IfConversion.cpp revision 1c9f91d7b2653dc730a80f49ac3b6e2b9d6ca236 
1//===-- IfConversion.cpp - Machine code if conversion pass. ---------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the Evan Cheng and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the machine instruction level if-conversion pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "ifcvt"
15#include "llvm/Function.h"
16#include "llvm/CodeGen/Passes.h"
17#include "llvm/CodeGen/MachineModuleInfo.h"
18#include "llvm/CodeGen/MachineFunctionPass.h"
19#include "llvm/Target/TargetInstrInfo.h"
20#include "llvm/Target/TargetLowering.h"
21#include "llvm/Target/TargetMachine.h"
22#include "llvm/Support/CommandLine.h"
23#include "llvm/Support/Debug.h"
24#include "llvm/ADT/DepthFirstIterator.h"
25#include "llvm/ADT/Statistic.h"
26using namespace llvm;
27
28namespace {
29  // Hidden options for help debugging.
30  cl::opt<int> IfCvtFnStart("ifcvt-fn-start", cl::init(-1), cl::Hidden);
31  cl::opt<int> IfCvtFnStop("ifcvt-fn-stop", cl::init(-1), cl::Hidden);
32  cl::opt<int> IfCvtLimit("ifcvt-limit", cl::init(-1), cl::Hidden);
33  cl::opt<bool> DisableSimple("disable-ifcvt-simple",
34                              cl::init(false), cl::Hidden);
35  cl::opt<bool> DisableSimpleF("disable-ifcvt-simple-false",
36                               cl::init(false), cl::Hidden);
37  cl::opt<bool> DisableTriangle("disable-ifcvt-triangle",
38                                cl::init(false), cl::Hidden);
39  cl::opt<bool> DisableTriangleF("disable-ifcvt-triangle-false",
40                                 cl::init(false), cl::Hidden);
41  cl::opt<bool> DisableTriangleFR("disable-ifcvt-triangle-false-rev",
42                                  cl::init(false), cl::Hidden);
43  cl::opt<bool> DisableDiamond("disable-ifcvt-diamond",
44                               cl::init(false), cl::Hidden);
45}
46
47STATISTIC(NumSimple,       "Number of simple if-conversions performed");
48STATISTIC(NumSimpleFalse,  "Number of simple (F) if-conversions performed");
49STATISTIC(NumTriangle,     "Number of triangle if-conversions performed");
50STATISTIC(NumTriangleFalse,"Number of triangle (F) if-conversions performed");
51STATISTIC(NumTriangleFRev, "Number of triangle (F/R) if-conversions performed");
52STATISTIC(NumDiamonds,     "Number of diamond if-conversions performed");
53STATISTIC(NumIfConvBBs,    "Number of if-converted blocks");
54
55namespace {
56  class IfConverter : public MachineFunctionPass {
57    enum BBICKind {
58      ICNotAnalyzed,   // BB has not been analyzed.
59      ICReAnalyze,     // BB must be re-analyzed.
60      ICNotClassfied,  // BB data valid, but not classified.
61      ICSimple,        // BB is entry of an one split, no rejoin sub-CFG.
62      ICSimpleFalse,   // Same as ICSimple, but on the false path.
63      ICTriangle,      // BB is entry of a triangle sub-CFG.
64      ICTriangleFalse, // Same as ICTriangle, but on the false path.
65      ICTriangleFRev,  // Same as ICTriangleFalse, but false path rev condition.
66      ICDiamond,       // BB is entry of a diamond sub-CFG.
67      ICChild,         // BB is part of the sub-CFG that'll be predicated.
68      ICDead           // BB cannot be if-converted again.
69    };
70
71    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
72    /// if-conversion feasibility analysis. This includes results from
73    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
74    /// classification, and common tail block of its successors (if it's a
75    /// diamond shape), its size, whether it's predicable, and whether any
76    /// instruction can clobber the 'would-be' predicate.
77    ///
78    /// Kind            - Type of block. See BBICKind.
79    /// NonPredSize     - Number of non-predicated instructions.
80    /// IsAnalyzable    - True if AnalyzeBranch() returns false.
81    /// ModifyPredicate - True if BB would modify the predicate (e.g. has
82    ///                   cmp, call, etc.)
83    /// BB              - Corresponding MachineBasicBlock.
84    /// TrueBB / FalseBB- See AnalyzeBranch().
85    /// BrCond          - Conditions for end of block conditional branches.
86    /// Predicate       - Predicate used in the BB.
87    struct BBInfo {
88      BBICKind Kind;
89      unsigned NonPredSize;
90      bool IsAnalyzable;
91      bool HasFallThrough;
92      bool ModifyPredicate;
93      MachineBasicBlock *BB;
94      MachineBasicBlock *TrueBB;
95      MachineBasicBlock *FalseBB;
96      MachineBasicBlock *TailBB;
97      std::vector<MachineOperand> BrCond;
98      std::vector<MachineOperand> Predicate;
99      BBInfo() : Kind(ICNotAnalyzed), NonPredSize(0),
100                 IsAnalyzable(false), HasFallThrough(false),
101                 ModifyPredicate(false),
102                 BB(0), TrueBB(0), FalseBB(0), TailBB(0) {}
103    };
104
105    /// Roots - Basic blocks that do not have successors. These are the starting
106    /// points of Graph traversal.
107    std::vector<MachineBasicBlock*> Roots;
108
109    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
110    /// basic block number.
111    std::vector<BBInfo> BBAnalysis;
112
113    const TargetLowering *TLI;
114    const TargetInstrInfo *TII;
115    bool MadeChange;
116  public:
117    static char ID;
118    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
119
120    virtual bool runOnMachineFunction(MachineFunction &MF);
121    virtual const char *getPassName() const { return "If converter"; }
122
123  private:
124    bool ReverseBranchCondition(BBInfo &BBI);
125    bool ValidSimple(BBInfo &TrueBBI) const;
126    bool ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
127                       bool FalseBranch = false) const;
128    bool ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const;
129    void ScanInstructions(BBInfo &BBI);
130    void AnalyzeBlock(MachineBasicBlock *BB);
131    bool FeasibilityAnalysis(BBInfo &BBI, std::vector<MachineOperand> &Cond,
132                             bool isTriangle = false, bool RevBranch = false);
133    bool AttemptRestructuring(BBInfo &BBI);
134    bool AnalyzeBlocks(MachineFunction &MF,
135                       std::vector<BBInfo*> &Candidates);
136    void ReTryPreds(MachineBasicBlock *BB);
137    void RemoveExtraEdges(BBInfo &BBI);
138    bool IfConvertSimple(BBInfo &BBI);
139    bool IfConvertTriangle(BBInfo &BBI);
140    bool IfConvertDiamond(BBInfo &BBI);
141    void PredicateBlock(BBInfo &BBI,
142                        std::vector<MachineOperand> &Cond,
143                        bool IgnoreTerm = false);
144    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
145
146    // blockAlwaysFallThrough - Block ends without a terminator.
147    bool blockAlwaysFallThrough(BBInfo &BBI) const {
148      return BBI.IsAnalyzable && BBI.TrueBB == NULL;
149    }
150
151    // IfcvtCandidateCmp - Used to sort if-conversion candidates.
152    static bool IfcvtCandidateCmp(BBInfo* C1, BBInfo* C2){
153      // Favor diamond over triangle, etc.
154      return (unsigned)C1->Kind < (unsigned)C2->Kind;
155    }
156  };
157  char IfConverter::ID = 0;
158}
159
160FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
161
162bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
163  TLI = MF.getTarget().getTargetLowering();
164  TII = MF.getTarget().getInstrInfo();
165  if (!TII) return false;
166
167  static int FnNum = -1;
168  DOUT << "\nIfcvt: function (" << ++FnNum <<  ") \'"
169       << MF.getFunction()->getName() << "\'";
170
171  if (FnNum < IfCvtFnStart || (IfCvtFnStop != -1 && FnNum > IfCvtFnStop)) {
172    DOUT << " skipped\n";
173    return false;
174  }
175  DOUT << "\n";
176
177  MF.RenumberBlocks();
178  BBAnalysis.resize(MF.getNumBlockIDs());
179
180  // Look for root nodes, i.e. blocks without successors.
181  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
182    if (I->succ_size() == 0)
183      Roots.push_back(I);
184
185  std::vector<BBInfo*> Candidates;
186  MadeChange = false;
187  while (IfCvtLimit == -1 || (int)NumIfConvBBs < IfCvtLimit) {
188    // Do an intial analysis for each basic block and finding all the potential
189    // candidates to perform if-convesion.
190    bool Change = AnalyzeBlocks(MF, Candidates);
191    while (!Candidates.empty()) {
192      BBInfo &BBI = *Candidates.back();
193      Candidates.pop_back();
194
195      bool RetVal = false;
196      switch (BBI.Kind) {
197      default: assert(false && "Unexpected!");
198        break;
199      case ICReAnalyze:
200        // One or more of 'children' have been modified, abort!
201      case ICDead:
202        // Block has been already been if-converted, abort!
