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      1 // Copyright 2014 the V8 project authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #ifndef V8_LAYOUT_DESCRIPTOR_H_
      6 #define V8_LAYOUT_DESCRIPTOR_H_
      7 
      8 #include <iosfwd>
      9 
     10 #include "src/objects.h"
     11 
     12 namespace v8 {
     13 namespace internal {
     14 
     15 // LayoutDescriptor is a bit vector defining which fields contain non-tagged
     16 // values. It could either be a fixed typed array (slow form) or a Smi
     17 // if the length fits (fast form).
     18 // Each bit in the layout represents a FIELD. The bits are referenced by
     19 // field_index which is a field number. If the bit is set then the corresponding
     20 // field contains a non-tagged value and therefore must be skipped by GC.
     21 // Otherwise the field is considered tagged. If the queried bit lays "outside"
     22 // of the descriptor then the field is also considered tagged.
     23 // Once a layout descriptor is created it is allowed only to append properties
     24 // to it.
     25 class LayoutDescriptor : public FixedTypedArray<Uint32ArrayTraits> {
     26  public:
     27   V8_INLINE bool IsTagged(int field_index);
     28 
     29   // Queries the contiguous region of fields that are either tagged or not.
     30   // Returns true if the given field is tagged or false otherwise and writes
     31   // the length of the contiguous region to |out_sequence_length|.
     32   // If the sequence is longer than |max_sequence_length| then
     33   // |out_sequence_length| is set to |max_sequence_length|.
     34   bool IsTagged(int field_index, int max_sequence_length,
     35                 int* out_sequence_length);
     36 
     37   // Returns true if this is a layout of the object having only tagged fields.
     38   V8_INLINE bool IsFastPointerLayout();
     39   V8_INLINE static bool IsFastPointerLayout(Object* layout_descriptor);
     40 
     41   // Returns true if the layout descriptor is in non-Smi form.
     42   V8_INLINE bool IsSlowLayout();
     43 
     44   V8_INLINE static LayoutDescriptor* cast(Object* object);
     45   V8_INLINE static const LayoutDescriptor* cast(const Object* object);
     46 
     47   V8_INLINE static LayoutDescriptor* cast_gc_safe(Object* object);
     48 
     49   // Builds layout descriptor optimized for given |map| by |num_descriptors|
     50   // elements of given descriptors array. The |map|'s descriptors could be
     51   // different.
     52   static Handle<LayoutDescriptor> New(Handle<Map> map,
     53                                       Handle<DescriptorArray> descriptors,
     54                                       int num_descriptors);
     55 
     56   // Modifies |map|'s layout descriptor or creates a new one if necessary by
     57   // appending property with |details| to it.
     58   static Handle<LayoutDescriptor> ShareAppend(Handle<Map> map,
     59                                               PropertyDetails details);
     60 
     61   // Creates new layout descriptor by appending property with |details| to
     62   // |map|'s layout descriptor and if it is still fast then returns it.
     63   // Otherwise the |full_layout_descriptor| is returned.
     64   static Handle<LayoutDescriptor> AppendIfFastOrUseFull(
     65       Handle<Map> map, PropertyDetails details,
     66       Handle<LayoutDescriptor> full_layout_descriptor);
     67 
     68   // Layout descriptor that corresponds to an object all fields of which are
     69   // tagged (FastPointerLayout).
     70   V8_INLINE static LayoutDescriptor* FastPointerLayout();
     71 
     72   // Check that this layout descriptor corresponds to given map.
     73   bool IsConsistentWithMap(Map* map, bool check_tail = false);
     74 
     75   // Trims this layout descriptor to given number of descriptors. This happens
     76   // only when corresponding descriptors array is trimmed.
     77   // The layout descriptor could be trimmed if it was slow or it could
     78   // become fast.
     79   LayoutDescriptor* Trim(Heap* heap, Map* map, DescriptorArray* descriptors,
     80                          int num_descriptors);
     81 
     82 #ifdef OBJECT_PRINT
     83   // For our gdb macros, we should perhaps change these in the future.
     84   void Print();
     85 
     86   void ShortPrint(std::ostream& os);
     87   void Print(std::ostream& os);  // NOLINT
     88 #endif
     89 
     90   // Capacity of layout descriptors in bits.
     91   V8_INLINE int capacity();
     92 
     93   static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length);
     94   LayoutDescriptor* SetTaggedForTesting(int field_index, bool tagged);
     95 
     96  private:
     97   static const int kNumberOfBits = 32;
     98 
     99   V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length);
    100   V8_INLINE static LayoutDescriptor* FromSmi(Smi* smi);
    101 
    102   V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
    103                                              PropertyDetails details);
    104 
    105   // Calculates minimal layout descriptor capacity required for given
    106   // |map|, |descriptors| and |num_descriptors|.
    107   V8_INLINE static int CalculateCapacity(Map* map, DescriptorArray* descriptors,
    108                                          int num_descriptors);
    109 
    110   // Calculates the length of the slow-mode backing store array by given layout
    111   // descriptor length.
    112   V8_INLINE static int GetSlowModeBackingStoreLength(int length);
    113 
    114   // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
    115   // and |num_descriptors|.
    116   V8_INLINE static LayoutDescriptor* Initialize(
    117       LayoutDescriptor* layout_descriptor, Map* map,
    118       DescriptorArray* descriptors, int num_descriptors);
    119 
    120   static Handle<LayoutDescriptor> EnsureCapacity(
    121       Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
    122       int new_capacity);
    123 
    124   // Returns false if requested field_index is out of bounds.
    125   V8_INLINE bool GetIndexes(int field_index, int* layout_word_index,
    126                             int* layout_bit_index);
    127 
    128   V8_INLINE MUST_USE_RESULT LayoutDescriptor* SetRawData(int field_index);
    129 
    130   V8_INLINE MUST_USE_RESULT LayoutDescriptor* SetTagged(int field_index,
    131                                                         bool tagged);
    132 };
    133 
    134 
    135 // LayoutDescriptorHelper is a helper class for querying layout descriptor
    136 // about whether the field at given offset is tagged or not.
    137 class LayoutDescriptorHelper {
    138  public:
    139   inline explicit LayoutDescriptorHelper(Map* map);
    140 
    141   bool all_fields_tagged() { return all_fields_tagged_; }
    142   inline bool IsTagged(int offset_in_bytes);
    143 
    144   // Queries the contiguous region of fields that are either tagged or not.
    145   // Returns true if fields starting at |offset_in_bytes| are tagged or false
    146   // otherwise and writes the offset of the end of the contiguous region to
    147   // |out_end_of_contiguous_region_offset|. The |end_offset| value is the
    148   // upper bound for |out_end_of_contiguous_region_offset|.
    149   bool IsTagged(int offset_in_bytes, int end_offset,
    150                 int* out_end_of_contiguous_region_offset);
    151 
    152  private:
    153   bool all_fields_tagged_;
    154   int header_size_;
    155   LayoutDescriptor* layout_descriptor_;
    156 };
    157 }  // namespace internal
    158 }  // namespace v8
    159 
    160 #endif  // V8_LAYOUT_DESCRIPTOR_H_
    161