tor-browser

hb-repacker.hh (18120B)

---

/*
* Copyright © 2020  Google, Inc.
*
*  This is part of HarfBuzz, a text shaping library.
*
* Permission is hereby granted, without written agreement and without
* license or royalty fees, to use, copy, modify, and distribute this
* software and its documentation for any purpose, provided that the
* above copyright notice and the following two paragraphs appear in
* all copies of this software.
*
* IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
* DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
* ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
* IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
* DAMAGE.
*
* THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
* BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
* FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
* ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
* PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
*
* Google Author(s): Garret Rieger
*/

#ifndef HB_REPACKER_HH
#define HB_REPACKER_HH

#include "hb-open-type.hh"
#include "hb-map.hh"
#include "hb-vector.hh"
#include "graph/graph.hh"
#include "graph/gsubgpos-graph.hh"
#include "graph/serialize.hh"

using graph::graph_t;

/*
* For a detailed writeup on the overflow resolution algorithm see:
* docs/repacker.md
*/

struct lookup_size_t
{
 unsigned lookup_index;
 size_t size;
 unsigned num_subtables;

 static int cmp (const void* a, const void* b)
 {
   return cmp ((const lookup_size_t*) a,
               (const lookup_size_t*) b);
 }

 static int cmp (const lookup_size_t* a, const lookup_size_t* b)
 {
   double subtables_per_byte_a = (double) a->num_subtables / (double) a->size;
   double subtables_per_byte_b = (double) b->num_subtables / (double) b->size;
   if (subtables_per_byte_a == subtables_per_byte_b) {
     return b->lookup_index - a->lookup_index;
   }

   double cmp = subtables_per_byte_b - subtables_per_byte_a;
   if (cmp < 0) return -1;
   if (cmp > 0) return 1;
   return 0;
 }
};

static inline
bool _presplit_subtables_if_needed (graph::gsubgpos_graph_context_t& ext_context)
{
 // For each lookup this will check the size of subtables and split them as needed
 // so that no subtable is at risk of overflowing. (where we support splitting for
 // that subtable type).
 //
 // TODO(grieger): de-dup newly added nodes as necessary. Probably just want a full de-dup
 //                pass after this processing is done. Not super necessary as splits are
 //                only done where overflow is likely, so de-dup probably will get undone
 //                later anyways.

 // The loop below can modify the contents of ext_context.lookups if new subtables are added
 // to a lookup during a split. So save the initial set of lookup indices so the iteration doesn't
 // risk access free'd memory if ext_context.lookups gets resized.
 hb_set_t lookup_indices(ext_context.lookups.keys ());
 for (unsigned lookup_index : lookup_indices)
 {
   graph::Lookup* lookup = ext_context.lookups.get(lookup_index);
   if (!lookup->split_subtables_if_needed (ext_context, lookup_index))
     return false;
 }

 return true;
}

/*
* Analyze the lookups in a GSUB/GPOS table and decide if any should be promoted
* to extension lookups.
*/
static inline
bool _promote_extensions_if_needed (graph::gsubgpos_graph_context_t& ext_context)
{
 // Simple Algorithm (v1, current):
 // 1. Calculate how many bytes each non-extension lookup consumes.
 // 2. Select up to 64k of those to remain as non-extension (greedy, highest subtables per byte first)
 // 3. Promote the rest.
 //
 // Advanced Algorithm (v2, not implemented):
 // 1. Perform connected component analysis using lookups as roots.
 // 2. Compute size of each connected component.
 // 3. Select up to 64k worth of connected components to remain as non-extensions.
 //    (greedy, highest subtables per byte first)
 // 4. Promote the rest.

 // TODO(garretrieger): support extension demotion, then consider all lookups. Requires advanced algo.
 // TODO(garretrieger): also support extension promotion during iterative resolution phase, then
 //                     we can use a less conservative threshold here.
 // TODO(grieger): skip this for the 24 bit case.
 if (!ext_context.lookups) return true;

 unsigned total_lookup_table_sizes = 0;
 hb_vector_t<lookup_size_t> lookup_sizes;
 lookup_sizes.alloc (ext_context.lookups.get_population (), true);

 for (unsigned lookup_index : ext_context.lookups.keys ())
 {
   const auto& lookup_v = ext_context.graph.vertices_[lookup_index];
   total_lookup_table_sizes += lookup_v.table_size ();

   const graph::Lookup* lookup = ext_context.lookups.get(lookup_index);
   hb_set_t visited;
   lookup_sizes.push (lookup_size_t {
       lookup_index,
       ext_context.graph.find_subgraph_size (lookup_index, visited),
       lookup->number_of_subtables (),
     });
 }

 lookup_sizes.qsort ();

 size_t lookup_list_size = ext_context.graph.vertices_[ext_context.lookup_list_index].table_size ();
 size_t l2_l3_size = lookup_list_size + total_lookup_table_sizes; // Lookup List + Lookups
 size_t l3_l4_size = total_lookup_table_sizes; // Lookups + SubTables
 size_t l4_plus_size = 0; // SubTables + their descendants

 // Start by assuming all lookups are using extension subtables, this size will be removed later
 // if it's decided to not make a lookup extension.
 for (auto p : lookup_sizes)
 {
   // TODO(garretrieger): this overestimates the extension subtables size because some extension subtables may be
   //                     reused. However, we can't correct this until we have connected component analysis in place.
   unsigned subtables_size = p.num_subtables * 8;
   l3_l4_size += subtables_size;
   l4_plus_size += subtables_size;
 }

 bool layers_full = false;
 for (auto p : lookup_sizes)
 {
   const graph::Lookup* lookup = ext_context.lookups.get(p.lookup_index);
   if (lookup->is_extension (ext_context.table_tag))
     // already an extension so size is counted by the loop above.
     continue;

   if (!layers_full)
   {
     size_t lookup_size = ext_context.graph.vertices_[p.lookup_index].table_size ();
     hb_set_t visited;
     size_t subtables_size = ext_context.graph.find_subgraph_size (p.lookup_index, visited, 1) - lookup_size;
     size_t remaining_size = p.size - subtables_size - lookup_size;

     l3_l4_size   += subtables_size;
     l3_l4_size   -= p.num_subtables * 8;
     l4_plus_size += subtables_size + remaining_size;

     if (l2_l3_size < (1 << 16)
         && l3_l4_size < (1 << 16)
         && l4_plus_size < (1 << 16)) continue; // this lookup fits within all layers groups

     layers_full = true;
   }

   if (!ext_context.lookups.get(p.lookup_index)->make_extension (ext_context, p.lookup_index))
     return false;
 }

 return true;
}

static inline
bool _try_isolating_subgraphs (const hb_vector_t<graph::overflow_record_t>& overflows,
                              graph_t& sorted_graph)
{
 unsigned space = 0;
 hb_set_t roots_to_isolate;

 for (int i = overflows.length - 1; i >= 0; i--)
 {
   const graph::overflow_record_t& r = overflows[i];

   unsigned root;
   unsigned overflow_space = sorted_graph.space_for (r.parent, &root);
   if (!overflow_space) continue;
   if (sorted_graph.num_roots_for_space (overflow_space) <= 1) continue;

   if (!space) {
     space = overflow_space;
   }

   if (space == overflow_space)
     roots_to_isolate.add(root);
 }

 if (!roots_to_isolate) return false;

 unsigned maximum_to_move = hb_max ((sorted_graph.num_roots_for_space (space) / 2u), 1u);
 if (roots_to_isolate.get_population () > maximum_to_move) {
   // Only move at most half of the roots in a space at a time.
   //
   // Note: this was ported from non-stable ids to stable ids. So to retain the same behaviour
   // with regards to which roots are removed from the set we need to remove them in the topological
   // order, not the object id order.
   int extra = roots_to_isolate.get_population () - maximum_to_move;
   for (unsigned id : sorted_graph.ordering_) {
     if (!extra) break;
     if (roots_to_isolate.has(id)) {
       roots_to_isolate.del(id);
       extra--;
     }
   }
 }

 DEBUG_MSG (SUBSET_REPACK, nullptr,
            "Overflow in space %u (%u roots). Moving %u roots to space %u.",
            space,
            sorted_graph.num_roots_for_space (space),
            roots_to_isolate.get_population (),
            sorted_graph.next_space ());

 sorted_graph.isolate_subgraph (roots_to_isolate);
 sorted_graph.move_to_new_space (roots_to_isolate);

 return true;
}

static inline
bool _resolve_shared_overflow(const hb_vector_t<graph::overflow_record_t>& overflows,
                             int overflow_index,
                             graph_t& sorted_graph)
{
 const graph::overflow_record_t& r = overflows[overflow_index];

