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 /// Helper for updating assignment tracking debug info when promoting allocas. 104 class AssignmentTrackingInfo { 105 /// DbgAssignIntrinsics linked to the alloca with at most one per variable 106 /// fragment. (i.e. not be a comprehensive set if there are multiple 107 /// dbg.assigns for one variable fragment). 108 SmallVector<DbgVariableIntrinsic *> DbgAssigns; 109 110 public: 111 void init(AllocaInst *AI) { 112 SmallSet<DebugVariable, 2> Vars; 113 for (DbgAssignIntrinsic *DAI : at::getAssignmentMarkers(AI)) { 114 if (Vars.insert(DebugVariable(DAI)).second) 115 DbgAssigns.push_back(DAI); 116 } 117 } 118 119 /// Update assignment tracking debug info given for the to-be-deleted store 120 /// \p ToDelete that stores to this alloca. 121 void updateForDeletedStore(StoreInst *ToDelete, DIBuilder &DIB) const { 122 // There's nothing to do if the alloca doesn't have any variables using 123 // assignment tracking. 124 if (DbgAssigns.empty()) { 125 assert(at::getAssignmentMarkers(ToDelete).empty()); 126 return; 127 } 128 129 // Just leave dbg.assign intrinsics in place and remember that we've seen 130 // one for each variable fragment. 131 SmallSet<DebugVariable, 2> VarHasDbgAssignForStore; 132 for (DbgAssignIntrinsic *DAI : at::getAssignmentMarkers(ToDelete)) 133 VarHasDbgAssignForStore.insert(DebugVariable(DAI)); 134 135 // It's possible for variables using assignment tracking to have no 136 // dbg.assign linked to this store. These are variables in DbgAssigns that 137 // are missing from VarHasDbgAssignForStore. Since there isn't a dbg.assign 138 // to mark the assignment - and the store is going to be deleted - insert a 139 // dbg.value to do that now. An untracked store may be either one that 140 // cannot be represented using assignment tracking (non-const offset or 141 // size) or one that is trackable but has had its DIAssignID attachment 142 // dropped accidentally. 143 for (auto *DAI : DbgAssigns) { 144 if (VarHasDbgAssignForStore.contains(DebugVariable(DAI))) 145 continue; 146 ConvertDebugDeclareToDebugValue(DAI, ToDelete, DIB); 147 } 148 } 149 150 /// Update assignment tracking debug info given for the newly inserted PHI \p 151 /// NewPhi. 152 void updateForNewPhi(PHINode *NewPhi, DIBuilder &DIB) const { 153 // Regardless of the position of dbg.assigns relative to stores, the 154 // incoming values into a new PHI should be the same for the (imaginary) 155 // debug-phi. 156 for (auto *DAI : DbgAssigns) 157 ConvertDebugDeclareToDebugValue(DAI, NewPhi, DIB); 158 } 159 160 void clear() { DbgAssigns.clear(); } 161 bool empty() { return DbgAssigns.empty(); } 162 }; 163 164 struct AllocaInfo { 165 using DbgUserVec = SmallVector<DbgVariableIntrinsic *, 1>; 166 167 SmallVector<BasicBlock *, 32> DefiningBlocks; 168 SmallVector<BasicBlock *, 32> UsingBlocks; 169 170 StoreInst *OnlyStore; 171 BasicBlock *OnlyBlock; 172 bool OnlyUsedInOneBlock; 173 174 /// Debug users of the alloca - does not include dbg.assign intrinsics. 175 DbgUserVec DbgUsers; 176 /// Helper to update assignment tracking debug info. 177 AssignmentTrackingInfo AssignmentTracking; 178 179 void clear() { 180 DefiningBlocks.clear(); 181 UsingBlocks.clear(); 182 OnlyStore = nullptr; 183 OnlyBlock = nullptr; 184 OnlyUsedInOneBlock = true; 185 DbgUsers.clear(); 186 AssignmentTracking.clear(); 187 } 188 189 /// Scan the uses of the specified alloca, filling in the AllocaInfo used 190 /// by the rest of the pass to reason about the uses of this alloca. 191 void AnalyzeAlloca(AllocaInst *AI) { 192 clear(); 193 194 // As we scan the uses of the alloca instruction, keep track of stores, 195 // and decide whether all of the loads and stores to the alloca are within 196 // the same basic block. 197 for (User *U : AI->users()) { 198 Instruction *User = cast<Instruction>(U); 199 200 if (StoreInst *SI = dyn_cast<StoreInst>(User)) { 201 // Remember the basic blocks which define new values for the alloca 202 DefiningBlocks.push_back(SI->getParent()); 203 OnlyStore = SI; 204 } else { 205 LoadInst *LI = cast<LoadInst>(User); 206 // Otherwise it must be a load instruction, keep track of variable 207 // reads. 208 UsingBlocks.push_back(LI->getParent()); 209 } 210 211 if (OnlyUsedInOneBlock) { 212 if (!OnlyBlock) 213 OnlyBlock = User->getParent(); 214 else if (OnlyBlock != User->getParent()) 215 OnlyUsedInOneBlock = false; 216 } 217 } 218 DbgUserVec AllDbgUsers; 219 findDbgUsers(AllDbgUsers, AI); 220 std::copy_if(AllDbgUsers.begin(), AllDbgUsers.end(), 221 std::back_inserter(DbgUsers), [](DbgVariableIntrinsic *DII) { 222 return !isa<DbgAssignIntrinsic>(DII); 223 }); 224 AssignmentTracking.init(AI); 225 } 226 }; 227 228 /// Data package used by RenamePass(). 