into a never-taken task-local bloom filter"),
-    }
-}
+impl<'a> ParallelPostorderDOMTraversal for ConstructFlows<'a> {}
 
-/// "Ancestors" in this context is inclusive of ourselves.
-fn insert_ancestors_into_bloom_filter(
-  bf: &mut BloomFilter, mut n: LayoutNode, layout_context: &LayoutContext) {
-    debug!("[{}] Inserting ancestors.", tid());
-    let mut ancestors = 0u;
-    loop {
-        ancestors += 1;
-
-        n.insert_into_bloom_filter(bf);
-        n = match n.layout_parent_node(layout_context.shared) {
-            None => break,
-            Some(p) => p,
-        };
+impl <'a> ParallelPreorderDOMTraversal for RecalcStyleForNode<'a> {
+    fn run_parallel(&mut self,
+                    unsafe_node: UnsafeLayoutNode,
+                    proxy: &mut WorkerProxy<*const SharedLayoutContext, UnsafeLayoutNode>) {
+        self.run_parallel_helper(unsafe_node,
+                                 proxy,
+                                 recalc_style,
+                                 construct_flows)
     }
-    debug!("[{}] Inserted {} ancestors.", tid(), ancestors);
 }
 
-fn recalc_style_for_node(mut unsafe_layout_node: UnsafeLayoutNode,
-                         proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeLayoutNode>) {
+fn recalc_style(unsafe_node: UnsafeLayoutNode,
+                proxy: &mut WorkerProxy<*const SharedLayoutContext, UnsafeLayoutNode>) {
     let shared_layout_context = unsafe { &**proxy.user_data() };
     let layout_context = LayoutContext::new(shared_layout_context);
-
-    // Get a real layout node.
-    let node: LayoutNode = unsafe {
-        layout_node_from_unsafe_layout_node(&unsafe_layout_node)
+    let mut recalc_style_for_node_traversal = RecalcStyleForNode {
+        layout_context: &layout_context,
     };
-
-    // Initialize layout data.
-    //
-    // FIXME(pcwalton): Stop allocating here. Ideally this should just be done by the HTML
-    // parser.
-    node.initialize_layout_data(layout_context.shared.layout_chan.clone());
-
-    // Get the parent node.
-    let parent_opt = node.layout_parent_node(layout_context.shared);
-
-    // Get the style bloom filter.
-    let bf = take_task_local_bloom_filter(parent_opt, &layout_context);
-
-    // Just needs to be wrapped in an option for `match_node`.
-    let some_bf = Some(bf);
-
-    if node.is_dirty() {
-        // First, check to see whether we can share a style with someone.
-        let style_sharing_candidate_cache = layout_context.style_sharing_candidate_cache();
-        let sharing_result = unsafe {
-            node.share_style_if_possible(style_sharing_candidate_cache,
-                                         parent_opt.clone())
-        };
-        // Otherwise, match and cascade selectors.
-        match sharing_result {
-            CannotShare(mut shareable) => {
-                let mut applicable_declarations = ApplicableDeclarations::new();
-
-                if node.is_element() {
-                    // Perform the CSS selector matching.
-                    let stylist = unsafe { &*layout_context.shared.stylist };
-                    node.match_node(stylist, &some_bf, &mut applicable_declarations, &mut shareable);
-                }
-
-                // Perform the CSS cascade.
-                unsafe {
-                    node.cascade_node(parent_opt,
-                                      &applicable_declarations,
-                                      layout_context.applicable_declarations_cache());
-                }
-
-                // Add ourselves to the LRU cache.
-                if shareable {
-                    style_sharing_candidate_cache.insert_if_possible(&node);
-                }
-            }
-            StyleWasShared(index) => style_sharing_candidate_cache.touch(index),
-        }
-    }
-
-    // Prepare for flow construction by counting the node's children and storing that count.
-    let mut child_count = 0u;
-    for _ in node.children() {
-        child_count += 1;
-    }
-    if child_count != 0 {
-        let mut layout_data_ref = node.mutate_layout_data();
-        match &mut *layout_data_ref {
-            &Some(ref mut layout_data) => {
-                layout_data.data.parallel.children_count.store(child_count as int, Relaxed)
-            }
-            &None => fail!("no layout data"),
-        }
-    }
-
-    // It can be `None` now.
