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/*
* Copyright (c) 2019-2020 Inria
* 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 the copyright holders 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.
*/
#ifndef __MEM_CACHE_COMPRESSORS_FREQUENT_VALUES_HH__
#define __MEM_CACHE_COMPRESSORS_FREQUENT_VALUES_HH__
#include <climits>
#include <cstdint>
#include <memory>
#include <vector>
#include "base/sat_counter.hh"
#include "base/types.hh"
#include "mem/cache/base.hh"
#include "mem/cache/compressors/base.hh"
#include "mem/cache/compressors/encoders/huffman.hh"
#include "mem/cache/prefetch/associative_set.hh"
#include "sim/eventq.hh"
#include "sim/probe/probe.hh"
struct FrequentValuesCompressorParams;
namespace Compressor {
/**
* This compressor samples the cache for a while, trying to define the
* most frequently used values. When these values are determined, they are
* associated to shorter representations (codes). Then the compressor can
* start its effective compression phase, in which occurrences of these
* values are substituted by their codes.
*/
class FrequentValues : public Base
{
private:
class CompData;
using DataUpdate = BaseCache::DataUpdate;
class FrequentValuesListener : public ProbeListenerArgBase<DataUpdate>
{
protected:
FrequentValues &parent;
public:
FrequentValuesListener(FrequentValues &_parent, ProbeManager *pm,
const std::string &name)
: ProbeListenerArgBase(pm, name), parent(_parent)
{
}
void notify(const DataUpdate &data_update) override;
};
std::vector<FrequentValuesListener*> listeners;
/** Whether Huffman encoding is applied to the VFT indices. */
const bool useHuffmanEncoding;
/** The encoder applied to the VFT indices. */
Encoder::Huffman encoder;
/** Number of bits in the saturating counters. */
const int counterBits;
/** Ticks needed to perform the CODE_GENERATION phase. */
const Tick codeGenerationTicks;
/** Whether an action must be performed when counters saturate. */
const bool checkSaturation;
/** Maximum number of VFT entries, and thus of codewords too. */
const unsigned numVFTEntries;
/** Number of samples in the sampling phase. */
const unsigned numSamples;
/** Number of samples taken so far. */
unsigned takenSamples;
/**
* The phase that the compressor is at. It assumes that sampling and
* code generation are done only once.
*/
enum Phase {SAMPLING, CODE_GENERATION, COMPRESSING};
Phase phase;
class VFTEntry : public TaggedEntry
{
public:
/**
* The value is stored as a 64 bit entry to accomodate for the
* uncompressed value. All real values must be 32 bits.
*/
uint64_t value;
/**
* The ideal counter width (in bits) is determined by the maximum
* number of times a given value appears in the cache
* (log2(cache_size / chunkSizeBits)). If smaller counters are used
* their values should be rescaled when saturated.
*/
SatCounter32 counter;
VFTEntry(std::size_t num_bits)
: TaggedEntry(), value(0), counter(num_bits)
{
}
void
invalidate() override
{
TaggedEntry::invalidate();
value = 0;
counter.reset();
}
};
/**
* The Value Frequency Table, a small cache that keeps track and estimates
* the frequency distribution of values in the cache.
*/
AssociativeSet<VFTEntry> VFT;
/**
* A pseudo value is used as the representation of uncompressed values.
* This value is a random value that is not present in the VFT. It is
* selected at the end of the sampling phase.
*/
uint64_t uncompressedValue;
/** Event to handle finishing code generation and starting compression. */
EventFunctionWrapper codeGenerationEvent;
/**
* Sample values from a packet, adding them to the VFT.
*
* @param data The line being sampled.
* @param is_invalidation whether this event comes from an invalidation.
*/
void sampleValues(const std::vector<uint64_t> &data,
bool is_invalidation);
/** End sampling phase and start the code generation. */
void generateCodes();
std::unique_ptr<Base::CompressionData> compress(
const std::vector<Chunk>& chunks, Cycles& comp_lat,
Cycles& decomp_lat) override;
void decompress(const CompressionData* comp_data, uint64_t* data) override;
public:
typedef FrequentValuesCompressorParams Params;
FrequentValues(const Params &p);
~FrequentValues() = default;
/**
* Process a notification event from the ProbeListener.
*
* @param data_update The data regarding the entry's contents update.
*/
void probeNotify(const DataUpdate &data_update);
void regProbeListeners() override;
};
class FrequentValues::CompData : public CompressionData
{
public:
/**
* A compressed value contains its encoding, and the compressed data
* itself.
*/
struct CompressedValue
{
/** The codeword.*/
Encoder::Code code;
/**
* Original value, stored both for when the codeword marks an
* uncompressed entry, and to verify correctness.
*/
uint64_t value;
CompressedValue(Encoder::Code _code, uint64_t _value)
: code(_code), value(_value)
{
}
};
/**
* The values contained in the original data, after being compressed
* sequentially.
*/
std::vector<CompressedValue> compressedValues;
};
} // namespace Compressor
#endif //__MEM_CACHE_COMPRESSORS_FREQUENT_VALUES_HH__