Auto merge of #9176 - bholley:hoist_traversal, r=SimonSapin

Hoist traversal code into style/

This should be the last major hoisting necessary to use the style system standalone. \o/

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11 files changed, 517 insertions, 407 deletions

diff --git a/components/layout/context.rs b/components/layout/context.rs
index a10fe334730..43c51147934 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -128,16 +128,6 @@ impl<'a> LayoutContext<'a> {
         self.cached_local_layout_context.font_context.borrow_mut()
     }
 
-    #[inline(always)]
-    pub fn applicable_declarations_cache(&self) -> RefMut<ApplicableDeclarationsCache> {
-        self.local_context().applicable_declarations_cache.borrow_mut()
-    }
-
-    #[inline(always)]
-    pub fn style_sharing_candidate_cache(&self) -> RefMut<StyleSharingCandidateCache> {
-        self.local_context().style_sharing_candidate_cache.borrow_mut()
-    }
-
     pub fn get_or_request_image(&self, url: Url, use_placeholder: UsePlaceholder)
                                 -> Option<Arc<Image>> {
         // See if the image is already available
diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index 0015cd3fc33..7c39d61db22 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -38,7 +38,7 @@ use msg::ParseErrorReporter;
 use msg::compositor_msg::Epoch;
 use msg::constellation_msg::{ConstellationChan, Failure, PipelineId};
 use net_traits::image_cache_task::{ImageCacheChan, ImageCacheResult, ImageCacheTask};
-use parallel::{self, WorkQueueData};
+use parallel;
 use profile_traits::mem::{self, Report, ReportKind, ReportsChan};
 use profile_traits::time::{TimerMetadataFrameType, TimerMetadataReflowType};
 use profile_traits::time::{self, TimerMetadata, profile};
@@ -67,6 +67,7 @@ use style::computed_values::{filter, mix_blend_mode};
 use style::context::{SharedStyleContext, StylistWrapper};
 use style::dom::{TDocument, TElement, TNode};
 use style::media_queries::{Device, MediaType};
+use style::parallel::WorkQueueData;
 use style::selector_matching::{Stylist, USER_OR_USER_AGENT_STYLESHEETS};
 use style::stylesheets::{CSSRuleIteratorExt, Stylesheet};
 use traversal::RecalcStyleAndConstructFlows;
@@ -1025,11 +1026,11 @@ impl LayoutTask {
                 // Perform CSS selector matching and flow construction.
                 match self.parallel_traversal {
                     None => {
-                        sequential::traverse_dom_preorder::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
+                        sequential::traverse_dom::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
                             node, &shared_layout_context);
                     }
                     Some(ref mut traversal) => {
-                        parallel::traverse_dom_preorder::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
+                        parallel::traverse_dom::<ServoLayoutNode, RecalcStyleAndConstructFlows>(
                             node, &shared_layout_context, traversal);
                     }
                 }
diff --git a/components/layout/parallel.rs b/components/layout/parallel.rs
index fa52a7efdb2..4ea427b11ee 100644
--- a/components/layout/parallel.rs
+++ b/components/layout/parallel.rs
@@ -11,28 +11,23 @@
 use context::{LayoutContext, SharedLayoutContext};
 use flow::{self, Flow, MutableFlowUtils, PostorderFlowTraversal, PreorderFlowTraversal};
 use flow_ref::{self, FlowRef};
-use gfx::display_list::OpaqueNode;
 use profile_traits::time::{self, TimerMetadata, profile};
 use std::mem;
 use std::sync::atomic::{AtomicIsize, Ordering};
-use style::dom::UnsafeNode;
-use traversal::PostorderNodeMutTraversal;
+use style::dom::{TNode, UnsafeNode};
+use style::parallel::{CHUNK_SIZE, WorkQueueData};
+use style::parallel::{run_queue_with_custom_work_data_type};
 use traversal::{AssignBSizesAndStoreOverflow, AssignISizes, BubbleISizes};
-use traversal::{BuildDisplayList, ComputeAbsolutePositions};
-use traversal::{DomTraversal, DomTraversalContext};
+use traversal::{BuildDisplayList, ComputeAbsolutePositions, PostorderNodeMutTraversal};
 use util::opts;
 use util::workqueue::{WorkQueue, WorkUnit, WorkerProxy};
-use wrapper::LayoutNode;
 
-const CHUNK_SIZE: usize = 64;
-
-pub struct WorkQueueData(usize, usize);
+pub use style::parallel::traverse_dom;
 
 #[allow(dead_code)]
 fn static_assertion(node: UnsafeNode) {
     unsafe {
         let _: UnsafeFlow = ::std::intrinsics::transmute(node);
-        let _: UnsafeNodeList = ::std::intrinsics::transmute(node);
     }
 }
 
@@ -55,18 +50,8 @@ pub fn borrowed_flow_to_unsafe_flow(flow: &Flow) -> UnsafeFlow {
     }
 }
 
-pub type UnsafeNodeList = (Box<Vec<UnsafeNode>>, OpaqueNode);
-
 pub type UnsafeFlowList = (Box<Vec<UnsafeNode>>, usize);
 
-pub type ChunkedDomTraversalFunction =
-    extern "Rust" fn(UnsafeNodeList,
-                     &mut WorkerProxy<SharedLayoutContext, UnsafeNodeList>);
-
-pub type DomTraversalFunction =
-    extern "Rust" fn(OpaqueNode, UnsafeNode,
-                     &mut WorkerProxy<SharedLayoutContext, UnsafeNodeList>);
-
 pub type ChunkedFlowTraversalFunction =
     extern "Rust" fn(UnsafeFlowList, &mut WorkerProxy<SharedLayoutContext, UnsafeFlowList>);
 
