Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

2 files changed, 1083 insertions, 0 deletions

diff --git a/servo/components/style/sharing/checks.rs b/servo/components/style/sharing/checks.rs
new file mode 100644
index 0000000000..a691ac5c76
--- /dev/null
+++ b/servo/components/style/sharing/checks.rs
@@ -0,0 +1,182 @@
+/* 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/. */
+
+//! Different checks done during the style sharing process in order to determine
+//! quickly whether it's worth to share style, and whether two different
+//! elements can indeed share the same style.
+
+use crate::bloom::StyleBloom;
+use crate::context::SharedStyleContext;
+use crate::dom::TElement;
+use crate::sharing::{StyleSharingCandidate, StyleSharingTarget};
+use selectors::NthIndexCache;
+
+/// Determines whether a target and a candidate have compatible parents for
+/// sharing.
+pub fn parents_allow_sharing<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    // If the identity of the parent style isn't equal, we can't share. We check
+    // this first, because the result is cached.
+    if target.parent_style_identity() != candidate.parent_style_identity() {
+        return false;
+    }
+
+    // Siblings can always share.
+    let parent = target.inheritance_parent().unwrap();
+    let candidate_parent = candidate.element.inheritance_parent().unwrap();
+    if parent == candidate_parent {
+        return true;
+    }
+
+    // Cousins are a bit more complicated.
+    //
+    // The fact that the candidate is here means that its element does not anchor
+    // the relative selector. However, it may have considered relative selector(s)
+    // to compute its style, i.e. there's a rule `<..> :has(<..>) <..> candidate`.
+    // In this case, evaluating style sharing requires evaluating the relative
+    // selector for the target anyway.
+    if candidate.considered_relative_selector {
+        return false;
+    }
+
+    // If a parent element was already styled and we traversed past it without
+    // restyling it, that may be because our clever invalidation logic was able
+    // to prove that the styles of that element would remain unchanged despite
+    // changes to the id or class attributes. However, style sharing relies in
+    // the strong guarantee that all the classes and ids up the respective parent
+    // chains are identical. As such, if we skipped styling for one (or both) of
+    // the parents on this traversal, we can't share styles across cousins.
+    //
+    // This is a somewhat conservative check. We could tighten it by having the
+    // invalidation logic explicitly flag elements for which it ellided styling.
+    let parent_data = parent.borrow_data().unwrap();
+    let candidate_parent_data = candidate_parent.borrow_data().unwrap();
+    if !parent_data.safe_for_cousin_sharing() || !candidate_parent_data.safe_for_cousin_sharing() {
+        return false;
+    }
+
+    true
+}
+
+/// Whether two elements have the same same style attribute (by pointer identity).
+pub fn have_same_style_attribute<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    match (target.style_attribute(), candidate.style_attribute()) {
+        (None, None) => true,
+        (Some(_), None) | (None, Some(_)) => false,
+        (Some(a), Some(b)) => &*a as *const _ == &*b as *const _,
+    }
+}
+
+/// Whether two elements have the same same presentational attributes.
+pub fn have_same_presentational_hints<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.pres_hints() == candidate.pres_hints()
+}
+
+/// Whether a given element has the same class attribute as a given candidate.
+///
+/// We don't try to share style across elements with different class attributes.
+pub fn have_same_class<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.class_list() == candidate.class_list()
+}
+
+/// Whether a given element has the same part attribute as a given candidate.
+///
+/// We don't try to share style across elements with different part attributes.
+pub fn have_same_parts<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.part_list() == candidate.part_list()
+}
+
+/// Whether a given element and a candidate match the same set of "revalidation"
+/// selectors.
+///
+/// Revalidation selectors are those that depend on the DOM structure, like
+/// :first-child, etc, or on attributes that we don't check off-hand (pretty
+/// much every attribute selector except `id` and `class`.
+#[inline]
+pub fn revalidate<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+    shared_context: &SharedStyleContext,
+    bloom: &StyleBloom<E>,
+    nth_index_cache: &mut NthIndexCache,
+) -> bool
+where
+    E: TElement,
+{
+    let stylist = &shared_context.stylist;
+
+    let for_element = target.revalidation_match_results(stylist, bloom, nth_index_cache);
+
+    let for_candidate = candidate.revalidation_match_results(stylist, bloom, nth_index_cache);
+
+    // This assert "ensures", to some extent, that the two candidates have
+    // matched the same rulehash buckets, and as such, that the bits we're
+    // comparing represent the same set of selectors.
+    debug_assert_eq!(for_element.len(), for_candidate.len());
+
+    for_element == for_candidate
+}
+
+/// Checks whether we might have rules for either of the two ids.
