src/cpu/pred/tournament.cc

   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 #include "base/intmath.hh"
  44 #include "cpu/pred/tournament.hh"
  45
  46 TournamentBP::TournamentBP(unsigned _localPredictorSize,
  47                            unsigned _localCtrBits,
  48                            unsigned _localHistoryTableSize,
  49                            unsigned _localHistoryBits,
  50                            unsigned _globalPredictorSize,
  51                            unsigned _globalHistoryBits,
  52                            unsigned _globalCtrBits,
  53                            unsigned _choicePredictorSize,
  54                            unsigned _choiceCtrBits,
  55                            unsigned _instShiftAmt)
  56     : localPredictorSize(_localPredictorSize),
  57       localCtrBits(_localCtrBits),
  58       localHistoryTableSize(_localHistoryTableSize),
  59       localHistoryBits(_localHistoryBits),
  60       globalPredictorSize(_globalPredictorSize),
  61       globalCtrBits(_globalCtrBits),
  62       globalHistoryBits(_globalHistoryBits),
  63       choicePredictorSize(_globalPredictorSize),
  64       choiceCtrBits(_choiceCtrBits),
  65       instShiftAmt(_instShiftAmt)
  66 {
  67     if (!isPowerOf2(localPredictorSize)) {
  68         fatal("Invalid local predictor size!\n");
  69     }
  70
  71     //Setup the array of counters for the local predictor
  72     localCtrs.resize(localPredictorSize);
  73
  74     for (int i = 0; i < localPredictorSize; ++i)
  75         localCtrs[i].setBits(localCtrBits);
  76
  77     localPredictorMask = floorPow2(localPredictorSize) - 1;
  78
  79     if (!isPowerOf2(localHistoryTableSize)) {
  80         fatal("Invalid local history table size!\n");
  81     }
  82
  83     //Setup the history table for the local table
  84     localHistoryTable.resize(localHistoryTableSize);
  85
  86     for (int i = 0; i < localHistoryTableSize; ++i)
  87         localHistoryTable[i] = 0;
  88
  89     // Setup the local history mask
  90     localHistoryMask = (1 << localHistoryBits) - 1;
  91
  92     if (!isPowerOf2(globalPredictorSize)) {
  93         fatal("Invalid global predictor size!\n");
  94     }
  95
  96     //Setup the array of counters for the global predictor
  97     globalCtrs.resize(globalPredictorSize);
  98
  99     for (int i = 0; i < globalPredictorSize; ++i)
 100         globalCtrs[i].setBits(globalCtrBits);
 101
 102     //Clear the global history
 103     globalHistory = 0;
 104     // Setup the global history mask
 105     globalHistoryMask = (1 << globalHistoryBits) - 1;
 106
 107     if (!isPowerOf2(choicePredictorSize)) {
 108         fatal("Invalid choice predictor size!\n");
 109     }
 110
 111     //Setup the array of counters for the choice predictor
 112     choiceCtrs.resize(choicePredictorSize);
 113
 114     for (int i = 0; i < choicePredictorSize; ++i)
 115         choiceCtrs[i].setBits(choiceCtrBits);
 116
 117     // @todo: Allow for different thresholds between the predictors.
 118     threshold = (1 << (localCtrBits - 1)) - 1;
 119 }
 120
 121 inline
 122 unsigned
 123 TournamentBP::calcLocHistIdx(Addr &branch_addr)
 124 {
 125     // Get low order bits after removing instruction offset.
 126     return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
 127 }
 128
 129 inline
 130 void
 131 TournamentBP::updateGlobalHistTaken()
 132 {
 133     globalHistory = (globalHistory << 1) | 1;
 134     globalHistory = globalHistory & globalHistoryMask;
 135 }
 136
 137 inline
 138 void
 139 TournamentBP::updateGlobalHistNotTaken()
 140 {
 141     globalHistory = (globalHistory << 1);
 142     globalHistory = globalHistory & globalHistoryMask;
 143 }
 144
 145 inline
 146 void
 147 TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
 148 {
 149     localHistoryTable[local_history_idx] =
 150         (localHistoryTable[local_history_idx] << 1) | 1;
 151 }
 152
 153 inline
 154 void
 155 TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
 156 {
 157     localHistoryTable[local_history_idx] =
 158         (localHistoryTable[local_history_idx] << 1);
 159 }
 160
 161
 162 void
 163 TournamentBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
 164 {
 165     unsigned local_history_idx = calcLocHistIdx(branch_addr);
 166     //Update Global History to Not Taken
 167     globalHistory = globalHistory & (globalHistoryMask - 1);
 168     //Update Local History to Not Taken
 169     localHistoryTable[local_history_idx] =
 170        localHistoryTable[local_history_idx] & (localPredictorMask - 1);
 171 }
 172
 173 bool
 174 TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
 175 {
 176     bool local_prediction;
 177     unsigned local_history_idx;
 178     unsigned local_predictor_idx;
 179
 180     bool global_prediction;
 181     bool choice_prediction;
 182
 183     //Lookup in the local predictor to get its branch prediction
 184     local_history_idx = calcLocHistIdx(branch_addr);
 185     local_predictor_idx = localHistoryTable[local_history_idx]
 186         & localPredictorMask;
 187     local_prediction = localCtrs[local_predictor_idx].read() > threshold;
 188
 189     //Lookup in the global predictor to get its branch prediction
 190     global_prediction = globalCtrs[globalHistory].read() > threshold;
 191
 192     //Lookup in the choice predictor to see which one to use
 193     choice_prediction = choiceCtrs[globalHistory].read() > threshold;
 194
 195     // Create BPHistory and pass it back to be recorded.
