Add support for WritableRegister policy in the register allocator.

runtime/vm/locations.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_LOCATIONS_H_
 #define VM_LOCATIONS_H_
 
 #include "vm/allocation.h"
 #include "vm/assembler.h"
 #include "vm/bitfield.h"
@@ skipping 78 matching lines @@
     ASSERT(&obj == &loc.constant());
     return loc;
   }
 
   const Object& constant() const {
     ASSERT(IsConstant());
     return *reinterpret_cast<const Object*>(value_ & ~kConstantMask);
   }
 
   // Unallocated locations.
-  // TODO(vegorov): writable register policy?
   enum Policy {
     kAny,
     kPrefersRegister,
     kRequiresRegister,
     kRequiresXmmRegister,
+    kWritableRegister,
     kSameAsFirstInput,
   };
 
   bool IsUnallocated() const {
     return kind() == kUnallocated;
   }
 
   bool IsRegisterBeneficial() {
     return !Equals(Any());
   }
@@ skipping 12 matching lines @@
   }
 
   static Location RequiresRegister() {
     return UnallocatedLocation(kRequiresRegister);
   }
 
   static Location RequiresXmmRegister() {
     return UnallocatedLocation(kRequiresXmmRegister);
   }
 
+  static Location WritableRegister() {
+    return UnallocatedLocation(kWritableRegister);
+  }
+
   // The location of the first input to the instruction will be
   // used to replace this unallocated location.
   static Location SameAsFirstInput() {
     return UnallocatedLocation(kSameAsFirstInput);
   }
 
   // Empty location. Used if there the location should be ignored.
   static Location NoLocation() {
     return Location();
   }
@@ skipping 263 matching lines @@
   BitmapBuilder* stack_bitmap_;
 
   const ContainsCall contains_call_;
   RegisterSet live_registers_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_LOCATIONS_H_