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https://github.com/mozilla/gecko-dev.git
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99fa3bca94
This is on top of #15888. Only the second commit needs review. We put the more recently used item last, so iterating then from left to right is pointless. Source-Repo: https://github.com/servo/servo Source-Revision: 7fa4a94bb154449702e5ae9422c3008a5195c714 --HG-- extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear extra : subtree_revision : fbf4742c4186ad27bd4d5ccd3e2319a5b0748b55
218 lines
8.3 KiB
Rust
218 lines
8.3 KiB
Rust
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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//! Implements parallel traversal over the DOM tree.
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//!
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//! This traversal is based on Rayon, and therefore its safety is largely
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//! verified by the type system.
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//!
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//! The primary trickiness and fine print for the above relates to the
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//! thread safety of the DOM nodes themselves. Accessing a DOM element
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//! concurrently on multiple threads is actually mostly "safe", since all
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//! the mutable state is protected by an AtomicRefCell, and so we'll
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//! generally panic if something goes wrong. Still, we try to to enforce our
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//! thread invariants at compile time whenever possible. As such, TNode and
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//! TElement are not Send, so ordinary style system code cannot accidentally
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//! share them with other threads. In the parallel traversal, we explicitly
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//! invoke |unsafe { SendNode::new(n) }| to put nodes in containers that may
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//! be sent to other threads. This occurs in only a handful of places and is
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//! easy to grep for. At the time of this writing, there is no other unsafe
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//! code in the parallel traversal.
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#![deny(missing_docs)]
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use context::TraversalStatistics;
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use dom::{OpaqueNode, SendNode, TElement, TNode};
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use rayon;
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use scoped_tls::ScopedTLS;
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use std::borrow::Borrow;
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use time;
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use traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
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/// The chunk size used to split the parallel traversal nodes.
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///
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/// We send each `CHUNK_SIZE` nodes as a different work unit to the work queue.
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pub const CHUNK_SIZE: usize = 64;
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/// A parallel top down traversal, generic over `D`.
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#[allow(unsafe_code)]
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pub fn traverse_dom<E, D>(traversal: &D,
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root: E,
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known_root_dom_depth: Option<usize>,
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token: PreTraverseToken,
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queue: &rayon::ThreadPool)
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where E: TElement,
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D: DomTraversal<E>,
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{
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let dump_stats = TraversalStatistics::should_dump();
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let start_time = if dump_stats { Some(time::precise_time_s()) } else { None };
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debug_assert!(traversal.is_parallel());
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// Handle Gecko's eager initial styling. We don't currently support it
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// in conjunction with bottom-up traversal. If we did, we'd need to put
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// it on the context to make it available to the bottom-up phase.
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let (nodes, depth) = if token.traverse_unstyled_children_only() {
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debug_assert!(!D::needs_postorder_traversal());
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let mut children = vec![];
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for kid in root.as_node().children() {
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if kid.as_element().map_or(false, |el| el.get_data().is_none()) {
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children.push(unsafe { SendNode::new(kid) });
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}
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}
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(children, known_root_dom_depth.map(|x| x + 1))
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} else {
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(vec![unsafe { SendNode::new(root.as_node()) }], known_root_dom_depth)
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};
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let traversal_data = PerLevelTraversalData {
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current_dom_depth: depth,
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};
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let tls = ScopedTLS::<D::ThreadLocalContext>::new(queue);
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let root = root.as_node().opaque();
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queue.install(|| {
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rayon::scope(|scope| {
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traverse_nodes(nodes, root, traversal_data, scope, traversal, &tls);
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});
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});
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// Dump statistics to stdout if requested.
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if dump_stats {
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let slots = unsafe { tls.unsafe_get() };
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let mut aggregate = slots.iter().fold(TraversalStatistics::default(), |acc, t| {
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match *t.borrow() {
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None => acc,
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Some(ref cx) => &cx.borrow().statistics + &acc,
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}
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});
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aggregate.finish(traversal, start_time.unwrap());
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println!("{}", aggregate);
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}
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}
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/// A parallel top-down DOM traversal.
