lib/CodeGen/MachineSSAUpdater.cpp

   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->getFirstTerminator(),
 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 }