loop_predictor.cc revision 14081 
1/*
2 * Copyright (c) 2014 The University of Wisconsin
3 *
4 * Copyright (c) 2006 INRIA (Institut National de Recherche en
5 * Informatique et en Automatique  / French National Research Institute
6 * for Computer Science and Applied Mathematics)
7 *
8 * All rights reserved.
9 *
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions are
12 * met: redistributions of source code must retain the above copyright
13 * notice, this list of conditions and the following disclaimer;
14 * redistributions in binary form must reproduce the above copyright
15 * notice, this list of conditions and the following disclaimer in the
16 * documentation and/or other materials provided with the distribution;
17 * neither the name of the copyright holders nor the names of its
18 * contributors may be used to endorse or promote products derived from
19 * this software without specific prior written permission.
20 *
21 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
22 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
23 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
24 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
25 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
26 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
27 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
28 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
29 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
30 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
31 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
32 *
33 * Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
34 * from André Seznec's code.
35 */
36
37#include "cpu/pred/loop_predictor.hh"
38
39#include "base/random.hh"
40#include "debug/LTage.hh"
41#include "params/LoopPredictor.hh"
42
43LoopPredictor::LoopPredictor(LoopPredictorParams *p)
44  : SimObject(p), logSizeLoopPred(p->logSizeLoopPred),
45    loopTableAgeBits(p->loopTableAgeBits),
46    loopTableConfidenceBits(p->loopTableConfidenceBits),
47    loopTableTagBits(p->loopTableTagBits),
48    loopTableIterBits(p->loopTableIterBits),
49    logLoopTableAssoc(p->logLoopTableAssoc),
50    confidenceThreshold((1 << loopTableConfidenceBits) - 1),
51    loopTagMask((1 << loopTableTagBits) - 1),
52    loopNumIterMask((1 << loopTableIterBits) - 1),
53    loopSetMask((1 << (logSizeLoopPred - logLoopTableAssoc)) - 1),
54    loopUseCounter(-1),
55    withLoopBits(p->withLoopBits),
56    useDirectionBit(p->useDirectionBit),
57    useSpeculation(p->useSpeculation),
58    useHashing(p->useHashing),
59    restrictAllocation(p->restrictAllocation),
60    initialLoopIter(p->initialLoopIter),
61    initialLoopAge(p->initialLoopAge),
62    optionalAgeReset(p->optionalAgeReset)
63{
64    assert(initialLoopAge <= ((1 << loopTableAgeBits) - 1));
65}
66
67void
68LoopPredictor::init()
69{
70    // we use uint16_t type for these vales, so they cannot be more than
71    // 16 bits
72    assert(loopTableTagBits <= 16);
73    assert(loopTableIterBits <= 16);
74
75    assert(logSizeLoopPred >= logLoopTableAssoc);
76
77    ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
78}
79
80LoopPredictor::BranchInfo*
81LoopPredictor::makeBranchInfo()
82{
83    return new BranchInfo();
84}
85
86int
87LoopPredictor::lindex(Addr pc_in, unsigned instShiftAmt) const
88{
89    // The loop table is implemented as a linear table
90    // If associativity is N (N being 1 << logLoopTableAssoc),
91    // the first N entries are for set 0, the next N entries are for set 1,
92    // and so on.
