On MIPS, keeps Object::null() in a register.

runtime/vm/constants_mips.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_CONSTANTS_MIPS_H_
 #define VM_CONSTANTS_MIPS_H_
 
 #include "platform/assert.h"
 
 namespace dart {
@@ skipping 121 matching lines @@
 
 // Architecture independent aliases.
 typedef FRegister FpuRegister;
 const FpuRegister FpuTMP = F0;
 const int kNumberOfFpuRegisters = kNumberOfFRegisters;
 const FpuRegister kNoFpuRegister = kNoFRegister;
 
 
 // Register aliases.
 const Register TMP1 = AT;  // Used as scratch register by assembler.
-const Register TMP2 = T9;  // Used as scratch register by assembler.
 const Register TMP = TMP1;  // Arch independent flow graph compiler needs a
                             // Register called TMP.
 const Register CTX = S6;  // Caches current context in generated code.
 const Register PP = S7;  // Caches object pool pointer in generated code.
 const Register SPREG = SP;  // Stack pointer register.
 const Register FPREG = FP;  // Frame pointer register.
 
+// NULLREG holds reinterpret_cast<intptr_t>(Object::null()).
+// TODO(zra): Is it worthwhile to devote a register to this? Investigate
+// performance effects when we are running on real hardware. Same with
+// CMPRES. Try moving CTX and PP to T8 and T9 and shifting kLastCpuRegister
+// down to S7.
+const Register NULLREG = T8;
+
 // The code that generates a comparison can be far away from the code that
 // generates the branch that uses the result of that comparison. In this case,
 // CMPRES is used for the result of the comparison.
-const Register CMPRES = T8;
+const Register CMPRES = T9;
 
 // Exception object is passed in this register to the catch handlers when an
 // exception is thrown.
 const Register kExceptionObjectReg = V0;
 
 // Stack trace object is passed in this register to the catch handlers when
 // an exception is thrown.
 const Register kStackTraceObjectReg = V1;
 
 
@@ skipping 375 matching lines @@
 #endif  // defined(DEBUG)
 
  private:
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Instr);
 };
 
 }  // namespace dart
 
 #endif  // VM_CONSTANTS_MIPS_H_