229 struct RenamePassData { 230 using ValVector = std::vector<Value *>; 231 using LocationVector = std::vector<DebugLoc>; 232 233 RenamePassData(BasicBlock *B, BasicBlock *P, ValVector V, LocationVector L) 234 : BB(B), Pred(P), Values(std::move(V)), Locations(std::move(L)) {} 235 236 BasicBlock *BB; 237 BasicBlock *Pred; 238 ValVector Values; 239 LocationVector Locations; 240 }; 241 242 /// This assigns and keeps a per-bb relative ordering of load/store 243 /// instructions in the block that directly load or store an alloca. 244 /// 245 /// This functionality is important because it avoids scanning large basic 246 /// blocks multiple times when promoting many allocas in the same block. 247 class LargeBlockInfo { 248 /// For each instruction that we track, keep the index of the 249 /// instruction. 250 /// 251 /// The index starts out as the number of the instruction from the start of 252 /// the block. 253 DenseMap<const Instruction *, unsigned> InstNumbers; 254 255 public: 256 257 /// This code only looks at accesses to allocas. 258 static bool isInterestingInstruction(const Instruction *I) { 259 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) || 260 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1))); 261 } 262 263 /// Get or calculate the index of the specified instruction. 264 unsigned getInstructionIndex(const Instruction *I) { 265 assert(isInterestingInstruction(I) && 266 "Not a load/store to/from an alloca?"); 267 268 // If we already have this instruction number, return it. 269 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I); 270 if (It != InstNumbers.end()) 271 return It->second; 272 273 // Scan the whole block to get the instruction. This accumulates 274 // information for every interesting instruction in the block, in order to 275 // avoid gratuitus rescans. 276 const BasicBlock *BB = I->getParent(); 277 unsigned InstNo = 0; 278 for (const Instruction &BBI : *BB) 279 if (isInterestingInstruction(&BBI)) 280 InstNumbers[&BBI] = InstNo++; 281 It = InstNumbers.find(I); 282 283 assert(It != InstNumbers.end() && "Didn't insert instruction?"); 284 return It->second; 285 } 286 287 void deleteValue(const Instruction *I) { InstNumbers.erase(I); } 288 289 void clear() { InstNumbers.clear(); } 290 }; 291 292 struct PromoteMem2Reg { 293 /// The alloca instructions being promoted. 294 std::vector<AllocaInst *> Allocas; 295 296 DominatorTree &DT; 297 DIBuilder DIB; 298 299 /// A cache of @llvm.assume intrinsics used by SimplifyInstruction. 300 AssumptionCache *AC; 301 302 const SimplifyQuery SQ; 303 304 /// Reverse mapping of Allocas. 305 DenseMap<AllocaInst *, unsigned> AllocaLookup; 306 307 /// The PhiNodes we're adding. 308 /// 309 /// That map is used to simplify some Phi nodes as we iterate over it, so 310 /// it should have deterministic iterators. We could use a MapVector, but 311 /// since we already maintain a map from BasicBlock* to a stable numbering 312 /// (BBNumbers), the DenseMap is more efficient (also supports removal). 313 DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes; 314 315 /// For each PHI node, keep track of which entry in Allocas it corresponds 316 /// to. 317 DenseMap<PHINode *, unsigned> PhiToAllocaMap; 318 319 /// For each alloca, we keep track of the dbg.declare intrinsic that 320 /// describes it, if any, so that we can convert it to a dbg.value 321 /// intrinsic if the alloca gets promoted. 322 SmallVector<AllocaInfo::DbgUserVec, 8> AllocaDbgUsers; 323 324 /// For each alloca, keep an instance of a helper class that gives us an easy 325 /// way to update assignment tracking debug info if the alloca is promoted. 326 SmallVector<AssignmentTrackingInfo, 8> AllocaATInfo; 327 328 /// The set of basic blocks the renamer has already visited. 329 SmallPtrSet<BasicBlock *, 16> Visited; 330 331 /// Contains a stable numbering of basic blocks to avoid non-determinstic 332 /// behavior. 333 DenseMap<BasicBlock *, unsigned> BBNumbers; 334 335 /// Lazily compute the number of predecessors a block has. 336 DenseMap<const BasicBlock *, unsigned> BBNumPreds; 337 338 public: 339 PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 340 AssumptionCache *AC) 341 : Allocas(Allocas.begin(), Allocas.end()), DT(DT), 342 DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false), 343 AC(AC), SQ(DT.getRoot()->getParent()->getParent()->getDataLayout(), 344 nullptr, &DT, AC) {} 345 346 void run(); 347 348 private: 349 void RemoveFromAllocasList(unsigned &AllocaIdx) { 350 Allocas[AllocaIdx] = Allocas.back(); 351 Allocas.pop_back(); 352 --AllocaIdx; 353 } 354 355 unsigned getNumPreds(const BasicBlock *BB) { 356 unsigned &NP = BBNumPreds[BB]; 357 if (NP == 0) 358 NP = pred_size(BB) + 1; 359 return NP - 1; 360 } 361 362 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info, 363 const SmallPtrSetImpl<BasicBlock *> &DefBlocks, 364 