93    // Thus, this function calculates the set and then it gets left shifted
94    // by logLoopTableAssoc in order to return the index of the first of the
95    // N entries of the set
96    Addr pc = pc_in >> instShiftAmt;
97    if (useHashing) {
98        pc ^= pc_in;
99    }
100    return ((pc & loopSetMask) << logLoopTableAssoc);
101}
102
103int
104LoopPredictor::finallindex(int index, int lowPcBits, int way) const
105{
106    return (useHashing ? (index ^ ((lowPcBits >> way) << logLoopTableAssoc)) :
107                         (index))
108           + way;
109}
110
111//loop prediction: only used if high confidence
112bool
113LoopPredictor::getLoop(Addr pc, BranchInfo* bi, bool speculative,
114                       unsigned instShiftAmt) const
115{
116    bi->loopHit = -1;
117    bi->loopPredValid = false;
118    bi->loopIndex = lindex(pc, instShiftAmt);
119
120    if (useHashing) {
121        unsigned pcShift = logSizeLoopPred - logLoopTableAssoc;
122        bi->loopIndexB = (pc >> pcShift) & loopSetMask;
123        bi->loopTag = (pc >> pcShift) ^ (pc >> (pcShift + loopTableTagBits));
124        bi->loopTag &= loopTagMask;
125    } else {
126        unsigned pcShift = instShiftAmt + logSizeLoopPred - logLoopTableAssoc;
127        bi->loopTag = (pc >> pcShift) & loopTagMask;
128        // bi->loopIndexB is not used without hash
129    }
130
131    for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
132        int idx = finallindex(bi->loopIndex, bi->loopIndexB, i);
133        if (ltable[idx].tag == bi->loopTag) {
134            bi->loopHit = i;
135            bi->loopPredValid = calcConf(idx);
136
137            uint16_t iter = speculative ? ltable[idx].currentIterSpec
138                                        : ltable[idx].currentIter;
139
140            if ((iter + 1) == ltable[idx].numIter) {
141                return useDirectionBit ? !(ltable[idx].dir) : false;
142            } else {
143                return useDirectionBit ? (ltable[idx].dir) : true;
144            }
145        }
146    }
147    return false;
148}
149
150bool
151LoopPredictor::calcConf(int index) const
152{
153    return ltable[index].confidence == confidenceThreshold;
154}
155
156void
157LoopPredictor::specLoopUpdate(bool taken, BranchInfo* bi)
158{
159    if (bi->loopHit>=0) {
160        int index = finallindex(bi->loopIndex, bi->loopIndexB, bi->loopHit);
161        if (taken != ltable[index].dir) {
162            ltable[index].currentIterSpec = 0;
163        } else {
164            ltable[index].currentIterSpec =
165                (ltable[index].currentIterSpec + 1) & loopNumIterMask;
166        }
167    }
168}
169
170bool
171LoopPredictor::optionalAgeInc() const
172{
173    return false;
174}
175
176void
177LoopPredictor::loopUpdate(Addr pc, bool taken, BranchInfo* bi, bool tage_pred)
178{
179    int idx = finallindex(bi->loopIndex, bi->loopIndexB, bi->loopHit);
180    if (bi->loopHit >= 0) {
181        //already a hit
182        if (bi->loopPredValid) {
183            if (taken != bi->loopPred) {
184                // free the entry
185                ltable[idx].numIter = 0;
186                ltable[idx].age = 0;
187                ltable[idx].confidence = 0;
188                ltable[idx].currentIter = 0;
189                return;
190            } else if (bi->loopPred != tage_pred || optionalAgeInc()) {
191                DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
192                unsignedCtrUpdate(ltable[idx].age, true, loopTableAgeBits);
193            }
194        }
195
196        ltable[idx].currentIter =
197            (ltable[idx].currentIter + 1) & loopNumIterMask;
198        if (ltable[idx].currentIter > ltable[idx].numIter) {
199            ltable[idx].confidence = 0;
200            if (ltable[idx].numIter != 0) {
201                // free the entry
202                ltable[idx].numIter = 0;
203                if (optionalAgeReset) {
204                    ltable[idx].age = 0;
205                }
206            }
207        }
208
209        if (taken != (useDirectionBit ? ltable[idx].dir : true)) {
210            if (ltable[idx].currentIter == ltable[idx].numIter) {
211                DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
212                unsignedCtrUpdate(ltable[idx].confidence, true,
213                                  loopTableConfidenceBits);
214                //just do not predict when the loop count is 1 or 2
215                if (ltable[idx].numIter < 3) {
216                    // free the entry
217                    ltable[idx].dir = taken; // ignored if no useDirectionBit
218                    ltable[idx].numIter = 0;
219                    ltable[idx].age = 0;
220                    ltable[idx].confidence = 0;
221                }
222            } else {
223                DPRINTF(LTage, "Loop End predicted incorrectly:%lx\n", pc);
224                if (ltable[idx].numIter == 0) {
225                    // first complete nest;
