MachineSSAUpdater.cpp revision c688eb17b601d4820bd645ce7d189b3e20af0fab
1//===- MachineSSAUpdater.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 MachineSSAUpdater class. It's based on SSAUpdater 11// class in lib/Transforms/Utils. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/CodeGen/MachineSSAUpdater.h" 16#include "llvm/CodeGen/MachineInstr.h" 17#include "llvm/CodeGen/MachineInstrBuilder.h" 18#include "llvm/CodeGen/MachineRegisterInfo.h" 19#include "llvm/Target/TargetInstrInfo.h" 20#include "llvm/Target/TargetMachine.h" 21#include "llvm/Target/TargetRegisterInfo.h" 22#include "llvm/ADT/DenseMap.h" 23#include "llvm/Support/Debug.h" 24#include "llvm/Support/ErrorHandling.h" 25#include "llvm/Support/raw_ostream.h" 26using namespace llvm; 27 28typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy; 29typedef std::vector<std::pair<MachineBasicBlock*, unsigned> > 30 IncomingPredInfoTy; 31 32static AvailableValsTy &getAvailableVals(void *AV) { 33 return *static_cast<AvailableValsTy*>(AV); 34} 35 36static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) { 37 return *static_cast<IncomingPredInfoTy*>(IPI); 38} 39 40 41MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF, 42 SmallVectorImpl<MachineInstr*> *NewPHI) 43 : AV(0), IPI(0), InsertedPHIs(NewPHI) { 44 TII = MF.getTarget().getInstrInfo(); 45 MRI = &MF.getRegInfo(); 46} 47 48MachineSSAUpdater::~MachineSSAUpdater() { 49 delete &getAvailableVals(AV); 50 delete &getIncomingPredInfo(IPI); 51} 52 53/// Initialize - Reset this object to get ready for a new set of SSA 54/// updates. ProtoValue is the value used to name PHI nodes. 55void MachineSSAUpdater::Initialize(unsigned V) { 56 if (AV == 0) 57 AV = new AvailableValsTy(); 58 else 59 getAvailableVals(AV).clear(); 60 61 if (IPI == 0) 62 IPI = new IncomingPredInfoTy(); 63 else 64 getIncomingPredInfo(IPI).clear(); 65 66 VR = V; 67 VRC = MRI->getRegClass(VR); 68} 69 70/// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for 71/// the specified block. 72bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const { 73 return getAvailableVals(AV).count(BB); 74} 75 76/// AddAvailableValue - Indicate that a rewritten value is available in the 77/// specified block with the specified value. 78void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) { 79 getAvailableVals(AV)[BB] = V; 80} 81 82/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is 83/// live at the end of the specified block. 84unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) { 85 return GetValueAtEndOfBlockInternal(BB); 86} 87 88static 89unsigned LookForIdenticalPHI(MachineBasicBlock *BB, 90 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> &PredValues) { 91 if (BB->empty()) 92 return 0; 93 94 MachineBasicBlock::iterator I = BB->front(); 95 if (I->getOpcode() != TargetInstrInfo::PHI) 96 return 0; 97 98 AvailableValsTy AVals; 99 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 100 AVals[PredValues[i].first] = PredValues[i].second; 101 while (I != BB->end() && I->getOpcode() == TargetInstrInfo::PHI) { 102 bool Same = true; 103 for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) { 104 unsigned SrcReg = I->getOperand(i).getReg(); 105 MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB(); 106 if (AVals[SrcBB] != SrcReg) { 107 Same = false; 108 break; 109 } 110 } 111 if (Same) 112 return I->getOperand(0).getReg(); 113 ++I; 114 } 115 return 0; 116} 117 118/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define 119/// a value of the given register class at the start of the specified basic 120/// block. It returns the virtual register defined by the instruction. 121static 122MachineInstr *InsertNewDef(unsigned Opcode, 123 MachineBasicBlock *BB, MachineBasicBlock::iterator I, 124 const TargetRegisterClass *RC, 125 MachineRegisterInfo *MRI, const TargetInstrInfo *TII) { 126 unsigned NewVR = MRI->createVirtualRegister(RC); 127 return BuildMI(*BB, I, DebugLoc::getUnknownLoc(), TII->get(Opcode), NewVR); 128} 129 130/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 131/// is live in the middle of the specified block. 132/// 133/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 134/// important case: if there is a definition of the rewritten value after the 135/// 'use' in BB. Consider code like this: 136/// 137/// X1 = ... 138/// SomeBB: 139/// use(X) 140/// X2 = ... 141/// br Cond, SomeBB, OutBB 142/// 143/// In this case, there are two values (X1 and X2) added to the AvailableVals 144/// set by the client of the rewriter, and those values are both live out of 145/// their respective blocks. However, the use of X happens in the *middle* of 146/// a block. Because of this, we need to insert a new PHI node in SomeBB to 147/// merge the appropriate values, and this value isn't live out of the block. 