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/DenseSet.h"
29#include "llvm/ADT/SmallSet.h"
30#include "llvm/ADT/SetVector.h"
31#include "llvm/ADT/Statistic.h"
32using namespace llvm;
33
34STATISTIC(NumTails     , "Number of tails duplicated");
35STATISTIC(NumTailDups  , "Number of tail duplicated blocks");
36STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
37STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
38STATISTIC(NumAddedPHIs , "Number of phis added");
39
40// Heuristic for tail duplication.
41static cl::opt<unsigned>
42TailDuplicateSize("tail-dup-size",
43                  cl::desc("Maximum instructions to consider tail duplicating"),
44                  cl::init(2), cl::Hidden);
45
46static cl::opt<bool>
47TailDupVerify("tail-dup-verify",
48              cl::desc("Verify sanity of PHI instructions during taildup"),
49              cl::init(false), cl::Hidden);
50
51static cl::opt<unsigned>
52TailDupLimit("tail-dup-limit", cl::init(~0U), cl::Hidden);
53
54typedef std::vector<std::pair<MachineBasicBlock*,unsigned> > AvailableValsTy;
55
56namespace {
57  /// TailDuplicatePass - Perform tail duplication.
58  class TailDuplicatePass : public MachineFunctionPass {
59    const TargetInstrInfo *TII;
60    MachineModuleInfo *MMI;
61    MachineRegisterInfo *MRI;
62    bool PreRegAlloc;
63
64    // SSAUpdateVRs - A list of virtual registers for which to update SSA form.
65    SmallVector<unsigned, 16> SSAUpdateVRs;
66
67    // SSAUpdateVals - For each virtual register in SSAUpdateVals keep a list of
68    // source virtual registers.
69    DenseMap<unsigned, AvailableValsTy> SSAUpdateVals;
70
71  public:
72    static char ID;
73    explicit TailDuplicatePass() :
74      MachineFunctionPass(ID), PreRegAlloc(false) {}
75
76    virtual bool runOnMachineFunction(MachineFunction &MF);
77
78  private:
79    void AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
80                           MachineBasicBlock *BB);
81    void ProcessPHI(MachineInstr *MI, MachineBasicBlock *TailBB,
82                    MachineBasicBlock *PredBB,
83                    DenseMap<unsigned, unsigned> &LocalVRMap,
84                    SmallVector<std::pair<unsigned,unsigned>, 4> &Copies,
85                    const DenseSet<unsigned> &UsedByPhi,
86                    bool Remove);
87    void DuplicateInstruction(MachineInstr *MI,
88                              MachineBasicBlock *TailBB,
89                              MachineBasicBlock *PredBB,
90                              MachineFunction &MF,
91                              DenseMap<unsigned, unsigned> &LocalVRMap,
92                              const DenseSet<unsigned> &UsedByPhi);
93    void UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
94                              SmallVector<MachineBasicBlock*, 8> &TDBBs,
95                              SmallSetVector<MachineBasicBlock*, 8> &Succs);
96    bool TailDuplicateBlocks(MachineFunction &MF);
97    bool shouldTailDuplicate(const MachineFunction &MF,
98                             bool IsSimple, MachineBasicBlock &TailBB);
99    bool isSimpleBB(MachineBasicBlock *TailBB);
100    bool canCompletelyDuplicateBB(MachineBasicBlock &BB);
101    bool duplicateSimpleBB(MachineBasicBlock *TailBB,
102                           SmallVector<MachineBasicBlock*, 8> &TDBBs,
103                           const DenseSet<unsigned> &RegsUsedByPhi,
104                           SmallVector<MachineInstr*, 16> &Copies);
105    bool TailDuplicate(MachineBasicBlock *TailBB,
106                       bool IsSimple,
107                       MachineFunction &MF,
108                       SmallVector<MachineBasicBlock*, 8> &TDBBs,
109                       SmallVector<MachineInstr*, 16> &Copies);
110    bool TailDuplicateAndUpdate(MachineBasicBlock *MBB,
111                                bool IsSimple,
112                                MachineFunction &MF);
113
114    void RemoveDeadBlock(MachineBasicBlock *MBB);
115  };
116
117  char TailDuplicatePass::ID = 0;
118}
119
120char &llvm::TailDuplicateID = TailDuplicatePass::ID;
121
122INITIALIZE_PASS(TailDuplicatePass, "tailduplication", "Tail Duplication",
123                false, false)
124
125bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
126  TII = MF.getTarget().getInstrInfo();
127  MRI = &MF.getRegInfo();
