use murmur3 finisher to improve font hash efficiency

src/core/SkGlyphCache.h

Index: src/core/SkGlyphCache.h
diff --git a/src/core/SkGlyphCache.h b/src/core/SkGlyphCache.h
index bb34a7d977411784971c9658793ad787e7197237..fc19ba0e7d82b2e8f92edc0ac77bd891960462a4 100644
--- a/src/core/SkGlyphCache.h
+++ b/src/core/SkGlyphCache.h
@@ -23,6 +23,10 @@ class SkPaint;
 
 class SkGlyphCache_Globals;
 
+// Enable this locally to add stats for hash-table hit rates. It also extends the dump()
+// output to show those stats.
+//#define SK_GLYPHCACHE_TRACK_HASH_STATS
+
 /** \class SkGlyphCache
 
     This class represents a strike: a specific combination of typeface, size,
@@ -101,6 +105,8 @@ public:
         return fScalerContext->isSubpixel();
     }
 
+    void dump() const;
+
     /*  AuxProc/Data allow a client to associate data with this cache entry.
         Multiple clients can use this, as their data is keyed with a function
         pointer. In addition to serving as a key, the function pointer is called
@@ -145,6 +151,8 @@ public:
         return VisitCache(typeface, desc, DetachProc, NULL);
     }
 
+    static void Dump();
+
 #ifdef SK_DEBUG
     void validate() const;
 #else
@@ -204,15 +212,29 @@ private:
     // no reason to use the same kHashCount as fGlyphHash, but we do for now
     CharGlyphRec    fCharToGlyphHash[kHashCount];
 
-    static inline unsigned ID2HashIndex(uint32_t id) {
-        id ^= id >> 16;
-        id ^= id >> 8;
-        return id & kHashMask;
+    static inline unsigned ID2HashIndex(uint32_t h) {

[mtklein, 2015/01/27 17:10:12]

Call SkChecksum::Mix()?

[reed1, 2015/01/27 18:33:33]

I think its slight overkill (as the comment below says) so I'm leaning to not
call the full Mix.

+        // apply (partial) Murmur3 finisher
+        h ^= h >> 16;
+        h *= 0x85ebca6b;
+#if 0
+        // This part of the Murmur3 finisher does not seems necessary, i.e. it does not seem
+        // to measurably improve our hash-hit efficiency, so we leave it out.
+        h ^= h >> 13;
+        h *= 0xc2b2ae35;
+#endif
+        h ^= h >> 16;
+        return h & kHashMask;
     }
 
     // used to track (approx) how much ram is tied-up in this cache
     size_t  fMemoryUsed;
 
+
+#ifdef SK_GLYPHCACHE_TRACK_HASH_STATS
+    int fHashHitCount;
+    int fHashMissCount;
+#endif
+
     struct AuxProcRec {
         AuxProcRec* fNext;
         void (*fProc)(void*);