TailDuplication.cpp revision 33b465877255cc75241216817247c61374958a36 
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");
37STATISTIC(NumAddedPHIs , "Number of phis added");
38
39// Heuristic for tail duplication.
40static cl::opt<unsigned>
41TailDuplicateSize("tail-dup-size",
42                  cl::desc("Maximum instructions to consider tail duplicating"),
43                  cl::init(2), cl::Hidden);
44
45static cl::opt<bool>
46TailDupVerify("tail-dup-verify",
47              cl::desc("Verify sanity of PHI instructions during taildup"),
48              cl::init(false), cl::Hidden);
49
50static cl::opt<unsigned>
51TailDupLimit("tail-dup-limit", cl::init(~0U), cl::Hidden);
52
53typedef std::vector<std::pair<MachineBasicBlock*,unsigned> > AvailableValsTy;
54
55namespace {
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                             MachineBasicBlock &TailBB);
99    bool TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
100                       SmallVector<MachineBasicBlock*, 8> &TDBBs,
101                       SmallVector<MachineInstr*, 16> &Copies);
102    void RemoveDeadBlock(MachineBasicBlock *MBB);
103  };
104
105  char TailDuplicatePass::ID = 0;
106}
107
108FunctionPass *llvm::createTailDuplicatePass(bool PreRegAlloc) {
109  return new TailDuplicatePass(PreRegAlloc);
110}
111
112bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
113  TII = MF.getTarget().getInstrInfo();
114  MRI = &MF.getRegInfo();
115  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
116
117  bool MadeChange = false;
118  while (TailDuplicateBlocks(MF))
119    MadeChange = true;
120
121  return MadeChange;
122}
123
124static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
125  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
126    MachineBasicBlock *MBB = I;
127    SmallSetVector<MachineBasicBlock*, 8> Preds(MBB->pred_begin(),
128                                                MBB->pred_end());
129    MachineBasicBlock::iterator MI = MBB->begin();
130    while (MI != MBB->end()) {
131      if (!MI->isPHI())
132        break;
133      for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
134             PE = Preds.end(); PI != PE; ++PI) {
135        MachineBasicBlock *PredBB = *PI;
136        bool Found = false;
137        for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
138          MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
139          if (PHIBB == PredBB) {
140            Found = true;
141            break;
142          }
143        }
144        if (!Found) {
145          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
146          dbgs() << "  missing input from predecessor BB#"
147                 << PredBB->getNumber() << '\n';
148          llvm_unreachable(0);
149        }
150      }
151
152      for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
153        MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
154        if (CheckExtra && !Preds.count(PHIBB)) {
155          dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber()
156                 << ": " << *MI;
157          dbgs() << "  extra input from predecessor BB#"
158                 << PHIBB->getNumber() << '\n';
159          llvm_unreachable(0);
160        }
161        if (PHIBB->getNumber() < 0) {
162          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
163          dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
164          llvm_unreachable(0);
165        }
166      }
167      ++MI;
168    }
169  }
170}
171
172/// TailDuplicateBlocks - Look for small blocks that are unconditionally
173/// branched to and do not fall through. Tail-duplicate their instructions
174/// into their predecessors to eliminate (dynamic) branches.
175bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
176  bool MadeChange = false;
177
178  if (PreRegAlloc && TailDupVerify) {
179    DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
180    VerifyPHIs(MF, true);
181  }
182
183  SmallVector<MachineInstr*, 8> NewPHIs;
184  MachineSSAUpdater SSAUpdate(MF, &NewPHIs);
185
186  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
187    MachineBasicBlock *MBB = I++;
188
189    if (NumTails == TailDupLimit)
190      break;
191
192    // Save the successors list.
193    SmallSetVector<MachineBasicBlock*, 8> Succs(MBB->succ_begin(),
194                                                MBB->succ_end());
195
196    SmallVector<MachineBasicBlock*, 8> TDBBs;
197    SmallVector<MachineInstr*, 16> Copies;
198    if (TailDuplicate(MBB, MF, TDBBs, Copies)) {
199      ++NumTails;
200
201      // TailBB's immediate successors are now successors of those predecessors
202      // which duplicated TailBB. Add the predecessors as sources to the PHI
203      // instructions.
