Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 401 insertions, 0 deletions

diff --git a/servo/components/style/bloom.rs b/servo/components/style/bloom.rs
new file mode 100644
index 0000000000..824acb7114
--- /dev/null
+++ b/servo/components/style/bloom.rs
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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.
+    }
+}