VM [KERNEL] Elide null-initialization of the fields

runtime/vm/kernel_to_il.cc

Index: runtime/vm/kernel_to_il.cc
diff --git a/runtime/vm/kernel_to_il.cc b/runtime/vm/kernel_to_il.cc
index 1e0a4064281dc05de46799efd2c132fd3fa5ad7c..e94448b88651b16d91d7bcb844d881007985294d 100644
--- a/runtime/vm/kernel_to_il.cc
+++ b/runtime/vm/kernel_to_il.cc
@@ -4073,6 +4073,28 @@ JoinEntryInstr* FlowGraphBuilder::BuildJoinEntry() {
 }
 
 
+Fragment FlowGraphBuilder::TranslateFieldInitializer(Field* kernel_field,
+                                                     Expression* init) {
+  dart::Field& field =
+      dart::Field::ZoneHandle(Z, H.LookupFieldByKernelField(kernel_field));
+  Fragment instructions;
+  bool elide_initialization = false;
+  instructions += LoadLocal(scopes_->this_variable);
+  Fragment initializer = TranslateExpression(init);
+  instructions += initializer;
+  // Elide initialization if the expression is null.
+  if (initializer.entry->IsConstant() &&
+      initializer.entry == initializer.current) {
+    ConstantInstr* inst = initializer.entry->AsConstant();
+    if (inst->value().IsNull()) {
+      elide_initialization = true;
+    }
+  }
+  instructions += StoreInstanceFieldGuarded(field, true);
+  return elide_initialization ? Fragment() : instructions;

[Florian Schneider, 2017/03/30 19:42:59]

dbc: If the explicit store is not present, you have to record the null-store
using Field::RecordStore for field type guards:

field.RecordStore(Object::null_object());

We do that in the parser in all places where we don't emit an explicit field
initializer.

[Dmitry Olshansky, 2017/03/31 14:19:50]

Done.

+}
+
+
 Fragment FlowGraphBuilder::TranslateInitializers(
     Class* kernel_klass,
     List<Initializer>* initializers) {
@@ -4086,13 +4108,8 @@ Fragment FlowGraphBuilder::TranslateInitializers(
     Field* kernel_field = kernel_klass->fields()[i];
     Expression* init = kernel_field->initializer();
     if (!kernel_field->IsStatic() && init != NULL) {
-      dart::Field& field =
-          dart::Field::ZoneHandle(Z, H.LookupFieldByKernelField(kernel_field));
-
       EnterScope(kernel_field);
-      instructions += LoadLocal(scopes_->this_variable);
-      instructions += TranslateExpression(init);
-      instructions += StoreInstanceFieldGuarded(field, true);
+      instructions += TranslateFieldInitializer(kernel_field, init);
       ExitScope(kernel_field);
     }
   }
@@ -4107,12 +4124,7 @@ Fragment FlowGraphBuilder::TranslateInitializers(
     Initializer* initializer = (*initializers)[i];
     if (initializer->IsFieldInitializer()) {
       FieldInitializer* init = FieldInitializer::Cast(initializer);
-      dart::Field& field =
-          dart::Field::ZoneHandle(Z, H.LookupFieldByKernelField(init->field()));
-
-      instructions += LoadLocal(scopes_->this_variable);
-      instructions += TranslateExpression(init->value());
-      instructions += StoreInstanceFieldGuarded(field, true);
+      instructions += TranslateFieldInitializer(init->field(), init->value());
     } else if (initializer->IsSuperInitializer()) {
       SuperInitializer* init = SuperInitializer::Cast(initializer);