Auto merge of #18595 - bholley:nth_index_cache, r=emilio

Implement an nth-index cache

https://bugzilla.mozilla.org/show_bug.cgi?id=1334730

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5 files changed, 135 insertions, 33 deletions

diff --git a/components/selectors/context.rs b/components/selectors/context.rs
index 46991d349ce..f0da3b14ef6 100644
--- a/components/selectors/context.rs
+++ b/components/selectors/context.rs
@@ -4,6 +4,7 @@
 
 use attr::CaseSensitivity;
 use bloom::BloomFilter;
+use nth_index_cache::NthIndexCache;
 
 /// What kind of selector matching mode we should use.
 ///
@@ -69,12 +70,6 @@ impl QuirksMode {
     }
 }
 
-/// A cache to speed up matching of nth-index-like selectors.
-///
-/// NB: This is just a dummy type right now, it will be fleshed out in later patches.
-#[derive(Default)]
-pub struct NthIndexCache(usize);
-
 /// Data associated with the matching process for a element.  This context is
 /// used across many selectors for an element, so it's not appropriate for
 /// transient data that applies to only a single selector.
@@ -84,7 +79,7 @@ pub struct MatchingContext<'a> {
     /// Input with the bloom filter used to fast-reject selectors.
     pub bloom_filter: Option<&'a BloomFilter>,
     /// An optional cache to speed up nth-index-like selectors.
-    nth_index_cache: Option<&'a mut NthIndexCache>,
+    pub nth_index_cache: Option<&'a mut NthIndexCache>,
     /// Input that controls how matching for links is handled.
     pub visited_handling: VisitedHandlingMode,
     /// Output that records whether we encountered a "relevant link" while
diff --git a/components/selectors/lib.rs b/components/selectors/lib.rs
index 2184dfc64e7..0805a53041f 100644
--- a/components/selectors/lib.rs
+++ b/components/selectors/lib.rs
@@ -24,6 +24,7 @@ pub mod bloom;
 mod builder;
 pub mod context;
 pub mod matching;
+mod nth_index_cache;
 pub mod parser;
 #[cfg(test)] mod size_of_tests;
 #[cfg(any(test, feature = "gecko_like_types"))] pub mod gecko_like_types;
@@ -31,5 +32,6 @@ pub mod sink;
 mod tree;
 pub mod visitor;
 
+pub use nth_index_cache::NthIndexCache;
 pub use parser::{SelectorImpl, Parser, SelectorList};
-pub use tree::Element;
+pub use tree::{Element, OpaqueElement};
diff --git a/components/selectors/matching.rs b/components/selectors/matching.rs
index afff6cd1e92..ff0dd40b484 100644
--- a/components/selectors/matching.rs
+++ b/components/selectors/matching.rs
@@ -4,6 +4,7 @@
 
 use attr::{ParsedAttrSelectorOperation, AttrSelectorOperation, NamespaceConstraint};
 use bloom::{BLOOM_HASH_MASK, BloomFilter};
+use nth_index_cache::NthIndexCacheInner;
 use parser::{AncestorHashes, Combinator, Component, LocalName};
 use parser::{Selector, SelectorImpl, SelectorIter, SelectorList};
 use std::borrow::Borrow;
@@ -787,7 +788,20 @@ fn matches_generic_nth_child<E, F>(element: &E,
         HAS_SLOW_SELECTOR_LATER_SIBLINGS
     });
 
-    let index = nth_child_index(element, is_of_type, is_from_end);
+    // Grab a reference to the appropriate cache.
+    let mut cache = context.shared.nth_index_cache.as_mut().map(|c| {
+        c.get(is_of_type, is_from_end)
+    });
+
+    // Lookup or compute the index.
+    let index = if let Some(i) = cache.as_mut().and_then(|c| c.lookup(element.opaque())) {
+        i
+    } else {
+        let i = nth_child_index(element, is_of_type, is_from_end, cache.as_mut().map(|s| &mut **s));
+        cache.as_mut().map(|c| c.insert(element.opaque(), i));
+        i
+    };
+    debug_assert_eq!(index, nth_child_index(element, is_of_type, is_from_end, None), "invalid cache");
 
     // Is there a non-negative integer n such that An+B=index?
     match index.checked_sub(b) {
@@ -800,40 +814,59 @@ fn matches_generic_nth_child<E, F>(element: &E,
 }
 
