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

src/ia32/macro-assembler-ia32.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 2517 matching lines @@
            &fail,
            DONT_DO_SMI_CHECK);
   mov(descriptors, FieldOperand(temp, TransitionArray::kDescriptorsOffset));
   jmp(&ok);
   bind(&fail);
   mov(descriptors, isolate()->factory()->empty_descriptor_array());
   bind(&ok);
 }
 
 
+void MacroAssembler::EnumLength(Register dst, Register map) {
+  mov(dst, FieldOperand(map, Map::kBitField3Offset));
+  DecodeField<Map::EnumLengthBits>(dst);
+}
+
+
 void MacroAssembler::LoadPowerOf2(XMMRegister dst,
                                   Register scratch,
                                   int power) {
   ASSERT(is_uintn(power + HeapNumber::kExponentBias,
                   HeapNumber::kExponentBits));
   mov(scratch, Immediate(power + HeapNumber::kExponentBias));
   movd(dst, scratch);
   psllq(dst, HeapNumber::kMantissaBits);
 }
 
@@ skipping 325 matching lines @@
     mov(length, Operand(bitmap_scratch, MemoryChunk::kLiveBytesOffset));
     cmp(length, Operand(bitmap_scratch, MemoryChunk::kSizeOffset));
     Check(less_equal, "Live Bytes Count overflow chunk size");
   }
 
   bind(&done);
 }
 
 
 void MacroAssembler::CheckEnumCache(Label* call_runtime) {
-  Label next;
+  Label next, start;
   mov(ecx, eax);
+
+  // Check if the enum length field is properly initialized, indicating that
+  // there is an enum cache.
+  mov(ebx, FieldOperand(ecx, HeapObject::kMapOffset));
+
+  EnumLength(edx, ebx);
+  cmp(edx, Immediate(Map::kInvalidEnumCache));
+  j(equal, call_runtime);
+
+  jmp(&start);
+
   bind(&next);
+  mov(ebx, FieldOperand(ecx, HeapObject::kMapOffset));
 
-  // Check that there are no elements.  Register ecx contains the
-  // current JS object we've reached through the prototype chain.
-  cmp(FieldOperand(ecx, JSObject::kElementsOffset),
-      isolate()->factory()->empty_fixed_array());
+  // For all objects but the receiver, check that the cache is empty.
+  EnumLength(edx, ebx);
+  cmp(edx, Immediate(0));
   j(not_equal, call_runtime);
 
-  // Check that instance descriptors are not empty so that we can
-  // check for an enum cache.  Leave the map in ebx for the subsequent
-  // prototype load.
-  mov(ebx, FieldOperand(ecx, HeapObject::kMapOffset));
-  mov(edx, FieldOperand(ebx, Map::kTransitionsOrBackPointerOffset));
-  CheckMap(edx,
-           isolate()->factory()->fixed_array_map(),
-           call_runtime,
-           DONT_DO_SMI_CHECK);
+  bind(&start);
 
-  mov(edx, FieldOperand(edx, TransitionArray::kDescriptorsOffset));
-  cmp(edx, isolate()->factory()->empty_descriptor_array());
-  j(equal, call_runtime);
-
-  // Check that there is an enum cache in the non-empty instance
-  // descriptors (edx).  This is the case if the next enumeration
-  // index field does not contain a smi.
-  mov(edx, FieldOperand(edx, DescriptorArray::kEnumCacheOffset));
-  JumpIfSmi(edx, call_runtime);
-
-  // For all objects but the receiver, check that the cache is empty.
-  Label check_prototype;
-  cmp(ecx, eax);
-  j(equal, &check_prototype, Label::kNear);
-  mov(edx, FieldOperand(edx, DescriptorArray::kEnumCacheBridgeCacheOffset));
-  cmp(edx, isolate()->factory()->empty_fixed_array());
+  // Check that there are no elements. Register rcx contains the current JS
+  // object we've reached through the prototype chain.
+  mov(ecx, FieldOperand(ecx, JSObject::kElementsOffset));
+  cmp(ecx, isolate()->factory()->empty_fixed_array());
   j(not_equal, call_runtime);
 
-  // Load the prototype from the map and loop if non-null.
-  bind(&check_prototype);
   mov(ecx, FieldOperand(ebx, Map::kPrototypeOffset));
   cmp(ecx, isolate()->factory()->null_value());
   j(not_equal, &next);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32