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 loopUseCounter(0),
62 withLoopBits(params->withLoopBits)
63{
64 // we use uint16_t type for these vales, so they cannot be more than
65 // 16 bits
66 assert(loopTableTagBits <= 16);
67 assert(loopTableIterBits <= 16);
68
69 assert(logSizeLoopPred >= logLoopTableAssoc);
70
71 ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
72}
73
74int
75LTAGE::lindex(Addr pc_in) const
76{
77 // The loop table is implemented as a linear table
78 // If associativity is N (N being 1 << logLoopTableAssoc),
79 // the first N entries are for set 0, the next N entries are for set 1,
80 // and so on.
81 // Thus, this function calculates the set and then it gets left shifted
82 // by logLoopTableAssoc in order to return the index of the first of the
83 // N entries of the set
84 Addr mask = (ULL(1) << (logSizeLoopPred - logLoopTableAssoc)) - 1;
85 return (((pc_in >> instShiftAmt) & mask) << logLoopTableAssoc);
86}
87
88//loop prediction: only used if high confidence
89bool
90LTAGE::getLoop(Addr pc, LTageBranchInfo* bi) const
91{
92 bi->loopHit = -1;
93 bi->loopPredValid = false;
94 bi->loopIndex = lindex(pc);
95 unsigned pcShift = instShiftAmt + logSizeLoopPred - logLoopTableAssoc;
96 bi->loopTag = ((pc) >> pcShift) & loopTagMask;
97
98 for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
99 if (ltable[bi->loopIndex + i].tag == bi->loopTag) {
100 bi->loopHit = i;
101 bi->loopPredValid =
102 ltable[bi->loopIndex + i].confidence == confidenceThreshold;
103 bi->currentIter = ltable[bi->loopIndex + i].currentIterSpec;
104 if (ltable[bi->loopIndex + i].currentIterSpec + 1 ==
105 ltable[bi->loopIndex + i].numIter) {
106 return !(ltable[bi->loopIndex + i].dir);
107 }else {
108 return (ltable[bi->loopIndex + i].dir);
109 }
110 }
111 }
112 return false;
113}
114
115void
116LTAGE::specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
117{
118 if (bi->loopHit>=0) {
119 int index = lindex(pc);
120 if (taken != ltable[index].dir) {
121 ltable[index].currentIterSpec = 0;
122 } else {
123 ltable[index].currentIterSpec =
124 (ltable[index].currentIterSpec + 1) & loopNumIterMask;
125 }
126 }
127}
128
129void
130LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
131{
132 int idx = bi->loopIndex + bi->loopHit;
133 if (bi->loopHit >= 0) {
134 //already a hit
135 if (bi->loopPredValid) {
136 if (taken != bi->loopPred) {
137 // free the entry
138 ltable[idx].numIter = 0;
139 ltable[idx].age = 0;
140 ltable[idx].confidence = 0;
141 ltable[idx].currentIter = 0;
142 return;
143 } else if (bi->loopPred != bi->tagePred) {
144 DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
145 unsignedCtrUpdate(ltable[idx].age, true, loopTableAgeBits);
146 }
147 }
148
149 ltable[idx].currentIter =
150 (ltable[idx].currentIter + 1) & loopNumIterMask;
151 if (ltable[idx].currentIter > ltable[idx].numIter) {
152 ltable[idx].confidence = 0;
153 if (ltable[idx].numIter != 0) {
154 // free the entry
155 ltable[idx].numIter = 0;
156 ltable[idx].age = 0;
157 ltable[idx].confidence = 0;
158 }
159 }
160
161 if (taken != ltable[idx].dir) {
162 if (ltable[idx].currentIter == ltable[idx].numIter) {
163 DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
164
165 unsignedCtrUpdate(ltable[idx].confidence, true,
166 loopTableConfidenceBits);
167 //just do not predict when the loop count is 1 or 2
