Introduce a PrototypeIterator class and use it for prototype access

src/prototype.h

+// Copyright 2014 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.
+
+#ifndef V8_PROTOTYPE_H_
+#define V8_PROTOTYPE_H_
+
+#include "src/isolate.h"
+#include "src/objects.h"
+
+namespace v8 {
+namespace internal {
+
+/**
+ * A class to uniformly access the prototype of any Object and walk its
+ * prototype chain.
+ *
+ * The PrototypeIterator can either start at the prototype (default), or
+ * include the receiver itself. If a PrototypeIterator is constructed for a
+ * Map, it will always start at the prototype.
+ *
+ * The PrototypeIterator can either run to the null_value(), the first
+ * non-hidden prototype, or a given object.
+ */
+class PrototypeIterator {
+ public:
+  enum WhereToStart { START_AT_RECEIVER, START_AT_PROTOTYPE };
+
+  enum WhereToEnd { END_AT_NULL, END_AT_NON_HIDDEN };
+
+  PrototypeIterator(Isolate* isolate, Handle<Object> receiver,
+                    WhereToStart where_to_start = START_AT_PROTOTYPE)
+      : did_jump_to_prototype_chain_(false),
+        object_(NULL),
+        handle_(receiver),
+        isolate_(isolate) {
+    CHECK(!handle_.is_null());
+    if (where_to_start == START_AT_PROTOTYPE) {
+      Advance();
+    }
+  }
+  PrototypeIterator(Isolate* isolate, Object* receiver,
+                    WhereToStart where_to_start = START_AT_PROTOTYPE)
+      : did_jump_to_prototype_chain_(false),
+        object_(receiver),
+        isolate_(isolate) {
+    if (where_to_start == START_AT_PROTOTYPE) {
+      Advance();
+    }
+  }
+  explicit PrototypeIterator(Map* receiver_map)
+      : did_jump_to_prototype_chain_(true),
+        object_(receiver_map->prototype()),
+        isolate_(receiver_map->GetIsolate()) {}
+  ~PrototypeIterator() {}
+
+  Object* GetCurrent() const {
+    ASSERT(handle_.is_null());
+    return object_;
+  }
+  static Handle<Object> GetCurrent(const PrototypeIterator& iterator) {
+    ASSERT(!iterator.handle_.is_null());
+    return iterator.handle_;
+  }
+  void Advance() {
+    if (!did_jump_to_prototype_chain_) {
+      did_jump_to_prototype_chain_ = true;
+      if (handle_.is_null()) {
+        object_ = object_->GetRootMap(isolate_)->prototype();
+      } else {
+        handle_ = handle(handle_->GetRootMap(isolate_)->prototype(), isolate_);
+      }
+    } else {

[Toon Verwaest, 2014/07/11 09:41:28]

What do we do in case of null_value / JSProxy? It seems like getting the
prototype of null via the map will result in null as well, but what about
proxies?

We should make those 2 cornercases explicit, and either return null_value as a
result, or ASSERT that we don't call Advance on them.

+      if (handle_.is_null()) {
+        object_ = HeapObject::cast(object_)->map()->prototype();
+      } else {
+        handle_ =
+            handle(HeapObject::cast(*handle_)->map()->prototype(), isolate_);
+      }
+    }
+  }
+  bool IsAtEnd(WhereToEnd where_to_end = END_AT_NULL) const {
+    if (handle_.is_null()) {
+      return object_->IsNull() ||
+             (did_jump_to_prototype_chain_ &&
+              where_to_end == END_AT_NON_HIDDEN &&
+              !HeapObject::cast(object_)->map()->is_hidden_prototype());
+    } else {
+      return handle_->IsNull() ||

[Toon Verwaest, 2014/07/11 09:41:28]

Should either be true for JSProxy, or we should ASSERT that we don't call this
for JSProxy.

+             (did_jump_to_prototype_chain_ &&
+              where_to_end == END_AT_NON_HIDDEN &&
+              !Handle<HeapObject>::cast(handle_)->map()->is_hidden_prototype());
+    }
+  }
+  bool IsAtEnd(Object* final_object) {
+    ASSERT(handle_.is_null());
+    return object_->IsNull() || object_ == final_object;
+  }
+  bool IsAtEnd(Handle<Object> final_object) {
+    ASSERT(!handle_.is_null());
+    return handle_->IsNull() || *handle_ == *final_object;
+  }
+
+ private:
+  bool did_jump_to_prototype_chain_;
+  Object* object_;
+  Handle<Object> handle_;
+  Isolate* isolate_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrototypeIterator);
+};
+
+
+}  // namespace internal
+
+}  // namespace v8
+
+#endif  // V8_PROTOTYPE_H_