[snapshot] support multiple contexts in the same snapshot.

src/snapshot/startup-serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 #define V8_SNAPSHOT_STARTUP_SERIALIZER_H_
 
 #include <bitset>
 #include "include/v8.h"
 #include "src/snapshot/serializer.h"
 
 namespace v8 {
 namespace internal {
 
 class StartupSerializer : public Serializer {
  public:
   StartupSerializer(
       Isolate* isolate,
       v8::SnapshotCreator::FunctionCodeHandling function_code_handling);
   ~StartupSerializer() override;
 
   // Serialize the current state of the heap.  The order is:
   // 1) Immortal immovable roots
   // 2) Remaining strong references.
   // 3) Partial snapshot cache.
   // 4) Weak references (e.g. the string table).
   void SerializeStrongReferences();
   void SerializeWeakReferencesAndDeferred();
 
+  int PartialSnapshotCacheIndex(HeapObject* o);
+
  private:
+  class PartialCacheIndexMap : public AddressMapBase {
+   public:
+    PartialCacheIndexMap()
+        : map_(base::HashMap::PointersMatch), next_index_(0) {}
+
+    // Lookup object in the map. Return its index if found, or create
+    // a new entry with new_index as value, and return kInvalidIndex.
+    bool LookupOrInsert(HeapObject* obj, int* index_out) {
+      base::HashMap::Entry* entry = LookupEntry(&map_, obj, false);
+      if (entry != NULL) {
+        *index_out = GetValue(entry);
+        return true;
+      }
+      *index_out = next_index_;
+      SetValue(LookupEntry(&map_, obj, true), next_index_++);
+      return false;
+    }
+
+   private:
+    base::HashMap map_;
+    int next_index_;
+
+    DISALLOW_COPY_AND_ASSIGN(PartialCacheIndexMap);
+  };
+
   // The StartupSerializer has to serialize the root array, which is slightly
   // different.
   void VisitPointers(Object** start, Object** end) override;
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
   void Synchronize(VisitorSynchronization::SyncTag tag) override;
 
   // Some roots should not be serialized, because their actual value depends on
   // absolute addresses and they are reset after deserialization, anyway.
   // In the first pass over the root list, we only serialize immortal immovable
   // roots. In the second pass, we serialize the rest.
   bool RootShouldBeSkipped(int root_index);
 
   bool clear_function_code_;
   bool serializing_builtins_;
   bool serializing_immortal_immovables_roots_;
   std::bitset<Heap::kStrongRootListLength> root_has_been_serialized_;
+  PartialCacheIndexMap partial_cache_index_map_;
   DISALLOW_COPY_AND_ASSIGN(StartupSerializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_STARTUP_SERIALIZER_H_