[OT] Get list of lookup subtables once and loop through them

This speeds up shaping the Amiri font by over 15%.

This was primarily needed for my work on OpenType GX, since
we will be collecting only sublookups that are "active" for
current font instance; but it's a nice boost in general as
well.

We might, in the future, collect subtables in the lookup_accel.
That would also allow us to do a per-subtbale set-digest, which
should speed things up some more, specially for ContextChainFormat3
lookups...  Amiri, for example, contains one lookup with 53
subtables!

src/hb-ot-layout.cc

@@ -902,20 +902,79 @@ struct GPOSProxy
 };
 
 
-template <typename Obj>
+struct hb_get_subtables_context_t :
+       OT::hb_dispatch_context_t<hb_get_subtables_context_t, hb_void_t, HB_DEBUG_APPLY>
+{
+  template <typename Type>
+  static inline bool apply_to (const void *obj, OT::hb_apply_context_t *c)
+  {
+    const Type *typed_obj = (const Type *) obj;
+    return typed_obj->apply (c);
+  }
+
+  typedef bool (*hb_apply_func_t) (const void *obj, OT::hb_apply_context_t *c);
+
+  struct hb_applicable_t
+  {
+    inline void init (const void *obj_, hb_apply_func_t apply_func_)
+    {
+      obj = obj_;
+      apply_func = apply_func_;
+    }
+
+    inline bool apply (OT::hb_apply_context_t *c) const { return apply_func (obj, c); }
+
+    private:
+    const void *obj;
+    hb_apply_func_t apply_func;
+  };
+
+  typedef hb_auto_array_t<hb_applicable_t> array_t;
+
+  /* Dispatch interface. */
+  inline const char *get_name (void) { return "GET_SUBTABLES"; }
+  template <typename T>
+  inline return_t dispatch (const T &obj)
+  {
+    hb_applicable_t *entry = array.push();
+    if (likely (entry))
+      entry->init (&obj, apply_to<T>);
+    return HB_VOID;
+  }
+  static return_t default_return_value (void) { return HB_VOID; }
+  bool stop_sublookup_iteration (return_t r HB_UNUSED) const { return false; }
+
+  hb_get_subtables_context_t (array_t &array_) :
+			      array (array_),
+			      debug_depth (0) {}
+
+  array_t &array;
+  unsigned int debug_depth;
+};
+
 static inline bool
 apply_forward (OT::hb_apply_context_t *c,
-	       const Obj &obj,
-	       const hb_ot_layout_lookup_accelerator_t &accel)
+	       const hb_ot_layout_lookup_accelerator_t &accel,
+	       const hb_get_subtables_context_t::array_t &subtables)
 {
   bool ret = false;
   hb_buffer_t *buffer = c->buffer;
   while (buffer->idx < buffer->len && !buffer->in_error)
   {
+    bool applied = false;
     if (accel.may_have (buffer->cur().codepoint) &&
 	(buffer->cur().mask & c->lookup_mask) &&
-	c->check_glyph_property (&buffer->cur(), c->lookup_props) &&
-	obj.apply (c))
+	c->check_glyph_property (&buffer->cur(), c->lookup_props))
+     {
+       for (unsigned int i = 0; i < subtables.len; i++)
+         if (subtables[i].apply (c))
+	 {
+	   applied = true;
+	   break;
+	 }
+     }
+
+    if (applied)
       ret = true;
     else
       buffer->next_glyph ();
@@ -923,11 +982,10 @@ apply_forward (OT::hb_apply_context_t *c,
   return ret;
 }
 
-template <typename Obj>
 static inline bool
 apply_backward (OT::hb_apply_context_t *c,
-		const Obj &obj,
-		const hb_ot_layout_lookup_accelerator_t &accel)
+	       const hb_ot_layout_lookup_accelerator_t &accel,
+	       const hb_get_subtables_context_t::array_t &subtables)
 {
   bool ret = false;
   hb_buffer_t *buffer = c->buffer;
@@ -935,9 +993,15 @@ apply_backward (OT::hb_apply_context_t *c,
   {
     if (accel.may_have (buffer->cur().codepoint) &&
 	(buffer->cur().mask & c->lookup_mask) &&
-	c->check_glyph_property (&buffer->cur(), c->lookup_props) &&
-	obj.apply (c))
-      ret = true;
+	c->check_glyph_property (&buffer->cur(), c->lookup_props))
+    {
+     for (unsigned int i = 0; i < subtables.len; i++)
+       if (subtables[i].apply (c))
+       {
+	 ret = true;
+	 break;
+       }
+    }
     /* The reverse lookup doesn't "advance" cursor (for good reason). */
     buffer->idx--;
 
@@ -946,26 +1010,6 @@ apply_backward (OT::hb_apply_context_t *c,
   return ret;
 }
 
-struct hb_apply_forward_context_t :
-       OT::hb_dispatch_context_t<hb_apply_forward_context_t, bool, HB_DEBUG_APPLY>
-{
-  inline const char *get_name (void) { return "APPLY_FWD"; }
-  template <typename T>
-  inline return_t dispatch (const T &obj) { return apply_forward (c, obj, accel); }
-  static return_t default_return_value (void) { return false; }
-  bool stop_sublookup_iteration (return_t r HB_UNUSED) const { return true; }
-
-  hb_apply_forward_context_t (OT::hb_apply_context_t *c_,
-			      const hb_ot_layout_lookup_accelerator_t &accel_) :
-				c (c_),
-				accel (accel_),
-				debug_depth (0) {}
-
-  OT::hb_apply_context_t *c;
-  const hb_ot_layout_lookup_accelerator_t &accel;
-  unsigned int debug_depth;
-};
-
 template <typename Proxy>
 static inline void
 apply_string (OT::hb_apply_context_t *c,
@@ -979,6 +1023,10 @@ apply_string (OT::hb_apply_context_t *c,
 
   c->set_lookup_props (lookup.get_props ());
 
+  hb_get_subtables_context_t::array_t subtables;
+  hb_get_subtables_context_t c_get_subtables (subtables);
+  lookup.dispatch (&c_get_subtables);
+
   if (likely (!lookup.is_reverse ()))
   {
     /* in/out forward substitution/positioning */
@@ -987,13 +1035,7 @@ apply_string (OT::hb_apply_context_t *c,
     buffer->idx = 0;
 
     bool ret;
-    if (lookup.get_subtable_count () == 1)
-    {
-      hb_apply_forward_context_t c_forward (c, accel);
-      ret = lookup.dispatch (&c_forward);
-    }
-    else
-      ret = apply_forward (c, lookup, accel);
+    ret = apply_forward (c, accel, subtables);
     if (ret)
     {
       if (!Proxy::inplace)
@@ -1009,7 +1051,7 @@ apply_string (OT::hb_apply_context_t *c,
       buffer->remove_output ();
     buffer->idx = buffer->len - 1;
 
-    apply_backward (c, lookup, accel);
+    apply_backward (c, accel, subtables);
   }
 }