summaryrefslogtreecommitdiffstats
path: root/third_party/jpeg-xl/lib/jxl/dec_bit_reader.h
blob: ea71d759faed5160859a9b2f770ccdb562fe3453 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
// Copyright (c) the JPEG XL Project Authors. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#ifndef LIB_JXL_DEC_BIT_READER_H_
#define LIB_JXL_DEC_BIT_READER_H_

// Bounds-checked bit reader; 64-bit buffer with support for deferred refills
// and switching to reading byte-aligned words.

#include <stddef.h>
#include <stdint.h>
#include <string.h>  // memcpy

#ifdef __BMI2__
#include <immintrin.h>
#endif

#include "lib/jxl/base/byte_order.h"
#include "lib/jxl/base/common.h"
#include "lib/jxl/base/compiler_specific.h"
#include "lib/jxl/base/span.h"
#include "lib/jxl/base/status.h"

namespace jxl {

// Reads bits previously written to memory by BitWriter. Uses unaligned 8-byte
// little-endian loads.
class BitReader {
 public:
  static constexpr size_t kMaxBitsPerCall = 56;

  // Constructs an invalid BitReader, to be overwritten before usage.
  BitReader()
      : buf_(0),
        bits_in_buf_(0),
        next_byte_{nullptr},
        end_minus_8_{nullptr},
        first_byte_(nullptr) {}
  BitReader(const BitReader&) = delete;

  // bytes need not be aligned nor padded!
  template <class ArrayLike>
  explicit BitReader(const ArrayLike& bytes)
      : buf_(0),
        bits_in_buf_(0),
        next_byte_(bytes.data()),
        // Assumes first_byte_ >= 8.
        end_minus_8_(bytes.data() - 8 + bytes.size()),
        first_byte_(bytes.data()) {
    Refill();
  }
  ~BitReader() {
    // Close() must be called before destroying an initialized bit reader.
    // Invalid bit readers will have a nullptr in first_byte_.
    JXL_ASSERT(close_called_ || !first_byte_);
  }

  // Move operator needs to invalidate the other BitReader such that it is
  // irrelevant if we call Close() on it or not.
  BitReader& operator=(BitReader&& other) noexcept {
    // Ensure the current instance was already closed, before we overwrite it
    // with other.
    JXL_ASSERT(close_called_ || !first_byte_);

    JXL_DASSERT(!other.close_called_);
    buf_ = other.buf_;
    bits_in_buf_ = other.bits_in_buf_;
    next_byte_ = other.next_byte_;
    end_minus_8_ = other.end_minus_8_;
    first_byte_ = other.first_byte_;
    overread_bytes_ = other.overread_bytes_;
    close_called_ = other.close_called_;

    other.first_byte_ = nullptr;
    other.next_byte_ = nullptr;
    return *this;
  }
  BitReader& operator=(const BitReader& other) = delete;

  // For time-critical reads, refills can be shared by multiple reads.
  // Based on variant 4 (plus bounds-checking), see
  // fgiesen.wordpress.com/2018/02/20/reading-bits-in-far-too-many-ways-part-2/
  JXL_INLINE void Refill() {
    if (JXL_UNLIKELY(next_byte_ > end_minus_8_)) {
      BoundsCheckedRefill();
    } else {
      // It's safe to load 64 bits; insert valid (possibly nonzero) bits above
      // bits_in_buf_. The shift requires bits_in_buf_ < 64.
      buf_ |= LoadLE64(next_byte_) << bits_in_buf_;

      // Advance by bytes fully absorbed into the buffer.
      next_byte_ += (63 - bits_in_buf_) >> 3;

      // We absorbed a multiple of 8 bits, so the lower 3 bits of bits_in_buf_
      // must remain unchanged, otherwise the next refill's shifted bits will
      // not align with buf_. Set the three upper bits so the result >= 56.
      bits_in_buf_ |= 56;
      JXL_DASSERT(56 <= bits_in_buf_ && bits_in_buf_ < 64);
    }
  }

  // Returns the bits that would be returned by Read without calling Advance().
  // It is legal to PEEK at more bits than present in the bitstream (required
  // by Huffman), and those bits will be zero.
  template <size_t N>
  JXL_INLINE uint64_t PeekFixedBits() const {
    static_assert(N <= kMaxBitsPerCall, "Reading too many bits in one call.");
    JXL_DASSERT(!close_called_);
    return buf_ & ((1ULL << N) - 1);
  }

  JXL_INLINE uint64_t PeekBits(size_t nbits) const {
    JXL_DASSERT(nbits <= kMaxBitsPerCall);
    JXL_DASSERT(!close_called_);

