1// Copyright 2005 Google Inc. All Rights Reserved.
2//
3// Redistribution and use in source and binary forms, with or without
4// modification, are permitted provided that the following conditions are
5// met:
6//
7// * Redistributions of source code must retain the above copyright
8// notice, this list of conditions and the following disclaimer.
9// * Redistributions in binary form must reproduce the above
10// copyright notice, this list of conditions and the following disclaimer
11// in the documentation and/or other materials provided with the
12// distribution.
13// * Neither the name of Google Inc. nor the names of its
14// contributors may be used to endorse or promote products derived from
15// this software without specific prior written permission.
16//
17// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
18// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
19// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
20// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
21// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
22// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
23// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
24// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
25// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
26// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
27// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
28
29#include "snappy.h"
30#include "snappy-internal.h"
31#include "snappy-sinksource.h"
32
33#if !defined(SNAPPY_HAVE_SSSE3)
34// __SSSE3__ is defined by GCC and Clang. Visual Studio doesn't target SIMD
35// support between SSE2 and AVX (so SSSE3 instructions require AVX support), and
36// defines __AVX__ when AVX support is available.
37#if defined(__SSSE3__) || defined(__AVX__)
38#define SNAPPY_HAVE_SSSE3 1
39#else
40#define SNAPPY_HAVE_SSSE3 0
41#endif
42#endif // !defined(SNAPPY_HAVE_SSSE3)
43
44#if !defined(SNAPPY_HAVE_BMI2)
45// __BMI2__ is defined by GCC and Clang. Visual Studio doesn't target BMI2
46// specifically, but it does define __AVX2__ when AVX2 support is available.
47// Fortunately, AVX2 was introduced in Haswell, just like BMI2.
48//
49// BMI2 is not defined as a subset of AVX2 (unlike SSSE3 and AVX above). So,
50// GCC and Clang can build code with AVX2 enabled but BMI2 disabled, in which
51// case issuing BMI2 instructions results in a compiler error.
52#if defined(__BMI2__) || (defined(_MSC_VER) && defined(__AVX2__))
53#define SNAPPY_HAVE_BMI2 1
54#else
55#define SNAPPY_HAVE_BMI2 0
56#endif
57#endif // !defined(SNAPPY_HAVE_BMI2)
58
59#if SNAPPY_HAVE_SSSE3
60// Please do not replace with <x86intrin.h>. or with headers that assume more
61// advanced SSE versions without checking with all the OWNERS.
62#include <tmmintrin.h>
63#endif
64
65#if SNAPPY_HAVE_BMI2
66// Please do not replace with <x86intrin.h>. or with headers that assume more
67// advanced SSE versions without checking with all the OWNERS.
68#include <immintrin.h>
69#endif
70
71#include <stdio.h>
72
73#include <algorithm>
74#include <string>
75#include <vector>
76
77namespace snappy {
78
79using internal::COPY_1_BYTE_OFFSET;
80using internal::COPY_2_BYTE_OFFSET;
81using internal::LITERAL;
82using internal::char_table;
83using internal::kMaximumTagLength;
84
85// Any hash function will produce a valid compressed bitstream, but a good
86// hash function reduces the number of collisions and thus yields better
87// compression for compressible input, and more speed for incompressible
88// input. Of course, it doesn't hurt if the hash function is reasonably fast
89// either, as it gets called a lot.
90static inline uint32 HashBytes(uint32 bytes, int shift) {
91 uint32 kMul = 0x1e35a7bd;
92 return (bytes * kMul) >> shift;
93}
94static inline uint32 Hash(const char* p, int shift) {
95 return HashBytes(UNALIGNED_LOAD32(p), shift);
96}
97
98size_t MaxCompressedLength(size_t source_len) {
99 // Compressed data can be defined as:
100 // compressed := item* literal*
101 // item := literal* copy
102 //
103 // The trailing literal sequence has a space blowup of at most 62/60
104 // since a literal of length 60 needs one tag byte + one extra byte
105 // for length information.
106 //
107 // Item blowup is trickier to measure. Suppose the "copy" op copies
108 // 4 bytes of data. Because of a special check in the encoding code,
109 // we produce a 4-byte copy only if the offset is < 65536. Therefore
110 // the copy op takes 3 bytes to encode, and this type of item leads
111 // to at most the 62/60 blowup for representing literals.
112 //
113 // Suppose the "copy" op copies 5 bytes of data. If the offset is big
114 // enough, it will take 5 bytes to encode the copy op. Therefore the
115 // worst case here is a one-byte literal followed by a five-byte copy.
116 // I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
117 //
118 // This last factor dominates the blowup, so the final estimate is:
119 return 32 + source_len + source_len/6;
120}
121
122namespace {
123
124void UnalignedCopy64(const void* src, void* dst) {
125 char tmp[8];
126 memcpy(tmp, src, 8);
127 memcpy(dst, tmp, 8);
128}
129
130void UnalignedCopy128(const void* src, void* dst) {
131 // memcpy gets vectorized when the appropriate compiler options are used.
132 // For example, x86 compilers targeting SSE2+ will optimize to an SSE2 load
133 // and store.
134 char tmp[16];
135 memcpy(tmp, src, 16);
136 memcpy(dst, tmp, 16);
137}
138
139// Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) a byte at a time. Used
140// for handling COPY operations where the input and output regions may overlap.
141// For example, suppose:
142// src == "ab"
143// op == src + 2
144// op_limit == op + 20
145// After IncrementalCopySlow(src, op, op_limit), the result will have eleven
146// copies of "ab"
147// ababababababababababab
148// Note that this does not match the semantics of either memcpy() or memmove().
149inline char* IncrementalCopySlow(const char* src, char* op,
150 char* const op_limit) {
151 // TODO: Remove pragma when LLVM is aware this
152 // function is only called in cold regions and when cold regions don't get
153 // vectorized or unrolled.
154#ifdef __clang__
155#pragma clang loop unroll(disable)
156#endif
157 while (op < op_limit) {
158 *op++ = *src++;
159 }
160 return op_limit;
161}
162
163#if SNAPPY_HAVE_SSSE3
164
165// This is a table of shuffle control masks that can be used as the source
166// operand for PSHUFB to permute the contents of the destination XMM register
167// into a repeating byte pattern.
168alignas(16) const char pshufb_fill_patterns[7][16] = {
169 {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
170 {0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1},
171 {0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0},
172 {0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3},
173 {0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0},
174 {0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3},
175 {0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6, 0, 1},
176};
177
178#endif // SNAPPY_HAVE_SSSE3
179
180// Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) but faster than
181// IncrementalCopySlow. buf_limit is the address past the end of the writable
182// region of the buffer.
183inline char* IncrementalCopy(const char* src, char* op, char* const op_limit,
184 char* const buf_limit) {
185 // Terminology:
186 //
187 // slop = buf_limit - op
188 // pat = op - src
189 // len = limit - op
190 assert(src < op);
191 assert(op <= op_limit);
192 assert(op_limit <= buf_limit);
193 // NOTE: The compressor always emits 4 <= len <= 64. It is ok to assume that
194 // to optimize this function but we have to also handle other cases in case
195 // the input does not satisfy these conditions.
196
197 size_t pattern_size = op - src;
198 // The cases are split into different branches to allow the branch predictor,
199 // FDO, and static prediction hints to work better. For each input we list the
200 // ratio of invocations that match each condition.
201 //
202 // input slop < 16 pat < 8 len > 16
203 // ------------------------------------------
204 // html|html4|cp 0% 1.01% 27.73%
205 // urls 0% 0.88% 14.79%
206 // jpg 0% 64.29% 7.14%
207 // pdf 0% 2.56% 58.06%
208 // txt[1-4] 0% 0.23% 0.97%
209 // pb 0% 0.96% 13.88%
210 // bin 0.01% 22.27% 41.17%
211 //
212 // It is very rare that we don't have enough slop for doing block copies. It
213 // is also rare that we need to expand a pattern. Small patterns are common
214 // for incompressible formats and for those we are plenty fast already.
