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

src/x64/full-codegen-x64.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 2624 matching lines @@
   __ LoadInstanceDescriptors(rbx, rbx);
   __ movq(rcx, FieldOperand(rbx, FixedArray::kLengthOffset));
   // rbx: descriptor array
   // rcx: length of descriptor array
   // Calculate the end of the descriptor array.
   SmiIndex index = masm_->SmiToIndex(rdx, rcx, kPointerSizeLog2);
   __ lea(rcx,
          Operand(
              rbx, index.reg, index.scale, FixedArray::kHeaderSize));
   // Calculate location of the first key name.
-  __ addq(rbx,
-          Immediate(FixedArray::kHeaderSize +
-                    DescriptorArray::kFirstIndex * kPointerSize));
+  __ addq(rbx, 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);
   __ movq(rdx, FieldOperand(rbx, 0));
   __ Cmp(rdx, FACTORY->value_of_symbol());
   __ j(equal, if_false);
-  __ addq(rbx, Immediate(kPointerSize));
+  __ addq(rbx, Immediate(DescriptorArray::kDescriptorSize * kPointerSize));
   __ bind(&entry);
   __ cmpq(rbx, rcx);
   __ j(not_equal, &loop);
 
   // Reload map as register rbx was used as temporary above.
   __ movq(rbx, FieldOperand(rax, 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.
   __ movq(rcx, FieldOperand(rbx, Map::kPrototypeOffset));
@@ skipping 1875 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64