    // Slightly faster but requires BMI2. It is infeasible to make the many
    // callers reside between begin/end_target, especially because only the
    // callers in dec_ans are time-critical. Therefore only enabled if the
    // entire binary is compiled for (and thus requires) BMI2.
#if defined(__BMI2__) && defined(__x86_64__)
    return _bzhi_u64(buf_, nbits);
#else
    const uint64_t mask = (1ULL << nbits) - 1;
    return buf_ & mask;
#endif
  }

  // Removes bits from the buffer. Need not match the previous Peek size, but
  // the buffer must contain at least num_bits (this prevents consuming more
  // than the total number of bits).
  JXL_INLINE void Consume(size_t num_bits) {
    JXL_DASSERT(!close_called_);
    JXL_DASSERT(bits_in_buf_ >= num_bits);
#ifdef JXL_CRASH_ON_ERROR
    // When JXL_CRASH_ON_ERROR is defined, it is a fatal error to read more bits
    // than available in the stream. A non-zero overread_bytes_ implies that
    // next_byte_ is already at the end of the stream, so we don't need to
    // check that.
    JXL_ASSERT(bits_in_buf_ >= num_bits + overread_bytes_ * kBitsPerByte);
#endif
    bits_in_buf_ -= num_bits;
    buf_ >>= num_bits;
  }

  JXL_INLINE uint64_t ReadBits(size_t nbits) {
    JXL_DASSERT(!close_called_);
    Refill();
    const uint64_t bits = PeekBits(nbits);
    Consume(nbits);
    return bits;
  }

  template <size_t N>
  JXL_INLINE uint64_t ReadFixedBits() {
    JXL_DASSERT(!close_called_);
    Refill();
    const uint64_t bits = PeekFixedBits<N>();
    Consume(N);
    return bits;
  }

  // Equivalent to calling ReadFixedBits(1) `skip` times, but much faster.
  // `skip` is typically large.
  void SkipBits(size_t skip) {
    JXL_DASSERT(!close_called_);
    // Buffer is large enough - don't zero buf_ below.
    if (JXL_UNLIKELY(skip <= bits_in_buf_)) {
      Consume(skip);
      return;
    }

    // First deduct what we can satisfy from the buffer
    skip -= bits_in_buf_;
    bits_in_buf_ = 0;
    // Not enough to call Advance - that may leave some bits in the buffer
    // which were previously ABOVE bits_in_buf.
    buf_ = 0;

    // Skip whole bytes
    const size_t whole_bytes = skip / kBitsPerByte;
    skip %= kBitsPerByte;
    if (JXL_UNLIKELY(whole_bytes >
                     static_cast<size_t>(end_minus_8_ + 8 - next_byte_))) {
      // This is already an overflow condition (skipping past the end of the bit
      // stream). However if we increase next_byte_ too much we risk overflowing
      // that value and potentially making it valid again (next_byte_ < end).
      // This will set next_byte_ to the end of the stream and still consume
      // some bits in overread_bytes_, however the TotalBitsConsumed() will be
      // incorrect (still larger than the TotalBytes()).
      next_byte_ = end_minus_8_ + 8;
      skip += kBitsPerByte;
    } else {
      next_byte_ += whole_bytes;
    }

    Refill();
    Consume(skip);
  }

  size_t TotalBitsConsumed() const {
    const size_t bytes_read = static_cast<size_t>(next_byte_ - first_byte_);
    return (bytes_read + overread_bytes_) * kBitsPerByte - bits_in_buf_;
  }

  Status JumpToByteBoundary() {
    const size_t remainder = TotalBitsConsumed() % kBitsPerByte;
    if (remainder == 0) return true;
    if (JXL_UNLIKELY(ReadBits(kBitsPerByte - remainder) != 0)) {
      return JXL_FAILURE("Non-zero padding bits");
    }
    return true;
  }

  // For interoperability with other bitreaders (for resuming at
  // non-byte-aligned positions).
  const uint8_t* FirstByte() const { return first_byte_; }
  size_t TotalBytes() const {
    return static_cast<size_t>(end_minus_8_ + 8 - first_byte_);
  }

  // Returns span of the remaining (unconsumed) bytes, e.g. for passing to
  // external decoders such as Brotli.
  Span<const uint8_t> GetSpan() const {
    JXL_DASSERT(first_byte_ != nullptr);
    JXL_ASSERT(TotalBitsConsumed() % kBitsPerByte == 0);
    const size_t offset = TotalBitsConsumed() / kBitsPerByte;  // no remainder
    JXL_ASSERT(offset <= TotalBytes());
    return Bytes(first_byte_ + offset, TotalBytes() - offset);
  }

