TailDuplication.cpp revision c3f507f98a0747bd256e1c13536060b6fc5c4b62 
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"
31using namespace llvm;
32
33STATISTIC(NumTails     , "Number of tails duplicated");
34STATISTIC(NumTailDups  , "Number of tail duplicated blocks");
35STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
36STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
37
38// Heuristic for tail duplication.
39static cl::opt<unsigned>
40TailDuplicateSize("tail-dup-size",
41                  cl::desc("Maximum instructions to consider tail duplicating"),
42                  cl::init(2), cl::Hidden);
43
44static cl::opt<bool>
45TailDupVerify("tail-dup-verify",
46              cl::desc("Verify sanity of PHI instructions during taildup"),
47              cl::init(false), cl::Hidden);
48
49static cl::opt<unsigned>
50TailDupLimit("tail-dup-limit", cl::init(~0U), cl::Hidden);
51
52typedef std::vector<std::pair<MachineBasicBlock*,unsigned> > AvailableValsTy;
53
54namespace {
55  /// TailDuplicatePass - Perform tail duplication.
56  class TailDuplicatePass : public MachineFunctionPass {
57    bool PreRegAlloc;
58    const TargetInstrInfo *TII;
59    MachineModuleInfo *MMI;
60    MachineRegisterInfo *MRI;
61
62    // SSAUpdateVRs - A list of virtual registers for which to update SSA form.
63    SmallVector<unsigned, 16> SSAUpdateVRs;
64
65    // SSAUpdateVals - For each virtual register in SSAUpdateVals keep a list of
66    // source virtual registers.
67    DenseMap<unsigned, AvailableValsTy> SSAUpdateVals;
68
69  public:
70    static char ID;
71    explicit TailDuplicatePass(bool PreRA) :
72      MachineFunctionPass(ID), PreRegAlloc(PreRA) {}
73
74    virtual bool runOnMachineFunction(MachineFunction &MF);
75    virtual const char *getPassName() const { return "Tail Duplication"; }
76
77  private:
78    void AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
79                           MachineBasicBlock *BB);
80    void ProcessPHI(MachineInstr *MI, MachineBasicBlock *TailBB,
81                    MachineBasicBlock *PredBB,
82                    DenseMap<unsigned, unsigned> &LocalVRMap,
83                    SmallVector<std::pair<unsigned,unsigned>, 4> &Copies);
84    void DuplicateInstruction(MachineInstr *MI,
85                              MachineBasicBlock *TailBB,
86                              MachineBasicBlock *PredBB,
87                              MachineFunction &MF,
88                              DenseMap<unsigned, unsigned> &LocalVRMap);
89    void UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
90                              SmallVector<MachineBasicBlock*, 8> &TDBBs,
91                              SmallSetVector<MachineBasicBlock*, 8> &Succs);
92    bool TailDuplicateBlocks(MachineFunction &MF);
93    bool TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
94                       SmallVector<MachineBasicBlock*, 8> &TDBBs,
95                       SmallVector<MachineInstr*, 16> &Copies);
96    void RemoveDeadBlock(MachineBasicBlock *MBB);
97  };
98
99  char TailDuplicatePass::ID = 0;
100}
101
102FunctionPass *llvm::createTailDuplicatePass(bool PreRegAlloc) {
103  return new TailDuplicatePass(PreRegAlloc);
104}
105
106bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
107  TII = MF.getTarget().getInstrInfo();
108  MRI = &MF.getRegInfo();
109  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
110
111  bool MadeChange = false;
112  while (TailDuplicateBlocks(MF))
113    MadeChange = true;
114
115  return MadeChange;
116}
117
118static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
119  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
120    MachineBasicBlock *MBB = I;
121    SmallSetVector<MachineBasicBlock*, 8> Preds(MBB->pred_begin(),
122                                                MBB->pred_end());
123    MachineBasicBlock::iterator MI = MBB->begin();
124    while (MI != MBB->end()) {
125      if (!MI->isPHI())
126        break;
127      for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
128             PE = Preds.end(); PI != PE; ++PI) {
129        MachineBasicBlock *PredBB = *PI;
130        bool Found = false;
131        for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
132          MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
133          if (PHIBB == PredBB) {
134            Found = true;
135            break;
136          }
137        }
138        if (!Found) {
139          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
140          dbgs() << "  missing input from predecessor BB#"
141                 << PredBB->getNumber() << '\n';
142          llvm_unreachable(0);
143        }
144      }
145
146      for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
147        MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
148        if (CheckExtra && !Preds.count(PHIBB)) {
149          // This is not a hard error.
