318 lines
14 KiB
C
318 lines
14 KiB
C
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// Copyright 2008 Google Inc. All Rights Reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above
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// copyright notice, this list of conditions and the following disclaimer
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// in the documentation and/or other materials provided with the
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// distribution.
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// * Neither the name of Google Inc. nor the names of its
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// contributors may be used to endorse or promote products derived from
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// this software without specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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//
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// Internals shared between the Snappy implementation and its unittest.
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#ifndef THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
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#define THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
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#include "snappy-stubs-internal.h"
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namespace snappy {
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namespace internal {
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// Working memory performs a single allocation to hold all scratch space
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// required for compression.
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class WorkingMemory {
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public:
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explicit WorkingMemory(size_t input_size);
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~WorkingMemory();
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// Allocates and clears a hash table using memory in "*this",
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// stores the number of buckets in "*table_size" and returns a pointer to
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// the base of the hash table.
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uint16_t* GetHashTable(size_t fragment_size, int* table_size) const;
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char* GetScratchInput() const { return input_; }
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char* GetScratchOutput() const { return output_; }
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private:
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char* mem_; // the allocated memory, never nullptr
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size_t size_; // the size of the allocated memory, never 0
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uint16_t* table_; // the pointer to the hashtable
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char* input_; // the pointer to the input scratch buffer
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char* output_; // the pointer to the output scratch buffer
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// No copying
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WorkingMemory(const WorkingMemory&);
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void operator=(const WorkingMemory&);
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};
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// Flat array compression that does not emit the "uncompressed length"
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// prefix. Compresses "input" string to the "*op" buffer.
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//
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// REQUIRES: "input_length <= kBlockSize"
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// REQUIRES: "op" points to an array of memory that is at least
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// "MaxCompressedLength(input_length)" in size.
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// REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
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// REQUIRES: "table_size" is a power of two
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//
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// Returns an "end" pointer into "op" buffer.
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// "end - op" is the compressed size of "input".
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char* CompressFragment(const char* input,
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size_t input_length,
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char* op,
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uint16_t* table,
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const int table_size);
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// Find the largest n such that
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//
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// s1[0,n-1] == s2[0,n-1]
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// and n <= (s2_limit - s2).
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//
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// Return make_pair(n, n < 8).
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// Does not read *s2_limit or beyond.
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// Does not read *(s1 + (s2_limit - s2)) or beyond.
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// Requires that s2_limit >= s2.
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//
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// In addition populate *data with the next 5 bytes from the end of the match.
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// This is only done if 8 bytes are available (s2_limit - s2 >= 8). The point is
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// that on some arch's this can be done faster in this routine than subsequent
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// loading from s2 + n.
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//
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// Separate implementation for 64-bit, little-endian cpus.
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#if !defined(SNAPPY_IS_BIG_ENDIAN) && \
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(defined(__x86_64__) || defined(_M_X64) || defined(ARCH_PPC) || defined(ARCH_ARM))
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static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit,
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uint64_t* data) {
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assert(s2_limit >= s2);
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size_t matched = 0;
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// This block isn't necessary for correctness; we could just start looping
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// immediately. As an optimization though, it is useful. It creates some not
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// uncommon code paths that determine, without extra effort, whether the match
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// length is less than 8. In short, we are hoping to avoid a conditional
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// branch, and perhaps get better code layout from the C++ compiler.
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if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
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uint64_t a1 = UNALIGNED_LOAD64(s1);
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uint64_t a2 = UNALIGNED_LOAD64(s2);
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if (SNAPPY_PREDICT_TRUE(a1 != a2)) {
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// This code is critical for performance. The reason is that it determines
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// how much to advance `ip` (s2). This obviously depends on both the loads
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// from the `candidate` (s1) and `ip`. Furthermore the next `candidate`
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// depends on the advanced `ip` calculated here through a load, hash and
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// new candidate hash lookup (a lot of cycles). This makes s1 (ie.
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// `candidate`) the variable that limits throughput. This is the reason we
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// go through hoops to have this function update `data` for the next iter.
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// The straightforward code would use *data, given by
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//
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// *data = UNALIGNED_LOAD64(s2 + matched_bytes) (Latency of 5 cycles),
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//
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// as input for the hash table lookup to find next candidate. However
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// this forces the load on the data dependency chain of s1, because
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// matched_bytes directly depends on s1. However matched_bytes is 0..7, so
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// we can also calculate *data by
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//
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// *data = AlignRight(UNALIGNED_LOAD64(s2), UNALIGNED_LOAD64(s2 + 8),
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// matched_bytes);
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//
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// The loads do not depend on s1 anymore and are thus off the bottleneck.
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// The straightforward implementation on x86_64 would be to use
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//
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// shrd rax, rdx, cl (cl being matched_bytes * 8)
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//
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// unfortunately shrd with a variable shift has a 4 cycle latency. So this
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// only wins 1 cycle. The BMI2 shrx instruction is a 1 cycle variable
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// shift instruction but can only shift 64 bits. If we focus on just
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// obtaining the least significant 4 bytes, we can obtain this by
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//
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// *data = ConditionalMove(matched_bytes < 4, UNALIGNED_LOAD64(s2),
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// UNALIGNED_LOAD64(s2 + 4) >> ((matched_bytes & 3) * 8);
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//
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// Writen like above this is not a big win, the conditional move would be
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// a cmp followed by a cmov (2 cycles) followed by a shift (1 cycle).
