1 //===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file promotes memory references to be register references. It promotes 11 // alloca instructions which only have loads and stores as uses. An alloca is 12 // transformed by using iterated dominator frontiers to place PHI nodes, then 13 // traversing the function in depth-first order to rewrite loads and stores as 14 // appropriate. 15 // 16 // The algorithm used here is based on: 17 // 18 // Sreedhar and Gao. A linear time algorithm for placing phi-nodes. 19 // In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of 20 // Programming Languages 21 // POPL '95. ACM, New York, NY, 62-73. 22 // 23 // It has been modified to not explicitly use the DJ graph data structure and to 24 // directly compute pruned SSA using per-variable liveness information. 25 // 26 //===----------------------------------------------------------------------===// 27 28 #include "llvm/Transforms/Utils/PromoteMemToReg.h" 29 #include "llvm/ADT/ArrayRef.h" 30 #include "llvm/ADT/DenseMap.h" 31 #include "llvm/ADT/STLExtras.h" 32 #include "llvm/ADT/SmallPtrSet.h" 33 #include "llvm/ADT/SmallVector.h" 34 #include "llvm/ADT/Statistic.h" 35 #include "llvm/Analysis/AliasSetTracker.h" 36 #include "llvm/Analysis/InstructionSimplify.h" 37 #include "llvm/Analysis/ValueTracking.h" 38 #include "llvm/IR/CFG.h" 39 #include "llvm/IR/Constants.h" 40 #include "llvm/IR/DIBuilder.h" 41 #include "llvm/IR/DebugInfo.h" 42 #include "llvm/IR/DerivedTypes.h" 43 #include "llvm/IR/Dominators.h" 44 #include "llvm/IR/Function.h" 45 #include "llvm/IR/Instructions.h" 46 #include "llvm/IR/IntrinsicInst.h" 47 #include "llvm/IR/Metadata.h" 48 #include "llvm/Transforms/Utils/Local.h" 49 #include <algorithm> 50 #include <queue> 51 using namespace llvm; 52 53 #define DEBUG_TYPE "mem2reg" 54 55 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block"); 56 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store"); 57 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed"); 58 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted"); 59 60 bool llvm::isAllocaPromotable(const AllocaInst *AI) { 61 // FIXME: If the memory unit is of pointer or integer type, we can permit 62 // assignments to subsections of the memory unit. 63 unsigned AS = AI->getType()->getAddressSpace(); 64 65 // Only allow direct and non-volatile loads and stores... 66 for (const User *U : AI->users()) { 67 if (const LoadInst *LI = dyn_cast<LoadInst>(U)) { 68 // Note that atomic loads can be transformed; atomic semantics do 69 // not have any meaning for a local alloca. 70 if (LI->isVolatile()) 71 return false; 72 } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { 73 if (SI->getOperand(0) == AI) 74 return false; // Don't allow a store OF the AI, only INTO the AI. 75 // Note that atomic stores can be transformed; atomic semantics do 76 // not have any meaning for a local alloca. 77 if (SI->isVolatile()) 78 return false; 79 } else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) { 80 if (II->getIntrinsicID() != Intrinsic::lifetime_start && 81 II->getIntrinsicID() != Intrinsic::lifetime_end) 82 return false; 83 } else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) { 84 if (BCI->getType() != Type::getInt8PtrTy(U->getContext(), AS)) 85 return false; 86 if (!onlyUsedByLifetimeMarkers(BCI)) 87 return false; 88 } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) { 89 if (GEPI->getType() != Type::getInt8PtrTy(U->getContext(), AS)) 90 return false; 91 if (!GEPI->hasAllZeroIndices()) 92 return false; 93 if (!onlyUsedByLifetimeMarkers(GEPI)) 94 return false; 95 } else { 96 return false; 97 } 98 } 99 100 return true; 101 } 102 103 namespace { 104 105 struct AllocaInfo { 106 SmallVector<BasicBlock *, 32> DefiningBlocks; 107 SmallVector<BasicBlock *, 32> UsingBlocks; 108 109 StoreInst *OnlyStore; 110 BasicBlock *OnlyBlock; 111 bool OnlyUsedInOneBlock; 112 113 Value *AllocaPointerVal; 114 DbgDeclareInst *DbgDeclare; 115 116 void clear() { 117 DefiningBlocks.clear(); 118 UsingBlocks.clear(); 119 OnlyStore = nullptr; 120 OnlyBlock = nullptr; 121 OnlyUsedInOneBlock = true; 122 AllocaPointerVal = nullptr; 123 DbgDeclare = nullptr; 124 } 125 126 /// Scan the uses of the specified alloca, filling in the AllocaInfo used 127 /// by the rest of the pass to reason about the uses of this alloca. 128 void AnalyzeAlloca(AllocaInst *AI) { 129 clear(); 130 131 // As we scan the uses of the alloca instruction, keep track of stores, 132 // and decide whether all of the loads and stores to the alloca are within 133 // the same basic block. 134 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) { 135 Instruction *User = cast<Instruction>(*UI++); 136 137 if (StoreInst *SI = dyn_cast<StoreInst>(User)) { 138 // Remember the basic blocks which define new values for the alloca 139 DefiningBlocks.push_back(SI->getParent()); 140 AllocaPointerVal = SI->getOperand(0); 141 OnlyStore = SI; 142 } else { 143 LoadInst *LI = cast<LoadInst>(User); 144 // Otherwise it must be a load instruction, keep track of variable 145 // reads. 