215 // Lengths are normally not greater than 16 but they vary depending on the
216 // input. In general if we always predict len <= 16 it would be an ok
217 // prediction.
218 //
219 // In order to be fast we want a pattern >= 8 bytes and an unrolled loop
220 // copying 2x 8 bytes at a time.
221
222 // Handle the uncommon case where pattern is less than 8 bytes.
223 if (SNAPPY_PREDICT_FALSE(pattern_size < 8)) {
224#if SNAPPY_HAVE_SSSE3
225 // Load the first eight bytes into an 128-bit XMM register, then use PSHUFB
226 // to permute the register's contents in-place into a repeating sequence of
227 // the first "pattern_size" bytes.
228 // For example, suppose:
229 // src == "abc"
230 // op == op + 3
231 // After _mm_shuffle_epi8(), "pattern" will have five copies of "abc"
232 // followed by one byte of slop: abcabcabcabcabca.
233 //
234 // The non-SSE fallback implementation suffers from store-forwarding stalls
235 // because its loads and stores partly overlap. By expanding the pattern
236 // in-place, we avoid the penalty.
237 if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 16)) {
238 const __m128i shuffle_mask = _mm_load_si128(
239 reinterpret_cast<const __m128i*>(pshufb_fill_patterns)
240 + pattern_size - 1);
241 const __m128i pattern = _mm_shuffle_epi8(
242 _mm_loadl_epi64(reinterpret_cast<const __m128i*>(src)), shuffle_mask);
243 // Uninitialized bytes are masked out by the shuffle mask.
244 // TODO: remove annotation and macro defs once MSan is fixed.
245 SNAPPY_ANNOTATE_MEMORY_IS_INITIALIZED(&pattern, sizeof(pattern));
246 pattern_size *= 16 / pattern_size;
247 char* op_end = std::min(op_limit, buf_limit - 15);
248 while (op < op_end) {
249 _mm_storeu_si128(reinterpret_cast<__m128i*>(op), pattern);
250 op += pattern_size;
251 }
252 if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
253 }
254 return IncrementalCopySlow(src, op, op_limit);
255#else // !SNAPPY_HAVE_SSSE3
256 // If plenty of buffer space remains, expand the pattern to at least 8
257 // bytes. The way the following loop is written, we need 8 bytes of buffer
258 // space if pattern_size >= 4, 11 bytes if pattern_size is 1 or 3, and 10
259 // bytes if pattern_size is 2. Precisely encoding that is probably not
260 // worthwhile; instead, invoke the slow path if we cannot write 11 bytes
261 // (because 11 are required in the worst case).
262 if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 11)) {
263 while (pattern_size < 8) {
264 UnalignedCopy64(src, op);
265 op += pattern_size;
266 pattern_size *= 2;
267 }
268 if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
269 } else {
270 return IncrementalCopySlow(src, op, op_limit);
271 }
272#endif // SNAPPY_HAVE_SSSE3
273 }
274 assert(pattern_size >= 8);
275
276 // Copy 2x 8 bytes at a time. Because op - src can be < 16, a single
277 // UnalignedCopy128 might overwrite data in op. UnalignedCopy64 is safe
278 // because expanding the pattern to at least 8 bytes guarantees that
279 // op - src >= 8.
280 //
281 // Typically, the op_limit is the gating factor so try to simplify the loop
282 // based on that.
283 if (SNAPPY_PREDICT_TRUE(op_limit <= buf_limit - 16)) {
284 // There is at least one, and at most four 16-byte blocks. Writing four
285 // conditionals instead of a loop allows FDO to layout the code with respect
286 // to the actual probabilities of each length.
287 // TODO: Replace with loop with trip count hint.
288 UnalignedCopy64(src, op);
289 UnalignedCopy64(src + 8, op + 8);
290
291 if (op + 16 < op_limit) {
292 UnalignedCopy64(src + 16, op + 16);
293 UnalignedCopy64(src + 24, op + 24);
294 }
295 if (op + 32 < op_limit) {
296 UnalignedCopy64(src + 32, op + 32);
297 UnalignedCopy64(src + 40, op + 40);
298 }
299 if (op + 48 < op_limit) {
300 UnalignedCopy64(src + 48, op + 48);
301 UnalignedCopy64(src + 56, op + 56);
302 }
303 return op_limit;
304 }
305
306 // Fall back to doing as much as we can with the available slop in the
307 // buffer. This code path is relatively cold however so we save code size by
308 // avoiding unrolling and vectorizing.
309 //
310 // TODO: Remove pragma when when cold regions don't get vectorized
311 // or unrolled.
312#ifdef __clang__
313#pragma clang loop unroll(disable)
314#endif
315 for (char *op_end = buf_limit - 16; op < op_end; op += 16, src += 16) {
316 UnalignedCopy64(src, op);
317 UnalignedCopy64(src + 8, op + 8);
318 }
319 if (op >= op_limit)
320 return op_limit;
321
322 // We only take this branch if we didn't have enough slop and we can do a
323 // single 8 byte copy.
324 if (SNAPPY_PREDICT_FALSE(op <= buf_limit - 8)) {
325 UnalignedCopy64(src, op);
326 src += 8;
327 op += 8;
328 }
329 return IncrementalCopySlow(src, op, op_limit);
330}
331
332} // namespace
333
334template <bool allow_fast_path>
335static inline char* EmitLiteral(char* op,
336 const char* literal,
337 int len) {
338 // The vast majority of copies are below 16 bytes, for which a
339 // call to memcpy is overkill. This fast path can sometimes
340 // copy up to 15 bytes too much, but that is okay in the
341 // main loop, since we have a bit to go on for both sides:
342 //
343 // - The input will always have kInputMarginBytes = 15 extra
344 // available bytes, as long as we're in the main loop, and
345 // if not, allow_fast_path = false.
346 // - The output will always have 32 spare bytes (see
347 // MaxCompressedLength).
348 assert(len > 0); // Zero-length literals are disallowed
349 int n = len - 1;
350 if (allow_fast_path && len <= 16) {
351 // Fits in tag byte
352 *op++ = LITERAL | (n << 2);
353
354 UnalignedCopy128(literal, op);
355 return op + len;
356 }
357
358 if (n < 60) {
359 // Fits in tag byte
360 *op++ = LITERAL | (n << 2);
361 } else {
362 int count = (Bits::Log2Floor(n) >> 3) + 1;
363 assert(count >= 1);
364 assert(count <= 4);
365 *op++ = LITERAL | ((59 + count) << 2);
366 // Encode in upcoming bytes.
367 // Write 4 bytes, though we may care about only 1 of them. The output buffer
368 // is guaranteed to have at least 3 more spaces left as 'len >= 61' holds
369 // here and there is a memcpy of size 'len' below.
370 LittleEndian::Store32(op, n);
371 op += count;
372 }
373 memcpy(op, literal, len);
374 return op + len;
375}
376
377template <bool len_less_than_12>
378static inline char* EmitCopyAtMost64(char* op, size_t offset, size_t len) {
379 assert(len <= 64);
380 assert(len >= 4);
381 assert(offset < 65536);
382 assert(len_less_than_12 == (len < 12));
383
384 if (len_less_than_12 && SNAPPY_PREDICT_TRUE(offset < 2048)) {
385 // offset fits in 11 bits. The 3 highest go in the top of the first byte,
386 // and the rest go in the second byte.
