Reapply descriptor array sharing.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2902 matching lines @@
   addsd(dst, scratch);
   bind(&done);
 }
 
 
 void MacroAssembler::LoadInstanceDescriptors(Register map,
                                              Register descriptors) {
   Register temp = descriptors;
   movq(temp, FieldOperand(map, Map::kTransitionsOrBackPointerOffset));
 
-  Label ok, fail;
+  Label ok, fail, load_from_back_pointer;
   CheckMap(temp,
            isolate()->factory()->fixed_array_map(),
            &fail,
            DONT_DO_SMI_CHECK);
-  movq(descriptors, FieldOperand(temp, TransitionArray::kDescriptorsOffset));
+  movq(temp, FieldOperand(temp, TransitionArray::kDescriptorsPointerOffset));
+  movq(descriptors, FieldOperand(temp, JSGlobalPropertyCell::kValueOffset));
   jmp(&ok);
+
   bind(&fail);
+  CompareRoot(temp, Heap::kUndefinedValueRootIndex);
+  j(not_equal, &load_from_back_pointer, Label::kNear);
   Move(descriptors, isolate()->factory()->empty_descriptor_array());
+  jmp(&ok);
+
+  bind(&load_from_back_pointer);
+  movq(temp, FieldOperand(temp, Map::kTransitionsOrBackPointerOffset));
+  movq(temp, FieldOperand(temp, TransitionArray::kDescriptorsPointerOffset));
+  movq(descriptors, FieldOperand(temp, JSGlobalPropertyCell::kValueOffset));
+
   bind(&ok);
 }
 
 
+void MacroAssembler::NumberOfOwnDescriptors(Register dst, Register map) {
+  movq(dst, FieldOperand(map, Map::kBitField3Offset));
+  DecodeField<Map::NumberOfOwnDescriptorsBits>(dst);
+}
+
+
 void MacroAssembler::EnumLength(Register dst, Register map) {
   STATIC_ASSERT(Map::EnumLengthBits::kShift == 0);
   movq(dst, FieldOperand(map, Map::kBitField3Offset));
   Move(kScratchRegister, Smi::FromInt(Map::EnumLengthBits::kMask));
   and_(dst, kScratchRegister);
 }
 
 
 void MacroAssembler::DispatchMap(Register obj,
                                  Handle<Map> map,
@@ skipping 1610 matching lines @@
 
   movq(rcx, FieldOperand(rbx, Map::kPrototypeOffset));
   cmpq(rcx, null_value);
   j(not_equal, &next);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64