Revert of Serializer: support all alignment kinds. (patchset #3 id:40001 of...

Revert of Serializer: support all alignment kinds. (patchset #3 id:40001 of https://codereview.chromium.org/1179873003/)

Reason for revert:
[Sheriff] Breaks arm debug compile:
http://build.chromium.org/p/client.v8/builders/V8%20Arm%20-%20debug%20builder/builds/4308

Original issue's description:
> 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}

TBR=jochen@chromium.org,bbudge@chromium.org,yangguo@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4178

Review URL: https://codereview.chromium.org/1173253006

Cr-Commit-Position: refs/heads/master@{#29045}

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()->GetConstantHole());
+                        graph()->GetConstantUndefined());
 
   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(the_hole_value(), SKIP_WRITE_BARRIER);
+  WeakCell::cast(result)->set_next(undefined_value(), SKIP_WRITE_BARRIER);
   return result;
 }
 
@@ -3706,18 +3706,19 @@ 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* the_hole = heap()->the_hole_value();
+  HeapObject* undefined = heap()->undefined_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(the_hole, SKIP_WRITE_BARRIER);
+    weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);
   }
   heap()->set_encountered_weak_cells(Smi::FromInt(0));
 }
 
 
 void MarkCompactCollector::AbortWeakCells() {
-  Object* the_hole = heap()->the_hole_value();
+  Object* undefined = heap()->undefined_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(the_hole, SKIP_WRITE_BARRIER);
+    weak_cell->set_next(undefined, 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* the_hole = heap->the_hole_value();
+  Object* undefined = heap->undefined_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() == the_hole && !weak_cell->cleared()) {
+  if (weak_cell->next() == undefined && !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.cc

@@ -753,8 +753,6 @@ HeapObject* Deserializer::PostProcessNewObject(HeapObject* obj, int space) {
       new_code_objects_.Add(Code::cast(obj));
     }
   }
-  // Check alignment.
-  DCHECK_EQ(0, Heap::GetFillToAlign(obj->address(), obj->RequiredAlignment()));
   return obj;
 }
 
@@ -771,14 +769,8 @@ HeapObject* Deserializer::GetBackReferencedObject(int space) {
     uint32_t chunk_index = back_reference.chunk_index();
     DCHECK_LE(chunk_index, current_chunk_[space]);
     uint32_t chunk_offset = back_reference.chunk_offset();
-    Address address = reservations_[space][chunk_index].start + chunk_offset;
-    if (next_alignment_ != kWordAligned) {
-      int padding = Heap::GetFillToAlign(address, next_alignment_);
-      next_alignment_ = kWordAligned;
-      DCHECK(padding == 0 || HeapObject::FromAddress(address)->IsFiller());
-      address += padding;
-    }
-    obj = HeapObject::FromAddress(address);
+    obj = HeapObject::FromAddress(reservations_[space][chunk_index].start +
+                                  chunk_offset);
   }
   if (deserializing_user_code() && obj->IsInternalizedString()) {
     obj = String::cast(obj)->GetForwardedInternalizedString();
@@ -796,25 +788,22 @@ HeapObject* Deserializer::GetBackReferencedObject(int space) {
 void Deserializer::ReadObject(int space_number, Object** write_back) {
   Address address;
   HeapObject* obj;
-  int size = source_.GetInt() << kObjectAlignmentBits;
-
-  if (next_alignment_ != kWordAligned) {
-    int reserved = size + Heap::GetMaximumFillToAlign(next_alignment_);
-    address = Allocate(space_number, reserved);
-    obj = HeapObject::FromAddress(address);
-    // If one of the following assertions fails, then we are deserializing an
-    // aligned object when the filler maps have not been deserialized yet.
-    // We require filler maps as padding to align the object.
-    Heap* heap = isolate_->heap();
-    DCHECK(heap->free_space_map()->IsMap());
-    DCHECK(heap->one_pointer_filler_map()->IsMap());
-    DCHECK(heap->two_pointer_filler_map()->IsMap());
-    obj = heap->AlignWithFiller(obj, size, reserved, next_alignment_);
+  int next_int = source_.GetInt();
+
+  bool double_align = false;
+#ifndef V8_HOST_ARCH_64_BIT
+  double_align = next_int == kDoubleAlignmentSentinel;
+  if (double_align) next_int = source_.GetInt();
+#endif
+
+  DCHECK_NE(kDoubleAlignmentSentinel, next_int);
+  int size = next_int << kObjectAlignmentBits;
+  int reserved_size = size + (double_align ? kPointerSize : 0);
+  address = Allocate(space_number, reserved_size);
+  obj = HeapObject::FromAddress(address);
+  if (double_align) {
+    obj = isolate_->heap()->DoubleAlignForDeserialization(obj, reserved_size);
     address = obj->address();
-    next_alignment_ = kWordAligned;
-  } else {
-    address = Allocate(space_number, size);
-    obj = HeapObject::FromAddress(address);
   }
 
