SSAUpdater.cpp revision 562b84b3aea359d1f918184e355da82bf05eb290
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#define DEBUG_TYPE "ssaupdater" 15#include "llvm/Constants.h" 16#include "llvm/Instructions.h" 17#include "llvm/ADT/DenseMap.h" 18#include "llvm/Analysis/InstructionSimplify.h" 19#include "llvm/Support/AlignOf.h" 20#include "llvm/Support/Allocator.h" 21#include "llvm/Support/CFG.h" 22#include "llvm/Support/Debug.h" 23#include "llvm/Support/raw_ostream.h" 24#include "llvm/Transforms/Utils/SSAUpdater.h" 25#include "llvm/Transforms/Utils/SSAUpdaterImpl.h" 26using namespace llvm; 27 28typedef DenseMap<BasicBlock*, Value*> AvailableValsTy; 29static AvailableValsTy &getAvailableVals(void *AV) { 30 return *static_cast<AvailableValsTy*>(AV); 31} 32 33SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI) 34 : AV(0), ProtoType(0), ProtoName(), InsertedPHIs(NewPHI) {} 35 36SSAUpdater::~SSAUpdater() { 37 delete &getAvailableVals(AV); 38} 39 40/// Initialize - Reset this object to get ready for a new set of SSA 41/// updates with type 'Ty'. PHI nodes get a name based on 'Name'. 42void SSAUpdater::Initialize(const Type *Ty, StringRef Name) { 43 if (AV == 0) 44 AV = new AvailableValsTy(); 45 else 46 getAvailableVals(AV).clear(); 47 ProtoType = Ty; 48 ProtoName = Name; 49} 50 51/// HasValueForBlock - Return true if the SSAUpdater already has a value for 52/// the specified block. 53bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const { 54 return getAvailableVals(AV).count(BB); 55} 56 57/// AddAvailableValue - Indicate that a rewritten value is available in the 58/// specified block with the specified value. 59void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) { 60 assert(ProtoType != 0 && "Need to initialize SSAUpdater"); 61 assert(ProtoType == V->getType() && 62 "All rewritten values must have the same type"); 63 getAvailableVals(AV)[BB] = V; 64} 65 66/// IsEquivalentPHI - Check if PHI has the same incoming value as specified 67/// in ValueMapping for each predecessor block. 68static bool IsEquivalentPHI(PHINode *PHI, 69 DenseMap<BasicBlock*, Value*> &ValueMapping) { 70 unsigned PHINumValues = PHI->getNumIncomingValues(); 71 if (PHINumValues != ValueMapping.size()) 72 return false; 73 74 // Scan the phi to see if it matches. 75 for (unsigned i = 0, e = PHINumValues; i != e; ++i) 76 if (ValueMapping[PHI->getIncomingBlock(i)] != 77 PHI->getIncomingValue(i)) { 78 return false; 79 } 80 81 return true; 82} 83 84/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is 85/// live at the end of the specified block. 86Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 87 Value *Res = GetValueAtEndOfBlockInternal(BB); 88 return Res; 89} 90 91/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 92/// is live in the middle of the specified block. 93/// 94/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 95/// important case: if there is a definition of the rewritten value after the 96/// 'use' in BB. Consider code like this: 97/// 98/// X1 = ... 99/// SomeBB: 100/// use(X) 101/// X2 = ... 102/// br Cond, SomeBB, OutBB 103/// 104/// In this case, there are two values (X1 and X2) added to the AvailableVals 105/// set by the client of the rewriter, and those values are both live out of 106/// their respective blocks. However, the use of X happens in the *middle* of 107/// a block. Because of this, we need to insert a new PHI node in SomeBB to 108/// merge the appropriate values, and this value isn't live out of the block. 109/// 110Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) { 111 // If there is no definition of the renamed variable in this block, just use 112 // GetValueAtEndOfBlock to do our work. 113 if (!HasValueForBlock(BB)) 114 return GetValueAtEndOfBlock(BB); 115 116 // Otherwise, we have the hard case. Get the live-in values for each 117 // predecessor. 118 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues; 119 Value *SingularValue = 0; 120 121 // We can get our predecessor info by walking the pred_iterator list, but it 122 // is relatively slow. If we already have PHI nodes in this block, walk one 123 // of them to get the predecessor list instead. 124 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 125 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 126 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 127 Value *PredVal = GetValueAtEndOfBlock(PredBB); 128 PredValues.push_back(std::make_pair(PredBB, PredVal)); 129 130 // Compute SingularValue. 