src/mem/cache/compressors/base.hh

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/** @file
 * Definition of a basic cache compressor.
 * A cache compressor must consist of a compression and a decompression
 * methods. It must also be aware of the size of an uncompressed cache
 * line.
 */

#ifndef __MEM_CACHE_COMPRESSORS_BASE_HH__
#define __MEM_CACHE_COMPRESSORS_BASE_HH__

#include <cstdint>

#include "base/statistics.hh"
#include "base/types.hh"
#include "sim/sim_object.hh"

class CacheBlk;
struct BaseCacheCompressorParams;

/**
 * Base cache compressor interface. Every cache compressor must implement a
 * compression and a decompression method.
 */
class BaseCacheCompressor : public SimObject
{
  protected:
    /**
     * This compressor must be able to access the protected functions of
     * its sub-compressors.
     */
    friend class MultiCompressor;

    /**
     * Forward declaration of compression data. Every new compressor must
     * create a new compression data based on it.
     */
    class CompressionData;

    /**
     * Uncompressed cache line size (in bytes).
     */
    const std::size_t blkSize;

    /**
     * Size in bytes at which a compression is classified as bad and therefore
     * the compressed block is restored to its uncompressed format.
     */
    const std::size_t sizeThreshold;

    struct BaseCacheCompressorStats : public Stats::Group
    {
        const BaseCacheCompressor& compressor;

        BaseCacheCompressorStats(BaseCacheCompressor& compressor);

        void regStats() override;

        /** Number of compressions performed. */
        Stats::Scalar compressions;

        /** Number of blocks that were compressed to this power of two size. */
        Stats::Vector compressionSize;

        /** Total compressed data size, in number of bits. */
        Stats::Scalar compressionSizeBits;

        /** Average data size after compression, in number of bits. */
        Stats::Formula avgCompressionSizeBits;

        /** Number of decompressions performed. */
        Stats::Scalar decompressions;
    } stats;

    /**
     * Apply the compression process to the cache line.
     * Returns the number of cycles used by the compressor, however it is
     * usually covered by a good pipelined execution, and is currently ignored.
     * The decompression latency is also returned, in order to avoid
     * increasing simulation time and memory consumption.
     *
     * @param cache_line The cache line to be compressed.
     * @param comp_lat Compression latency in number of cycles.
     * @param decomp_lat Decompression latency in number of cycles.
     * @return Cache line after compression.
     */
    virtual std::unique_ptr<CompressionData> compress(
        const uint64_t* cache_line, Cycles& comp_lat, Cycles& decomp_lat) = 0;

    /**
     * Apply the decompression process to the compressed data.
     *
     * @param comp_data Compressed cache line.
     * @param cache_line The cache line to be decompressed.
     */
    virtual void decompress(const CompressionData* comp_data,
                              uint64_t* cache_line) = 0;

  public:
    /** Convenience typedef. */
     typedef BaseCacheCompressorParams Params;

    /**
     * Default constructor.
     */
    BaseCacheCompressor(const Params *p);

    /**
     * Default destructor.
     */
    virtual ~BaseCacheCompressor() {};

    /**
     * Apply the compression process to the cache line. Ignores compression
     * cycles.
     *
     * @param data The cache line to be compressed.
     * @param comp_lat Compression latency in number of cycles.
     * @param decomp_lat Decompression latency in number of cycles.
     * @param comp_size_bits Compressed data size (in bits).
     */
    void compress(const uint64_t* data, Cycles& comp_lat,
                  Cycles& decomp_lat, std::size_t& comp_size_bits);

    /**
     * Get the decompression latency if the block is compressed. Latency is 0
     * otherwise.
     *
     * @param blk The compressed block.
     */
    Cycles getDecompressionLatency(const CacheBlk* blk);

    /**
     * Set the decompression latency of compressed block.
     *
     * @param blk The compressed block.
     * @param lat The decompression latency.
     */
    static void setDecompressionLatency(CacheBlk* blk, const Cycles lat);

    /**
     * Set the size of the compressed block, in bits.
     *
     * @param blk The compressed block.
     * @param size_bits The block size.
     */
    static void setSizeBits(CacheBlk* blk, const std::size_t size_bits);
};

class BaseCacheCompressor::CompressionData {
  private:
    /**
     * Compressed cache line size (in bits).
     */
    std::size_t _size;

  public:
    /**
     * Default constructor.
     */
    CompressionData();

    /**
     * Virtual destructor. Without it unique_ptr will cause mem leak.
     */
    virtual ~CompressionData();

    /**
     * Set compression size (in bits).
     *
     * @param size Compressed data size.
     */
    void setSizeBits(std::size_t size);

    /**
     * Get compression size (in bits).
     *
     * @return Compressed data size.
     */
    std::size_t getSizeBits() const;

    /**
     * Get compression size (in bytes).
     *
     * @return Compressed data size.
     */
    std::size_t getSize() const;
};

#endif //__MEM_CACHE_COMPRESSORS_BASE_HH__