// 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_LAYOUT_DESCRIPTOR_H_ #define V8_LAYOUT_DESCRIPTOR_H_ #include <iosfwd> #include "src/objects/fixed-array.h" namespace v8 { namespace internal { // LayoutDescriptor is a bit vector defining which fields contain non-tagged // values. It could either be a fixed typed array (slow form) or a Smi // if the length fits (fast form). // Each bit in the layout represents a FIELD. The bits are referenced by // field_index which is a field number. If the bit is set then the corresponding // field contains a non-tagged value and therefore must be skipped by GC. // Otherwise the field is considered tagged. If the queried bit lays "outside" // of the descriptor then the field is also considered tagged. // Once a layout descriptor is created it is allowed only to append properties // to it. GC uses layout descriptors to iterate objects. Avoid heap pointers // in a layout descriptor because they can lead to data races in GC when // GC moves objects in parallel. class LayoutDescriptor : public ByteArray { public: V8_INLINE bool IsTagged(int field_index); // Queries the contiguous region of fields that are either tagged or not. // Returns true if the given field is tagged or false otherwise and writes // the length of the contiguous region to |out_sequence_length|. // If the sequence is longer than |max_sequence_length| then // |out_sequence_length| is set to |max_sequence_length|. bool IsTagged(int field_index, int max_sequence_length, int* out_sequence_length); // Returns true if this is a layout of the object having only tagged fields. V8_INLINE bool IsFastPointerLayout(); V8_INLINE static bool IsFastPointerLayout(Object* layout_descriptor); // Returns true if the layout descriptor is in non-Smi form. V8_INLINE bool IsSlowLayout(); V8_INLINE static LayoutDescriptor* cast(Object* object); V8_INLINE static const LayoutDescriptor* cast(const Object* object); V8_INLINE static LayoutDescriptor* cast_gc_safe(Object* object); // Builds layout descriptor optimized for given |map| by |num_descriptors| // elements of given descriptors array. The |map|'s descriptors could be // different. static Handle<LayoutDescriptor> New(Isolate* isolate, Handle<Map> map, Handle<DescriptorArray> descriptors, int num_descriptors); // Modifies |map|'s layout descriptor or creates a new one if necessary by // appending property with |details| to it. static Handle<LayoutDescriptor> ShareAppend(Isolate* isolate, Handle<Map> map, PropertyDetails details); // Creates new layout descriptor by appending property with |details| to // |map|'s layout descriptor and if it is still fast then returns it. // Otherwise the |full_layout_descriptor| is returned. static Handle<LayoutDescriptor> AppendIfFastOrUseFull( Isolate* isolate, Handle<Map> map, PropertyDetails details, Handle<LayoutDescriptor> full_layout_descriptor); // Layout descriptor that corresponds to an object all fields of which are // tagged (FastPointerLayout). V8_INLINE static LayoutDescriptor* FastPointerLayout(); // Check that this layout descriptor corresponds to given map. bool IsConsistentWithMap(Map map, bool check_tail = false); // Trims this layout descriptor to given number of descriptors. This happens // only when corresponding descriptors array is trimmed. // The layout descriptor could be trimmed if it was slow or it could // become fast. LayoutDescriptor* Trim(Heap* heap, Map map, DescriptorArray* descriptors, int num_descriptors); #ifdef OBJECT_PRINT // For our gdb macros, we should perhaps change these in the future. void Print(); void ShortPrint(std::ostream& os); void Print(std::ostream& os); // NOLINT #endif // Capacity of layout descriptors in bits. V8_INLINE int capacity(); static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length); LayoutDescriptor* SetTaggedForTesting(int field_index, bool tagged); private: // Exclude sign-bit to simplify encoding. static constexpr int kBitsInSmiLayout = SmiValuesAre32Bits() ? 32 : kSmiValueSize - 1; static const int kBitsPerLayoutWord = 32; V8_INLINE int number_of_layout_words(); V8_INLINE uint32_t get_layout_word(int index) const; V8_INLINE void set_layout_word(int index, uint32_t value); V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length); V8_INLINE static LayoutDescriptor* FromSmi(Smi smi); V8_INLINE static bool InobjectUnboxedField(int inobject_properties, PropertyDetails details); // Calculates minimal layout descriptor capacity required for given // |map|, |descriptors| and |num_descriptors|. V8_INLINE static int CalculateCapacity(Map map, DescriptorArray* descriptors, int num_descriptors); // Calculates the length of the slow-mode backing store array by given layout // descriptor length. V8_INLINE static int GetSlowModeBackingStoreLength(int length); // Fills in clean |layout_descriptor| according to given |map|, |descriptors| // and |num_descriptors|. V8_INLINE static LayoutDescriptor* Initialize( LayoutDescriptor* layout_descriptor, Map map, DescriptorArray* descriptors, int num_descriptors); static Handle<LayoutDescriptor> EnsureCapacity( Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor, int new_capacity); // Returns false if requested field_index is out of bounds. V8_INLINE bool GetIndexes(int field_index, int* layout_word_index, int* layout_bit_index); V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor* SetRawData(int field_index); V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor* SetTagged(int field_index, bool tagged); }; // LayoutDescriptorHelper is a helper class for querying layout descriptor // about whether the field at given offset is tagged or not. class LayoutDescriptorHelper { public: inline explicit LayoutDescriptorHelper(Map map); bool all_fields_tagged() { return all_fields_tagged_; } inline bool IsTagged(int offset_in_bytes); // Queries the contiguous region of fields that are either tagged or not. // Returns true if fields starting at |offset_in_bytes| are tagged or false // otherwise and writes the offset of the end of the contiguous region to // |out_end_of_contiguous_region_offset|. The |end_offset| value is the // upper bound for |out_end_of_contiguous_region_offset|. bool IsTagged(int offset_in_bytes, int end_offset, int* out_end_of_contiguous_region_offset); private: bool all_fields_tagged_; int header_size_; LayoutDescriptor* layout_descriptor_; }; } // namespace internal } // namespace v8 #endif // V8_LAYOUT_DESCRIPTOR_H_