tournament.cc revision 11098 
1/*
2 * Copyright (c) 2011, 2014 ARM Limited
3 * All rights reserved
4 *
5 * The license below extends only to copyright in the software and shall
6 * not be construed as granting a license to any other intellectual
7 * property including but not limited to intellectual property relating
8 * to a hardware implementation of the functionality of the software
9 * licensed hereunder.  You may use the software subject to the license
10 * terms below provided that you ensure that this notice is replicated
11 * unmodified and in its entirety in all distributions of the software,
12 * modified or unmodified, in source code or in binary form.
13 *
14 * Copyright (c) 2004-2006 The Regents of The University of Michigan
15 * All rights reserved.
16 *
17 * Redistribution and use in source and binary forms, with or without
18 * modification, are permitted provided that the following conditions are
19 * met: redistributions of source code must retain the above copyright
20 * notice, this list of conditions and the following disclaimer;
21 * redistributions in binary form must reproduce the above copyright
22 * notice, this list of conditions and the following disclaimer in the
23 * documentation and/or other materials provided with the distribution;
24 * neither the name of the copyright holders nor the names of its
25 * contributors may be used to endorse or promote products derived from
26 * this software without specific prior written permission.
27 *
28 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
29 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
30 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
31 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
32 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
33 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
34 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
35 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
36 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
37 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
38 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
39 *
40 * Authors: Kevin Lim
41 */
42
43#include "base/bitfield.hh"
44#include "base/intmath.hh"
45#include "cpu/pred/tournament.hh"
46
47TournamentBP::TournamentBP(const TournamentBPParams *params)
48    : BPredUnit(params),
49      localPredictorSize(params->localPredictorSize),
50      localCtrBits(params->localCtrBits),
51      localHistoryTableSize(params->localHistoryTableSize),
52      localHistoryBits(ceilLog2(params->localPredictorSize)),
53      globalPredictorSize(params->globalPredictorSize),
54      globalCtrBits(params->globalCtrBits),
55      globalHistoryBits(
56          ceilLog2(params->globalPredictorSize) >
57          ceilLog2(params->choicePredictorSize) ?
58          ceilLog2(params->globalPredictorSize) :
59          ceilLog2(params->choicePredictorSize)),
60      choicePredictorSize(params->choicePredictorSize),
61      choiceCtrBits(params->choiceCtrBits)
62{
63    if (!isPowerOf2(localPredictorSize)) {
64        fatal("Invalid local predictor size!\n");
65    }
66
67    if (!isPowerOf2(globalPredictorSize)) {
68        fatal("Invalid global predictor size!\n");
69    }
70
71    //Set up the array of counters for the local predictor
72    localCtrs.resize(localPredictorSize);
73
74    for (int i = 0; i < localPredictorSize; ++i)
75        localCtrs[i].setBits(localCtrBits);
76
77    localPredictorMask = mask(localHistoryBits);
78
79    if (!isPowerOf2(localHistoryTableSize)) {
80        fatal("Invalid local history table size!\n");
81    }
82
83    //Setup the history table for the local table
84    localHistoryTable.resize(localHistoryTableSize);
85
86    for (int i = 0; i < localHistoryTableSize; ++i)
87        localHistoryTable[i] = 0;
88
89    //Setup the array of counters for the global predictor
90    globalCtrs.resize(globalPredictorSize);
91
92    for (int i = 0; i < globalPredictorSize; ++i)
93        globalCtrs[i].setBits(globalCtrBits);
94
95    //Clear the global history
96    globalHistory = 0;
97    // Set up the global history mask
98    // this is equivalent to mask(log2(globalPredictorSize)
99    globalHistoryMask = globalPredictorSize - 1;
100
101    if (!isPowerOf2(choicePredictorSize)) {
102        fatal("Invalid choice predictor size!\n");
103    }
104
105    // Set up choiceHistoryMask
106    // this is equivalent to mask(log2(choicePredictorSize)
107    choiceHistoryMask = choicePredictorSize - 1;
108
109    //Setup the array of counters for the choice predictor
110    choiceCtrs.resize(choicePredictorSize);
111
112    for (int i = 0; i < choicePredictorSize; ++i)
113        choiceCtrs[i].setBits(choiceCtrBits);
114
115    //Set up historyRegisterMask
116    historyRegisterMask = mask(globalHistoryBits);
117
118    //Check that predictors don't use more bits than they have available
119    if (globalHistoryMask > historyRegisterMask) {
120        fatal("Global predictor too large for global history bits!\n");
121    }
122    if (choiceHistoryMask > historyRegisterMask) {
123        fatal("Choice predictor too large for global history bits!\n");
124    }
125
126    if (globalHistoryMask < historyRegisterMask &&
127        choiceHistoryMask < historyRegisterMask) {
128        inform("More global history bits than required by predictors\n");
129    }
130
131    // Set thresholds for the three predictors' counters
132    // This is equivalent to (2^(Ctr))/2 - 1
133    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
134    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
135    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
136}
137
138inline
139unsigned
140TournamentBP::calcLocHistIdx(Addr &branch_addr)
141{
142    // Get low order bits after removing instruction offset.
