Stop slicing selectors when noting dependencies, and match with an offset instead.

MozReview-Commit-ID: KLqmdRKygO0

components/selectors/matching.rs

@@ -160,6 +160,7 @@ pub fn matches_selector_list<E>(selector_list: &SelectorList<E::Impl>,
 {
     selector_list.0.iter().any(|selector_and_hashes| {
         matches_selector(&selector_and_hashes.selector,
+                         0,
                          &selector_and_hashes.hashes,
                          element,
                          context,
@@ -333,8 +334,16 @@ enum SelectorMatchingResult {
 }
 
 /// Matches a selector, fast-rejecting against a bloom filter.
+///
+/// We accept an offset to allow consumers to represent and match against partial
+/// selectors (indexed from the right). We use this API design, rather than
+/// having the callers pass a SelectorIter, because creating a SelectorIter
+/// requires dereferencing the selector to get the length, which adds an
+/// unncessary cache miss for cases when we can fast-reject with AncestorHashes
+/// (which the caller can store inline with the selector pointer).
 #[inline(always)]
 pub fn matches_selector<E, F>(selector: &Selector<E::Impl>,
+                              offset: usize,
                               hashes: &AncestorHashes,
                               element: &E,
                               context: &mut MatchingContext,
@@ -350,11 +359,12 @@ pub fn matches_selector<E, F>(selector: &Selector<E::Impl>,
         }
     }
 
-    matches_complex_selector(selector, element, context, flags_setter)
+    matches_complex_selector(selector, offset, element, context, flags_setter)
 }
 
 /// Matches a complex selector.
 pub fn matches_complex_selector<E, F>(complex_selector: &Selector<E::Impl>,
+                                      offset: usize,
                                       element: &E,
                                       context: &mut MatchingContext,
                                       flags_setter: &mut F)
@@ -362,11 +372,15 @@ pub fn matches_complex_selector<E, F>(complex_selector: &Selector<E::Impl>,
     where E: Element,
           F: FnMut(&E, ElementSelectorFlags),
 {
-    let mut iter = complex_selector.iter();
+    let mut iter = if offset == 0 {
+        complex_selector.iter()
+    } else {
+        complex_selector.iter_from(offset)
+    };
 
     if cfg!(debug_assertions) {
         if context.matching_mode == MatchingMode::ForStatelessPseudoElement {
-            assert!(complex_selector.iter().any(|c| {
+            assert!(iter.clone().any(|c| {
                 matches!(*c, Component::PseudoElement(..))
             }));
         }

components/selectors/parser.rs

@@ -177,9 +177,13 @@ pub struct AncestorHashes(pub [u32; NUM_ANCESTOR_HASHES]);
 
 impl AncestorHashes {
     pub fn new<Impl: SelectorImpl>(s: &Selector<Impl>) -> Self {
+        Self::from_iter(s.iter())
+    }
+
+    pub fn from_iter<Impl: SelectorImpl>(iter: SelectorIter<Impl>) -> Self {
         let mut hashes = [0; NUM_ANCESTOR_HASHES];
         // Compute ancestor hashes for the bloom filter.
-        let mut hash_iter = s.iter_ancestors()
+        let mut hash_iter = AncestorIter::new(iter)
                              .map(|x| x.ancestor_hash())
                              .filter(|x| x.is_some())
                              .map(|x| x.unwrap());
@@ -356,6 +360,16 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         }
     }
 
+    pub fn iter_from(&self, offset: usize) -> SelectorIter<Impl> {
+        // Note: selectors are stored left-to-right but logical order is right-to-left.
+        let slice = self.0.as_ref();
+        let iter = slice[..(slice.len() - offset)].iter().rev();
+        SelectorIter {
+            iter: iter,
+            next_combinator: None,
+        }
+    }
+
     /// Returns an iterator over the entire sequence of simple selectors and combinators,
     /// from right to left.
     pub fn iter_raw(&self) -> Rev<slice::Iter<Component<Impl>>> {
@@ -368,28 +382,6 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         self.0.iter()
     }
 
-    /// Returns an iterator over ancestor simple selectors. All combinators and
-    /// non-ancestor simple selectors will be skipped.
-    pub fn iter_ancestors(&self) -> AncestorIter<Impl> {
-        AncestorIter::new(self.iter())
-    }
-
-    /// Returns a Selector identical to |self| but with the rightmost |index| entries
-    /// removed.
-    pub fn slice_from(&self, index: usize) -> Self {
-        // Note that we convert the slice_from to slice_to because selectors are
-        // stored left-to-right but logical order is right-to-left.
-        Selector(self.0.clone().slice_to(self.0.len() - index), self.1)
-    }
-
-    /// Returns a Selector identical to |self| but with the leftmost |len() - index|
-    /// entries removed.
-    pub fn slice_to(&self, index: usize) -> Self {
-        // Note that we convert the slice_to to slice_from because selectors are
-        // stored left-to-right but logical order is right-to-left.
-        Selector(self.0.clone().slice_from(self.0.len() - index), self.1)
-    }
-
     /// Creates a Selector from a vec of Components. Used in tests.
     pub fn from_vec(vec: Vec<Component<Impl>>, specificity_and_flags: u32) -> Self {
         Selector(ArcSlice::new(vec.into_boxed_slice()), specificity_and_flags)