src/cpu/o3/bpred_unit_impl.hh

   1 /*
   2  * Copyright (c) 2004-2005 The Regents of The University of Michigan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
   7  * met: redistributions of source code must retain the above copyright
   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  *
  28  * Authors: Kevin Lim
  29  */
  30
  31 #include <list>
  32 #include <vector>
  33
  34 #include "base/trace.hh"
  35 #include "base/traceflags.hh"
  36 #include "cpu/o3/bpred_unit.hh"
  37
  38 using namespace std;
  39
  40 template<class Impl>
  41 BPredUnit<Impl>::BPredUnit(Params *params)
  42   : BTB(params->BTBEntries,
  43         params->BTBTagSize,
  44         params->instShiftAmt)
  45 {
  46     // Setup the selected predictor.
  47     if (params->predType == "local") {
  48         localBP = new LocalBP(params->localPredictorSize,
  49                               params->localCtrBits,
  50                               params->instShiftAmt);
  51         predictor = Local;
  52     } else if (params->predType == "tournament") {
  53         tournamentBP = new TournamentBP(params->localPredictorSize,
  54                                         params->localCtrBits,
  55                                         params->localHistoryTableSize,
  56                                         params->localHistoryBits,
  57                                         params->globalPredictorSize,
  58                                         params->globalHistoryBits,
  59                                         params->globalCtrBits,
  60                                         params->choicePredictorSize,
  61                                         params->choiceCtrBits,
  62                                         params->instShiftAmt);
  63         predictor = Tournament;
  64     } else {
  65         fatal("Invalid BP selected!");
  66     }
  67
  68     for (int i=0; i < Impl::MaxThreads; i++)
  69         RAS[i].init(params->RASSize);
  70 }
  71
  72 template <class Impl>
  73 void
  74 BPredUnit<Impl>::regStats()
  75 {
  76     lookups
  77         .name(name() + ".BPredUnit.lookups")
  78         .desc("Number of BP lookups")
  79         ;
  80
  81     condPredicted
  82         .name(name() + ".BPredUnit.condPredicted")
  83         .desc("Number of conditional branches predicted")
  84         ;
  85
  86     condIncorrect
  87         .name(name() + ".BPredUnit.condIncorrect")
  88         .desc("Number of conditional branches incorrect")
  89         ;
  90
  91     BTBLookups
  92         .name(name() + ".BPredUnit.BTBLookups")
  93         .desc("Number of BTB lookups")
  94         ;
  95
  96     BTBHits
  97         .name(name() + ".BPredUnit.BTBHits")
  98         .desc("Number of BTB hits")
  99         ;
 100
 101     BTBCorrect
 102         .name(name() + ".BPredUnit.BTBCorrect")
 103         .desc("Number of correct BTB predictions (this stat may not "
 104               "work properly.")
 105         ;
 106
 107     usedRAS
 108         .name(name() + ".BPredUnit.usedRAS")
 109         .desc("Number of times the RAS was used to get a target.")
 110         ;
 111
 112     RASIncorrect
 113         .name(name() + ".BPredUnit.RASInCorrect")
 114         .desc("Number of incorrect RAS predictions.")
 115         ;
 116 }
 117
 118 template <class Impl>
 119 void
 120 BPredUnit<Impl>::switchOut()
 121 {
 122     // Clear any state upon switch out.
 123     for (int i = 0; i < Impl::MaxThreads; ++i) {
 124         squash(0, i);
 125     }
 126 }
 127
 128 template <class Impl>
 129 void
 130 BPredUnit<Impl>::takeOverFrom()
 131 {
 132     // Can reset all predictor state, but it's not necessarily better
 133     // than leaving it be.
 134 /*
 135     for (int i = 0; i < Impl::MaxThreads; ++i)
 136         RAS[i].reset();
 137
 138     BP.reset();
 139     BTB.reset();
 140 */
 141 }
 142
 143 template <class Impl>
 144 bool
 145 BPredUnit<Impl>::predict(DynInstPtr &inst, Addr &PC, unsigned tid)
 146 {
 147     // See if branch predictor predicts taken.
 148     // If so, get its target addr either from the BTB or the RAS.
 149     // Save off record of branch stuff so the RAS can be fixed
 150     // up once it's done.
 151
 152     using TheISA::MachInst;
 153
 154     bool pred_taken = false;
 155     Addr target;
 156
 157     ++lookups;
 158
 159     void *bp_history = NULL;
 160
 161     if (inst->isUncondCtrl()) {
 162         DPRINTF(Fetch, "BranchPred: [tid:%i] Unconditional control.\n", tid);
 163         pred_taken = true;
 164         // Tell the BP there was an unconditional branch.
