bi_mode.cc revision 11793:ef606668d247 
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 "cpu/pred/bi_mode.hh"
36
37#include "base/bitfield.hh"
38#include "base/intmath.hh"
39
40BiModeBP::BiModeBP(const BiModeBPParams *params)
41    : BPredUnit(params),
42      globalHistoryReg(params->numThreads, 0),
43      globalHistoryBits(ceilLog2(params->globalPredictorSize)),
44      choicePredictorSize(params->choicePredictorSize),
45      choiceCtrBits(params->choiceCtrBits),
46      globalPredictorSize(params->globalPredictorSize),
47      globalCtrBits(params->globalCtrBits)
48{
49    if (!isPowerOf2(choicePredictorSize))
50        fatal("Invalid choice predictor size.\n");
51    if (!isPowerOf2(globalPredictorSize))
52        fatal("Invalid global history predictor size.\n");
53
54    choiceCounters.resize(choicePredictorSize);
55    takenCounters.resize(globalPredictorSize);
56    notTakenCounters.resize(globalPredictorSize);
57
58    for (int i = 0; i < choicePredictorSize; ++i) {
59        choiceCounters[i].setBits(choiceCtrBits);
60    }
61    for (int i = 0; i < globalPredictorSize; ++i) {
62        takenCounters[i].setBits(globalCtrBits);
63        notTakenCounters[i].setBits(globalCtrBits);
64    }
65
66    historyRegisterMask = mask(globalHistoryBits);
67    choiceHistoryMask = choicePredictorSize - 1;
68    globalHistoryMask = globalPredictorSize - 1;
69
70    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
71    takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
72    notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
73}
74
75/*
76 * For an unconditional branch we set its history such that
77 * everything is set to taken. I.e., its choice predictor
78 * chooses the taken array and the taken array predicts taken.
79 */
80void
81BiModeBP::uncondBranch(ThreadID tid, Addr pc, void * &bpHistory)
82{
83    BPHistory *history = new BPHistory;
84    history->globalHistoryReg = globalHistoryReg[tid];
85    history->takenUsed = true;
86    history->takenPred = true;
87    history->notTakenPred = true;
88    history->finalPred = true;
89    bpHistory = static_cast<void*>(history);
90    updateGlobalHistReg(tid, true);
91}
92
93void
94BiModeBP::squash(ThreadID tid, void *bpHistory)
95{
96    BPHistory *history = static_cast<BPHistory*>(bpHistory);
97    globalHistoryReg[tid] = history->globalHistoryReg;
98
99    delete history;
100}
101
102/*
103 * Here we lookup the actual branch prediction. We use the PC to
104 * identify the bias of a particular branch, which is based on the
105 * prediction in the choice array. A hash of the global history
106 * register and a branch's PC is used to index into both the taken
107 * and not-taken predictors, which both present a prediction. The
108 * choice array's prediction is used to select between the two
109 * direction predictors for the final branch prediction.
110 */
111bool
112BiModeBP::lookup(ThreadID tid, Addr branchAddr, void * &bpHistory)
113{
114    unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
115                                & choiceHistoryMask);
116    unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
117                                ^ globalHistoryReg[tid])
118                                & globalHistoryMask);
119
120    assert(choiceHistoryIdx < choicePredictorSize);
121    assert(globalHistoryIdx < globalPredictorSize);
122
123    bool choicePrediction = choiceCounters[choiceHistoryIdx].read()
124                            > choiceThreshold;
125    bool takenGHBPrediction = takenCounters[globalHistoryIdx].read()
126                              > takenThreshold;
127    bool notTakenGHBPrediction = notTakenCounters[globalHistoryIdx].read()
128                                 > notTakenThreshold;
129    bool finalPrediction;
130
131    BPHistory *history = new BPHistory;
132    history->globalHistoryReg = globalHistoryReg[tid];
133    history->takenUsed = choicePrediction;
134    history->takenPred = takenGHBPrediction;
135    history->notTakenPred = notTakenGHBPrediction;
136
137    if (choicePrediction) {
138        finalPrediction = takenGHBPrediction;
139    } else {
140        finalPrediction = notTakenGHBPrediction;
141    }
142
143    history->finalPred = finalPrediction;
144    bpHistory = static_cast<void*>(history);
145    updateGlobalHistReg(tid, finalPrediction);
146
147    return finalPrediction;
148}
149
150void
151BiModeBP::btbUpdate(ThreadID tid, Addr branchAddr, void * &bpHistory)
152{
153    globalHistoryReg[tid] &= (historyRegisterMask & ~ULL(1));
154}
155
156/* Only the selected direction predictor will be updated with the final
157 * outcome; the status of the unselected one will not be altered. The choice
158 * predictor is always updated with the branch outcome, except when the
159 * choice is opposite to the branch outcome but the selected counter of
160 * the direction predictors makes a correct final prediction.
