While finding the valueOf descriptor, only look at the key entries.

src/ia32/full-codegen-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 2652 matching lines @@
   __ LoadInstanceDescriptors(ebx, ebx);
   __ mov(ecx, FieldOperand(ebx, FixedArray::kLengthOffset));
   // ebx: descriptor array
   // ecx: length of descriptor array
   // Calculate the end of the descriptor array.
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize == 1);
   STATIC_ASSERT(kPointerSize == 4);
   __ lea(ecx, Operand(ebx, ecx, times_2, FixedArray::kHeaderSize));
   // Calculate location of the first key name.
-  __ add(ebx,
-         Immediate(FixedArray::kHeaderSize +
-                   DescriptorArray::kFirstIndex * kPointerSize));
+  __ add(ebx, Immediate(DescriptorArray::kFirstOffset));
   // Loop through all the keys in the descriptor array. If one of these is the
   // symbol valueOf the result is false.
   Label entry, loop;
   __ jmp(&entry);
   __ bind(&loop);
   __ mov(edx, FieldOperand(ebx, 0));
   __ cmp(edx, FACTORY->value_of_symbol());
   __ j(equal, if_false);
-  __ add(ebx, Immediate(kPointerSize));
+  __ add(ebx, Immediate(DescriptorArray::kDescriptorSize * kPointerSize));
   __ bind(&entry);
   __ cmp(ebx, ecx);
   __ j(not_equal, &loop);
 
   // Reload map as register ebx was used as temporary above.
   __ mov(ebx, FieldOperand(eax, HeapObject::kMapOffset));
 
   // If a valueOf property is not found on the object check that it's
   // prototype is the un-modified String prototype. If not result is false.
   __ mov(ecx, FieldOperand(ebx, Map::kPrototypeOffset));
@@ skipping 1850 matching lines @@
   *stack_depth = 0;
   *context_length = 0;
   return previous_;
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32