lib/CodeGen/TailDuplication.cpp

   1 //===-- TailDuplication.cpp - Duplicate blocks into predecessors' tails ---===//
   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 pass duplicates basic blocks ending in unconditional branches into
  11 // the tails of their predecessors.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #define DEBUG_TYPE "tailduplication"
  16 #include "llvm/Function.h"
  17 #include "llvm/CodeGen/Passes.h"
  18 #include "llvm/CodeGen/MachineModuleInfo.h"
  19 #include "llvm/CodeGen/MachineFunctionPass.h"
  20 #include "llvm/CodeGen/MachineInstrBuilder.h"
  21 #include "llvm/CodeGen/MachineRegisterInfo.h"
  22 #include "llvm/CodeGen/MachineSSAUpdater.h"
  23 #include "llvm/Target/TargetInstrInfo.h"
  24 #include "llvm/Support/CommandLine.h"
  25 #include "llvm/Support/Debug.h"
  26 #include "llvm/Support/ErrorHandling.h"
  27 #include "llvm/Support/raw_ostream.h"
  28 #include "llvm/ADT/SmallSet.h"
  29 #include "llvm/ADT/SetVector.h"
  30 #include "llvm/ADT/Statistic.h"
  31 using namespace llvm;
  32
  33 STATISTIC(NumTails     , "Number of tails duplicated");
  34 STATISTIC(NumTailDups  , "Number of tail duplicated blocks");
  35 STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
  36 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
  37 STATISTIC(NumAddedPHIs , "Number of phis added");
  38
  39 // Heuristic for tail duplication.
  40 static cl::opt<unsigned>
  41 TailDuplicateSize("tail-dup-size",
  42                   cl::desc("Maximum instructions to consider tail duplicating"),
  43                   cl::init(2), cl::Hidden);
  44
  45 static cl::opt<bool>
  46 TailDupVerify("tail-dup-verify",
  47               cl::desc("Verify sanity of PHI instructions during taildup"),
  48               cl::init(false), cl::Hidden);
  49
  50 static cl::opt<unsigned>
  51 TailDupLimit("tail-dup-limit", cl::init(~0U), cl::Hidden);
  52
  53 typedef std::vector<std::pair<MachineBasicBlock*,unsigned> > AvailableValsTy;
  54
  55 namespace {
  56   /// TailDuplicatePass - Perform tail duplication.
  57   class TailDuplicatePass : public MachineFunctionPass {
  58     bool PreRegAlloc;
  59     const TargetInstrInfo *TII;
  60     MachineModuleInfo *MMI;
  61     MachineRegisterInfo *MRI;
  62
  63     // SSAUpdateVRs - A list of virtual registers for which to update SSA form.
  64     SmallVector<unsigned, 16> SSAUpdateVRs;
  65
  66     // SSAUpdateVals - For each virtual register in SSAUpdateVals keep a list of
  67     // source virtual registers.
  68     DenseMap<unsigned, AvailableValsTy> SSAUpdateVals;
  69
  70   public:
  71     static char ID;
  72     explicit TailDuplicatePass(bool PreRA) :
  73       MachineFunctionPass(ID), PreRegAlloc(PreRA) {}
  74
  75     virtual bool runOnMachineFunction(MachineFunction &MF);
  76     virtual const char *getPassName() const { return "Tail Duplication"; }
  77
  78   private:
  79     void AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
  80                            MachineBasicBlock *BB);
  81     void ProcessPHI(MachineInstr *MI, MachineBasicBlock *TailBB,
  82                     MachineBasicBlock *PredBB,
  83                     DenseMap<unsigned, unsigned> &LocalVRMap,
  84                     SmallVector<std::pair<unsigned,unsigned>, 4> &Copies,
  85                     const DenseSet<unsigned> &UsedByPhi,
  86                     bool Remove);
  87     void DuplicateInstruction(MachineInstr *MI,
  88                               MachineBasicBlock *TailBB,
  89                               MachineBasicBlock *PredBB,
  90                               MachineFunction &MF,
  91                               DenseMap<unsigned, unsigned> &LocalVRMap,
  92                               const DenseSet<unsigned> &UsedByPhi);
  93     void UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
  94                               SmallVector<MachineBasicBlock*, 8> &TDBBs,
  95                               SmallSetVector<MachineBasicBlock*, 8> &Succs);
  96     bool TailDuplicateBlocks(MachineFunction &MF);
  97     bool shouldTailDuplicate(const MachineFunction &MF,
  98                              bool IsSimple, MachineBasicBlock &TailBB);
  99     bool isSimpleBB(MachineBasicBlock *TailBB);
 100     bool canCompletelyDuplicateBB(MachineBasicBlock &BB);
 101     bool duplicateSimpleBB(MachineBasicBlock *TailBB,
 102                            SmallVector<MachineBasicBlock*, 8> &TDBBs,
