style: Add a TLS-based style struct caching mechanism.

components/style/rule_cache.rs

@@ -0,0 +1,143 @@
+/* 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/. */
+
+//! A cache from rule node to computed values, in order to cache reset
+//! properties.
+
+use fnv::FnvHashMap;
+use logical_geometry::WritingMode;
+use properties::{ComputedValues, StyleBuilder};
+use rule_tree::StrongRuleNode;
+use servo_arc::Arc;
+use smallvec::SmallVec;
+use values::computed::NonNegativeLength;
+
+/// The conditions for caching and matching a style in the rule cache.
+#[derive(Clone, Debug, Default)]
+pub struct RuleCacheConditions {
+    uncacheable: bool,
+    font_size: Option<NonNegativeLength>,
+    writing_mode: Option<WritingMode>,
+}
+
+impl RuleCacheConditions {
+    /// Sets the style as depending in the font-size value.
+    pub fn set_font_size_dependency(&mut self, font_size: NonNegativeLength) {
+        debug_assert!(self.font_size.map_or(true, |f| f == font_size));
+        self.font_size = Some(font_size);
+    }
+
+    /// Sets the style as uncacheable.
+    pub fn set_uncacheable(&mut self) {
+        self.uncacheable = true;
+    }
+
+    /// Sets the style as depending in the writing-mode value `writing_mode`.
+    pub fn set_writing_mode_dependency(&mut self, writing_mode: WritingMode) {
+        debug_assert!(self.writing_mode.map_or(true, |wm| wm == writing_mode));
+        self.writing_mode = Some(writing_mode);
+    }
+
+    /// Returns whether the current style's reset properties are cacheable.
+    fn cacheable(&self) -> bool {
+        !self.uncacheable
+    }
+
+    /// Returns whether `style` matches the conditions.
+    fn matches(&self, style: &StyleBuilder) -> bool {
+        if self.uncacheable {
+            return false;
+        }
+
+        if let Some(fs) = self.font_size {
+            if style.get_font().clone_font_size() != fs {
+                return false;
+            }
+        }
+
+        if let Some(wm) = self.writing_mode {
+            if style.writing_mode != wm {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
+/// A TLS cache from rules matched to computed values.
+pub struct RuleCache {
+    // FIXME(emilio): Consider using LRUCache or something like that?
+    map: FnvHashMap<StrongRuleNode, SmallVec<[(RuleCacheConditions, Arc<ComputedValues>); 1]>>,
+}
+
+impl RuleCache {
+    /// Creates an empty `RuleCache`.
+    pub fn new() -> Self {
+        Self {
+            map: FnvHashMap::default(),
+        }
+    }
+
+    /// Finds a node in the properties matched cache.
+    ///
+    /// This needs to receive a `StyleBuilder` with the `early` properties
+    /// already applied.
+    pub fn find(
+        &self,
+        builder_with_early_props: &StyleBuilder,
+    ) -> Option<&ComputedValues> {
+        if builder_with_early_props.is_style_if_visited() {
+            // FIXME(emilio): We can probably do better, does it matter much?
+            return None;
+        }
+
+        let rules = match builder_with_early_props.rules {
+            Some(ref rules) => rules,
+            None => return None,
+        };
+
+        self.map.get(rules).and_then(|cached_values| {
+            for &(ref conditions, ref values) in cached_values.iter() {
+                if conditions.matches(builder_with_early_props) {
+                    debug!("Using cached reset style with conditions {:?}", conditions);
+                    return Some(&**values)
+                }
+            }
+            None
+        })
+    }
+
+    /// Inserts a node into the rules cache if possible.
+    ///
+    /// Returns whether the style was inserted into the cache.
+    pub fn insert_if_possible(
+        &mut self,
+        style: &Arc<ComputedValues>,
+        conditions: &RuleCacheConditions,
+    ) -> bool {
+        if !conditions.cacheable() {
+            return false;
+        }
+
+        if style.is_style_if_visited() {
+            // FIXME(emilio): We can probably do better, does it matter much?
+            return false;
+        }
+
+        let rules = match style.rules {
+            Some(ref r) => r.clone(),
+            None => return false,
+        };
+
+        debug!("Inserting cached reset style with conditions {:?}", conditions);
+        self.map
+            .entry(rules)
+            .or_insert_with(SmallVec::new)
+            .push((conditions.clone(), style.clone()));
+
+        true
+    }
+
+}