 // Find all of the parents in overflowing links that link to this
 // same child node. We will then try duplicating the child node and
 // re-assigning all of these parents to the duplicate.
 hb_set_t parents;
 parents.add(r.parent);
 for (int i = overflow_index - 1; i >= 0; i--) {
   const graph::overflow_record_t& r2 = overflows[i];
   if (r2.child == r.child) {
     parents.add(r2.parent);
   }
 }

 unsigned result = sorted_graph.duplicate(&parents, r.child);
 if (result == (unsigned) -1 && parents.get_population() > 2) {
   // All links to the child are overflowing, so we can't include all
   // in the duplication. Remove one parent from the duplication.
   // Remove the lowest index parent, which will be the closest to the child.
   parents.del(parents.get_min());
   result = sorted_graph.duplicate(&parents, r.child);
 }

 if (result == (unsigned) -1) return false;

 if (parents.get_population() > 1) {
   // If the duplicated node has more than one parent pre-emptively raise it's priority to the maximum.
   // This will place it close to the parents. Node's with only one parent, don't need this as normal overflow
   // resolution will raise priority if needed.
   //
   // Reasoning: most of the parents to this child are likely at the same layer in the graph. Duplicating
   // the child will theoretically allow it to be placed closer to it's parents. However, due to the shortest
   // distance sort by default it's placement will remain in the same layer, thus it will remain in roughly the
   // same position (and distance from parents) as the original child node. The overflow resolution will attempt
   // to move nodes closer, but only for non-shared nodes. Since this node is shared, it will simply be given
   // further duplication which defeats the attempt to duplicate with multiple parents. To fix this we
   // pre-emptively raise priority now which allows the duplicated node to pack into the same layer as it's parents.
   sorted_graph.vertices_[result].give_max_priority();
 }

 return true;
}

static inline
bool _process_overflows (const hb_vector_t<graph::overflow_record_t>& overflows,
                        hb_set_t& priority_bumped_parents,
                        graph_t& sorted_graph)
{
 bool resolution_attempted = false;

 // Try resolving the furthest overflows first.
 for (int i = overflows.length - 1; i >= 0; i--)
 {
   const graph::overflow_record_t& r = overflows[i];
   const auto& child = sorted_graph.vertices_[r.child];
   if (child.is_shared ())
   {
     // The child object is shared, we may be able to eliminate the overflow
     // by duplicating it.
     if (_resolve_shared_overflow(overflows, i, sorted_graph))
       return true;

     // Sometimes we can't duplicate a node which looks shared because it's not actually shared
     // (eg. all links from the same parent) in this case continue on to other resolution options.
   }

   if (child.is_leaf () && !priority_bumped_parents.has (r.parent))
   {
     // This object is too far from it's parent, attempt to move it closer.
     //
     // TODO(garretrieger): initially limiting this to leaf's since they can be
     //                     moved closer with fewer consequences. However, this can
     //                     likely can be used for non-leafs as well.
     // TODO(garretrieger): also try lowering priority of the parent. Make it
     //                     get placed further up in the ordering, closer to it's children.
     //                     this is probably preferable if the total size of the parent object
     //                     is < then the total size of the children (and the parent can be moved).
     //                     Since in that case moving the parent will cause a smaller increase in
     //                     the length of other offsets.
     if (sorted_graph.raise_childrens_priority (r.parent)) {
       priority_bumped_parents.add (r.parent);
       resolution_attempted = true;
     }
     continue;
   }

   // TODO(garretrieger): add additional offset resolution strategies
   // - Promotion to extension lookups.
   // - Table splitting.
 }

 return resolution_attempted;
}

inline bool
hb_resolve_graph_overflows (hb_tag_t table_tag,
                           unsigned max_rounds ,
                           bool always_recalculate_extensions,
                           graph_t& sorted_graph /* IN/OUT */)
{
 DEBUG_MSG (SUBSET_REPACK, nullptr, "Repacking %c%c%c%c.", HB_UNTAG(table_tag));
 sorted_graph.sort_shortest_distance ();
 if (sorted_graph.in_error ())
 {
   DEBUG_MSG (SUBSET_REPACK, nullptr, "Sorted graph in error state after initial sort.");
   return false;
 }

 bool will_overflow = graph::will_overflow (sorted_graph);
 if (!will_overflow)
   return true;

