SSAUpdater.cpp revision 0bef562ea253878ee92a1eaf6db05b0c2edfa74c
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/Instructions.h" 16#include "llvm/ADT/DenseMap.h" 17#include "llvm/Support/CFG.h" 18#include "llvm/Support/Debug.h" 19#include "llvm/Support/ValueHandle.h" 20#include "llvm/Support/raw_ostream.h" 21using namespace llvm; 22 23typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy; 24typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > > 25 IncomingPredInfoTy; 26 27static AvailableValsTy &getAvailableVals(void *AV) { 28 return *static_cast<AvailableValsTy*>(AV); 29} 30 31static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) { 32 return *static_cast<IncomingPredInfoTy*>(IPI); 33} 34 35 36SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI) 37 : AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {} 38 39SSAUpdater::~SSAUpdater() { 40 delete &getAvailableVals(AV); 41 delete &getIncomingPredInfo(IPI); 42} 43 44/// Initialize - Reset this object to get ready for a new set of SSA 45/// updates. ProtoValue is the value used to name PHI nodes. 46void SSAUpdater::Initialize(Value *ProtoValue) { 47 if (AV == 0) 48 AV = new AvailableValsTy(); 49 else 50 getAvailableVals(AV).clear(); 51 52 if (IPI == 0) 53 IPI = new IncomingPredInfoTy(); 54 else 55 getIncomingPredInfo(IPI).clear(); 56 PrototypeValue = ProtoValue; 57} 58 59/// HasValueForBlock - Return true if the SSAUpdater already has a value for 60/// the specified block. 61bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const { 62 return getAvailableVals(AV).count(BB); 63} 64 65/// AddAvailableValue - Indicate that a rewritten value is available in the 66/// specified block with the specified value. 67void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) { 68 assert(PrototypeValue != 0 && "Need to initialize SSAUpdater"); 69 assert(PrototypeValue->getType() == V->getType() && 70 "All rewritten values must have the same type"); 71 getAvailableVals(AV)[BB] = V; 72} 73 74/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is 75/// live at the end of the specified block. 76Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 77 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State"); 78 Value *Res = GetValueAtEndOfBlockInternal(BB); 79 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State"); 80 return Res; 81} 82 83/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 84/// is live in the middle of the specified block. 85/// 86/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 87/// important case: if there is a definition of the rewritten value after the 88/// 'use' in BB. Consider code like this: 89/// 90/// X1 = ... 91/// SomeBB: 92/// use(X) 93/// X2 = ... 94/// br Cond, SomeBB, OutBB 95/// 96/// In this case, there are two values (X1 and X2) added to the AvailableVals 97/// set by the client of the rewriter, and those values are both live out of 98/// their respective blocks. However, the use of X happens in the *middle* of 99/// a block. Because of this, we need to insert a new PHI node in SomeBB to 100/// merge the appropriate values, and this value isn't live out of the block. 101/// 102Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) { 103 // If there is no definition of the renamed variable in this block, just use 104 // GetValueAtEndOfBlock to do our work. 105 if (!getAvailableVals(AV).count(BB)) 106 return GetValueAtEndOfBlock(BB); 107 108 // Otherwise, we have the hard case. Get the live-in values for each 109 // predecessor. 110 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues; 111 Value *SingularValue = 0; 112 113 // We can get our predecessor info by walking the pred_iterator list, but it 114 // is relatively slow. If we already have PHI nodes in this block, walk one 115 // of them to get the predecessor list instead. 116 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 117 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 118 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 119 Value *PredVal = GetValueAtEndOfBlock(PredBB); 120 PredValues.push_back(std::make_pair(PredBB, PredVal)); 121 122 // Compute SingularValue. 123 if (i == 0) 124 SingularValue = PredVal; 125 else if (PredVal != SingularValue) 126 SingularValue = 0; 127 } 128 } else { 129 bool isFirstPred = true; 130 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 131 BasicBlock *PredBB = *PI; 132 Value *PredVal = GetValueAtEndOfBlock(PredBB); 133 PredValues.push_back(std::make_pair(PredBB, PredVal)); 134 135 // Compute SingularValue. 136 if (isFirstPred) { 137 SingularValue = PredVal; 138 isFirstPred = false; 139 } else if (PredVal != SingularValue) 140 SingularValue = 0; 141 } 142 } 143 144 // If there are no predecessors, just return undef. 145 if (PredValues.empty()) 146 return UndefValue::get(PrototypeValue->getType()); 147 148 // Otherwise, if all the merged values are the same, just use it. 149 if (SingularValue != 0) 150 return SingularValue; 151 152 // Otherwise, we do need a PHI: insert one now. 153 PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(), 154 PrototypeValue->getName(), 155 &BB->front()); 156 InsertedPHI->reserveOperandSpace(PredValues.size()); 157 158 // Fill in all the predecessors of the PHI. 159 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 160 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first); 161 162 // See if the PHI node can be merged to a single value. This can happen in 163 // loop cases when we get a PHI of itself and one other value. 164 if (Value *ConstVal = InsertedPHI->hasConstantValue()) { 165 InsertedPHI->eraseFromParent(); 166 return ConstVal; 167 } 168 169 // If the client wants to know about all new instructions, tell it. 170 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 171 172 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n"); 173 return InsertedPHI; 174} 175 176/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 177/// which use their value in the corresponding predecessor. 178void SSAUpdater::RewriteUse(Use &U) { 179 Instruction *User = cast<Instruction>(U.getUser()); 180 BasicBlock *UseBB = User->getParent(); 181 if (PHINode *UserPN = dyn_cast<PHINode>(User)) 182 UseBB = UserPN->getIncomingBlock(U); 183 184 U.set(GetValueInMiddleOfBlock(UseBB)); 185} 186 187 188/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry 189/// for the specified BB and if so, return it. If not, construct SSA form by 190/// walking predecessors inserting PHI nodes as needed until we get to a block 191/// where the value is available. 192/// 193Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) { 194 AvailableValsTy &AvailableVals = getAvailableVals(AV); 195 196 // Query AvailableVals by doing an insertion of null. 197 std::pair<AvailableValsTy::iterator, bool> InsertRes = 198 AvailableVals.insert(std::make_pair(BB, WeakVH())); 199 200 // Handle the case when the insertion fails because we have already seen BB. 201 if (!InsertRes.second) { 202 // If the insertion failed, there are two cases. The first case is that the 203 // value is already available for the specified block. If we get this, just 204 // return the value. 205 if (InsertRes.first->second != 0) 206 return InsertRes.first->second; 207 208 // Otherwise, if the value we find is null, then this is the value is not 209 // known but it is being computed elsewhere in our recursion. This means 210 // that we have a cycle. Handle this by inserting a PHI node and returning 211 // it. When we get back to the first instance of the recursion we will fill 212 // in the PHI node. 213 return InsertRes.first->second = 214 PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(), 215 &BB->front()); 216 } 217 218 // Okay, the value isn't in the map and we just inserted a null in the entry 219 // to indicate that we're processing the block. Since we have no idea what 220 // value is in this block, we have to recurse through our predecessors. 221 // 222 // While we're walking our predecessors, we keep track of them in a vector, 223 // then insert a PHI node in the end if we actually need one. We could use a 224 // smallvector here, but that would take a lot of stack space for every level 225 // of the recursion, just use IncomingPredInfo as an explicit stack. 226 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI); 227 unsigned FirstPredInfoEntry = IncomingPredInfo.size(); 228 229 // As we're walking the predecessors, keep track of whether they are all 230 // producing the same value. If so, this value will capture it, if not, it 231 // will get reset to null. We distinguish the no-predecessor case explicitly 232 // below. 233 TrackingVH<Value> SingularValue; 234 235 // We can get our predecessor info by walking the pred_iterator list, but it 236 // is relatively slow. If we already have PHI nodes in this block, walk one 237 // of them to get the predecessor list instead. 238 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) { 239 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) { 240 BasicBlock *PredBB = SomePhi->getIncomingBlock(i); 241 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB); 242 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal)); 243 244 // Compute SingularValue. 245 if (i == 0) 246 SingularValue = PredVal; 247 else if (PredVal != SingularValue) 248 SingularValue = 0; 249 } 250 } else { 251 bool isFirstPred = true; 252 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 253 BasicBlock *PredBB = *PI; 254 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB); 255 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal)); 256 257 // Compute SingularValue. 258 if (isFirstPred) { 259 SingularValue = PredVal; 260 isFirstPred = false; 261 } else if (PredVal != SingularValue) 262 SingularValue = 0; 263 } 264 } 265 266 // If there are no predecessors, then we must have found an unreachable block 267 // just return 'undef'. Since there are no predecessors, InsertRes must not 268 // be invalidated. 269 if (IncomingPredInfo.size() == FirstPredInfoEntry) 270 return InsertRes.first->second = UndefValue::get(PrototypeValue->getType()); 271 272 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If 273 /// this block is involved in a loop, a no-entry PHI node will have been 274 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted 275 /// above. 276 TrackingVH<Value> &InsertedVal = AvailableVals[BB]; 277 278 // If all the predecessor values are the same then we don't need to insert a 279 // PHI. This is the simple and common case. 280 if (SingularValue) { 281 // If a PHI node got inserted, replace it with the singlar value and delete 282 // it. 283 if (InsertedVal) { 284 PHINode *OldVal = cast<PHINode>(InsertedVal); 285 // Be careful about dead loops. These RAUW's also update InsertedVal. 286 if (InsertedVal != SingularValue) 287 OldVal->replaceAllUsesWith(SingularValue); 288 else 289 OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType())); 290 OldVal->eraseFromParent(); 291 } else { 292 InsertedVal = SingularValue; 293 } 294 295 // Drop the entries we added in IncomingPredInfo to restore the stack. 296 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 297 IncomingPredInfo.end()); 298 return InsertedVal; 299 } 300 301 // Otherwise, we do need a PHI: insert one now if we don't already have one. 302 if (InsertedVal == 0) 303 InsertedVal = PHINode::Create(PrototypeValue->getType(), 304 PrototypeValue->getName(), &BB->front()); 305 306 PHINode *InsertedPHI = cast<PHINode>(InsertedVal); 307 InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry); 308 309 // Fill in all the predecessors of the PHI. 310 for (IncomingPredInfoTy::iterator I = 311 IncomingPredInfo.begin()+FirstPredInfoEntry, 312 E = IncomingPredInfo.end(); I != E; ++I) 313 InsertedPHI->addIncoming(I->second, I->first); 314 315 // Drop the entries we added in IncomingPredInfo to restore the stack. 316 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 317 IncomingPredInfo.end()); 318 319 // See if the PHI node can be merged to a single value. This can happen in 320 // loop cases when we get a PHI of itself and one other value. 321 if (Value *ConstVal = InsertedPHI->hasConstantValue()) { 322 InsertedPHI->replaceAllUsesWith(ConstVal); 323 InsertedPHI->eraseFromParent(); 324 InsertedVal = ConstVal; 325 } else { 326 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n"); 327 328 // If the client wants to know about all new instructions, tell it. 329 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 330 } 331 332 return InsertedVal; 333} 334 335 336