// Copyright 2008 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. // // Internals shared between the Snappy implementation and its unittest. #ifndef THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_ #define THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_ #include "snappy-stubs-internal.h" namespace snappy { namespace internal { // Working memory performs a single allocation to hold all scratch space // required for compression. class WorkingMemory { public: explicit WorkingMemory(size_t input_size); ~WorkingMemory(); // Allocates and clears a hash table using memory in "*this", // stores the number of buckets in "*table_size" and returns a pointer to // the base of the hash table. uint16_t* GetHashTable(size_t fragment_size, int* table_size) const; char* GetScratchInput() const { return input_; } char* GetScratchOutput() const { return output_; } private: char* mem_; // the allocated memory, never nullptr size_t size_; // the size of the allocated memory, never 0 uint16_t* table_; // the pointer to the hashtable char* input_; // the pointer to the input scratch buffer char* output_; // the pointer to the output scratch buffer // No copying WorkingMemory(const WorkingMemory&); void operator=(const WorkingMemory&); }; // Flat array compression that does not emit the "uncompressed length" // prefix. Compresses "input" string to the "*op" buffer. // // REQUIRES: "input_length <= kBlockSize" // REQUIRES: "op" points to an array of memory that is at least // "MaxCompressedLength(input_length)" 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". char* CompressFragment(const char* input, size_t input_length, char* op, uint16_t* table, const int table_size); // Find the largest n such that // // s1[0,n-1] == s2[0,n-1] // and n <= (s2_limit - s2). // // Return make_pair(n, n < 8). // Does not read *s2_limit or beyond. // Does not read *(s1 + (s2_limit - s2)) or beyond. // Requires that s2_limit >= s2. // // In addition populate *data with the next 5 bytes from the end of the match. // This is only done if 8 bytes are available (s2_limit - s2 >= 8). The point is // that on some arch's this can be done faster in this routine than subsequent // loading from s2 + n. // // Separate implementation for 64-bit, little-endian cpus. #if !defined(SNAPPY_IS_BIG_ENDIAN) && \ (defined(__x86_64__) || defined(_M_X64) || defined(ARCH_PPC) || defined(ARCH_ARM)) static inline std::pair FindMatchLength(const char* s1, const char* s2, const char* s2_limit, uint64_t* data) { assert(s2_limit >= s2); size_t matched = 0; // This block isn't necessary for correctness; we could just start looping // immediately. As an optimization though, it is useful. It creates some not // uncommon code paths that determine, without extra effort, whether the match // length is less than 8. In short, we are hoping to avoid a conditional // branch, and perhaps get better code layout from the C++ compiler. if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) { uint64_t a1 = UNALIGNED_LOAD64(s1); uint64_t a2 = UNALIGNED_LOAD64(s2); if (SNAPPY_PREDICT_TRUE(a1 != a2)) { // This code is critical for performance. The reason is that it determines // how much to advance `ip` (s2). This obviously depends on both the loads // from the `candidate` (s1) and `ip`. Furthermore the next `candidate` // depends on the advanced `ip` calculated here through a load, hash and // new candidate hash lookup (a lot of cycles). This makes s1 (ie. // `candidate`) the variable that limits throughput. This is the reason we // go through hoops to have this function update `data` for the next iter. // The straightforward code would use *data, given by // // *data = UNALIGNED_LOAD64(s2 + matched_bytes) (Latency of 5 cycles), // // as input for the hash table lookup to find next candidate. However // this forces the load on the data dependency chain of s1, because // matched_bytes directly depends on s1. However matched_bytes is 0..7, so // we can also calculate *data by // // *data = AlignRight(UNALIGNED_LOAD64(s2), UNALIGNED_LOAD64(s2 + 8), // matched_bytes); // // The loads do not depend on s1 anymore and are thus off the bottleneck. // The straightforward implementation on x86_64 would be to use // // shrd rax, rdx, cl (cl being matched_bytes * 8) // // unfortunately shrd with a variable shift has a 4 cycle latency. So this // only wins 1 cycle. The BMI2 shrx instruction is a 1 cycle variable // shift instruction but can only shift 64 bits. If we focus on just // obtaining the least significant 4 bytes, we can obtain this by // // *data = ConditionalMove(matched_bytes < 4, UNALIGNED_LOAD64(s2), // UNALIGNED_LOAD64(s2 + 4) >> ((matched_bytes & 3) * 8); // // Writen like above this is not a big win, the conditional move would be // a cmp followed by a cmov (2 cycles) followed by a shift (1 cycle). // However matched_bytes < 4 is equal to // static_cast(xorval) != 0. Writen that way, the conditional // move (2 cycles) can execute in parallel with FindLSBSetNonZero64 // (tzcnt), which takes 3 cycles. uint64_t xorval = a1 ^ a2; int shift = Bits::FindLSBSetNonZero64(xorval); size_t matched_bytes = shift >> 3; #ifndef __x86_64__ *data = UNALIGNED_LOAD64(s2 + matched_bytes); #else // Ideally this would just be // // a2 = static_cast(xorval) == 