lib/CodeGen/MachineBasicBlock.cpp

   1 //===-- llvm/CodeGen/MachineBasicBlock.cpp ----------------------*- C++ -*-===//
   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 // Collect the sequence of machine instructions for a basic block.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/CodeGen/MachineBasicBlock.h"
  15 #include "llvm/BasicBlock.h"
  16 #include "llvm/CodeGen/LiveVariables.h"
  17 #include "llvm/CodeGen/MachineDominators.h"
  18 #include "llvm/CodeGen/MachineFunction.h"
  19 #include "llvm/CodeGen/MachineLoopInfo.h"
  20 #include "llvm/MC/MCAsmInfo.h"
  21 #include "llvm/MC/MCContext.h"
  22 #include "llvm/Target/TargetRegisterInfo.h"
  23 #include "llvm/Target/TargetData.h"
  24 #include "llvm/Target/TargetInstrDesc.h"
  25 #include "llvm/Target/TargetInstrInfo.h"
  26 #include "llvm/Target/TargetMachine.h"
  27 #include "llvm/Assembly/Writer.h"
  28 #include "llvm/ADT/SmallString.h"
  29 #include "llvm/ADT/SmallPtrSet.h"
  30 #include "llvm/Support/Debug.h"
  31 #include "llvm/Support/LeakDetector.h"
  32 #include "llvm/Support/raw_ostream.h"
  33 #include <algorithm>
  34 using namespace llvm;
  35
  36 MachineBasicBlock::MachineBasicBlock(MachineFunction &mf, const BasicBlock *bb)
  37   : BB(bb), Number(-1), xParent(&mf), Alignment(0), IsLandingPad(false),
  38     AddressTaken(false) {
  39   Insts.Parent = this;
  40 }
  41
  42 MachineBasicBlock::~MachineBasicBlock() {
  43   LeakDetector::removeGarbageObject(this);
  44 }
  45
  46 /// getSymbol - Return the MCSymbol for this basic block.
  47 ///
  48 MCSymbol *MachineBasicBlock::getSymbol() const {
  49   const MachineFunction *MF = getParent();
  50   MCContext &Ctx = MF->getContext();
  51   const char *Prefix = Ctx.getAsmInfo().getPrivateGlobalPrefix();
  52   return Ctx.GetOrCreateSymbol(Twine(Prefix) + "BB" +
  53                                Twine(MF->getFunctionNumber()) + "_" +
  54                                Twine(getNumber()));
  55 }
  56
  57
  58 raw_ostream &llvm::operator<<(raw_ostream &OS, const MachineBasicBlock &MBB) {
  59   MBB.print(OS);
  60   return OS;
  61 }
  62
  63 /// addNodeToList (MBB) - When an MBB is added to an MF, we need to update the
  64 /// parent pointer of the MBB, the MBB numbering, and any instructions in the
  65 /// MBB to be on the right operand list for registers.
  66 ///
  67 /// MBBs start out as #-1. When a MBB is added to a MachineFunction, it
  68 /// gets the next available unique MBB number. If it is removed from a
  69 /// MachineFunction, it goes back to being #-1.
  70 void ilist_traits<MachineBasicBlock>::addNodeToList(MachineBasicBlock *N) {
  71   MachineFunction &MF = *N->getParent();
  72   N->Number = MF.addToMBBNumbering(N);
  73
  74   // Make sure the instructions have their operands in the reginfo lists.
  75   MachineRegisterInfo &RegInfo = MF.getRegInfo();
  76   for (MachineBasicBlock::iterator I = N->begin(), E = N->end(); I != E; ++I)
  77     I->AddRegOperandsToUseLists(RegInfo);
  78
  79   LeakDetector::removeGarbageObject(N);
  80 }
  81
  82 void ilist_traits<MachineBasicBlock>::removeNodeFromList(MachineBasicBlock *N) {
  83   N->getParent()->removeFromMBBNumbering(N->Number);
  84   N->Number = -1;
  85   LeakDetector::addGarbageObject(N);
  86 }
  87
  88
  89 /// addNodeToList (MI) - When we add an instruction to a basic block
  90 /// list, we update its parent pointer and add its operands from reg use/def
  91 /// lists if appropriate.
