MachineSSAUpdater.cpp revision 723be602ecdfde30a50c3afe1780575fdf8f0f56
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 88/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define 89/// a value of the given register class at the start of the specified basic 90/// block. It returns the virtual register defined by the instruction. 91static 92MachineInstr *InsertNewDef(unsigned Opcode, 93 MachineBasicBlock *BB, MachineBasicBlock::iterator I, 94 const TargetRegisterClass *RC, 95 MachineRegisterInfo *MRI, const TargetInstrInfo *TII) { 96 unsigned NewVR = MRI->createVirtualRegister(RC); 97 return BuildMI(*BB, I, I->getDebugLoc(), TII->get(Opcode), NewVR); 98} 99 100 101/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 102/// is live in the middle of the specified block. 103/// 104/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 105/// important case: if there is a definition of the rewritten value after the 106/// 'use' in BB. Consider code like this: 107/// 108/// X1 = ... 109/// SomeBB: 110/// use(X) 111/// X2 = ... 112/// br Cond, SomeBB, OutBB 113/// 114/// In this case, there are two values (X1 and X2) added to the AvailableVals 115/// set by the client of the rewriter, and those values are both live out of 116/// their respective blocks. However, the use of X happens in the *middle* of 117/// a block. Because of this, we need to insert a new PHI node in SomeBB to 118/// merge the appropriate values, and this value isn't live out of the block. 119/// 120unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) { 121 // If there is no definition of the renamed variable in this block, just use 122 // GetValueAtEndOfBlock to do our work. 123 if (!getAvailableVals(AV).count(BB)) 124 return GetValueAtEndOfBlock(BB); 125 126 // If there are no predecessors, just return undef. 127 if (BB->pred_empty()) 128 return ~0U; // Sentinel value representing undef. 129 130 // Otherwise, we have the hard case. Get the live-in values for each 131 // predecessor. 132 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues; 133 unsigned SingularValue = 0; 134 135 bool isFirstPred = true; 136 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 137 E = BB->pred_end(); PI != E; ++PI) { 138 MachineBasicBlock *PredBB = *PI; 139 unsigned PredVal = GetValueAtEndOfBlockInternal(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 // Otherwise, if all the merged values are the same, just use it. 151 if (SingularValue != 0) 152 return SingularValue; 153 154 // Otherwise, we do need a PHI: insert one now. 155 MachineInstr *InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB, 156 BB->front(), VRC, MRI, TII); 157 158 // Fill in all the predecessors of the PHI. 159 MachineInstrBuilder MIB(InsertedPHI); 160 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 161 MIB.addReg(PredValues[i].second).addMBB(PredValues[i].first); 162 163 // See if the PHI node can be merged to a single value. This can happen in 164 // loop cases when we get a PHI of itself and one other value. 165 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) { 166 InsertedPHI->eraseFromParent(); 167 return ConstVal; 168 } 169 170 // If the client wants to know about all new instructions, tell it. 171 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 172 173 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n"); 174 return InsertedPHI->getOperand(0).getReg(); 175} 176 177static 178MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI, 179 MachineOperand *U) { 180 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) { 181 if (&MI->getOperand(i) == U) 182 return MI->getOperand(i+1).getMBB(); 183 } 184 185 llvm_unreachable("MachineOperand::getParent() failure?"); 186 return 0; 187} 188 189/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 190/// which use their value in the corresponding predecessor. 191void MachineSSAUpdater::RewriteUse(MachineOperand &U) { 192 MachineInstr *UseMI = U.getParent(); 193 unsigned NewVR = 0; 194 if (UseMI->getOpcode() == TargetInstrInfo::PHI) { 195 MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U); 196 NewVR = GetValueAtEndOfBlock(SourceBB); 197 // Insert an implicit_def to represent an undef value. 198 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF, 199 SourceBB,SourceBB->getFirstTerminator(), 200 VRC, MRI, TII); 201 NewVR = NewDef->getOperand(0).getReg(); 202 } else { 203 NewVR = GetValueInMiddleOfBlock(UseMI->getParent()); 204 if (NewVR == ~0U) { 205 // Insert an implicit_def to represent an undef value. 206 MachineInstr *NewDef = InsertNewDef(TargetInstrInfo::IMPLICIT_DEF, 207 UseMI->getParent(), UseMI, 208 VRC, MRI, TII); 209 NewVR = NewDef->getOperand(0).getReg(); 210 } 211 } 212 213 U.setReg(NewVR); 214} 215 216/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry 217/// for the specified BB and if so, return it. If not, construct SSA form by 218/// walking predecessors inserting PHI nodes as needed until we get to a block 219/// where the value is available. 