Serializer: support all alignment kinds.

We use an alignment prefix for unusual alignment kinds (not kWordAligned).
This will cause new objects to be aligned in ReadObject, and back references
to be aligned to skip padding.

The motivation to change the undefined next sentinel in WeakCell is this:
When the deserializer aligns an object, it requires filler maps to already
exist to create filler objects as padding. However, deserializing the
filler map leads to deserializing NaN, which as heap number is aligned:
filler map > meta map > weak cell cache > undefined > NaN
If we use the-hole instead of undefined as sentinel, it works.

R=jochen@chromium.org,bbudge@chromium.org
BUG=v8:4178
LOG=N

Committed: https://crrev.com/2146ab75387d47eef6582bd8c2d0cfc6c42b68b6
Cr-Commit-Position: refs/heads/master@{#29044}

Review URL: https://codereview.chromium.org/1179873003

Cr-Commit-Position: refs/heads/master@{#29047}

src/code-stubs-hydrogen.cc

@@ -641,7 +641,7 @@ HValue* CodeStubGraphBuilder<CreateWeakCellStub>::BuildCodeStub() {
   HInstruction* value = GetParameter(CreateWeakCellDescriptor::kValueIndex);
   Add<HStoreNamedField>(object, HObjectAccess::ForWeakCellValue(), value);
   Add<HStoreNamedField>(object, HObjectAccess::ForWeakCellNext(),
-                        graph()->GetConstantUndefined());
+                        graph()->GetConstantHole());
 
   HInstruction* feedback_vector =
       GetParameter(CreateWeakCellDescriptor::kVectorIndex);

src/heap/heap.cc

@@ -3102,7 +3102,7 @@ AllocationResult Heap::AllocateWeakCell(HeapObject* value) {
   }
   result->set_map_no_write_barrier(weak_cell_map());
   WeakCell::cast(result)->initialize(value);
-  WeakCell::cast(result)->set_next(undefined_value(), SKIP_WRITE_BARRIER);
+  WeakCell::cast(result)->set_next(the_hole_value(), SKIP_WRITE_BARRIER);
   return result;
 }
 
@@ -3706,19 +3706,18 @@ AllocationResult Heap::AllocateByteArray(int length, PretenureFlag pretenure) {
 void Heap::CreateFillerObjectAt(Address addr, int size) {
   if (size == 0) return;
   HeapObject* filler = HeapObject::FromAddress(addr);
-  // At this point, we may be deserializing the heap from a snapshot, and
-  // none of the maps have been created yet and are NULL.
   if (size == kPointerSize) {
     filler->set_map_no_write_barrier(raw_unchecked_one_pointer_filler_map());
-    DCHECK(filler->map() == NULL || filler->map() == one_pointer_filler_map());
   } else if (size == 2 * kPointerSize) {
     filler->set_map_no_write_barrier(raw_unchecked_two_pointer_filler_map());
-    DCHECK(filler->map() == NULL || filler->map() == two_pointer_filler_map());
   } else {
     filler->set_map_no_write_barrier(raw_unchecked_free_space_map());
-    DCHECK(filler->map() == NULL || filler->map() == free_space_map());
     FreeSpace::cast(filler)->nobarrier_set_size(size);
   }
+  // At this point, we may be deserializing the heap from a snapshot, and
+  // none of the maps have been created yet and are NULL.
+  DCHECK((filler->map() == NULL && !deserialization_complete_) ||
+         filler->map()->IsMap());
 }
 
 

src/heap/mark-compact.cc

@@ -2637,7 +2637,7 @@ void MarkCompactCollector::AbortWeakCollections() {
 
 
 void MarkCompactCollector::ProcessAndClearWeakCells() {
-  HeapObject* undefined = heap()->undefined_value();
+  HeapObject* the_hole = heap()->the_hole_value();
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   while (weak_cell_obj != Smi::FromInt(0)) {
     WeakCell* weak_cell = reinterpret_cast<WeakCell*>(weak_cell_obj);
@@ -2672,19 +2672,19 @@ void MarkCompactCollector::ProcessAndClearWeakCells() {
       RecordSlot(slot, slot, *slot);
     }
     weak_cell_obj = weak_cell->next();
-    weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);
+    weak_cell->set_next(the_hole, SKIP_WRITE_BARRIER);
   }
   heap()->set_encountered_weak_cells(Smi::FromInt(0));
 }
 