  92 void ilist_traits<MachineInstr>::addNodeToList(MachineInstr *N) {
  93   assert(N->getParent() == 0 && "machine instruction already in a basic block");
  94   N->setParent(Parent);
  95
  96   // Add the instruction's register operands to their corresponding
  97   // use/def lists.
  98   MachineFunction *MF = Parent->getParent();
  99   N->AddRegOperandsToUseLists(MF->getRegInfo());
 100
 101   LeakDetector::removeGarbageObject(N);
 102 }
 103
 104 /// removeNodeFromList (MI) - When we remove an instruction from a basic block
 105 /// list, we update its parent pointer and remove its operands from reg use/def
 106 /// lists if appropriate.
 107 void ilist_traits<MachineInstr>::removeNodeFromList(MachineInstr *N) {
 108   assert(N->getParent() != 0 && "machine instruction not in a basic block");
 109
 110   // Remove from the use/def lists.
 111   N->RemoveRegOperandsFromUseLists();
 112
 113   N->setParent(0);
 114
 115   LeakDetector::addGarbageObject(N);
 116 }
 117
 118 /// transferNodesFromList (MI) - When moving a range of instructions from one
 119 /// MBB list to another, we need to update the parent pointers and the use/def
 120 /// lists.
 121 void ilist_traits<MachineInstr>::
 122 transferNodesFromList(ilist_traits<MachineInstr> &fromList,
 123                       MachineBasicBlock::iterator first,
 124                       MachineBasicBlock::iterator last) {
 125   assert(Parent->getParent() == fromList.Parent->getParent() &&
 126         "MachineInstr parent mismatch!");
 127
 128   // Splice within the same MBB -> no change.
 129   if (Parent == fromList.Parent) return;
 130
 131   // If splicing between two blocks within the same function, just update the
 132   // parent pointers.
 133   for (; first != last; ++first)
 134     first->setParent(Parent);
 135 }
 136
 137 void ilist_traits<MachineInstr>::deleteNode(MachineInstr* MI) {
 138   assert(!MI->getParent() && "MI is still in a block!");
 139   Parent->getParent()->DeleteMachineInstr(MI);
 140 }
 141
 142 MachineBasicBlock::iterator MachineBasicBlock::getFirstNonPHI() {
 143   iterator I = begin();
 144   while (I != end() && I->isPHI())
 145     ++I;
 146   return I;
 147 }
 148
 149 MachineBasicBlock::iterator MachineBasicBlock::getFirstTerminator() {
 150   iterator I = end();
 151   while (I != begin() && (--I)->getDesc().isTerminator())
 152     ; /*noop */
 153   if (I != end() && !I->getDesc().isTerminator()) ++I;
 154   return I;
 155 }
 156
 157 void MachineBasicBlock::dump() const {
 158   print(dbgs());
 159 }
 160
 161 static inline void OutputReg(raw_ostream &os, unsigned RegNo,
 162                              const TargetRegisterInfo *TRI = 0) {
 163   if (RegNo != 0 && TargetRegisterInfo::isPhysicalRegister(RegNo)) {
 164     if (TRI)
 165       os << " %" << TRI->get(RegNo).Name;
 166     else
 167       os << " %physreg" << RegNo;
 168   } else
 169     os << " %reg" << RegNo;
 170 }
 171
 172 StringRef MachineBasicBlock::getName() const {
 173   if (const BasicBlock *LBB = getBasicBlock())
 174     return LBB->getName();
 175   else
 176     return "(null)";
 177 }
 178
 179 void MachineBasicBlock::print(raw_ostream &OS) const {
 180   const MachineFunction *MF = getParent();
 181   if (!MF) {
 182     OS << "Can't print out MachineBasicBlock because parent MachineFunction"
 183        << " is null\n";
 184     return;
 185   }
 186
 187   if (Alignment) { OS << "Alignment " << Alignment << "\n"; }
 188
 189   OS << "BB#" << getNumber() << ": ";
 190
 191   const char *Comma = "";
 192   if (const BasicBlock *LBB = getBasicBlock()) {
 193     OS << Comma << "derived from LLVM BB ";
 194     WriteAsOperand(OS, LBB, /*PrintType=*/false);
 195     Comma = ", ";
 196   }
 197   if (isLandingPad()) { OS << Comma << "EH LANDING PAD"; Comma = ", "; }
 198   if (hasAddressTaken()) { OS << Comma << "ADDRESS TAKEN"; Comma = ", "; }
 199   OS << '\n';
 200
 201   const TargetRegisterInfo *TRI = MF->getTarget().getRegisterInfo();
 202   if (!livein_empty()) {
 203     OS << "    Live Ins:";
 204     for (livein_iterator I = livein_begin(),E = livein_end(); I != E; ++I)
 205       OutputReg(OS, *I, TRI);
 206     OS << '\n';
 207   }
 208   // Print the preds of this block according to the CFG.
