-
Notifications
You must be signed in to change notification settings - Fork 1
/
perceptron_hybrid.cc
265 lines (200 loc) · 7.63 KB
/
perceptron_hybrid.cc
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
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
#include "predictor.h"
#include <bitset>
#include <stdint.h>
#define PERC_TABLE_SIZE 2048
#define GSHARE_TABLE_SIZE 131072
#define CHOICE_TABLE_SIZE 16384
#define THRESHOLD 46 // Floor(1.93 * HIST_LEN + 14) as specified in paper
#define HIST_LEN 17 // Optimal value as determined by paper
#define PHT_CTR_MAX 3
#define PHT_CTR_INIT 2
/////////////// STORAGE BUDGET JUSTIFICATION ////////////////
// Total storage budget: 64KB = 524288 bits
// Total bits in history register: 17 bits
// Total Gshare table storage: 262144 bits
// Total number of weights per perceptron: 17 + 1 = 18
// Total bits to store weight: 1 + log(46) = 1 + 5 = 6
// Total number of perceptrons: 2048
// Total perceptron table size = 2048 * 18 * 6 = 221184 bits
// Total choice table size: 16384 * 2 = 32768 bits
// Total size = History register size + Gshare table size +
// Perceptron table size + Choice table size
// + Perceptron steps counter size = 524287 bits
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
PREDICTOR::PREDICTOR(void){
// Given the history length and table size, construct the ghr
// and perceptron and gshare PHT tables. Initialize the choice table.
ghr = bitset<HIST_LEN>();
perceptronSteps = 0;
// Initialize the choice table.
for(UINT32 iii=0; iii < CHOICE_TABLE_SIZE; iii++){
choiceTable[iii] = bitset<2>();
}
// Initialize the PHT.
pht = new UINT32[GSHARE_TABLE_SIZE];
for(UINT32 ii=0; ii< GSHARE_TABLE_SIZE; ii++){
pht[ii]=PHT_CTR_INIT;
}
// Initialize each entry in the perceptron table to a value of
// zero. Initialize number of steps executed for each perceptron to zero.
for(UINT32 i=0; i < PERC_TABLE_SIZE; i++){
for(UINT32 j=0; j < HIST_LEN; j++){
perceptronTable[i][j] = 0;
}
}
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
bool PREDICTOR::GetPrediction(UINT32 PC){
// If the highest bit in the saturating counter is 1, choose
// gshare. Otherwise, choose the perceptron predictor.
UINT32 counterIndex = HashPC(PC, CHOICE_TABLE_SIZE);
if (choiceTable[counterIndex][1] == 1){
return GetGsharePrediction(PC);
}
else{
return GetPerceptronPrediction(PC);
}
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
void PREDICTOR::UpdatePredictor(UINT32 PC, bool resolveDir, bool predDir, UINT32 branchTarget){
// Update saturating counter based on current state and outcome.
UINT32 counterIndex = HashPC(PC, CHOICE_TABLE_SIZE);
// Decrement if perceptron got it right or gshare got it wrong.
if ((choiceTable[counterIndex][1] == 0 && resolveDir == predDir) || (choiceTable[counterIndex][1] == 1 && resolveDir != predDir)){
if(choiceTable[counterIndex][1] == 1 && choiceTable[counterIndex][0] == 1){
choiceTable[counterIndex].set(0,0);
}
else if(choiceTable[counterIndex][1] == 1 && choiceTable[counterIndex][0] == 0){
choiceTable[counterIndex].set(0,1);
choiceTable[counterIndex].set(1,0);
}
else if(choiceTable[counterIndex][1] == 0 && choiceTable[counterIndex][0] == 1){
choiceTable[counterIndex].set(0,0);
}
}
else{
// Increment if perceptron got it wrong or gshare got it right.
if ((choiceTable[counterIndex][1] == 1 && resolveDir == predDir) || (choiceTable[counterIndex][1] == 0 && resolveDir != predDir)){
if(choiceTable[counterIndex][1] == 1 && choiceTable[counterIndex][0] == 0){
choiceTable[counterIndex].set(0,1);
}
else if(choiceTable[counterIndex][1] == 0 && choiceTable[counterIndex][0] == 1){
choiceTable[counterIndex].set(1,1);
choiceTable[counterIndex].set(0,0);
}
else if(choiceTable[counterIndex][1] == 0 && choiceTable[counterIndex][0] == 0){
choiceTable[counterIndex].set(0,1);
}
}
}
UpdatePerceptronPredictor(PC, resolveDir, predDir);
UpdateGsharePredictor(PC, resolveDir);
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
void PREDICTOR::TrackOtherInst(UINT32 PC, OpType opType, UINT32 branchTarget){
// This function is called for instructions which are not
// conditional branches, just in case someone decides to design
// a predictor that uses information from such instructions.
// We expect most contestants to leave this function untouched.
return;
}
/////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////
UINT32 PREDICTOR::HashPC(UINT32 PC, UINT32 hashLimit){
// Hash the PC so that it can be used as an index for the perceptron table.
UINT32 PCend = PC % hashLimit;
UINT32 ghrend = ((UINT32)ghr.to_ulong()) % hashLimit;
return PCend ^ ghrend;
}
bool PREDICTOR::GetGsharePrediction(UINT32 PC){
UINT32 phtIndex = HashPC(PC, GSHARE_TABLE_SIZE);
UINT32 phtCounter = pht[phtIndex];
if(phtCounter > PHT_CTR_MAX/2){
return TAKEN;
}else{
return NOT_TAKEN;
}
}
void PREDICTOR::UpdateGsharePredictor(UINT32 PC, bool resolveDir){
UINT32 phtIndex = HashPC(PC, GSHARE_TABLE_SIZE);
UINT32 phtCounter = pht[phtIndex];
// update the PHT
if(resolveDir == TAKEN){
pht[phtIndex] = SatIncrement(phtCounter, PHT_CTR_MAX);
}else{
pht[phtIndex] = SatDecrement(phtCounter);
}
// update the GHR
ghr = (ghr << 1);
if(resolveDir == TAKEN){
ghr.set(0, 1);
}
else{
ghr.set(0, 0);
}
}
bool PREDICTOR::GetPerceptronPrediction(UINT32 PC){
UINT32 perceptronIndex = HashPC(PC, PERC_TABLE_SIZE);
INT32 prediction = 0;
// Calculate prediction based on selected perceptron and global history.
// First add the bias, then all other weights.
prediction += perceptronTable[perceptronIndex][0];
for(UINT32 i=1; i < HIST_LEN + 1; i++){
// If history bit is taken, add the weight to the prediction.
// Else, subtract the weight.
if(ghr[i - 1] == 1){
prediction += perceptronTable[perceptronIndex][i];
}
else{
prediction -= perceptronTable[perceptronIndex][i];
}
}
// Update perceptron steps to absolute value of the prediction.
perceptronSteps = abs(prediction);
// If the prediction is negative, predict not taken. If it is positive,
// predict taken. Assume zero is positive.
if(prediction >= 0){
return TAKEN;
}
else{
return NOT_TAKEN;
}
}
void PREDICTOR::UpdatePerceptronPredictor(UINT32 PC, bool resolveDir, bool predDir){
UINT32 perceptronIndex = HashPC(PC, PERC_TABLE_SIZE);
// Update the perceptron table entry only if the threshold has not been
// reached or the predicted and true outcomes disagree. Update the bias first, then the weights.
if(resolveDir != predDir || perceptronSteps <= THRESHOLD){
// If the branch was taken, increment the bias value. Else, decrement it.
if(resolveDir == TAKEN){
perceptronTable[perceptronIndex][0]++;
}
else{
perceptronTable[perceptronIndex][0]--;
}
// Update the weights.
for(UINT32 i = 1; i < HIST_LEN + 1; i++){
// If the branch outcome matches the history bit, increment the weight value.
// Else, decrement the weight value.
if((resolveDir == TAKEN && ghr[i - 1] == 1) || (resolveDir == NOT_TAKEN && ghr[i - 1] == 0)){
perceptronTable[perceptronIndex][i]++;
}
else{
perceptronTable[perceptronIndex][i]--;
}
}
}
// update the GHR by shifting left and setting new bit.
ghr = (ghr << 1);
if(resolveDir == TAKEN){
ghr.set(0, 1);
}
else{
ghr.set(0, 0);
}
}