dolysis/snappy
3
0
Fork 0
mirror of https://github.com/google/snappy.git synced 2024-11-28 05:44:33 +00:00
Code Issues Packages Projects Releases Wiki Activity
snappy/snappy.cc

1688 lines
59 KiB
C++
Raw Blame History

// Copyright 2005 Google Inc. All Rights Reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "snappy.h"
#include "snappy-internal.h"
#include "snappy-sinksource.h"
#if !defined(SNAPPY_HAVE_SSSE3)
// __SSSE3__ is defined by GCC and Clang. Visual Studio doesn't target SIMD
// support between SSE2 and AVX (so SSSE3 instructions require AVX support), and
// defines __AVX__ when AVX support is available.
#if defined(__SSSE3__) || defined(__AVX__)
#define SNAPPY_HAVE_SSSE3 1
#else
#define SNAPPY_HAVE_SSSE3 0
#endif
#endif // !defined(SNAPPY_HAVE_SSSE3)
#if !defined(SNAPPY_HAVE_BMI2)
// __BMI2__ is defined by GCC and Clang. Visual Studio doesn't target BMI2
// specifically, but it does define __AVX2__ when AVX2 support is available.
// Fortunately, AVX2 was introduced in Haswell, just like BMI2.
//
// BMI2 is not defined as a subset of AVX2 (unlike SSSE3 and AVX above). So,
// GCC and Clang can build code with AVX2 enabled but BMI2 disabled, in which
// case issuing BMI2 instructions results in a compiler error.
#if defined(__BMI2__) || (defined(_MSC_VER) && defined(__AVX2__))
#define SNAPPY_HAVE_BMI2 1
#else
#define SNAPPY_HAVE_BMI2 0
#endif
#endif // !defined(SNAPPY_HAVE_BMI2)
#if SNAPPY_HAVE_SSSE3
// Please do not replace with <x86intrin.h>. or with headers that assume more
// advanced SSE versions without checking with all the OWNERS.
#include <tmmintrin.h>
#endif
#if SNAPPY_HAVE_BMI2
// Please do not replace with <x86intrin.h>. or with headers that assume more
// advanced SSE versions without checking with all the OWNERS.
#include <immintrin.h>
#endif
#include <stdio.h>
#include <algorithm>
#include <string>
#include <vector>
namespace snappy {
using internal::char_table;
using internal::COPY_1_BYTE_OFFSET;
using internal::COPY_2_BYTE_OFFSET;
using internal::COPY_4_BYTE_OFFSET;
using internal::kMaximumTagLength;
using internal::LITERAL;
// Any hash function will produce a valid compressed bitstream, but a good
// hash function reduces the number of collisions and thus yields better
// compression for compressible input, and more speed for incompressible
// input. Of course, it doesn't hurt if the hash function is reasonably fast
// either, as it gets called a lot.
static inline uint32 HashBytes(uint32 bytes, int shift) {
uint32 kMul = 0x1e35a7bd;
return (bytes * kMul) >> shift;
}
static inline uint32 Hash(const char* p, int shift) {
return HashBytes(UNALIGNED_LOAD32(p), shift);
}
size_t MaxCompressedLength(size_t source_len) {
// Compressed data can be defined as:
// compressed := item* literal*
// item := literal* copy
//
// The trailing literal sequence has a space blowup of at most 62/60
// since a literal of length 60 needs one tag byte + one extra byte
// for length information.
//
// Item blowup is trickier to measure. Suppose the "copy" op copies
// 4 bytes of data. Because of a special check in the encoding code,
// we produce a 4-byte copy only if the offset is < 65536. Therefore
// the copy op takes 3 bytes to encode, and this type of item leads
// to at most the 62/60 blowup for representing literals.
//
// Suppose the "copy" op copies 5 bytes of data. If the offset is big
// enough, it will take 5 bytes to encode the copy op. Therefore the
// worst case here is a one-byte literal followed by a five-byte copy.
// I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
//
// This last factor dominates the blowup, so the final estimate is:
return 32 + source_len + source_len/6;
}
namespace {
void UnalignedCopy64(const void* src, void* dst) {
char tmp[8];
memcpy(tmp, src, 8);
memcpy(dst, tmp, 8);
}
void UnalignedCopy128(const void* src, void* dst) {
// memcpy gets vectorized when the appropriate compiler options are used.
// For example, x86 compilers targeting SSE2+ will optimize to an SSE2 load
// and store.
char tmp[16];
memcpy(tmp, src, 16);
memcpy(dst, tmp, 16);
}
// Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) a byte at a time. Used
// for handling COPY operations where the input and output regions may overlap.
// For example, suppose:
// src == "ab"
// op == src + 2
// op_limit == op + 20
// After IncrementalCopySlow(src, op, op_limit), the result will have eleven
// copies of "ab"
// ababababababababababab
// Note that this does not match the semantics of either memcpy() or memmove().
inline char* IncrementalCopySlow(const char* src, char* op,
char* const op_limit) {
// TODO: Remove pragma when LLVM is aware this
// function is only called in cold regions and when cold regions don't get
// vectorized or unrolled.
#ifdef __clang__
#pragma clang loop unroll(disable)
#endif
while (op < op_limit) {
*op++ = *src++;
}
return op_limit;
}
#if SNAPPY_HAVE_SSSE3
// This is a table of shuffle control masks that can be used as the source
// operand for PSHUFB to permute the contents of the destination XMM register
// into a repeating byte pattern.
alignas(16) const char pshufb_fill_patterns[7][16] = {
{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1},
{0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0},
{0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3},
{0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0},
{0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3},
{0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6, 0, 1},
};
#endif // SNAPPY_HAVE_SSSE3
// Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) but faster than
// IncrementalCopySlow. buf_limit is the address past the end of the writable
// region of the buffer.
inline char* IncrementalCopy(const char* src, char* op, char* const op_limit,
char* const buf_limit) {
// Terminology:
//
// slop = buf_limit - op
// pat = op - src
// len = limit - op
assert(src < op);
assert(op <= op_limit);
assert(op_limit <= buf_limit);
// NOTE: The copy tags use 3 or 6 bits to store the copy length, so len <= 64.
assert(op_limit - op <= 64);
// NOTE: In practice the compressor always emits len >= 4, so it is ok to
// assume that to optimize this function, but this is not guaranteed by the
// compression format, so we have to also handle len < 4 in case the input
// does not satisfy these conditions.
size_t pattern_size = op - src;
// The cases are split into different branches to allow the branch predictor,
// FDO, and static prediction hints to work better. For each input we list the
// ratio of invocations that match each condition.
