src/cpu/o3/bpred_unit.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 #ifndef __CPU_O3_BPRED_UNIT_HH__
  32 #define __CPU_O3_BPRED_UNIT_HH__
  33
  34 #include "base/statistics.hh"
  35 #include "cpu/inst_seq.hh"
  36
  37 #include "cpu/o3/2bit_local_pred.hh"
  38 #include "cpu/o3/btb.hh"
  39 #include "cpu/o3/ras.hh"
  40 #include "cpu/o3/tournament_pred.hh"
  41
  42 #include "sim/host.hh"
  43
  44 #include <list>
  45
  46 /**
  47  * Basically a wrapper class to hold both the branch predictor
  48  * and the BTB.
  49  */
  50 template<class Impl>
  51 class BPredUnit
  52 {
  53   private:
  54     typedef typename Impl::Params Params;
  55     typedef typename Impl::DynInstPtr DynInstPtr;
  56
  57     enum PredType {
  58         Local,
  59         Tournament
  60     };
  61
  62     PredType predictor;
  63
  64   public:
  65
  66     /**
  67      * @param params The params object, that has the size of the BP and BTB.
  68      */
  69     BPredUnit(Params *params);
  70
  71     /**
  72      * Registers statistics.
  73      */
  74     void regStats();
  75
  76     void switchOut();
  77
  78     void takeOverFrom();
  79
  80     /**
  81      * Predicts whether or not the instruction is a taken branch, and the
  82      * target of the branch if it is taken.
  83      * @param inst The branch instruction.
  84      * @param PC The predicted PC is passed back through this parameter.
  85      * @param tid The thread id.
  86      * @return Returns if the branch is taken or not.
  87      */
  88     bool predict(DynInstPtr &inst, Addr &PC, unsigned tid);
  89
  90     // @todo: Rename this function.
  91     void BPUncond(void * &bp_history);
  92
  93     /**
  94      * Tells the branch predictor to commit any updates until the given
  95      * sequence number.
  96      * @param done_sn The sequence number to commit any older updates up until.
  97      * @param tid The thread id.
  98      */
  99     void update(const InstSeqNum &done_sn, unsigned tid);
 100
 101     /**
 102      * Squashes all outstanding updates until a given sequence number.
 103      * @param squashed_sn The sequence number to squash any younger updates up
 104      * until.
 105      * @param tid The thread id.
 106      */
 107     void squash(const InstSeqNum &squashed_sn, unsigned tid);
 108
 109     /**
 110      * Squashes all outstanding updates until a given sequence number, and
 111      * corrects that sn's update with the proper address and taken/not taken.
 112      * @param squashed_sn The sequence number to squash any younger updates up
 113      * until.
 114      * @param corr_target The correct branch target.
 115      * @param actually_taken The correct branch direction.
 116      * @param tid The thread id.
 117      */
 118     void squash(const InstSeqNum &squashed_sn, const Addr &corr_target,
 119                 bool actually_taken, unsigned tid);
 120
 121     /**
 122      * @param bp_history Pointer to the history object.  The predictor
 123      * will need to update any state and delete the object.
 124      */
 125     void BPSquash(void *bp_history);
 126
 127     /**
 128      * Looks up a given PC in the BP to see if it is taken or not taken.
 129      * @param inst_PC The PC to look up.
 130      * @param bp_history Pointer that will be set to an object that
 131      * has the branch predictor state associated with the lookup.
 132      * @return Whether the branch is taken or not taken.
 133      */
 134     bool BPLookup(Addr &inst_PC, void * &bp_history);
 135
 136     /**
 137      * Looks up a given PC in the BTB to see if a matching entry exists.
 138      * @param inst_PC The PC to look up.
 139      * @return Whether the BTB contains the given PC.
 140      */
 141     bool BTBValid(Addr &inst_PC)
 142     { return BTB.valid(inst_PC, 0); }
 143
 144     /**
 145      * Looks up a given PC in the BTB to get the predicted target.
 146      * @param inst_PC The PC to look up.
 147      * @return The address of the target of the branch.
