1 //===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===// 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 implements the SSAUpdater class. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Utils/SSAUpdater.h" 15 #include "llvm/ADT/DenseMap.h" 16 #include "llvm/ADT/TinyPtrVector.h" 17 #include "llvm/Analysis/InstructionSimplify.h" 18 #include "llvm/IR/CFG.h" 19 #include "llvm/IR/Constants.h" 20 #include "llvm/IR/Instructions.h" 21 #include "llvm/IR/IntrinsicInst.h" 22 #include "llvm/IR/Module.h" 23 #include "llvm/Support/Debug.h" 24 #include "llvm/Support/raw_ostream.h" 25 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 26 #include "llvm/Transforms/Utils/Local.h" 27 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h" 28 29 using namespace llvm; 30 31 #define DEBUG_TYPE "ssaupdater" 32 33 typedef DenseMap<BasicBlock*, Value*> AvailableValsTy; 34 static AvailableValsTy &getAvailableVals(void *AV) { 35 return *static_cast<AvailableValsTy*>(AV); 36 } 37 38 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI) 39 : AV(nullptr), ProtoType(nullptr), ProtoName(), InsertedPHIs(NewPHI) {} 40 41 SSAUpdater::~SSAUpdater() { 42 delete static_cast<AvailableValsTy*>(AV); 43 } 44 45 void SSAUpdater::Initialize(Type *Ty, StringRef Name) { 46 if (!AV) 47 AV = new AvailableValsTy(); 48 else 49 getAvailableVals(AV).clear(); 50 ProtoType = Ty; 51 ProtoName = Name; 52 } 53 54 bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const { 55 return getAvailableVals(AV).count(BB); 56 } 57 58 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) { 59 assert(ProtoType && "Need to initialize SSAUpdater"); 60 assert(ProtoType == V->getType() && 61 "All rewritten values must have the same type"); 62 getAvailableVals(AV)[BB] = V; 63 } 64 65 static bool IsEquivalentPHI(PHINode *PHI, 66 SmallDenseMap<BasicBlock*, Value*, 8> &ValueMapping) { 67 unsigned PHINumValues = PHI->getNumIncomingValues(); 68 if (PHINumValues != ValueMapping.size()) 69 return false; 70 71 // Scan the phi to see if it matches. 72 for (unsigned i = 0, e = PHINumValues; i != e; ++i) 73 if (ValueMapping[PHI->getIncomingBlock(i)] != 74 PHI->getIncomingValue(i)) { 75 return false; 76 } 77 78 return true; 79 } 80 81 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 82 Value *Res = GetValueAtEndOfBlockInternal(BB); 83 return Res; 84 } 85 86 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) { 87 // If there is no definition of the renamed variable in this block, just use 88 // GetValueAtEndOfBlock to do our work. 89 if (!HasValueForBlock(BB)) 90 return GetValueAtEndOfBlock(BB); 91 92 // Otherwise, we have the hard case. Get the live-in values for each 93 // predecessor. 94 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues; 95 Value *SingularValue = nullptr; 96 97 // We can get our predecessor info by walking the pred_iterator list, but it 98 // is relatively slow. If we already have PHI nodes in this block, walk one 99 // of them to get the predecessor list instead. 100 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 101 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 102 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 103 Value *PredVal = GetValueAtEndOfBlock(PredBB); 104 PredValues.push_back(std::make_pair(PredBB, PredVal)); 105 106 // Compute SingularValue. 107 if (i == 0) 108 SingularValue = PredVal; 109 else if (PredVal != SingularValue) 110 SingularValue = nullptr; 111 } 112 } else { 113 bool isFirstPred = true; 114 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 115 BasicBlock *PredBB = *PI; 116 Value *PredVal = GetValueAtEndOfBlock(PredBB); 117 PredValues.push_back(std::make_pair(PredBB, PredVal)); 118 119 // Compute SingularValue. 120 if (isFirstPred) { 121 SingularValue = PredVal; 122 isFirstPred = false; 123 } else if (PredVal != SingularValue) 124 SingularValue = nullptr; 125 } 126 } 127 128 // If there are no predecessors, just return undef. 129 if (PredValues.empty()) 130 return UndefValue::get(ProtoType); 131 132 // Otherwise, if all the merged values are the same, just use it. 133 if (SingularValue) 134 return SingularValue; 135 136 // Otherwise, we do need a PHI: check to see if we already have one available 137 // in this block that produces the right value. 