Tree shaker:

runtime/vm/clustered_snapshot.h

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_CLUSTERED_SNAPSHOT_H_
 #define RUNTIME_VM_CLUSTERED_SNAPSHOT_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
 #include "vm/datastream.h"
 #include "vm/exceptions.h"
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/hash_map.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
 #include "vm/snapshot.h"
 #include "vm/version.h"
 #include "vm/visitor.h"
 
+#if defined(DEBUG)
+#define SNAPSHOT_BACKTRACE
+#endif

[siva, 2017/04/18 21:17:58]

If this is going to be always on when DEBUG is defined why have this extra
define, why not just use #if defined(DEBUG) for the code that is used for
snapshot backtraces.

+
 namespace dart {
 
 // Forward declarations.
 class Serializer;
 class Deserializer;
 class ObjectStore;
 
 // For full snapshots, we use a clustered snapshot format that trades longer
 // serialization time for faster deserialization time and smaller snapshots.
 // Objects are clustered by class to allow writing type information once per
@@ skipping 114 matching lines @@
       } else {
         SmiObjectIdPair new_pair;
         new_pair.smi_ = smi;
         new_pair.id_ = next_ref_index_;
         smi_ids_.Insert(new_pair);
       }
     }
     next_ref_index_++;
   }
 
-  void Push(RawObject* object) {
-    if (!object->IsHeapObject()) {
-      RawSmi* smi = Smi::RawCast(object);
-      if (smi_ids_.Lookup(smi) == NULL) {
-        SmiObjectIdPair pair;
-        pair.smi_ = smi;
-        pair.id_ = 1;
-        smi_ids_.Insert(pair);
-        stack_.Add(object);
-        num_written_objects_++;
-      }
-      return;
-    }
-
-    if (object->IsCode() && !Snapshot::IncludesCode(kind_)) {
-      return;  // Do not trace, will write null.
-    }
-
-    if (object->IsSendPort()) {
-      // TODO(rmacnak): Do a better job of resetting fields in precompilation
-      // and assert this is unreachable.
-      return;  // Do not trace, will write null.
-    }
-
-    intptr_t id = heap_->GetObjectId(object);
-    if (id == 0) {
-      heap_->SetObjectId(object, 1);
-      ASSERT(heap_->GetObjectId(object) != 0);
-      stack_.Add(object);
-      num_written_objects_++;
-    }
-  }
+  void Push(RawObject* object);
 
   void AddUntracedRef() { num_written_objects_++; }
 
   void Trace(RawObject* object);
 
+  void UnexpectedObject(RawObject* object, const char* message);
+#if defined(SNAPSHOT_BACKTRACE)
+  RawObject* ParentOf(const Object& object);
+#endif
+
   SerializationCluster* NewClusterForClass(intptr_t cid);
 
   void ReserveHeader() {
     // Make room for recording snapshot buffer size.
     stream_.set_current(stream_.buffer() + Snapshot::kHeaderSize);
   }
 
   void FillHeader(Snapshot::Kind kind) {
     int64_t* data = reinterpret_cast<int64_t*>(stream_.buffer());
     data[Snapshot::kLengthIndex] = stream_.bytes_written();
@@ skipping 71 matching lines @@
   WriteStream stream_;
   ImageWriter* image_writer_;
   SerializationCluster** clusters_by_cid_;
   GrowableArray<RawObject*> stack_;
   intptr_t num_cids_;
   intptr_t num_base_objects_;
   intptr_t num_written_objects_;
   intptr_t next_ref_index_;
   SmiObjectIdMap smi_ids_;
 
+#if defined(SNAPSHOT_BACKTRACE)
+  RawObject* current_parent_;
+  GrowableArray<Object*> parent_pairs_;
+#endif
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(Serializer);
 };
 
 
 class Deserializer : public StackResource {
  public:
   Deserializer(Thread* thread,
                Snapshot::Kind kind,
                const uint8_t* buffer,
                intptr_t size,
@@ skipping 167 matching lines @@
   intptr_t size_;
   const uint8_t* instructions_buffer_;
   const uint8_t* data_buffer_;
 
   DISALLOW_COPY_AND_ASSIGN(FullSnapshotReader);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_CLUSTERED_SNAPSHOT_H_