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 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. GC uses layout descriptors to iterate objects. Avoid heap pointers
25 // in a layout descriptor because they can lead to data races in GC when
26 // GC moves objects in parallel.
27 class LayoutDescriptor : public ByteArray {
28  public:
29   V8_INLINE bool IsTagged(int field_index);
30 
31   // Queries the contiguous region of fields that are either tagged or not.
32   // Returns true if the given field is tagged or false otherwise and writes
33   // the length of the contiguous region to |out_sequence_length|.
34   // If the sequence is longer than |max_sequence_length| then
35   // |out_sequence_length| is set to |max_sequence_length|.
36   bool IsTagged(int field_index, int max_sequence_length,
37                 int* out_sequence_length);
38 
39   // Returns true if this is a layout of the object having only tagged fields.
40   V8_INLINE bool IsFastPointerLayout();
41   V8_INLINE static bool IsFastPointerLayout(Object* layout_descriptor);
42 
43   // Returns true if the layout descriptor is in non-Smi form.
44   V8_INLINE bool IsSlowLayout();
45 
46   V8_INLINE static LayoutDescriptor* cast(Object* object);
47   V8_INLINE static const LayoutDescriptor* cast(const Object* object);
48 
49   V8_INLINE static LayoutDescriptor* cast_gc_safe(Object* object);
50 
51   // Builds layout descriptor optimized for given |map| by |num_descriptors|
52   // elements of given descriptors array. The |map|'s descriptors could be
53   // different.
54   static Handle<LayoutDescriptor> New(Isolate* isolate, Handle<Map> map,
55                                       Handle<DescriptorArray> descriptors,
56                                       int num_descriptors);
57 
58   // Modifies |map|'s layout descriptor or creates a new one if necessary by
59   // appending property with |details| to it.
60   static Handle<LayoutDescriptor> ShareAppend(Isolate* isolate, Handle<Map> map,
61                                               PropertyDetails details);
62 
63   // Creates new layout descriptor by appending property with |details| to
64   // |map|'s layout descriptor and if it is still fast then returns it.
65   // Otherwise the |full_layout_descriptor| is returned.
66   static Handle<LayoutDescriptor> AppendIfFastOrUseFull(
67       Isolate* isolate, Handle<Map> map, PropertyDetails details,
68       Handle<LayoutDescriptor> full_layout_descriptor);
69 
70   // Layout descriptor that corresponds to an object all fields of which are
71   // tagged (FastPointerLayout).
72   V8_INLINE static LayoutDescriptor* FastPointerLayout();
73 
74   // Check that this layout descriptor corresponds to given map.
75   bool IsConsistentWithMap(Map* map, bool check_tail = false);
76 
77   // Trims this layout descriptor to given number of descriptors. This happens
78   // only when corresponding descriptors array is trimmed.
79   // The layout descriptor could be trimmed if it was slow or it could
80   // become fast.
81   LayoutDescriptor* Trim(Heap* heap, Map* map, DescriptorArray* descriptors,
82                          int num_descriptors);
83 
84 #ifdef OBJECT_PRINT
85   // For our gdb macros, we should perhaps change these in the future.
86   void Print();
87 
88   void ShortPrint(std::ostream& os);
89   void Print(std::ostream& os);  // NOLINT
90 #endif
91 
92   // Capacity of layout descriptors in bits.
93   V8_INLINE int capacity();
94 
95   static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length);
96   LayoutDescriptor* SetTaggedForTesting(int field_index, bool tagged);
97 
98  private:
99   // Exclude sign-bit to simplify encoding.
100   static constexpr int kBitsInSmiLayout =
101       SmiValuesAre32Bits() ? 32 : kSmiValueSize - 1;
102 
103   static const int kBitsPerLayoutWord = 32;
104 
105   V8_INLINE int number_of_layout_words();
106   V8_INLINE uint32_t get_layout_word(int index) const;
107   V8_INLINE void set_layout_word(int index, uint32_t value);
108 
109   V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length);
110   V8_INLINE static LayoutDescriptor* FromSmi(Smi* smi);
111 
112   V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
113                                              PropertyDetails details);
114 
115   // Calculates minimal layout descriptor capacity required for given
116   // |map|, |descriptors| and |num_descriptors|.
117   V8_INLINE static int CalculateCapacity(Map* map, DescriptorArray* descriptors,
118                                          int num_descriptors);
119 
120   // Calculates the length of the slow-mode backing store array by given layout
121   // descriptor length.
122   V8_INLINE static int GetSlowModeBackingStoreLength(int length);
123 
124   // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
125   // and |num_descriptors|.
126   V8_INLINE static LayoutDescriptor* Initialize(
127       LayoutDescriptor* layout_descriptor, Map* map,
128       DescriptorArray* descriptors, int num_descriptors);
129 
130   static Handle<LayoutDescriptor> EnsureCapacity(
131       Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
132       int new_capacity);
133 
134   // Returns false if requested field_index is out of bounds.
135   V8_INLINE bool GetIndexes(int field_index, int* layout_word_index,
136                             int* layout_bit_index);
137 
138   V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor* SetRawData(int field_index);
139 
140   V8_INLINE V8_WARN_UNUSED_RESULT LayoutDescriptor* SetTagged(int field_index,
141                                                               bool tagged);
142 };
143 
144 
145 // LayoutDescriptorHelper is a helper class for querying layout descriptor
146 // about whether the field at given offset is tagged or not.
147 class LayoutDescriptorHelper {
148  public:
149   inline explicit LayoutDescriptorHelper(Map* map);
150 
all_fields_tagged()151   bool all_fields_tagged() { return all_fields_tagged_; }
152   inline bool IsTagged(int offset_in_bytes);
153 
154   // Queries the contiguous region of fields that are either tagged or not.
155   // Returns true if fields starting at |offset_in_bytes| are tagged or false
156   // otherwise and writes the offset of the end of the contiguous region to
157   // |out_end_of_contiguous_region_offset|. The |end_offset| value is the
158   // upper bound for |out_end_of_contiguous_region_offset|.
159   bool IsTagged(int offset_in_bytes, int end_offset,
160                 int* out_end_of_contiguous_region_offset);
161 
162  private:
163   bool all_fields_tagged_;
164   int header_size_;
165   LayoutDescriptor* layout_descriptor_;
166 };
167 }  // namespace internal
168 }  // namespace v8
169 
170 #endif  // V8_LAYOUT_DESCRIPTOR_H_
171