IfConversion.cpp revision 9618bcaf7d3e6fd4a26fab47efb0bd2409b69e0f 
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      ICNotClassfied,  // BB data valid, but not classified.
59      ICSimple,        // BB is entry of an one split, no rejoin sub-CFG.
60      ICSimpleFalse,   // Same as ICSimple, but on the false path.
61      ICTriangle,      // BB is entry of a triangle sub-CFG.
62      ICTriangleFalse, // Same as ICTriangle, but on the false path.
63      ICTriangleFRev,  // Same as ICTriangleFalse, but false path rev condition.
64      ICDiamond        // BB is entry of a diamond sub-CFG.
65    };
66
67    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
68    /// if-conversion feasibility analysis. This includes results from
69    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
70    /// classification, and common tail block of its successors (if it's a
71    /// diamond shape), its size, whether it's predicable, and whether any
72    /// instruction can clobber the 'would-be' predicate.
73    ///
74    /// Kind            - Type of block. See BBICKind.
75    /// IsDone          - True if BB is not to be considered for ifcvt.
76    /// IsBeingAnalyzed - True if BB is currently being analyzed.
77    /// IsAnalyzed      - True if BB has been analyzed (info is still valid).
78    /// IsEnqueued      - True if BB has been enqueued to be ifcvt'ed.
79    /// IsBrAnalyzable  - True if AnalyzeBranch() returns false.
80    /// HasFallThrough  - True if BB may fallthrough to the following BB.
81    /// IsUnpredicable  - True if BB is known to be unpredicable.
82    /// ClobbersPredicate- True if BB would modify the predicate (e.g. has
83    ///                   cmp, call, etc.)
84    /// NonPredSize     - Number of non-predicated instructions.
85    /// BB              - Corresponding MachineBasicBlock.
86    /// TrueBB / FalseBB- See AnalyzeBranch().
87    /// BrCond          - Conditions for end of block conditional branches.
88    /// Predicate       - Predicate used in the BB.
89    struct BBInfo {
90      BBICKind Kind;
91      bool IsDone          : 1;
92      bool IsBeingAnalyzed : 1;
93      bool IsAnalyzed      : 1;
94      bool IsEnqueued      : 1;
95      bool IsBrAnalyzable  : 1;
96      bool HasFallThrough  : 1;
97      bool IsUnpredicable  : 1;
98      bool ClobbersPred    : 1;
99      unsigned NonPredSize;
100      MachineBasicBlock *BB;
101      MachineBasicBlock *TrueBB;
102      MachineBasicBlock *FalseBB;
103      MachineBasicBlock *TailBB;
104      std::vector<MachineOperand> BrCond;
105      std::vector<MachineOperand> Predicate;
106      BBInfo() : Kind(ICNotClassfied), IsDone(false), IsBeingAnalyzed(false),
107                 IsAnalyzed(false), IsEnqueued(false), IsBrAnalyzable(false),
108                 HasFallThrough(false), IsUnpredicable(false),
109                 ClobbersPred(false), NonPredSize(0),
110                 BB(0), TrueBB(0), FalseBB(0), TailBB(0) {}
111    };
112
113    /// Roots - Basic blocks that do not have successors. These are the starting
114    /// points of Graph traversal.
115    std::vector<MachineBasicBlock*> Roots;
116
117    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
118    /// basic block number.
119    std::vector<BBInfo> BBAnalysis;
120
121    const TargetLowering *TLI;
122    const TargetInstrInfo *TII;
123    bool MadeChange;
124  public:
125    static char ID;
126    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
127
128    virtual bool runOnMachineFunction(MachineFunction &MF);
129    virtual const char *getPassName() const { return "If converter"; }
130
131  private:
132    bool ReverseBranchCondition(BBInfo &BBI);
133    bool ValidSimple(BBInfo &TrueBBI) const;
134    bool ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
135                       bool FalseBranch = false) const;
136    bool ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const;
137    void ScanInstructions(BBInfo &BBI);
138    BBInfo &AnalyzeBlock(MachineBasicBlock *BB);
139    bool FeasibilityAnalysis(BBInfo &BBI, std::vector<MachineOperand> &Cond,
140                             bool isTriangle = false, bool RevBranch = false);
141    bool AttemptRestructuring(BBInfo &BBI);
142    bool AnalyzeBlocks(MachineFunction &MF,
143                       std::vector<BBInfo*> &Candidates);
144    void ReTryPreds(MachineBasicBlock *BB);
145    void RemoveExtraEdges(BBInfo &BBI);
146    bool IfConvertSimple(BBInfo &BBI);
147    bool IfConvertTriangle(BBInfo &BBI);
148    bool IfConvertDiamond(BBInfo &BBI);
149    void PredicateBlock(BBInfo &BBI,
150                        std::vector<MachineOperand> &Cond,
151                        bool IgnoreTerm = false);
152    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
153
154    // blockAlwaysFallThrough - Block ends without a terminator.