143    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
144}
145
146inline
147void
148TournamentBP::updateGlobalHistTaken()
149{
150    globalHistory = (globalHistory << 1) | 1;
151    globalHistory = globalHistory & historyRegisterMask;
152}
153
154inline
155void
156TournamentBP::updateGlobalHistNotTaken()
157{
158    globalHistory = (globalHistory << 1);
159    globalHistory = globalHistory & historyRegisterMask;
160}
161
162inline
163void
164TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
165{
166    localHistoryTable[local_history_idx] =
167        (localHistoryTable[local_history_idx] << 1) | 1;
168}
169
170inline
171void
172TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
173{
174    localHistoryTable[local_history_idx] =
175        (localHistoryTable[local_history_idx] << 1);
176}
177
178
179void
180TournamentBP::btbUpdate(Addr branch_addr, void * &bp_history)
181{
182    unsigned local_history_idx = calcLocHistIdx(branch_addr);
183    //Update Global History to Not Taken (clear LSB)
184    globalHistory &= (historyRegisterMask & ~ULL(1));
185    //Update Local History to Not Taken
186    localHistoryTable[local_history_idx] =
187       localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
188}
189
190bool
191TournamentBP::lookup(Addr branch_addr, void * &bp_history)
192{
193    bool local_prediction;
194    unsigned local_history_idx;
195    unsigned local_predictor_idx;
196
197    bool global_prediction;
198    bool choice_prediction;
199
200    //Lookup in the local predictor to get its branch prediction
201    local_history_idx = calcLocHistIdx(branch_addr);
202    local_predictor_idx = localHistoryTable[local_history_idx]
203        & localPredictorMask;
204    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
205
206    //Lookup in the global predictor to get its branch prediction
207    global_prediction =
208      globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;
209
210    //Lookup in the choice predictor to see which one to use
211    choice_prediction =
212      choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;
213
214    // Create BPHistory and pass it back to be recorded.
215    BPHistory *history = new BPHistory;
216    history->globalHistory = globalHistory;
217    history->localPredTaken = local_prediction;
218    history->globalPredTaken = global_prediction;
219    history->globalUsed = choice_prediction;
220    history->localHistoryIdx = local_history_idx;
221    history->localHistory = local_predictor_idx;
222    bp_history = (void *)history;
223
224    assert(local_history_idx < localHistoryTableSize);
225
226    // Commented code is for doing speculative update of counters and
227    // all histories.
228    if (choice_prediction) {
229        if (global_prediction) {
230            updateGlobalHistTaken();
231            updateLocalHistTaken(local_history_idx);
232            return true;
233        } else {
234            updateGlobalHistNotTaken();
235            updateLocalHistNotTaken(local_history_idx);
236            return false;
237        }
238    } else {
239        if (local_prediction) {
240            updateGlobalHistTaken();
241            updateLocalHistTaken(local_history_idx);
242            return true;
243        } else {
244            updateGlobalHistNotTaken();
245            updateLocalHistNotTaken(local_history_idx);
246            return false;
247        }
248    }
249}
250
251void
252TournamentBP::uncondBranch(Addr pc, void * &bp_history)
253{
254    // Create BPHistory and pass it back to be recorded.
255    BPHistory *history = new BPHistory;
256    history->globalHistory = globalHistory;
257    history->localPredTaken = true;
258    history->globalPredTaken = true;
259    history->globalUsed = true;
260    history->localHistoryIdx = invalidPredictorIndex;
261    history->localHistory = invalidPredictorIndex;
262    bp_history = static_cast<void *>(history);
263
264    updateGlobalHistTaken();
265}
266
267void
268TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
269                     bool squashed)
270{
271    unsigned local_history_idx;
272    unsigned local_predictor_idx M5_VAR_USED;
273    unsigned local_predictor_hist;
274
275    // Get the local predictor's current prediction
276    local_history_idx = calcLocHistIdx(branch_addr);
277    local_predictor_hist = localHistoryTable[local_history_idx];
278    local_predictor_idx = local_predictor_hist & localPredictorMask;
279
280    if (bp_history) {
281        BPHistory *history = static_cast<BPHistory *>(bp_history);
282        // Update may also be called if the Branch target is incorrect even if
283        // the prediction is correct. In that case do not update the counters.
284        bool historyPred = false;
285        unsigned old_local_pred_index = history->localHistory &
286                localPredictorMask;
287
288        bool old_local_pred_valid = history->localHistory !=
289            invalidPredictorIndex;
290
291        assert(old_local_pred_index < localPredictorSize);
292
293        if (history->globalUsed) {
294           historyPred = history->globalPredTaken;
295        } else {
296           historyPred = history->localPredTaken;
297        }
298        if (historyPred != taken || !squashed) {
299            // Update the choice predictor to tell it which one was correct if
300            // there was a prediction.
301            if (history->localPredTaken != history->globalPredTaken) {
302                 // If the local prediction matches the actual outcome,
303                 // decerement the counter.  Otherwise increment the
304                 // counter.
305                 unsigned choice_predictor_idx =
306                   history->globalHistory & choiceHistoryMask;
307                 if (history->localPredTaken == taken) {
308                     choiceCtrs[choice_predictor_idx].decrement();
309                 } else if (history->globalPredTaken == taken) {
310                     choiceCtrs[choice_predictor_idx].increment();
311                 }
312
313             }
314
315             // Update the counters and local history with the proper
316             // resolution of the branch.  Global history is updated
317             // speculatively and restored upon squash() calls, so it does not
318             // need to be updated.
319             unsigned global_predictor_idx =
320               history->globalHistory & globalHistoryMask;
321             if (taken) {
322                  globalCtrs[global_predictor_idx].increment();
323                  if (old_local_pred_valid) {
324                          localCtrs[old_local_pred_index].increment();
325                  }
326             } else {
327                  globalCtrs[global_predictor_idx].decrement();
328                  if (old_local_pred_valid) {
329                          localCtrs[old_local_pred_index].decrement();
330                  }
331             }
332        }
333        if (squashed) {
334             if (taken) {
335                globalHistory = (history->globalHistory << 1) | 1;
336                globalHistory = globalHistory & historyRegisterMask;
337                if (old_local_pred_valid) {
338                    localHistoryTable[local_history_idx] =
339                     (history->localHistory << 1) | 1;
340                }
341             } else {
342                globalHistory = (history->globalHistory << 1);
343                globalHistory = globalHistory & historyRegisterMask;
344                if (old_local_pred_valid) {
345                     localHistoryTable[local_history_idx] =
346                     history->localHistory << 1;
347                }
348             }
349
350        } else {
351            // We're done with this history, now delete it.
352            delete history;
353        }
354    }
355
356    assert(local_history_idx < localHistoryTableSize);
357
358
359}
360
361void
362TournamentBP::retireSquashed(void *bp_history)
363{
364    BPHistory *history = static_cast<BPHistory *>(bp_history);
365    delete history;
366}
367
368void
369TournamentBP::squash(void *bp_history)
370{
371    BPHistory *history = static_cast<BPHistory *>(bp_history);
372
373    // Restore global history to state prior to this branch.
374    globalHistory = history->globalHistory;
375
376    // Restore local history
377    if (history->localHistoryIdx != invalidPredictorIndex) {
378        localHistoryTable[history->localHistoryIdx] = history->localHistory;
379    }
380
381    // Delete this BPHistory now that we're done with it.
382    delete history;
383}
384
385TournamentBP*
386TournamentBPParams::create()
387{
388    return new TournamentBP(this);
389}
390
391#ifdef DEBUG
392int
393TournamentBP::BPHistory::newCount = 0;
394#endif
395