+#[inline]
+pub fn may_match_different_id_rules<E>(
+    shared_context: &SharedStyleContext,
+    element: E,
+    candidate: E,
+) -> bool
+where
+    E: TElement,
+{
+    let element_id = element.id();
+    let candidate_id = candidate.id();
+
+    if element_id == candidate_id {
+        return false;
+    }
+
+    let stylist = &shared_context.stylist;
+
+    let may_have_rules_for_element = match element_id {
+        Some(id) => stylist.may_have_rules_for_id(id, element),
+        None => false,
+    };
+
+    if may_have_rules_for_element {
+        return true;
+    }
+
+    match candidate_id {
+        Some(id) => stylist.may_have_rules_for_id(id, candidate),
+        None => false,
+    }
+}
diff --git a/servo/components/style/sharing/mod.rs b/servo/components/style/sharing/mod.rs
new file mode 100644
index 0000000000..31a9587a85
--- /dev/null
+++ b/servo/components/style/sharing/mod.rs
@@ -0,0 +1,901 @@
+/* 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/. */
+
+//! Code related to the style sharing cache, an optimization that allows similar
+//! nodes to share style without having to run selector matching twice.
+//!
+//! The basic setup is as follows.  We have an LRU cache of style sharing
+//! candidates.  When we try to style a target element, we first check whether
+//! we can quickly determine that styles match something in this cache, and if
+//! so we just use the cached style information.  This check is done with a
+//! StyleBloom filter set up for the target element, which may not be a correct
+//! state for the cached candidate element if they're cousins instead of
+//! siblings.
+//!
+//! The complicated part is determining that styles match.  This is subject to
+//! the following constraints:
+//!
+//! 1) The target and candidate must be inheriting the same styles.
+//! 2) The target and candidate must have exactly the same rules matching them.
+//! 3) The target and candidate must have exactly the same non-selector-based
+//!    style information (inline styles, presentation hints).
+//! 4) The target and candidate must have exactly the same rules matching their
+//!    pseudo-elements, because an element's style data points to the style
+//!    data for its pseudo-elements.
+//!
+//! These constraints are satisfied in the following ways:
+//!
+//! * We check that the parents of the target and the candidate have the same
+//!   computed style.  This addresses constraint 1.
+//!
+//! * We check that the target and candidate have the same inline style and
+//!   presentation hint declarations.  This addresses constraint 3.
+//!
+//! * We ensure that a target matches a candidate only if they have the same
+//!   matching result for all selectors that target either elements or the
+//!   originating elements of pseudo-elements.  This addresses constraint 4
+//!   (because it prevents a target that has pseudo-element styles from matching
+//!   a candidate that has different pseudo-element styles) as well as
+//!   constraint 2.
+//!
+//! The actual checks that ensure that elements match the same rules are
+//! conceptually split up into two pieces.  First, we do various checks on
+//! elements that make sure that the set of possible rules in all selector maps
+//! in the stylist (for normal styling and for pseudo-elements) that might match
+//! the two elements is the same.  For example, we enforce that the target and
+//! candidate must have the same localname and namespace.  Second, we have a
+//! selector map of "revalidation selectors" that the stylist maintains that we
+//! actually match against the target and candidate and then check whether the
+//! two sets of results were the same.  Due to the up-front selector map checks,
+//! we know that the target and candidate will be matched against the same exact
+//! set of revalidation selectors, so the match result arrays can be compared
+//! directly.
+//!
+//! It's very important that a selector be added to the set of revalidation
+//! selectors any time there are two elements that could pass all the up-front
+//! checks but match differently against some ComplexSelector in the selector.
+//! If that happens, then they can have descendants that might themselves pass
+//! the up-front checks but would have different matching results for the
+//! selector in question.  In this case, "descendants" includes pseudo-elements,
+//! so there is a single selector map of revalidation selectors that includes
+//! both selectors targeting elements and selectors targeting pseudo-element
+//! originating elements.  We ensure that the pseudo-element parts of all these
+//! selectors are effectively stripped off, so that matching them all against
+//! elements makes sense.
+
+use crate::applicable_declarations::ApplicableDeclarationBlock;
+use crate::bloom::StyleBloom;
+use crate::computed_value_flags::ComputedValueFlags;
+use crate::context::{SharedStyleContext, StyleContext};
+use crate::dom::{SendElement, TElement};
+use crate::properties::ComputedValues;
+use crate::rule_tree::StrongRuleNode;
+use crate::style_resolver::{PrimaryStyle, ResolvedElementStyles};
+use crate::stylist::Stylist;
+use crate::values::AtomIdent;
+use atomic_refcell::{AtomicRefCell, AtomicRefMut};
+use owning_ref::OwningHandle;
+use selectors::matching::{NeedsSelectorFlags, VisitedHandlingMode};
+use selectors::NthIndexCache;
+use servo_arc::Arc;
+use smallbitvec::SmallBitVec;
+use smallvec::SmallVec;
+use std::marker::PhantomData;
+use std::mem::{self, ManuallyDrop};
+use std::ops::Deref;
+use std::ptr::NonNull;
+use uluru::LRUCache;
+
+mod checks;
+
+/// The amount of nodes that the style sharing candidate cache should hold at
+/// most.
