1 //===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file implements the MachineSSAUpdater class. It's based on SSAUpdater 10 // class in lib/Transforms/Utils. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/CodeGen/MachineSSAUpdater.h" 15 #include "llvm/ADT/DenseMap.h" 16 #include "llvm/ADT/SmallVector.h" 17 #include "llvm/CodeGen/MachineBasicBlock.h" 18 #include "llvm/CodeGen/MachineFunction.h" 19 #include "llvm/CodeGen/MachineInstr.h" 20 #include "llvm/CodeGen/MachineInstrBuilder.h" 21 #include "llvm/CodeGen/MachineOperand.h" 22 #include "llvm/CodeGen/MachineRegisterInfo.h" 23 #include "llvm/CodeGen/TargetInstrInfo.h" 24 #include "llvm/CodeGen/TargetOpcodes.h" 25 #include "llvm/CodeGen/TargetSubtargetInfo.h" 26 #include "llvm/IR/DebugLoc.h" 27 #include "llvm/Support/Debug.h" 28 #include "llvm/Support/ErrorHandling.h" 29 #include "llvm/Support/raw_ostream.h" 30 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h" 31 #include <utility> 32 33 using namespace llvm; 34 35 #define DEBUG_TYPE "machine-ssaupdater" 36 37 using AvailableValsTy = DenseMap<MachineBasicBlock *, unsigned>; 38 39 static AvailableValsTy &getAvailableVals(void *AV) { 40 return *static_cast<AvailableValsTy*>(AV); 41 } 42 43 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF, 44 SmallVectorImpl<MachineInstr*> *NewPHI) 45 : InsertedPHIs(NewPHI), TII(MF.getSubtarget().getInstrInfo()), 46 MRI(&MF.getRegInfo()) {} 47 48 MachineSSAUpdater::~MachineSSAUpdater() { 49 delete static_cast<AvailableValsTy*>(AV); 50 } 51 52 /// Initialize - Reset this object to get ready for a new set of SSA 53 /// updates. ProtoValue is the value used to name PHI nodes. 54 void MachineSSAUpdater::Initialize(unsigned V) { 55 if (!AV) 56 AV = new AvailableValsTy(); 57 else 58 getAvailableVals(AV).clear(); 59 60 VR = V; 61 VRC = MRI->getRegClass(VR); 62 } 63 64 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for 65 /// the specified block. 66 bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const { 67 return getAvailableVals(AV).count(BB); 68 } 69 70 /// AddAvailableValue - Indicate that a rewritten value is available in the 71 /// specified block with the specified value. 72 void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) { 73 getAvailableVals(AV)[BB] = V; 74 } 75 76 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is 77 /// live at the end of the specified block. 78 unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) { 79 return GetValueAtEndOfBlockInternal(BB); 80 } 81 82 static 83 unsigned LookForIdenticalPHI(MachineBasicBlock *BB, 84 SmallVectorImpl<std::pair<MachineBasicBlock *, unsigned>> &PredValues) { 85 if (BB->empty()) 86 return 0; 87 88 MachineBasicBlock::iterator I = BB->begin(); 89 if (!I->isPHI()) 90 return 0; 91 92 AvailableValsTy AVals; 93 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 94 AVals[PredValues[i].first] = PredValues[i].second; 95 while (I != BB->end() && I->isPHI()) { 96 bool Same = true; 97 for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) { 98 Register SrcReg = I->getOperand(i).getReg(); 99 MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB(); 100 if (AVals[SrcBB] != SrcReg) { 101 Same = false; 102 break; 103 } 104 } 105 if (Same) 106 return I->getOperand(0).getReg(); 107 ++I; 108 } 109 return 0; 110 } 111 112 /// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define 113 /// a value of the given register class at the start of the specified basic 114 /// block. It returns the virtual register defined by the instruction. 115 static 116 MachineInstrBuilder InsertNewDef(unsigned Opcode, 117 MachineBasicBlock *BB, MachineBasicBlock::iterator I, 118 const TargetRegisterClass *RC, 119 MachineRegisterInfo *MRI, 120 const TargetInstrInfo *TII) { 121 Register NewVR = MRI->createVirtualRegister(RC); 122 return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR); 123 } 124 125 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that 126 /// is live in the middle of the specified block. 127 /// 128 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one 129 /// important case: if there is a definition of the rewritten value after the 130 /// 'use' in BB. Consider code like this: 131 /// 132 /// X1 = ... 