Dynamically determine optimal instance size....

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 1558 matching lines @@
   inline Object* InObjectPropertyAt(int index);
   inline Object* InObjectPropertyAtPut(int index,
                                        Object* value,
                                        WriteBarrierMode mode
                                        = UPDATE_WRITE_BARRIER);
 
   // initializes the body after properties slot, properties slot is
   // initialized by set_properties
   // Note: this call does not update write barrier, it is caller's
   // reponsibility to ensure that *v* can be collected without WB here.
-  inline void InitializeBody(int object_size);
+  inline void InitializeBody(int object_size, Object* value);
 
   // Check whether this object references another object
   bool ReferencesObject(Object* obj);
 
   // Casting.
   static inline JSObject* cast(Object* obj);
 
   // Disalow further properties to be added to the object.
   MUST_USE_RESULT Object* PreventExtensions();
 
@@ skipping 1554 matching lines @@
       set_bit_field2(bit_field2() | (1 << kHasFastElements));
     } else {
       set_bit_field2(bit_field2() & ~(1 << kHasFastElements));
     }
   }
 
   inline bool has_fast_elements() {
     return ((1 << kHasFastElements) & bit_field2()) != 0;
   }
 
+  // Tells whether the map is attached to SharedFunctionInfo
+  // (for inobject slack tracking).
+  inline void set_attached_to_shared_function_info(bool value);
+
+  inline bool attached_to_shared_function_info();
+
   // Tells whether the instance needs security checks when accessing its
   // properties.
   inline void set_is_access_check_needed(bool access_check_needed);
   inline bool is_access_check_needed();
 
   // [prototype]: implicit prototype object.
   DECL_ACCESSORS(prototype, Object)
 
   // [constructor]: points back to the function responsible for this map.
   DECL_ACCESSORS(constructor, Object)
 
+  inline JSFunction* unchecked_constructor();
+
   // [instance descriptors]: describes the object.
   DECL_ACCESSORS(instance_descriptors, DescriptorArray)
 
   // [stub cache]: contains stubs compiled for this map.
   DECL_ACCESSORS(code_cache, Object)
 
   MUST_USE_RESULT Object* CopyDropDescriptors();
 
   MUST_USE_RESULT Object* CopyNormalized(PropertyNormalizationMode mode);
 
@@ skipping 58 matching lines @@
   // Dispatched behavior.
 #ifdef DEBUG
   void MapPrint();
   void MapVerify();
   void NormalizedMapVerify();
 #endif
 
   inline int visitor_id();
   inline void set_visitor_id(int visitor_id);
 
+  typedef void (*TraverseCallback)(Map* map, void* data);
+
+  void TraverseTransitionTree(TraverseCallback callback, void* data);
+
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // Layout description.
   static const int kInstanceSizesOffset = HeapObject::kHeaderSize;
   static const int kInstanceAttributesOffset = kInstanceSizesOffset + kIntSize;
   static const int kPrototypeOffset = kInstanceAttributesOffset + kIntSize;
   static const int kConstructorOffset = kPrototypeOffset + kPointerSize;
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
@@ skipping 33 matching lines @@
   static const int kHasIndexedInterceptor = 4;
   static const int kIsUndetectable = 5;
   static const int kHasInstanceCallHandler = 6;
   static const int kIsAccessCheckNeeded = 7;
 
   // Bit positions for bit field 2
   static const int kIsExtensible = 0;
   static const int kFunctionWithPrototype = 1;
   static const int kHasFastElements = 2;
   static const int kStringWrapperSafeForDefaultValueOf = 3;
+  static const int kAttachedToSharedFunctionInfo = 4;
 
   // Layout of the default cache. It holds alternating name and code objects.
   static const int kCodeCacheEntrySize = 2;
   static const int kCodeCacheEntryNameOffset = 0;
   static const int kCodeCacheEntryCodeOffset = 1;
 
   typedef FixedBodyDescriptor<kPointerFieldsBeginOffset,
                               kPointerFieldsEndOffset,
                               kSize> BodyDescriptor;
 
@@ skipping 134 matching lines @@
   inline void set_formal_parameter_count(int value);
 
   // Set the formal parameter count so the function code will be
   // 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);
 