//
// input slop < 16 pat < 8 len > 16
// ------------------------------------------
// html|html4|cp 0% 1.01% 27.73%
// urls 0% 0.88% 14.79%
// jpg 0% 64.29% 7.14%
// pdf 0% 2.56% 58.06%
// txt[1-4] 0% 0.23% 0.97%
// pb 0% 0.96% 13.88%
// bin 0.01% 22.27% 41.17%
//
// It is very rare that we don't have enough slop for doing block copies. It
// is also rare that we need to expand a pattern. Small patterns are common
// for incompressible formats and for those we are plenty fast already.
// Lengths are normally not greater than 16 but they vary depending on the
// input. In general if we always predict len <= 16 it would be an ok
// prediction.
//
// In order to be fast we want a pattern >= 8 bytes and an unrolled loop
// copying 2x 8 bytes at a time.
// Handle the uncommon case where pattern is less than 8 bytes.
if (SNAPPY_PREDICT_FALSE(pattern_size < 8)) {
#if SNAPPY_HAVE_SSSE3
// Load the first eight bytes into an 128-bit XMM register, then use PSHUFB
// to permute the register's contents in-place into a repeating sequence of
// the first "pattern_size" bytes.
// For example, suppose:
// src == "abc"
// op == op + 3
// After _mm_shuffle_epi8(), "pattern" will have five copies of "abc"
// followed by one byte of slop: abcabcabcabcabca.
//
// The non-SSE fallback implementation suffers from store-forwarding stalls
// because its loads and stores partly overlap. By expanding the pattern
// in-place, we avoid the penalty.
if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 16)) {
const __m128i shuffle_mask = _mm_load_si128(
reinterpret_cast<const __m128i*>(pshufb_fill_patterns)
+ pattern_size - 1);
const __m128i pattern = _mm_shuffle_epi8(
_mm_loadl_epi64(reinterpret_cast<const __m128i*>(src)), shuffle_mask);
// Uninitialized bytes are masked out by the shuffle mask.
// TODO: remove annotation and macro defs once MSan is fixed.
SNAPPY_ANNOTATE_MEMORY_IS_INITIALIZED(&pattern, sizeof(pattern));
pattern_size *= 16 / pattern_size;
char* op_end = std::min(op_limit, buf_limit - 15);
while (op < op_end) {
_mm_storeu_si128(reinterpret_cast<__m128i*>(op), pattern);
op += pattern_size;
}
if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
}
return IncrementalCopySlow(src, op, op_limit);
#else // !SNAPPY_HAVE_SSSE3
// If plenty of buffer space remains, expand the pattern to at least 8
// bytes. The way the following loop is written, we need 8 bytes of buffer
// space if pattern_size >= 4, 11 bytes if pattern_size is 1 or 3, and 10
// bytes if pattern_size is 2. Precisely encoding that is probably not
// worthwhile; instead, invoke the slow path if we cannot write 11 bytes
// (because 11 are required in the worst case).
if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 11)) {
while (pattern_size < 8) {
UnalignedCopy64(src, op);
op += pattern_size;
pattern_size *= 2;
}
if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
} else {
return IncrementalCopySlow(src, op, op_limit);
}
#endif // SNAPPY_HAVE_SSSE3
}
assert(pattern_size >= 8);
// Copy 2x 8 bytes at a time. Because op - src can be < 16, a single
// UnalignedCopy128 might overwrite data in op. UnalignedCopy64 is safe
// because expanding the pattern to at least 8 bytes guarantees that
// op - src >= 8.
//
// Typically, the op_limit is the gating factor so try to simplify the loop
// based on that.
if (SNAPPY_PREDICT_TRUE(op_limit <= buf_limit - 16)) {
// There is at least one, and at most four 16-byte blocks. Writing four
// conditionals instead of a loop allows FDO to layout the code with respect
// to the actual probabilities of each length.
// TODO: Replace with loop with trip count hint.
UnalignedCopy64(src, op);
UnalignedCopy64(src + 8, op + 8);
if (op + 16 < op_limit) {
UnalignedCopy64(src + 16, op + 16);
UnalignedCopy64(src + 24, op + 24);
}
if (op + 32 < op_limit) {
UnalignedCopy64(src + 32, op + 32);
UnalignedCopy64(src + 40, op + 40);
}
if (op + 48 < op_limit) {
UnalignedCopy64(src + 48, op + 48);
UnalignedCopy64(src + 56, op + 56);
}
return op_limit;
}
// Fall back to doing as much as we can with the available slop in the
// buffer. This code path is relatively cold however so we save code size by
// avoiding unrolling and vectorizing.
//
// TODO: Remove pragma when when cold regions don't get vectorized
// or unrolled.
#ifdef __clang__
#pragma clang loop unroll(disable)
#endif
for (char *op_end = buf_limit - 16; op < op_end; op += 16, src += 16) {
UnalignedCopy64(src, op);
UnalignedCopy64(src + 8, op + 8);
}
if (op >= op_limit)
return op_limit;
// We only take this branch if we didn't have enough slop and we can do a
// single 8 byte copy.
if (SNAPPY_PREDICT_FALSE(op <= buf_limit - 8)) {
UnalignedCopy64(src, op);
src += 8;
op += 8;
}
return IncrementalCopySlow(src, op, op_limit);
}
} // namespace
template <bool allow_fast_path>
static inline char* EmitLiteral(char* op,
const char* literal,
int len) {
// The vast majority of copies are below 16 bytes, for which a
// call to memcpy is overkill. This fast path can sometimes
// copy up to 15 bytes too much, but that is okay in the
// main loop, since we have a bit to go on for both sides:
//
// - The input will always have kInputMarginBytes = 15 extra
// available bytes, as long as we're in the main loop, and
// if not, allow_fast_path = false.
// - The output will always have 32 spare bytes (see
// MaxCompressedLength).
assert(len > 0); // Zero-length literals are disallowed
int n = len - 1;
if (allow_fast_path && len <= 16) {
// Fits in tag byte
*op++ = LITERAL | (n << 2);
UnalignedCopy128(literal, op);
return op + len;
}
if (n < 60) {
// Fits in tag byte
*op++ = LITERAL | (n << 2);
} else {
int count = (Bits::Log2Floor(n) >> 3) + 1;
assert(count >= 1);
assert(count <= 4);
*op++ = LITERAL | ((59 + count) << 2);
// Encode in upcoming bytes.
// Write 4 bytes, though we may care about only 1 of them. The output buffer
// is guaranteed to have at least 3 more spaces left as 'len >= 61' holds
// here and there is a memcpy of size 'len' below.
LittleEndian::Store32(op, n);
op += count;
}
memcpy(op, literal, len);
return op + len;
}
template <bool len_less_than_12>
static inline char* EmitCopyAtMost64(char* op, size_t offset, size_t len) {
assert(len <= 64);
assert(len >= 4);
assert(offset < 65536);
assert(len_less_than_12 == (len < 12));
if (len_less_than_12 && SNAPPY_PREDICT_TRUE(offset < 2048)) {
// offset fits in 11 bits. The 3 highest go in the top of the first byte,
// and the rest go in the second byte.
