1/**
2 * Copyright (c) 2018 Metempsy Technology Consulting
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are
7 * met: redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer;

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48 signatureTable(p->signature_table_assoc, p->signature_table_entries,
49 p->signature_table_indexing_policy,
50 p->signature_table_replacement_policy),
51 patternTable(p->pattern_table_assoc, p->pattern_table_entries,
52 p->pattern_table_indexing_policy,
53 p->pattern_table_replacement_policy,
54 PatternEntry(stridesPerPatternEntry))
55{
56 fatal_if(prefetchConfidenceThreshold < 0,
57 "The prefetch confidence threshold must be greater than 0\n");
58 fatal_if(prefetchConfidenceThreshold > 1,
59 "The prefetch confidence threshold must be less than 1\n");
60 fatal_if(lookaheadConfidenceThreshold < 0,
61 "The lookahead confidence threshold must be greater than 0\n");
62 fatal_if(lookaheadConfidenceThreshold > 1,
63 "The lookahead confidence threshold must be less than 1\n");
64}
65
66SignaturePathPrefetcher::PatternStrideEntry &
67SignaturePathPrefetcher::PatternEntry::getStrideEntry(stride_t stride,
68 uint8_t max_counter_value)
69{
70 PatternStrideEntry *pstride_entry = findStride(stride);
71 if (pstride_entry == nullptr) {

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89 pstride_entry = victim_pstride_entry;
90 pstride_entry->counter = 0;
91 pstride_entry->stride = stride;
92 }
93 return *pstride_entry;
94}
95
96void
97SignaturePathPrefetcher::addPrefetch(Addr ppn, stride_t last_block,
98 stride_t delta, double path_confidence, signature_t signature,
99 bool is_secure, std::vector<AddrPriority> &addresses)
100{
101 stride_t block = last_block + delta;
102
103 Addr pf_ppn;
104 stride_t pf_block;
105 if (block < 0) {
106 stride_t num_cross_pages = 1 + (-block) / (pageBytes/blkSize);
107 if (num_cross_pages > ppn) {
108 // target address smaller than page 0, ignore this request;
109 return;
110 }
111 pf_ppn = ppn - num_cross_pages;
112 pf_block = block + (pageBytes/blkSize) * num_cross_pages;
113 handlePageCrossingLookahead(signature, last_block, delta,
114 path_confidence);
115 } else if (block >= (pageBytes/blkSize)) {
116 stride_t num_cross_pages = block / (pageBytes/blkSize);
117 if (MaxAddr/pageBytes < (ppn + num_cross_pages)) {
118 // target address goes beyond MaxAddr, ignore this request;
119 return;
120 }
121 pf_ppn = ppn + num_cross_pages;
122 pf_block = block - (pageBytes/blkSize) * num_cross_pages;
123 handlePageCrossingLookahead(signature, last_block, delta,
124 path_confidence);
125 } else {
126 pf_ppn = ppn;
127 pf_block = block;
128 }
129
130 Addr new_addr = pf_ppn * pageBytes;
131 new_addr += pf_block * (Addr)blkSize;
132
133 DPRINTF(HWPrefetch, "Queuing prefetch to %#x.\n", new_addr);
134 addresses.push_back(AddrPriority(new_addr, 0));
135}
136
137void
138SignaturePathPrefetcher::handleSignatureTableMiss(stride_t current_block,
139 signature_t &new_signature, double &new_conf, stride_t &new_stride)
140{
141 new_signature = current_block;
142 new_conf = 1.0;
143 new_stride = current_block;
144}
145
146void
147SignaturePathPrefetcher::increasePatternEntryCounter(
148 PatternEntry &pattern_entry, PatternStrideEntry &pstride_entry)
149{
150 if (pstride_entry.counter < maxCounterValue) {
151 pstride_entry.counter += 1;
152 }
153}
154
155void
156SignaturePathPrefetcher::updatePatternTable(Addr signature, stride_t stride)
157{
158 assert(stride != 0);
159 // The pattern table is indexed by signatures
160 PatternEntry &p_entry = getPatternEntry(signature);
161 PatternStrideEntry &ps_entry = p_entry.getStrideEntry(stride,
162 maxCounterValue);
163 increasePatternEntryCounter(p_entry, ps_entry);
164}
165
166SignaturePathPrefetcher::SignatureEntry &
167SignaturePathPrefetcher::getSignatureEntry(Addr ppn, bool is_secure,
168 stride_t block, bool &miss, stride_t &stride,
169 double &initial_confidence)
170{
171 SignatureEntry* signature_entry = signatureTable.findEntry(ppn, is_secure);
172 if (signature_entry != nullptr) {
173 signatureTable.accessEntry(signature_entry);
174 miss = false;
175 stride = block - signature_entry->lastBlock;
176 } else {
177 signature_entry = signatureTable.findVictim(ppn);
178 assert(signature_entry != nullptr);
179
180 // Sets signature_entry->signature, initial_confidence, and stride
181 handleSignatureTableMiss(block, signature_entry->signature,
182 initial_confidence, stride);
183
184 signatureTable.insertEntry(ppn, is_secure, signature_entry);
185 miss = true;
186 }
187 signature_entry->lastBlock = block;
188 return *signature_entry;
189}
190
191SignaturePathPrefetcher::PatternEntry &
192SignaturePathPrefetcher::getPatternEntry(Addr signature)
193{
194 PatternEntry* pattern_entry = patternTable.findEntry(signature, false);
195 if (pattern_entry != nullptr) {

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200 pattern_entry = patternTable.findVictim(signature);
201 assert(pattern_entry != nullptr);
202
203 patternTable.insertEntry(signature, false, pattern_entry);
204 }
205 return *pattern_entry;
206}
207
208double
209SignaturePathPrefetcher::calculatePrefetchConfidence(PatternEntry const &sig,
210 PatternStrideEntry const &entry) const
211{
212 return ((double) entry.counter) / maxCounterValue;
213}
214
215double
216SignaturePathPrefetcher::calculateLookaheadConfidence(PatternEntry const &sig,
217 PatternStrideEntry const &lookahead) const
218{
219 double lookahead_confidence;
220 if (lookahead.counter == maxCounterValue) {
221 /**
222 * maximum confidence is 0.95, guaranteeing that
223 * current confidence will eventually fall beyond
224 * the threshold
225 */
226 lookahead_confidence = 0.95;
227 } else {
228 lookahead_confidence = ((double) lookahead.counter / maxCounterValue);
229 }
230 return lookahead_confidence;
231}
232
233void
234SignaturePathPrefetcher::calculatePrefetch(const PrefetchInfo &pfi,
235 std::vector<AddrPriority> &addresses)
236{
237 Addr request_addr = pfi.getAddr();
238 Addr ppn = request_addr / pageBytes;
239 stride_t current_block = (request_addr % pageBytes) / blkSize;
240 stride_t stride;
241 bool is_secure = pfi.isSecure();
242 double initial_confidence = 1.0;
243
244 // Get the SignatureEntry of this page to:
245 // - compute the current stride
246 // - obtain the current signature of accesses
247 bool miss;
248 SignatureEntry &signature_entry = getSignatureEntry(ppn, is_secure,
249 current_block, miss, stride, initial_confidence);
250
251 if (miss) {
252 // No history for this page, can't continue
253 return;
254 }
255
256 if (stride == 0) {
257 // Can't continue with a stride 0
258 return;
259 }
260
261 // Update the confidence of the current signature
262 updatePatternTable(signature_entry.signature, stride);
263
264 // Update the current SignatureEntry signature
265 signature_entry.signature =
266 updateSignature(signature_entry.signature, stride);
267
268 signature_t current_signature = signature_entry.signature;
269 double current_confidence = initial_confidence;
270 stride_t current_stride = signature_entry.lastBlock;
271
272 // Look for prefetch candidates while the current path confidence is
273 // high enough
274 while (current_confidence > lookaheadConfidenceThreshold) {
275 // With the updated signature, attempt to generate prefetches
276 // - search the PatternTable and select all entries with enough
277 // confidence, these are prefetch candidates
278 // - select the entry with the highest counter as the "lookahead"
279 PatternEntry *current_pattern_entry =
280 patternTable.findEntry(current_signature, false);
281 PatternStrideEntry const *lookahead = nullptr;
282 if (current_pattern_entry != nullptr) {
283 uint8_t max_counter = 0;
284 for (auto const &entry : current_pattern_entry->strideEntries) {
285 //select the entry with the maximum counter value as lookahead
286 if (max_counter < entry.counter) {
287 max_counter = entry.counter;
288 lookahead = &entry;
289 }
290 double prefetch_confidence =
291 calculatePrefetchConfidence(*current_pattern_entry, entry);
292
293 if (prefetch_confidence >= prefetchConfidenceThreshold) {
294 assert(entry.stride != 0);
295 //prefetch candidate
296 addPrefetch(ppn, current_stride, entry.stride,
297 current_confidence, current_signature,
298 is_secure, addresses);
299 }
300 }
301 }
302
303 if (lookahead != nullptr) {
304 current_confidence *= calculateLookaheadConfidence(
305 *current_pattern_entry, *lookahead);
306 current_signature =
307 updateSignature(current_signature, lookahead->stride);
308 current_stride += lookahead->stride;
309 } else {
310 current_confidence = 0.0;
311 }
312 }
313
314 auxiliaryPrefetcher(ppn, current_block, is_secure, addresses);
315}
316
317void
318SignaturePathPrefetcher::auxiliaryPrefetcher(Addr ppn, stride_t current_block,
319 bool is_secure, std::vector<AddrPriority> &addresses)
320{
321 if (addresses.empty()) {
322 // Enable the next line prefetcher if no prefetch candidates are found
323 addPrefetch(ppn, current_block, 1, 0.0 /* unused*/, 0 /* unused */,
324 is_secure, addresses);
325 }
326}
327
328SignaturePathPrefetcher*
329SignaturePathPrefetcherParams::create()
330{
331 return new SignaturePathPrefetcher(this);
332}