155    bool blockAlwaysFallThrough(BBInfo &BBI) const {
156      return BBI.IsBrAnalyzable && BBI.TrueBB == NULL;
157    }
158
159    // IfcvtCandidateCmp - Used to sort if-conversion candidates.
160    static bool IfcvtCandidateCmp(BBInfo* C1, BBInfo* C2){
161      // Favor diamond over triangle, etc.
162      return (unsigned)C1->Kind < (unsigned)C2->Kind;
163    }
164  };
165  char IfConverter::ID = 0;
166}
167
168FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
169
170bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
171  TLI = MF.getTarget().getTargetLowering();
172  TII = MF.getTarget().getInstrInfo();
173  if (!TII) return false;
174
175  static int FnNum = -1;
176  DOUT << "\nIfcvt: function (" << ++FnNum <<  ") \'"
177       << MF.getFunction()->getName() << "\'";
178
179  if (FnNum < IfCvtFnStart || (IfCvtFnStop != -1 && FnNum > IfCvtFnStop)) {
180    DOUT << " skipped\n";
181    return false;
182  }
183  DOUT << "\n";
184
185  MF.RenumberBlocks();
186  BBAnalysis.resize(MF.getNumBlockIDs());
187
188  // Look for root nodes, i.e. blocks without successors.
189  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
190    if (I->succ_size() == 0)
191      Roots.push_back(I);
192
193  std::vector<BBInfo*> Candidates;
194  MadeChange = false;
195  while (IfCvtLimit == -1 || (int)NumIfConvBBs < IfCvtLimit) {
196    // Do an intial analysis for each basic block and finding all the potential
197    // candidates to perform if-convesion.
198    bool Change = AnalyzeBlocks(MF, Candidates);
199    while (!Candidates.empty()) {
200      BBInfo &BBI = *Candidates.back();
201      Candidates.pop_back();
202
203      // If the block has been evicted out of the queue or it has already been
204      // marked dead (due to it being predicated), then skip it.
205      if (!BBI.IsEnqueued || BBI.IsDone)
206        continue;
207
208      bool RetVal = false;
209      switch (BBI.Kind) {
210      default: assert(false && "Unexpected!");
211        break;
212      case ICSimple:
213      case ICSimpleFalse: {
214        bool isFalse = BBI.Kind == ICSimpleFalse;
215        if ((isFalse && DisableSimpleF) || (!isFalse && DisableSimple)) break;
216        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
217             << "): BB#" << BBI.BB->getNumber() << " ("
218             << ((BBI.Kind == ICSimpleFalse)
219                 ? BBI.FalseBB->getNumber() : BBI.TrueBB->getNumber()) << ") ";
220        RetVal = IfConvertSimple(BBI);
221        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
222        if (RetVal)
223          if (isFalse) NumSimpleFalse++;
224          else         NumSimple++;
225       break;
226      }
227      case ICTriangle:
228      case ICTriangleFalse:
229      case ICTriangleFRev: {
230        bool isFalse = BBI.Kind == ICTriangleFalse;
231        bool isFalseRev = BBI.Kind == ICTriangleFRev;
232        if (DisableTriangle && !isFalse && !isFalseRev) break;
233        if (DisableTriangleF && isFalse) break;
234        if (DisableTriangleFR && isFalseRev) break;
235        DOUT << "Ifcvt (Triangle";
236        if (isFalse)
237          DOUT << " false";
238        if (isFalseRev)
239          DOUT << " false/rev";
240        DOUT << "): BB#" << BBI.BB->getNumber() << " (T:"
241             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber()
242             << ") ";
243        RetVal = IfConvertTriangle(BBI);
244        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
245        if (RetVal) {
246          if (isFalseRev)   NumTriangleFRev++;
247          else if (isFalse) NumTriangleFalse++;
248          else              NumTriangle++;
249        }
250        break;
251      }