*op++ = COPY_1_BYTE_OFFSET + ((len - 4) << 2) + ((offset >> 3) & 0xe0);
*op++ = offset & 0xff;
} else {
// Write 4 bytes, though we only care about 3 of them. The output buffer
// is required to have some slack, so the extra byte won't overrun it.
uint32 u = COPY_2_BYTE_OFFSET + ((len - 1) << 2) + (offset << 8);
LittleEndian::Store32(op, u);
op += 3;
}
return op;
}
template <bool len_less_than_12>
static inline char* EmitCopy(char* op, size_t offset, size_t len) {
assert(len_less_than_12 == (len < 12));
if (len_less_than_12) {
return EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
} else {
// A special case for len <= 64 might help, but so far measurements suggest
// it's in the noise.
// Emit 64 byte copies but make sure to keep at least four bytes reserved.
while (SNAPPY_PREDICT_FALSE(len >= 68)) {
op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 64);
len -= 64;
}
// One or two copies will now finish the job.
if (len > 64) {
op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 60);
len -= 60;
}
// Emit remainder.
if (len < 12) {
op = EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
} else {
op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, len);
}
return op;
}
}
bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
uint32 v = 0;
const char* limit = start + n;
if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
*result = v;
return true;
} else {
return false;
}
}
namespace {
uint32 CalculateTableSize(uint32 input_size) {
static_assert(
kMaxHashTableSize >= kMinHashTableSize,
"kMaxHashTableSize should be greater or equal to kMinHashTableSize.");
if (input_size > kMaxHashTableSize) {
return kMaxHashTableSize;
}
if (input_size < kMinHashTableSize) {
return kMinHashTableSize;
}
// This is equivalent to Log2Ceiling(input_size), assuming input_size > 1.
// 2 << Log2Floor(x - 1) is equivalent to 1 << (1 + Log2Floor(x - 1)).
return 2u << Bits::Log2Floor(input_size - 1);
}
} // namespace
namespace internal {
WorkingMemory::WorkingMemory(size_t input_size) {
const size_t max_fragment_size = std::min(input_size, kBlockSize);
const size_t table_size = CalculateTableSize(max_fragment_size);
size_ = table_size * sizeof(*table_) + max_fragment_size +
MaxCompressedLength(max_fragment_size);
mem_ = std::allocator<char>().allocate(size_);
table_ = reinterpret_cast<uint16*>(mem_);
input_ = mem_ + table_size * sizeof(*table_);
output_ = input_ + max_fragment_size;
}
WorkingMemory::~WorkingMemory() {
std::allocator<char>().deallocate(mem_, size_);
}
uint16* WorkingMemory::GetHashTable(size_t fragment_size,
int* table_size) const {
const size_t htsize = CalculateTableSize(fragment_size);
memset(table_, 0, htsize * sizeof(*table_));
*table_size = htsize;
return table_;
}
} // end namespace internal
// For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(p), offset) will
// equal UNALIGNED_LOAD32(p + offset). Motivation: On x86-64 hardware we have
// empirically found that overlapping loads such as
// UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
// are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
//
// We have different versions for 64- and 32-bit; ideally we would avoid the
// two functions and just inline the UNALIGNED_LOAD64 call into
// GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever
// enough to avoid loading the value multiple times then. For 64-bit, the load
// is done when GetEightBytesAt() is called, whereas for 32-bit, the load is
// done at GetUint32AtOffset() time.
#ifdef ARCH_K8
typedef uint64 EightBytesReference;
static inline EightBytesReference GetEightBytesAt(const char* ptr) {
return UNALIGNED_LOAD64(ptr);
}
static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
assert(offset >= 0);
assert(offset <= 4);
return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
}
#else
typedef const char* EightBytesReference;
static inline EightBytesReference GetEightBytesAt(const char* ptr) {
return ptr;
}
static inline uint32 GetUint32AtOffset(const char* v, int offset) {
assert(offset >= 0);
assert(offset <= 4);
return UNALIGNED_LOAD32(v + offset);
}
#endif
// Flat array compression that does not emit the "uncompressed length"
// prefix. Compresses "input" string to the "*op" buffer.
//
// REQUIRES: "input" is at most "kBlockSize" bytes long.
// REQUIRES: "op" points to an array of memory that is at least
// "MaxCompressedLength(input.size())" in size.
// REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
// REQUIRES: "table_size" is a power of two
//
// Returns an "end" pointer into "op" buffer.
// "end - op" is the compressed size of "input".
namespace internal {
char* CompressFragment(const char* input,
size_t input_size,
char* op,
uint16* table,
const int table_size) {
// "ip" is the input pointer, and "op" is the output pointer.
const char* ip = input;
assert(input_size <= kBlockSize);
assert((table_size & (table_size - 1)) == 0); // table must be power of two
const int shift = 32 - Bits::Log2Floor(table_size);
assert(static_cast<int>(kuint32max >> shift) == table_size - 1);
const char* ip_end = input + input_size;
const char* base_ip = ip;
// Bytes in [next_emit, ip) will be emitted as literal bytes. Or
// [next_emit, ip_end) after the main loop.
const char* next_emit = ip;
const size_t kInputMarginBytes = 15;
if (SNAPPY_PREDICT_TRUE(input_size >= kInputMarginBytes)) {
const char* ip_limit = input + input_size - kInputMarginBytes;
for (uint32 next_hash = Hash(++ip, shift); ; ) {
assert(next_emit < ip);
// The body of this loop calls EmitLiteral once and then EmitCopy one or
// more times. (The exception is that when we're close to exhausting
// the input we goto emit_remainder.)
//
// In the first iteration of this loop we're just starting, so
// there's nothing to copy, so calling EmitLiteral once is
// necessary. And we only start a new iteration when the
// current iteration has determined that a call to EmitLiteral will
// precede the next call to EmitCopy (if any).
//
// Step 1: Scan forward in the input looking for a 4-byte-long match.
// If we get close to exhausting the input then goto emit_remainder.
//
// Heuristic match skipping: If 32 bytes are scanned with no matches
// found, start looking only at every other byte. If 32 more bytes are
// scanned (or skipped), look at every third byte, etc.. When a match is
// found, immediately go back to looking at every byte. This is a small
// loss (~5% performance, ~0.1% density) for compressible data due to more
// bookkeeping, but for non-compressible data (such as JPEG) it's a huge
// win since the compressor quickly "realizes" the data is incompressible
// and doesn't bother looking for matches everywhere.
