tournament.cc revision 8487:c7982323e834 
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 _globalHistoryBits,
40                           unsigned _globalCtrBits,
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}
108
109inline
110unsigned
111TournamentBP::calcLocHistIdx(Addr &branch_addr)
112{
113    // Get low order bits after removing instruction offset.
114    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
115}
116
117inline
118void
119TournamentBP::updateGlobalHistTaken()
120{
121    globalHistory = (globalHistory << 1) | 1;
122    globalHistory = globalHistory & globalHistoryMask;
123}
124
125inline
126void
127TournamentBP::updateGlobalHistNotTaken()
128{
129    globalHistory = (globalHistory << 1);
130    globalHistory = globalHistory & globalHistoryMask;
131}
132
133inline
134void
135TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
136{
137    localHistoryTable[local_history_idx] =
138        (localHistoryTable[local_history_idx] << 1) | 1;
139}
140
141inline
142void
143TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
144{
145    localHistoryTable[local_history_idx] =
146        (localHistoryTable[local_history_idx] << 1);
147}
148
149bool
150TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
151{
152    bool local_prediction;
153    unsigned local_history_idx;
154    unsigned local_predictor_idx;
155
156    bool global_prediction;
157    bool choice_prediction;
158
159    //Lookup in the local predictor to get its branch prediction
160    local_history_idx = calcLocHistIdx(branch_addr);
161    local_predictor_idx = localHistoryTable[local_history_idx]
162        & localPredictorMask;
163    local_prediction = localCtrs[local_predictor_idx].read() > threshold;
164
165    //Lookup in the global predictor to get its branch prediction
166    global_prediction = globalCtrs[globalHistory].read() > threshold;
167
168    //Lookup in the choice predictor to see which one to use
169    choice_prediction = choiceCtrs[globalHistory].read() > threshold;
170
171    // Create BPHistory and pass it back to be recorded.
172    BPHistory *history = new BPHistory;
173    history->globalHistory = globalHistory;
174    history->localPredTaken = local_prediction;
175    history->globalPredTaken = global_prediction;
176    history->globalUsed = choice_prediction;
177    bp_history = (void *)history;
178
179    assert(globalHistory < globalPredictorSize &&
180           local_history_idx < localHistoryTableSize &&
181           local_predictor_idx < localPredictorSize);
182
183    // Commented code is for doing speculative update of counters and
184    // all histories.
185    if (choice_prediction) {
186        if (global_prediction) {
187//            updateHistoriesTaken(local_history_idx);
188//            globalCtrs[globalHistory].increment();
189//            localCtrs[local_history_idx].increment();
190            updateGlobalHistTaken();
191            return true;
192        } else {
193//            updateHistoriesNotTaken(local_history_idx);
194//            globalCtrs[globalHistory].decrement();
195//            localCtrs[local_history_idx].decrement();
196            updateGlobalHistNotTaken();
197            return false;
198        }
199    } else {
200        if (local_prediction) {
201//            updateHistoriesTaken(local_history_idx);
202//            globalCtrs[globalHistory].increment();
203//            localCtrs[local_history_idx].increment();
204            updateGlobalHistTaken();
205            return true;
206        } else {
207//            updateHistoriesNotTaken(local_history_idx);
208//            globalCtrs[globalHistory].decrement();
209//            localCtrs[local_history_idx].decrement();
210            updateGlobalHistNotTaken();
211            return false;
212        }
213    }
214}
215
216void
217TournamentBP::uncondBr(void * &bp_history)
218{
219    // Create BPHistory and pass it back to be recorded.
220    BPHistory *history = new BPHistory;
221    history->globalHistory = globalHistory;
222    history->localPredTaken = true;
223    history->globalPredTaken = true;
224    history->globalUsed = 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[history->globalHistory].decrement();
252            } else if (history->globalPredTaken == taken){
253                choiceCtrs[history->globalHistory].increment();
254            }
255
256        }
257
258        // Update the counters and local history with the proper
259        // resolution of the branch.  Global history is updated
260        // speculatively and restored upon squash() calls, so it does not
261        // need to be updated.
262        if (taken) {
263               localCtrs[local_predictor_idx].increment();
264               globalCtrs[history->globalHistory].increment();
265
266               updateLocalHistTaken(local_history_idx);
267        } else {
268               localCtrs[local_predictor_idx].decrement();
269               globalCtrs[history->globalHistory].decrement();
270
271               updateLocalHistNotTaken(local_history_idx);
272       }
273
274       bool mispredict = false;
275
276       //global predictor used and mispredicted
277       if (history->globalUsed && history->globalPredTaken != taken)
278           mispredict = true;
279       //local predictor used and mispredicted
280       else if (!history->globalUsed && history->localPredTaken != taken)
281           mispredict = true;
282
283       if (mispredict) {
284           if (taken) {
285              globalHistory = globalHistory | 1;
286           } else {
287              unsigned mask = globalHistoryMask - 1;
288              globalHistory = globalHistory & mask;
289           }
290
291        }
292        // We're done with this history, now delete it.
293        delete history;
294    }
295
296    assert(globalHistory < globalPredictorSize &&
297           local_history_idx < localHistoryTableSize &&
298           local_predictor_idx < localPredictorSize);
299
300
301}
302
303void
304TournamentBP::squash(void *bp_history)
305{
306    BPHistory *history = static_cast<BPHistory *>(bp_history);
307
308    // Restore global history to state prior to this branch.
309    globalHistory = history->globalHistory;
310
311    // Delete this BPHistory now that we're done with it.
312    delete history;
313}
314
315#ifdef DEBUG
316int
317TournamentBP::BPHistory::newCount = 0;
318#endif
319