-    let mut bf = some_bf;
-
-    // Before running the children, we need to insert our nodes into the bloom
-    // filter.
-    debug!("[{}] + {:X}", tid(), unsafe_layout_node.val0());
-    bf.as_mut().map(|bf| node.insert_into_bloom_filter(bf));
-
-    // It's *very* important that this block is in a separate scope to the block above,
-    // to avoid a data race that can occur (github issue #2308). The block above issues
-    // a borrow on the node layout data. That borrow must be dropped before the child
-    // nodes are actually pushed into the work queue. Otherwise, it's possible for a child
-    // node to get into construct_flows() and move up it's parent hierarchy, which can call
-    // borrow on the layout data before it is dropped from the block above.
-    if child_count != 0 {
-        // Enqueue kids.
-        for kid in node.children() {
-            proxy.push(WorkUnit {
-                fun: recalc_style_for_node,
-                data: layout_node_to_unsafe_layout_node(&kid),
-            });
-        }
-    } else {
-        // If we got here, we're a leaf. Start construction of flows for this node.
-        construct_flows(&mut unsafe_layout_node, &mut bf, &layout_context);
-    }
-
-    bf.map(|bf| put_task_local_bloom_filter(bf, &unsafe_layout_node, &layout_context));
+    recalc_style_for_node_traversal.run_parallel(unsafe_node, proxy)
 }
 
-fn construct_flows<'a>(unsafe_layout_node: &mut UnsafeLayoutNode,
-                       parent_bf: &mut Option<BloomFilter>,
-                       layout_context: &'a LayoutContext<'a>) {
-    loop {
-        // Get a real layout node.
-        let node: LayoutNode = unsafe {
-            layout_node_from_unsafe_layout_node(&*unsafe_layout_node)
-        };
-
-        // Construct flows for this node.
-        {
-            let node = ThreadSafeLayoutNode::new(&node);
-            let mut flow_constructor = FlowConstructor::new(layout_context);
-            flow_constructor.process(&node);
-
-            // Reset the layout damage in this node. It's been propagated to the
-            // flow by the flow constructor.
-            node.set_restyle_damage(RestyleDamage::empty());
-        }
-
-        unsafe {
-            node.set_dirty(false);
-            node.set_dirty_descendants(false);
-        }
-
-        // Reset the count of children for the next traversal.
-        //
-        // FIXME(pcwalton): Use children().len() when the implementation of that is efficient.
-        let mut child_count = 0u;
-        for _ in node.children() {
-            child_count += 1
-        }
-        {
-            let mut layout_data_ref = node.mutate_layout_data();
-            match &mut *layout_data_ref {
-                &Some(ref mut layout_data) => {
-                    layout_data.data.parallel.children_count.store(child_count as int, Relaxed)
-                }
-                &None => fail!("no layout data"),
-            }
-        }
-
-        // If this is the reflow root, we're done.
-        let opaque_node: OpaqueNode = OpaqueNodeMethods::from_layout_node(&node);
-        if layout_context.shared.reflow_root == opaque_node {
-            debug!("[{}] - {:X}, and deleting BF.", tid(), unsafe_layout_node.val0());
-            *parent_bf = None;
-            break;
-        } else {
-            debug!("[{}] - {:X}", tid(), unsafe_layout_node.val0());
-            parent_bf.as_mut().map(|parent_bf| node.remove_from_bloom_filter(parent_bf));
-        }
-
-        // Otherwise, enqueue the parent.
-        match node.parent_node() {
-            Some(parent) => {
-
-                // No, we're not at the root yet. Then are we the last sibling of our parent?
-                // If so, we can continue on with our parent; otherwise, we've gotta wait.