146 UsingBlocks.push_back(LI->getParent()); 147 AllocaPointerVal = LI; 148 } 149 150 if (OnlyUsedInOneBlock) { 151 if (!OnlyBlock) 152 OnlyBlock = User->getParent(); 153 else if (OnlyBlock != User->getParent()) 154 OnlyUsedInOneBlock = false; 155 } 156 } 157 158 DbgDeclare = FindAllocaDbgDeclare(AI); 159 } 160 }; 161 162 // Data package used by RenamePass() 163 class RenamePassData { 164 public: 165 typedef std::vector<Value *> ValVector; 166 167 RenamePassData() : BB(nullptr), Pred(nullptr), Values() {} 168 RenamePassData(BasicBlock *B, BasicBlock *P, const ValVector &V) 169 : BB(B), Pred(P), Values(V) {} 170 BasicBlock *BB; 171 BasicBlock *Pred; 172 ValVector Values; 173 174 void swap(RenamePassData &RHS) { 175 std::swap(BB, RHS.BB); 176 std::swap(Pred, RHS.Pred); 177 Values.swap(RHS.Values); 178 } 179 }; 180 181 /// \brief This assigns and keeps a per-bb relative ordering of load/store 182 /// instructions in the block that directly load or store an alloca. 183 /// 184 /// This functionality is important because it avoids scanning large basic 185 /// blocks multiple times when promoting many allocas in the same block. 186 class LargeBlockInfo { 187 /// \brief For each instruction that we track, keep the index of the 188 /// instruction. 189 /// 190 /// The index starts out as the number of the instruction from the start of 191 /// the block. 192 DenseMap<const Instruction *, unsigned> InstNumbers; 193 194 public: 195 196 /// This code only looks at accesses to allocas. 197 static bool isInterestingInstruction(const Instruction *I) { 198 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) || 199 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1))); 200 } 201 202 /// Get or calculate the index of the specified instruction. 203 unsigned getInstructionIndex(const Instruction *I) { 204 assert(isInterestingInstruction(I) && 205 "Not a load/store to/from an alloca?"); 206 207 // If we already have this instruction number, return it. 208 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I); 209 if (It != InstNumbers.end()) 210 return It->second; 211 212 // Scan the whole block to get the instruction. This accumulates 213 // information for every interesting instruction in the block, in order to 214 // avoid gratuitus rescans. 215 const BasicBlock *BB = I->getParent(); 216 unsigned InstNo = 0; 217 for (BasicBlock::const_iterator BBI = BB->begin(), E = BB->end(); BBI != E; 218 ++BBI) 219 if (isInterestingInstruction(BBI)) 220 InstNumbers[BBI] = InstNo++; 221 It = InstNumbers.find(I); 222 223 assert(It != InstNumbers.end() && "Didn't insert instruction?"); 224 return It->second; 225 } 226 227 void deleteValue(const Instruction *I) { InstNumbers.erase(I); } 228 229 void clear() { InstNumbers.clear(); } 230 }; 231 232 struct PromoteMem2Reg { 233 /// The alloca instructions being promoted. 234 std::vector<AllocaInst *> Allocas; 235 DominatorTree &DT; 236 DIBuilder DIB; 237 238 /// An AliasSetTracker object to update. If null, don't update it. 239 AliasSetTracker *AST; 240 241 /// Reverse mapping of Allocas. 242 DenseMap<AllocaInst *, unsigned> AllocaLookup; 243 244 /// \brief The PhiNodes we're adding. 245 /// 246 /// That map is used to simplify some Phi nodes as we iterate over it, so 247 /// it should have deterministic iterators. We could use a MapVector, but 248 /// since we already maintain a map from BasicBlock* to a stable numbering 249 /// (BBNumbers), the DenseMap is more efficient (also supports removal). 250 DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes; 251 252 /// For each PHI node, keep track of which entry in Allocas it corresponds 253 /// to. 254 DenseMap<PHINode *, unsigned> PhiToAllocaMap; 255 256 /// If we are updating an AliasSetTracker, then for each alloca that is of 257 /// pointer type, we keep track of what to copyValue to the inserted PHI 258 /// nodes here. 259 std::vector<Value *> PointerAllocaValues; 260 261 /// For each alloca, we keep track of the dbg.declare intrinsic that 262 /// describes it, if any, so that we can convert it to a dbg.value 263 /// intrinsic if the alloca gets promoted. 264 SmallVector<DbgDeclareInst *, 8> AllocaDbgDeclares; 265 266 /// The set of basic blocks the renamer has already visited. 267 /// 268 SmallPtrSet<BasicBlock *, 16> Visited; 269 270 /// Contains a stable numbering of basic blocks to avoid non-determinstic 271 /// behavior. 272 DenseMap<BasicBlock *, unsigned> BBNumbers; 273 274 /// Maps DomTreeNodes to their level in the dominator tree. 275 DenseMap<DomTreeNode *, unsigned> DomLevels; 276 277 /// Lazily compute the number of predecessors a block has. 278 DenseMap<const BasicBlock *, unsigned> BBNumPreds; 279 280 public: 281 PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 282 AliasSetTracker *AST) 283 : Allocas(Allocas.begin(), Allocas.end()), DT(DT), 284 DIB(*DT.getRoot()->getParent()->getParent()), AST(AST) {} 285 286 void run(); 287 288 private: 289 void RemoveFromAllocasList(unsigned &AllocaIdx) { 290 Allocas[AllocaIdx] = Allocas.back(); 291 Allocas.pop_back(); 292 --AllocaIdx; 293 } 294 295 unsigned getNumPreds(const BasicBlock *BB) { 296 unsigned &NP = BBNumPreds[BB]; 297 if (NP == 0) 298 NP = std::distance(pred_begin(BB), pred_end(BB)) + 1; 299 return NP - 1; 300 } 301 302 void DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum, 303 AllocaInfo &Info); 304 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info, 305 const SmallPtrSet<BasicBlock *, 32> &DefBlocks, 306 SmallPtrSet<BasicBlock *, 32> &LiveInBlocks); 307 void RenamePass(BasicBlock *BB, BasicBlock *Pred, 308 RenamePassData::ValVector &IncVals, 309 std::vector<RenamePassData> &Worklist); 310 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version); 311 }; 312 313 } // end of anonymous namespace 314 315 static void removeLifetimeIntrinsicUsers(AllocaInst *AI) { 316 // Knowing that this alloca is promotable, we know that it's safe to kill all 317 // instructions except for load and store. 318 319 for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) { 320 Instruction *I = cast<Instruction>(*UI); 321 ++UI; 322 if (isa<LoadInst>(I) || isa<StoreInst>(I)) 323 continue; 324 325 if (!I->getType()->isVoidTy()) { 326 // The only users of this bitcast/GEP instruction are lifetime intrinsics. 327 // Follow the use/def chain to erase them now instead of leaving it for 328 // dead code elimination later. 329 for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) { 330 Instruction *Inst = cast<Instruction>(*UUI); 331 ++UUI; 332 Inst->eraseFromParent(); 333 } 334 } 335 I->eraseFromParent(); 336 } 337 } 338 339 /// \brief Rewrite as many loads as possible given a single store. 340 /// 341 /// When there is only a single store, we can use the domtree to trivially 342 /// replace all of the dominated loads with the stored value. Do so, and return 343 /// true if this has successfully promoted the alloca entirely. If this returns 344 /// false there were some loads which were not dominated by the single store 345 /// and thus must be phi-ed with undef. We fall back to the standard alloca 346 /// promotion algorithm in that case. 347 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info, 348 LargeBlockInfo &LBI, 349 DominatorTree &DT, 350 AliasSetTracker *AST) { 351 StoreInst *OnlyStore = Info.OnlyStore; 352 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0)); 353 BasicBlock *StoreBB = OnlyStore->getParent(); 354 int StoreIndex = -1; 355 356 // Clear out UsingBlocks. We will reconstruct it here if needed. 357 Info.UsingBlocks.clear(); 358 359 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) { 360 Instruction *UserInst = cast<Instruction>(*UI++); 361 if (!isa<LoadInst>(UserInst)) { 362 assert(UserInst == OnlyStore && "Should only have load/stores"); 363 continue; 364 } 365 LoadInst *LI = cast<LoadInst>(UserInst); 366 367 // Okay, if we have a load from the alloca, we want to replace it with the 368 // only value stored to the alloca. We can do this if the value is 369 // dominated by the store. If not, we use the rest of the mem2reg machinery 370 // to insert the phi nodes as needed. 371 if (!StoringGlobalVal) { // Non-instructions are always dominated. 372 if (LI->getParent() == StoreBB) { 373 // If we have a use that is in the same block as the store, compare the 374 // indices of the two instructions to see which one came first. If the 375 // load came before the store, we can't handle it. 376 if (StoreIndex == -1) 377 StoreIndex = LBI.getInstructionIndex(OnlyStore); 378 379 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) { 380 // Can't handle this load, bail out. 381 Info.UsingBlocks.push_back(StoreBB); 382 continue; 383 } 384 385 } else if (LI->getParent() != StoreBB && 386 !DT.dominates(StoreBB, LI->getParent())) { 387 // If the load and store are in different blocks, use BB dominance to 388 // check their relationships. If the store doesn't dom the use, bail 389 // out. 390 Info.UsingBlocks.push_back(LI->getParent()); 391 continue; 392 } 393 } 394 395 // Otherwise, we *can* safely rewrite this load. 396 Value *ReplVal = OnlyStore->getOperand(0); 397 // If the replacement value is the load, this must occur in unreachable 398 // code. 399 if (ReplVal == LI) 400 ReplVal = UndefValue::get(LI->getType()); 401 LI->replaceAllUsesWith(ReplVal); 402 if (AST && LI->getType()->isPointerTy()) 403 AST->deleteValue(LI); 404 LI->eraseFromParent(); 405 LBI.deleteValue(LI); 406 } 407 408 // Finally, after the scan, check to see if the store is all that is left. 409 if (!Info.UsingBlocks.empty()) 410 return false; // If not, we'll have to fall back for the remainder. 411 412 // Record debuginfo for the store and remove the declaration's 413 // debuginfo. 414 if (DbgDeclareInst *DDI = Info.DbgDeclare) { 415 DIBuilder DIB(*AI->getParent()->getParent()->getParent()); 416 ConvertDebugDeclareToDebugValue(DDI, Info.OnlyStore, DIB); 417 DDI->eraseFromParent(); 418 LBI.deleteValue(DDI); 419 } 420 // Remove the (now dead) store and alloca. 421 Info.OnlyStore->eraseFromParent(); 422 LBI.deleteValue(Info.OnlyStore); 423 424 if (AST) 425 AST->deleteValue(AI); 426 AI->eraseFromParent(); 427 LBI.deleteValue(AI); 428 return true; 429 } 430 431 /// Many allocas are only used within a single basic block. If this is the 432 /// case, avoid traversing the CFG and inserting a lot of potentially useless 433 /// PHI nodes by just performing a single linear pass over the basic block 434 /// using the Alloca. 435 /// 436 /// If we cannot promote this alloca (because it is read before it is written), 437 /// return true. This is necessary in cases where, due to control flow, the 438 /// alloca is potentially undefined on some control flow paths. e.g. code like 439 /// this is potentially correct: 440 /// 441 /// for (...) { if (c) { A = undef; undef = B; } } 442 /// 443 /// ... so long as A is not used before undef is set. 444 static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info, 445 LargeBlockInfo &LBI, 446 AliasSetTracker *AST) { 447 // The trickiest case to handle is when we have large blocks. Because of this, 448 // this code is optimized assuming that large blocks happen. This does not 449 // significantly pessimize the small block case. This uses LargeBlockInfo to 450 // make it efficient to get the index of various operations in the block. 451 452 // Walk the use-def list of the alloca, getting the locations of all stores. 453 typedef SmallVector<std::pair<unsigned, StoreInst *>, 64> StoresByIndexTy; 454 StoresByIndexTy StoresByIndex; 455 456 for (User *U : AI->users()) 457 if (StoreInst *SI = dyn_cast<StoreInst>(U)) 458 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI)); 459 460 // Sort the stores by their index, making it efficient to do a lookup with a 461 // binary search. 462 std::sort(StoresByIndex.begin(), StoresByIndex.end(), less_first()); 463 464 // Walk all of the loads from this alloca, replacing them with the nearest 465 // store above them, if any. 466 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) { 467 LoadInst *LI = dyn_cast<LoadInst>(*UI++); 468 if (!LI) 469 continue; 470 471 unsigned LoadIdx = LBI.getInstructionIndex(LI); 472 473 // Find the nearest store that has a lower index than this load. 474 StoresByIndexTy::iterator I = 475 std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(), 476 std::make_pair(LoadIdx, 477 static_cast<StoreInst *>(nullptr)), 478 less_first()); 479 480 if (I == StoresByIndex.begin()) 481 // If there is no store before this load, the load takes the undef value. 482 LI->replaceAllUsesWith(UndefValue::get(LI->getType())); 483 else 484 // Otherwise, there was a store before this load, the load takes its value. 485 LI->replaceAllUsesWith(std::prev(I)->second->getOperand(0)); 486 487 if (AST && LI->getType()->isPointerTy()) 488 AST->deleteValue(LI); 489 LI->eraseFromParent(); 490 LBI.deleteValue(LI); 491 } 492 493 // Remove the (now dead) stores and alloca. 494 while (!AI->use_empty()) { 495 StoreInst *SI = cast<StoreInst>(AI->user_back()); 496 // Record debuginfo for the store before removing it. 497 if (DbgDeclareInst *DDI = Info.DbgDeclare) { 498 DIBuilder DIB(*AI->getParent()->getParent()->getParent()); 499 ConvertDebugDeclareToDebugValue(DDI, SI, DIB); 500 } 501 SI->eraseFromParent(); 502 LBI.deleteValue(SI); 503 } 504 505 if (AST) 506 AST->deleteValue(AI); 507 AI->eraseFromParent(); 508 LBI.deleteValue(AI); 509 510 // The alloca's debuginfo can be removed as well. 511 if (DbgDeclareInst *DDI = Info.DbgDeclare) { 512 DDI->eraseFromParent(); 513 LBI.deleteValue(DDI); 514 } 515 516 ++NumLocalPromoted; 517 } 518 519 void PromoteMem2Reg::run() { 520 Function &F = *DT.getRoot()->getParent(); 521 522 if (AST) 523 PointerAllocaValues.resize(Allocas.size()); 524 AllocaDbgDeclares.resize(Allocas.size()); 525 526 AllocaInfo Info; 527 LargeBlockInfo LBI; 528 529 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) { 530 AllocaInst *AI = Allocas[AllocaNum]; 531 532 assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!"); 533 assert(AI->getParent()->getParent() == &F && 534 "All allocas should be in the same function, which is same as DF!"); 535 536 removeLifetimeIntrinsicUsers(AI); 537 538 if (AI->use_empty()) { 539 // If there are no uses of the alloca, just delete it now. 540 if (AST) 541 AST->deleteValue(AI); 542 AI->eraseFromParent(); 543 544 // Remove the alloca from the Allocas list, since it has been processed 545 RemoveFromAllocasList(AllocaNum); 546 ++NumDeadAlloca; 547 continue; 548 } 549 550 // Calculate the set of read and write-locations for each alloca. This is 551 // analogous to finding the 'uses' and 'definitions' of each variable. 