ltage.cc revision 13493 
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/* @file
38 * Implementation of a L-TAGE branch predictor
39 */
40
41#include "cpu/pred/ltage.hh"
42
43#include "base/intmath.hh"
44#include "base/logging.hh"
45#include "base/random.hh"
46#include "base/trace.hh"
47#include "debug/Fetch.hh"
48#include "debug/LTage.hh"
49
50LTAGE::LTAGE(const LTAGEParams *params)
51  : TAGE(params),
52    logSizeLoopPred(params->logSizeLoopPred),
53    loopTableAgeBits(params->loopTableAgeBits),
54    loopTableConfidenceBits(params->loopTableConfidenceBits),
55    loopTableTagBits(params->loopTableTagBits),
56    loopTableIterBits(params->loopTableIterBits),
57    logLoopTableAssoc(params->logLoopTableAssoc),
58    confidenceThreshold((1 << loopTableConfidenceBits) - 1),
59    loopTagMask((1 << loopTableTagBits) - 1),
60    loopNumIterMask((1 << loopTableIterBits) - 1),
61    loopSetMask((1 << (logSizeLoopPred - logLoopTableAssoc)) - 1),
62    loopUseCounter(0),
63    withLoopBits(params->withLoopBits),
64    useDirectionBit(params->useDirectionBit),
65    useSpeculation(params->useSpeculation),
66    useHashing(params->useHashing)
67{
68    // we use uint16_t type for these vales, so they cannot be more than
69    // 16 bits
70    assert(loopTableTagBits <= 16);
71    assert(loopTableIterBits <= 16);
72
73    assert(logSizeLoopPred >= logLoopTableAssoc);
74
75    ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
76}
77
78int
79LTAGE::lindex(Addr pc_in) const
80{
81    // The loop table is implemented as a linear table
82    // If associativity is N (N being 1 << logLoopTableAssoc),
83    // the first N entries are for set 0, the next N entries are for set 1,
84    // and so on.
85    // Thus, this function calculates the set and then it gets left shifted
86    // by logLoopTableAssoc in order to return the index of the first of the
87    // N entries of the set
88    Addr mask = (ULL(1) << (logSizeLoopPred - logLoopTableAssoc)) - 1;
89    Addr pc = pc_in >> instShiftAmt;
90    if (useHashing) {
91        // copied from TAGE-SC-L
92        // (http://www.jilp.org/cbp2016/code/AndreSeznecLimited.tar.gz)
93        pc ^= (pc_in >> (instShiftAmt + logLoopTableAssoc));
94    }
95    return ((pc & mask) << logLoopTableAssoc);
96}
97
98int
99LTAGE::finallindex(int index, int lowPcBits, int way) const
100{
101    // copied from TAGE-SC-L
102    // (http://www.jilp.org/cbp2016/code/AndreSeznecLimited.tar.gz)
103    return (useHashing ? (index ^ ((lowPcBits >> way) << logLoopTableAssoc)) :
104                         (index))
105           + way;
106}
107
108//loop prediction: only used if high confidence
109bool
110LTAGE::getLoop(Addr pc, LTageBranchInfo* bi, bool speculative) const
111{
112    bi->loopHit = -1;
113    bi->loopPredValid = false;
114    bi->loopIndex = lindex(pc);
115    unsigned pcShift = instShiftAmt + logSizeLoopPred - logLoopTableAssoc;
116    bi->loopTag = ((pc) >> pcShift) & loopTagMask;
117
118    if (useHashing) {
119        bi->loopTag ^= ((pc >> (pcShift + logSizeLoopPred)) & loopTagMask);
120        bi->loopLowPcBits = (pc >> pcShift) & loopSetMask;
121    }
122
123    for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
124        int idx = finallindex(bi->loopIndex, bi->loopLowPcBits, i);
125        if (ltable[idx].tag == bi->loopTag) {
126            bi->loopHit = i;
127            bi->loopPredValid =
128                ltable[idx].confidence == confidenceThreshold;
129
130            uint16_t iter = speculative ? ltable[idx].currentIterSpec
