Swap bitfield3 and backpointer.

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 cda7549df5e24bc2b8ef53478101b2cb85ef1522..5053d6768a66cefd0a21d358cba1475cd17413f8 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2853,11 +2853,21 @@ void MacroAssembler::ClampDoubleToUint8(XMMRegister input_reg,
 void MacroAssembler::LoadInstanceDescriptors(Register map,
                                              Register descriptors) {
   movq(descriptors, FieldOperand(map,
-                                 Map::kInstanceDescriptorsOrBitField3Offset));
-  Label not_smi;
-  JumpIfNotSmi(descriptors, &not_smi, Label::kNear);
+                                 Map::kInstanceDescriptorsOrBackPointerOffset));
+
+  Label ok, fail;
+  CheckMap(descriptors,
+           isolate()->factory()->fixed_array_map(),
+           &fail,
+           DONT_DO_SMI_CHECK);
+  jmp(&ok);
+  bind(&fail);
   Move(descriptors, isolate()->factory()->empty_descriptor_array());
-  bind(&not_smi);
+  bind(&ok);
+
+  if (emit_debug_code()) {
+    AbortIfNotFixedArray(descriptors);
+  }
 }
 
 
@@ -2905,6 +2915,19 @@ void MacroAssembler::AbortIfNotSmi(const Operand& object) {
 }
 
 
+void MacroAssembler::AbortIfNotFixedArray(Register object) {
+  Label ok, fail;
+  CheckMap(object,
+           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::AbortIfNotZeroExtended(Register int32_register) {
   ASSERT(!int32_register.is(kScratchRegister));
   movq(kScratchRegister, 0x100000000l, RelocInfo::NONE);
@@ -4457,8 +4480,16 @@ void MacroAssembler::CheckEnumCache(Register null_value, Label* call_runtime) {
   // check for an enum cache.  Leave the map in rbx for the subsequent
   // prototype load.
   movq(rbx, FieldOperand(rcx, HeapObject::kMapOffset));
-  movq(rdx, FieldOperand(rbx, Map::kInstanceDescriptorsOrBitField3Offset));
-  JumpIfSmi(rdx, call_runtime);
+  movq(rdx, FieldOperand(rbx, Map::kInstanceDescriptorsOrBackPointerOffset));
+
+  CheckMap(rdx,
+           isolate()->factory()->fixed_array_map(),
+           call_runtime,
+           DONT_DO_SMI_CHECK);
+
+  if (emit_debug_code()) {
+    AbortIfNotFixedArray(rdx);
+  }
 
   // Check that there is an enum cache in the non-empty instance
   // descriptors (rdx).  This is the case if the next enumeration