-                unsafe {
-                    match *parent.borrow_layout_data_unchecked() {
-                        Some(ref parent_layout_data) => {
-                            *unsafe_layout_node = layout_node_to_unsafe_layout_node(&parent);
-
-                            let parent_layout_data: &mut LayoutDataWrapper = mem::transmute(parent_layout_data);
-                            if parent_layout_data.data
-                                                 .parallel
-                                                 .children_count
-                                                 .fetch_sub(1, SeqCst) == 1 {
-                                // We were the last child of our parent. Construct flows for our
-                                // parent.
-                            } else {
-                                // Get out of here and find another node to work on.
-                                break
-                            }
-                        }
-                        None => fail!("no layout data for parent?!"),
-                    }
-                }
-            }
-            None => fail!("no parent and weren't at reflow root?!"),
-        }
-    }
+fn construct_flows(unsafe_node: UnsafeLayoutNode,
+                   proxy: &mut WorkerProxy<*const SharedLayoutContext, UnsafeLayoutNode>) {
+    let shared_layout_context = unsafe { &**proxy.user_data() };
+    let layout_context = LayoutContext::new(shared_layout_context);
+    let mut construct_flows_traversal = ConstructFlows {
+        layout_context: &layout_context,
+    };
+    construct_flows_traversal.run_parallel(unsafe_node, proxy)
 }
 
 fn assign_inline_sizes(unsafe_flow: UnsafeFlow,
-                 proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
+                       proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
     let shared_layout_context = unsafe { &**proxy.user_data() };
     let layout_context = LayoutContext::new(shared_layout_context);
-    let mut assign_inline_sizes_traversal = AssignISizesTraversal {
+    let mut assign_inline_sizes_traversal = AssignISizes {
         layout_context: &layout_context,
     };
     assign_inline_sizes_traversal.run_parallel(unsafe_flow, proxy)
@@ -511,7 +366,7 @@ fn assign_block_sizes_and_store_overflow(unsafe_flow: UnsafeFlow,
                                      proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
     let shared_layout_context = unsafe { &**proxy.user_data() };
     let layout_context = LayoutContext::new(shared_layout_context);
-    let mut assign_block_sizes_traversal = AssignBSizesAndStoreOverflowTraversal {
+    let mut assign_block_sizes_traversal = AssignBSizesAndStoreOverflow {
         layout_context: &layout_context,
     };
     assign_block_sizes_traversal.run_parallel(unsafe_flow, proxy)
@@ -629,21 +484,19 @@ fn build_display_list(mut unsafe_flow: UnsafeFlow,
     }
 }
 
-pub fn recalc_style_for_subtree(root_node: &LayoutNode,
-                                shared_layout_context: &SharedLayoutContext,
-                                queue: &mut WorkQueue<*const SharedLayoutContext,UnsafeLayoutNode>) {
-    debug!("[{}] Style Recalc START", tid());
+pub fn traverse_dom_preorder(root: LayoutNode,
+                             shared_layout_context: &SharedLayoutContext,
+                             queue: &mut WorkQueue<*const SharedLayoutContext, UnsafeLayoutNode>) {
     queue.data = shared_layout_context as *const _;
 
-    // Enqueue the root node.
     queue.push(WorkUnit {
-        fun: recalc_style_for_node,
-        data: layout_node_to_unsafe_layout_node(root_node),
+        fun:  recalc_style,
+        data: layout_node_to_unsafe_layout_node(&root),
     });
 
     queue.run();
 
-    queue.data = ptr::null()
+    queue.data = ptr::null();
 }
 
 pub fn traverse_flow_tree_preorder(root: &mut FlowRef,
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
new file mode 100644
index 00000000000..a4417b55d45
--- /dev/null
+++ b/components/layout/traversal.rs
@@ -0,0 +1,333 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Traversals over the DOM and flow trees, running the layout computations.