 165         BPUncond(bp_history);
 166     } else {
 167         ++condPredicted;
 168
 169         pred_taken = BPLookup(PC, bp_history);
 170
 171         DPRINTF(Fetch, "BranchPred: [tid:%i]: Branch predictor predicted %i "
 172                 "for PC %#x\n",
 173                 tid, pred_taken, inst->readPC());
 174     }
 175
 176     PredictorHistory predict_record(inst->seqNum, PC, pred_taken,
 177                                     bp_history, tid);
 178
 179     // Now lookup in the BTB or RAS.
 180     if (pred_taken) {
 181         if (inst->isReturn()) {
 182             ++usedRAS;
 183
 184             // If it's a function return call, then look up the address
 185             // in the RAS.
 186             target = RAS[tid].top();
 187
 188             // Record the top entry of the RAS, and its index.
 189             predict_record.usedRAS = true;
 190             predict_record.RASIndex = RAS[tid].topIdx();
 191             predict_record.RASTarget = target;
 192
 193             assert(predict_record.RASIndex < 16);
 194
 195             RAS[tid].pop();
 196
 197             DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %#x is a return, "
 198                     "RAS predicted target: %#x, RAS index: %i.\n",
 199                     tid, inst->readPC(), target, predict_record.RASIndex);
 200         } else {
 201             ++BTBLookups;
 202
 203             if (inst->isCall()) {
 204                 RAS[tid].push(PC + sizeof(MachInst));
 205
 206                 // Record that it was a call so that the top RAS entry can
 207                 // be popped off if the speculation is incorrect.
 208                 predict_record.wasCall = true;
 209
 210                 DPRINTF(Fetch, "BranchPred: [tid:%i] Instruction %#x was a call"
 211                         ", adding %#x to the RAS.\n",
 212                         tid, inst->readPC(), PC + sizeof(MachInst));
 213             }
 214
 215             if (BTB.valid(PC, tid)) {
 216                 ++BTBHits;
 217
 218                 // If it's not a return, use the BTB to get the target addr.
 219                 target = BTB.lookup(PC, tid);
 220
 221                 DPRINTF(Fetch, "BranchPred: [tid:%i]: Instruction %#x predicted"
 222                         " target is %#x.\n",
 223                         tid, inst->readPC(), target);
 224
 225             } else {
 226                 DPRINTF(Fetch, "BranchPred: [tid:%i]: BTB doesn't have a "
 227                         "valid entry.\n",tid);
 228                 pred_taken = false;
 229             }
 230
 231         }
 232     }
 233
 234     if (pred_taken) {
 235         // Set the PC and the instruction's predicted target.
 236         PC = target;
 237         inst->setPredTarg(target);
 238     } else {
 239         PC = PC + sizeof(MachInst);
 240         inst->setPredTarg(PC);
 241     }
 242
 243     predHist[tid].push_front(predict_record);
 244
 245     DPRINTF(Fetch, "[tid:%i] predHist.size(): %i\n", tid, predHist[tid].size());
 246
 247     return pred_taken;
 248 }
 249
 250 template <class Impl>
 251 void
 252 BPredUnit<Impl>::update(const InstSeqNum &done_sn, unsigned tid)
 253 {
 254     DPRINTF(Fetch, "BranchPred: [tid:%i]: Commiting branches until sequence"
 255             "number %lli.\n", tid, done_sn);
 256
 257     while (!predHist[tid].empty() &&
 258            predHist[tid].back().seqNum <= done_sn) {
 259         // Update the branch predictor with the correct results.
 260         BPUpdate(predHist[tid].back().PC,
 261                  predHist[tid].back().predTaken,
 262                  predHist[tid].back().bpHistory);
 263
 264         predHist[tid].pop_back();
 265     }
 266 }
 267
 268 template <class Impl>
 269 void
 270 BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn, unsigned tid)
 271 {
 272     History &pred_hist = predHist[tid];
 273
 274     while (!pred_hist.empty() &&
 275            pred_hist.front().seqNum > squashed_sn) {
 276         if (pred_hist.front().usedRAS) {
 277             DPRINTF(Fetch, "BranchPred: [tid:%i]: Restoring top of RAS to: %i,"
 278                     " target: %#x.\n",
 279                     tid,
 280                     pred_hist.front().RASIndex,
 281                     pred_hist.front().RASTarget);
 282
 283             RAS[tid].restore(pred_hist.front().RASIndex,
 284                              pred_hist.front().RASTarget);
 285
 286         } else if (pred_hist.front().wasCall) {
 287             DPRINTF(Fetch, "BranchPred: [tid:%i]: Removing speculative entry "
 288                     "added to the RAS.\n",tid);
 289
 290             RAS[tid].pop();
 291         }
 292
 293         // This call should delete the bpHistory.