 209   if (!pred_empty()) {
 210     OS << "    Predecessors according to CFG:";
 211     for (const_pred_iterator PI = pred_begin(), E = pred_end(); PI != E; ++PI)
 212       OS << " BB#" << (*PI)->getNumber();
 213     OS << '\n';
 214   }
 215
 216   for (const_iterator I = begin(); I != end(); ++I) {
 217     OS << '\t';
 218     I->print(OS, &getParent()->getTarget());
 219   }
 220
 221   // Print the successors of this block according to the CFG.
 222   if (!succ_empty()) {
 223     OS << "    Successors according to CFG:";
 224     for (const_succ_iterator SI = succ_begin(), E = succ_end(); SI != E; ++SI)
 225       OS << " BB#" << (*SI)->getNumber();
 226     OS << '\n';
 227   }
 228 }
 229
 230 void MachineBasicBlock::removeLiveIn(unsigned Reg) {
 231   std::vector<unsigned>::iterator I =
 232     std::find(LiveIns.begin(), LiveIns.end(), Reg);
 233   assert(I != LiveIns.end() && "Not a live in!");
 234   LiveIns.erase(I);
 235 }
 236
 237 bool MachineBasicBlock::isLiveIn(unsigned Reg) const {
 238   livein_iterator I = std::find(livein_begin(), livein_end(), Reg);
 239   return I != livein_end();
 240 }
 241
 242 void MachineBasicBlock::moveBefore(MachineBasicBlock *NewAfter) {
 243   getParent()->splice(NewAfter, this);
 244 }
 245
 246 void MachineBasicBlock::moveAfter(MachineBasicBlock *NewBefore) {
 247   MachineFunction::iterator BBI = NewBefore;
 248   getParent()->splice(++BBI, this);
 249 }
 250
 251 void MachineBasicBlock::updateTerminator() {
 252   const TargetInstrInfo *TII = getParent()->getTarget().getInstrInfo();
 253   // A block with no successors has no concerns with fall-through edges.
 254   if (this->succ_empty()) return;
 255
 256   MachineBasicBlock *TBB = 0, *FBB = 0;
 257   SmallVector<MachineOperand, 4> Cond;
 258   DebugLoc dl;  // FIXME: this is nowhere
 259   bool B = TII->AnalyzeBranch(*this, TBB, FBB, Cond);
 260   (void) B;
 261   assert(!B && "UpdateTerminators requires analyzable predecessors!");
 262   if (Cond.empty()) {
 263     if (TBB) {
 264       // The block has an unconditional branch. If its successor is now
 265       // its layout successor, delete the branch.
 266       if (isLayoutSuccessor(TBB))
 267         TII->RemoveBranch(*this);
 268     } else {
 269       // The block has an unconditional fallthrough. If its successor is not
 270       // its layout successor, insert a branch.
