Ben Murdoch | 4a90d5f | 2016-03-22 12:00:34 +0000 | [diff] [blame] | 1 | // Copyright 2013 the V8 project authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style license that can be |
| 3 | // found in the LICENSE file. |
| 4 | |
| 5 | #include "src/crankshaft/hydrogen-bce.h" |
| 6 | |
| 7 | namespace v8 { |
| 8 | namespace internal { |
| 9 | |
| 10 | |
| 11 | // We try to "factor up" HBoundsCheck instructions towards the root of the |
| 12 | // dominator tree. |
| 13 | // For now we handle checks where the index is like "exp + int32value". |
| 14 | // If in the dominator tree we check "exp + v1" and later (dominated) |
| 15 | // "exp + v2", if v2 <= v1 we can safely remove the second check, and if |
| 16 | // v2 > v1 we can use v2 in the 1st check and again remove the second. |
| 17 | // To do so we keep a dictionary of all checks where the key if the pair |
| 18 | // "exp, length". |
| 19 | // The class BoundsCheckKey represents this key. |
| 20 | class BoundsCheckKey : public ZoneObject { |
| 21 | public: |
| 22 | HValue* IndexBase() const { return index_base_; } |
| 23 | HValue* Length() const { return length_; } |
| 24 | |
| 25 | uint32_t Hash() { |
| 26 | return static_cast<uint32_t>(index_base_->Hashcode() ^ length_->Hashcode()); |
| 27 | } |
| 28 | |
| 29 | static BoundsCheckKey* Create(Zone* zone, |
| 30 | HBoundsCheck* check, |
| 31 | int32_t* offset) { |
| 32 | if (!check->index()->representation().IsSmiOrInteger32()) return NULL; |
| 33 | |
| 34 | HValue* index_base = NULL; |
| 35 | HConstant* constant = NULL; |
| 36 | bool is_sub = false; |
| 37 | |
| 38 | if (check->index()->IsAdd()) { |
| 39 | HAdd* index = HAdd::cast(check->index()); |
| 40 | if (index->left()->IsConstant()) { |
| 41 | constant = HConstant::cast(index->left()); |
| 42 | index_base = index->right(); |
| 43 | } else if (index->right()->IsConstant()) { |
| 44 | constant = HConstant::cast(index->right()); |
| 45 | index_base = index->left(); |
| 46 | } |
| 47 | } else if (check->index()->IsSub()) { |
| 48 | HSub* index = HSub::cast(check->index()); |
| 49 | is_sub = true; |
| 50 | if (index->right()->IsConstant()) { |
| 51 | constant = HConstant::cast(index->right()); |
| 52 | index_base = index->left(); |
| 53 | } |
| 54 | } else if (check->index()->IsConstant()) { |
| 55 | index_base = check->block()->graph()->GetConstant0(); |
| 56 | constant = HConstant::cast(check->index()); |
| 57 | } |
| 58 | |
| 59 | if (constant != NULL && constant->HasInteger32Value() && |
| 60 | constant->Integer32Value() != kMinInt) { |
| 61 | *offset = is_sub ? - constant->Integer32Value() |
| 62 | : constant->Integer32Value(); |
| 63 | } else { |
| 64 | *offset = 0; |
| 65 | index_base = check->index(); |
| 66 | } |
| 67 | |
| 68 | return new(zone) BoundsCheckKey(index_base, check->length()); |
| 69 | } |
| 70 | |
| 71 | private: |
| 72 | BoundsCheckKey(HValue* index_base, HValue* length) |
| 73 | : index_base_(index_base), |
| 74 | length_(length) { } |
| 75 | |
| 76 | HValue* index_base_; |
| 77 | HValue* length_; |
| 78 | |
| 79 | DISALLOW_COPY_AND_ASSIGN(BoundsCheckKey); |
| 80 | }; |
| 81 | |
| 82 | |
| 83 | // Data about each HBoundsCheck that can be eliminated or moved. |
| 84 | // It is the "value" in the dictionary indexed by "base-index, length" |
