Reduce field type tracking to a single stable map per field.

BUG=

Review URL: https://codereview.chromium.org/930983003

Cr-Commit-Position: refs/heads/master@{#26679}

src/objects.cc

@@ -2326,18 +2326,8 @@ void Map::UpdateFieldType(int descriptor, Handle<Name> name,
 Handle<HeapType> Map::GeneralizeFieldType(Handle<HeapType> type1,
                                           Handle<HeapType> type2,
                                           Isolate* isolate) {
-  static const int kMaxClassesPerFieldType = 5;
   if (type1->NowIs(type2)) return type2;
   if (type2->NowIs(type1)) return type1;
-  if (type1->NowStable() && type2->NowStable()) {
-    Handle<HeapType> type = HeapType::Union(type1, type2, isolate);
-    if (type->NumClasses() <= kMaxClassesPerFieldType) {
-      DCHECK(type->NowStable());
-      DCHECK(type1->NowIs(type));
-      DCHECK(type2->NowIs(type));
-      return type;
-    }
-  }
   return HeapType::Any(isolate);
 }
 

src/types.cc

@@ -575,10 +575,7 @@ bool TypeImpl<Config>::NowIs(TypeImpl* that) {
 template<class Config>
 bool TypeImpl<Config>::NowStable() {
   DisallowHeapAllocation no_allocation;
-  for (Iterator<i::Map> it = this->Classes(); !it.Done(); it.Advance()) {
-    if (!it.Current()->is_stable()) return false;
-  }
-  return true;
+  return !this->IsClass() || this->AsClass()->Map()->is_stable();
 }
 
 

test/cctest/test-migrations.cc

@@ -694,7 +694,7 @@ TEST(GeneralizeRepresentationHeapObjectToHeapObject) {
   Isolate* isolate = CcTest::i_isolate();
   Handle<HeapType> any_type = HeapType::Any(isolate);
 
-  const int kMaxClassesPerFieldType = 5;
+  const int kMaxClassesPerFieldType = 1;
   Handle<HeapType> current_type =
       HeapType::Class(Map::Create(isolate, 0), isolate);
 
@@ -1070,7 +1070,7 @@ TEST(ReconfigureDataFieldAttribute_GeneralizeRepresentationHeapObjToHeapObj) {
   Isolate* isolate = CcTest::i_isolate();
   Handle<HeapType> any_type = HeapType::Any(isolate);
 
-  const int kMaxClassesPerFieldType = 5;
+  const int kMaxClassesPerFieldType = 1;
   Handle<HeapType> current_type =
       HeapType::Class(Map::Create(isolate, 0), isolate);