 103                            const DenseSet<unsigned> &RegsUsedByPhi,
 104                            SmallVector<MachineInstr*, 16> &Copies);
 105     bool TailDuplicate(MachineBasicBlock *TailBB,
 106                        bool IsSimple,
 107                        MachineFunction &MF,
 108                        SmallVector<MachineBasicBlock*, 8> &TDBBs,
 109                        SmallVector<MachineInstr*, 16> &Copies);
 110     bool TailDuplicateAndUpdate(MachineBasicBlock *MBB,
 111                                 bool IsSimple,
 112                                 MachineFunction &MF);
 113
 114     void RemoveDeadBlock(MachineBasicBlock *MBB);
 115   };
 116
 117   char TailDuplicatePass::ID = 0;
 118 }
 119
 120 FunctionPass *llvm::createTailDuplicatePass(bool PreRegAlloc) {
 121   return new TailDuplicatePass(PreRegAlloc);
 122 }
 123
 124 bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
 125   TII = MF.getTarget().getInstrInfo();
 126   MRI = &MF.getRegInfo();
 127   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
 128
 129   bool MadeChange = false;
 130   while (TailDuplicateBlocks(MF))
 131     MadeChange = true;
 132
 133   return MadeChange;
 134 }
 135
 136 static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
 137   for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
 138     MachineBasicBlock *MBB = I;
 139     SmallSetVector<MachineBasicBlock*, 8> Preds(MBB->pred_begin(),
 140                                                 MBB->pred_end());
 141     MachineBasicBlock::iterator MI = MBB->begin();
 142     while (MI != MBB->end()) {
 143       if (!MI->isPHI())
 144         break;
 145       for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
 146              PE = Preds.end(); PI != PE; ++PI) {
 147         MachineBasicBlock *PredBB = *PI;
 148         bool Found = false;
 149         for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
 150           MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
 151           if (PHIBB == PredBB) {
 152             Found = true;
 153             break;
 154           }
 155         }
 156         if (!Found) {
 157           dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
 158           dbgs() << "  missing input from predecessor BB#"
 159                  << PredBB->getNumber() << '\n';
 160           llvm_unreachable(0);
 161         }
 162       }
 163
 164       for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
 165         MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
 166         if (CheckExtra && !Preds.count(PHIBB)) {
 167           dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber()
 168                  << ": " << *MI;
 169           dbgs() << "  extra input from predecessor BB#"
 170                  << PHIBB->getNumber() << '\n';
 171           llvm_unreachable(0);
 172         }
 173         if (PHIBB->getNumber() < 0) {
 174           dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
 175           dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
 176           llvm_unreachable(0);
 177         }
 178       }
 179       ++MI;
 180     }
 181   }
 182 }
 183
 184 /// TailDuplicateAndUpdate - Tail duplicate the block and cleanup.
 185 bool
 186 TailDuplicatePass::TailDuplicateAndUpdate(MachineBasicBlock *MBB,
 187                                           bool IsSimple,
 188                                           MachineFunction &MF) {
 189   // Save the successors list.
 190   SmallSetVector<MachineBasicBlock*, 8> Succs(MBB->succ_begin(),
 191                                               MBB->succ_end());
 192
 193   SmallVector<MachineBasicBlock*, 8> TDBBs;
 194   SmallVector<MachineInstr*, 16> Copies;
 195   if (!TailDuplicate(MBB, IsSimple, MF, TDBBs, Copies))
 196     return false;
 197
 198   ++NumTails;
 199
 200   SmallVector<MachineInstr*, 8> NewPHIs;
 201   MachineSSAUpdater SSAUpdate(MF, &NewPHIs);
 202
 203   // TailBB's immediate successors are now successors of those predecessors
 204   // which duplicated TailBB. Add the predecessors as sources to the PHI
 205   // instructions.
 206   bool isDead = MBB->pred_empty() && !MBB->hasAddressTaken();
 207   if (PreRegAlloc)
 208     UpdateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
 209
 210   // If it is dead, remove it.
 211   if (isDead) {
 212     NumInstrDups -= MBB->size();
 213     RemoveDeadBlock(MBB);
 214     ++NumDeadBlocks;
 215   }
 216
 217   // Update SSA form.
 218   if (!SSAUpdateVRs.empty()) {
 219     for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
 220       unsigned VReg = SSAUpdateVRs[i];
 221       SSAUpdate.Initialize(VReg);
 222
 223       // If the original definition is still around, add it as an available
 224       // value.
 225       MachineInstr *DefMI = MRI->getVRegDef(VReg);
 226       MachineBasicBlock *DefBB = 0;
 227       if (DefMI) {
 228         DefBB = DefMI->getParent();
 229         SSAUpdate.AddAvailableValue(DefBB, VReg);
 230       }
 231
 232       // Add the new vregs as available values.
 233       DenseMap<unsigned, AvailableValsTy>::iterator LI =
 234         SSAUpdateVals.find(VReg);
 235       for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
 236         MachineBasicBlock *SrcBB = LI->second[j].first;
 237         unsigned SrcReg = LI->second[j].second;
 238         SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
 239       }
 240
 241       // Rewrite uses that are outside of the original def's block.
 242       MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
 243       while (UI != MRI->use_end()) {
 244         MachineOperand &UseMO = UI.getOperand();
 245         MachineInstr *UseMI = &*UI;
 246         ++UI;
 247         if (UseMI->isDebugValue()) {
 248           // SSAUpdate can replace the use with an undef. That creates
 249           // a debug instruction that is a kill.
 250           // FIXME: Should it SSAUpdate job to delete debug instructions
 251           // instead of replacing the use with undef?
 252           UseMI->eraseFromParent();
 253           continue;
 254         }
 255         if (UseMI->getParent() == DefBB && !UseMI->isPHI())
 256           continue;
 257         SSAUpdate.RewriteUse(UseMO);
 258       }
 259     }
 260
 261     SSAUpdateVRs.clear();
 262     SSAUpdateVals.clear();
 263   }
 264
 265   // Eliminate some of the copies inserted by tail duplication to maintain
 266   // SSA form.
 267   for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
 268     MachineInstr *Copy = Copies[i];
 269     if (!Copy->isCopy())
 270       continue;
 271     unsigned Dst = Copy->getOperand(0).getReg();
 272     unsigned Src = Copy->getOperand(1).getReg();
 273     MachineRegisterInfo::use_iterator UI = MRI->use_begin(Src);
 274     if (++UI == MRI->use_end()) {
 275       // Copy is the only use. Do trivial copy propagation here.
 276       MRI->replaceRegWith(Dst, Src);
 277       Copy->eraseFromParent();
 278     }
 279   }
 280
 281   if (NewPHIs.size())
 282     NumAddedPHIs += NewPHIs.size();
 283
 284   return true;
 285 }
 286
 287 /// TailDuplicateBlocks - Look for small blocks that are unconditionally
 288 /// branched to and do not fall through. Tail-duplicate their instructions
 289 /// into their predecessors to eliminate (dynamic) branches.
 290 bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
 291   bool MadeChange = false;
 292
 293   if (PreRegAlloc && TailDupVerify) {
 294     DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
 295     VerifyPHIs(MF, true);
 296   }
 297
 298   for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
 299     MachineBasicBlock *MBB = I++;
 300
 301     if (NumTails == TailDupLimit)
 302       break;
 303
 304     bool IsSimple = isSimpleBB(MBB);
 305
 306     if (!shouldTailDuplicate(MF, IsSimple, *MBB))
 307       continue;
 308
 309     MadeChange |= TailDuplicateAndUpdate(MBB, IsSimple, MF);
 310   }
 311
 312   if (PreRegAlloc && TailDupVerify)
 313     VerifyPHIs(MF, false);
 314
 315   return MadeChange;
 316 }
 317
 318 static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
 319                          const MachineRegisterInfo *MRI) {
 320   for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
 321          UE = MRI->use_end(); UI != UE; ++UI) {
 322     MachineInstr *UseMI = &*UI;
 323     if (UseMI->isDebugValue())
 324       continue;
 325     if (UseMI->getParent() != BB)
 326       return true;
 327   }
 328   return false;
 329 }
 330
 331 static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
 332   for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
 333     if (MI->getOperand(i+1).getMBB() == SrcBB)
 334       return i;
 335   return 0;
 336 }
 337
 338
 339 // Remember which registers are used by phis in this block. This is
 340 // used to determine which registers are liveout while modifying the
 341 // block (which is why we need to copy the information).
 342 static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
 343                               DenseSet<unsigned> *UsedByPhi) {
 344   for(MachineBasicBlock::const_iterator I = BB.begin(), E = BB.end();
 345       I != E; ++I) {
 346     const MachineInstr &MI = *I;
 347     if (!MI.isPHI())
 348       break;
 349     for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
 350       unsigned SrcReg = MI.getOperand(i).getReg();
 351       UsedByPhi->insert(SrcReg);
 352     }
 353   }
 354 }
 355
 356 /// AddSSAUpdateEntry - Add a definition and source virtual registers pair for
 357 /// SSA update.
