1 // Copyright 2013 the V8 project authors. All rights reserved. 2 // Redistribution and use in source and binary forms, with or without 3 // modification, are permitted provided that the following conditions are 4 // met: 5 // 6 // * Redistributions of source code must retain the above copyright 7 // notice, this list of conditions and the following disclaimer. 8 // * Redistributions in binary form must reproduce the above 9 // copyright notice, this list of conditions and the following 10 // disclaimer in the documentation and/or other materials provided 11 // with the distribution. 12 // * Neither the name of Google Inc. nor the names of its 13 // contributors may be used to endorse or promote products derived 14 // from this software without specific prior written permission. 15 // 16 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 17 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 18 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 19 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 20 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 21 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 22 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 26 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 28 #include "hydrogen-bch.h" 29 30 namespace v8 { 31 namespace internal { 32 33 /* 34 * This class is a table with one element for eack basic block. 35 * 36 * It is used to check if, inside one loop, all execution paths contain 37 * a bounds check for a particular [index, length] combination. 38 * The reason is that if there is a path that stays in the loop without 39 * executing a check then the check cannot be hoisted out of the loop (it 40 * would likely fail and cause a deopt for no good reason). 41 * We also check is there are paths that exit the loop early, and if yes we 42 * perform the hoisting only if graph()->use_optimistic_licm() is true. 43 * The reason is that such paths are realtively common and harmless (like in 44 * a "search" method that scans an array until an element is found), but in 45 * some cases they could cause a deopt if we hoist the check so this is a 46 * situation we need to detect. 47 */ 48 class InductionVariableBlocksTable BASE_EMBEDDED { 49 public: 50 class Element { 51 public: 52 static const int kNoBlock = -1; 53 54 HBasicBlock* block() { return block_; } 55 void set_block(HBasicBlock* block) { block_ = block; } 56 bool is_start() { return is_start_; } 57 bool is_proper_exit() { return is_proper_exit_; } 58 bool is_in_loop() { return is_in_loop_; } 59 bool has_check() { return has_check_; } 60 void set_has_check() { has_check_ = true; } 61 InductionVariableLimitUpdate* additional_limit() { 62 return &additional_limit_; 63 } 64 65 /* 66 * Initializes the table element for a given loop (identified by its 67 * induction variable). 68 */ 69 void InitializeLoop(InductionVariableData* data) { 70 ASSERT(data->limit() != NULL); 71 HLoopInformation* loop = data->phi()->block()->current_loop(); 72 is_start_ = (block() == loop->loop_header()); 73 is_proper_exit_ = (block() == data->induction_exit_target()); 74 is_in_loop_ = loop->IsNestedInThisLoop(block()->current_loop()); 75 has_check_ = false; 76 } 77 78 // Utility methods to iterate over dominated blocks. 79 void ResetCurrentDominatedBlock() { current_dominated_block_ = kNoBlock; } 80 HBasicBlock* CurrentDominatedBlock() { 81 ASSERT(current_dominated_block_ != kNoBlock); 82 return current_dominated_block_ < block()->dominated_blocks()->length() ? 83 block()->dominated_blocks()->at(current_dominated_block_) : NULL; 84 } 85 HBasicBlock* NextDominatedBlock() { 86 current_dominated_block_++; 87 return CurrentDominatedBlock(); 88 } 89 90 Element() 91 : block_(NULL), is_start_(false), is_proper_exit_(false), 92 has_check_(false), additional_limit_(), 93 current_dominated_block_(kNoBlock) {} 94 95 private: 96 HBasicBlock* block_; 97 bool is_start_; 98 bool is_proper_exit_; 99 bool is_in_loop_; 100 bool has_check_; 101 InductionVariableLimitUpdate additional_limit_; 102 int current_dominated_block_; 103 }; 104 105 HGraph* graph() { return graph_; } 106 HBasicBlock* loop_header() { return loop_header_; } 107 Element* at(int index) { return &(elements_.at(index)); } 108 Element* at(HBasicBlock* block) { return at(block->block_id()); } 109 110 void AddCheckAt(HBasicBlock* block) { 111 at(block->block_id())->set_has_check(); 112 } 113 114 /* 115 * Initializes the table for a given loop (identified by its induction 116 * variable). 117 */ 118 void InitializeLoop(InductionVariableData* data) { 119 for (int i = 0; i < graph()->blocks()->length(); i++) { 120 at(i)->InitializeLoop(data); 121 } 122 loop_header_ = data->phi()->block()->current_loop()->loop_header(); 123 } 124 125 126 enum Hoistability { 127 HOISTABLE, 128 OPTIMISTICALLY_HOISTABLE, 129 NOT_HOISTABLE 130 }; 131 132 /* 133 * This method checks if it is appropriate to hoist the bounds checks on an 134 * induction variable out of the loop. 135 * The problem is that in the loop code graph there could be execution paths 136 * where the check is not performed, but hoisting the check has the same 137 * semantics as performing it at every loop iteration, which could cause 138 * unnecessary check failures (which would mean unnecessary deoptimizations). 139 * The method returns OK if there are no paths that perform an iteration 140 * (loop back to the header) without meeting a check, or UNSAFE is set if 141 * early exit paths are found. 142 */ 143 Hoistability CheckHoistability() { 144 for (int i = 0; i < elements_.length(); i++) { 145 at(i)->ResetCurrentDominatedBlock(); 146 } 147 bool unsafe = false; 148 149 HBasicBlock* current = loop_header(); 150 while (current != NULL) { 151 HBasicBlock* next; 152 153 if (at(current)->has_check() || !at(current)->is_in_loop()) { 154 // We found a check or we reached a dominated block out of the loop, 155 // therefore this block is safe and we can backtrack. 156 next = NULL; 157 } else { 158 for (int i = 0; i < current->end()->SuccessorCount(); i ++) { 159 Element* successor = at(current->end()->SuccessorAt(i)); 160 161 if (!successor->is_in_loop()) { 162 if (!successor->is_proper_exit()) { 163 // We found a path that exits the loop early, and is not the exit 164 // related to the induction limit, therefore hoisting checks is 165 // an optimistic assumption. 166 unsafe = true; 167 } 168 } 169 170 if (successor->is_start()) { 171 // We found a path that does one loop iteration without meeting any 172 // check, therefore hoisting checks would be likely to cause 173 // unnecessary deopts. 174 return NOT_HOISTABLE; 175 } 176 } 177 178 next = at(current)->NextDominatedBlock(); 179 } 180 181 // If we have no next block we need to backtrack the tree traversal. 182 while (next == NULL) { 183 current = current->dominator(); 184 if (current != NULL) { 185 next = at(current)->NextDominatedBlock(); 186 } else { 187 // We reached the root: next stays NULL. 188 next = NULL; 189 break; 190 } 191 } 192 193 current = next; 194 } 195 196 return unsafe ? OPTIMISTICALLY_HOISTABLE : HOISTABLE; 197 } 198 199 explicit InductionVariableBlocksTable(HGraph* graph) 200 : graph_(graph), loop_header_(NULL), 201 elements_(graph->blocks()->length(), graph->zone()) { 202 for (int i = 0; i < graph->blocks()->length(); i++) { 203 Element element; 204 element.set_block(graph->blocks()->at(i)); 205 elements_.Add(element, graph->zone()); 206 ASSERT(at(i)->block()->block_id() == i); 207 } 208 } 209 210 // Tries to hoist a check out of its induction loop. 211 void ProcessRelatedChecks( 212 InductionVariableData::InductionVariableCheck* check, 213 InductionVariableData* data) { 214 HValue* length = check->check()->length(); 215 check->set_processed(); 216 HBasicBlock* header = 217 data->phi()->block()->current_loop()->loop_header(); 218 HBasicBlock* pre_header = header->predecessors()->at(0); 219 // Check that the limit is defined in the loop preheader. 220 if (!data->limit()->IsInteger32Constant()) { 221 HBasicBlock* limit_block = data->limit()->block(); 222 if (limit_block != pre_header && 223 !limit_block->Dominates(pre_header)) { 224 return; 225 } 226 } 227 // Check that the length and limit have compatible representations. 228 if (!(data->limit()->representation().Equals( 229 length->representation()) || 230 data->limit()->IsInteger32Constant())) { 231 return; 232 } 233 // Check that the length is defined in the loop preheader. 234 if (check->check()->length()->block() != pre_header && 235 !check->check()->length()->block()->Dominates(pre_header)) { 236 return; 237 } 238 239 // Add checks to the table. 240 for (InductionVariableData::InductionVariableCheck* current_check = check; 241 current_check != NULL; 242 current_check = current_check->next()) { 243 if (current_check->check()->length() != length) continue; 244 245 AddCheckAt(current_check->check()->block()); 246 current_check->set_processed(); 247 } 248 249 // Check that we will not cause unwanted deoptimizations. 250 Hoistability hoistability = CheckHoistability(); 251 if (hoistability == NOT_HOISTABLE || 252 (hoistability == OPTIMISTICALLY_HOISTABLE && 253 !graph()->use_optimistic_licm())) { 254 return; 255 } 256 257 // We will do the hoisting, but we must see if the limit is "limit" or if 258 // all checks are done on constants: if all check are done against the same 259 // constant limit we will use that instead of the induction limit. 260 bool has_upper_constant_limit = true; 261 InductionVariableData::InductionVariableCheck* current_check = check; 262 int32_t upper_constant_limit = 263 current_check != NULL && current_check->HasUpperLimit() ? 