150          dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber()
151                 << ": " << *MI;
152          dbgs() << "  extra input from predecessor BB#"
153                 << PHIBB->getNumber() << '\n';
154        }
155        if (PHIBB->getNumber() < 0) {
156          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
157          dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
158          llvm_unreachable(0);
159        }
160      }
161      ++MI;
162    }
163  }
164}
165
166/// TailDuplicateBlocks - Look for small blocks that are unconditionally
167/// branched to and do not fall through. Tail-duplicate their instructions
168/// into their predecessors to eliminate (dynamic) branches.
169bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
170  bool MadeChange = false;
171
172  if (PreRegAlloc && TailDupVerify) {
173    DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
174    VerifyPHIs(MF, true);
175  }
176
177  SmallVector<MachineInstr*, 8> NewPHIs;
178  MachineSSAUpdater SSAUpdate(MF, &NewPHIs);
179
180  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
181    MachineBasicBlock *MBB = I++;
182
183    if (NumTails == TailDupLimit)
184      break;
185
186    // Only duplicate blocks that end with unconditional branches.
187    if (MBB->canFallThrough())
188      continue;
189
190    // Save the successors list.
191    SmallSetVector<MachineBasicBlock*, 8> Succs(MBB->succ_begin(),
192                                                MBB->succ_end());
193
194    SmallVector<MachineBasicBlock*, 8> TDBBs;
195    SmallVector<MachineInstr*, 16> Copies;
196    if (TailDuplicate(MBB, MF, TDBBs, Copies)) {
197      ++NumTails;
198
199      // TailBB's immediate successors are now successors of those predecessors
200      // which duplicated TailBB. Add the predecessors as sources to the PHI
201      // instructions.
202      bool isDead = MBB->pred_empty();
203      if (PreRegAlloc)
204        UpdateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
205
206      // If it is dead, remove it.
207      if (isDead) {
208        NumInstrDups -= MBB->size();
209        RemoveDeadBlock(MBB);
210        ++NumDeadBlocks;
211      }
212
213      // Update SSA form.
214      if (!SSAUpdateVRs.empty()) {
215        for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
216          unsigned VReg = SSAUpdateVRs[i];
217          SSAUpdate.Initialize(VReg);
218
219          // If the original definition is still around, add it as an available
220          // value.
221          MachineInstr *DefMI = MRI->getVRegDef(VReg);
222          MachineBasicBlock *DefBB = 0;
223          if (DefMI) {
224            DefBB = DefMI->getParent();
225            SSAUpdate.AddAvailableValue(DefBB, VReg);
226          }
227
228          // Add the new vregs as available values.
229          DenseMap<unsigned, AvailableValsTy>::iterator LI =
230            SSAUpdateVals.find(VReg);
231          for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
232            MachineBasicBlock *SrcBB = LI->second[j].first;
233            unsigned SrcReg = LI->second[j].second;
234            SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
235          }
236
237          // Rewrite uses that are outside of the original def's block.
238          MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
239          while (UI != MRI->use_end()) {
240            MachineOperand &UseMO = UI.getOperand();
241            MachineInstr *UseMI = &*UI;
242            ++UI;
243            if (UseMI->getParent() == DefBB)
244              continue;
245            SSAUpdate.RewriteUse(UseMO);
246          }
247        }
248
249        SSAUpdateVRs.clear();
250        SSAUpdateVals.clear();
251      }
252
253      // Eliminate some of the copies inserted by tail duplication to maintain
254      // SSA form.
255      for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
256        MachineInstr *Copy = Copies[i];
257        if (!Copy->isCopy())
258          continue;
259        unsigned Dst = Copy->getOperand(0).getReg();
260        unsigned Src = Copy->getOperand(1).getReg();
261        MachineRegisterInfo::use_iterator UI = MRI->use_begin(Src);
262        if (++UI == MRI->use_end()) {
263          // Copy is the only use. Do trivial copy propagation here.
264          MRI->replaceRegWith(Dst, Src);
265          Copy->eraseFromParent();
266        }
267      }
268
269      if (PreRegAlloc && TailDupVerify)
270        VerifyPHIs(MF, false);
271      MadeChange = true;
272    }
273  }
274
275  return MadeChange;
276}
277
278static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
279                         const MachineRegisterInfo *MRI) {
280  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
281         UE = MRI->use_end(); UI != UE; ++UI) {
282    MachineInstr *UseMI = &*UI;
283    if (UseMI->getParent() != BB)
284      return true;
285  }
286  return false;
287}
288
289static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
290  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
291    if (MI->getOperand(i+1).getMBB() == SrcBB)
292      return i;
293  return 0;
294}
295
296/// AddSSAUpdateEntry - Add a definition and source virtual registers pair for
297/// SSA update.