552 Info.AnalyzeAlloca(AI); 553 554 // If there is only a single store to this value, replace any loads of 555 // it that are directly dominated by the definition with the value stored. 556 if (Info.DefiningBlocks.size() == 1) { 557 if (rewriteSingleStoreAlloca(AI, Info, LBI, DT, AST)) { 558 // The alloca has been processed, move on. 559 RemoveFromAllocasList(AllocaNum); 560 ++NumSingleStore; 561 continue; 562 } 563 } 564 565 // If the alloca is only read and written in one basic block, just perform a 566 // linear sweep over the block to eliminate it. 567 if (Info.OnlyUsedInOneBlock) { 568 promoteSingleBlockAlloca(AI, Info, LBI, AST); 569 570 // The alloca has been processed, move on. 571 RemoveFromAllocasList(AllocaNum); 572 continue; 573 } 574 575 // If we haven't computed dominator tree levels, do so now. 576 if (DomLevels.empty()) { 577 SmallVector<DomTreeNode *, 32> Worklist; 578 579 DomTreeNode *Root = DT.getRootNode(); 580 DomLevels[Root] = 0; 581 Worklist.push_back(Root); 582 583 while (!Worklist.empty()) { 584 DomTreeNode *Node = Worklist.pop_back_val(); 585 unsigned ChildLevel = DomLevels[Node] + 1; 586 for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end(); 587 CI != CE; ++CI) { 588 DomLevels[*CI] = ChildLevel; 589 Worklist.push_back(*CI); 590 } 591 } 592 } 593 594 // If we haven't computed a numbering for the BB's in the function, do so 595 // now. 596 if (BBNumbers.empty()) { 597 unsigned ID = 0; 598 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 599 BBNumbers[I] = ID++; 600 } 601 602 // If we have an AST to keep updated, remember some pointer value that is 603 // stored into the alloca. 604 if (AST) 605 PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal; 606 607 // Remember the dbg.declare intrinsic describing this alloca, if any. 608 if (Info.DbgDeclare) 609 AllocaDbgDeclares[AllocaNum] = Info.DbgDeclare; 610 611 // Keep the reverse mapping of the 'Allocas' array for the rename pass. 612 AllocaLookup[Allocas[AllocaNum]] = AllocaNum; 613 614 // At this point, we're committed to promoting the alloca using IDF's, and 615 // the standard SSA construction algorithm. Determine which blocks need PHI 616 // nodes and see if we can optimize out some work by avoiding insertion of 617 // dead phi nodes. 618 DetermineInsertionPoint(AI, AllocaNum, Info); 619 } 620 621 if (Allocas.empty()) 622 return; // All of the allocas must have been trivial! 623 624 LBI.clear(); 625 626 // Set the incoming values for the basic block to be null values for all of 627 // the alloca's. We do this in case there is a load of a value that has not 628 // been stored yet. In this case, it will get this null value. 629 // 630 RenamePassData::ValVector Values(Allocas.size()); 631 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) 632 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType()); 633 634 // Walks all basic blocks in the function performing the SSA rename algorithm 635 // and inserting the phi nodes we marked as necessary 636 // 637 std::vector<RenamePassData> RenamePassWorkList; 638 RenamePassWorkList.push_back(RenamePassData(F.begin(), nullptr, Values)); 639 do { 640 RenamePassData RPD; 641 RPD.swap(RenamePassWorkList.back()); 642 RenamePassWorkList.pop_back(); 643 // RenamePass may add new worklist entries. 644 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList); 645 } while (!RenamePassWorkList.empty()); 646 647 // The renamer uses the Visited set to avoid infinite loops. Clear it now. 648 Visited.clear(); 649 650 // Remove the allocas themselves from the function. 651 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) { 652 Instruction *A = Allocas[i]; 653 654 // If there are any uses of the alloca instructions left, they must be in 655 // unreachable basic blocks that were not processed by walking the dominator 656 // tree. Just delete the users now. 657 if (!A->use_empty()) 658 A->replaceAllUsesWith(UndefValue::get(A->getType())); 659 if (AST) 660 AST->deleteValue(A); 661 A->eraseFromParent(); 662 } 663 664 // Remove alloca's dbg.declare instrinsics from the function. 665 for (unsigned i = 0, e = AllocaDbgDeclares.size(); i != e; ++i) 666 if (DbgDeclareInst *DDI = AllocaDbgDeclares[i]) 667 DDI->eraseFromParent(); 668 669 // Loop over all of the PHI nodes and see if there are any that we can get 670 // rid of because they merge all of the same incoming values. This can 671 // happen due to undef values coming into the PHI nodes. This process is 672 // iterative, because eliminating one PHI node can cause others to be removed. 673 bool EliminatedAPHI = true; 674 while (EliminatedAPHI) { 675 EliminatedAPHI = false; 676 677 // Iterating over NewPhiNodes is deterministic, so it is safe to try to 678 // simplify and RAUW them as we go. If it was not, we could add uses to 679 // the values we replace with in a non-deterministic order, thus creating 680 // non-deterministic def->use chains. 681 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 682 I = NewPhiNodes.begin(), 683 E = NewPhiNodes.end(); 684 I != E;) { 685 PHINode *PN = I->second; 686 687 // If this PHI node merges one value and/or undefs, get the value. 688 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, &DT)) { 689 if (AST && PN->getType()->isPointerTy()) 690 AST->deleteValue(PN); 691 PN->replaceAllUsesWith(V); 692 PN->eraseFromParent(); 693 NewPhiNodes.erase(I++); 694 EliminatedAPHI = true; 695 continue; 696 } 697 ++I; 698 } 699 } 700 701 // At this point, the renamer has added entries to PHI nodes for all reachable 702 // code. Unfortunately, there may be unreachable blocks which the renamer 703 // hasn't traversed. If this is the case, the PHI nodes may not 704 // have incoming values for all predecessors. Loop over all PHI nodes we have 705 // created, inserting undef values if they are missing any incoming values. 706 // 707 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 708 I = NewPhiNodes.begin(), 709 E = NewPhiNodes.end(); 710 I != E; ++I) { 711 // We want to do this once per basic block. As such, only process a block 712 // when we find the PHI that is the first entry in the block. 713 PHINode *SomePHI = I->second; 714 BasicBlock *BB = SomePHI->getParent(); 715 if (&BB->front() != SomePHI) 716 continue; 717 718 // Only do work here if there the PHI nodes are missing incoming values. We 719 // know that all PHI nodes that were inserted in a block will have the same 720 // number of incoming values, so we can just check any of them. 721 if (SomePHI->getNumIncomingValues() == getNumPreds(BB)) 722 continue; 723 724 // Get the preds for BB. 725 SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB)); 726 727 // Ok, now we know that all of the PHI nodes are missing entries for some 728 // basic blocks. Start by sorting the incoming predecessors for efficient 729 // access. 730 std::sort(Preds.begin(), Preds.end()); 731 732 // Now we loop through all BB's which have entries in SomePHI and remove 733 // them from the Preds list. 734 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) { 735 // Do a log(n) search of the Preds list for the entry we want. 736 SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound( 737 Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i)); 738 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) && 739 "PHI node has entry for a block which is not a predecessor!"); 740 741 // Remove the entry 742 Preds.erase(EntIt); 743 } 744 745 // At this point, the blocks left in the preds list must have dummy 746 // entries inserted into every PHI nodes for the block. Update all the phi 747 // nodes in this block that we are inserting (there could be phis before 748 // mem2reg runs). 749 unsigned NumBadPreds = SomePHI->getNumIncomingValues(); 750 BasicBlock::iterator BBI = BB->begin(); 751 while ((SomePHI = dyn_cast<PHINode>(BBI++)) && 752 SomePHI->getNumIncomingValues() == NumBadPreds) { 753 Value *UndefVal = UndefValue::get(SomePHI->getType()); 754 for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred) 755 SomePHI->addIncoming(UndefVal, Preds[pred]); 756 } 757 } 758 759 NewPhiNodes.clear(); 760 } 761 762 /// \brief Determine which blocks the value is live in. 763 /// 764 /// These are blocks which lead to uses. Knowing this allows us to avoid 765 /// inserting PHI nodes into blocks which don't lead to uses (thus, the 766 /// inserted phi nodes would be dead). 767 void PromoteMem2Reg::ComputeLiveInBlocks( 768 AllocaInst *AI, AllocaInfo &Info, 769 const SmallPtrSet<BasicBlock *, 32> &DefBlocks, 770 SmallPtrSet<BasicBlock *, 32> &LiveInBlocks) { 771 772 // To determine liveness, we must iterate through the predecessors of blocks 773 // where the def is live. Blocks are added to the worklist if we need to 774 // check their predecessors. Start with all the using blocks. 775 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(), 776 Info.UsingBlocks.end()); 777 778 // If any of the using blocks is also a definition block, check to see if the 779 // definition occurs before or after the use. If it happens before the use, 780 // the value isn't really live-in. 781 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) { 782 BasicBlock *BB = LiveInBlockWorklist[i]; 783 if (!DefBlocks.count(BB)) 784 continue; 785 786 // Okay, this is a block that both uses and defines the value. If the first 787 // reference to the alloca is a def (store), then we know it isn't live-in. 788 for (BasicBlock::iterator I = BB->begin();; ++I) { 789 if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 790 if (SI->getOperand(1) != AI) 791 continue; 792 793 // We found a store to the alloca before a load. The alloca is not 794 // actually live-in here. 795 LiveInBlockWorklist[i] = LiveInBlockWorklist.back(); 796 LiveInBlockWorklist.pop_back(); 797 --i, --e; 798 break; 799 } 800 801 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 802 if (LI->getOperand(0) != AI) 803 continue; 804 805 // Okay, we found a load before a store to the alloca. It is actually 806 // live into this block. 807 break; 808 } 809 } 810 } 811 812 // Now that we have a set of blocks where the phi is live-in, recursively add 813 // their predecessors until we find the full region the value is live. 814 while (!LiveInBlockWorklist.empty()) { 815 BasicBlock *BB = LiveInBlockWorklist.pop_back_val(); 816 817 // The block really is live in here, insert it into the set. If already in 818 // the set, then it has already been processed. 819 if (!LiveInBlocks.insert(BB)) 820 continue; 821 822 // Since the value is live into BB, it is either defined in a predecessor or 823 // live into it to. Add the preds to the worklist unless they are a 824 // defining block. 825 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 826 BasicBlock *P = *PI; 827 828 // The value is not live into a predecessor if it defines the value. 829 if (DefBlocks.count(P)) 830 continue; 831 832 // Otherwise it is, add to the worklist. 833 LiveInBlockWorklist.push_back(P); 834 } 835 } 836 } 837 838 /// At this point, we're committed to promoting the alloca using IDF's, and the 839 /// standard SSA construction algorithm. Determine which blocks need phi nodes 840 /// and see if we can optimize out some work by avoiding insertion of dead phi 841 /// nodes. 842 void PromoteMem2Reg::DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum, 843 AllocaInfo &Info) { 844 // Unique the set of defining blocks for efficient lookup. 845 SmallPtrSet<BasicBlock *, 32> DefBlocks; 846 DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end()); 847 848 // Determine which blocks the value is live in. These are blocks which lead 849 // to uses. 850 SmallPtrSet<BasicBlock *, 32> LiveInBlocks; 851 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks); 852 853 // Use a priority queue keyed on dominator tree level so that inserted nodes 854 // are handled from the bottom of the dominator tree upwards. 855 typedef std::pair<DomTreeNode *, unsigned> DomTreeNodePair; 856 typedef std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>, 857 less_second> IDFPriorityQueue; 858 IDFPriorityQueue PQ; 859 860 for (SmallPtrSet<BasicBlock *, 32>::const_iterator I = DefBlocks.begin(), 861 E = DefBlocks.end(); 862 I != E; ++I) { 863 if (DomTreeNode *Node = DT.getNode(*I)) 864 PQ.push(std::make_pair(Node, DomLevels[Node])); 865 } 866 867 SmallVector<std::pair<unsigned, BasicBlock *>, 32> DFBlocks; 868 SmallPtrSet<DomTreeNode *, 32> Visited; 869 SmallVector<DomTreeNode *, 32> Worklist; 870 while (!PQ.empty()) { 871 DomTreeNodePair RootPair = PQ.top(); 872 PQ.pop(); 873 DomTreeNode *Root = RootPair.first; 874 unsigned RootLevel = RootPair.second; 875 876 // Walk all dominator tree children of Root, inspecting their CFG edges with 877 // targets elsewhere on the dominator tree. Only targets whose level is at 878 // most Root's level are added to the iterated dominance frontier of the 879 // definition set. 880 881 Worklist.clear(); 882 Worklist.push_back(Root); 883 884 while (!Worklist.empty()) { 885 DomTreeNode *Node = Worklist.pop_back_val(); 886 BasicBlock *BB = Node->getBlock(); 887 888 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; 889 ++SI) { 890 DomTreeNode *SuccNode = DT.getNode(*SI); 891 892 // Quickly skip all CFG edges that are also dominator tree edges instead 893 // of catching them below. 894 if (SuccNode->getIDom() == Node) 895 continue; 896 897 unsigned SuccLevel = DomLevels[SuccNode]; 898 if (SuccLevel > RootLevel) 899 continue; 900 901 if (!Visited.insert(SuccNode)) 902 continue; 903 904 BasicBlock *SuccBB = SuccNode->getBlock(); 905 if (!LiveInBlocks.count(SuccBB)) 906 continue; 907 908 DFBlocks.push_back(std::make_pair(BBNumbers[SuccBB], SuccBB)); 909 if (!DefBlocks.count(SuccBB)) 910 PQ.push(std::make_pair(SuccNode, SuccLevel)); 911 } 912 913 for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end(); CI != CE; 914 ++CI) { 915 if (!Visited.count(*CI)) 916 Worklist.push_back(*CI); 917 } 918 } 919 } 920 921 if (DFBlocks.size() > 1) 922 std::sort(DFBlocks.begin(), DFBlocks.end()); 923 924 unsigned CurrentVersion = 0; 925 for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i) 926 QueuePhiNode(DFBlocks[i].second, AllocaNum, CurrentVersion); 927 } 928 929 /// \brief Queue a phi-node to be added to a basic-block for a specific Alloca. 930 /// 931 /// Returns true if there wasn't already a phi-node for that variable 932 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo, 933 unsigned &Version) { 934 // Look up the basic-block in question. 