+///
+/// The cache size was chosen by measuring style sharing and resulting
+/// performance on a few pages; sizes up to about 32 were giving good sharing
+/// improvements (e.g. 3x fewer styles having to be resolved than at size 8) and
+/// slight performance improvements.  Sizes larger than 32 haven't really been
+/// tested.
+pub const SHARING_CACHE_SIZE: usize = 32;
+
+/// Opaque pointer type to compare ComputedValues identities.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct OpaqueComputedValues(NonNull<()>);
+
+unsafe impl Send for OpaqueComputedValues {}
+unsafe impl Sync for OpaqueComputedValues {}
+
+impl OpaqueComputedValues {
+    fn from(cv: &ComputedValues) -> Self {
+        let p =
+            unsafe { NonNull::new_unchecked(cv as *const ComputedValues as *const () as *mut ()) };
+        OpaqueComputedValues(p)
+    }
+
+    fn eq(&self, cv: &ComputedValues) -> bool {
+        Self::from(cv) == *self
+    }
+}
+
+/// Some data we want to avoid recomputing all the time while trying to share
+/// style.
+#[derive(Debug, Default)]
+pub struct ValidationData {
+    /// The class list of this element.
+    ///
+    /// TODO(emilio): Maybe check whether rules for these classes apply to the
+    /// element?
+    class_list: Option<SmallVec<[AtomIdent; 5]>>,
+
+    /// The part list of this element.
+    ///
+    /// TODO(emilio): Maybe check whether rules with these part names apply to
+    /// the element?
+    part_list: Option<SmallVec<[AtomIdent; 5]>>,
+
+    /// The list of presentational attributes of the element.
+    pres_hints: Option<SmallVec<[ApplicableDeclarationBlock; 5]>>,
+
+    /// The pointer identity of the parent ComputedValues.
+    parent_style_identity: Option<OpaqueComputedValues>,
+
+    /// The cached result of matching this entry against the revalidation
+    /// selectors.
+    revalidation_match_results: Option<SmallBitVec>,
+}
+
+impl ValidationData {
+    /// Move the cached data to a new instance, and return it.
+    pub fn take(&mut self) -> Self {
+        mem::replace(self, Self::default())
+    }
+
+    /// Get or compute the list of presentational attributes associated with
+    /// this element.
+    pub fn pres_hints<E>(&mut self, element: E) -> &[ApplicableDeclarationBlock]
+    where
+        E: TElement,
+    {
+        self.pres_hints.get_or_insert_with(|| {
+            let mut pres_hints = SmallVec::new();
+            element.synthesize_presentational_hints_for_legacy_attributes(
+                VisitedHandlingMode::AllLinksUnvisited,
+                &mut pres_hints,
+            );
+            pres_hints
+        })
+    }
+
+    /// Get or compute the part-list associated with this element.
+    pub fn part_list<E>(&mut self, element: E) -> &[AtomIdent]
+    where
+        E: TElement,
+    {
+        if !element.has_part_attr() {
+            return &[];
+        }
+        self.part_list.get_or_insert_with(|| {
+            let mut list = SmallVec::<[_; 5]>::new();
+            element.each_part(|p| list.push(p.clone()));
+            // See below for the reasoning.
+            if !list.spilled() {
+                list.sort_unstable_by_key(|a| a.get_hash());
+            }
+            list
+        })
+    }
+
+    /// Get or compute the class-list associated with this element.
+    pub fn class_list<E>(&mut self, element: E) -> &[AtomIdent]
+    where
+        E: TElement,
+    {
+        self.class_list.get_or_insert_with(|| {
+            let mut list = SmallVec::<[_; 5]>::new();
+            element.each_class(|c| list.push(c.clone()));
+            // Assuming there are a reasonable number of classes (we use the
+            // inline capacity as "reasonable number"), sort them to so that
+            // we don't mistakenly reject sharing candidates when one element
+            // has "foo bar" and the other has "bar foo".
+            if !list.spilled() {
+                list.sort_unstable_by_key(|a| a.get_hash());
+            }
+            list
+        })
+    }
+
+    /// Get or compute the parent style identity.
