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
167