+
+use css::node_style::StyledNode;
+use css::matching::{ApplicableDeclarations, CannotShare, MatchMethods, StyleWasShared};
+use construct::FlowConstructor;
+use context::LayoutContext;
+use flow::{Flow, MutableFlowUtils, PreorderFlowTraversal, PostorderFlowTraversal};
+use flow;
+use incremental::RestyleDamage;
+use wrapper::{layout_node_to_unsafe_layout_node, LayoutNode};
+use wrapper::{PostorderNodeMutTraversal, ThreadSafeLayoutNode, UnsafeLayoutNode};
+use wrapper::{PreorderDOMTraversal, PostorderDOMTraversal};
+
+use servo_util::bloom::BloomFilter;
+use servo_util::tid::tid;
+use style;
+use style::TNode;
+
+/// Every time we do another layout, the old bloom filters are invalid. This is
+/// detected by ticking a generation number every layout.
+type Generation = uint;
+
+/// A pair of the bloom filter used for css selector matching, and the node to
+/// which it applies. This is used to efficiently do `Descendant` selector
+/// matches. Thanks to the bloom filter, we can avoid walking up the tree
+/// looking for ancestors that aren't there in the majority of cases.
+///
+/// As we walk down the DOM tree a task-local bloom filter is built of all the
+/// CSS `SimpleSelector`s which are part of a `Descendant` compound selector
+/// (i.e. paired with a `Descendant` combinator, in the `next` field of a
+/// `CompoundSelector`.
+///
+/// Before a `Descendant` selector match is tried, it's compared against the
+/// bloom filter. If the bloom filter can exclude it, the selector is quickly
+/// rejected.
+///
+/// When done styling a node, all selectors previously inserted into the filter
+/// are removed.
+///
+/// Since a work-stealing queue is used for styling, sometimes, the bloom filter
+/// will no longer be the for the parent of the node we're currently on. When
+/// this happens, the task local bloom filter will be thrown away and rebuilt.
+local_data_key!(style_bloom: (BloomFilter, UnsafeLayoutNode, Generation))
+
+/// Returns the task local bloom filter.
+///
+/// If one does not exist, a new one will be made for you. If it is out of date,
+/// it will be thrown out and a new one will be made for you.
+fn take_task_local_bloom_filter(
+  parent_node: Option<LayoutNode>,
+  layout_context: &LayoutContext)
+      -> BloomFilter {
+
+    let new_bloom =
+        |p: Option<LayoutNode>| -> BloomFilter {
+            let mut bf = BloomFilter::new(style::RECOMMENDED_SELECTOR_BLOOM_FILTER_SIZE);
+            p.map(|p| insert_ancestors_into_bloom_filter(&mut bf, p, layout_context));
+            if p.is_none() {
+                debug!("[{}] No parent, but new bloom filter!", tid());
+            }
+            bf
+        };
+
+    match (parent_node, style_bloom.replace(None)) {
+        // Root node. Needs new bloom filter.
+        (None,     _  ) => new_bloom(None),
+        // No bloom filter for this thread yet.
+        (Some(p), None) => new_bloom(Some(p)),
+        // Found cached bloom filter.
+        (Some(p), Some((bf, old_node, old_generation))) => {
+            // Hey, the cached parent is our parent! We can reuse the bloom filter.
+            if old_node == layout_node_to_unsafe_layout_node(&p) &&
+                old_generation == layout_context.shared.generation {
+                debug!("[{}] Parent matches (={}). Reusing bloom filter.", tid(), old_node.val0());
+                bf
+            // Oh no. the cached parent is stale. I guess we need a new one...
+            } else {
+                new_bloom(Some(p))
+            }
+        },
+    }
+}
+
+fn put_task_local_bloom_filter(bf: BloomFilter, unsafe_node: &UnsafeLayoutNode, layout_context: &LayoutContext) {
+    match style_bloom.replace(Some((bf, *unsafe_node, layout_context.shared.generation))) {
+        None => {},
+        Some(_) => fail!("Putting into a never-taken task-local bloom filter"),
+    }
+}
+
+/// "Ancestors" in this context is inclusive of ourselves.