   isolate_->heap()->OnAllocationEvent(obj, size);
@@ -1009,17 +998,14 @@ bool Deserializer::ReadData(Object** current, Object** limit, int source_space,
   FOUR_CASES(byte_code + 8)               \
   FOUR_CASES(byte_code + 12)
 
-#define SINGLE_CASE(where, how, within, space) \
-  CASE_STATEMENT(where, how, within, space)    \
-  CASE_BODY(where, how, within, space)
-
       // Deserialize a new object and write a pointer to it to the current
       // object.
       ALL_SPACES(kNewObject, kPlain, kStartOfObject)
       // Support for direct instruction pointers in functions.  It's an inner
       // pointer because it points at the entry point, not at the start of the
       // code object.
-      SINGLE_CASE(kNewObject, kPlain, kInnerPointer, CODE_SPACE)
+      CASE_STATEMENT(kNewObject, kPlain, kInnerPointer, CODE_SPACE)
+      CASE_BODY(kNewObject, kPlain, kInnerPointer, CODE_SPACE)
       // Deserialize a new code object and write a pointer to its first
       // instruction to the current code object.
       ALL_SPACES(kNewObject, kFromCode, kInnerPointer)
@@ -1050,33 +1036,45 @@ bool Deserializer::ReadData(Object** current, Object** limit, int source_space,
       ALL_SPACES(kBackrefWithSkip, kPlain, kInnerPointer)
       // Find an object in the roots array and write a pointer to it to the
       // current object.
-      SINGLE_CASE(kRootArray, kPlain, kStartOfObject, 0)
+      CASE_STATEMENT(kRootArray, kPlain, kStartOfObject, 0)
+      CASE_BODY(kRootArray, kPlain, kStartOfObject, 0)
 #if defined(V8_TARGET_ARCH_MIPS) || defined(V8_TARGET_ARCH_MIPS64) || \
     defined(V8_TARGET_ARCH_PPC) || V8_EMBEDDED_CONSTANT_POOL
       // Find an object in the roots array and write a pointer to it to in code.
-      SINGLE_CASE(kRootArray, kFromCode, kStartOfObject, 0)
+      CASE_STATEMENT(kRootArray, kFromCode, kStartOfObject, 0)
+      CASE_BODY(kRootArray, kFromCode, kStartOfObject, 0)
 #endif
       // Find an object in the partial snapshots cache and write a pointer to it
       // to the current object.
-      SINGLE_CASE(kPartialSnapshotCache, kPlain, kStartOfObject, 0)
+      CASE_STATEMENT(kPartialSnapshotCache, kPlain, kStartOfObject, 0)
+      CASE_BODY(kPartialSnapshotCache, kPlain, kStartOfObject, 0)
       // Find an code entry in the partial snapshots cache and
       // write a pointer to it to the current object.
-      SINGLE_CASE(kPartialSnapshotCache, kPlain, kInnerPointer, 0)
+      CASE_STATEMENT(kPartialSnapshotCache, kPlain, kInnerPointer, 0)
+      CASE_BODY(kPartialSnapshotCache, kPlain, kInnerPointer, 0)
       // Find an external reference and write a pointer to it to the current
       // object.
-      SINGLE_CASE(kExternalReference, kPlain, kStartOfObject, 0)
+      CASE_STATEMENT(kExternalReference, kPlain, kStartOfObject, 0)
+      CASE_BODY(kExternalReference, kPlain, kStartOfObject, 0)
       // Find an external reference and write a pointer to it in the current
       // code object.
-      SINGLE_CASE(kExternalReference, kFromCode, kStartOfObject, 0)
+      CASE_STATEMENT(kExternalReference, kFromCode, kStartOfObject, 0)
+      CASE_BODY(kExternalReference, kFromCode, kStartOfObject, 0)
       // Find an object in the attached references and write a pointer to it to
       // the current object.
-      SINGLE_CASE(kAttachedReference, kPlain, kStartOfObject, 0)
-      SINGLE_CASE(kAttachedReference, kPlain, kInnerPointer, 0)
-      SINGLE_CASE(kAttachedReference, kFromCode, kInnerPointer, 0)
+      CASE_STATEMENT(kAttachedReference, kPlain, kStartOfObject, 0)
+      CASE_BODY(kAttachedReference, kPlain, kStartOfObject, 0)
+      CASE_STATEMENT(kAttachedReference, kPlain, kInnerPointer, 0)
+      CASE_BODY(kAttachedReference, kPlain, kInnerPointer, 0)
+      CASE_STATEMENT(kAttachedReference, kFromCode, kInnerPointer, 0)
+      CASE_BODY(kAttachedReference, kFromCode, kInnerPointer, 0)
       // Find a builtin and write a pointer to it to the current object.
-      SINGLE_CASE(kBuiltin, kPlain, kStartOfObject, 0)
-      SINGLE_CASE(kBuiltin, kPlain, kInnerPointer, 0)
-      SINGLE_CASE(kBuiltin, kFromCode, kInnerPointer, 0)
+      CASE_STATEMENT(kBuiltin, kPlain, kStartOfObject, 0)
+      CASE_BODY(kBuiltin, kPlain, kStartOfObject, 0)
+      CASE_STATEMENT(kBuiltin, kPlain, kInnerPointer, 0)
+      CASE_BODY(kBuiltin, kPlain, kInnerPointer, 0)
+      CASE_STATEMENT(kBuiltin, kFromCode, kInnerPointer, 0)
+      CASE_BODY(kBuiltin, kFromCode, kInnerPointer, 0)
 