131 if (i == 0) 132 SingularValue = PredVal; 133 else if (PredVal != SingularValue) 134 SingularValue = 0; 135 } 136 } else { 137 bool isFirstPred = true; 138 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 139 BasicBlock *PredBB = *PI; 140 Value *PredVal = GetValueAtEndOfBlock(PredBB); 141 PredValues.push_back(std::make_pair(PredBB, PredVal)); 142 143 // Compute SingularValue. 144 if (isFirstPred) { 145 SingularValue = PredVal; 146 isFirstPred = false; 147 } else if (PredVal != SingularValue) 148 SingularValue = 0; 149 } 150 } 151 152 // If there are no predecessors, just return undef. 153 if (PredValues.empty()) 154 return UndefValue::get(ProtoType); 155 156 // Otherwise, if all the merged values are the same, just use it. 157 if (SingularValue != 0) 158 return SingularValue; 159 160 // Otherwise, we do need a PHI: check to see if we already have one available 161 // in this block that produces the right value. 162 if (isa<PHINode>(BB->begin())) { 163 DenseMap<BasicBlock*, Value*> ValueMapping(PredValues.begin(), 164 PredValues.end()); 165 PHINode *SomePHI; 166 for (BasicBlock::iterator It = BB->begin(); 167 (SomePHI = dyn_cast<PHINode>(It)); ++It) { 168 if (IsEquivalentPHI(SomePHI, ValueMapping)) 169 return SomePHI; 170 } 171 } 172 173 // Ok, we have no way out, insert a new one now. 174 PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(), 175 ProtoName, &BB->front()); 176 177 // Fill in all the predecessors of the PHI. 178 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 179 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first); 180 181 // See if the PHI node can be merged to a single value. This can happen in 182 // loop cases when we get a PHI of itself and one other value. 183 if (Value *V = SimplifyInstruction(InsertedPHI)) { 184 InsertedPHI->eraseFromParent(); 185 return V; 186 } 187 188 // If the client wants to know about all new instructions, tell it. 189 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 190 191 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 192 return InsertedPHI; 193} 194 195/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 196/// which use their value in the corresponding predecessor. 197void SSAUpdater::RewriteUse(Use &U) { 198 Instruction *User = cast<Instruction>(U.getUser()); 199 200 Value *V; 201 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 202 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 203 else 204 V = GetValueInMiddleOfBlock(User->getParent()); 205 206 U.set(V); 207} 208 209/// RewriteUseAfterInsertions - Rewrite a use, just like RewriteUse. However, 210/// this version of the method can rewrite uses in the same block as a 211/// definition, because it assumes that all uses of a value are below any 212/// inserted values. 213void SSAUpdater::RewriteUseAfterInsertions(Use &U) { 214 Instruction *User = cast<Instruction>(U.getUser()); 215 216 Value *V; 217 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 218 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 219 else 220 V = GetValueAtEndOfBlock(User->getParent()); 221 222 U.set(V); 223} 224 225/// PHIiter - Iterator for PHI operands. This is used for the PHI_iterator 226/// in the SSAUpdaterImpl template. 227namespace { 228 class PHIiter { 229 private: 230 PHINode *PHI; 231 unsigned idx; 232 233 public: 234 explicit PHIiter(PHINode *P) // begin iterator 235 : PHI(P), idx(0) {} 236 PHIiter(PHINode *P, bool) // end iterator 237 : PHI(P), idx(PHI->getNumIncomingValues()) {} 238 239 PHIiter &operator++() { ++idx; return *this; } 240 bool operator==(const PHIiter& x) const { return idx == x.idx; } 241 bool operator!=(const PHIiter& x) const { return !operator==(x); } 242 Value *getIncomingValue() { return PHI->getIncomingValue(idx); } 243 BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); } 244 }; 245} 246 247/// SSAUpdaterTraits<SSAUpdater> - Traits for the SSAUpdaterImpl template, 248/// specialized for SSAUpdater. 249namespace llvm { 250template<> 251class SSAUpdaterTraits<SSAUpdater> { 252public: 253 typedef BasicBlock BlkT; 254 typedef Value *ValT; 255 typedef PHINode PhiT; 256 257 typedef succ_iterator BlkSucc_iterator; 258 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); } 259 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); } 260 261 typedef PHIiter PHI_iterator; 262 static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); } 263 static inline PHI_iterator PHI_end(PhiT *PHI) { 264 return PHI_iterator(PHI, true); 265 } 266 267 /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds 268 /// vector, set Info->NumPreds, and allocate space in Info->Preds. 