bpred_unit.hh revision 13957 
1/*
2 * Copyright (c) 2011-2012, 2014 ARM Limited
3 * Copyright (c) 2010 The University of Edinburgh
4 * All rights reserved
5 *
6 * The license below extends only to copyright in the software and shall
7 * not be construed as granting a license to any other intellectual
8 * property including but not limited to intellectual property relating
9 * to a hardware implementation of the functionality of the software
10 * licensed hereunder.  You may use the software subject to the license
11 * terms below provided that you ensure that this notice is replicated
12 * unmodified and in its entirety in all distributions of the software,
13 * modified or unmodified, in source code or in binary form.
14 *
15 * Copyright (c) 2004-2005 The Regents of The University of Michigan
16 * All rights reserved.
17 *
18 * Redistribution and use in source and binary forms, with or without
19 * modification, are permitted provided that the following conditions are
20 * met: redistributions of source code must retain the above copyright
21 * notice, this list of conditions and the following disclaimer;
22 * redistributions in binary form must reproduce the above copyright
23 * notice, this list of conditions and the following disclaimer in the
24 * documentation and/or other materials provided with the distribution;
25 * neither the name of the copyright holders nor the names of its
26 * contributors may be used to endorse or promote products derived from
27 * this software without specific prior written permission.
28 *
29 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
30 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
31 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
32 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
33 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
34 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
35 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
36 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
37 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
38 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
39 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
40 *
41 * Authors: Kevin Lim
42 *          Korey Sewell
43 *          Timothy M. Jones
44 *          Nilay Vaish
45 */
46
47#ifndef __CPU_PRED_BPRED_UNIT_HH__
48#define __CPU_PRED_BPRED_UNIT_HH__
49
50#include <deque>
51
52#include "base/statistics.hh"
53#include "base/types.hh"
54#include "cpu/pred/btb.hh"
55#include "cpu/pred/indirect.hh"
56#include "cpu/pred/ras.hh"
57#include "cpu/inst_seq.hh"
58#include "cpu/static_inst.hh"
59#include "params/BranchPredictor.hh"
60#include "sim/probe/pmu.hh"
61#include "sim/sim_object.hh"
62
63/**
64 * Basically a wrapper class to hold both the branch predictor
65 * and the BTB.
66 */
67class BPredUnit : public SimObject
68{
69  public:
70      typedef BranchPredictorParams Params;
71    /**
72     * @param params The params object, that has the size of the BP and BTB.
73     */
74    BPredUnit(const Params *p);
75
76    /**
77     * Registers statistics.
78     */
79    void regStats() override;
80
81    void regProbePoints() override;
82
83    /** Perform sanity checks after a drain. */
84    void drainSanityCheck() const;
85
86    /**
87     * Predicts whether or not the instruction is a taken branch, and the
88     * target of the branch if it is taken.
89     * @param inst The branch instruction.
90     * @param PC The predicted PC is passed back through this parameter.
91     * @param tid The thread id.
92     * @return Returns if the branch is taken or not.
93     */
94    bool predict(const StaticInstPtr &inst, const InstSeqNum &seqNum,
95                 TheISA::PCState &pc, ThreadID tid);
96
97    // @todo: Rename this function.
98    virtual void uncondBranch(ThreadID tid, Addr pc, void * &bp_history) = 0;
99
100    /**
101     * Tells the branch predictor to commit any updates until the given
102     * sequence number.
103     * @param done_sn The sequence number to commit any older updates up until.
104     * @param tid The thread id.
105     */
106    void update(const InstSeqNum &done_sn, ThreadID tid);
107
108    /**
109     * Squashes all outstanding updates until a given sequence number.
110     * @param squashed_sn The sequence number to squash any younger updates up
111     * until.
112     * @param tid The thread id.
113     */
114    void squash(const InstSeqNum &squashed_sn, ThreadID tid);
115
116    /**
117     * Squashes all outstanding updates until a given sequence number, and
118     * corrects that sn's update with the proper address and taken/not taken.
119     * @param squashed_sn The sequence number to squash any younger updates up
120     * until.
