tournament.cc revision 11783:f94c14fd6561 
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      globalHistory(params->numThreads, 0),
56      globalHistoryBits(
57          ceilLog2(params->globalPredictorSize) >
58          ceilLog2(params->choicePredictorSize) ?
59          ceilLog2(params->globalPredictorSize) :
60          ceilLog2(params->choicePredictorSize)),
61      choicePredictorSize(params->choicePredictorSize),
62      choiceCtrBits(params->choiceCtrBits)
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    // Set up the global history mask
97    // this is equivalent to mask(log2(globalPredictorSize)
98    globalHistoryMask = globalPredictorSize - 1;
99
100    if (!isPowerOf2(choicePredictorSize)) {
101        fatal("Invalid choice predictor size!\n");
102    }
103
104    // Set up choiceHistoryMask
105    // this is equivalent to mask(log2(choicePredictorSize)
106    choiceHistoryMask = choicePredictorSize - 1;
107
108    //Setup the array of counters for the choice predictor
109    choiceCtrs.resize(choicePredictorSize);
110
111    for (int i = 0; i < choicePredictorSize; ++i)
112        choiceCtrs[i].setBits(choiceCtrBits);
113
114    //Set up historyRegisterMask
115    historyRegisterMask = mask(globalHistoryBits);
116
117    //Check that predictors don't use more bits than they have available
118    if (globalHistoryMask > historyRegisterMask) {
119        fatal("Global predictor too large for global history bits!\n");
120    }
121    if (choiceHistoryMask > historyRegisterMask) {
122        fatal("Choice predictor too large for global history bits!\n");
123    }
124
125    if (globalHistoryMask < historyRegisterMask &&
126        choiceHistoryMask < historyRegisterMask) {
127        inform("More global history bits than required by predictors\n");
128    }
129
130    // Set thresholds for the three predictors' counters
131    // This is equivalent to (2^(Ctr))/2 - 1
132    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
133    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
134    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
135}
136
137inline
138unsigned
139TournamentBP::calcLocHistIdx(Addr &branch_addr)
140{
141    // Get low order bits after removing instruction offset.
142    return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
143}
144
145inline
146void
147TournamentBP::updateGlobalHistTaken(ThreadID tid)
148{
149    globalHistory[tid] = (globalHistory[tid] << 1) | 1;
150    globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
151}
152
153inline
154void
155TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
156{
157    globalHistory[tid] = (globalHistory[tid] << 1);
158    globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
159}
160
161inline
162void
163TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
164{
165    localHistoryTable[local_history_idx] =
166        (localHistoryTable[local_history_idx] << 1) | 1;
167}
168
169inline
170void
171TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
172{
173    localHistoryTable[local_history_idx] =
174        (localHistoryTable[local_history_idx] << 1);
175}
176
177
178void
179TournamentBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
180{
181    unsigned local_history_idx = calcLocHistIdx(branch_addr);
182    //Update Global History to Not Taken (clear LSB)
183    globalHistory[tid] &= (historyRegisterMask & ~ULL(1));
184    //Update Local History to Not Taken
185    localHistoryTable[local_history_idx] =
186       localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
187}
188
189bool
190TournamentBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
191{
192    bool local_prediction;
193    unsigned local_history_idx;
194    unsigned local_predictor_idx;
195
196    bool global_prediction;
197    bool choice_prediction;
198
199    //Lookup in the local predictor to get its branch prediction
200    local_history_idx = calcLocHistIdx(branch_addr);
201    local_predictor_idx = localHistoryTable[local_history_idx]
202        & localPredictorMask;
203    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
204
205    //Lookup in the global predictor to get its branch prediction
206    global_prediction = globalThreshold <
207      globalCtrs[globalHistory[tid] & globalHistoryMask].read();
208
209    //Lookup in the choice predictor to see which one to use
210    choice_prediction = choiceThreshold <
211      choiceCtrs[globalHistory[tid] & choiceHistoryMask].read();
212
213    // Create BPHistory and pass it back to be recorded.
214    BPHistory *history = new BPHistory;
215    history->globalHistory = globalHistory[tid];
216    history->localPredTaken = local_prediction;
217    history->globalPredTaken = global_prediction;
218    history->globalUsed = choice_prediction;
219    history->localHistoryIdx = local_history_idx;
220    history->localHistory = local_predictor_idx;
221    bp_history = (void *)history;
222
223    assert(local_history_idx < localHistoryTableSize);
224
225    // Speculative update of the global history and the
226    // selected local history.