//
// The "skip" variable keeps track of how many bytes there are since the
// last match; dividing it by 32 (ie. right-shifting by five) gives the
// number of bytes to move ahead for each iteration.
uint32 skip = 32;
const char* next_ip = ip;
const char* candidate;
do {
ip = next_ip;
uint32 hash = next_hash;
assert(hash == Hash(ip, shift));
uint32 bytes_between_hash_lookups = skip >> 5;
skip += bytes_between_hash_lookups;
next_ip = ip + bytes_between_hash_lookups;
if (SNAPPY_PREDICT_FALSE(next_ip > ip_limit)) {
goto emit_remainder;
}
next_hash = Hash(next_ip, shift);
candidate = base_ip + table[hash];
assert(candidate >= base_ip);
assert(candidate < ip);
table[hash] = ip - base_ip;
} while (SNAPPY_PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
UNALIGNED_LOAD32(candidate)));
// Step 2: A 4-byte match has been found. We'll later see if more
// than 4 bytes match. But, prior to the match, input
// bytes [next_emit, ip) are unmatched. Emit them as "literal bytes."
assert(next_emit + 16 <= ip_end);
op = EmitLiteral</*allow_fast_path=*/true>(op, next_emit, ip - next_emit);
// Step 3: Call EmitCopy, and then see if another EmitCopy could
// be our next move. Repeat until we find no match for the
// input immediately after what was consumed by the last EmitCopy call.
//
// If we exit this loop normally then we need to call EmitLiteral next,
// though we don't yet know how big the literal will be. We handle that
// by proceeding to the next iteration of the main loop. We also can exit
// this loop via goto if we get close to exhausting the input.
EightBytesReference input_bytes;
uint32 candidate_bytes = 0;
do {
// We have a 4-byte match at ip, and no need to emit any
// "literal bytes" prior to ip.
const char* base = ip;
std::pair<size_t, bool> p =
FindMatchLength(candidate + 4, ip + 4, ip_end);
size_t matched = 4 + p.first;
ip += matched;
size_t offset = base - candidate;
assert(0 == memcmp(base, candidate, matched));
if (p.second) {
op = EmitCopy</*len_less_than_12=*/true>(op, offset, matched);
} else {
op = EmitCopy</*len_less_than_12=*/false>(op, offset, matched);
}
next_emit = ip;
if (SNAPPY_PREDICT_FALSE(ip >= ip_limit)) {
goto emit_remainder;
}
// We are now looking for a 4-byte match again. We read
// table[Hash(ip, shift)] for that. To improve compression,
// we also update table[Hash(ip - 1, shift)] and table[Hash(ip, shift)].
input_bytes = GetEightBytesAt(ip - 1);
uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
table[prev_hash] = ip - base_ip - 1;
uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
candidate = base_ip + table[cur_hash];
candidate_bytes = UNALIGNED_LOAD32(candidate);
table[cur_hash] = ip - base_ip;
} while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
++ip;
}
}
emit_remainder:
// Emit the remaining bytes as a literal
if (next_emit < ip_end) {
op = EmitLiteral</*allow_fast_path=*/false>(op, next_emit,
ip_end - next_emit);
}
return op;
}
} // end namespace internal
// Called back at avery compression call to trace parameters and sizes.
static inline void Report(const char *algorithm, size_t compressed_size,
size_t uncompressed_size) {}
// Signature of output types needed by decompression code.
// The decompression code is templatized on a type that obeys this
// signature so that we do not pay virtual function call overhead in
// the middle of a tight decompression loop.
//
// class DecompressionWriter {
// public:
// // Called before decompression
// void SetExpectedLength(size_t length);
//
// // Called after decompression
// bool CheckLength() const;
//
// // Called repeatedly during decompression
// bool Append(const char* ip, size_t length);
// bool AppendFromSelf(uint32 offset, size_t length);
//
// // The rules for how TryFastAppend differs from Append are somewhat
// // convoluted:
// //
// // - TryFastAppend is allowed to decline (return false) at any
// // time, for any reason -- just "return false" would be
// // a perfectly legal implementation of TryFastAppend.
// // The intention is for TryFastAppend to allow a fast path
// // in the common case of a small append.
// // - TryFastAppend is allowed to read up to <available> bytes
// // from the input buffer, whereas Append is allowed to read
// // <length>. However, if it returns true, it must leave
// // at least five (kMaximumTagLength) bytes in the input buffer
// // afterwards, so that there is always enough space to read the
// // next tag without checking for a refill.
// // - TryFastAppend must always return decline (return false)
// // if <length> is 61 or more, as in this case the literal length is not
// // decoded fully. In practice, this should not be a big problem,
// // as it is unlikely that one would implement a fast path accepting
// // this much data.
// //
// bool TryFastAppend(const char* ip, size_t available, size_t length);
// };
static inline uint32 ExtractLowBytes(uint32 v, int n) {
assert(n >= 0);
assert(n <= 4);
#if SNAPPY_HAVE_BMI2
return _bzhi_u32(v, 8 * n);
#else
// This needs to be wider than uint32 otherwise `mask << 32` will be
// undefined.
uint64 mask = 0xffffffff;
return v & ~(mask << (8 * n));
#endif
}
static inline bool LeftShiftOverflows(uint8 value, uint32 shift) {
assert(shift < 32);
static const uint8 masks[] = {
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, //
0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe};
return (value & masks[shift]) != 0;
}
// Helper class for decompression
class SnappyDecompressor {
private:
Source* reader_; // Underlying source of bytes to decompress
const char* ip_; // Points to next buffered byte
const char* ip_limit_; // Points just past buffered bytes
uint32 peeked_; // Bytes peeked from reader (need to skip)
bool eof_; // Hit end of input without an error?
char scratch_[kMaximumTagLength]; // See RefillTag().
// Ensure that all of the tag metadata for the next tag is available
// in [ip_..ip_limit_-1]. Also ensures that [ip,ip+4] is readable even
// if (ip_limit_ - ip_ < 5).
//
// Returns true on success, false on error or end of input.
bool RefillTag();
public:
explicit SnappyDecompressor(Source* reader)
: reader_(reader),
ip_(NULL),
ip_limit_(NULL),
peeked_(0),
eof_(false) {
}
~SnappyDecompressor() {
// Advance past any bytes we peeked at from the reader
reader_->Skip(peeked_);
}
// Returns true iff we have hit the end of the input without an error.
bool eof() const {
return eof_;
}
// Read the uncompressed length stored at the start of the compressed data.
// On success, stores the length in *result and returns true.
