super_blk.cc

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#include "mem/cache/tags/super_blk.hh"
 
#include <climits>
#include <cmath>
 
#include "base/bitfield.hh"
 
namespace gem5
{
 
CompressionBlk::CompressionBlk()
    : SectorSubBlk(), _size(0), _decompressionLatency(0), _compressed(false)
{
}
 
CacheBlk&
CompressionBlk::operator=(CacheBlk&& other)
{
    operator=(std::move(static_cast<CompressionBlk&&>(other)));
    return *this;
}
 
CompressionBlk&
CompressionBlk::operator=(CompressionBlk&& other)
{
    // Copy internal variables; if moving, that means we had an expansion or
    // contraction, and therefore the size is no longer valid, so it is not
    // moved
    setDecompressionLatency(other.getDecompressionLatency());
    if (other.isCompressed()) {
        setCompressed();
    } else {
        setUncompressed();
    }
 
    CacheBlk::operator=(std::move(other));
    return *this;
}
 
bool
CompressionBlk::isCompressed() const
{
    return _compressed;
}
 
void
CompressionBlk::setCompressed()
{
    _compressed = true;
}
 
void
CompressionBlk::setUncompressed()
{
    _compressed = false;
}
 
std::size_t
CompressionBlk::getSizeBits() const
{
    return _size;
}
 
void
CompressionBlk::setSizeBits(const std::size_t size)
{
    _size = size;
 
    SuperBlk* superblock = static_cast<SuperBlk*>(getSectorBlock());
    const uint8_t compression_factor =
        superblock->calculateCompressionFactor(size);
    superblock->setCompressionFactor(compression_factor);
 
    // Either this function is called after an insertion, or an update.
    // If somebody else is present in the block, keep the superblock's
    // compressibility. Otherwise, check if it can co-allocate
    const uint8_t num_valid = superblock->getNumValid();
    assert(num_valid >= 1);
    if (num_valid == 1) {
        if (compression_factor != 1) {
            setCompressed();
        } else {
            setUncompressed();
        }
    } else {
        if (superblock->isCompressed(this)) {
            setCompressed();
        } else {
            setUncompressed();
        }
    }
}
 
Cycles
CompressionBlk::getDecompressionLatency() const
{
    return _decompressionLatency;
}
 
void
CompressionBlk::setDecompressionLatency(const Cycles lat)
{
    _decompressionLatency = lat;
}
 
void
CompressionBlk::invalidate()
{
    SectorSubBlk::invalidate();
    setUncompressed();
}
 
CompressionBlk::OverwriteType
CompressionBlk::checkExpansionContraction(const std::size_t size) const
{
    // An expansion happens when a block passes from a compressible state
    // to a less compressible state (e.g., blkSize/4 to (blkSize/2 or blkSize),
    // or blkSize/2 to blkSize). A contraction happens when a block passes
    // from a less compressible state to a more compressible state (i.e., the
    // opposite of expansion)
    const SuperBlk* superblock =
        static_cast<const SuperBlk*>(getSectorBlock());
    const uint8_t prev_cf = superblock->getCompressionFactor();
    const uint8_t new_cf = superblock->calculateCompressionFactor(size);
    return (new_cf < prev_cf) ? DATA_EXPANSION :
        ((new_cf > prev_cf) ? DATA_CONTRACTION : UNCHANGED);
}
 
std::string
CompressionBlk::print() const
{
    return csprintf("%s compressed: %d size: %llu decompression latency: %d",
                    SectorSubBlk::print(), isCompressed(), getSizeBits(),
                    getDecompressionLatency());
}
 
SuperBlk::SuperBlk()
    : SectorBlk(), blkSize(0), compressionFactor(1)
{
}
 
void
SuperBlk::invalidate()
{
    SectorBlk::invalidate();
    compressionFactor = 1;
}
 
bool
SuperBlk::isCompressed(const CompressionBlk* ignored_blk) const
{
    for (const auto& blk : blks) {
        if (blk->isValid() && (blk != ignored_blk)) {
            return static_cast<CompressionBlk*>(blk)->isCompressed();
        }
    }
 
    // An invalid block is seen as compressed
    return true;
}
 
bool
SuperBlk::canCoAllocate(const std::size_t compressed_size) const
{
    // A YACC-like (Sardashti et al., 2016) co-allocation function: at most
    // numBlocksPerSector blocks that compress at least to fit in the space
    // allocated by its compression factor can share a superblock
    return (getNumValid() < getCompressionFactor()) &&
        (compressed_size <= (blkSize * CHAR_BIT) / getCompressionFactor());
}
 
void
SuperBlk::setBlkSize(const std::size_t blk_size)
{
    assert(blkSize == 0);
    blkSize = blk_size;
}
 
uint8_t
SuperBlk::calculateCompressionFactor(const std::size_t size) const
{
    // The number of blocks per sector determines the maximum comp factor.
    // If the compressed size is worse than the uncompressed size, we assume
    // the size is the uncompressed size, and thus the compression factor is 1
    const std::size_t blk_size_bits = CHAR_BIT * blkSize;
    const std::size_t compression_factor = (size > blk_size_bits) ? 1 :
        ((size == 0) ? blk_size_bits :
        alignToPowerOfTwo(std::floor(double(blk_size_bits) / size)));
    return std::min(compression_factor, blks.size());
}
 
uint8_t
SuperBlk::getCompressionFactor() const
{
    return compressionFactor;
}
 
void
SuperBlk::setCompressionFactor(const uint8_t compression_factor)
{
    // Either the block is alone, in which case the compression factor
    // must be set, or it co-allocates with someone with a worse or
    // equal compression factor, in which case it should not be updated
    if (getNumValid() <= 1) {
        compressionFactor = compression_factor;
    }
}
 
std::string
SuperBlk::print() const
{
    return csprintf("CF: %d %s", getCompressionFactor(), SectorBlk::print());
}
 
} // namespace gem5