 271       TBB = *succ_begin();
 272       if (!isLayoutSuccessor(TBB))
 273         TII->InsertBranch(*this, TBB, 0, Cond, dl);
 274     }
 275   } else {
 276     if (FBB) {
 277       // The block has a non-fallthrough conditional branch. If one of its
 278       // successors is its layout successor, rewrite it to a fallthrough
 279       // conditional branch.
 280       if (isLayoutSuccessor(TBB)) {
 281         if (TII->ReverseBranchCondition(Cond))
 282           return;
 283         TII->RemoveBranch(*this);
 284         TII->InsertBranch(*this, FBB, 0, Cond, dl);
 285       } else if (isLayoutSuccessor(FBB)) {
 286         TII->RemoveBranch(*this);
 287         TII->InsertBranch(*this, TBB, 0, Cond, dl);
 288       }
 289     } else {
 290       // The block has a fallthrough conditional branch.
 291       MachineBasicBlock *MBBA = *succ_begin();
 292       MachineBasicBlock *MBBB = *llvm::next(succ_begin());
 293       if (MBBA == TBB) std::swap(MBBB, MBBA);
 294       if (isLayoutSuccessor(TBB)) {
 295         if (TII->ReverseBranchCondition(Cond)) {
 296           // We can't reverse the condition, add an unconditional branch.
 297           Cond.clear();
 298           TII->InsertBranch(*this, MBBA, 0, Cond, dl);
 299           return;
 300         }
 301         TII->RemoveBranch(*this);
 302         TII->InsertBranch(*this, MBBA, 0, Cond, dl);
 303       } else if (!isLayoutSuccessor(MBBA)) {
 304         TII->RemoveBranch(*this);
 305         TII->InsertBranch(*this, TBB, MBBA, Cond, dl);
 306       }
 307     }
 308   }
 309 }
 310
 311 void MachineBasicBlock::addSuccessor(MachineBasicBlock *succ) {
 312   Successors.push_back(succ);
 313   succ->addPredecessor(this);
 314 }
 315
 316 void MachineBasicBlock::removeSuccessor(MachineBasicBlock *succ) {
 317   succ->removePredecessor(this);
 318   succ_iterator I = std::find(Successors.begin(), Successors.end(), succ);
 319   assert(I != Successors.end() && "Not a current successor!");
 320   Successors.erase(I);
 321 }
 322
 323 MachineBasicBlock::succ_iterator
 324 MachineBasicBlock::removeSuccessor(succ_iterator I) {
 325   assert(I != Successors.end() && "Not a current successor!");
 326   (*I)->removePredecessor(this);
 327   return Successors.erase(I);
 328 }
 329
 330 void MachineBasicBlock::addPredecessor(MachineBasicBlock *pred) {
 331   Predecessors.push_back(pred);
 332 }
 333
 334 void MachineBasicBlock::removePredecessor(MachineBasicBlock *pred) {
 335   std::vector<MachineBasicBlock *>::iterator I =
 336     std::find(Predecessors.begin(), Predecessors.end(), pred);
 337   assert(I != Predecessors.end() && "Pred is not a predecessor of this block!");
 338   Predecessors.erase(I);
 339 }
 340
 341 void MachineBasicBlock::transferSuccessors(MachineBasicBlock *fromMBB) {
 342   if (this == fromMBB)
 343     return;
 344
 345   while (!fromMBB->succ_empty()) {
 346     MachineBasicBlock *Succ = *fromMBB->succ_begin();
 347     addSuccessor(Succ);
 348     fromMBB->removeSuccessor(Succ);
 349   }
 350 }
 351
 352 void
 353 MachineBasicBlock::transferSuccessorsAndUpdatePHIs(MachineBasicBlock *fromMBB) {
 354   if (this == fromMBB)
 355     return;
 356
 357   while (!fromMBB->succ_empty()) {
 358     MachineBasicBlock *Succ = *fromMBB->succ_begin();
 359     addSuccessor(Succ);
 360     fromMBB->removeSuccessor(Succ);
 361
 362     // Fix up any PHI nodes in the successor.
