Merge pull request #3710 from googlefonts/24bit_repacking

[subset] Prepare the repacker for handling 24bit offsets in GSUB/GPOS.

src/graph/graph.hh

@@ -265,28 +265,64 @@ struct graph_t
   }
 
   /*
-   * Assign unique space numbers to each connected subgraph of 32 bit offset(s).
+   * Finds the set of nodes (placed into roots) that should be assigned unique spaces.
+   * More specifically this looks for the top most 24 bit or 32 bit links in the graph.
+   * Some special casing is done that is specific to the layout of GSUB/GPOS tables.
    */
-  bool assign_32bit_spaces ()
+  void find_space_roots (hb_set_t& visited, hb_set_t& roots)
   {
-    unsigned root_index = root_idx ();
-    hb_set_t visited;
-    hb_set_t roots;
-    for (unsigned i = 0; i <= root_index; i++)
+    int root_index = (int) root_idx ();
+    for (int i = root_index; i >= 0; i--)
     {
+      if (visited.has (i)) continue;
+
       // Only real links can form 32 bit spaces
       for (auto& l : vertices_[i].obj.real_links)
       {
-        if (l.width == 4 && !l.is_signed)
+        if (l.is_signed || l.width < 3)
+          continue;
+
+        if (i == root_index && l.width == 3)
+          // Ignore 24bit links from the root node, this skips past the single 24bit
+          // pointer to the lookup list.
+          continue;
+
+        if (l.width == 3)
         {
-          roots.add (l.objidx);
-          find_subgraph (l.objidx, visited);
+          // A 24bit offset forms a root, unless there is 32bit offsets somewhere
+          // in it's subgraph, then those become the roots instead. This is to make sure
+          // that extension subtables beneath a 24bit lookup become the spaces instead
+          // of the offset to the lookup.
+          hb_set_t sub_roots;
+          find_32bit_roots (l.objidx, sub_roots);
+          if (sub_roots) {
+            for (unsigned sub_root_idx : sub_roots) {
+              roots.add (sub_root_idx);
+              find_subgraph (sub_root_idx, visited);
+            }
+            continue;
+          }
         }
+
+        roots.add (l.objidx);
+        find_subgraph (l.objidx, visited);
       }
     }
+  }
 
-    // Mark everything not in the subgraphs of 32 bit roots as visited.
-    // This prevents 32 bit subgraphs from being connected via nodes not in the 32 bit subgraphs.
+  /*
+   * Assign unique space numbers to each connected subgraph of 24 bit and/or 32 bit offset(s).
+   * Currently, this is implemented specifically tailored to the structure of a GPOS/GSUB
+   * (including with 24bit offsets) table.
+   */
+  bool assign_spaces ()
+  {
+    hb_set_t visited;
+    hb_set_t roots;
+    find_space_roots (visited, roots);
+
+    // Mark everything not in the subgraphs of the roots as visited. This prevents
+    // subgraphs from being connected via nodes not in those subgraphs.
     visited.invert ();
 
     if (!roots) return false;
@@ -422,6 +458,22 @@ struct graph_t
       find_subgraph (link.objidx, subgraph);
   }
 
+  /*
+   * Finds the topmost children of 32bit offsets in the subgraph starting
+   * at node_idx. Found indices are placed into 'found'.
+   */
+  void find_32bit_roots (unsigned node_idx, hb_set_t& found)
+  {
+    for (const auto& link : vertices_[node_idx].obj.all_links ())
+    {
+      if (!link.is_signed && link.width == 4) {
+        found.add (link.objidx);
+        continue;
+      }
+      find_32bit_roots (link.objidx, found);
+    }
+  }
+
   /*
    * duplicates all nodes in the subgraph reachable from node_idx. Does not re-assign
    * links. index_map is updated with mappings from old id to new id. If a duplication has already
@@ -622,7 +674,7 @@ struct graph_t
  private:
 
