Add fast-path to GSUB to check coverage

Shaves a good 10% off DejaVu Sans with simple Latin text for me.
Now, DejaVu is very ChainContext-intensive, but it's also a very
popular font!

diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index 6a4199d..caaaa85 100644 (file)
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -968,6 +968,17 @@ struct SubstLookupSubTable
   inline bool apply (hb_apply_context_t *c, unsigned int lookup_type) const
   {
     TRACE_APPLY ();
+    if (likely (lookup_type < Context) ||
+       (hb_in_range<unsigned int> (lookup_type, Context, ChainContext) &&
+        hb_in_range<unsigned int> (u.header.sub_format, 1, 2)))
+    {
+      /* Fast path, for most that have coverage in the same place.
+       * Note that ReverseChainSingle can also go through this but
+       * it's not worth the effort. */
+      hb_codepoint_t glyph_id = c->buffer->cur().codepoint;
+      unsigned int index = (this+u.header.coverage) (glyph_id);
+      if (likely (index == NOT_COVERED)) return TRACE_RETURN (false);
+    }
     switch (lookup_type) {
     case Single:               return TRACE_RETURN (u.single.apply (c));
     case Multiple:             return TRACE_RETURN (u.multiple.apply (c));
@@ -998,7 +1009,10 @@ struct SubstLookupSubTable
 
   private:
   union {
-  USHORT                       sub_format;
+  struct {
+    USHORT                     sub_format;
+    OffsetTo<Coverage>         coverage;
+  } header;
   SingleSubst                  single;
   MultipleSubst                        multiple;
   AlternateSubst               alternate;
@@ -1009,7 +1023,7 @@ struct SubstLookupSubTable
   ReverseChainSingleSubst      reverseChainContextSingle;
   } u;
   public:
-  DEFINE_SIZE_UNION (2, sub_format);
+  DEFINE_SIZE_UNION (2, header.sub_format);
 };