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks); 365 void RenamePass(BasicBlock *BB, BasicBlock *Pred, 366 RenamePassData::ValVector &IncVals, 367 RenamePassData::LocationVector &IncLocs, 368 std::vector<RenamePassData> &Worklist); 369 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version); 370 }; 371 372 } // end anonymous namespace 373 374 /// Given a LoadInst LI this adds assume(LI != null) after it. 375 static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) { 376 Function *AssumeIntrinsic = 377 Intrinsic::getDeclaration(LI->getModule(), Intrinsic::assume); 378 ICmpInst *LoadNotNull = new ICmpInst(ICmpInst::ICMP_NE, LI, 379 Constant::getNullValue(LI->getType())); 380 LoadNotNull->insertAfter(LI); 381 CallInst *CI = CallInst::Create(AssumeIntrinsic, {LoadNotNull}); 382 CI->insertAfter(LoadNotNull); 383 AC->registerAssumption(cast<AssumeInst>(CI)); 384 } 385 386 static void convertMetadataToAssumes(LoadInst *LI, Value *Val, 387 const DataLayout &DL, AssumptionCache *AC, 388 const DominatorTree *DT) { 389 // If the load was marked as nonnull we don't want to lose that information 390 // when we erase this Load. So we preserve it with an assume. As !nonnull 391 // returns poison while assume violations are immediate undefined behavior, 392 // we can only do this if the value is known non-poison. 393 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) && 394 LI->getMetadata(LLVMContext::MD_noundef) && 395 !isKnownNonZero(Val, DL, 0, AC, LI, DT)) 396 addAssumeNonNull(AC, LI); 397 } 398 399 static void removeIntrinsicUsers(AllocaInst *AI) { 400 // Knowing that this alloca is promotable, we know that it's safe to kill all 401 // instructions except for load and store. 402 403 for (Use &U : llvm::make_early_inc_range(AI->uses())) { 404 Instruction *I = cast<Instruction>(U.getUser()); 405 if (isa<LoadInst>(I) || isa<StoreInst>(I)) 406 continue; 407 408 // Drop the use of AI in droppable instructions. 409 if (I->isDroppable()) { 410 I->dropDroppableUse(U); 411 continue; 412 } 413 414 if (!I->getType()->isVoidTy()) { 415 // The only users of this bitcast/GEP instruction are lifetime intrinsics. 416 // Follow the use/def chain to erase them now instead of leaving it for 417 // dead code elimination later. 418 for (Use &UU : llvm::make_early_inc_range(I->uses())) { 419 Instruction *Inst = cast<Instruction>(UU.getUser()); 420 421 // Drop the use of I in droppable instructions. 422 if (Inst->isDroppable()) { 423 Inst->dropDroppableUse(UU); 424 continue; 425 } 426 Inst->eraseFromParent(); 427 } 428 } 429 I->eraseFromParent(); 430 } 431 } 432 433 /// Rewrite as many loads as possible given a single store. 434 /// 435 /// When there is only a single store, we can use the domtree to trivially 436 /// replace all of the dominated loads with the stored value. Do so, and return 437 /// true if this has successfully promoted the alloca entirely. If this returns 438 /// false there were some loads which were not dominated by the single store 439 /// and thus must be phi-ed with undef. We fall back to the standard alloca 440 /// promotion algorithm in that case. 441 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info, 442 LargeBlockInfo &LBI, const DataLayout &DL, 443 DominatorTree &DT, AssumptionCache *AC) { 444 StoreInst *OnlyStore = Info.OnlyStore; 445 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0)); 446 BasicBlock *StoreBB = OnlyStore->getParent(); 447 int StoreIndex = -1; 448 449 // Clear out UsingBlocks. We will reconstruct it here if needed. 450 Info.UsingBlocks.clear(); 451 452 for (User *U : make_early_inc_range(AI->users())) { 453 Instruction *UserInst = cast<Instruction>(U); 454 if (UserInst == OnlyStore) 455 continue; 456 LoadInst *LI = cast<LoadInst>(UserInst); 457 458 // Okay, if we have a load from the alloca, we want to replace it with the 459 // only value stored to the alloca. We can do this if the value is 460 // dominated by the store. If not, we use the rest of the mem2reg machinery 461 // to insert the phi nodes as needed. 462 if (!StoringGlobalVal) { // Non-instructions are always dominated. 463 if (LI->getParent() == StoreBB) { 464 // If we have a use that is in the same block as the store, compare the 465 // indices of the two instructions to see which one came first. If the 466 // load came before the store, we can't handle it. 467 if (StoreIndex == -1) 468 StoreIndex = LBI.getInstructionIndex(OnlyStore); 469 470 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) { 471 // Can't handle this load, bail out. 472 Info.UsingBlocks.push_back(StoreBB); 473 continue; 474 } 475 } else if (!DT.dominates(StoreBB, LI->getParent())) { 476 // If the load and store are in different blocks, use BB dominance to 477 // check their relationships. If the store doesn't dom the use, bail 478 // out. 479 Info.UsingBlocks.push_back(LI->getParent()); 480 continue; 481 } 482 } 483 484 // Otherwise, we *can* safely rewrite this load. 485 Value *ReplVal = OnlyStore->getOperand(0); 486 // If the replacement value is the load, this must occur in unreachable 487 // code. 488 if (ReplVal == LI) 489 ReplVal = PoisonValue::get(LI->getType()); 490 491 convertMetadataToAssumes(LI, ReplVal, DL, AC, &DT); 492 LI->replaceAllUsesWith(ReplVal); 493 LI->eraseFromParent(); 494 LBI.deleteValue(LI); 495 } 496 497 // Finally, after the scan, check to see if the store is all that is left. 498 if (!Info.UsingBlocks.empty()) 499 return false; // If not, we'll have to fall back for the remainder. 500 501 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false); 502 // Update assignment tracking info for the store we're going to delete. 503 Info.AssignmentTracking.updateForDeletedStore(Info.OnlyStore, DIB); 504 505 // Record debuginfo for the store and remove the declaration's 506 // debuginfo. 507 for (DbgVariableIntrinsic *DII : Info.DbgUsers) { 508 if (DII->isAddressOfVariable()) { 509 ConvertDebugDeclareToDebugValue(DII, Info.OnlyStore, DIB); 510 DII->eraseFromParent(); 511 } else if (DII->getExpression()->startsWithDeref()) { 512 DII->eraseFromParent(); 513 } 514 } 515 516 // Remove dbg.assigns linked to the alloca as these are now redundant. 517 at::deleteAssignmentMarkers(AI); 518 519 // Remove the (now dead) store and alloca. 520 Info.OnlyStore->eraseFromParent(); 521 LBI.deleteValue(Info.OnlyStore); 522 523 AI->eraseFromParent(); 524 return true; 525 } 526 527 /// Many allocas are only used within a single basic block. If this is the 528 /// case, avoid traversing the CFG and inserting a lot of potentially useless 529 /// PHI nodes by just performing a single linear pass over the basic block 530 /// using the Alloca. 531 /// 532 /// If we cannot promote this alloca (because it is read before it is written), 533 /// return false. This is necessary in cases where, due to control flow, the 534 /// alloca is undefined only on some control flow paths. e.g. code like 535 /// this is correct in LLVM IR: 536 /// // A is an alloca with no stores so far 537 /// for (...) { 538 /// int t = *A; 539 /// if (!first_iteration) 540 /// use(t); 541 /// *A = 42; 542 /// } 543 static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info, 544 LargeBlockInfo &LBI, 545 const DataLayout &DL, 546 DominatorTree &DT, 547 AssumptionCache *AC) { 548 // The trickiest case to handle is when we have large blocks. Because of this, 549 // this code is optimized assuming that large blocks happen. This does not 550 // significantly pessimize the small block case. This uses LargeBlockInfo to 551 // make it efficient to get the index of various operations in the block. 552 553 // Walk the use-def list of the alloca, getting the locations of all stores. 554 using StoresByIndexTy = SmallVector<std::pair<unsigned, StoreInst *>, 64>; 555 StoresByIndexTy StoresByIndex; 556 557 for (User *U : AI->users()) 558 if (StoreInst *SI = dyn_cast<StoreInst>(U)) 559 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI)); 560 561 // Sort the stores by their index, making it efficient to do a lookup with a 562 // binary search. 563 llvm::sort(StoresByIndex, less_first()); 564 565 // Walk all of the loads from this alloca, replacing them with the nearest 566 // store above them, if any. 567 for (User *U : make_early_inc_range(AI->users())) { 568 LoadInst *LI = dyn_cast<LoadInst>(U); 569 if (!LI) 570 continue; 571 572 unsigned LoadIdx = LBI.getInstructionIndex(LI); 573 574 // Find the nearest store that has a lower index than this load. 575 StoresByIndexTy::iterator I = llvm::lower_bound( 576 StoresByIndex, 577 std::make_pair(LoadIdx, static_cast<StoreInst *>(nullptr)), 578 less_first()); 579 Value *ReplVal; 580 if (I == StoresByIndex.begin()) { 581 if (StoresByIndex.empty()) 582 // If there are no stores, the load takes the undef value. 583 ReplVal = UndefValue::get(LI->getType()); 584 else 585 // There is no store before this load, bail out (load may be affected 586 // by the following stores - see main comment). 587 return false; 588 } else { 589 // Otherwise, there was a store before this load, the load takes its 590 // value. 591 ReplVal = std::prev(I)->second->getOperand(0); 592 } 593 594 convertMetadataToAssumes(LI, ReplVal, DL, AC, &DT); 595 596 // If the replacement value is the load, this must occur in unreachable 597 // code. 598 if (ReplVal == LI) 599 ReplVal = PoisonValue::get(LI->getType()); 600 601 LI->replaceAllUsesWith(ReplVal); 602 LI->eraseFromParent(); 603 LBI.deleteValue(LI); 604 } 605 606 // Remove the (now dead) stores and alloca. 607 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false); 608 while (!AI->use_empty()) { 609 StoreInst *SI = cast<StoreInst>(AI->user_back()); 610 // Update assignment tracking info for the store we're going to delete. 611 Info.AssignmentTracking.updateForDeletedStore(SI, DIB); 612 // Record debuginfo for the store before removing it. 613 for (DbgVariableIntrinsic *DII : Info.DbgUsers) { 614 if (DII->isAddressOfVariable()) { 615 ConvertDebugDeclareToDebugValue(DII, SI, DIB); 616 } 617 } 618 SI->eraseFromParent(); 619 LBI.deleteValue(SI); 620 } 621 622 // Remove dbg.assigns linked to the alloca as these are now redundant. 623 at::deleteAssignmentMarkers(AI); 624 AI->eraseFromParent(); 625 626 // The alloca's debuginfo can be removed as well. 627 for (DbgVariableIntrinsic *DII : Info.DbgUsers) 628 if (DII->isAddressOfVariable() || DII->getExpression()->startsWithDeref()) 629 DII->eraseFromParent(); 630 631 ++NumLocalPromoted; 632 return true; 633 } 634 635 void PromoteMem2Reg::run() { 636 Function &F = *DT.getRoot()->getParent(); 637 638 AllocaDbgUsers.resize(Allocas.size()); 639 AllocaATInfo.resize(Allocas.size()); 640 641 AllocaInfo Info; 642 LargeBlockInfo LBI; 643 ForwardIDFCalculator IDF(DT); 644 645 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) { 646 AllocaInst *AI = Allocas[AllocaNum]; 647 648 assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!"); 649 assert(AI->getParent()->getParent() == &F && 650 "All allocas should be in the same function, which is same as DF!"); 651 652 removeIntrinsicUsers(AI); 653 654 if (AI->use_empty()) { 655 // If there are no uses of the alloca, just delete it now. 656 AI->eraseFromParent(); 657 658 // Remove the alloca from the Allocas list, since it has been processed 659 RemoveFromAllocasList(AllocaNum); 660 ++NumDeadAlloca; 661 continue; 662 } 663 664 // Calculate the set of read and write-locations for each alloca. This is 665 // analogous to finding the 'uses' and 'definitions' of each variable. 666 Info.AnalyzeAlloca(AI); 667 668 // If there is only a single store to this value, replace any loads of 669 // it that are directly dominated by the definition with the value stored. 670 if (Info.DefiningBlocks.size() == 1) { 671 if (rewriteSingleStoreAlloca(AI, Info, LBI, SQ.DL, DT, AC)) { 672 // The alloca has been processed, move on. 673 RemoveFromAllocasList(AllocaNum); 674 ++NumSingleStore; 675 continue; 676 } 677 } 678 679 // If the alloca is only read and written in one basic block, just perform a 680 // linear sweep over the block to eliminate it. 681 if (Info.OnlyUsedInOneBlock && 682 promoteSingleBlockAlloca(AI, Info, LBI, SQ.DL, DT, AC)) { 683 // The alloca has been processed, move on. 684 RemoveFromAllocasList(AllocaNum); 685 continue; 686 } 687 688 // If we haven't computed a numbering for the BB's in the function, do so 689 // now. 690 if (BBNumbers.empty()) { 691 unsigned ID = 0; 692 for (auto &BB : F) 693 BBNumbers[&BB] = ID++; 694 } 695 696 // Remember the dbg.declare intrinsic describing this alloca, if any. 697 if (!Info.DbgUsers.empty()) 698 AllocaDbgUsers[AllocaNum] = Info.DbgUsers; 699 if (!Info.AssignmentTracking.empty()) 700 AllocaATInfo[AllocaNum] = Info.AssignmentTracking; 701 702 // Keep the reverse mapping of the 'Allocas' array for the rename pass. 703 AllocaLookup[Allocas[AllocaNum]] = AllocaNum; 704 705 // Unique the set of defining blocks for efficient lookup. 706 SmallPtrSet<BasicBlock *, 32> DefBlocks(Info.DefiningBlocks.begin(), 707 Info.DefiningBlocks.end()); 708 709 // Determine which blocks the value is live in. These are blocks which lead 710 // to uses. 711 SmallPtrSet<BasicBlock *, 32> LiveInBlocks; 712 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks); 713 714 // At this point, we're committed to promoting the alloca using IDF's, and 715 // the standard SSA construction algorithm. Determine which blocks need phi 716 // nodes and see if we can optimize out some work by avoiding insertion of 717 // dead phi nodes. 718 IDF.setLiveInBlocks(LiveInBlocks); 719 IDF.setDefiningBlocks(DefBlocks); 720 SmallVector<BasicBlock *, 32> PHIBlocks; 721 IDF.calculate(PHIBlocks); 722 llvm::sort(PHIBlocks, [this](BasicBlock *A, BasicBlock *B) { 723 return BBNumbers.find(A)->second < BBNumbers.find(B)->second; 724 }); 725 726 unsigned CurrentVersion = 0; 727 for (BasicBlock *BB : PHIBlocks) 728 QueuePhiNode(BB, AllocaNum, CurrentVersion); 729 } 730 731 if (Allocas.empty()) 732 return; // All of the allocas must have been trivial! 733 734 LBI.clear(); 735 736 // Set the incoming values for the basic block to be null values for all of 737 // the alloca's. We do this in case there is a load of a value that has not 738 // been stored yet. In this case, it will get this null value. 739 RenamePassData::ValVector Values(Allocas.size()); 740 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) 741 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType()); 742 743 // When handling debug info, treat all incoming values as if they have unknown 744 // locations until proven otherwise. 