tournament_pred.hh

来自「M5,一个功能强大的多处理器系统模拟器.很多针对处理器架构,性能的研究都使用它作」· HH 代码 · 共 221 行

/*
 * Copyright (c) 2004, 2005, 2006
 * The Regents of The University of Michigan
 * All Rights Reserved
 *
 * This code is part of the M5 simulator.
 *
 * Permission is granted to use, copy, create derivative works and
 * redistribute this software and such derivative works for any
 * purpose, so long as the copyright notice above, this grant of
 * permission, and the disclaimer below appear in all copies made; and
 * so long as the name of The University of Michigan is not used in
 * any advertising or publicity pertaining to the use or distribution
 * of this software without specific, written prior authorization.
 *
 * THIS SOFTWARE IS PROVIDED AS IS, WITHOUT REPRESENTATION FROM THE
 * UNIVERSITY OF MICHIGAN AS TO ITS FITNESS FOR ANY PURPOSE, AND
 * WITHOUT WARRANTY BY THE UNIVERSITY OF MICHIGAN OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE. THE REGENTS OF THE UNIVERSITY OF MICHIGAN SHALL NOT BE
 * LIABLE FOR ANY DAMAGES, INCLUDING DIRECT, SPECIAL, INDIRECT,
 * INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WITH RESPECT TO ANY CLAIM
 * ARISING OUT OF OR IN CONNECTION WITH THE USE OF THE SOFTWARE, EVEN
 * IF IT HAS BEEN OR IS HEREAFTER ADVISED OF THE POSSIBILITY OF SUCH
 * DAMAGES.
 *
 * Authors: Kevin T. Lim
 */

#ifndef __CPU_O3_TOURNAMENT_PRED_HH__
#define __CPU_O3_TOURNAMENT_PRED_HH__

#include "cpu/o3/sat_counter.hh"
#include "sim/host.hh"
#include <vector>

/**
 * Implements a tournament branch predictor, hopefully identical to the one
 * used in the 21264.  It has a local predictor, which uses a local history
 * table to index into a table of counters, and a global predictor, which
 * uses a global history to index into a table of counters.  A choice
 * predictor chooses between the two.  Only the global history register
 * is speculatively updated, the rest are updated upon branches committing
 * or misspeculating.
 */
class TournamentBP
{
  public:
    /**
     * Default branch predictor constructor.
     */
    TournamentBP(unsigned localPredictorSize,
                 unsigned localCtrBits,
                 unsigned localHistoryTableSize,
                 unsigned localHistoryBits,
                 unsigned globalPredictorSize,
                 unsigned globalHistoryBits,
                 unsigned globalCtrBits,
                 unsigned choicePredictorSize,
                 unsigned choiceCtrBits,
                 unsigned instShiftAmt);

    /**
     * Looks up the given address in the branch predictor and returns
     * a true/false value as to whether it is taken.  Also creates a
     * BPHistory object to store any state it will need on squash/update.
     * @param branch_addr The address of the branch to look up.
     * @param bp_history Pointer that will be set to the BPHistory object.
     * @return Whether or not the branch is taken.
     */
    bool lookup(Addr &branch_addr, void * &bp_history);

    /**
     * Records that there was an unconditional branch, and modifies
     * the bp history to point to an object that has the previous
     * global history stored in it.
     * @param bp_history Pointer that will be set to the BPHistory object.
     */
    void uncondBr(void * &bp_history);

    /**
     * Updates the branch predictor with the actual result of a branch.
     * @param branch_addr The address of the branch to update.
     * @param taken Whether or not the branch was taken.
     * @param bp_history Pointer to the BPHistory object that was created
     * when the branch was predicted.
     */
    void update(Addr &branch_addr, bool taken, void *bp_history);

    /**
     * Restores the global branch history on a squash.
     * @param bp_history Pointer to the BPHistory object that has the
     * previous global branch history in it.
     */
    void squash(void *bp_history);

    /** Returns the global history. */
    inline unsigned readGlobalHist() { return globalHistory; }

  private:
    /**
     * Returns if the branch should be taken or not, given a counter
     * value.
     * @param count The counter value.
     */
    inline bool getPrediction(uint8_t &count);

    /**
     * Returns the local history index, given a branch address.
     * @param branch_addr The branch's PC address.
     */
    inline unsigned calcLocHistIdx(Addr &branch_addr);

    /** Updates global history as taken. */
    inline void updateGlobalHistTaken();

    /** Updates global history as not taken. */
    inline void updateGlobalHistNotTaken();

    /**
     * Updates local histories as taken.
     * @param local_history_idx The local history table entry that
     * will be updated.
     */
    inline void updateLocalHistTaken(unsigned local_history_idx);

    /**
     * Updates local histories as not taken.
     * @param local_history_idx The local history table entry that
     * will be updated.
     */
    inline void updateLocalHistNotTaken(unsigned local_history_idx);

    /**
     * The branch history information that is created upon predicting
     * a branch.  It will be passed back upon updating and squashing,
     * when the BP can use this information to update/restore its
     * state properly.
     */
    struct BPHistory {
#ifdef DEBUG
        BPHistory()
        { newCount++; }
        ~BPHistory()
        { newCount--; }

        static int newCount;
#endif
        unsigned globalHistory;
        bool localPredTaken;
        bool globalPredTaken;
        bool globalUsed;
    };

    /** Local counters. */
    std::vector<SatCounter> localCtrs;

    /** Size of the local predictor. */
    unsigned localPredictorSize;

    /** Mask to get the proper index bits into the predictor. */
    unsigned localPredictorMask;

    /** Number of bits of the local predictor's counters. */
    unsigned localCtrBits;

    /** Array of local history table entries. */
    std::vector<unsigned> localHistoryTable;

    /** Size of the local history table. */
    unsigned localHistoryTableSize;

    /** Number of bits for each entry of the local history table.
     *  @todo Doesn't this come from the size of the local predictor?
     */
    unsigned localHistoryBits;

    /** Mask to get the proper local history. */
    unsigned localHistoryMask;

    /** Array of counters that make up the global predictor. */
    std::vector<SatCounter> globalCtrs;

    /** Size of the global predictor. */
    unsigned globalPredictorSize;

    /** Number of bits of the global predictor's counters. */
    unsigned globalCtrBits;

    /** Global history register. */
    unsigned globalHistory;

    /** Number of bits for the global history. */
    unsigned globalHistoryBits;

    /** Mask to get the proper global history. */
    unsigned globalHistoryMask;

    /** Array of counters that make up the choice predictor. */
    std::vector<SatCounter> choiceCtrs;

    /** Size of the choice predictor (identical to the global predictor). */
    unsigned choicePredictorSize;

    /** Number of bits of the choice predictor's counters. */
    unsigned choiceCtrBits;

    /** Number of bits to shift the instruction over to get rid of the word
     *  offset.
     */
    unsigned instShiftAmt;

    /** Threshold for the counter value; above the threshold is taken,
     *  equal to or below the threshold is not taken.
     */
    unsigned threshold;
};

#endif // __CPU_O3_TOURNAMENT_PRED_HH__