SSAUpdater.cpp revision cdbd99262286e96729007ac535cd430ecb3d38ac
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/Instructions.h" 16#include "llvm/ADT/DenseMap.h" 17#include "llvm/Analysis/InstructionSimplify.h" 18#include "llvm/Support/AlignOf.h" 19#include "llvm/Support/Allocator.h" 20#include "llvm/Support/CFG.h" 21#include "llvm/Support/Debug.h" 22#include "llvm/Support/raw_ostream.h" 23#include "llvm/Transforms/Utils/SSAUpdater.h" 24#include "llvm/Transforms/Utils/SSAUpdaterImpl.h" 25using namespace llvm; 26 27typedef DenseMap<BasicBlock*, Value*> AvailableValsTy; 28static AvailableValsTy &getAvailableVals(void *AV) { 29 return *static_cast<AvailableValsTy*>(AV); 30} 31 32SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI) 33 : AV(0), ProtoType(0), ProtoName(), InsertedPHIs(NewPHI) {} 34 35SSAUpdater::~SSAUpdater() { 36 delete &getAvailableVals(AV); 37} 38 39/// Initialize - Reset this object to get ready for a new set of SSA 40/// updates with type 'Ty'. PHI nodes get a name based on 'Name'. 41void SSAUpdater::Initialize(const Type *Ty, StringRef Name) { 42 if (AV == 0) 43 AV = new AvailableValsTy(); 44 else 45 getAvailableVals(AV).clear(); 46 ProtoType = Ty; 47 ProtoName = Name; 48} 49 50/// HasValueForBlock - Return true if the SSAUpdater already has a value for 51/// the specified block. 52bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const { 53 return getAvailableVals(AV).count(BB); 54} 55 56/// AddAvailableValue - Indicate that a rewritten value is available in the 57/// specified block with the specified value. 58void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) { 59 assert(ProtoType != 0 && "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/// IsEquivalentPHI - Check if PHI has the same incoming value as specified 66/// in ValueMapping for each predecessor block. 67static bool IsEquivalentPHI(PHINode *PHI, 68 DenseMap<BasicBlock*, Value*> &ValueMapping) { 69 unsigned PHINumValues = PHI->getNumIncomingValues(); 70 if (PHINumValues != ValueMapping.size()) 71 return false; 72 73 // Scan the phi to see if it matches. 74 for (unsigned i = 0, e = PHINumValues; i != e; ++i) 75 if (ValueMapping[PHI->getIncomingBlock(i)] != 76 PHI->getIncomingValue(i)) { 77 return false; 78 } 79 80 return true; 81} 82 83/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is 84/// live at the end of the specified block. 85Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 86 Value *Res = GetValueAtEndOfBlockInternal(BB); 87 return Res; 88} 89 90/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 91/// is live in the middle of the specified block. 92/// 93/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 94/// important case: if there is a definition of the rewritten value after the 95/// 'use' in BB. Consider code like this: 96/// 97/// X1 = ... 98/// SomeBB: 99/// use(X) 100/// X2 = ... 101/// br Cond, SomeBB, OutBB 102/// 103/// In this case, there are two values (X1 and X2) added to the AvailableVals 104/// set by the client of the rewriter, and those values are both live out of 105/// their respective blocks. However, the use of X happens in the *middle* of 106/// a block. Because of this, we need to insert a new PHI node in SomeBB to 107/// merge the appropriate values, and this value isn't live out of the block. 108/// 109Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) { 110 // If there is no definition of the renamed variable in this block, just use 111 // GetValueAtEndOfBlock to do our work. 112 if (!HasValueForBlock(BB)) 113 return GetValueAtEndOfBlock(BB); 114 115 // Otherwise, we have the hard case. Get the live-in values for each 116 // predecessor. 117 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues; 118 Value *SingularValue = 0; 119 120 // We can get our predecessor info by walking the pred_iterator list, but it 121 // is relatively slow. If we already have PHI nodes in this block, walk one 122 // of them to get the predecessor list instead. 123 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 124 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 125 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 126 Value *PredVal = GetValueAtEndOfBlock(PredBB); 127 PredValues.push_back(std::make_pair(PredBB, PredVal)); 128 129 // Compute SingularValue. 130 if (i == 0) 131 SingularValue = PredVal; 132 else if (PredVal != SingularValue) 133 SingularValue = 0; 134 } 135 } else { 136 bool isFirstPred = true; 137 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 138 BasicBlock *PredBB = *PI; 139 Value *PredVal = GetValueAtEndOfBlock(PredBB); 140 PredValues.push_back(std::make_pair(PredBB, PredVal)); 141 142 // Compute SingularValue. 143 if (isFirstPred) { 144 SingularValue = PredVal; 145 isFirstPred = false; 146 } else if (PredVal != SingularValue) 147 SingularValue = 0; 148 } 149 } 150 151 // If there are no predecessors, just return undef. 152 if (PredValues.empty()) 153 return UndefValue::get(ProtoType); 154 155 // Otherwise, if all the merged values are the same, just use it. 156 if (SingularValue != 0) 157 return SingularValue; 158 159 // Otherwise, we do need a PHI: check to see if we already have one available 160 // in this block that produces the right value. 