tournament.cc revision 13626 
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 "cpu/pred/tournament.hh"
44
45#include "base/bitfield.hh"
46#include "base/intmath.hh"
47
48TournamentBP::TournamentBP(const TournamentBPParams *params)
49    : BPredUnit(params),
50      localPredictorSize(params->localPredictorSize),
51      localCtrBits(params->localCtrBits),
52      localHistoryTableSize(params->localHistoryTableSize),
53      localHistoryBits(ceilLog2(params->localPredictorSize)),
54      globalPredictorSize(params->globalPredictorSize),
55      globalCtrBits(params->globalCtrBits),
56      globalHistory(params->numThreads, 0),
57      globalHistoryBits(
58          ceilLog2(params->globalPredictorSize) >
59          ceilLog2(params->choicePredictorSize) ?
60          ceilLog2(params->globalPredictorSize) :
61          ceilLog2(params->choicePredictorSize)),
62      choicePredictorSize(params->choicePredictorSize),
63      choiceCtrBits(params->choiceCtrBits)
64{
65    if (!isPowerOf2(localPredictorSize)) {
66        fatal("Invalid local predictor size!\n");
67    }
68
69    if (!isPowerOf2(globalPredictorSize)) {
70        fatal("Invalid global predictor size!\n");
71    }
72
73    //Set up the array of counters for the local predictor
74    localCtrs.resize(localPredictorSize);
75
76    for (int i = 0; i < localPredictorSize; ++i)
77        localCtrs[i].setBits(localCtrBits);
78
79    localPredictorMask = mask(localHistoryBits);
80
81    if (!isPowerOf2(localHistoryTableSize)) {
82        fatal("Invalid local history table size!\n");
83    }
84
85    //Setup the history table for the local table
86    localHistoryTable.resize(localHistoryTableSize);
87
88    for (int i = 0; i < localHistoryTableSize; ++i)
89        localHistoryTable[i] = 0;
90
91    //Setup the array of counters for the global predictor
92    globalCtrs.resize(globalPredictorSize);
93
94    for (int i = 0; i < globalPredictorSize; ++i)
95        globalCtrs[i].setBits(globalCtrBits);
96
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(ThreadID tid)
149{
150    globalHistory[tid] = (globalHistory[tid] << 1) | 1;
151    globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
152}
153
154inline
155void
156TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
157{
158    globalHistory[tid] = (globalHistory[tid] << 1);
159    globalHistory[tid] = globalHistory[tid] & 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(ThreadID tid, 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[tid] &= (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(ThreadID tid, 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 = globalThreshold <
208      globalCtrs[globalHistory[tid] & globalHistoryMask].read();
209
210    //Lookup in the choice predictor to see which one to use
211    choice_prediction = choiceThreshold <
212      choiceCtrs[globalHistory[tid] & choiceHistoryMask].read();
213
214    // Create BPHistory and pass it back to be recorded.
215    BPHistory *history = new BPHistory;
216    history->globalHistory = globalHistory[tid];
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    // Speculative update of the global history and the
227    // selected local history.
228    if (choice_prediction) {
229        if (global_prediction) {
230            updateGlobalHistTaken(tid);
231            updateLocalHistTaken(local_history_idx);
232            return true;
233        } else {
234            updateGlobalHistNotTaken(tid);
235            updateLocalHistNotTaken(local_history_idx);
236            return false;
237        }
238    } else {
239        if (local_prediction) {
240            updateGlobalHistTaken(tid);
241            updateLocalHistTaken(local_history_idx);
242            return true;
243        } else {
244            updateGlobalHistNotTaken(tid);
245            updateLocalHistNotTaken(local_history_idx);
246            return false;
247        }
248    }
249}
250
251void
252TournamentBP::uncondBranch(ThreadID tid, 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[tid];
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(tid);
265}
266
267void
268TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
269                     void *bp_history, bool squashed,
270                     const StaticInstPtr & inst, Addr corrTarget)
271{
272    assert(bp_history);
273
274    BPHistory *history = static_cast<BPHistory *>(bp_history);
275
276    unsigned local_history_idx = calcLocHistIdx(branch_addr);
277
278    assert(local_history_idx < localHistoryTableSize);
279
280    // Unconditional branches do not use local history.
281    bool old_local_pred_valid = history->localHistory !=
282            invalidPredictorIndex;
283
284    // If this is a misprediction, restore the speculatively
285    // updated state (global history register and local history)
286    // and update again.
287    if (squashed) {
288        // Global history restore and update
289        globalHistory[tid] = (history->globalHistory << 1) | taken;
290        globalHistory[tid] &= historyRegisterMask;
291
292        // Local history restore and update.
293        if (old_local_pred_valid) {
294            localHistoryTable[local_history_idx] =
295                        (history->localHistory << 1) | taken;
296        }
297
298        return;
299    }
300
301    unsigned old_local_pred_index = history->localHistory &
302        localPredictorMask;
303
304    assert(old_local_pred_index < localPredictorSize);
305
306    // Update the choice predictor to tell it which one was correct if
307    // there was a prediction.
308    if (history->localPredTaken != history->globalPredTaken &&
309        old_local_pred_valid)
310    {
311         // If the local prediction matches the actual outcome,
312         // decrement the counter. Otherwise increment the
313         // counter.
314         unsigned choice_predictor_idx =
315           history->globalHistory & choiceHistoryMask;
316         if (history->localPredTaken == taken) {
317             choiceCtrs[choice_predictor_idx].decrement();
318         } else if (history->globalPredTaken == taken) {
319             choiceCtrs[choice_predictor_idx].increment();
320         }
321    }
322
323    // Update the counters with the proper
324    // resolution of the branch. Histories are updated
325    // speculatively, restored upon squash() calls, and
326    // recomputed upon update(squash = true) calls,
327    // so they do not need to be updated.
328    unsigned global_predictor_idx =
329            history->globalHistory & globalHistoryMask;
330    if (taken) {
331          globalCtrs[global_predictor_idx].increment();
332          if (old_local_pred_valid) {
333                 localCtrs[old_local_pred_index].increment();
334          }
335    } else {
336          globalCtrs[global_predictor_idx].decrement();
337          if (old_local_pred_valid) {
338              localCtrs[old_local_pred_index].decrement();
339          }
340    }
341
342    // We're done with this history, now delete it.
343    delete history;
344}
345
346void
347TournamentBP::squash(ThreadID tid, void *bp_history)
348{
349    BPHistory *history = static_cast<BPHistory *>(bp_history);
350
351    // Restore global history to state prior to this branch.
352    globalHistory[tid] = history->globalHistory;
353
354    // Restore local history
355    if (history->localHistoryIdx != invalidPredictorIndex) {
356        localHistoryTable[history->localHistoryIdx] = history->localHistory;
357    }
358
359    // Delete this BPHistory now that we're done with it.
360    delete history;
361}
362
363TournamentBP*
364TournamentBPParams::create()
365{
366    return new TournamentBP(this);
367}
368
369unsigned
370TournamentBP::getGHR(ThreadID tid, void *bp_history) const
371{
372    return static_cast<BPHistory *>(bp_history)->globalHistory;
373}
374
375#ifdef DEBUG
376int
377TournamentBP::BPHistory::newCount = 0;
378#endif
379