src/mem/cache/compressors/base.cc

   1 /*
   2  * Copyright (c) 2018 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
   7  * met: redistributions of source code must retain the above copyright
   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  * Authors: Daniel Carvalho
  29  */
  30
  31 /** @file
  32  * Definition of a basic cache compressor.
  33  */
  34
  35 #include "mem/cache/compressors/base.hh"
  36
  37 #include <algorithm>
  38 #include <cstdint>
  39 #include <string>
  40
  41 #include "debug/CacheComp.hh"
  42 #include "mem/cache/tags/super_blk.hh"
  43 #include "params/BaseCacheCompressor.hh"
  44
  45 // Uncomment this line if debugging compression
  46 //#define DEBUG_COMPRESSION
  47
  48 BaseCacheCompressor::CompressionData::CompressionData()
  49     : _size(0)
  50 {
  51 }
  52
  53 BaseCacheCompressor::CompressionData::~CompressionData()
  54 {
  55 }
  56
  57 void
  58 BaseCacheCompressor::CompressionData::setSizeBits(std::size_t size)
  59 {
  60     _size = size;
  61 }
  62
  63 std::size_t
  64 BaseCacheCompressor::CompressionData::getSizeBits() const
  65 {
  66     return _size;
  67 }
  68
  69 std::size_t
  70 BaseCacheCompressor::CompressionData::getSize() const
  71 {
  72     return std::ceil(_size/8);
  73 }
  74
  75 BaseCacheCompressor::BaseCacheCompressor(const Params *p)
  76     : SimObject(p), blkSize(p->block_size), sizeThreshold(p->size_threshold)
  77 {
  78     fatal_if(blkSize < sizeThreshold, "Compressed data must fit in a block");
  79 }
  80
  81 void
  82 BaseCacheCompressor::compress(const uint64_t* data, Cycles& comp_lat,
  83                               Cycles& decomp_lat, std::size_t& comp_size_bits)
  84 {
  85     // Apply compression
  86     std::unique_ptr<CompressionData> comp_data =
  87         compress(data, comp_lat, decomp_lat);
  88
  89     // If we are in debug mode apply decompression just after the compression.
  90     // If the results do not match, we've got an error
  91     #ifdef DEBUG_COMPRESSION
  92     uint64_t decomp_data[blkSize/8];
  93
  94     // Apply decompression
  95     decompress(comp_data.get(), decomp_data);
  96
  97     // Check if decompressed line matches original cache line
  98     fatal_if(std::memcmp(data, decomp_data, blkSize),
  99              "Decompressed line does not match original line.");
 100     #endif
 101
 102     // Get compression size. If compressed size is greater than the size
 103     // threshold, the compression is seen as unsuccessful
 104     comp_size_bits = comp_data->getSizeBits();
 105     if (comp_size_bits >= sizeThreshold * 8) {
 106         comp_size_bits = blkSize * 8;
 107     }
 108
 109     // Update stats
 110     compressionSize[std::ceil(std::log2(comp_size_bits))]++;
 111
 112     // Print debug information
 113     DPRINTF(CacheComp, "Compressed cache line from %d to %d bits. " \
 114             "Compression latency: %llu, decompression latency: %llu\n",
 115             blkSize*8, comp_size_bits, comp_lat, decomp_lat);
 116 }
 117
 118 Cycles
 119 BaseCacheCompressor::getDecompressionLatency(const CacheBlk* blk) const
 120 {
 121     const CompressionBlk* comp_blk = static_cast<const CompressionBlk*>(blk);
 122
 123     // If block is compressed, return its decompression latency
 124     if (comp_blk && comp_blk->isCompressed()){
 125         const Cycles decomp_lat = comp_blk->getDecompressionLatency();
 126         DPRINTF(CacheComp, "Decompressing block: %s (%d cycles)\n",
 127                 comp_blk->print(), decomp_lat);
 128         return decomp_lat;
 129     }
 130
 131     // Block is not compressed, so there is no decompression latency
 132     return Cycles(0);
 133 }
 134
 135 void
 136 BaseCacheCompressor::setDecompressionLatency(CacheBlk* blk, const Cycles lat)
 137 {
 138     // Sanity check
 139     assert(blk != nullptr);
 140
 141     // Assign latency
 142     static_cast<CompressionBlk*>(blk)->setDecompressionLatency(lat);
 143 }
 144
 145 void
 146 BaseCacheCompressor::setSizeBits(CacheBlk* blk, const std::size_t size_bits)
 147 {
 148     // Sanity check
 149     assert(blk != nullptr);
 150
 151     // Assign size
 152     static_cast<CompressionBlk*>(blk)->setSizeBits(size_bits);
 153 }
 154
 155 void
 156 BaseCacheCompressor::regStats()
 157 {
 158     SimObject::regStats();
 159
 160     compressionSize
 161         .init(std::log2(blkSize*8) + 1)
 162         .name(name() + ".compression_size")
 163         .desc("Number of blocks that were compressed to this power of" \
 164               "two size.")
 165         ;
 166
 167     for (unsigned i = 0; i <= std::log2(blkSize*8); ++i) {
 168         compressionSize.subname(i, std::to_string(1 << i));
 169         compressionSize.subdesc(i, "Number of blocks that compressed to fit " \
 170                                    "in " + std::to_string(1 << i) + " bits");
 171     }
 172 }
 173