298void TailDuplicatePass::AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
299                                          MachineBasicBlock *BB) {
300  DenseMap<unsigned, AvailableValsTy>::iterator LI= SSAUpdateVals.find(OrigReg);
301  if (LI != SSAUpdateVals.end())
302    LI->second.push_back(std::make_pair(BB, NewReg));
303  else {
304    AvailableValsTy Vals;
305    Vals.push_back(std::make_pair(BB, NewReg));
306    SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
307    SSAUpdateVRs.push_back(OrigReg);
308  }
309}
310
311/// ProcessPHI - Process PHI node in TailBB by turning it into a copy in PredBB.
312/// Remember the source register that's contributed by PredBB and update SSA
313/// update map.
314void TailDuplicatePass::ProcessPHI(MachineInstr *MI,
315                                   MachineBasicBlock *TailBB,
316                                   MachineBasicBlock *PredBB,
317                                   DenseMap<unsigned, unsigned> &LocalVRMap,
318                         SmallVector<std::pair<unsigned,unsigned>, 4> &Copies) {
319  unsigned DefReg = MI->getOperand(0).getReg();
320  unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
321  assert(SrcOpIdx && "Unable to find matching PHI source?");
322  unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
323  const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
324  LocalVRMap.insert(std::make_pair(DefReg, SrcReg));
325
326  // Insert a copy from source to the end of the block. The def register is the
327  // available value liveout of the block.
328  unsigned NewDef = MRI->createVirtualRegister(RC);
329  Copies.push_back(std::make_pair(NewDef, SrcReg));
330  if (isDefLiveOut(DefReg, TailBB, MRI))
331    AddSSAUpdateEntry(DefReg, NewDef, PredBB);
332
333  // Remove PredBB from the PHI node.
334  MI->RemoveOperand(SrcOpIdx+1);
335  MI->RemoveOperand(SrcOpIdx);
336  if (MI->getNumOperands() == 1)
337    MI->eraseFromParent();
338}
339
340/// DuplicateInstruction - Duplicate a TailBB instruction to PredBB and update
341/// the source operands due to earlier PHI translation.
342void TailDuplicatePass::DuplicateInstruction(MachineInstr *MI,
343                                     MachineBasicBlock *TailBB,
344                                     MachineBasicBlock *PredBB,
345                                     MachineFunction &MF,
346                                     DenseMap<unsigned, unsigned> &LocalVRMap) {
347  MachineInstr *NewMI = TII->duplicate(MI, MF);
348  for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
349    MachineOperand &MO = NewMI->getOperand(i);
350    if (!MO.isReg())
351      continue;
352    unsigned Reg = MO.getReg();
353    if (!TargetRegisterInfo::isVirtualRegister(Reg))
354      continue;
355    if (MO.isDef()) {
356      const TargetRegisterClass *RC = MRI->getRegClass(Reg);
357      unsigned NewReg = MRI->createVirtualRegister(RC);
358      MO.setReg(NewReg);
359      LocalVRMap.insert(std::make_pair(Reg, NewReg));
360      if (isDefLiveOut(Reg, TailBB, MRI))
361        AddSSAUpdateEntry(Reg, NewReg, PredBB);
362    } else {
363      DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
364      if (VI != LocalVRMap.end())
365        MO.setReg(VI->second);
366    }
367  }
368  PredBB->insert(PredBB->end(), NewMI);
369}
370
371/// UpdateSuccessorsPHIs - After FromBB is tail duplicated into its predecessor
372/// blocks, the successors have gained new predecessors. Update the PHI
373/// instructions in them accordingly.
374void
375TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
376                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
377                                  SmallSetVector<MachineBasicBlock*,8> &Succs) {
378  for (SmallSetVector<MachineBasicBlock*, 8>::iterator SI = Succs.begin(),
379         SE = Succs.end(); SI != SE; ++SI) {
380    MachineBasicBlock *SuccBB = *SI;
381    for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
382         II != EE; ++II) {
383      if (!II->isPHI())
384        break;
385      unsigned Idx = 0;
386      for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
387        MachineOperand &MO = II->getOperand(i+1);
388        if (MO.getMBB() == FromBB) {
389          Idx = i;
390          break;
391        }
392      }
393
394      assert(Idx != 0);
395      MachineOperand &MO0 = II->getOperand(Idx);
396      unsigned Reg = MO0.getReg();
397      if (isDead) {
398        // Folded into the previous BB.
399        // There could be duplicate phi source entries. FIXME: Should sdisel
400        // or earlier pass fixed this?
