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demboyz/external/snappy-1.1.9/snappy.cc

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// 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-internal.h"
#include "snappy-sinksource.h"
#include "snappy.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 <algorithm>
#include <array>
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <string>
#include <utility>
#include <vector>
namespace snappy {
namespace {
// The amount of slop bytes writers are using for unconditional copies.
constexpr int kSlopBytes = 64;
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;
// We translate the information encoded in a tag through a lookup table to a
// format that requires fewer instructions to decode. Effectively we store
// the length minus the tag part of the offset. The lowest significant byte
// thus stores the length. While total length - offset is given by
// entry - ExtractOffset(type). The nice thing is that the subtraction
// immediately sets the flags for the necessary check that offset >= length.
// This folds the cmp with sub. We engineer the long literals and copy-4 to
// always fail this check, so their presence doesn't affect the fast path.
// To prevent literals from triggering the guard against offset < length (offset
// does not apply to literals) the table is giving them a spurious offset of
// 256.
inline constexpr int16_t MakeEntry(int16_t len, int16_t offset) {
return len - (offset << 8);
}
inline constexpr int16_t LengthMinusOffset(int data, int type) {
return type == 3 ? 0xFF // copy-4 (or type == 3)
: type == 2 ? MakeEntry(data + 1, 0) // copy-2
: type == 1 ? MakeEntry((data & 7) + 4, data >> 3) // copy-1
: data < 60 ? MakeEntry(data + 1, 1) // note spurious offset.
: 0xFF; // long literal
}
inline constexpr int16_t LengthMinusOffset(uint8_t tag) {
return LengthMinusOffset(tag >> 2, tag & 3);
}
template <size_t... Ints>
struct index_sequence {};
template <std::size_t N, size_t... Is>
struct make_index_sequence : make_index_sequence<N - 1, N - 1, Is...> {};
template <size_t... Is>
struct make_index_sequence<0, Is...> : index_sequence<Is...> {};
template <size_t... seq>
constexpr std::array<int16_t, 256> MakeTable(index_sequence<seq...>) {
return std::array<int16_t, 256>{LengthMinusOffset(seq)...};
}
// We maximally co-locate the two tables so that only one register needs to be
// reserved for the table address.
struct {
alignas(64) const std::array<int16_t, 256> length_minus_offset;
uint32_t extract_masks[4]; // Used for extracting offset based on tag type.
} table = {MakeTable(make_index_sequence<256>{}), {0, 0xFF, 0xFFFF, 0}};
// 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.
inline uint32_t HashBytes(uint32_t bytes, uint32_t mask) {
constexpr uint32_t kMagic = 0x1e35a7bd;
return ((kMagic * bytes) >> (32 - kMaxHashTableBits)) & mask;
}
} // namespace
size_t MaxCompressedLength(size_t source_bytes) {
// 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_bytes + source_bytes / 6;
}
namespace {
void UnalignedCopy64(const void* src, void* dst) {
char tmp[8];
std::memcpy(tmp, src, 8);
std::memcpy(dst, tmp, 8);
}
void UnalignedCopy128(const void* src, void* dst) {
// std::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];
std::memcpy(tmp, src, 16);
std::memcpy(dst, tmp, 16);
}
template <bool use_16bytes_chunk>
inline void ConditionalUnalignedCopy128(const char* src, char* dst) {
if (use_16bytes_chunk) {
UnalignedCopy128(src, dst);
} else {
UnalignedCopy64(src, dst);
UnalignedCopy64(src + 8, dst + 8);
}
}
// 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 std::memcpy() or
// std::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
// Computes the bytes for shuffle control mask (please read comments on
// 'pattern_generation_masks' as well) for the given index_offset and
// pattern_size. For example, when the 'offset' is 6, it will generate a
// repeating pattern of size 6. So, the first 16 byte indexes will correspond to
// the pattern-bytes {0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3} and the
// next 16 byte indexes will correspond to the pattern-bytes {4, 5, 0, 1, 2, 3,
// 4, 5, 0, 1, 2, 3, 4, 5, 0, 1}. These byte index sequences are generated by
// calling MakePatternMaskBytes(0, 6, index_sequence<16>()) and
// MakePatternMaskBytes(16, 6, index_sequence<16>()) respectively.
template <size_t... indexes>
inline constexpr std::array<char, sizeof...(indexes)> MakePatternMaskBytes(
int index_offset, int pattern_size, index_sequence<indexes...>) {
return {static_cast<char>((index_offset + indexes) % pattern_size)...};
}
// Computes the shuffle control mask bytes array for given pattern-sizes and
// returns an array.
