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

src/mips/full-codegen-mips.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 2749 matching lines @@
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize == 1);
   STATIC_ASSERT(kPointerSize == 4);
   __ Addu(a2, t0, Operand(FixedArray::kHeaderSize - kHeapObjectTag));
   __ sll(t1, a3, kPointerSizeLog2 - kSmiTagSize);
   __ Addu(a2, a2, t1);
 
   // Calculate location of the first key name.
   __ Addu(t0,
           t0,
-          Operand(FixedArray::kHeaderSize - kHeapObjectTag +
-                  DescriptorArray::kFirstIndex * kPointerSize));
+          Operand(DescriptorArray::kFirstOffset - kHeapObjectTag));
   // Loop through all the keys in the descriptor array. If one of these is the
   // symbol valueOf the result is false.
   Label entry, loop;
   // The use of t2 to store the valueOf symbol asumes that it is not otherwise
   // used in the loop below.
   __ LoadRoot(t2, Heap::kvalue_of_symbolRootIndex);
   __ jmp(&entry);
   __ bind(&loop);
   __ lw(a3, MemOperand(t0, 0));
   __ Branch(if_false, eq, a3, Operand(t2));
-  __ Addu(t0, t0, Operand(kPointerSize));
+  __ Addu(t0, t0, Operand(DescriptorArray::kDescriptorSize * kPointerSize));
   __ bind(&entry);
   __ Branch(&loop, ne, t0, Operand(a2));
 
   // 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.
   __ lw(a2, FieldMemOperand(a1, Map::kPrototypeOffset));
   __ JumpIfSmi(a2, if_false);
   __ lw(a2, FieldMemOperand(a2, HeapObject::kMapOffset));
   __ lw(a3, ContextOperand(cp, Context::GLOBAL_OBJECT_INDEX));
   __ lw(a3, FieldMemOperand(a3, GlobalObject::kNativeContextOffset));
@@ skipping 1818 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS