src/cpu/pred/tournament.cc

   1 /*
   2  * Copyright (c) 2011, 2014 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
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  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;
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  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,
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  38  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  39  */
  40
  41 #include "cpu/pred/tournament.hh"
  42
  43 #include "base/bitfield.hh"
  44 #include "base/intmath.hh"
  45
  46 TournamentBP::TournamentBP(const TournamentBPParams &params)
  47     : BPredUnit(params),
  48       localPredictorSize(params.localPredictorSize),
  49       localCtrBits(params.localCtrBits),
  50       localCtrs(localPredictorSize, SatCounter8(localCtrBits)),
  51       localHistoryTableSize(params.localHistoryTableSize),
  52       localHistoryBits(ceilLog2(params.localPredictorSize)),
  53       globalPredictorSize(params.globalPredictorSize),
  54       globalCtrBits(params.globalCtrBits),
  55       globalCtrs(globalPredictorSize, SatCounter8(globalCtrBits)),
  56       globalHistory(params.numThreads, 0),
  57       globalHistoryBits(
  58           ceilLog2(params.globalPredictorSize) >
  59           ceilLog2(params.choicePredictorSize) ?
  60           ceilLog2(params.globalPredictorSize) :
  61           ceilLog2(params.choicePredictorSize)),
  62       choicePredictorSize(params.choicePredictorSize),
  63       choiceCtrBits(params.choiceCtrBits),
  64       choiceCtrs(choicePredictorSize, SatCounter8(choiceCtrBits))
  65 {
  66     if (!isPowerOf2(localPredictorSize)) {
  67         fatal("Invalid local predictor size!\n");
  68     }
  69
  70     if (!isPowerOf2(globalPredictorSize)) {
  71         fatal("Invalid global predictor size!\n");
  72     }
  73
  74     localPredictorMask = mask(localHistoryBits);
  75
  76     if (!isPowerOf2(localHistoryTableSize)) {
  77         fatal("Invalid local history table size!\n");
  78     }
  79
  80     //Setup the history table for the local table
  81     localHistoryTable.resize(localHistoryTableSize);
  82
  83     for (int i = 0; i < localHistoryTableSize; ++i)
  84         localHistoryTable[i] = 0;
  85
  86     // Set up the global history mask
  87     // this is equivalent to mask(log2(globalPredictorSize)
  88     globalHistoryMask = globalPredictorSize - 1;
  89
  90     if (!isPowerOf2(choicePredictorSize)) {
  91         fatal("Invalid choice predictor size!\n");
  92     }
  93
  94     // Set up choiceHistoryMask
  95     // this is equivalent to mask(log2(choicePredictorSize)
  96     choiceHistoryMask = choicePredictorSize - 1;
  97
  98     //Set up historyRegisterMask
  99     historyRegisterMask = mask(globalHistoryBits);
 100
 101     //Check that predictors don't use more bits than they have available
 102     if (globalHistoryMask > historyRegisterMask) {
 103         fatal("Global predictor too large for global history bits!\n");
 104     }
 105     if (choiceHistoryMask > historyRegisterMask) {
 106         fatal("Choice predictor too large for global history bits!\n");
 107     }
 108
 109     if (globalHistoryMask < historyRegisterMask &&
 110         choiceHistoryMask < historyRegisterMask) {
 111         inform("More global history bits than required by predictors\n");
 112     }
 113
 114     // Set thresholds for the three predictors' counters
 115     // This is equivalent to (2^(Ctr))/2 - 1
 116     localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
 117     globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
 118     choiceThreshold = (ULL(1) << (choiceCtrBits - 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(ThreadID tid)
 132 {
 133     globalHistory[tid] = (globalHistory[tid] << 1) | 1;
 134     globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
 135 }
 136
 137 inline
 138 void
 139 TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
 140 {
 141     globalHistory[tid] = (globalHistory[tid] << 1);
 142     globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
 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(ThreadID tid, Addr branch_addr, void * &bp_history)
 164 {
 165     unsigned local_history_idx = calcLocHistIdx(branch_addr);
 166     //Update Global History to Not Taken (clear LSB)
 167     globalHistory[tid] &= (historyRegisterMask & ~ULL(1));
 168     //Update Local History to Not Taken
 169     localHistoryTable[local_history_idx] =
 170        localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
 171 }
 172
 173 bool
 174 TournamentBP::lookup(ThreadID tid, 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] > localThreshold;
 188
 189     //Lookup in the global predictor to get its branch prediction
 190     global_prediction = globalThreshold <
 191       globalCtrs[globalHistory[tid] & globalHistoryMask];
 192
 193     //Lookup in the choice predictor to see which one to use
 194     choice_prediction = choiceThreshold <
 195       choiceCtrs[globalHistory[tid] & choiceHistoryMask];
 196
 197     // Create BPHistory and pass it back to be recorded.
