Swap bitfield3 and backpointer.

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 933399e00f43bd62c65993cb1b820d0035aa2cc7..cac8ffb5129d8be2c3de037e88d6f28dfaa124b6 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -3014,6 +3014,20 @@ void MacroAssembler::AbortIfNotSmi(Register object) {
 }
 
 
+void MacroAssembler::AbortIfNotFixedArray(Register object, Register scratch) {
+  Label ok, fail;
+  CheckMap(object,
+           scratch,
+           isolate()->factory()->fixed_array_map(),
+           &fail,
+           DONT_DO_SMI_CHECK);
+  jmp(&ok);
+  bind(&fail);
+  Abort("Operand is not a fixed array");
+  bind(&ok);
+}
+
+
 void MacroAssembler::AbortIfNotString(Register object) {
   STATIC_ASSERT(kSmiTag == 0);
   tst(object, Operand(kSmiTagMask));
@@ -3673,13 +3687,25 @@ void MacroAssembler::ClampDoubleToUint8(Register result_reg,
 
 
 void MacroAssembler::LoadInstanceDescriptors(Register map,
-                                             Register descriptors) {
+                                             Register descriptors,
+                                             Register scratch) {
   ldr(descriptors,
-      FieldMemOperand(map, Map::kInstanceDescriptorsOrBitField3Offset));
-  Label not_smi;
-  JumpIfNotSmi(descriptors, &not_smi);
+      FieldMemOperand(map, Map::kInstanceDescriptorsOrBackPointerOffset));
+
+  Label ok, fail;
+  CheckMap(descriptors,
+           scratch,
+           isolate()->factory()->fixed_array_map(),
+           &fail,
+           DONT_DO_SMI_CHECK);
+  jmp(&ok);
+  bind(&fail);
   mov(descriptors, Operand(FACTORY->empty_descriptor_array()));
-  bind(&not_smi);
+  bind(&ok);
+
+  if (emit_debug_code()) {
+    AbortIfNotFixedArray(descriptors, scratch);
+  }
 }
 
 
@@ -3704,8 +3730,17 @@ void MacroAssembler::CheckEnumCache(Register null_value, Label* call_runtime) {
   // check for an enum cache.  Leave the map in r2 for the subsequent
   // prototype load.
   ldr(r2, FieldMemOperand(r1, HeapObject::kMapOffset));
-  ldr(r3, FieldMemOperand(r2, Map::kInstanceDescriptorsOrBitField3Offset));
-  JumpIfSmi(r3, call_runtime);
+  ldr(r3, FieldMemOperand(r2, Map::kInstanceDescriptorsOrBackPointerOffset));
+
+  CheckMap(r3,
+           r7,
+           isolate()->factory()->fixed_array_map(),
+           call_runtime,
+           DONT_DO_SMI_CHECK);
+
+  if (emit_debug_code()) {
+    AbortIfNotFixedArray(r3, r7);
+  }
 
   // Check that there is an enum cache in the non-empty instance
   // descriptors (r3).  This is the case if the next enumeration