252      case ICDiamond:
253        if (DisableDiamond) break;
254        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
255             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber();
256        if (BBI.TailBB)
257          DOUT << "," << BBI.TailBB->getNumber() ;
258        DOUT << ") ";
259        RetVal = IfConvertDiamond(BBI);
260        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
261        if (RetVal) NumDiamonds++;
262        break;
263      }
264      Change |= RetVal;
265
266      if (IfCvtLimit != -1 && (int)NumIfConvBBs >= IfCvtLimit)
267        break;
268    }
269
270    if (!Change)
271      break;
272    MadeChange |= Change;
273  }
274
275  Roots.clear();
276  BBAnalysis.clear();
277
278  return MadeChange;
279}
280
281static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
282                                         MachineBasicBlock *TrueBB) {
283  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
284         E = BB->succ_end(); SI != E; ++SI) {
285    MachineBasicBlock *SuccBB = *SI;
286    if (SuccBB != TrueBB)
287      return SuccBB;
288  }
289  return NULL;
290}
291
292bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
293  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
294    TII->RemoveBranch(*BBI.BB);
295    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
296    std::swap(BBI.TrueBB, BBI.FalseBB);
297    return true;
298  }
299  return false;
300}
301
302/// getNextBlock - Returns the next block in the function blocks ordering. If
303/// it is the end, returns NULL.
304static inline MachineBasicBlock *getNextBlock(MachineBasicBlock *BB) {
305  MachineFunction::iterator I = BB;
306  MachineFunction::iterator E = BB->getParent()->end();
307  if (++I == E)
308    return NULL;
309  return I;
310}
311
312/// ValidSimple - Returns true if the 'true' block (along with its
313/// predecessor) forms a valid simple shape for ifcvt.
314bool IfConverter::ValidSimple(BBInfo &TrueBBI) const {
315  if (TrueBBI.IsBeingAnalyzed)
316    return false;
317
318  return !blockAlwaysFallThrough(TrueBBI) &&
319    TrueBBI.BrCond.size() == 0 && TrueBBI.BB->pred_size() == 1;
320}
321
322/// ValidTriangle - Returns true if the 'true' and 'false' blocks (along
323/// with their common predecessor) forms a valid triangle shape for ifcvt.
324bool IfConverter::ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
325                                bool FalseBranch) const {
326  if (TrueBBI.IsBeingAnalyzed)
327    return false;
328
329  if (TrueBBI.BB->pred_size() != 1)
330    return false;
331
332  MachineBasicBlock *TExit = FalseBranch ? TrueBBI.FalseBB : TrueBBI.TrueBB;
333  if (!TExit && blockAlwaysFallThrough(TrueBBI)) {
334    MachineFunction::iterator I = TrueBBI.BB;
335    if (++I == TrueBBI.BB->getParent()->end())
336      return false;
337    TExit = I;
338  }
339  return TExit && TExit == FalseBBI.BB;
340}
341
342/// ValidDiamond - Returns true if the 'true' and 'false' blocks (along
343/// with their common predecessor) forms a valid diamond shape for ifcvt.
344bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const {
345  if (TrueBBI.IsBeingAnalyzed || FalseBBI.IsBeingAnalyzed)
346    return false;
347
348  MachineBasicBlock *TT = TrueBBI.TrueBB;
349  MachineBasicBlock *FT = FalseBBI.TrueBB;
350
351  if (!TT && blockAlwaysFallThrough(TrueBBI))
352    TT = getNextBlock(TrueBBI.BB);
353  if (!FT && blockAlwaysFallThrough(FalseBBI))
354    FT = getNextBlock(FalseBBI.BB);
355  if (TT != FT)
356    return false;
357  if (TT == NULL && (TrueBBI.IsBrAnalyzable || FalseBBI.IsBrAnalyzable))
358    return false;
359  // FIXME: Allow false block to have an early exit?