227    if (choice_prediction) {
228        if (global_prediction) {
229            updateGlobalHistTaken(tid);
230            updateLocalHistTaken(local_history_idx);
231            return true;
232        } else {
233            updateGlobalHistNotTaken(tid);
234            updateLocalHistNotTaken(local_history_idx);
235            return false;
236        }
237    } else {
238        if (local_prediction) {
239            updateGlobalHistTaken(tid);
240            updateLocalHistTaken(local_history_idx);
241            return true;
242        } else {
243            updateGlobalHistNotTaken(tid);
244            updateLocalHistNotTaken(local_history_idx);
245            return false;
246        }
247    }
248}
249
250void
251TournamentBP::uncondBranch(ThreadID tid, Addr pc, void * &bp_history)
252{
253    // Create BPHistory and pass it back to be recorded.
254    BPHistory *history = new BPHistory;
255    history->globalHistory = globalHistory[tid];
256    history->localPredTaken = true;
257    history->globalPredTaken = true;
258    history->globalUsed = true;
259    history->localHistoryIdx = invalidPredictorIndex;
260    history->localHistory = invalidPredictorIndex;
261    bp_history = static_cast<void *>(history);
262
263    updateGlobalHistTaken(tid);
264}
265
266void
267TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
268                     void *bp_history, bool squashed)
269{
270    assert(bp_history);
271
272    BPHistory *history = static_cast<BPHistory *>(bp_history);
273
274    unsigned local_history_idx = calcLocHistIdx(branch_addr);
275
276    assert(local_history_idx < localHistoryTableSize);
277
278    // Unconditional branches do not use local history.
279    bool old_local_pred_valid = history->localHistory !=
280            invalidPredictorIndex;
281
282    // If this is a misprediction, restore the speculatively
283    // updated state (global history register and local history)
284    // and update again.
285    if (squashed) {
286        // Global history restore and update
287        globalHistory[tid] = (history->globalHistory << 1) | taken;
288        globalHistory[tid] &= historyRegisterMask;
289
290        // Local history restore and update.
291        if (old_local_pred_valid) {
292            localHistoryTable[local_history_idx] =
293                        (history->localHistory << 1) | taken;
294        }
295
296        return;
297    }
298
299    unsigned old_local_pred_index = history->localHistory &
300        localPredictorMask;
301
302    assert(old_local_pred_index < localPredictorSize);
303
304    // Update the choice predictor to tell it which one was correct if
305    // there was a prediction.
306    if (history->localPredTaken != history->globalPredTaken &&
307        old_local_pred_valid)
308    {
309         // If the local prediction matches the actual outcome,
310         // decrement the counter. Otherwise increment the
311         // counter.
312         unsigned choice_predictor_idx =
313           history->globalHistory & choiceHistoryMask;
314         if (history->localPredTaken == taken) {
315             choiceCtrs[choice_predictor_idx].decrement();
316         } else if (history->globalPredTaken == taken) {
317             choiceCtrs[choice_predictor_idx].increment();
318         }
319    }
320
321    // Update the counters with the proper
322    // resolution of the branch. Histories are updated
323    // speculatively, restored upon squash() calls, and
324    // recomputed upon update(squash = true) calls,
325    // so they do not need to be updated.
326    unsigned global_predictor_idx =
327            history->globalHistory & globalHistoryMask;
328    if (taken) {
329          globalCtrs[global_predictor_idx].increment();
330          if (old_local_pred_valid) {
331                 localCtrs[old_local_pred_index].increment();
332          }
333    } else {
334          globalCtrs[global_predictor_idx].decrement();
335          if (old_local_pred_valid) {
336              localCtrs[old_local_pred_index].decrement();
337          }
338    }
339
340    // We're done with this history, now delete it.
341    delete history;
342}
343
344void
345TournamentBP::squash(ThreadID tid, void *bp_history)
346{
347    BPHistory *history = static_cast<BPHistory *>(bp_history);
348
349    // Restore global history to state prior to this branch.
350    globalHistory[tid] = history->globalHistory;
351
352    // Restore local history
353    if (history->localHistoryIdx != invalidPredictorIndex) {
354        localHistoryTable[history->localHistoryIdx] = history->localHistory;
355    }
356
357    // Delete this BPHistory now that we're done with it.
358    delete history;
359}
360
361TournamentBP*
362TournamentBPParams::create()
363{
364    return new TournamentBP(this);
365}
366
367unsigned
368TournamentBP::getGHR(ThreadID tid, void *bp_history) const
369{
370    return static_cast<BPHistory *>(bp_history)->globalHistory;
371}
372
373#ifdef DEBUG
374int
375TournamentBP::BPHistory::newCount = 0;
376#endif
377