+    pub fn parent_style_identity<E>(&mut self, el: E) -> OpaqueComputedValues
+    where
+        E: TElement,
+    {
+        self.parent_style_identity
+            .get_or_insert_with(|| {
+                let parent = el.inheritance_parent().unwrap();
+                let values =
+                    OpaqueComputedValues::from(parent.borrow_data().unwrap().styles.primary());
+                values
+            })
+            .clone()
+    }
+
+    /// Computes the revalidation results if needed, and returns it.
+    /// Inline so we know at compile time what bloom_known_valid is.
+    #[inline]
+    fn revalidation_match_results<E>(
+        &mut self,
+        element: E,
+        stylist: &Stylist,
+        bloom: &StyleBloom<E>,
+        nth_index_cache: &mut NthIndexCache,
+        bloom_known_valid: bool,
+        needs_selector_flags: NeedsSelectorFlags,
+    ) -> &SmallBitVec
+    where
+        E: TElement,
+    {
+        self.revalidation_match_results.get_or_insert_with(|| {
+            // The bloom filter may already be set up for our element.
+            // If it is, use it.  If not, we must be in a candidate
+            // (i.e. something in the cache), and the element is one
+            // of our cousins, not a sibling.  In that case, we'll
+            // just do revalidation selector matching without a bloom
+            // filter, to avoid thrashing the filter.
+            let bloom_to_use = if bloom_known_valid {
+                debug_assert_eq!(bloom.current_parent(), element.traversal_parent());
+                Some(bloom.filter())
+            } else {
+                if bloom.current_parent() == element.traversal_parent() {
+                    Some(bloom.filter())
+                } else {
+                    None
+                }
+            };
+            stylist.match_revalidation_selectors(
+                element,
+                bloom_to_use,
+                nth_index_cache,
+                needs_selector_flags,
+            )
+        })
+    }
+}
+
+/// Information regarding a style sharing candidate, that is, an entry in the
+/// style sharing cache.
+///
+/// Note that this information is stored in TLS and cleared after the traversal,
+/// and once here, the style information of the element is immutable, so it's
+/// safe to access.
+///
+/// Important: If you change the members/layout here, You need to do the same for
+/// FakeCandidate below.
+#[derive(Debug)]
+pub struct StyleSharingCandidate<E: TElement> {
+    /// The element.
+    element: E,
+    validation_data: ValidationData,
+    considered_relative_selector: bool,
+}
+
+struct FakeCandidate {
+    _element: usize,
+    _validation_data: ValidationData,
+    _considered_relative_selector: bool,
+}
+
+impl<E: TElement> Deref for StyleSharingCandidate<E> {
+    type Target = E;
+
+    fn deref(&self) -> &Self::Target {
+        &self.element
+    }
+}
+
+impl<E: TElement> StyleSharingCandidate<E> {
+    /// Get the classlist of this candidate.
+    fn class_list(&mut self) -> &[AtomIdent] {
+        self.validation_data.class_list(self.element)
+    }
+
+    /// Get the part list of this candidate.
+    fn part_list(&mut self) -> &[AtomIdent] {
+        self.validation_data.part_list(self.element)
+    }
+
+    /// Get the pres hints of this candidate.
+    fn pres_hints(&mut self) -> &[ApplicableDeclarationBlock] {
+        self.validation_data.pres_hints(self.element)
+    }
+
+    /// Get the parent style identity.
+    fn parent_style_identity(&mut self) -> OpaqueComputedValues {
+        self.validation_data.parent_style_identity(self.element)
+    }
+
+    /// Compute the bit vector of revalidation selector match results
+    /// for this candidate.
+    fn revalidation_match_results(
+        &mut self,
+        stylist: &Stylist,
+        bloom: &StyleBloom<E>,
+        nth_index_cache: &mut NthIndexCache,
+    ) -> &SmallBitVec {
+        self.validation_data.revalidation_match_results(
+            self.element,
+            stylist,
+            bloom,
+            nth_index_cache,
+            /* bloom_known_valid = */ false,
+            // The candidate must already have the right bits already, if
+            // needed.
+            NeedsSelectorFlags::No,
+        )
+    }
+}
+
+impl<E: TElement> PartialEq<StyleSharingCandidate<E>> for StyleSharingCandidate<E> {
+    fn eq(&self, other: &Self) -> bool {
+        self.element == other.element
+    }
+}
+
+/// An element we want to test against the style sharing cache.
+pub struct StyleSharingTarget<E: TElement> {
+    element: E,
+    validation_data: ValidationData,
+}
+
+impl<E: TElement> Deref for StyleSharingTarget<E> {
+    type Target = E;
+
+    fn deref(&self) -> &Self::Target {
+        &self.element
+    }
+}
+
+impl<E: TElement> StyleSharingTarget<E> {
+    /// Trivially construct a new StyleSharingTarget to test against the cache.