// On failure, returns false.
bool ReadUncompressedLength(uint32* result) {
assert(ip_ == NULL); // Must not have read anything yet
// Length is encoded in 1..5 bytes
*result = 0;
uint32 shift = 0;
while (true) {
if (shift >= 32) return false;
size_t n;
const char* ip = reader_->Peek(&n);
if (n == 0) return false;
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
reader_->Skip(1);
uint32 val = c & 0x7f;
if (LeftShiftOverflows(static_cast<uint8>(val), shift)) return false;
*result |= val << shift;
if (c < 128) {
break;
}
shift += 7;
}
return true;
}
// Process the next item found in the input.
// Returns true if successful, false on error or end of input.
template <class Writer>
#if defined(__GNUC__) && defined(__x86_64__)
__attribute__((aligned(32)))
#endif
void DecompressAllTags(Writer* writer) {
// In x86, pad the function body to start 16 bytes later. This function has
// a couple of hotspots that are highly sensitive to alignment: we have
// observed regressions by more than 20% in some metrics just by moving the
// exact same code to a different position in the benchmark binary.
//
// Putting this code on a 32-byte-aligned boundary + 16 bytes makes us hit
// the "lucky" case consistently. Unfortunately, this is a very brittle
// workaround, and future differences in code generation may reintroduce
// this regression. If you experience a big, difficult to explain, benchmark
// performance regression here, first try removing this hack.
#if defined(__GNUC__) && defined(__x86_64__)
// Two 8-byte "NOP DWORD ptr [EAX + EAX*1 + 00000000H]" instructions.
asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
#endif
const char* ip = ip_;
// We could have put this refill fragment only at the beginning of the loop.
// However, duplicating it at the end of each branch gives the compiler more
// scope to optimize the <ip_limit_ - ip> expression based on the local
// context, which overall increases speed.
#define MAYBE_REFILL() \
if (ip_limit_ - ip < kMaximumTagLength) { \
ip_ = ip; \
if (!RefillTag()) return; \
ip = ip_; \
}
MAYBE_REFILL();
for ( ;; ) {
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
// Ratio of iterations that have LITERAL vs non-LITERAL for different
// inputs.
//
// input LITERAL NON_LITERAL
// -----------------------------------
// html|html4|cp 23% 77%
// urls 36% 64%
// jpg 47% 53%
// pdf 19% 81%
// txt[1-4] 25% 75%
// pb 24% 76%
// bin 24% 76%
if (SNAPPY_PREDICT_FALSE((c & 0x3) == LITERAL)) {
size_t literal_length = (c >> 2) + 1u;
if (writer->TryFastAppend(ip, ip_limit_ - ip, literal_length)) {
assert(literal_length < 61);
ip += literal_length;
// NOTE: There is no MAYBE_REFILL() here, as TryFastAppend()
// will not return true unless there's already at least five spare
// bytes in addition to the literal.
continue;
}
if (SNAPPY_PREDICT_FALSE(literal_length >= 61)) {
// Long literal.
const size_t literal_length_length = literal_length - 60;
literal_length =
ExtractLowBytes(LittleEndian::Load32(ip), literal_length_length) +
1;
ip += literal_length_length;
}
size_t avail = ip_limit_ - ip;
while (avail < literal_length) {
if (!writer->Append(ip, avail)) return;
literal_length -= avail;
reader_->Skip(peeked_);
size_t n;
ip = reader_->Peek(&n);
avail = n;
peeked_ = avail;
if (avail == 0) return; // Premature end of input
ip_limit_ = ip + avail;
}
if (!writer->Append(ip, literal_length)) {
return;
}
ip += literal_length;
MAYBE_REFILL();
} else {
// If we ignore branch prediction misses, then the throughput of the
// loop over tags is limited by the data dependency chain carried by
// ip. The amount ip advances per iteration depends on the value c.
// For COPY_1 it's 2 bytes, for COPY_2 it's 3 bytes and for COPY_4 it's
// 5 bytes. We could be branch free and use information from the entry
// in char_table to advance ip, but that would lead to the following
// critical data dependency chain
// ip -> c = *ip++ -> entry = char_table[c] ->
// advance = entry >> 11 -> ip += advance
// which is 5 + 5 + 1 + 1 = 12 cycles of latency. Meaning the processor
// will never be able to execute the loop faster then 12 cycles per
// iteration. It's better to speculate it's not a COPY_4, which is an
// uncommon tag, in which case we get the data dependency chain
// ip -> c = *ip++ -> c &= 3 -> ip += c
// which is 5 + 1 + 1 = 7 cycles of latency.
if (SNAPPY_PREDICT_FALSE((c & 3) == COPY_4_BYTE_OFFSET)) {
const size_t copy_offset = LittleEndian::Load32(ip);
const size_t length = (c >> 2) + 1;
ip += 4;
if (!writer->AppendFromSelf(copy_offset, length)) return;
} else {
const size_t entry = char_table[c];
const size_t trailer =
ExtractLowBytes(LittleEndian::Load32(ip), c & 3);
const size_t length = entry & 0xff;
ip += (c & 3);
// copy_offset/256 is encoded in bits 8..10. By just fetching
// those bits, we get copy_offset (since the bit-field starts at
// bit 8).
const size_t copy_offset = entry & 0x700;
if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
return;
}
}
MAYBE_REFILL();
}
}
#undef MAYBE_REFILL
}
};
bool SnappyDecompressor::RefillTag() {
const char* ip = ip_;
if (ip == ip_limit_) {
// Fetch a new fragment from the reader
reader_->Skip(peeked_); // All peeked bytes are used up
size_t n;
ip = reader_->Peek(&n);
peeked_ = n;
eof_ = (n == 0);
if (eof_) return false;
ip_limit_ = ip + n;
}
// Read the tag character
assert(ip < ip_limit_);
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
const uint32 entry = char_table[c];
const uint32 needed = (entry >> 11) + 1; // +1 byte for 'c'
assert(needed <= sizeof(scratch_));
// Read more bytes from reader if needed
uint32 nbuf = ip_limit_ - ip;
if (nbuf < needed) {
// Stitch together bytes from ip and reader to form the word
// contents. We store the needed bytes in "scratch_". They
// will be consumed immediately by the caller since we do not
// read more than we need.
memmove(scratch_, ip, nbuf);
reader_->Skip(peeked_); // All peeked bytes are used up
peeked_ = 0;
while (nbuf < needed) {
size_t length;
const char* src = reader_->Peek(&length);
if (length == 0) return false;
uint32 to_add = std::min<uint32>(needed - nbuf, length);
memcpy(scratch_ + nbuf, src, to_add);
nbuf += to_add;
reader_->Skip(to_add);
}
assert(nbuf == needed);
ip_ = scratch_;
ip_limit_ = scratch_ + needed;
} else if (nbuf < kMaximumTagLength) {
// Have enough bytes, but move into scratch_ so that we do not
// read past end of input
memmove(scratch_, ip, nbuf);
reader_->Skip(peeked_); // All peeked bytes are used up
peeked_ = 0;
ip_ = scratch_;
ip_limit_ = scratch_ + nbuf;
} else {
// Pass pointer to buffer returned by reader_.
