1 //===-- sanitizer_ring_buffer.h ---------------------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Simple ring buffer.
10 //
11 //===----------------------------------------------------------------------===//
12 #ifndef SANITIZER_RING_BUFFER_H
13 #define SANITIZER_RING_BUFFER_H
14
15 #include "sanitizer_common.h"
16
17 namespace __sanitizer {
18 // RingBuffer<T>: fixed-size ring buffer optimized for speed of push().
19 // T should be a POD type and sizeof(T) should be divisible by sizeof(void*).
20 // At creation, all elements are zero.
21 template<class T>
22 class RingBuffer {
23 public:
24 COMPILER_CHECK(sizeof(T) % sizeof(void *) == 0);
25 static RingBuffer *New(uptr Size) {
26 void *Ptr = MmapOrDie(SizeInBytes(Size), "RingBuffer");
27 RingBuffer *RB = reinterpret_cast<RingBuffer*>(Ptr);
28 uptr End = reinterpret_cast<uptr>(Ptr) + SizeInBytes(Size);
29 RB->last_ = RB->next_ = reinterpret_cast<T*>(End - sizeof(T));
30 return RB;
31 }
32 void Delete() {
33 UnmapOrDie(this, SizeInBytes(size()));
34 }
35 uptr size() const {
36 return last_ + 1 -
37 reinterpret_cast<T *>(reinterpret_cast<uptr>(this) +
38 2 * sizeof(T *));
39 }
40
41 static uptr SizeInBytes(uptr Size) {
42 return Size * sizeof(T) + 2 * sizeof(T*);
43 }
44
45 uptr SizeInBytes() { return SizeInBytes(size()); }
46
47 void push(T t) {
48 *next_ = t;
49 next_--;
50 // The condition below works only if sizeof(T) is divisible by sizeof(T*).
51 if (next_ <= reinterpret_cast<T*>(&next_))
52 next_ = last_;
53 }
54
55 T operator[](uptr Idx) const {
56 CHECK_LT(Idx, size());
57 sptr IdxNext = Idx + 1;
58 if (IdxNext > last_ - next_)
59 IdxNext -= size();
60 return next_[IdxNext];
61 }
62
63 private:
64 RingBuffer() {}
65 ~RingBuffer() {}
66 RingBuffer(const RingBuffer&) = delete;
67
68 // Data layout:
69 // LNDDDDDDDD
70 // D: data elements.
71 // L: last_, always points to the last data element.
72 // N: next_, initially equals to last_, is decremented on every push,
73 // wraps around if it's less or equal than its own address.
74 T *last_;
75 T *next_;
76 T data_[1]; // flexible array.
77 };
78
79 // A ring buffer with externally provided storage that encodes its state in 8
80 // bytes. Has significant constraints on size and alignment of storage.
81 // See a comment in hwasan/hwasan_thread_list.h for the motivation behind this.
82 #if SANITIZER_WORDSIZE == 64
83 template <class T>
84 class CompactRingBuffer {
85 // Top byte of long_ stores the buffer size in pages.
86 // Lower bytes store the address of the next buffer element.
87 static constexpr int kPageSizeBits = 12;
88 static constexpr int kSizeShift = 56;
89 static constexpr int kSizeBits = 64 - kSizeShift;
90 static constexpr uptr kNextMask = (1ULL << kSizeShift) - 1;
91
92 uptr GetStorageSize() const { return (long_ >> kSizeShift) << kPageSizeBits; }
93
94 static uptr SignExtend(uptr x) { return ((sptr)x) << kSizeBits >> kSizeBits; }
95
96 void Init(void *storage, uptr size) {
97 CHECK_EQ(sizeof(CompactRingBuffer<T>), sizeof(void *));
98 CHECK(IsPowerOfTwo(size));
99 CHECK_GE(size, 1 << kPageSizeBits);
100 CHECK_LE(size, 128 << kPageSizeBits);
101 CHECK_EQ(size % 4096, 0);
102 CHECK_EQ(size % sizeof(T), 0);
103 uptr st = (uptr)storage;
104 CHECK_EQ(st % (size * 2), 0);
105 CHECK_EQ(st, SignExtend(st & kNextMask));
106 long_ = (st & kNextMask) | ((size >> kPageSizeBits) << kSizeShift);
107 }
108
109 void SetNext(const T *next) {
110 long_ = (long_ & ~kNextMask) | ((uptr)next & kNextMask);
111 }
112
113 public:
114 CompactRingBuffer(void *storage, uptr size) {
115 Init(storage, size);
116 }
117
118 // A copy constructor of sorts.
119 CompactRingBuffer(const CompactRingBuffer &other, void *storage) {
120 uptr size = other.GetStorageSize();
121 internal_memcpy(storage, other.StartOfStorage(), size);
122 Init(storage, size);
123 uptr Idx = other.Next() - (const T *)other.StartOfStorage();
124 SetNext((const T *)storage + Idx);
125 }
126
127 T *Next() const { return (T *)(SignExtend(long_ & kNextMask)); }
128
129 void *StartOfStorage() const {
130 return (void *)((uptr)Next() & ~(GetStorageSize() - 1));
131 }
132
133 void *EndOfStorage() const {
134 return (void *)((uptr)StartOfStorage() + GetStorageSize());
135 }
136
137 uptr size() const { return GetStorageSize() / sizeof(T); }
138
139 void push(T t) {
140 T *next = Next();
141 *next = t;
142 next++;
143 next = (T *)((uptr)next & ~GetStorageSize());
144 SetNext(next);
145 }
146
147 const T &operator[](uptr Idx) const {
148 CHECK_LT(Idx, size());
149 const T *Begin = (const T *)StartOfStorage();
150 sptr StorageIdx = Next() - Begin;
151 StorageIdx -= (sptr)(Idx + 1);
152 if (StorageIdx < 0)
153 StorageIdx += size();
154 return Begin[StorageIdx];
155 }
156
157 public:
158 ~CompactRingBuffer() {}
159 CompactRingBuffer(const CompactRingBuffer &) = delete;
160
161 uptr long_;
162 };
163 #endif
164 } // namespace __sanitizer
165
166 #endif // SANITIZER_RING_BUFFER_H