Reland of "Enable inobject double fields unboxing for 64-bit archs."

src/layout-descriptor-inl.h

 // Copyright 2014 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.
 
 #ifndef V8_LAYOUT_DESCRIPTOR_INL_H_
 #define V8_LAYOUT_DESCRIPTOR_INL_H_
 
 #include "src/layout-descriptor.h"
 
 namespace v8 {
@@ skipping 102 matching lines @@
     uint32_t value = get_scalar(layout_word_index);
     return (value & layout_mask) == 0;
   } else {
     uint32_t value = static_cast<uint32_t>(Smi::cast(this)->value());
     return (value & layout_mask) == 0;
   }
 }
 
 
 bool LayoutDescriptor::IsFastPointerLayout() {
-  return IsSmi() && (Smi::cast(this)->value() == 0);
+  return this == FastPointerLayout();
 }
 
 
+bool LayoutDescriptor::IsFastPointerLayout(Object* layout_descriptor) {
+  return layout_descriptor == FastPointerLayout();
+}
+
+
 bool LayoutDescriptor::IsSlowLayout() { return !IsSmi(); }
 
 
 int LayoutDescriptor::capacity() {
   return IsSlowLayout() ? (length() * kNumberOfBits) : kSmiValueSize;
 }
 
 
 LayoutDescriptor* LayoutDescriptor::cast_gc_safe(Object* object) {
   if (object->IsSmi()) {
@@ skipping 39 matching lines @@
   // Object headers do not contain non-tagged fields.
   if (offset_in_bytes < header_size_) return true;
   int field_index = (offset_in_bytes - header_size_) / kPointerSize;
 
   return layout_descriptor_->IsTagged(field_index);
 }
 }
 }  // namespace v8::internal
 
 #endif  // V8_LAYOUT_DESCRIPTOR_INL_H_