 358 void TailDuplicatePass::AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
 359                                           MachineBasicBlock *BB) {
 360   DenseMap<unsigned, AvailableValsTy>::iterator LI= SSAUpdateVals.find(OrigReg);
 361   if (LI != SSAUpdateVals.end())
 362     LI->second.push_back(std::make_pair(BB, NewReg));
 363   else {
 364     AvailableValsTy Vals;
 365     Vals.push_back(std::make_pair(BB, NewReg));
 366     SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
 367     SSAUpdateVRs.push_back(OrigReg);
 368   }
 369 }
 370
 371 /// ProcessPHI - Process PHI node in TailBB by turning it into a copy in PredBB.
 372 /// Remember the source register that's contributed by PredBB and update SSA
 373 /// update map.
 374 void TailDuplicatePass::ProcessPHI(MachineInstr *MI,
 375                                    MachineBasicBlock *TailBB,
 376                                    MachineBasicBlock *PredBB,
 377                                    DenseMap<unsigned, unsigned> &LocalVRMap,
 378                            SmallVector<std::pair<unsigned,unsigned>, 4> &Copies,
 379                                    const DenseSet<unsigned> &RegsUsedByPhi,
 380                                    bool Remove) {
 381   unsigned DefReg = MI->getOperand(0).getReg();
 382   unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
 383   assert(SrcOpIdx && "Unable to find matching PHI source?");
 384   unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
 385   const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
 386   LocalVRMap.insert(std::make_pair(DefReg, SrcReg));
 387
 388   // Insert a copy from source to the end of the block. The def register is the
 389   // available value liveout of the block.
 390   unsigned NewDef = MRI->createVirtualRegister(RC);
 391   Copies.push_back(std::make_pair(NewDef, SrcReg));
 392   if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
 393     AddSSAUpdateEntry(DefReg, NewDef, PredBB);
 394
 395   if (!Remove)
 396     return;
 397
 398   // Remove PredBB from the PHI node.
 399   MI->RemoveOperand(SrcOpIdx+1);
 400   MI->RemoveOperand(SrcOpIdx);
 401   if (MI->getNumOperands() == 1)
 402     MI->eraseFromParent();
 403 }
 404
 405 /// DuplicateInstruction - Duplicate a TailBB instruction to PredBB and update
 406 /// the source operands due to earlier PHI translation.
 407 void TailDuplicatePass::DuplicateInstruction(MachineInstr *MI,
 408                                      MachineBasicBlock *TailBB,
 409                                      MachineBasicBlock *PredBB,
 410                                      MachineFunction &MF,
 411                                      DenseMap<unsigned, unsigned> &LocalVRMap,
 412                                      const DenseSet<unsigned> &UsedByPhi) {
 413   MachineInstr *NewMI = TII->duplicate(MI, MF);
 414   for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
 415     MachineOperand &MO = NewMI->getOperand(i);
 416     if (!MO.isReg())
 417       continue;
 418     unsigned Reg = MO.getReg();
 419     if (!TargetRegisterInfo::isVirtualRegister(Reg))
 420       continue;
 421     if (MO.isDef()) {
 422       const TargetRegisterClass *RC = MRI->getRegClass(Reg);
 423       unsigned NewReg = MRI->createVirtualRegister(RC);
 424       MO.setReg(NewReg);
 425       LocalVRMap.insert(std::make_pair(Reg, NewReg));
 426       if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
 427         AddSSAUpdateEntry(Reg, NewReg, PredBB);
 428     } else {
 429       DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
 430       if (VI != LocalVRMap.end())
 431         MO.setReg(VI->second);
 432     }
 433   }
 434   PredBB->insert(PredBB->end(), NewMI);
 435 }
 436
 437 /// UpdateSuccessorsPHIs - After FromBB is tail duplicated into its predecessor
 438 /// blocks, the successors have gained new predecessors. Update the PHI
 439 /// instructions in them accordingly.
 440 void
 441 TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
 442                                   SmallVector<MachineBasicBlock*, 8> &TDBBs,
 443                                   SmallSetVector<MachineBasicBlock*,8> &Succs) {
 444   for (SmallSetVector<MachineBasicBlock*, 8>::iterator SI = Succs.begin(),
 445          SE = Succs.end(); SI != SE; ++SI) {
 446     MachineBasicBlock *SuccBB = *SI;
 447     for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
 448          II != EE; ++II) {
 449       if (!II->isPHI())
 450         break;
 451       unsigned Idx = 0;
 452       for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
 453         MachineOperand &MO = II->getOperand(i+1);
 454         if (MO.getMBB() == FromBB) {
 455           Idx = i;
 456           break;
 457         }
 458       }
 459
 460       assert(Idx != 0);
 461       MachineOperand &MO0 = II->getOperand(Idx);
 462       unsigned Reg = MO0.getReg();
 463       if (isDead) {
 464         // Folded into the previous BB.
 465         // There could be duplicate phi source entries. FIXME: Should sdisel
 466         // or earlier pass fixed this?
 467         for (unsigned i = II->getNumOperands()-2; i != Idx; i -= 2) {
 468           MachineOperand &MO = II->getOperand(i+1);
 469           if (MO.getMBB() == FromBB) {
 470             II->RemoveOperand(i+1);
 471             II->RemoveOperand(i);
 472           }
 473         }
 474       } else
 475         Idx = 0;
 476
 477       // If Idx is set, the operands at Idx and Idx+1 must be removed.
 478       // We reuse the location to avoid expensive RemoveOperand calls.
 479
 480       DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
 481       if (LI != SSAUpdateVals.end()) {
 482         // This register is defined in the tail block.
 483         for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
 484           MachineBasicBlock *SrcBB = LI->second[j].first;
 485           // If we didn't duplicate a bb into a particular predecessor, we
 486           // might still have added an entry to SSAUpdateVals to correcly
 487           // recompute SSA. If that case, avoid adding a dummy extra argument
 488           // this PHI.
 489           if (!SrcBB->isSuccessor(SuccBB))
 490             continue;
 491
 492           unsigned SrcReg = LI->second[j].second;
 493           if (Idx != 0) {
 494             II->getOperand(Idx).setReg(SrcReg);
 495             II->getOperand(Idx+1).setMBB(SrcBB);
 496             Idx = 0;
 497           } else {
 498             II->addOperand(MachineOperand::CreateReg(SrcReg, false));
 499             II->addOperand(MachineOperand::CreateMBB(SrcBB));
 500           }
 501         }
 502       } else {
 503         // Live in tail block, must also be live in predecessors.
 504         for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
 505           MachineBasicBlock *SrcBB = TDBBs[j];
 506           if (Idx != 0) {
 507             II->getOperand(Idx).setReg(Reg);
 508             II->getOperand(Idx+1).setMBB(SrcBB);
 509             Idx = 0;
 510           } else {
 511             II->addOperand(MachineOperand::CreateReg(Reg, false));
 512             II->addOperand(MachineOperand::CreateMBB(SrcBB));
 513           }
 514         }
 515       }
 516       if (Idx != 0) {
 517         II->RemoveOperand(Idx+1);
 518         II->RemoveOperand(Idx);
 519       }
 520     }
 521   }
 522 }
 523
 524 /// shouldTailDuplicate - Determine if it is profitable to duplicate this block.
 525 bool
 526 TailDuplicatePass::shouldTailDuplicate(const MachineFunction &MF,
 527                                        bool IsSimple,
 528                                        MachineBasicBlock &TailBB) {
 529   // Only duplicate blocks that end with unconditional branches.
 530   if (TailBB.canFallThrough())
 531     return false;
 532
 533   // Don't try to tail-duplicate single-block loops.
 534   if (TailBB.isSuccessor(&TailBB))
 535     return false;
 536
 537   // Set the limit on the cost to duplicate. When optimizing for size,
 538   // duplicate only one, because one branch instruction can be eliminated to
 539   // compensate for the duplication.
 540   unsigned MaxDuplicateCount;
 541   if (TailDuplicateSize.getNumOccurrences() == 0 &&
 542       MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
 543     MaxDuplicateCount = 1;
 544   else
 545     MaxDuplicateCount = TailDuplicateSize;
 546
 547   // If the target has hardware branch prediction that can handle indirect
 548   // branches, duplicating them can often make them predictable when there
 549   // are common paths through the code.  The limit needs to be high enough
 550   // to allow undoing the effects of tail merging and other optimizations
 551   // that rearrange the predecessors of the indirect branch.
 552
 553   bool HasIndirectbr = false;
 554   if (!TailBB.empty())
 555     HasIndirectbr = TailBB.back().getDesc().isIndirectBranch();
 556
 557   if (HasIndirectbr && PreRegAlloc)
 558     MaxDuplicateCount = 20;
 559
 560   // Check the instructions in the block to determine whether tail-duplication
 561   // is invalid or unlikely to be profitable.