121     * @param corr_target The correct branch target.
122     * @param actually_taken The correct branch direction.
123     * @param tid The thread id.
124     */
125    void squash(const InstSeqNum &squashed_sn,
126                const TheISA::PCState &corr_target,
127                bool actually_taken, ThreadID tid);
128
129    /**
130     * @param bp_history Pointer to the history object.  The predictor
131     * will need to update any state and delete the object.
132     */
133    virtual void squash(ThreadID tid, void *bp_history) = 0;
134
135    /**
136     * Looks up a given PC in the BP to see if it is taken or not taken.
137     * @param inst_PC The PC to look up.
138     * @param bp_history Pointer that will be set to an object that
139     * has the branch predictor state associated with the lookup.
140     * @return Whether the branch is taken or not taken.
141     */
142    virtual bool lookup(ThreadID tid, Addr instPC, void * &bp_history) = 0;
143
144     /**
145     * If a branch is not taken, because the BTB address is invalid or missing,
146     * this function sets the appropriate counter in the global and local
147     * predictors to not taken.
148     * @param inst_PC The PC to look up the local predictor.
149     * @param bp_history Pointer that will be set to an object that
150     * has the branch predictor state associated with the lookup.
151     */
152    virtual void btbUpdate(ThreadID tid, Addr instPC, void * &bp_history) = 0;
153
154    /**
155     * Looks up a given PC in the BTB to see if a matching entry exists.
156     * @param inst_PC The PC to look up.
157     * @return Whether the BTB contains the given PC.
158     */
159    bool BTBValid(Addr instPC)
160    { return BTB.valid(instPC, 0); }
161
162    /**
163     * Looks up a given PC in the BTB to get the predicted target.
164     * @param inst_PC The PC to look up.
165     * @return The address of the target of the branch.
166     */
167    TheISA::PCState BTBLookup(Addr instPC)
168    { return BTB.lookup(instPC, 0); }
169
170    /**
171     * Updates the BP with taken/not taken information.
172     * @param inst_PC The branch's PC that will be updated.
173     * @param taken Whether the branch was taken or not taken.
174     * @param bp_history Pointer to the branch predictor state that is
175     * associated with the branch lookup that is being updated.
176     * @param squashed Set to true when this function is called during a
177     * squash operation.
178     * @param inst Static instruction information
179     * @param corrTarget The resolved target of the branch (only needed
180     * for squashed branches)
181     * @todo Make this update flexible enough to handle a global predictor.
182     */
183    virtual void update(ThreadID tid, Addr instPC, bool taken,
184                   void *bp_history, bool squashed,
185                   const StaticInstPtr & inst = StaticInst::nullStaticInstPtr,
186                   Addr corrTarget = MaxAddr) = 0;
187    /**
188     * Updates the BTB with the target of a branch.
189     * @param inst_PC The branch's PC that will be updated.
190     * @param target_PC The branch's target that will be added to the BTB.
191     */
192    void BTBUpdate(Addr instPC, const TheISA::PCState &target)
193    { BTB.update(instPC, target, 0); }
194
195
196    void dump();
197
198  private:
199    struct PredictorHistory {
200        /**
201         * Makes a predictor history struct that contains any
202         * information needed to update the predictor, BTB, and RAS.
203         */
204        PredictorHistory(const InstSeqNum &seq_num, Addr instPC,
205                         bool pred_taken, void *bp_history,
206                         void *indirect_history, ThreadID _tid,
207                         const StaticInstPtr & inst)
208            : seqNum(seq_num), pc(instPC), bpHistory(bp_history),
209              indirectHistory(indirect_history), RASTarget(0), RASIndex(0),
210              tid(_tid), predTaken(pred_taken), usedRAS(0), pushedRAS(0),
211              wasCall(0), wasReturn(0), wasIndirect(0), target(MaxAddr),
212              inst(inst)
213        {}
214
215        bool operator==(const PredictorHistory &entry) const {
216            return this->seqNum == entry.seqNum;
217        }
218
219        /** The sequence number for the predictor history entry. */
220        InstSeqNum seqNum;
221
222        /** The PC associated with the sequence number. */
223        Addr pc;
224
225        /** Pointer to the history object passed back from the branch
226         * predictor.  It is used to update or restore state of the
227         * branch predictor.