401        for (unsigned i = II->getNumOperands()-2; i != Idx; i -= 2) {
402          MachineOperand &MO = II->getOperand(i+1);
403          if (MO.getMBB() == FromBB) {
404            II->RemoveOperand(i+1);
405            II->RemoveOperand(i);
406          }
407        }
408      } else
409        Idx = 0;
410
411      // If Idx is set, the operands at Idx and Idx+1 must be removed.
412      // We reuse the location to avoid expensive RemoveOperand calls.
413
414      DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
415      if (LI != SSAUpdateVals.end()) {
416        // This register is defined in the tail block.
417        for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
418          MachineBasicBlock *SrcBB = LI->second[j].first;
419          unsigned SrcReg = LI->second[j].second;
420          if (Idx != 0) {
421            II->getOperand(Idx).setReg(SrcReg);
422            II->getOperand(Idx+1).setMBB(SrcBB);
423            Idx = 0;
424          } else {
425            II->addOperand(MachineOperand::CreateReg(SrcReg, false));
426            II->addOperand(MachineOperand::CreateMBB(SrcBB));
427          }
428        }
429      } else {
430        // Live in tail block, must also be live in predecessors.
431        for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
432          MachineBasicBlock *SrcBB = TDBBs[j];
433          if (Idx != 0) {
434            II->getOperand(Idx).setReg(Reg);
435            II->getOperand(Idx+1).setMBB(SrcBB);
436            Idx = 0;
437          } else {
438            II->addOperand(MachineOperand::CreateReg(Reg, false));
439            II->addOperand(MachineOperand::CreateMBB(SrcBB));
440          }
441        }
442      }
443      if (Idx != 0) {
444        II->RemoveOperand(Idx+1);
445        II->RemoveOperand(Idx);
446      }
447    }
448  }
449}
450
451/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
452/// of its predecessors.
453bool
454TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
455                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
456                                 SmallVector<MachineInstr*, 16> &Copies) {
457  // Set the limit on the number of instructions to duplicate, with a default
458  // of one less than the tail-merge threshold. When optimizing for size,
459  // duplicate only one, because one branch instruction can be eliminated to
460  // compensate for the duplication.
461  unsigned MaxDuplicateCount;
462  if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
463    MaxDuplicateCount = 1;
464  else
465    MaxDuplicateCount = TailDuplicateSize;
466
467  if (PreRegAlloc) {
468    if (TailBB->empty())
469      return false;
470    const TargetInstrDesc &TID = TailBB->back().getDesc();
471    // Pre-regalloc tail duplication hurts compile time and doesn't help
472    // much except for indirect branches and returns.
473    if (!TID.isIndirectBranch() && !TID.isReturn())
474      return false;
475    // If the target has hardware branch prediction that can handle indirect
476    // branches, duplicating them can often make them predictable when there
477    // are common paths through the code.  The limit needs to be high enough
478    // to allow undoing the effects of tail merging and other optimizations
479    // that rearrange the predecessors of the indirect branch.
480    MaxDuplicateCount = 20;
481  }
482
483  // Don't try to tail-duplicate single-block loops.
484  if (TailBB->isSuccessor(TailBB))
485    return false;
486
487  // Check the instructions in the block to determine whether tail-duplication
488  // is invalid or unlikely to be profitable.
489  unsigned InstrCount = 0;
490  bool HasCall = false;
491  for (MachineBasicBlock::iterator I = TailBB->begin();
492       I != TailBB->end(); ++I) {
493    // Non-duplicable things shouldn't be tail-duplicated.
494    if (I->getDesc().isNotDuplicable()) return false;
495    // Do not duplicate 'return' instructions if this is a pre-regalloc run.
496    // A return may expand into a lot more instructions (e.g. reload of callee
497    // saved registers) after PEI.
498    if (PreRegAlloc && I->getDesc().isReturn()) return false;
499    // Don't duplicate more than the threshold.
500    if (InstrCount == MaxDuplicateCount) return false;
501    // Remember if we saw a call.
502    if (I->getDesc().isCall()) HasCall = true;
503    if (!I->isPHI() && !I->isDebugValue())
504      InstrCount += 1;
505  }
506  // Heuristically, don't tail-duplicate calls if it would expand code size,
507  // as it's less likely to be worth the extra cost.
508  if (InstrCount > 1 && (PreRegAlloc && HasCall))
509    return false;
510
511  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
512
513  // Iterate through all the unique predecessors and tail-duplicate this
514  // block into them, if possible. Copying the list ahead of time also
515  // avoids trouble with the predecessor list reallocating.
