Optimize glyph cache access

Loading a glyph from the cache is a very hot operation in the renderer.
The original implementation would first check if a glyph was loaded and
then call `get()` which would have to search a second time. This showed
up as a very slow point in profiles.

This patch addresses glyph cache access in two ways: by using a faster
hasher optimized for small keys (fnv), and by using the entry API for
fetching a cached glyph. The `fnv` hasher is faster than the default and
is very efficient for small keys. Using the entry API on the HashMap
means only 1 lookup instead of two. The entry API has a downside where
the key needs to get cloned on fetches.

Reducing the GlyphKey width to 64-bits helps in both areas. Copying an
8-byte wide type is very cheap and thus limits downside of the entry
API. The small width also helps with the hasher performance.

Over all, this patch reduced typical render times by several hundred
microseconds on a 2013 MacBook Pro with a full screen terminal full of
text.

1 files changed, 40 insertions, 43 deletions

diff --git a/src/renderer/mod.rs b/src/renderer/mod.rs
index a9833822..fe4ff29f 100644
--- a/src/renderer/mod.rs
+++ b/src/renderer/mod.rs
@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 use std::collections::HashMap;
+use std::hash::BuildHasherDefault;
 use std::fs::File;
 use std::io::{self, Read};
 use std::mem::size_of;
@@ -20,20 +21,19 @@ use std::ptr;
 use std::sync::mpsc;
 
 use cgmath;
+use fnv::FnvHasher;
 use font::{self, Rasterizer, Rasterize, RasterizedGlyph, FontDesc, GlyphKey, FontKey};
 use gl::types::*;
 use gl;
-use notify::{Watcher as WatcherApi, RecommendedWatcher as Watcher, op};
 use index::{Line, Column, RangeInclusive};
-
-use window::{Size, Pixels};
+use notify::{Watcher as WatcherApi, RecommendedWatcher as Watcher, op};
 
 use ansi::{Color, NamedColor};
-
 use config::{Config, ColorList};
 use term::{self, cell, IndexedCell, Cell};
+use window::{Size, Pixels};
 
-use super::Rgb;
+use Rgb;
 
 // Shader paths for live reload
 static TEXT_SHADER_F_PATH: &'static str = concat!(env!("CARGO_MANIFEST_DIR"), "/res/text.f.glsl");
@@ -136,7 +136,7 @@ pub struct Glyph {
 /// representations of the same code point.
 pub struct GlyphCache {
     /// Cache of buffered glyphs
-    cache: HashMap<GlyphKey, Glyph>,
+    cache: HashMap<GlyphKey, Glyph, BuildHasherDefault<FnvHasher>>,
 
     /// Rasterizer for loading new glyphs
     rasterizer: Rasterizer,
@@ -215,7 +215,7 @@ impl GlyphCache {
         };
 
         let mut cache = GlyphCache {
-            cache: HashMap::new(),
+            cache: HashMap::default(),
             rasterizer: rasterizer,
             font_size: font.size(),
             font_key: regular,
@@ -251,26 +251,24 @@ impl GlyphCache {
     fn load_and_cache_glyph<L>(&mut self, glyph_key: GlyphKey, loader: &mut L)
         where L: LoadGlyph
     {
-        if let Ok(rasterized) = self.rasterizer.get_glyph(&glyph_key) {
-            let glyph = loader.load_glyph(&rasterized);
-            self.cache.insert(glyph_key, glyph);
-        }
+        let rasterized = self.rasterizer.get_glyph(&glyph_key)
+            .unwrap_or_else(|_| Default::default());
+
+        let glyph = loader.load_glyph(&rasterized);
+        self.cache.insert(glyph_key, glyph);
     }
 
-    pub fn get<L>(&mut self, glyph_key: &GlyphKey, loader: &mut L) -> Option<&Glyph>
+    pub fn get<'a, L>(&'a mut self, glyph_key: &GlyphKey, loader: &mut L) -> &'a Glyph
         where L: LoadGlyph
     {
-        // Return glyph if it's already loaded
-        // hi borrowck
-        {
-            if self.cache.contains_key(glyph_key) {
-                return self.cache.get(glyph_key);
-            }
-        }
-
-        // Rasterize and load the glyph
-        self.load_and_cache_glyph(glyph_key.to_owned(), loader);
-        self.cache.get(glyph_key)
+        let rasterizer = &mut self.rasterizer;
+        self.cache
+            .entry(*glyph_key)
+            .or_insert_with(|| {
+                let rasterized = rasterizer.get_glyph(&glyph_key)
+                    .unwrap_or_else(|_| Default::default());
+                loader.load_glyph(&rasterized)
+            })
     }
 }
 
@@ -817,26 +815,25 @@ impl<'a> RenderApi<'a> {
                 c: cell.c
             };
 
-            // Add cell to batch if glyph available
-            glyph_cache.get(&glyph_key, self)
-                .map(|glyph| self.add_render_item(&cell, glyph))
-                .and_then(|_| {
-                    // FIXME This is a super hacky way to do underlined text. During
-                    //       a time crunch to release 0.1, this seemed like a really
-                    //       easy, clean hack.
-                    if cell.flags.contains(cell::UNDERLINE) {
-                        let glyph_key = GlyphKey {
-                            font_key: font_key,
-                            size: glyph_cache.font_size,
-                            c: '_'
-                        };
-
-                        glyph_cache.get(&glyph_key, self)
-                    } else {
-                        None
-                    }
-                })
-                .map(|underscore| self.add_render_item(&cell, underscore));
+            // Add cell to batch
+            {
+                let glyph = glyph_cache.get(&glyph_key, self);
+                self.add_render_item(&cell, glyph);
+            }
+
+            // FIXME This is a super hacky way to do underlined text. During
+            //       a time crunch to release 0.1, this seemed like a really
+            //       easy, clean hack.
+            if cell.flags.contains(cell::UNDERLINE) {
+                let glyph_key = GlyphKey {
+                    font_key: font_key,
+                    size: glyph_cache.font_size,
+                    c: '_'
+                };
+
+                let underscore = glyph_cache.get(&glyph_key, self);
+                self.add_render_item(&cell, underscore);
+            }
         }
     }
 }