| 85 | // (the key is BoundsCheckKey). |
| 86 | // We scan the code with a dominator tree traversal. |
| 87 | // Traversing the dominator tree we keep a stack (implemented as a singly |
| 88 | // linked list) of "data" for each basic block that contains a relevant check |
| 89 | // with the same key (the dictionary holds the head of the list). |
| 90 | // We also keep all the "data" created for a given basic block in a list, and |
| 91 | // use it to "clean up" the dictionary when backtracking in the dominator tree |
| 92 | // traversal. |
| 93 | // Doing this each dictionary entry always directly points to the check that |
| 94 | // is dominating the code being examined now. |
| 95 | // We also track the current "offset" of the index expression and use it to |
| 96 | // decide if any check is already "covered" (so it can be removed) or not. |
| 97 | class BoundsCheckBbData: public ZoneObject { |
| 98 | public: |
| 99 | BoundsCheckKey* Key() const { return key_; } |
| 100 | int32_t LowerOffset() const { return lower_offset_; } |
| 101 | int32_t UpperOffset() const { return upper_offset_; } |
| 102 | HBasicBlock* BasicBlock() const { return basic_block_; } |
| 103 | HBoundsCheck* LowerCheck() const { return lower_check_; } |
| 104 | HBoundsCheck* UpperCheck() const { return upper_check_; } |
| 105 | BoundsCheckBbData* NextInBasicBlock() const { return next_in_bb_; } |
| 106 | BoundsCheckBbData* FatherInDominatorTree() const { return father_in_dt_; } |
| 107 | |
| 108 | bool OffsetIsCovered(int32_t offset) const { |
| 109 | return offset >= LowerOffset() && offset <= UpperOffset(); |
| 110 | } |
| 111 | |
| 112 | bool HasSingleCheck() { return lower_check_ == upper_check_; } |
| 113 | |
| 114 | void UpdateUpperOffsets(HBoundsCheck* check, int32_t offset) { |
| 115 | BoundsCheckBbData* data = FatherInDominatorTree(); |
| 116 | while (data != NULL && data->UpperCheck() == check) { |
| 117 | DCHECK(data->upper_offset_ < offset); |
| 118 | data->upper_offset_ = offset; |
| 119 | data = data->FatherInDominatorTree(); |
| 120 | } |
| 121 | } |
| 122 | |
| 123 | void UpdateLowerOffsets(HBoundsCheck* check, int32_t offset) { |
| 124 | BoundsCheckBbData* data = FatherInDominatorTree(); |
| 125 | while (data != NULL && data->LowerCheck() == check) { |
| 126 | DCHECK(data->lower_offset_ > offset); |
| 127 | data->lower_offset_ = offset; |
| 128 | data = data->FatherInDominatorTree(); |
| 129 | } |
| 130 | } |
| 131 | |
| 132 | // The goal of this method is to modify either upper_offset_ or |
| 133 | // lower_offset_ so that also new_offset is covered (the covered |
| 134 | // range grows). |
| 135 | // |
| 136 | // The precondition is that new_check follows UpperCheck() and |
| 137 | // LowerCheck() in the same basic block, and that new_offset is not |
| 138 | // covered (otherwise we could simply remove new_check). |
| 139 | // |
| 140 | // If HasSingleCheck() is true then new_check is added as "second check" |
| 141 | // (either upper or lower; note that HasSingleCheck() becomes false). |
| 142 | // Otherwise one of the current checks is modified so that it also covers |
| 143 | // new_offset, and new_check is removed. |
| 144 | void CoverCheck(HBoundsCheck* new_check, |
| 145 | int32_t new_offset) { |