148/// 149unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) { 150 // If there is no definition of the renamed variable in this block, just use 151 // GetValueAtEndOfBlock to do our work. 152 if (!getAvailableVals(AV).count(BB)) 153 return GetValueAtEndOfBlockInternal(BB); 154 155 // If there are no predecessors, just return undef. 156 if (BB->pred_empty()) { 157 // Insert an implicit_def to represent an undef value. 158 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF, 159 BB, BB->getFirstTerminator(), 160 VRC, MRI, TII); 161 return NewDef->getOperand(0).getReg(); 162 } 163 164 // Otherwise, we have the hard case. Get the live-in values for each 165 // predecessor. 166 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues; 167 unsigned SingularValue = 0; 168 169 bool isFirstPred = true; 170 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 171 E = BB->pred_end(); PI != E; ++PI) { 172 MachineBasicBlock *PredBB = *PI; 173 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB); 174 PredValues.push_back(std::make_pair(PredBB, PredVal)); 175 176 // Compute SingularValue. 177 if (isFirstPred) { 178 SingularValue = PredVal; 179 isFirstPred = false; 180 } else if (PredVal != SingularValue) 181 SingularValue = 0; 182 } 183 184 // Otherwise, if all the merged values are the same, just use it. 185 if (SingularValue != 0) 186 return SingularValue; 187 188 // If an identical PHI is already in BB, just reuse it. 189 unsigned DupPHI = LookForIdenticalPHI(BB, PredValues); 190 if (DupPHI) 191 return DupPHI; 192 193 // Otherwise, we do need a PHI: insert one now. 194 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front(); 195 MachineInstr *InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB, 196 Loc, VRC, MRI, TII); 197 198 // Fill in all the predecessors of the PHI. 199 MachineInstrBuilder MIB(InsertedPHI); 200 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 201 MIB.addReg(PredValues[i].second).addMBB(PredValues[i].first); 202 203 // See if the PHI node can be merged to a single value. This can happen in 204 // loop cases when we get a PHI of itself and one other value. 205 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) { 206 InsertedPHI->eraseFromParent(); 207 return ConstVal; 208 } 209 210 // If the client wants to know about all new instructions, tell it. 211 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 212 213 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 214 return InsertedPHI->getOperand(0).getReg(); 215} 216 217static 218MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI, 219 MachineOperand *U) { 220 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) { 221 if (&MI->getOperand(i) == U) 222 return MI->getOperand(i+1).getMBB(); 223 } 224 225 llvm_unreachable("MachineOperand::getParent() failure?"); 226 return 0; 227} 228 229/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 230/// which use their value in the corresponding predecessor. 231void MachineSSAUpdater::RewriteUse(MachineOperand &U) { 232 MachineInstr *UseMI = U.getParent(); 233 unsigned NewVR = 0; 234 if (UseMI->getOpcode() == TargetInstrInfo::PHI) { 235 MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U); 236 NewVR = GetValueAtEndOfBlockInternal(SourceBB); 237 } else { 238 NewVR = GetValueInMiddleOfBlock(UseMI->getParent()); 239 } 240 241 U.setReg(NewVR); 242} 243 244void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) { 245 MRI->replaceRegWith(OldReg, NewReg); 246 247 AvailableValsTy &AvailableVals = getAvailableVals(AV); 248 for (DenseMap<MachineBasicBlock*, unsigned>::iterator 249 I = AvailableVals.begin(), E = AvailableVals.end(); I != E; ++I) 250 if (I->second == OldReg) 251 I->second = NewReg; 252} 253 254/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry 255/// for the specified BB and if so, return it. If not, construct SSA form by 256/// walking predecessors inserting PHI nodes as needed until we get to a block 257/// where the value is available. 258/// 259unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){ 260 AvailableValsTy &AvailableVals = getAvailableVals(AV); 261 262 // Query AvailableVals by doing an insertion of null. 263 std::pair<AvailableValsTy::iterator, bool> InsertRes = 264 AvailableVals.insert(std::make_pair(BB, 0)); 265 266 // Handle the case when the insertion fails because we have already seen BB. 267 if (!InsertRes.second) { 268 // If the insertion failed, there are two cases. The first case is that the 269 // value is already available for the specified block. If we get this, just 270 // return the value. 