 294         BPSquash(pred_hist.front().bpHistory);
 295
 296         pred_hist.pop_front();
 297     }
 298
 299 }
 300
 301 template <class Impl>
 302 void
 303 BPredUnit<Impl>::squash(const InstSeqNum &squashed_sn,
 304                         const Addr &corr_target,
 305                         const bool actually_taken,
 306                         unsigned tid)
 307 {
 308     // Now that we know that a branch was mispredicted, we need to undo
 309     // all the branches that have been seen up until this branch and
 310     // fix up everything.
 311
 312     History &pred_hist = predHist[tid];
 313
 314     ++condIncorrect;
 315
 316     DPRINTF(Fetch, "BranchPred: [tid:%i]: Squashing from sequence number %i, "
 317             "setting target to %#x.\n",
 318             tid, squashed_sn, corr_target);
 319
 320     squash(squashed_sn, tid);
 321
 322     // If there's a squash due to a syscall, there may not be an entry
 323     // corresponding to the squash.  In that case, don't bother trying to
 324     // fix up the entry.
 325     if (!pred_hist.empty()) {
 326         assert(pred_hist.front().seqNum == squashed_sn);
 327         if (pred_hist.front().usedRAS) {
 328             ++RASIncorrect;
 329         }
 330
 331         BPUpdate(pred_hist.front().PC, actually_taken,
 332                  pred_hist.front().bpHistory);
 333
 334         BTB.update(pred_hist.front().PC, corr_target, tid);
 335         pred_hist.pop_front();
 336     }
 337 }
 338
 339 template <class Impl>
 340 void
 341 BPredUnit<Impl>::BPUncond(void * &bp_history)
 342 {
 343     // Only the tournament predictor cares about unconditional branches.
 344     if (predictor == Tournament) {
 345         tournamentBP->uncondBr(bp_history);
 346     }
 347 }
 348
 349 template <class Impl>
 350 void
 351 BPredUnit<Impl>::BPSquash(void *bp_history)
 352 {
 353     if (predictor == Local) {
 354         localBP->squash(bp_history);
 355     } else if (predictor == Tournament) {
 356         tournamentBP->squash(bp_history);
 357     } else {
 358         panic("Predictor type is unexpected value!");
 359     }
 360 }
 361
 362 template <class Impl>
 363 bool
 364 BPredUnit<Impl>::BPLookup(Addr &inst_PC, void * &bp_history)
 365 {
 366     if (predictor == Local) {
 367         return localBP->lookup(inst_PC, bp_history);
 368     } else if (predictor == Tournament) {
 369         return tournamentBP->lookup(inst_PC, bp_history);
 370     } else {
 371         panic("Predictor type is unexpected value!");
 372     }
 373 }
 374
 375 template <class Impl>
 376 void
 377 BPredUnit<Impl>::BPUpdate(Addr &inst_PC, bool taken, void *bp_history)
 378 {
 379     if (predictor == Local) {
 380         localBP->update(inst_PC, taken, bp_history);
 381     } else if (predictor == Tournament) {
 382         tournamentBP->update(inst_PC, taken, bp_history);
 383     } else {
 384         panic("Predictor type is unexpected value!");
 385     }
 386 }
 387
 388 template <class Impl>
 389 void
 390 BPredUnit<Impl>::dump()
 391 {
 392     typename History::iterator pred_hist_it;
 393
 394     for (int i = 0; i < Impl::MaxThreads; ++i) {
 395         if (!predHist[i].empty()) {
 396             pred_hist_it = predHist[i].begin();
 397
 398             cprintf("predHist[%i].size(): %i\n", i, predHist[i].size());
 399
 400             while (pred_hist_it != predHist[i].end()) {
 401                 cprintf("[sn:%lli], PC:%#x, tid:%i, predTaken:%i, "
 402                         "bpHistory:%#x\n",
 403                         (*pred_hist_it).seqNum, (*pred_hist_it).PC,
 404                         (*pred_hist_it).tid, (*pred_hist_it).predTaken,
 405                         (*pred_hist_it).bpHistory);
 406                 pred_hist_it++;
 407             }
 408
 409             cprintf("\n");
 410         }
 411     }
 412 }