+fn insert_ancestors_into_bloom_filter(
+  bf: &mut BloomFilter, mut n: LayoutNode, layout_context: &LayoutContext) {
+    debug!("[{}] Inserting ancestors.", tid());
+    let mut ancestors = 0u;
+    loop {
+        ancestors += 1;
+
+        n.insert_into_bloom_filter(bf);
+        n = match n.layout_parent_node(layout_context.shared) {
+            None => break,
+            Some(p) => p,
+        };
+    }
+    debug!("[{}] Inserted {} ancestors.", tid(), ancestors);
+}
+
+/// The recalc-style-for-node traversal, which styles each node and must run before
+/// layout computation. This computes the styles applied to each node.
+pub struct RecalcStyleForNode<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> PreorderDOMTraversal for RecalcStyleForNode<'a> {
+    #[inline]
+    fn process(&self, node: LayoutNode) {
+        // Initialize layout data.
+        //
+        // FIXME(pcwalton): Stop allocating here. Ideally this should just be done by the HTML
+        // parser.
+        node.initialize_layout_data(self.layout_context.shared.layout_chan.clone());
+
+        // Get the parent node.
+        let parent_opt = node.layout_parent_node(self.layout_context.shared);
+
+        // Get the style bloom filter.
+        let bf = take_task_local_bloom_filter(parent_opt, self.layout_context);
+
+        // Just needs to be wrapped in an option for `match_node`.
+        let some_bf = Some(bf);
+
+        if node.is_dirty() {
+            // First, check to see whether we can share a style with someone.
+            let style_sharing_candidate_cache = self.layout_context.style_sharing_candidate_cache();
+            let sharing_result = unsafe {
+                node.share_style_if_possible(style_sharing_candidate_cache,
+                                             parent_opt.clone())
+            };
+            // Otherwise, match and cascade selectors.
+            match sharing_result {
+                CannotShare(mut shareable) => {
+                    let mut applicable_declarations = ApplicableDeclarations::new();
+
+                    if node.is_element() {
+                        // Perform the CSS selector matching.
+                        let stylist = unsafe { &*self.layout_context.shared.stylist };
+                        node.match_node(stylist, &some_bf, &mut applicable_declarations, &mut shareable);
+                    }
+
+                    // Perform the CSS cascade.
+                    unsafe {
+                        node.cascade_node(parent_opt,
+                                          &applicable_declarations,
+                                          self.layout_context.applicable_declarations_cache());
+                    }
+
+                    // Add ourselves to the LRU cache.
+                    if shareable {
+                        style_sharing_candidate_cache.insert_if_possible(&node);
+                    }
+                }
+                StyleWasShared(index) => style_sharing_candidate_cache.touch(index),
+            }
+        }
+
+        let mut bf = some_bf.unwrap();
+
+        let unsafe_layout_node = layout_node_to_unsafe_layout_node(&node);
+
+        // Before running the children, we need to insert our nodes into the bloom
+        // filter.
+        debug!("[{}] + {:X}", tid(), unsafe_layout_node.val0());
+        node.insert_into_bloom_filter(&mut bf);
+
+        // NB: flow construction updates the bloom filter on the way up.
+        put_task_local_bloom_filter(bf, &unsafe_layout_node, self.layout_context);
+    }
+}
+
+/// The flow construction traversal, which builds flows for styled nodes.
+pub struct ConstructFlows<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> PostorderDOMTraversal for ConstructFlows<'a> {
+    #[inline]
+    fn process(&self, node: LayoutNode) {
+        // Construct flows for this node.
+        {
+            let node = ThreadSafeLayoutNode::new(&node);
+            let mut flow_constructor = FlowConstructor::new(self.layout_context);
+            flow_constructor.process(&node);
+
+            // Reset the layout damage in this node. It's been propagated to the
+            // flow by the flow constructor.
+            node.set_restyle_damage(RestyleDamage::empty());
+        }
+
+        unsafe {
+            node.set_dirty(false);
+            node.set_dirty_descendants(false);
+        }
+
+        let unsafe_layout_node = layout_node_to_unsafe_layout_node(&node);
+
+        let (mut bf, old_node, old_generation) =
+            style_bloom
+            .replace(None)
+            .expect("The bloom filter should have been set by style recalc.");
+
+        assert_eq!(old_node, unsafe_layout_node);
+        assert_eq!(old_generation, self.layout_context.shared.generation);
+
+        match node.layout_parent_node(self.layout_context.shared) {
+            None => {
+                debug!("[{}] - {:X}, and deleting BF.", tid(), unsafe_layout_node.val0());
+                // If this is the reflow root, eat the task-local bloom filter.