138 if (isa<PHINode>(BB->begin())) { 139 SmallDenseMap<BasicBlock*, Value*, 8> ValueMapping(PredValues.begin(), 140 PredValues.end()); 141 PHINode *SomePHI; 142 for (BasicBlock::iterator It = BB->begin(); 143 (SomePHI = dyn_cast<PHINode>(It)); ++It) { 144 if (IsEquivalentPHI(SomePHI, ValueMapping)) 145 return SomePHI; 146 } 147 } 148 149 // Ok, we have no way out, insert a new one now. 150 PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(), 151 ProtoName, &BB->front()); 152 153 // Fill in all the predecessors of the PHI. 154 for (const auto &PredValue : PredValues) 155 InsertedPHI->addIncoming(PredValue.second, PredValue.first); 156 157 // See if the PHI node can be merged to a single value. This can happen in 158 // loop cases when we get a PHI of itself and one other value. 159 if (Value *V = 160 SimplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) { 161 InsertedPHI->eraseFromParent(); 162 return V; 163 } 164 165 // Set the DebugLoc of the inserted PHI, if available. 166 DebugLoc DL; 167 if (const Instruction *I = BB->getFirstNonPHI()) 168 DL = I->getDebugLoc(); 169 InsertedPHI->setDebugLoc(DL); 170 171 // If the client wants to know about all new instructions, tell it. 172 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 173 174 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 175 return InsertedPHI; 176 } 177 178 void SSAUpdater::RewriteUse(Use &U) { 179 Instruction *User = cast<Instruction>(U.getUser()); 180 181 Value *V; 182 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 183 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 184 else 185 V = GetValueInMiddleOfBlock(User->getParent()); 186 187 // Notify that users of the existing value that it is being replaced. 188 Value *OldVal = U.get(); 189 if (OldVal != V && OldVal->hasValueHandle()) 190 ValueHandleBase::ValueIsRAUWd(OldVal, V); 191 192 U.set(V); 193 } 194 195 void SSAUpdater::RewriteUseAfterInsertions(Use &U) { 196 Instruction *User = cast<Instruction>(U.getUser()); 197 198 Value *V; 199 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 200 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 201 else 202 V = GetValueAtEndOfBlock(User->getParent()); 203 204 U.set(V); 205 } 206 207 namespace llvm { 208 template<> 209 class SSAUpdaterTraits<SSAUpdater> { 210 public: 211 typedef BasicBlock BlkT; 212 typedef Value *ValT; 213 typedef PHINode PhiT; 214 215 typedef succ_iterator BlkSucc_iterator; 216 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); } 217 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); } 218 219 class PHI_iterator { 220 private: 221 PHINode *PHI; 222 unsigned idx; 223 224 public: 225 explicit PHI_iterator(PHINode *P) // begin iterator 226 : PHI(P), idx(0) {} 227 PHI_iterator(PHINode *P, bool) // end iterator 228 : PHI(P), idx(PHI->getNumIncomingValues()) {} 229 230 PHI_iterator &operator++() { ++idx; return *this; } 231 bool operator==(const PHI_iterator& x) const { return idx == x.idx; } 232 bool operator!=(const PHI_iterator& x) const { return !operator==(x); } 233 Value *getIncomingValue() { return PHI->getIncomingValue(idx); } 234 BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); } 235 }; 236 237 static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); } 238 static PHI_iterator PHI_end(PhiT *PHI) { 239 return PHI_iterator(PHI, true); 240 } 241 242 /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds 243 /// vector, set Info->NumPreds, and allocate space in Info->Preds. 244 static void FindPredecessorBlocks(BasicBlock *BB, 245 SmallVectorImpl<BasicBlock*> *Preds) { 246 // We can get our predecessor info by walking the pred_iterator list, 247 // but it is relatively slow. If we already have PHI nodes in this 248 // block, walk one of them to get the predecessor list instead. 249 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 250 Preds->append(SomePhi->block_begin(), SomePhi->block_end()); 251 } else { 252 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 253 Preds->push_back(*PI); 254 } 255 } 256 257 /// GetUndefVal - Get an undefined value of the same type as the value 258 /// being handled. 