loop_predictor.hh

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/*
 * Copyright (c) 2022-2023 The University of Edinburgh
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 *
 * The license below extends only to copyright in the software and shall
 * not be construed as granting a license to any other intellectual
 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
 * licensed hereunder.  You may use the software subject to the license
 * terms below provided that you ensure that this notice is replicated
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 * modified or unmodified, in source code or in binary form.
 *
 * Copyright (c) 2006 INRIA (Institut National de Recherche en
 * Informatique et en Automatique  / French National Research Institute
 * for Computer Science and Applied Mathematics)
 *
 * All rights reserved.
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 * met: redistributions of source code must retain the above copyright
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#ifndef __CPU_PRED_LOOP_PREDICTOR_HH__
#define __CPU_PRED_LOOP_PREDICTOR_HH__
 
#include "base/random.hh"
#include "base/statistics.hh"
#include "base/types.hh"
#include "sim/sim_object.hh"
 
namespace gem5
{
 
struct LoopPredictorParams;
 
namespace branch_prediction
{
 
class LoopPredictor : public SimObject
{
  protected:
    const unsigned logSizeLoopPred;
    const unsigned loopTableAgeBits;
    const unsigned loopTableConfidenceBits;
    const unsigned loopTableTagBits;
    const unsigned loopTableIterBits;
    const unsigned logLoopTableAssoc;
    const uint8_t confidenceThreshold;
    const uint16_t loopTagMask;
    const uint16_t loopNumIterMask;
    const int loopSetMask;
 
    mutable Random::RandomPtr rng = Random::genRandom();
 
    // Prediction Structures
    // Loop Predictor Entry
    struct LoopEntry
    {
        uint16_t numIter;
        uint16_t currentIter;
        uint16_t currentIterSpec; // only for useSpeculation
        uint8_t confidence;
        uint16_t tag;
        uint8_t age;
        bool dir; // only for useDirectionBit
 
        LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
                      confidence(0), tag(0), age(0), dir(0) { }
        LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0), {…}
    };
    struct LoopEntry {…};
 
    LoopEntry *ltable;
 
    int8_t loopUseCounter;
    unsigned withLoopBits;
 
    const bool useDirectionBit;
    const bool useSpeculation;
    const bool useHashing;
    const bool restrictAllocation;
    const unsigned initialLoopIter;
    const unsigned initialLoopAge;
    const bool optionalAgeReset;
 
    struct LoopPredictorStats : public statistics::Group
    {
        LoopPredictorStats(statistics::Group *parent);
        statistics::Scalar used;
        statistics::Scalar correct;
        statistics::Scalar wrong;
    } stats;
    struct LoopPredictorStats : public statistics::Group {…};
 
    static inline void unsignedCtrUpdate(uint8_t &ctr, bool up, unsigned nbits)
    {
        assert(nbits <= sizeof(uint8_t) << 3);
        if (up) {
            if (ctr < ((1 << nbits) - 1))
                ctr++;
        } else {
            if (ctr)
                ctr--;
        }
    }
    static inline void unsignedCtrUpdate(uint8_t &ctr, bool up, unsigned nbits) {…}
    static inline void signedCtrUpdate(int8_t &ctr, bool up, unsigned nbits)
    {
        if (up) {
            if (ctr < ((1 << (nbits - 1)) - 1))
                ctr++;
        } else {
            if (ctr > -(1 << (nbits - 1)))
                ctr--;
        }
    }
    static inline void signedCtrUpdate(int8_t &ctr, bool up, unsigned nbits) {…}
  public:
    // Primary branch history entry
    struct BranchInfo
    {
        uint16_t loopTag;
        uint16_t currentIter;
 
        bool loopPred;
        bool loopPredValid;
        bool loopPredUsed;
        int  loopIndex;
        int  loopIndexB;  // only for useHashing
        int loopHit;
        bool predTaken;
 
        BranchInfo()
            : loopTag(0), currentIter(0),
              loopPred(false),
              loopPredValid(false), loopIndex(0), loopIndexB(0), loopHit(0),
              predTaken(false)
        {}
        BranchInfo() {…}
    };
    struct BranchInfo {…};
 
    int lindex(Addr pc_in, unsigned instShiftAmt) const;
 
    int finallindex(int lindex, int lowPcBits, int way) const;
 
    bool getLoop(Addr pc, BranchInfo* bi, bool speculative,
                 unsigned instShiftAmt) const;
 
    void loopUpdate(Addr pc, bool Taken, BranchInfo* bi, bool tage_pred);
 
    void specLoopUpdate(bool taken, BranchInfo* bi);
 
    void condBranchUpdate(ThreadID tid, Addr branch_pc, bool taken,
        bool tage_pred, BranchInfo* bi, unsigned instShiftAmt);
 
    bool loopPredict(
        ThreadID tid, Addr branch_pc, bool cond_branch,
        BranchInfo* bi, bool prev_pred_taken, unsigned instShiftAmt);
 
    void updateStats(bool taken, BranchInfo* bi);
 
    void squashLoop(BranchInfo * bi);
 
    void squash(ThreadID tid, BranchInfo *bi);
 
    virtual bool calcConf(int index) const;
 
    virtual bool optionalAgeInc() const;
 
    virtual BranchInfo *makeBranchInfo();
 
    int8_t getLoopUseCounter() const
    {
        return loopUseCounter;
    }
    int8_t getLoopUseCounter() const {…}
 
    void init() override;
 
    LoopPredictor(const LoopPredictorParams &p);
 
    size_t getSizeInBits() const;
};
class LoopPredictor : public SimObject {…};
 
} // namespace branch_prediction
} // namespace gem5
 
#endif//__CPU_PRED_LOOP_PREDICTOR_HH__