tournament.cc revision 6226 
1/*
2 * Copyright (c) 2004-2006 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: Kevin Lim
29 */
30
31#include "base/intmath.hh"
32#include "cpu/pred/tournament.hh"
33
34TournamentBP::TournamentBP(unsigned _localPredictorSize,
35                           unsigned _localCtrBits,
36                           unsigned _localHistoryTableSize,
37                           unsigned _localHistoryBits,
38                           unsigned _globalPredictorSize,
39                           unsigned _globalCtrBits,
40                           unsigned _globalHistoryBits,
41                           unsigned _choicePredictorSize,
42                           unsigned _choiceCtrBits,
43                           unsigned _instShiftAmt)
44    : localPredictorSize(_localPredictorSize),
45      localCtrBits(_localCtrBits),
46      localHistoryTableSize(_localHistoryTableSize),
47      localHistoryBits(_localHistoryBits),
48      globalPredictorSize(_globalPredictorSize),
49      globalCtrBits(_globalCtrBits),
50      globalHistoryBits(_globalHistoryBits),
51      choicePredictorSize(_globalPredictorSize),
52      choiceCtrBits(_choiceCtrBits),
53      instShiftAmt(_instShiftAmt)
54{
55    if (!isPowerOf2(localPredictorSize)) {
56        fatal("Invalid local predictor size!\n");
57    }
58
59    //Setup the array of counters for the local predictor
60    localCtrs.resize(localPredictorSize);
61
62    for (int i = 0; i < localPredictorSize; ++i)
63        localCtrs[i].setBits(localCtrBits);
64
65    localPredictorMask = floorPow2(localPredictorSize) - 1;
66
67    if (!isPowerOf2(localHistoryTableSize)) {
68        fatal("Invalid local history table size!\n");
69    }
70
71    //Setup the history table for the local table
72    localHistoryTable.resize(localHistoryTableSize);
73
74    for (int i = 0; i < localHistoryTableSize; ++i)
75        localHistoryTable[i] = 0;
76
77    // Setup the local history mask
78    localHistoryMask = (1 << localHistoryBits) - 1;
79
80    if (!isPowerOf2(globalPredictorSize)) {
81        fatal("Invalid global predictor size!\n");
82    }
83
84    //Setup the array of counters for the global predictor
85    globalCtrs.resize(globalPredictorSize);
86
87    for (int i = 0; i < globalPredictorSize; ++i)
88        globalCtrs[i].setBits(globalCtrBits);
89
90    //Clear the global history
91    globalHistory = 0;
92    // Setup the global history mask
93    globalHistoryMask = (1 << globalHistoryBits) - 1;
94
95    if (!isPowerOf2(choicePredictorSize)) {
96        fatal("Invalid choice predictor size!\n");
97    }
98
99    //Setup the array of counters for the choice predictor
100    choiceCtrs.resize(choicePredictorSize);
101
102    for (int i = 0; i < choicePredictorSize; ++i)
103        choiceCtrs[i].setBits(choiceCtrBits);
104
105    // @todo: Allow for different thresholds between the predictors.
106    threshold = (1 << (localCtrBits - 1)) - 1;
107    threshold = threshold / 2;
108}
109
110inline
111unsigned
112TournamentBP::calcLocHistIdx(Addr &branch_addr)
113{
114    // Get low order bits after removing instruction offset.
115    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
116}
117
118inline
119void
120TournamentBP::updateGlobalHistTaken()
121{
122    globalHistory = (globalHistory << 1) | 1;
123    globalHistory = globalHistory & globalHistoryMask;
124}
125
126inline
127void
128TournamentBP::updateGlobalHistNotTaken()
129{
130    globalHistory = (globalHistory << 1);
131    globalHistory = globalHistory & globalHistoryMask;
132}
133
134inline
135void
136TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
137{
138    localHistoryTable[local_history_idx] =
139        (localHistoryTable[local_history_idx] << 1) | 1;
140}
141
142inline
143void
144TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
145{
146    localHistoryTable[local_history_idx] =
147        (localHistoryTable[local_history_idx] << 1);
148}
149
150bool
151TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
152{
153    bool local_prediction;
154    unsigned local_history_idx;
155    unsigned local_predictor_idx;
156
157    bool global_prediction;
158    bool choice_prediction;
159
160    //Lookup in the local predictor to get its branch prediction
161    local_history_idx = calcLocHistIdx(branch_addr);
162    local_predictor_idx = localHistoryTable[local_history_idx]
163        & localPredictorMask;
164    local_prediction = localCtrs[local_predictor_idx].read() > threshold;
165
166    //Lookup in the global predictor to get its branch prediction
167    global_prediction = globalCtrs[globalHistory].read() > threshold;
168
169    //Lookup in the choice predictor to see which one to use
170    choice_prediction = choiceCtrs[globalHistory].read() > threshold;
171
172    // Create BPHistory and pass it back to be recorded.
173    BPHistory *history = new BPHistory;
174    history->globalHistory = globalHistory;
175    history->localPredTaken = local_prediction;
176    history->globalPredTaken = global_prediction;
177    history->globalUsed = choice_prediction;
178    bp_history = (void *)history;
179
180    assert(globalHistory < globalPredictorSize &&
181           local_history_idx < localHistoryTableSize &&
182           local_predictor_idx < localPredictorSize);
183
184    // Commented code is for doing speculative update of counters and
185    // all histories.
186    if (choice_prediction) {
187        if (global_prediction) {
188//            updateHistoriesTaken(local_history_idx);
189//            globalCtrs[globalHistory].increment();
190//            localCtrs[local_history_idx].increment();
191            updateGlobalHistTaken();
192            return true;
193        } else {
194//            updateHistoriesNotTaken(local_history_idx);
195//            globalCtrs[globalHistory].decrement();
196//            localCtrs[local_history_idx].decrement();
197            updateGlobalHistNotTaken();
198            return false;
199        }
200    } else {
201        if (local_prediction) {
202//            updateHistoriesTaken(local_history_idx);
203//            globalCtrs[globalHistory].increment();
204//            localCtrs[local_history_idx].increment();
205            updateGlobalHistTaken();
206            return true;
207        } else {
208//            updateHistoriesNotTaken(local_history_idx);
209//            globalCtrs[globalHistory].decrement();
210//            localCtrs[local_history_idx].decrement();
211            updateGlobalHistNotTaken();
212            return false;
213        }
214    }
215}
216
217void
218TournamentBP::uncondBr(void * &bp_history)
219{
220    // Create BPHistory and pass it back to be recorded.
221    BPHistory *history = new BPHistory;
222    history->globalHistory = globalHistory;
223    history->localPredTaken = true;
224    history->globalPredTaken = true;
225    bp_history = static_cast<void *>(history);
226
227    updateGlobalHistTaken();
228}
229
230void
231TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
232{
233    unsigned local_history_idx;
234    unsigned local_predictor_idx;
235    unsigned local_predictor_hist;
236
237    // Get the local predictor's current prediction
238    local_history_idx = calcLocHistIdx(branch_addr);
239    local_predictor_hist = localHistoryTable[local_history_idx];
240    local_predictor_idx = local_predictor_hist & localPredictorMask;
241
242    // Update the choice predictor to tell it which one was correct if
243    // there was a prediction.
244    if (bp_history) {
245        BPHistory *history = static_cast<BPHistory *>(bp_history);
246        if (history->localPredTaken != history->globalPredTaken) {
247            // If the local prediction matches the actual outcome,
248            // decerement the counter.  Otherwise increment the
249            // counter.
250            if (history->localPredTaken == taken) {
251                choiceCtrs[globalHistory].decrement();
252            } else if (history->globalPredTaken == taken){
253                choiceCtrs[globalHistory].increment();
254            }
255        }
256
257        // We're done with this history, now delete it.
258        delete history;
259    }
260
261    assert(globalHistory < globalPredictorSize &&
262           local_history_idx < localHistoryTableSize &&
263           local_predictor_idx < localPredictorSize);
264
265    // Update the counters and local history with the proper
266    // resolution of the branch.  Global history is updated
267    // speculatively and restored upon squash() calls, so it does not
268    // need to be updated.
269    if (taken) {
270        localCtrs[local_predictor_idx].increment();
271        globalCtrs[globalHistory].increment();
272
273        updateLocalHistTaken(local_history_idx);
274    } else {
275        localCtrs[local_predictor_idx].decrement();
276        globalCtrs[globalHistory].decrement();
277
278        updateLocalHistNotTaken(local_history_idx);
279    }
280}
281
282void
283TournamentBP::squash(void *bp_history)
284{
285    BPHistory *history = static_cast<BPHistory *>(bp_history);
286
287    // Restore global history to state prior to this branch.
288    globalHistory = history->globalHistory;
289
290    // Delete this BPHistory now that we're done with it.
291    delete history;
292}
293
294#ifdef DEBUG
295int
296TournamentBP::BPHistory::newCount = 0;
297#endif
298