@@ -232,104 +217,6 @@ impl<'a> ParallelPreorderFlowTraversal for ComputeAbsolutePositions<'a> {
 
 impl<'a> ParallelPostorderFlowTraversal for BuildDisplayList<'a> {}
 
-/// A parallel top-down DOM traversal.
-#[inline(always)]
-fn top_down_dom<'ln, N, T>(unsafe_nodes: UnsafeNodeList,
-                           proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeNodeList>)
-                           where N: LayoutNode<'ln>, T: DomTraversal<'ln, N> {
-    let shared_layout_context = proxy.user_data();
-    let layout_context = LayoutContext::new(shared_layout_context);
-    let traversal_context = DomTraversalContext {
-        layout_context: &layout_context,
-        root: unsafe_nodes.1,
-    };
-
-    let mut discovered_child_nodes = Vec::new();
-    for unsafe_node in *unsafe_nodes.0 {
-        // Get a real layout node.
-        let node = unsafe { N::from_unsafe(&unsafe_node) };
-
-        // Perform the appropriate traversal.
-        T::process_preorder(&traversal_context, node);
-
-        let child_count = node.children_count();
-
-        // Reset the count of children.
-        {
-            let data = node.mutate_data().unwrap();
-            data.parallel.children_count.store(child_count as isize,
-                                               Ordering::Relaxed);
-        }
-
-        // Possibly enqueue the children.
-        if child_count != 0 {
-            for kid in node.children() {
-                discovered_child_nodes.push(kid.to_unsafe())
-            }
-        } else {
-            // If there were no more children, start walking back up.
-            bottom_up_dom::<N, T>(unsafe_nodes.1, unsafe_node, proxy)
-        }
-    }
-
-    for chunk in discovered_child_nodes.chunks(CHUNK_SIZE) {
-        proxy.push(WorkUnit {
-            fun:  top_down_dom::<N, T>,
-            data: (box chunk.iter().cloned().collect(), unsafe_nodes.1),
-        });
-    }
-}
-
-/// 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 bottom_up_dom<'ln, N, T>(root: OpaqueNode,
-                            unsafe_node: UnsafeNode,
-                            proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeNodeList>)
-                            where N: LayoutNode<'ln>, T: DomTraversal<'ln, N> {
-    let shared_layout_context = proxy.user_data();
-    let layout_context = LayoutContext::new(shared_layout_context);
-    let traversal_context = DomTraversalContext {
-        layout_context: &layout_context,
-        root: root,
-    };
-
-    // Get a real layout node.
-    let mut node = unsafe { N::from_unsafe(&unsafe_node) };
-    loop {
-        // Perform the appropriate operation.
-        T::process_postorder(&traversal_context, node);
-
-        let parent = match node.layout_parent_node(traversal_context.root) {
-            None => break,
-            Some(parent) => parent,
-        };
-
-        let parent_data = unsafe {
-            &*parent.borrow_data_unchecked().unwrap()
-        };
-
-        if parent_data
-            .parallel
-            .children_count
-            .fetch_sub(1, Ordering::Relaxed) != 1 {
-            // Get out of here and find another node to work on.
-            break
-        }
-
-        // We were the last child of our parent. Construct flows for our parent.
-        node = parent;
-    }
-}
-
 fn assign_inline_sizes(unsafe_flows: UnsafeFlowList,
                        proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeFlowList>) {
     let shared_layout_context = proxy.user_data();
@@ -372,31 +259,6 @@ fn build_display_list(unsafe_flow: UnsafeFlow,
     build_display_list_traversal.run_parallel(unsafe_flow);
 }
 
-fn run_queue_with_custom_work_data_type<To, F>(
-        queue: &mut WorkQueue<SharedLayoutContext, WorkQueueData>,
-        callback: F,
-        shared_layout_context: &SharedLayoutContext)
-        where To: 'static + Send, F: FnOnce(&mut WorkQueue<SharedLayoutContext, To>) {
-    let queue: &mut WorkQueue<SharedLayoutContext, To> = unsafe {
-        mem::transmute(queue)
-    };
-    callback(queue);
-    queue.run(shared_layout_context);
-}
-
-pub fn traverse_dom_preorder<'ln, N, T>(
-                             root: N,
-                             shared_layout_context: &SharedLayoutContext,
-                             queue: &mut WorkQueue<SharedLayoutContext, WorkQueueData>)
-                             where N: LayoutNode<'ln>, T: DomTraversal<'ln, N> {
-    run_queue_with_custom_work_data_type(queue, |queue| {
-        queue.push(WorkUnit {
-            fun:  top_down_dom::<N, T>,
-            data: (box vec![root.to_unsafe()], root.opaque()),
-        });
-    }, shared_layout_context);
-}
-
 pub fn traverse_flow_tree_preorder(
         root: &mut FlowRef,
         profiler_metadata: Option<TimerMetadata>,
diff --git a/components/layout/sequential.rs b/components/layout/sequential.rs
index 310560e2fa7..ad332f30fdd 100644
--- a/components/layout/sequential.rs
+++ b/components/layout/sequential.rs
@@ -12,36 +12,13 @@ use flow::{self, Flow, ImmutableFlowUtils, InorderFlowTraversal, MutableFlowUtil
 use flow_ref::{self, FlowRef};
 use fragment::FragmentBorderBoxIterator;
 use generated_content::ResolveGeneratedContent;
-use traversal::PostorderNodeMutTraversal;
+use style::dom::TNode;
+use style::traversal::DomTraversalContext;
 use traversal::{AssignBSizesAndStoreOverflow, AssignISizes};
-use traversal::{BubbleISizes, BuildDisplayList, ComputeAbsolutePositions};
-use traversal::{DomTraversal, DomTraversalContext};
+use traversal::{BubbleISizes, BuildDisplayList, ComputeAbsolutePositions, PostorderNodeMutTraversal};
 use util::opts;
-use wrapper::LayoutNode;
-
-pub fn traverse_dom_preorder<'ln, N, T>(root: N,
-                                        shared_layout_context: &SharedLayoutContext)
-                                        where N: LayoutNode<'ln>,
-                                              T: DomTraversal<'ln, N> {
-    fn doit<'a, 'ln, N, T>(context: &'a DomTraversalContext<'a>, node: N)
-                           where N: LayoutNode<'ln>, T: DomTraversal<'ln, N> {
-        T::process_preorder(context, node);
-
-        for kid in node.children() {
-            doit::<N, T>(context, kid);
-        }
-
-        T::process_postorder(context, node);
-    }
 
-    let layout_context = LayoutContext::new(shared_layout_context);
-    let traversal_context = DomTraversalContext {
-        layout_context: &layout_context,
-        root: root.opaque(),
-    };
-
-    doit::<N, T>(&traversal_context, root);
-}
+pub use style::sequential::traverse_dom;
 
 pub fn resolve_generated_content(root: &mut FlowRef, shared_layout_context: &SharedLayoutContext) {
     fn doit(flow: &mut Flow, level: u32, traversal: &mut ResolveGeneratedContent) {
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index b740e7dbcf3..c42ba491732 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -5,142 +5,68 @@
 //! Traversals over the DOM and flow trees, running the layout computations.
 
 use construct::FlowConstructor;
-use context::LayoutContext;
+use context::{LayoutContext, SharedLayoutContext};
 use flow::{PostorderFlowTraversal, PreorderFlowTraversal};
 use flow::{self, Flow};
 use gfx::display_list::OpaqueNode;
 use incremental::{BUBBLE_ISIZES, REFLOW, REFLOW_OUT_OF_FLOW, REPAINT, RestyleDamage};
 use script::layout_interface::ReflowGoal;
-use selectors::bloom::BloomFilter;
-use std::cell::RefCell;
 use std::mem;
 use style::context::StyleContext;
-use style::dom::{TRestyleDamage, UnsafeNode};
-use style::matching::{ApplicableDeclarations, ElementMatchMethods, MatchMethods, StyleSharingResult};
+use style::matching::MatchMethods;
+use style::traversal::{DomTraversalContext, STYLE_BLOOM};
+use style::traversal::{put_task_local_bloom_filter, recalc_style_at};
 use util::opts;
 use util::tid::tid;
 use wrapper::{LayoutNode, ThreadSafeLayoutNode};
 
-/// Every time we do another layout, the old bloom filters are invalid. This is
-/// detected by ticking a generation number every layout.
-type Generation = u32;
-
-/// 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.
-thread_local!(
-    static STYLE_BLOOM: RefCell<Option<(Box<BloomFilter>, UnsafeNode, Generation)>> = RefCell::new(None));
-
-/// 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 cleared and reused.
-fn take_task_local_bloom_filter<'ln, N>(parent_node: Option<N>,
-                                        root: OpaqueNode,
-                                        layout_context: &LayoutContext)
-                                        -> Box<BloomFilter>
-                                        where N: LayoutNode<'ln> {
-    STYLE_BLOOM.with(|style_bloom| {
-        match (parent_node, style_bloom.borrow_mut().take()) {
-            // Root node. Needs new bloom filter.
-            (None,     _  ) => {
-                debug!("[{}] No parent, but new bloom filter!", tid());
-                box BloomFilter::new()
-            }
-            // No bloom filter for this thread yet.
-            (Some(parent), None) => {
-                let mut bloom_filter = box BloomFilter::new();
-                insert_ancestors_into_bloom_filter(&mut bloom_filter, parent, root);
-                bloom_filter
-            }
-            // Found cached bloom filter.
-            (Some(parent), Some((mut bloom_filter, old_node, old_generation))) => {
-                if old_node == parent.to_unsafe() &&
-                    old_generation == layout_context.shared_context().generation {
-                    // Hey, the cached parent is our parent! We can reuse the bloom filter.
-                    debug!("[{}] Parent matches (={}). Reusing bloom filter.", tid(), old_node.0);
-                } else {
-                    // Oh no. the cached parent is stale. I guess we need a new one. Reuse the existing
-                    // allocation to avoid malloc churn.
-                    bloom_filter.clear();
-                    insert_ancestors_into_bloom_filter(&mut bloom_filter, parent, root);
-                }
-                bloom_filter
-            },
+pub struct RecalcStyleAndConstructFlows<'lc> {
+    context: LayoutContext<'lc>,
+    root: OpaqueNode,
+}
+
+impl<'lc, 'ln, N: LayoutNode<'ln>> DomTraversalContext<'ln, N> for RecalcStyleAndConstructFlows<'lc> {
+    type SharedContext = SharedLayoutContext;
+    #[allow(unsafe_code)]
+    fn new<'a>(shared: &'a Self::SharedContext, root: OpaqueNode) -> Self {
+        // FIXME(bholley): This transmutation from &'a to &'lc is very unfortunate, but I haven't
+        // found a way to avoid it despite spending several days on it (and consulting Manishearth,
+        // brson, and nmatsakis).
+        //
+        // The crux of the problem is that parameterizing DomTraversalContext on the lifetime of
+        // the SharedContext doesn't work for a variety of reasons [1]. However, the code in
+        // parallel.rs needs to be able to use the DomTraversalContext trait (or something similar)
+        // to stack-allocate a struct (a generalized LayoutContext<'a>) that holds a borrowed
+        // SharedContext, which means that the struct needs to be parameterized on a lifetime.
+        // Given the aforementioned constraint, the only way to accomplish this is to avoid
+        // propagating the borrow lifetime from the struct to the trait, but that means that the
+        // new() method on the trait cannot require the lifetime of its argument to match the
+        // lifetime of the Self object it creates.
+        //
+        // This could be solved with an associated type with an unbound lifetime parameter, but
+        // that would require higher-kinded types, which don't exist yet and probably aren't coming
+        // for a while.
+        //
+        // So we transmute. :-( This is safe because the DomTravesalContext is stack-allocated on
+        // the worker thread while processing a WorkUnit, whereas the borrowed SharedContext is
+        // live for the entire duration of the restyle. This really could _almost_ compile: all
+        // we'd need to do is change the signature to to |new<'a: 'lc>|, and everything would
+        // work great. But we can't do that, because that would cause a mismatch with the signature
+        // in the trait we're implementing, and we can't mention 'lc in that trait at all for the
+        // reasons described above.
+        //
+        // [1] For example, the WorkQueue type needs to be parameterized on the concrete type of
+        // DomTraversalContext::SharedContext, and the WorkQueue lifetime is similar to that of the
+        // LayoutTask, generally much longer than that of a given SharedLayoutContext borrow.
+        let shared_lc: &'lc SharedLayoutContext = unsafe { mem::transmute(shared) };
+        RecalcStyleAndConstructFlows {
+            context: LayoutContext::new(shared_lc),
+            root: root,
         }
-    })
-}
-
-fn put_task_local_bloom_filter(bf: Box<BloomFilter>,
-                               unsafe_node: &UnsafeNode,
-                               layout_context: &LayoutContext) {
-    STYLE_BLOOM.with(move |style_bloom| {
-        assert!(style_bloom.borrow().is_none(),
-                "Putting into a never-taken task-local bloom filter");
-        *style_bloom.borrow_mut() = Some((bf, *unsafe_node, layout_context.shared_context().generation));
-    })
-}
-
-/// "Ancestors" in this context is inclusive of ourselves.
-fn insert_ancestors_into_bloom_filter<'ln, N>(bf: &mut Box<BloomFilter>,
-                                              mut n: N,
-                                              root: OpaqueNode)
-                                              where N: LayoutNode<'ln> {
-    debug!("[{}] Inserting ancestors.", tid());
-    let mut ancestors = 0;
-    loop {
-        ancestors += 1;
-
-        n.insert_into_bloom_filter(&mut **bf);
-        n = match n.layout_parent_node(root) {
-            None => break,
-            Some(p) => p,
-        };
     }
-    debug!("[{}] Inserted {} ancestors.", tid(), ancestors);
-}
 
-#[derive(Copy, Clone)]
-pub struct DomTraversalContext<'a> {
-    pub layout_context: &'a LayoutContext<'a>,
-    pub root: OpaqueNode,
-}
-
-pub trait DomTraversal<'ln, N: LayoutNode<'ln>>  {
-    fn process_preorder<'a>(context: &'a DomTraversalContext<'a>, node: N);
-    fn process_postorder<'a>(context: &'a DomTraversalContext<'a>, node: N);
-}
-
-/// FIXME(bholley): I added this now to demonstrate the usefulness of the new design.
-/// This is currently unused, but will be used shortly.
-#[allow(dead_code)]
-pub struct RecalcStyleOnly;
-impl<'ln, N: LayoutNode<'ln>> DomTraversal<'ln, N> for RecalcStyleOnly {
-    fn process_preorder<'a>(context: &'a DomTraversalContext<'a>, node: N) { recalc_style_at(context, node); }
-    fn process_postorder<'a>(_: &'a DomTraversalContext<'a>, _: N) {}
-}
-
-pub struct RecalcStyleAndConstructFlows;
-impl<'ln, N: LayoutNode<'ln>> DomTraversal<'ln, N> for RecalcStyleAndConstructFlows {
-    fn process_preorder<'a>(context: &'a DomTraversalContext<'a>, node: N) { recalc_style_at(context, node); }
-    fn process_postorder<'a>(context: &'a DomTraversalContext<'a>, node: N) { construct_flows_at(context, node); }
+    fn process_preorder(&self, node: N) { recalc_style_at(&self.context, self.root, node); }
+    fn process_postorder(&self, node: N) { construct_flows_at(&self.context, self.root, node); }
 }
 
 /// A bottom-up, parallelizable traversal.
@@ -149,107 +75,10 @@ pub trait PostorderNodeMutTraversal<'ln, ConcreteThreadSafeLayoutNode: ThreadSaf
     fn process(&mut self, node: &ConcreteThreadSafeLayoutNode) -> bool;
 }
 
