[subset-perf] Cache a glyph map for gsub.

This allows us in some cases to avoid using glyph_set_gsub as a filter.

src/hb-ot-layout-common.hh

@@ -1717,14 +1717,12 @@ struct Coverage
   bool subset (hb_subset_context_t *c) const
   {
     TRACE_SUBSET (this);
-    const hb_set_t &glyphset = *c->plan->glyphset_gsub ();
-    const hb_map_t &glyph_map = *c->plan->glyph_map;
-
     auto it =
     + iter ()
-    | hb_filter (glyphset)
-    | hb_map_retains_sorting (glyph_map)
+    | hb_filter (c->plan->glyph_map_gsub)
+    | hb_map_retains_sorting (c->plan->glyph_map_gsub)
     ;
+
     // Cache the iterator result as it will be iterated multiple times
     // by the serialize code below.
     hb_sorted_vector_t<hb_codepoint_t> glyphs (it);

src/hb-subset-plan.cc

@@ -420,6 +420,20 @@ _populate_gids_to_retain (hb_subset_plan_t* plan,
 #endif
 }
 
+static void
+_create_glyph_map_gsub (const hb_set_t* glyph_set_gsub,
+                        const hb_map_t* glyph_map,
+                        hb_map_t* out)
+{
+  + hb_iter (glyph_set_gsub)
+  | hb_map ([&] (hb_codepoint_t gid) {
+    return hb_pair_t<hb_codepoint_t, hb_codepoint_t> (gid,
+                                                      glyph_map->get (gid));
+  })
+  | hb_sink (out)
+  ;
+}
+
 static void
 _create_old_gid_to_new_gid_map (const hb_face_t *face,
 				bool		 retain_gids,
@@ -518,6 +532,7 @@ hb_subset_plan_create_or_fail (hb_face_t	 *face,
   plan->codepoint_to_glyph = hb_map_create ();
   plan->glyph_map = hb_map_create ();
   plan->reverse_glyph_map = hb_map_create ();
+  plan->glyph_map_gsub = hb_map_create ();
   plan->gsub_lookups = hb_map_create ();
   plan->gpos_lookups = hb_map_create ();
 
@@ -552,6 +567,11 @@ hb_subset_plan_create_or_fail (hb_face_t	 *face,
 				  plan->reverse_glyph_map,
 				  &plan->_num_output_glyphs);
 
+  _create_glyph_map_gsub (
+      plan->_glyphset_gsub,
+      plan->glyph_map,
+      plan->glyph_map_gsub);
+
   // Now that we have old to new gid map update the unicode to new gid list.
   for (unsigned i = 0; i < plan->unicode_to_new_gid_list.length; i++)
   {
@@ -594,6 +614,7 @@ hb_subset_plan_destroy (hb_subset_plan_t *plan)
   hb_map_destroy (plan->codepoint_to_glyph);
   hb_map_destroy (plan->glyph_map);
   hb_map_destroy (plan->reverse_glyph_map);
+  hb_map_destroy (plan->glyph_map_gsub);
   hb_set_destroy (plan->_glyphset);
   hb_set_destroy (plan->_glyphset_gsub);
   hb_set_destroy (plan->_glyphset_mathed);

src/hb-subset-plan.hh

@@ -70,6 +70,7 @@ struct hb_subset_plan_t
   // Old -> New glyph id mapping
   hb_map_t *glyph_map;
   hb_map_t *reverse_glyph_map;
+  hb_map_t *glyph_map_gsub;
 
   // Plan is only good for a specific source/dest so keep them with it
   hb_face_t *source;