Reapply descriptor array sharing.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 1b0f2fa2d4fb196b3184ef9cf9e8ba8c6fe7ac82..6759b19a28868e5c3c0ce134aea8e6211531b3de 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2920,19 +2920,36 @@ void MacroAssembler::LoadInstanceDescriptors(Register map,
   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));