161 if (isa<PHINode>(BB->begin())) { 162 DenseMap<BasicBlock*, Value*> ValueMapping(PredValues.begin(), 163 PredValues.end()); 164 PHINode *SomePHI; 165 for (BasicBlock::iterator It = BB->begin(); 166 (SomePHI = dyn_cast<PHINode>(It)); ++It) { 167 if (IsEquivalentPHI(SomePHI, ValueMapping)) 168 return SomePHI; 169 } 170 } 171 172 // Ok, we have no way out, insert a new one now. 173 PHINode *InsertedPHI = PHINode::Create(ProtoType, ProtoName, &BB->front()); 174 InsertedPHI->reserveOperandSpace(PredValues.size()); 175 176 // Fill in all the predecessors of the PHI. 177 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 178 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first); 179 180 // See if the PHI node can be merged to a single value. This can happen in 181 // loop cases when we get a PHI of itself and one other value. 182 if (Value *V = SimplifyInstruction(InsertedPHI)) { 183 InsertedPHI->eraseFromParent(); 184 return V; 185 } 186 187 // If the client wants to know about all new instructions, tell it. 188 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 189 190 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 191 return InsertedPHI; 192} 193 194/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 195/// which use their value in the corresponding predecessor. 196void SSAUpdater::RewriteUse(Use &U) { 197 Instruction *User = cast<Instruction>(U.getUser()); 198 199 Value *V; 200 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 201 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 202 else 203 V = GetValueInMiddleOfBlock(User->getParent()); 204 205 U.set(V); 206} 207 208/// RewriteUseAfterInsertions - Rewrite a use, just like RewriteUse. However, 209/// this version of the method can rewrite uses in the same block as a 210/// definition, because it assumes that all uses of a value are below any 211/// inserted values. 212void SSAUpdater::RewriteUseAfterInsertions(Use &U) { 213 Instruction *User = cast<Instruction>(U.getUser()); 214 215 Value *V; 216 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 217 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U)); 218 else 219 V = GetValueAtEndOfBlock(User->getParent()); 220 221 U.set(V); 222} 223 224/// PHIiter - Iterator for PHI operands. This is used for the PHI_iterator 225/// in the SSAUpdaterImpl template. 226namespace { 227 class PHIiter { 228 private: 229 PHINode *PHI; 230 unsigned idx; 231 232 public: 233 explicit PHIiter(PHINode *P) // begin iterator 234 : PHI(P), idx(0) {} 235 PHIiter(PHINode *P, bool) // end iterator 236 : PHI(P), idx(PHI->getNumIncomingValues()) {} 237 238 PHIiter &operator++() { ++idx; return *this; } 239 bool operator==(const PHIiter& x) const { return idx == x.idx; } 240 bool operator!=(const PHIiter& x) const { return !operator==(x); } 241 Value *getIncomingValue() { return PHI->getIncomingValue(idx); } 242 BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); } 243 }; 244} 245 246/// SSAUpdaterTraits<SSAUpdater> - Traits for the SSAUpdaterImpl template, 247/// specialized for SSAUpdater. 248namespace llvm { 249template<> 250class SSAUpdaterTraits<SSAUpdater> { 251public: 252 typedef BasicBlock BlkT; 253 typedef Value *ValT; 254 typedef PHINode PhiT; 255 256 typedef succ_iterator BlkSucc_iterator; 257 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); } 258 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); } 259 260 typedef PHIiter PHI_iterator; 261 static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); } 262 static inline PHI_iterator PHI_end(PhiT *PHI) { 263 return PHI_iterator(PHI, true); 264 } 265 266 /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds 267 /// vector, set Info->NumPreds, and allocate space in Info->Preds. 268 static void FindPredecessorBlocks(BasicBlock *BB, 269 SmallVectorImpl<BasicBlock*> *Preds) { 270 // We can get our predecessor info by walking the pred_iterator list, 271 // but it is relatively slow. If we already have PHI nodes in this 272 // block, walk one of them to get the predecessor list instead. 273 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 274 for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI) 275 Preds->push_back(SomePhi->getIncomingBlock(PI)); 276 } else { 277 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 278 Preds->push_back(*PI); 279 } 280 } 281 282 /// GetUndefVal - Get an undefined value of the same type as the value 283 /// being handled. 284 static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) { 285 return UndefValue::get(Updater->ProtoType); 286 } 287 288 /// CreateEmptyPHI - Create a new PHI instruction in the specified block. 289 /// Reserve space for the operands but do not fill them in yet. 290 static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds, 291 SSAUpdater *Updater) { 292 PHINode *PHI = PHINode::Create(Updater->ProtoType, Updater->ProtoName, 293 &BB->front()); 294 PHI->reserveOperandSpace(NumPreds); 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