Factor out the bottom-up postorder traversal logic.

components/style/parallel.rs

@@ -121,18 +121,8 @@ fn top_down_dom<'a, 'scope, E, D>(nodes: &'a [SendNode<E::ConcreteNode>],
                 });
             }
 
-            // Reset the count of children if we need to do a bottom-up traversal
+            traversal.handle_postorder_traversal(&mut *tlc, root, node,
-            // after the top up.
+                                                 children_to_process);
-            if D::needs_postorder_traversal() {
-                if children_to_process == 0 {
-                    // If there were no more children, start walking back up.
-                    bottom_up_dom(traversal, &mut *tlc, root, node)
-                } else {
-                    // Otherwise record the number of children to process when the
-                    // time comes.
-                    node.as_element().unwrap().store_children_to_process(children_to_process);
-                }
-            }
         }
     }
 
@@ -173,45 +163,3 @@ fn traverse_nodes<'a, 'scope, E, D>(nodes: Vec<SendNode<E::ConcreteNode>>, root:
         })
     }
 }
-
-/// 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<E, D>(traversal: &D,
-                       thread_local: &mut D::ThreadLocalContext,
-                       root: OpaqueNode,
-                       mut node: E::ConcreteNode)
-    where E: TElement,
-          D: DomTraversal<E>,
-{
-    loop {
-        // Perform the appropriate operation.
-        traversal.process_postorder(thread_local, node);
-
-        if node.opaque() == root {
-            break;
-        }
-
-        let parent = match node.parent_element() {
-            None => unreachable!("How can this happen after the break above?"),
-            Some(parent) => parent,
-        };
-
-        let remaining = parent.did_process_child();
-        if remaining != 0 {
-            // 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.as_node();
-    }
-}

components/style/traversal.rs

@@ -9,7 +9,7 @@
 use atomic_refcell::{AtomicRefCell, AtomicRefMut};
 use context::{SharedStyleContext, StyleContext, ThreadLocalStyleContext};
 use data::{ElementData, ElementStyles, StoredRestyleHint};
-use dom::{DirtyDescendants, NodeInfo, TElement, TNode};
+use dom::{DirtyDescendants, NodeInfo, OpaqueNode, TElement, TNode};
 use matching::{MatchMethods, StyleSharingBehavior};
 use restyle_hints::{RESTYLE_DESCENDANTS, RESTYLE_SELF};
 use selector_parser::RestyleDamage;
@@ -136,6 +136,56 @@ pub trait DomTraversal<E: TElement> : Sync {
     /// If it's false, then process_postorder has no effect at all.
     fn needs_postorder_traversal() -> bool { true }
 
+    /// Handles the postorder step of the traversal, if it exists, by bubbling
+    /// up the parent chain.
+    ///
+    /// 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. This makes it safe to
+    /// call durign the parallel traversal.
+    fn handle_postorder_traversal(&self,
+                                  thread_local: &mut Self::ThreadLocalContext,
+                                  root: OpaqueNode,
+                                  mut node: E::ConcreteNode,
+                                  children_to_process: isize)
+    {
+        // If the postorder step is a no-op, don't bother.
+        if !Self::needs_postorder_traversal() {
+            return;
+        }
+
+        if children_to_process == 0 {
+            // We are a leaf. Walk up the chain.
+            loop {
+                self.process_postorder(thread_local, node);
+                if node.opaque() == root {
+                    break;
+                }
+                let parent = node.parent_element().unwrap();
+                let remaining = parent.did_process_child();
+                if remaining != 0 {
+                    // The parent has other unprocessed descendants. We only
+                    // perform postorder processing after the last descendant
+                    // has been processed.
+                    break
+                }
+
+                node = parent.as_node();
+            }
+        } else {
+            // Otherwise record the number of children to process when the
+            // time comes.
+            node.as_element().unwrap()
+                .store_children_to_process(children_to_process);
+        }
+    }
+
     /// Must be invoked before traversing the root element to determine whether
     /// a traversal is needed. Returns a token that allows the caller to prove
     /// that the call happened.