TailDuplication.cpp revision 217b0a9b15146f981fab0cfafb651a50235cf8c0 
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/MachineRegisterInfo.h"
21#include "llvm/CodeGen/MachineSSAUpdater.h"
22#include "llvm/Target/TargetInstrInfo.h"
23#include "llvm/Support/CommandLine.h"
24#include "llvm/Support/Debug.h"
25#include "llvm/Support/ErrorHandling.h"
26#include "llvm/Support/raw_ostream.h"
27#include "llvm/ADT/SmallSet.h"
28#include "llvm/ADT/SetVector.h"
29#include "llvm/ADT/Statistic.h"
30using namespace llvm;
31
32STATISTIC(NumTails     , "Number of tails duplicated");
33STATISTIC(NumTailDups  , "Number of tail duplicated blocks");
34STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
35STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
36
37// Heuristic for tail duplication.
38static cl::opt<unsigned>
39TailDuplicateSize("tail-dup-size",
40                  cl::desc("Maximum instructions to consider tail duplicating"),
41                  cl::init(2), cl::Hidden);
42
43static cl::opt<bool>
44TailDupVerify("tail-dup-verify",
45              cl::desc("Verify sanity of PHI instructions during taildup"),
46              cl::init(false), cl::Hidden);
47
48static cl::opt<unsigned>
49TailDupLimit("tail-dup-limit", cl::init(~0U), cl::Hidden);
50
51typedef std::vector<std::pair<MachineBasicBlock*,unsigned> > AvailableValsTy;
52
53namespace {
54  /// TailDuplicatePass - Perform tail duplication.
55  class TailDuplicatePass : public MachineFunctionPass {
56    bool PreRegAlloc;
57    const TargetInstrInfo *TII;
58    MachineModuleInfo *MMI;
59    MachineRegisterInfo *MRI;
60
61    // SSAUpdateVRs - A list of virtual registers for which to update SSA form.
62    SmallVector<unsigned, 16> SSAUpdateVRs;
63
64    // SSAUpdateVals - For each virtual register in SSAUpdateVals keep a list of
65    // source virtual registers.
66    DenseMap<unsigned, AvailableValsTy> SSAUpdateVals;
67
68  public:
69    static char ID;
70    explicit TailDuplicatePass(bool PreRA) :
71      MachineFunctionPass(&ID), PreRegAlloc(PreRA) {}
72
73    virtual bool runOnMachineFunction(MachineFunction &MF);
74    virtual const char *getPassName() const { return "Tail Duplication"; }
75
76  private:
77    void AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
78                           MachineBasicBlock *BB);
79    void ProcessPHI(MachineInstr *MI, MachineBasicBlock *TailBB,
80                    MachineBasicBlock *PredBB,
81                    DenseMap<unsigned, unsigned> &LocalVRMap,
82                    SmallVector<std::pair<unsigned,unsigned>, 4> &Copies);
83    void DuplicateInstruction(MachineInstr *MI,
84                              MachineBasicBlock *TailBB,
85                              MachineBasicBlock *PredBB,
86                              MachineFunction &MF,
87                              DenseMap<unsigned, unsigned> &LocalVRMap);
88    void UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
89                              SmallVector<MachineBasicBlock*, 8> &TDBBs,
90                              SmallSetVector<MachineBasicBlock*, 8> &Succs);
91    bool TailDuplicateBlocks(MachineFunction &MF);
92    bool TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
93                       SmallVector<MachineBasicBlock*, 8> &TDBBs,
94                       SmallVector<MachineInstr*, 16> &Copies);
95    void RemoveDeadBlock(MachineBasicBlock *MBB);
96  };
97
98  char TailDuplicatePass::ID = 0;
99}
100
101FunctionPass *llvm::createTailDuplicatePass(bool PreRegAlloc) {
102  return new TailDuplicatePass(PreRegAlloc);
103}
104
105bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
106  TII = MF.getTarget().getInstrInfo();
107  MRI = &MF.getRegInfo();
108  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
109
110  bool MadeChange = false;
111  while (TailDuplicateBlocks(MF))
112    MadeChange = true;
113
114  return MadeChange;
115}
116
117static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
118  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
119    MachineBasicBlock *MBB = I;
120    SmallSetVector<MachineBasicBlock*, 8> Preds(MBB->pred_begin(),
121                                                MBB->pred_end());
122    MachineBasicBlock::iterator MI = MBB->begin();
123    while (MI != MBB->end()) {
124      if (!MI->isPHI())
125        break;
126      for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
127             PE = Preds.end(); PI != PE; ++PI) {
128        MachineBasicBlock *PredBB = *PI;
129        bool Found = false;
130        for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
131          MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
132          if (PHIBB == PredBB) {
133            Found = true;
134            break;
135          }
136        }
137        if (!Found) {
138          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
139          dbgs() << "  missing input from predecessor BB#"
140                 << PredBB->getNumber() << '\n';
141          llvm_unreachable(0);
142        }
143      }
144
145      for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
146        MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
147        if (CheckExtra && !Preds.count(PHIBB)) {
148          // This is not a hard error.