269 static void FindPredecessorBlocks(BasicBlock *BB, 270 SmallVectorImpl<BasicBlock*> *Preds) { 271 // We can get our predecessor info by walking the pred_iterator list, 272 // but it is relatively slow. If we already have PHI nodes in this 273 // block, walk one of them to get the predecessor list instead. 274 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 275 for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI) 276 Preds->push_back(SomePhi->getIncomingBlock(PI)); 277 } else { 278 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 279 Preds->push_back(*PI); 280 } 281 } 282 283 /// GetUndefVal - Get an undefined value of the same type as the value 284 /// being handled. 285 static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) { 286 return UndefValue::get(Updater->ProtoType); 287 } 288 289 /// CreateEmptyPHI - Create a new PHI instruction in the specified block. 290 /// Reserve space for the operands but do not fill them in yet. 291 static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds, 292 SSAUpdater *Updater) { 293 PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds, 294 Updater->ProtoName, &BB->front()); 295 return PHI; 296 } 297 298 /// AddPHIOperand - Add the specified value as an operand of the PHI for 299 /// the specified predecessor block. 300 static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) { 301 PHI->addIncoming(Val, Pred); 302 } 303 304 /// InstrIsPHI - Check if an instruction is a PHI. 305 /// 306 static PHINode *InstrIsPHI(Instruction *I) { 307 return dyn_cast<PHINode>(I); 308 } 309 310 /// ValueIsPHI - Check if a value is a PHI. 311 /// 312 static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) { 313 return dyn_cast<PHINode>(Val); 314 } 315 316 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source 317 /// operands, i.e., it was just added. 318 static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) { 319 PHINode *PHI = ValueIsPHI(Val, Updater); 320 if (PHI && PHI->getNumIncomingValues() == 0) 321 return PHI; 322 return 0; 323 } 324 325 /// GetPHIValue - For the specified PHI instruction, return the value 326 /// that it defines. 327 static Value *GetPHIValue(PHINode *PHI) { 328 return PHI; 329 } 330}; 331 332} // End llvm namespace 333 334/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry 335/// for the specified BB and if so, return it. If not, construct SSA form by 336/// first calculating the required placement of PHIs and then inserting new 337/// PHIs where needed. 338Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) { 339 AvailableValsTy &AvailableVals = getAvailableVals(AV); 340 if (Value *V = AvailableVals[BB]) 341 return V; 342 343 SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs); 344 return Impl.GetValue(BB); 345} 346 347//===----------------------------------------------------------------------===// 348// LoadAndStorePromoter Implementation 349//===----------------------------------------------------------------------===// 350 351LoadAndStorePromoter:: 352LoadAndStorePromoter(const SmallVectorImpl<Instruction*> &Insts, 353 SSAUpdater &S, StringRef BaseName) : SSA(S) { 354 if (Insts.empty()) return; 355 356 Value *SomeVal; 357 if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0])) 358 SomeVal = LI; 359 else 360 SomeVal = cast<StoreInst>(Insts[0])->getOperand(0); 361 362 if (BaseName.empty()) 363 BaseName = SomeVal->getName(); 364 SSA.Initialize(SomeVal->getType(), BaseName); 365} 366 367 368void LoadAndStorePromoter:: 369run(const SmallVectorImpl<Instruction*> &Insts) const { 370 371 // First step: bucket up uses of the alloca by the block they occur in. 372 // This is important because we have to handle multiple defs/uses in a block 373 // ourselves: SSAUpdater is purely for cross-block references. 374 // FIXME: Want a TinyVector<Instruction*> since there is often 0/1 element. 375 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock; 376 377 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 378 Instruction *User = Insts[i]; 379 UsesByBlock[User->getParent()].push_back(User); 380 } 381 382 // Okay, now we can iterate over all the blocks in the function with uses, 383 // processing them. Keep track of which loads are loading a live-in value. 384 // Walk the uses in the use-list order to be determinstic. 