128  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
129  PreRegAlloc = MRI->isSSA();
130
131  bool MadeChange = false;
132  while (TailDuplicateBlocks(MF))
133    MadeChange = true;
134
135  return MadeChange;
136}
137
138static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
139  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
140    MachineBasicBlock *MBB = I;
141    SmallSetVector<MachineBasicBlock*, 8> Preds(MBB->pred_begin(),
142                                                MBB->pred_end());
143    MachineBasicBlock::iterator MI = MBB->begin();
144    while (MI != MBB->end()) {
145      if (!MI->isPHI())
146        break;
147      for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
148             PE = Preds.end(); PI != PE; ++PI) {
149        MachineBasicBlock *PredBB = *PI;
150        bool Found = false;
151        for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
152          MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
153          if (PHIBB == PredBB) {
154            Found = true;
155            break;
156          }
157        }
158        if (!Found) {
159          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
160          dbgs() << "  missing input from predecessor BB#"
161                 << PredBB->getNumber() << '\n';
162          llvm_unreachable(0);
163        }
164      }
165
166      for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
167        MachineBasicBlock *PHIBB = MI->getOperand(i+1).getMBB();
168        if (CheckExtra && !Preds.count(PHIBB)) {
169          dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber()
170                 << ": " << *MI;
171          dbgs() << "  extra input from predecessor BB#"
172                 << PHIBB->getNumber() << '\n';
173          llvm_unreachable(0);
174        }
175        if (PHIBB->getNumber() < 0) {
176          dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
177          dbgs() << "  non-existing BB#" << PHIBB->getNumber() << '\n';
178          llvm_unreachable(0);
179        }
180      }
181      ++MI;
182    }
183  }
184}
185
186/// TailDuplicateAndUpdate - Tail duplicate the block and cleanup.
187bool
188TailDuplicatePass::TailDuplicateAndUpdate(MachineBasicBlock *MBB,
189                                          bool IsSimple,
190                                          MachineFunction &MF) {
191  // Save the successors list.
192  SmallSetVector<MachineBasicBlock*, 8> Succs(MBB->succ_begin(),
193                                              MBB->succ_end());
194
195  SmallVector<MachineBasicBlock*, 8> TDBBs;
196  SmallVector<MachineInstr*, 16> Copies;
197  if (!TailDuplicate(MBB, IsSimple, MF, TDBBs, Copies))
198    return false;
199
200  ++NumTails;
201
202  SmallVector<MachineInstr*, 8> NewPHIs;
203  MachineSSAUpdater SSAUpdate(MF, &NewPHIs);
204
205  // TailBB's immediate successors are now successors of those predecessors
206  // which duplicated TailBB. Add the predecessors as sources to the PHI
207  // instructions.
208  bool isDead = MBB->pred_empty() && !MBB->hasAddressTaken();
209  if (PreRegAlloc)
210    UpdateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
211
212  // If it is dead, remove it.
213  if (isDead) {
214    NumInstrDups -= MBB->size();
215    RemoveDeadBlock(MBB);
216    ++NumDeadBlocks;
217  }
218
219  // Update SSA form.
220  if (!SSAUpdateVRs.empty()) {
221    for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
222      unsigned VReg = SSAUpdateVRs[i];
223      SSAUpdate.Initialize(VReg);
224
225      // If the original definition is still around, add it as an available
226      // value.
227      MachineInstr *DefMI = MRI->getVRegDef(VReg);
228      MachineBasicBlock *DefBB = 0;
229      if (DefMI) {
230        DefBB = DefMI->getParent();
231        SSAUpdate.AddAvailableValue(DefBB, VReg);
232      }
233
234      // Add the new vregs as available values.
235      DenseMap<unsigned, AvailableValsTy>::iterator LI =
236        SSAUpdateVals.find(VReg);
237      for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
238        MachineBasicBlock *SrcBB = LI->second[j].first;
239        unsigned SrcReg = LI->second[j].second;
240        SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
241      }
242
243      // Rewrite uses that are outside of the original def's block.