-/// The recalc-style-for-node traversal, which styles each node and must run before
-/// layout computation. This computes the styles applied to each node.
-#[inline]
-#[allow(unsafe_code)]
-fn recalc_style_at<'a, 'ln, N: LayoutNode<'ln>> (context: &'a DomTraversalContext<'a>, node: N) {
-    // Initialize layout data.
-    //
-    // FIXME(pcwalton): Stop allocating here. Ideally this should just be done by the HTML
-    // parser.
-    node.initialize_data();
-
-    // Get the parent node.
-    let parent_opt = node.layout_parent_node(context.root);
-
-    // Get the style bloom filter.
-    let mut bf = take_task_local_bloom_filter(parent_opt, context.root, context.layout_context);
-
-    let nonincremental_layout = opts::get().nonincremental_layout;
-    if nonincremental_layout || node.is_dirty() {
-        // Remove existing CSS styles from nodes whose content has changed (e.g. text changed),
-        // to force non-incremental reflow.
-        if node.has_changed() {
-            let node = node.to_threadsafe();
-            node.unstyle();
-        }
-
-        // Check to see whether we can share a style with someone.
-        let style_sharing_candidate_cache =
-            &mut context.layout_context.style_sharing_candidate_cache();
-
-        let sharing_result = match node.as_element() {
-            Some(element) => {
-                unsafe {
-                    element.share_style_if_possible(style_sharing_candidate_cache,
-                                                    parent_opt.clone())
-                }
-            },
-            None => StyleSharingResult::CannotShare,
-        };
-
-        // Otherwise, match and cascade selectors.
-        match sharing_result {
-            StyleSharingResult::CannotShare => {
-                let mut applicable_declarations = ApplicableDeclarations::new();
-
-                let shareable_element = match node.as_element() {
-                    Some(element) => {
-                        // Perform the CSS selector matching.
-                        let stylist = unsafe { &*context.layout_context.shared_context().stylist.0 };
-                        if element.match_element(stylist,
-                                                 Some(&*bf),
-                                                 &mut applicable_declarations) {
-                            Some(element)
-                        } else {
-                            None
-                        }
-                    },
-                    None => {
-                        if node.has_changed() {
-                            node.set_restyle_damage(N::ConcreteRestyleDamage::rebuild_and_reflow())
-                        }
-                        None
-                    },
-                };
-
-                // Perform the CSS cascade.
-                unsafe {
-                    node.cascade_node(&context.layout_context.shared.style_context,
-                                      parent_opt,
-                                      &applicable_declarations,
-                                      &mut context.layout_context.applicable_declarations_cache(),
-                                      &context.layout_context.shared_context().new_animations_sender);
-                }
-
-                // Add ourselves to the LRU cache.
-                if let Some(element) = shareable_element {
-                    style_sharing_candidate_cache.insert_if_possible(&element);
-                }
-            }
-            StyleSharingResult::StyleWasShared(index, damage) => {
-                style_sharing_candidate_cache.touch(index);
-                node.set_restyle_damage(damage);
-            }
-        }
-    }
-
-    let unsafe_layout_node = node.to_unsafe();
-
-    // Before running the children, we need to insert our nodes into the bloom
-    // filter.
-    debug!("[{}] + {:X}", tid(), unsafe_layout_node.0);
-    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, context.layout_context);
-}
-
 /// The flow construction traversal, which builds flows for styled nodes.
 #[inline]
 #[allow(unsafe_code)]
-fn construct_flows_at<'a, 'ln, N: LayoutNode<'ln>>(context: &'a DomTraversalContext<'a>, node: N) {
+fn construct_flows_at<'a, 'ln, N: LayoutNode<'ln>>(context: &'a LayoutContext<'a>, root: OpaqueNode, node: N) {
     // Construct flows for this node.
     {
         let tnode = node.to_threadsafe();
@@ -257,7 +86,7 @@ fn construct_flows_at<'a, 'ln, N: LayoutNode<'ln>>(context: &'a DomTraversalCont
         // Always reconstruct if incremental layout is turned off.
         let nonincremental_layout = opts::get().nonincremental_layout;
         if nonincremental_layout || node.has_dirty_descendants() {
-            let mut flow_constructor = FlowConstructor::new(context.layout_context);
+            let mut flow_constructor = FlowConstructor::new(context);
             if nonincremental_layout || !flow_constructor.repair_if_possible(&tnode) {
                 flow_constructor.process(&tnode);
                 debug!("Constructed flow for {:x}: {:x}",
@@ -286,9 +115,9 @@ fn construct_flows_at<'a, 'ln, N: LayoutNode<'ln>>(context: &'a DomTraversalCont
         });
 
     assert_eq!(old_node, unsafe_layout_node);
-    assert_eq!(old_generation, context.layout_context.shared_context().generation);
+    assert_eq!(old_generation, context.shared_context().generation);
 
-    match node.layout_parent_node(context.root) {
+    match node.layout_parent_node(root) {
         None => {
             debug!("[{}] - {:X}, and deleting BF.", tid(), unsafe_layout_node.0);
             // If this is the reflow root, eat the task-local bloom filter.
@@ -297,7 +126,7 @@ fn construct_flows_at<'a, 'ln, N: LayoutNode<'ln>>(context: &'a DomTraversalCont
             // Otherwise, put it back, but remove this node.
             node.remove_from_bloom_filter(&mut *bf);
             let unsafe_parent = parent.to_unsafe();
-            put_task_local_bloom_filter(bf, &unsafe_parent, context.layout_context);
+            put_task_local_bloom_filter(bf, &unsafe_parent, &context.shared_context());
         },
     };
 }
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index 4a3ca347848..aa947937d0b 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -275,6 +275,14 @@ impl<'ln> TNode<'ln> for ServoLayoutNode<'ln> {
             self.node.next_sibling_ref().map(|node| self.new_with_this_lifetime(&node))
         }
     }
+
+    fn style(&self) -> Ref<Arc<ComputedValues>> {
+        Ref::map(self.borrow_data().unwrap(), |data| data.style.as_ref().unwrap())
+    }
+
+    fn unstyle(self) {
+        self.mutate_data().unwrap().style = None;
+    }
 }
 
 impl<'ln> LayoutNode<'ln> for ServoLayoutNode<'ln> {
@@ -696,6 +704,8 @@ pub trait ThreadSafeLayoutNode<'ln> : Clone + Copy + Sized {
 
     /// Returns the style results for the given node. If CSS selector matching
     /// has not yet been performed, fails.
+    ///
+    /// Unlike the version on TNode, this handles pseudo-elements.
     #[inline]
     fn style(&self) -> Ref<Arc<ComputedValues>> {
         Ref::map(self.borrow_layout_data().unwrap(), |data| {
@@ -709,6 +719,8 @@ pub trait ThreadSafeLayoutNode<'ln> : Clone + Copy + Sized {
     }
 
     /// Removes the style from this node.
+    ///
+    /// Unlike the version on TNode, this handles pseudo-elements.
     fn unstyle(self) {
         let mut data = self.mutate_layout_data().unwrap();
         let style =
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 40933b03a7e..f85484e0c70 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -156,6 +156,14 @@ pub trait TNode<'ln> : Sized + Copy + Clone {
     fn prev_sibling(&self) -> Option<Self>;
 
     fn next_sibling(&self) -> Option<Self>;
+
+
+    /// Returns the style results for the given node. If CSS selector matching
+    /// has not yet been performed, fails.
+    fn style(&self) -> Ref<Arc<ComputedValues>>;
+
+    /// Removes the style from this node.
+    fn unstyle(self);
 }
 
 pub trait TDocument<'ld> : Sized + Copy + Clone {
diff --git a/components/style/lib.rs b/components/style/lib.rs
index e010a92f0b2..0f0dfca30cf 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -53,10 +53,13 @@ pub mod dom;
 pub mod font_face;
 pub mod matching;
 pub mod media_queries;
+pub mod parallel;
 pub mod parser;
 pub mod restyle_hints;
 pub mod selector_matching;
+pub mod sequential;
 pub mod stylesheets;
+pub mod traversal;
 #[macro_use]
 #[allow(non_camel_case_types)]
 pub mod values;
diff --git a/components/style/parallel.rs b/components/style/parallel.rs
new file mode 100644
index 00000000000..cb8e8260a21
--- /dev/null
+++ b/components/style/parallel.rs
@@ -0,0 +1,141 @@
+/* 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/. */
+
+//! Implements parallel traversal over the DOM tree.
+//!
+//! This code is highly unsafe. Keep this file small and easy to audit.
+
+#![allow(unsafe_code)]
+
+use dom::{OpaqueNode, TNode, UnsafeNode};
+use std::mem;
+use std::sync::atomic::Ordering;
+use traversal::DomTraversalContext;
+use util::workqueue::{WorkQueue, WorkUnit, WorkerProxy};
+
+#[allow(dead_code)]
+fn static_assertion(node: UnsafeNode) {
+    unsafe {
+        let _: UnsafeNodeList = ::std::intrinsics::transmute(node);
+    }
+}
+
+pub type UnsafeNodeList = (Box<Vec<UnsafeNode>>, OpaqueNode);
+
+pub const CHUNK_SIZE: usize = 64;
+
+pub struct WorkQueueData(usize, usize);
+
+pub fn run_queue_with_custom_work_data_type<To, F, SharedContext: Sync>(
+        queue: &mut WorkQueue<SharedContext, WorkQueueData>,
+        callback: F,
+        shared: &SharedContext)
+        where To: 'static + Send, F: FnOnce(&mut WorkQueue<SharedContext, To>) {
+    let queue: &mut WorkQueue<SharedContext, To> = unsafe {
+        mem::transmute(queue)
+    };
+    callback(queue);
+    queue.run(shared);
+}
+
+pub fn traverse_dom<'ln, N, C>(root: N,
+                               queue_data: &C::SharedContext,
+                               queue: &mut WorkQueue<C::SharedContext, WorkQueueData>)
+                               where N: TNode<'ln>, C: DomTraversalContext<'ln, N> {
+    run_queue_with_custom_work_data_type(queue, |queue| {
+        queue.push(WorkUnit {
+            fun:  top_down_dom::<N, C>,
+            data: (box vec![root.to_unsafe()], root.opaque()),
+        });
+    }, queue_data);
+}
+
+/// A parallel top-down DOM traversal.
+#[inline(always)]
+fn top_down_dom<'ln, N, C>(unsafe_nodes: UnsafeNodeList,
+                           proxy: &mut WorkerProxy<C::SharedContext, UnsafeNodeList>)
+                           where N: TNode<'ln>, C: DomTraversalContext<'ln, N> {
+    let context = C::new(proxy.user_data(), unsafe_nodes.1);
+
+    let mut discovered_child_nodes = Vec::new();
+    for unsafe_node in *unsafe_nodes.0 {
+        // Get a real layout node.
+        let node = unsafe { N::from_unsafe(&unsafe_node) };
+
+        // Perform the appropriate traversal.
+        context.process_preorder(node);
+
+        let child_count = node.children_count();
+
+        // Reset the count of children.
+        {
+            let data = node.mutate_data().unwrap();
+            data.parallel.children_count.store(child_count as isize,
+                                               Ordering::Relaxed);
+        }
+
+        // Possibly enqueue the children.
+        if child_count != 0 {
+            for kid in node.children() {
+                discovered_child_nodes.push(kid.to_unsafe())
+            }
+        } else {
+            // If there were no more children, start walking back up.
+            bottom_up_dom::<N, C>(unsafe_nodes.1, unsafe_node, proxy)
+        }
+    }
+
+    for chunk in discovered_child_nodes.chunks(CHUNK_SIZE) {
+        proxy.push(WorkUnit {
+            fun:  top_down_dom::<N, C>,
+            data: (box chunk.iter().cloned().collect(), unsafe_nodes.1),
+        });
+    }
+}
+
+/// 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 bottom_up_dom<'ln, N, C>(root: OpaqueNode,
+                            unsafe_node: UnsafeNode,
+                            proxy: &mut WorkerProxy<C::SharedContext, UnsafeNodeList>)
+                            where N: TNode<'ln>, C: DomTraversalContext<'ln, N> {
+    let context = C::new(proxy.user_data(), root);
+
+    // Get a real layout node.
+    let mut node = unsafe { N::from_unsafe(&unsafe_node) };
+    loop {
+        // Perform the appropriate operation.
+        context.process_postorder(node);
+
+        let parent = match node.layout_parent_node(root) {
+            None => break,
+            Some(parent) => parent,
+        };
+
+        let parent_data = unsafe {
+            &*parent.borrow_data_unchecked().unwrap()
+        };
+
+        if parent_data
+            .parallel
+            .children_count
+            .fetch_sub(1, Ordering::Relaxed) != 1 {
+            // Get out of here and find another node to work on.
+            break
+        }
+
+        // We were the last child of our parent. Construct flows for our parent.
+        node = parent;
+    }
+}
+
diff --git a/components/style/sequential.rs b/components/style/sequential.rs
new file mode 100644
index 00000000000..9dfbb0126c1
--- /dev/null
+++ b/components/style/sequential.rs
@@ -0,0 +1,28 @@
+/* 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/. */
+
+//! Implements sequential traversal over the DOM tree.
+
+use dom::TNode;
+use traversal::DomTraversalContext;
+
+pub fn traverse_dom<'ln, N, C>(root: N,
+                               shared: &C::SharedContext)
+                               where N: TNode<'ln>,
+                                     C: DomTraversalContext<'ln, N> {
+    fn doit<'a, 'ln, N, C>(context: &'a C, node: N)
+                           where N: TNode<'ln>, C: DomTraversalContext<'ln, N> {
+        context.process_preorder(node);
+
+        for kid in node.children() {
+            doit::<N, C>(context, kid);
+        }
+
+        context.process_postorder(node);
+    }
+
+    let context = C::new(shared, root.opaque());
+    doit::<N, C>(&context, root);
+}
+
diff --git a/components/style/traversal.rs b/components/style/traversal.rs
new file mode 100644
index 00000000000..c3307aa2593
--- /dev/null
+++ b/components/style/traversal.rs
@@ -0,0 +1,259 @@
+/* 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/. */
+
+use context::{LocalStyleContext, SharedStyleContext, StyleContext};
+use dom::{OpaqueNode, TNode, TRestyleDamage, UnsafeNode};
+use matching::{ApplicableDeclarations, ElementMatchMethods, MatchMethods, StyleSharingResult};
+use selectors::bloom::BloomFilter;
+use std::cell::RefCell;
+use std::mem;
+use std::rc::Rc;
+use util::opts;
+use util::tid::tid;
+
+/// Every time we do another layout, the old bloom filters are invalid. This is
+/// detected by ticking a generation number every layout.
+pub type Generation = u32;
+
+/// 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.
+thread_local!(
+    pub static STYLE_BLOOM: RefCell<Option<(Box<BloomFilter>, UnsafeNode, Generation)>> = RefCell::new(None));
+
+/// 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 cleared and reused.
+fn take_task_local_bloom_filter<'ln, N>(parent_node: Option<N>,
+                                        root: OpaqueNode,
+                                        context: &SharedStyleContext)
+                                        -> Box<BloomFilter>
+                                        where N: TNode<'ln> {
+    STYLE_BLOOM.with(|style_bloom| {
+        match (parent_node, style_bloom.borrow_mut().take()) {
+            // Root node. Needs new bloom filter.
+            (None,     _  ) => {
+                debug!("[{}] No parent, but new bloom filter!", tid());
+                box BloomFilter::new()
+            }
+            // No bloom filter for this thread yet.
+            (Some(parent), None) => {
+                let mut bloom_filter = box BloomFilter::new();
+                insert_ancestors_into_bloom_filter(&mut bloom_filter, parent, root);
+                bloom_filter
+            }
+            // Found cached bloom filter.
+            (Some(parent), Some((mut bloom_filter, old_node, old_generation))) => {
+                if old_node == parent.to_unsafe() &&
+                    old_generation == context.generation {
+                    // Hey, the cached parent is our parent! We can reuse the bloom filter.
+                    debug!("[{}] Parent matches (={}). Reusing bloom filter.", tid(), old_node.0);
+                } else {
+                    // Oh no. the cached parent is stale. I guess we need a new one. Reuse the existing
+                    // allocation to avoid malloc churn.
+                    bloom_filter.clear();
+                    insert_ancestors_into_bloom_filter(&mut bloom_filter, parent, root);
+                }
+                bloom_filter
+            },
+        }
+    })
+}
+
+pub fn put_task_local_bloom_filter(bf: Box<BloomFilter>,
+                               unsafe_node: &UnsafeNode,
+                               context: &SharedStyleContext) {
+    STYLE_BLOOM.with(move |style_bloom| {
+        assert!(style_bloom.borrow().is_none(),
+                "Putting into a never-taken task-local bloom filter");
+        *style_bloom.borrow_mut() = Some((bf, *unsafe_node, context.generation));
+    })
+}
+
+/// "Ancestors" in this context is inclusive of ourselves.
+fn insert_ancestors_into_bloom_filter<'ln, N>(bf: &mut Box<BloomFilter>,
+                                              mut n: N,
+                                              root: OpaqueNode)
+                                              where N: TNode<'ln> {
+    debug!("[{}] Inserting ancestors.", tid());
+    let mut ancestors = 0;
+    loop {
+        ancestors += 1;
+
+        n.insert_into_bloom_filter(&mut **bf);
+        n = match n.layout_parent_node(root) {
+            None => break,
+            Some(p) => p,
+        };
+    }
+    debug!("[{}] Inserted {} ancestors.", tid(), ancestors);
+}
+
+pub trait DomTraversalContext<'ln, N: TNode<'ln>>  {
+    type SharedContext: Sync + 'static;
