Use mutable heapnumbers to store doubles in fields.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 08e0dae1a15a4fe00bc52b1bfd5631725b5978a6..ef52d87380d00c96a8501749d4c046d52c9b8704 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2028,9 +2028,7 @@ LInstruction* LChunkBuilder::DoStoreContextSlot(HStoreContextSlot* instr) {
 
 LInstruction* LChunkBuilder::DoLoadNamedField(HLoadNamedField* instr) {
   LOperand* obj = UseRegisterAtStart(instr->object());
-  LOperand* temp = instr->representation().IsDouble() ? TempRegister() : NULL;
-  ASSERT(temp == NULL || FLAG_track_double_fields);
-  return DefineAsRegister(new(zone()) LLoadNamedField(obj, temp));
+  return DefineAsRegister(new(zone()) LLoadNamedField(obj));
 }
 
 
@@ -2250,6 +2248,9 @@ LInstruction* LChunkBuilder::DoStoreNamedField(HStoreNamedField* instr) {
     val = UseRegisterOrConstant(instr->value());
   } else if (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());
   }
@@ -2260,8 +2261,7 @@ LInstruction* LChunkBuilder::DoStoreNamedField(HStoreNamedField* instr) {
       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;