360  return (TrueBBI.BB->pred_size() == 1 &&
361          FalseBBI.BB->pred_size() == 1 &&
362          !TrueBBI.FalseBB && !FalseBBI.FalseBB);
363}
364
365/// ScanInstructions - Scan all the instructions in the block to determine if
366/// the block is predicable. In most cases, that means all the instructions
367/// in the block has M_PREDICABLE flag. Also checks if the block contains any
368/// instruction which can clobber a predicate (e.g. condition code register).
369/// If so, the block is not predicable unless it's the last instruction.
370void IfConverter::ScanInstructions(BBInfo &BBI) {
371  if (BBI.IsDone)
372    return;
373
374  // First analyze the end of BB branches.
375  BBI.TrueBB = BBI.FalseBB;
376  BBI.BrCond.clear();
377  BBI.IsBrAnalyzable =
378    !TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
379  BBI.HasFallThrough = BBI.IsBrAnalyzable && BBI.FalseBB == NULL;
380
381  if (BBI.BrCond.size()) {
382    // No false branch. This BB must end with a conditional branch and a
383    // fallthrough.
384    if (!BBI.FalseBB)
385      BBI.FalseBB = findFalseBlock(BBI.BB, BBI.TrueBB);
386    assert(BBI.FalseBB && "Expected to find the fallthrough block!");
387  }
388
389  // Then scan all the instructions.
390  BBI.NonPredSize = 0;
391  BBI.ClobbersPred = false;
392  bool SeenCondBr = false;
393  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
394       I != E; ++I) {
395    const TargetInstrDescriptor *TID = I->getInstrDescriptor();
396    bool isPredicated = TII->isPredicated(I);
397    bool isCondBr = BBI.IsBrAnalyzable &&
398      (TID->Flags & M_BRANCH_FLAG) != 0 && (TID->Flags & M_BARRIER_FLAG) == 0;
399
400    if (!isPredicated && !isCondBr)
401      BBI.NonPredSize++;
402
403    if (BBI.ClobbersPred && !isPredicated) {
404      // Predicate modification instruction should end the block (except for
405      // already predicated instructions and end of block branches).
406      if (isCondBr) {
407        SeenCondBr = true;
408
409        // Conditional branches is not predicable. But it may be eliminated.
410        continue;
411      }
412
413      // Predicate may have been modified, the subsequent (currently)
414      // unpredocated instructions cannot be correctly predicated.
415      BBI.IsUnpredicable = true;
416      return;
417    }
418
419    if (TID->Flags & M_CLOBBERS_PRED)
420      BBI.ClobbersPred = true;
421
422    if (!I->isPredicable()) {
423      BBI.IsUnpredicable = true;
424      return;
425    }
426  }
427}
428
429/// FeasibilityAnalysis - Determine if the block is a suitable candidate to be
430/// predicated by the specified predicate.
431bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
432                                      std::vector<MachineOperand> &Pred,
433                                      bool isTriangle, bool RevBranch) {
434  // Forget about it if it's unpredicable.
435  if (BBI.IsUnpredicable)
436    return false;
437
438  // If the block is dead, or it is going to be the entry block of a sub-CFG
439  // that will be if-converted, then it cannot be predicated.
440  if (BBI.IsDone || BBI.IsEnqueued)
441    return false;
442
443  // Check predication threshold.
444  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
445    return false;
446
447  // If it is already predicated, check if its predicate subsumes the new
448  // predicate.
449  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Pred))
450    return false;
451
452  if (BBI.BrCond.size()) {
453    if (!isTriangle)
454      return false;
455
456    // Test predicate subsumsion.
457    std::vector<MachineOperand> RevPred(Pred);
458    std::vector<MachineOperand> Cond(BBI.BrCond);
459    if (RevBranch) {
460      if (TII->ReverseBranchCondition(Cond))
461        return false;
462    }
463    if (TII->ReverseBranchCondition(RevPred) ||
464        !TII->SubsumesPredicate(Cond, RevPred))
465      return false;
466  }
467
468  return true;
469}
470
471/// AnalyzeBlock - Analyze the structure of the sub-CFG starting from
472/// the specified block. Record its successors and whether it looks like an
473/// if-conversion candidate.
