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