244      MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
245      while (UI != MRI->use_end()) {
246        MachineOperand &UseMO = UI.getOperand();
247        MachineInstr *UseMI = &*UI;
248        ++UI;
249        if (UseMI->isDebugValue()) {
250          // SSAUpdate can replace the use with an undef. That creates
251          // a debug instruction that is a kill.
252          // FIXME: Should it SSAUpdate job to delete debug instructions
253          // instead of replacing the use with undef?
254          UseMI->eraseFromParent();
255          continue;
256        }
257        if (UseMI->getParent() == DefBB && !UseMI->isPHI())
258          continue;
259        SSAUpdate.RewriteUse(UseMO);
260      }
261    }
262
263    SSAUpdateVRs.clear();
264    SSAUpdateVals.clear();
265  }
266
267  // Eliminate some of the copies inserted by tail duplication to maintain
268  // SSA form.
269  for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
270    MachineInstr *Copy = Copies[i];
271    if (!Copy->isCopy())
272      continue;
273    unsigned Dst = Copy->getOperand(0).getReg();
274    unsigned Src = Copy->getOperand(1).getReg();
275    MachineRegisterInfo::use_iterator UI = MRI->use_begin(Src);
276    if (++UI == MRI->use_end()) {
277      // Copy is the only use. Do trivial copy propagation here.
278      MRI->replaceRegWith(Dst, Src);
279      Copy->eraseFromParent();
280    }
281  }
282
283  if (NewPHIs.size())
284    NumAddedPHIs += NewPHIs.size();
285
286  return true;
287}
288
289/// TailDuplicateBlocks - Look for small blocks that are unconditionally
290/// branched to and do not fall through. Tail-duplicate their instructions
291/// into their predecessors to eliminate (dynamic) branches.
292bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
293  bool MadeChange = false;
294
295  if (PreRegAlloc && TailDupVerify) {
296    DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
297    VerifyPHIs(MF, true);
298  }
299
300  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
301    MachineBasicBlock *MBB = I++;
302
303    if (NumTails == TailDupLimit)
304      break;
305
306    bool IsSimple = isSimpleBB(MBB);
307
308    if (!shouldTailDuplicate(MF, IsSimple, *MBB))
309      continue;
310
311    MadeChange |= TailDuplicateAndUpdate(MBB, IsSimple, MF);
312  }
313
314  if (PreRegAlloc && TailDupVerify)
315    VerifyPHIs(MF, false);
316
317  return MadeChange;
318}
319
320static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
321                         const MachineRegisterInfo *MRI) {
322  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
323         UE = MRI->use_end(); UI != UE; ++UI) {
324    MachineInstr *UseMI = &*UI;
325    if (UseMI->isDebugValue())
326      continue;
327    if (UseMI->getParent() != BB)
328      return true;
329  }
330  return false;
331}
332
333static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
334  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
335    if (MI->getOperand(i+1).getMBB() == SrcBB)
336      return i;
337  return 0;
338}
339
340
341// Remember which registers are used by phis in this block. This is
342// used to determine which registers are liveout while modifying the
343// block (which is why we need to copy the information).
344static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
345                              DenseSet<unsigned> *UsedByPhi) {
346  for(MachineBasicBlock::const_iterator I = BB.begin(), E = BB.end();
347      I != E; ++I) {
348    const MachineInstr &MI = *I;
349    if (!MI.isPHI())
350      break;
351    for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
352      unsigned SrcReg = MI.getOperand(i).getReg();
353      UsedByPhi->insert(SrcReg);
354    }
355  }
356}
357
358/// AddSSAUpdateEntry - Add a definition and source virtual registers pair for
359/// SSA update.
360void TailDuplicatePass::AddSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
361                                          MachineBasicBlock *BB) {
362  DenseMap<unsigned, AvailableValsTy>::iterator LI= SSAUpdateVals.find(OrigReg);
363  if (LI != SSAUpdateVals.end())
364    LI->second.push_back(std::make_pair(BB, NewReg));
365  else {
366    AvailableValsTy Vals;
367    Vals.push_back(std::make_pair(BB, NewReg));
368    SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
369    SSAUpdateVRs.push_back(OrigReg);
370  }
371}
372
373/// ProcessPHI - Process PHI node in TailBB by turning it into a copy in PredBB.