935 PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)]; 936 937 // If the BB already has a phi node added for the i'th alloca then we're done! 938 if (PN) 939 return false; 940 941 // Create a PhiNode using the dereferenced type... and add the phi-node to the 942 // BasicBlock. 943 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB), 944 Allocas[AllocaNo]->getName() + "." + Twine(Version++), 945 BB->begin()); 946 ++NumPHIInsert; 947 PhiToAllocaMap[PN] = AllocaNo; 948 949 if (AST && PN->getType()->isPointerTy()) 950 AST->copyValue(PointerAllocaValues[AllocaNo], PN); 951 952 return true; 953 } 954 955 /// \brief Recursively traverse the CFG of the function, renaming loads and 956 /// stores to the allocas which we are promoting. 957 /// 958 /// IncomingVals indicates what value each Alloca contains on exit from the 959 /// predecessor block Pred. 960 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred, 961 RenamePassData::ValVector &IncomingVals, 962 std::vector<RenamePassData> &Worklist) { 963 NextIteration: 964 // If we are inserting any phi nodes into this BB, they will already be in the 965 // block. 966 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) { 967 // If we have PHI nodes to update, compute the number of edges from Pred to 968 // BB. 969 if (PhiToAllocaMap.count(APN)) { 970 // We want to be able to distinguish between PHI nodes being inserted by 971 // this invocation of mem2reg from those phi nodes that already existed in 972 // the IR before mem2reg was run. We determine that APN is being inserted 973 // because it is missing incoming edges. All other PHI nodes being 974 // inserted by this pass of mem2reg will have the same number of incoming 975 // operands so far. Remember this count. 976 unsigned NewPHINumOperands = APN->getNumOperands(); 977 978 unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB); 979 assert(NumEdges && "Must be at least one edge from Pred to BB!"); 980 981 // Add entries for all the phis. 982 BasicBlock::iterator PNI = BB->begin(); 983 do { 984 unsigned AllocaNo = PhiToAllocaMap[APN]; 985 986 // Add N incoming values to the PHI node. 987 for (unsigned i = 0; i != NumEdges; ++i) 988 APN->addIncoming(IncomingVals[AllocaNo], Pred); 989 990 // The currently active variable for this block is now the PHI. 991 IncomingVals[AllocaNo] = APN; 992 993 // Get the next phi node. 994 ++PNI; 995 APN = dyn_cast<PHINode>(PNI); 996 if (!APN) 997 break; 998 999 // Verify that it is missing entries. If not, it is not being inserted 1000 // by this mem2reg invocation so we want to ignore it. 1001 } while (APN->getNumOperands() == NewPHINumOperands); 1002 } 1003 } 1004 1005 // Don't revisit blocks. 1006 if (!Visited.insert(BB)) 1007 return; 1008 1009 for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II);) { 1010 Instruction *I = II++; // get the instruction, increment iterator 1011 1012 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 1013 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand()); 1014 if (!Src) 1015 continue; 1016 1017 DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src); 1018 if (AI == AllocaLookup.end()) 1019 continue; 1020 1021 Value *V = IncomingVals[AI->second]; 1022 1023 // Anything using the load now uses the current value. 1024 LI->replaceAllUsesWith(V); 1025 if (AST && LI->getType()->isPointerTy()) 1026 AST->deleteValue(LI); 1027 BB->getInstList().erase(LI); 1028 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 1029 // Delete this instruction and mark the name as the current holder of the 1030 // value 1031 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand()); 1032 if (!Dest) 1033 continue; 1034 1035 DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest); 1036 if (ai == AllocaLookup.end()) 1037 continue; 1038 1039 // what value were we writing? 1040 IncomingVals[ai->second] = SI->getOperand(0); 1041 // Record debuginfo for the store before removing it. 1042 if (DbgDeclareInst *DDI = AllocaDbgDeclares[ai->second]) 1043 ConvertDebugDeclareToDebugValue(DDI, SI, DIB); 1044 BB->getInstList().erase(SI); 1045 } 1046 } 1047 1048 // 'Recurse' to our successors. 1049 succ_iterator I = succ_begin(BB), E = succ_end(BB); 1050 if (I == E) 1051 return; 1052 1053 // Keep track of the successors so we don't visit the same successor twice 1054 SmallPtrSet<BasicBlock *, 8> VisitedSuccs; 1055 1056 // Handle the first successor without using the worklist. 1057 VisitedSuccs.insert(*I); 1058 Pred = BB; 1059 BB = *I; 1060 ++I; 1061 1062 for (; I != E; ++I) 1063 if (VisitedSuccs.insert(*I)) 1064 Worklist.push_back(RenamePassData(*I, Pred, IncomingVals)); 1065 1066 goto NextIteration; 1067 } 1068 1069 void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 1070 AliasSetTracker *AST) { 1071 // If there is nothing to do, bail out... 1072 if (Allocas.empty()) 1073 return; 1074 1075 PromoteMem2Reg(Allocas, DT, AST).run(); 1076 } 1077