bpred_unit.hh revision 5529 
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
46class DerivO3CPUParams;
47
48/**
49 * Basically a wrapper class to hold both the branch predictor
50 * and the BTB.
51 */
52template<class Impl>
53class BPredUnit
54{
55  private:
56    typedef typename Impl::DynInstPtr DynInstPtr;
57
58    enum PredType {
59        Local,
60        Tournament
61    };
62
63    PredType predictor;
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    /**
73     * Registers statistics.
74     */
75    void regStats();
76
77    void switchOut();
78
79    void takeOverFrom();
80
81    /**
82     * Predicts whether or not the instruction is a taken branch, and the
83     * target of the branch if it is taken.
84     * @param inst The branch instruction.
85     * @param PC The predicted PC is passed back through this parameter.
86     * @param tid The thread id.
87     * @return Returns if the branch is taken or not.
88     */
89    bool predict(DynInstPtr &inst, Addr &PC, unsigned tid);
90
91    // @todo: Rename this function.
92    void BPUncond(void * &bp_history);
93
94    /**
95     * Tells the branch predictor to commit any updates until the given
96     * sequence number.
97     * @param done_sn The sequence number to commit any older updates up until.
98     * @param tid The thread id.
99     */
100    void update(const InstSeqNum &done_sn, unsigned tid);
101
102    /**
103     * Squashes all outstanding updates until a given sequence number.
104     * @param squashed_sn The sequence number to squash any younger updates up
105     * until.
106     * @param tid The thread id.
107     */
108    void squash(const InstSeqNum &squashed_sn, unsigned tid);
109
110    /**
111     * Squashes all outstanding updates until a given sequence number, and
112     * corrects that sn's update with the proper address and taken/not taken.
113     * @param squashed_sn The sequence number to squash any younger updates up
114     * until.
115     * @param corr_target The correct branch target.
116     * @param actually_taken The correct branch direction.
117     * @param tid The thread id.
118     */
119    void squash(const InstSeqNum &squashed_sn, const Addr &corr_target,
120                bool actually_taken, unsigned tid);
121
122    /**
123     * @param bp_history Pointer to the history object.  The predictor
124     * will need to update any state and delete the object.
125     */
126    void BPSquash(void *bp_history);
127
128    /**
129     * Looks up a given PC in the BP to see if it is taken or not taken.
130     * @param inst_PC The PC to look up.
131     * @param bp_history Pointer that will be set to an object that
132     * has the branch predictor state associated with the lookup.
133     * @return Whether the branch is taken or not taken.
134     */
135    bool BPLookup(Addr &inst_PC, void * &bp_history);
136
137    /**
138     * Looks up a given PC in the BTB to see if a matching entry exists.
139     * @param inst_PC The PC to look up.
140     * @return Whether the BTB contains the given PC.
141     */
142    bool BTBValid(Addr &inst_PC)
143    { return BTB.valid(inst_PC, 0); }
144
145    /**
146     * Looks up a given PC in the BTB to get the predicted target.
147     * @param inst_PC The PC to look up.
148     * @return The address of the target of the branch.
149     */
150    Addr BTBLookup(Addr &inst_PC)
151    { return BTB.lookup(inst_PC, 0); }
152
153    /**
154     * Updates the BP with taken/not taken information.
155     * @param inst_PC The branch's PC that will be updated.
156     * @param taken Whether the branch was taken or not taken.
157     * @param bp_history Pointer to the branch predictor state that is
158     * associated with the branch lookup that is being updated.
159     * @todo Make this update flexible enough to handle a global predictor.
160     */
161    void BPUpdate(Addr &inst_PC, bool taken, void *bp_history);
162
163    /**
164     * Updates the BTB with the target of a branch.
165     * @param inst_PC The branch's PC that will be updated.
166     * @param target_PC The branch's target that will be added to the BTB.
167     */
168    void BTBUpdate(Addr &inst_PC, Addr &target_PC)
169    { BTB.update(inst_PC, target_PC,0); }
170
171    void dump();
172
173  private:
174    struct PredictorHistory {
175        /**
176         * Makes a predictor history struct that contains any
177         * information needed to update the predictor, BTB, and RAS.
178         */
179        PredictorHistory(const InstSeqNum &seq_num, const Addr &inst_PC,
180                         const bool pred_taken, void *bp_history,
181                         const unsigned _tid)
182            : seqNum(seq_num), PC(inst_PC), RASTarget(0),
183              RASIndex(0), tid(_tid), predTaken(pred_taken), usedRAS(0),
184              wasCall(0), bpHistory(bp_history)
185        { }
186
187        /** The sequence number for the predictor history entry. */
188        InstSeqNum seqNum;
189
190        /** The PC associated with the sequence number. */
191        Addr PC;
192
193        /** The RAS target (only valid if a return). */
194        Addr RASTarget;
195
196        /** The RAS index of the instruction (only valid if a call). */
197        unsigned RASIndex;
198
199        /** The thread id. */
200        unsigned tid;
201
202        /** Whether or not it was predicted taken. */
203        bool predTaken;
204
205        /** Whether or not the RAS was used. */
206        bool usedRAS;
207
208        /** Whether or not the instruction was a call. */
209        bool wasCall;
210
211        /** Pointer to the history object passed back from the branch
212         * predictor.  It is used to update or restore state of the
213         * branch predictor.
214         */
215        void *bpHistory;
216    };
217
218    typedef std::list<PredictorHistory> History;
219
220    /**
221     * The per-thread predictor history. This is used to update the predictor
222     * as instructions are committed, or restore it to the proper state after
223     * a squash.
224     */
225    History predHist[Impl::MaxThreads];
226
227    /** The local branch predictor. */
228    LocalBP *localBP;
229
230    /** The tournament branch predictor. */
231    TournamentBP *tournamentBP;
232
233    /** The BTB. */
234    DefaultBTB BTB;
235
236    /** The per-thread return address stack. */
237    ReturnAddrStack RAS[Impl::MaxThreads];
238
239    /** Stat for number of BP lookups. */
240    Stats::Scalar<> lookups;
241    /** Stat for number of conditional branches predicted. */
242    Stats::Scalar<> condPredicted;
243    /** Stat for number of conditional branches predicted incorrectly. */
244    Stats::Scalar<> condIncorrect;
245    /** Stat for number of BTB lookups. */
246    Stats::Scalar<> BTBLookups;
247    /** Stat for number of BTB hits. */
248    Stats::Scalar<> BTBHits;
249    /** Stat for number of times the BTB is correct. */
250    Stats::Scalar<> BTBCorrect;
251    /** Stat for number of times the RAS is used to get a target. */
252    Stats::Scalar<> usedRAS;
253    /** Stat for number of times the RAS is incorrect. */
254    Stats::Scalar<> RASIncorrect;
255};
256
257#endif // __CPU_O3_BPRED_UNIT_HH__
258