374/// Remember the source register that's contributed by PredBB and update SSA
375/// update map.
376void TailDuplicatePass::ProcessPHI(MachineInstr *MI,
377                                   MachineBasicBlock *TailBB,
378                                   MachineBasicBlock *PredBB,
379                                   DenseMap<unsigned, unsigned> &LocalVRMap,
380                           SmallVector<std::pair<unsigned,unsigned>, 4> &Copies,
381                                   const DenseSet<unsigned> &RegsUsedByPhi,
382                                   bool Remove) {
383  unsigned DefReg = MI->getOperand(0).getReg();
384  unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
385  assert(SrcOpIdx && "Unable to find matching PHI source?");
386  unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
387  const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
388  LocalVRMap.insert(std::make_pair(DefReg, SrcReg));
389
390  // Insert a copy from source to the end of the block. The def register is the
391  // available value liveout of the block.
392  unsigned NewDef = MRI->createVirtualRegister(RC);
393  Copies.push_back(std::make_pair(NewDef, SrcReg));
394  if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
395    AddSSAUpdateEntry(DefReg, NewDef, PredBB);
396
397  if (!Remove)
398    return;
399
400  // Remove PredBB from the PHI node.
401  MI->RemoveOperand(SrcOpIdx+1);
402  MI->RemoveOperand(SrcOpIdx);
403  if (MI->getNumOperands() == 1)
404    MI->eraseFromParent();
405}
406
407/// DuplicateInstruction - Duplicate a TailBB instruction to PredBB and update
408/// the source operands due to earlier PHI translation.
409void TailDuplicatePass::DuplicateInstruction(MachineInstr *MI,
410                                     MachineBasicBlock *TailBB,
411                                     MachineBasicBlock *PredBB,
412                                     MachineFunction &MF,
413                                     DenseMap<unsigned, unsigned> &LocalVRMap,
414                                     const DenseSet<unsigned> &UsedByPhi) {
415  MachineInstr *NewMI = TII->duplicate(MI, MF);
416  for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
417    MachineOperand &MO = NewMI->getOperand(i);
418    if (!MO.isReg())
419      continue;
420    unsigned Reg = MO.getReg();
421    if (!TargetRegisterInfo::isVirtualRegister(Reg))
422      continue;
423    if (MO.isDef()) {
424      const TargetRegisterClass *RC = MRI->getRegClass(Reg);
425      unsigned NewReg = MRI->createVirtualRegister(RC);
426      MO.setReg(NewReg);
427      LocalVRMap.insert(std::make_pair(Reg, NewReg));
428      if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
429        AddSSAUpdateEntry(Reg, NewReg, PredBB);
430    } else {
431      DenseMap<unsigned, unsigned>::iterator VI = LocalVRMap.find(Reg);
432      if (VI != LocalVRMap.end())
433        MO.setReg(VI->second);
434    }
435  }
436  PredBB->insert(PredBB->instr_end(), NewMI);
437}
438
439/// UpdateSuccessorsPHIs - After FromBB is tail duplicated into its predecessor
440/// blocks, the successors have gained new predecessors. Update the PHI
441/// instructions in them accordingly.
442void
443TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
444                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
445                                  SmallSetVector<MachineBasicBlock*,8> &Succs) {
446  for (SmallSetVector<MachineBasicBlock*, 8>::iterator SI = Succs.begin(),
447         SE = Succs.end(); SI != SE; ++SI) {
448    MachineBasicBlock *SuccBB = *SI;
449    for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end();
450         II != EE; ++II) {
451      if (!II->isPHI())
452        break;
453      unsigned Idx = 0;
454      for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) {
455        MachineOperand &MO = II->getOperand(i+1);
456        if (MO.getMBB() == FromBB) {
457          Idx = i;
458          break;
459        }
460      }
461
462      assert(Idx != 0);
463      MachineOperand &MO0 = II->getOperand(Idx);
464      unsigned Reg = MO0.getReg();
465      if (isDead) {
466        // Folded into the previous BB.
467        // There could be duplicate phi source entries. FIXME: Should sdisel
468        // or earlier pass fixed this?
