tournament.hh revision 11434 
1/*
2 * Copyright (c) 2011, 2014 ARM Limited
3 * All rights reserved
4 *
5 * The license below extends only to copyright in the software and shall
6 * not be construed as granting a license to any other intellectual
7 * property including but not limited to intellectual property relating
8 * to a hardware implementation of the functionality of the software
9 * licensed hereunder.  You may use the software subject to the license
10 * terms below provided that you ensure that this notice is replicated
11 * unmodified and in its entirety in all distributions of the software,
12 * modified or unmodified, in source code or in binary form.
13 *
14 * Copyright (c) 2004-2006 The Regents of The University of Michigan
15 * All rights reserved.
16 *
17 * Redistribution and use in source and binary forms, with or without
18 * modification, are permitted provided that the following conditions are
19 * met: redistributions of source code must retain the above copyright
20 * notice, this list of conditions and the following disclaimer;
21 * redistributions in binary form must reproduce the above copyright
22 * notice, this list of conditions and the following disclaimer in the
23 * documentation and/or other materials provided with the distribution;
24 * neither the name of the copyright holders nor the names of its
25 * contributors may be used to endorse or promote products derived from
26 * this software without specific prior written permission.
27 *
28 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
29 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
30 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
31 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
32 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
33 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
34 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
35 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
36 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
37 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
38 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
39 *
40 * Authors: Kevin Lim
41 *          Timothy M. Jones
42 *          Nilay Vaish
43 */
44
45#ifndef __CPU_PRED_TOURNAMENT_PRED_HH__
46#define __CPU_PRED_TOURNAMENT_PRED_HH__
47
48#include <vector>
49
50#include "base/types.hh"
51#include "cpu/pred/bpred_unit.hh"
52#include "cpu/pred/sat_counter.hh"
53#include "params/TournamentBP.hh"
54
55/**
56 * Implements a tournament branch predictor, hopefully identical to the one
57 * used in the 21264.  It has a local predictor, which uses a local history
58 * table to index into a table of counters, and a global predictor, which
59 * uses a global history to index into a table of counters.  A choice
60 * predictor chooses between the two.  Only the global history register
61 * is speculatively updated, the rest are updated upon branches committing
62 * or misspeculating.
63 */
64class TournamentBP : public BPredUnit
65{
66  public:
67    /**
68     * Default branch predictor constructor.
69     */
70    TournamentBP(const TournamentBPParams *params);
71
72    /**
73     * Looks up the given address in the branch predictor and returns
74     * a true/false value as to whether it is taken.  Also creates a
75     * BPHistory object to store any state it will need on squash/update.
76     * @param branch_addr The address of the branch to look up.
77     * @param bp_history Pointer that will be set to the BPHistory object.
78     * @return Whether or not the branch is taken.
79     */
80    bool lookup(ThreadID tid, Addr branch_addr, void * &bp_history);
81
82    /**
83     * Records that there was an unconditional branch, and modifies
84     * the bp history to point to an object that has the previous
85     * global history stored in it.
86     * @param bp_history Pointer that will be set to the BPHistory object.
87     */
88    void uncondBranch(ThreadID tid, Addr pc, void * &bp_history);
89    /**
90     * Updates the branch predictor to Not Taken if a BTB entry is
91     * invalid or not found.
92     * @param branch_addr The address of the branch to look up.
93     * @param bp_history Pointer to any bp history state.
94     * @return Whether or not the branch is taken.
95     */
96    void btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history);
97    /**
98     * Updates the branch predictor with the actual result of a branch.
99     * @param branch_addr The address of the branch to update.
100     * @param taken Whether or not the branch was taken.
101     * @param bp_history Pointer to the BPHistory object that was created
102     * when the branch was predicted.
103     * @param squashed is set when this function is called during a squash
104     * operation.
105     */
106    void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
107                bool squashed);
108
109    void retireSquashed(ThreadID tid, void *bp_history);
110
111    /**
112     * Restores the global branch history on a squash.
113     * @param bp_history Pointer to the BPHistory object that has the
114     * previous global branch history in it.
115     */
116    void squash(ThreadID tid, void *bp_history);
117
118    unsigned getGHR(ThreadID tid, void *bp_history) const;
119
120  private:
121    /**
122     * Returns if the branch should be taken or not, given a counter
123     * value.
124     * @param count The counter value.
125     */
126    inline bool getPrediction(uint8_t &count);
127
128    /**
129     * Returns the local history index, given a branch address.
130     * @param branch_addr The branch's PC address.
131     */
132    inline unsigned calcLocHistIdx(Addr &branch_addr);
133
134    /** Updates global history as taken. */
135    inline void updateGlobalHistTaken(ThreadID tid);
136
137    /** Updates global history as not taken. */
138    inline void updateGlobalHistNotTaken(ThreadID tid);
139
140    /**
141     * Updates local histories as taken.
142     * @param local_history_idx The local history table entry that
143     * will be updated.
144     */
145    inline void updateLocalHistTaken(unsigned local_history_idx);
146
147    /**
148     * Updates local histories as not taken.
149     * @param local_history_idx The local history table entry that
150     * will be updated.
151     */
152    inline void updateLocalHistNotTaken(unsigned local_history_idx);
153
154    /**
155     * The branch history information that is created upon predicting
156     * a branch.  It will be passed back upon updating and squashing,
157     * when the BP can use this information to update/restore its
158     * state properly.
159     */
160    struct BPHistory {
161#ifdef DEBUG
162        BPHistory()
163        { newCount++; }
164        ~BPHistory()
165        { newCount--; }
166
167        static int newCount;
168#endif
169        unsigned globalHistory;
170        unsigned localHistoryIdx;
171        unsigned localHistory;
172        bool localPredTaken;
173        bool globalPredTaken;
174        bool globalUsed;
175    };
176
177    /** Flag for invalid predictor index */
178    static const int invalidPredictorIndex = -1;
179    /** Local counters. */
180    std::vector<SatCounter> localCtrs;
181
182    /** Number of counters in the local predictor. */
183    unsigned localPredictorSize;
184
185    /** Mask to truncate values stored in the local history table. */
186    unsigned localPredictorMask;
187
188    /** Number of bits of the local predictor's counters. */
189    unsigned localCtrBits;
190
191    /** Array of local history table entries. */
192    std::vector<unsigned> localHistoryTable;
193
194    /** Number of entries in the local history table. */
195    unsigned localHistoryTableSize;
196
197    /** Number of bits for each entry of the local history table. */
198    unsigned localHistoryBits;
199
200    /** Array of counters that make up the global predictor. */
201    std::vector<SatCounter> globalCtrs;
202
203    /** Number of entries in the global predictor. */
204    unsigned globalPredictorSize;
205
206    /** Number of bits of the global predictor's counters. */
207    unsigned globalCtrBits;
208
209    /** Global history register. Contains as much history as specified by
210     *  globalHistoryBits. Actual number of bits used is determined by
211     *  globalHistoryMask and choiceHistoryMask. */
212    std::vector<unsigned> globalHistory;
213
214    /** Number of bits for the global history. Determines maximum number of
215        entries in global and choice predictor tables. */
216    unsigned globalHistoryBits;
217
218    /** Mask to apply to globalHistory to access global history table.
219     *  Based on globalPredictorSize.*/
220    unsigned globalHistoryMask;
221
222    /** Mask to apply to globalHistory to access choice history table.
223     *  Based on choicePredictorSize.*/
224    unsigned choiceHistoryMask;
225
226    /** Mask to control how much history is stored. All of it might not be
227     *  used. */
228    unsigned historyRegisterMask;
229
230    /** Array of counters that make up the choice predictor. */
231    std::vector<SatCounter> choiceCtrs;
232
233    /** Number of entries in the choice predictor. */
234    unsigned choicePredictorSize;
235
236    /** Number of bits in the choice predictor's counters. */
237    unsigned choiceCtrBits;
238
239    /** Thresholds for the counter value; above the threshold is taken,
240     *  equal to or below the threshold is not taken.
241     */
242    unsigned localThreshold;
243    unsigned globalThreshold;
244    unsigned choiceThreshold;
245};
246
247#endif // __CPU_PRED_TOURNAMENT_PRED_HH__
248