474IfConverter::BBInfo &IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
475  BBInfo &BBI = BBAnalysis[BB->getNumber()];
476
477  if (BBI.IsAnalyzed || BBI.IsBeingAnalyzed)
478    return BBI;
479
480  BBI.BB = BB;
481  BBI.IsBeingAnalyzed = true;
482  BBI.Kind = ICNotClassfied;
483
484  ScanInstructions(BBI);
485
486  // Unanalyable or ends with fallthrough or unconditional branch.
487  if (!BBI.IsBrAnalyzable || BBI.BrCond.size() == 0) {
488    BBI.IsBeingAnalyzed = false;
489    BBI.IsAnalyzed = true;
490    return BBI;
491  }
492
493  // Do not ifcvt if either path is a back edge to the entry block.
494  if (BBI.TrueBB == BB || BBI.FalseBB == BB) {
495    BBI.IsBeingAnalyzed = false;
496    BBI.IsAnalyzed = true;
497    return BBI;
498  }
499
500  BBInfo &TrueBBI  = AnalyzeBlock(BBI.TrueBB);
501  BBInfo &FalseBBI = AnalyzeBlock(BBI.FalseBB);
502
503  if (TrueBBI.IsDone && FalseBBI.IsDone) {
504    BBI.IsBeingAnalyzed = false;
505    BBI.IsAnalyzed = true;
506    return BBI;
507  }
508
509  std::vector<MachineOperand> RevCond(BBI.BrCond);
510  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
511
512  if (CanRevCond && ValidDiamond(TrueBBI, FalseBBI) &&
513      !(TrueBBI.ClobbersPred && FalseBBI.ClobbersPred) &&
514      FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
515      FeasibilityAnalysis(FalseBBI, RevCond)) {
516    // Diamond:
517    //   EBB
518    //   / \_
519    //  |   |
520    // TBB FBB
521    //   \ /
522    //  TailBB
523    // Note TailBB can be empty.
524    BBI.Kind = ICDiamond;
525    BBI.TailBB = TrueBBI.TrueBB;
526  } else {
527    // FIXME: Consider duplicating if BB is small.
528    if (ValidTriangle(TrueBBI, FalseBBI) &&
529        FeasibilityAnalysis(TrueBBI, BBI.BrCond, true)) {
530      // Triangle:
531      //   EBB
532      //   | \_
533      //   |  |
534      //   | TBB
535      //   |  /
536      //   FBB
537      BBI.Kind = ICTriangle;
538    } else if (ValidSimple(TrueBBI) &&
539               FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
540      // Simple (split, no rejoin):
541      //   EBB
542      //   | \_
543      //   |  |
544      //   | TBB---> exit
545      //   |
546      //   FBB
547      BBI.Kind = ICSimple;
548    } else if (CanRevCond) {
549      // Try the other path...
550      if (ValidTriangle(FalseBBI, TrueBBI) &&
551          FeasibilityAnalysis(FalseBBI, RevCond, true)) {
552        BBI.Kind = ICTriangleFalse;
553      } else if (ValidTriangle(FalseBBI, TrueBBI, true) &&
554                 FeasibilityAnalysis(FalseBBI, RevCond, true, true)) {
555        BBI.Kind = ICTriangleFRev;
556      } else if (ValidSimple(FalseBBI) &&
557                 FeasibilityAnalysis(FalseBBI, RevCond)) {
558        BBI.Kind = ICSimpleFalse;
559      }
560    }
561  }
562
563  BBI.IsBeingAnalyzed = false;
564  BBI.IsAnalyzed = true;
565  return BBI;
566}
567
568/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
569/// expose more if-conversion opportunities. e.g.
570///
571///                cmp
572///                b le BB1
573///                /  \____
574///               /        |
575///             cmp        |
576///             b eq BB1   |
577///              /  \____  |
578///             /        \ |
579///                      BB1
580///  ==>
581///
582///                cmp
583///                b eq BB1
584///                /  \____
585///               /        |
586///             cmp        |
587///             b le BB1   |
588///              /  \____  |
589///             /        \ |
590///                      BB1
591bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
592  return false;
593}
594
595/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
596/// candidates. It returns true if any CFG restructuring is done to expose more
597/// if-conversion opportunities.
598bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
599                                std::vector<BBInfo*> &Candidates) {
600  bool Change = false;
601  std::set<MachineBasicBlock*> Visited;
602  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
603    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
604           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
605      MachineBasicBlock *BB = *I;
606      BBInfo &BBI = AnalyzeBlock(BB);
607      switch (BBI.Kind) {
608        case ICSimple:
609        case ICSimpleFalse:
610        case ICTriangle:
611        case ICDiamond:
612          Candidates.push_back(&BBI);
613          break;
614        default:
615          Change |= AttemptRestructuring(BBI);
616          break;
617      }
618    }
619  }
620
621  // Sort to favor more complex ifcvt scheme.
622  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
623
624  return Change;
625}
626
627/// canFallThroughTo - Returns true either if ToBB is the next block after BB or
628/// that all the intervening blocks are empty (given BB can fall through to its
629/// next block).
630static bool canFallThroughTo(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
631  MachineFunction::iterator I = BB;
632  MachineFunction::iterator TI = ToBB;
633  MachineFunction::iterator E = BB->getParent()->end();
634  while (++I != TI)
635    if (I == E || !I->empty())
636      return false;
637  return true;
638}
639
640/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
641/// to determine if it can be if-converted.
642void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
643  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
644         E = BB->pred_end(); PI != E; ++PI) {
645    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
646    if (!PBBI.IsDone && PBBI.Kind == ICNotClassfied) {
647      assert(PBBI.IsEnqueued && "Unexpected");
648      PBBI.IsAnalyzed = false;
649    }
650  }
651}
652
653/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
654///
655static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
656                               const TargetInstrInfo *TII) {
657  std::vector<MachineOperand> NoCond;
658  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
659}
660
661/// RemoveExtraEdges - Remove true / false edges if either / both are no longer
662/// successors.
663void IfConverter::RemoveExtraEdges(BBInfo &BBI) {
664  MachineBasicBlock *TBB = NULL, *FBB = NULL;
665  std::vector<MachineOperand> Cond;
666  bool isAnalyzable = !TII->AnalyzeBranch(*BBI.BB, TBB, FBB, Cond);
667  bool CanFallthrough = isAnalyzable && (TBB == NULL || FBB == NULL);
668  if (BBI.TrueBB && BBI.BB->isSuccessor(BBI.TrueBB))
669    if (!(BBI.TrueBB == TBB || BBI.TrueBB == FBB ||
670          (CanFallthrough && getNextBlock(BBI.BB) == BBI.TrueBB)))
671      BBI.BB->removeSuccessor(BBI.TrueBB);
672  if (BBI.FalseBB && BBI.BB->isSuccessor(BBI.FalseBB))
673    if (!(BBI.FalseBB == TBB || BBI.FalseBB == FBB ||
674          (CanFallthrough && getNextBlock(BBI.BB) == BBI.FalseBB)))
675      BBI.BB->removeSuccessor(BBI.FalseBB);
676}
677
678/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
679///
680bool IfConverter::IfConvertSimple(BBInfo &BBI) {
681  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
682  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
683  BBInfo *CvtBBI = &TrueBBI;
684  BBInfo *NextBBI = &FalseBBI;
685
686  std::vector<MachineOperand> Cond(BBI.BrCond);
687  if (BBI.Kind == ICSimpleFalse) {
688    std::swap(CvtBBI, NextBBI);
689    TII->ReverseBranchCondition(Cond);
690  }
691
692  PredicateBlock(*CvtBBI, Cond);
693
694  // Merge converted block into entry block.
695  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
696  MergeBlocks(BBI, *CvtBBI);
697
698  bool IterIfcvt = true;
699  if (!canFallThroughTo(BBI.BB, NextBBI->BB)) {
700    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
701    BBI.HasFallThrough = false;
702    // Now ifcvt'd block will look like this:
703    // BB:
704    // ...
705    // t, f = cmp
706    // if t op
707    // b BBf
708    //
709    // We cannot further ifcvt this block because the unconditional branch
710    // will have to be predicated on the new condition, that will not be
711    // available if cmp executes.
712    IterIfcvt = false;
713  }
714
715  RemoveExtraEdges(BBI);
716
717  // Update block info. BB can be iteratively if-converted.
718  if (!IterIfcvt)
719    BBI.IsDone = true;
720  ReTryPreds(BBI.BB);
721  CvtBBI->IsDone = true;
722
723  // FIXME: Must maintain LiveIns.
724  return true;
725}
726
727/// IfConvertTriangle - If convert a triangle sub-CFG.
728///
729bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
730  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
731  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
732  BBInfo *CvtBBI = &TrueBBI;
733  BBInfo *NextBBI = &FalseBBI;
734
735  std::vector<MachineOperand> Cond(BBI.BrCond);
736  if (BBI.Kind == ICTriangleFalse || BBI.Kind == ICTriangleFRev) {
737    if (BBI.Kind == ICTriangleFRev) {
738      ReverseBranchCondition(*NextBBI);
739      // False BB has been changed, modify its predecessors (except for this
740      // one) so they don't get ifcvt'ed based on bad intel.
741      for (MachineBasicBlock::pred_iterator PI = NextBBI->BB->pred_begin(),
742             E = NextBBI->BB->pred_end(); PI != E; ++PI) {
743        MachineBasicBlock *PBB = *PI;
744        if (PBB == BBI.BB)
745          continue;
746        BBInfo &PBBI = BBAnalysis[PBB->getNumber()];
747        if (PBBI.IsEnqueued)
748          PBBI.IsEnqueued = false;
749      }
750    }
751    std::swap(CvtBBI, NextBBI);
752    TII->ReverseBranchCondition(Cond);
753  }
754
755  // Predicate the 'true' block after removing its branch.
756  TrueBBI.NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
757  PredicateBlock(*CvtBBI, Cond);
758
759  // If 'true' block has a 'false' successor, add an exit branch to it.
760  bool HasEarlyExit = CvtBBI->FalseBB != NULL;
761  if (HasEarlyExit) {
762    std::vector<MachineOperand> RevCond(CvtBBI->BrCond);
763    if (TII->ReverseBranchCondition(RevCond))
764      assert(false && "Unable to reverse branch condition!");
765    TII->InsertBranch(*CvtBBI->BB, CvtBBI->FalseBB, NULL, RevCond);
766  }
767
768  // Now merge the entry of the triangle with the true block.
769  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
770  MergeBlocks(BBI, *CvtBBI);
771
772  // Merge in the 'false' block if the 'false' block has no other
773  // predecessors. Otherwise, add a unconditional branch from to 'false'.
774  bool FalseBBDead = false;
775  bool IterIfcvt = true;
776  bool isFallThrough = canFallThroughTo(BBI.BB, NextBBI->BB);
777  if (!isFallThrough) {
778    // Only merge them if the true block does not fallthrough to the false
779    // block. By not merging them, we make it possible to iteratively
780    // ifcvt the blocks.
781    if (!HasEarlyExit && NextBBI->BB->pred_size() == 1) {
782      MergeBlocks(BBI, *NextBBI);
783      FalseBBDead = true;
784    } else {
785      InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
786      BBI.HasFallThrough = false;
787    }
788    // Mixed predicated and unpredicated code. This cannot be iteratively
789    // predicated.
790    IterIfcvt = false;
791  }
792
793  RemoveExtraEdges(BBI);
794
795  // Update block info. BB can be iteratively if-converted.
796  if (!IterIfcvt)
797    BBI.IsDone = true;
798  ReTryPreds(BBI.BB);
799  CvtBBI->IsDone = true;
800  if (FalseBBDead)
801    NextBBI->IsDone = true;
802
803  // FIXME: Must maintain LiveIns.
804  return true;
805}
806
807/// IfConvertDiamond - If convert a diamond sub-CFG.
808///
809bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
810  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
811  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
812
813  SmallVector<MachineInstr*, 2> Dups;
814  if (!BBI.TailBB) {
815    // No common merge block. Check if the terminators (e.g. return) are
816    // the same or predicable.
817    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
818    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
819    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
820      if (TT->isIdenticalTo(FT))
821        Dups.push_back(TT);  // Will erase these later.
822      else if (!TT->isPredicable() && !FT->isPredicable())
823        return false; // Can't if-convert. Abort!
824      ++TT;
825      ++FT;
826    }
827
828    // One of the two pathes have more terminators, make sure they are
829    // all predicable.
830    while (TT != BBI.TrueBB->end()) {
831      if (!TT->isPredicable()) {
832        return false; // Can't if-convert. Abort!
