src/mem/cache/compressors/base.cc

   1 /*
   2  * Copyright (c) 2018-2020 Inria
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
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   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  */
  28
  29 /** @file
  30  * Definition of a basic cache compressor.
  31  */
  32
  33 #include "mem/cache/compressors/base.hh"
  34
  35 #include <algorithm>
  36 #include <cmath>
  37 #include <cstdint>
  38 #include <string>
  39
  40 #include "base/trace.hh"
  41 #include "debug/CacheComp.hh"
  42 #include "mem/cache/tags/super_blk.hh"
  43 #include "params/BaseCacheCompressor.hh"
  44
  45 namespace Compressor {
  46
  47 // Uncomment this line if debugging compression
  48 //#define DEBUG_COMPRESSION
  49
  50 Base::CompressionData::CompressionData()
  51     : _size(0)
  52 {
  53 }
  54
  55 Base::CompressionData::~CompressionData()
  56 {
  57 }
  58
  59 void
  60 Base::CompressionData::setSizeBits(std::size_t size)
  61 {
  62     _size = size;
  63 }
  64
  65 std::size_t
  66 Base::CompressionData::getSizeBits() const
  67 {
  68     return _size;
  69 }
  70
  71 std::size_t
  72 Base::CompressionData::getSize() const
  73 {
  74     return std::ceil(_size/8);
  75 }
  76
  77 Base::Base(const Params *p)
  78   : SimObject(p), blkSize(p->block_size), sizeThreshold(p->size_threshold),
  79     stats(*this)
  80 {
  81     fatal_if(blkSize < sizeThreshold, "Compressed data must fit in a block");
  82 }
  83
  84 void
  85 Base::compress(const uint64_t* data, Cycles& comp_lat,
  86                               Cycles& decomp_lat, std::size_t& comp_size_bits)
  87 {
  88     // Apply compression
  89     std::unique_ptr<CompressionData> comp_data =
  90         compress(data, comp_lat, decomp_lat);
  91
  92     // If we are in debug mode apply decompression just after the compression.
  93     // If the results do not match, we've got an error
  94     #ifdef DEBUG_COMPRESSION
  95     uint64_t decomp_data[blkSize/8];
  96
  97     // Apply decompression
  98     decompress(comp_data.get(), decomp_data);
  99
 100     // Check if decompressed line matches original cache line
 101     fatal_if(std::memcmp(data, decomp_data, blkSize),
 102              "Decompressed line does not match original line.");
 103     #endif
 104
 105     // Get compression size. If compressed size is greater than the size
 106     // threshold, the compression is seen as unsuccessful
 107     comp_size_bits = comp_data->getSizeBits();
 108     if (comp_size_bits >= sizeThreshold * 8) {
 109         comp_size_bits = blkSize * 8;
 110     }
 111
 112     // Update stats
 113     stats.compressions++;
 114     stats.compressionSizeBits += comp_size_bits;
 115     stats.compressionSize[std::ceil(std::log2(comp_size_bits))]++;
 116
 117     // Print debug information
 118     DPRINTF(CacheComp, "Compressed cache line from %d to %d bits. " \
 119             "Compression latency: %llu, decompression latency: %llu\n",
 120             blkSize*8, comp_size_bits, comp_lat, decomp_lat);
 121 }
 122
 123 Cycles
 124 Base::getDecompressionLatency(const CacheBlk* blk)
 125 {
 126     const CompressionBlk* comp_blk = static_cast<const CompressionBlk*>(blk);
 127
 128     // If block is compressed, return its decompression latency
 129     if (comp_blk && comp_blk->isCompressed()){
 130         const Cycles decomp_lat = comp_blk->getDecompressionLatency();
 131         DPRINTF(CacheComp, "Decompressing block: %s (%d cycles)\n",
 132                 comp_blk->print(), decomp_lat);
 133         stats.decompressions += 1;
 134         return decomp_lat;
 135     }
 136
 137     // Block is not compressed, so there is no decompression latency
 138     return Cycles(0);
 139 }
 140
 141 void
 142 Base::setDecompressionLatency(CacheBlk* blk, const Cycles lat)
 143 {
 144     // Sanity check
 145     assert(blk != nullptr);
 146
 147     // Assign latency
 148     static_cast<CompressionBlk*>(blk)->setDecompressionLatency(lat);
 149 }
 150
 151 void
 152 Base::setSizeBits(CacheBlk* blk, const std::size_t size_bits)
 153 {
 154     // Sanity check
 155     assert(blk != nullptr);
 156
 157     // Assign size
 158     static_cast<CompressionBlk*>(blk)->setSizeBits(size_bits);
 159 }
 160
 161 Base::BaseStats::BaseStats(Base& _compressor)
 162   : Stats::Group(&_compressor), compressor(_compressor),
 163     compressions(this, "compressions",
 164         "Total number of compressions"),
 165     compressionSize(this, "compression_size",
 166         "Number of blocks that were compressed to this power of two size"),
 167     compressionSizeBits(this, "compression_size_bits",
 168         "Total compressed data size, in bits"),
 169     avgCompressionSizeBits(this, "avg_compression_size_bits",
 170         "Average compression size, in bits"),
 171     decompressions(this, "total_decompressions",
 172         "Total number of decompressions")
 173 {
 174 }
 175
 176 void
 177 Base::BaseStats::regStats()
 178 {
 179     Stats::Group::regStats();
 180
 181     compressionSize.init(std::log2(compressor.blkSize*8) + 1);
 182     for (unsigned i = 0; i <= std::log2(compressor.blkSize*8); ++i) {
 183         std::string str_i = std::to_string(1 << i);
 184         compressionSize.subname(i, str_i);
 185         compressionSize.subdesc(i,
 186             "Number of blocks that compressed to fit in " + str_i + " bits");
 187     }
 188
 189     avgCompressionSizeBits.flags(Stats::total | Stats::nozero | Stats::nonan);
 190     avgCompressionSizeBits = compressionSizeBits / compressions;
 191 }
 192
 193 } // namespace Compressor