259 static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) { 260 return UndefValue::get(Updater->ProtoType); 261 } 262 263 /// CreateEmptyPHI - Create a new PHI instruction in the specified block. 264 /// Reserve space for the operands but do not fill them in yet. 265 static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds, 266 SSAUpdater *Updater) { 267 PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds, 268 Updater->ProtoName, &BB->front()); 269 return PHI; 270 } 271 272 /// AddPHIOperand - Add the specified value as an operand of the PHI for 273 /// the specified predecessor block. 274 static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) { 275 PHI->addIncoming(Val, Pred); 276 } 277 278 /// InstrIsPHI - Check if an instruction is a PHI. 279 /// 280 static PHINode *InstrIsPHI(Instruction *I) { 281 return dyn_cast<PHINode>(I); 282 } 283 284 /// ValueIsPHI - Check if a value is a PHI. 285 /// 286 static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) { 287 return dyn_cast<PHINode>(Val); 288 } 289 290 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source 291 /// operands, i.e., it was just added. 292 static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) { 293 PHINode *PHI = ValueIsPHI(Val, Updater); 294 if (PHI && PHI->getNumIncomingValues() == 0) 295 return PHI; 296 return nullptr; 297 } 298 299 /// GetPHIValue - For the specified PHI instruction, return the value 300 /// that it defines. 301 static Value *GetPHIValue(PHINode *PHI) { 302 return PHI; 303 } 304 }; 305 306 } // End llvm namespace 307 308 /// Check to see if AvailableVals has an entry for the specified BB and if so, 309 /// return it. If not, construct SSA form by first calculating the required 310 /// placement of PHIs and then inserting new PHIs where needed. 311 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) { 312 AvailableValsTy &AvailableVals = getAvailableVals(AV); 313 if (Value *V = AvailableVals[BB]) 314 return V; 315 316 SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs); 317 return Impl.GetValue(BB); 318 } 319 320 //===----------------------------------------------------------------------===// 321 // LoadAndStorePromoter Implementation 322 //===----------------------------------------------------------------------===// 323 324 LoadAndStorePromoter:: 325 LoadAndStorePromoter(ArrayRef<const Instruction*> Insts, 326 SSAUpdater &S, StringRef BaseName) : SSA(S) { 327 if (Insts.empty()) return; 328 329 const Value *SomeVal; 330 if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0])) 331 SomeVal = LI; 332 else 333 SomeVal = cast<StoreInst>(Insts[0])->getOperand(0); 334 335 if (BaseName.empty()) 336 BaseName = SomeVal->getName(); 337 SSA.Initialize(SomeVal->getType(), BaseName); 338 } 339 340 341 void LoadAndStorePromoter:: 342 run(const SmallVectorImpl<Instruction*> &Insts) const { 343 344 // First step: bucket up uses of the alloca by the block they occur in. 345 // This is important because we have to handle multiple defs/uses in a block 346 // ourselves: SSAUpdater is purely for cross-block references. 347 DenseMap<BasicBlock*, TinyPtrVector<Instruction*> > UsesByBlock; 348 349 for (Instruction *User : Insts) 350 UsesByBlock[User->getParent()].push_back(User); 351 352 // Okay, now we can iterate over all the blocks in the function with uses, 353 // processing them. Keep track of which loads are loading a live-in value. 354 // Walk the uses in the use-list order to be determinstic. 355 SmallVector<LoadInst*, 32> LiveInLoads; 356 DenseMap<Value*, Value*> ReplacedLoads; 357 358 for (Instruction *User : Insts) { 359 BasicBlock *BB = User->getParent(); 360 TinyPtrVector<Instruction*> &BlockUses = UsesByBlock[BB]; 361 362 // If this block has already been processed, ignore this repeat use. 363 if (BlockUses.empty()) continue; 364 365 // Okay, this is the first use in the block. If this block just has a 366 // single user in it, we can rewrite it trivially. 367 if (BlockUses.size() == 1) { 368 // If it is a store, it is a trivial def of the value in the block. 