1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
|
/*
* Copyright (c) 2014 The Regents of The University of Michigan
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met: redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer;
* redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution;
* neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* Authors: Anthony Gutierrez
*/
/* @file
* Implementation of a bi-mode branch predictor
*/
#include "base/bitfield.hh"
#include "base/intmath.hh"
#include "cpu/pred/bi_mode.hh"
BiModeBP::BiModeBP(const Params *params)
: BPredUnit(params), instShiftAmt(params->instShiftAmt),
globalHistoryReg(0),
globalHistoryBits(ceilLog2(params->globalPredictorSize)),
choicePredictorSize(params->choicePredictorSize),
choiceCtrBits(params->choiceCtrBits),
globalPredictorSize(params->globalPredictorSize),
globalCtrBits(params->globalCtrBits)
{
if (!isPowerOf2(choicePredictorSize))
fatal("Invalid choice predictor size.\n");
if (!isPowerOf2(globalPredictorSize))
fatal("Invalid global history predictor size.\n");
choiceCounters.resize(choicePredictorSize);
takenCounters.resize(globalPredictorSize);
notTakenCounters.resize(globalPredictorSize);
for (int i = 0; i < choicePredictorSize; ++i) {
choiceCounters[i].setBits(choiceCtrBits);
}
for (int i = 0; i < globalPredictorSize; ++i) {
takenCounters[i].setBits(globalCtrBits);
notTakenCounters[i].setBits(globalCtrBits);
}
historyRegisterMask = mask(globalHistoryBits);
choiceHistoryMask = choicePredictorSize - 1;
globalHistoryMask = globalPredictorSize - 1;
choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
}
/*
* For an unconditional branch we set its history such that
* everything is set to taken. I.e., its choice predictor
* chooses the taken array and the taken array predicts taken.
*/
void
BiModeBP::uncondBranch(void * &bpHistory)
{
BPHistory *history = new BPHistory;
history->globalHistoryReg = globalHistoryReg;
history->takenUsed = true;
history->takenPred = true;
history->notTakenPred = true;
history->finalPred = true;
bpHistory = static_cast<void*>(history);
updateGlobalHistReg(true);
}
void
BiModeBP::squash(void *bpHistory)
{
BPHistory *history = static_cast<BPHistory*>(bpHistory);
globalHistoryReg = history->globalHistoryReg;
delete history;
}
/*
* Here we lookup the actual branch prediction. We use the PC to
* identify the bias of a particular branch, which is based on the
* prediction in the choice array. A hash of the global history
* register and a branch's PC is used to index into both the taken
* and not-taken predictors, which both present a prediction. The
* choice array's prediction is used to select between the two
* direction predictors for the final branch prediction.
*/
bool
BiModeBP::lookup(Addr branchAddr, void * &bpHistory)
{
unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
& choiceHistoryMask);
unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
^ globalHistoryReg)
& globalHistoryMask);
assert(choiceHistoryIdx < choicePredictorSize);
assert(globalHistoryIdx < globalPredictorSize);
bool choicePrediction = choiceCounters[choiceHistoryIdx].read()
> choiceThreshold;
bool takenGHBPrediction = takenCounters[globalHistoryIdx].read()
> takenThreshold;
bool notTakenGHBPrediction = notTakenCounters[globalHistoryIdx].read()
> notTakenThreshold;
bool finalPrediction;
BPHistory *history = new BPHistory;
history->globalHistoryReg = globalHistoryReg;
history->takenUsed = choicePrediction;
history->takenPred = takenGHBPrediction;
history->notTakenPred = notTakenGHBPrediction;
if (choicePrediction) {
finalPrediction = takenGHBPrediction;
} else {
finalPrediction = notTakenGHBPrediction;
}
history->finalPred = finalPrediction;
bpHistory = static_cast<void*>(history);
updateGlobalHistReg(finalPrediction);
return finalPrediction;
}
void
BiModeBP::btbUpdate(Addr branchAddr, void * &bpHistory)
{
globalHistoryReg &= (historyRegisterMask & ~ULL(1));
}
/* Only the selected direction predictor will be updated with the final
* outcome; the status of the unselected one will not be altered. The choice
* predictor is always updated with the branch outcome, except when the
* choice is opposite to the branch outcome but the selected counter of
* the direction predictors makes a correct final prediction.
*/
void
BiModeBP::update(Addr branchAddr, bool taken, void *bpHistory, bool squashed)
{
if (bpHistory) {
BPHistory *history = static_cast<BPHistory*>(bpHistory);
unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
& choiceHistoryMask);
unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
^ history->globalHistoryReg)
& globalHistoryMask);
assert(choiceHistoryIdx < choicePredictorSize);
assert(globalHistoryIdx < globalPredictorSize);
if (history->takenUsed) {
// if the taken array's prediction was used, update it
if (taken) {
takenCounters[globalHistoryIdx].increment();
} else {
takenCounters[globalHistoryIdx].decrement();
}
} else {
// if the not-taken array's prediction was used, update it
if (taken) {
notTakenCounters[globalHistoryIdx].increment();
} else {
notTakenCounters[globalHistoryIdx].decrement();
}
}
if (history->finalPred == taken) {
/* If the final prediction matches the actual branch's
* outcome and the choice predictor matches the final
* outcome, we update the choice predictor, otherwise it
* is not updated. While the designers of the bi-mode
* predictor don't explicity say why this is done, one
* can infer that it is to preserve the choice predictor's
* bias with respect to the branch being predicted; afterall,
* the whole point of the bi-mode predictor is to identify the
* atypical case when a branch deviates from its bias.
*/
if (history->finalPred == history->takenUsed) {
if (taken) {
choiceCounters[choiceHistoryIdx].increment();
} else {
choiceCounters[choiceHistoryIdx].decrement();
}
}
} else {
// always update the choice predictor on an incorrect prediction
if (taken) {
choiceCounters[choiceHistoryIdx].increment();
} else {
choiceCounters[choiceHistoryIdx].decrement();
}
}
if (squashed) {
if (taken) {
globalHistoryReg = (history->globalHistoryReg << 1) | 1;
} else {
globalHistoryReg = (history->globalHistoryReg << 1);
}
globalHistoryReg &= historyRegisterMask;
} else {
delete history;
}
}
}
void
BiModeBP::retireSquashed(void *bp_history)
{
BPHistory *history = static_cast<BPHistory*>(bp_history);
delete history;
}
void
BiModeBP::updateGlobalHistReg(bool taken)
{
globalHistoryReg = taken ? (globalHistoryReg << 1) | 1 :
(globalHistoryReg << 1);
globalHistoryReg &= historyRegisterMask;
}
|
