[turbofan] Non-dictionary prototype maps are always stable.

src/compiler/js-native-context-specialization.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/js-native-context-specialization.h"
 
 #include "src/accessors.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
 #include "src/compiler/js-graph.h"
@@ skipping 551 matching lines @@
           (this_control_count == 1)
               ? this_controls.front()
               : graph()->NewNode(common()->Merge(this_control_count),
                                  this_control_count, &this_controls.front());
     }
 
     // Determine actual holder and perform prototype chain checks.
     Handle<JSObject> holder;
     if (access_info.holder().ToHandle(&holder)) {
       this_value = jsgraph()->Constant(holder);
-      for (auto i = access_info.receiver_type()->Classes(); !i.Done();
-           i.Advance()) {
-        Handle<Map> map = i.Current();
-        PrototypeIterator j(map);
-        while (true) {
-          // Check that the {prototype} still has the same map. For stable
-          // maps, we can add a stability dependency on the prototype map;
-          // for everything else we need to perform a map check at runtime.
-          Handle<JSReceiver> prototype =
-              PrototypeIterator::GetCurrent<JSReceiver>(j);
-          if (prototype->map()->is_stable()) {
-            dependencies()->AssumeMapStable(
-                handle(prototype->map(), isolate()));
-          } else {
-            Node* prototype_map = this_effect = graph()->NewNode(
-                simplified()->LoadField(AccessBuilder::ForMap()),
-                jsgraph()->Constant(prototype), this_effect, this_control);
-            Node* check = graph()->NewNode(
-                simplified()->ReferenceEqual(Type::Internal()), prototype_map,
-                jsgraph()->Constant(handle(prototype->map(), isolate())));
-            Node* branch = graph()->NewNode(common()->Branch(BranchHint::kTrue),
-                                            check, this_control);
-            exit_controls.push_back(
-                graph()->NewNode(common()->IfFalse(), branch));
-            this_control = graph()->NewNode(common()->IfTrue(), branch);
-          }
-          // Stop once we get to the holder.
-          if (prototype.is_identical_to(holder)) break;
-          j.Advance();
-        }
-      }
+      AssumePrototypesStable(receiver_type, holder);
     }
 
     // Generate the actual property access.
     if (access_info.IsDataConstant()) {
       this_value = jsgraph()->Constant(access_info.constant());
     } else {
       // TODO(bmeurer): This is sort of adhoc, and must be refactored into some
       // common code once we also have support for stores.
       DCHECK(access_info.IsDataField());
       FieldIndex const field_index = access_info.field_index();
@@ skipping 162 matching lines @@
       this_control =
           (this_control_count == 1)
               ? this_controls.front()
               : graph()->NewNode(common()->Merge(this_control_count),
                                  this_control_count, &this_controls.front());
     }
 
     // Determine actual holder and perform prototype chain checks.
     Handle<JSObject> holder;
     if (access_info.holder().ToHandle(&holder)) {
-      for (auto i = access_info.receiver_type()->Classes(); !i.Done();
-           i.Advance()) {
-        Handle<Map> map = i.Current();
-        PrototypeIterator j(map);
-        while (true) {
-          // Check that the {prototype} still has the same map. For stable
-          // maps, we can add a stability dependency on the prototype map;
-          // for everything else we need to perform a map check at runtime.
-          Handle<JSReceiver> prototype =
-              PrototypeIterator::GetCurrent<JSReceiver>(j);
-          if (prototype->map()->is_stable()) {
-            dependencies()->AssumeMapStable(
-                handle(prototype->map(), isolate()));
-          } else {
-            Node* prototype_map = this_effect = graph()->NewNode(
-                simplified()->LoadField(AccessBuilder::ForMap()),
-                jsgraph()->Constant(prototype), this_effect, this_control);
-            Node* check = graph()->NewNode(
-                simplified()->ReferenceEqual(Type::Internal()), prototype_map,
-                jsgraph()->Constant(handle(prototype->map(), isolate())));
-            Node* branch = graph()->NewNode(common()->Branch(BranchHint::kTrue),
-                                            check, this_control);
-            exit_controls.push_back(
-                graph()->NewNode(common()->IfFalse(), branch));
-            this_control = graph()->NewNode(common()->IfTrue(), branch);
-          }
-          // Stop once we get to the holder.
-          if (prototype.is_identical_to(holder)) break;
-          j.Advance();
-        }
-      }
+      AssumePrototypesStable(receiver_type, holder);
     }
 

[Toon Verwaest, 2015/10/23 13:45:40]

What Object.prototype.x? Object.prototype has a null prototype, there won't be a
holder, but we also don't load a property from the map.

[Benedikt Meurer, 2015/10/23 13:56:26]

NotFound case is not implemented.

     // Generate the actual property access.
     if (access_info.IsDataConstant()) {
       Node* check =
           graph()->NewNode(simplified()->ReferenceEqual(Type::Tagged()), value,
                            jsgraph()->Constant(access_info.constant()));
       Node* branch = graph()->NewNode(common()->Branch(BranchHint::kTrue),
                                       check, this_control);
       exit_controls.push_back(graph()->NewNode(common()->IfFalse(), branch));
       this_control = graph()->NewNode(common()->IfTrue(), branch);
     } else {
@@ skipping 116 matching lines @@
   }
   Handle<Context> script_context = ScriptContextTable::GetContext(
       script_context_table, lookup_result.context_index);
   result->context = script_context;
   result->immutable = IsImmutableVariableMode(lookup_result.mode);
   result->index = lookup_result.slot_index;
   return true;
 }
 
 
+void JSNativeContextSpecialization::AssumePrototypesStable(
+    Type* receiver_type, Handle<JSObject> holder) {
+  // Determine actual holder and perform prototype chain checks.
+  for (auto i = receiver_type->Classes(); !i.Done(); i.Advance()) {
+    Handle<Map> const map = i.Current();
+    for (PrototypeIterator j(map);; j.Advance()) {

[Jarin, 2015/10/23 13:09:34]

Does not it make sense to dcheck that we are not at end here?

+      // Check that the {prototype} still has the same map.  All prototype
+      // maps are guaranteed to be stable, so it's sufficient to add a
+      // stability dependency here.
+      Handle<JSReceiver> const prototype =
+          PrototypeIterator::GetCurrent<JSReceiver>(j);
+      dependencies()->AssumeMapStable(handle(prototype->map(), isolate()));
+      // Stop once we get to the holder.
+      if (prototype.is_identical_to(holder)) break;
+    }
+  }
+}
+
+
 Graph* JSNativeContextSpecialization::graph() const {
   return jsgraph()->graph();
 }
 
 
 Isolate* JSNativeContextSpecialization::isolate() const {
   return jsgraph()->isolate();
 }
 
 
@@ skipping 17 matching lines @@
 }
 
 
 SimplifiedOperatorBuilder* JSNativeContextSpecialization::simplified() const {
   return jsgraph()->simplified();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8