 363     for (MachineBasicBlock::iterator MI = Succ->begin(), ME = Succ->end();
 364          MI != ME && MI->isPHI(); ++MI)
 365       for (unsigned i = 2, e = MI->getNumOperands()+1; i != e; i += 2) {
 366         MachineOperand &MO = MI->getOperand(i);
 367         if (MO.getMBB() == fromMBB)
 368           MO.setMBB(this);
 369       }
 370   }
 371 }
 372
 373 bool MachineBasicBlock::isSuccessor(const MachineBasicBlock *MBB) const {
 374   std::vector<MachineBasicBlock *>::const_iterator I =
 375     std::find(Successors.begin(), Successors.end(), MBB);
 376   return I != Successors.end();
 377 }
 378
 379 bool MachineBasicBlock::isLayoutSuccessor(const MachineBasicBlock *MBB) const {
 380   MachineFunction::const_iterator I(this);
 381   return llvm::next(I) == MachineFunction::const_iterator(MBB);
 382 }
 383
 384 bool MachineBasicBlock::canFallThrough() {
 385   MachineFunction::iterator Fallthrough = this;
 386   ++Fallthrough;
 387   // If FallthroughBlock is off the end of the function, it can't fall through.
 388   if (Fallthrough == getParent()->end())
 389     return false;
 390
 391   // If FallthroughBlock isn't a successor, no fallthrough is possible.
 392   if (!isSuccessor(Fallthrough))
 393     return false;
 394
 395   // Analyze the branches, if any, at the end of the block.
 396   MachineBasicBlock *TBB = 0, *FBB = 0;
 397   SmallVector<MachineOperand, 4> Cond;
 398   const TargetInstrInfo *TII = getParent()->getTarget().getInstrInfo();
 399   if (TII->AnalyzeBranch(*this, TBB, FBB, Cond)) {
 400     // If we couldn't analyze the branch, examine the last instruction.
 401     // If the block doesn't end in a known control barrier, assume fallthrough
 402     // is possible. The isPredicable check is needed because this code can be
 403     // called during IfConversion, where an instruction which is normally a
 404     // Barrier is predicated and thus no longer an actual control barrier. This
 405     // is over-conservative though, because if an instruction isn't actually
 406     // predicated we could still treat it like a barrier.
 407     return empty() || !back().getDesc().isBarrier() ||
 408            back().getDesc().isPredicable();
 409   }
 410
 411   // If there is no branch, control always falls through.
 412   if (TBB == 0) return true;
 413
 414   // If there is some explicit branch to the fallthrough block, it can obviously
 415   // reach, even though the branch should get folded to fall through implicitly.
 416   if (MachineFunction::iterator(TBB) == Fallthrough ||
 417       MachineFunction::iterator(FBB) == Fallthrough)
 418     return true;
 419
 420   // If it's an unconditional branch to some block not the fall through, it
 421   // doesn't fall through.
 422   if (Cond.empty()) return false;
 423
 424   // Otherwise, if it is conditional and has no explicit false block, it falls
 425   // through.
 426   return FBB == 0;
 427 }
 428
 429 MachineBasicBlock *
 430 MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ, Pass *P) {
 431   MachineFunction *MF = getParent();
 432   DebugLoc dl;  // FIXME: this is nowhere
 433
 434   // We may need to update this's terminator, but we can't do that if AnalyzeBranch
 435   // fails. If this uses a jump table, we won't touch it.
 436   const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
 437   MachineBasicBlock *TBB = 0, *FBB = 0;
 438   SmallVector<MachineOperand, 4> Cond;
 439   if (TII->AnalyzeBranch(*this, TBB, FBB, Cond))
 440     return NULL;
 441
 442   MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
 443   MF->insert(llvm::next(MachineFunction::iterator(this)), NMBB);
 444   DEBUG(dbgs() << "PHIElimination splitting critical edge:"
 445         " BB#" << getNumber()
 446         << " -- BB#" << NMBB->getNumber()
 447         << " -- BB#" << Succ->getNumber() << '\n');
 448
 449   ReplaceUsesOfBlockWith(Succ, NMBB);
 450   updateTerminator();
 451
 452   // Insert unconditional "jump Succ" instruction in NMBB if necessary.