| 146 | DCHECK(new_check->index()->representation().IsSmiOrInteger32()); |
| 147 | bool keep_new_check = false; |
| 148 | |
| 149 | if (new_offset > upper_offset_) { |
| 150 | upper_offset_ = new_offset; |
| 151 | if (HasSingleCheck()) { |
| 152 | keep_new_check = true; |
| 153 | upper_check_ = new_check; |
| 154 | } else { |
| 155 | TightenCheck(upper_check_, new_check, new_offset); |
| 156 | UpdateUpperOffsets(upper_check_, upper_offset_); |
| 157 | } |
| 158 | } else if (new_offset < lower_offset_) { |
| 159 | lower_offset_ = new_offset; |
| 160 | if (HasSingleCheck()) { |
| 161 | keep_new_check = true; |
| 162 | lower_check_ = new_check; |
| 163 | } else { |
| 164 | TightenCheck(lower_check_, new_check, new_offset); |
| 165 | UpdateLowerOffsets(lower_check_, lower_offset_); |
| 166 | } |
| 167 | } else { |
| 168 | // Should never have called CoverCheck() in this case. |
| 169 | UNREACHABLE(); |
| 170 | } |
| 171 | |
| 172 | if (!keep_new_check) { |
| 173 | if (FLAG_trace_bce) { |
| 174 | base::OS::Print("Eliminating check #%d after tightening\n", |
| 175 | new_check->id()); |
| 176 | } |
| 177 | new_check->block()->graph()->isolate()->counters()-> |
| 178 | bounds_checks_eliminated()->Increment(); |
| 179 | new_check->DeleteAndReplaceWith(new_check->ActualValue()); |
| 180 | } else { |
| 181 | HBoundsCheck* first_check = new_check == lower_check_ ? upper_check_ |
| 182 | : lower_check_; |
| 183 | if (FLAG_trace_bce) { |
| 184 | base::OS::Print("Moving second check #%d after first check #%d\n", |
| 185 | new_check->id(), first_check->id()); |
| 186 | } |
| 187 | // The length is guaranteed to be live at first_check. |
| 188 | DCHECK(new_check->length() == first_check->length()); |
| 189 | HInstruction* old_position = new_check->next(); |
| 190 | new_check->Unlink(); |
| 191 | new_check->InsertAfter(first_check); |
| 192 | MoveIndexIfNecessary(new_check->index(), new_check, old_position); |
| 193 | } |
| 194 | } |
| 195 | |
| 196 | BoundsCheckBbData(BoundsCheckKey* key, |
| 197 | int32_t lower_offset, |
| 198 | int32_t upper_offset, |
| 199 | HBasicBlock* bb, |
| 200 | HBoundsCheck* lower_check, |
| 201 | HBoundsCheck* upper_check, |
| 202 | BoundsCheckBbData* next_in_bb, |
| 203 | BoundsCheckBbData* father_in_dt) |
| 204 | : key_(key), |
| 205 | lower_offset_(lower_offset), |
| 206 | upper_offset_(upper_offset), |
| 207 | basic_block_(bb), |
| 208 | lower_check_(lower_check), |
| 209 | upper_check_(upper_check), |
| 210 | next_in_bb_(next_in_bb), |
| 211 | father_in_dt_(father_in_dt) { } |
| 212 | |
| 213 | private: |
| 214 | BoundsCheckKey* key_; |
| 215 | int32_t lower_offset_; |
| 216 | int32_t upper_offset_; |
| 217 | HBasicBlock* basic_block_; |
| 218 | HBoundsCheck* lower_check_; |
| 219 | HBoundsCheck* upper_check_; |
| 220 | BoundsCheckBbData* next_in_bb_; |
| 221 | BoundsCheckBbData* father_in_dt_; |
| 222 | |
| 223 | void MoveIndexIfNecessary(HValue* index_raw, |
| 224 | HBoundsCheck* insert_before, |
| 225 | HInstruction* end_of_scan_range) { |
| 226 | // index_raw can be HAdd(index_base, offset), HSub(index_base, offset), |
| 227 | // HConstant(offset) or index_base directly. |
| 228 | // In the latter case, no need to move anything. |
| 229 | if (index_raw->IsAdd() || index_raw->IsSub()) { |
| 230 | HArithmeticBinaryOperation* index = |