387 *op++ = COPY_1_BYTE_OFFSET + ((len - 4) << 2) + ((offset >> 3) & 0xe0);
388 *op++ = offset & 0xff;
389 } else {
390 // Write 4 bytes, though we only care about 3 of them. The output buffer
391 // is required to have some slack, so the extra byte won't overrun it.
392 uint32 u = COPY_2_BYTE_OFFSET + ((len - 1) << 2) + (offset << 8);
393 LittleEndian::Store32(op, u);
394 op += 3;
395 }
396 return op;
397}
398
399template <bool len_less_than_12>
400static inline char* EmitCopy(char* op, size_t offset, size_t len) {
401 assert(len_less_than_12 == (len < 12));
402 if (len_less_than_12) {
403 return EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
404 } else {
405 // A special case for len <= 64 might help, but so far measurements suggest
406 // it's in the noise.
407
408 // Emit 64 byte copies but make sure to keep at least four bytes reserved.
409 while (SNAPPY_PREDICT_FALSE(len >= 68)) {
410 op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 64);
411 len -= 64;
412 }
413
414 // One or two copies will now finish the job.
415 if (len > 64) {
416 op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 60);
417 len -= 60;
418 }
419
420 // Emit remainder.
421 if (len < 12) {
422 op = EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
423 } else {
424 op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, len);
425 }
426 return op;
427 }
428}
429
430bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
431 uint32 v = 0;
432 const char* limit = start + n;
433 if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
434 *result = v;
435 return true;
436 } else {
437 return false;
438 }
439}
440
441namespace {
442uint32 CalculateTableSize(uint32 input_size) {
443 static_assert(
444 kMaxHashTableSize >= kMinHashTableSize,
445 "kMaxHashTableSize should be greater or equal to kMinHashTableSize.");
446 if (input_size > kMaxHashTableSize) {
447 return kMaxHashTableSize;
448 }
449 if (input_size < kMinHashTableSize) {
450 return kMinHashTableSize;
451 }
452 // This is equivalent to Log2Ceiling(input_size), assuming input_size > 1.
453 // 2 << Log2Floor(x - 1) is equivalent to 1 << (1 + Log2Floor(x - 1)).
454 return 2u << Bits::Log2Floor(input_size - 1);
455}
456} // namespace
457
458namespace internal {
459WorkingMemory::WorkingMemory(size_t input_size) {
460 const size_t max_fragment_size = std::min(input_size, kBlockSize);
461 const size_t table_size = CalculateTableSize(max_fragment_size);
462 size_ = table_size * sizeof(*table_) + max_fragment_size +
463 MaxCompressedLength(max_fragment_size);
464 mem_ = std::allocator<char>().allocate(size_);
465 table_ = reinterpret_cast<uint16*>(mem_);
466 input_ = mem_ + table_size * sizeof(*table_);
467 output_ = input_ + max_fragment_size;
468}
469
470WorkingMemory::~WorkingMemory() {
471 std::allocator<char>().deallocate(mem_, size_);
472}
473
474uint16* WorkingMemory::GetHashTable(size_t fragment_size,
475 int* table_size) const {
476 const size_t htsize = CalculateTableSize(fragment_size);
477 memset(table_, 0, htsize * sizeof(*table_));
478 *table_size = htsize;
479 return table_;
480}
481} // end namespace internal
482
483// For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(p), offset) will
484// equal UNALIGNED_LOAD32(p + offset). Motivation: On x86-64 hardware we have
485// empirically found that overlapping loads such as
486// UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
487// are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
488//
489// We have different versions for 64- and 32-bit; ideally we would avoid the
490// two functions and just inline the UNALIGNED_LOAD64 call into
491// GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever
492// enough to avoid loading the value multiple times then. For 64-bit, the load
493// is done when GetEightBytesAt() is called, whereas for 32-bit, the load is
494// done at GetUint32AtOffset() time.
495
496#ifdef ARCH_K8
497
498typedef uint64 EightBytesReference;
499
500static inline EightBytesReference GetEightBytesAt(const char* ptr) {
501 return UNALIGNED_LOAD64(ptr);
502}
503
504static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
505 assert(offset >= 0);
506 assert(offset <= 4);
507 return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
508}
509
510#else
511
512typedef const char* EightBytesReference;
513
514static inline EightBytesReference GetEightBytesAt(const char* ptr) {
515 return ptr;
516}
517
518static inline uint32 GetUint32AtOffset(const char* v, int offset) {
519 assert(offset >= 0);
520 assert(offset <= 4);
521 return UNALIGNED_LOAD32(v + offset);
522}
523
524#endif
525
526// Flat array compression that does not emit the "uncompressed length"
527// prefix. Compresses "input" string to the "*op" buffer.
528//
529// REQUIRES: "input" is at most "kBlockSize" bytes long.
530// REQUIRES: "op" points to an array of memory that is at least
531// "MaxCompressedLength(input.size())" in size.
532// REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
533// REQUIRES: "table_size" is a power of two
534//
535// Returns an "end" pointer into "op" buffer.
536// "end - op" is the compressed size of "input".
537namespace internal {
538char* CompressFragment(const char* input,
539 size_t input_size,
540 char* op,
541 uint16* table,
542 const int table_size) {
543 // "ip" is the input pointer, and "op" is the output pointer.
544 const char* ip = input;
545 assert(input_size <= kBlockSize);
546 assert((table_size & (table_size - 1)) == 0); // table must be power of two
547 const int shift = 32 - Bits::Log2Floor(table_size);
548 assert(static_cast<int>(kuint32max >> shift) == table_size - 1);
549 const char* ip_end = input + input_size;
550 const char* base_ip = ip;
551 // Bytes in [next_emit, ip) will be emitted as literal bytes. Or
552 // [next_emit, ip_end) after the main loop.
553 const char* next_emit = ip;
554
555 const size_t kInputMarginBytes = 15;
556 if (SNAPPY_PREDICT_TRUE(input_size >= kInputMarginBytes)) {
557 const char* ip_limit = input + input_size - kInputMarginBytes;
558
559 for (uint32 next_hash = Hash(++ip, shift); ; ) {
560 assert(next_emit < ip);
561 // The body of this loop calls EmitLiteral once and then EmitCopy one or
562 // more times. (The exception is that when we're close to exhausting
563 // the input we goto emit_remainder.)
564 //
565 // In the first iteration of this loop we're just starting, so
566 // there's nothing to copy, so calling EmitLiteral once is
567 // necessary. And we only start a new iteration when the
568 // current iteration has determined that a call to EmitLiteral will
569 // precede the next call to EmitCopy (if any).
570 //
571 // Step 1: Scan forward in the input looking for a 4-byte-long match.
572 // If we get close to exhausting the input then goto emit_remainder.
573 //
574 // Heuristic match skipping: If 32 bytes are scanned with no matches
575 // found, start looking only at every other byte. If 32 more bytes are
576 // scanned (or skipped), look at every third byte, etc.. When a match is
577 // found, immediately go back to looking at every byte. This is a small
578 // loss (~5% performance, ~0.1% density) for compressible data due to more
579 // bookkeeping, but for non-compressible data (such as JPEG) it's a huge
580 // win since the compressor quickly "realizes" the data is incompressible
581 // and doesn't bother looking for matches everywhere.
582 //
583 // The "skip" variable keeps track of how many bytes there are since the
584 // last match; dividing it by 32 (ie. right-shifting by five) gives the
585 // number of bytes to move ahead for each iteration.
