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+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! The style bloom filter is used as an optimization when matching deep
+//! descendant selectors.
+
+#![deny(missing_docs)]
+
+use crate::dom::{SendElement, TElement};
+use atomic_refcell::{AtomicRefCell, AtomicRefMut};
+use owning_ref::OwningHandle;
+use selectors::bloom::BloomFilter;
+use servo_arc::Arc;
+use smallvec::SmallVec;
+use std::mem::ManuallyDrop;
+
+thread_local! {
+ /// Bloom filters are large allocations, so we store them in thread-local storage
+ /// such that they can be reused across style traversals. StyleBloom is responsible
+ /// for ensuring that the bloom filter is zeroed when it is dropped.
+ ///
+ /// We intentionally leak this from TLS because we don't have the guarantee
+ /// of TLS destructors to run in worker threads.
+ ///
+ /// We could change this once https://github.com/rayon-rs/rayon/issues/688
+ /// is fixed, hopefully.
+ static BLOOM_KEY: ManuallyDrop<Arc<AtomicRefCell<BloomFilter>>> =
+ ManuallyDrop::new(Arc::new_leaked(Default::default()));
+}
+
+/// A struct that allows us to fast-reject deep descendant selectors avoiding
+/// selector-matching.
+///
+/// This is implemented using a counting bloom filter, and it's a standard
+/// optimization. See Gecko's `AncestorFilter`, and Blink's and WebKit's
+/// `SelectorFilter`.
+///
+/// The constraints for Servo's style system are a bit different compared to
+/// traditional style systems given Servo does a parallel breadth-first
+/// traversal instead of a sequential depth-first traversal.
+///
+/// This implies that we need to track a bit more state than other browsers to
+/// ensure we're doing the correct thing during the traversal, and being able to
+/// apply this optimization effectively.
+///
+/// Concretely, we have a bloom filter instance per worker thread, and we track
+/// the current DOM depth in order to find a common ancestor when it doesn't
+/// match the previous element we've styled.
+///
+/// This is usually a pretty fast operation (we use to be one level deeper than
+/// the previous one), but in the case of work-stealing, we may needed to push
+/// and pop multiple elements.
+///
+/// See the `insert_parents_recovering`, where most of the magic happens.
+///
+/// Regarding thread-safety, this struct is safe because:
+///
+/// * We clear this after a restyle.
+/// * The DOM shape and attributes (and every other thing we access here) are
+/// immutable during a restyle.
+///
+pub struct StyleBloom<E: TElement> {
+ /// A handle to the bloom filter from the thread upon which this StyleBloom
+ /// was created. We use AtomicRefCell so that this is all |Send|, which allows
+ /// StyleBloom to live in ThreadLocalStyleContext, which is dropped from the
+ /// parent thread.
+ filter: OwningHandle<Arc<AtomicRefCell<BloomFilter>>, AtomicRefMut<'static, BloomFilter>>,
+
+ /// The stack of elements that this bloom filter contains, along with the
+ /// number of hashes pushed for each element.
+ elements: SmallVec<[PushedElement<E>; 16]>,
+
+ /// Stack of hashes that have been pushed onto this filter.
+ pushed_hashes: SmallVec<[u32; 64]>,
+}
+
+/// The very rough benchmarks in the selectors crate show clear()
+/// costing about 25 times more than remove_hash(). We use this to implement
+/// clear() more efficiently when only a small number of hashes have been
+/// pushed.
+///
+/// One subtly to note is that remove_hash() will not touch the value
+/// if the filter overflowed. However, overflow can only occur if we
+/// get 255 collisions on the same hash value, and 25 < 255.
+const MEMSET_CLEAR_THRESHOLD: usize = 25;
+
+struct PushedElement<E: TElement> {
+ /// The element that was pushed.
+ element: SendElement<E>,
+
+ /// The number of hashes pushed for the element.
+ num_hashes: usize,
+}
+
+impl<E: TElement> PushedElement<E> {
+ fn new(el: E, num_hashes: usize) -> Self {
+ PushedElement {
+ element: unsafe { SendElement::new(el) },
+ num_hashes,
+ }
+ }
+}
+
+/// Returns whether the attribute name is excluded from the bloom filter.
+///
+/// We do this for attributes that are very common but not commonly used in
+/// selectors.
+#[inline]
+pub fn is_attr_name_excluded_from_filter(atom: &crate::Atom) -> bool {
+ *atom == atom!("class") || *atom == atom!("id") || *atom == atom!("style")
+}
+
+/// Gather all relevant hash for fast-reject filters from an element.
