1/*
2 * Copyright (c) 2014 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 * Authors: Anthony Gutierrez
29 */
30
31/* @file
32 * Implementation of a bi-mode branch predictor
33 */
34
35#include "base/bitfield.hh"
36#include "base/intmath.hh"
37#include "cpu/pred/bi_mode.hh"
38
39BiModeBP::BiModeBP(const BiModeBPParams *params)
40 : BPredUnit(params),
41 globalHistoryReg(params->numThreads, 0),
42 globalHistoryBits(ceilLog2(params->globalPredictorSize)),
43 choicePredictorSize(params->choicePredictorSize),
44 choiceCtrBits(params->choiceCtrBits),
45 globalPredictorSize(params->globalPredictorSize),
46 globalCtrBits(params->globalCtrBits)
47{
48 if (!isPowerOf2(choicePredictorSize))
49 fatal("Invalid choice predictor size.\n");
50 if (!isPowerOf2(globalPredictorSize))
51 fatal("Invalid global history predictor size.\n");
52
53 choiceCounters.resize(choicePredictorSize);
54 takenCounters.resize(globalPredictorSize);
55 notTakenCounters.resize(globalPredictorSize);
56
57 for (int i = 0; i < choicePredictorSize; ++i) {
58 choiceCounters[i].setBits(choiceCtrBits);
59 }
60 for (int i = 0; i < globalPredictorSize; ++i) {
61 takenCounters[i].setBits(globalCtrBits);
62 notTakenCounters[i].setBits(globalCtrBits);
63 }
64
65 historyRegisterMask = mask(globalHistoryBits);
66 choiceHistoryMask = choicePredictorSize - 1;
67 globalHistoryMask = globalPredictorSize - 1;
68
69 choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
70 takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
71 notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
72}
73
74/*
75 * For an unconditional branch we set its history such that
76 * everything is set to taken. I.e., its choice predictor
77 * chooses the taken array and the taken array predicts taken.
78 */
79void
80BiModeBP::uncondBranch(ThreadID tid, Addr pc, void * &bpHistory)
81{
82 BPHistory *history = new BPHistory;
83 history->globalHistoryReg = globalHistoryReg[tid];
84 history->takenUsed = true;
85 history->takenPred = true;
86 history->notTakenPred = true;
87 history->finalPred = true;
88 bpHistory = static_cast<void*>(history);
89 updateGlobalHistReg(tid, true);
90}
91
92void
93BiModeBP::squash(ThreadID tid, void *bpHistory)
94{
95 BPHistory *history = static_cast<BPHistory*>(bpHistory);
96 globalHistoryReg[tid] = history->globalHistoryReg;
97
98 delete history;
99}
100
101/*
102 * Here we lookup the actual branch prediction. We use the PC to
103 * identify the bias of a particular branch, which is based on the
104 * prediction in the choice array. A hash of the global history
105 * register and a branch's PC is used to index into both the taken
106 * and not-taken predictors, which both present a prediction. The
107 * choice array's prediction is used to select between the two
108 * direction predictors for the final branch prediction.
109 */
110bool
111BiModeBP::lookup(ThreadID tid, Addr branchAddr, void * &bpHistory)
112{
113 unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
114 & choiceHistoryMask);
115 unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
116 ^ globalHistoryReg[tid])
117 & globalHistoryMask);
118
119 assert(choiceHistoryIdx < choicePredictorSize);
120 assert(globalHistoryIdx < globalPredictorSize);
121
122 bool choicePrediction = choiceCounters[choiceHistoryIdx].read()
123 > choiceThreshold;
124 bool takenGHBPrediction = takenCounters[globalHistoryIdx].read()
125 > takenThreshold;
126 bool notTakenGHBPrediction = notTakenCounters[globalHistoryIdx].read()
127 > notTakenThreshold;
128 bool finalPrediction;
129
130 BPHistory *history = new BPHistory;
131 history->globalHistoryReg = globalHistoryReg[tid];
132 history->takenUsed = choicePrediction;
133 history->takenPred = takenGHBPrediction;
134 history->notTakenPred = notTakenGHBPrediction;
135
136 if (choicePrediction) {
137 finalPrediction = takenGHBPrediction;
138 } else {
139 finalPrediction = notTakenGHBPrediction;
140 }
141
142 history->finalPred = finalPrediction;
143 bpHistory = static_cast<void*>(history);
144 updateGlobalHistReg(tid, finalPrediction);
145
146 return finalPrediction;
147}
148
149void
150BiModeBP::btbUpdate(ThreadID tid, Addr branchAddr, void * &bpHistory)
151{
152 globalHistoryReg[tid] &= (historyRegisterMask & ~ULL(1));
153}
154
155/* Only the selected direction predictor will be updated with the final
156 * outcome; the status of the unselected one will not be altered. The choice
157 * predictor is always updated with the branch outcome, except when the
158 * choice is opposite to the branch outcome but the selected counter of
159 * the direction predictors makes a correct final prediction.
160 */
161void
162BiModeBP::update(ThreadID tid, Addr branchAddr, bool taken, void *bpHistory,
163 bool squashed)
164{
165 assert(bpHistory);
166
167 BPHistory *history = static_cast<BPHistory*>(bpHistory);
168
169 // We do not update the counters speculatively on a squash.
170 // We just restore the global history register.
171 if (squashed) {
172 globalHistoryReg[tid] = (history->globalHistoryReg << 1) | taken;
173 return;
174 }
175
176 unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
177 & choiceHistoryMask);
178 unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
179 ^ history->globalHistoryReg)
180 & globalHistoryMask);
181
182 assert(choiceHistoryIdx < choicePredictorSize);
183 assert(globalHistoryIdx < globalPredictorSize);
184
185 if (history->takenUsed) {
186 // if the taken array's prediction was used, update it
187 if (taken) {
188 takenCounters[globalHistoryIdx].increment();
189 } else {
190 takenCounters[globalHistoryIdx].decrement();
191 }
192 } else {
193 // if the not-taken array's prediction was used, update it
194 if (taken) {
195 notTakenCounters[globalHistoryIdx].increment();
196 } else {
197 notTakenCounters[globalHistoryIdx].decrement();
198 }
199 }
200
201 if (history->finalPred == taken) {
202 /* If the final prediction matches the actual branch's
203 * outcome and the choice predictor matches the final
204 * outcome, we update the choice predictor, otherwise it
205 * is not updated. While the designers of the bi-mode
206 * predictor don't explicity say why this is done, one
207 * can infer that it is to preserve the choice predictor's
208 * bias with respect to the branch being predicted; afterall,
209 * the whole point of the bi-mode predictor is to identify the
210 * atypical case when a branch deviates from its bias.
211 */
212 if (history->finalPred == history->takenUsed) {
213 if (taken) {
214 choiceCounters[choiceHistoryIdx].increment();
215 } else {
216 choiceCounters[choiceHistoryIdx].decrement();
217 }
218 }
219 } else {
220 // always update the choice predictor on an incorrect prediction
221 if (taken) {
222 choiceCounters[choiceHistoryIdx].increment();
223 } else {
224 choiceCounters[choiceHistoryIdx].decrement();
225 }
226 }
227
228 delete history;
229}
230
231unsigned
232BiModeBP::getGHR(ThreadID tid, void *bp_history) const
233{
234 return static_cast<BPHistory*>(bp_history)->globalHistoryReg;
235}
236
237void
238BiModeBP::updateGlobalHistReg(ThreadID tid, bool taken)
239{
240 globalHistoryReg[tid] = taken ? (globalHistoryReg[tid] << 1) | 1 :
241 (globalHistoryReg[tid] << 1);
242 globalHistoryReg[tid] &= historyRegisterMask;
243}
244
245BiModeBP*
246BiModeBPParams::create()
247{
248 return new BiModeBP(this);
249}