745 RenamePassData::LocationVector Locations(Allocas.size()); 746 747 // Walks all basic blocks in the function performing the SSA rename algorithm 748 // and inserting the phi nodes we marked as necessary 749 std::vector<RenamePassData> RenamePassWorkList; 750 RenamePassWorkList.emplace_back(&F.front(), nullptr, std::move(Values), 751 std::move(Locations)); 752 do { 753 RenamePassData RPD = std::move(RenamePassWorkList.back()); 754 RenamePassWorkList.pop_back(); 755 // RenamePass may add new worklist entries. 756 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RPD.Locations, RenamePassWorkList); 757 } while (!RenamePassWorkList.empty()); 758 759 // The renamer uses the Visited set to avoid infinite loops. Clear it now. 760 Visited.clear(); 761 762 // Remove the allocas themselves from the function. 763 for (Instruction *A : Allocas) { 764 // Remove dbg.assigns linked to the alloca as these are now redundant. 765 at::deleteAssignmentMarkers(A); 766 // If there are any uses of the alloca instructions left, they must be in 767 // unreachable basic blocks that were not processed by walking the dominator 768 // tree. Just delete the users now. 769 if (!A->use_empty()) 770 A->replaceAllUsesWith(PoisonValue::get(A->getType())); 771 A->eraseFromParent(); 772 } 773 774 // Remove alloca's dbg.declare intrinsics from the function. 775 for (auto &DbgUsers : AllocaDbgUsers) { 776 for (auto *DII : DbgUsers) 777 if (DII->isAddressOfVariable() || DII->getExpression()->startsWithDeref()) 778 DII->eraseFromParent(); 779 } 780 781 // Loop over all of the PHI nodes and see if there are any that we can get 782 // rid of because they merge all of the same incoming values. This can 783 // happen due to undef values coming into the PHI nodes. This process is 784 // iterative, because eliminating one PHI node can cause others to be removed. 785 bool EliminatedAPHI = true; 786 while (EliminatedAPHI) { 787 EliminatedAPHI = false; 788 789 // Iterating over NewPhiNodes is deterministic, so it is safe to try to 790 // simplify and RAUW them as we go. If it was not, we could add uses to 791 // the values we replace with in a non-deterministic order, thus creating 792 // non-deterministic def->use chains. 793 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 794 I = NewPhiNodes.begin(), 795 E = NewPhiNodes.end(); 796 I != E;) { 797 PHINode *PN = I->second; 798 799 // If this PHI node merges one value and/or undefs, get the value. 800 if (Value *V = simplifyInstruction(PN, SQ)) { 801 PN->replaceAllUsesWith(V); 802 PN->eraseFromParent(); 803 NewPhiNodes.erase(I++); 804 EliminatedAPHI = true; 805 continue; 806 } 807 ++I; 808 } 809 } 810 811 // At this point, the renamer has added entries to PHI nodes for all reachable 812 // code. Unfortunately, there may be unreachable blocks which the renamer 813 // hasn't traversed. If this is the case, the PHI nodes may not 814 // have incoming values for all predecessors. Loop over all PHI nodes we have 815 // created, inserting undef values if they are missing any incoming values. 816 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator 817 I = NewPhiNodes.begin(), 818 E = NewPhiNodes.end(); 819 I != E; ++I) { 820 // We want to do this once per basic block. As such, only process a block 821 // when we find the PHI that is the first entry in the block. 822 PHINode *SomePHI = I->second; 823 BasicBlock *BB = SomePHI->getParent(); 824 if (&BB->front() != SomePHI) 825 continue; 826 827 // Only do work here if there the PHI nodes are missing incoming values. We 828 // know that all PHI nodes that were inserted in a block will have the same 829 // number of incoming values, so we can just check any of them. 830 if (SomePHI->getNumIncomingValues() == getNumPreds(BB)) 831 continue; 832 833 // Get the preds for BB. 834 SmallVector<BasicBlock *, 16> Preds(predecessors(BB)); 835 836 // Ok, now we know that all of the PHI nodes are missing entries for some 837 // basic blocks. Start by sorting the incoming predecessors for efficient 838 // access. 839 auto CompareBBNumbers = [this](BasicBlock *A, BasicBlock *B) { 840 return BBNumbers.find(A)->second < BBNumbers.find(B)->second; 841 }; 842 llvm::sort(Preds, CompareBBNumbers); 843 844 // Now we loop through all BB's which have entries in SomePHI and remove 845 // them from the Preds list. 846 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) { 847 // Do a log(n) search of the Preds list for the entry we want. 848 SmallVectorImpl<BasicBlock *>::iterator EntIt = llvm::lower_bound( 849 Preds, SomePHI->getIncomingBlock(i), CompareBBNumbers); 850 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) && 851 "PHI node has entry for a block which is not a predecessor!"); 852 853 // Remove the entry 854 Preds.erase(EntIt); 855 } 856 857 // At this point, the blocks left in the preds list must have dummy 858 // entries inserted into every PHI nodes for the block. Update all the phi 859 // nodes in this block that we are inserting (there could be phis before 860 // mem2reg runs). 