 #[inline]
+fn same_type<E: Element>(a: &E, b: &E) -> bool {
+    a.get_local_name() == b.get_local_name() &&
+    a.get_namespace() == b.get_namespace()
+}
+
+#[inline]
 fn nth_child_index<E>(
     element: &E,
     is_of_type: bool,
     is_from_end: bool,
+    mut cache: Option<&mut NthIndexCacheInner>,
 ) -> i32
 where
     E: Element,
 {
-    let mut index: i32 = 1;
-    let mut next_sibling = if is_from_end {
-        element.next_sibling_element()
-    } else {
-        element.prev_sibling_element()
-    };
-
-    loop {
-        let sibling = match next_sibling {
-            None => break,
-            Some(next_sibling) => next_sibling
-        };
+    // The traversal mostly processes siblings left to right. So when we walk
+    // siblings to the right when computing NthLast/NthLastOfType we're unlikely
+    // to get cache hits along the way. As such, we take the hit of walking the
+    // siblings to the left checking the cache in the is_from_end case (this
+    // matches what Gecko does). The indices-from-the-left is handled during the
+    // regular look further below.
+    if let Some(ref mut c) = cache {
+        if is_from_end && !c.is_empty() {
+            let mut index: i32 = 1;
+            let mut curr = element.clone();
+            while let Some(e) = curr.prev_sibling_element() {
+                curr = e;
+                if !is_of_type || same_type(element, &curr) {
+                    if let Some(i) = c.lookup(curr.opaque()) {
+                        return i - index;
+                    }
+                    index += 1;
+                }
+            }
+        }
+    }
 
-        if is_of_type {
-            if element.get_local_name() == sibling.get_local_name() &&
-                element.get_namespace() == sibling.get_namespace() {
-                index += 1;
+    let mut index: i32 = 1;
+    let mut curr = element.clone();
+    let next = |e: E| if is_from_end { e.next_sibling_element() } else { e.prev_sibling_element() };
+    while let Some(e) = next(curr) {
+        curr = e;
+        if !is_of_type || same_type(element, &curr) {
+            // If we're computing indices from the left, check each element in the
+            // cache. We handle the indices-from-the-right case at the top of this
+            // function.
+            if !is_from_end {
+                if let Some(i) = cache.as_mut().and_then(|c| c.lookup(curr.opaque())) {
+                    return i + index
+                }
             }
-        } else {
-          index += 1;
+            index += 1;
         }
-        next_sibling = if is_from_end {
-            sibling.next_sibling_element()
-        } else {
-            sibling.prev_sibling_element()
-        };
     }
 
     index
diff --git a/components/selectors/nth_index_cache.rs b/components/selectors/nth_index_cache.rs
new file mode 100644
index 00000000000..ebae7a6033a
--- /dev/null
+++ b/components/selectors/nth_index_cache.rs
@@ -0,0 +1,56 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use fnv::FnvHashMap;
+use tree::OpaqueElement;
+
+/// A cache to speed up matching of nth-index-like selectors.
+///
+/// See [1] for some discussion around the design tradeoffs.
+///
+/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1401855#c3
+#[derive(Default)]
+pub struct NthIndexCache {
+    nth: NthIndexCacheInner,
+    nth_last: NthIndexCacheInner,
+    nth_of_type: NthIndexCacheInner,
+    nth_last_of_type: NthIndexCacheInner,
+}
+
+impl NthIndexCache {
+    /// Gets the appropriate cache for the given parameters.
+    pub fn get(
+        &mut self,
+        is_of_type: bool,
+        is_from_end: bool
+    ) -> &mut NthIndexCacheInner {
+        match (is_of_type, is_from_end) {
+            (false, false) => &mut self.nth,
+            (false, true) => &mut self.nth_last,
+            (true, false) => &mut self.nth_of_type,
+            (true, true) => &mut self.nth_last_of_type,
+        }
+    }
+}
+
+/// The concrete per-pseudo-class cache.
+#[derive(Default)]
+pub struct NthIndexCacheInner(FnvHashMap<OpaqueElement, i32>);
+
+impl NthIndexCacheInner {
+    /// Does a lookup for a given element in the cache.
+    pub fn lookup(&mut self, el: OpaqueElement) -> Option<i32> {
+        self.0.get(&el).map(|x| *x)
+    }
+
+    /// Inserts an entry into the cache.
+    pub fn insert(&mut self, element: OpaqueElement, index: i32) {
+        self.0.insert(element, index);
+    }
+
+    /// Returns whether the cache is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
diff --git a/components/selectors/tree.rs b/components/selectors/tree.rs
index 23238237b2f..958311ad0d8 100644
--- a/components/selectors/tree.rs
+++ b/components/selectors/tree.rs
@@ -8,11 +8,27 @@
 use attr::{AttrSelectorOperation, NamespaceConstraint, CaseSensitivity};
 use matching::{ElementSelectorFlags, LocalMatchingContext, MatchingContext, RelevantLinkStatus};
 use parser::SelectorImpl;
+use servo_arc::NonZeroPtrMut;
 use std::fmt::Debug;
 
-pub trait Element: Sized + Debug {
+/// Opaque representation of an Element, for identity comparisons. We use
+/// NonZeroPtrMut to get the NonZero optimization.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub struct OpaqueElement(pub NonZeroPtrMut<()>);
+
+impl OpaqueElement {
+    /// Creates a new OpaqueElement from an arbitrarily-typed pointer.
+    pub fn new<T>(ptr: *const T) -> Self {
+        OpaqueElement(NonZeroPtrMut::new(ptr as *const () as *mut ()))
+    }
+}
+
+pub trait Element: Sized + Clone + Debug {
     type Impl: SelectorImpl;
 
+    /// Converts self into an opaque representation.
+    fn opaque(&self) -> OpaqueElement;
+
     fn parent_element(&self) -> Option<Self>;
 
     /// The parent of a given pseudo-element, after matching a pseudo-element