Correctly handle clearing of deprecated field types.

BUG=v8:4027
LOG=NO

Review URL: https://codereview.chromium.org/1086063003

Cr-Commit-Position: refs/heads/master@{#27837}

src/objects.cc

@@ -2389,10 +2389,17 @@ void Map::GeneralizeFieldType(Handle<Map> map, int modify_index,
   Handle<DescriptorArray> descriptors(
       field_owner->instance_descriptors(), isolate);
   DCHECK_EQ(*old_field_type, descriptors->GetFieldType(modify_index));
-
-  // Determine the generalized new field type.
-  new_field_type = Map::GeneralizeFieldType(
-      old_field_type, new_field_type, isolate);
+  bool old_field_type_was_cleared =
+      old_field_type->Is(HeapType::None()) && old_representation.IsHeapObject();
+
+  // Determine the generalized new field type. Conservatively assume type Any
+  // for cleared field types because the cleared type could have been a
+  // deprecated map and there still could be live instances with a non-
+  // deprecated version of the map.
+  new_field_type =
+      old_field_type_was_cleared
+          ? HeapType::Any(isolate)
+          : Map::GeneralizeFieldType(old_field_type, new_field_type, isolate);
 
   PropertyDetails details = descriptors->GetDetails(modify_index);
   Handle<Name> name(descriptors->GetKey(modify_index));

test/mjsunit/regress/regress-4027.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --expose-gc
+
+function Inner() {
+  this.inner_name = "inner";
+}
+
+function Boom() {
+  this.boom = "boom";
+}
+
+function Outer() {
+  this.outer_name = "outer";
+}
+
+function SetInner(inner, value) {
+  inner.prop = value;
+}
+
+function SetOuter(outer, value) {
+  outer.inner = value;
+}
+
+var inner1 = new Inner();
+var inner2 = new Inner();
+
+SetInner(inner1, 10);
+SetInner(inner2, 10);
+
+var outer1 = new Outer();
+var outer2 = new Outer();
+var outer3 = new Outer();
+
+SetOuter(outer1, inner1);
+SetOuter(outer1, inner1);
+SetOuter(outer1, inner1);
+
+SetOuter(outer2, inner2);
+SetOuter(outer2, inner2);
+SetOuter(outer2, inner2);
+
+SetOuter(outer3, inner2);
+SetOuter(outer3, inner2);
+SetOuter(outer3, inner2);
+
+
+SetInner(inner2, 6.5);
+
+outer1 = null;
+inner1 = null;
+
+gc();
+
+var boom = new Boom();
+SetOuter(outer2, boom);
+
+gc();