+  // Inobject slack tracking is the way to reclaim unused inobject space.
+  //
+  // The instance size is initially determined by adding some slack to
+  // expected_nof_properties (to allow for a few extra properties added
+  // after the constructor). There is no guarantee that the extra space
+  // will not be wasted.
+  //
+  // Here is the algorithm to reclaim the unused inobject space:
+  // - Detect the first constructor call for this SharedFunctionInfo.
+  //   When it happens enter the "in progress" state: remember the
+  //   constructor's initial_map and install a special construct stub that
+  //   counts constructor calls.
+  // - While the tracking is in progress create objects filled with
+  //   one_pointer_filler_map instead of undefined_value. This way they can be
+  //   resized quickly and safely.
+  // - Once enough (kGenerousAllocationCount) objects have been created
+  //   compute the 'slack' (traverse the map transition tree starting from the
+  //   initial_map and find the lowest value of unused_property_fields).
+  // - Traverse the transition tree again and decrease the instance size
+  //   of every map. Existing objects will resize automatically (they are
+  //   filled with one_pointer_filler_map). All further allocations will
+  //   use the adjusted instance size.
+  // - Decrease expected_nof_properties so that an allocations made from
+  //   another context will use the adjusted instance size too.
+  // - Exit "in progress" state by clearing the reference to the initial_map
+  //   and setting the regular construct stub (generic or inline).
+  //
+  //  The above is the main event sequence. Two special cases are possible
+  //  while the tracking is in progress:
+  //
+  // - GC occurs.
+  //   Check if the initial_map is referenced by any live objects (except this
+  //   SharedFunctionInfo). If it is, continue tracking as usual.
+  //   If it is not, clear the reference and reset the tracking state. The
+  //   tracking will be initiated again on the next constructor call.
+  //
+  // - The constructor is called from another context.

[Vitaly Repeshko, 2010/09/22 14:40:25]

As we discussed this probably applies also to the case when a few closures from
a single context share a SharedFunctionInfo. If yes, please update the comment.

[Vladislav Kaznacheev, 2010/09/23 08:38:16]

Done.

+  //   Immediately complete the tracking, perform all the necessary changes
+  //   to maps. This is  necessary because there is no efficient way to track
+  //   object creation in multiple contexts.
+  //   Proceed to create an object in the current context (with the adjusted
+  //   size).
+  //
+  //  Important: inobject slack tracking is not attempted during the snapshot
+  //  creation.
+
+  static const int kGenerousAllocationCount = 16;
+
+  // [construction_count]: Counter for constructor calls made during
+  // the tracking phase.
+  inline int construction_count();
+  inline void set_construction_count(int value);
+
+  // [initial_map]: initial map of the first function called as a constructor.
+  // Saved for the duration of the tracking phase.
+  // This is a weak link (GC resets it to undefined_value if no other live
+  // object reference this map).
+  DECL_ACCESSORS(initial_map, Object)
+
+  // True if the initial_map is not undefined and the countdown stub is
+  // installed.
+  inline bool IsInobjectSlackTrackingInProgress();
+
+  // Starts the tracking.
+  // Stores the initial map and installs the countdown stub.
+  // IsInobjectSlackTrackingInProgress is normally true after this call,
+  // except when tracking have not been started (e.g. the map has no unused
+  // properties or the snapshot is being built).
+  void StartInobjectSlackTracking(Map* map);
+
+  // Completes the tracking.
+  // IsInobjectSlackTrackingInProgress is false after this call.
+  void CompleteInobjectSlackTracking();
+
+  // Clears the initial_map before the GC marking phase to ensure the reference
+  // is weak. IsInobjectSlackTrackingInProgress is false after this call.
+  void DetachInitialMap();
+
+  // Restores the link to the initial map after the GC marking phase.
+  // IsInobjectSlackTrackingInProgress is true after this call.
+  void AttachInitialMap(Map* map);
+
+  // False if there are definitely no live objects created from this function.
+  // True if live objects _may_ exist (existence not guaranteed).
+  // May go back from true to false after GC.
+  inline bool live_objects_may_exist();
+
+  inline void set_live_objects_may_exist(bool value);
+
   // [instance class name]: class name for instances.
   DECL_ACCESSORS(instance_class_name, Object)
 