+    pub fn new(element: E) -> Self {
+        Self {
+            element: element,
+            validation_data: ValidationData::default(),
+        }
+    }
+
+    fn class_list(&mut self) -> &[AtomIdent] {
+        self.validation_data.class_list(self.element)
+    }
+
+    fn part_list(&mut self) -> &[AtomIdent] {
+        self.validation_data.part_list(self.element)
+    }
+
+    /// Get the pres hints of this candidate.
+    fn pres_hints(&mut self) -> &[ApplicableDeclarationBlock] {
+        self.validation_data.pres_hints(self.element)
+    }
+
+    /// Get the parent style identity.
+    fn parent_style_identity(&mut self) -> OpaqueComputedValues {
+        self.validation_data.parent_style_identity(self.element)
+    }
+
+    fn revalidation_match_results(
+        &mut self,
+        stylist: &Stylist,
+        bloom: &StyleBloom<E>,
+        nth_index_cache: &mut NthIndexCache,
+    ) -> &SmallBitVec {
+        // It's important to set the selector flags. Otherwise, if we succeed in
+        // sharing the style, we may not set the slow selector flags for the
+        // right elements (which may not necessarily be |element|), causing
+        // missed restyles after future DOM mutations.
+        //
+        // Gecko's test_bug534804.html exercises this. A minimal testcase is:
+        // <style> #e:empty + span { ... } </style>
+        // <span id="e">
+        //   <span></span>
+        // </span>
+        // <span></span>
+        //
+        // The style sharing cache will get a hit for the second span. When the
+        // child span is subsequently removed from the DOM, missing selector
+        // flags would cause us to miss the restyle on the second span.
+        self.validation_data.revalidation_match_results(
+            self.element,
+            stylist,
+            bloom,
+            nth_index_cache,
+            /* bloom_known_valid = */ true,
+            NeedsSelectorFlags::Yes,
+        )
+    }
+
+    /// Attempts to share a style with another node.
+    pub fn share_style_if_possible(
+        &mut self,
+        context: &mut StyleContext<E>,
+    ) -> Option<ResolvedElementStyles> {
+        let cache = &mut context.thread_local.sharing_cache;
+        let shared_context = &context.shared;
+        let bloom_filter = &context.thread_local.bloom_filter;
+        let nth_index_cache = &mut context.thread_local.nth_index_cache;
+
+        if cache.dom_depth != bloom_filter.matching_depth() {
+            debug!(
+                "Can't share style, because DOM depth changed from {:?} to {:?}, element: {:?}",
+                cache.dom_depth,
+                bloom_filter.matching_depth(),
+                self.element
+            );
+            return None;
+        }
+        debug_assert_eq!(
+            bloom_filter.current_parent(),
+            self.element.traversal_parent()
+        );
+
+        cache.share_style_if_possible(shared_context, bloom_filter, nth_index_cache, self)
+    }
+
+    /// Gets the validation data used to match against this target, if any.
+    pub fn take_validation_data(&mut self) -> ValidationData {
+        self.validation_data.take()
+    }
+}
+
+struct SharingCacheBase<Candidate> {
+    entries: LRUCache<Candidate, SHARING_CACHE_SIZE>,
+}
+
+impl<Candidate> Default for SharingCacheBase<Candidate> {
+    fn default() -> Self {
+        Self {
+            entries: LRUCache::default(),
+        }
+    }
+}
+
+impl<Candidate> SharingCacheBase<Candidate> {
+    fn clear(&mut self) {
+        self.entries.clear();
+    }
+
+    fn is_empty(&self) -> bool {
+        self.entries.len() == 0
+    }
+}
+
+impl<E: TElement> SharingCache<E> {
+    fn insert(
+        &mut self,
+        element: E,
+        considered_relative_selector: bool,
+        validation_data_holder: Option<&mut StyleSharingTarget<E>>,
+    ) {
+        let validation_data = match validation_data_holder {
+            Some(v) => v.take_validation_data(),
+            None => ValidationData::default(),
+        };
+        self.entries.insert(StyleSharingCandidate {
+            element,
+            considered_relative_selector,
+            validation_data,
+        });
+    }
+}
+
+/// Style sharing caches are are large allocations, so we store them in thread-local
+/// storage such that they can be reused across style traversals. Ideally, we'd just
+/// stack-allocate these buffers with uninitialized memory, but right now rustc can't
+/// avoid memmoving the entire cache during setup, which gets very expensive. See
+/// issues like [1] and [2].