+    fn new<'a>(&'a Self::SharedContext, OpaqueNode) -> Self;
+    fn process_preorder(&self, node: N);
+    fn process_postorder(&self, node: N);
+}
+
+/// FIXME(bholley): I added this now to demonstrate the usefulness of the new design.
+/// This is currently unused, but will be used shortly.
+
+#[allow(dead_code)]
+pub struct StandaloneStyleContext<'a> {
+    pub shared: &'a SharedStyleContext,
+    cached_local_style_context: Rc<LocalStyleContext>,
+}
+
+impl<'a> StandaloneStyleContext<'a> {
+    pub fn new(_: &'a SharedStyleContext) -> Self { panic!("Not implemented") }
+}
+
+impl<'a> StyleContext<'a> for StandaloneStyleContext<'a> {
+    fn shared_context(&self) -> &'a SharedStyleContext {
+        &self.shared
+    }
+
+    fn local_context(&self) -> &LocalStyleContext {
+        &self.cached_local_style_context
+    }
+}
+
+#[allow(dead_code)]
+pub struct RecalcStyleOnly<'lc> {
+    context: StandaloneStyleContext<'lc>,
+    root: OpaqueNode,
+}
+
+impl<'lc, 'ln, N: TNode<'ln>> DomTraversalContext<'ln, N> for RecalcStyleOnly<'lc> {
+    type SharedContext = SharedStyleContext;
+    #[allow(unsafe_code)]
+    fn new<'a>(shared: &'a Self::SharedContext, root: OpaqueNode) -> Self {
+        // See the comment in RecalcStyleAndConstructFlows::new for an explanation of why this is
+        // necessary.
+        let shared_lc: &'lc SharedStyleContext = unsafe { mem::transmute(shared) };
+        RecalcStyleOnly {
+            context: StandaloneStyleContext::new(shared_lc),
+            root: root,
+        }
+    }
+
+    fn process_preorder(&self, node: N) { recalc_style_at(&self.context, self.root, node); }
+    fn process_postorder(&self, _: N) {}
+}
+
+/// The recalc-style-for-node traversal, which styles each node and must run before
+/// layout computation. This computes the styles applied to each node.
+#[inline]
+#[allow(unsafe_code)]
+pub fn recalc_style_at<'a, 'ln, N: TNode<'ln>, C: StyleContext<'a>> (context: &'a C, root: OpaqueNode, node: N) {
+    // Initialize layout data.
+    //
+    // FIXME(pcwalton): Stop allocating here. Ideally this should just be done by the HTML
+    // parser.
+    node.initialize_data();
+
+    // Get the parent node.
+    let parent_opt = node.layout_parent_node(root);
+
+    // Get the style bloom filter.
+    let mut bf = take_task_local_bloom_filter(parent_opt, root, context.shared_context());
+
+    let nonincremental_layout = opts::get().nonincremental_layout;
+    if nonincremental_layout || node.is_dirty() {
+        // Remove existing CSS styles from nodes whose content has changed (e.g. text changed),
+        // to force non-incremental reflow.
+        if node.has_changed() {
+            node.unstyle();
+        }
+
+        // Check to see whether we can share a style with someone.
+        let style_sharing_candidate_cache =
+            &mut context.local_context().style_sharing_candidate_cache.borrow_mut();
+
+        let sharing_result = match node.as_element() {
+            Some(element) => {
+                unsafe {
+                    element.share_style_if_possible(style_sharing_candidate_cache,
+                                                    parent_opt.clone())
+                }
+            },
+            None => StyleSharingResult::CannotShare,
+        };
+
+        // Otherwise, match and cascade selectors.
+        match sharing_result {
+            StyleSharingResult::CannotShare => {
+                let mut applicable_declarations = ApplicableDeclarations::new();
+
+                let shareable_element = match node.as_element() {
+                    Some(element) => {
+                        // Perform the CSS selector matching.
+                        let stylist = unsafe { &*context.shared_context().stylist.0 };
+                        if element.match_element(stylist,
+                                                 Some(&*bf),
+                                                 &mut applicable_declarations) {
+                            Some(element)
+                        } else {
+                            None
+                        }
+                    },
+                    None => {
+                        if node.has_changed() {
+                            node.set_restyle_damage(N::ConcreteRestyleDamage::rebuild_and_reflow())
+                        }
+                        None
+                    },
+                };
+
+                // Perform the CSS cascade.
+                unsafe {
+                    node.cascade_node(&context.shared_context(),
+                                      parent_opt,
+                                      &applicable_declarations,
+                                      &mut context.local_context().applicable_declarations_cache.borrow_mut(),
+                                      &context.shared_context().new_animations_sender);
+                }
+
+                // Add ourselves to the LRU cache.
+                if let Some(element) = shareable_element {
+                    style_sharing_candidate_cache.insert_if_possible(&element);
+                }
+            }
+            StyleSharingResult::StyleWasShared(index, damage) => {
+                style_sharing_candidate_cache.touch(index);
+                node.set_restyle_damage(damage);
+            }
+        }
+    }
+
+    let unsafe_layout_node = node.to_unsafe();
+
+    // Before running the children, we need to insert our nodes into the bloom
+    // filter.
+    debug!("[{}] + {:X}", tid(), unsafe_layout_node.0);
+    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, context.shared_context());
+}
+