 
 void MarkCompactCollector::AbortWeakCells() {
-  Object* undefined = heap()->undefined_value();
+  Object* the_hole = heap()->the_hole_value();
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   while (weak_cell_obj != Smi::FromInt(0)) {
     WeakCell* weak_cell = reinterpret_cast<WeakCell*>(weak_cell_obj);
     weak_cell_obj = weak_cell->next();
-    weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);
+    weak_cell->set_next(the_hole, SKIP_WRITE_BARRIER);
   }
   heap()->set_encountered_weak_cells(Smi::FromInt(0));
 }

src/heap/objects-visiting-inl.h

@@ -329,11 +329,11 @@ void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
                                                         HeapObject* object) {
   Heap* heap = map->GetHeap();
   WeakCell* weak_cell = reinterpret_cast<WeakCell*>(object);
-  Object* undefined = heap->undefined_value();
+  Object* the_hole = heap->the_hole_value();
   // Enqueue weak cell in linked list of encountered weak collections.
   // We can ignore weak cells with cleared values because they will always
   // contain smi zero.
-  if (weak_cell->next() == undefined && !weak_cell->cleared()) {
+  if (weak_cell->next() == the_hole && !weak_cell->cleared()) {
     weak_cell->set_next(heap->encountered_weak_cells(),
                         UPDATE_WEAK_WRITE_BARRIER);
     heap->set_encountered_weak_cells(weak_cell);

src/snapshot/serialize.h

@@ -324,12 +324,14 @@ class SerializerDeserializer: public ObjectVisitor {
     // 0x07        Unused (including 0x27, 0x47, 0x67).
     // 0x08..0x0c  Reference to previous object from space.
     kBackref = 0x08,
+    // 0x0d        Unused (including 0x2d, 0x4d, 0x6d).
     // 0x0e        Unused (including 0x2e, 0x4e, 0x6e).
     // 0x0f        Unused (including 0x2f, 0x4f, 0x6f).
     // 0x10..0x14  Reference to previous object from space after skip.
     kBackrefWithSkip = 0x10,
+    // 0x15        Unused (including 0x35, 0x55, 0x75).
     // 0x16        Unused (including 0x36, 0x56, 0x76).
-    // 0x17        Unused (including 0x37, 0x57, 0x77).
+    // 0x17        Misc (including 0x37, 0x57, 0x77).
     // 0x18        Root array item.
     kRootArray = 0x18,
     // 0x19        Object in the partial snapshot cache.
@@ -384,14 +386,18 @@ class SerializerDeserializer: public ObjectVisitor {
   // is an indication that the snapshot and the VM do not fit together.
   // Examine the build process for architecture, version or configuration
   // mismatches.
-  static const int kSynchronize = 0x5d;
+  static const int kSynchronize = 0x17;
   // Used for the source code of the natives, which is in the executable, but
   // is referred to from external strings in the snapshot.
-  static const int kNativesStringResource = 0x5e;
+  static const int kNativesStringResource = 0x37;
   // Raw data of variable length.
-  static const int kVariableRawData = 0x7d;
+  static const int kVariableRawData = 0x57;
   // Repeats of variable length.
-  static const int kVariableRepeat = 0x7e;
+  static const int kVariableRepeat = 0x77;
+  // Alignment prefixes 0x7d..0x7f
+  static const int kAlignmentPrefix = 0x7d;
+
+  // 0x5d..0x5f unused
 
   // ---------- byte code range 0x80..0xff ----------
   // First 32 root array items.
@@ -515,7 +521,8 @@ class Deserializer: public SerializerDeserializer {
         magic_number_(data->GetMagicNumber()),
         external_reference_table_(NULL),
         deserialized_large_objects_(0),
-        deserializing_user_code_(false) {
+        deserializing_user_code_(false),
+        next_alignment_(kWordAligned) {
     DecodeReservation(data->Reservations());
   }
 
@@ -602,6 +609,8 @@ class Deserializer: public SerializerDeserializer {
 
   bool deserializing_user_code_;
 
+  AllocationAlignment next_alignment_;
+
   DISALLOW_COPY_AND_ASSIGN(Deserializer);
 };
 
@@ -706,6 +715,9 @@ class Serializer : public SerializerDeserializer {
 
   void PutBackReference(HeapObject* object, BackReference reference);
 
+  // Emit alignment prefix if necessary, return required padding space in bytes.
+  int PutAlignmentPrefix(HeapObject* object);
+
   // Returns true if the object was successfully serialized.
   bool SerializeKnownObject(HeapObject* obj, HowToCode how_to_code,
                             WhereToPoint where_to_point, int skip);