+pub fn each_relevant_element_hash<E, F>(element: E, mut f: F)
+where
+ E: TElement,
+ F: FnMut(u32),
+{
+ f(element.local_name().get_hash());
+ f(element.namespace().get_hash());
+
+ if let Some(id) = element.id() {
+ f(id.get_hash());
+ }
+
+ element.each_class(|class| f(class.get_hash()));
+
+ element.each_attr_name(|name| {
+ if !is_attr_name_excluded_from_filter(name) {
+ f(name.get_hash())
+ }
+ });
+}
+
+impl<E: TElement> Drop for StyleBloom<E> {
+ fn drop(&mut self) {
+ // Leave the reusable bloom filter in a zeroed state.
+ self.clear();
+ }
+}
+
+impl<E: TElement> StyleBloom<E> {
+ /// Create an empty `StyleBloom`. Because StyleBloom acquires the thread-
+ /// local filter buffer, creating multiple live StyleBloom instances at
+ /// the same time on the same thread will panic.
+
+ // Forced out of line to limit stack frame sizes after extra inlining from
+ // https://github.com/rust-lang/rust/pull/43931
+ //
+ // See https://github.com/servo/servo/pull/18420#issuecomment-328769322
+ #[inline(never)]
+ pub fn new() -> Self {
+ let bloom_arc = BLOOM_KEY.with(|b| Arc::clone(&*b));
+ let filter =
+ OwningHandle::new_with_fn(bloom_arc, |x| unsafe { x.as_ref() }.unwrap().borrow_mut());
+ debug_assert!(
+ filter.is_zeroed(),
+ "Forgot to zero the bloom filter last time"
+ );
+ StyleBloom {
+ filter,
+ elements: Default::default(),
+ pushed_hashes: Default::default(),
+ }
+ }
+
+ /// Return the bloom filter used properly by the `selectors` crate.
+ pub fn filter(&self) -> &BloomFilter {
+ &*self.filter
+ }
+
+ /// Push an element to the bloom filter, knowing that it's a child of the
+ /// last element parent.
+ pub fn push(&mut self, element: E) {
+ if cfg!(debug_assertions) {
+ if self.elements.is_empty() {
+ assert!(element.traversal_parent().is_none());
+ }
+ }
+ self.push_internal(element);
+ }
+
+ /// Same as `push`, but without asserting, in order to use it from
+ /// `rebuild`.
+ fn push_internal(&mut self, element: E) {
+ let mut count = 0;
+ each_relevant_element_hash(element, |hash| {
+ count += 1;
+ self.filter.insert_hash(hash);
+ self.pushed_hashes.push(hash);
+ });
+ self.elements.push(PushedElement::new(element, count));
+ }
+
+ /// Pop the last element in the bloom filter and return it.
+ #[inline]
+ fn pop(&mut self) -> Option<E> {
+ let PushedElement {
+ element,
+ num_hashes,
+ } = self.elements.pop()?;
+ let popped_element = *element;
+
+ // Verify that the pushed hashes match the ones we'd get from the element.
+ let mut expected_hashes = vec![];
+ if cfg!(debug_assertions) {
+ each_relevant_element_hash(popped_element, |hash| expected_hashes.push(hash));
+ }
+
+ for _ in 0..num_hashes {
+ let hash = self.pushed_hashes.pop().unwrap();
+ debug_assert_eq!(expected_hashes.pop().unwrap(), hash);
+ self.filter.remove_hash(hash);
+ }
+
+ Some(popped_element)
+ }
+
+ /// Returns the DOM depth of elements that can be correctly
+ /// matched against the bloom filter (that is, the number of
+ /// elements in our list).
+ pub fn matching_depth(&self) -> usize {
+ self.elements.len()
+ }
+
+ /// Clears the bloom filter.
+ pub fn clear(&mut self) {
+ self.elements.clear();
+
+ if self.pushed_hashes.len() > MEMSET_CLEAR_THRESHOLD {
+ self.filter.clear();
+ self.pushed_hashes.clear();
+ } else {
+ for hash in self.pushed_hashes.drain(..) {
+ self.filter.remove_hash(hash);
+ }
+ debug_assert!(self.filter.is_zeroed());
+ }
+ }
+
+ /// Rebuilds the bloom filter up to the parent of the given element.
+ pub fn rebuild(&mut self, mut element: E) {
+ self.clear();
+
+ let mut parents_to_insert = SmallVec::<[E; 16]>::new();
+ while let Some(parent) = element.traversal_parent() {
+ parents_to_insert.push(parent);
+ element = parent;
+ }
+
+ for parent in parents_to_insert.drain(..).rev() {
+ self.push(parent);
+ }
+ }
+
+ /// In debug builds, asserts that all the parents of `element` are in the
+ /// bloom filter.
+ ///
+ /// Goes away in release builds.