   // [function data]: This field holds some additional data for function.
   // Currently it either has FunctionTemplateInfo to make benefit the API
   // or Smi identifying a custom call generator.
   // 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)
@@ skipping 131 matching lines @@
   static const int kCodeOffset = kNameOffset + kPointerSize;
   static const int kScopeInfoOffset = kCodeOffset + kPointerSize;
   static const int kConstructStubOffset = kScopeInfoOffset + kPointerSize;
   static const int kInstanceClassNameOffset =
       kConstructStubOffset + kPointerSize;
   static const int kFunctionDataOffset =
       kInstanceClassNameOffset + kPointerSize;
   static const int kScriptOffset = kFunctionDataOffset + kPointerSize;
   static const int kDebugInfoOffset = kScriptOffset + kPointerSize;
   static const int kInferredNameOffset = kDebugInfoOffset + kPointerSize;
+  static const int kInitialMapOffset =
+      kInferredNameOffset + kPointerSize;
   static const int kThisPropertyAssignmentsOffset =
-      kInferredNameOffset + kPointerSize;
+      kInitialMapOffset + kPointerSize;
 #if V8_HOST_ARCH_32_BIT
   // Smi fields.
   static const int kLengthOffset =
       kThisPropertyAssignmentsOffset + kPointerSize;
   static const int kFormalParameterCountOffset = kLengthOffset + kPointerSize;
   static const int kExpectedNofPropertiesOffset =
       kFormalParameterCountOffset + kPointerSize;
   static const int kNumLiteralsOffset =
       kExpectedNofPropertiesOffset + kPointerSize;
   static const int kStartPositionAndTypeOffset =
       kNumLiteralsOffset + kPointerSize;
   static const int kEndPositionOffset =
       kStartPositionAndTypeOffset + kPointerSize;
   static const int kFunctionTokenPositionOffset =
       kEndPositionOffset + kPointerSize;
   static const int kCompilerHintsOffset =
       kFunctionTokenPositionOffset + kPointerSize;
   static const int kThisPropertyAssignmentsCountOffset =
       kCompilerHintsOffset + kPointerSize;
   // Total size.
   static const int kSize = kThisPropertyAssignmentsCountOffset + kPointerSize;
 #else
   // The only reason to use smi fields instead of int fields
-  // is to allow interation without maps decoding during
+  // is to allow iteration without maps decoding during
   // garbage collections.
   // To avoid wasting space on 64-bit architectures we use
   // the following trick: we group integer fields into pairs
   // First integer in each pair is shifted left by 1.
   // By doing this we guarantee that LSB of each kPointerSize aligned
   // word is not set and thus this word cannot be treated as pointer
   // to HeapObject during old space traversal.
   static const int kLengthOffset =
       kThisPropertyAssignmentsOffset + kPointerSize;
   static const int kFormalParameterCountOffset =
@@ skipping 14 matching lines @@
   static const int kCompilerHintsOffset =
       kFunctionTokenPositionOffset + kIntSize;
 
   static const int kThisPropertyAssignmentsCountOffset =
       kCompilerHintsOffset + kIntSize;
 
   // Total size.
   static const int kSize = kThisPropertyAssignmentsCountOffset + kIntSize;
 
 #endif
+
+  // The construction counter for inobject slack tracking is stored in the
+  // most significant byte of compiler_hints which is otherwise unused.
+  // Its offset depends on the endian-ness of the architecture.
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+  static const int kConstructionCountOffset = kCompilerHintsOffset + 3;
+#elif __BYTE_ORDER == __BIG_ENDIAN
+  static const int kConstructionCountOffset = kCompilerHintsOffset + 0;
+#else
+#error Unknown byte ordering
+#endif
+
   static const int kAlignedSize = POINTER_SIZE_ALIGN(kSize);
 
   typedef FixedBodyDescriptor<kNameOffset,
                               kThisPropertyAssignmentsOffset + kPointerSize,
                               kSize> BodyDescriptor;
 
  private:
   // Bit positions in start_position_and_type.
   // The source code start position is in the 30 most significant bits of
   // the start_position_and_type field.
   static const int kIsExpressionBit = 0;
   static const int kIsTopLevelBit   = 1;
   static const int kStartPositionShift = 2;
   static const int kStartPositionMask = ~((1 << kStartPositionShift) - 1);
 
   // Bit positions in compiler_hints.
   static const int kHasOnlySimpleThisPropertyAssignments = 0;
   static const int kTryFullCodegen = 1;
   static const int kAllowLazyCompilation = 2;
-  static const int kCodeAgeShift = 3;
+  static const int kLiveObjectsMayExist = 3;
+  static const int kCodeAgeShift = 4;
   static const int kCodeAgeMask = 7;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(SharedFunctionInfo);
 };
 
 
 // JSFunction describes JavaScript functions.
 class JSFunction: public JSObject {
  public:
   // [prototype_or_initial_map]:
@@ skipping 1880 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_