fix -Wunused-but-set-variable for gcc-4.6 on x64

src/x64/stub-cache-x64.cc

 // Copyright 2011 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 1071 matching lines @@
                                         Register scratch1,
                                         Register scratch2,
                                         Register scratch3,
                                         Object* value,
                                         String* name,
                                         Label* miss) {
   // Check that the receiver isn't a smi.
   __ JumpIfSmi(receiver, miss);
 
   // Check that the maps haven't changed.
-  Register reg =
-      CheckPrototypes(object, receiver, holder,
-                      scratch1, scratch2, scratch3, name, miss);
+  CheckPrototypes(object, receiver, holder,
+                  scratch1, scratch2, scratch3, name, miss);
 
   // Return the constant value.
   __ Move(rax, Handle<Object>(value));
   __ ret(0);
 }
 
 
 void StubCompiler::GenerateLoadInterceptor(JSObject* object,
                                            JSObject* interceptor_holder,
                                            LookupResult* lookup,
@@ skipping 2634 matching lines @@
 
   __ bind(&maybe_nan);
   // Could be NaN or Infinity. If fraction is not zero, it's NaN, otherwise
   // it's an Infinity, and the non-NaN code path applies.
   __ j(greater, &is_nan, Label::kNear);
   __ cmpl(FieldOperand(rax, HeapNumber::kValueOffset), Immediate(0));
   __ j(zero, &not_nan);
   __ bind(&is_nan);
   // Convert all NaNs to the same canonical NaN value when they are stored in
   // the double array.
-  ExternalReference canonical_nan_reference =
-      ExternalReference::address_of_canonical_non_hole_nan();
   __ Set(kScratchRegister, BitCast<uint64_t>(
       FixedDoubleArray::canonical_not_the_hole_nan_as_double()));
   __ movq(xmm0, kScratchRegister);
   __ jmp(&have_double_value, Label::kNear);
 
   __ bind(&smi_value);
   // Value is a smi. convert to a double and store.
   __ SmiToInteger32(rax, rax);
   __ push(rax);
   __ fild_s(Operand(rsp, 0));
   __ pop(rax);
   __ SmiToInteger32(rcx, rcx);
   __ fstp_d(FieldOperand(rdi, rcx, times_8, FixedDoubleArray::kHeaderSize));
   __ ret(0);
 
   // Handle store cache miss, replacing the ic with the generic stub.
   __ bind(&miss_force_generic);
   Handle<Code> ic_force_generic =
       masm->isolate()->builtins()->KeyedStoreIC_MissForceGeneric();
   __ jmp(ic_force_generic, RelocInfo::CODE_TARGET);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64