Allow constants to be deleted by reinserting them into the graph as needed.

src/hydrogen.cc

 // Copyright 2013 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 616 matching lines @@
 
 #endif
 
 
 HConstant* HGraph::GetConstant(SetOncePointer<HConstant>* pointer,
                                int32_t value) {
   if (!pointer->is_set()) {
     // Can't pass GetInvalidContext() to HConstant::New, because that will
     // recursively call GetConstant
     HConstant* constant = HConstant::New(zone(), NULL, value);
-    constant->InsertAfter(GetConstantUndefined());
+    constant->InsertAfter(entry_block()->first());
     pointer->set(constant);
+    return constant;
   }
-  return pointer->get();
+  return ReinsertConstantIfNecessary(pointer->get());
 }
 
 
+HConstant* HGraph::ReinsertConstantIfNecessary(HConstant* constant) {
+  if (!constant->IsLinked()) {
+    // The constant was removed from the graph. Reinsert.
+    constant->ClearFlag(HValue::kIsDead);
+    constant->InsertAfter(entry_block()->first());
+  }
+  return constant;
+}
+
+
 HConstant* HGraph::GetConstant0() {
   return GetConstant(&constant_0_, 0);
 }
 
 
 HConstant* HGraph::GetConstant1() {
   return GetConstant(&constant_1_, 1);
 }
 
 
 HConstant* HGraph::GetConstantMinus1() {
   return GetConstant(&constant_minus1_, -1);
 }
 
 
 #define DEFINE_GET_CONSTANT(Name, name, htype, boolean_value)                  \
 HConstant* HGraph::GetConstant##Name() {                                       \
   if (!constant_##name##_.is_set()) {                                          \
     HConstant* constant = new(zone()) HConstant(                               \
         Unique<Object>::CreateImmovable(isolate()->factory()->name##_value()), \
         Representation::Tagged(),                                              \
         htype,                                                                 \
         false,                                                                 \
         true,                                                                  \
         false,                                                                 \
         boolean_value);                                                        \
-    constant->InsertAfter(GetConstantUndefined());                             \
+    constant->InsertAfter(entry_block()->first());                             \
     constant_##name##_.set(constant);                                          \
   }                                                                            \
-  return constant_##name##_.get();                                             \
+  return ReinsertConstantIfNecessary(constant_##name##_.get());                \
 }
 
 
+DEFINE_GET_CONSTANT(Undefined, undefined, HType::Tagged(), false)
 DEFINE_GET_CONSTANT(True, true, HType::Boolean(), true)
 DEFINE_GET_CONSTANT(False, false, HType::Boolean(), false)
 DEFINE_GET_CONSTANT(Hole, the_hole, HType::Tagged(), false)
 DEFINE_GET_CONSTANT(Null, null, HType::Tagged(), false)
 
 
 #undef DEFINE_GET_CONSTANT
 
 
 HConstant* HGraph::GetInvalidContext() {
@@ skipping 2512 matching lines @@
   }
   return call;
 }
 
 
 void HOptimizedGraphBuilder::SetUpScope(Scope* scope) {
   // First special is HContext.
   HInstruction* context = Add<HContext>();
   environment()->BindContext(context);
 
-  HConstant* undefined_constant = HConstant::cast(Add<HConstant>(
-      isolate()->factory()->undefined_value()));
-  graph()->set_undefined_constant(undefined_constant);
-
   // Create an arguments object containing the initial parameters.  Set the
   // initial values of parameters including "this" having parameter index 0.
   ASSERT_EQ(scope->num_parameters() + 1, environment()->parameter_count());
   HArgumentsObject* arguments_object =
       New<HArgumentsObject>(environment()->parameter_count());
   for (int i = 0; i < environment()->parameter_count(); ++i) {
     HInstruction* parameter = Add<HParameter>(i);
     arguments_object->AddArgument(parameter, zone());
     environment()->Bind(i, parameter);
   }
   AddInstruction(arguments_object);
   graph()->SetArgumentsObject(arguments_object);
 
+  HConstant* undefined_constant = graph()->GetConstantUndefined();
   // Initialize specials and locals to undefined.
   for (int i = environment()->parameter_count() + 1;
        i < environment()->length();
        ++i) {
     environment()->Bind(i, undefined_constant);
   }
 
   // Handle the arguments and arguments shadow variables specially (they do
   // not have declarations).
   if (scope->arguments() != NULL) {
@@ skipping 6605 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal