1 //===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file promotes memory references to be register references. It promotes 10 // alloca instructions which only have loads and stores as uses. An alloca is 11 // transformed by using iterated dominator frontiers to place PHI nodes, then 12 // traversing the function in depth-first order to rewrite loads and stores as 13 // appropriate. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "llvm/ADT/ArrayRef.h" 18 #include "llvm/ADT/DenseMap.h" 19 #include "llvm/ADT/STLExtras.h" 20 #include "llvm/ADT/SmallPtrSet.h" 21 #include "llvm/ADT/SmallVector.h" 22 #include "llvm/ADT/Statistic.h" 23 #include "llvm/ADT/Twine.h" 24 #include "llvm/Analysis/AssumptionCache.h" 25 #include "llvm/Analysis/InstructionSimplify.h" 26 #include "llvm/Analysis/IteratedDominanceFrontier.h" 27 #include "llvm/Analysis/ValueTracking.h" 28 #include "llvm/IR/BasicBlock.h" 29 #include "llvm/IR/CFG.h" 30 #include "llvm/IR/Constant.h" 31 #include "llvm/IR/Constants.h" 32 #include "llvm/IR/DIBuilder.h" 33 #include "llvm/IR/DebugInfo.h" 34 #include "llvm/IR/Dominators.h" 35 #include "llvm/IR/Function.h" 36 #include "llvm/IR/InstrTypes.h" 37 #include "llvm/IR/Instruction.h" 38 #include "llvm/IR/Instructions.h" 39 #include "llvm/IR/IntrinsicInst.h" 40 #include "llvm/IR/Intrinsics.h" 41 #include "llvm/IR/LLVMContext.h" 42 #include "llvm/IR/Module.h" 43 #include "llvm/IR/Type.h" 44 #include "llvm/IR/User.h" 45 #include "llvm/Support/Casting.h" 46 #include "llvm/Transforms/Utils/Local.h" 47 #include "llvm/Transforms/Utils/PromoteMemToReg.h" 48 #include <algorithm> 49 #include <cassert> 50 #include <iterator> 51 #include <utility> 52 #include <vector> 53 54 using namespace llvm; 55 56 #define DEBUG_TYPE "mem2reg" 57 58 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block"); 59 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store"); 60 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed"); 61 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted"); 62 63 bool llvm::isAllocaPromotable(const AllocaInst *AI) { 64 // Only allow direct and non-volatile loads and stores... 65 for (const User *U : AI->users()) { 66 if (const LoadInst *LI = dyn_cast<LoadInst>(U)) { 67 // Note that atomic loads can be transformed; atomic semantics do 68 // not have any meaning for a local alloca. 69 if (LI->isVolatile() || LI->getType() != AI->getAllocatedType()) 70 return false; 71 } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { 72 if (SI->getValueOperand() == AI || 73 SI->getValueOperand()->getType() != AI->getAllocatedType()) 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->isLifetimeStartOrEnd() && !II->isDroppable()) 81 return false; 82 } else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) { 83 if (!onlyUsedByLifetimeMarkersOrDroppableInsts(BCI)) 84 return false; 85 } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) { 86 if (!GEPI->hasAllZeroIndices()) 87 return false; 88 if (!onlyUsedByLifetimeMarkersOrDroppableInsts(GEPI)) 89 return false; 90 } else if (const AddrSpaceCastInst *ASCI = dyn_cast<AddrSpaceCastInst>(U)) { 91 if (!onlyUsedByLifetimeMarkers(ASCI)) 92 return false; 93 } else { 94 return false; 95 } 96 } 97 98 return true; 99 } 100 101 namespace { 102 103 struct AllocaInfo { 104 using DbgUserVec = SmallVector<DbgVariableIntrinsic *, 1>; 105 106 SmallVector<BasicBlock *, 32> DefiningBlocks; 107 SmallVector<BasicBlock *, 32> UsingBlocks; 108 109 StoreInst *OnlyStore; 110 BasicBlock *OnlyBlock; 111 bool OnlyUsedInOneBlock; 112 113 DbgUserVec DbgUsers; 114 115 void clear() { 116 DefiningBlocks.clear(); 117 UsingBlocks.clear(); 118 OnlyStore = nullptr; 119 OnlyBlock = nullptr; 120 OnlyUsedInOneBlock = true; 121 DbgUsers.clear(); 122 } 123 124 /// Scan the uses of the specified alloca, filling in the AllocaInfo used 125 /// by the rest of the pass to reason about the uses of this alloca. 126 void AnalyzeAlloca(AllocaInst *AI) { 127 clear(); 128 129 // As we scan the uses of the alloca instruction, keep track of stores, 130 // and decide whether all of the loads and stores to the alloca are within 131 // the same basic block. 132 for (User *U : AI->users()) { 133 Instruction *User = cast<Instruction>(U); 134 135 if (StoreInst *SI = dyn_cast<StoreInst>(User)) { 136 // Remember the basic blocks which define new values for the alloca 137 DefiningBlocks.push_back(SI->getParent()); 138 OnlyStore = SI; 139 } else { 140 LoadInst *LI = cast<LoadInst>(User); 141 // Otherwise it must be a load instruction, keep track of variable 142 // reads. 143 UsingBlocks.push_back(LI->getParent()); 144 } 145 146 if (OnlyUsedInOneBlock) { 147 if (!OnlyBlock) 148 OnlyBlock = User->getParent(); 149 else if (OnlyBlock != User->getParent()) 150 OnlyUsedInOneBlock = false; 151 } 152 } 153 154 findDbgUsers(DbgUsers, AI); 155 } 156 }; 157 158 /// Data package used by RenamePass(). 159 struct RenamePassData { 160 using ValVector = std::vector<Value *>; 161 using LocationVector = std::vector<DebugLoc>; 162 163 RenamePassData(BasicBlock *B, BasicBlock *P, ValVector V, LocationVector L) 164 : BB(B), Pred(P), Values(std::move(V)), Locations(std::move(L)) {} 165 166 BasicBlock *BB; 167 BasicBlock *Pred; 168 ValVector Values; 169 LocationVector Locations; 170 }; 171 172 /// This assigns and keeps a per-bb relative ordering of load/store 173 /// instructions in the block that directly load or store an alloca. 174 /// 175 /// This functionality is important because it avoids scanning large basic 176 /// blocks multiple times when promoting many allocas in the same block. 177 class LargeBlockInfo { 178 /// For each instruction that we track, keep the index of the 179 /// instruction. 180 /// 181 /// The index starts out as the number of the instruction from the start of 182 /// the block. 183 DenseMap<const Instruction *, unsigned> InstNumbers; 184 185 public: 186 187 /// This code only looks at accesses to allocas. 188 static bool isInterestingInstruction(const Instruction *I) { 189 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) || 190 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1))); 191 } 192 193 /// Get or calculate the index of the specified instruction. 194 unsigned getInstructionIndex(const Instruction *I) { 195 assert(isInterestingInstruction(I) && 196 "Not a load/store to/from an alloca?"); 197 198 // If we already have this instruction number, return it. 199 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I); 200 if (It != InstNumbers.end()) 201 return It->second; 202 203 // Scan the whole block to get the instruction. This accumulates 204 // information for every interesting instruction in the block, in order to 205 // avoid gratuitus rescans. 206 const BasicBlock *BB = I->getParent(); 207 unsigned InstNo = 0; 208 for (const Instruction &BBI : *BB) 209 if (isInterestingInstruction(&BBI)) 210 InstNumbers[&BBI] = InstNo++; 211 It = InstNumbers.find(I); 212 213 assert(It != InstNumbers.end() && "Didn't insert instruction?"); 214 return It->second; 215 } 216 217 void deleteValue(const Instruction *I) { InstNumbers.erase(I); } 218 219 void clear() { InstNumbers.clear(); } 220 }; 221 222 struct PromoteMem2Reg { 223 /// The alloca instructions being promoted. 224 std::vector<AllocaInst *> Allocas; 225 226 DominatorTree &DT; 227 DIBuilder DIB; 228 229 /// A cache of @llvm.assume intrinsics used by SimplifyInstruction. 230 AssumptionCache *AC; 231 232 const SimplifyQuery SQ; 233 234 /// Reverse mapping of Allocas. 235 DenseMap<AllocaInst *, unsigned> AllocaLookup; 236 237 /// The PhiNodes we're adding. 238 /// 239 /// That map is used to simplify some Phi nodes as we iterate over it, so 240 /// it should have deterministic iterators. We could use a MapVector, but 241 /// since we already maintain a map from BasicBlock* to a stable numbering 242 /// (BBNumbers), the DenseMap is more efficient (also supports removal). 243 DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes; 244 245 /// For each PHI node, keep track of which entry in Allocas it corresponds 246 /// to. 247 DenseMap<PHINode *, unsigned> PhiToAllocaMap; 248 249 /// For each alloca, we keep track of the dbg.declare intrinsic that 250 /// describes it, if any, so that we can convert it to a dbg.value 251 /// intrinsic if the alloca gets promoted. 252 SmallVector<AllocaInfo::DbgUserVec, 8> AllocaDbgUsers; 253 254 /// The set of basic blocks the renamer has already visited. 255 SmallPtrSet<BasicBlock *, 16> Visited; 256 257 /// Contains a stable numbering of basic blocks to avoid non-determinstic 258 /// behavior. 259 DenseMap<BasicBlock *, unsigned> BBNumbers; 260 261 /// Lazily compute the number of predecessors a block has. 262 DenseMap<const BasicBlock *, unsigned> BBNumPreds; 263 264 public: 265 PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 266 AssumptionCache *AC) 267 : Allocas(Allocas.begin(), Allocas.end()), DT(DT), 268 DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false), 269 AC(AC), SQ(DT.getRoot()->getParent()->getParent()->getDataLayout(), 270 nullptr, &DT, AC) {} 271 272 void run(); 273 274 private: 275 void RemoveFromAllocasList(unsigned &AllocaIdx) { 276 Allocas[AllocaIdx] = Allocas.back(); 277 Allocas.pop_back(); 278 --AllocaIdx; 279 } 280 281 unsigned getNumPreds(const BasicBlock *BB) { 282 unsigned &NP = BBNumPreds[BB]; 283 if (NP == 0) 284 NP = pred_size(BB) + 1; 285 return NP - 1; 286 } 287 288 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info, 289 const SmallPtrSetImpl<BasicBlock *> &DefBlocks, 290 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks); 291 void RenamePass(BasicBlock *BB, BasicBlock *Pred, 292 RenamePassData::ValVector &IncVals, 293 RenamePassData::LocationVector &IncLocs, 294 std::vector<RenamePassData> &Worklist); 295 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version); 296 }; 297 298 } // end anonymous namespace 299 300 /// Given a LoadInst LI this adds assume(LI != null) after it. 301 static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) { 302 Function *AssumeIntrinsic = 303 Intrinsic::getDeclaration(LI->getModule(), Intrinsic::assume); 304 ICmpInst *LoadNotNull = new ICmpInst(ICmpInst::ICMP_NE, LI, 305 Constant::getNullValue(LI->getType())); 306 LoadNotNull->insertAfter(LI); 307 CallInst *CI = CallInst::Create(AssumeIntrinsic, {LoadNotNull}); 308 CI->insertAfter(LoadNotNull); 309 AC->registerAssumption(cast<AssumeInst>(CI)); 310 } 311 312 static void removeIntrinsicUsers(AllocaInst *AI) { 313 // Knowing that this alloca is promotable, we know that it's safe to kill all 314 // instructions except for load and store. 315 316 for (Use &U : llvm::make_early_inc_range(AI->uses())) { 317 Instruction *I = cast<Instruction>(U.getUser()); 318 if (isa<LoadInst>(I) || isa<StoreInst>(I)) 319 continue; 320 321 // Drop the use of AI in droppable instructions. 322 if (I->isDroppable()) { 323 I->dropDroppableUse(U); 324 continue; 325 } 326 327 if (!I->getType()->isVoidTy()) { 328 // The only users of this bitcast/GEP instruction are lifetime intrinsics. 329 // Follow the use/def chain to erase them now instead of leaving it for 330 // dead code elimination later. 331 for (Use &UU : llvm::make_early_inc_range(I->uses())) { 332 Instruction *Inst = cast<Instruction>(UU.getUser()); 333 334 // Drop the use of I in droppable instructions. 335 if (Inst->isDroppable()) { 336 Inst->dropDroppableUse(UU); 337 continue; 338 } 339 Inst->eraseFromParent(); 340 } 341 } 342 I->eraseFromParent(); 343 } 344 } 345 346 /// Rewrite as many loads as possible given a single store. 347 /// 348 /// When there is only a single store, we can use the domtree to trivially 349 /// replace all of the dominated loads with the stored value. Do so, and return 350 /// true if this has successfully promoted the alloca entirely. If this returns 351 /// false there were some loads which were not dominated by the single store 352 /// and thus must be phi-ed with undef. We fall back to the standard alloca 353 /// promotion algorithm in that case. 354 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info, 355 LargeBlockInfo &LBI, const DataLayout &DL, 356 DominatorTree &DT, AssumptionCache *AC) { 357 StoreInst *OnlyStore = Info.OnlyStore; 358 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0)); 359 BasicBlock *StoreBB = OnlyStore->getParent(); 360 int StoreIndex = -1; 361 362 // Clear out UsingBlocks. We will reconstruct it here if needed. 363 Info.UsingBlocks.clear(); 364 365 for (User *U : make_early_inc_range(AI->users())) { 366 Instruction *UserInst = cast<Instruction>(U); 367 if (UserInst == OnlyStore) 368 continue; 369 LoadInst *LI = cast<LoadInst>(UserInst); 370 371 // Okay, if we have a load from the alloca, we want to replace it with the 372 // only value stored to the alloca. We can do this if the value is 373 // dominated by the store. If not, we use the rest of the mem2reg machinery 374 // to insert the phi nodes as needed. 375 if (!StoringGlobalVal) { // Non-instructions are always dominated. 376 if (LI->getParent() == StoreBB) { 377 // If we have a use that is in the same block as the store, compare the 378 // indices of the two instructions to see which one came first. If the 379 // load came before the store, we can't handle it. 380 if (StoreIndex == -1) 381 StoreIndex = LBI.getInstructionIndex(OnlyStore); 382 383 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) { 384 // Can't handle this load, bail out. 385 Info.UsingBlocks.push_back(StoreBB); 386 continue; 387 } 388 } else if (!DT.dominates(StoreBB, LI->getParent())) { 389 // If the load and store are in different blocks, use BB dominance to 390 // check their relationships. If the store doesn't dom the use, bail 391 // out. 392 Info.UsingBlocks.push_back(LI->getParent()); 393 continue; 394 } 395 } 396 397 // Otherwise, we *can* safely rewrite this load. 398 Value *ReplVal = OnlyStore->getOperand(0); 399 // If the replacement value is the load, this must occur in unreachable 400 // code. 401 if (ReplVal == LI) 402 ReplVal = PoisonValue::get(LI->getType()); 403 404 // If the load was marked as nonnull we don't want to lose 405 // that information when we erase this Load. So we preserve 406 // it with an assume. 