gfx: Perform more aggressive caching in

`FontContext::get_layout_font_group_for_style()`.

There are several optimizations here:

* We make font families atoms, to allow for quicker comparisons.

* We precalculate an FNV hash of the relevant fields of the font style
  structure.

* When obtaining a platform font group, we first check pointer equality
  for the font style. If there's no match, we go to the FNV hash. Only
  if both caches miss do we construct and cache a font group. Note that
  individual fonts are *also* cached; thus there are two layers of
  caching here.

15% improvement in total layout thread time for Facebook Timeline.

components/gfx/Cargo.toml

@@ -66,6 +66,9 @@ branch = "upstream-2014-06-16"
 [dependencies.script_traits]
 path = "../script_traits"
 
+[dependencies.string_cache]
+git = "https://github.com/servo/string-cache"
+
 [dependencies]
 url = "0.2.16"
 time = "0.1.12"

components/gfx/font_cache_task.rs

@@ -9,16 +9,17 @@ use platform::font_list::get_last_resort_font_families;
 use platform::font_context::FontContextHandle;
 
 use collections::str::Str;
+use font_template::{FontTemplate, FontTemplateDescriptor};
+use net::resource_task::{ResourceTask, load_whole_resource};
+use platform::font_template::FontTemplateData;
 use std::borrow::ToOwned;
 use std::collections::HashMap;
 use std::sync::Arc;
 use std::sync::mpsc::{Sender, Receiver, channel};
-use font_template::{FontTemplate, FontTemplateDescriptor};
-use platform::font_template::FontTemplateData;
-use net::resource_task::{ResourceTask, load_whole_resource};
-use util::task::spawn_named;
-use util::str::LowercaseString;
+use string_cache::Atom;
 use style::font_face::Source;
+use util::str::LowercaseString;
+use util::task::spawn_named;
 
 /// A list of font templates that make up a given font family.
 struct FontFamily {
@@ -77,7 +78,7 @@ impl FontFamily {
 pub enum Command {
     GetFontTemplate(String, FontTemplateDescriptor, Sender<Reply>),
     GetLastResortFontTemplate(FontTemplateDescriptor, Sender<Reply>),
-    AddWebFont(String, Source, Sender<()>),
+    AddWebFont(Atom, Source, Sender<()>),
     Exit(Sender<()>),
 }
 
@@ -315,7 +316,7 @@ impl FontCacheTask {
         }
     }
 
-    pub fn add_web_font(&self, family: String, src: Source) {
+    pub fn add_web_font(&self, family: Atom, src: Source) {
         let (response_chan, response_port) = channel();
         self.chan.send(Command::AddWebFont(family, src, response_chan)).unwrap();
         response_port.recv().unwrap();

components/gfx/font_context.rs

@@ -7,19 +7,24 @@ use font::SpecifiedFontStyle;
 use platform::font_context::FontContextHandle;
 use style::computed_values::{font_style, font_variant};
 
+use font::FontHandleMethods;
 use font_cache_task::FontCacheTask;
 use font_template::FontTemplateDescriptor;
-use platform::font_template::FontTemplateData;
-use font::FontHandleMethods;
 use platform::font::FontHandle;
-use util::cache::HashCache;
-use util::smallvec::{SmallVec, SmallVec8};
-use util::geometry::Au;
+use platform::font_template::FontTemplateData;
 use util::arc_ptr_eq;
+use util::cache::HashCache;
+use util::fnv::FnvHasher;
+use util::geometry::Au;
+use util::smallvec::{SmallVec, SmallVec8};
 
-use std::borrow::ToOwned;
-use std::rc::Rc;
+use std::borrow::{self, ToOwned};
 use std::cell::RefCell;
+use std::collections::HashMap;
+use std::collections::hash_state::DefaultState;
+use std::default::Default;
+use std::hash::{Hash, Hasher};
+use std::rc::Rc;
 use std::sync::Arc;
 
 use azure::AzFloat;
@@ -76,8 +81,8 @@ pub struct FontContext {
     /// per frame. TODO: Make this weak when incremental redraw is done.
     paint_font_cache: Vec<PaintFontCacheEntry>,
 
