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/Support/Debug.h" 23 #include "llvm/Support/raw_ostream.h" 24 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 25 #include "llvm/Transforms/Utils/Local.h" 26 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h" 27 28 using namespace llvm; 29 30 #define DEBUG_TYPE "ssaupdater" 31 32 typedef DenseMap<BasicBlock*, Value*> AvailableValsTy; 33 static AvailableValsTy &getAvailableVals(void *AV) { 34 return *static_cast<AvailableValsTy*>(AV); 35 } 36 37 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI) 38 : AV(nullptr), ProtoType(nullptr), ProtoName(), InsertedPHIs(NewPHI) {} 39 40 SSAUpdater::~SSAUpdater() { 41 delete static_cast<AvailableValsTy*>(AV); 42 } 43 44 void SSAUpdater::Initialize(Type *Ty, StringRef Name) { 45 if (!AV) 46 AV = new AvailableValsTy(); 47 else 48 getAvailableVals(AV).clear(); 49 ProtoType = Ty; 50 ProtoName = Name; 51 } 52 53 bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const { 54 return getAvailableVals(AV).count(BB); 55 } 56 57 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) { 58 assert(ProtoType && "Need to initialize SSAUpdater"); 59 assert(ProtoType == V->getType() && 60 "All rewritten values must have the same type"); 61 getAvailableVals(AV)[BB] = V; 62 } 63 64 static bool IsEquivalentPHI(PHINode *PHI, 65 SmallDenseMap<BasicBlock*, Value*, 8> &ValueMapping) { 66 unsigned PHINumValues = PHI->getNumIncomingValues(); 67 if (PHINumValues != ValueMapping.size()) 68 return false; 69 70 // Scan the phi to see if it matches. 71 for (unsigned i = 0, e = PHINumValues; i != e; ++i) 72 if (ValueMapping[PHI->getIncomingBlock(i)] != 73 PHI->getIncomingValue(i)) { 74 return false; 75 } 76 77 return true; 78 } 79 80 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 81 Value *Res = GetValueAtEndOfBlockInternal(BB); 82 return Res; 83 } 84 85 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) { 86 // If there is no definition of the renamed variable in this block, just use 87 // GetValueAtEndOfBlock to do our work. 88 if (!HasValueForBlock(BB)) 89 return GetValueAtEndOfBlock(BB); 90 91 // Otherwise, we have the hard case. Get the live-in values for each 92 // predecessor. 93 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues; 94 Value *SingularValue = nullptr; 95 96 // We can get our predecessor info by walking the pred_iterator list, but it 97 // is relatively slow. If we already have PHI nodes in this block, walk one 98 // of them to get the predecessor list instead. 99 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 100 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 101 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 102 Value *PredVal = GetValueAtEndOfBlock(PredBB); 103 PredValues.push_back(std::make_pair(PredBB, PredVal)); 104 105 // Compute SingularValue. 106 if (i == 0) 107 SingularValue = PredVal; 108 else if (PredVal != SingularValue) 109 SingularValue = nullptr; 110 } 111 } else { 112 bool isFirstPred = true; 113 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 114 BasicBlock *PredBB = *PI; 115 Value *PredVal = GetValueAtEndOfBlock(PredBB); 116 PredValues.push_back(std::make_pair(PredBB, PredVal)); 117 118 // Compute SingularValue. 119 if (isFirstPred) { 120 SingularValue = PredVal; 121 isFirstPred = false; 122 } else if (PredVal != SingularValue) 123 SingularValue = nullptr; 124 } 125 } 126 127 // If there are no predecessors, just return undef. 128 if (PredValues.empty()) 129 return UndefValue::get(ProtoType); 130 131 // Otherwise, if all the merged values are the same, just use it. 132 if (SingularValue) 133 return SingularValue; 134 135 // Otherwise, we do need a PHI: check to see if we already have one available 136 // in this block that produces the right value. 