bpred_unit.hh revision 8842 
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
44struct DerivO3CPUParams;
45
46/**
47 * Basically a wrapper class to hold both the branch predictor
48 * and the BTB.
49 */
50template<class Impl>
51class 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     * If a branch is not taken, because the BTB address is invalid or missing,
142     * this function sets the appropriate counter in the global and local
143     * predictors to not taken.
144     * @param inst_PC The PC to look up the local predictor.
145     * @param bp_history Pointer that will be set to an object that
146     * has the branch predictor state associated with the lookup.
147     */
148    void BPBTBUpdate(Addr instPC, void * &bp_history);
149
150    /**
151     * Looks up a given PC in the BTB to see if a matching entry exists.
152     * @param inst_PC The PC to look up.
153     * @return Whether the BTB contains the given PC.
154     */
155    bool BTBValid(Addr instPC)
156    { return BTB.valid(instPC, 0); }
157
158    /**
159     * Looks up a given PC in the BTB to get the predicted target.
160     * @param inst_PC The PC to look up.
161     * @return The address of the target of the branch.
162     */
163    TheISA::PCState BTBLookup(Addr instPC)
164    { return BTB.lookup(instPC, 0); }
165
166    /**
167     * Updates the BP with taken/not taken information.
168     * @param inst_PC The branch's PC that will be updated.
169     * @param taken Whether the branch was taken or not taken.
170     * @param bp_history Pointer to the branch predictor state that is
171     * associated with the branch lookup that is being updated.
172     * @param squashed Set to true when this function is called during a
173     * squash operation.
174     * @todo Make this update flexible enough to handle a global predictor.
175     */
176    void BPUpdate(Addr instPC, bool taken, void *bp_history, bool squashed);
177
178    /**
179     * Updates the BTB with the target of a branch.
180     * @param inst_PC The branch's PC that will be updated.
181     * @param target_PC The branch's target that will be added to the BTB.
182     */
183    void BTBUpdate(Addr instPC, const TheISA::PCState &target)
184    { BTB.update(instPC, target, 0); }
185
186    void dump();
187
188  private:
189    struct PredictorHistory {
190        /**
191         * Makes a predictor history struct that contains any
192         * information needed to update the predictor, BTB, and RAS.
193         */
194        PredictorHistory(const InstSeqNum &seq_num, Addr instPC,
195                         bool pred_taken, void *bp_history,
196                         ThreadID _tid)
197            : seqNum(seq_num), pc(instPC), RASTarget(0), RASIndex(0),
198              tid(_tid), predTaken(pred_taken), usedRAS(0),
199              wasCall(0), bpHistory(bp_history)
200        {}
201
202        bool operator==(const PredictorHistory &entry) const {
203            return this->seqNum == entry.seqNum;
204        }
205
206        /** The sequence number for the predictor history entry. */
207        InstSeqNum seqNum;
208
209        /** The PC associated with the sequence number. */
210        Addr pc;
211
212        /** The RAS target (only valid if a return). */
213        TheISA::PCState RASTarget;
214
215        /** The RAS index of the instruction (only valid if a call). */
216        unsigned RASIndex;
217
218        /** The thread id. */
219        ThreadID tid;
220
221        /** Whether or not it was predicted taken. */
222        bool predTaken;
223
224        /** Whether or not the RAS was used. */
225        bool usedRAS;
226
227        /** Whether or not the instruction was a call. */
228        bool wasCall;
229
230        /** Pointer to the history object passed back from the branch
231         * predictor.  It is used to update or restore state of the
232         * branch predictor.
233         */
234        void *bpHistory;
235    };
236
237    typedef std::list<PredictorHistory> History;
238    typedef typename History::iterator HistoryIt;
239
240    /**
241     * The per-thread predictor history. This is used to update the predictor
242     * as instructions are committed, or restore it to the proper state after
243     * a squash.
244     */
245    History predHist[Impl::MaxThreads];
246
247    /** The local branch predictor. */
248    LocalBP *localBP;
249
250    /** The tournament branch predictor. */
251    TournamentBP *tournamentBP;
252
253    /** The BTB. */
254    DefaultBTB BTB;
255
256    /** The per-thread return address stack. */
257    ReturnAddrStack RAS[Impl::MaxThreads];
258
259    /** Stat for number of BP lookups. */
260    Stats::Scalar lookups;
261    /** Stat for number of conditional branches predicted. */
262    Stats::Scalar condPredicted;
263    /** Stat for number of conditional branches predicted incorrectly. */
264    Stats::Scalar condIncorrect;
265    /** Stat for number of BTB lookups. */
266    Stats::Scalar BTBLookups;
267    /** Stat for number of BTB hits. */
268    Stats::Scalar BTBHits;
269    /** Stat for number of times the BTB is correct. */
270    Stats::Scalar BTBCorrect;
271    /** Stat for number of times the RAS is used to get a target. */
272    Stats::Scalar usedRAS;
273    /** Stat for number of times the RAS is incorrect. */
274    Stats::Scalar RASIncorrect;
275};
276
277#endif // __CPU_O3_BPRED_UNIT_HH__
278