   /*
-   * Returns the numbers of incoming edges that are 32bits wide.
+   * Returns the numbers of incoming edges that are 24 or 32 bits wide.
    */
   unsigned wide_parents (unsigned node_idx, hb_set_t& parents) const
   {
@@ -636,7 +688,9 @@ struct graph_t
       // Only real links can be wide
       for (const auto& l : vertices_[p].obj.real_links)
       {
-        if (l.objidx == node_idx && l.width == 4 && !l.is_signed)
+        if (l.objidx == node_idx
+            && (l.width == 3 || l.width == 4)
+            && !l.is_signed)
         {
           count++;
           parents.add (p);

src/hb-repacker.hh

@@ -172,7 +172,7 @@ hb_resolve_overflows (const T& packed,
       && will_overflow)
   {
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
-    if (sorted_graph.assign_32bit_spaces ())
+    if (sorted_graph.assign_spaces ())
       sorted_graph.sort_shortest_distance ();
   }
 

src/test-repacker.cc

@@ -57,6 +57,14 @@ static void add_offset (unsigned id,
   c->add_link (*offset, id);
 }
 
+static void add_24_offset (unsigned id,
+                           hb_serialize_context_t* c)
+{
+  OT::Offset24* offset = c->start_embed<OT::Offset24> ();
+  c->extend_min (offset);
+  c->add_link (*offset, id);
+}
+
 static void add_wide_offset (unsigned id,
                              hb_serialize_context_t* c)
 {
@@ -812,6 +820,51 @@ populate_serializer_virtual_link (hb_serialize_context_t* c)
   c->end_serialize();
 }
 
+static void
+populate_serializer_with_24_and_32_bit_offsets (hb_serialize_context_t* c)
+{
+  std::string large_string(60000, 'a');
+  c->start_serialize<char> ();
+
+  unsigned obj_f = add_object ("f", 1, c);
+  unsigned obj_g = add_object ("g", 1, c);
+  unsigned obj_j = add_object ("j", 1, c);
+  unsigned obj_k = add_object ("k", 1, c);
+
+  start_object (large_string.c_str (), 40000, c);
+  add_offset (obj_f, c);
+  unsigned obj_c = c->pop_pack (false);
+
+  start_object (large_string.c_str (), 40000, c);
+  add_offset (obj_g, c);
+  unsigned obj_d = c->pop_pack (false);
+
+  start_object (large_string.c_str (), 40000, c);
+  add_offset (obj_j, c);
+  unsigned obj_h = c->pop_pack (false);
+
+  start_object (large_string.c_str (), 40000, c);
+  add_offset (obj_k, c);
+  unsigned obj_i = c->pop_pack (false);
+
+  start_object ("e", 1, c);
+  add_wide_offset (obj_h, c);
+  add_wide_offset (obj_i, c);
+  unsigned obj_e = c->pop_pack (false);
+
+  start_object ("b", 1, c);
+  add_24_offset (obj_c, c);
+  add_24_offset (obj_d, c);
+  add_24_offset (obj_e, c);
+  unsigned obj_b = c->pop_pack (false);
+
+  start_object ("a", 1, c);
+  add_24_offset (obj_b, c);
+  c->pop_pack (false);
+
+  c->end_serialize();
+}
+
 static void test_sort_shortest ()
 {
   size_t buffer_size = 100;
@@ -1129,6 +1182,36 @@ static void test_resolve_overflows_via_isolation_spaces ()
   hb_blob_destroy (out);
 }
 
+static void test_resolve_mixed_overflows_via_isolation_spaces ()
+{
+  size_t buffer_size = 200000;
+  void* buffer = malloc (buffer_size);
+  hb_serialize_context_t c (buffer, buffer_size);
+  populate_serializer_with_24_and_32_bit_offsets (&c);
+  graph_t graph (c.object_graph ());
+
+  assert (c.offset_overflow ());
+  hb_blob_t* out = hb_resolve_overflows (c.object_graph (), HB_TAG ('G', 'S', 'U', 'B'), 0);
+  assert (out);
+  hb_bytes_t result = out->as_bytes ();
+
+  unsigned expected_length =
+      // Objects
+      7 +
+      4 * 40000;
+
+  expected_length +=
+      // Links
+      2 * 4 +  // 32
+      4 * 3 +  // 24
+      4 * 2;   // 16
+
+  assert (result.length == expected_length);
+
+  free (buffer);
+  hb_blob_destroy (out);
+}
+
 static void test_resolve_overflows_via_splitting_spaces ()
 {
   size_t buffer_size = 160000;
@@ -1270,6 +1353,7 @@ main (int argc, char **argv)
   test_resolve_overflows_via_isolating_16bit_space_2 ();
   test_resolve_overflows_via_splitting_spaces ();
   test_resolve_overflows_via_splitting_spaces_2 ();
+  test_resolve_mixed_overflows_via_isolation_spaces ();
   test_duplicate_leaf ();
   test_duplicate_interior ();
   test_virtual_link ();