131                                        : ltable[idx].currentIter;
132
133            if ((iter + 1) == ltable[idx].numIter) {
134                return useDirectionBit ? !(ltable[idx].dir) : false;
135            } else {
136                return useDirectionBit ? (ltable[idx].dir) : true;
137            }
138        }
139    }
140    return false;
141}
142
143void
144LTAGE::specLoopUpdate(bool taken, LTageBranchInfo* bi)
145{
146    if (bi->loopHit>=0) {
147        int index = finallindex(bi->loopIndex, bi->loopLowPcBits, bi->loopHit);
148        if (taken != ltable[index].dir) {
149            ltable[index].currentIterSpec = 0;
150        } else {
151            ltable[index].currentIterSpec =
152                (ltable[index].currentIterSpec + 1) & loopNumIterMask;
153        }
154    }
155}
156
157void
158LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
159{
160    int idx = finallindex(bi->loopIndex, bi->loopLowPcBits, bi->loopHit);
161    if (bi->loopHit >= 0) {
162        //already a hit
163        if (bi->loopPredValid) {
164            if (taken != bi->loopPred) {
165                // free the entry
166                ltable[idx].numIter = 0;
167                ltable[idx].age = 0;
168                ltable[idx].confidence = 0;
169                ltable[idx].currentIter = 0;
170                return;
171            } else if (bi->loopPred != bi->tagePred) {
172                DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
173                unsignedCtrUpdate(ltable[idx].age, true, loopTableAgeBits);
174            }
175        }
176
177        ltable[idx].currentIter =
178            (ltable[idx].currentIter + 1) & loopNumIterMask;
179        if (ltable[idx].currentIter > ltable[idx].numIter) {
180            ltable[idx].confidence = 0;
181            if (ltable[idx].numIter != 0) {
182                // free the entry
183                ltable[idx].numIter = 0;
184                ltable[idx].age = 0;
185                ltable[idx].confidence = 0;
186            }
187        }
188
189        if (taken != (useDirectionBit ? ltable[idx].dir : true)) {
190            if (ltable[idx].currentIter == ltable[idx].numIter) {
191                DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
192
193                unsignedCtrUpdate(ltable[idx].confidence, true,
194                                  loopTableConfidenceBits);
195                //just do not predict when the loop count is 1 or 2
196                if (ltable[idx].numIter < 3) {
197                    // free the entry
198                    ltable[idx].dir = taken; // ignored if no useDirectionBit
199                    ltable[idx].numIter = 0;
200                    ltable[idx].age = 0;
201                    ltable[idx].confidence = 0;
202                }
203            } else {
204                DPRINTF(LTage, "Loop End predicted incorrectly:%lx\n", pc);
205                if (ltable[idx].numIter == 0) {
206                    // first complete nest;
207                    ltable[idx].confidence = 0;
208                    ltable[idx].numIter = ltable[idx].currentIter;
209                } else {
210                    //not the same number of iterations as last time: free the
211                    //entry
212                    ltable[idx].numIter = 0;
213                    ltable[idx].age = 0;
214                    ltable[idx].confidence = 0;
215                }
216            }
217            ltable[idx].currentIter = 0;
218        }
219
220    } else if (useDirectionBit ?