149          dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber()
150                 << ": " << *MI;
151          dbgs() << "  extra input from predecessor BB#"
152                 << PHIBB->getNumber() << '\n';
153        }
154        if (PHIBB->getNumber() < 0) {
155          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
156          dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
157          llvm_unreachable(0);
158        }
159      }
160      ++MI;
161    }
162  }
163}
164
165/// TailDuplicateBlocks - Look for small blocks that are unconditionally
166/// branched to and do not fall through. Tail-duplicate their instructions
167/// into their predecessors to eliminate (dynamic) branches.
168bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
169  bool MadeChange = false;
170
171  if (PreRegAlloc && TailDupVerify) {
172    DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
173    VerifyPHIs(MF, true);
174  }
175
176  SmallVector<MachineInstr*, 8> NewPHIs;
177  MachineSSAUpdater SSAUpdate(MF, &NewPHIs);
178
179  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
180    MachineBasicBlock *MBB = I++;
181
182    if (NumTails == TailDupLimit)
183      break;
184
185    // Only duplicate blocks that end with unconditional branches.
186    if (MBB->canFallThrough())
187      continue;
188
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, MF, TDBBs, Copies)) {
196      ++NumTails;
197
198      // TailBB's immediate successors are now successors of those predecessors
199      // which duplicated TailBB. Add the predecessors as sources to the PHI
200      // instructions.
201      bool isDead = MBB->pred_empty();
202      if (PreRegAlloc)
203        UpdateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
204
205      // If it is dead, remove it.
206      if (isDead) {
207        NumInstrDups -= MBB->size();
208        RemoveDeadBlock(MBB);
209        ++NumDeadBlocks;
210      }
211
212      // Update SSA form.
213      if (!SSAUpdateVRs.empty()) {
214        for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
215          unsigned VReg = SSAUpdateVRs[i];
216          SSAUpdate.Initialize(VReg);
217
218          // If the original definition is still around, add it as an available
219          // value.
220          MachineInstr *DefMI = MRI->getVRegDef(VReg);
221          MachineBasicBlock *DefBB = 0;
222          if (DefMI) {
223            DefBB = DefMI->getParent();
224            SSAUpdate.AddAvailableValue(DefBB, VReg);
225          }
226
227          // Add the new vregs as available values.
228          DenseMap<unsigned, AvailableValsTy>::iterator LI =
229            SSAUpdateVals.find(VReg);
230          for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
231            MachineBasicBlock *SrcBB = LI->second[j].first;
232            unsigned SrcReg = LI->second[j].second;
233            SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
234          }
235
236          // Rewrite uses that are outside of the original def's block.