template <size_t... pattern_sizes_minus_one>
inline constexpr std::array<std::array<char, sizeof(__m128i)>,
sizeof...(pattern_sizes_minus_one)>
MakePatternMaskBytesTable(int index_offset,
index_sequence<pattern_sizes_minus_one...>) {
return {MakePatternMaskBytes(
index_offset, pattern_sizes_minus_one + 1,
make_index_sequence</*indexes=*/sizeof(__m128i)>())...};
}
// This is an array 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) constexpr std::array<std::array<char, sizeof(__m128i)>,
16> pattern_generation_masks =
MakePatternMaskBytesTable(
/*index_offset=*/0,
/*pattern_sizes_minus_one=*/make_index_sequence<16>());
// Similar to 'pattern_generation_masks', this table is used to "rotate" the
// pattern so that we can copy the *next 16 bytes* consistent with the pattern.
// Basically, pattern_reshuffle_masks is a continuation of
// pattern_generation_masks. It follows that, pattern_reshuffle_masks is same as
// pattern_generation_masks for offsets 1, 2, 4, 8 and 16.
alignas(16) constexpr std::array<std::array<char, sizeof(__m128i)>,
16> pattern_reshuffle_masks =
MakePatternMaskBytesTable(
/*index_offset=*/16,
/*pattern_sizes_minus_one=*/make_index_sequence<16>());
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
static inline __m128i LoadPattern(const char* src, const size_t pattern_size) {
__m128i generation_mask = _mm_load_si128(reinterpret_cast<const __m128i*>(
pattern_generation_masks[pattern_size - 1].data()));
// Uninitialized bytes are masked out by the shuffle mask.
// TODO: remove annotation and macro defs once MSan is fixed.
SNAPPY_ANNOTATE_MEMORY_IS_INITIALIZED(src + pattern_size, 16 - pattern_size);
return _mm_shuffle_epi8(
_mm_loadu_si128(reinterpret_cast<const __m128i*>(src)), generation_mask);
}
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
static inline std::pair<__m128i /* pattern */, __m128i /* reshuffle_mask */>
LoadPatternAndReshuffleMask(const char* src, const size_t pattern_size) {
__m128i pattern = LoadPattern(src, pattern_size);
// This mask will generate the next 16 bytes in-place. Doing so enables us to
// write data by at most 4 _mm_storeu_si128.
//
// For example, suppose pattern is: abcdefabcdefabcd
// Shuffling with this mask will generate: efabcdefabcdefab
// Shuffling again will generate: cdefabcdefabcdef
__m128i reshuffle_mask = _mm_load_si128(reinterpret_cast<const __m128i*>(
pattern_reshuffle_masks[pattern_size - 1].data()));
return {pattern, reshuffle_mask};
}
#endif // SNAPPY_HAVE_SSSE3
// Fallback for when we need to copy while extending the pattern, for example
// copying 10 bytes from 3 positions back abc -> abcabcabcabca.
//
// REQUIRES: [dst - offset, dst + 64) is a valid address range.
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
static inline bool Copy64BytesWithPatternExtension(char* dst, size_t offset) {
#if SNAPPY_HAVE_SSSE3
if (SNAPPY_PREDICT_TRUE(offset <= 16)) {
switch (offset) {
case 0:
return false;
case 1: {
std::memset(dst, dst[-1], 64);
return true;
}
case 2:
case 4:
case 8:
case 16: {
__m128i pattern = LoadPattern(dst - offset, offset);
for (int i = 0; i < 4; i++) {
_mm_storeu_si128(reinterpret_cast<__m128i*>(dst + 16 * i), pattern);
}
return true;
}
default: {
auto pattern_and_reshuffle_mask =
LoadPatternAndReshuffleMask(dst - offset, offset);
__m128i pattern = pattern_and_reshuffle_mask.first;
__m128i reshuffle_mask = pattern_and_reshuffle_mask.second;
for (int i = 0; i < 4; i++) {
_mm_storeu_si128(reinterpret_cast<__m128i*>(dst + 16 * i), pattern);
pattern = _mm_shuffle_epi8(pattern, reshuffle_mask);
}
return true;
}
}
}
#else
if (SNAPPY_PREDICT_TRUE(offset < 16)) {
if (SNAPPY_PREDICT_FALSE(offset == 0)) return false;
// Extend the pattern to the first 16 bytes.
for (int i = 0; i < 16; i++) dst[i] = dst[i - offset];
// Find a multiple of pattern >= 16.
static std::array<uint8_t, 16> pattern_sizes = []() {
std::array<uint8_t, 16> res;
for (int i = 1; i < 16; i++) res[i] = (16 / i + 1) * i;
return res;
}();
offset = pattern_sizes[offset];
for (int i = 1; i < 4; i++) {
std::memcpy(dst + i * 16, dst + i * 16 - offset, 16);
}
return true;
}
#endif // SNAPPY_HAVE_SSSE3
// Very rare.
for (int i = 0; i < 4; i++) {
std::memcpy(dst + i * 16, dst + i * 16 - offset, 16);
}
return true;
}
// Copy [src, src+(op_limit-op)) to [op, op_limit) 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) {
#if SNAPPY_HAVE_SSSE3
constexpr int big_pattern_size_lower_bound = 16;
#else
constexpr int big_pattern_size_lower_bound = 8;
#endif
// Terminology:
//
// slop = buf_limit - op
// pat = op - src
// len = op_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 >= 16 bytes (or 8 bytes in non-SSE)
// and an unrolled loop copying 1x 16 bytes (or 2x 8 bytes in non-SSE) at a
// time.
// Handle the uncommon case where pattern is less than 16 (or 8 in non-SSE)
// bytes.
if (pattern_size < big_pattern_size_lower_bound) {
#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.
// 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 - 15)) {
auto pattern_and_reshuffle_mask =
LoadPatternAndReshuffleMask(src, pattern_size);
__m128i pattern = pattern_and_reshuffle_mask.first;
__m128i reshuffle_mask = pattern_and_reshuffle_mask.second;
// 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.
_mm_storeu_si128(reinterpret_cast<__m128i*>(op), pattern);
if (op + 16 < op_limit) {
pattern = _mm_shuffle_epi8(pattern, reshuffle_mask);
_mm_storeu_si128(reinterpret_cast<__m128i*>(op + 16), pattern);
}
if (op + 32 < op_limit) {
pattern = _mm_shuffle_epi8(pattern, reshuffle_mask);
_mm_storeu_si128(reinterpret_cast<__m128i*>(op + 32), pattern);
}
if (op + 48 < op_limit) {
pattern = _mm_shuffle_epi8(pattern, reshuffle_mask);
_mm_storeu_si128(reinterpret_cast<__m128i*>(op + 48), pattern);
}
return op_limit;
}
char* const op_end = buf_limit - 15;
if (SNAPPY_PREDICT_TRUE(op < op_end)) {
auto pattern_and_reshuffle_mask =
LoadPatternAndReshuffleMask(src, pattern_size);
__m128i pattern = pattern_and_reshuffle_mask.first;
__m128i reshuffle_mask = pattern_and_reshuffle_mask.second;
// 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
do {
_mm_storeu_si128(reinterpret_cast<__m128i*>(op), pattern);
pattern = _mm_shuffle_epi8(pattern, reshuffle_mask);
op += 16;
} while (SNAPPY_PREDICT_TRUE(op < op_end));
}
return IncrementalCopySlow(op - pattern_size, 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 >= big_pattern_size_lower_bound);
constexpr bool use_16bytes_chunk = big_pattern_size_lower_bound == 16;
// Copy 1x 16 bytes (or 2x 8 bytes in non-SSE) at a time. Because op - src can
// be < 16 in non-SSE, 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 - 15)) {
// 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.
ConditionalUnalignedCopy128<use_16bytes_chunk>(src, op);
if (op + 16 < op_limit) {
ConditionalUnalignedCopy128<use_16bytes_chunk>(src + 16, op + 16);
}
if (op + 32 < op_limit) {
ConditionalUnalignedCopy128<use_16bytes_chunk>(src + 32, op + 32);
}
if (op + 48 < op_limit) {
ConditionalUnalignedCopy128<use_16bytes_chunk>(src + 48, op + 48);
}
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) {
ConditionalUnalignedCopy128<use_16bytes_chunk>(src, op);
}
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 std::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 std::memcpy() of size 'len' below.
LittleEndian::Store32(op, n);
op += count;
}
std::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) {
uint32_t u = (len << 2) + (offset << 8);
uint32_t copy1 = COPY_1_BYTE_OFFSET - (4 << 2) + ((offset >> 3) & 0xe0);
uint32_t copy2 = COPY_2_BYTE_OFFSET - (1 << 2);
// It turns out that offset < 2048 is a difficult to predict branch.
// `perf record` shows this is the highest percentage of branch misses in
// benchmarks. This code produces branch free code, the data dependency
// chain that bottlenecks the throughput is so long that a few extra
// instructions are completely free (IPC << 6 because of data deps).
u += offset < 2048 ? copy1 : copy2;
LittleEndian::Store32(op, u);
op += offset < 2048 ? 2 : 3;
} 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_t 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_t v = 0;
const char* limit = start + n;
if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
*result = v;
return true;
} else {
return false;
}
}
namespace {
uint32_t CalculateTableSize(uint32_t 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_t*>(mem_);
input_ = mem_ + table_size * sizeof(*table_);
output_ = input_ + max_fragment_size;
}
WorkingMemory::~WorkingMemory() {
std::allocator<char>().deallocate(mem_, size_);
}
uint16_t* 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
// 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_t* 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 uint32_t mask = table_size - 1;
const char* ip_end = input + input_size;
const char* base_ip = ip;
const size_t kInputMarginBytes = 15;
if (SNAPPY_PREDICT_TRUE(input_size >= kInputMarginBytes)) {
const char* ip_limit = input + input_size - kInputMarginBytes;
for (uint32_t preload = LittleEndian::Load32(ip + 1);;) {
// 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++;
uint64_t data = LittleEndian::Load64(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_t skip = 32;
const char* candidate;
if (ip_limit - ip >= 16) {
auto delta = ip - base_ip;
for (int j = 0; j < 4; ++j) {
for (int k = 0; k < 4; ++k) {
int i = 4 * j + k;
// These for-loops are meant to be unrolled. So we can freely
// special case the first iteration to use the value already
// loaded in preload.
uint32_t dword = i == 0 ? preload : static_cast<uint32_t>(data);
assert(dword == LittleEndian::Load32(ip + i));
uint32_t hash = HashBytes(dword, mask);
candidate = base_ip + table[hash];
assert(candidate >= base_ip);
assert(candidate < ip + i);
table[hash] = delta + i;
if (SNAPPY_PREDICT_FALSE(LittleEndian::Load32(candidate) == dword)) {
*op = LITERAL | (i << 2);
UnalignedCopy128(next_emit, op + 1);
ip += i;
op = op + i + 2;
goto emit_match;
}
data >>= 8;
}
data = LittleEndian::Load64(ip + 4 * j + 4);
}
ip += 16;
skip += 16;
}
while (true) {
assert(static_cast<uint32_t>(data) == LittleEndian::Load32(ip));
uint32_t hash = HashBytes(data, mask);
uint32_t bytes_between_hash_lookups = skip >> 5;
skip += bytes_between_hash_lookups;
const char* next_ip = ip + bytes_between_hash_lookups;
if (SNAPPY_PREDICT_FALSE(next_ip > ip_limit)) {
ip = next_emit;
goto emit_remainder;
}
candidate = base_ip + table[hash];
assert(candidate >= base_ip);
assert(candidate < ip);
table[hash] = ip - base_ip;
if (SNAPPY_PREDICT_FALSE(static_cast<uint32_t>(data) ==
LittleEndian::Load32(candidate))) {
break;
}
data = LittleEndian::Load32(next_ip);
ip = next_ip;
}
// 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.
emit_match:
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, &data);
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);
}
if (SNAPPY_PREDICT_FALSE(ip >= ip_limit)) {
goto emit_remainder;
}
// Expect 5 bytes to match
assert((data & 0xFFFFFFFFFF) ==
(LittleEndian::Load64(ip) & 0xFFFFFFFFFF));
// 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, mask)] and table[Hash(ip, mask)].
table[HashBytes(LittleEndian::Load32(ip - 1), mask)] = ip - base_ip - 1;
uint32_t hash = HashBytes(data, mask);
candidate = base_ip + table[hash];
table[hash] = ip - base_ip;
// Measurements on the benchmarks have shown the following probabilities
// for the loop to exit (ie. avg. number of iterations is reciprocal).
// BM_Flat/6 txt1 p = 0.3-0.4
// BM_Flat/7 txt2 p = 0.35
// BM_Flat/8 txt3 p = 0.3-0.4
// BM_Flat/9 txt3 p = 0.34-0.4
// BM_Flat/10 pb p = 0.4
// BM_Flat/11 gaviota p = 0.1
// BM_Flat/12 cp p = 0.5
// BM_Flat/13 c p = 0.3
} while (static_cast<uint32_t>(data) == LittleEndian::Load32(candidate));
// Because the least significant 5 bytes matched, we can utilize data
// for the next iteration.
preload = data >> 8;
}
}
emit_remainder:
// Emit the remaining bytes as a literal
if (ip < ip_end) {
op = EmitLiteral</*allow_fast_path=*/false>(op, ip, ip_end - ip);
}
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) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)algorithm;
(void)compressed_size;
(void)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);
//
// // For performance a writer may choose to donate the cursor variable to the
// // decompression function. The decompression will inject it in all its
// // function calls to the writer. Keeping the important output cursor as a
// // function local stack variable allows the compiler to keep it in
// // register, which greatly aids performance by avoiding loads and stores of
// // this variable in the fast path loop iterations.
// T GetOutputPtr() const;
//
// // At end of decompression the loop donates the ownership of the cursor
// // variable back to the writer by calling this function.
// void SetOutputPtr(T op);
//
// // Called after decompression
// bool CheckLength() const;
//
// // Called repeatedly during decompression
// // Each function get a pointer to the op (output pointer), that the writer
// // can use and update. Note it's important that these functions get fully
// // inlined so that no actual address of the local variable needs to be
// // taken.
// bool Append(const char* ip, size_t length, T* op);
// bool AppendFromSelf(uint32_t offset, size_t length, T* op);
//
// // 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, T* op);
// };
static inline uint32_t ExtractLowBytes(uint32_t 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_t otherwise `mask << 32` will be
// undefined.
uint64_t mask = 0xffffffff;
return v & ~(mask << (8 * n));
#endif
}
static inline bool LeftShiftOverflows(uint8_t value, uint32_t shift) {
assert(shift < 32);
static const uint8_t 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;
}
inline bool Copy64BytesWithPatternExtension(ptrdiff_t dst, size_t offset) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)dst;
return offset != 0;
}
void MemCopy(char* dst, const uint8_t* src, size_t size) {
std::memcpy(dst, src, size);
}
void MemCopy(ptrdiff_t dst, const uint8_t* src, size_t size) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)dst;
(void)src;
(void)size;
}
void MemMove(char* dst, const void* src, size_t size) {
std::memmove(dst, src, size);
}
void MemMove(ptrdiff_t dst, const void* src, size_t size) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)dst;
(void)src;
(void)size;
}
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
size_t AdvanceToNextTag(const uint8_t** ip_p, size_t* tag) {
const uint8_t*& ip = *ip_p;
// This section is crucial for the throughput of the decompression loop.
// The latency of an iteration is fundamentally constrained by the
// following data chain on ip.
// ip -> c = Load(ip) -> ip1 = ip + 1 + (c & 3) -> ip = ip1 or ip2
// ip2 = ip + 2 + (c >> 2)
// This amounts to 8 cycles.
// 5 (load) + 1 (c & 3) + 1 (lea ip1, [ip + (c & 3) + 1]) + 1 (cmov)
size_t literal_len = *tag >> 2;
size_t tag_type = *tag;
bool is_literal;
#if defined(__GNUC__) && defined(__x86_64__)
// TODO clang misses the fact that the (c & 3) already correctly
// sets the zero flag.
asm("and $3, %k[tag_type]\n\t"
: [tag_type] "+r"(tag_type), "=@ccz"(is_literal));
#else
tag_type &= 3;
is_literal = (tag_type == 0);
#endif
// TODO
// This is code is subtle. Loading the values first and then cmov has less
// latency then cmov ip and then load. However clang would move the loads
// in an optimization phase, volatile prevents this transformation.
// Note that we have enough slop bytes (64) that the loads are always valid.
size_t tag_literal =
static_cast<const volatile uint8_t*>(ip)[1 + literal_len];
size_t tag_copy = static_cast<const volatile uint8_t*>(ip)[tag_type];
*tag = is_literal ? tag_literal : tag_copy;
const uint8_t* ip_copy = ip + 1 + tag_type;
const uint8_t* ip_literal = ip + 2 + literal_len;
ip = is_literal ? ip_literal : ip_copy;
#if defined(__GNUC__) && defined(__x86_64__)
// TODO Clang is "optimizing" zero-extension (a totally free
// operation) this means that after the cmov of tag, it emits another movzb
// tag, byte(tag). It really matters as it's on the core chain. This dummy
// asm, persuades clang to do the zero-extension at the load (it's automatic)
// removing the expensive movzb.
asm("" ::"r"(tag_copy));
#endif
return tag_type;
}
// Extract the offset for copy-1 and copy-2 returns 0 for literals or copy-4.
inline uint32_t ExtractOffset(uint32_t val, size_t tag_type) {
return val & table.extract_masks[tag_type];
};
// Core decompression loop, when there is enough data available.
// Decompresses the input buffer [ip, ip_limit) into the output buffer
// [op, op_limit_min_slop). Returning when either we are too close to the end
// of the input buffer, or we exceed op_limit_min_slop or when a exceptional
// tag is encountered (literal of length > 60) or a copy-4.
// Returns {ip, op} at the points it stopped decoding.
// TODO This function probably does not need to be inlined, as it
// should decode large chunks at a time. This allows runtime dispatch to
// implementations based on CPU capability (BMI2 / perhaps 32 / 64 byte memcpy).
template <typename T>
std::pair<const uint8_t*, ptrdiff_t> DecompressBranchless(
const uint8_t* ip, const uint8_t* ip_limit, ptrdiff_t op, T op_base,
ptrdiff_t op_limit_min_slop) {
// We unroll the inner loop twice so we need twice the spare room.
op_limit_min_slop -= kSlopBytes;
if (2 * (kSlopBytes + 1) < ip_limit - ip && op < op_limit_min_slop) {
const uint8_t* const ip_limit_min_slop = ip_limit - 2 * kSlopBytes - 1;
ip++;
// ip points just past the tag and we are touching at maximum kSlopBytes
// in an iteration.
size_t tag = ip[-1];
do {
// The throughput is limited by instructions, unrolling the inner loop
// twice reduces the amount of instructions checking limits and also
// leads to reduced mov's.
for (int i = 0; i < 2; i++) {
const uint8_t* old_ip = ip;
assert(tag == ip[-1]);
// For literals tag_type = 0, hence we will always obtain 0 from
// ExtractLowBytes. For literals offset will thus be kLiteralOffset.
ptrdiff_t len_min_offset = table.length_minus_offset[tag];
size_t tag_type = AdvanceToNextTag(&ip, &tag);
uint32_t next = LittleEndian::Load32(old_ip);
size_t len = len_min_offset & 0xFF;
len_min_offset -= ExtractOffset(next, tag_type);
if (SNAPPY_PREDICT_FALSE(len_min_offset > 0)) {
if (SNAPPY_PREDICT_FALSE(len & 0x80)) {
// Exceptional case (long literal or copy 4).
// Actually doing the copy here is negatively impacting the main
// loop due to compiler incorrectly allocating a register for
// this fallback. Hence we just break.
break_loop:
ip = old_ip;
goto exit;
}
// Only copy-1 or copy-2 tags can get here.
assert(tag_type == 1 || tag_type == 2);
std::ptrdiff_t delta = op + len_min_offset - len;
// Guard against copies before the buffer start.
if (SNAPPY_PREDICT_FALSE(delta < 0 ||
!Copy64BytesWithPatternExtension(
op_base + op, len - len_min_offset))) {
goto break_loop;
}
op += len;
continue;
}
std::ptrdiff_t delta = op + len_min_offset - len;
if (SNAPPY_PREDICT_FALSE(delta < 0)) {
#if defined(__GNUC__) && defined(__x86_64__)
// TODO
// When validating, both code path reduced to `op += len`. Ie. this
// becomes effectively
//
// if (delta < 0) if (tag_type != 0) goto break_loop;
// op += len;
//
// The compiler interchanges the predictable and almost always false
// first if-statement with the completely unpredictable second
// if-statement, putting an unpredictable branch on every iteration.
// This empty asm is worth almost 2x, which I think qualifies for an
// award for the most load-bearing empty statement.
asm("");
#endif
// Due to the spurious offset in literals have this will trigger
// at the start of a block when op is still smaller than 256.
if (tag_type != 0) goto break_loop;
MemCopy(op_base + op, old_ip, 64);
op += len;
continue;
}
// For copies we need to copy from op_base + delta, for literals
// we need to copy from ip instead of from the stream.
const void* from =
tag_type ? reinterpret_cast<void*>(op_base + delta) : old_ip;
MemMove(op_base + op, from, 64);
op += len;
}
} while (ip < ip_limit_min_slop && op < op_limit_min_slop);
exit:
ip--;
assert(ip <= ip_limit);
}
return {ip, op};
}
// 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
// If ip < ip_limit_min_maxtaglen_ it's safe to read kMaxTagLength from
// buffer.
const char* ip_limit_min_maxtaglen_;
uint32_t 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();
void ResetLimit(const char* ip) {
ip_limit_min_maxtaglen_ =
ip_limit_ - std::min<ptrdiff_t>(ip_limit_ - ip, kMaximumTagLength - 1);
}
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_t* result) {
assert(ip_ == NULL); // Must not have read anything yet
// Length is encoded in 1..5 bytes
*result = 0;
uint32_t 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_t val = c & 0x7f;
if (LeftShiftOverflows(static_cast<uint8_t>(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) {
const char* ip = ip_;
ResetLimit(ip);
auto op = writer->GetOutputPtr();
// 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 (SNAPPY_PREDICT_FALSE(ip >= ip_limit_min_maxtaglen_)) { \
ip_ = ip; \
if (SNAPPY_PREDICT_FALSE(!RefillTag())) goto exit; \
ip = ip_; \
ResetLimit(ip); \
} \
preload = static_cast<uint8_t>(*ip)
// At the start of the for loop below the least significant byte of preload
// contains the tag.
uint32_t preload;
MAYBE_REFILL();
for (;;) {
{
ptrdiff_t op_limit_min_slop;
auto op_base = writer->GetBase(&op_limit_min_slop);
if (op_base) {
auto res =
DecompressBranchless(reinterpret_cast<const uint8_t*>(ip),
reinterpret_cast<const uint8_t*>(ip_limit_),
op - op_base, op_base, op_limit_min_slop);
ip = reinterpret_cast<const char*>(res.first);
op = op_base + res.second;
MAYBE_REFILL();
}
}
const uint8_t c = static_cast<uint8_t>(preload);
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, &op)) {
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.
preload = static_cast<uint8_t>(*ip);
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, &op)) goto exit;
literal_length -= avail;
reader_->Skip(peeked_);
size_t n;
ip = reader_->Peek(&n);
avail = n;
peeked_ = avail;
if (avail == 0) goto exit;
ip_limit_ = ip + avail;
ResetLimit(ip);
}
if (!writer->Append(ip, literal_length, &op)) goto exit;
ip += literal_length;
MAYBE_REFILL();
} else {
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, &op)) goto exit;
} else {
const ptrdiff_t entry = table.length_minus_offset[c];
preload = LittleEndian::Load32(ip);
const uint32_t trailer = ExtractLowBytes(preload, c & 3);
const uint32_t length = entry & 0xff;
assert(length > 0);
// 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 uint32_t copy_offset = trailer - entry + length;
if (!writer->AppendFromSelf(copy_offset, length, &op)) goto exit;
ip += (c & 3);
// By using the result of the previous load we reduce the critical
// dependency chain of ip to 4 cycles.
preload >>= (c & 3) * 8;
if (ip < ip_limit_min_maxtaglen_) continue;
}
MAYBE_REFILL();
}
}
#undef MAYBE_REFILL
exit:
writer->SetOutputPtr(op);
}
};
constexpr uint32_t CalculateNeeded(uint8_t tag) {
return ((tag & 3) == 0 && tag >= (60 * 4))
? (tag >> 2) - 58
: (0x05030201 >> ((tag * 8) & 31)) & 0xFF;
}
#if __cplusplus >= 201402L
constexpr bool VerifyCalculateNeeded() {
for (int i = 0; i < 1; i++) {
if (CalculateNeeded(i) != (char_table[i] >> 11) + 1) return false;
}
return true;
}
// Make sure CalculateNeeded is correct by verifying it against the established
// table encoding the number of added bytes needed.
static_assert(VerifyCalculateNeeded(), "");
#endif // c++14
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));
// At this point make sure that the data for the next tag is consecutive.
// For copy 1 this means the next 2 bytes (tag and 1 byte offset)
// For copy 2 the next 3 bytes (tag and 2 byte offset)
// For copy 4 the next 5 bytes (tag and 4 byte offset)
// For all small literals we only need 1 byte buf for literals 60...63 the
// length is encoded in 1...4 extra bytes.
const uint32_t needed = CalculateNeeded(c);
assert(needed <= sizeof(scratch_));
// Read more bytes from reader if needed
uint32_t 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.
std::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_t to_add = std::min<uint32_t>(needed - nbuf, length);
std::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
std::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_t 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_t compressed_len,
uint32_t 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_t* 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();
std::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);
std::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_t* 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, char**) {
if (total_written_ + len > output_limit_) {
return false;
}
return AppendNoCheck(ip, len);
}
char* GetOutputPtr() { return nullptr; }
char* GetBase(ptrdiff_t*) { return nullptr; }
void SetOutputPtr(char* op) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)op;
}
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_);
std::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,
char**) {
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, char**) {
// 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;
}
assert(to_copy > 0);
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_;
// If op < op_limit_min_slop_ then it's safe to unconditionally write
// kSlopBytes starting at op.
char* op_limit_min_slop_;
public:
inline explicit SnappyArrayWriter(char* dst)
: base_(dst),
op_(dst),
op_limit_(dst),
op_limit_min_slop_(dst) {} // Safe default see invariant.
inline void SetExpectedLength(size_t len) {
op_limit_ = op_ + len;
// Prevent pointer from being past the buffer.
op_limit_min_slop_ = op_limit_ - std::min<size_t>(kSlopBytes - 1, len);
}
inline bool CheckLength() const { return op_ == op_limit_; }
char* GetOutputPtr() { return op_; }
char* GetBase(ptrdiff_t* op_limit_min_slop) {
*op_limit_min_slop = op_limit_min_slop_ - base_;
return base_;
}
void SetOutputPtr(char* op) { op_ = op; }
inline bool Append(const char* ip, size_t len, char** op_p) {
char* op = *op_p;
const size_t space_left = op_limit_ - op;
if (space_left < len) return false;
std::memcpy(op, ip, len);
*op_p = op + len;
return true;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t len,
char** op_p) {
char* op = *op_p;
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_p = op + len;
return true;
} else {
return false;
}
}
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
inline bool AppendFromSelf(size_t offset, size_t len, char** op_p) {
assert(len > 0);
char* const op = *op_p;
assert(op >= base_);
char* const op_end = op + len;
// Check if we try to append from before the start of the buffer.