 196     BPHistory *history = new BPHistory;
 197     history->globalHistory = globalHistory;
 198     history->localPredTaken = local_prediction;
 199     history->globalPredTaken = global_prediction;
 200     history->globalUsed = choice_prediction;
 201     history->localHistory = local_predictor_idx;
 202     bp_history = (void *)history;
 203
 204     assert(globalHistory < globalPredictorSize &&
 205            local_history_idx < localHistoryTableSize &&
 206            local_predictor_idx < localPredictorSize);
 207
 208     // Commented code is for doing speculative update of counters and
 209     // all histories.
 210     if (choice_prediction) {
 211         if (global_prediction) {
 212             updateGlobalHistTaken();
 213             updateLocalHistTaken(local_history_idx);
 214             return true;
 215         } else {
 216             updateGlobalHistNotTaken();
 217             updateLocalHistNotTaken(local_history_idx);
 218             return false;
 219         }
 220     } else {
 221         if (local_prediction) {
 222             updateGlobalHistTaken();
 223             updateLocalHistTaken(local_history_idx);
 224             return true;
 225         } else {
 226             updateGlobalHistNotTaken();
 227             updateLocalHistNotTaken(local_history_idx);
 228             return false;
 229         }
 230     }
 231 }
 232
 233 void
 234 TournamentBP::uncondBr(void * &bp_history)
 235 {
 236     // Create BPHistory and pass it back to be recorded.
 237     BPHistory *history = new BPHistory;
 238     history->globalHistory = globalHistory;
 239     history->localPredTaken = true;
 240     history->globalPredTaken = true;
 241     history->globalUsed = true;
 242     history->localHistory = invalidPredictorIndex;
 243     bp_history = static_cast<void *>(history);
 244
 245     updateGlobalHistTaken();
 246 }
 247
 248 void
 249 TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
 250                      bool squashed)
 251 {
 252     unsigned local_history_idx;
 253     unsigned local_predictor_idx M5_VAR_USED;
 254     unsigned local_predictor_hist;
 255
 256     // Get the local predictor's current prediction
 257     local_history_idx = calcLocHistIdx(branch_addr);
 258     local_predictor_hist = localHistoryTable[local_history_idx];
 259     local_predictor_idx = local_predictor_hist & localPredictorMask;
 260
 261     if (bp_history) {
 262         BPHistory *history = static_cast<BPHistory *>(bp_history);
 263         // Update may also be called if the Branch target is incorrect even if
 264         // the prediction is correct. In that case do not update the counters.
 265         bool historyPred = false;
 266         unsigned old_local_pred_index = history->localHistory
 267                       & localPredictorMask;
 268         if (history->globalUsed) {
 269            historyPred = history->globalPredTaken;
 270         } else {
 271            historyPred = history->localPredTaken;
 272         }
 273         if (historyPred != taken || !squashed) {
 274             // Update the choice predictor to tell it which one was correct if
 275             // there was a prediction.
 276             if (history->localPredTaken != history->globalPredTaken) {
 277                  // If the local prediction matches the actual outcome,
 278                  // decerement the counter.  Otherwise increment the
 279                  // counter.
 280                  if (history->localPredTaken == taken) {
 281                      choiceCtrs[history->globalHistory].decrement();
 282                  } else if (history->globalPredTaken == taken) {
 283                      choiceCtrs[history->globalHistory].increment();
 284                  }
 285
 286              }
 287
 288              // Update the counters and local history with the proper
 289              // resolution of the branch.  Global history is updated
 290              // speculatively and restored upon squash() calls, so it does not
 291              // need to be updated.
 292              if (taken) {
 293                   globalCtrs[history->globalHistory].increment();
 294                   if (old_local_pred_index != invalidPredictorIndex) {
 295                           localCtrs[old_local_pred_index].increment();
 296                   }
 297              } else {
 298                   globalCtrs[history->globalHistory].decrement();
 299                   if (old_local_pred_index != invalidPredictorIndex) {
 300                           localCtrs[old_local_pred_index].decrement();
 301                   }
 302              }
 303         }
 304         if (squashed) {
 305              if (taken) {
 306                 globalHistory = (history->globalHistory << 1) | 1;
 307                 globalHistory = globalHistory & globalHistoryMask;
 308                 if (old_local_pred_index != invalidPredictorIndex) {
 309                     localHistoryTable[old_local_pred_index] =
 310                      (history->localHistory << 1) | 1;
 311                 }
 312              } else {
 313                 globalHistory = (history->globalHistory << 1);
 314                 globalHistory = globalHistory & globalHistoryMask;
 315                 if (old_local_pred_index != invalidPredictorIndex) {
 316                      localHistoryTable[old_local_pred_index] =
 317                      history->localHistory << 1;
 318                 }
 319              }
 320
 321         }
 322         // We're done with this history, now delete it.
 323         delete history;
 324
 325     }
 326
 327     assert(globalHistory < globalPredictorSize &&
 328            local_history_idx < localHistoryTableSize &&
 329            local_predictor_idx < localPredictorSize);
 330
 331
 332 }
 333
 334 void
 335 TournamentBP::squash(void *bp_history)
 336 {
 337     BPHistory *history = static_cast<BPHistory *>(bp_history);
 338
 339     // Restore global history to state prior to this branch.
 340     globalHistory = history->globalHistory;
 341
 342     // Delete this BPHistory now that we're done with it.
 343     delete history;
 344 }
 345
 346 #ifdef DEBUG
 347 int
 348 TournamentBP::BPHistory::newCount = 0;
 349 #endif