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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;
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.
static BLOOM_KEY: Arc<AtomicRefCell<BloomFilter>> =
Arc::new(AtomicRefCell::new(BloomFilter::new()));
}
/// 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,
}
}
}
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()));
}
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| b.clone());
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: 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 true if the bloom filter is empty.
pub fn is_empty(&self) -> bool {
self.elements.is_empty()
}
/// 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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