237          MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
238          while (UI != MRI->use_end()) {
239            MachineOperand &UseMO = UI.getOperand();
240            MachineInstr *UseMI = &*UI;
241            ++UI;
242            if (UseMI->getParent() == DefBB)
243              continue;
244            SSAUpdate.RewriteUse(UseMO);
245          }
246        }
247
248        SSAUpdateVRs.clear();
249        SSAUpdateVals.clear();
250      }
251
252      // Eliminate some of the copies inserted by tail duplication to maintain
253      // SSA form.
254      for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
255        MachineInstr *Copy = Copies[i];
256        unsigned Src, Dst, SrcSR, DstSR;
257        if (TII->isMoveInstr(*Copy, Src, Dst, SrcSR, DstSR)) {
258          MachineRegisterInfo::use_iterator UI = MRI->use_begin(Src);
259          if (++UI == MRI->use_end()) {
260            // Copy is the only use. Do trivial copy propagation here.
261            MRI->replaceRegWith(Dst, Src);
262            Copy->eraseFromParent();
263          }
264        }
265      }
266
267      if (PreRegAlloc && TailDupVerify)
268        VerifyPHIs(MF, false);
269      MadeChange = true;
270    }
271  }
272
273  return MadeChange;
274}
275
276static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
277                         const MachineRegisterInfo *MRI) {
278  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
279         UE = MRI->use_end(); UI != UE; ++UI) {
280    MachineInstr *UseMI = &*UI;
281    if (UseMI->getParent() != BB)
282      return true;
283  }
284  return false;
285}
286
287static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
288  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
289    if (MI->getOperand(i+1).getMBB() == SrcBB)
290      return i;
291  return 0;
292}
293
294/// AddSSAUpdateEntry - Add a definition and source virtual registers pair for
295/// SSA update.
296void TailDuplicatePass::AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
297                                          MachineBasicBlock *BB) {
298  DenseMap<unsigned, AvailableValsTy>::iterator LI= SSAUpdateVals.find(OrigReg);
299  if (LI != SSAUpdateVals.end())
300    LI->second.push_back(std::make_pair(BB, NewReg));
301  else {
302    AvailableValsTy Vals;
303    Vals.push_back(std::make_pair(BB, NewReg));
304    SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
305    SSAUpdateVRs.push_back(OrigReg);
306  }
307}
308
309/// ProcessPHI - Process PHI node in TailBB by turning it into a copy in PredBB.
310/// Remember the source register that's contributed by PredBB and update SSA
311/// update map.
312void TailDuplicatePass::ProcessPHI(MachineInstr *MI,
313                                   MachineBasicBlock *TailBB,
314                                   MachineBasicBlock *PredBB,
315                                   DenseMap<unsigned, unsigned> &LocalVRMap,
316                         SmallVector<std::pair<unsigned,unsigned>, 4> &Copies) {
317  unsigned DefReg = MI->getOperand(0).getReg();
318  unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
319  assert(SrcOpIdx && "Unable to find matching PHI source?");
320  unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
321  const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
322  LocalVRMap.insert(std::make_pair(DefReg, SrcReg));
323
324  // Insert a copy from source to the end of the block. The def register is the
325  // available value liveout of the block.
326  unsigned NewDef = MRI->createVirtualRegister(RC);
327  Copies.push_back(std::make_pair(NewDef, SrcReg));
328  if (isDefLiveOut(DefReg, TailBB, MRI))
329    AddSSAUpdateEntry(DefReg, NewDef, PredBB);
330
331  // Remove PredBB from the PHI node.
332  MI->RemoveOperand(SrcOpIdx+1);
333  MI->RemoveOperand(SrcOpIdx);
334  if (MI->getNumOperands() == 1)
335    MI->eraseFromParent();
336}
337
338/// DuplicateInstruction - Duplicate a TailBB instruction to PredBB and update
339/// the source operands due to earlier PHI translation.