ip_ = ip;
}
return true;
}
template <typename Writer>
static bool InternalUncompress(Source* r, Writer* writer) {
// Read the uncompressed length from the front of the compressed input
SnappyDecompressor decompressor(r);
uint32 uncompressed_len = 0;
if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
return InternalUncompressAllTags(&decompressor, writer, r->Available(),
uncompressed_len);
}
template <typename Writer>
static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
Writer* writer,
uint32 compressed_len,
uint32 uncompressed_len) {
Report("snappy_uncompress", compressed_len, uncompressed_len);
writer->SetExpectedLength(uncompressed_len);
// Process the entire input
decompressor->DecompressAllTags(writer);
writer->Flush();
return (decompressor->eof() && writer->CheckLength());
}
bool GetUncompressedLength(Source* source, uint32* result) {
SnappyDecompressor decompressor(source);
return decompressor.ReadUncompressedLength(result);
}
size_t Compress(Source* reader, Sink* writer) {
size_t written = 0;
size_t N = reader->Available();
const size_t uncompressed_size = N;
char ulength[Varint::kMax32];
char* p = Varint::Encode32(ulength, N);
writer->Append(ulength, p-ulength);
written += (p - ulength);
internal::WorkingMemory wmem(N);
while (N > 0) {
// Get next block to compress (without copying if possible)
size_t fragment_size;
const char* fragment = reader->Peek(&fragment_size);
assert(fragment_size != 0); // premature end of input
const size_t num_to_read = std::min(N, kBlockSize);
size_t bytes_read = fragment_size;
size_t pending_advance = 0;
if (bytes_read >= num_to_read) {
// Buffer returned by reader is large enough
pending_advance = num_to_read;
fragment_size = num_to_read;
} else {
char* scratch = wmem.GetScratchInput();
memcpy(scratch, fragment, bytes_read);
reader->Skip(bytes_read);
while (bytes_read < num_to_read) {
fragment = reader->Peek(&fragment_size);
size_t n = std::min<size_t>(fragment_size, num_to_read - bytes_read);
memcpy(scratch + bytes_read, fragment, n);
bytes_read += n;
reader->Skip(n);
}
assert(bytes_read == num_to_read);
fragment = scratch;
fragment_size = num_to_read;
}
assert(fragment_size == num_to_read);
// Get encoding table for compression
int table_size;
uint16* table = wmem.GetHashTable(num_to_read, &table_size);
// Compress input_fragment and append to dest
const int max_output = MaxCompressedLength(num_to_read);
// Need a scratch buffer for the output, in case the byte sink doesn't
// have room for us directly.
// Since we encode kBlockSize regions followed by a region
// which is <= kBlockSize in length, a previously allocated
// scratch_output[] region is big enough for this iteration.
char* dest = writer->GetAppendBuffer(max_output, wmem.GetScratchOutput());
char* end = internal::CompressFragment(fragment, fragment_size, dest, table,
table_size);
writer->Append(dest, end - dest);
written += (end - dest);
N -= num_to_read;
reader->Skip(pending_advance);
}
Report("snappy_compress", written, uncompressed_size);
return written;
}
// -----------------------------------------------------------------------
// IOVec interfaces
// -----------------------------------------------------------------------
// A type that writes to an iovec.
// Note that this is not a "ByteSink", but a type that matches the
// Writer template argument to SnappyDecompressor::DecompressAllTags().
class SnappyIOVecWriter {
private:
// output_iov_end_ is set to iov + count and used to determine when
// the end of the iovs is reached.
const struct iovec* output_iov_end_;
#if !defined(NDEBUG)
const struct iovec* output_iov_;
#endif // !defined(NDEBUG)
// Current iov that is being written into.
const struct iovec* curr_iov_;
// Pointer to current iov's write location.
char* curr_iov_output_;
// Remaining bytes to write into curr_iov_output.
size_t curr_iov_remaining_;
// Total bytes decompressed into output_iov_ so far.
size_t total_written_;
// Maximum number of bytes that will be decompressed into output_iov_.
size_t output_limit_;
static inline char* GetIOVecPointer(const struct iovec* iov, size_t offset) {
return reinterpret_cast<char*>(iov->iov_base) + offset;
}
public:
// Does not take ownership of iov. iov must be valid during the
// entire lifetime of the SnappyIOVecWriter.
inline SnappyIOVecWriter(const struct iovec* iov, size_t iov_count)
: output_iov_end_(iov + iov_count),
#if !defined(NDEBUG)
output_iov_(iov),
#endif // !defined(NDEBUG)
curr_iov_(iov),
curr_iov_output_(iov_count ? reinterpret_cast<char*>(iov->iov_base)
: nullptr),
curr_iov_remaining_(iov_count ? iov->iov_len : 0),
total_written_(0),
output_limit_(-1) {}
inline void SetExpectedLength(size_t len) {
output_limit_ = len;
}
inline bool CheckLength() const {
return total_written_ == output_limit_;
}
inline bool Append(const char* ip, size_t len) {
if (total_written_ + len > output_limit_) {
return false;
}
return AppendNoCheck(ip, len);
}
inline bool AppendNoCheck(const char* ip, size_t len) {
while (len > 0) {
if (curr_iov_remaining_ == 0) {
// This iovec is full. Go to the next one.
if (curr_iov_ + 1 >= output_iov_end_) {
return false;
}
++curr_iov_;
curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
curr_iov_remaining_ = curr_iov_->iov_len;
}
const size_t to_write = std::min(len, curr_iov_remaining_);
memcpy(curr_iov_output_, ip, to_write);
curr_iov_output_ += to_write;
curr_iov_remaining_ -= to_write;
total_written_ += to_write;
ip += to_write;
len -= to_write;
}
return true;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
const size_t space_left = output_limit_ - total_written_;
if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16 &&
curr_iov_remaining_ >= 16) {
// Fast path, used for the majority (about 95%) of invocations.