| 231 | HArithmeticBinaryOperation::cast(index_raw); |
| 232 | HValue* left_input = index->left(); |
| 233 | HValue* right_input = index->right(); |
| 234 | HValue* context = index->context(); |
| 235 | bool must_move_index = false; |
| 236 | bool must_move_left_input = false; |
| 237 | bool must_move_right_input = false; |
| 238 | bool must_move_context = false; |
| 239 | for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) { |
| 240 | if (cursor == left_input) must_move_left_input = true; |
| 241 | if (cursor == right_input) must_move_right_input = true; |
| 242 | if (cursor == context) must_move_context = true; |
| 243 | if (cursor == index) must_move_index = true; |
| 244 | if (cursor->previous() == NULL) { |
| 245 | cursor = cursor->block()->dominator()->end(); |
| 246 | } else { |
| 247 | cursor = cursor->previous(); |
| 248 | } |
| 249 | } |
| 250 | if (must_move_index) { |
| 251 | index->Unlink(); |
| 252 | index->InsertBefore(insert_before); |
| 253 | } |
| 254 | // The BCE algorithm only selects mergeable bounds checks that share |
| 255 | // the same "index_base", so we'll only ever have to move constants. |
| 256 | if (must_move_left_input) { |
| 257 | HConstant::cast(left_input)->Unlink(); |
| 258 | HConstant::cast(left_input)->InsertBefore(index); |
| 259 | } |
| 260 | if (must_move_right_input) { |
| 261 | HConstant::cast(right_input)->Unlink(); |
| 262 | HConstant::cast(right_input)->InsertBefore(index); |
| 263 | } |
| 264 | if (must_move_context) { |
| 265 | // Contexts are always constants. |
| 266 | HConstant::cast(context)->Unlink(); |
| 267 | HConstant::cast(context)->InsertBefore(index); |
| 268 | } |
| 269 | } else if (index_raw->IsConstant()) { |
| 270 | HConstant* index = HConstant::cast(index_raw); |
| 271 | bool must_move = false; |
| 272 | for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) { |
| 273 | if (cursor == index) must_move = true; |
| 274 | if (cursor->previous() == NULL) { |
| 275 | cursor = cursor->block()->dominator()->end(); |
| 276 | } else { |
| 277 | cursor = cursor->previous(); |
| 278 | } |
| 279 | } |
| 280 | if (must_move) { |
| 281 | index->Unlink(); |
| 282 | index->InsertBefore(insert_before); |
| 283 | } |
| 284 | } |
| 285 | } |
| 286 | |
| 287 | void TightenCheck(HBoundsCheck* original_check, |
| 288 | HBoundsCheck* tighter_check, |
| 289 | int32_t new_offset) { |
| 290 | DCHECK(original_check->length() == tighter_check->length()); |
| 291 | MoveIndexIfNecessary(tighter_check->index(), original_check, tighter_check); |
| 292 | original_check->ReplaceAllUsesWith(original_check->index()); |
| 293 | original_check->SetOperandAt(0, tighter_check->index()); |
| 294 | if (FLAG_trace_bce) { |
| 295 | base::OS::Print("Tightened check #%d with offset %d from #%d\n", |
| 296 | original_check->id(), new_offset, tighter_check->id()); |
| 297 | } |
| 298 | } |
| 299 | |
| 300 | DISALLOW_COPY_AND_ASSIGN(BoundsCheckBbData); |
| 301 | }; |
| 302 | |
| 303 | |
| 304 | static bool BoundsCheckKeyMatch(void* key1, void* key2) { |
| 305 | BoundsCheckKey* k1 = static_cast<BoundsCheckKey*>(key1); |
| 306 | BoundsCheckKey* k2 = static_cast<BoundsCheckKey*>(key2); |
| 307 | return k1->IndexBase() == k2->IndexBase() && k1->Length() == k2->Length(); |
| 308 | } |