340void TailDuplicatePass::DuplicateInstruction(MachineInstr *MI,
341                                     MachineBasicBlock *TailBB,
342                                     MachineBasicBlock *PredBB,
343                                     MachineFunction &MF,
344                                     DenseMap<unsigned, unsigned> &LocalVRMap) {
345  MachineInstr *NewMI = TII->duplicate(MI, MF);
346  for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
347    MachineOperand &MO = NewMI->getOperand(i);
348    if (!MO.isReg())
349      continue;
350    unsigned Reg = MO.getReg();
351    if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
352      continue;
353    if (MO.isDef()) {
354      const TargetRegisterClass *RC = MRI->getRegClass(Reg);
355      unsigned NewReg = MRI->createVirtualRegister(RC);
356      MO.setReg(NewReg);
357      LocalVRMap.insert(std::make_pair(Reg, NewReg));
358      if (isDefLiveOut(Reg, TailBB, MRI))
359        AddSSAUpdateEntry(Reg, NewReg, PredBB);
360    } else {
361      DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
362      if (VI != LocalVRMap.end())
363        MO.setReg(VI->second);
364    }
365  }
366  PredBB->insert(PredBB->end(), NewMI);
367}
368
369/// UpdateSuccessorsPHIs - After FromBB is tail duplicated into its predecessor
370/// blocks, the successors have gained new predecessors. Update the PHI
371/// instructions in them accordingly.
372void
373TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
374                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
375                                  SmallSetVector<MachineBasicBlock*,8> &Succs) {
376  for (SmallSetVector<MachineBasicBlock*, 8>::iterator SI = Succs.begin(),
377         SE = Succs.end(); SI != SE; ++SI) {
378    MachineBasicBlock *SuccBB = *SI;
379    for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
380         II != EE; ++II) {
381      if (!II->isPHI())
382        break;
383      unsigned Idx = 0;
384      for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
385        MachineOperand &MO = II->getOperand(i+1);
386        if (MO.getMBB() == FromBB) {
387          Idx = i;
388          break;
389        }
390      }
391
392      assert(Idx != 0);
393      MachineOperand &MO0 = II->getOperand(Idx);
394      unsigned Reg = MO0.getReg();
395      if (isDead) {
396        // Folded into the previous BB.
397        // There could be duplicate phi source entries. FIXME: Should sdisel
398        // or earlier pass fixed this?
399        for (unsigned i = II->getNumOperands()-2; i != Idx; i -= 2) {
400          MachineOperand &MO = II->getOperand(i+1);
401          if (MO.getMBB() == FromBB) {
402            II->RemoveOperand(i+1);
403            II->RemoveOperand(i);
404          }
405        }
406      } else
407        Idx = 0;
408
409      // If Idx is set, the operands at Idx and Idx+1 must be removed.
410      // We reuse the location to avoid expensive RemoveOperand calls.
411
412      DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
413      if (LI != SSAUpdateVals.end()) {
414        // This register is defined in the tail block.
415        for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
416          MachineBasicBlock *SrcBB = LI->second[j].first;
417          unsigned SrcReg = LI->second[j].second;
418          if (Idx != 0) {
419            II->getOperand(Idx).setReg(SrcReg);
420            II->getOperand(Idx+1).setMBB(SrcBB);
421            Idx = 0;
422          } else {
423            II->addOperand(MachineOperand::CreateReg(SrcReg, false));
424            II->addOperand(MachineOperand::CreateMBB(SrcBB));
425          }
426        }
427      } else {
428        // Live in tail block, must also be live in predecessors.
429        for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
430          MachineBasicBlock *SrcBB = TDBBs[j];
431          if (Idx != 0) {
432            II->getOperand(Idx).setReg(Reg);
433            II->getOperand(Idx+1).setMBB(SrcBB);
434            Idx = 0;
435          } else {
436            II->addOperand(MachineOperand::CreateReg(Reg, false));
437            II->addOperand(MachineOperand::CreateMBB(SrcBB));
438          }
439        }
440      }
441      if (Idx != 0) {
442        II->RemoveOperand(Idx+1);
443        II->RemoveOperand(Idx);
444      }
445    }
446  }
447}
448
449/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
450/// of its predecessors.