407 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) && 408 !isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT)) 409 addAssumeNonNull(AC, LI); 410 411 LI->replaceAllUsesWith(ReplVal); 412 LI->eraseFromParent(); 413 LBI.deleteValue(LI); 414 } 415 416 // Finally, after the scan, check to see if the store is all that is left. 417 if (!Info.UsingBlocks.empty()) 418 return false; // If not, we'll have to fall back for the remainder. 419 420 // Record debuginfo for the store and remove the declaration's 421 // debuginfo. 422 for (DbgVariableIntrinsic *DII : Info.DbgUsers) { 423 if (DII->isAddressOfVariable()) { 424 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false); 425 ConvertDebugDeclareToDebugValue(DII, Info.OnlyStore, DIB); 426 DII->eraseFromParent(); 427 } else if (DII->getExpression()->startsWithDeref()) { 428 DII->eraseFromParent(); 429 } 430 } 431 // Remove the (now dead) store and alloca. 432 Info.OnlyStore->eraseFromParent(); 433 LBI.deleteValue(Info.OnlyStore); 434 435 AI->eraseFromParent(); 436 return true; 437 } 438 439 /// Many allocas are only used within a single basic block. If this is the 440 /// case, avoid traversing the CFG and inserting a lot of potentially useless 441 /// PHI nodes by just performing a single linear pass over the basic block 442 /// using the Alloca. 443 /// 444 /// If we cannot promote this alloca (because it is read before it is written), 445 /// return false. This is necessary in cases where, due to control flow, the 446 /// alloca is undefined only on some control flow paths. e.g. code like 447 /// this is correct in LLVM IR: 448 /// // A is an alloca with no stores so far 449 /// for (...) { 450 /// int t = *A; 451 /// if (!first_iteration) 452 /// use(t); 453 /// *A = 42; 454 /// } 455 static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info, 456 LargeBlockInfo &LBI, 457 const DataLayout &DL, 458 DominatorTree &DT, 459 AssumptionCache *AC) { 460 // The trickiest case to handle is when we have large blocks. Because of this, 461 // this code is optimized assuming that large blocks happen. This does not 462 // significantly pessimize the small block case. This uses LargeBlockInfo to 463 // make it efficient to get the index of various operations in the block. 464 465 // Walk the use-def list of the alloca, getting the locations of all stores. 466 using StoresByIndexTy = SmallVector<std::pair<unsigned, StoreInst *>, 64>; 467 StoresByIndexTy StoresByIndex; 468 469 for (User *U : AI->users()) 470 if (StoreInst *SI = dyn_cast<StoreInst>(U)) 471 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI)); 472 473 // Sort the stores by their index, making it efficient to do a lookup with a 474 // binary search. 475 llvm::sort(StoresByIndex, less_first()); 476 477 // Walk all of the loads from this alloca, replacing them with the nearest 478 // store above them, if any. 479 for (User *U : make_early_inc_range(AI->users())) { 480 LoadInst *LI = dyn_cast<LoadInst>(U); 481 if (!LI) 482 continue; 483 484 unsigned LoadIdx = LBI.getInstructionIndex(LI); 485 486 // Find the nearest store that has a lower index than this load. 487 StoresByIndexTy::iterator I = llvm::lower_bound( 488 StoresByIndex, 489 std::make_pair(LoadIdx, static_cast<StoreInst *>(nullptr)), 490 less_first()); 491 if (I == StoresByIndex.begin()) { 492 if (StoresByIndex.empty()) 493 // If there are no stores, the load takes the undef value. 494 LI->replaceAllUsesWith(UndefValue::get(LI->getType())); 495 else 496 // There is no store before this load, bail out (load may be affected 497 // by the following stores - see main comment). 498 return false; 499 } else { 500 // Otherwise, there was a store before this load, the load takes its value. 501 // Note, if the load was marked as nonnull we don't want to lose that 502 // information when we erase it. So we preserve it with an assume. 503 Value *ReplVal = std::prev(I)->second->getOperand(0); 504 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) && 505 !isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT)) 506 addAssumeNonNull(AC, LI); 507 508 // If the replacement value is the load, this must occur in unreachable 509 // code. 510 if (ReplVal == LI) 511 ReplVal = PoisonValue::get(LI->getType()); 512 513 LI->replaceAllUsesWith(ReplVal); 514 } 515 516 LI->eraseFromParent(); 517 LBI.deleteValue(LI); 518 } 519 520 // Remove the (now dead) stores and alloca. 521 while (!AI->use_empty()) { 522 StoreInst *SI = cast<StoreInst>(AI->user_back()); 523 // Record debuginfo for the store before removing it. 524 for (DbgVariableIntrinsic *DII : Info.DbgUsers) { 525 if (DII->isAddressOfVariable()) { 526 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false); 527 ConvertDebugDeclareToDebugValue(DII, SI, DIB); 528 } 529 } 530 SI->eraseFromParent(); 531 LBI.deleteValue(SI); 532 } 533 534 AI->eraseFromParent(); 535 536 // The alloca's debuginfo can be removed as well. 537 for (DbgVariableIntrinsic *DII : Info.DbgUsers) 538 if (DII->isAddressOfVariable() || DII->getExpression()->startsWithDeref()) 539 DII->eraseFromParent(); 540 541 ++NumLocalPromoted; 542 return true; 543 } 544 545 void PromoteMem2Reg::run() { 546 Function &F = *DT.getRoot()->getParent(); 547 548 AllocaDbgUsers.resize(Allocas.size()); 549 550 AllocaInfo Info; 551 LargeBlockInfo LBI; 552 ForwardIDFCalculator IDF(DT); 553 554 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) { 555 AllocaInst *AI = Allocas[AllocaNum]; 556 557 assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!"); 558 assert(AI->getParent()->getParent() == &F && 559 "All allocas should be in the same function, which is same as DF!"); 560 561 removeIntrinsicUsers(AI); 562 563 if (AI->use_empty()) { 564 // If there are no uses of the alloca, just delete it now. 565 AI->eraseFromParent(); 566 567 // Remove the alloca from the Allocas list, since it has been processed 568 RemoveFromAllocasList(AllocaNum); 569 ++NumDeadAlloca; 570 continue; 571 } 572 573 // Calculate the set of read and write-locations for each alloca. This is 574 // analogous to finding the 'uses' and 'definitions' of each variable. 575 Info.AnalyzeAlloca(AI); 576 577 // If there is only a single store to this value, replace any loads of 578 // it that are directly dominated by the definition with the value stored. 579 if (Info.DefiningBlocks.size() == 1) { 580 if (rewriteSingleStoreAlloca(AI, Info, LBI, SQ.DL, DT, AC)) { 581 // The alloca has been processed, move on. 582 RemoveFromAllocasList(AllocaNum); 583 ++NumSingleStore; 584 continue; 585 } 586 } 587 588 // If the alloca is only read and written in one basic block, just perform a 589 // linear sweep over the block to eliminate it. 590 if (Info.OnlyUsedInOneBlock && 591 promoteSingleBlockAlloca(AI, Info, LBI, SQ.DL, DT, AC)) { 592 // The alloca has been processed, move on. 593 RemoveFromAllocasList(AllocaNum); 594 continue; 595 } 596 597 // If we haven't computed a numbering for the BB's in the function, do so 598 // now. 599 if (BBNumbers.empty()) { 600 unsigned ID = 0; 601 for (auto &BB : F) 602 BBNumbers[&BB] = ID++; 603 } 604 605 // Remember the dbg.declare intrinsic describing this alloca, if any. 606 if (!Info.DbgUsers.empty()) 607 AllocaDbgUsers[AllocaNum] = Info.DbgUsers; 608 609 // Keep the reverse mapping of the 'Allocas' array for the rename pass. 610 AllocaLookup[Allocas[AllocaNum]] = AllocaNum; 611 612 // Unique the set of defining blocks for efficient lookup. 613 SmallPtrSet<BasicBlock *, 32> DefBlocks(Info.DefiningBlocks.begin(), 614 Info.DefiningBlocks.end()); 615 616 // Determine which blocks the value is live in. These are blocks which lead 617 // to uses. 618 SmallPtrSet<BasicBlock *, 32> LiveInBlocks; 619 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks); 620 621 // At this point, we're committed to promoting the alloca using IDF's, and 622 // the standard SSA construction algorithm. Determine which blocks need phi 623 // nodes and see if we can optimize out some work by avoiding insertion of 624 // dead phi nodes. 625 IDF.setLiveInBlocks(LiveInBlocks); 626 IDF.setDefiningBlocks(DefBlocks); 627 SmallVector<BasicBlock *, 32> PHIBlocks; 628 IDF.calculate(PHIBlocks); 629 llvm::sort(PHIBlocks, [this](BasicBlock *A, BasicBlock *B) { 630 return BBNumbers.find(A)->second < BBNumbers.find(B)->second; 631 }); 632 633 unsigned CurrentVersion = 0; 634 for (BasicBlock *BB : PHIBlocks) 635 QueuePhiNode(BB, AllocaNum, CurrentVersion); 636 } 637 638 if (Allocas.empty()) 639 return; // All of the allocas must have been trivial! 640 641 LBI.clear(); 642 643 // Set the incoming values for the basic block to be null values for all of 644 // the alloca's. We do this in case there is a load of a value that has not 645 // been stored yet. In this case, it will get this null value. 646 RenamePassData::ValVector Values(Allocas.size()); 647 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) 648 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType()); 649 650 // When handling debug info, treat all incoming values as if they have unknown 651 // locations until proven otherwise. 