src/cpu/pred/tournament.hh

   1 /*
   2  * Copyright (c) 2011 ARM Limited
   3  * All rights reserved
   4  *
   5  * The license below extends only to copyright in the software and shall
   6  * not be construed as granting a license to any other intellectual
   7  * property including but not limited to intellectual property relating
   8  * to a hardware implementation of the functionality of the software
   9  * licensed hereunder.  You may use the software subject to the license
  10  * terms below provided that you ensure that this notice is replicated
  11  * unmodified and in its entirety in all distributions of the software,
  12  * modified or unmodified, in source code or in binary form.
  13  *
  14  * Copyright (c) 2004-2006 The Regents of The University of Michigan
  15  * All rights reserved.
  16  *
  17  * Redistribution and use in source and binary forms, with or without
  18  * modification, are permitted provided that the following conditions are
  19  * met: redistributions of source code must retain the above copyright
  20  * notice, this list of conditions and the following disclaimer;
  21  * redistributions in binary form must reproduce the above copyright
  22  * notice, this list of conditions and the following disclaimer in the
  23  * documentation and/or other materials provided with the distribution;
  24  * neither the name of the copyright holders nor the names of its
  25  * contributors may be used to endorse or promote products derived from
  26  * this software without specific prior written permission.
  27  *
  28  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  29  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  30  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  31  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  32  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  33  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  34  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  35  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  36  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  37  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  38  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  39  *
  40  * Authors: Kevin Lim
  41  */
  42
  43 #ifndef __CPU_O3_TOURNAMENT_PRED_HH__
  44 #define __CPU_O3_TOURNAMENT_PRED_HH__
  45
  46 #include <vector>
  47
  48 #include "base/types.hh"
  49 #include "cpu/o3/sat_counter.hh"
  50
  51 /**
  52  * Implements a tournament branch predictor, hopefully identical to the one
  53  * used in the 21264.  It has a local predictor, which uses a local history
  54  * table to index into a table of counters, and a global predictor, which
  55  * uses a global history to index into a table of counters.  A choice
  56  * predictor chooses between the two.  Only the global history register
  57  * is speculatively updated, the rest are updated upon branches committing
  58  * or misspeculating.
  59  */
  60 class TournamentBP
  61 {
  62   public:
  63     /**
  64      * Default branch predictor constructor.
  65      */
  66     TournamentBP(unsigned localPredictorSize,
  67                  unsigned localCtrBits,
  68                  unsigned localHistoryTableSize,
  69                  unsigned localHistoryBits,
  70                  unsigned globalPredictorSize,
  71                  unsigned globalHistoryBits,
  72                  unsigned globalCtrBits,
  73                  unsigned choicePredictorSize,
  74                  unsigned choiceCtrBits,
  75                  unsigned instShiftAmt);
  76
  77     /**
  78      * Looks up the given address in the branch predictor and returns
  79      * a true/false value as to whether it is taken.  Also creates a
  80      * BPHistory object to store any state it will need on squash/update.
  81      * @param branch_addr The address of the branch to look up.
  82      * @param bp_history Pointer that will be set to the BPHistory object.
  83      * @return Whether or not the branch is taken.
  84      */
  85     bool lookup(Addr &branch_addr, void * &bp_history);
  86
  87     /**
  88      * Records that there was an unconditional branch, and modifies
  89      * the bp history to point to an object that has the previous
  90      * global history stored in it.
  91      * @param bp_history Pointer that will be set to the BPHistory object.
  92      */
  93     void uncondBr(void * &bp_history);
  94     /**
  95      * Updates the branch predictor to Not Taken if a BTB entry is
  96      * invalid or not found.
  97      * @param branch_addr The address of the branch to look up.
  98      * @param bp_history Pointer to any bp history state.
  99      * @return Whether or not the branch is taken.
 100      */
 101     void BTBUpdate(Addr &branch_addr, void * &bp_history);
 102     /**
 103      * Updates the branch predictor with the actual result of a branch.
 104      * @param branch_addr The address of the branch to update.
 105      * @param taken Whether or not the branch was taken.
 106      * @param bp_history Pointer to the BPHistory object that was created
 107      * when the branch was predicted.
 108      * @param squashed is set when this function is called during a squash
 109      * operation.