 198     BPHistory *history = new BPHistory;
 199     history->globalHistory = globalHistory[tid];
 200     history->localPredTaken = local_prediction;
 201     history->globalPredTaken = global_prediction;
 202     history->globalUsed = choice_prediction;
 203     history->localHistoryIdx = local_history_idx;
 204     history->localHistory = local_predictor_idx;
 205     bp_history = (void *)history;
 206
 207     assert(local_history_idx < localHistoryTableSize);
 208
 209     // Speculative update of the global history and the
 210     // selected local history.
 211     if (choice_prediction) {
 212         if (global_prediction) {
 213             updateGlobalHistTaken(tid);
 214             updateLocalHistTaken(local_history_idx);
 215             return true;
 216         } else {
 217             updateGlobalHistNotTaken(tid);
 218             updateLocalHistNotTaken(local_history_idx);
 219             return false;
 220         }
 221     } else {
 222         if (local_prediction) {
 223             updateGlobalHistTaken(tid);
 224             updateLocalHistTaken(local_history_idx);
 225             return true;
 226         } else {
 227             updateGlobalHistNotTaken(tid);
 228             updateLocalHistNotTaken(local_history_idx);
 229             return false;
 230         }
 231     }
 232 }
 233
 234 void
 235 TournamentBP::uncondBranch(ThreadID tid, Addr pc, void * &bp_history)
 236 {
 237     // Create BPHistory and pass it back to be recorded.
 238     BPHistory *history = new BPHistory;
 239     history->globalHistory = globalHistory[tid];
 240     history->localPredTaken = true;
 241     history->globalPredTaken = true;
 242     history->globalUsed = true;
 243     history->localHistoryIdx = invalidPredictorIndex;
 244     history->localHistory = invalidPredictorIndex;
 245     bp_history = static_cast<void *>(history);
 246
 247     updateGlobalHistTaken(tid);
 248 }
 249
 250 void
 251 TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
 252                      void *bp_history, bool squashed,
 253                      const StaticInstPtr & inst, Addr corrTarget)
 254 {
 255     assert(bp_history);
 256
 257     BPHistory *history = static_cast<BPHistory *>(bp_history);
 258
 259     unsigned local_history_idx = calcLocHistIdx(branch_addr);
 260
 261     assert(local_history_idx < localHistoryTableSize);
 262
 263     // Unconditional branches do not use local history.
 264     bool old_local_pred_valid = history->localHistory !=
 265             invalidPredictorIndex;
 266
 267     // If this is a misprediction, restore the speculatively
 268     // updated state (global history register and local history)
 269     // and update again.
 270     if (squashed) {
 271         // Global history restore and update
 272         globalHistory[tid] = (history->globalHistory << 1) | taken;
 273         globalHistory[tid] &= historyRegisterMask;
 274
 275         // Local history restore and update.
 276         if (old_local_pred_valid) {
 277             localHistoryTable[local_history_idx] =
 278                         (history->localHistory << 1) | taken;
 279         }
 280
 281         return;
 282     }
 283
 284     unsigned old_local_pred_index = history->localHistory &
 285         localPredictorMask;
 286
 287     assert(old_local_pred_index < localPredictorSize);
 288
 289     // Update the choice predictor to tell it which one was correct if
 290     // there was a prediction.
 291     if (history->localPredTaken != history->globalPredTaken &&
 292         old_local_pred_valid)
 293     {
 294         // If the local prediction matches the actual outcome,
 295         // decrement the counter. Otherwise increment the
 296         // counter.
 297         unsigned choice_predictor_idx =
 298             history->globalHistory & choiceHistoryMask;
 299         if (history->localPredTaken == taken) {
 300             choiceCtrs[choice_predictor_idx]--;
 301         } else if (history->globalPredTaken == taken) {
 302             choiceCtrs[choice_predictor_idx]++;
 303         }
 304     }
 305
 306     // Update the counters with the proper
 307     // resolution of the branch. Histories are updated
 308     // speculatively, restored upon squash() calls, and
 309     // recomputed upon update(squash = true) calls,
 310     // so they do not need to be updated.
 311     unsigned global_predictor_idx =
 312             history->globalHistory & globalHistoryMask;
 313     if (taken) {
 314         globalCtrs[global_predictor_idx]++;
 315         if (old_local_pred_valid) {
 316             localCtrs[old_local_pred_index]++;
 317         }
 318     } else {
 319         globalCtrs[global_predictor_idx]--;
 320         if (old_local_pred_valid) {
 321             localCtrs[old_local_pred_index]--;
 322         }
 323     }
 324
 325     // We're done with this history, now delete it.
 326     delete history;
 327 }
 328
 329 void
 330 TournamentBP::squash(ThreadID tid, void *bp_history)
 331 {
 332     BPHistory *history = static_cast<BPHistory *>(bp_history);
 333
 334     // Restore global history to state prior to this branch.
 335     globalHistory[tid] = history->globalHistory;
 336
 337     // Restore local history
 338     if (history->localHistoryIdx != invalidPredictorIndex) {
 339         localHistoryTable[history->localHistoryIdx] = history->localHistory;
 340     }
 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