516  bool Changed = false;
517  SmallSetVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
518                                              TailBB->pred_end());
519  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
520       PE = Preds.end(); PI != PE; ++PI) {
521    MachineBasicBlock *PredBB = *PI;
522
523    assert(TailBB != PredBB &&
524           "Single-block loop should have been rejected earlier!");
525    if (PredBB->succ_size() > 1) continue;
526
527    MachineBasicBlock *PredTBB, *PredFBB;
528    SmallVector<MachineOperand, 4> PredCond;
529    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
530      continue;
531    if (!PredCond.empty())
532      continue;
533    // EH edges are ignored by AnalyzeBranch.
534    if (PredBB->succ_size() != 1)
535      continue;
536    // Don't duplicate into a fall-through predecessor (at least for now).
537    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
538      continue;
539
540    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
541                 << "From Succ: " << *TailBB);
542
543    TDBBs.push_back(PredBB);
544
545    // Remove PredBB's unconditional branch.
546    TII->RemoveBranch(*PredBB);
547
548    // Clone the contents of TailBB into PredBB.
549    DenseMap<unsigned, unsigned> LocalVRMap;
550    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
551    MachineBasicBlock::iterator I = TailBB->begin();
552    while (I != TailBB->end()) {
553      MachineInstr *MI = &*I;
554      ++I;
555      if (MI->isPHI()) {
556        // Replace the uses of the def of the PHI with the register coming
557        // from PredBB.
558        ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos);
559      } else {
560        // Replace def of virtual registers with new registers, and update
561        // uses with PHI source register or the new registers.
562        DuplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap);
563      }
564    }
565    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
566    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
567      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
568                               TII->get(TargetOpcode::COPY),
569                               CopyInfos[i].first).addReg(CopyInfos[i].second));
570    }
571    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
572
573    // Update the CFG.
574    PredBB->removeSuccessor(PredBB->succ_begin());
575    assert(PredBB->succ_empty() &&
576           "TailDuplicate called on block with multiple successors!");
577    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
578           E = TailBB->succ_end(); I != E; ++I)
579      PredBB->addSuccessor(*I);
580
581    Changed = true;
582    ++NumTailDups;
583  }
584
585  // If TailBB was duplicated into all its predecessors except for the prior
586  // block, which falls through unconditionally, move the contents of this
587  // block into the prior block.
588  MachineBasicBlock *PrevBB = prior(MachineFunction::iterator(TailBB));
589  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
590  SmallVector<MachineOperand, 4> PriorCond;
591  bool PriorUnAnalyzable =
592    TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true);
593  // This has to check PrevBB->succ_size() because EH edges are ignored by
594  // AnalyzeBranch.
595  if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
596      TailBB->pred_size() == 1 && PrevBB->succ_size() == 1 &&
597      !TailBB->hasAddressTaken()) {
598    DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
599          << "From MBB: " << *TailBB);
600    if (PreRegAlloc) {
601      DenseMap<unsigned, unsigned> LocalVRMap;
602      SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
603      MachineBasicBlock::iterator I = TailBB->begin();
604      // Process PHI instructions first.
605      while (I != TailBB->end() && I->isPHI()) {
606        // Replace the uses of the def of the PHI with the register coming
607        // from PredBB.
608        MachineInstr *MI = &*I++;
609        ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos);
610        if (MI->getParent())
611          MI->eraseFromParent();
612      }
613
614      // Now copy the non-PHI instructions.
615      while (I != TailBB->end()) {
616        // Replace def of virtual registers with new registers, and update
617        // uses with PHI source register or the new registers.
618        MachineInstr *MI = &*I++;
619        DuplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap);
620        MI->eraseFromParent();
621      }
622      MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
623      for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
624        Copies.push_back(BuildMI(*PrevBB, Loc, DebugLoc(),
625                                 TII->get(TargetOpcode::COPY),
626                                 CopyInfos[i].first)
627                           .addReg(CopyInfos[i].second));
628      }
629    } else {
630      // No PHIs to worry about, just splice the instructions over.
631      PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
632    }
633    PrevBB->removeSuccessor(PrevBB->succ_begin());
634    assert(PrevBB->succ_empty());
635    PrevBB->transferSuccessors(TailBB);
636    TDBBs.push_back(PrevBB);
637    Changed = true;
638  }
639
640  return Changed;
641}
642
643/// RemoveDeadBlock - Remove the specified dead machine basic block from the
644/// function, updating the CFG.
645void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
646  assert(MBB->pred_empty() && "MBB must be dead!");
647  DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
648
649  // Remove all successors.
650  while (!MBB->succ_empty())
651    MBB->removeSuccessor(MBB->succ_end()-1);
652
653  // Remove the block.
654  MBB->eraseFromParent();
655}
656
657