| 309 | |
| 310 | |
| 311 | BoundsCheckTable::BoundsCheckTable(Zone* zone) |
| 312 | : ZoneHashMap(BoundsCheckKeyMatch, ZoneHashMap::kDefaultHashMapCapacity, |
| 313 | ZoneAllocationPolicy(zone)) { } |
| 314 | |
| 315 | |
| 316 | BoundsCheckBbData** BoundsCheckTable::LookupOrInsert(BoundsCheckKey* key, |
| 317 | Zone* zone) { |
| 318 | return reinterpret_cast<BoundsCheckBbData**>( |
| 319 | &(ZoneHashMap::LookupOrInsert(key, key->Hash(), |
| 320 | ZoneAllocationPolicy(zone))->value)); |
| 321 | } |
| 322 | |
| 323 | |
| 324 | void BoundsCheckTable::Insert(BoundsCheckKey* key, |
| 325 | BoundsCheckBbData* data, |
| 326 | Zone* zone) { |
| 327 | ZoneHashMap::LookupOrInsert(key, key->Hash(), ZoneAllocationPolicy(zone)) |
| 328 | ->value = data; |
| 329 | } |
| 330 | |
| 331 | |
| 332 | void BoundsCheckTable::Delete(BoundsCheckKey* key) { |
| 333 | Remove(key, key->Hash()); |
| 334 | } |
| 335 | |
| 336 | |
| 337 | class HBoundsCheckEliminationState { |
| 338 | public: |
| 339 | HBasicBlock* block_; |
| 340 | BoundsCheckBbData* bb_data_list_; |
| 341 | int index_; |
| 342 | }; |
| 343 | |
| 344 | |
| 345 | // Eliminates checks in bb and recursively in the dominated blocks. |
| 346 | // Also replace the results of check instructions with the original value, if |
| 347 | // the result is used. This is safe now, since we don't do code motion after |
| 348 | // this point. It enables better register allocation since the value produced |
| 349 | // by check instructions is really a copy of the original value. |
| 350 | void HBoundsCheckEliminationPhase::EliminateRedundantBoundsChecks( |
| 351 | HBasicBlock* entry) { |
| 352 | // Allocate the stack. |
| 353 | HBoundsCheckEliminationState* stack = |
| 354 | zone()->NewArray<HBoundsCheckEliminationState>(graph()->blocks()->length()); |
| 355 | |
| 356 | // Explicitly push the entry block. |
| 357 | stack[0].block_ = entry; |
| 358 | stack[0].bb_data_list_ = PreProcessBlock(entry); |
| 359 | stack[0].index_ = 0; |
| 360 | int stack_depth = 1; |
| 361 | |
| 362 | // Implement depth-first traversal with a stack. |
| 363 | while (stack_depth > 0) { |
| 364 | int current = stack_depth - 1; |
| 365 | HBoundsCheckEliminationState* state = &stack[current]; |
| 366 | const ZoneList<HBasicBlock*>* children = state->block_->dominated_blocks(); |
| 367 | |
| 368 | if (state->index_ < children->length()) { |
| 369 | // Recursively visit children blocks. |
| 370 | HBasicBlock* child = children->at(state->index_++); |
| 371 | int next = stack_depth++; |
| 372 | stack[next].block_ = child; |
| 373 | stack[next].bb_data_list_ = PreProcessBlock(child); |
| 374 | stack[next].index_ = 0; |
| 375 | } else { |
| 376 | // Finished with all children; post process the block. |
| 377 | PostProcessBlock(state->block_, state->bb_data_list_); |
| 378 | stack_depth--; |
| 379 | } |
| 380 | } |
| 381 | } |
| 382 | |
| 383 | |
| 384 | BoundsCheckBbData* HBoundsCheckEliminationPhase::PreProcessBlock( |
| 385 | HBasicBlock* bb) { |
| 386 | BoundsCheckBbData* bb_data_list = NULL; |
| 387 | |
| 388 | for (HInstructionIterator it(bb); !it.Done(); it.Advance()) { |
| 389 | HInstruction* i = it.Current(); |
| 390 | if (!i->IsBoundsCheck()) continue; |
| 391 | |
| 392 | HBoundsCheck* check = HBoundsCheck::cast(i); |
