Cache font style struct addresses in a separate font group cache.

On http://en.wikipedia.org/wiki/Spotted_hyena, I was seeing a 100% miss
rate in the first fast cache lookup and 45% of total layout time in this
function. After making this change, the first cache lookup almost always
succeeds, and the time spent in this function drops to 8%.

components/gfx/font_context.rs

@@ -82,6 +82,8 @@ pub struct FontContext {
     /// per frame. TODO: Make this weak when incremental redraw is done.
     paint_font_cache: Vec<PaintFontCacheEntry>,
 
+    layout_font_group_address_cache: HashMap<usize, Rc<FontGroup>, DefaultState<FnvHasher>>,
+
     layout_font_group_cache:
         HashMap<LayoutFontGroupCacheKey, Rc<FontGroup>, DefaultState<FnvHasher>>,
 
@@ -97,6 +99,7 @@ impl FontContext {
             layout_font_cache: vec!(),
             fallback_font_cache: vec!(),
             paint_font_cache: vec!(),
+            layout_font_group_address_cache: HashMap::with_hash_state(Default::default()),
             layout_font_group_cache: HashMap::with_hash_state(Default::default()),
             epoch: 0,
         }
@@ -144,6 +147,7 @@ impl FontContext {
         self.layout_font_cache.clear();
         self.fallback_font_cache.clear();
         self.paint_font_cache.clear();
+        self.layout_font_group_address_cache.clear();
         self.layout_font_group_cache.clear();
         self.epoch = current_epoch
     }
@@ -152,21 +156,21 @@ impl FontContext {
     /// a cached font if this font instance has already been used by
     /// this context.
     pub fn layout_font_group_for_style(&mut self, style: Arc<SpecifiedFontStyle>)
-                                            -> Rc<FontGroup> {
+                                       -> Rc<FontGroup> {
         self.expire_font_caches_if_necessary();
 
         let address = &*style as *const SpecifiedFontStyle as usize;
-        if let Some(ref cached_font_group) = self.layout_font_group_cache.get(&address) {
+        if let Some(ref cached_font_group) = self.layout_font_group_address_cache.get(&address) {
             return (*cached_font_group).clone()
         }
 
         let layout_font_group_cache_key = LayoutFontGroupCacheKey {
             pointer: style.clone(),
             size: style.font_size,
-            address: address,
         };
         if let Some(ref cached_font_group) = self.layout_font_group_cache.get(
                 &layout_font_group_cache_key) {
+            self.layout_font_group_address_cache.insert(address, (*cached_font_group).clone());
             return (*cached_font_group).clone()
         }
 
@@ -275,6 +279,7 @@ impl FontContext {
 
         let font_group = Rc::new(FontGroup::new(fonts));
         self.layout_font_group_cache.insert(layout_font_group_cache_key, font_group.clone());
+        self.layout_font_group_address_cache.insert(address, font_group.clone());
         font_group
     }
 
@@ -316,7 +321,6 @@ impl HeapSizeOf for FontContext {
 struct LayoutFontGroupCacheKey {
     pointer: Arc<SpecifiedFontStyle>,
     size: Au,
-    address: usize,
 }
 
 impl PartialEq for LayoutFontGroupCacheKey {
@@ -333,12 +337,6 @@ impl Hash for LayoutFontGroupCacheKey {
     }
 }
 
-impl borrow::Borrow<usize> for LayoutFontGroupCacheKey {
-    fn borrow(&self) -> &usize {
-        &self.address
-    }
-}
-
 #[inline]
 pub fn invalidate_font_caches() {
     FONT_CACHE_EPOCH.fetch_add(1, Ordering::SeqCst);