203        break;
204      case ICSimple:
205      case ICSimpleFalse: {
206        bool isFalse = BBI.Kind == ICSimpleFalse;
207        if ((isFalse && DisableSimpleF) || (!isFalse && DisableSimple)) break;
208        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
209             << "): BB#" << BBI.BB->getNumber() << " ("
210             << ((BBI.Kind == ICSimpleFalse)
211                 ? BBI.FalseBB->getNumber() : BBI.TrueBB->getNumber()) << ") ";
212        RetVal = IfConvertSimple(BBI);
213        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
214        if (RetVal)
215          if (isFalse) NumSimpleFalse++;
216          else         NumSimple++;
217       break;
218      }
219      case ICTriangle:
220      case ICTriangleFalse:
221      case ICTriangleFRev:
222        bool isFalse = BBI.Kind == ICTriangleFalse;
223        bool isFalseRev = BBI.Kind == ICTriangleFRev;
224        if (DisableTriangle && !isFalse && !isFalseRev) break;
225        if (DisableTriangleF && isFalse) break;
226        if (DisableTriangleFR && isFalseRev) break;
227        DOUT << "Ifcvt (Triangle";
228        if (isFalse)
229          DOUT << " false";
230        if (isFalseRev)
231          DOUT << " false/rev";
232        DOUT << "): BB#" << BBI.BB->getNumber() << " (T:"
233             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber()
234             << ") ";
235        RetVal = IfConvertTriangle(BBI);
236        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
237        if (RetVal) {
238          if (isFalseRev)   NumTriangleFRev++;
239          else if (isFalse) NumTriangleFalse++;
240          else              NumTriangle++;
241        }
242        break;
243      case ICDiamond:
244        if (DisableDiamond) break;
245        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
246             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber();
247        if (BBI.TailBB)
248          DOUT << "," << BBI.TailBB->getNumber() ;
249        DOUT << ") ";
250        RetVal = IfConvertDiamond(BBI);
251        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
252        if (RetVal) NumDiamonds++;
253        break;
254      }
255      Change |= RetVal;
256
257      if (IfCvtLimit != -1 && (int)NumIfConvBBs >= IfCvtLimit)
258        break;
259    }
260
261    if (!Change)
262      break;
263    MadeChange |= Change;
264  }
265
266  Roots.clear();
267  BBAnalysis.clear();
268
269  return MadeChange;
270}
271
272static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
273                                         MachineBasicBlock *TrueBB) {
274  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
275         E = BB->succ_end(); SI != E; ++SI) {
276    MachineBasicBlock *SuccBB = *SI;
277    if (SuccBB != TrueBB)
278      return SuccBB;
279  }
280  return NULL;
281}
282
283bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
284  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
285    TII->RemoveBranch(*BBI.BB);
286    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
287    std::swap(BBI.TrueBB, BBI.FalseBB);
288    return true;
289  }
290  return false;
291}
292
293/// getNextBlock - Returns the next block in the function blocks ordering. If
294/// it is the end, returns NULL.
295static inline MachineBasicBlock *getNextBlock(MachineBasicBlock *BB) {
296  MachineFunction::iterator I = BB;
297  MachineFunction::iterator E = BB->getParent()->end();
298  if (++I == E)
299    return NULL;
300  return I;
301}
302
303/// ValidSimple - Returns true if the 'true' block (along with its
304/// predecessor) forms a valid simple shape for ifcvt.
305bool IfConverter::ValidSimple(BBInfo &TrueBBI) const {
306  return !blockAlwaysFallThrough(TrueBBI) &&
307    TrueBBI.BrCond.size() == 0 && TrueBBI.BB->pred_size() == 1;
308}
309
310/// ValidTriangle - Returns true if the 'true' and 'false' blocks (along
311/// with their common predecessor) forms a valid triangle shape for ifcvt.
312bool IfConverter::ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
313                                bool FalseBranch) const {
314  if (TrueBBI.BB->pred_size() != 1)
315    return false;
316
317  MachineBasicBlock *TExit = FalseBranch ? TrueBBI.FalseBB : TrueBBI.TrueBB;
318  if (!TExit && blockAlwaysFallThrough(TrueBBI)) {
319    MachineFunction::iterator I = TrueBBI.BB;
320    if (++I == TrueBBI.BB->getParent()->end())
321      return false;
322    TExit = I;
323  }
324  return TExit && TExit == FalseBBI.BB;
325}
326
327/// ValidDiamond - Returns true if the 'true' and 'false' blocks (along
328/// with their common predecessor) forms a valid diamond shape for ifcvt.
329bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const {
330  MachineBasicBlock *TT = TrueBBI.TrueBB;
331  MachineBasicBlock *FT = FalseBBI.TrueBB;
332
333  if (!TT && blockAlwaysFallThrough(TrueBBI))
334    TT = getNextBlock(TrueBBI.BB);
335  if (!FT && blockAlwaysFallThrough(FalseBBI))
336    FT = getNextBlock(FalseBBI.BB);
337  if (TT != FT)
338    return false;
339  if (TT == NULL && (TrueBBI.IsAnalyzable || FalseBBI.IsAnalyzable))
340    return false;
341  // FIXME: Allow false block to have an early exit?
342  return (TrueBBI.BB->pred_size() == 1 &&
343          FalseBBI.BB->pred_size() == 1 &&
344          !TrueBBI.FalseBB && !FalseBBI.FalseBB);
345}
346
347/// AnalyzeBlock - Analyze the structure of the sub-CFG starting from
348/// the specified block. Record its successors and whether it looks like an
349/// if-conversion candidate.
350void IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
351  BBInfo &BBI = BBAnalysis[BB->getNumber()];
352
353  if (BBI.Kind == ICReAnalyze) {
354    BBI.BrCond.clear();
355    BBI.TrueBB = BBI.FalseBB = NULL;
356  } else {
357    if (BBI.Kind != ICNotAnalyzed)
358      return;  // Already analyzed.
359    BBI.BB = BB;
360    BBI.NonPredSize = std::distance(BB->begin(), BB->end());
361  }
362
363  // Look for 'root' of a simple (non-nested) triangle or diamond.
364  BBI.Kind = ICNotClassfied;
365  BBI.IsAnalyzable =
366    !TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
367  BBI.HasFallThrough = BBI.IsAnalyzable && BBI.FalseBB == NULL;
368  // Unanalyable or ends with fallthrough or unconditional branch.
369  if (!BBI.IsAnalyzable || BBI.BrCond.size() == 0)
370    return;
371  // Do not ifcvt if either path is a back edge to the entry block.
372  if (BBI.TrueBB == BB || BBI.FalseBB == BB)
373    return;
374
375  AnalyzeBlock(BBI.TrueBB);
376  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
377
378  // No false branch. This BB must end with a conditional branch and a
379  // fallthrough.
380  if (!BBI.FalseBB)
381    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
382  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
383
384  AnalyzeBlock(BBI.FalseBB);
385  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
386
387  // If both paths are dead, then forget about it.
388  if (TrueBBI.Kind == ICDead && FalseBBI.Kind == ICDead) {
389    BBI.Kind = ICDead;
390    return;
391  }
392
393  // Look for more opportunities to if-convert a triangle. Try to restructure
394  // the CFG to form a triangle with the 'false' path.
395  std::vector<MachineOperand> RevCond(BBI.BrCond);
396  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
397
398  if (CanRevCond && ValidDiamond(TrueBBI, FalseBBI) &&
399      !(TrueBBI.ModifyPredicate && FalseBBI.ModifyPredicate) &&
400      FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
401      FeasibilityAnalysis(FalseBBI, RevCond)) {
402    // Diamond:
403    //   EBB
404    //   / \_
405    //  |   |
406    // TBB FBB
407    //   \ /
408    //  TailBB
409    // Note TailBB can be empty.
410    BBI.Kind = ICDiamond;
411    TrueBBI.Kind = FalseBBI.Kind = ICChild;
412    BBI.TailBB = TrueBBI.TrueBB;
413  } else {
414    // FIXME: Consider duplicating if BB is small.
415    if (ValidTriangle(TrueBBI, FalseBBI) &&
416        FeasibilityAnalysis(TrueBBI, BBI.BrCond, true)) {
417      // Triangle:
418      //   EBB
419      //   | \_
420      //   |  |
421      //   | TBB
422      //   |  /
423      //   FBB
424      BBI.Kind = ICTriangle;
425      TrueBBI.Kind = ICChild;
426    } else if (ValidSimple(TrueBBI) &&
427               FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
428      // Simple (split, no rejoin):
429      //   EBB
430      //   | \_
431      //   |  |
432      //   | TBB---> exit
433      //   |
434      //   FBB
435      BBI.Kind = ICSimple;
436      TrueBBI.Kind = ICChild;
437    } else if (CanRevCond) {
438      // Try the other path...
439      if (ValidTriangle(FalseBBI, TrueBBI) &&
440          FeasibilityAnalysis(FalseBBI, RevCond, true)) {
441        BBI.Kind = ICTriangleFalse;
442        FalseBBI.Kind = ICChild;
443      } else if (ValidTriangle(FalseBBI, TrueBBI, true) &&
444                 FeasibilityAnalysis(FalseBBI, RevCond, true, true)) {
445        BBI.Kind = ICTriangleFRev;
446        FalseBBI.Kind = ICChild;
447      } else if (ValidSimple(FalseBBI) &&
448                 FeasibilityAnalysis(FalseBBI, RevCond)) {
449        BBI.Kind = ICSimpleFalse;
450        FalseBBI.Kind = ICChild;
451      }
452    }
453  }
454  return;
455}
456
457/// FeasibilityAnalysis - Determine if the block is predicable. In most
458/// cases, that means all the instructions in the block has M_PREDICABLE flag.
459/// Also checks if the block contains any instruction which can clobber a
460/// predicate (e.g. condition code register). If so, the block is not
461/// predicable unless it's the last instruction.
462bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
463                                      std::vector<MachineOperand> &Pred,
464                                      bool isTriangle, bool RevBranch) {
465  // If the block is dead, or it is going to be the entry block of a sub-CFG
466  // that will be if-converted, then it cannot be predicated.
467  if (BBI.Kind != ICNotAnalyzed &&
468      BBI.Kind != ICNotClassfied &&
469      BBI.Kind != ICChild)
470    return false;
471
472  // Check predication threshold.
473  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
474    return false;
475
476  // If it is already predicated, check if its predicate subsumes the new
477  // predicate.
478  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Pred))
479    return false;
480
481  bool SeenPredMod = false;
482  bool SeenCondBr = false;
483  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
484       I != E; ++I) {
485    const TargetInstrDescriptor *TID = I->getInstrDescriptor();
486    if (SeenPredMod) {
487      // Predicate modification instruction should end the block (except for
488      // already predicated instructions and end of block branches).
489      if (!TII->isPredicated(I)) {
490        // This is the 'true' block of a triangle, i.e. its 'true' block is
491        // the same as the 'false' block of the entry. So false positive
492        // is ok.
493        if (isTriangle && !SeenCondBr && BBI.IsAnalyzable &&
494            (TID->Flags & M_BRANCH_FLAG) != 0 &&
495            (TID->Flags & M_BARRIER_FLAG) == 0) {
496          // This is the first conditional branch, test predicate subsumsion.
497          std::vector<MachineOperand> RevPred(Pred);
498          std::vector<MachineOperand> Cond(BBI.BrCond);
499          if (RevBranch) {
500            if (TII->ReverseBranchCondition(Cond))
501              return false;
502          }
503          if (TII->ReverseBranchCondition(RevPred) ||
504              !TII->SubsumesPredicate(Cond, RevPred))
505            return false;
506          SeenCondBr = true;
507          continue;  // Conditional branches is not predicable.
508        }
509        return false;
510      }
511    }
512
513    if (TID->Flags & M_CLOBBERS_PRED) {
514      BBI.ModifyPredicate = true;
515      SeenPredMod = true;
516    }
517
518    if (!I->isPredicable())
519      return false;
520  }
521
522  return true;
523}
524
525/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
526/// expose more if-conversion opportunities. e.g.
527///
528///                cmp
529///                b le BB1
530///                /  \____
531///               /        |
532///             cmp        |
533///             b eq BB1   |
534///              /  \____  |
535///             /        \ |
536///                      BB1
537///  ==>
538///
539///                cmp
540///                b eq BB1
541///                /  \____
542///               /        |
543///             cmp        |
544///             b le BB1   |
545///              /  \____  |
546///             /        \ |
547///                      BB1
548bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
549  return false;
550}
551
552/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
553/// candidates. It returns true if any CFG restructuring is done to expose more
554/// if-conversion opportunities.
555bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
556                                std::vector<BBInfo*> &Candidates) {
557  bool Change = false;
558  std::set<MachineBasicBlock*> Visited;
559  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
560    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
561           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
562      MachineBasicBlock *BB = *I;
563      AnalyzeBlock(BB);
564      BBInfo &BBI = BBAnalysis[BB->getNumber()];
565      switch (BBI.Kind) {
566        case ICSimple:
567        case ICSimpleFalse:
568        case ICTriangle:
569        case ICDiamond:
570          Candidates.push_back(&BBI);
571          break;
572        default:
573          Change |= AttemptRestructuring(BBI);
574          break;
575      }
576    }
577  }
578
579  // Sort to favor more complex ifcvt scheme.
580  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
581
582  return Change;
583}
584
585/// canFallThroughTo - Returns true either if ToBB is the next block after BB or
586/// that all the intervening blocks are empty (given BB can fall through to its
587/// next block).