 110      */
 111     void update(Addr &branch_addr, bool taken, void *bp_history, bool squashed);
 112
 113     /**
 114      * Restores the global branch history on a squash.
 115      * @param bp_history Pointer to the BPHistory object that has the
 116      * previous global branch history in it.
 117      */
 118     void squash(void *bp_history);
 119
 120     /** Returns the global history. */
 121     inline unsigned readGlobalHist() { return globalHistory; }
 122
 123   private:
 124     /**
 125      * Returns if the branch should be taken or not, given a counter
 126      * value.
 127      * @param count The counter value.
 128      */
 129     inline bool getPrediction(uint8_t &count);
 130
 131     /**
 132      * Returns the local history index, given a branch address.
 133      * @param branch_addr The branch's PC address.
 134      */
 135     inline unsigned calcLocHistIdx(Addr &branch_addr);
 136
 137     /** Updates global history as taken. */
 138     inline void updateGlobalHistTaken();
 139
 140     /** Updates global history as not taken. */
 141     inline void updateGlobalHistNotTaken();
 142
 143     /**
 144      * Updates local histories as taken.
 145      * @param local_history_idx The local history table entry that
 146      * will be updated.
 147      */
 148     inline void updateLocalHistTaken(unsigned local_history_idx);
 149
 150     /**
 151      * Updates local histories as not taken.
 152      * @param local_history_idx The local history table entry that
 153      * will be updated.
 154      */
 155     inline void updateLocalHistNotTaken(unsigned local_history_idx);
 156
 157     /**
 158      * The branch history information that is created upon predicting
 159      * a branch.  It will be passed back upon updating and squashing,
 160      * when the BP can use this information to update/restore its
 161      * state properly.
 162      */
 163     struct BPHistory {
 164 #ifdef DEBUG
 165         BPHistory()
 166         { newCount++; }
 167         ~BPHistory()
 168         { newCount--; }
 169
 170         static int newCount;
 171 #endif
 172         unsigned globalHistory;
 173         unsigned localHistory;
 174         bool localPredTaken;
 175         bool globalPredTaken;
 176         bool globalUsed;
 177     };
 178
 179     /** Flag for invalid predictor index */
 180     static const int invalidPredictorIndex = -1;
 181     /** Local counters. */
 182     std::vector<SatCounter> localCtrs;
 183
 184     /** Size of the local predictor. */
 185     unsigned localPredictorSize;
 186
 187     /** Mask to get the proper index bits into the predictor. */
 188     unsigned localPredictorMask;
 189
 190     /** Number of bits of the local predictor's counters. */
 191     unsigned localCtrBits;
 192
 193     /** Array of local history table entries. */
 194     std::vector<unsigned> localHistoryTable;
 195
 196     /** Size of the local history table. */
 197     unsigned localHistoryTableSize;
 198
 199     /** Number of bits for each entry of the local history table.
 200      *  @todo Doesn't this come from the size of the local predictor?
 201      */
 202     unsigned localHistoryBits;
 203
 204     /** Mask to get the proper local history. */
 205     unsigned localHistoryMask;
 206
 207     /** Array of counters that make up the global predictor. */
 208     std::vector<SatCounter> globalCtrs;
 209
 210     /** Size of the global predictor. */
 211     unsigned globalPredictorSize;
 212
 213     /** Number of bits of the global predictor's counters. */
 214     unsigned globalCtrBits;
 215
 216     /** Global history register. */
 217     unsigned globalHistory;
 218
 219     /** Number of bits for the global history. */
 220     unsigned globalHistoryBits;
 221
 222     /** Mask to get the proper global history. */
 223     unsigned globalHistoryMask;
 224
 225     /** Array of counters that make up the choice predictor. */
 226     std::vector<SatCounter> choiceCtrs;
 227
 228     /** Size of the choice predictor (identical to the global predictor). */
 229     unsigned choicePredictorSize;
 230
 231     /** Number of bits of the choice predictor's counters. */
 232     unsigned choiceCtrBits;
 233
 234     /** Number of bits to shift the instruction over to get rid of the word
 235      *  offset.
 236      */
 237     unsigned instShiftAmt;
 238
 239     /** Threshold for the counter value; above the threshold is taken,
 240      *  equal to or below the threshold is not taken.
 241      */
 242     unsigned threshold;
 243 };
 244
 245 #endif // __CPU_O3_TOURNAMENT_PRED_HH__