586 uint32 skip = 32;
587
588 const char* next_ip = ip;
589 const char* candidate;
590 do {
591 ip = next_ip;
592 uint32 hash = next_hash;
593 assert(hash == Hash(ip, shift));
594 uint32 bytes_between_hash_lookups = skip >> 5;
595 skip += bytes_between_hash_lookups;
596 next_ip = ip + bytes_between_hash_lookups;
597 if (SNAPPY_PREDICT_FALSE(next_ip > ip_limit)) {
598 goto emit_remainder;
599 }
600 next_hash = Hash(next_ip, shift);
601 candidate = base_ip + table[hash];
602 assert(candidate >= base_ip);
603 assert(candidate < ip);
604
605 table[hash] = ip - base_ip;
606 } while (SNAPPY_PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
607 UNALIGNED_LOAD32(candidate)));
608
609 // Step 2: A 4-byte match has been found. We'll later see if more
610 // than 4 bytes match. But, prior to the match, input
611 // bytes [next_emit, ip) are unmatched. Emit them as "literal bytes."
612 assert(next_emit + 16 <= ip_end);
613 op = EmitLiteral</*allow_fast_path=*/true>(op, next_emit, ip - next_emit);
614
615 // Step 3: Call EmitCopy, and then see if another EmitCopy could
616 // be our next move. Repeat until we find no match for the
617 // input immediately after what was consumed by the last EmitCopy call.
618 //
619 // If we exit this loop normally then we need to call EmitLiteral next,
620 // though we don't yet know how big the literal will be. We handle that
621 // by proceeding to the next iteration of the main loop. We also can exit
622 // this loop via goto if we get close to exhausting the input.
623 EightBytesReference input_bytes;
624 uint32 candidate_bytes = 0;
625
626 do {
627 // We have a 4-byte match at ip, and no need to emit any
628 // "literal bytes" prior to ip.
629 const char* base = ip;
630 std::pair<size_t, bool> p =
631 FindMatchLength(candidate + 4, ip + 4, ip_end);
632 size_t matched = 4 + p.first;
633 ip += matched;
634 size_t offset = base - candidate;
635 assert(0 == memcmp(base, candidate, matched));
636 if (p.second) {
637 op = EmitCopy</*len_less_than_12=*/true>(op, offset, matched);
638 } else {
639 op = EmitCopy</*len_less_than_12=*/false>(op, offset, matched);
640 }
641 next_emit = ip;
642 if (SNAPPY_PREDICT_FALSE(ip >= ip_limit)) {
643 goto emit_remainder;
644 }
645 // We are now looking for a 4-byte match again. We read
646 // table[Hash(ip, shift)] for that. To improve compression,
647 // we also update table[Hash(ip - 1, shift)] and table[Hash(ip, shift)].
648 input_bytes = GetEightBytesAt(ip - 1);
649 uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
650 table[prev_hash] = ip - base_ip - 1;
651 uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
652 candidate = base_ip + table[cur_hash];
653 candidate_bytes = UNALIGNED_LOAD32(candidate);
654 table[cur_hash] = ip - base_ip;
655 } while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
656
657 next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
658 ++ip;
659 }
660 }
661
662 emit_remainder:
663 // Emit the remaining bytes as a literal
664 if (next_emit < ip_end) {
665 op = EmitLiteral</*allow_fast_path=*/false>(op, next_emit,
666 ip_end - next_emit);
667 }
668
669 return op;
670}
671} // end namespace internal
672
673// Called back at avery compression call to trace parameters and sizes.
674static inline void Report(const char *algorithm, size_t compressed_size,
675 size_t uncompressed_size) {}
676
677// Signature of output types needed by decompression code.
678// The decompression code is templatized on a type that obeys this
679// signature so that we do not pay virtual function call overhead in
680// the middle of a tight decompression loop.
681//
682// class DecompressionWriter {
683// public:
684// // Called before decompression
685// void SetExpectedLength(size_t length);
686//
687// // Called after decompression
688// bool CheckLength() const;
689//
690// // Called repeatedly during decompression
691// bool Append(const char* ip, size_t length);
692// bool AppendFromSelf(uint32 offset, size_t length);
693//
694// // The rules for how TryFastAppend differs from Append are somewhat
695// // convoluted:
696// //
697// // - TryFastAppend is allowed to decline (return false) at any
698// // time, for any reason -- just "return false" would be
699// // a perfectly legal implementation of TryFastAppend.
700// // The intention is for TryFastAppend to allow a fast path
701// // in the common case of a small append.
702// // - TryFastAppend is allowed to read up to <available> bytes
703// // from the input buffer, whereas Append is allowed to read
704// // <length>. However, if it returns true, it must leave
705// // at least five (kMaximumTagLength) bytes in the input buffer
706// // afterwards, so that there is always enough space to read the
707// // next tag without checking for a refill.
708// // - TryFastAppend must always return decline (return false)
709// // if <length> is 61 or more, as in this case the literal length is not
710// // decoded fully. In practice, this should not be a big problem,
711// // as it is unlikely that one would implement a fast path accepting
712// // this much data.
713// //
714// bool TryFastAppend(const char* ip, size_t available, size_t length);
715// };
716
717static inline uint32 ExtractLowBytes(uint32 v, int n) {
718 assert(n >= 0);
719 assert(n <= 4);
720#if SNAPPY_HAVE_BMI2
721 return _bzhi_u32(v, 8 * n);
722#else
723 // This needs to be wider than uint32 otherwise `mask << 32` will be
724 // undefined.
725 uint64 mask = 0xffffffff;
726 return v & ~(mask << (8 * n));
727#endif
728}
729
730static inline bool LeftShiftOverflows(uint8 value, uint32 shift) {
731 assert(shift < 32);
732 static const uint8 masks[] = {
733 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
734 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
735 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
736 0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe};
737 return (value & masks[shift]) != 0;
738}
739
740// Helper class for decompression
741class SnappyDecompressor {
742 private:
743 Source* reader_; // Underlying source of bytes to decompress
744 const char* ip_; // Points to next buffered byte
745 const char* ip_limit_; // Points just past buffered bytes
746 uint32 peeked_; // Bytes peeked from reader (need to skip)
747 bool eof_; // Hit end of input without an error?
748 char scratch_[kMaximumTagLength]; // See RefillTag().
749
750 // Ensure that all of the tag metadata for the next tag is available
751 // in [ip_..ip_limit_-1]. Also ensures that [ip,ip+4] is readable even
752 // if (ip_limit_ - ip_ < 5).
753 //
754 // Returns true on success, false on error or end of input.
755 bool RefillTag();
756
757 public:
758 explicit SnappyDecompressor(Source* reader)
759 : reader_(reader),
760 ip_(NULL),
761 ip_limit_(NULL),
762 peeked_(0),
763 eof_(false) {
764 }
765
766 ~SnappyDecompressor() {
767 // Advance past any bytes we peeked at from the reader
768 reader_->Skip(peeked_);
769 }
770
771 // Returns true iff we have hit the end of the input without an error.
772 bool eof() const {
773 return eof_;
774 }
775
776 // Read the uncompressed length stored at the start of the compressed data.
777 // On success, stores the length in *result and returns true.
778 // On failure, returns false.
779 bool ReadUncompressedLength(uint32* result) {
780 assert(ip_ == NULL); // Must not have read anything yet
781 // Length is encoded in 1..5 bytes
782 *result = 0;
783 uint32 shift = 0;
784 while (true) {
785 if (shift >= 32) return false;
786 size_t n;
787 const char* ip = reader_->Peek(&n);
788 if (n == 0) return false;
789 const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
790 reader_->Skip(1);
791 uint32 val = c & 0x7f;
792 if (LeftShiftOverflows(static_cast<uint8>(val), shift)) return false;
793 *result |= val << shift;
794 if (c < 128) {
795 break;
796 }
797 shift += 7;
798 }
799 return true;
800 }
801
802 // Process the next item found in the input.
803 // Returns true if successful, false on error or end of input.