UnalignedCopy128(ip, curr_iov_output_);
curr_iov_output_ += len;
curr_iov_remaining_ -= len;
total_written_ += len;
return true;
}
return false;
}
inline bool AppendFromSelf(size_t offset, size_t len) {
// See SnappyArrayWriter::AppendFromSelf for an explanation of
// the "offset - 1u" trick.
if (offset - 1u >= total_written_) {
return false;
}
const size_t space_left = output_limit_ - total_written_;
if (len > space_left) {
return false;
}
// Locate the iovec from which we need to start the copy.
const iovec* from_iov = curr_iov_;
size_t from_iov_offset = curr_iov_->iov_len - curr_iov_remaining_;
while (offset > 0) {
if (from_iov_offset >= offset) {
from_iov_offset -= offset;
break;
}
offset -= from_iov_offset;
--from_iov;
#if !defined(NDEBUG)
assert(from_iov >= output_iov_);
#endif // !defined(NDEBUG)
from_iov_offset = from_iov->iov_len;
}
// Copy <len> bytes starting from the iovec pointed to by from_iov_index to
// the current iovec.
while (len > 0) {
assert(from_iov <= curr_iov_);
if (from_iov != curr_iov_) {
const size_t to_copy =
std::min(from_iov->iov_len - from_iov_offset, len);
AppendNoCheck(GetIOVecPointer(from_iov, from_iov_offset), to_copy);
len -= to_copy;
if (len > 0) {
++from_iov;
from_iov_offset = 0;
}
} else {
size_t to_copy = curr_iov_remaining_;
if (to_copy == 0) {
// This iovec is full. Go to the next one.
if (curr_iov_ + 1 >= output_iov_end_) {
return false;
}
++curr_iov_;
curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
curr_iov_remaining_ = curr_iov_->iov_len;
continue;
}
if (to_copy > len) {
to_copy = len;
}
IncrementalCopy(GetIOVecPointer(from_iov, from_iov_offset),
curr_iov_output_, curr_iov_output_ + to_copy,
curr_iov_output_ + curr_iov_remaining_);
curr_iov_output_ += to_copy;
curr_iov_remaining_ -= to_copy;
from_iov_offset += to_copy;
total_written_ += to_copy;
len -= to_copy;
}
}
return true;
}
inline void Flush() {}
};
bool RawUncompressToIOVec(const char* compressed, size_t compressed_length,
const struct iovec* iov, size_t iov_cnt) {
ByteArraySource reader(compressed, compressed_length);
return RawUncompressToIOVec(&reader, iov, iov_cnt);
}
bool RawUncompressToIOVec(Source* compressed, const struct iovec* iov,
size_t iov_cnt) {
SnappyIOVecWriter output(iov, iov_cnt);
return InternalUncompress(compressed, &output);
}
// -----------------------------------------------------------------------
// Flat array interfaces
// -----------------------------------------------------------------------
// A type that writes to a flat array.
// Note that this is not a "ByteSink", but a type that matches the
// Writer template argument to SnappyDecompressor::DecompressAllTags().
class SnappyArrayWriter {
private:
char* base_;
char* op_;
char* op_limit_;
public:
inline explicit SnappyArrayWriter(char* dst)
: base_(dst),
op_(dst),
op_limit_(dst) {
}
inline void SetExpectedLength(size_t len) {
op_limit_ = op_ + len;
}
inline bool CheckLength() const {
return op_ == op_limit_;
}
inline bool Append(const char* ip, size_t len) {
char* op = op_;
const size_t space_left = op_limit_ - op;
if (space_left < len) {
return false;
}
memcpy(op, ip, len);
op_ = op + len;
return true;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
char* op = op_;
const size_t space_left = op_limit_ - op;
if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16) {
// Fast path, used for the majority (about 95%) of invocations.
UnalignedCopy128(ip, op);
op_ = op + len;
return true;
} else {
return false;
}
}
inline bool AppendFromSelf(size_t offset, size_t len) {
char* const op_end = op_ + len;
// Check if we try to append from before the start of the buffer.
// Normally this would just be a check for "produced < offset",
// but "produced <= offset - 1u" is equivalent for every case
// except the one where offset==0, where the right side will wrap around
// to a very big number. This is convenient, as offset==0 is another
// invalid case that we also want to catch, so that we do not go
// into an infinite loop.
if (Produced() <= offset - 1u || op_end > op_limit_) return false;
op_ = IncrementalCopy(op_ - offset, op_, op_end, op_limit_);
return true;
}
inline size_t Produced() const {
assert(op_ >= base_);
return op_ - base_;
}
inline void Flush() {}
};
bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
ByteArraySource reader(compressed, n);
return RawUncompress(&reader, uncompressed);
}
bool RawUncompress(Source* compressed, char* uncompressed) {
SnappyArrayWriter output(uncompressed);
return InternalUncompress(compressed, &output);
}
bool Uncompress(const char* compressed, size_t n, std::string* uncompressed) {
size_t ulength;
if (!GetUncompressedLength(compressed, n, &ulength)) {
return false;
}
// On 32-bit builds: max_size() < kuint32max. Check for that instead
// of crashing (e.g., consider externally specified compressed data).
if (ulength > uncompressed->max_size()) {
return false;
}
STLStringResizeUninitialized(uncompressed, ulength);
return RawUncompress(compressed, n, string_as_array(uncompressed));
}
// A Writer that drops everything on the floor and just does validation
class SnappyDecompressionValidator {
private:
size_t expected_;
size_t produced_;
public:
inline SnappyDecompressionValidator() : expected_(0), produced_(0) { }
inline void SetExpectedLength(size_t len) {
expected_ = len;
}
inline bool CheckLength() const {
return expected_ == produced_;
}
inline bool Append(const char* ip, size_t len) {
produced_ += len;
return produced_ <= expected_;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
return false;
}
inline bool AppendFromSelf(size_t offset, size_t len) {
// See SnappyArrayWriter::AppendFromSelf for an explanation of
// the "offset - 1u" trick.
if (produced_ <= offset - 1u) return false;
produced_ += len;
return produced_ <= expected_;
}
inline void Flush() {}
};
bool IsValidCompressedBuffer(const char* compressed, size_t n) {
ByteArraySource reader(compressed, n);
SnappyDecompressionValidator writer;
return InternalUncompress(&reader, &writer);
}
bool IsValidCompressed(Source* compressed) {
SnappyDecompressionValidator writer;
return InternalUncompress(compressed, &writer);
}
void RawCompress(const char* input,
size_t input_length,
char* compressed,
size_t* compressed_length) {
ByteArraySource reader(input, input_length);
UncheckedByteArraySink writer(compressed);
Compress(&reader, &writer);
// Compute how many bytes were added
*compressed_length = (writer.CurrentDestination() - compressed);
}
size_t Compress(const char* input, size_t input_length,
std::string* compressed) {
// Pre-grow the buffer to the max length of the compressed output
STLStringResizeUninitialized(compressed, MaxCompressedLength(input_length));
size_t compressed_length;
RawCompress(input, input_length, string_as_array(compressed),
&compressed_length);
compressed->resize(compressed_length);
return compressed_length;
}
// -----------------------------------------------------------------------
// Sink interface
// -----------------------------------------------------------------------
// A type that decompresses into a Sink. The template parameter
// Allocator must export one method "char* Allocate(int size);", which
// allocates a buffer of "size" and appends that to the destination.
template <typename Allocator>
class SnappyScatteredWriter {
Allocator allocator_;
// We need random access into the data generated so far. Therefore
// we keep track of all of the generated data as an array of blocks.
