cpu/o3/2bit_local_pred.cc

/*
 * Copyright (c) 2004-2005 The Regents of The University of Michigan
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * 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;
 * redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution;
 * 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
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "base/trace.hh"
#include "cpu/o3/2bit_local_pred.hh"

DefaultBP::DefaultBP(unsigned _localPredictorSize,
                     unsigned _localCtrBits,
                     unsigned _instShiftAmt)
    : localPredictorSize(_localPredictorSize),
      localCtrBits(_localCtrBits),
      instShiftAmt(_instShiftAmt)
{
    // Should do checks here to make sure sizes are correct (powers of 2).

    // Setup the index mask.
    indexMask = localPredictorSize - 1;

    DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);

    // Setup the array of counters for the local predictor.
    localCtrs = new SatCounter[localPredictorSize];

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

    DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
            localPredictorSize);

    DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);

    DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
            instShiftAmt);
}

bool
DefaultBP::lookup(Addr &branch_addr)
{
    bool taken;
    uint8_t local_prediction;
    unsigned local_predictor_idx = getLocalIndex(branch_addr);

    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
            local_predictor_idx);

    assert(local_predictor_idx < localPredictorSize);

    local_prediction = localCtrs[local_predictor_idx].read();

    DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
            (int)local_prediction);

    taken = getPrediction(local_prediction);

#if 0
    // Speculative update.
    if (taken) {
        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
        localCtrs[local_predictor_idx].increment();
    } else {
        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
        localCtrs[local_predictor_idx].decrement();
    }
#endif

    return taken;
}

void
DefaultBP::update(Addr &branch_addr, bool taken)
{
    unsigned local_predictor_idx;

    // Update the local predictor.
    local_predictor_idx = getLocalIndex(branch_addr);

    DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
            local_predictor_idx);

    assert(local_predictor_idx < localPredictorSize);

    if (taken) {
        DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
        localCtrs[local_predictor_idx].increment();
    } else {
        DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
        localCtrs[local_predictor_idx].decrement();
    }
}

inline
bool
DefaultBP::getPrediction(uint8_t &count)
{
    // Get the MSB of the count
    return (count >> (localCtrBits - 1));
}

inline
unsigned
DefaultBP::getLocalIndex(Addr &branch_addr)
{
    return (branch_addr >> instShiftAmt) & indexMask;
}