+            }
+            Some(parent) => {
+                // Otherwise, put it back, but remove this node.
+                node.remove_from_bloom_filter(&mut bf);
+                let unsafe_parent = layout_node_to_unsafe_layout_node(&parent);
+                put_task_local_bloom_filter(bf, &unsafe_parent, self.layout_context);
+            },
+        };
+    }
+}
+
+/// The flow tree verification traversal. This is only on in debug builds.
+#[cfg(debug)]
+struct FlowTreeVerification;
+
+#[cfg(debug)]
+impl PreorderFlow for FlowTreeVerification {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        let base = flow::base(flow);
+        if !base.flags.is_leaf() && !base.flags.is_nonleaf() {
+            println("flow tree verification failed: flow wasn't a leaf or a nonleaf!");
+            flow.dump();
+            fail!("flow tree verification failed")
+        }
+        true
+    }
+}
+
+/// The bubble-inline-sizes traversal, the first part of layout computation. This computes preferred
+/// and intrinsic inline-sizes and bubbles them up the tree.
+pub struct BubbleISizes<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> PostorderFlowTraversal for BubbleISizes<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.bubble_inline_sizes(self.layout_context);
+        true
+    }
+
+    // FIXME: We can't prune until we start reusing flows
+    /*
+    #[inline]
+    fn should_prune(&mut self, flow: &mut Flow) -> bool {
+        flow::mut_base(flow).restyle_damage.lacks(BubbleISizes)
+    }
+    */
+}
+
+/// The assign-inline-sizes traversal. In Gecko this corresponds to `Reflow`.
+pub struct AssignISizes<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> PreorderFlowTraversal for AssignISizes<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.assign_inline_sizes(self.layout_context);
+        true
+    }
+}
+
+/// The assign-block-sizes-and-store-overflow traversal, the last (and most expensive) part of layout
+/// computation. Determines the final block-sizes for all layout objects, computes positions, and
+/// computes overflow regions. In Gecko this corresponds to `FinishAndStoreOverflow`.
+pub struct AssignBSizesAndStoreOverflow<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> PostorderFlowTraversal for AssignBSizesAndStoreOverflow<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.assign_block_size(self.layout_context);
+        // Skip store-overflow for absolutely positioned flows. That will be
+        // done in a separate traversal.
+        if !flow.is_store_overflow_delayed() {
+            flow.store_overflow(self.layout_context);
+        }
+        true
+    }
+
+    #[inline]
+    fn should_process(&mut self, flow: &mut Flow) -> bool {
+        !flow::base(flow).flags.impacted_by_floats()
+    }
+}
+
+/// The display list construction traversal.
+pub struct BuildDisplayList<'a> {
+    pub layout_context: &'a LayoutContext<'a>,
+}
+
+impl<'a> BuildDisplayList<'a> {
+    #[inline]
+    pub fn process(&mut self, flow: &mut Flow) {
+        flow.compute_absolute_position();
+
+        for kid in flow::mut_base(flow).child_iter() {
+            if !kid.is_absolutely_positioned() {
+                self.process(kid)
+            }
+        }
+
+        for absolute_descendant_link in flow::mut_base(flow).abs_descendants.iter() {
+            self.process(absolute_descendant_link)
+        }
+
+        flow.build_display_list(self.layout_context)
+    }
+}
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index 2cc58e6e7b0..0a10bf0315d 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -888,3 +888,15 @@ pub unsafe fn layout_node_from_unsafe_layout_node(node: &UnsafeLayoutNode) -> La
     let (node, _) = *node;
     mem::transmute(node)
 }
+
+/// A top-down traversal.
+pub trait PreorderDOMTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&self, _node: LayoutNode);
+}
+
+/// A bottom-up traversal, with a optional in-order pass.
+pub trait PostorderDOMTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&self, _node: LayoutNode);
+}