First take on custom call generators.

src/objects.h

 // Copyright 2006-2009 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 3168 matching lines @@
   // called without using argument adaptor frames.
   inline void DontAdaptArguments();
 
   // [expected_nof_properties]: Expected number of properties for the function.
   inline int expected_nof_properties();
   inline void set_expected_nof_properties(int value);
 
   // [instance class name]: class name for instances.
   DECL_ACCESSORS(instance_class_name, Object)
 
-  // [function data]: This field has been added for make benefit the API.
+  // [function data]: This field holds some additional data for function.
+  // Currently it either has FunctionTemplateInfo to make benefit the API
+  // or Proxy wrapping CustomCallGenerator.
   // In the long run we don't want all functions to have this field but
   // we can fix that when we have a better model for storing hidden data
   // on objects.
   DECL_ACCESSORS(function_data, Object)
 
+  inline bool IsApiFunction();
+  inline FunctionTemplateInfo* get_api_func_data();
+  inline bool HasCustomCallGenerator();
+
   // [script info]: Script from which the function originates.
   DECL_ACCESSORS(script, Object)
 
   // [start_position_and_type]: Field used to store both the source code
   // position, whether or not the function is a function expression,
   // and whether or not the function is a toplevel function. The two
   // least significants bit indicates whether the function is an
   // expression and the rest contains the source code position.
   inline int start_position_and_type();
   inline void set_start_position_and_type(int value);
@@ skipping 87 matching lines @@
   // Constants.
   static const int kDontAdaptArgumentsSentinel = -1;
 
   // Layout description.
   // Pointer fields.
   static const int kNameOffset = HeapObject::kHeaderSize;
   static const int kCodeOffset = kNameOffset + kPointerSize;
   static const int kConstructStubOffset = kCodeOffset + kPointerSize;
   static const int kInstanceClassNameOffset =
       kConstructStubOffset + kPointerSize;
-  static const int kExternalReferenceDataOffset =
+  static const int kFunctionDataOffset =
       kInstanceClassNameOffset + kPointerSize;
-  static const int kScriptOffset = kExternalReferenceDataOffset + kPointerSize;
+  static const int kScriptOffset = kFunctionDataOffset + kPointerSize;
   static const int kDebugInfoOffset = kScriptOffset + kPointerSize;
   static const int kInferredNameOffset = kDebugInfoOffset + kPointerSize;
   static const int kThisPropertyAssignmentsOffset =
       kInferredNameOffset + kPointerSize;
   // Integer fields.
   static const int kLengthOffset =
       kThisPropertyAssignmentsOffset + kPointerSize;
   static const int kFormalParameterCountOffset = kLengthOffset + kIntSize;
   static const int kExpectedNofPropertiesOffset =
       kFormalParameterCountOffset + kIntSize;
@@ skipping 1768 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_