221                ((bi->loopPredValid ? bi->loopPred : bi->tagePred) != taken) :
222                taken) {
223        //try to allocate an entry on taken branch
224        int nrand = random_mt.random<int>();
225        for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
226            int loop_hit = (nrand + i) & ((1 << logLoopTableAssoc) - 1);
227            idx = bi->loopIndex + loop_hit;
228            if (ltable[idx].age == 0) {
229                DPRINTF(LTage, "Allocating loop pred entry for branch %lx\n",
230                        pc);
231                ltable[idx].dir = !taken; // ignored if no useDirectionBit
232                ltable[idx].tag = bi->loopTag;
233                ltable[idx].numIter = 0;
234                ltable[idx].age = (1 << loopTableAgeBits) - 1;
235                ltable[idx].confidence = 0;
236                ltable[idx].currentIter = 1;
237                break;
238
239            }
240            else
241                ltable[idx].age--;
242        }
243    }
244
245}
246
247//prediction
248bool
249LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
250{
251    LTageBranchInfo *bi = new LTageBranchInfo(nHistoryTables+1);
252    b = (void*)(bi);
253
254    bool pred_taken = tagePredict(tid, branch_pc, cond_branch, bi);
255
256    if (cond_branch) {
257        // loop prediction
258        bi->loopPred = getLoop(branch_pc, bi, useSpeculation);
259
260        if ((loopUseCounter >= 0) && bi->loopPredValid) {
261            pred_taken = bi->loopPred;
262            bi->provider = LOOP;
263        }
264        DPRINTF(LTage, "Predict for %lx: taken?:%d, loopTaken?:%d, "
265                "loopValid?:%d, loopUseCounter:%d, tagePred:%d, altPred:%d\n",
266                branch_pc, pred_taken, bi->loopPred, bi->loopPredValid,
267                loopUseCounter, bi->tagePred, bi->altTaken);
268
269        if (useSpeculation) {
270            specLoopUpdate(pred_taken, bi);
271        }
272    }
273
274    return pred_taken;
275}
276
277void
278LTAGE::condBranchUpdate(Addr branch_pc, bool taken,
279                        TageBranchInfo* tage_bi, int nrand)
280{
281    LTageBranchInfo* bi = static_cast<LTageBranchInfo*>(tage_bi);
282
283    if (useSpeculation) {
284        // recalculate loop prediction without speculation
285        // It is ok to overwrite the loop prediction fields in bi
286        // as the stats have already been updated with the previous
287        // values
288        bi->loopPred = getLoop(branch_pc, bi, false);
289    }
290
291    if (bi->loopPredValid) {
292        if (bi->tagePred != bi->loopPred) {
293            ctrUpdate(loopUseCounter,
294                      (bi->loopPred == taken),
295                      withLoopBits);
296        }
297    }
298
299    loopUpdate(branch_pc, taken, bi);
300
301    TAGE::condBranchUpdate(branch_pc, taken, bi, nrand);
302}
303
304void
305LTAGE::squash(ThreadID tid, bool taken, void *bp_history)
306{
307    TAGE::squash(tid, taken, bp_history);
308
309    LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
310
311    if (bi->condBranch) {
312        if (bi->loopHit >= 0) {
313            int idx = finallindex(bi->loopIndex,
314                                  bi->loopLowPcBits,
315                                  bi->loopHit);
316            ltable[idx].currentIterSpec = bi->currentIter;
317        }
318    }
319}
320
321void
322LTAGE::squash(ThreadID tid, void *bp_history)
323{
324    LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
325    if (bi->condBranch) {
326        if (bi->loopHit >= 0) {
327            int idx = finallindex(bi->loopIndex,
328                                  bi->loopLowPcBits,
329                                  bi->loopHit);
330            ltable[idx].currentIterSpec = bi->currentIter;
331        }
332    }
333
334    TAGE::squash(tid, bp_history);
335}
336
337
338void
339LTAGE::updateStats(bool taken, TageBranchInfo* bi)
340{
341    TAGE::updateStats(taken, bi);
342
343    LTageBranchInfo * ltage_bi = static_cast<LTageBranchInfo *>(bi);
344
345    if (ltage_bi->provider == LOOP) {
346        if (taken == ltage_bi->loopPred) {
347            loopPredictorCorrect++;
348        } else {
349            loopPredictorWrong++;
350        }
351    }
352}
353
354
355
356void
357LTAGE::regStats()
358{
359    TAGE::regStats();
360
361    loopPredictorCorrect
362        .name(name() + ".loopPredictorCorrect")
363        .desc("Number of times the loop predictor is the provider and "
364              "the prediction is correct");
365
366    loopPredictorWrong
367        .name(name() + ".loopPredictorWrong")
368        .desc("Number of times the loop predictor is the provier and "
369              "the prediction is wrong");
370}
371
372
373
374LTAGE*
375LTAGEParams::create()
376{
377    return new LTAGE(this);
378}
379