1 // Copyright 2018 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_OBJECTS_EMBEDDER_DATA_SLOT_H_
6 #define V8_OBJECTS_EMBEDDER_DATA_SLOT_H_
7 
8 #include <utility>
9 
10 #include "src/common/assert-scope.h"
11 #include "src/common/globals.h"
12 #include "src/objects/slots.h"
13 
14 // Has to be the last include (doesn't have include guards):
15 #include "src/objects/object-macros.h"
16 
17 namespace v8 {
18 namespace internal {
19 
20 class EmbedderDataArray;
21 class JSObject;
22 class Object;
23 
24 // An EmbedderDataSlot instance describes a kEmbedderDataSlotSize field ("slot")
25 // holding an embedder data which may contain raw aligned pointer or a tagged
26 // pointer (smi or heap object).
27 // Its address() is the address of the slot.
28 // The slot's contents can be read and written using respective load_XX() and
29 // store_XX() methods.
30 // Storing heap object through this slot may require triggering write barriers
31 // so this operation must be done via static store_tagged() methods.
32 class EmbedderDataSlot
33     : public SlotBase<EmbedderDataSlot, Address, kTaggedSize> {
34  public:
EmbedderDataSlot()35   EmbedderDataSlot() : SlotBase(kNullAddress) {}
36   V8_INLINE EmbedderDataSlot(EmbedderDataArray array, int entry_index);
37   V8_INLINE EmbedderDataSlot(JSObject object, int embedder_field_index);
38 
39 #if defined(V8_TARGET_BIG_ENDIAN) && defined(V8_COMPRESS_POINTERS)
40   static constexpr int kTaggedPayloadOffset = kTaggedSize;
41 #else
42   static constexpr int kTaggedPayloadOffset = 0;
43 #endif
44 
45 #ifdef V8_COMPRESS_POINTERS
46   // The raw payload is located in the other "tagged" part of the full pointer
47   // and cotains the upper part of aligned address. The raw part is not expected
48   // to look like a tagged value.
49   // When V8_HEAP_SANDBOX is defined the raw payload contains an index into the
50   // external pointer table.
51   static constexpr int kRawPayloadOffset = kTaggedSize - kTaggedPayloadOffset;
52 #endif
53   static constexpr int kRequiredPtrAlignment = kSmiTagSize;
54 
55   // Opaque type used for storing raw embedder data.
56   using RawData = Address;
57 
58   V8_INLINE void AllocateExternalPointerEntry(Isolate* isolate);
59 
60   V8_INLINE Object load_tagged() const;
61   V8_INLINE void store_smi(Smi value);
62 
63   // Setting an arbitrary tagged value requires triggering a write barrier
64   // which requires separate object and offset values, therefore these static
65   // functions also has the target object parameter.
66   static V8_INLINE void store_tagged(EmbedderDataArray array, int entry_index,
67                                      Object value);
68   static V8_INLINE void store_tagged(JSObject object, int embedder_field_index,
69                                      Object value);
70 
71   // Tries reinterpret the value as an aligned pointer and sets *out_result to
72   // the pointer-like value. Note, that some Smis could still look like an
73   // aligned pointers.
74   // Returns true on success.
75   // When V8 heap sandbox is enabled, calling this method when the raw part of
76   // the slot does not contain valid external pointer table index is undefined
77   // behaviour and most likely result in crashes.
78   V8_INLINE bool ToAlignedPointer(Isolate* isolate, void** out_result) const;
79 
80   // Same as ToAlignedPointer() but with a workaround for V8 heap sandbox.
81   // When V8 heap sandbox is enabled, this method doesn't crash when the raw
82   // part of the slot contains "undefined" instead of a correct external table
83   // entry index (see Factory::InitializeJSObjectBody() for details).
84   // Returns true when the external pointer table index was pointing to a valid
85   // entry, otherwise false.
86   //
87   // Call this function if you are not sure whether the slot contains valid
88   // external pointer or not.
89   V8_INLINE bool ToAlignedPointerSafe(Isolate* isolate,
90                                       void** out_result) const;
91 
92   // Returns true if the pointer was successfully stored or false it the pointer
93   // was improperly aligned.
94   V8_INLINE V8_WARN_UNUSED_RESULT bool store_aligned_pointer(Isolate* isolate,
95                                                              void* ptr);
96 
97   V8_INLINE RawData load_raw(Isolate* isolate,
98                              const DisallowGarbageCollection& no_gc) const;
99   V8_INLINE void store_raw(Isolate* isolate, RawData data,
100                            const DisallowGarbageCollection& no_gc);
101 
102  private:
103   // Stores given value to the embedder data slot in a concurrent-marker
104   // friendly manner (tagged part of the slot is written atomically).
105   V8_INLINE void gc_safe_store(Isolate* isolate, Address value);
106 };
107 
108 }  // namespace internal
109 }  // namespace v8
110 
111 #include "src/objects/object-macros-undef.h"
112 
113 #endif  // V8_OBJECTS_EMBEDDER_DATA_SLOT_H_
114