264 current_check->upper_limit() : 0; 265 while (current_check != NULL) { 266 if (check->HasUpperLimit()) { 267 if (check->upper_limit() != upper_constant_limit) { 268 has_upper_constant_limit = false; 269 } 270 } else { 271 has_upper_constant_limit = false; 272 } 273 274 current_check->check()->block()->graph()->isolate()->counters()-> 275 bounds_checks_eliminated()->Increment(); 276 current_check->check()->set_skip_check(); 277 current_check = current_check->next(); 278 } 279 280 // Choose the appropriate limit. 281 Zone* zone = graph()->zone(); 282 HValue* context = graph()->GetInvalidContext(); 283 HValue* limit = data->limit(); 284 if (has_upper_constant_limit) { 285 HConstant* new_limit = HConstant::New(zone, context, 286 upper_constant_limit); 287 new_limit->InsertBefore(pre_header->end()); 288 limit = new_limit; 289 } 290 291 // If necessary, redefine the limit in the preheader. 292 if (limit->IsInteger32Constant() && 293 limit->block() != pre_header && 294 !limit->block()->Dominates(pre_header)) { 295 HConstant* new_limit = HConstant::New(zone, context, 296 limit->GetInteger32Constant()); 297 new_limit->InsertBefore(pre_header->end()); 298 limit = new_limit; 299 } 300 301 // Do the hoisting. 302 HBoundsCheck* hoisted_check = HBoundsCheck::New( 303 zone, context, limit, check->check()->length()); 304 hoisted_check->InsertBefore(pre_header->end()); 305 hoisted_check->set_allow_equality(true); 306 hoisted_check->block()->graph()->isolate()->counters()-> 307 bounds_checks_hoisted()->Increment(); 308 } 309 310 void CollectInductionVariableData(HBasicBlock* bb) { 311 bool additional_limit = false; 312 313 for (int i = 0; i < bb->phis()->length(); i++) { 314 HPhi* phi = bb->phis()->at(i); 315 phi->DetectInductionVariable(); 316 } 317 318 additional_limit = InductionVariableData::ComputeInductionVariableLimit( 319 bb, at(bb)->additional_limit()); 320 321 if (additional_limit) { 322 at(bb)->additional_limit()->updated_variable-> 323 UpdateAdditionalLimit(at(bb)->additional_limit()); 324 } 325 326 for (HInstruction* i = bb->first(); i != NULL; i = i->next()) { 327 if (!i->IsBoundsCheck()) continue; 328 HBoundsCheck* check = HBoundsCheck::cast(i); 329 InductionVariableData::BitwiseDecompositionResult decomposition; 330 InductionVariableData::DecomposeBitwise(check->index(), &decomposition); 331 if (!decomposition.base->IsPhi()) continue; 332 HPhi* phi = HPhi::cast(decomposition.base); 333 334 if (!phi->IsInductionVariable()) continue; 335 InductionVariableData* data = phi->induction_variable_data(); 336 337 // For now ignore loops decrementing the index. 338 if (data->increment() <= 0) continue; 339 if (!data->LowerLimitIsNonNegativeConstant()) continue; 340 341 // TODO(mmassi): skip OSR values for check->length(). 342 if (check->length() == data->limit() || 343 check->length() == data->additional_upper_limit()) { 344 check->block()->graph()->isolate()->counters()-> 345 bounds_checks_eliminated()->Increment(); 346 check->set_skip_check(); 347 continue; 348 } 349 350 if (!phi->IsLimitedInductionVariable()) continue; 351 352 int32_t limit = data->ComputeUpperLimit(decomposition.and_mask, 353 decomposition.or_mask); 354 phi->induction_variable_data()->AddCheck(check, limit); 355 } 356 357 for (int i = 0; i < bb->dominated_blocks()->length(); i++) { 358 CollectInductionVariableData(bb->dominated_blocks()->at(i)); 359 } 360 361 if (additional_limit) { 362 at(bb->block_id())->additional_limit()->updated_variable-> 363 UpdateAdditionalLimit(at(bb->block_id())->additional_limit()); 364 } 365 } 366 367 void EliminateRedundantBoundsChecks(HBasicBlock* bb) { 368 for (int i = 0; i < bb->phis()->length(); i++) { 369 HPhi* phi = bb->phis()->at(i); 370 if (!phi->IsLimitedInductionVariable()) continue; 371 372 InductionVariableData* induction_data = phi->induction_variable_data(); 373 InductionVariableData::ChecksRelatedToLength* current_length_group = 374 induction_data->checks(); 375 while (current_length_group != NULL) { 376 current_length_group->CloseCurrentBlock(); 377 InductionVariableData::InductionVariableCheck* current_base_check = 378 current_length_group->checks(); 379 InitializeLoop(induction_data); 380 381 while (current_base_check != NULL) { 382 ProcessRelatedChecks(current_base_check, induction_data); 383 while (current_base_check != NULL && 384 current_base_check->processed()) { 385 current_base_check = current_base_check->next(); 386 } 387 } 388 389 current_length_group = current_length_group->next(); 390 } 391 } 392 } 393 394 private: 395 HGraph* graph_; 396 HBasicBlock* loop_header_; 397 ZoneList<Element> elements_; 398 }; 399 400 401 void HBoundsCheckHoistingPhase::HoistRedundantBoundsChecks() { 402 InductionVariableBlocksTable table(graph()); 403 table.CollectInductionVariableData(graph()->entry_block()); 404 for (int i = 0; i < graph()->blocks()->length(); i++) { 405 table.EliminateRedundantBoundsChecks(graph()->blocks()->at(i)); 406 } 407 } 408 409 } } // namespace v8::internal 410 411
