[snapshot] support multiple contexts in the same snapshot.

src/snapshot/startup-serializer.cc

 // 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.
 
 #include "src/snapshot/startup-serializer.h"
 
 #include "src/objects-inl.h"
 #include "src/v8threads.h"
 
 namespace v8 {
@@ skipping 72 matching lines @@
   // This comes right after serialization of the partial snapshot, where we
   // add entries to the partial snapshot cache of the startup snapshot. Add
   // one entry with 'undefined' to terminate the partial snapshot cache.
   Object* undefined = isolate()->heap()->undefined_value();
   VisitPointer(&undefined);
   isolate()->heap()->IterateWeakRoots(this, VISIT_ALL);
   SerializeDeferredObjects();
   Pad();
 }
 
+int StartupSerializer::PartialSnapshotCacheIndex(HeapObject* heap_object) {
+  int index;
+  if (!partial_cache_index_map_.LookupOrInsert(heap_object, &index)) {
+    // This object is not part of the partial snapshot cache yet. Add it to the
+    // startup snapshot so we can refer to it via partial snapshot index from
+    // the partial snapshot.
+    VisitPointer(reinterpret_cast<Object**>(&heap_object));
+  }
+  return index;
+}
+
 void StartupSerializer::Synchronize(VisitorSynchronization::SyncTag tag) {
   // We expect the builtins tag after builtins have been serialized.
   DCHECK(!serializing_builtins_ || tag == VisitorSynchronization::kBuiltins);
   serializing_builtins_ = (tag == VisitorSynchronization::kHandleScope);
   sink_.Put(kSynchronize, "Synchronize");
 }
 
 void StartupSerializer::SerializeStrongReferences() {
   Isolate* isolate = this->isolate();
   // No active threads.
@@ skipping 56 matching lines @@
   if (root_index == Heap::kStackLimitRootIndex ||
       root_index == Heap::kRealStackLimitRootIndex) {
     return true;
   }
   return Heap::RootIsImmortalImmovable(root_index) !=
          serializing_immortal_immovables_roots_;
 }
 
 }  // namespace internal
 }  // namespace v8