src/mem/cache/replacement_policies/brrip_rp.cc

/**
 * Copyright (c) 2018-2020 Inria
 * All rights reserved.
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 * modification, are permitted provided that the following conditions are
 * met: redistributions of source code must retain the above copyright
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 * 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
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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#include "mem/cache/replacement_policies/brrip_rp.hh"

#include <cassert>
#include <memory>

#include "base/logging.hh" // For fatal_if
#include "base/random.hh"
#include "params/BRRIPRP.hh"

namespace gem5
{

GEM5_DEPRECATED_NAMESPACE(ReplacementPolicy, replacement_policy);
namespace replacement_policy
{

BRRIP::BRRIP(const Params &p)
  : Base(p), numRRPVBits(p.num_bits), hitPriority(p.hit_priority),
    btp(p.btp)
{
    fatal_if(numRRPVBits <= 0, "There should be at least one bit per RRPV.\n");
}

void
BRRIP::invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
{
    std::shared_ptr<BRRIPReplData> casted_replacement_data =
        std::static_pointer_cast<BRRIPReplData>(replacement_data);

    // Invalidate entry
    casted_replacement_data->valid = false;
}

void
BRRIP::touch(const std::shared_ptr<ReplacementData>& replacement_data) const
{
    std::shared_ptr<BRRIPReplData> casted_replacement_data =
        std::static_pointer_cast<BRRIPReplData>(replacement_data);

    // Update RRPV if not 0 yet
    // Every hit in HP mode makes the entry the last to be evicted, while
    // in FP mode a hit makes the entry less likely to be evicted
    if (hitPriority) {
        casted_replacement_data->rrpv.reset();
    } else {
        casted_replacement_data->rrpv--;
    }
}

void
BRRIP::reset(const std::shared_ptr<ReplacementData>& replacement_data) const
{
    std::shared_ptr<BRRIPReplData> casted_replacement_data =
        std::static_pointer_cast<BRRIPReplData>(replacement_data);

    // Reset RRPV
    // Replacement data is inserted as "long re-reference" if lower than btp,
    // "distant re-reference" otherwise
    casted_replacement_data->rrpv.saturate();
    if (random_mt.random<unsigned>(1, 100) <= btp) {
        casted_replacement_data->rrpv--;
    }

    // Mark entry as ready to be used
    casted_replacement_data->valid = true;
}

ReplaceableEntry*
BRRIP::getVictim(const ReplacementCandidates& candidates) const
{
    // There must be at least one replacement candidate
    assert(candidates.size() > 0);

    // Use first candidate as dummy victim
    ReplaceableEntry* victim = candidates[0];

    // Store victim->rrpv in a variable to improve code readability
    int victim_RRPV = std::static_pointer_cast<BRRIPReplData>(
                        victim->replacementData)->rrpv;

    // Visit all candidates to find victim
    for (const auto& candidate : candidates) {
        std::shared_ptr<BRRIPReplData> candidate_repl_data =
            std::static_pointer_cast<BRRIPReplData>(
                candidate->replacementData);

        // Stop searching for victims if an invalid entry is found
        if (!candidate_repl_data->valid) {
            return candidate;
        }

        // Update victim entry if necessary
        int candidate_RRPV = candidate_repl_data->rrpv;
        if (candidate_RRPV > victim_RRPV) {
            victim = candidate;
            victim_RRPV = candidate_RRPV;
        }
    }

    // Get difference of victim's RRPV to the highest possible RRPV in
    // order to update the RRPV of all the other entries accordingly
    int diff = std::static_pointer_cast<BRRIPReplData>(
        victim->replacementData)->rrpv.saturate();

    // No need to update RRPV if there is no difference
    if (diff > 0){
        // Update RRPV of all candidates
        for (const auto& candidate : candidates) {
            std::static_pointer_cast<BRRIPReplData>(
                candidate->replacementData)->rrpv += diff;
        }
    }

    return victim;
}

std::shared_ptr<ReplacementData>
BRRIP::instantiateEntry()
{
    return std::shared_ptr<ReplacementData>(new BRRIPReplData(numRRPVBits));
}

} // namespace replacement_policy
} // namespace gem5