137 if (isa<PHINode>(BB->begin())) { 138 SmallDenseMap<BasicBlock*, Value*, 8> ValueMapping(PredValues.begin(), 139 PredValues.end()); 140 PHINode *SomePHI; 141 for (BasicBlock::iterator It = BB->begin(); 142 (SomePHI = dyn_cast<PHINode>(It)); ++It) { 143 if (IsEquivalentPHI(SomePHI, ValueMapping)) 144 return SomePHI; 145 } 146 } 147 148 // Ok, we have no way out, insert a new one now. 149 PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(), 150 ProtoName, &BB->front()); 151 152 // Fill in all the predecessors of the PHI. 153 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 154 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first); 155 156 // See if the PHI node can be merged to a single value. This can happen in 157 // loop cases when we get a PHI of itself and one other value. 158 if (Value *V = SimplifyInstruction(InsertedPHI)) { 159 InsertedPHI->eraseFromParent(); 160 return V; 161 } 162 163 // Set the DebugLoc of the inserted PHI, if available. 164 DebugLoc DL; 165 if (const Instruction *I = BB->getFirstNonPHI()) 166 DL = I->getDebugLoc(); 167 InsertedPHI->setDebugLoc(DL); 168 169 // If the client wants to know about all new instructions, tell it. 170 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 171 172 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 173 return InsertedPHI; 174 } 175 176 void SSAUpdater::RewriteUse(Use &U) { 177 Instruction *User = cast<Instruction>(U.getUser()); 178 179 Value *V; 180 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 181 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 182 else 183 V = GetValueInMiddleOfBlock(User->getParent()); 184 185 // Notify that users of the existing value that it is being replaced. 186 Value *OldVal = U.get(); 187 if (OldVal != V && OldVal->hasValueHandle()) 188 ValueHandleBase::ValueIsRAUWd(OldVal, V); 189 190 U.set(V); 191 } 192 193 void SSAUpdater::RewriteUseAfterInsertions(Use &U) { 194 Instruction *User = cast<Instruction>(U.getUser()); 195 196 Value *V; 197 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 198 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 199 else 200 V = GetValueAtEndOfBlock(User->getParent()); 201 202 U.set(V); 203 } 204 205 namespace llvm { 206 template<> 207 class SSAUpdaterTraits<SSAUpdater> { 208 public: 209 typedef BasicBlock BlkT; 210 typedef Value *ValT; 211 typedef PHINode PhiT; 212 213 typedef succ_iterator BlkSucc_iterator; 214 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); } 215 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); } 216 217 class PHI_iterator { 218 private: 219 PHINode *PHI; 220 unsigned idx; 221 222 public: 223 explicit PHI_iterator(PHINode *P) // begin iterator 224 : PHI(P), idx(0) {} 225 PHI_iterator(PHINode *P, bool) // end iterator 226 : PHI(P), idx(PHI->getNumIncomingValues()) {} 227 228 PHI_iterator &operator++() { ++idx; return *this; } 229 bool operator==(const PHI_iterator& x) const { return idx == x.idx; } 230 bool operator!=(const PHI_iterator& x) const { return !operator==(x); } 231 Value *getIncomingValue() { return PHI->getIncomingValue(idx); } 232 BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); } 233 }; 234 235 static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); } 236 static PHI_iterator PHI_end(PhiT *PHI) { 237 return PHI_iterator(PHI, true); 238 } 239 240 /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds 241 /// vector, set Info->NumPreds, and allocate space in Info->Preds. 242 static void FindPredecessorBlocks(BasicBlock *BB, 243 SmallVectorImpl<BasicBlock*> *Preds) { 244 // We can get our predecessor info by walking the pred_iterator list, 245 // but it is relatively slow. If we already have PHI nodes in this 246 // block, walk one of them to get the predecessor list instead. 247 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 248 for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI) 249 Preds->push_back(SomePhi->getIncomingBlock(PI)); 250 } else { 251 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 252 Preds->push_back(*PI); 253 } 254 } 255 256 /// GetUndefVal - Get an undefined value of the same type as the value 257 /// being handled. 258 static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) { 259 return UndefValue::get(Updater->ProtoType); 260 } 261 262 /// CreateEmptyPHI - Create a new PHI instruction in the specified block. 