385 SmallVector<LoadInst*, 32> LiveInLoads; 386 DenseMap<Value*, Value*> ReplacedLoads; 387 388 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 389 Instruction *User = Insts[i]; 390 BasicBlock *BB = User->getParent(); 391 std::vector<Instruction*> &BlockUses = UsesByBlock[BB]; 392 393 // If this block has already been processed, ignore this repeat use. 394 if (BlockUses.empty()) continue; 395 396 // Okay, this is the first use in the block. If this block just has a 397 // single user in it, we can rewrite it trivially. 398 if (BlockUses.size() == 1) { 399 // If it is a store, it is a trivial def of the value in the block. 400 if (StoreInst *SI = dyn_cast<StoreInst>(User)) 401 SSA.AddAvailableValue(BB, SI->getOperand(0)); 402 else 403 // Otherwise it is a load, queue it to rewrite as a live-in load. 404 LiveInLoads.push_back(cast<LoadInst>(User)); 405 BlockUses.clear(); 406 continue; 407 } 408 409 // Otherwise, check to see if this block is all loads. 410 bool HasStore = false; 411 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) { 412 if (isa<StoreInst>(BlockUses[i])) { 413 HasStore = true; 414 break; 415 } 416 } 417 418 // If so, we can queue them all as live in loads. We don't have an 419 // efficient way to tell which on is first in the block and don't want to 420 // scan large blocks, so just add all loads as live ins. 421 if (!HasStore) { 422 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) 423 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i])); 424 BlockUses.clear(); 425 continue; 426 } 427 428 // Otherwise, we have mixed loads and stores (or just a bunch of stores). 429 // Since SSAUpdater is purely for cross-block values, we need to determine 430 // the order of these instructions in the block. If the first use in the 431 // block is a load, then it uses the live in value. The last store defines 432 // the live out value. We handle this by doing a linear scan of the block. 433 Value *StoredValue = 0; 434 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 435 if (LoadInst *L = dyn_cast<LoadInst>(II)) { 436 // If this is a load from an unrelated pointer, ignore it. 437 if (!isInstInList(L, Insts)) continue; 438 439 // If we haven't seen a store yet, this is a live in use, otherwise 440 // use the stored value. 441 if (StoredValue) { 442 replaceLoadWithValue(L, StoredValue); 443 L->replaceAllUsesWith(StoredValue); 444 ReplacedLoads[L] = StoredValue; 445 } else { 446 LiveInLoads.push_back(L); 447 } 448 continue; 449 } 450 451 if (StoreInst *S = dyn_cast<StoreInst>(II)) { 452 // If this is a store to an unrelated pointer, ignore it. 453 if (!isInstInList(S, Insts)) continue; 454 455 // Remember that this is the active value in the block. 456 StoredValue = S->getOperand(0); 457 } 458 } 459 460 // The last stored value that happened is the live-out for the block. 461 assert(StoredValue && "Already checked that there is a store in block"); 462 SSA.AddAvailableValue(BB, StoredValue); 463 BlockUses.clear(); 464 } 465 466 // Okay, now we rewrite all loads that use live-in values in the loop, 467 // inserting PHI nodes as necessary. 468 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) { 469 LoadInst *ALoad = LiveInLoads[i]; 470 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent()); 471 replaceLoadWithValue(ALoad, NewVal); 472 473 // Avoid assertions in unreachable code. 474 if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType()); 475 ALoad->replaceAllUsesWith(NewVal); 476 ReplacedLoads[ALoad] = NewVal; 477 } 478 479 // Allow the client to do stuff before we start nuking things. 480 doExtraRewritesBeforeFinalDeletion(); 481 482 // Now that everything is rewritten, delete the old instructions from the 483 // function. They should all be dead now. 484 for (unsigned i = 0, e = Insts.size(); i != e; ++i) { 485 Instruction *User = Insts[i]; 486 487 // If this is a load that still has uses, then the load must have been added 488 // as a live value in the SSAUpdate data structure for a block (e.g. because 489 // the loaded value was stored later). In this case, we need to recursively 490 // propagate the updates until we get to the real value. 491 if (!User->use_empty()) { 492 Value *NewVal = ReplacedLoads[User]; 493 assert(NewVal && "not a replaced load?"); 494 495 // Propagate down to the ultimate replacee. The intermediately loads 496 // could theoretically already have been deleted, so we don't want to 497 // dereference the Value*'s. 498 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal); 499 while (RLI != ReplacedLoads.end()) { 500 NewVal = RLI->second; 501 RLI = ReplacedLoads.find(NewVal); 502 } 503 504 replaceLoadWithValue(cast<LoadInst>(User), NewVal); 505 User->replaceAllUsesWith(NewVal); 506 } 507 508 instructionDeleted(User); 509 User->eraseFromParent(); 510 } 511} 512