804 template <class Writer>
805#if defined(__GNUC__) && defined(__x86_64__)
806 __attribute__((aligned(32)))
807#endif
808 void DecompressAllTags(Writer* writer) {
809 // In x86, pad the function body to start 16 bytes later. This function has
810 // a couple of hotspots that are highly sensitive to alignment: we have
811 // observed regressions by more than 20% in some metrics just by moving the
812 // exact same code to a different position in the benchmark binary.
813 //
814 // Putting this code on a 32-byte-aligned boundary + 16 bytes makes us hit
815 // the "lucky" case consistently. Unfortunately, this is a very brittle
816 // workaround, and future differences in code generation may reintroduce
817 // this regression. If you experience a big, difficult to explain, benchmark
818 // performance regression here, first try removing this hack.
819#if defined(__GNUC__) && defined(__x86_64__)
820 // Two 8-byte "NOP DWORD ptr [EAX + EAX*1 + 00000000H]" instructions.
821 asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
822 asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
823#endif
824
825 const char* ip = ip_;
826 // We could have put this refill fragment only at the beginning of the loop.
827 // However, duplicating it at the end of each branch gives the compiler more
828 // scope to optimize the <ip_limit_ - ip> expression based on the local
829 // context, which overall increases speed.
830 #define MAYBE_REFILL() \
831 if (ip_limit_ - ip < kMaximumTagLength) { \
832 ip_ = ip; \
833 if (!RefillTag()) return; \
834 ip = ip_; \
835 }
836
837 MAYBE_REFILL();
838 for ( ;; ) {
839 const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
840
841 // Ratio of iterations that have LITERAL vs non-LITERAL for different
842 // inputs.
843 //
844 // input LITERAL NON_LITERAL
845 // -----------------------------------
846 // html|html4|cp 23% 77%
847 // urls 36% 64%
848 // jpg 47% 53%
849 // pdf 19% 81%
850 // txt[1-4] 25% 75%
851 // pb 24% 76%
852 // bin 24% 76%
853 if (SNAPPY_PREDICT_FALSE((c & 0x3) == LITERAL)) {
854 size_t literal_length = (c >> 2) + 1u;
855 if (writer->TryFastAppend(ip, ip_limit_ - ip, literal_length)) {
856 assert(literal_length < 61);
857 ip += literal_length;
858 // NOTE: There is no MAYBE_REFILL() here, as TryFastAppend()
859 // will not return true unless there's already at least five spare
860 // bytes in addition to the literal.
861 continue;
862 }
863 if (SNAPPY_PREDICT_FALSE(literal_length >= 61)) {
864 // Long literal.
865 const size_t literal_length_length = literal_length - 60;
866 literal_length =
867 ExtractLowBytes(LittleEndian::Load32(ip), literal_length_length) +
868 1;
869 ip += literal_length_length;
870 }
871
872 size_t avail = ip_limit_ - ip;
873 while (avail < literal_length) {
874 if (!writer->Append(ip, avail)) return;
875 literal_length -= avail;
876 reader_->Skip(peeked_);
877 size_t n;
878 ip = reader_->Peek(&n);
879 avail = n;
880 peeked_ = avail;
881 if (avail == 0) return; // Premature end of input
882 ip_limit_ = ip + avail;
883 }
884 if (!writer->Append(ip, literal_length)) {
885 return;
886 }
887 ip += literal_length;
888 MAYBE_REFILL();
889 } else {
890 const size_t entry = char_table[c];
891 const size_t trailer =
892 ExtractLowBytes(LittleEndian::Load32(ip), entry >> 11);
893 const size_t length = entry & 0xff;
894 ip += entry >> 11;
895
896 // copy_offset/256 is encoded in bits 8..10. By just fetching
897 // those bits, we get copy_offset (since the bit-field starts at
898 // bit 8).
899 const size_t copy_offset = entry & 0x700;
900 if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
901 return;
902 }
903 MAYBE_REFILL();
904 }
905 }
906
907#undef MAYBE_REFILL
908 }
909};
910
911bool SnappyDecompressor::RefillTag() {
912 const char* ip = ip_;
913 if (ip == ip_limit_) {
914 // Fetch a new fragment from the reader
915 reader_->Skip(peeked_); // All peeked bytes are used up
916 size_t n;
917 ip = reader_->Peek(&n);
918 peeked_ = n;
919 eof_ = (n == 0);
920 if (eof_) return false;
921 ip_limit_ = ip + n;
922 }
923
924 // Read the tag character
925 assert(ip < ip_limit_);
926 const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
927 const uint32 entry = char_table[c];
928 const uint32 needed = (entry >> 11) + 1; // +1 byte for 'c'
929 assert(needed <= sizeof(scratch_));
930
931 // Read more bytes from reader if needed
932 uint32 nbuf = ip_limit_ - ip;
933 if (nbuf < needed) {
934 // Stitch together bytes from ip and reader to form the word
935 // contents. We store the needed bytes in "scratch_". They
936 // will be consumed immediately by the caller since we do not
937 // read more than we need.
938 memmove(scratch_, ip, nbuf);
939 reader_->Skip(peeked_); // All peeked bytes are used up
940 peeked_ = 0;
941 while (nbuf < needed) {
942 size_t length;
943 const char* src = reader_->Peek(&length);
944 if (length == 0) return false;
945 uint32 to_add = std::min<uint32>(needed - nbuf, length);
946 memcpy(scratch_ + nbuf, src, to_add);
947 nbuf += to_add;
948 reader_->Skip(to_add);
949 }
950 assert(nbuf == needed);
951 ip_ = scratch_;
952 ip_limit_ = scratch_ + needed;
953 } else if (nbuf < kMaximumTagLength) {
954 // Have enough bytes, but move into scratch_ so that we do not
955 // read past end of input
956 memmove(scratch_, ip, nbuf);
957 reader_->Skip(peeked_); // All peeked bytes are used up
958 peeked_ = 0;
959 ip_ = scratch_;
960 ip_limit_ = scratch_ + nbuf;
961 } else {
962 // Pass pointer to buffer returned by reader_.
963 ip_ = ip;
964 }
965 return true;
966}
967
968template <typename Writer>
969static bool InternalUncompress(Source* r, Writer* writer) {
970 // Read the uncompressed length from the front of the compressed input
971 SnappyDecompressor decompressor(r);
972 uint32 uncompressed_len = 0;
973 if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
974
975 return InternalUncompressAllTags(&decompressor, writer, r->Available(),
976 uncompressed_len);
977}
978
979template <typename Writer>
980static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
981 Writer* writer,
982 uint32 compressed_len,
983 uint32 uncompressed_len) {
984 Report("snappy_uncompress", compressed_len, uncompressed_len);
985
986 writer->SetExpectedLength(uncompressed_len);
987
988 // Process the entire input
989 decompressor->DecompressAllTags(writer);
990 writer->Flush();
991 return (decompressor->eof() && writer->CheckLength());
992}
993
994bool GetUncompressedLength(Source* source, uint32* result) {
995 SnappyDecompressor decompressor(source);
996 return decompressor.ReadUncompressedLength(result);
997}
998
999size_t Compress(Source* reader, Sink* writer) {
1000 size_t written = 0;
1001 size_t N = reader->Available();
1002 const size_t uncompressed_size = N;
1003 char ulength[Varint::kMax32];
1004 char* p = Varint::Encode32(ulength, N);
1005 writer->Append(ulength, p-ulength);
1006 written += (p - ulength);
1007
1008 internal::WorkingMemory wmem(N);
1009
1010 while (N > 0) {
1011 // Get next block to compress (without copying if possible)
1012 size_t fragment_size;
1013 const char* fragment = reader->Peek(&fragment_size);
1014 assert(fragment_size != 0); // premature end of input
1015 const size_t num_to_read = std::min(N, kBlockSize);
1016 size_t bytes_read = fragment_size;
1017
1018 size_t pending_advance = 0;
1019 if (bytes_read >= num_to_read) {
1020 // Buffer returned by reader is large enough
1021 pending_advance = num_to_read;
1022 fragment_size = num_to_read;
1023 } else {
1024 char* scratch = wmem.GetScratchInput();
1025 memcpy(scratch, fragment, bytes_read);
1026 reader->Skip(bytes_read);
1027
1028 while (bytes_read < num_to_read) {
1029 fragment = reader->Peek(&fragment_size);
1030 size_t n = std::min<size_t>(fragment_size, num_to_read - bytes_read);
1031 memcpy(scratch + bytes_read, fragment, n);
1032 bytes_read += n;
1033 reader->Skip(n);
1034 }
1035 assert(bytes_read == num_to_read);
1036 fragment = scratch;
1037 fragment_size = num_to_read;
1038 }
1039 assert(fragment_size == num_to_read);
1040
1041 // Get encoding table for compression
1042 int table_size;
1043 uint16* table = wmem.GetHashTable(num_to_read, &table_size);
1044
1045 // Compress input_fragment and append to dest
1046 const int max_output = MaxCompressedLength(num_to_read);
1047
1048 // Need a scratch buffer for the output, in case the byte sink doesn't
1049 // have room for us directly.