228         */
229        void *bpHistory;
230
231        void *indirectHistory;
232
233        /** The RAS target (only valid if a return). */
234        TheISA::PCState RASTarget;
235
236        /** The RAS index of the instruction (only valid if a call). */
237        unsigned RASIndex;
238
239        /** The thread id. */
240        ThreadID tid;
241
242        /** Whether or not it was predicted taken. */
243        bool predTaken;
244
245        /** Whether or not the RAS was used. */
246        bool usedRAS;
247
248        /* Whether or not the RAS was pushed */
249        bool pushedRAS;
250
251        /** Whether or not the instruction was a call. */
252        bool wasCall;
253
254        /** Whether or not the instruction was a return. */
255        bool wasReturn;
256
257        /** Wether this instruction was an indirect branch */
258        bool wasIndirect;
259
260        /** Target of the branch. First it is predicted, and fixed later
261         *  if necessary
262         */
263        Addr target;
264
265        /** The branch instrction */
266        const StaticInstPtr inst;
267    };
268
269    typedef std::deque<PredictorHistory> History;
270
271    /** Number of the threads for which the branch history is maintained. */
272    const unsigned numThreads;
273
274
275    /**
276     * The per-thread predictor history. This is used to update the predictor
277     * as instructions are committed, or restore it to the proper state after
278     * a squash.
279     */
280    std::vector<History> predHist;
281
282    /** The BTB. */
283    DefaultBTB BTB;
284
285    /** The per-thread return address stack. */
286    std::vector<ReturnAddrStack> RAS;
287
288    /** The indirect target predictor. */
289    IndirectPredictor * iPred;
290
291    /** Stat for number of BP lookups. */
292    Stats::Scalar lookups;
293    /** Stat for number of conditional branches predicted. */
294    Stats::Scalar condPredicted;
295    /** Stat for number of conditional branches predicted incorrectly. */
296    Stats::Scalar condIncorrect;
297    /** Stat for number of BTB lookups. */
298    Stats::Scalar BTBLookups;
299    /** Stat for number of BTB hits. */
300    Stats::Scalar BTBHits;
301    /** Stat for number of times the BTB is correct. */
302    Stats::Scalar BTBCorrect;
303    /** Stat for percent times an entry in BTB found. */
304    Stats::Formula BTBHitPct;
305    /** Stat for number of times the RAS is used to get a target. */
306    Stats::Scalar usedRAS;
307    /** Stat for number of times the RAS is incorrect. */
308    Stats::Scalar RASIncorrect;
309
310    /** Stat for the number of indirect target lookups.*/
311    Stats::Scalar indirectLookups;
312    /** Stat for the number of indirect target hits.*/
313    Stats::Scalar indirectHits;
314    /** Stat for the number of indirect target misses.*/
315    Stats::Scalar indirectMisses;
316    /** Stat for the number of indirect target mispredictions.*/
317    Stats::Scalar indirectMispredicted;
318
319  protected:
320    /** Number of bits to shift instructions by for predictor addresses. */
321    const unsigned instShiftAmt;
322
323    /**
324     * @{
325     * @name PMU Probe points.
326     */
327
328    /**
329     * Helper method to instantiate probe points belonging to this
330     * object.
331     *
332     * @param name Name of the probe point.
333     * @return A unique_ptr to the new probe point.
334     */
335    ProbePoints::PMUUPtr pmuProbePoint(const char *name);
336
337
338    /**
339     * Branches seen by the branch predictor
340     *
341     * @note This counter includes speculative branches.
342     */
343    ProbePoints::PMUUPtr ppBranches;
344
345    /** Miss-predicted branches */
346    ProbePoints::PMUUPtr ppMisses;
347
348    /** @} */
349};
350
351#endif // __CPU_PRED_BPRED_UNIT_HH__
352