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 localCtrBits,
  67                  unsigned localHistoryTableSize,
  68                  unsigned localHistoryBits,
  69                  unsigned globalPredictorSize,
  70                  unsigned globalHistoryBits,
  71                  unsigned globalCtrBits,
  72                  unsigned choicePredictorSize,
  73                  unsigned choiceCtrBits,
  74                  unsigned instShiftAmt);
  75
  76     /**
  77      * Looks up the given address in the branch predictor and returns
  78      * a true/false value as to whether it is taken.  Also creates a
  79      * BPHistory object to store any state it will need on squash/update.
  80      * @param branch_addr The address of the branch to look up.
  81      * @param bp_history Pointer that will be set to the BPHistory object.
  82      * @return Whether or not the branch is taken.
  83      */
  84     bool lookup(Addr &branch_addr, void * &bp_history);
  85
  86     /**
  87      * Records that there was an unconditional branch, and modifies
  88      * the bp history to point to an object that has the previous
  89      * global history stored in it.
  90      * @param bp_history Pointer that will be set to the BPHistory object.
  91      */
  92     void uncondBr(void * &bp_history);
  93     /**
  94      * Updates the branch predictor to Not Taken if a BTB entry is
  95      * invalid or not found.
  96      * @param branch_addr The address of the branch to look up.
  97      * @param bp_history Pointer to any bp history state.
  98      * @return Whether or not the branch is taken.
  99      */
 100     void BTBUpdate(Addr &branch_addr, void * &bp_history);
 101     /**
 102      * Updates the branch predictor with the actual result of a branch.
 103      * @param branch_addr The address of the branch to update.
 104      * @param taken Whether or not the branch was taken.
 105      * @param bp_history Pointer to the BPHistory object that was created
 106      * when the branch was predicted.
 107      * @param squashed is set when this function is called during a squash
 108      * operation.
 109      */
 110     void update(Addr &branch_addr, bool taken, void *bp_history, bool squashed);
 111
 112     /**
 113      * Restores the global branch history on a squash.
 114      * @param bp_history Pointer to the BPHistory object that has the
 115      * previous global branch history in it.
 116      */
 117     void squash(void *bp_history);
 118
 119     /** Returns the global history. */
 120     inline unsigned readGlobalHist() { return globalHistory; }
 121
 122   private:
 123     /**
 124      * Returns if the branch should be taken or not, given a counter
 125      * value.
 126      * @param count The counter value.
 127      */
 128     inline bool getPrediction(uint8_t &count);
 129
 130     /**
 131      * Returns the local history index, given a branch address.
 132      * @param branch_addr The branch's PC address.
 133      */
 134     inline unsigned calcLocHistIdx(Addr &branch_addr);
 135
 136     /** Updates global history as taken. */
 137     inline void updateGlobalHistTaken();
 138
 139     /** Updates global history as not taken. */
 140     inline void updateGlobalHistNotTaken();
 141
 142     /**
 143      * Updates local histories as taken.
 144      * @param local_history_idx The local history table entry that
 145      * will be updated.
 146      */
 147     inline void updateLocalHistTaken(unsigned local_history_idx);
 148
 149     /**
 150      * Updates local histories as not taken.
 151      * @param local_history_idx The local history table entry that
 152      * will be updated.
 153      */
 154     inline void updateLocalHistNotTaken(unsigned local_history_idx);
 155
 156     /**
 157      * The branch history information that is created upon predicting
 158      * a branch.  It will be passed back upon updating and squashing,
 159      * when the BP can use this information to update/restore its
 160      * state properly.
 161      */
 162     struct BPHistory {
 163 #ifdef DEBUG
 164         BPHistory()
 165         { newCount++; }
 166         ~BPHistory()
 167         { newCount--; }
 168
 169         static int newCount;
 170 #endif
 171         unsigned globalHistory;
 172         unsigned localHistory;
 173         bool localPredTaken;
 174         bool globalPredTaken;
 175         bool globalUsed;
 176     };
 177
 178     /** Flag for invalid predictor index */
 179     static const int invalidPredictorIndex = -1;
 180     /** Local counters. */
 181     std::vector<SatCounter> localCtrs;
 182
 183     /** Number of counters in the local predictor. */
 184     unsigned localPredictorSize;
 185
 186     /** Mask to truncate values stored in the local history table. */
 187     unsigned localPredictorMask;
 188
 189     /** Number of bits of the local predictor's counters. */
 190     unsigned localCtrBits;
 191
 192     /** Array of local history table entries. */
 193     std::vector<unsigned> localHistoryTable;
 194
 195     /** Number of entries in the local history table. */
 196     unsigned localHistoryTableSize;
 197
 198     /** Number of bits for each entry of the local history table. */
 199     unsigned localHistoryBits;
 200
 201     /** Array of counters that make up the global predictor. */
 202     std::vector<SatCounter> globalCtrs;
 203
 204     /** Number of entries in the global predictor. */
 205     unsigned globalPredictorSize;
 206
 207     /** Number of bits of the global predictor's counters. */
 208     unsigned globalCtrBits;
 209
 210     /** Global history register. Contains as much history as specified by
 211      *  globalHistoryBits. Actual number of bits used is determined by
 212      *  globalHistoryMask and choiceHistoryMask. */
 213     unsigned globalHistory;
 214
 215     /** Number of bits for the global history. Determines maximum number of
 216         entries in global and choice predictor tables. */
 217     unsigned globalHistoryBits;
 218
 219     /** Mask to apply to globalHistory to access global history table.
 220      *  Based on globalPredictorSize.*/
 221     unsigned globalHistoryMask;
 222
 223     /** Mask to apply to globalHistory to access choice history table.
 224      *  Based on choicePredictorSize.*/
 225     unsigned choiceHistoryMask;
 226
 227     /** Mask to control how much history is stored. All of it might not be
 228      *  used. */
 229     unsigned historyRegisterMask;
 230
 231     /** Array of counters that make up the choice predictor. */
 232     std::vector<SatCounter> choiceCtrs;
 233
 234     /** Number of entries in the choice predictor. */
 235     unsigned choicePredictorSize;
 236
 237     /** Number of bits in the choice predictor's counters. */
 238     unsigned choiceCtrBits;
 239
 240     /** Number of bits to shift the instruction over to get rid of the word
 241      *  offset.
 242      */
 243     unsigned instShiftAmt;
 244
 245     /** Thresholds for the counter value; above the threshold is taken,
 246      *  equal to or below the threshold is not taken.
 247      */
 248     unsigned localThreshold;
 249     unsigned globalThreshold;
 250     unsigned choiceThreshold;
 251 };
 252
 253 #endif // __CPU_O3_TOURNAMENT_PRED_HH__