Use mutable heapnumbers to store doubles in fields.

src/arm/lithium-arm.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 2300 matching lines @@
   if (needs_write_barrier) {
     obj = instr->is_in_object()
         ? UseRegister(instr->object())
         : UseTempRegister(instr->object());
   } else {
     obj = needs_write_barrier_for_map
         ? UseRegister(instr->object())
         : UseRegisterAtStart(instr->object());
   }
 
-  LOperand* val =
-      needs_write_barrier ||
-      (FLAG_track_fields && instr->field_representation().IsSmi())
-          ? UseTempRegister(instr->value()) : UseRegister(instr->value());
+  LOperand* val;
+  if (needs_write_barrier ||
+      (FLAG_track_fields && instr->field_representation().IsSmi())) {
+    val = UseTempRegister(instr->value());
+  } else if (FLAG_track_double_fields &&
+             instr->field_representation().IsDouble()) {
+    val = UseRegisterAtStart(instr->value());
+  } else {
+    val = UseRegister(instr->value());
+  }
 
   // We need a temporary register for write barrier of the map field.
   LOperand* temp = needs_write_barrier_for_map ? TempRegister() : NULL;
 
   LStoreNamedField* result = new(zone()) LStoreNamedField(obj, val, temp);
-  if ((FLAG_track_fields && instr->field_representation().IsSmi()) ||
-      (FLAG_track_double_fields && instr->field_representation().IsDouble())) {
+  if (FLAG_track_fields && instr->field_representation().IsSmi()) {
     return AssignEnvironment(result);
   }
   return result;
 }
 
 
 LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
   LOperand* obj = UseFixed(instr->object(), r1);
   LOperand* val = UseFixed(instr->value(), r0);
 
@@ skipping 279 matching lines @@
 
 
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseRegister(instr->index());
   return DefineAsRegister(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal