tournament.cc revision 10330 
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 Params *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      instShiftAmt(params->instShiftAmt)
63{
64    if (!isPowerOf2(localPredictorSize)) {
65        fatal("Invalid local predictor size!\n");
66    }
67
68    if (!isPowerOf2(globalPredictorSize)) {
69        fatal("Invalid global predictor size!\n");
70    }
71
72    //Set up the array of counters for the local predictor
73    localCtrs.resize(localPredictorSize);
74
75    for (int i = 0; i < localPredictorSize; ++i)
76        localCtrs[i].setBits(localCtrBits);
77
78    localPredictorMask = mask(localHistoryBits);
79
80    if (!isPowerOf2(localHistoryTableSize)) {
81        fatal("Invalid local history table size!\n");
82    }
83
84    //Setup the history table for the local table
85    localHistoryTable.resize(localHistoryTableSize);
86
87    for (int i = 0; i < localHistoryTableSize; ++i)
88        localHistoryTable[i] = 0;
89
90    //Setup the array of counters for the global predictor
91    globalCtrs.resize(globalPredictorSize);
92
93    for (int i = 0; i < globalPredictorSize; ++i)
94        globalCtrs[i].setBits(globalCtrBits);
95
96    //Clear the global history
97    globalHistory = 0;
98    // Set up the global history mask
99    // this is equivalent to mask(log2(globalPredictorSize)
100    globalHistoryMask = globalPredictorSize - 1;
101
102    if (!isPowerOf2(choicePredictorSize)) {
103        fatal("Invalid choice predictor size!\n");
104    }
105
106    // Set up choiceHistoryMask
107    // this is equivalent to mask(log2(choicePredictorSize)
108    choiceHistoryMask = choicePredictorSize - 1;
109
110    //Setup the array of counters for the choice predictor
111    choiceCtrs.resize(choicePredictorSize);
112
113    for (int i = 0; i < choicePredictorSize; ++i)
114        choiceCtrs[i].setBits(choiceCtrBits);
115
116    //Set up historyRegisterMask
117    historyRegisterMask = mask(globalHistoryBits);
118
119    //Check that predictors don't use more bits than they have available
120    if (globalHistoryMask > historyRegisterMask) {
121        fatal("Global predictor too large for global history bits!\n");
122    }
123    if (choiceHistoryMask > historyRegisterMask) {
124        fatal("Choice predictor too large for global history bits!\n");
125    }
126
127    if (globalHistoryMask < historyRegisterMask &&
128        choiceHistoryMask < historyRegisterMask) {
129        inform("More global history bits than required by predictors\n");
130    }
131
132    // Set thresholds for the three predictors' counters
133    // This is equivalent to (2^(Ctr))/2 - 1
134    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
135    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
136    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
137}
138
139inline
140unsigned
141TournamentBP::calcLocHistIdx(Addr &branch_addr)
142{
143    // Get low order bits after removing instruction offset.
144    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
145}
146
147inline
148void
149TournamentBP::updateGlobalHistTaken()
150{
151    globalHistory = (globalHistory << 1) | 1;
152    globalHistory = globalHistory & historyRegisterMask;
153}
154
155inline
156void
157TournamentBP::updateGlobalHistNotTaken()
158{
159    globalHistory = (globalHistory << 1);
160    globalHistory = globalHistory & historyRegisterMask;
161}
162
163inline
164void
165TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
166{
167    localHistoryTable[local_history_idx] =
168        (localHistoryTable[local_history_idx] << 1) | 1;
169}
170
171inline
172void
173TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
174{
175    localHistoryTable[local_history_idx] =
176        (localHistoryTable[local_history_idx] << 1);
177}
178
179
180void
181TournamentBP::btbUpdate(Addr branch_addr, void * &bp_history)
182{
183    unsigned local_history_idx = calcLocHistIdx(branch_addr);
184    //Update Global History to Not Taken (clear LSB)
185    globalHistory &= (historyRegisterMask & ~ULL(1));
186    //Update Local History to Not Taken
187    localHistoryTable[local_history_idx] =
188       localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
189}
190
191bool
192TournamentBP::lookup(Addr branch_addr, void * &bp_history)
193{
194    bool local_prediction;
195    unsigned local_history_idx;
196    unsigned local_predictor_idx;
197
198    bool global_prediction;
199    bool choice_prediction;
200
201    //Lookup in the local predictor to get its branch prediction
202    local_history_idx = calcLocHistIdx(branch_addr);
203    local_predictor_idx = localHistoryTable[local_history_idx]
204        & localPredictorMask;
205    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
206
207    //Lookup in the global predictor to get its branch prediction
208    global_prediction =
209      globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;
210
211    //Lookup in the choice predictor to see which one to use
212    choice_prediction =
213      choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;
214
215    // Create BPHistory and pass it back to be recorded.
216    BPHistory *history = new BPHistory;
217    history->globalHistory = globalHistory;
218    history->localPredTaken = local_prediction;
219    history->globalPredTaken = global_prediction;
220    history->globalUsed = choice_prediction;
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(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->localHistory = invalidPredictorIndex;
261    bp_history = static_cast<void *>(history);
262
263    updateGlobalHistTaken();
264}
265
266void
267TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
268                     bool squashed)
269{
270    unsigned local_history_idx;
271    unsigned local_predictor_idx M5_VAR_USED;
272    unsigned local_predictor_hist;
273
274    // Get the local predictor's current prediction
275    local_history_idx = calcLocHistIdx(branch_addr);
276    local_predictor_hist = localHistoryTable[local_history_idx];
277    local_predictor_idx = local_predictor_hist & localPredictorMask;
278
279    if (bp_history) {
280        BPHistory *history = static_cast<BPHistory *>(bp_history);
281        // Update may also be called if the Branch target is incorrect even if
282        // the prediction is correct. In that case do not update the counters.
283        bool historyPred = false;
284        unsigned old_local_pred_index = history->localHistory &
285                localPredictorMask;
286
287        bool old_local_pred_valid = history->localHistory !=
288            invalidPredictorIndex;
289
290        assert(old_local_pred_index < localPredictorSize);
291
292        if (history->globalUsed) {
293           historyPred = history->globalPredTaken;
294        } else {
295           historyPred = history->localPredTaken;
296        }
297        if (historyPred != taken || !squashed) {
298            // Update the choice predictor to tell it which one was correct if
299            // there was a prediction.
300            if (history->localPredTaken != history->globalPredTaken) {
301                 // If the local prediction matches the actual outcome,
302                 // decerement the counter.  Otherwise increment the
303                 // counter.
304                 unsigned choice_predictor_idx =
305                   history->globalHistory & choiceHistoryMask;
306                 if (history->localPredTaken == taken) {
307                     choiceCtrs[choice_predictor_idx].decrement();
308                 } else if (history->globalPredTaken == taken) {
309                     choiceCtrs[choice_predictor_idx].increment();
310                 }
311
312             }
313
314             // Update the counters and local history with the proper
315             // resolution of the branch.  Global history is updated
316             // speculatively and restored upon squash() calls, so it does not
317             // need to be updated.
318             unsigned global_predictor_idx =
319               history->globalHistory & globalHistoryMask;
320             if (taken) {
321                  globalCtrs[global_predictor_idx].increment();
322                  if (old_local_pred_valid) {
323                          localCtrs[old_local_pred_index].increment();
324                  }
325             } else {
326                  globalCtrs[global_predictor_idx].decrement();
327                  if (old_local_pred_valid) {
328                          localCtrs[old_local_pred_index].decrement();
329                  }
330             }
331        }
332        if (squashed) {
333             if (taken) {
334                globalHistory = (history->globalHistory << 1) | 1;
335                globalHistory = globalHistory & historyRegisterMask;
336                if (old_local_pred_valid) {
337                    localHistoryTable[local_history_idx] =
338                     (history->localHistory << 1) | 1;
339                }
340             } else {
341                globalHistory = (history->globalHistory << 1);
342                globalHistory = globalHistory & historyRegisterMask;
343                if (old_local_pred_valid) {
344                     localHistoryTable[local_history_idx] =
345                     history->localHistory << 1;
346                }
347             }
348
349        } else {
350            // We're done with this history, now delete it.
351            delete history;
352        }
353    }
354
355    assert(local_history_idx < localHistoryTableSize);
356
357
358}
359
360void
361TournamentBP::retireSquashed(void *bp_history)
362{
363    BPHistory *history = static_cast<BPHistory *>(bp_history);
364    delete history;
365}
366
367void
368TournamentBP::squash(void *bp_history)
369{
370    BPHistory *history = static_cast<BPHistory *>(bp_history);
371
372    // Restore global history to state prior to this branch.
373    globalHistory = history->globalHistory;
374
375    // Delete this BPHistory now that we're done with it.
376    delete history;
377}
378
379#ifdef DEBUG
380int
381TournamentBP::BPHistory::newCount = 0;
382#endif
383