Add mutable double boxes for fields.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
===================================================================
--- runtime/vm/flow_graph_optimizer.cc	(revision 31125)
+++ runtime/vm/flow_graph_optimizer.cc	(working copy)
@@ -3506,6 +3506,33 @@
 }
 
 
+void FlowGraphOptimizer::VisitStoreInstanceField(
+    StoreInstanceFieldInstr* instr) {
+  if (instr->IsUnboxedStore()) {
+    ASSERT(instr->initialization_);
+    // Determine if this field should be unboxed based on the usage of getter
+    // and setter functions.
+    const Field& field = Field::ZoneHandle(instr->field().raw());
+    const String& field_name = String::Handle(field.name());
+    class Class& owner = Class::Handle(field.owner());
+    const Function& getter =
+        Function::Handle(owner.LookupGetterFunction(field_name));
+    const Function& setter =
+        Function::Handle(owner.LookupSetterFunction(field_name));

[srdjan, 2013/12/13 22:12:06]

Below is an important heuristic, add brief comment.

[Florian Schneider, 2013/12/16 13:08:09]

Done.

+    bool result = !getter.IsNull()
+               && !setter.IsNull()
+               && setter.usage_counter() > 0

[srdjan, 2013/12/13 18:13:17]

Add parentheses here and below.

[Florian Schneider, 2013/12/16 13:08:09]

Done.

+               && 10 * setter.usage_counter() > getter.usage_counter();

[srdjan, 2013/12/13 22:12:06]

put 10 into a flag to experiment with ?

[Florian Schneider, 2013/12/16 13:08:09]

Done.

+    if (!result) {
+      field.set_is_unboxing_candidate(false);
+      field.DeoptimizeDependentCode();
+    } else {
+      FlowGraph::AddToGuardedFields(flow_graph_->guarded_fields(), &field);
+    }
+  }
+}
+
+
 bool FlowGraphOptimizer::TryInlineInstanceSetter(InstanceCallInstr* instr,
                                                  const ICData& unary_ic_data) {
   ASSERT((unary_ic_data.NumberOfChecks() > 0) &&
@@ -3536,7 +3563,7 @@
   // Inline implicit instance setter.
   const String& field_name =
       String::Handle(Field::NameFromSetter(instr->function_name()));
-  const Field& field = Field::Handle(GetField(class_id, field_name));
+  const Field& field = Field::ZoneHandle(GetField(class_id, field_name));
   ASSERT(!field.IsNull());
 
   if (InstanceCallNeedsClassCheck(instr)) {
@@ -3567,6 +3594,11 @@
       new Value(instr->ArgumentAt(0)),
       new Value(instr->ArgumentAt(1)),
       needs_store_barrier);
+
+  if (store->IsUnboxedStore()) {
+    FlowGraph::AddToGuardedFields(flow_graph_->guarded_fields(), &field);
+  }
+
   // Discard the environment from the original instruction because the store
   // can't deoptimize.
   instr->RemoveEnvironment();