133 /// SomeBB: 134 /// use(X) 135 /// X2 = ... 136 /// br Cond, SomeBB, OutBB 137 /// 138 /// In this case, there are two values (X1 and X2) added to the AvailableVals 139 /// set by the client of the rewriter, and those values are both live out of 140 /// their respective blocks. However, the use of X happens in the *middle* of 141 /// a block. Because of this, we need to insert a new PHI node in SomeBB to 142 /// merge the appropriate values, and this value isn't live out of the block. 143 unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) { 144 // If there is no definition of the renamed variable in this block, just use 145 // GetValueAtEndOfBlock to do our work. 146 if (!HasValueForBlock(BB)) 147 return GetValueAtEndOfBlockInternal(BB); 148 149 // If there are no predecessors, just return undef. 150 if (BB->pred_empty()) { 151 // Insert an implicit_def to represent an undef value. 152 MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF, 153 BB, BB->getFirstTerminator(), 154 VRC, MRI, TII); 155 return NewDef->getOperand(0).getReg(); 156 } 157 158 // Otherwise, we have the hard case. Get the live-in values for each 159 // predecessor. 160 SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues; 161 unsigned SingularValue = 0; 162 163 bool isFirstPred = true; 164 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 165 E = BB->pred_end(); PI != E; ++PI) { 166 MachineBasicBlock *PredBB = *PI; 167 unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB); 168 PredValues.push_back(std::make_pair(PredBB, PredVal)); 169 170 // Compute SingularValue. 171 if (isFirstPred) { 172 SingularValue = PredVal; 173 isFirstPred = false; 174 } else if (PredVal != SingularValue) 175 SingularValue = 0; 176 } 177 178 // Otherwise, if all the merged values are the same, just use it. 179 if (SingularValue != 0) 180 return SingularValue; 181 182 // If an identical PHI is already in BB, just reuse it. 183 unsigned DupPHI = LookForIdenticalPHI(BB, PredValues); 184 if (DupPHI) 185 return DupPHI; 186 187 // Otherwise, we do need a PHI: insert one now. 188 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin(); 189 MachineInstrBuilder InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB, 190 Loc, VRC, MRI, TII); 191 192 // Fill in all the predecessors of the PHI. 193 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) 194 InsertedPHI.addReg(PredValues[i].second).addMBB(PredValues[i].first); 195 196 // See if the PHI node can be merged to a single value. This can happen in 197 // loop cases when we get a PHI of itself and one other value. 198 if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) { 199 InsertedPHI->eraseFromParent(); 200 return ConstVal; 201 } 202 203 // If the client wants to know about all new instructions, tell it. 204 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); 205 206 LLVM_DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n"); 207 return InsertedPHI->getOperand(0).getReg(); 208 } 209 210 static 211 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI, 212 MachineOperand *U) { 213 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) { 214 if (&MI->getOperand(i) == U) 215 return MI->getOperand(i+1).getMBB(); 216 } 217 218 llvm_unreachable("MachineOperand::getParent() failure?"); 219 } 220 221 /// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes, 222 /// which use their value in the corresponding predecessor. 223 void MachineSSAUpdater::RewriteUse(MachineOperand &U) { 224 MachineInstr *UseMI = U.getParent(); 225 unsigned NewVR = 0; 226 if (UseMI->isPHI()) { 227 MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U); 228 NewVR = GetValueAtEndOfBlockInternal(SourceBB); 229 } else { 230 NewVR = GetValueInMiddleOfBlock(UseMI->getParent()); 231 } 232 233 U.setReg(NewVR); 234 } 235 236 /// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl 237 /// template, specialized for MachineSSAUpdater. 238 namespace llvm { 239 240 template<> 241 class SSAUpdaterTraits<MachineSSAUpdater> { 242 public: 243 using BlkT = MachineBasicBlock; 244 using ValT = unsigned; 245 using PhiT = MachineInstr; 246 using BlkSucc_iterator = MachineBasicBlock::succ_iterator; 247 248 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); } 249 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); } 250 251 /// Iterator for PHI operands. 