204      bool isDead = MBB->pred_empty();
205      if (PreRegAlloc)
206        UpdateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
207
208      // If it is dead, remove it.
209      if (isDead) {
210        NumInstrDups -= MBB->size();
211        RemoveDeadBlock(MBB);
212        ++NumDeadBlocks;
213      }
214
215      // Update SSA form.
216      if (!SSAUpdateVRs.empty()) {
217        for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
218          unsigned VReg = SSAUpdateVRs[i];
219          SSAUpdate.Initialize(VReg);
220
221          // If the original definition is still around, add it as an available
222          // value.
223          MachineInstr *DefMI = MRI->getVRegDef(VReg);
224          MachineBasicBlock *DefBB = 0;
225          if (DefMI) {
226            DefBB = DefMI->getParent();
227            SSAUpdate.AddAvailableValue(DefBB, VReg);
228          }
229
230          // Add the new vregs as available values.
231          DenseMap<unsigned, AvailableValsTy>::iterator LI =
232            SSAUpdateVals.find(VReg);
233          for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
234            MachineBasicBlock *SrcBB = LI->second[j].first;
235            unsigned SrcReg = LI->second[j].second;
236            SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
237          }
238
239          // Rewrite uses that are outside of the original def's block.
240          MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
241          while (UI != MRI->use_end()) {
242            MachineOperand &UseMO = UI.getOperand();
243            MachineInstr *UseMI = &*UI;
244            ++UI;
245            if (UseMI->getParent() == DefBB && !UseMI->isPHI())
246              continue;
247            SSAUpdate.RewriteUse(UseMO);
248          }
249        }
250
251        SSAUpdateVRs.clear();
252        SSAUpdateVals.clear();
253      }
254
255      // Eliminate some of the copies inserted by tail duplication to maintain
256      // SSA form.
257      for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
258        MachineInstr *Copy = Copies[i];
259        if (!Copy->isCopy())
260          continue;
261        unsigned Dst = Copy->getOperand(0).getReg();
262        unsigned Src = Copy->getOperand(1).getReg();
263        MachineRegisterInfo::use_iterator UI = MRI->use_begin(Src);
264        if (++UI == MRI->use_end()) {
265          // Copy is the only use. Do trivial copy propagation here.
266          MRI->replaceRegWith(Dst, Src);
267          Copy->eraseFromParent();
268        }
269      }
270
271      if (PreRegAlloc && TailDupVerify)
272        VerifyPHIs(MF, false);
273      MadeChange = true;
274    }
275  }
276  NumAddedPHIs += NewPHIs.size();
277
278  return MadeChange;
279}
280
281static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
282                         const MachineRegisterInfo *MRI) {
283  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
284         UE = MRI->use_end(); UI != UE; ++UI) {
285    MachineInstr *UseMI = &*UI;
286    if (UseMI->getParent() != BB)
287      return true;
288  }
289  return false;
290}
291
292static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
293  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
294    if (MI->getOperand(i+1).getMBB() == SrcBB)
295      return i;
296  return 0;
297}
298
299
300// Remember which registers are used by phis in this block. This is
301// used to determine which registers are liveout while modifying the
302// block (which is why we need to copy the information).
303static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
304                              DenseSet<unsigned> *UsedByPhi) {
305  for(MachineBasicBlock::const_iterator I = BB.begin(), E = BB.end();
306      I != E; ++I) {
307    const MachineInstr &MI = *I;
308    if (!MI.isPHI())
309      break;
310    for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
311      unsigned SrcReg = MI.getOperand(i).getReg();
312      UsedByPhi->insert(SrcReg);
313    }
314  }
315}
316
317/// AddSSAUpdateEntry - Add a definition and source virtual registers pair for
318/// SSA update.
319void TailDuplicatePass::AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
320                                          MachineBasicBlock *BB) {
321  DenseMap<unsigned, AvailableValsTy>::iterator LI= SSAUpdateVals.find(OrigReg);
322  if (LI != SSAUpdateVals.end())
323    LI->second.push_back(std::make_pair(BB, NewReg));
324  else {
325    AvailableValsTy Vals;
326    Vals.push_back(std::make_pair(BB, NewReg));
327    SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
328    SSAUpdateVRs.push_back(OrigReg);
329  }
330}
331
332/// ProcessPHI - Process PHI node in TailBB by turning it into a copy in PredBB.
333/// Remember the source register that's contributed by PredBB and update SSA
334/// update map.
335void TailDuplicatePass::ProcessPHI(MachineInstr *MI,
336                                   MachineBasicBlock *TailBB,
337                                   MachineBasicBlock *PredBB,
338                                   DenseMap<unsigned, unsigned> &LocalVRMap,
339                           SmallVector<std::pair<unsigned,unsigned>, 4> &Copies,
340                                   const DenseSet<unsigned> &RegsUsedByPhi,
341                                   bool Remove) {
342  unsigned DefReg = MI->getOperand(0).getReg();
343  unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
344  assert(SrcOpIdx && "Unable to find matching PHI source?");
345  unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
346  const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
347  LocalVRMap.insert(std::make_pair(DefReg, SrcReg));
348
349  // Insert a copy from source to the end of the block. The def register is the
350  // available value liveout of the block.
351  unsigned NewDef = MRI->createVirtualRegister(RC);
352  Copies.push_back(std::make_pair(NewDef, SrcReg));
353  if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
354    AddSSAUpdateEntry(DefReg, NewDef, PredBB);
355
356  if (!Remove)
357    return;
358
359  // Remove PredBB from the PHI node.
360  MI->RemoveOperand(SrcOpIdx+1);
361  MI->RemoveOperand(SrcOpIdx);
362  if (MI->getNumOperands() == 1)
363    MI->eraseFromParent();
364}
365
366/// DuplicateInstruction - Duplicate a TailBB instruction to PredBB and update
367/// the source operands due to earlier PHI translation.
368void TailDuplicatePass::DuplicateInstruction(MachineInstr *MI,
369                                     MachineBasicBlock *TailBB,
370                                     MachineBasicBlock *PredBB,
371                                     MachineFunction &MF,
372                                     DenseMap<unsigned, unsigned> &LocalVRMap,
373                                     const DenseSet<unsigned> &UsedByPhi) {
374  MachineInstr *NewMI = TII->duplicate(MI, MF);
375  for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
376    MachineOperand &MO = NewMI->getOperand(i);
377    if (!MO.isReg())
378      continue;
379    unsigned Reg = MO.getReg();
380    if (!TargetRegisterInfo::isVirtualRegister(Reg))
381      continue;
382    if (MO.isDef()) {
383      const TargetRegisterClass *RC = MRI->getRegClass(Reg);
384      unsigned NewReg = MRI->createVirtualRegister(RC);
385      MO.setReg(NewReg);
386      LocalVRMap.insert(std::make_pair(Reg, NewReg));
387      if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
388        AddSSAUpdateEntry(Reg, NewReg, PredBB);
389    } else {
390      DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
391      if (VI != LocalVRMap.end())
392        MO.setReg(VI->second);
393    }
394  }
395  PredBB->insert(PredBB->end(), NewMI);
396}
397
398/// UpdateSuccessorsPHIs - After FromBB is tail duplicated into its predecessor
399/// blocks, the successors have gained new predecessors. Update the PHI
400/// instructions in them accordingly.
401void
402TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
403                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
404                                  SmallSetVector<MachineBasicBlock*,8> &Succs) {
405  for (SmallSetVector<MachineBasicBlock*, 8>::iterator SI = Succs.begin(),
406         SE = Succs.end(); SI != SE; ++SI) {
407    MachineBasicBlock *SuccBB = *SI;
408    for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
409         II != EE; ++II) {
410      if (!II->isPHI())
411        break;
412      unsigned Idx = 0;
413      for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
414        MachineOperand &MO = II->getOperand(i+1);
415        if (MO.getMBB() == FromBB) {
416          Idx = i;
417          break;
418        }
419      }
420
421      assert(Idx != 0);
422      MachineOperand &MO0 = II->getOperand(Idx);
423      unsigned Reg = MO0.getReg();
424      if (isDead) {
425        // Folded into the previous BB.
426        // There could be duplicate phi source entries. FIXME: Should sdisel
427        // or earlier pass fixed this?
428        for (unsigned i = II->getNumOperands()-2; i != Idx; i -= 2) {
429          MachineOperand &MO = II->getOperand(i+1);
430          if (MO.getMBB() == FromBB) {
431            II->RemoveOperand(i+1);
432            II->RemoveOperand(i);
433          }
434        }
435      } else
436        Idx = 0;
437
438      // If Idx is set, the operands at Idx and Idx+1 must be removed.
439      // We reuse the location to avoid expensive RemoveOperand calls.
440
441      DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
442      if (LI != SSAUpdateVals.end()) {
443        // This register is defined in the tail block.
444        for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
445          MachineBasicBlock *SrcBB = LI->second[j].first;
446          // If we didn't duplicate a bb into a particular predecessor, we
447          // might still have added an entry to SSAUpdateVals to correcly
448          // recompute SSA. If that case, avoid adding a dummy extra argument
449          // this PHI.
450          if (!SrcBB->isSuccessor(SuccBB))
451            continue;
452
453          unsigned SrcReg = LI->second[j].second;
454          if (Idx != 0) {
455            II->getOperand(Idx).setReg(SrcReg);
456            II->getOperand(Idx+1).setMBB(SrcBB);
457            Idx = 0;
458          } else {
459            II->addOperand(MachineOperand::CreateReg(SrcReg, false));
460            II->addOperand(MachineOperand::CreateMBB(SrcBB));
461          }
462        }
463      } else {
464        // Live in tail block, must also be live in predecessors.
465        for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
466          MachineBasicBlock *SrcBB = TDBBs[j];
467          if (Idx != 0) {
468            II->getOperand(Idx).setReg(Reg);
469            II->getOperand(Idx+1).setMBB(SrcBB);
470            Idx = 0;
471          } else {
472            II->addOperand(MachineOperand::CreateReg(Reg, false));
473            II->addOperand(MachineOperand::CreateMBB(SrcBB));
474          }
475        }
476      }
477      if (Idx != 0) {
478        II->RemoveOperand(Idx+1);
479        II->RemoveOperand(Idx);
480      }
481    }
482  }
483}
484
485/// shouldTailDuplicate - Determine if it is profitable to duplicate this block.
486bool
487TailDuplicatePass::shouldTailDuplicate(const MachineFunction &MF,
488                                       MachineBasicBlock &TailBB) {
489  // Only duplicate blocks that end with unconditional branches.
490  if (TailBB.canFallThrough())
491    return false;
492
493  // Set the limit on the cost to duplicate. When optimizing for size,
494  // duplicate only one, because one branch instruction can be eliminated to
495  // compensate for the duplication.
496  unsigned MaxDuplicateCount;
497  if (TailDuplicateSize.getNumOccurrences() == 0 &&
498      MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
499    MaxDuplicateCount = 1;
500  else
501    MaxDuplicateCount = TailDuplicateSize;
502
503  if (PreRegAlloc) {
504    if (TailBB.empty())
505      return false;
506    const TargetInstrDesc &TID = TailBB.back().getDesc();
507    // Pre-regalloc tail duplication hurts compile time and doesn't help
508    // much except for indirect branches.
509    if (!TID.isIndirectBranch())
510      return false;
511    // If the target has hardware branch prediction that can handle indirect
512    // branches, duplicating them can often make them predictable when there
513    // are common paths through the code.  The limit needs to be high enough
514    // to allow undoing the effects of tail merging and other optimizations
515    // that rearrange the predecessors of the indirect branch.
516    MaxDuplicateCount = 20;
517  }
518
519  // Don't try to tail-duplicate single-block loops.
520  if (TailBB.isSuccessor(&TailBB))
521    return false;
522
523  // Check the instructions in the block to determine whether tail-duplication
524  // is invalid or unlikely to be profitable.
525  unsigned InstrCount = 0;
526  bool HasCall = false;
527  for (MachineBasicBlock::const_iterator I = TailBB.begin(); I != TailBB.end();
528       ++I) {
529    // Non-duplicable things shouldn't be tail-duplicated.
530    if (I->getDesc().isNotDuplicable()) return false;
531    // Do not duplicate 'return' instructions if this is a pre-regalloc run.
532    // A return may expand into a lot more instructions (e.g. reload of callee
533    // saved registers) after PEI.
534    if (PreRegAlloc && I->getDesc().isReturn()) return false;
535    // Don't duplicate more than the threshold.
536    if (InstrCount == MaxDuplicateCount) return false;
537    // Remember if we saw a call.
538    if (I->getDesc().isCall()) HasCall = true;
539    if (!I->isPHI() && !I->isDebugValue())
540      InstrCount += 1;
541  }
542  // Don't tail-duplicate calls before register allocation. Calls presents a
543  // barrier to register allocation so duplicating them may end up increasing
544  // spills.
545  if (InstrCount > 1 && (PreRegAlloc && HasCall))
546    return false;
547
548  return true;
549}
550
551/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
552/// of its predecessors.
553bool
554TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
555                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
556                                 SmallVector<MachineInstr*, 16> &Copies) {
557  if (!shouldTailDuplicate(MF, *TailBB))
558    return false;
559
560  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
561
562  // Iterate through all the unique predecessors and tail-duplicate this
563  // block into them, if possible. Copying the list ahead of time also
564  // avoids trouble with the predecessor list reallocating.
565  bool Changed = false;
566  SmallSetVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
567                                              TailBB->pred_end());
568  DenseSet<unsigned> UsedByPhi;
569  getRegsUsedByPHIs(*TailBB, &UsedByPhi);
570  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
571       PE = Preds.end(); PI != PE; ++PI) {
572    MachineBasicBlock *PredBB = *PI;
573
574    assert(TailBB != PredBB &&
575           "Single-block loop should have been rejected earlier!");
576    // EH edges are ignored by AnalyzeBranch.
577    if (PredBB->succ_size() > 1)
578      continue;
579
580    MachineBasicBlock *PredTBB, *PredFBB;
581    SmallVector<MachineOperand, 4> PredCond;
582    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
583      continue;
584    if (!PredCond.empty())
585      continue;
586    // Don't duplicate into a fall-through predecessor (at least for now).
587    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
588      continue;
589
590    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
591                 << "From Succ: " << *TailBB);
592
593    TDBBs.push_back(PredBB);
594
595    // Remove PredBB's unconditional branch.
596    TII->RemoveBranch(*PredBB);
597
598    // Clone the contents of TailBB into PredBB.
599    DenseMap<unsigned, unsigned> LocalVRMap;
600    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
601    MachineBasicBlock::iterator I = TailBB->begin();
602    while (I != TailBB->end()) {
603      MachineInstr *MI = &*I;
604      ++I;
605      if (MI->isPHI()) {
606        // Replace the uses of the def of the PHI with the register coming
607        // from PredBB.
608        ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
609      } else {
610        // Replace def of virtual registers with new registers, and update
611        // uses with PHI source register or the new registers.
612        DuplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap, UsedByPhi);
613      }
614    }
615    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
616    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
617      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
618                               TII->get(TargetOpcode::COPY),
619                               CopyInfos[i].first).addReg(CopyInfos[i].second));
620    }
621    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
622
623    // Update the CFG.
624    PredBB->removeSuccessor(PredBB->succ_begin());
625    assert(PredBB->succ_empty() &&
626           "TailDuplicate called on block with multiple successors!");
627    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
628           E = TailBB->succ_end(); I != E; ++I)
629      PredBB->addSuccessor(*I);
630
631    Changed = true;
632    ++NumTailDups;
633  }
634
635  // If TailBB was duplicated into all its predecessors except for the prior
636  // block, which falls through unconditionally, move the contents of this
637  // block into the prior block.