+///
+/// Given that the cache stores entries of type TElement, we transmute to usize
+/// before storing in TLS. This is safe as long as we make sure to empty the cache
+/// before we let it go.
+///
+/// [1] https://github.com/rust-lang/rust/issues/42763
+/// [2] https://github.com/rust-lang/rust/issues/13707
+type SharingCache<E> = SharingCacheBase<StyleSharingCandidate<E>>;
+type TypelessSharingCache = SharingCacheBase<FakeCandidate>;
+type StoredSharingCache = Arc<AtomicRefCell<TypelessSharingCache>>;
+
+thread_local! {
+    // See the comment on bloom.rs about why do we leak this.
+    static SHARING_CACHE_KEY: ManuallyDrop<StoredSharingCache> =
+        ManuallyDrop::new(Arc::new_leaked(Default::default()));
+}
+
+/// An LRU cache of the last few nodes seen, so that we can aggressively try to
+/// reuse their styles.
+///
+/// Note that this cache is flushed every time we steal work from the queue, so
+/// storing nodes here temporarily is safe.
+pub struct StyleSharingCache<E: TElement> {
+    /// The LRU cache, with the type cast away to allow persisting the allocation.
+    cache_typeless: OwningHandle<StoredSharingCache, AtomicRefMut<'static, TypelessSharingCache>>,
+    /// Bind this structure to the lifetime of E, since that's what we effectively store.
+    marker: PhantomData<SendElement<E>>,
+    /// The DOM depth we're currently at.  This is used as an optimization to
+    /// clear the cache when we change depths, since we know at that point
+    /// nothing in the cache will match.
+    dom_depth: usize,
+}
+
+impl<E: TElement> Drop for StyleSharingCache<E> {
+    fn drop(&mut self) {
+        self.clear();
+    }
+}
+
+impl<E: TElement> StyleSharingCache<E> {
+    #[allow(dead_code)]
+    fn cache(&self) -> &SharingCache<E> {
+        let base: &TypelessSharingCache = &*self.cache_typeless;
+        unsafe { mem::transmute(base) }
+    }
+
+    fn cache_mut(&mut self) -> &mut SharingCache<E> {
+        let base: &mut TypelessSharingCache = &mut *self.cache_typeless;
+        unsafe { mem::transmute(base) }
+    }
+
+    /// Create a new style sharing candidate cache.
+
+    // 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 {
+        assert_eq!(
+            mem::size_of::<SharingCache<E>>(),
+            mem::size_of::<TypelessSharingCache>()
+        );
+        assert_eq!(
+            mem::align_of::<SharingCache<E>>(),
+            mem::align_of::<TypelessSharingCache>()
+        );
+        let cache_arc = SHARING_CACHE_KEY.with(|c| Arc::clone(&*c));
+        let cache =
+            OwningHandle::new_with_fn(cache_arc, |x| unsafe { x.as_ref() }.unwrap().borrow_mut());
+        debug_assert!(cache.is_empty());
+
+        StyleSharingCache {
+            cache_typeless: cache,
+            marker: PhantomData,
+            dom_depth: 0,
+        }
+    }
+
+    /// Tries to insert an element in the style sharing cache.
+    ///
+    /// Fails if we know it should never be in the cache.
+    ///
+    /// NB: We pass a source for the validation data, rather than the data itself,
+    /// to avoid memmoving at each function call. See rust issue #42763.
+    pub fn insert_if_possible(
+        &mut self,
+        element: &E,
+        style: &PrimaryStyle,
+        validation_data_holder: Option<&mut StyleSharingTarget<E>>,
+        dom_depth: usize,
+        shared_context: &SharedStyleContext,
+    ) {
+        let parent = match element.traversal_parent() {
+            Some(element) => element,
+            None => {
+                debug!("Failing to insert to the cache: no parent element");
+                return;
+            },
+        };
+
+        if !element.matches_user_and_content_rules() {
+            debug!("Failing to insert into the cache: no tree rules:");
+            return;
+        }
+
+        // We can't share style across shadow hosts right now, because they may
+        // match different :host rules.
+        //
+        // TODO(emilio): We could share across the ones that don't have :host
+        // rules or have the same.
+        if element.shadow_root().is_some() {
+            debug!("Failing to insert into the cache: Shadow Host");
+            return;
+        }
+
+        // If the element has running animations, we can't share style.
+        //
+        // This is distinct from the specifies_{animations,transitions} check below,
+        // because:
+        //   * Animations can be triggered directly via the Web Animations API.
+        //   * Our computed style can still be affected by animations after we no
+        //     longer match any animation rules, since removing animations involves
+        //     a sequential task and an additional traversal.