861 unsigned NumBadPreds = SomePHI->getNumIncomingValues(); 862 BasicBlock::iterator BBI = BB->begin(); 863 while ((SomePHI = dyn_cast<PHINode>(BBI++)) && 864 SomePHI->getNumIncomingValues() == NumBadPreds) { 865 Value *UndefVal = UndefValue::get(SomePHI->getType()); 866 for (BasicBlock *Pred : Preds) 867 SomePHI->addIncoming(UndefVal, Pred); 868 } 869 } 870 871 NewPhiNodes.clear(); 872 } 873 874 /// Determine which blocks the value is live in. 875 /// 876 /// These are blocks which lead to uses. Knowing this allows us to avoid 877 /// inserting PHI nodes into blocks which don't lead to uses (thus, the 878 /// inserted phi nodes would be dead). 879 void PromoteMem2Reg::ComputeLiveInBlocks( 880 AllocaInst *AI, AllocaInfo &Info, 881 const SmallPtrSetImpl<BasicBlock *> &DefBlocks, 882 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) { 883 // To determine liveness, we must iterate through the predecessors of blocks 884 // where the def is live. Blocks are added to the worklist if we need to 885 // check their predecessors. Start with all the using blocks. 886 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(), 887 Info.UsingBlocks.end()); 888 889 // If any of the using blocks is also a definition block, check to see if the 890 // definition occurs before or after the use. If it happens before the use, 891 // the value isn't really live-in. 892 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) { 893 BasicBlock *BB = LiveInBlockWorklist[i]; 894 if (!DefBlocks.count(BB)) 895 continue; 896 897 // Okay, this is a block that both uses and defines the value. If the first 898 // reference to the alloca is a def (store), then we know it isn't live-in. 899 for (BasicBlock::iterator I = BB->begin();; ++I) { 900 if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 901 if (SI->getOperand(1) != AI) 902 continue; 903 904 // We found a store to the alloca before a load. The alloca is not 905 // actually live-in here. 906 LiveInBlockWorklist[i] = LiveInBlockWorklist.back(); 907 LiveInBlockWorklist.pop_back(); 908 --i; 909 --e; 910 break; 911 } 912 913 if (LoadInst *LI = dyn_cast<LoadInst>(I)) 914 // Okay, we found a load before a store to the alloca. It is actually 915 // live into this block. 916 if (LI->getOperand(0) == AI) 917 break; 918 } 919 } 920 921 // Now that we have a set of blocks where the phi is live-in, recursively add 922 // their predecessors until we find the full region the value is live. 923 while (!LiveInBlockWorklist.empty()) { 924 BasicBlock *BB = LiveInBlockWorklist.pop_back_val(); 925 926 // The block really is live in here, insert it into the set. If already in 927 // the set, then it has already been processed. 928 if (!LiveInBlocks.insert(BB).second) 929 continue; 930 931 // Since the value is live into BB, it is either defined in a predecessor or 932 // live into it to. Add the preds to the worklist unless they are a 933 // defining block. 934 for (BasicBlock *P : predecessors(BB)) { 935 // The value is not live into a predecessor if it defines the value. 936 if (DefBlocks.count(P)) 937 continue; 938 939 // Otherwise it is, add to the worklist. 940 LiveInBlockWorklist.push_back(P); 941 } 942 } 943 } 944 945 /// Queue a phi-node to be added to a basic-block for a specific Alloca. 946 /// 947 /// Returns true if there wasn't already a phi-node for that variable 948 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo, 949 unsigned &Version) { 950 // Look up the basic-block in question. 951 PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)]; 952 953 // If the BB already has a phi node added for the i'th alloca then we're done! 954 if (PN) 955 return false; 956 957 // Create a PhiNode using the dereferenced type... and add the phi-node to the 958 // BasicBlock. 959 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB), 960 Allocas[AllocaNo]->getName() + "." + Twine(Version++), 961 &BB->front()); 962 ++NumPHIInsert; 963 PhiToAllocaMap[PN] = AllocaNo; 964 return true; 965 } 966 967 /// Update the debug location of a phi. \p ApplyMergedLoc indicates whether to 968 /// create a merged location incorporating \p DL, or to set \p DL directly. 969 static void updateForIncomingValueLocation(PHINode *PN, DebugLoc DL, 970 bool ApplyMergedLoc) { 971 if (ApplyMergedLoc) 972 PN->applyMergedLocation(PN->getDebugLoc(), DL); 973 else 974 PN->setDebugLoc(DL); 975 } 976 977 /// Recursively traverse the CFG of the function, renaming loads and 978 /// stores to the allocas which we are promoting. 