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// However matched_bytes < 4 is equal to
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// static_cast<uint32_t>(xorval) != 0. Writen that way, the conditional
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// move (2 cycles) can execute in parallel with FindLSBSetNonZero64
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// (tzcnt), which takes 3 cycles.
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uint64_t xorval = a1 ^ a2;
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int shift = Bits::FindLSBSetNonZero64(xorval);
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size_t matched_bytes = shift >> 3;
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#ifndef __x86_64__
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*data = UNALIGNED_LOAD64(s2 + matched_bytes);
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#else
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// Ideally this would just be
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//
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// a2 = static_cast<uint32_t>(xorval) == 0 ? a3 : a2;
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//
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// However clang correctly infers that the above statement participates on
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// a critical data dependency chain and thus, unfortunately, refuses to
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// use a conditional move (it's tuned to cut data dependencies). In this
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// case there is a longer parallel chain anyway AND this will be fairly
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// unpredictable.
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uint64_t a3 = UNALIGNED_LOAD64(s2 + 4);
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asm("testl %k2, %k2\n\t"
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"cmovzq %1, %0\n\t"
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: "+r"(a2)
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: "r"(a3), "r"(xorval));
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*data = a2 >> (shift & (3 * 8));
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#endif
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return std::pair<size_t, bool>(matched_bytes, true);
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} else {
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matched = 8;
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s2 += 8;
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}
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}
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// Find out how long the match is. We loop over the data 64 bits at a
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// time until we find a 64-bit block that doesn't match; then we find
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// the first non-matching bit and use that to calculate the total
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// length of the match.
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while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
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uint64_t a1 = UNALIGNED_LOAD64(s1 + matched);
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uint64_t a2 = UNALIGNED_LOAD64(s2);
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if (a1 == a2) {
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s2 += 8;
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matched += 8;
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} else {
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uint64_t xorval = a1 ^ a2;
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int shift = Bits::FindLSBSetNonZero64(xorval);
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size_t matched_bytes = shift >> 3;
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#ifndef __x86_64__
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*data = UNALIGNED_LOAD64(s2 + matched_bytes);
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#else
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uint64_t a3 = UNALIGNED_LOAD64(s2 + 4);
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asm("testl %k2, %k2\n\t"
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"cmovzq %1, %0\n\t"
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: "+r"(a2)
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: "r"(a3), "r"(xorval));
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*data = a2 >> (shift & (3 * 8));
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#endif
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matched += matched_bytes;
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assert(matched >= 8);
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return std::pair<size_t, bool>(matched, false);
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}
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}
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while (SNAPPY_PREDICT_TRUE(s2 < s2_limit)) {
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if (s1[matched] == *s2) {
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++s2;
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++matched;
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} else {
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if (s2 <= s2_limit - 8) {
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*data = UNALIGNED_LOAD64(s2);
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}
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return std::pair<size_t, bool>(matched, matched < 8);
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}
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}
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return std::pair<size_t, bool>(matched, matched < 8);
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}
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#else
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static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
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const char* s2,
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const char* s2_limit,
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uint64_t* data) {
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// Implementation based on the x86-64 version, above.
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assert(s2_limit >= s2);
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int matched = 0;
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while (s2 <= s2_limit - 4 &&
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UNALIGNED_LOAD32(s2) == UNALIGNED_LOAD32(s1 + matched)) {
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s2 += 4;
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matched += 4;
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}
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if (LittleEndian::IsLittleEndian() && s2 <= s2_limit - 4) {
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uint32_t x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
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int matching_bits = Bits::FindLSBSetNonZero(x);
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matched += matching_bits >> 3;
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s2 += matching_bits >> 3;
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} else {
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while ((s2 < s2_limit) && (s1[matched] == *s2)) {
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++s2;
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++matched;
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}
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}
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if (s2 <= s2_limit - 8) *data = LittleEndian::Load64(s2);
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return std::pair<size_t, bool>(matched, matched < 8);
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}
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#endif
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// Lookup tables for decompression code. Give --snappy_dump_decompression_table
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// to the unit test to recompute char_table.
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enum {
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LITERAL = 0,
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COPY_1_BYTE_OFFSET = 1, // 3 bit length + 3 bits of offset in opcode
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COPY_2_BYTE_OFFSET = 2,
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COPY_4_BYTE_OFFSET = 3
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};
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static const int kMaximumTagLength = 5; // COPY_4_BYTE_OFFSET plus the actual offset.
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// Data stored per entry in lookup table:
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// Range Bits-used Description
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// ------------------------------------
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// 1..64 0..7 Literal/copy length encoded in opcode byte
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// 0..7 8..10 Copy offset encoded in opcode byte / 256
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// 0..4 11..13 Extra bytes after opcode
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//
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// We use eight bits for the length even though 7 would have sufficed
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// because of efficiency reasons:
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// (1) Extracting a byte is faster than a bit-field
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// (2) It properly aligns copy offset so we do not need a <<8
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static constexpr uint16_t char_table[256] = {
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// clang-format off
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0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
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0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
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0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
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0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
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0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
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0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
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0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
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0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
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0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
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0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
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0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
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0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
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0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
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0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
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0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
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0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
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0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
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0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
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0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
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0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
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0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
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0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
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0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
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0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
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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,
|
||
|
|
// clang-format on
|
||
|
|
};
|
||
|
|
|
||
|
|
} // end namespace internal
|
||
|
|
} // end namespace snappy
|
||
|
|
|
||
|
|
#endif // THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
|