220/// 221unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){ 222 AvailableValsTy &AvailableVals = getAvailableVals(AV); 223 224 // Query AvailableVals by doing an insertion of null. 225 std::pair<AvailableValsTy::iterator, bool> InsertRes = 226 AvailableVals.insert(std::make_pair(BB, 0)); 227 228 // Handle the case when the insertion fails because we have already seen BB. 229 if (!InsertRes.second) { 230 // If the insertion failed, there are two cases. The first case is that the 231 // value is already available for the specified block. If we get this, just 232 // return the value. 233 if (InsertRes.first->second != 0) 234 return InsertRes.first->second; 235 236 // Otherwise, if the value we find is null, then this is the value is not 237 // known but it is being computed elsewhere in our recursion. This means 238 // that we have a cycle. Handle this by inserting a PHI node and returning 239 // it. When we get back to the first instance of the recursion we will fill 240 // in the PHI node. 241 MachineInstr *NewPHI = InsertNewDef(TargetInstrInfo::PHI, BB, BB->front(), 242 VRC, MRI,TII); 243 unsigned NewVR = NewPHI->getOperand(0).getReg(); 244 InsertRes.first->second = NewVR; 245 return NewVR; 246 } 247 248 // If there are no predecessors, then we must have found an unreachable block 249 // just return 'undef'. Since there are no predecessors, InsertRes must not 250 // be invalidated. 251 if (BB->pred_empty()) 252 return InsertRes.first->second = ~0U; // Sentinel value representing undef. 253 254 // Okay, the value isn't in the map and we just inserted a null in the entry 255 // to indicate that we're processing the block. Since we have no idea what 256 // value is in this block, we have to recurse through our predecessors. 257 // 258 // While we're walking our predecessors, we keep track of them in a vector, 259 // then insert a PHI node in the end if we actually need one. We could use a 260 // smallvector here, but that would take a lot of stack space for every level 261 // of the recursion, just use IncomingPredInfo as an explicit stack. 262 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI); 263 unsigned FirstPredInfoEntry = IncomingPredInfo.size(); 264 265 // As we're walking the predecessors, keep track of whether they are all 266 // producing the same value. If so, this value will capture it, if not, it 267 // will get reset to null. We distinguish the no-predecessor case explicitly 268 // below. 269 unsigned SingularValue = 0; 270 bool isFirstPred = true; 271 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 272 E = BB->pred_end(); PI != E; ++PI) { 273 MachineBasicBlock *PredBB = *PI; 274 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB); 275 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal)); 276 277 // Compute SingularValue. 278 if (isFirstPred) { 279 SingularValue = PredVal; 280 isFirstPred = false; 281 } else if (PredVal != SingularValue) 282 SingularValue = 0; 283 } 284 285 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If 286 /// this block is involved in a loop, a no-entry PHI node will have been 287 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted 288 /// above. 289 unsigned &InsertedVal = AvailableVals[BB]; 290 291 // If all the predecessor values are the same then we don't need to insert a 292 // PHI. This is the simple and common case. 293 if (SingularValue) { 294 // If a PHI node got inserted, replace it with the singlar value and delete 295 // it. 296 if (InsertedVal) { 297 MachineInstr *OldVal = MRI->getVRegDef(InsertedVal); 298 // Be careful about dead loops. These RAUW's also update InsertedVal. 299 assert(InsertedVal != SingularValue && "Dead loop?"); 300 MRI->replaceRegWith(InsertedVal, SingularValue); 301 OldVal->eraseFromParent(); 302 } 303 304 InsertedVal = SingularValue; 305 306 // Drop the entries we added in IncomingPredInfo to restore the stack. 307 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 308 IncomingPredInfo.end()); 309 return InsertedVal; 310 } 311 312 313 // Otherwise, we do need a PHI: insert one now if we don't already have one. 314 MachineInstr *InsertedPHI; 315 if (InsertedVal == 0) { 316 InsertedPHI = InsertNewDef(TargetInstrInfo::PHI, BB, BB->front(), 317 VRC, MRI, TII); 318 InsertedVal = InsertedPHI->getOperand(0).getReg(); 319 } else { 320 InsertedPHI = MRI->getVRegDef(InsertedVal); 321 } 322 323 // Fill in all the predecessors of the PHI. 324 bool IsUndef = true; 325 MachineInstrBuilder MIB(InsertedPHI); 326 for (IncomingPredInfoTy::iterator I = 327 IncomingPredInfo.begin()+FirstPredInfoEntry, 328 E = IncomingPredInfo.end(); I != E; ++I) { 329 if (I->second == ~0U) 330 continue; 331 IsUndef = false; 332 MIB.addReg(I->second).addMBB(I->first); 333 } 334 335 // Drop the entries we added in IncomingPredInfo to restore the stack. 336 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, 337 IncomingPredInfo.end()); 338 339 // See if the PHI node can be merged to a single value. This can happen in 340 // loop cases when we get a PHI of itself and one other value. 341 if (IsUndef) { 342 InsertedPHI->eraseFromParent(); 343 InsertedVal = ~0U; 344 } else if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) { 345 MRI->replaceRegWith(InsertedVal, ConstVal); 346 InsertedPHI->eraseFromParent(); 347 InsertedVal = ConstVal; 348 } else { 349 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n"); 350 351 // If the client wants to know about all new instructions, tell it. 352 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 353 } 354 355 return InsertedVal; 356} 357