263 /// Reserve space for the operands but do not fill them in yet. 264 static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds, 265 SSAUpdater *Updater) { 266 PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds, 267 Updater->ProtoName, &BB->front()); 268 return PHI; 269 } 270 271 /// AddPHIOperand - Add the specified value as an operand of the PHI for 272 /// the specified predecessor block. 273 static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) { 274 PHI->addIncoming(Val, Pred); 275 } 276 277 /// InstrIsPHI - Check if an instruction is a PHI. 278 /// 279 static PHINode *InstrIsPHI(Instruction *I) { 280 return dyn_cast<PHINode>(I); 281 } 282 283 /// ValueIsPHI - Check if a value is a PHI. 284 /// 285 static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) { 286 return dyn_cast<PHINode>(Val); 287 } 288 289 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source 290 /// operands, i.e., it was just added. 291 static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) { 292 PHINode *PHI = ValueIsPHI(Val, Updater); 293 if (PHI && PHI->getNumIncomingValues() == 0) 294 return PHI; 295 return nullptr; 296 } 297 298 /// GetPHIValue - For the specified PHI instruction, return the value 299 /// that it defines. 300 static Value *GetPHIValue(PHINode *PHI) { 301 return PHI; 302 } 303 }; 304 305 } // End llvm namespace 306 307 /// Check to see if AvailableVals has an entry for the specified BB and if so, 308 /// return it. If not, construct SSA form by first calculating the required 309 /// placement of PHIs and then inserting new PHIs where needed. 310 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) { 311 AvailableValsTy &AvailableVals = getAvailableVals(AV); 312 if (Value *V = AvailableVals[BB]) 313 return V; 314 315 SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs); 316 return Impl.GetValue(BB); 317 } 318 319 //===----------------------------------------------------------------------===// 320 // LoadAndStorePromoter Implementation 321 //===----------------------------------------------------------------------===// 322 323 LoadAndStorePromoter:: 324 LoadAndStorePromoter(const SmallVectorImpl<Instruction*> &Insts, 325 SSAUpdater &S, StringRef BaseName) : SSA(S) { 326 if (Insts.empty()) return; 327 328 Value *SomeVal; 329 if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0])) 330 SomeVal = LI; 331 else 332 SomeVal = cast<StoreInst>(Insts[0])->getOperand(0); 333 334 if (BaseName.empty()) 335 BaseName = SomeVal->getName(); 336 SSA.Initialize(SomeVal->getType(), BaseName); 337 } 338 339 340 void LoadAndStorePromoter:: 341 run(const SmallVectorImpl<Instruction*> &Insts) const { 342 343 // First step: bucket up uses of the alloca by the block they occur in. 344 // This is important because we have to handle multiple defs/uses in a block 345 // ourselves: SSAUpdater is purely for cross-block references. 346 DenseMap<BasicBlock*, TinyPtrVector<Instruction*> > UsesByBlock; 347 348 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 349 Instruction *User = Insts[i]; 350 UsesByBlock[User->getParent()].push_back(User); 351 } 352 353 // Okay, now we can iterate over all the blocks in the function with uses, 354 // processing them. Keep track of which loads are loading a live-in value. 355 // Walk the uses in the use-list order to be determinstic. 356 SmallVector<LoadInst*, 32> LiveInLoads; 357 DenseMap<Value*, Value*> ReplacedLoads; 358 359 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 360 Instruction *User = Insts[i]; 361 BasicBlock *BB = User->getParent(); 362 TinyPtrVector<Instruction*> &BlockUses = UsesByBlock[BB]; 363 364 // If this block has already been processed, ignore this repeat use. 365 if (BlockUses.empty()) continue; 366 367 // Okay, this is the first use in the block. If this block just has a 368 // single user in it, we can rewrite it trivially. 369 if (BlockUses.size() == 1) { 370 // If it is a store, it is a trivial def of the value in the block. 371 if (StoreInst *SI = dyn_cast<StoreInst>(User)) { 372 updateDebugInfo(SI); 373 SSA.AddAvailableValue(BB, SI->getOperand(0)); 374 } else 375 // Otherwise it is a load, queue it to rewrite as a live-in load. 376 LiveInLoads.push_back(cast<LoadInst>(User)); 377 BlockUses.clear(); 378 continue; 379 } 380 381 // Otherwise, check to see if this block is all loads. 