+        if element.has_animations(shared_context) {
+            debug!("Failing to insert to the cache: running animations");
+            return;
+        }
+
+        // If this element was considered for matching a relative selector, we can't
+        // share style, as that requires evaluating the relative selector in the
+        // first place. A couple notes:
+        // - This means that a document that contains a standalone `:has()`
+        //   rule would basically turn style sharing off.
+        // - Since the flag is set on the element, we may be overly pessimistic:
+        //   For example, given `<div class="foo"><div class="bar"></div></div>`,
+        //   if we run into a `.foo:has(.bar) .baz` selector, we refuse any selector
+        //   matching `.foo`, even if `:has()` may not even be used. Ideally we'd
+        //   have something like `RelativeSelectorConsidered::RightMost`, but the
+        //   element flag is required for invalidation, and this reduces more tracking.
+        if element.anchors_relative_selector() {
+            debug!("Failing to insert to the cache: may anchor relative selector");
+            return;
+        }
+
+        // In addition to the above running animations check, we also need to
+        // check CSS animation and transition styles since it's possible that
+        // we are about to create CSS animations/transitions.
+        //
+        // These are things we don't check in the candidate match because they
+        // are either uncommon or expensive.
+        let ui_style = style.style().get_ui();
+        if ui_style.specifies_transitions() {
+            debug!("Failing to insert to the cache: transitions");
+            return;
+        }
+
+        if ui_style.specifies_animations() {
+            debug!("Failing to insert to the cache: animations");
+            return;
+        }
+
+        debug!(
+            "Inserting into cache: {:?} with parent {:?}",
+            element, parent
+        );
+
+        if self.dom_depth != dom_depth {
+            debug!(
+                "Clearing cache because depth changed from {:?} to {:?}, element: {:?}",
+                self.dom_depth, dom_depth, element
+            );
+            self.clear();
+            self.dom_depth = dom_depth;
+        }
+        self.cache_mut().insert(
+            *element,
+            style
+                .style
+                .0
+                .flags
+                .intersects(ComputedValueFlags::CONSIDERED_RELATIVE_SELECTOR),
+            validation_data_holder,
+        );
+    }
+
+    /// Clear the style sharing candidate cache.
+    pub fn clear(&mut self) {
+        self.cache_mut().clear();
+    }
+
+    /// Attempts to share a style with another node.
+    fn share_style_if_possible(
+        &mut self,
+        shared_context: &SharedStyleContext,
+        bloom_filter: &StyleBloom<E>,
+        nth_index_cache: &mut NthIndexCache,
+        target: &mut StyleSharingTarget<E>,
+    ) -> Option<ResolvedElementStyles> {
+        if shared_context.options.disable_style_sharing_cache {
+            debug!(
+                "{:?} Cannot share style: style sharing cache disabled",
+                target.element
+            );
+            return None;
+        }
+
+        if target.inheritance_parent().is_none() {
+            debug!(
+                "{:?} Cannot share style: element has no parent",
+                target.element
+            );
+            return None;
+        }
+
+        if !target.matches_user_and_content_rules() {
+            debug!("{:?} Cannot share style: content rules", target.element);
+            return None;
+        }
+
+        self.cache_mut().entries.lookup(|candidate| {
+            Self::test_candidate(
+                target,
+                candidate,
+                &shared_context,
+                bloom_filter,
+                nth_index_cache,
+                shared_context,
+            )
+        })
+    }
+
+    fn test_candidate(
+        target: &mut StyleSharingTarget<E>,
+        candidate: &mut StyleSharingCandidate<E>,
+        shared: &SharedStyleContext,
+        bloom: &StyleBloom<E>,
+        nth_index_cache: &mut NthIndexCache,
+        shared_context: &SharedStyleContext,
+    ) -> Option<ResolvedElementStyles> {
+        debug_assert!(target.matches_user_and_content_rules());
+
+        // Check that we have the same parent, or at least that the parents
+        // share styles and permit sharing across their children. The latter
+        // check allows us to share style between cousins if the parents
+        // shared style.
+        if !checks::parents_allow_sharing(target, candidate) {
+            trace!("Miss: Parent");
+            return None;
+        }
+
+        if target.local_name() != candidate.element.local_name() {
+            trace!("Miss: Local Name");
+            return None;
+        }
+
+        if target.namespace() != candidate.element.namespace() {
+            trace!("Miss: Namespace");
+            return None;
+        }
+
+        // We do not ignore visited state here, because Gecko needs to store
+        // extra bits on visited styles, so these contexts cannot be shared.