652 RenamePassData::LocationVector Locations(Allocas.size()); 653 654 // Walks all basic blocks in the function performing the SSA rename algorithm 655 // and inserting the phi nodes we marked as necessary 656 std::vector<RenamePassData> RenamePassWorkList; 657 RenamePassWorkList.emplace_back(&F.front(), nullptr, std::move(Values), 658 std::move(Locations)); 659 do { 660 RenamePassData RPD = std::move(RenamePassWorkList.back()); 661 RenamePassWorkList.pop_back(); 662 // RenamePass may add new worklist entries. 663 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RPD.Locations, RenamePassWorkList); 664 } while (!RenamePassWorkList.empty()); 665 666 // The renamer uses the Visited set to avoid infinite loops. Clear it now. 667 Visited.clear(); 668 669 // Remove the allocas themselves from the function. 670 for (Instruction *A : Allocas) { 671 // If there are any uses of the alloca instructions left, they must be in 672 // unreachable basic blocks that were not processed by walking the dominator 673 // tree. Just delete the users now. 674 if (!A->use_empty()) 675 A->replaceAllUsesWith(PoisonValue::get(A->getType())); 676 A->eraseFromParent(); 677 } 678 679 // Remove alloca's dbg.declare intrinsics from the function. 680 for (auto &DbgUsers : AllocaDbgUsers) { 681 for (auto *DII : DbgUsers) 682 if (DII->isAddressOfVariable() || DII->getExpression()->startsWithDeref()) 683 DII->eraseFromParent(); 684 } 685 686 // Loop over all of the PHI nodes and see if there are any that we can get 687 // rid of because they merge all of the same incoming values. This can 688 // happen due to undef values coming into the PHI nodes. This process is 689 // iterative, because eliminating one PHI node can cause others to be removed. 690 bool EliminatedAPHI = true; 691 while (EliminatedAPHI) { 692 EliminatedAPHI = false; 693 694 // Iterating over NewPhiNodes is deterministic, so it is safe to try to 695 // simplify and RAUW them as we go. If it was not, we could add uses to 696 // the values we replace with in a non-deterministic order, thus creating 697 // non-deterministic def->use chains. 698 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 699 I = NewPhiNodes.begin(), 700 E = NewPhiNodes.end(); 701 I != E;) { 702 PHINode *PN = I->second; 703 704 // If this PHI node merges one value and/or undefs, get the value. 705 if (Value *V = simplifyInstruction(PN, SQ)) { 706 PN->replaceAllUsesWith(V); 707 PN->eraseFromParent(); 708 NewPhiNodes.erase(I++); 709 EliminatedAPHI = true; 710 continue; 711 } 712 ++I; 713 } 714 } 715 716 // At this point, the renamer has added entries to PHI nodes for all reachable 717 // code. Unfortunately, there may be unreachable blocks which the renamer 718 // hasn't traversed. If this is the case, the PHI nodes may not 719 // have incoming values for all predecessors. Loop over all PHI nodes we have 720 // created, inserting undef values if they are missing any incoming values. 721 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 722 I = NewPhiNodes.begin(), 723 E = NewPhiNodes.end(); 724 I != E; ++I) { 725 // We want to do this once per basic block. As such, only process a block 726 // when we find the PHI that is the first entry in the block. 727 PHINode *SomePHI = I->second; 728 BasicBlock *BB = SomePHI->getParent(); 729 if (&BB->front() != SomePHI) 730 continue; 731 732 // Only do work here if there the PHI nodes are missing incoming values. We 733 // know that all PHI nodes that were inserted in a block will have the same 734 // number of incoming values, so we can just check any of them. 735 if (SomePHI->getNumIncomingValues() == getNumPreds(BB)) 736 continue; 737 738 // Get the preds for BB. 739 SmallVector<BasicBlock *, 16> Preds(predecessors(BB)); 740 741 // Ok, now we know that all of the PHI nodes are missing entries for some 742 // basic blocks. Start by sorting the incoming predecessors for efficient 743 // access. 744 auto CompareBBNumbers = [this](BasicBlock *A, BasicBlock *B) { 745 return BBNumbers.find(A)->second < BBNumbers.find(B)->second; 746 }; 747 llvm::sort(Preds, CompareBBNumbers); 748 749 // Now we loop through all BB's which have entries in SomePHI and remove 750 // them from the Preds list. 751 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) { 752 // Do a log(n) search of the Preds list for the entry we want. 753 SmallVectorImpl<BasicBlock *>::iterator EntIt = llvm::lower_bound( 754 Preds, SomePHI->getIncomingBlock(i), CompareBBNumbers); 755 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) && 756 "PHI node has entry for a block which is not a predecessor!"); 757 758 // Remove the entry 759 Preds.erase(EntIt); 760 } 761 762 // At this point, the blocks left in the preds list must have dummy 763 // entries inserted into every PHI nodes for the block. Update all the phi 764 // nodes in this block that we are inserting (there could be phis before 765 // mem2reg runs). 