588static bool canFallThroughTo(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
589  MachineFunction::iterator I = BB;
590  MachineFunction::iterator TI = ToBB;
591  MachineFunction::iterator E = BB->getParent()->end();
592  while (++I != TI)
593    if (I == E || !I->empty())
594      return false;
595  return true;
596}
597
598/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
599/// to determine if it can be if-converted.
600void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
601  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
602         E = BB->pred_end(); PI != E; ++PI) {
603    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
604    if (PBBI.Kind == ICNotClassfied)
605      PBBI.Kind = ICReAnalyze;
606  }
607}
608
609/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
610///
611static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
612                               const TargetInstrInfo *TII) {
613  std::vector<MachineOperand> NoCond;
614  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
615}
616
617/// RemoveExtraEdges - Remove true / false edges if either / both are no longer
618/// successors.
619void IfConverter::RemoveExtraEdges(BBInfo &BBI) {
620  MachineBasicBlock *TBB = NULL, *FBB = NULL;
621  std::vector<MachineOperand> Cond;
622  bool isAnalyzable = !TII->AnalyzeBranch(*BBI.BB, TBB, FBB, Cond);
623  bool CanFallthrough = isAnalyzable && (TBB == NULL || FBB == NULL);
624  if (BBI.TrueBB && BBI.BB->isSuccessor(BBI.TrueBB))
625    if (!(BBI.TrueBB == TBB || BBI.TrueBB == FBB ||
626          (CanFallthrough && getNextBlock(BBI.BB) == BBI.TrueBB)))
627      BBI.BB->removeSuccessor(BBI.TrueBB);
628  if (BBI.FalseBB && BBI.BB->isSuccessor(BBI.FalseBB))
629    if (!(BBI.FalseBB == TBB || BBI.FalseBB == FBB ||
630          (CanFallthrough && getNextBlock(BBI.BB) == BBI.FalseBB)))
631      BBI.BB->removeSuccessor(BBI.FalseBB);
632}
633
634/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
635///
636bool IfConverter::IfConvertSimple(BBInfo &BBI) {
637  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
638  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
639  BBInfo *CvtBBI = &TrueBBI;
640  BBInfo *NextBBI = &FalseBBI;
641
642  std::vector<MachineOperand> Cond(BBI.BrCond);
643  if (BBI.Kind == ICSimpleFalse) {
644    std::swap(CvtBBI, NextBBI);
645    TII->ReverseBranchCondition(Cond);
646  }
647
648  PredicateBlock(*CvtBBI, Cond);
649
650  // Merge converted block into entry block.
651  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
652  MergeBlocks(BBI, *CvtBBI);
653
654  bool IterIfcvt = true;
655  if (!canFallThroughTo(BBI.BB, NextBBI->BB)) {
656    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
657    BBI.HasFallThrough = false;
658    // Now ifcvt'd block will look like this:
659    // BB:
660    // ...
661    // t, f = cmp
662    // if t op
663    // b BBf
664    //
665    // We cannot further ifcvt this block because the unconditional branch
666    // will have to be predicated on the new condition, that will not be
667    // available if cmp executes.
668    IterIfcvt = false;
669  }
670
671  RemoveExtraEdges(BBI);
672
673  // Update block info. BB can be iteratively if-converted.
674  if (IterIfcvt)
675    BBI.Kind = ICReAnalyze;
676  else
677    BBI.Kind = ICDead;
678  ReTryPreds(BBI.BB);
679  CvtBBI->Kind = ICDead;
680
681  // FIXME: Must maintain LiveIns.
682  return true;
683}
684
685/// IfConvertTriangle - If convert a triangle sub-CFG.
686///
687bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
688  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
689  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
690  BBInfo *CvtBBI = &TrueBBI;
691  BBInfo *NextBBI = &FalseBBI;
692
693  std::vector<MachineOperand> Cond(BBI.BrCond);
694  if (BBI.Kind == ICTriangleFalse || BBI.Kind == ICTriangleFRev) {
695    if (BBI.Kind == ICTriangleFRev) {
696      ReverseBranchCondition(*NextBBI);
697      // False BB has been changed, modify its predecessors (except for this
698      // one) so they don't get ifcvt'ed based on bad intel.
699      for (MachineBasicBlock::pred_iterator PI = NextBBI->BB->pred_begin(),
700             E = NextBBI->BB->pred_end(); PI != E; ++PI) {
701        MachineBasicBlock *PBB = *PI;
702        if (PBB == BBI.BB)
703          continue;
704        BBInfo &PBBI = BBAnalysis[PBB->getNumber()];
705        PBBI.Kind = ICReAnalyze;
706      }
707    }
708    std::swap(CvtBBI, NextBBI);
709    TII->ReverseBranchCondition(Cond);
710  }
711
712  // Predicate the 'true' block after removing its branch.
713  TrueBBI.NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
714  PredicateBlock(*CvtBBI, Cond);
715
716  // If 'true' block has a 'false' successor, add an exit branch to it.
717  bool HasEarlyExit = CvtBBI->FalseBB != NULL;
718  if (HasEarlyExit) {
719    std::vector<MachineOperand> RevCond(CvtBBI->BrCond);
720    if (TII->ReverseBranchCondition(RevCond))
721      assert(false && "Unable to reverse branch condition!");
722    TII->InsertBranch(*CvtBBI->BB, CvtBBI->FalseBB, NULL, RevCond);
723  }
724
725  // Now merge the entry of the triangle with the true block.
726  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
727  MergeBlocks(BBI, *CvtBBI);
728
729  // Merge in the 'false' block if the 'false' block has no other
730  // predecessors. Otherwise, add a unconditional branch from to 'false'.
731  bool FalseBBDead = false;
732  bool IterIfcvt = true;
733  bool isFallThrough = canFallThroughTo(BBI.BB, NextBBI->BB);
734  if (!isFallThrough) {
735    // Only merge them if the true block does not fallthrough to the false
736    // block. By not merging them, we make it possible to iteratively
737    // ifcvt the blocks.
738    if (!HasEarlyExit && NextBBI->BB->pred_size() == 1) {
739      MergeBlocks(BBI, *NextBBI);
740      FalseBBDead = true;
741    } else {
742      InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
743      BBI.HasFallThrough = false;
744    }
745    // Mixed predicated and unpredicated code. This cannot be iteratively
746    // predicated.
747    IterIfcvt = false;
748  }
749
750  RemoveExtraEdges(BBI);
751
752  // Update block info. BB can be iteratively if-converted.
753  if (IterIfcvt)
754    BBI.Kind = ICReAnalyze;
755  else
756    BBI.Kind = ICDead;
757  ReTryPreds(BBI.BB);
758  CvtBBI->Kind = ICDead;
759  if (FalseBBDead)
760    NextBBI->Kind = ICDead;
761
762  // FIXME: Must maintain LiveIns.
763  return true;
764}
765
766/// IfConvertDiamond - If convert a diamond sub-CFG.
767///
768bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
769  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
770  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
771
772  SmallVector<MachineInstr*, 2> Dups;
773  if (!BBI.TailBB) {
774    // No common merge block. Check if the terminators (e.g. return) are
775    // the same or predicable.
776    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
777    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
778    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
779      if (TT->isIdenticalTo(FT))
780        Dups.push_back(TT);  // Will erase these later.
781      else if (!TT->isPredicable() && !FT->isPredicable())
782        return false; // Can't if-convert. Abort!
783      ++TT;
784      ++FT;
785    }
786
787    // One of the two pathes have more terminators, make sure they are
788    // all predicable.
789    while (TT != BBI.TrueBB->end()) {
790      if (!TT->isPredicable()) {
791        return false; // Can't if-convert. Abort!
792      }
793      ++TT;
794    }
795    while (FT != BBI.FalseBB->end()) {
796      if (!FT->isPredicable()) {
797        return false; // Can't if-convert. Abort!
798      }
799      ++FT;
800    }
801  }
802
803  // Remove the duplicated instructions from the 'true' block.
804  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
805    Dups[i]->eraseFromParent();
806    --TrueBBI.NonPredSize;
807  }
808
809  // Merge the 'true' and 'false' blocks by copying the instructions
810  // from the 'false' block to the 'true' block. That is, unless the true
811  // block would clobber the predicate, in that case, do the opposite.
812  BBInfo *BBI1 = &TrueBBI;
813  BBInfo *BBI2 = &FalseBBI;
814  std::vector<MachineOperand> RevCond(BBI.BrCond);
815  TII->ReverseBranchCondition(RevCond);
816  std::vector<MachineOperand> *Cond1 = &BBI.BrCond;
817  std::vector<MachineOperand> *Cond2 = &RevCond;
818  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
819  // Either the block fallthrough to another block or it ends with a
820  // return. If it's the former, add a branch to its successor.
821  bool NeedBr1 = !BBI1->TrueBB && BBI1->BB->succ_size();
822  bool NeedBr2 = !BBI2->TrueBB && BBI2->BB->succ_size();
823
824  if ((TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate) ||
825      (!TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate &&
826       NeedBr1 && !NeedBr2)) {
827    std::swap(BBI1, BBI2);
828    std::swap(Cond1, Cond2);
829    std::swap(NeedBr1, NeedBr2);
830  }
831
832  // Predicate the 'true' block after removing its branch.