469        for (unsigned i = II->getNumOperands()-2; i != Idx; i -= 2) {
470          MachineOperand &MO = II->getOperand(i+1);
471          if (MO.getMBB() == FromBB) {
472            II->RemoveOperand(i+1);
473            II->RemoveOperand(i);
474          }
475        }
476      } else
477        Idx = 0;
478
479      // If Idx is set, the operands at Idx and Idx+1 must be removed.
480      // We reuse the location to avoid expensive RemoveOperand calls.
481
482      DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
483      if (LI != SSAUpdateVals.end()) {
484        // This register is defined in the tail block.
485        for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
486          MachineBasicBlock *SrcBB = LI->second[j].first;
487          // If we didn't duplicate a bb into a particular predecessor, we
488          // might still have added an entry to SSAUpdateVals to correcly
489          // recompute SSA. If that case, avoid adding a dummy extra argument
490          // this PHI.
491          if (!SrcBB->isSuccessor(SuccBB))
492            continue;
493
494          unsigned SrcReg = LI->second[j].second;
495          if (Idx != 0) {
496            II->getOperand(Idx).setReg(SrcReg);
497            II->getOperand(Idx+1).setMBB(SrcBB);
498            Idx = 0;
499          } else {
500            II->addOperand(MachineOperand::CreateReg(SrcReg, false));
501            II->addOperand(MachineOperand::CreateMBB(SrcBB));
502          }
503        }
504      } else {
505        // Live in tail block, must also be live in predecessors.
506        for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
507          MachineBasicBlock *SrcBB = TDBBs[j];
508          if (Idx != 0) {
509            II->getOperand(Idx).setReg(Reg);
510            II->getOperand(Idx+1).setMBB(SrcBB);
511            Idx = 0;
512          } else {
513            II->addOperand(MachineOperand::CreateReg(Reg, false));
514            II->addOperand(MachineOperand::CreateMBB(SrcBB));
515          }
516        }
517      }
518      if (Idx != 0) {
519        II->RemoveOperand(Idx+1);
520        II->RemoveOperand(Idx);
521      }
522    }
523  }
524}
525
526/// shouldTailDuplicate - Determine if it is profitable to duplicate this block.
527bool
528TailDuplicatePass::shouldTailDuplicate(const MachineFunction &MF,
529                                       bool IsSimple,
530                                       MachineBasicBlock &TailBB) {
531  // Only duplicate blocks that end with unconditional branches.
532  if (TailBB.canFallThrough())
533    return false;
534
535  // Don't try to tail-duplicate single-block loops.
536  if (TailBB.isSuccessor(&TailBB))
537    return false;
538
539  // Set the limit on the cost to duplicate. When optimizing for size,
540  // duplicate only one, because one branch instruction can be eliminated to
541  // compensate for the duplication.
542  unsigned MaxDuplicateCount;
543  if (TailDuplicateSize.getNumOccurrences() == 0 &&
544      MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
545    MaxDuplicateCount = 1;
546  else
547    MaxDuplicateCount = TailDuplicateSize;
548
549  // If the target has hardware branch prediction that can handle indirect
550  // branches, duplicating them can often make them predictable when there
551  // are common paths through the code.  The limit needs to be high enough
552  // to allow undoing the effects of tail merging and other optimizations
553  // that rearrange the predecessors of the indirect branch.
554
555  bool HasIndirectbr = false;
556  if (!TailBB.empty())
557    HasIndirectbr = TailBB.back().isIndirectBranch();
558
559  if (HasIndirectbr && PreRegAlloc)
560    MaxDuplicateCount = 20;
561
562  // Check the instructions in the block to determine whether tail-duplication
563  // is invalid or unlikely to be profitable.
564  unsigned InstrCount = 0;
565  for (MachineBasicBlock::iterator I = TailBB.begin(); I != TailBB.end(); ++I) {
566    // Non-duplicable things shouldn't be tail-duplicated.
567    if (I->isNotDuplicable())
568      return false;
569
570    // Do not duplicate 'return' instructions if this is a pre-regalloc run.
571    // A return may expand into a lot more instructions (e.g. reload of callee
572    // saved registers) after PEI.
573    if (PreRegAlloc && I->isReturn())
574      return false;
575
576    // Avoid duplicating calls before register allocation. Calls presents a
577    // barrier to register allocation so duplicating them may end up increasing
578    // spills.