 453   NMBB->addSuccessor(Succ);
 454   if (!NMBB->isLayoutSuccessor(Succ)) {
 455     Cond.clear();
 456     MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, Succ, NULL, Cond, dl);
 457   }
 458
 459   // Fix PHI nodes in Succ so they refer to NMBB instead of this
 460   for (MachineBasicBlock::iterator i = Succ->begin(), e = Succ->end();
 461        i != e && i->isPHI(); ++i)
 462     for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
 463       if (i->getOperand(ni+1).getMBB() == this)
 464         i->getOperand(ni+1).setMBB(NMBB);
 465
 466   if (LiveVariables *LV =
 467         P->getAnalysisIfAvailable<LiveVariables>())
 468     LV->addNewBlock(NMBB, this, Succ);
 469
 470   if (MachineDominatorTree *MDT =
 471         P->getAnalysisIfAvailable<MachineDominatorTree>())
 472     MDT->addNewBlock(NMBB, this);
 473
 474   if (MachineLoopInfo *MLI =
 475         P->getAnalysisIfAvailable<MachineLoopInfo>())
 476     if (MachineLoop *TIL = MLI->getLoopFor(this)) {
 477       // If one or the other blocks were not in a loop, the new block is not
 478       // either, and thus LI doesn't need to be updated.
 479       if (MachineLoop *DestLoop = MLI->getLoopFor(Succ)) {
 480         if (TIL == DestLoop) {
 481           // Both in the same loop, the NMBB joins loop.
 482           DestLoop->addBasicBlockToLoop(NMBB, MLI->getBase());
 483         } else if (TIL->contains(DestLoop)) {
 484           // Edge from an outer loop to an inner loop.  Add to the outer loop.
 485           TIL->addBasicBlockToLoop(NMBB, MLI->getBase());
 486         } else if (DestLoop->contains(TIL)) {
 487           // Edge from an inner loop to an outer loop.  Add to the outer loop.
 488           DestLoop->addBasicBlockToLoop(NMBB, MLI->getBase());
 489         } else {
 490           // Edge from two loops with no containment relation.  Because these
 491           // are natural loops, we know that the destination block must be the
 492           // header of its loop (adding a branch into a loop elsewhere would
 493           // create an irreducible loop).
 494           assert(DestLoop->getHeader() == Succ &&
 495                  "Should not create irreducible loops!");
 496           if (MachineLoop *P = DestLoop->getParentLoop())
 497             P->addBasicBlockToLoop(NMBB, MLI->getBase());
 498         }
 499       }
 500     }
 501
 502   return NMBB;
 503 }
 504
 505 /// removeFromParent - This method unlinks 'this' from the containing function,
 506 /// and returns it, but does not delete it.
 507 MachineBasicBlock *MachineBasicBlock::removeFromParent() {
 508   assert(getParent() && "Not embedded in a function!");
 509   getParent()->remove(this);
 510   return this;
 511 }
 512
 513
 514 /// eraseFromParent - This method unlinks 'this' from the containing function,
 515 /// and deletes it.
 516 void MachineBasicBlock::eraseFromParent() {
 517   assert(getParent() && "Not embedded in a function!");
 518   getParent()->erase(this);
 519 }
 520
 521
 522 /// ReplaceUsesOfBlockWith - Given a machine basic block that branched to
 523 /// 'Old', change the code and CFG so that it branches to 'New' instead.
 524 void MachineBasicBlock::ReplaceUsesOfBlockWith(MachineBasicBlock *Old,
 525                                                MachineBasicBlock *New) {
 526   assert(Old != New && "Cannot replace self with self!");
 527
 528   MachineBasicBlock::iterator I = end();
 529   while (I != begin()) {
 530     --I;
 531     if (!I->getDesc().isTerminator()) break;
 532
 533     // Scan the operands of this machine instruction, replacing any uses of Old
 534     // with New.
