Reapply descriptor array sharing.

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index 2a677be525855425b814bb88c8add2440b2ba363..d2663102657a5f6eb04d189435b1582fc7a9d3fa 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -3703,20 +3703,37 @@ void MacroAssembler::LoadInstanceDescriptors(Register map,
   Register temp = descriptors;
   ldr(temp, FieldMemOperand(map, Map::kTransitionsOrBackPointerOffset));
 
-  Label ok, fail;
+  Label ok, fail, load_from_back_pointer;
   CheckMap(temp,
            scratch,
            isolate()->factory()->fixed_array_map(),
            &fail,
            DONT_DO_SMI_CHECK);
-  ldr(descriptors, FieldMemOperand(temp, TransitionArray::kDescriptorsOffset));
+  ldr(temp, FieldMemOperand(temp, TransitionArray::kDescriptorsPointerOffset));
+  ldr(descriptors, FieldMemOperand(temp, JSGlobalPropertyCell::kValueOffset));
   jmp(&ok);
+
   bind(&fail);
+  CompareRoot(temp, Heap::kUndefinedValueRootIndex);
+  b(ne, &load_from_back_pointer);
   mov(descriptors, Operand(FACTORY->empty_descriptor_array()));
+  jmp(&ok);
+
+  bind(&load_from_back_pointer);
+  ldr(temp, FieldMemOperand(temp, Map::kTransitionsOrBackPointerOffset));
+  ldr(temp, FieldMemOperand(temp, TransitionArray::kDescriptorsPointerOffset));
+  ldr(descriptors, FieldMemOperand(temp, JSGlobalPropertyCell::kValueOffset));
+
   bind(&ok);
 }
 
 
+void MacroAssembler::NumberOfOwnDescriptors(Register dst, Register map) {
+  ldr(dst, FieldMemOperand(map, Map::kBitFieldOffset));
+  DecodeField<Map::NumberOfOwnDescriptorsBits>(dst);
+}
+
+
 void MacroAssembler::EnumLength(Register dst, Register map) {
   STATIC_ASSERT(Map::EnumLengthBits::kShift == 0);
   ldr(dst, FieldMemOperand(map, Map::kBitField3Offset));