766 unsigned NumBadPreds = SomePHI->getNumIncomingValues(); 767 BasicBlock::iterator BBI = BB->begin(); 768 while ((SomePHI = dyn_cast<PHINode>(BBI++)) && 769 SomePHI->getNumIncomingValues() == NumBadPreds) { 770 Value *UndefVal = UndefValue::get(SomePHI->getType()); 771 for (BasicBlock *Pred : Preds) 772 SomePHI->addIncoming(UndefVal, Pred); 773 } 774 } 775 776 NewPhiNodes.clear(); 777 } 778 779 /// Determine which blocks the value is live in. 780 /// 781 /// These are blocks which lead to uses. Knowing this allows us to avoid 782 /// inserting PHI nodes into blocks which don't lead to uses (thus, the 783 /// inserted phi nodes would be dead). 784 void PromoteMem2Reg::ComputeLiveInBlocks( 785 AllocaInst *AI, AllocaInfo &Info, 786 const SmallPtrSetImpl<BasicBlock *> &DefBlocks, 787 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) { 788 // To determine liveness, we must iterate through the predecessors of blocks 789 // where the def is live. Blocks are added to the worklist if we need to 790 // check their predecessors. Start with all the using blocks. 791 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(), 792 Info.UsingBlocks.end()); 793 794 // If any of the using blocks is also a definition block, check to see if the 795 // definition occurs before or after the use. If it happens before the use, 796 // the value isn't really live-in. 797 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) { 798 BasicBlock *BB = LiveInBlockWorklist[i]; 799 if (!DefBlocks.count(BB)) 800 continue; 801 802 // Okay, this is a block that both uses and defines the value. If the first 803 // reference to the alloca is a def (store), then we know it isn't live-in. 804 for (BasicBlock::iterator I = BB->begin();; ++I) { 805 if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 806 if (SI->getOperand(1) != AI) 807 continue; 808 809 // We found a store to the alloca before a load. The alloca is not 810 // actually live-in here. 811 LiveInBlockWorklist[i] = LiveInBlockWorklist.back(); 812 LiveInBlockWorklist.pop_back(); 813 --i; 814 --e; 815 break; 816 } 817 818 if (LoadInst *LI = dyn_cast<LoadInst>(I)) 819 // Okay, we found a load before a store to the alloca. It is actually 820 // live into this block. 821 if (LI->getOperand(0) == AI) 822 break; 823 } 824 } 825 826 // Now that we have a set of blocks where the phi is live-in, recursively add 827 // their predecessors until we find the full region the value is live. 828 while (!LiveInBlockWorklist.empty()) { 829 BasicBlock *BB = LiveInBlockWorklist.pop_back_val(); 830 831 // The block really is live in here, insert it into the set. If already in 832 // the set, then it has already been processed. 833 if (!LiveInBlocks.insert(BB).second) 834 continue; 835 836 // Since the value is live into BB, it is either defined in a predecessor or 837 // live into it to. Add the preds to the worklist unless they are a 838 // defining block. 839 for (BasicBlock *P : predecessors(BB)) { 840 // The value is not live into a predecessor if it defines the value. 841 if (DefBlocks.count(P)) 842 continue; 843 844 // Otherwise it is, add to the worklist. 845 LiveInBlockWorklist.push_back(P); 846 } 847 } 848 } 849 850 /// Queue a phi-node to be added to a basic-block for a specific Alloca. 851 /// 852 /// Returns true if there wasn't already a phi-node for that variable 853 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo, 854 unsigned &Version) { 855 // Look up the basic-block in question. 856 PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)]; 857 858 // If the BB already has a phi node added for the i'th alloca then we're done! 859 if (PN) 860 return false; 861 862 // Create a PhiNode using the dereferenced type... and add the phi-node to the 863 // BasicBlock. 864 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB), 865 Allocas[AllocaNo]->getName() + "." + Twine(Version++), 866 &BB->front()); 867 ++NumPHIInsert; 868 PhiToAllocaMap[PN] = AllocaNo; 869 return true; 870 } 871 872 /// Update the debug location of a phi. \p ApplyMergedLoc indicates whether to 873 /// create a merged location incorporating \p DL, or to set \p DL directly. 874 static void updateForIncomingValueLocation(PHINode *PN, DebugLoc DL, 875 bool ApplyMergedLoc) { 876 if (ApplyMergedLoc) 877 PN->applyMergedLocation(PN->getDebugLoc(), DL); 878 else 879 PN->setDebugLoc(DL); 880 } 881 882 /// Recursively traverse the CFG of the function, renaming loads and 883 /// stores to the allocas which we are promoting. 884 /// 885 /// IncomingVals indicates what value each Alloca contains on exit from the 886 /// predecessor block Pred. 887 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred, 888 RenamePassData::ValVector &IncomingVals, 889 RenamePassData::LocationVector &IncomingLocs, 890 std::vector<RenamePassData> &Worklist) { 891 NextIteration: 892 // If we are inserting any phi nodes into this BB, they will already be in the 893 // block. 894 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) { 895 // If we have PHI nodes to update, compute the number of edges from Pred to 896 // BB. 897 if (PhiToAllocaMap.count(APN)) { 898 // We want to be able to distinguish between PHI nodes being inserted by 899 // this invocation of mem2reg from those phi nodes that already existed in 900 // the IR before mem2reg was run. We determine that APN is being inserted 901 // because it is missing incoming edges. All other PHI nodes being 902 // inserted by this pass of mem2reg will have the same number of incoming 903 // operands so far. Remember this count. 904 unsigned NewPHINumOperands = APN->getNumOperands(); 905 906 unsigned NumEdges = llvm::count(successors(Pred), BB); 907 assert(NumEdges && "Must be at least one edge from Pred to BB!"); 908 909 // Add entries for all the phis. 910 BasicBlock::iterator PNI = BB->begin(); 911 do { 912 unsigned AllocaNo = PhiToAllocaMap[APN]; 913 914 // Update the location of the phi node. 915 updateForIncomingValueLocation(APN, IncomingLocs[AllocaNo], 916 APN->getNumIncomingValues() > 0); 917 918 // Add N incoming values to the PHI node. 919 for (unsigned i = 0; i != NumEdges; ++i) 920 APN->addIncoming(IncomingVals[AllocaNo], Pred); 921 922 // The currently active variable for this block is now the PHI. 923 IncomingVals[AllocaNo] = APN; 924 for (DbgVariableIntrinsic *DII : AllocaDbgUsers[AllocaNo]) 925 if (DII->isAddressOfVariable()) 926 ConvertDebugDeclareToDebugValue(DII, APN, DIB); 927 928 // Get the next phi node. 929 ++PNI; 930 APN = dyn_cast<PHINode>(PNI); 931 if (!APN) 932 break; 933 934 // Verify that it is missing entries. If not, it is not being inserted 935 // by this mem2reg invocation so we want to ignore it. 936 } while (APN->getNumOperands() == NewPHINumOperands); 937 } 938 } 939 940 // Don't revisit blocks. 941 if (!Visited.insert(BB).second) 942 return; 943 944 for (BasicBlock::iterator II = BB->begin(); !II->isTerminator();) { 945 Instruction *I = &*II++; // get the instruction, increment iterator 946 947 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 948 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand()); 949 if (!Src) 950 continue; 951 952 DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src); 953 if (AI == AllocaLookup.end()) 954 continue; 955 956 Value *V = IncomingVals[AI->second]; 957 958 // If the load was marked as nonnull we don't want to lose 959 // that information when we erase this Load. So we preserve 960 // it with an assume. 961 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) && 962 !isKnownNonZero(V, SQ.DL, 0, AC, LI, &DT)) 963 addAssumeNonNull(AC, LI); 964 965 // Anything using the load now uses the current value. 966 LI->replaceAllUsesWith(V); 967 BB->getInstList().erase(LI); 968 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 969 // Delete this instruction and mark the name as the current holder of the 970 // value 971 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand()); 972 if (!Dest) 973 continue; 974 975 DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest); 976 if (ai == AllocaLookup.end()) 977 continue; 978 979 // what value were we writing? 980 unsigned AllocaNo = ai->second; 981 IncomingVals[AllocaNo] = SI->getOperand(0); 982 983 // Record debuginfo for the store before removing it. 984 IncomingLocs[AllocaNo] = SI->getDebugLoc(); 985 for (DbgVariableIntrinsic *DII : AllocaDbgUsers[ai->second]) 986 if (DII->isAddressOfVariable()) 987 ConvertDebugDeclareToDebugValue(DII, SI, DIB); 988 BB->getInstList().erase(SI); 989 } 990 } 991 992 // 'Recurse' to our successors. 993 succ_iterator I = succ_begin(BB), E = succ_end(BB); 994 if (I == E) 995 return; 996 997 // Keep track of the successors so we don't visit the same successor twice 998 SmallPtrSet<BasicBlock *, 8> VisitedSuccs; 999 1000 // Handle the first successor without using the worklist. 1001 VisitedSuccs.insert(*I); 1002 Pred = BB; 1003 BB = *I; 1004 ++I; 1005 1006 for (; I != E; ++I) 1007 if (VisitedSuccs.insert(*I).second) 1008 Worklist.emplace_back(*I, Pred, IncomingVals, IncomingLocs); 1009 1010 goto NextIteration; 1011 } 1012 1013 void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 1014 AssumptionCache *AC) { 1015 // If there is nothing to do, bail out... 1016 if (Allocas.empty()) 1017 return; 1018 1019 PromoteMem2Reg(Allocas, DT, AC).run(); 1020 } 1021