979 /// 980 /// IncomingVals indicates what value each Alloca contains on exit from the 981 /// predecessor block Pred. 982 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred, 983 RenamePassData::ValVector &IncomingVals, 984 RenamePassData::LocationVector &IncomingLocs, 985 std::vector<RenamePassData> &Worklist) { 986 NextIteration: 987 // If we are inserting any phi nodes into this BB, they will already be in the 988 // block. 989 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) { 990 // If we have PHI nodes to update, compute the number of edges from Pred to 991 // BB. 992 if (PhiToAllocaMap.count(APN)) { 993 // We want to be able to distinguish between PHI nodes being inserted by 994 // this invocation of mem2reg from those phi nodes that already existed in 995 // the IR before mem2reg was run. We determine that APN is being inserted 996 // because it is missing incoming edges. All other PHI nodes being 997 // inserted by this pass of mem2reg will have the same number of incoming 998 // operands so far. Remember this count. 999 unsigned NewPHINumOperands = APN->getNumOperands(); 1000 1001 unsigned NumEdges = llvm::count(successors(Pred), BB); 1002 assert(NumEdges && "Must be at least one edge from Pred to BB!"); 1003 1004 // Add entries for all the phis. 1005 BasicBlock::iterator PNI = BB->begin(); 1006 do { 1007 unsigned AllocaNo = PhiToAllocaMap[APN]; 1008 1009 // Update the location of the phi node. 1010 updateForIncomingValueLocation(APN, IncomingLocs[AllocaNo], 1011 APN->getNumIncomingValues() > 0); 1012 1013 // Add N incoming values to the PHI node. 1014 for (unsigned i = 0; i != NumEdges; ++i) 1015 APN->addIncoming(IncomingVals[AllocaNo], Pred); 1016 1017 // The currently active variable for this block is now the PHI. 1018 IncomingVals[AllocaNo] = APN; 1019 AllocaATInfo[AllocaNo].updateForNewPhi(APN, DIB); 1020 for (DbgVariableIntrinsic *DII : AllocaDbgUsers[AllocaNo]) 1021 if (DII->isAddressOfVariable()) 1022 ConvertDebugDeclareToDebugValue(DII, APN, DIB); 1023 1024 // Get the next phi node. 1025 ++PNI; 1026 APN = dyn_cast<PHINode>(PNI); 1027 if (!APN) 1028 break; 1029 1030 // Verify that it is missing entries. If not, it is not being inserted 1031 // by this mem2reg invocation so we want to ignore it. 1032 } while (APN->getNumOperands() == NewPHINumOperands); 1033 } 1034 } 1035 1036 // Don't revisit blocks. 1037 if (!Visited.insert(BB).second) 1038 return; 1039 1040 for (BasicBlock::iterator II = BB->begin(); !II->isTerminator();) { 1041 Instruction *I = &*II++; // get the instruction, increment iterator 1042 1043 if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 1044 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand()); 1045 if (!Src) 1046 continue; 1047 1048 DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src); 1049 if (AI == AllocaLookup.end()) 1050 continue; 1051 1052 Value *V = IncomingVals[AI->second]; 1053 convertMetadataToAssumes(LI, V, SQ.DL, AC, &DT); 1054 1055 // Anything using the load now uses the current value. 1056 LI->replaceAllUsesWith(V); 1057 LI->eraseFromParent(); 1058 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 1059 // Delete this instruction and mark the name as the current holder of the 1060 // value 1061 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand()); 1062 if (!Dest) 1063 continue; 1064 1065 DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest); 1066 if (ai == AllocaLookup.end()) 1067 continue; 1068 1069 // what value were we writing? 1070 unsigned AllocaNo = ai->second; 1071 IncomingVals[AllocaNo] = SI->getOperand(0); 1072 1073 // Record debuginfo for the store before removing it. 1074 IncomingLocs[AllocaNo] = SI->getDebugLoc(); 1075 AllocaATInfo[AllocaNo].updateForDeletedStore(SI, DIB); 1076 for (DbgVariableIntrinsic *DII : AllocaDbgUsers[ai->second]) 1077 if (DII->isAddressOfVariable()) 1078 ConvertDebugDeclareToDebugValue(DII, SI, DIB); 1079 SI->eraseFromParent(); 1080 } 1081 } 1082 1083 // 'Recurse' to our successors. 1084 succ_iterator I = succ_begin(BB), E = succ_end(BB); 1085 if (I == E) 1086 return; 1087 1088 // Keep track of the successors so we don't visit the same successor twice 1089 SmallPtrSet<BasicBlock *, 8> VisitedSuccs; 1090 1091 // Handle the first successor without using the worklist. 1092 VisitedSuccs.insert(*I); 1093 Pred = BB; 1094 BB = *I; 1095 ++I; 1096 1097 for (; I != E; ++I) 1098 if (VisitedSuccs.insert(*I).second) 1099 Worklist.emplace_back(*I, Pred, IncomingVals, IncomingLocs); 1100 1101 goto NextIteration; 1102 } 1103 1104 void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT, 1105 AssumptionCache *AC) { 1106 // If there is nothing to do, bail out... 1107 if (Allocas.empty()) 1108 return; 1109 1110 PromoteMem2Reg(Allocas, DT, AC).run(); 1111 } 1112