1050
1051 // Since we encode kBlockSize regions followed by a region
1052 // which is <= kBlockSize in length, a previously allocated
1053 // scratch_output[] region is big enough for this iteration.
1054 char* dest = writer->GetAppendBuffer(max_output, wmem.GetScratchOutput());
1055 char* end = internal::CompressFragment(fragment, fragment_size, dest, table,
1056 table_size);
1057 writer->Append(dest, end - dest);
1058 written += (end - dest);
1059
1060 N -= num_to_read;
1061 reader->Skip(pending_advance);
1062 }
1063
1064 Report("snappy_compress", written, uncompressed_size);
1065
1066 return written;
1067}
1068
1069// -----------------------------------------------------------------------
1070// IOVec interfaces
1071// -----------------------------------------------------------------------
1072
1073// A type that writes to an iovec.
1074// Note that this is not a "ByteSink", but a type that matches the
1075// Writer template argument to SnappyDecompressor::DecompressAllTags().
1076class SnappyIOVecWriter {
1077 private:
1078 // output_iov_end_ is set to iov + count and used to determine when
1079 // the end of the iovs is reached.
1080 const struct iovec* output_iov_end_;
1081
1082#if !defined(NDEBUG)
1083 const struct iovec* output_iov_;
1084#endif // !defined(NDEBUG)
1085
1086 // Current iov that is being written into.
1087 const struct iovec* curr_iov_;
1088
1089 // Pointer to current iov's write location.
1090 char* curr_iov_output_;
1091
1092 // Remaining bytes to write into curr_iov_output.
1093 size_t curr_iov_remaining_;
1094
1095 // Total bytes decompressed into output_iov_ so far.
1096 size_t total_written_;
1097
1098 // Maximum number of bytes that will be decompressed into output_iov_.
1099 size_t output_limit_;
1100
1101 static inline char* GetIOVecPointer(const struct iovec* iov, size_t offset) {
1102 return reinterpret_cast<char*>(iov->iov_base) + offset;
1103 }
1104
1105 public:
1106 // Does not take ownership of iov. iov must be valid during the
1107 // entire lifetime of the SnappyIOVecWriter.
1108 inline SnappyIOVecWriter(const struct iovec* iov, size_t iov_count)
1109 : output_iov_end_(iov + iov_count),
1110#if !defined(NDEBUG)
1111 output_iov_(iov),
1112#endif // !defined(NDEBUG)
1113 curr_iov_(iov),
1114 curr_iov_output_(iov_count ? reinterpret_cast<char*>(iov->iov_base)
1115 : nullptr),
1116 curr_iov_remaining_(iov_count ? iov->iov_len : 0),
1117 total_written_(0),
1118 output_limit_(-1) {}
1119
1120 inline void SetExpectedLength(size_t len) {
1121 output_limit_ = len;
1122 }
1123
1124 inline bool CheckLength() const {
1125 return total_written_ == output_limit_;
1126 }
1127
1128 inline bool Append(const char* ip, size_t len) {
1129 if (total_written_ + len > output_limit_) {
1130 return false;
1131 }
1132
1133 return AppendNoCheck(ip, len);
1134 }
1135
1136 inline bool AppendNoCheck(const char* ip, size_t len) {
1137 while (len > 0) {
1138 if (curr_iov_remaining_ == 0) {
1139 // This iovec is full. Go to the next one.
1140 if (curr_iov_ + 1 >= output_iov_end_) {
1141 return false;
1142 }
1143 ++curr_iov_;
1144 curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
1145 curr_iov_remaining_ = curr_iov_->iov_len;
1146 }
1147
1148 const size_t to_write = std::min(len, curr_iov_remaining_);
1149 memcpy(curr_iov_output_, ip, to_write);
1150 curr_iov_output_ += to_write;
1151 curr_iov_remaining_ -= to_write;
1152 total_written_ += to_write;
1153 ip += to_write;
1154 len -= to_write;
1155 }
1156
1157 return true;
1158 }
1159
1160 inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
1161 const size_t space_left = output_limit_ - total_written_;
1162 if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16 &&
1163 curr_iov_remaining_ >= 16) {
1164 // Fast path, used for the majority (about 95%) of invocations.
1165 UnalignedCopy128(ip, curr_iov_output_);
1166 curr_iov_output_ += len;
1167 curr_iov_remaining_ -= len;
1168 total_written_ += len;
1169 return true;
1170 }
1171
1172 return false;
1173 }
1174
1175 inline bool AppendFromSelf(size_t offset, size_t len) {
1176 // See SnappyArrayWriter::AppendFromSelf for an explanation of
1177 // the "offset - 1u" trick.
1178 if (offset - 1u >= total_written_) {
1179 return false;
1180 }
1181 const size_t space_left = output_limit_ - total_written_;
1182 if (len > space_left) {
1183 return false;
1184 }
1185
1186 // Locate the iovec from which we need to start the copy.
1187 const iovec* from_iov = curr_iov_;
1188 size_t from_iov_offset = curr_iov_->iov_len - curr_iov_remaining_;
1189 while (offset > 0) {
1190 if (from_iov_offset >= offset) {
1191 from_iov_offset -= offset;
1192 break;
1193 }
1194
1195 offset -= from_iov_offset;
1196 --from_iov;
1197#if !defined(NDEBUG)
1198 assert(from_iov >= output_iov_);
1199#endif // !defined(NDEBUG)
1200 from_iov_offset = from_iov->iov_len;
1201 }
1202
1203 // Copy <len> bytes starting from the iovec pointed to by from_iov_index to
1204 // the current iovec.
1205 while (len > 0) {
1206 assert(from_iov <= curr_iov_);
1207 if (from_iov != curr_iov_) {
1208 const size_t to_copy =
1209 std::min(from_iov->iov_len - from_iov_offset, len);
1210 AppendNoCheck(GetIOVecPointer(from_iov, from_iov_offset), to_copy);
1211 len -= to_copy;
1212 if (len > 0) {
1213 ++from_iov;
1214 from_iov_offset = 0;
1215 }
1216 } else {
1217 size_t to_copy = curr_iov_remaining_;
1218 if (to_copy == 0) {
1219 // This iovec is full. Go to the next one.
