tournament.hh revision 9360 
1/*
2 * Copyright (c) 2011 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 */
42
43#ifndef __CPU_O3_TOURNAMENT_PRED_HH__
44#define __CPU_O3_TOURNAMENT_PRED_HH__
45
46#include <vector>
47
48#include "base/types.hh"
49#include "cpu/o3/sat_counter.hh"
50
51/**
52 * Implements a tournament branch predictor, hopefully identical to the one
53 * used in the 21264.  It has a local predictor, which uses a local history
54 * table to index into a table of counters, and a global predictor, which
55 * uses a global history to index into a table of counters.  A choice
56 * predictor chooses between the two.  Only the global history register
57 * is speculatively updated, the rest are updated upon branches committing
58 * or misspeculating.
59 */
60class TournamentBP
61{
62  public:
63    /**
64     * Default branch predictor constructor.
65     */
66    TournamentBP(unsigned localCtrBits,
67                 unsigned localHistoryTableSize,
68                 unsigned localHistoryBits,
69                 unsigned globalPredictorSize,
70                 unsigned globalHistoryBits,
71                 unsigned globalCtrBits,
72                 unsigned choicePredictorSize,
73                 unsigned choiceCtrBits,
74                 unsigned instShiftAmt);
75
76    /**
77     * Looks up the given address in the branch predictor and returns
78     * a true/false value as to whether it is taken.  Also creates a
79     * BPHistory object to store any state it will need on squash/update.
80     * @param branch_addr The address of the branch to look up.
81     * @param bp_history Pointer that will be set to the BPHistory object.
82     * @return Whether or not the branch is taken.
83     */
84    bool lookup(Addr &branch_addr, void * &bp_history);
85
86    /**
87     * Records that there was an unconditional branch, and modifies
88     * the bp history to point to an object that has the previous
89     * global history stored in it.
90     * @param bp_history Pointer that will be set to the BPHistory object.
91     */
92    void uncondBr(void * &bp_history);
93    /**
94     * Updates the branch predictor to Not Taken if a BTB entry is
95     * invalid or not found.
96     * @param branch_addr The address of the branch to look up.
97     * @param bp_history Pointer to any bp history state.
98     * @return Whether or not the branch is taken.
99     */
100    void BTBUpdate(Addr &branch_addr, void * &bp_history);
101    /**
102     * Updates the branch predictor with the actual result of a branch.
103     * @param branch_addr The address of the branch to update.
104     * @param taken Whether or not the branch was taken.
105     * @param bp_history Pointer to the BPHistory object that was created
106     * when the branch was predicted.
107     * @param squashed is set when this function is called during a squash
108     * operation.
109     */
110    void update(Addr &branch_addr, bool taken, void *bp_history, bool squashed);
111
112    /**
113     * Restores the global branch history on a squash.
114     * @param bp_history Pointer to the BPHistory object that has the
115     * previous global branch history in it.
116     */
117    void squash(void *bp_history);
118
119    /** Returns the global history. */
120    inline unsigned readGlobalHist() { return globalHistory; }
121
122  private:
123    /**
124     * Returns if the branch should be taken or not, given a counter
125     * value.
126     * @param count The counter value.
127     */
128    inline bool getPrediction(uint8_t &count);
129
130    /**
131     * Returns the local history index, given a branch address.
132     * @param branch_addr The branch's PC address.
133     */
134    inline unsigned calcLocHistIdx(Addr &branch_addr);
135
136    /** Updates global history as taken. */
137    inline void updateGlobalHistTaken();
138
139    /** Updates global history as not taken. */
140    inline void updateGlobalHistNotTaken();
141
142    /**
143     * Updates local histories as taken.
144     * @param local_history_idx The local history table entry that
145     * will be updated.
146     */
147    inline void updateLocalHistTaken(unsigned local_history_idx);
148
149    /**
150     * Updates local histories as not taken.
151     * @param local_history_idx The local history table entry that
152     * will be updated.
153     */
154    inline void updateLocalHistNotTaken(unsigned local_history_idx);
155
156    /**
157     * The branch history information that is created upon predicting
158     * a branch.  It will be passed back upon updating and squashing,
159     * when the BP can use this information to update/restore its
160     * state properly.
161     */
162    struct BPHistory {
163#ifdef DEBUG
164        BPHistory()
165        { newCount++; }
166        ~BPHistory()
167        { newCount--; }
168
169        static int newCount;
170#endif
171        unsigned globalHistory;
172        unsigned localHistory;
173        bool localPredTaken;
174        bool globalPredTaken;
175        bool globalUsed;
176    };
177
178    /** Flag for invalid predictor index */
179    static const int invalidPredictorIndex = -1;
180    /** Local counters. */
181    std::vector<SatCounter> localCtrs;
182
183    /** Number of counters in the local predictor. */
184    unsigned localPredictorSize;
185
186    /** Mask to truncate values stored in the local history table. */
187    unsigned localPredictorMask;
188
189    /** Number of bits of the local predictor's counters. */
190    unsigned localCtrBits;
191
192    /** Array of local history table entries. */
193    std::vector<unsigned> localHistoryTable;
194
195    /** Number of entries in the local history table. */
196    unsigned localHistoryTableSize;
197
198    /** Number of bits for each entry of the local history table. */
199    unsigned localHistoryBits;
200
201    /** Array of counters that make up the global predictor. */
202    std::vector<SatCounter> globalCtrs;
203
204    /** Number of entries in the global predictor. */
205    unsigned globalPredictorSize;
206
207    /** Number of bits of the global predictor's counters. */
208    unsigned globalCtrBits;
209
210    /** Global history register. Contains as much history as specified by
211     *  globalHistoryBits. Actual number of bits used is determined by
212     *  globalHistoryMask and choiceHistoryMask. */
213    unsigned globalHistory;
214
215    /** Number of bits for the global history. Determines maximum number of
216        entries in global and choice predictor tables. */
217    unsigned globalHistoryBits;
218
219    /** Mask to apply to globalHistory to access global history table.
220     *  Based on globalPredictorSize.*/
221    unsigned globalHistoryMask;
222
223    /** Mask to apply to globalHistory to access choice history table.
224     *  Based on choicePredictorSize.*/
225    unsigned choiceHistoryMask;
226
227    /** Mask to control how much history is stored. All of it might not be
228     *  used. */
229    unsigned historyRegisterMask;
230
231    /** Array of counters that make up the choice predictor. */
232    std::vector<SatCounter> choiceCtrs;
233
234    /** Number of entries in the choice predictor. */
235    unsigned choicePredictorSize;
236
237    /** Number of bits in the choice predictor's counters. */
238    unsigned choiceCtrBits;
239
240    /** Number of bits to shift the instruction over to get rid of the word
241     *  offset.
242     */
243    unsigned instShiftAmt;
244
245    /** Thresholds for the counter value; above the threshold is taken,
246     *  equal to or below the threshold is not taken.
247     */
248    unsigned localThreshold;
249    unsigned globalThreshold;
250    unsigned choiceThreshold;
251};
252
253#endif // __CPU_O3_TOURNAMENT_PRED_HH__
254