tournament.hh revision 8842:a02932e2e73d 
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
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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 localPredictorSize,
67                 unsigned localCtrBits,
68                 unsigned localHistoryTableSize,
69                 unsigned localHistoryBits,
70                 unsigned globalPredictorSize,
71                 unsigned globalHistoryBits,
72                 unsigned globalCtrBits,
73                 unsigned choicePredictorSize,
74                 unsigned choiceCtrBits,
75                 unsigned instShiftAmt);
76
77    /**
78     * Looks up the given address in the branch predictor and returns
79     * a true/false value as to whether it is taken.  Also creates a
80     * BPHistory object to store any state it will need on squash/update.
81     * @param branch_addr The address of the branch to look up.
82     * @param bp_history Pointer that will be set to the BPHistory object.
83     * @return Whether or not the branch is taken.
84     */
85    bool lookup(Addr &branch_addr, void * &bp_history);
86
87    /**
88     * Records that there was an unconditional branch, and modifies
89     * the bp history to point to an object that has the previous
90     * global history stored in it.
91     * @param bp_history Pointer that will be set to the BPHistory object.
92     */
93    void uncondBr(void * &bp_history);
94    /**
95     * Updates the branch predictor to Not Taken if a BTB entry is
96     * invalid or not found.
97     * @param branch_addr The address of the branch to look up.
98     * @param bp_history Pointer to any bp history state.
99     * @return Whether or not the branch is taken.
100     */
101    void BTBUpdate(Addr &branch_addr, void * &bp_history);
102    /**
103     * Updates the branch predictor with the actual result of a branch.
104     * @param branch_addr The address of the branch to update.
105     * @param taken Whether or not the branch was taken.
106     * @param bp_history Pointer to the BPHistory object that was created
107     * when the branch was predicted.
108     * @param squashed is set when this function is called during a squash
109     * operation.
110     */
111    void update(Addr &branch_addr, bool taken, void *bp_history, bool squashed);
112
113    /**
114     * Restores the global branch history on a squash.
115     * @param bp_history Pointer to the BPHistory object that has the
116     * previous global branch history in it.
117     */
118    void squash(void *bp_history);
119
120    /** Returns the global history. */
121    inline unsigned readGlobalHist() { return globalHistory; }
122
123  private:
124    /**
125     * Returns if the branch should be taken or not, given a counter
126     * value.
127     * @param count The counter value.
128     */
129    inline bool getPrediction(uint8_t &count);
130
131    /**
132     * Returns the local history index, given a branch address.
133     * @param branch_addr The branch's PC address.
134     */
135    inline unsigned calcLocHistIdx(Addr &branch_addr);
136
137    /** Updates global history as taken. */
138    inline void updateGlobalHistTaken();
139
140    /** Updates global history as not taken. */
141    inline void updateGlobalHistNotTaken();
142
143    /**
144     * Updates local histories as taken.
145     * @param local_history_idx The local history table entry that
146     * will be updated.
147     */
148    inline void updateLocalHistTaken(unsigned local_history_idx);
149
150    /**
151     * Updates local histories as not taken.
152     * @param local_history_idx The local history table entry that
153     * will be updated.
154     */
155    inline void updateLocalHistNotTaken(unsigned local_history_idx);
156
157    /**
158     * The branch history information that is created upon predicting
159     * a branch.  It will be passed back upon updating and squashing,
160     * when the BP can use this information to update/restore its
161     * state properly.
162     */
163    struct BPHistory {
164#ifdef DEBUG
165        BPHistory()
166        { newCount++; }
167        ~BPHistory()
168        { newCount--; }
169
170        static int newCount;
171#endif
172        unsigned globalHistory;
173        unsigned localHistory;
174        bool localPredTaken;
175        bool globalPredTaken;
176        bool globalUsed;
177    };
178
179    /** Flag for invalid predictor index */
180    static const int invalidPredictorIndex = -1;
181    /** Local counters. */
182    std::vector<SatCounter> localCtrs;
183
184    /** Size of the local predictor. */
185    unsigned localPredictorSize;
186
187    /** Mask to get the proper index bits into the predictor. */
188    unsigned localPredictorMask;
189
190    /** Number of bits of the local predictor's counters. */
191    unsigned localCtrBits;
192
193    /** Array of local history table entries. */
194    std::vector<unsigned> localHistoryTable;
195
196    /** Size of the local history table. */
197    unsigned localHistoryTableSize;
198
199    /** Number of bits for each entry of the local history table.
200     *  @todo Doesn't this come from the size of the local predictor?
201     */
202    unsigned localHistoryBits;
203
204    /** Mask to get the proper local history. */
205    unsigned localHistoryMask;
206
207    /** Array of counters that make up the global predictor. */
208    std::vector<SatCounter> globalCtrs;
209
210    /** Size of the global predictor. */
211    unsigned globalPredictorSize;
212
213    /** Number of bits of the global predictor's counters. */
214    unsigned globalCtrBits;
215
216    /** Global history register. */
217    unsigned globalHistory;
218
219    /** Number of bits for the global history. */
220    unsigned globalHistoryBits;
221
222    /** Mask to get the proper global history. */
223    unsigned globalHistoryMask;
224
225    /** Array of counters that make up the choice predictor. */
226    std::vector<SatCounter> choiceCtrs;
227
228    /** Size of the choice predictor (identical to the global predictor). */
229    unsigned choicePredictorSize;
230
231    /** Number of bits of the choice predictor's counters. */
232    unsigned choiceCtrBits;
233
234    /** Number of bits to shift the instruction over to get rid of the word
235     *  offset.
236     */
237    unsigned instShiftAmt;
238
239    /** Threshold for the counter value; above the threshold is taken,
240     *  equal to or below the threshold is not taken.
241     */
242    unsigned threshold;
243};
244
245#endif // __CPU_O3_TOURNAMENT_PRED_HH__
246