xref: /gem5/src/cpu/pred/2bit_local.hh

/*
 * Copyright (c) 2004-2006 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
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 */

#ifndef __CPU_O3_2BIT_LOCAL_PRED_HH__
#define __CPU_O3_2BIT_LOCAL_PRED_HH__

// For Addr type.
#include "arch/isa_traits.hh"
#include "cpu/o3/sat_counter.hh"

#include <vector>

/**
 * Implements a local predictor that uses the PC to index into a table of
 * counters.  Note that any time a pointer to the bp_history is given, it
 * should be NULL using this predictor because it does not have any branch
 * predictor state that needs to be recorded or updated; the update can be
 * determined solely by the branch being taken or not taken.
 */
class LocalBP
{
  public:
    /**
     * Default branch predictor constructor.
     * @param localPredictorSize Size of the local predictor.
     * @param localCtrBits Number of bits per counter.
     * @param instShiftAmt Offset amount for instructions to ignore alignment.
     */
    LocalBP(unsigned localPredictorSize, unsigned localCtrBits,
            unsigned instShiftAmt);

    /**
     * Looks up the given address in the branch predictor and returns
     * a true/false value as to whether it is taken.
     * @param branch_addr The address of the branch to look up.
     * @param bp_history Pointer to any bp history state.
     * @return Whether or not the branch is taken.
     */
    bool lookup(Addr &branch_addr, 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.
     */
    void update(Addr &branch_addr, bool taken, void *bp_history);

    void squash(void *bp_history)
    { assert(bp_history == NULL); }

    void reset();

  private:
    /**
     *  Returns the taken/not taken prediction given the value of the
     *  counter.
     *  @param count The value of the counter.
     *  @return The prediction based on the counter value.
     */
    inline bool getPrediction(uint8_t &count);

    /** Calculates the local index based on the PC. */
    inline unsigned getLocalIndex(Addr &PC);

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

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

    /** Number of sets. */
    unsigned localPredictorSets;

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

    /** Number of bits to shift the PC when calculating index. */
    unsigned instShiftAmt;

    /** Mask to get index bits. */
    unsigned indexMask;
};

#endif // __CPU_O3_2BIT_LOCAL_PRED_HH__