stride.cc

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#include "mem/cache/prefetch/stride.hh"

#include <cassert>

#include "base/intmath.hh"
#include "base/logging.hh"
#include "base/random.hh"
#include "base/trace.hh"
#include "debug/HWPrefetch.hh"
#include "mem/cache/prefetch/associative_set_impl.hh"
#include "mem/cache/replacement_policies/base.hh"
#include "params/StridePrefetcher.hh"

namespace Prefetcher {

Stride::StrideEntry::StrideEntry(const SatCounter& init_confidence)
  : TaggedEntry(), confidence(init_confidence)
{
    invalidate();
}

void
Stride::StrideEntry::invalidate()
{
    lastAddr = 0;
    stride = 0;
    confidence.reset();
}

Stride::Stride(const StridePrefetcherParams *p)
  : Queued(p),
    initConfidence(p->confidence_counter_bits, p->initial_confidence),
    threshConf(p->confidence_threshold/100.0),
    useMasterId(p->use_master_id),
    degree(p->degree),
    pcTableInfo(p->table_assoc, p->table_entries, p->table_indexing_policy,
        p->table_replacement_policy)
{
}

Stride::PCTable*
Stride::findTable(int context)
{
    // Check if table for given context exists
    auto it = pcTables.find(context);
    if (it != pcTables.end())
        return &it->second;

    // If table does not exist yet, create one
    return allocateNewContext(context);
}

Stride::PCTable*
Stride::allocateNewContext(int context)
{
    // Create new table
    auto insertion_result = pcTables.insert(std::make_pair(context,
        PCTable(pcTableInfo.assoc, pcTableInfo.numEntries,
        pcTableInfo.indexingPolicy, pcTableInfo.replacementPolicy,
        StrideEntry(initConfidence))));

    DPRINTF(HWPrefetch, "Adding context %i with stride entries\n", context);

    // Get iterator to new pc table, and then return a pointer to the new table
    return &(insertion_result.first->second);
}

void
Stride::calculatePrefetch(const PrefetchInfo &pfi,
                                    std::vector<AddrPriority> &addresses)
{
    if (!pfi.hasPC()) {
        DPRINTF(HWPrefetch, "Ignoring request with no PC.\n");
        return;
    }

    // Get required packet info
    Addr pf_addr = pfi.getAddr();
    Addr pc = pfi.getPC();
    bool is_secure = pfi.isSecure();
    MasterID master_id = useMasterId ? pfi.getMasterId() : 0;

    // Get corresponding pc table
    PCTable* pcTable = findTable(master_id);

    // Search for entry in the pc table
    StrideEntry *entry = pcTable->findEntry(pc, is_secure);

    if (entry != nullptr) {
        pcTable->accessEntry(entry);

        // Hit in table
        int new_stride = pf_addr - entry->lastAddr;
        bool stride_match = (new_stride == entry->stride);

        // Adjust confidence for stride entry
        if (stride_match && new_stride != 0) {
            entry->confidence++;
        } else {
            entry->confidence--;
            // If confidence has dropped below the threshold, train new stride
            if (entry->confidence.calcSaturation() < threshConf) {
                entry->stride = new_stride;
            }
        }

        DPRINTF(HWPrefetch, "Hit: PC %x pkt_addr %x (%s) stride %d (%s), "
                "conf %d\n", pc, pf_addr, is_secure ? "s" : "ns",
                new_stride, stride_match ? "match" : "change",
                (int)entry->confidence);

        entry->lastAddr = pf_addr;

        // Abort prefetch generation if below confidence threshold
        if (entry->confidence.calcSaturation() < threshConf) {
            return;
        }

        // Generate up to degree prefetches
        for (int d = 1; d <= degree; d++) {
            // Round strides up to atleast 1 cacheline
            int prefetch_stride = new_stride;
            if (abs(new_stride) < blkSize) {
                prefetch_stride = (new_stride < 0) ? -blkSize : blkSize;
            }

            Addr new_addr = pf_addr + d * prefetch_stride;
            addresses.push_back(AddrPriority(new_addr, 0));
        }
    } else {
        // Miss in table
        DPRINTF(HWPrefetch, "Miss: PC %x pkt_addr %x (%s)\n", pc, pf_addr,
                is_secure ? "s" : "ns");

        StrideEntry* entry = pcTable->findVictim(pc);

        // Insert new entry's data
        entry->lastAddr = pf_addr;
        pcTable->insertEntry(pc, is_secure, entry);
    }
}

inline uint32_t
StridePrefetcherHashedSetAssociative::extractSet(const Addr pc) const
{
    const Addr hash1 = pc >> 1;
    const Addr hash2 = hash1 >> tagShift;
    return (hash1 ^ hash2) & setMask;
}

Addr
StridePrefetcherHashedSetAssociative::extractTag(const Addr addr) const
{
    return addr;
}

} // namespace Prefetcher

Prefetcher::StridePrefetcherHashedSetAssociative*
StridePrefetcherHashedSetAssociativeParams::create()
{
    return new Prefetcher::StridePrefetcherHashedSetAssociative(this);
}

Prefetcher::Stride*
StridePrefetcherParams::create()
{
    return new Prefetcher::Stride(this);
}