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
15namespace v8 {
16namespace 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.
30class V8_EXPORT_PRIVATE 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_CAST(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// LayoutDescriptorHelper is a helper class for querying layout descriptor
149// about whether the field at given offset is tagged or not.
150class LayoutDescriptorHelper {
151 public:
152 inline explicit LayoutDescriptorHelper(Map map);
153
154 bool all_fields_tagged() { return all_fields_tagged_; }
155 inline bool IsTagged(int offset_in_bytes);
156
157 // Queries the contiguous region of fields that are either tagged or not.
158 // Returns true if fields starting at |offset_in_bytes| are tagged or false
159 // otherwise and writes the offset of the end of the contiguous region to
160 // |out_end_of_contiguous_region_offset|. The |end_offset| value is the
161 // upper bound for |out_end_of_contiguous_region_offset|.
162 V8_EXPORT_PRIVATE bool IsTagged(int offset_in_bytes, int end_offset,
163 int* out_end_of_contiguous_region_offset);
164
165 private:
166 bool all_fields_tagged_;
167 int header_size_;
168 LayoutDescriptor layout_descriptor_;
169};
170} // namespace internal
171} // namespace v8
172
173#include "src/objects/object-macros-undef.h"
174
175#endif // V8_LAYOUT_DESCRIPTOR_H_
176