1220 if (curr_iov_ + 1 >= output_iov_end_) {
1221 return false;
1222 }
1223 ++curr_iov_;
1224 curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
1225 curr_iov_remaining_ = curr_iov_->iov_len;
1226 continue;
1227 }
1228 if (to_copy > len) {
1229 to_copy = len;
1230 }
1231
1232 IncrementalCopy(GetIOVecPointer(from_iov, from_iov_offset),
1233 curr_iov_output_, curr_iov_output_ + to_copy,
1234 curr_iov_output_ + curr_iov_remaining_);
1235 curr_iov_output_ += to_copy;
1236 curr_iov_remaining_ -= to_copy;
1237 from_iov_offset += to_copy;
1238 total_written_ += to_copy;
1239 len -= to_copy;
1240 }
1241 }
1242
1243 return true;
1244 }
1245
1246 inline void Flush() {}
1247};
1248
1249bool RawUncompressToIOVec(const char* compressed, size_t compressed_length,
1250 const struct iovec* iov, size_t iov_cnt) {
1251 ByteArraySource reader(compressed, compressed_length);
1252 return RawUncompressToIOVec(&reader, iov, iov_cnt);
1253}
1254
1255bool RawUncompressToIOVec(Source* compressed, const struct iovec* iov,
1256 size_t iov_cnt) {
1257 SnappyIOVecWriter output(iov, iov_cnt);
1258 return InternalUncompress(compressed, &output);
1259}
1260
1261// -----------------------------------------------------------------------
1262// Flat array interfaces
1263// -----------------------------------------------------------------------
1264
1265// A type that writes to a flat array.
1266// Note that this is not a "ByteSink", but a type that matches the
1267// Writer template argument to SnappyDecompressor::DecompressAllTags().
1268class SnappyArrayWriter {
1269 private:
1270 char* base_;
1271 char* op_;
1272 char* op_limit_;
1273
1274 public:
1275 inline explicit SnappyArrayWriter(char* dst)
1276 : base_(dst),
1277 op_(dst),
1278 op_limit_(dst) {
1279 }
1280
1281 inline void SetExpectedLength(size_t len) {
1282 op_limit_ = op_ + len;
1283 }
1284
1285 inline bool CheckLength() const {
1286 return op_ == op_limit_;
1287 }
1288
1289 inline bool Append(const char* ip, size_t len) {
1290 char* op = op_;
1291 const size_t space_left = op_limit_ - op;
1292 if (space_left < len) {
1293 return false;
1294 }
1295 memcpy(op, ip, len);
1296 op_ = op + len;
1297 return true;
1298 }
1299
1300 inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
1301 char* op = op_;
1302 const size_t space_left = op_limit_ - op;
1303 if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16) {
1304 // Fast path, used for the majority (about 95%) of invocations.
1305 UnalignedCopy128(ip, op);
1306 op_ = op + len;
1307 return true;
1308 } else {
1309 return false;
1310 }
1311 }
1312
1313 inline bool AppendFromSelf(size_t offset, size_t len) {
1314 char* const op_end = op_ + len;
1315
1316 // Check if we try to append from before the start of the buffer.
1317 // Normally this would just be a check for "produced < offset",
1318 // but "produced <= offset - 1u" is equivalent for every case
1319 // except the one where offset==0, where the right side will wrap around
1320 // to a very big number. This is convenient, as offset==0 is another
1321 // invalid case that we also want to catch, so that we do not go
1322 // into an infinite loop.
1323 if (Produced() <= offset - 1u || op_end > op_limit_) return false;
1324 op_ = IncrementalCopy(op_ - offset, op_, op_end, op_limit_);
1325
1326 return true;
1327 }
1328 inline size_t Produced() const {
1329 assert(op_ >= base_);
1330 return op_ - base_;
1331 }
1332 inline void Flush() {}
1333};
1334
1335bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
1336 ByteArraySource reader(compressed, n);
1337 return RawUncompress(&reader, uncompressed);
1338}
1339
1340bool RawUncompress(Source* compressed, char* uncompressed) {
1341 SnappyArrayWriter output(uncompressed);
1342 return InternalUncompress(compressed, &output);
1343}
1344
1345bool Uncompress(const char* compressed, size_t n, std::string* uncompressed) {
1346 size_t ulength;
1347 if (!GetUncompressedLength(compressed, n, &ulength)) {
1348 return false;
1349 }
1350 // On 32-bit builds: max_size() < kuint32max. Check for that instead
1351 // of crashing (e.g., consider externally specified compressed data).
1352 if (ulength > uncompressed->max_size()) {
1353 return false;
1354 }
1355 STLStringResizeUninitialized(uncompressed, ulength);
1356 return RawUncompress(compressed, n, string_as_array(uncompressed));
1357}
1358
1359// A Writer that drops everything on the floor and just does validation
1360class SnappyDecompressionValidator {
1361 private:
1362 size_t expected_;
1363 size_t produced_;
1364
1365 public:
1366 inline SnappyDecompressionValidator() : expected_(0), produced_(0) { }
1367 inline void SetExpectedLength(size_t len) {
1368 expected_ = len;
1369 }
1370 inline bool CheckLength() const {
1371 return expected_ == produced_;
1372 }
1373 inline bool Append(const char* ip, size_t len) {
1374 produced_ += len;
1375 return produced_ <= expected_;
1376 }
1377 inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
1378 return false;
1379 }
1380 inline bool AppendFromSelf(size_t offset, size_t len) {
1381 // See SnappyArrayWriter::AppendFromSelf for an explanation of
1382 // the "offset - 1u" trick.
1383 if (produced_ <= offset - 1u) return false;
1384 produced_ += len;
1385 return produced_ <= expected_;
1386 }
1387 inline void Flush() {}
1388};
1389
1390bool IsValidCompressedBuffer(const char* compressed, size_t n) {
1391 ByteArraySource reader(compressed, n);
1392 SnappyDecompressionValidator writer;
1393 return InternalUncompress(&reader, &writer);
1394}
1395
1396bool IsValidCompressed(Source* compressed) {
1397 SnappyDecompressionValidator writer;
1398 return InternalUncompress(compressed, &writer);
1399}
1400
1401void RawCompress(const char* input,
1402 size_t input_length,
1403 char* compressed,
1404 size_t* compressed_length) {
1405 ByteArraySource reader(input, input_length);
1406 UncheckedByteArraySink writer(compressed);
1407 Compress(&reader, &writer);
1408
1409 // Compute how many bytes were added
1410 *compressed_length = (writer.CurrentDestination() - compressed);
1411}
1412
1413size_t Compress(const char* input, size_t input_length,
1414 std::string* compressed) {
1415 // Pre-grow the buffer to the max length of the compressed output
1416 STLStringResizeUninitialized(compressed, MaxCompressedLength(input_length));
1417
1418 size_t compressed_length;
1419 RawCompress(input, input_length, string_as_array(compressed),
1420 &compressed_length);
1421 compressed->resize(compressed_length);
1422 return compressed_length;
1423}
1424
1425// -----------------------------------------------------------------------
1426// Sink interface
1427// -----------------------------------------------------------------------
1428
1429// A type that decompresses into a Sink. The template parameter
1430// Allocator must export one method "char* Allocate(int size);", which
1431// allocates a buffer of "size" and appends that to the destination.
1432template <typename Allocator>
1433class SnappyScatteredWriter {
1434 Allocator allocator_;
1435
1436 // We need random access into the data generated so far. Therefore
1437 // we keep track of all of the generated data as an array of blocks.
1438 // All of the blocks except the last have length kBlockSize.
