style: Allow sharing style for elements with presentational hints.

components/style/sharing/checks.rs

@@ -12,7 +12,6 @@ use element_state::*;
 use selectors::bloom::BloomFilter;
 use selectors::matching::StyleRelations;
 use sharing::{StyleSharingCandidate, StyleSharingTarget};
-use sink::ForgetfulSink;
 use stylearc::Arc;
 
 /// Determines, based on the results of selector matching, whether it's worth to
@@ -23,8 +22,7 @@ pub fn relations_are_shareable(relations: &StyleRelations) -> bool {
     use selectors::matching::*;
     !relations.intersects(AFFECTED_BY_ID_SELECTOR |
                           AFFECTED_BY_PSEUDO_ELEMENTS |
-                          AFFECTED_BY_STYLE_ATTRIBUTE |
-                          AFFECTED_BY_PRESENTATIONAL_HINTS)
+                          AFFECTED_BY_STYLE_ATTRIBUTE)
 }
 
 /// Whether, given two elements, they have pointer-equal computed values.
@@ -51,12 +49,13 @@ pub fn same_computed_values<E>(first: Option<E>, second: Option<E>) -> bool
 /// We consider not worth to share style with an element that has presentational
 /// hints, both because implementing the code that compares that the hints are
 /// equal is somewhat annoying, and also because it'd be expensive enough.
-pub fn has_presentational_hints<E>(element: E) -> bool
+pub fn have_same_presentational_hints<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>
+) -> bool
     where E: TElement,
 {
-    let mut hints = ForgetfulSink::new();
-    element.synthesize_presentational_hints_for_legacy_attributes(&mut hints);
-    !hints.is_empty()
+    target.pres_hints() == candidate.pres_hints()
 }
 
 /// Whether a given element has the same class attribute than a given candidate.

components/style/sharing/mod.rs

@@ -15,10 +15,9 @@ use matching::{ChildCascadeRequirement, MatchMethods};
 use properties::ComputedValues;
 use selectors::bloom::BloomFilter;
 use selectors::matching::{ElementSelectorFlags, StyleRelations};
-use sink::ForgetfulSink;
 use smallvec::SmallVec;
 use std::ops::Deref;
-use stylist::Stylist;
+use stylist::{ApplicableDeclarationBlock, Stylist};
 
 mod checks;
 
@@ -45,6 +44,9 @@ pub struct CachedStyleSharingData {
     /// a similar fashion as what Boris is doing for the ID attribute.
     class_list: Option<SmallVec<[Atom; 5]>>,
 
+    /// The list of presentational attributes of the element.
+    pres_hints: Option<SmallVec<[ApplicableDeclarationBlock; 5]>>,
+
     /// The cached result of matching this entry against the revalidation
     /// selectors.
     revalidation_match_results: Option<BitVec>,
@@ -56,6 +58,7 @@ impl CachedStyleSharingData {
     pub fn new() -> Self {
         Self {
             class_list: None,
+            pres_hints: None,
             revalidation_match_results: None,
         }
     }
@@ -64,10 +67,24 @@ impl CachedStyleSharingData {
     pub fn take(&mut self) -> Self {
         Self {
             class_list: self.class_list.take(),
+            pres_hints: self.pres_hints.take(),
             revalidation_match_results: self.revalidation_match_results.take(),
         }
     }
 
+    /// 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,
+    {
+        if self.pres_hints.is_none() {
+            let mut pres_hints = SmallVec::new();
+            element.synthesize_presentational_hints_for_legacy_attributes(&mut pres_hints);
+            self.pres_hints = Some(pres_hints);
+        }
+        &*self.pres_hints.as_ref().unwrap()
+    }
+
     /// Get or compute the class-list associated with this element.
     pub fn class_list<E>(&mut self, element: E) -> &[Atom]
         where E: TElement,
@@ -121,6 +138,11 @@ impl<E: TElement> StyleSharingCandidate<E> {
         self.cache.class_list(*self.element)
     }
 
+    /// Get the pres hints of this candidate.
+    fn pres_hints(&mut self) -> &[ApplicableDeclarationBlock] {
+        self.cache.pres_hints(*self.element)
+    }
+
     /// Get the classlist of this candidate.
     fn revalidation_match_results(
         &mut self,
@@ -167,6 +189,11 @@ impl<E: TElement> StyleSharingTarget<E> {
         self.cache.class_list(self.element)
     }
 
+    /// Get the pres hints of this candidate.
+    fn pres_hints(&mut self) -> &[ApplicableDeclarationBlock] {
+        self.cache.pres_hints(self.element)
+    }
+
     fn revalidation_match_results(
         &mut self,
         stylist: &Stylist,
@@ -307,7 +334,9 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
                               element: &E,
                               style: &ComputedValues,
                               relations: StyleRelations,
-                              cache: CachedStyleSharingData) {
+                              mut cache: CachedStyleSharingData) {
+        use selectors::matching::AFFECTED_BY_PRESENTATIONAL_HINTS;
+
         let parent = match element.parent_element() {
             Some(element) => element,
             None => {
@@ -328,14 +357,6 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
             return;
         }
 
-        // Make sure we noted any presentational hints in the StyleRelations.
-        if cfg!(debug_assertions) {
-            let mut hints = ForgetfulSink::new();
-            element.synthesize_presentational_hints_for_legacy_attributes(&mut hints);
-            debug_assert!(hints.is_empty(),
-                          "Style relations should not be shareable!");
-        }
-
         let box_style = style.get_box();
         if box_style.specifies_transitions() {
             debug!("Failing to insert to the cache: transitions");
@@ -347,6 +368,13 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
             return;
         }
 
+        // Take advantage of the information we've learned during
+        // selector-matching.
+        if !relations.intersects(AFFECTED_BY_PRESENTATIONAL_HINTS) {
+            debug_assert!(cache.pres_hints.as_ref().map_or(true, |v| v.is_empty()));
+            cache.pres_hints = Some(SmallVec::new());
+        }
+
         debug!("Inserting into cache: {:?} with parent {:?}", element, parent);
 
         self.cache.insert(StyleSharingCandidate {
@@ -532,7 +560,7 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
             miss!(Class)
         }
 
-        if checks::has_presentational_hints(target.element) {
+        if !checks::have_same_presentational_hints(target, candidate) {
             miss!(PresHints)
         }