579    if (PreRegAlloc && I->isCall())
580      return false;
581
582    if (!I->isPHI() && !I->isDebugValue())
583      InstrCount += 1;
584
585    if (InstrCount > MaxDuplicateCount)
586      return false;
587  }
588
589  if (HasIndirectbr && PreRegAlloc)
590    return true;
591
592  if (IsSimple)
593    return true;
594
595  if (!PreRegAlloc)
596    return true;
597
598  return canCompletelyDuplicateBB(TailBB);
599}
600
601/// isSimpleBB - True if this BB has only one unconditional jump.
602bool
603TailDuplicatePass::isSimpleBB(MachineBasicBlock *TailBB) {
604  if (TailBB->succ_size() != 1)
605    return false;
606  if (TailBB->pred_empty())
607    return false;
608  MachineBasicBlock::iterator I = TailBB->begin();
609  MachineBasicBlock::iterator E = TailBB->end();
610  while (I != E && I->isDebugValue())
611    ++I;
612  if (I == E)
613    return true;
614  return I->isUnconditionalBranch();
615}
616
617static bool
618bothUsedInPHI(const MachineBasicBlock &A,
619              SmallPtrSet<MachineBasicBlock*, 8> SuccsB) {
620  for (MachineBasicBlock::const_succ_iterator SI = A.succ_begin(),
621         SE = A.succ_end(); SI != SE; ++SI) {
622    MachineBasicBlock *BB = *SI;
623    if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
624      return true;
625  }
626
627  return false;
628}
629
630bool
631TailDuplicatePass::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
632  SmallPtrSet<MachineBasicBlock*, 8> Succs(BB.succ_begin(), BB.succ_end());
633
634  for (MachineBasicBlock::pred_iterator PI = BB.pred_begin(),
635       PE = BB.pred_end(); PI != PE; ++PI) {
636    MachineBasicBlock *PredBB = *PI;
637
638    if (PredBB->succ_size() > 1)
639      return false;
640
641    MachineBasicBlock *PredTBB = NULL, *PredFBB = NULL;
642    SmallVector<MachineOperand, 4> PredCond;
643    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
644      return false;
645
646    if (!PredCond.empty())
647      return false;
648  }
649  return true;
650}
651
652bool
653TailDuplicatePass::duplicateSimpleBB(MachineBasicBlock *TailBB,
654                                     SmallVector<MachineBasicBlock*, 8> &TDBBs,
655                                     const DenseSet<unsigned> &UsedByPhi,
656                                     SmallVector<MachineInstr*, 16> &Copies) {
657  SmallPtrSet<MachineBasicBlock*, 8> Succs(TailBB->succ_begin(),
658                                           TailBB->succ_end());
659  SmallVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
660                                           TailBB->pred_end());
661  bool Changed = false;
662  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
663       PE = Preds.end(); PI != PE; ++PI) {
664    MachineBasicBlock *PredBB = *PI;
665
666    if (PredBB->getLandingPadSuccessor())
667      continue;
668
669    if (bothUsedInPHI(*PredBB, Succs))
670      continue;
671
672    MachineBasicBlock *PredTBB = NULL, *PredFBB = NULL;
673    SmallVector<MachineOperand, 4> PredCond;
674    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
675      continue;
676
677    Changed = true;
678    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
679                 << "From simple Succ: " << *TailBB);
680
681    MachineBasicBlock *NewTarget = *TailBB->succ_begin();
682    MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(PredBB));
683
684    // Make PredFBB explicit.
685    if (PredCond.empty())
686      PredFBB = PredTBB;
687
688    // Make fall through explicit.
689    if (!PredTBB)
690      PredTBB = NextBB;
691    if (!PredFBB)
692      PredFBB = NextBB;
693
694    // Redirect
695    if (PredFBB == TailBB)
696      PredFBB = NewTarget;
697    if (PredTBB == TailBB)
698      PredTBB = NewTarget;
699
700    // Make the branch unconditional if possible
701    if (PredTBB == PredFBB) {
702      PredCond.clear();
703      PredFBB = NULL;
704    }
705
706    // Avoid adding fall through branches.
707    if (PredFBB == NextBB)
708      PredFBB = NULL;
709    if (PredTBB == NextBB && PredFBB == NULL)
710      PredTBB = NULL;
711
712    TII->RemoveBranch(*PredBB);
713
714    if (PredTBB)
715      TII->InsertBranch(*PredBB, PredTBB, PredFBB, PredCond, DebugLoc());
716
717    PredBB->removeSuccessor(TailBB);
718    unsigned NumSuccessors = PredBB->succ_size();
719    assert(NumSuccessors <= 1);
720    if (NumSuccessors == 0 || *PredBB->succ_begin() != NewTarget)
721      PredBB->addSuccessor(NewTarget);
722
723    TDBBs.push_back(PredBB);
724  }
725  return Changed;
726}
727
728/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
729/// of its predecessors.
730bool
731TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
732                                 bool IsSimple,
733                                 MachineFunction &MF,
734                                 SmallVector<MachineBasicBlock*, 8> &TDBBs,
735                                 SmallVector<MachineInstr*, 16> &Copies) {
736  DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
737
738  DenseSet<unsigned> UsedByPhi;
739  getRegsUsedByPHIs(*TailBB, &UsedByPhi);
740
741  if (IsSimple)
742    return duplicateSimpleBB(TailBB, TDBBs, UsedByPhi, Copies);
743
744  // Iterate through all the unique predecessors and tail-duplicate this
745  // block into them, if possible. Copying the list ahead of time also
746  // avoids trouble with the predecessor list reallocating.
747  bool Changed = false;
748  SmallSetVector<MachineBasicBlock*, 8> Preds(TailBB->pred_begin(),
749                                              TailBB->pred_end());
750  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
751       PE = Preds.end(); PI != PE; ++PI) {
752    MachineBasicBlock *PredBB = *PI;
753
754    assert(TailBB != PredBB &&
755           "Single-block loop should have been rejected earlier!");
756    // EH edges are ignored by AnalyzeBranch.
757    if (PredBB->succ_size() > 1)
758      continue;
759
760    MachineBasicBlock *PredTBB, *PredFBB;
761    SmallVector<MachineOperand, 4> PredCond;
762    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
763      continue;
764    if (!PredCond.empty())
765      continue;
766    // Don't duplicate into a fall-through predecessor (at least for now).
767    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
768      continue;
769
770    DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
771                 << "From Succ: " << *TailBB);
772
773    TDBBs.push_back(PredBB);
774
775    // Remove PredBB's unconditional branch.
776    TII->RemoveBranch(*PredBB);
777
778    // Clone the contents of TailBB into PredBB.
779    DenseMap<unsigned, unsigned> LocalVRMap;
780    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
781    // Use instr_iterator here to properly handle bundles, e.g.
782    // ARM Thumb2 IT block.
783    MachineBasicBlock::instr_iterator I = TailBB->instr_begin();
784    while (I != TailBB->instr_end()) {
785      MachineInstr *MI = &*I;
786      ++I;
787      if (MI->isPHI()) {
788        // Replace the uses of the def of the PHI with the register coming
789        // from PredBB.
790        ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
791      } else {
792        // Replace def of virtual registers with new registers, and update
793        // uses with PHI source register or the new registers.
794        DuplicateInstruction(MI, TailBB, PredBB, MF, LocalVRMap, UsedByPhi);
795      }
796    }
797    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
798    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
799      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
800                               TII->get(TargetOpcode::COPY),
801                               CopyInfos[i].first).addReg(CopyInfos[i].second));
802    }
803
804    // Simplify
805    TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);
806
807    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
808
809    // Update the CFG.
810    PredBB->removeSuccessor(PredBB->succ_begin());
811    assert(PredBB->succ_empty() &&
812           "TailDuplicate called on block with multiple successors!");
813    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
814           E = TailBB->succ_end(); I != E; ++I)
815      PredBB->addSuccessor(*I);
816
817    Changed = true;
818    ++NumTailDups;
819  }
820
821  // If TailBB was duplicated into all its predecessors except for the prior
822  // block, which falls through unconditionally, move the contents of this
823  // block into the prior block.
824  MachineBasicBlock *PrevBB = prior(MachineFunction::iterator(TailBB));
825  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
826  SmallVector<MachineOperand, 4> PriorCond;
827  // This has to check PrevBB->succ_size() because EH edges are ignored by
828  // AnalyzeBranch.
829  if (PrevBB->succ_size() == 1 &&
830      !TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
831      PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 &&
832      !TailBB->hasAddressTaken()) {
833    DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
834          << "From MBB: " << *TailBB);
835    if (PreRegAlloc) {
836      DenseMap<unsigned, unsigned> LocalVRMap;
837      SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
838      MachineBasicBlock::iterator I = TailBB->begin();
839      // Process PHI instructions first.
840      while (I != TailBB->end() && I->isPHI()) {
841        // Replace the uses of the def of the PHI with the register coming
842        // from PredBB.
843        MachineInstr *MI = &*I++;
844        ProcessPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
845        if (MI->getParent())
846          MI->eraseFromParent();
847      }
848
849      // Now copy the non-PHI instructions.
850      while (I != TailBB->end()) {
851        // Replace def of virtual registers with new registers, and update
852        // uses with PHI source register or the new registers.
853        MachineInstr *MI = &*I++;
854        assert(!MI->isBundle() && "Not expecting bundles before regalloc!");
855        DuplicateInstruction(MI, TailBB, PrevBB, MF, LocalVRMap, UsedByPhi);
856        MI->eraseFromParent();
857      }
858      MachineBasicBlock::iterator Loc = PrevBB->getFirstTerminator();
859      for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
860        Copies.push_back(BuildMI(*PrevBB, Loc, DebugLoc(),
861                                 TII->get(TargetOpcode::COPY),
862                                 CopyInfos[i].first)
863                           .addReg(CopyInfos[i].second));
864      }
865    } else {
866      // No PHIs to worry about, just splice the instructions over.
867      PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
868    }
869    PrevBB->removeSuccessor(PrevBB->succ_begin());
870    assert(PrevBB->succ_empty());
871    PrevBB->transferSuccessors(TailBB);
872    TDBBs.push_back(PrevBB);
873    Changed = true;
874  }
875
876  // If this is after register allocation, there are no phis to fix.
877  if (!PreRegAlloc)
878    return Changed;
879
880  // If we made no changes so far, we are safe.
881  if (!Changed)
882    return Changed;
883
884
885  // Handle the nasty case in that we duplicated a block that is part of a loop
886  // into some but not all of its predecessors. For example:
887  //    1 -> 2 <-> 3                 |
888  //          \                      |
889  //           \---> rest            |
890  // if we duplicate 2 into 1 but not into 3, we end up with
891  // 12 -> 3 <-> 2 -> rest           |
892  //   \             /               |
893  //    \----->-----/                |
894  // If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
895  // with a phi in 3 (which now dominates 2).
896  // What we do here is introduce a copy in 3 of the register defined by the
897  // phi, just like when we are duplicating 2 into 3, but we don't copy any
898  // real instructions or remove the 3 -> 2 edge from the phi in 2.
899  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
900       PE = Preds.end(); PI != PE; ++PI) {
901    MachineBasicBlock *PredBB = *PI;
902    if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end())
903      continue;
904
905    // EH edges
906    if (PredBB->succ_size() != 1)
907      continue;
908
909    DenseMap<unsigned, unsigned> LocalVRMap;
910    SmallVector<std::pair<unsigned,unsigned>, 4> CopyInfos;
911    MachineBasicBlock::iterator I = TailBB->begin();
912    // Process PHI instructions first.
913    while (I != TailBB->end() && I->isPHI()) {
914      // Replace the uses of the def of the PHI with the register coming
915      // from PredBB.
916      MachineInstr *MI = &*I++;
917      ProcessPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
918    }
919    MachineBasicBlock::iterator Loc = PredBB->getFirstTerminator();
920    for (unsigned i = 0, e = CopyInfos.size(); i != e; ++i) {
921      Copies.push_back(BuildMI(*PredBB, Loc, DebugLoc(),
922                               TII->get(TargetOpcode::COPY),
923                               CopyInfos[i].first).addReg(CopyInfos[i].second));
924    }
925  }
926
927  return Changed;
928}
929
930/// RemoveDeadBlock - Remove the specified dead machine basic block from the
931/// function, updating the CFG.
932void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
933  assert(MBB->pred_empty() && "MBB must be dead!");
934  DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
935
936  // Remove all successors.
937  while (!MBB->succ_empty())
938    MBB->removeSuccessor(MBB->succ_end()-1);
939
940  // Remove the block.
941  MBB->eraseFromParent();
942}
943