1439 std::vector<char*> blocks_;
1440 size_t expected_;
1441
1442 // Total size of all fully generated blocks so far
1443 size_t full_size_;
1444
1445 // Pointer into current output block
1446 char* op_base_; // Base of output block
1447 char* op_ptr_; // Pointer to next unfilled byte in block
1448 char* op_limit_; // Pointer just past block
1449
1450 inline size_t Size() const {
1451 return full_size_ + (op_ptr_ - op_base_);
1452 }
1453
1454 bool SlowAppend(const char* ip, size_t len);
1455 bool SlowAppendFromSelf(size_t offset, size_t len);
1456
1457 public:
1458 inline explicit SnappyScatteredWriter(const Allocator& allocator)
1459 : allocator_(allocator),
1460 full_size_(0),
1461 op_base_(NULL),
1462 op_ptr_(NULL),
1463 op_limit_(NULL) {
1464 }
1465
1466 inline void SetExpectedLength(size_t len) {
1467 assert(blocks_.empty());
1468 expected_ = len;
1469 }
1470
1471 inline bool CheckLength() const {
1472 return Size() == expected_;
1473 }
1474
1475 // Return the number of bytes actually uncompressed so far
1476 inline size_t Produced() const {
1477 return Size();
1478 }
1479
1480 inline bool Append(const char* ip, size_t len) {
1481 size_t avail = op_limit_ - op_ptr_;
1482 if (len <= avail) {
1483 // Fast path
1484 memcpy(op_ptr_, ip, len);
1485 op_ptr_ += len;
1486 return true;
1487 } else {
1488 return SlowAppend(ip, len);
1489 }
1490 }
1491
1492 inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
1493 char* op = op_ptr_;
1494 const int space_left = op_limit_ - op;
1495 if (length <= 16 && available >= 16 + kMaximumTagLength &&
1496 space_left >= 16) {
1497 // Fast path, used for the majority (about 95%) of invocations.
1498 UnalignedCopy128(ip, op);
1499 op_ptr_ = op + length;
1500 return true;
1501 } else {
1502 return false;
1503 }
1504 }
1505
1506 inline bool AppendFromSelf(size_t offset, size_t len) {
1507 char* const op_end = op_ptr_ + len;
1508 // See SnappyArrayWriter::AppendFromSelf for an explanation of
1509 // the "offset - 1u" trick.
1510 if (SNAPPY_PREDICT_TRUE(offset - 1u < op_ptr_ - op_base_ &&
1511 op_end <= op_limit_)) {
1512 // Fast path: src and dst in current block.
1513 op_ptr_ = IncrementalCopy(op_ptr_ - offset, op_ptr_, op_end, op_limit_);
1514 return true;
1515 }
1516 return SlowAppendFromSelf(offset, len);
1517 }
1518
1519 // Called at the end of the decompress. We ask the allocator
1520 // write all blocks to the sink.
1521 inline void Flush() { allocator_.Flush(Produced()); }
1522};
1523
1524template<typename Allocator>
1525bool SnappyScatteredWriter<Allocator>::SlowAppend(const char* ip, size_t len) {
1526 size_t avail = op_limit_ - op_ptr_;
1527 while (len > avail) {
1528 // Completely fill this block
1529 memcpy(op_ptr_, ip, avail);
1530 op_ptr_ += avail;
1531 assert(op_limit_ - op_ptr_ == 0);
1532 full_size_ += (op_ptr_ - op_base_);
1533 len -= avail;
1534 ip += avail;
1535
1536 // Bounds check
1537 if (full_size_ + len > expected_) {
1538 return false;
1539 }
1540
1541 // Make new block
1542 size_t bsize = std::min<size_t>(kBlockSize, expected_ - full_size_);
1543 op_base_ = allocator_.Allocate(bsize);
1544 op_ptr_ = op_base_;
1545 op_limit_ = op_base_ + bsize;
1546 blocks_.push_back(op_base_);
1547 avail = bsize;
1548 }
1549
1550 memcpy(op_ptr_, ip, len);
1551 op_ptr_ += len;
1552 return true;
1553}
1554
1555template<typename Allocator>
1556bool SnappyScatteredWriter<Allocator>::SlowAppendFromSelf(size_t offset,
1557 size_t len) {
1558 // Overflow check
1559 // See SnappyArrayWriter::AppendFromSelf for an explanation of
1560 // the "offset - 1u" trick.
1561 const size_t cur = Size();
1562 if (offset - 1u >= cur) return false;
1563 if (expected_ - cur < len) return false;
1564
1565 // Currently we shouldn't ever hit this path because Compress() chops the
1566 // input into blocks and does not create cross-block copies. However, it is
1567 // nice if we do not rely on that, since we can get better compression if we
1568 // allow cross-block copies and thus might want to change the compressor in
1569 // the future.
1570 size_t src = cur - offset;
1571 while (len-- > 0) {
1572 char c = blocks_[src >> kBlockLog][src & (kBlockSize-1)];
1573 Append(&c, 1);
1574 src++;
1575 }
1576 return true;
1577}
1578
1579class SnappySinkAllocator {
1580 public:
1581 explicit SnappySinkAllocator(Sink* dest): dest_(dest) {}
1582 ~SnappySinkAllocator() {}
1583
1584 char* Allocate(int size) {
1585 Datablock block(new char[size], size);
1586 blocks_.push_back(block);
1587 return block.data;
1588 }
1589
1590 // We flush only at the end, because the writer wants
1591 // random access to the blocks and once we hand the
1592 // block over to the sink, we can't access it anymore.
1593 // Also we don't write more than has been actually written
1594 // to the blocks.
1595 void Flush(size_t size) {
1596 size_t size_written = 0;
1597 size_t block_size;
1598 for (int i = 0; i < blocks_.size(); ++i) {
1599 block_size = std::min<size_t>(blocks_[i].size, size - size_written);
1600 dest_->AppendAndTakeOwnership(blocks_[i].data, block_size,
1601 &SnappySinkAllocator::Deleter, NULL);
1602 size_written += block_size;
1603 }
1604 blocks_.clear();
1605 }
1606
1607 private:
1608 struct Datablock {
1609 char* data;
1610 size_t size;
1611 Datablock(char* p, size_t s) : data(p), size(s) {}
1612 };
1613
1614 static void Deleter(void* arg, const char* bytes, size_t size) {
1615 delete[] bytes;
1616 }
1617
1618 Sink* dest_;
1619 std::vector<Datablock> blocks_;
1620
1621 // Note: copying this object is allowed
1622};
1623
1624size_t UncompressAsMuchAsPossible(Source* compressed, Sink* uncompressed) {
1625 SnappySinkAllocator allocator(uncompressed);
1626 SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
1627 InternalUncompress(compressed, &writer);
1628 return writer.Produced();
1629}
1630
1631bool Uncompress(Source* compressed, Sink* uncompressed) {
1632 // Read the uncompressed length from the front of the compressed input
1633 SnappyDecompressor decompressor(compressed);
1634 uint32 uncompressed_len = 0;
1635 if (!decompressor.ReadUncompressedLength(&uncompressed_len)) {
1636 return false;
1637 }
1638
1639 char c;
1640 size_t allocated_size;
1641 char* buf = uncompressed->GetAppendBufferVariable(
1642 1, uncompressed_len, &c, 1, &allocated_size);
1643
1644 const size_t compressed_len = compressed->Available();
1645 // If we can get a flat buffer, then use it, otherwise do block by block
1646 // uncompression
1647 if (allocated_size >= uncompressed_len) {
1648 SnappyArrayWriter writer(buf);
1649 bool result = InternalUncompressAllTags(&decompressor, &writer,
1650 compressed_len, uncompressed_len);
1651 uncompressed->Append(buf, writer.Produced());
1652 return result;
1653 } else {
1654 SnappySinkAllocator allocator(uncompressed);
1655 SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
1656 return InternalUncompressAllTags(&decompressor, &writer, compressed_len,
1657 uncompressed_len);
1658 }
1659}
1660
1661} // namespace snappy
1662