  // Returns whether all the bits read so far have been within the input bounds.
  // When reading past the EOF, the Read*() and Consume() functions return zeros
  // but flag a failure when calling Close() without checking this function.
  Status AllReadsWithinBounds() {
    // Mark up to which point the user checked the out of bounds condition. If
    // the user handles the condition at higher level (e.g. fetch more bytes
    // from network, return a custom JXL_FAILURE, ...), Close() should not
    // output a debug error (which would break tests with JXL_CRASH_ON_ERROR
    // even when legitimately handling the situation at higher level). This is
    // used by Bundle::CanRead.
    checked_out_of_bounds_bits_ = TotalBitsConsumed();
    if (TotalBitsConsumed() > TotalBytes() * kBitsPerByte) {
      return false;
    }
    return true;
  }

  // Close the bit reader and return whether all the previous reads were
  // successful. Close must be called once.
  Status Close() {
    JXL_DASSERT(!close_called_);
    close_called_ = true;
    if (!first_byte_) return true;
    if (TotalBitsConsumed() > checked_out_of_bounds_bits_ &&
        TotalBitsConsumed() > TotalBytes() * kBitsPerByte) {
      return JXL_FAILURE("Read more bits than available in the bit_reader");
    }
    return true;
  }

 private:
  // Separate function avoids inlining this relatively cold code into callers.
  JXL_NOINLINE void BoundsCheckedRefill() {
    const uint8_t* end = end_minus_8_ + 8;

    // Read whole bytes until we have [56, 64) bits (same as LoadLE64)
    for (; bits_in_buf_ < 64 - kBitsPerByte; bits_in_buf_ += kBitsPerByte) {
      if (next_byte_ >= end) break;
      buf_ |= static_cast<uint64_t>(*next_byte_++) << bits_in_buf_;
    }
    JXL_DASSERT(bits_in_buf_ < 64);

    // Add extra bytes as 0 at the end of the stream in the bit_buffer_. If
    // these bits are read, Close() will return a failure.
    size_t extra_bytes = (63 - bits_in_buf_) / kBitsPerByte;
    overread_bytes_ += extra_bytes;
    bits_in_buf_ += extra_bytes * kBitsPerByte;

    JXL_DASSERT(bits_in_buf_ < 64);
    JXL_DASSERT(bits_in_buf_ >= 56);
  }

  JXL_NOINLINE uint32_t BoundsCheckedReadByteAlignedWord() {
    if (next_byte_ + 1 < end_minus_8_ + 8) {
      uint32_t ret = LoadLE16(next_byte_);
      next_byte_ += 2;
      return ret;
    }
    overread_bytes_ += 2;
    return 0;
  }

  uint64_t buf_;
  size_t bits_in_buf_;  // [0, 64)
  const uint8_t* JXL_RESTRICT next_byte_;
  const uint8_t* end_minus_8_;  // for refill bounds check
  const uint8_t* first_byte_;   // for GetSpan

  // Number of bytes past the end that were loaded into the buf_. These bytes
  // are not read from memory, but instead assumed 0. It is an error (likely due
  // to an invalid stream) to Consume() more bits than specified in the range
  // passed to the constructor.
  uint64_t overread_bytes_{0};
  bool close_called_{false};

  uint64_t checked_out_of_bounds_bits_{0};
};

// Closes a BitReader when the BitReaderScopedCloser goes out of scope. When
// closing the bit reader, if the status result was failure it sets this failure
// to the passed variable pointer. Typical usage.
//
// Status ret = true;
// {
//   BitReader reader(...);
//   BitReaderScopedCloser reader_closer(&reader, &ret);
//
//   // ... code that can return errors here ...
// }
// // ... more code that doesn't use the BitReader.
// return ret;

class BitReaderScopedCloser {
 public:
  BitReaderScopedCloser(BitReader* reader, Status* status)
      : reader_(reader), status_(status) {
    JXL_DASSERT(reader_ != nullptr);
    JXL_DASSERT(status_ != nullptr);
  }
  ~BitReaderScopedCloser() {
    if (reader_ != nullptr) {
      Status close_ret = reader_->Close();
      if (!close_ret) *status_ = close_ret;
    }
  }
  void CloseAndSuppressError() {
    JXL_ASSERT(reader_ != nullptr);
    (void)reader_->Close();
    reader_ = nullptr;
  }
  BitReaderScopedCloser(const BitReaderScopedCloser&) = delete;

 private:
  BitReader* reader_;
  Status* status_;
};

}  // namespace jxl

#endif  // LIB_JXL_DEC_BIT_READER_H_