451bool
452TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
453                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
454                                 SmallVector<MachineInstr*, 16> &Copies) {
455  // Set the limit on the number of instructions to duplicate, with a default
456  // of one less than the tail-merge threshold. When optimizing for size,
457  // duplicate only one, because one branch instruction can be eliminated to
458  // compensate for the duplication.
459  unsigned MaxDuplicateCount;
460  if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
461    MaxDuplicateCount = 1;
462  else
463    MaxDuplicateCount = TailDuplicateSize;
464
465  if (PreRegAlloc) {
466      // Pre-regalloc tail duplication hurts compile time and doesn't help
467      // much except for indirect branches.
468    if (TailBB->empty() || !TailBB->back().getDesc().isIndirectBranch())
469      return false;
470    // If the target has hardware branch prediction that can handle indirect
471    // branches, duplicating them can often make them predictable when there
472    // are common paths through the code.  The limit needs to be high enough
473    // to allow undoing the effects of tail merging and other optimizations
474    // that rearrange the predecessors of the indirect branch.
475    MaxDuplicateCount = 20;
476  }
477
478  // Don't try to tail-duplicate single-block loops.
479  if (TailBB->isSuccessor(TailBB))
480    return false;
481
482  // Check the instructions in the block to determine whether tail-duplication
483  // is invalid or unlikely to be profitable.
484  unsigned InstrCount = 0;
485  bool HasCall = false;
486  for (MachineBasicBlock::iterator I = TailBB->begin();
487       I != TailBB->end(); ++I) {
488    // Non-duplicable things shouldn't be tail-duplicated.
489    if (I->getDesc().isNotDuplicable()) return false;
490    // Do not duplicate 'return' instructions if this is a pre-regalloc run.
491    // A return may expand into a lot more instructions (e.g. reload of callee
492    // saved registers) after PEI.
493    if (PreRegAlloc && I->getDesc().isReturn()) return false;
494    // Don't duplicate more than the threshold.
495    if (InstrCount == MaxDuplicateCount) return false;
496    // Remember if we saw a call.
497    if (I->getDesc().isCall()) HasCall = true;
498    if (!I->isPHI())
499      InstrCount += 1;
500  }
501  // Heuristically, don't tail-duplicate calls if it would expand code size,
502  // as it's less likely to be worth the extra cost.
503  if (InstrCount > 1 && HasCall)
504    return false;
505
506  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
507
508  // Iterate through all the unique predecessors and tail-duplicate this
509  // block into them, if possible. Copying the list ahead of time also
510  // avoids trouble with the predecessor list reallocating.
511  bool Changed = false;
512  SmallSetVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
513                                              TailBB->pred_end());
514  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
515       PE = Preds.end(); PI != PE; ++PI) {
516    MachineBasicBlock *PredBB = *PI;
517
518    assert(TailBB != PredBB &&
519           "Single-block loop should have been rejected earlier!");
520    if (PredBB->succ_size() > 1) continue;
521
522    MachineBasicBlock *PredTBB, *PredFBB;
523    SmallVector<MachineOperand, 4> PredCond;
524    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
525      continue;
526    if (!PredCond.empty())
527      continue;
528    // EH edges are ignored by AnalyzeBranch.
529    if (PredBB->succ_size() != 1)
530      continue;
531    // Don't duplicate into a fall-through predecessor (at least for now).
532    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
533      continue;
534
535    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
536                 << "From Succ: " << *TailBB);
537
538    TDBBs.push_back(PredBB);
539
540    // Remove PredBB's unconditional branch.
541    TII->RemoveBranch(*PredBB);
542
543    // Clone the contents of TailBB into PredBB.
544    DenseMap<unsigned, unsigned> LocalVRMap;
545    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
546    MachineBasicBlock::iterator I = TailBB->begin();
547    while (I != TailBB->end()) {
548      MachineInstr *MI = &*I;
549      ++I;
550      if (MI->isPHI()) {
551        // Replace the uses of the def of the PHI with the register coming
552        // from PredBB.
553        ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos);
554      } else {
555        // Replace def of virtual registers with new registers, and update
556        // uses with PHI source register or the new registers.
557        DuplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap);
558      }
559    }
560    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
561    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
562      const TargetRegisterClass *RC = MRI->getRegClass(CopyInfos[i].first);
563      TII->copyRegToReg(*PredBB, Loc, CopyInfos[i].first,
564                        CopyInfos[i].second, RC,RC);
565      MachineInstr *CopyMI = prior(Loc);
566      Copies.push_back(CopyMI);
567    }
568    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
569
570    // Update the CFG.
571    PredBB->removeSuccessor(PredBB->succ_begin());
572    assert(PredBB->succ_empty() &&
573           "TailDuplicate called on block with multiple successors!");
574    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
575           E = TailBB->succ_end(); I != E; ++I)
576      PredBB->addSuccessor(*I);
577
578    Changed = true;
579    ++NumTailDups;
580  }
581
582  // If TailBB was duplicated into all its predecessors except for the prior
583  // block, which falls through unconditionally, move the contents of this
584  // block into the prior block.
585  MachineBasicBlock *PrevBB = prior(MachineFunction::iterator(TailBB));
586  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
587  SmallVector<MachineOperand, 4> PriorCond;
588  bool PriorUnAnalyzable =
589    TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true);
590  // This has to check PrevBB->succ_size() because EH edges are ignored by
591  // AnalyzeBranch.
592  if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
593      TailBB->pred_size() == 1 && PrevBB->succ_size() == 1 &&
594      !TailBB->hasAddressTaken()) {
595    DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
596          << "From MBB: " << *TailBB);
597    if (PreRegAlloc) {
598      DenseMap<unsigned, unsigned> LocalVRMap;
599      SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
600      MachineBasicBlock::iterator I = TailBB->begin();
601      // Process PHI instructions first.
602      while (I != TailBB->end() && I->isPHI()) {
603        // Replace the uses of the def of the PHI with the register coming
604        // from PredBB.
605        MachineInstr *MI = &*I++;
606        ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos);
607        if (MI->getParent())
608          MI->eraseFromParent();
609      }
610
611      // Now copy the non-PHI instructions.
612      while (I != TailBB->end()) {
613        // Replace def of virtual registers with new registers, and update
614        // uses with PHI source register or the new registers.
615        MachineInstr *MI = &*I++;
616        DuplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap);
617        MI->eraseFromParent();
618      }
619      MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
620      for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
621        const TargetRegisterClass *RC = MRI->getRegClass(CopyInfos[i].first);
622        TII->copyRegToReg(*PrevBB, Loc, CopyInfos[i].first,
623                          CopyInfos[i].second, RC, RC);
624        MachineInstr *CopyMI = prior(Loc);
625        Copies.push_back(CopyMI);
626      }
627    } else {
628      // No PHIs to worry about, just splice the instructions over.
629      PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
630    }
631    PrevBB->removeSuccessor(PrevBB->succ_begin());
632    assert(PrevBB->succ_empty());
633    PrevBB->transferSuccessors(TailBB);
634    TDBBs.push_back(PrevBB);
635    Changed = true;
636  }
637
638  return Changed;
639}
640
641/// RemoveDeadBlock - Remove the specified dead machine basic block from the
642/// function, updating the CFG.
643void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
644  assert(MBB->pred_empty() && "MBB must be dead!");
645  DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
646
647  // Remove all successors.
648  while (!MBB->succ_empty())
649    MBB->removeSuccessor(MBB->succ_end()-1);
650
651  // If there are any labels in the basic block, unregister them from
652  // MachineModuleInfo.
653  if (MMI && !MBB->empty()) {
654    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
655         I != E; ++I) {
656      if (I->isLabel())
657        // The label ID # is always operand #0, an immediate.
658        MMI->InvalidateLabel(I->getOperand(0).getImm());
659    }
660  }
661
662  // Remove the block.
663  MBB->eraseFromParent();
664}
665
666