 #undef CASE_STATEMENT
 #undef CASE_BODY
@@ -1171,15 +1169,6 @@ bool Deserializer::ReadData(Object** current, Object** limit, int source_space,
         break;
       }
 
-      case kAlignmentPrefix:
-      case kAlignmentPrefix + 1:
-      case kAlignmentPrefix + 2: {
-        DCHECK_EQ(kWordAligned, next_alignment_);
-        next_alignment_ =
-            static_cast<AllocationAlignment>(data - (kAlignmentPrefix - 1));
-        break;
-      }
-
       STATIC_ASSERT(kNumberOfRootArrayConstants == Heap::kOldSpaceRoots);
       STATIC_ASSERT(kNumberOfRootArrayConstants == 32);
       SIXTEEN_CASES(kRootArrayConstantsWithSkip)
@@ -1246,7 +1235,6 @@ bool Deserializer::ReadData(Object** current, Object** limit, int source_space,
 
 #undef SIXTEEN_CASES
 #undef FOUR_CASES
-#undef SINGLE_CASE
 
       default:
         CHECK(false);
@@ -1574,7 +1562,6 @@ bool Serializer::SerializeKnownObject(HeapObject* obj, HowToCode how_to_code,
         PrintF("\n");
       }
 
-      PutAlignmentPrefix(obj);
       AllocationSpace space = back_reference.space();
       if (skip == 0) {
         sink_->Put(kBackref + how_to_code + where_to_point + space, "BackRef");
@@ -1668,18 +1655,6 @@ void Serializer::PutBackReference(HeapObject* object, BackReference reference) {
 }
 
 
-int Serializer::PutAlignmentPrefix(HeapObject* object) {
-  AllocationAlignment alignment = object->RequiredAlignment();
-  if (alignment != kWordAligned) {
-    DCHECK(1 <= alignment && alignment <= 3);
-    byte prefix = (kAlignmentPrefix - 1) + alignment;
-    sink_->Put(prefix, "Alignment");
-    return Heap::GetMaximumFillToAlign(alignment);
-  }
-  return 0;
-}
-
-
 void PartialSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
                                         WhereToPoint where_to_point, int skip) {
   if (obj->IsMap()) {
@@ -1761,10 +1736,21 @@ void Serializer::ObjectSerializer::SerializePrologue(AllocationSpace space,
     }
     back_reference = serializer_->AllocateLargeObject(size);
   } else {
-    int fill = serializer_->PutAlignmentPrefix(object_);
-    back_reference = serializer_->Allocate(space, size + fill);
+    bool needs_double_align = false;
+    // TODO(bbudge): Generalize to other alignment constraints.
+    if (object_->RequiredAlignment() == kDoubleAligned) {
+      // Add wriggle room for double alignment padding.
+      back_reference = serializer_->Allocate(space, size + kPointerSize);
+      needs_double_align = true;
+    } else {
+      back_reference = serializer_->Allocate(space, size);
+    }
     sink_->Put(kNewObject + reference_representation_ + space, "NewObject");
-    sink_->PutInt(size >> kObjectAlignmentBits, "ObjectSizeInWords");
+    if (needs_double_align)
+      sink_->PutInt(kDoubleAlignmentSentinel, "DoubleAlignSentinel");
+    int encoded_size = size >> kObjectAlignmentBits;
+    DCHECK_NE(kDoubleAlignmentSentinel, encoded_size);
+    sink_->PutInt(encoded_size, "ObjectSizeInWords");
   }
 
 #ifdef OBJECT_PRINT

src/snapshot/serialize.h

@@ -324,14 +324,12 @@ 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        Misc (including 0x37, 0x57, 0x77).
+    // 0x17        Unused (including 0x37, 0x57, 0x77).
     // 0x18        Root array item.
     kRootArray = 0x18,
     // 0x19        Object in the partial snapshot cache.
@@ -386,18 +384,14 @@ 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 = 0x17;
+  static const int kSynchronize = 0x5d;
   // 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 = 0x37;
+  static const int kNativesStringResource = 0x5e;
   // Raw data of variable length.
-  static const int kVariableRawData = 0x57;
+  static const int kVariableRawData = 0x7d;
   // Repeats of variable length.
-  static const int kVariableRepeat = 0x77;
-  // Alignment prefixes 0x7d..0x7f
-  static const int kAlignmentPrefix = 0x7d;
-
-  // 0x5d..0x5f unused
+  static const int kVariableRepeat = 0x7e;
 
   // ---------- byte code range 0x80..0xff ----------
   // First 32 root array items.
@@ -521,8 +515,7 @@ class Deserializer: public SerializerDeserializer {
         magic_number_(data->GetMagicNumber()),
         external_reference_table_(NULL),
         deserialized_large_objects_(0),
-        deserializing_user_code_(false),
-        next_alignment_(kWordAligned) {
+        deserializing_user_code_(false) {
     DecodeReservation(data->Reservations());
   }
 
@@ -609,8 +602,6 @@ class Deserializer: public SerializerDeserializer {
 
   bool deserializing_user_code_;
 
-  AllocationAlignment next_alignment_;
-
   DISALLOW_COPY_AND_ASSIGN(Deserializer);
 };
 
@@ -715,9 +706,6 @@ 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);