tournament.hh revision 2356 
1/*
2 * Copyright (c) 2004-2006 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
29#ifndef __CPU_O3_TOURNAMENT_PRED_HH__
30#define __CPU_O3_TOURNAMENT_PRED_HH__
31
32// For Addr type.
33#include "arch/isa_traits.hh"
34#include "cpu/o3/sat_counter.hh"
35#include <vector>
36
37/**
38 * Implements a tournament branch predictor, hopefully identical to the one
39 * used in the 21264.  It has a local predictor, which uses a local history
40 * table to index into a table of counters, and a global predictor, which
41 * uses a global history to index into a table of counters.  A choice
42 * predictor chooses between the two.  Only the global history register
43 * is speculatively updated, the rest are updated upon branches committing
44 * or misspeculating.
45 */
46class TournamentBP
47{
48  public:
49    /**
50     * Default branch predictor constructor.
51     */
52    TournamentBP(unsigned localPredictorSize,
53                 unsigned localCtrBits,
54                 unsigned localHistoryTableSize,
55                 unsigned localHistoryBits,
56                 unsigned globalPredictorSize,
57                 unsigned globalHistoryBits,
58                 unsigned globalCtrBits,
59                 unsigned choicePredictorSize,
60                 unsigned choiceCtrBits,
61                 unsigned instShiftAmt);
62
63    /**
64     * Looks up the given address in the branch predictor and returns
65     * a true/false value as to whether it is taken.  Also creates a
66     * BPHistory object to store any state it will need on squash/update.
67     * @param branch_addr The address of the branch to look up.
68     * @param bp_history Pointer that will be set to the BPHistory object.
69     * @return Whether or not the branch is taken.
70     */
71    bool lookup(Addr &branch_addr, void * &bp_history);
72
73    /**
74     * Records that there was an unconditional branch, and modifies
75     * the bp history to point to an object that has the previous
76     * global history stored in it.
77     * @param bp_history Pointer that will be set to the BPHistory object.
78     */
79    void uncondBr(void * &bp_history);
80
81    /**
82     * Updates the branch predictor with the actual result of a branch.
83     * @param branch_addr The address of the branch to update.
84     * @param taken Whether or not the branch was taken.
85     * @param bp_history Pointer to the BPHistory object that was created
86     * when the branch was predicted.
87     */
88    void update(Addr &branch_addr, bool taken, void *bp_history);
89
90    /**
91     * Restores the global branch history on a squash.
92     * @param bp_history Pointer to the BPHistory object that has the
93     * previous global branch history in it.
94     */
95    void squash(void *bp_history);
96
97    /** Returns the global history. */
98    inline unsigned readGlobalHist() { return globalHistory; }
99
100  private:
101    /**
102     * Returns if the branch should be taken or not, given a counter
103     * value.
104     * @param count The counter value.
105     */
106    inline bool getPrediction(uint8_t &count);
107
108    /**
109     * Returns the local history index, given a branch address.
110     * @param branch_addr The branch's PC address.
111     */
112    inline unsigned calcLocHistIdx(Addr &branch_addr);
113
114    /** Updates global history as taken. */
115    inline void updateGlobalHistTaken();
116
117    /** Updates global history as not taken. */
118    inline void updateGlobalHistNotTaken();
119
120    /**
121     * Updates local histories as taken.
122     * @param local_history_idx The local history table entry that
123     * will be updated.
124     */
125    inline void updateLocalHistTaken(unsigned local_history_idx);
126
127    /**
128     * Updates local histories as not taken.
129     * @param local_history_idx The local history table entry that
130     * will be updated.
131     */
132    inline void updateLocalHistNotTaken(unsigned local_history_idx);
133
134    /**
135     * The branch history information that is created upon predicting
136     * a branch.  It will be passed back upon updating and squashing,
137     * when the BP can use this information to update/restore its
138     * state properly.
139     */
140    struct BPHistory {
141#ifdef DEBUG
142        BPHistory()
143        { newCount++; }
144        ~BPHistory()
145        { newCount--; }
146
147        static int newCount;
148#endif
149        unsigned globalHistory;
150        bool localPredTaken;
151        bool globalPredTaken;
152        bool globalUsed;
153    };
154
155    /** Local counters. */
156    std::vector<SatCounter> localCtrs;
157
158    /** Size of the local predictor. */
159    unsigned localPredictorSize;
160
161    /** Mask to get the proper index bits into the predictor. */
162    unsigned localPredictorMask;
163
164    /** Number of bits of the local predictor's counters. */
165    unsigned localCtrBits;
166
167    /** Array of local history table entries. */
168    std::vector<unsigned> localHistoryTable;
169
170    /** Size of the local history table. */
171    unsigned localHistoryTableSize;
172
173    /** Number of bits for each entry of the local history table.
174     *  @todo Doesn't this come from the size of the local predictor?
175     */
176    unsigned localHistoryBits;
177
178    /** Mask to get the proper local history. */
179    unsigned localHistoryMask;
180
181    /** Array of counters that make up the global predictor. */
182    std::vector<SatCounter> globalCtrs;
183
184    /** Size of the global predictor. */
185    unsigned globalPredictorSize;
186
187    /** Number of bits of the global predictor's counters. */
188    unsigned globalCtrBits;
189
190    /** Global history register. */
191    unsigned globalHistory;
192
193    /** Number of bits for the global history. */
194    unsigned globalHistoryBits;
195
196    /** Mask to get the proper global history. */
197    unsigned globalHistoryMask;
198
199    /** Array of counters that make up the choice predictor. */
200    std::vector<SatCounter> choiceCtrs;
201
202    /** Size of the choice predictor (identical to the global predictor). */
203    unsigned choicePredictorSize;
204
205    /** Number of bits of the choice predictor's counters. */
206    unsigned choiceCtrBits;
207
208    /** Number of bits to shift the instruction over to get rid of the word
209     *  offset.
210     */
211    unsigned instShiftAmt;
212
213    /** Threshold for the counter value; above the threshold is taken,
214     *  equal to or below the threshold is not taken.
215     */
216    unsigned threshold;
217};
218
219#endif // __CPU_O3_TOURNAMENT_PRED_HH__
220