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Diffstat (limited to 'other-licenses/snappy/src/snappy.cc')
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diff --git a/other-licenses/snappy/src/snappy.cc b/other-licenses/snappy/src/snappy.cc new file mode 100644 index 0000000000..57df3f11fc --- /dev/null +++ b/other-licenses/snappy/src/snappy.cc @@ -0,0 +1,2193 @@ +// 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 - offset)[i]; + // 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__) && defined(__GCC_ASM_FLAG_OUTPUTS__) + // 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 |