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 <list>
  35
  36 #include "base/statistics.hh"
  37 #include "base/types.hh"
  38 #include "cpu/pred/2bit_local.hh"
  39 #include "cpu/pred/btb.hh"
  40 #include "cpu/pred/ras.hh"
  41 #include "cpu/pred/tournament.hh"
  42 #include "cpu/inst_seq.hh"
  43
  44 class DerivO3CPUParams;
  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::DynInstPtr DynInstPtr;
  55
  56     enum PredType {
  57         Local,
  58         Tournament
  59     };
  60
  61     PredType predictor;
  62
  63     const std::string _name;
  64
  65   public:
  66
  67     /**
  68      * @param params The params object, that has the size of the BP and BTB.
  69      */
  70     BPredUnit(DerivO3CPUParams *params);
  71
  72     const std::string &name() const { return _name; }
  73
  74     /**
  75      * Registers statistics.
  76      */
  77     void regStats();
  78
  79     void switchOut();
  80
  81     void takeOverFrom();
  82
  83     /**
  84      * Predicts whether or not the instruction is a taken branch, and the
  85      * target of the branch if it is taken.
  86      * @param inst The branch instruction.
  87      * @param PC The predicted PC is passed back through this parameter.
  88      * @param tid The thread id.
  89      * @return Returns if the branch is taken or not.
  90      */
  91     bool predict(DynInstPtr &inst, TheISA::PCState &pc, ThreadID tid);
  92
  93     // @todo: Rename this function.
  94     void BPUncond(void * &bp_history);
  95
  96     /**
  97      * Tells the branch predictor to commit any updates until the given
  98      * sequence number.
  99      * @param done_sn The sequence number to commit any older updates up until.
 100      * @param tid The thread id.
 101      */
 102     void update(const InstSeqNum &done_sn, ThreadID tid);
 103
 104     /**
 105      * Squashes all outstanding updates until a given sequence number.
 106      * @param squashed_sn The sequence number to squash any younger updates up
 107      * until.
 108      * @param tid The thread id.
 109      */
 110     void squash(const InstSeqNum &squashed_sn, ThreadID tid);
 111
 112     /**
 113      * Squashes all outstanding updates until a given sequence number, and
 114      * corrects that sn's update with the proper address and taken/not taken.
 115      * @param squashed_sn The sequence number to squash any younger updates up
 116      * until.
 117      * @param corr_target The correct branch target.
 118      * @param actually_taken The correct branch direction.
 119      * @param tid The thread id.
 120      */
 121     void squash(const InstSeqNum &squashed_sn,
 122                 const TheISA::PCState &corr_target,
 123                 bool actually_taken, ThreadID tid);
 124
 125     /**
 126      * @param bp_history Pointer to the history object.  The predictor
 127      * will need to update any state and delete the object.
 128      */
 129     void BPSquash(void *bp_history);
 130
 131     /**
 132      * Looks up a given PC in the BP to see if it is taken or not taken.
 133      * @param inst_PC The PC to look up.
 134      * @param bp_history Pointer that will be set to an object that
 135      * has the branch predictor state associated with the lookup.
 136      * @return Whether the branch is taken or not taken.
 137      */
 138     bool BPLookup(Addr instPC, void * &bp_history);
 139
 140     /**
 141      * Looks up a given PC in the BTB to see if a matching entry exists.
 142      * @param inst_PC The PC to look up.
 143      * @return Whether the BTB contains the given PC.
 144      */
 145     bool BTBValid(Addr instPC)
 146     { return BTB.valid(instPC, 0); }
 147
 148     /**
 149      * Looks up a given PC in the BTB to get the predicted target.
 150      * @param inst_PC The PC to look up.
 151      * @return The address of the target of the branch.
 152      */
 153     TheISA::PCState BTBLookup(Addr instPC)
 154     { return BTB.lookup(instPC, 0); }
 155
 156     /**
 157      * Updates the BP with taken/not taken information.
 158      * @param inst_PC The branch's PC that will be updated.