 bool is_gsub_or_gpos = (table_tag == HB_OT_TAG_GPOS ||  table_tag == HB_OT_TAG_GSUB);
 graph::gsubgpos_graph_context_t ext_context (table_tag, sorted_graph);
 if (is_gsub_or_gpos && will_overflow)
 {
   DEBUG_MSG (SUBSET_REPACK, nullptr, "Applying GSUB/GPOS repacking specializations.");
   if (always_recalculate_extensions)
   {
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Splitting subtables if needed.");
     if (!_presplit_subtables_if_needed (ext_context)) {
       DEBUG_MSG (SUBSET_REPACK, nullptr, "Subtable splitting failed.");
       return false;
     }

     DEBUG_MSG (SUBSET_REPACK, nullptr, "Promoting lookups to extensions if needed.");
     if (!_promote_extensions_if_needed (ext_context)) {
       DEBUG_MSG (SUBSET_REPACK, nullptr, "Extensions promotion failed.");
       return false;
     }
   }

   DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
   if (sorted_graph.assign_spaces ())
     sorted_graph.sort_shortest_distance ();
   else
     sorted_graph.sort_shortest_distance_if_needed ();
 }

 unsigned round = 0;
 hb_vector_t<graph::overflow_record_t> overflows;
 // TODO(garretrieger): select a good limit for max rounds.
 while (!sorted_graph.in_error ()
        && graph::will_overflow (sorted_graph, &overflows)
        && round < max_rounds) {
   DEBUG_MSG (SUBSET_REPACK, nullptr, "=== Overflow resolution round %u ===", round);
   print_overflows (sorted_graph, overflows);

   hb_set_t priority_bumped_parents;

   if (!_try_isolating_subgraphs (overflows, sorted_graph))
   {
     // Don't count space isolation towards round limit. Only increment
     // round counter if space isolation made no changes.
     round++;
     if (!_process_overflows (overflows, priority_bumped_parents, sorted_graph))
     {
       DEBUG_MSG (SUBSET_REPACK, nullptr, "No resolution available :(");
       break;
     }
   }

   sorted_graph.sort_shortest_distance ();
 }

 if (sorted_graph.in_error ())
 {
   DEBUG_MSG (SUBSET_REPACK, nullptr, "Sorted graph in error state.");
   return false;
 }

 if (graph::will_overflow (sorted_graph))
 {
   if (is_gsub_or_gpos && !always_recalculate_extensions) {
     // If this a GSUB/GPOS table and we didn't try to extension promotion and table splitting then
     // as a last ditch effort, re-run the repacker with it enabled.
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Failed to find a resolution. Re-running with extension promotion and table splitting enabled.");
     return hb_resolve_graph_overflows (table_tag, max_rounds, true, sorted_graph);
   }

   DEBUG_MSG (SUBSET_REPACK, nullptr, "Offset overflow resolution failed.");
   return false;
 }

 return true;
}

/*
* Attempts to modify the topological sorting of the provided object graph to
* eliminate offset overflows in the links between objects of the graph. If a
* non-overflowing ordering is found the updated graph is serialized it into the
* provided serialization context.
*
* If necessary the structure of the graph may be modified in ways that do not
* affect the functionality of the graph. For example shared objects may be
* duplicated.
*
* For a detailed writeup describing how the algorithm operates see:
* docs/repacker.md
*/
template<typename T>
inline hb_blob_t*
hb_resolve_overflows (const T& packed,
                     hb_tag_t table_tag,
                     unsigned max_rounds = 32,
                     bool recalculate_extensions = false) {
 graph_t sorted_graph (packed);
 if (sorted_graph.in_error ())
 {
   // Invalid graph definition.
   return nullptr;
 }

 if (!sorted_graph.is_fully_connected ())
 {
   sorted_graph.print_orphaned_nodes ();
   return nullptr;
 }

 if (sorted_graph.in_error ())
 {
   // Allocations failed somewhere
   DEBUG_MSG (SUBSET_REPACK, nullptr,
              "Graph is in error, likely due to a memory allocation error.");
   return nullptr;
 }

 if (!hb_resolve_graph_overflows (table_tag, max_rounds, recalculate_extensions, sorted_graph))
   return nullptr;

 return graph::serialize (sorted_graph);
}

#endif /* HB_REPACKER_HH */