0 ? a3 : a2; // // However clang correctly infers that the above statement participates on // a critical data dependency chain and thus, unfortunately, refuses to // use a conditional move (it's tuned to cut data dependencies). In this // case there is a longer parallel chain anyway AND this will be fairly // unpredictable. uint64_t a3 = UNALIGNED_LOAD64(s2 + 4); asm("testl %k2, %k2\n\t" "cmovzq %1, %0\n\t" : "+r"(a2) : "r"(a3), "r"(xorval)); *data = a2 >> (shift & (3 * 8)); #endif return std::pair(matched_bytes, true); } else { matched = 8; s2 += 8; } } // Find out how long the match is. We loop over the data 64 bits at a // time until we find a 64-bit block that doesn't match; then we find // the first non-matching bit and use that to calculate the total // length of the match. while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) { uint64_t a1 = UNALIGNED_LOAD64(s1 + matched); uint64_t a2 = UNALIGNED_LOAD64(s2); if (a1 == a2) { s2 += 8; matched += 8; } else { uint64_t xorval = a1 ^ a2; int shift = Bits::FindLSBSetNonZero64(xorval); size_t matched_bytes = shift >> 3; #ifndef __x86_64__ *data = UNALIGNED_LOAD64(s2 + matched_bytes); #else uint64_t a3 = UNALIGNED_LOAD64(s2 + 4); asm("testl %k2, %k2\n\t" "cmovzq %1, %0\n\t" : "+r"(a2) : "r"(a3), "r"(xorval)); *data = a2 >> (shift & (3 * 8)); #endif matched += matched_bytes; assert(matched >= 8); return std::pair(matched, false); } } while (SNAPPY_PREDICT_TRUE(s2 < s2_limit)) { if (s1[matched] == *s2) { ++s2; ++matched; } else { if (s2 <= s2_limit - 8) { *data = UNALIGNED_LOAD64(s2); } return std::pair(matched, matched < 8); } } return std::pair(matched, matched < 8); } #else static inline std::pair FindMatchLength(const char* s1, const char* s2, const char* s2_limit, uint64_t* data) { // Implementation based on the x86-64 version, above. assert(s2_limit >= s2); int matched = 0; while (s2 <= s2_limit - 4 && UNALIGNED_LOAD32(s2) == UNALIGNED_LOAD32(s1 + matched)) { s2 += 4; matched += 4; } if (LittleEndian::IsLittleEndian() && s2 <= s2_limit - 4) { uint32_t x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched); int matching_bits = Bits::FindLSBSetNonZero(x); matched += matching_bits >> 3; s2 += matching_bits >> 3; } else { while ((s2 < s2_limit) && (s1[matched] == *s2)) { ++s2; ++matched; } } if (s2 <= s2_limit - 8) *data = LittleEndian::Load64(s2); return std::pair(matched, matched < 8); } #endif // Lookup tables for decompression code. Give --snappy_dump_decompression_table // to the unit test to recompute char_table. enum { LITERAL = 0, COPY_1_BYTE_OFFSET = 1, // 3 bit length + 3 bits of offset in opcode COPY_2_BYTE_OFFSET = 2, COPY_4_BYTE_OFFSET = 3 }; static const int kMaximumTagLength = 5; // COPY_4_BYTE_OFFSET plus the actual offset. // Data stored per entry in lookup table: // Range Bits-used Description // ------------------------------------ // 1..64 0..7 Literal/copy length encoded in opcode byte // 0..7 8..10 Copy offset encoded in opcode byte / 256 // 0..4 11..13 Extra bytes after opcode // // We use eight bits for the length even though 7 would have sufficed // because of efficiency reasons: // (1) Extracting a byte is faster than a bit-field // (2) It properly aligns copy offset so we do not need a <<8 static const uint16_t char_table[256] = { 0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002, 0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004, 0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006, 0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008, 0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a, 0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c, 0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e, 0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010, 0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012, 0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014, 0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016, 0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018, 0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a, 0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c, 0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e, 0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020, 0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022, 0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024, 0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026, 0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028, 0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a, 0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c, 0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e, 0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030, 0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032, 0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034, 0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036, 0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038, 0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a, 0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c, 0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e, 0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040 }; } // end namespace internal } // end namespace snappy #endif // THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_