+ pub fn assert_complete(&self, mut element: E) {
+ if cfg!(debug_assertions) {
+ let mut checked = 0;
+ while let Some(parent) = element.traversal_parent() {
+ assert_eq!(
+ parent,
+ *(self.elements[self.elements.len() - 1 - checked].element)
+ );
+ element = parent;
+ checked += 1;
+ }
+ assert_eq!(checked, self.elements.len());
+ }
+ }
+
+ /// Get the element that represents the chain of things inserted
+ /// into the filter right now. That chain is the given element
+ /// (if any) and its ancestors.
+ #[inline]
+ pub fn current_parent(&self) -> Option<E> {
+ self.elements.last().map(|ref el| *el.element)
+ }
+
+ /// Insert the parents of an element in the bloom filter, trying to recover
+ /// the filter if the last element inserted doesn't match.
+ ///
+ /// Gets the element depth in the dom, to make it efficient, or if not
+ /// provided always rebuilds the filter from scratch.
+ ///
+ /// Returns the new bloom filter depth, that the traversal code is
+ /// responsible to keep around if it wants to get an effective filter.
+ pub fn insert_parents_recovering(&mut self, element: E, element_depth: usize) {
+ // Easy case, we're in a different restyle, or we're empty.
+ if self.elements.is_empty() {
+ self.rebuild(element);
+ return;
+ }
+
+ let traversal_parent = match element.traversal_parent() {
+ Some(parent) => parent,
+ None => {
+ // Yay, another easy case.
+ self.clear();
+ return;
+ },
+ };
+
+ if self.current_parent() == Some(traversal_parent) {
+ // Ta da, cache hit, we're all done.
+ return;
+ }
+
+ if element_depth == 0 {
+ self.clear();
+ return;
+ }
+
+ // We should've early exited above.
+ debug_assert!(
+ element_depth != 0,
+ "We should have already cleared the bloom filter"
+ );
+ debug_assert!(!self.elements.is_empty(), "How! We should've just rebuilt!");
+
+ // Now the fun begins: We have the depth of the dom and the depth of the
+ // last element inserted in the filter, let's try to find a common
+ // parent.
+ //
+ // The current depth, that is, the depth of the last element inserted in
+ // the bloom filter, is the number of elements _minus one_, that is: if
+ // there's one element, it must be the root -> depth zero.
+ let mut current_depth = self.elements.len() - 1;
+
+ // If the filter represents an element too deep in the dom, we need to
+ // pop ancestors.
+ while current_depth > element_depth - 1 {
+ self.pop().expect("Emilio is bad at math");
+ current_depth -= 1;
+ }
+
+ // Now let's try to find a common parent in the bloom filter chain,
+ // starting with traversal_parent.
+ let mut common_parent = traversal_parent;
+ let mut common_parent_depth = element_depth - 1;
+
+ // Let's collect the parents we are going to need to insert once we've
+ // found the common one.
+ let mut parents_to_insert = SmallVec::<[E; 16]>::new();
+
+ // If the bloom filter still doesn't have enough elements, the common
+ // parent is up in the dom.
+ while common_parent_depth > current_depth {
+ // TODO(emilio): Seems like we could insert parents here, then
+ // reverse the slice.
+ parents_to_insert.push(common_parent);
+ common_parent = common_parent.traversal_parent().expect("We were lied to");
+ common_parent_depth -= 1;
+ }
+
+ // Now the two depths are the same.
+ debug_assert_eq!(common_parent_depth, current_depth);
+
+ // Happy case: The parents match, we only need to push the ancestors
+ // we've collected and we'll never enter in this loop.
+ //
+ // Not-so-happy case: Parent's don't match, so we need to keep going up
+ // until we find a common ancestor.
+ //
+ // Gecko currently models native anonymous content that conceptually
+ // hangs off the document (such as scrollbars) as a separate subtree
+ // from the document root.
+ //
+ // Thus it's possible with Gecko that we do not find any common
+ // ancestor.
+ while *(self.elements.last().unwrap().element) != common_parent {
+ parents_to_insert.push(common_parent);
+ self.pop().unwrap();
+ common_parent = match common_parent.traversal_parent() {
+ Some(parent) => parent,
+ None => {
+ debug_assert!(self.elements.is_empty());
+ if cfg!(feature = "gecko") {
+ break;
+ } else {
+ panic!("should have found a common ancestor");
+ }
+ },
+ }
+ }
+
+ // Now the parents match, so insert the stack of elements we have been
+ // collecting so far.
+ for parent in parents_to_insert.drain(..).rev() {
+ self.push(parent);
+ }
+
+ debug_assert_eq!(self.elements.len(), element_depth);
+
+ // We're done! Easy.
+ }
+}
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