bi_mode.cc revision 11429:cf5af0cc3be4 
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(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(Addr pc, void * &bpHistory)
81{
82    BPHistory *history = new BPHistory;
83    history->globalHistoryReg = globalHistoryReg;
84    history->takenUsed = true;
85    history->takenPred = true;
86    history->notTakenPred = true;
87    history->finalPred = true;
88    bpHistory = static_cast<void*>(history);
89    updateGlobalHistReg(true);
90}
91
92void
93BiModeBP::squash(void *bpHistory)
94{
95    BPHistory *history = static_cast<BPHistory*>(bpHistory);
96    globalHistoryReg = 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(Addr branchAddr, void * &bpHistory)
112{
113    unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
114                                & choiceHistoryMask);
115    unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
116                                ^ globalHistoryReg)
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;
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(finalPrediction);
145
146    return finalPrediction;
147}
148
149void
150BiModeBP::btbUpdate(Addr branchAddr, void * &bpHistory)
151{
152    globalHistoryReg &= (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(Addr branchAddr, bool taken, void *bpHistory, bool squashed)
163{
164    if (bpHistory) {
165        BPHistory *history = static_cast<BPHistory*>(bpHistory);
166
167        unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
168                                    & choiceHistoryMask);
169        unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
170                                    ^ history->globalHistoryReg)
171                                    & globalHistoryMask);
172
173        assert(choiceHistoryIdx < choicePredictorSize);
174        assert(globalHistoryIdx < globalPredictorSize);
175
176        if (history->takenUsed) {
177            // if the taken array's prediction was used, update it
178            if (taken) {
179                takenCounters[globalHistoryIdx].increment();
180            } else {
181                takenCounters[globalHistoryIdx].decrement();
182            }
183        } else {
184            // if the not-taken array's prediction was used, update it
185            if (taken) {
186                notTakenCounters[globalHistoryIdx].increment();
187            } else {
188                notTakenCounters[globalHistoryIdx].decrement();
189            }
190        }
191
192        if (history->finalPred == taken) {
193            /* If the final prediction matches the actual branch's
194             * outcome and the choice predictor matches the final
195             * outcome, we update the choice predictor, otherwise it
196             * is not updated. While the designers of the bi-mode
197             * predictor don't explicity say why this is done, one
198             * can infer that it is to preserve the choice predictor's
199             * bias with respect to the branch being predicted; afterall,
200             * the whole point of the bi-mode predictor is to identify the
201             * atypical case when a branch deviates from its bias.
202             */
203            if (history->finalPred == history->takenUsed) {
204                if (taken) {
205                    choiceCounters[choiceHistoryIdx].increment();
206                } else {
207                    choiceCounters[choiceHistoryIdx].decrement();
208                }
209            }
210        } else {
211            // always update the choice predictor on an incorrect prediction
212            if (taken) {
213                choiceCounters[choiceHistoryIdx].increment();
214            } else {
215                choiceCounters[choiceHistoryIdx].decrement();
216            }
217        }
218
219        if (squashed) {
220            if (taken) {
221                globalHistoryReg = (history->globalHistoryReg << 1) | 1;
222            } else {
223                globalHistoryReg = (history->globalHistoryReg << 1);
224            }
225            globalHistoryReg &= historyRegisterMask;
226        } else {
227            delete history;
228        }
229    }
230}
231
232void
233BiModeBP::retireSquashed(void *bp_history)
234{
235    BPHistory *history = static_cast<BPHistory*>(bp_history);
236    delete history;
237}
238
239void
240BiModeBP::updateGlobalHistReg(bool taken)
241{
242    globalHistoryReg = taken ? (globalHistoryReg << 1) | 1 :
243                               (globalHistoryReg << 1);
244    globalHistoryReg &= historyRegisterMask;
245}
246
247BiModeBP*
248BiModeBPParams::create()
249{
250    return new BiModeBP(this);
251}
252