271 if (InsertRes.first->second != 0) 272 return InsertRes.first->second; 273 274 // Otherwise, if the value we find is null, then this is the value is not 275 // known but it is being computed elsewhere in our recursion. This means 276 // that we have a cycle. Handle this by inserting a PHI node and returning 277 // it. When we get back to the first instance of the recursion we will fill 278 // in the PHI node. 279 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front(); 280 MachineInstr *NewPHI = InsertNewDef(TargetInstrInfo::PHI, BB, Loc, 281 VRC, MRI,TII); 282 unsigned NewVR = NewPHI->getOperand(0).getReg(); 283 InsertRes.first->second = NewVR; 284 return NewVR; 285 } 286 287 // If there are no predecessors, then we must have found an unreachable block 288 // just return 'undef'. Since there are no predecessors, InsertRes must not 289 // be invalidated. 290 if (BB->pred_empty()) { 291 // Insert an implicit_def to represent an undef value. 292 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF, 293 BB, BB->getFirstTerminator(), 294 VRC, MRI, TII); 295 return InsertRes.first->second = NewDef->getOperand(0).getReg(); 296 } 297 298 // Okay, the value isn't in the map and we just inserted a null in the entry 299 // to indicate that we're processing the block. Since we have no idea what 300 // value is in this block, we have to recurse through our predecessors. 301 // 302 // While we're walking our predecessors, we keep track of them in a vector, 303 // then insert a PHI node in the end if we actually need one. We could use a 304 // smallvector here, but that would take a lot of stack space for every level 305 // of the recursion, just use IncomingPredInfo as an explicit stack. 306 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI); 307 unsigned FirstPredInfoEntry = IncomingPredInfo.size(); 308 309 // As we're walking the predecessors, keep track of whether they are all 310 // producing the same value. If so, this value will capture it, if not, it 311 // will get reset to null. We distinguish the no-predecessor case explicitly 312 // below. 313 unsigned SingularValue = 0; 314 bool isFirstPred = true; 315 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 316 E = BB->pred_end(); PI != E; ++PI) { 317 MachineBasicBlock *PredBB = *PI; 318 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB); 319 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal)); 320 321 // Compute SingularValue. 322 if (isFirstPred) { 323 SingularValue = PredVal; 324 isFirstPred = false; 325 } else if (PredVal != SingularValue) 326 SingularValue = 0; 327 } 328 329 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If 330 /// this block is involved in a loop, a no-entry PHI node will have been 331 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted 332 /// above. 333 unsigned &InsertedVal = AvailableVals[BB]; 334 335 // If all the predecessor values are the same then we don't need to insert a 336 // PHI. This is the simple and common case. 337 if (SingularValue) { 338 // If a PHI node got inserted, replace it with the singlar value and delete 339 // it. 340 if (InsertedVal) { 341 MachineInstr *OldVal = MRI->getVRegDef(InsertedVal); 342 // Be careful about dead loops. These RAUW's also update InsertedVal. 343 assert(InsertedVal != SingularValue && "Dead loop?"); 344 ReplaceRegWith(InsertedVal, SingularValue); 345 OldVal->eraseFromParent(); 346 } 347 348 InsertedVal = SingularValue; 349 350 // Drop the entries we added in IncomingPredInfo to restore the stack. 351 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 352 IncomingPredInfo.end()); 353 return InsertedVal; 354 } 355 356 357 // Otherwise, we do need a PHI: insert one now if we don't already have one. 358 MachineInstr *InsertedPHI; 359 if (InsertedVal == 0) { 360 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front(); 361 InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB, Loc, 362 VRC, MRI, TII); 363 InsertedVal = InsertedPHI->getOperand(0).getReg(); 364 } else { 365 InsertedPHI = MRI->getVRegDef(InsertedVal); 366 } 367 368 // Fill in all the predecessors of the PHI. 369 MachineInstrBuilder MIB(InsertedPHI); 370 for (IncomingPredInfoTy::iterator I = 371 IncomingPredInfo.begin()+FirstPredInfoEntry, 372 E = IncomingPredInfo.end(); I != E; ++I) 373 MIB.addReg(I->second).addMBB(I->first); 374 375 // Drop the entries we added in IncomingPredInfo to restore the stack. 376 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 377 IncomingPredInfo.end()); 378 379 // See if the PHI node can be merged to a single value. This can happen in 380 // loop cases when we get a PHI of itself and one other value. 381 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) { 382 MRI->replaceRegWith(InsertedVal, ConstVal); 383 InsertedPHI->eraseFromParent(); 384 InsertedVal = ConstVal; 385 } else { 386 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 387 388 // If the client wants to know about all new instructions, tell it. 389 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 390 } 391 392 return InsertedVal; 393} 394