382 bool HasStore = false; 383 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) { 384 if (isa<StoreInst>(BlockUses[i])) { 385 HasStore = true; 386 break; 387 } 388 } 389 390 // If so, we can queue them all as live in loads. We don't have an 391 // efficient way to tell which on is first in the block and don't want to 392 // scan large blocks, so just add all loads as live ins. 393 if (!HasStore) { 394 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) 395 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i])); 396 BlockUses.clear(); 397 continue; 398 } 399 400 // Otherwise, we have mixed loads and stores (or just a bunch of stores). 401 // Since SSAUpdater is purely for cross-block values, we need to determine 402 // the order of these instructions in the block. If the first use in the 403 // block is a load, then it uses the live in value. The last store defines 404 // the live out value. We handle this by doing a linear scan of the block. 405 Value *StoredValue = nullptr; 406 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 407 if (LoadInst *L = dyn_cast<LoadInst>(II)) { 408 // If this is a load from an unrelated pointer, ignore it. 409 if (!isInstInList(L, Insts)) continue; 410 411 // If we haven't seen a store yet, this is a live in use, otherwise 412 // use the stored value. 413 if (StoredValue) { 414 replaceLoadWithValue(L, StoredValue); 415 L->replaceAllUsesWith(StoredValue); 416 ReplacedLoads[L] = StoredValue; 417 } else { 418 LiveInLoads.push_back(L); 419 } 420 continue; 421 } 422 423 if (StoreInst *SI = dyn_cast<StoreInst>(II)) { 424 // If this is a store to an unrelated pointer, ignore it. 425 if (!isInstInList(SI, Insts)) continue; 426 updateDebugInfo(SI); 427 428 // Remember that this is the active value in the block. 429 StoredValue = SI->getOperand(0); 430 } 431 } 432 433 // The last stored value that happened is the live-out for the block. 434 assert(StoredValue && "Already checked that there is a store in block"); 435 SSA.AddAvailableValue(BB, StoredValue); 436 BlockUses.clear(); 437 } 438 439 // Okay, now we rewrite all loads that use live-in values in the loop, 440 // inserting PHI nodes as necessary. 441 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) { 442 LoadInst *ALoad = LiveInLoads[i]; 443 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent()); 444 replaceLoadWithValue(ALoad, NewVal); 445 446 // Avoid assertions in unreachable code. 447 if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType()); 448 ALoad->replaceAllUsesWith(NewVal); 449 ReplacedLoads[ALoad] = NewVal; 450 } 451 452 // Allow the client to do stuff before we start nuking things. 453 doExtraRewritesBeforeFinalDeletion(); 454 455 // Now that everything is rewritten, delete the old instructions from the 456 // function. They should all be dead now. 457 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 458 Instruction *User = Insts[i]; 459 460 // If this is a load that still has uses, then the load must have been added 461 // as a live value in the SSAUpdate data structure for a block (e.g. because 462 // the loaded value was stored later). In this case, we need to recursively 463 // propagate the updates until we get to the real value. 464 if (!User->use_empty()) { 465 Value *NewVal = ReplacedLoads[User]; 466 assert(NewVal && "not a replaced load?"); 467 468 // Propagate down to the ultimate replacee. The intermediately loads 469 // could theoretically already have been deleted, so we don't want to 470 // dereference the Value*'s. 471 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal); 472 while (RLI != ReplacedLoads.end()) { 473 NewVal = RLI->second; 474 RLI = ReplacedLoads.find(NewVal); 475 } 476 477 replaceLoadWithValue(cast<LoadInst>(User), NewVal); 478 User->replaceAllUsesWith(NewVal); 479 } 480 481 instructionDeleted(User); 482 User->eraseFromParent(); 483 } 484 } 485 486 bool 487 LoadAndStorePromoter::isInstInList(Instruction *I, 488 const SmallVectorImpl<Instruction*> &Insts) 489 const { 490 return std::find(Insts.begin(), Insts.end(), I) != Insts.end(); 491 } 492