V8 API Reference, 7.2.502.16 (for Deno 0.2.4)
layout-descriptor.h
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/fixed-array.h"
11 
12 // Has to be the last include (doesn't have include guards):
13 #include "src/objects/object-macros.h"
14 
15 namespace v8 {
16 namespace internal {
17 
18 // LayoutDescriptor is a bit vector defining which fields contain non-tagged
19 // values. It could either be a fixed typed array (slow form) or a Smi
20 // if the length fits (fast form).
21 // Each bit in the layout represents a FIELD. The bits are referenced by
22 // field_index which is a field number. If the bit is set then the corresponding
23 // field contains a non-tagged value and therefore must be skipped by GC.
24 // Otherwise the field is considered tagged. If the queried bit lays "outside"
25 // of the descriptor then the field is also considered tagged.
26 // Once a layout descriptor is created it is allowed only to append properties
27 // to it. GC uses layout descriptors to iterate objects. Avoid heap pointers
28 // in a layout descriptor because they can lead to data races in GC when
29 // GC moves objects in parallel.
30 class LayoutDescriptor : public ByteArray {
31  public:
32  V8_INLINE bool IsTagged(int field_index);
33 
34  // Queries the contiguous region of fields that are either tagged or not.
35  // Returns true if the given field is tagged or false otherwise and writes
36  // the length of the contiguous region to |out_sequence_length|.
37  // If the sequence is longer than |max_sequence_length| then
38  // |out_sequence_length| is set to |max_sequence_length|.
39  bool IsTagged(int field_index, int max_sequence_length,
40  int* out_sequence_length);
41 
42  // Returns true if this is a layout of the object having only tagged fields.
43  V8_INLINE bool IsFastPointerLayout();
44  V8_INLINE static bool IsFastPointerLayout(Object* layout_descriptor);
45 
46  // Returns true if the layout descriptor is in non-Smi form.
47  V8_INLINE bool IsSlowLayout();
48 
49  DECL_CAST2(LayoutDescriptor)
50 
51  V8_INLINE static LayoutDescriptor cast_gc_safe(Object* object);
52 
53  // Builds layout descriptor optimized for given |map| by |num_descriptors|
54  // elements of given descriptors array. The |map|'s descriptors could be
55  // different.
56  static Handle<LayoutDescriptor> New(Isolate* isolate, Handle<Map> map,
57  Handle<DescriptorArray> descriptors,
58  int num_descriptors);
59 
60  // Modifies |map|'s layout descriptor or creates a new one if necessary by
61  // appending property with |details| to it.
62  static Handle<LayoutDescriptor> ShareAppend(Isolate* isolate, Handle<Map> map,
63  PropertyDetails details);
64 
65  // Creates new layout descriptor by appending property with |details| to
66  // |map|'s layout descriptor and if it is still fast then returns it.
67  // Otherwise the |full_layout_descriptor| is returned.
68  static Handle<LayoutDescriptor> AppendIfFastOrUseFull(
69  Isolate* isolate, Handle<Map> map, PropertyDetails details,
70  Handle<LayoutDescriptor> full_layout_descriptor);
71 
72  // Layout descriptor that corresponds to an object all fields of which are
73  // tagged (FastPointerLayout).
74  V8_INLINE static LayoutDescriptor FastPointerLayout();
75 
76  // Check that this layout descriptor corresponds to given map.
77  bool IsConsistentWithMap(Map map, bool check_tail = false);
78 
79  // Trims this layout descriptor to given number of descriptors. This happens
80  // only when corresponding descriptors array is trimmed.
81  // The layout descriptor could be trimmed if it was slow or it could
82  // become fast.
83  LayoutDescriptor Trim(Heap* heap, Map map, DescriptorArray* descriptors,
84  int num_descriptors);
85 
86 #ifdef OBJECT_PRINT
87  // For our gdb macros, we should perhaps change these in the future.
88  void Print();
89 
90  void ShortPrint(std::ostream& os);
91  void Print(std::ostream& os); // NOLINT
92 #endif
93 
94  // Capacity of layout descriptors in bits.
95  V8_INLINE int capacity();
96 
97  static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length);
98  LayoutDescriptor SetTaggedForTesting(int field_index, bool tagged);
99 
100  private:
101  // Exclude sign-bit to simplify encoding.
102  static constexpr int kBitsInSmiLayout =
103  SmiValuesAre32Bits() ? 32 : kSmiValueSize - 1;
104 
105  static const int kBitsPerLayoutWord = 32;
106 
107  V8_INLINE int number_of_layout_words();
108  V8_INLINE uint32_t get_layout_word(int index) const;
109  V8_INLINE void set_layout_word(int index, uint32_t value);
110 
111  V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length);
112  V8_INLINE static LayoutDescriptor FromSmi(Smi smi);
113 
114  V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
115  PropertyDetails details);
116 
117  // Calculates minimal layout descriptor capacity required for given
118  // |map|, |descriptors| and |num_descriptors|.
119  V8_INLINE static int CalculateCapacity(Map map, DescriptorArray* descriptors,
120  int num_descriptors);
121 
122  // Calculates the length of the slow-mode backing store array by given layout
123  // descriptor length.
124  V8_INLINE static int GetSlowModeBackingStoreLength(int length);
125 
126  // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
127  // and |num_descriptors|.
128  V8_INLINE static LayoutDescriptor Initialize(
129  LayoutDescriptor layout_descriptor, Map map, DescriptorArray* descriptors,
130  int num_descriptors);
131 
132  static Handle<LayoutDescriptor> EnsureCapacity(
133  Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
134  int new_capacity);
135 
136  // Returns false if requested field_index is out of bounds.
137  V8_INLINE bool GetIndexes(int field_index, int* layout_word_index,
138  int* layout_bit_index);
139 
140  V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor SetRawData(int field_index);
141 
142  V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor SetTagged(int field_index,
143  bool tagged);
144 
145  OBJECT_CONSTRUCTORS(LayoutDescriptor, ByteArray)
146 };
147 
148 
149 // LayoutDescriptorHelper is a helper class for querying layout descriptor
150 // about whether the field at given offset is tagged or not.
152  public:
153  inline explicit LayoutDescriptorHelper(Map map);
154 
155  bool all_fields_tagged() { return all_fields_tagged_; }
156  inline bool IsTagged(int offset_in_bytes);
157 
158  // Queries the contiguous region of fields that are either tagged or not.
159  // Returns true if fields starting at |offset_in_bytes| are tagged or false
160  // otherwise and writes the offset of the end of the contiguous region to
161  // |out_end_of_contiguous_region_offset|. The |end_offset| value is the
162  // upper bound for |out_end_of_contiguous_region_offset|.
163  bool IsTagged(int offset_in_bytes, int end_offset,
164  int* out_end_of_contiguous_region_offset);
165 
166  private:
167  bool all_fields_tagged_;
168  int header_size_;
169  LayoutDescriptor layout_descriptor_;
170 };
171 } // namespace internal
172 } // namespace v8
173 
174 #include "src/objects/object-macros-undef.h"
175 
176 #endif // V8_LAYOUT_DESCRIPTOR_H_
Definition: libplatform.h:13