Move Maps' back pointers from "transitions" to "constructor" field

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index e1817ba8b46435a234050b6c4db2746a52075827..ef0a0e941aa1b4cc9e8b01f37c7d989061c20c28 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -2464,14 +2464,14 @@ AllocationResult Heap::AllocateMap(InstanceType instance_type,
   Map* map = Map::cast(result);
   map->set_instance_type(instance_type);
   map->set_prototype(null_value(), SKIP_WRITE_BARRIER);
-  map->set_constructor(null_value(), SKIP_WRITE_BARRIER);
+  map->set_constructor_or_backpointer(null_value(), SKIP_WRITE_BARRIER);
   map->set_instance_size(instance_size);
   map->set_inobject_properties(0);
   map->set_pre_allocated_property_fields(0);
   map->set_code_cache(empty_fixed_array(), SKIP_WRITE_BARRIER);
   map->set_dependent_code(DependentCode::cast(empty_fixed_array()),
                           SKIP_WRITE_BARRIER);
-  map->init_back_pointer(undefined_value());
+  map->init_transitions(undefined_value());
   map->set_unused_property_fields(0);
   map->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
@@ -2605,7 +2605,7 @@ bool Heap::CreateInitialMaps() {
   // Fix the instance_descriptors for the existing maps.
   meta_map()->set_code_cache(empty_fixed_array());
   meta_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  meta_map()->init_back_pointer(undefined_value());
+  meta_map()->init_transitions(undefined_value());
   meta_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     meta_map()->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
@@ -2614,7 +2614,7 @@ bool Heap::CreateInitialMaps() {
   fixed_array_map()->set_code_cache(empty_fixed_array());
   fixed_array_map()->set_dependent_code(
       DependentCode::cast(empty_fixed_array()));
-  fixed_array_map()->init_back_pointer(undefined_value());
+  fixed_array_map()->init_transitions(undefined_value());
   fixed_array_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     fixed_array_map()->set_layout_descriptor(
@@ -2623,7 +2623,7 @@ bool Heap::CreateInitialMaps() {
 
   undefined_map()->set_code_cache(empty_fixed_array());
   undefined_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  undefined_map()->init_back_pointer(undefined_value());
+  undefined_map()->init_transitions(undefined_value());
   undefined_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     undefined_map()->set_layout_descriptor(
@@ -2632,7 +2632,7 @@ bool Heap::CreateInitialMaps() {
 
   null_map()->set_code_cache(empty_fixed_array());
   null_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  null_map()->init_back_pointer(undefined_value());
+  null_map()->init_transitions(undefined_value());
   null_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     null_map()->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
@@ -2641,7 +2641,7 @@ bool Heap::CreateInitialMaps() {
   constant_pool_array_map()->set_code_cache(empty_fixed_array());
   constant_pool_array_map()->set_dependent_code(
       DependentCode::cast(empty_fixed_array()));
-  constant_pool_array_map()->init_back_pointer(undefined_value());
+  constant_pool_array_map()->init_transitions(undefined_value());
   constant_pool_array_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     constant_pool_array_map()->set_layout_descriptor(
@@ -2650,19 +2650,19 @@ bool Heap::CreateInitialMaps() {
 
   // Fix prototype object for existing maps.
   meta_map()->set_prototype(null_value());
-  meta_map()->set_constructor(null_value());
+  meta_map()->set_constructor_or_backpointer(null_value());
 
   fixed_array_map()->set_prototype(null_value());
-  fixed_array_map()->set_constructor(null_value());
+  fixed_array_map()->set_constructor_or_backpointer(null_value());
 
   undefined_map()->set_prototype(null_value());
-  undefined_map()->set_constructor(null_value());
+  undefined_map()->set_constructor_or_backpointer(null_value());
 
   null_map()->set_prototype(null_value());
-  null_map()->set_constructor(null_value());
+  null_map()->set_constructor_or_backpointer(null_value());
 
   constant_pool_array_map()->set_prototype(null_value());
-  constant_pool_array_map()->set_constructor(null_value());
+  constant_pool_array_map()->set_constructor_or_backpointer(null_value());
 
   {  // Map allocation
 #define ALLOCATE_MAP(instance_type, size, field_name)               \
@@ -3849,9 +3849,9 @@ void Heap::InitializeJSObjectFromMap(JSObject* obj, FixedArray* properties,
   // Pre-allocated fields need to be initialized with undefined_value as well
   // so that object accesses before the constructor completes (e.g. in the
   // debugger) will not cause a crash.
-  if (map->constructor()->IsJSFunction() &&
-      JSFunction::cast(map->constructor())
-          ->IsInobjectSlackTrackingInProgress()) {
+  Object* constructor = map->GetConstructor();
+  if (constructor->IsJSFunction() &&
+      JSFunction::cast(constructor)->IsInobjectSlackTrackingInProgress()) {
     // We might want to shrink the object later.
     DCHECK(obj->GetInternalFieldCount() == 0);
     filler = Heap::one_pointer_filler_map();