if (SNAPPY_PREDICT_FALSE(static_cast<size_t>(op - base_) < offset))
return false;
if (SNAPPY_PREDICT_FALSE((kSlopBytes < 64 && len > kSlopBytes) ||
op >= op_limit_min_slop_ || offset < len)) {
if (op_end > op_limit_ || offset == 0) return false;
*op_p = IncrementalCopy(op - offset, op, op_end, op_limit_);
return true;
}
std::memmove(op, op - offset, kSlopBytes);
*op_p = op_end;
return true;
}
inline size_t Produced() const {
assert(op_ >= base_);
return op_ - base_;
}
inline void Flush() {}
};
bool RawUncompress(const char* compressed, size_t compressed_length,
char* uncompressed) {
ByteArraySource reader(compressed, compressed_length);
return RawUncompress(&reader, uncompressed);
}
bool RawUncompress(Source* compressed, char* uncompressed) {
SnappyArrayWriter output(uncompressed);
return InternalUncompress(compressed, &output);
}
bool Uncompress(const char* compressed, size_t compressed_length,
std::string* uncompressed) {
size_t ulength;
if (!GetUncompressedLength(compressed, compressed_length, &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, compressed_length,
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; }
size_t GetOutputPtr() { return produced_; }
size_t GetBase(ptrdiff_t* op_limit_min_slop) {
*op_limit_min_slop = std::numeric_limits<ptrdiff_t>::max() - kSlopBytes + 1;
return 1;
}
void SetOutputPtr(size_t op) { produced_ = op; }
inline bool CheckLength() const { return expected_ == produced_; }
inline bool Append(const char* ip, size_t len, size_t* produced) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)ip;
*produced += len;
return *produced <= expected_;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t length,
size_t* produced) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)ip;
(void)available;
(void)length;
(void)produced;
return false;
}
inline bool AppendFromSelf(size_t offset, size_t len, size_t* produced) {
// 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 compressed_length) {
ByteArraySource reader(compressed, compressed_length);
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
// If op < op_limit_min_slop_ then it's safe to unconditionally write
// kSlopBytes starting at op.
char* op_limit_min_slop_;
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),
op_limit_min_slop_(NULL) {}
char* GetOutputPtr() { return op_ptr_; }
char* GetBase(ptrdiff_t* op_limit_min_slop) {
*op_limit_min_slop = op_limit_min_slop_ - op_base_;
return op_base_;
}
void SetOutputPtr(char* op) { op_ptr_ = op; }
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, char** op_p) {
char* op = *op_p;
size_t avail = op_limit_ - op;
if (len <= avail) {
// Fast path
std::memcpy(op, ip, len);
*op_p = op + len;
return true;
} else {
op_ptr_ = op;
bool res = SlowAppend(ip, len);
*op_p = op_ptr_;
return res;
}
}
inline bool TryFastAppend(const char* ip, size_t available, size_t length,
char** op_p) {
char* op = *op_p;
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_p = op + length;
return true;
} else {
return false;
}
}
inline bool AppendFromSelf(size_t offset, size_t len, char** op_p) {
char* op = *op_p;
assert(op >= op_base_);
// Check if we try to append from before the start of the buffer.
if (SNAPPY_PREDICT_FALSE((kSlopBytes < 64 && len > kSlopBytes) ||
static_cast<size_t>(op - op_base_) < offset ||
op >= op_limit_min_slop_ || offset < len)) {
if (offset == 0) return false;
if (SNAPPY_PREDICT_FALSE(static_cast<size_t>(op - op_base_) < offset ||
op + len > op_limit_)) {
op_ptr_ = op;
bool res = SlowAppendFromSelf(offset, len);
*op_p = op_ptr_;
return res;
}
*op_p = IncrementalCopy(op - offset, op, op + len, op_limit_);
return true;
}
// Fast path
char* const op_end = op + len;
std::memmove(op, op - offset, kSlopBytes);
*op_p = op_end;
return true;
}
// 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
std::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;
op_limit_min_slop_ = op_limit_ - std::min<size_t>(kSlopBytes - 1, bsize);
blocks_.push_back(op_base_);
avail = bsize;
}
std::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.
// TODO Replace this with a properly optimized path. This is not
// triggered right now. But this is so super slow, that it would regress
// performance unacceptably if triggered.
size_t src = cur - offset;
char* op = op_ptr_;
while (len-- > 0) {
char c = blocks_[src >> kBlockLog][src & (kBlockSize - 1)];
if (!Append(&c, 1, &op)) {
op_ptr_ = op;
return false;
}
src++;
}
op_ptr_ = op;
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;
for (Datablock& block : blocks_) {
size_t block_size = std::min<size_t>(block.size, size - size_written);
dest_->AppendAndTakeOwnership(block.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) {
// TODO: Switch to [[maybe_unused]] when we can assume C++17.
(void)arg;
(void)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_t 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
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