161 */
162void
163BiModeBP::update(ThreadID tid, Addr branchAddr, bool taken, void *bpHistory,
164                 bool squashed)
165{
166    assert(bpHistory);
167
168    BPHistory *history = static_cast<BPHistory*>(bpHistory);
169
170    // We do not update the counters speculatively on a squash.
171    // We just restore the global history register.
172    if (squashed) {
173        globalHistoryReg[tid] = (history->globalHistoryReg << 1) | taken;
174        return;
175    }
176
177    unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
178                                & choiceHistoryMask);
179    unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
180                                ^ history->globalHistoryReg)
181                                & globalHistoryMask);
182
183    assert(choiceHistoryIdx < choicePredictorSize);
184    assert(globalHistoryIdx < globalPredictorSize);
185
186    if (history->takenUsed) {
187        // if the taken array's prediction was used, update it
188        if (taken) {
189            takenCounters[globalHistoryIdx].increment();
190        } else {
191            takenCounters[globalHistoryIdx].decrement();
192        }
193    } else {
194        // if the not-taken array's prediction was used, update it
195        if (taken) {
196            notTakenCounters[globalHistoryIdx].increment();
197        } else {
198            notTakenCounters[globalHistoryIdx].decrement();
199        }
200    }
201
202    if (history->finalPred == taken) {
203       /* If the final prediction matches the actual branch's
204        * outcome and the choice predictor matches the final
205        * outcome, we update the choice predictor, otherwise it
206        * is not updated. While the designers of the bi-mode
207        * predictor don't explicity say why this is done, one
208        * can infer that it is to preserve the choice predictor's
209        * bias with respect to the branch being predicted; afterall,
210        * the whole point of the bi-mode predictor is to identify the
211        * atypical case when a branch deviates from its bias.
212        */
213        if (history->finalPred == history->takenUsed) {
214            if (taken) {
215                choiceCounters[choiceHistoryIdx].increment();
216            } else {
217                choiceCounters[choiceHistoryIdx].decrement();
218            }
219        }
220    } else {
221        // always update the choice predictor on an incorrect prediction
222        if (taken) {
223            choiceCounters[choiceHistoryIdx].increment();
224        } else {
225            choiceCounters[choiceHistoryIdx].decrement();
226        }
227    }
228
229    delete history;
230}
231
232unsigned
233BiModeBP::getGHR(ThreadID tid, void *bp_history) const
234{
235    return static_cast<BPHistory*>(bp_history)->globalHistoryReg;
236}
237
238void
239BiModeBP::updateGlobalHistReg(ThreadID tid, bool taken)
240{
241    globalHistoryReg[tid] = taken ? (globalHistoryReg[tid] << 1) | 1 :
242                               (globalHistoryReg[tid] << 1);
243    globalHistoryReg[tid] &= historyRegisterMask;
244}
245
246BiModeBP*
247BiModeBPParams::create()
248{
249    return new BiModeBP(this);
250}
251