 535     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
 536       if (I->getOperand(i).isMBB() &&
 537           I->getOperand(i).getMBB() == Old)
 538         I->getOperand(i).setMBB(New);
 539   }
 540
 541   // Update the successor information.
 542   removeSuccessor(Old);
 543   addSuccessor(New);
 544 }
 545
 546 /// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the
 547 /// CFG to be inserted.  If we have proven that MBB can only branch to DestA and
 548 /// DestB, remove any other MBB successors from the CFG.  DestA and DestB can be
 549 /// null.
 550 ///
 551 /// Besides DestA and DestB, retain other edges leading to LandingPads
 552 /// (currently there can be only one; we don't check or require that here).
 553 /// Note it is possible that DestA and/or DestB are LandingPads.
 554 bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
 555                                              MachineBasicBlock *DestB,
 556                                              bool isCond) {
 557   // The values of DestA and DestB frequently come from a call to the
 558   // 'TargetInstrInfo::AnalyzeBranch' method. We take our meaning of the initial
 559   // values from there.
 560   //
 561   // 1. If both DestA and DestB are null, then the block ends with no branches
 562   //    (it falls through to its successor).
 563   // 2. If DestA is set, DestB is null, and isCond is false, then the block ends
 564   //    with only an unconditional branch.
 565   // 3. If DestA is set, DestB is null, and isCond is true, then the block ends
 566   //    with a conditional branch that falls through to a successor (DestB).
 567   // 4. If DestA and DestB is set and isCond is true, then the block ends with a
 568   //    conditional branch followed by an unconditional branch. DestA is the
 569   //    'true' destination and DestB is the 'false' destination.
 570
 571   bool Changed = false;
 572
 573   MachineFunction::iterator FallThru =
 574     llvm::next(MachineFunction::iterator(this));
 575
 576   if (DestA == 0 && DestB == 0) {
 577     // Block falls through to successor.
 578     DestA = FallThru;
 579     DestB = FallThru;
 580   } else if (DestA != 0 && DestB == 0) {
 581     if (isCond)
 582       // Block ends in conditional jump that falls through to successor.
 583       DestB = FallThru;
 584   } else {
 585     assert(DestA && DestB && isCond &&
 586            "CFG in a bad state. Cannot correct CFG edges");
 587   }
 588
 589   // Remove superfluous edges. I.e., those which aren't destinations of this
 590   // basic block, duplicate edges, or landing pads.
 591   SmallPtrSet<const MachineBasicBlock*, 8> SeenMBBs;
 592   MachineBasicBlock::succ_iterator SI = succ_begin();
 593   while (SI != succ_end()) {
 594     const MachineBasicBlock *MBB = *SI;
 595     if (!SeenMBBs.insert(MBB) ||
 596         (MBB != DestA && MBB != DestB && !MBB->isLandingPad())) {
 597       // This is a superfluous edge, remove it.
 598       SI = removeSuccessor(SI);
 599       Changed = true;
 600     } else {
 601       ++SI;
 602     }
 603   }
 604
 605   return Changed;
 606 }
 607
 608 /// findDebugLoc - find the next valid DebugLoc starting at MBBI, skipping
 609 /// any DBG_VALUE instructions.  Return UnknownLoc if there is none.
 610 DebugLoc
 611 MachineBasicBlock::findDebugLoc(MachineBasicBlock::iterator &MBBI) {
 612   DebugLoc DL;
 613   MachineBasicBlock::iterator E = end();
 614   if (MBBI != E) {
 615     // Skip debug declarations, we don't want a DebugLoc from them.
 616     MachineBasicBlock::iterator MBBI2 = MBBI;
 617     while (MBBI2 != E && MBBI2->isDebugValue())
 618       MBBI2++;
 619     if (MBBI2 != E)
 620       DL = MBBI2->getDebugLoc();
 621   }
 622   return DL;
 623 }
 624
 625 void llvm::WriteAsOperand(raw_ostream &OS, const MachineBasicBlock *MBB,
 626                           bool t) {
 627   OS << "BB#" << MBB->getNumber();
 628 }
 629