| 393 | int32_t offset = 0; |
| 394 | BoundsCheckKey* key = |
| 395 | BoundsCheckKey::Create(zone(), check, &offset); |
| 396 | if (key == NULL) continue; |
| 397 | BoundsCheckBbData** data_p = table_.LookupOrInsert(key, zone()); |
| 398 | BoundsCheckBbData* data = *data_p; |
| 399 | if (data == NULL) { |
| 400 | bb_data_list = new(zone()) BoundsCheckBbData(key, |
| 401 | offset, |
| 402 | offset, |
| 403 | bb, |
| 404 | check, |
| 405 | check, |
| 406 | bb_data_list, |
| 407 | NULL); |
| 408 | *data_p = bb_data_list; |
| 409 | if (FLAG_trace_bce) { |
| 410 | base::OS::Print("Fresh bounds check data for block #%d: [%d]\n", |
| 411 | bb->block_id(), offset); |
| 412 | } |
| 413 | } else if (data->OffsetIsCovered(offset)) { |
| 414 | bb->graph()->isolate()->counters()-> |
| 415 | bounds_checks_eliminated()->Increment(); |
| 416 | if (FLAG_trace_bce) { |
| 417 | base::OS::Print("Eliminating bounds check #%d, offset %d is covered\n", |
| 418 | check->id(), offset); |
| 419 | } |
| 420 | check->DeleteAndReplaceWith(check->ActualValue()); |
| 421 | } else if (data->BasicBlock() == bb) { |
| 422 | // TODO(jkummerow): I think the following logic would be preferable: |
| 423 | // if (data->Basicblock() == bb || |
| 424 | // graph()->use_optimistic_licm() || |
| 425 | // bb->IsLoopSuccessorDominator()) { |
| 426 | // data->CoverCheck(check, offset) |
| 427 | // } else { |
| 428 | // /* add pristine BCBbData like in (data == NULL) case above */ |
| 429 | // } |
| 430 | // Even better would be: distinguish between read-only dominator-imposed |
| 431 | // knowledge and modifiable upper/lower checks. |
| 432 | // What happens currently is that the first bounds check in a dominated |
| 433 | // block will stay around while any further checks are hoisted out, |
| 434 | // which doesn't make sense. Investigate/fix this in a future CL. |
| 435 | data->CoverCheck(check, offset); |
| 436 | } else if (graph()->use_optimistic_licm() || |
| 437 | bb->IsLoopSuccessorDominator()) { |
| 438 | int32_t new_lower_offset = offset < data->LowerOffset() |
| 439 | ? offset |
| 440 | : data->LowerOffset(); |
| 441 | int32_t new_upper_offset = offset > data->UpperOffset() |
| 442 | ? offset |
| 443 | : data->UpperOffset(); |
| 444 | bb_data_list = new(zone()) BoundsCheckBbData(key, |
| 445 | new_lower_offset, |
| 446 | new_upper_offset, |
| 447 | bb, |
| 448 | data->LowerCheck(), |
| 449 | data->UpperCheck(), |
| 450 | bb_data_list, |
| 451 | data); |
| 452 | if (FLAG_trace_bce) { |
| 453 | base::OS::Print("Updated bounds check data for block #%d: [%d - %d]\n", |
| 454 | bb->block_id(), new_lower_offset, new_upper_offset); |
| 455 | } |
| 456 | table_.Insert(key, bb_data_list, zone()); |
| 457 | } |
| 458 | } |
| 459 | |
| 460 | return bb_data_list; |
| 461 | } |
| 462 | |
| 463 | |
| 464 | void HBoundsCheckEliminationPhase::PostProcessBlock( |
| 465 | HBasicBlock* block, BoundsCheckBbData* data) { |
| 466 | while (data != NULL) { |
| 467 | if (data->FatherInDominatorTree()) { |
| 468 | table_.Insert(data->Key(), data->FatherInDominatorTree(), zone()); |
| 469 | } else { |
| 470 | table_.Delete(data->Key()); |
| 471 | } |
| 472 | data = data->NextInBasicBlock(); |
| 473 | } |
| 474 | } |
| 475 | |
| 476 | } // namespace internal |
| 477 | } // namespace v8 |