638  MachineBasicBlock *PrevBB = prior(MachineFunction::iterator(TailBB));
639  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
640  SmallVector<MachineOperand, 4> PriorCond;
641  // This has to check PrevBB->succ_size() because EH edges are ignored by
642  // AnalyzeBranch.
643  if (PrevBB->succ_size() == 1 &&
644      !TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
645      PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 &&
646      !TailBB->hasAddressTaken()) {
647    DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
648          << "From MBB: " << *TailBB);
649    if (PreRegAlloc) {
650      DenseMap<unsigned, unsigned> LocalVRMap;
651      SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
652      MachineBasicBlock::iterator I = TailBB->begin();
653      // Process PHI instructions first.
654      while (I != TailBB->end() && I->isPHI()) {
655        // Replace the uses of the def of the PHI with the register coming
656        // from PredBB.
657        MachineInstr *MI = &*I++;
658        ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
659        if (MI->getParent())
660          MI->eraseFromParent();
661      }
662
663      // Now copy the non-PHI instructions.
664      while (I != TailBB->end()) {
665        // Replace def of virtual registers with new registers, and update
666        // uses with PHI source register or the new registers.
667        MachineInstr *MI = &*I++;
668        DuplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap, UsedByPhi);
669        MI->eraseFromParent();
670      }
671      MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
672      for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
673        Copies.push_back(BuildMI(*PrevBB, Loc, DebugLoc(),
674                                 TII->get(TargetOpcode::COPY),
675                                 CopyInfos[i].first)
676                           .addReg(CopyInfos[i].second));
677      }
678    } else {
679      // No PHIs to worry about, just splice the instructions over.
680      PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
681    }
682    PrevBB->removeSuccessor(PrevBB->succ_begin());
683    assert(PrevBB->succ_empty());
684    PrevBB->transferSuccessors(TailBB);
685    TDBBs.push_back(PrevBB);
686    Changed = true;
687  }
688
689  // If this is after register allocation, there are no phis to fix.
690  if (!PreRegAlloc)
691    return Changed;
692
693  // If we made no changes so far, we are safe.
694  if (!Changed)
695    return Changed;
696
697
698  // Handle the nasty case in that we duplicated a block that is part of a loop
699  // into some but not all of its predecessors. For example:
700  //    1 -> 2 <-> 3                 |
701  //          \                      |
702  //           \---> rest            |
703  // if we duplicate 2 into 1 but not into 3, we end up with
704  // 12 -> 3 <-> 2 -> rest           |
705  //   \             /               |
706  //    \----->-----/                |
707  // If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
708  // with a phi in 3 (which now dominates 2).
709  // What we do here is introduce a copy in 3 of the register defined by the
710  // phi, just like when we are duplicating 2 into 3, but we don't copy any
711  // real instructions or remove the 3 -> 2 edge from the phi in 2.
712  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
713       PE = Preds.end(); PI != PE; ++PI) {
714    MachineBasicBlock *PredBB = *PI;
715    if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end())
716      continue;
717
718    // EH edges
719    if (PredBB->succ_size() != 1)
720      continue;
721
722    DenseMap<unsigned, unsigned> LocalVRMap;
723    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
724    MachineBasicBlock::iterator I = TailBB->begin();
725    // Process PHI instructions first.
726    while (I != TailBB->end() && I->isPHI()) {
727      // Replace the uses of the def of the PHI with the register coming
728      // from PredBB.
729      MachineInstr *MI = &*I++;
730      ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
731    }
732    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
733    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
734      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
735                               TII->get(TargetOpcode::COPY),
736                               CopyInfos[i].first).addReg(CopyInfos[i].second));
737    }
738  }
739
740  return Changed;
741}
742
743/// RemoveDeadBlock - Remove the specified dead machine basic block from the
744/// function, updating the CFG.
745void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
746  assert(MBB->pred_empty() && "MBB must be dead!");
747  DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
748
749  // Remove all successors.
750  while (!MBB->succ_empty())
751    MBB->removeSuccessor(MBB->succ_end()-1);
752
753  // Remove the block.
754  MBB->eraseFromParent();
755}
756