-    last_style: Option<Arc<SpecifiedFontStyle>>,
-    last_fontgroup: Option<Rc<FontGroup>>,
+    layout_font_group_cache:
+        HashMap<LayoutFontGroupCacheKey,Rc<FontGroup>,DefaultState<FnvHasher>>,
 }
 
 impl FontContext {
@@ -89,8 +94,7 @@ impl FontContext {
             layout_font_cache: vec!(),
             fallback_font_cache: vec!(),
             paint_font_cache: vec!(),
-            last_style: None,
-            last_fontgroup: None,
+            layout_font_group_cache: HashMap::with_hash_state(Default::default()),
         }
     }
 
@@ -132,14 +136,21 @@ impl FontContext {
     /// this context.
     pub fn get_layout_font_group_for_style(&mut self, style: Arc<SpecifiedFontStyle>)
                                             -> Rc<FontGroup> {
-        let matches = match self.last_style {
-            Some(ref last_style) => arc_ptr_eq(&style, last_style),
-            None => false,
-        };
-        if matches {
-            return self.last_fontgroup.as_ref().unwrap().clone();
+        let address = &*style as *const SpecifiedFontStyle as usize;
+        if let Some(ref cached_font_group) = self.layout_font_group_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) {
+                return (*cached_font_group).clone()
+            }
+
         // TODO: The font context holds a strong ref to the cached fonts
         // so they will never be released. Find out a good time to drop them.
 
@@ -147,6 +158,7 @@ impl FontContext {
                                                style.font_stretch,
                                                style.font_style == font_style::T::italic ||
                                                 style.font_style == font_style::T::oblique);
+
         let mut fonts = SmallVec8::new();
 
         for family in style.font_family.iter() {
@@ -160,7 +172,7 @@ impl FontContext {
                             break;
                         }
                         Some(ref cached_font_ref) => {
-                            let cached_font = cached_font_ref.borrow();
+                            let cached_font = (*cached_font_ref).borrow();
                             if cached_font.descriptor == desc &&
                                cached_font.requested_pt_size == style.font_size &&
                                cached_font.variant == style.font_variant {
@@ -243,8 +255,7 @@ impl FontContext {
         }
 
         let font_group = Rc::new(FontGroup::new(fonts));
-        self.last_style = Some(style);
-        self.last_fontgroup = Some(font_group.clone());
+        self.layout_font_group_cache.insert(layout_font_group_cache_key, font_group.clone());
         font_group
     }
 
@@ -275,3 +286,34 @@ impl FontContext {
         self.font_cache_task.clone()
     }
 }
+
+struct LayoutFontGroupCacheKey {
+    pointer: Arc<SpecifiedFontStyle>,
+    size: Au,
+    address: usize,
+}
+
+impl PartialEq for LayoutFontGroupCacheKey {
+    fn eq(&self, other: &LayoutFontGroupCacheKey) -> bool {
+        self.pointer.font_family == other.pointer.font_family &&
+            self.pointer.font_stretch == other.pointer.font_stretch &&
+            self.pointer.font_style == other.pointer.font_style &&
+            self.pointer.font_weight as u16 == other.pointer.font_weight as u16 &&
+            self.size == other.size
+    }
+}
+
+impl Eq for LayoutFontGroupCacheKey {}
+
+impl Hash for LayoutFontGroupCacheKey {
+    fn hash<H>(&self, hasher: &mut H) where H: Hasher {
+        self.pointer.hash.hash(hasher)
+    }
+}
+
+impl borrow::Borrow<usize> for LayoutFontGroupCacheKey {
+    fn borrow(&self) -> &usize {
+        &self.address
+    }
+}
+

components/gfx/font_template.rs

@@ -11,11 +11,11 @@ use std::borrow::ToOwned;
 use std::sync::{Arc, Weak};
 use style::computed_values::{font_stretch, font_weight};
 
-/// Describes how to select a font from a given family.
-/// This is very basic at the moment and needs to be
-/// expanded or refactored when we support more of the
-/// font styling parameters.
-#[derive(Clone, Copy)]
+/// Describes how to select a font from a given family. This is very basic at the moment and needs
+/// to be expanded or refactored when we support more of the font styling parameters.
+///
+/// NB: If you change this, you will need to update `style::properties::compute_font_hash()`.
+#[derive(Clone, Copy, Eq, Hash)]
 pub struct FontTemplateDescriptor {
     pub weight: font_weight::T,
     pub stretch: font_stretch::T,

components/gfx/lib.rs

@@ -35,6 +35,7 @@ extern crate net;
 #[macro_use]
 extern crate util;
 extern crate msg;
+extern crate string_cache;
 extern crate style;
 extern crate skia;
 extern crate time;