833  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
834  PredicateBlock(*BBI1, *Cond1);
835
836  // Add an early exit branch if needed.
837  if (NeedBr1)
838    TII->InsertBranch(*BBI1->BB, *BBI1->BB->succ_begin(), NULL, *Cond1);
839
840  // Predicate the 'false' block.
841  PredicateBlock(*BBI2, *Cond2, true);
842
843  // Add an unconditional branch from 'false' to to 'false' successor if it
844  // will not be the fallthrough block.
845  if (NeedBr2 && !NeedBr1) {
846    // If BBI2 isn't going to be merged in, then the existing fallthrough
847    // or branch is fine.
848    if (!canFallThroughTo(BBI.BB, *BBI2->BB->succ_begin())) {
849      InsertUncondBranch(BBI2->BB, *BBI2->BB->succ_begin(), TII);
850      BBI2->HasFallThrough = false;
851    }
852  }
853
854  // Keep them as two separate blocks if there is an early exit.
855  if (!NeedBr1)
856    MergeBlocks(*BBI1, *BBI2);
857
858  // Remove the conditional branch from entry to the blocks.
859  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
860
861  // Merge the combined block into the entry of the diamond.
862  MergeBlocks(BBI, *BBI1);
863
864  // 'True' and 'false' aren't combined, see if we need to add a unconditional
865  // branch to the 'false' block.
866  if (NeedBr1 && !canFallThroughTo(BBI.BB, BBI2->BB)) {
867    InsertUncondBranch(BBI.BB, BBI2->BB, TII);
868    BBI1->HasFallThrough = false;
869  }
870
871  // If the if-converted block fallthrough or unconditionally branch into the
872  // tail block, and the tail block does not have other predecessors, then
873  // fold the tail block in as well.
874  BBInfo *CvtBBI = NeedBr1 ? BBI2 : &BBI;
875  if (BBI.TailBB &&
876      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
877    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
878    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
879    MergeBlocks(*CvtBBI, TailBBI);
880    TailBBI.Kind = ICDead;
881  }
882
883  RemoveExtraEdges(BBI);
884
885  // Update block info.
886  BBI.Kind = ICDead;
887  TrueBBI.Kind = ICDead;
888  FalseBBI.Kind = ICDead;
889
890  // FIXME: Must maintain LiveIns.
891  return true;
892}
893
894/// PredicateBlock - Predicate every instruction in the block with the specified
895/// condition. If IgnoreTerm is true, skip over all terminator instructions.
896void IfConverter::PredicateBlock(BBInfo &BBI,
897                                 std::vector<MachineOperand> &Cond,
898                                 bool IgnoreTerm) {
899  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
900       I != E; ++I) {
901    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
902      continue;
903    if (TII->isPredicated(I))
904      continue;
905    if (!TII->PredicateInstruction(I, Cond)) {
906      cerr << "Unable to predicate " << *I << "!\n";
907      abort();
908    }
909  }
910
911  BBI.NonPredSize = 0;
912  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
913
914  NumIfConvBBs++;
915}
916
917/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
918///
919void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
920  ToBBI.BB->splice(ToBBI.BB->end(),
921                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
922
923  // Redirect all branches to FromBB to ToBB.
924  std::vector<MachineBasicBlock *> Preds(FromBBI.BB->pred_begin(),
925                                         FromBBI.BB->pred_end());
926  for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
927    MachineBasicBlock *Pred = Preds[i];
928    if (Pred == ToBBI.BB)
929      continue;
930    Pred->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
931  }
932
933  std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
934                                         FromBBI.BB->succ_end());
935  MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
936  MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : NULL;
937
938  for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
939    MachineBasicBlock *Succ = Succs[i];
940    // Fallthrough edge can't be transferred.
941    if (Succ == FallThrough)
942      continue;
943    FromBBI.BB->removeSuccessor(Succ);
944    if (!ToBBI.BB->isSuccessor(Succ))
945      ToBBI.BB->addSuccessor(Succ);
946  }
947
948  // Now FromBBI always fall through to the next block!
949  if (NBB)
950    FromBBI.BB->addSuccessor(NBB);
951
952  ToBBI.NonPredSize += FromBBI.NonPredSize;
953  FromBBI.NonPredSize = 0;
954
955  ToBBI.ModifyPredicate |= FromBBI.ModifyPredicate;
956  ToBBI.HasFallThrough = FromBBI.HasFallThrough;
957
958  std::copy(FromBBI.Predicate.begin(), FromBBI.Predicate.end(),
959            std::back_inserter(ToBBI.Predicate));
960  FromBBI.Predicate.clear();
961}
962