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 */
69 TournamentBP(const Params *params);
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(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 */
87 void uncondBranch(void * &bp_history);
88 void uncondBranch(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(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(Addr branch_addr, bool taken, void *bp_history, bool squashed);
107
108 void retireSquashed(void *bp_history);
109
110 /**
111 * Restores the global branch history on a squash.
112 * @param bp_history Pointer to the BPHistory object that has the
113 * previous global branch history in it.
114 */
115 void squash(void *bp_history);
116
117 /** Returns the global history. */
118 inline unsigned readGlobalHist() { return globalHistory; }
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();
136
137 /** Updates global history as not taken. */
138 inline void updateGlobalHistNotTaken();
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 localHistory;
171 bool localPredTaken;
172 bool globalPredTaken;
173 bool globalUsed;
174 };
175
176 /** Flag for invalid predictor index */
177 static const int invalidPredictorIndex = -1;
178 /** Local counters. */
179 std::vector<SatCounter> localCtrs;
180
181 /** Number of counters in the local predictor. */
182 unsigned localPredictorSize;
183
184 /** Mask to truncate values stored in the local history table. */
185 unsigned localPredictorMask;
186
187 /** Number of bits of the local predictor's counters. */
188 unsigned localCtrBits;
189
190 /** Array of local history table entries. */
191 std::vector<unsigned> localHistoryTable;
192
193 /** Number of entries in the local history table. */
194 unsigned localHistoryTableSize;
195
196 /** Number of bits for each entry of the local history table. */
197 unsigned localHistoryBits;
198
199 /** Array of counters that make up the global predictor. */
200 std::vector<SatCounter> globalCtrs;
201
202 /** Number of entries in the global predictor. */
203 unsigned globalPredictorSize;
204
205 /** Number of bits of the global predictor's counters. */
206 unsigned globalCtrBits;
207
208 /** Global history register. Contains as much history as specified by
209 * globalHistoryBits. Actual number of bits used is determined by
210 * globalHistoryMask and choiceHistoryMask. */
211 unsigned globalHistory;
212
213 /** Number of bits for the global history. Determines maximum number of
214 entries in global and choice predictor tables. */
215 unsigned globalHistoryBits;
216
217 /** Mask to apply to globalHistory to access global history table.
218 * Based on globalPredictorSize.*/
219 unsigned globalHistoryMask;
220
221 /** Mask to apply to globalHistory to access choice history table.
222 * Based on choicePredictorSize.*/
223 unsigned choiceHistoryMask;
224
225 /** Mask to control how much history is stored. All of it might not be
226 * used. */
227 unsigned historyRegisterMask;
228
229 /** Array of counters that make up the choice predictor. */
230 std::vector<SatCounter> choiceCtrs;
231
232 /** Number of entries in the choice predictor. */
233 unsigned choicePredictorSize;
234
235 /** Number of bits in the choice predictor's counters. */
236 unsigned choiceCtrBits;
237
237 /** Number of bits to shift the instruction over to get rid of the word
238 * offset.
239 */
240 unsigned instShiftAmt;
241
238 /** Thresholds for the counter value; above the threshold is taken,
239 * equal to or below the threshold is not taken.
240 */
241 unsigned localThreshold;
242 unsigned globalThreshold;
243 unsigned choiceThreshold;
244};
245
246#endif // __CPU_PRED_TOURNAMENT_PRED_HH__