 562   unsigned InstrCount = 0;
 563   for (MachineBasicBlock::const_iterator I = TailBB.begin(); I != TailBB.end();
 564        ++I) {
 565     // Non-duplicable things shouldn't be tail-duplicated.
 566     if (I->getDesc().isNotDuplicable())
 567       return false;
 568
 569     // Do not duplicate 'return' instructions if this is a pre-regalloc run.
 570     // A return may expand into a lot more instructions (e.g. reload of callee
 571     // saved registers) after PEI.
 572     if (PreRegAlloc && I->getDesc().isReturn())
 573       return false;
 574
 575     // Avoid duplicating calls before register allocation. Calls presents a
 576     // barrier to register allocation so duplicating them may end up increasing
 577     // spills.
 578     if (PreRegAlloc && I->getDesc().isCall())
 579       return false;
 580
 581     if (!I->isPHI() && !I->isDebugValue())
 582       InstrCount += 1;
 583
 584     if (InstrCount > MaxDuplicateCount)
 585       return false;
 586   }
 587
 588   if (HasIndirectbr && PreRegAlloc)
 589     return true;
 590
 591   if (IsSimple)
 592     return true;
 593
 594   if (!PreRegAlloc)
 595     return true;
 596
 597   return canCompletelyDuplicateBB(TailBB);
 598 }
 599
 600 /// isSimpleBB - True if this BB has only one unconditional jump.
 601 bool
 602 TailDuplicatePass::isSimpleBB(MachineBasicBlock *TailBB) {
 603   if (TailBB->succ_size() != 1)
 604     return false;
 605   if (TailBB->pred_empty())
 606     return false;
 607   MachineBasicBlock::iterator I = TailBB->begin();
 608   MachineBasicBlock::iterator E = TailBB->end();
 609   while (I != E && I->isDebugValue())
 610     ++I;
 611   if (I == E)
 612     return true;
 613   return I->getDesc().isUnconditionalBranch();
 614 }
 615
 616 static bool
 617 bothUsedInPHI(const MachineBasicBlock &A,
 618               SmallPtrSet<MachineBasicBlock*, 8> SuccsB) {
 619   for (MachineBasicBlock::const_succ_iterator SI = A.succ_begin(),
 620          SE = A.succ_end(); SI != SE; ++SI) {
 621     MachineBasicBlock *BB = *SI;
 622     if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
 623       return true;
 624   }
 625
 626   return false;
 627 }
 628
 629 bool
 630 TailDuplicatePass::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
 631   SmallPtrSet<MachineBasicBlock*, 8> Succs(BB.succ_begin(), BB.succ_end());
 632
 633   for (MachineBasicBlock::pred_iterator PI = BB.pred_begin(),
 634        PE = BB.pred_end(); PI != PE; ++PI) {
 635     MachineBasicBlock *PredBB = *PI;
 636
 637     if (PredBB->succ_size() > 1)
 638       return false;
 639
 640     MachineBasicBlock *PredTBB = NULL, *PredFBB = NULL;
 641     SmallVector<MachineOperand, 4> PredCond;
 642     if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
 643       return false;
 644
 645     if (!PredCond.empty())
 646       return false;
 647   }
 648   return true;
 649 }
 650
 651 bool
 652 TailDuplicatePass::duplicateSimpleBB(MachineBasicBlock *TailBB,
 653                                      SmallVector<MachineBasicBlock*, 8> &TDBBs,
 654                                      const DenseSet<unsigned> &UsedByPhi,
 655                                      SmallVector<MachineInstr*, 16> &Copies) {
 656   SmallPtrSet<MachineBasicBlock*, 8> Succs(TailBB->succ_begin(),
 657                                            TailBB->succ_end());
 658   SmallVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
 659                                            TailBB->pred_end());
 660   bool Changed = false;
 661   for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
 662        PE = Preds.end(); PI != PE; ++PI) {
 663     MachineBasicBlock *PredBB = *PI;
 664
 665     if (PredBB->getLandingPadSuccessor())
 666       continue;
 667
 668     if (bothUsedInPHI(*PredBB, Succs))
 669       continue;
 670
 671     MachineBasicBlock *PredTBB = NULL, *PredFBB = NULL;
 672     SmallVector<MachineOperand, 4> PredCond;
 673     if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
 674       continue;
 675
 676     Changed = true;
 677     DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
 678                  << "From simple Succ: " << *TailBB);
 679
 680     MachineBasicBlock *NewTarget = *TailBB->succ_begin();
 681     MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(PredBB));
 682
 683     // Make PredFBB explicit.
 684     if (PredCond.empty())
 685       PredFBB = PredTBB;
 686
 687     // Make fall through explicit.
 688     if (!PredTBB)
 689       PredTBB = NextBB;
 690     if (!PredFBB)
 691       PredFBB = NextBB;
 692
 693     // Redirect
 694     if (PredFBB == TailBB)
 695       PredFBB = NewTarget;
 696     if (PredTBB == TailBB)
 697       PredTBB = NewTarget;
 698
 699     // Make the branch unconditional if possible
 700     if (PredTBB == PredFBB) {
 701       PredCond.clear();
 702       PredFBB = NULL;
 703     }
 704
 705     // Avoid adding fall through branches.
 706     if (PredFBB == NextBB)
 707       PredFBB = NULL;
 708     if (PredTBB == NextBB && PredFBB == NULL)
 709       PredTBB = NULL;
 710
 711     TII->RemoveBranch(*PredBB);
 712
 713     if (PredTBB)
 714       TII->InsertBranch(*PredBB, PredTBB, PredFBB, PredCond, DebugLoc());
 715
 716     PredBB->removeSuccessor(TailBB);
 717     unsigned NumSuccessors = PredBB->succ_size();
 718     assert(NumSuccessors <= 1);
 719     if (NumSuccessors == 0 || *PredBB->succ_begin() != NewTarget)
 720       PredBB->addSuccessor(NewTarget);
 721
 722     TDBBs.push_back(PredBB);
 723   }
 724   return Changed;
 725 }
 726
 727 /// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
 728 /// of its predecessors.
 729 bool
 730 TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
 731                                  bool IsSimple,
 732                                  MachineFunction &MF,
 733                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
 734                                  SmallVector<MachineInstr*, 16> &Copies) {
 735   DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
 736
 737   DenseSet<unsigned> UsedByPhi;
 738   getRegsUsedByPHIs(*TailBB, &UsedByPhi);
 739
 740   if (IsSimple)
 741     return duplicateSimpleBB(TailBB, TDBBs, UsedByPhi, Copies);
 742
 743   // Iterate through all the unique predecessors and tail-duplicate this
 744   // block into them, if possible. Copying the list ahead of time also
 745   // avoids trouble with the predecessor list reallocating.
 746   bool Changed = false;
 747   SmallSetVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
 748                                               TailBB->pred_end());
 749   for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
 750        PE = Preds.end(); PI != PE; ++PI) {
 751     MachineBasicBlock *PredBB = *PI;
 752
 753     assert(TailBB != PredBB &&
 754            "Single-block loop should have been rejected earlier!");
 755     // EH edges are ignored by AnalyzeBranch.
 756     if (PredBB->succ_size() > 1)
 757       continue;
 758
 759     MachineBasicBlock *PredTBB, *PredFBB;
 760     SmallVector<MachineOperand, 4> PredCond;
 761     if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
 762       continue;
 763     if (!PredCond.empty())
 764       continue;
 765     // Don't duplicate into a fall-through predecessor (at least for now).
 766     if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
 767       continue;
 768
 769     DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
 770                  << "From Succ: " << *TailBB);
 771
 772     TDBBs.push_back(PredBB);
 773
 774     // Remove PredBB's unconditional branch.
 775     TII->RemoveBranch(*PredBB);
 776
 777     // Clone the contents of TailBB into PredBB.
 778     DenseMap<unsigned, unsigned> LocalVRMap;
 779     SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
 780     MachineBasicBlock::iterator I = TailBB->begin();
 781     while (I != TailBB->end()) {
 782       MachineInstr *MI = &*I;
 783       ++I;
 784       if (MI->isPHI()) {
 785         // Replace the uses of the def of the PHI with the register coming
 786         // from PredBB.
 787         ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
 788       } else {
 789         // Replace def of virtual registers with new registers, and update
 790         // uses with PHI source register or the new registers.
 791         DuplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap, UsedByPhi);
 792       }
 793     }
 794     MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
 795     for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
 796       Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
 797                                TII->get(TargetOpcode::COPY),
 798                                CopyInfos[i].first).addReg(CopyInfos[i].second));
 799     }
 800
 801     // Simplify
 802     TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);
 803
 804     NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
 805
 806     // Update the CFG.
 807     PredBB->removeSuccessor(PredBB->succ_begin());
 808     assert(PredBB->succ_empty() &&
 809            "TailDuplicate called on block with multiple successors!");
 810     for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
 811            E = TailBB->succ_end(); I != E; ++I)
 812       PredBB->addSuccessor(*I);
 813
 814     Changed = true;
 815     ++NumTailDups;
 816   }
 817
 818   // If TailBB was duplicated into all its predecessors except for the prior
 819   // block, which falls through unconditionally, move the contents of this
 820   // block into the prior block.