168 if (ltable[idx].numIter < 3) {
169 // free the entry
170 ltable[idx].dir = taken;
171 ltable[idx].numIter = 0;
172 ltable[idx].age = 0;
173 ltable[idx].confidence = 0;
174 }
175 } else {
176 DPRINTF(LTage, "Loop End predicted incorrectly:%lx\n", pc);
177 if (ltable[idx].numIter == 0) {
178 // first complete nest;
179 ltable[idx].confidence = 0;
180 ltable[idx].numIter = ltable[idx].currentIter;
181 } else {
182 //not the same number of iterations as last time: free the
183 //entry
184 ltable[idx].numIter = 0;
185 ltable[idx].age = 0;
186 ltable[idx].confidence = 0;
187 }
188 }
189 ltable[idx].currentIter = 0;
190 }
191
192 } else if (taken) {
193 //try to allocate an entry on taken branch
194 int nrand = random_mt.random<int>();
195 for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
196 int loop_hit = (nrand + i) & ((1 << logLoopTableAssoc) - 1);
197 idx = bi->loopIndex + loop_hit;
198 if (ltable[idx].age == 0) {
199 DPRINTF(LTage, "Allocating loop pred entry for branch %lx\n",
200 pc);
201 ltable[idx].dir = !taken;
202 ltable[idx].tag = bi->loopTag;
203 ltable[idx].numIter = 0;
204 ltable[idx].age = (1 << loopTableAgeBits) - 1;
205 ltable[idx].confidence = 0;
206 ltable[idx].currentIter = 1;
207 break;
208
209 }
210 else
211 ltable[idx].age--;
212 }
213 }
214
215}
216
217//prediction
218bool
219LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
220{
221 LTageBranchInfo *bi = new LTageBranchInfo(nHistoryTables+1);
222 b = (void*)(bi);
223
224 bool pred_taken = tagePredict(tid, branch_pc, cond_branch, bi);
225
226 if (cond_branch) {
227 bi->loopPred = getLoop(branch_pc, bi); // loop prediction
228
229 if ((loopUseCounter >= 0) && bi->loopPredValid) {
230 pred_taken = bi->loopPred;
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232 }
233 DPRINTF(LTage, "Predict for %lx: taken?:%d, loopTaken?:%d, "
234 "loopValid?:%d, loopUseCounter:%d, tagePred:%d, altPred:%d\n",
235 branch_pc, pred_taken, bi->loopPred, bi->loopPredValid,
236 loopUseCounter, bi->tagePred, bi->altTaken);
237 }
238
239 specLoopUpdate(branch_pc, pred_taken, bi);
240 return pred_taken;
241}
242
243void
244LTAGE::condBranchUpdate(Addr branch_pc, bool taken,
245 TageBranchInfo* tage_bi, int nrand)
246{
247 LTageBranchInfo* bi = static_cast<LTageBranchInfo*>(tage_bi);
248
249 // first update the loop predictor
250 loopUpdate(branch_pc, taken, bi);
251
252 if (bi->loopPredValid) {
253 if (bi->tagePred != bi->loopPred) {
254 ctrUpdate(loopUseCounter,
255 (bi->loopPred == taken),
256 withLoopBits);
257 }
258 }
259
260 TAGE::condBranchUpdate(branch_pc, taken, bi, nrand);
261}
262
263void
264LTAGE::squash(ThreadID tid, bool taken, void *bp_history)
265{
266 TAGE::squash(tid, taken, bp_history);
267
268 LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
269
270 if (bi->condBranch) {
271 if (bi->loopHit >= 0) {
272 int idx = bi->loopIndex + bi->loopHit;
273 ltable[idx].currentIterSpec = bi->currentIter;
274 }
275 }
276}
277
278void
279LTAGE::squash(ThreadID tid, void *bp_history)
280{
281 LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
282 if (bi->condBranch) {
283 if (bi->loopHit >= 0) {
284 int idx = bi->loopIndex + bi->loopHit;
285 ltable[idx].currentIterSpec = bi->currentIter;
286 }
287 }
288
289 TAGE::squash(tid, bp_history);
290}
291
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