// All of the blocks except the last have length kBlockSize.
std::vector<char*> blocks_;
size_t expected_;
// Total size of all fully generated blocks so far
size_t full_size_;
// Pointer into current output block
char* op_base_; // Base of output block
char* op_ptr_; // Pointer to next unfilled byte in block
char* op_limit_; // Pointer just past block
inline size_t Size() const {
return full_size_ + (op_ptr_ - op_base_);
}
bool SlowAppend(const char* ip, size_t len);
bool SlowAppendFromSelf(size_t offset, size_t len);
public:
inline explicit SnappyScatteredWriter(const Allocator& allocator)
: allocator_(allocator),
full_size_(0),
op_base_(NULL),
op_ptr_(NULL),
op_limit_(NULL) {
}
inline void SetExpectedLength(size_t len) {
assert(blocks_.empty());
expected_ = len;
}
inline bool CheckLength() const {
return Size() == expected_;
}
// Return the number of bytes actually uncompressed so far
inline size_t Produced() const {
return Size();
}
inline bool Append(const char* ip, size_t len) {
size_t avail = op_limit_ - op_ptr_;
if (len <= avail) {
// Fast path
memcpy(op_ptr_, ip, len);
op_ptr_ += len;
return true;
} else {
return SlowAppend(ip, len);
}
}
inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
char* op = op_ptr_;
const int space_left = op_limit_ - op;
if (length <= 16 && available >= 16 + kMaximumTagLength &&
space_left >= 16) {
// Fast path, used for the majority (about 95%) of invocations.
UnalignedCopy128(ip, op);
op_ptr_ = op + length;
return true;
} else {
return false;
}
}
inline bool AppendFromSelf(size_t offset, size_t len) {
char* const op_end = op_ptr_ + len;
// See SnappyArrayWriter::AppendFromSelf for an explanation of
// the "offset - 1u" trick.
if (SNAPPY_PREDICT_TRUE(offset - 1u < op_ptr_ - op_base_ &&
op_end <= op_limit_)) {
// Fast path: src and dst in current block.
op_ptr_ = IncrementalCopy(op_ptr_ - offset, op_ptr_, op_end, op_limit_);
return true;
}
return SlowAppendFromSelf(offset, len);
}
// Called at the end of the decompress. We ask the allocator
// write all blocks to the sink.
inline void Flush() { allocator_.Flush(Produced()); }
};
template<typename Allocator>
bool SnappyScatteredWriter<Allocator>::SlowAppend(const char* ip, size_t len) {
size_t avail = op_limit_ - op_ptr_;
while (len > avail) {
// Completely fill this block
memcpy(op_ptr_, ip, avail);
op_ptr_ += avail;
assert(op_limit_ - op_ptr_ == 0);
full_size_ += (op_ptr_ - op_base_);
len -= avail;
ip += avail;
// Bounds check
if (full_size_ + len > expected_) {
return false;
}
// Make new block
size_t bsize = std::min<size_t>(kBlockSize, expected_ - full_size_);
op_base_ = allocator_.Allocate(bsize);
op_ptr_ = op_base_;
op_limit_ = op_base_ + bsize;
blocks_.push_back(op_base_);
avail = bsize;
}
memcpy(op_ptr_, ip, len);
op_ptr_ += len;
return true;
}
template<typename Allocator>
bool SnappyScatteredWriter<Allocator>::SlowAppendFromSelf(size_t offset,
size_t len) {
// Overflow check
// See SnappyArrayWriter::AppendFromSelf for an explanation of
// the "offset - 1u" trick.
const size_t cur = Size();
if (offset - 1u >= cur) return false;
if (expected_ - cur < len) return false;
// Currently we shouldn't ever hit this path because Compress() chops the
// input into blocks and does not create cross-block copies. However, it is
// nice if we do not rely on that, since we can get better compression if we
// allow cross-block copies and thus might want to change the compressor in
// the future.
size_t src = cur - offset;
while (len-- > 0) {
char c = blocks_[src >> kBlockLog][src & (kBlockSize-1)];
Append(&c, 1);
src++;
}
return true;
}
class SnappySinkAllocator {
public:
explicit SnappySinkAllocator(Sink* dest): dest_(dest) {}
~SnappySinkAllocator() {}
char* Allocate(int size) {
Datablock block(new char[size], size);
blocks_.push_back(block);
return block.data;
}
// We flush only at the end, because the writer wants
// random access to the blocks and once we hand the
// block over to the sink, we can't access it anymore.
// Also we don't write more than has been actually written
// to the blocks.
void Flush(size_t size) {
size_t size_written = 0;
size_t block_size;
for (int i = 0; i < blocks_.size(); ++i) {
block_size = std::min<size_t>(blocks_[i].size, size - size_written);
dest_->AppendAndTakeOwnership(blocks_[i].data, block_size,
&SnappySinkAllocator::Deleter, NULL);
size_written += block_size;
}
blocks_.clear();
}
private:
struct Datablock {
char* data;
size_t size;
Datablock(char* p, size_t s) : data(p), size(s) {}
};
static void Deleter(void* arg, const char* bytes, size_t size) {
delete[] bytes;
}
Sink* dest_;
std::vector<Datablock> blocks_;
// Note: copying this object is allowed
};
size_t UncompressAsMuchAsPossible(Source* compressed, Sink* uncompressed) {
SnappySinkAllocator allocator(uncompressed);
SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
InternalUncompress(compressed, &writer);
return writer.Produced();
}
bool Uncompress(Source* compressed, Sink* uncompressed) {
// Read the uncompressed length from the front of the compressed input
SnappyDecompressor decompressor(compressed);
uint32 uncompressed_len = 0;
if (!decompressor.ReadUncompressedLength(&uncompressed_len)) {
return false;
}
char c;
size_t allocated_size;
char* buf = uncompressed->GetAppendBufferVariable(
1, uncompressed_len, &c, 1, &allocated_size);
const size_t compressed_len = compressed->Available();
// If we can get a flat buffer, then use it, otherwise do block by block
// uncompression
if (allocated_size >= uncompressed_len) {
SnappyArrayWriter writer(buf);
bool result = InternalUncompressAllTags(&decompressor, &writer,
compressed_len, uncompressed_len);
uncompressed->Append(buf, writer.Produced());
return result;
} else {
SnappySinkAllocator allocator(uncompressed);
SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
return InternalUncompressAllTags(&decompressor, &writer, compressed_len,
uncompressed_len);
}
}
} // namespace snappy
Reference in a new issue View git blame Copy permalink