CTX and TMP registers may not be allocated.

runtime/vm/constants_ia32.h

 // Copyright (c) 2012, 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_IA32_H_
 #define VM_CONSTANTS_IA32_H_
 
 #include "platform/assert.h"
 
 namespace dart {
@@ skipping 34 matching lines @@
   XMM4 = 4,
   XMM5 = 5,
   XMM6 = 6,
   XMM7 = 7,
   kNumberOfXmmRegisters = 8,
   kNoXmmRegister = -1  // Signals an illegal register.
 };
 
 
 // Register aliases.
-const Register CTX = ESI;  // Caches current context in generated code.
+const Register TMP = kNoRegister;  // No scratch register used by assembler.
+const Register CTX = ESI;          // Caches current context in generated code.
 
 // Exception object is passed in this register to the catch handlers when an
 // exception is thrown.
 const Register kExceptionObjectReg = EAX;
 
 // Stack trace object is passed in this register to the catch handlers when
 // an exception is thrown.
 const Register kStackTraceObjectReg = EDX;
 
 enum ScaleFactor {
@@ skipping 54 matching lines @@
 };
 
 
 // The largest multibyte nop we will emit.  This could go up to 15 if it
 // becomes important to us.
 const int MAX_NOP_SIZE = 8;
 
 }  // namespace dart
 
 #endif  // VM_CONSTANTS_IA32_H_