 821   MachineBasicBlock *PrevBB = prior(MachineFunction::iterator(TailBB));
 822   MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
 823   SmallVector<MachineOperand, 4> PriorCond;
 824   // This has to check PrevBB->succ_size() because EH edges are ignored by
 825   // AnalyzeBranch.
 826   if (PrevBB->succ_size() == 1 &&
 827       !TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
 828       PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 &&
 829       !TailBB->hasAddressTaken()) {
 830     DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
 831           << "From MBB: " << *TailBB);
 832     if (PreRegAlloc) {
 833       DenseMap<unsigned, unsigned> LocalVRMap;
 834       SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
 835       MachineBasicBlock::iterator I = TailBB->begin();
 836       // Process PHI instructions first.
 837       while (I != TailBB->end() && I->isPHI()) {
 838         // Replace the uses of the def of the PHI with the register coming
 839         // from PredBB.
 840         MachineInstr *MI = &*I++;
 841         ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
 842         if (MI->getParent())
 843           MI->eraseFromParent();
 844       }
 845
 846       // Now copy the non-PHI instructions.
 847       while (I != TailBB->end()) {
 848         // Replace def of virtual registers with new registers, and update
 849         // uses with PHI source register or the new registers.
 850         MachineInstr *MI = &*I++;
 851         DuplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap, UsedByPhi);
 852         MI->eraseFromParent();
 853       }
 854       MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
 855       for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
 856         Copies.push_back(BuildMI(*PrevBB, Loc, DebugLoc(),
 857                                  TII->get(TargetOpcode::COPY),
 858                                  CopyInfos[i].first)
 859                            .addReg(CopyInfos[i].second));
 860       }
 861     } else {
 862       // No PHIs to worry about, just splice the instructions over.
 863       PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
 864     }
 865     PrevBB->removeSuccessor(PrevBB->succ_begin());
 866     assert(PrevBB->succ_empty());
 867     PrevBB->transferSuccessors(TailBB);
 868     TDBBs.push_back(PrevBB);
 869     Changed = true;
 870   }
 871
 872   // If this is after register allocation, there are no phis to fix.
 873   if (!PreRegAlloc)
 874     return Changed;
 875
 876   // If we made no changes so far, we are safe.
 877   if (!Changed)
 878     return Changed;
 879
 880
 881   // Handle the nasty case in that we duplicated a block that is part of a loop
 882   // into some but not all of its predecessors. For example:
 883   //    1 -> 2 <-> 3                 |
 884   //          \                      |
 885   //           \---> rest            |
 886   // if we duplicate 2 into 1 but not into 3, we end up with
 887   // 12 -> 3 <-> 2 -> rest           |
 888   //   \             /               |
 889   //    \----->-----/                |
 890   // If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
 891   // with a phi in 3 (which now dominates 2).
 892   // What we do here is introduce a copy in 3 of the register defined by the
 893   // phi, just like when we are duplicating 2 into 3, but we don't copy any
 894   // real instructions or remove the 3 -> 2 edge from the phi in 2.
 895   for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
 896        PE = Preds.end(); PI != PE; ++PI) {
 897     MachineBasicBlock *PredBB = *PI;
 898     if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end())
 899       continue;
 900
 901     // EH edges
 902     if (PredBB->succ_size() != 1)
 903       continue;
 904
 905     DenseMap<unsigned, unsigned> LocalVRMap;
 906     SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
 907     MachineBasicBlock::iterator I = TailBB->begin();
 908     // Process PHI instructions first.
 909     while (I != TailBB->end() && I->isPHI()) {
 910       // Replace the uses of the def of the PHI with the register coming
 911       // from PredBB.
 912       MachineInstr *MI = &*I++;
 913       ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
 914     }
 915     MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
 916     for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
 917       Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
 918                                TII->get(TargetOpcode::COPY),
 919                                CopyInfos[i].first).addReg(CopyInfos[i].second));
 920     }
 921   }
 922
 923   return Changed;
 924 }
 925
 926 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
 927 /// function, updating the CFG.
 928 void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
 929   assert(MBB->pred_empty() && "MBB must be dead!");
 930   DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
 931
 932   // Remove all successors.
 933   while (!MBB->succ_empty())
 934     MBB->removeSuccessor(MBB->succ_end()-1);
 935
 936   // Remove the block.
 937   MBB->eraseFromParent();
 938 }