 159      * @param taken Whether the branch was taken or not taken.
 160      * @param bp_history Pointer to the branch predictor state that is
 161      * associated with the branch lookup that is being updated.
 162      * @todo Make this update flexible enough to handle a global predictor.
 163      */
 164     void BPUpdate(Addr instPC, bool taken, void *bp_history);
 165
 166     /**
 167      * Updates the BTB with the target of a branch.
 168      * @param inst_PC The branch's PC that will be updated.
 169      * @param target_PC The branch's target that will be added to the BTB.
 170      */
 171     void BTBUpdate(Addr instPC, const TheISA::PCState &target)
 172     { BTB.update(instPC, target, 0); }
 173
 174     void dump();
 175
 176   private:
 177     struct PredictorHistory {
 178         /**
 179          * Makes a predictor history struct that contains any
 180          * information needed to update the predictor, BTB, and RAS.
 181          */
 182         PredictorHistory(const InstSeqNum &seq_num, Addr instPC,
 183                          bool pred_taken, void *bp_history,
 184                          ThreadID _tid)
 185             : seqNum(seq_num), pc(instPC), RASTarget(0), RASIndex(0),
 186               tid(_tid), predTaken(pred_taken), usedRAS(0),
 187               wasCall(0), bpHistory(bp_history)
 188         {}
 189
 190         bool operator==(const PredictorHistory &entry) const {
 191             return this->seqNum == entry.seqNum;
 192         }
 193
 194         /** The sequence number for the predictor history entry. */
 195         InstSeqNum seqNum;
 196
 197         /** The PC associated with the sequence number. */
 198         Addr pc;
 199
 200         /** The RAS target (only valid if a return). */
 201         TheISA::PCState RASTarget;
 202
 203         /** The RAS index of the instruction (only valid if a call). */
 204         unsigned RASIndex;
 205
 206         /** The thread id. */
 207         ThreadID tid;
 208
 209         /** Whether or not it was predicted taken. */
 210         bool predTaken;
 211
 212         /** Whether or not the RAS was used. */
 213         bool usedRAS;
 214
 215         /** Whether or not the instruction was a call. */
 216         bool wasCall;
 217
 218         /** Pointer to the history object passed back from the branch
 219          * predictor.  It is used to update or restore state of the
 220          * branch predictor.
 221          */
 222         void *bpHistory;
 223     };
 224
 225     typedef std::list<PredictorHistory> History;
 226     typedef typename History::iterator HistoryIt;
 227
 228     /**
 229      * The per-thread predictor history. This is used to update the predictor
 230      * as instructions are committed, or restore it to the proper state after
 231      * a squash.
 232      */
 233     History predHist[Impl::MaxThreads];
 234
 235     /** The local branch predictor. */
 236     LocalBP *localBP;
 237
 238     /** The tournament branch predictor. */
 239     TournamentBP *tournamentBP;
 240
 241     /** The BTB. */
 242     DefaultBTB BTB;
 243
 244     /** The per-thread return address stack. */
 245     ReturnAddrStack RAS[Impl::MaxThreads];
 246
 247     /** Stat for number of BP lookups. */
 248     Stats::Scalar lookups;
 249     /** Stat for number of conditional branches predicted. */
 250     Stats::Scalar condPredicted;
 251     /** Stat for number of conditional branches predicted incorrectly. */
 252     Stats::Scalar condIncorrect;
 253     /** Stat for number of BTB lookups. */
 254     Stats::Scalar BTBLookups;
 255     /** Stat for number of BTB hits. */
 256     Stats::Scalar BTBHits;
 257     /** Stat for number of times the BTB is correct. */
 258     Stats::Scalar BTBCorrect;
 259     /** Stat for number of times the RAS is used to get a target. */
 260     Stats::Scalar usedRAS;
 261     /** Stat for number of times the RAS is incorrect. */
 262     Stats::Scalar RASIncorrect;
 263 };
 264
 265 #endif // __CPU_O3_BPRED_UNIT_HH__