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#[inline(always)]
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#[allow(unsafe_code)]
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fn top_down_dom<'a, 'scope, E, D>(nodes: &'a [SendNode<E::ConcreteNode>],
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root: OpaqueNode,
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mut traversal_data: PerLevelTraversalData,
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scope: &'a rayon::Scope<'scope>,
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traversal: &'scope D,
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tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
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where E: TElement + 'scope,
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D: DomTraversal<E>,
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{
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let mut discovered_child_nodes = vec![];
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{
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// Scope the borrow of the TLS so that the borrow is dropped before
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// potentially traversing a child on this thread.
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let mut tlc = tls.ensure(|| traversal.create_thread_local_context());
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for n in nodes {
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// Perform the appropriate traversal.
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let node = **n;
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let mut children_to_process = 0isize;
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traversal.process_preorder(&mut traversal_data, &mut *tlc, node);
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if let Some(el) = node.as_element() {
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traversal.traverse_children(&mut *tlc, el, |_tlc, kid| {
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children_to_process += 1;
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discovered_child_nodes.push(unsafe { SendNode::new(kid) })
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});
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}
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// Reset the count of children if we need to do a bottom-up traversal
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// after the top up.
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if D::needs_postorder_traversal() {
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if children_to_process == 0 {
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// If there were no more children, start walking back up.
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bottom_up_dom(traversal, &mut *tlc, root, node)
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} else {
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// Otherwise record the number of children to process when the
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// time comes.
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node.as_element().unwrap().store_children_to_process(children_to_process);
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}
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}
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}
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}
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if let Some(ref mut depth) = traversal_data.current_dom_depth {
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*depth += 1;
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}
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traverse_nodes(discovered_child_nodes, root, traversal_data, scope, traversal, tls);
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}
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fn traverse_nodes<'a, 'scope, E, D>(nodes: Vec<SendNode<E::ConcreteNode>>, root: OpaqueNode,
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traversal_data: PerLevelTraversalData,
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scope: &'a rayon::Scope<'scope>,
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traversal: &'scope D,
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tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
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where E: TElement + 'scope,
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D: DomTraversal<E>,
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{
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if nodes.is_empty() {
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return;
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}
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// Optimization: traverse directly and avoid a heap-allocating spawn() call if
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// we're only pushing one work unit.
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if nodes.len() <= CHUNK_SIZE {
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let nodes = nodes.into_boxed_slice();
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top_down_dom(&nodes, root, traversal_data, scope, traversal, tls);
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return;
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}
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// General case.
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for chunk in nodes.chunks(CHUNK_SIZE) {
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let nodes = chunk.iter().cloned().collect::<Vec<_>>().into_boxed_slice();
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let traversal_data = traversal_data.clone();
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scope.spawn(move |scope| {
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let nodes = nodes;
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top_down_dom(&nodes, root, traversal_data, scope, traversal, tls)
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})
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}
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}
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/// Process current node and potentially traverse its ancestors.
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///
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/// If we are the last child that finished processing, recursively process
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/// our parent. Else, stop. Also, stop at the root.
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///
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/// Thus, if we start with all the leaves of a tree, we end up traversing
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/// the whole tree bottom-up because each parent will be processed exactly
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/// once (by the last child that finishes processing).
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///
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/// The only communication between siblings is that they both
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/// fetch-and-subtract the parent's children count.
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fn bottom_up_dom<E, D>(traversal: &D,
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thread_local: &mut D::ThreadLocalContext,
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root: OpaqueNode,
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mut node: E::ConcreteNode)
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where E: TElement,
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D: DomTraversal<E>,
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{
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loop {
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// Perform the appropriate operation.
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traversal.process_postorder(thread_local, node);
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if node.opaque() == root {
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break;
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}
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let parent = match node.parent_element() {
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None => unreachable!("How can this happen after the break above?"),
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Some(parent) => parent,
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};
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let remaining = parent.did_process_child();
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if remaining != 0 {
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// Get out of here and find another node to work on.
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break
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}
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// We were the last child of our parent. Construct flows for our parent.
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node = parent.as_node();
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}
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}
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