 148      */
 149     Addr BTBLookup(Addr &inst_PC)
 150     { return BTB.lookup(inst_PC, 0); }
 151
 152     /**
 153      * Updates the BP with taken/not taken information.
 154      * @param inst_PC The branch's PC that will be updated.
 155      * @param taken Whether the branch was taken or not taken.
 156      * @param bp_history Pointer to the branch predictor state that is
 157      * associated with the branch lookup that is being updated.
 158      * @todo Make this update flexible enough to handle a global predictor.
 159      */
 160     void BPUpdate(Addr &inst_PC, bool taken, void *bp_history);
 161
 162     /**
 163      * Updates the BTB with the target of a branch.
 164      * @param inst_PC The branch's PC that will be updated.
 165      * @param target_PC The branch's target that will be added to the BTB.
 166      */
 167     void BTBUpdate(Addr &inst_PC, Addr &target_PC)
 168     { BTB.update(inst_PC, target_PC,0); }
 169
 170     void dump();
 171
 172   private:
 173     struct PredictorHistory {
 174         /**
 175          * Makes a predictor history struct that contains any
 176          * information needed to update the predictor, BTB, and RAS.
 177          */
 178         PredictorHistory(const InstSeqNum &seq_num, const Addr &inst_PC,
 179                          const bool pred_taken, void *bp_history,
 180                          const unsigned _tid)
 181             : seqNum(seq_num), PC(inst_PC), RASTarget(0),
 182               RASIndex(0), tid(_tid), predTaken(pred_taken), usedRAS(0),
 183               wasCall(0), bpHistory(bp_history)
 184         { }
 185
 186         /** The sequence number for the predictor history entry. */
 187         InstSeqNum seqNum;
 188
 189         /** The PC associated with the sequence number. */
 190         Addr PC;
 191
 192         /** The RAS target (only valid if a return). */
 193         Addr RASTarget;
 194
 195         /** The RAS index of the instruction (only valid if a call). */
 196         unsigned RASIndex;
 197
 198         /** The thread id. */
 199         unsigned tid;
 200
 201         /** Whether or not it was predicted taken. */
 202         bool predTaken;
 203
 204         /** Whether or not the RAS was used. */
 205         bool usedRAS;
 206
 207         /** Whether or not the instruction was a call. */
 208         bool wasCall;
 209
 210         /** Pointer to the history object passed back from the branch
 211          * predictor.  It is used to update or restore state of the
 212          * branch predictor.
 213          */
 214         void *bpHistory;
 215     };
 216
 217     typedef std::list<PredictorHistory> History;
 218
 219     /**
 220      * The per-thread predictor history. This is used to update the predictor
 221      * as instructions are committed, or restore it to the proper state after
 222      * a squash.
 223      */
 224     History predHist[Impl::MaxThreads];
 225
 226     /** The local branch predictor. */
 227     LocalBP *localBP;
 228
 229     /** The tournament branch predictor. */
 230     TournamentBP *tournamentBP;
 231
 232     /** The BTB. */
 233     DefaultBTB BTB;
 234
 235     /** The per-thread return address stack. */
 236     ReturnAddrStack RAS[Impl::MaxThreads];
 237
 238     /** Stat for number of BP lookups. */
 239     Stats::Scalar<> lookups;
 240     /** Stat for number of conditional branches predicted. */
 241     Stats::Scalar<> condPredicted;
 242     /** Stat for number of conditional branches predicted incorrectly. */
 243     Stats::Scalar<> condIncorrect;
 244     /** Stat for number of BTB lookups. */
 245     Stats::Scalar<> BTBLookups;
 246     /** Stat for number of BTB hits. */
 247     Stats::Scalar<> BTBHits;
 248     /** Stat for number of times the BTB is correct. */
 249     Stats::Scalar<> BTBCorrect;
 250     /** Stat for number of times the RAS is used to get a target. */
 251     Stats::Scalar<> usedRAS;
 252     /** Stat for number of times the RAS is incorrect. */
 253     Stats::Scalar<> RASIncorrect;
 254 };
 255
 256 #endif // __CPU_O3_BPRED_UNIT_HH__