+        if target.element.state() != candidate.state() {
+            trace!("Miss: User and Author State");
+            return None;
+        }
+
+        if target.is_link() != candidate.element.is_link() {
+            trace!("Miss: Link");
+            return None;
+        }
+
+        // If two elements belong to different shadow trees, different rules may
+        // apply to them, from the respective trees.
+        if target.element.containing_shadow() != candidate.element.containing_shadow() {
+            trace!("Miss: Different containing shadow roots");
+            return None;
+        }
+
+        // If the elements are not assigned to the same slot they could match
+        // different ::slotted() rules in the slot scope.
+        //
+        // If two elements are assigned to different slots, even within the same
+        // shadow root, they could match different rules, due to the slot being
+        // assigned to yet another slot in another shadow root.
+        if target.element.assigned_slot() != candidate.element.assigned_slot() {
+            // TODO(emilio): We could have a look at whether the shadow roots
+            // actually have slotted rules and such.
+            trace!("Miss: Different assigned slots");
+            return None;
+        }
+
+        if target.element.shadow_root().is_some() {
+            trace!("Miss: Shadow host");
+            return None;
+        }
+
+        if target.element.has_animations(shared_context) {
+            trace!("Miss: Has Animations");
+            return None;
+        }
+
+        if target.matches_user_and_content_rules() !=
+            candidate.element.matches_user_and_content_rules()
+        {
+            trace!("Miss: User and Author Rules");
+            return None;
+        }
+
+        // It's possible that there are no styles for either id.
+        if checks::may_match_different_id_rules(shared, target.element, candidate.element) {
+            trace!("Miss: ID Attr");
+            return None;
+        }
+
+        if !checks::have_same_style_attribute(target, candidate) {
+            trace!("Miss: Style Attr");
+            return None;
+        }
+
+        if !checks::have_same_class(target, candidate) {
+            trace!("Miss: Class");
+            return None;
+        }
+
+        if !checks::have_same_presentational_hints(target, candidate) {
+            trace!("Miss: Pres Hints");
+            return None;
+        }
+
+        if !checks::have_same_parts(target, candidate) {
+            trace!("Miss: Shadow parts");
+            return None;
+        }
+
+        if !checks::revalidate(target, candidate, shared, bloom, nth_index_cache) {
+            trace!("Miss: Revalidation");
+            return None;
+        }
+
+        debug!(
+            "Sharing allowed between {:?} and {:?}",
+            target.element, candidate.element
+        );
+        Some(candidate.element.borrow_data().unwrap().share_styles())
+    }
+
+    /// Attempts to find an element in the cache with the given primary rule
+    /// node and parent.
+    ///
+    /// FIXME(emilio): re-measure this optimization, and remove if it's not very
+    /// useful... It's probably not worth the complexity / obscure bugs.
+    pub fn lookup_by_rules(
+        &mut self,
+        shared_context: &SharedStyleContext,
+        inherited: &ComputedValues,
+        rules: &StrongRuleNode,
+        visited_rules: Option<&StrongRuleNode>,
+        target: E,
+    ) -> Option<PrimaryStyle> {
+        if shared_context.options.disable_style_sharing_cache {
+            return None;
+        }
+
+        self.cache_mut().entries.lookup(|candidate| {
+            debug_assert_ne!(candidate.element, target);
+            if !candidate.parent_style_identity().eq(inherited) {
+                return None;
+            }
+            let data = candidate.element.borrow_data().unwrap();
+            let style = data.styles.primary();
+            if style.rules.as_ref() != Some(&rules) {
+                return None;
+            }
+            if style.visited_rules() != visited_rules {
+                return None;
+            }
+            // NOTE(emilio): We only need to check name / namespace because we
+            // do name-dependent style adjustments, like the display: contents
+            // to display: none adjustment.
+            if target.namespace() != candidate.element.namespace() {
+                return None;
+            }
+            if target.local_name() != candidate.element.local_name() {
+                return None;
+            }
+            // Rule nodes and styles are computed independent of the element's
+            // actual visitedness, but at the end of the cascade (in
+            // `adjust_for_visited`) we do store the visitedness as a flag in
+            // style.  (This is a subtle change from initial visited work that
+            // landed when computed values were fused, see
+            // https://bugzilla.mozilla.org/show_bug.cgi?id=1381635.)
+            // So at the moment, we need to additionally compare visitedness,
+            // since that is not accounted for by rule nodes alone.
+            // FIXME(jryans): This seems like it breaks the constant time
+            // requirements of visited, assuming we get a cache hit on only one
+            // of unvisited vs. visited.
+            // TODO(emilio): We no longer have such a flag, remove this check.
+            if target.is_visited_link() != candidate.element.is_visited_link() {
+                return None;
+            }
+
+            Some(data.share_primary_style())
+        })
+    }
+}