252 class PHI_iterator { 253 private: 254 MachineInstr *PHI; 255 unsigned idx; 256 257 public: 258 explicit PHI_iterator(MachineInstr *P) // begin iterator 259 : PHI(P), idx(1) {} 260 PHI_iterator(MachineInstr *P, bool) // end iterator 261 : PHI(P), idx(PHI->getNumOperands()) {} 262 263 PHI_iterator &operator++() { idx += 2; return *this; } 264 bool operator==(const PHI_iterator& x) const { return idx == x.idx; } 265 bool operator!=(const PHI_iterator& x) const { return !operator==(x); } 266 267 unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); } 268 269 MachineBasicBlock *getIncomingBlock() { 270 return PHI->getOperand(idx+1).getMBB(); 271 } 272 }; 273 274 static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); } 275 276 static inline PHI_iterator PHI_end(PhiT *PHI) { 277 return PHI_iterator(PHI, true); 278 } 279 280 /// FindPredecessorBlocks - Put the predecessors of BB into the Preds 281 /// vector. 282 static void FindPredecessorBlocks(MachineBasicBlock *BB, 283 SmallVectorImpl<MachineBasicBlock*> *Preds){ 284 for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(), 285 E = BB->pred_end(); PI != E; ++PI) 286 Preds->push_back(*PI); 287 } 288 289 /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register. 290 /// Add it into the specified block and return the register. 291 static unsigned GetUndefVal(MachineBasicBlock *BB, 292 MachineSSAUpdater *Updater) { 293 // Insert an implicit_def to represent an undef value. 294 MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF, 295 BB, BB->getFirstNonPHI(), 296 Updater->VRC, Updater->MRI, 297 Updater->TII); 298 return NewDef->getOperand(0).getReg(); 299 } 300 301 /// CreateEmptyPHI - Create a PHI instruction that defines a new register. 302 /// Add it into the specified block and return the register. 303 static unsigned CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds, 304 MachineSSAUpdater *Updater) { 305 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin(); 306 MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc, 307 Updater->VRC, Updater->MRI, 308 Updater->TII); 309 return PHI->getOperand(0).getReg(); 310 } 311 312 /// AddPHIOperand - Add the specified value as an operand of the PHI for 313 /// the specified predecessor block. 314 static void AddPHIOperand(MachineInstr *PHI, unsigned Val, 315 MachineBasicBlock *Pred) { 316 MachineInstrBuilder(*Pred->getParent(), PHI).addReg(Val).addMBB(Pred); 317 } 318 319 /// InstrIsPHI - Check if an instruction is a PHI. 320 static MachineInstr *InstrIsPHI(MachineInstr *I) { 321 if (I && I->isPHI()) 322 return I; 323 return nullptr; 324 } 325 326 /// ValueIsPHI - Check if the instruction that defines the specified register 327 /// is a PHI instruction. 328 static MachineInstr *ValueIsPHI(unsigned Val, MachineSSAUpdater *Updater) { 329 return InstrIsPHI(Updater->MRI->getVRegDef(Val)); 330 } 331 332 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source 333 /// operands, i.e., it was just added. 334 static MachineInstr *ValueIsNewPHI(unsigned Val, MachineSSAUpdater *Updater) { 335 MachineInstr *PHI = ValueIsPHI(Val, Updater); 336 if (PHI && PHI->getNumOperands() <= 1) 337 return PHI; 338 return nullptr; 339 } 340 341 /// GetPHIValue - For the specified PHI instruction, return the register 342 /// that it defines. 343 static unsigned GetPHIValue(MachineInstr *PHI) { 344 return PHI->getOperand(0).getReg(); 345 } 346 }; 347 348 } // end namespace llvm 349 350 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry 351 /// for the specified BB and if so, return it. If not, construct SSA form by 352 /// first calculating the required placement of PHIs and then inserting new 353 /// PHIs where needed. 354 unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){ 355 AvailableValsTy &AvailableVals = getAvailableVals(AV); 356 if (unsigned V = AvailableVals[BB]) 357 return V; 358 359 SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs); 360 return Impl.GetValue(BB); 361 } 362