226                    ltable[idx].confidence = 0;
227                    ltable[idx].numIter = ltable[idx].currentIter;
228                } else {
229                    //not the same number of iterations as last time: free the
230                    //entry
231                    ltable[idx].numIter = 0;
232                    if (optionalAgeReset) {
233                        ltable[idx].age = 0;
234                    }
235                    ltable[idx].confidence = 0;
236                }
237            }
238            ltable[idx].currentIter = 0;
239        }
240
241    } else if (useDirectionBit ? (bi->predTaken != taken) : taken) {
242        if ((random_mt.random<int>() & 3) == 0 || !restrictAllocation) {
243            //try to allocate an entry on taken branch
244            int nrand = random_mt.random<int>();
245            for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
246                int loop_hit = (nrand + i) & ((1 << logLoopTableAssoc) - 1);
247                idx = finallindex(bi->loopIndex, bi->loopIndexB, loop_hit);
248                if (ltable[idx].age == 0) {
249                    DPRINTF(LTage,
250                            "Allocating loop pred entry for branch %lx\n",
251                            pc);
252                    ltable[idx].dir = !taken; // ignored if no useDirectionBit
253                    ltable[idx].tag = bi->loopTag;
254                    ltable[idx].numIter = 0;
255                    ltable[idx].age = initialLoopAge;
256                    ltable[idx].confidence = 0;
257                    ltable[idx].currentIter = initialLoopIter;
258                    break;
259
260                } else {
261                    ltable[idx].age--;
262                }
263                if (restrictAllocation) {
264                    break;
265                }
266            }
267        }
268    }
269}
270
271bool
272LoopPredictor::loopPredict(ThreadID tid, Addr branch_pc, bool cond_branch,
273                   BranchInfo* bi, bool prev_pred_taken, unsigned instShiftAmt)
274{
275    bool pred_taken = prev_pred_taken;
276    if (cond_branch) {
277        // loop prediction
278        bi->loopPred = getLoop(branch_pc, bi, useSpeculation, instShiftAmt);
279
280        if ((loopUseCounter >= 0) && bi->loopPredValid) {
281            pred_taken = bi->loopPred;
282            bi->loopPredUsed = true;
283        }
284
285        if (useSpeculation) {
286            specLoopUpdate(pred_taken, bi);
287        }
288    }
289
290    return pred_taken;
291}
292
293void
294LoopPredictor::squash(ThreadID tid, BranchInfo *bi)
295{
296    if (bi->loopHit >= 0) {
297        int idx = finallindex(bi->loopIndex,
298                bi->loopIndexB,
299                bi->loopHit);
300        ltable[idx].currentIterSpec = bi->currentIter;
301    }
302}
303
304void
305LoopPredictor::squashLoop(BranchInfo* bi)
306{
307    if (bi->loopHit >= 0) {
308        int idx = finallindex(bi->loopIndex,
309                bi->loopIndexB,
310                bi->loopHit);
311        ltable[idx].currentIterSpec = bi->currentIter;
312    }
313}
314
315void
316LoopPredictor::updateStats(bool taken, BranchInfo* bi)
317{
318    if (taken == bi->loopPred) {
319        loopPredictorCorrect++;
320    } else {
321        loopPredictorWrong++;
322    }
323}
324
325void
326LoopPredictor::condBranchUpdate(ThreadID tid, Addr branch_pc, bool taken,
327                                bool tage_pred, BranchInfo* bi,
328                                unsigned instShiftAmt)
329{
330    if (useSpeculation) {
331        // recalculate loop prediction without speculation
332        // It is ok to overwrite the loop prediction fields in bi
333        // as the stats have already been updated with the previous
334        // values
335        bi->loopPred = getLoop(branch_pc, bi, false, instShiftAmt);
336    }
337
338    if (bi->loopPredValid) {
339        if (bi->predTaken != bi->loopPred) {
340            signedCtrUpdate(loopUseCounter,
341                      (bi->loopPred == taken),
342                      withLoopBits);
343        }
344    }
345
346    loopUpdate(branch_pc, taken, bi, tage_pred);
347}
348
349void
350LoopPredictor::regStats()
351{
352    loopPredictorCorrect
353        .name(name() + ".loopPredictorCorrect")
354        .desc("Number of times the loop predictor is the provider and "
355              "the prediction is correct");
356
357    loopPredictorWrong
358        .name(name() + ".loopPredictorWrong")
359        .desc("Number of times the loop predictor is the provider and "
360              "the prediction is wrong");
361}
362
363size_t
364LoopPredictor::getSizeInBits() const
365{
366    return (1ULL << logSizeLoopPred) *
367        ((useSpeculation ? 3 : 2) * loopTableIterBits +
368        loopTableConfidenceBits + loopTableTagBits +
369        loopTableAgeBits + useDirectionBit);
370}
371
372LoopPredictor *
373LoopPredictorParams::create()
374{
375    return new LoopPredictor(this);
376}
377