Use a special EnumLength field to indicate number of valid enum cache values.

src/arm/full-codegen-arm.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 1107 matching lines @@
 
   // Get the set of properties to enumerate.
   __ bind(&call_runtime);
   __ push(r0);  // Duplicate the enumerable object on the stack.
   __ CallRuntime(Runtime::kGetPropertyNamesFast, 1);
 
   // If we got a map from the runtime call, we can do a fast
   // modification check. Otherwise, we got a fixed array, and we have
   // to do a slow check.
   Label fixed_array;
-  __ mov(r2, r0);
-  __ ldr(r1, FieldMemOperand(r2, HeapObject::kMapOffset));
+  __ ldr(r2, FieldMemOperand(r0, HeapObject::kMapOffset));
   __ LoadRoot(ip, Heap::kMetaMapRootIndex);
-  __ cmp(r1, ip);
+  __ cmp(r2, ip);
   __ b(ne, &fixed_array);
 
   // We got a map in register r0. Get the enumeration cache from it.
+  Label no_descriptors;
   __ bind(&use_cache);
-  __ LoadInstanceDescriptors(r0, r1, r2);
-  __ ldr(r1, FieldMemOperand(r1, DescriptorArray::kEnumCacheOffset));
-  __ ldr(r2, FieldMemOperand(r1, DescriptorArray::kEnumCacheBridgeCacheOffset));
+
+  __ EnumLength(r1, r0);
+  __ cmp(r1, Operand(Smi::FromInt(0)));
+  __ b(eq, &no_descriptors);
+
+  __ LoadInstanceDescriptors(r0, r2, r4);
+  __ ldr(r2, FieldMemOperand(r2, DescriptorArray::kEnumCacheOffset));
+  __ ldr(r2, FieldMemOperand(r2, DescriptorArray::kEnumCacheBridgeCacheOffset));
 
   // Set up the four remaining stack slots.
   __ push(r0);  // Map.
-  __ ldr(r1, FieldMemOperand(r2, FixedArray::kLengthOffset));
   __ mov(r0, Operand(Smi::FromInt(0)));
   // Push enumeration cache, enumeration cache length (as smi) and zero.
   __ Push(r2, r1, r0);
   __ jmp(&loop);
 
+  __ bind(&no_descriptors);
+  __ Drop(1);
+  __ jmp(&exit);
+
   // We got a fixed array in register r0. Iterate through that.
   Label non_proxy;
   __ bind(&fixed_array);
 
   Handle<JSGlobalPropertyCell> cell =
       isolate()->factory()->NewJSGlobalPropertyCell(
           Handle<Object>(
               Smi::FromInt(TypeFeedbackCells::kForInFastCaseMarker)));
   RecordTypeFeedbackCell(stmt->ForInFeedbackId(), cell);
   __ LoadHeapObject(r1, cell);
@@ skipping 3411 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM