gem5/src/cpu/pred/tournament.cc

/*
 * Copyright (c) 2011, 2014 ARM Limited
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 * modification, are permitted provided that the following conditions are
 * met: redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer;
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 * this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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 * Authors: Kevin Lim
 */

#include "base/bitfield.hh"
#include "base/intmath.hh"
#include "cpu/pred/tournament.hh"

TournamentBP::TournamentBP(const TournamentBPParams *params)
    : BPredUnit(params),
      localPredictorSize(params->localPredictorSize),
      localCtrBits(params->localCtrBits),
      localHistoryTableSize(params->localHistoryTableSize),
      localHistoryBits(ceilLog2(params->localPredictorSize)),
      globalPredictorSize(params->globalPredictorSize),
      globalCtrBits(params->globalCtrBits),
      globalHistoryBits(
          ceilLog2(params->globalPredictorSize) >
          ceilLog2(params->choicePredictorSize) ?
          ceilLog2(params->globalPredictorSize) :
          ceilLog2(params->choicePredictorSize)),
      choicePredictorSize(params->choicePredictorSize),
      choiceCtrBits(params->choiceCtrBits)
{
    if (!isPowerOf2(localPredictorSize)) {
        fatal("Invalid local predictor size!\n");
    }

    if (!isPowerOf2(globalPredictorSize)) {
        fatal("Invalid global predictor size!\n");
    }

    //Set up the array of counters for the local predictor
    localCtrs.resize(localPredictorSize);

    for (int i = 0; i < localPredictorSize; ++i)
        localCtrs[i].setBits(localCtrBits);

    localPredictorMask = mask(localHistoryBits);

    if (!isPowerOf2(localHistoryTableSize)) {
        fatal("Invalid local history table size!\n");
    }

    //Setup the history table for the local table
    localHistoryTable.resize(localHistoryTableSize);

    for (int i = 0; i < localHistoryTableSize; ++i)
        localHistoryTable[i] = 0;

    //Setup the array of counters for the global predictor
    globalCtrs.resize(globalPredictorSize);

    for (int i = 0; i < globalPredictorSize; ++i)
        globalCtrs[i].setBits(globalCtrBits);

    //Clear the global history
    globalHistory = 0;
    // Set up the global history mask
    // this is equivalent to mask(log2(globalPredictorSize)
    globalHistoryMask = globalPredictorSize - 1;

    if (!isPowerOf2(choicePredictorSize)) {
        fatal("Invalid choice predictor size!\n");
    }

    // Set up choiceHistoryMask
    // this is equivalent to mask(log2(choicePredictorSize)
    choiceHistoryMask = choicePredictorSize - 1;

    //Setup the array of counters for the choice predictor
    choiceCtrs.resize(choicePredictorSize);

    for (int i = 0; i < choicePredictorSize; ++i)
        choiceCtrs[i].setBits(choiceCtrBits);

    //Set up historyRegisterMask
    historyRegisterMask = mask(globalHistoryBits);

    //Check that predictors don't use more bits than they have available
    if (globalHistoryMask > historyRegisterMask) {
        fatal("Global predictor too large for global history bits!\n");
    }
    if (choiceHistoryMask > historyRegisterMask) {
        fatal("Choice predictor too large for global history bits!\n");
    }

    if (globalHistoryMask < historyRegisterMask &&
        choiceHistoryMask < historyRegisterMask) {
        inform("More global history bits than required by predictors\n");
    }

    // Set thresholds for the three predictors' counters
    // This is equivalent to (2^(Ctr))/2 - 1
    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
}

inline
unsigned
TournamentBP::calcLocHistIdx(Addr &branch_addr)
{
    // Get low order bits after removing instruction offset.
    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
}

inline
void
TournamentBP::updateGlobalHistTaken()
{
    globalHistory = (globalHistory << 1) | 1;
    globalHistory = globalHistory & historyRegisterMask;
}

inline
void
TournamentBP::updateGlobalHistNotTaken()
{
    globalHistory = (globalHistory << 1);
    globalHistory = globalHistory & historyRegisterMask;
}

inline
void
TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
{
    localHistoryTable[local_history_idx] =
        (localHistoryTable[local_history_idx] << 1) | 1;
}

inline
void
TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
{
    localHistoryTable[local_history_idx] =
        (localHistoryTable[local_history_idx] << 1);
}


void
TournamentBP::btbUpdate(Addr branch_addr, void * &bp_history)
{
    unsigned local_history_idx = calcLocHistIdx(branch_addr);
    //Update Global History to Not Taken (clear LSB)
    globalHistory &= (historyRegisterMask & ~ULL(1));
    //Update Local History to Not Taken
    localHistoryTable[local_history_idx] =
       localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
}

bool
TournamentBP::lookup(Addr branch_addr, void * &bp_history)
{
    bool local_prediction;
    unsigned local_history_idx;
    unsigned local_predictor_idx;

    bool global_prediction;
    bool choice_prediction;

    //Lookup in the local predictor to get its branch prediction
    local_history_idx = calcLocHistIdx(branch_addr);
    local_predictor_idx = localHistoryTable[local_history_idx]
        & localPredictorMask;
    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;

    //Lookup in the global predictor to get its branch prediction
    global_prediction =
      globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;

    //Lookup in the choice predictor to see which one to use
    choice_prediction =
      choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;

    // Create BPHistory and pass it back to be recorded.
    BPHistory *history = new BPHistory;
    history->globalHistory = globalHistory;
    history->localPredTaken = local_prediction;
    history->globalPredTaken = global_prediction;
    history->globalUsed = choice_prediction;
    history->localHistoryIdx = local_history_idx;
    history->localHistory = local_predictor_idx;
    bp_history = (void *)history;

    assert(local_history_idx < localHistoryTableSize);

    // Commented code is for doing speculative update of counters and
    // all histories.
    if (choice_prediction) {
        if (global_prediction) {
            updateGlobalHistTaken();
            updateLocalHistTaken(local_history_idx);
            return true;
        } else {
            updateGlobalHistNotTaken();
            updateLocalHistNotTaken(local_history_idx);
            return false;
        }
    } else {
        if (local_prediction) {
            updateGlobalHistTaken();
            updateLocalHistTaken(local_history_idx);
            return true;
        } else {
            updateGlobalHistNotTaken();
            updateLocalHistNotTaken(local_history_idx);
            return false;
        }
    }
}

void
TournamentBP::uncondBranch(Addr pc, void * &bp_history)
{
    // Create BPHistory and pass it back to be recorded.
    BPHistory *history = new BPHistory;
    history->globalHistory = globalHistory;
    history->localPredTaken = true;
    history->globalPredTaken = true;
    history->globalUsed = true;
    history->localHistoryIdx = invalidPredictorIndex;
    history->localHistory = invalidPredictorIndex;
    bp_history = static_cast<void *>(history);

    updateGlobalHistTaken();
}

void
TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
                     bool squashed)
{
    unsigned local_history_idx;
    unsigned local_predictor_idx M5_VAR_USED;
    unsigned local_predictor_hist;

    // Get the local predictor's current prediction
    local_history_idx = calcLocHistIdx(branch_addr);
    local_predictor_hist = localHistoryTable[local_history_idx];
    local_predictor_idx = local_predictor_hist & localPredictorMask;

    if (bp_history) {
        BPHistory *history = static_cast<BPHistory *>(bp_history);
        // Update may also be called if the Branch target is incorrect even if
        // the prediction is correct. In that case do not update the counters.
        bool historyPred = false;
        unsigned old_local_pred_index = history->localHistory &
                localPredictorMask;

        bool old_local_pred_valid = history->localHistory !=
            invalidPredictorIndex;

        assert(old_local_pred_index < localPredictorSize);

        if (history->globalUsed) {
           historyPred = history->globalPredTaken;
        } else {
           historyPred = history->localPredTaken;
        }
        if (historyPred != taken || !squashed) {
            // Update the choice predictor to tell it which one was correct if
            // there was a prediction.
            if (history->localPredTaken != history->globalPredTaken) {
                 // If the local prediction matches the actual outcome,
                 // decerement the counter.  Otherwise increment the
                 // counter.
                 unsigned choice_predictor_idx =
                   history->globalHistory & choiceHistoryMask;
                 if (history->localPredTaken == taken) {
                     choiceCtrs[choice_predictor_idx].decrement();
                 } else if (history->globalPredTaken == taken) {
                     choiceCtrs[choice_predictor_idx].increment();
                 }

             }

             // Update the counters and local history with the proper
             // resolution of the branch.  Global history is updated
             // speculatively and restored upon squash() calls, so it does not
             // need to be updated.
             unsigned global_predictor_idx =
               history->globalHistory & globalHistoryMask;
             if (taken) {
                  globalCtrs[global_predictor_idx].increment();
                  if (old_local_pred_valid) {
                          localCtrs[old_local_pred_index].increment();
                  }
             } else {
                  globalCtrs[global_predictor_idx].decrement();
                  if (old_local_pred_valid) {
                          localCtrs[old_local_pred_index].decrement();
                  }
             }
        }
        if (squashed) {
             if (taken) {
                globalHistory = (history->globalHistory << 1) | 1;
                globalHistory = globalHistory & historyRegisterMask;
                if (old_local_pred_valid) {
                    localHistoryTable[local_history_idx] =
                     (history->localHistory << 1) | 1;
                }
             } else {
                globalHistory = (history->globalHistory << 1);
                globalHistory = globalHistory & historyRegisterMask;
                if (old_local_pred_valid) {
                     localHistoryTable[local_history_idx] =
                     history->localHistory << 1;
                }
             }

        } else {
            // We're done with this history, now delete it.
            delete history;
        }
    }

    assert(local_history_idx < localHistoryTableSize);


}

void
TournamentBP::retireSquashed(void *bp_history)
{
    BPHistory *history = static_cast<BPHistory *>(bp_history);
    delete history;
}

void
TournamentBP::squash(void *bp_history)
{
    BPHistory *history = static_cast<BPHistory *>(bp_history);

    // Restore global history to state prior to this branch.
    globalHistory = history->globalHistory;

    // Restore local history
    if (history->localHistoryIdx != invalidPredictorIndex) {
        localHistoryTable[history->localHistoryIdx] = history->localHistory;
    }

    // Delete this BPHistory now that we're done with it.
    delete history;
}

TournamentBP*
TournamentBPParams::create()
{
    return new TournamentBP(this);
}

unsigned
TournamentBP::getGHR(void *bp_history) const
{
    return static_cast<BPHistory *>(bp_history)->globalHistory;
}

#ifdef DEBUG
int
TournamentBP::BPHistory::newCount = 0;
#endif