369 if (StoreInst *SI = dyn_cast<StoreInst>(User)) { 370 updateDebugInfo(SI); 371 SSA.AddAvailableValue(BB, SI->getOperand(0)); 372 } else 373 // Otherwise it is a load, queue it to rewrite as a live-in load. 374 LiveInLoads.push_back(cast<LoadInst>(User)); 375 BlockUses.clear(); 376 continue; 377 } 378 379 // Otherwise, check to see if this block is all loads. 380 bool HasStore = false; 381 for (Instruction *I : BlockUses) { 382 if (isa<StoreInst>(I)) { 383 HasStore = true; 384 break; 385 } 386 } 387 388 // If so, we can queue them all as live in loads. We don't have an 389 // efficient way to tell which on is first in the block and don't want to 390 // scan large blocks, so just add all loads as live ins. 391 if (!HasStore) { 392 for (Instruction *I : BlockUses) 393 LiveInLoads.push_back(cast<LoadInst>(I)); 394 BlockUses.clear(); 395 continue; 396 } 397 398 // Otherwise, we have mixed loads and stores (or just a bunch of stores). 399 // Since SSAUpdater is purely for cross-block values, we need to determine 400 // the order of these instructions in the block. If the first use in the 401 // block is a load, then it uses the live in value. The last store defines 402 // the live out value. We handle this by doing a linear scan of the block. 403 Value *StoredValue = nullptr; 404 for (Instruction &I : *BB) { 405 if (LoadInst *L = dyn_cast<LoadInst>(&I)) { 406 // If this is a load from an unrelated pointer, ignore it. 407 if (!isInstInList(L, Insts)) continue; 408 409 // If we haven't seen a store yet, this is a live in use, otherwise 410 // use the stored value. 411 if (StoredValue) { 412 replaceLoadWithValue(L, StoredValue); 413 L->replaceAllUsesWith(StoredValue); 414 ReplacedLoads[L] = StoredValue; 415 } else { 416 LiveInLoads.push_back(L); 417 } 418 continue; 419 } 420 421 if (StoreInst *SI = dyn_cast<StoreInst>(&I)) { 422 // If this is a store to an unrelated pointer, ignore it. 423 if (!isInstInList(SI, Insts)) continue; 424 updateDebugInfo(SI); 425 426 // Remember that this is the active value in the block. 427 StoredValue = SI->getOperand(0); 428 } 429 } 430 431 // The last stored value that happened is the live-out for the block. 432 assert(StoredValue && "Already checked that there is a store in block"); 433 SSA.AddAvailableValue(BB, StoredValue); 434 BlockUses.clear(); 435 } 436 437 // Okay, now we rewrite all loads that use live-in values in the loop, 438 // inserting PHI nodes as necessary. 439 for (LoadInst *ALoad : LiveInLoads) { 440 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent()); 441 replaceLoadWithValue(ALoad, NewVal); 442 443 // Avoid assertions in unreachable code. 444 if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType()); 445 ALoad->replaceAllUsesWith(NewVal); 446 ReplacedLoads[ALoad] = NewVal; 447 } 448 449 // Allow the client to do stuff before we start nuking things. 450 doExtraRewritesBeforeFinalDeletion(); 451 452 // Now that everything is rewritten, delete the old instructions from the 453 // function. They should all be dead now. 454 for (Instruction *User : Insts) { 455 // If this is a load that still has uses, then the load must have been added 456 // as a live value in the SSAUpdate data structure for a block (e.g. because 457 // the loaded value was stored later). In this case, we need to recursively 458 // propagate the updates until we get to the real value. 459 if (!User->use_empty()) { 460 Value *NewVal = ReplacedLoads[User]; 461 assert(NewVal && "not a replaced load?"); 462 463 // Propagate down to the ultimate replacee. The intermediately loads 464 // could theoretically already have been deleted, so we don't want to 465 // dereference the Value*'s. 466 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal); 467 while (RLI != ReplacedLoads.end()) { 468 NewVal = RLI->second; 469 RLI = ReplacedLoads.find(NewVal); 470 } 471 472 replaceLoadWithValue(cast<LoadInst>(User), NewVal); 473 User->replaceAllUsesWith(NewVal); 474 } 475 476 instructionDeleted(User); 477 User->eraseFromParent(); 478 } 479 } 480 481 bool 482 LoadAndStorePromoter::isInstInList(Instruction *I, 483 const SmallVectorImpl<Instruction*> &Insts) 484 const { 485 return std::find(Insts.begin(), Insts.end(), I) != Insts.end(); 486 } 487