tournament.cc revision 9480 
1/*
2 * Copyright (c) 2011 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 Params *params)
48    : BPredUnit(params),
49      localCtrBits(params->localCtrBits),
50      localHistoryTableSize(params->localHistoryTableSize),
51      localHistoryBits(params->localHistoryBits),
52      globalPredictorSize(params->globalPredictorSize),
53      globalCtrBits(params->globalCtrBits),
54      globalHistoryBits(params->globalHistoryBits),
55      choicePredictorSize(params->choicePredictorSize),
56      choiceCtrBits(params->choiceCtrBits),
57      instShiftAmt(params->instShiftAmt)
58{
59    localPredictorSize = ULL(1) << localHistoryBits;
60
61    //Set up the array of counters for the local predictor
62    localCtrs.resize(localPredictorSize);
63
64    for (int i = 0; i < localPredictorSize; ++i)
65        localCtrs[i].setBits(localCtrBits);
66
67    localPredictorMask = mask(localHistoryBits);
68
69    if (!isPowerOf2(localHistoryTableSize)) {
70        fatal("Invalid local history table size!\n");
71    }
72
73    //Setup the history table for the local table
74    localHistoryTable.resize(localHistoryTableSize);
75
76    for (int i = 0; i < localHistoryTableSize; ++i)
77        localHistoryTable[i] = 0;
78
79    if (!isPowerOf2(globalPredictorSize)) {
80        fatal("Invalid global predictor size!\n");
81    }
82
83    //Setup the array of counters for the global predictor
84    globalCtrs.resize(globalPredictorSize);
85
86    for (int i = 0; i < globalPredictorSize; ++i)
87        globalCtrs[i].setBits(globalCtrBits);
88
89    //Clear the global history
90    globalHistory = 0;
91    // Set up the global history mask
92    // this is equivalent to mask(log2(globalPredictorSize)
93    globalHistoryMask = globalPredictorSize - 1;
94
95    if (!isPowerOf2(choicePredictorSize)) {
96        fatal("Invalid choice predictor size!\n");
97    }
98
99    // Set up choiceHistoryMask
100    // this is equivalent to mask(log2(choicePredictorSize)
101    choiceHistoryMask = choicePredictorSize - 1;
102
103    //Setup the array of counters for the choice predictor
104    choiceCtrs.resize(choicePredictorSize);
105
106    for (int i = 0; i < choicePredictorSize; ++i)
107        choiceCtrs[i].setBits(choiceCtrBits);
108
109    //Set up historyRegisterMask
110    historyRegisterMask = mask(globalHistoryBits);
111
112    //Check that predictors don't use more bits than they have available
113    if (globalHistoryMask > historyRegisterMask) {
114        fatal("Global predictor too large for global history bits!\n");
115    }
116    if (choiceHistoryMask > historyRegisterMask) {
117        fatal("Choice predictor too large for global history bits!\n");
118    }
119
120    if (globalHistoryMask < historyRegisterMask &&
121        choiceHistoryMask < historyRegisterMask) {
122        inform("More global history bits than required by predictors\n");
123    }
124
125    // Set thresholds for the three predictors' counters
126    // This is equivalent to (2^(Ctr))/2 - 1
127    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
128    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
129    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
130}
131
132inline
133unsigned
134TournamentBP::calcLocHistIdx(Addr &branch_addr)
135{
136    // Get low order bits after removing instruction offset.
137    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
138}
139
140inline
141void
142TournamentBP::updateGlobalHistTaken()
143{
144    globalHistory = (globalHistory << 1) | 1;
145    globalHistory = globalHistory & historyRegisterMask;
146}
147
148inline
149void
150TournamentBP::updateGlobalHistNotTaken()
151{
152    globalHistory = (globalHistory << 1);
153    globalHistory = globalHistory & historyRegisterMask;
154}
155
156inline
157void
158TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
159{
160    localHistoryTable[local_history_idx] =
161        (localHistoryTable[local_history_idx] << 1) | 1;
162}
163
164inline
165void
166TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
167{
168    localHistoryTable[local_history_idx] =
169        (localHistoryTable[local_history_idx] << 1);
170}
171
172
173void
174TournamentBP::btbUpdate(Addr branch_addr, void * &bp_history)
175{
176    unsigned local_history_idx = calcLocHistIdx(branch_addr);
177    //Update Global History to Not Taken (clear LSB)
178    globalHistory &= (historyRegisterMask & ~ULL(1));
179    //Update Local History to Not Taken
180    localHistoryTable[local_history_idx] =
181       localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
182}
183
184bool
185TournamentBP::lookup(Addr branch_addr, void * &bp_history)
186{
187    bool local_prediction;
188    unsigned local_history_idx;
189    unsigned local_predictor_idx;
190
191    bool global_prediction;
192    bool choice_prediction;
193
194    //Lookup in the local predictor to get its branch prediction
195    local_history_idx = calcLocHistIdx(branch_addr);
196    local_predictor_idx = localHistoryTable[local_history_idx]
197        & localPredictorMask;
198    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
199
200    //Lookup in the global predictor to get its branch prediction
201    global_prediction =
202      globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;
203
204    //Lookup in the choice predictor to see which one to use
205    choice_prediction =
206      choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;
207
208    // Create BPHistory and pass it back to be recorded.
209    BPHistory *history = new BPHistory;
210    history->globalHistory = globalHistory;
211    history->localPredTaken = local_prediction;
212    history->globalPredTaken = global_prediction;
213    history->globalUsed = choice_prediction;
214    history->localHistory = local_predictor_idx;
215    bp_history = (void *)history;
216
217    assert(local_history_idx < localHistoryTableSize);
218
219    // Commented code is for doing speculative update of counters and
220    // all histories.
221    if (choice_prediction) {
222        if (global_prediction) {
223            updateGlobalHistTaken();
224            updateLocalHistTaken(local_history_idx);
225            return true;
226        } else {
227            updateGlobalHistNotTaken();
228            updateLocalHistNotTaken(local_history_idx);
229            return false;
230        }
231    } else {
232        if (local_prediction) {
233            updateGlobalHistTaken();
234            updateLocalHistTaken(local_history_idx);
235            return true;
236        } else {
237            updateGlobalHistNotTaken();
238            updateLocalHistNotTaken(local_history_idx);
239            return false;
240        }
241    }
242}
243
244void
245TournamentBP::uncondBranch(void * &bp_history)
246{
247    // Create BPHistory and pass it back to be recorded.
248    BPHistory *history = new BPHistory;
249    history->globalHistory = globalHistory;
250    history->localPredTaken = true;
251    history->globalPredTaken = true;
252    history->globalUsed = true;
253    history->localHistory = invalidPredictorIndex;
254    bp_history = static_cast<void *>(history);
255
256    updateGlobalHistTaken();
257}
258
259void
260TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
261                     bool squashed)
262{
263    unsigned local_history_idx;
264    unsigned local_predictor_idx M5_VAR_USED;
265    unsigned local_predictor_hist;
266
267    // Get the local predictor's current prediction
268    local_history_idx = calcLocHistIdx(branch_addr);
269    local_predictor_hist = localHistoryTable[local_history_idx];
270    local_predictor_idx = local_predictor_hist & localPredictorMask;
271
272    if (bp_history) {
273        BPHistory *history = static_cast<BPHistory *>(bp_history);
274        // Update may also be called if the Branch target is incorrect even if
275        // the prediction is correct. In that case do not update the counters.
276        bool historyPred = false;
277        unsigned old_local_pred_index = history->localHistory &
278                localPredictorMask;
279
280        bool old_local_pred_valid = history->localHistory !=
281            invalidPredictorIndex;
282
283        assert(old_local_pred_index < localPredictorSize);
284
285        if (history->globalUsed) {
286           historyPred = history->globalPredTaken;
287        } else {
288           historyPred = history->localPredTaken;
289        }
290        if (historyPred != taken || !squashed) {
291            // Update the choice predictor to tell it which one was correct if
292            // there was a prediction.
293            if (history->localPredTaken != history->globalPredTaken) {
294                 // If the local prediction matches the actual outcome,
295                 // decerement the counter.  Otherwise increment the
296                 // counter.
297                 unsigned choice_predictor_idx =
298                   history->globalHistory & choiceHistoryMask;
299                 if (history->localPredTaken == taken) {
300                     choiceCtrs[choice_predictor_idx].decrement();
301                 } else if (history->globalPredTaken == taken) {
302                     choiceCtrs[choice_predictor_idx].increment();
303                 }
304
305             }
306
307             // Update the counters and local history with the proper
308             // resolution of the branch.  Global history is updated
309             // speculatively and restored upon squash() calls, so it does not
310             // need to be updated.
311             unsigned global_predictor_idx =
312               history->globalHistory & globalHistoryMask;
313             if (taken) {
314                  globalCtrs[global_predictor_idx].increment();
315                  if (old_local_pred_valid) {
316                          localCtrs[old_local_pred_index].increment();
317                  }
318             } else {
319                  globalCtrs[global_predictor_idx].decrement();
320                  if (old_local_pred_valid) {
321                          localCtrs[old_local_pred_index].decrement();
322                  }
323             }
324        }
325        if (squashed) {
326             if (taken) {
327                globalHistory = (history->globalHistory << 1) | 1;
328                globalHistory = globalHistory & historyRegisterMask;
329                if (old_local_pred_valid) {
330                    localHistoryTable[local_history_idx] =
331                     (history->localHistory << 1) | 1;
332                }
333             } else {
334                globalHistory = (history->globalHistory << 1);
335                globalHistory = globalHistory & historyRegisterMask;
336                if (old_local_pred_valid) {
337                     localHistoryTable[local_history_idx] =
338                     history->localHistory << 1;
339                }
340             }
341
342        }
343        // We're done with this history, now delete it.
344        delete history;
345
346    }
347
348    assert(local_history_idx < localHistoryTableSize);
349
350
351}
352
353void
354TournamentBP::squash(void *bp_history)
355{
356    BPHistory *history = static_cast<BPHistory *>(bp_history);
357
358    // Restore global history to state prior to this branch.
359    globalHistory = history->globalHistory;
360
361    // Delete this BPHistory now that we're done with it.
362    delete history;
363}
364
365#ifdef DEBUG
366int
367TournamentBP::BPHistory::newCount = 0;
368#endif
369