833      }
834      ++TT;
835    }
836    while (FT != BBI.FalseBB->end()) {
837      if (!FT->isPredicable()) {
838        return false; // Can't if-convert. Abort!
839      }
840      ++FT;
841    }
842  }
843
844  // Remove the duplicated instructions from the 'true' block.
845  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
846    Dups[i]->eraseFromParent();
847    --TrueBBI.NonPredSize;
848  }
849
850  // Merge the 'true' and 'false' blocks by copying the instructions
851  // from the 'false' block to the 'true' block. That is, unless the true
852  // block would clobber the predicate, in that case, do the opposite.
853  BBInfo *BBI1 = &TrueBBI;
854  BBInfo *BBI2 = &FalseBBI;
855  std::vector<MachineOperand> RevCond(BBI.BrCond);
856  TII->ReverseBranchCondition(RevCond);
857  std::vector<MachineOperand> *Cond1 = &BBI.BrCond;
858  std::vector<MachineOperand> *Cond2 = &RevCond;
859  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
860  // Either the block fallthrough to another block or it ends with a
861  // return. If it's the former, add a branch to its successor.
862  bool NeedBr1 = !BBI1->TrueBB && BBI1->BB->succ_size();
863  bool NeedBr2 = !BBI2->TrueBB && BBI2->BB->succ_size();
864
865  if ((TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred) ||
866      (!TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred &&
867       NeedBr1 && !NeedBr2)) {
868    std::swap(BBI1, BBI2);
869    std::swap(Cond1, Cond2);
870    std::swap(NeedBr1, NeedBr2);
871  }
872
873  // Predicate the 'true' block after removing its branch.
874  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
875  PredicateBlock(*BBI1, *Cond1);
876
877  // Add an early exit branch if needed.
878  if (NeedBr1)
879    TII->InsertBranch(*BBI1->BB, *BBI1->BB->succ_begin(), NULL, *Cond1);
880
881  // Predicate the 'false' block.
882  PredicateBlock(*BBI2, *Cond2, true);
883
884  // Add an unconditional branch from 'false' to to 'false' successor if it
885  // will not be the fallthrough block.
886  if (NeedBr2 && !NeedBr1) {
887    // If BBI2 isn't going to be merged in, then the existing fallthrough
888    // or branch is fine.
889    if (!canFallThroughTo(BBI.BB, *BBI2->BB->succ_begin())) {
890      InsertUncondBranch(BBI2->BB, *BBI2->BB->succ_begin(), TII);
891      BBI2->HasFallThrough = false;
892    }
893  }
894
895  // Keep them as two separate blocks if there is an early exit.
896  if (!NeedBr1)
897    MergeBlocks(*BBI1, *BBI2);
898
899  // Remove the conditional branch from entry to the blocks.
900  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
901
902  // Merge the combined block into the entry of the diamond.
903  MergeBlocks(BBI, *BBI1);
904
905  // 'True' and 'false' aren't combined, see if we need to add a unconditional
906  // branch to the 'false' block.
907  if (NeedBr1 && !canFallThroughTo(BBI.BB, BBI2->BB)) {
908    InsertUncondBranch(BBI.BB, BBI2->BB, TII);
909    BBI1->HasFallThrough = false;
910  }
911
912  // If the if-converted block fallthrough or unconditionally branch into the
913  // tail block, and the tail block does not have other predecessors, then
914  // fold the tail block in as well.
915  BBInfo *CvtBBI = NeedBr1 ? BBI2 : &BBI;
916  if (BBI.TailBB &&
917      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
918    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
919    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
920    MergeBlocks(*CvtBBI, TailBBI);
921    TailBBI.IsDone = true;
922  }
923
924  RemoveExtraEdges(BBI);
925
926  // Update block info.
927  BBI.IsDone = TrueBBI.IsDone = FalseBBI.IsDone = true;
928
929  // FIXME: Must maintain LiveIns.
930  return true;
931}
932
933/// PredicateBlock - Predicate every instruction in the block with the specified
934/// condition. If IgnoreTerm is true, skip over all terminator instructions.
935void IfConverter::PredicateBlock(BBInfo &BBI,
936                                 std::vector<MachineOperand> &Cond,
937                                 bool IgnoreTerm) {
938  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
939       I != E; ++I) {
940    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
941      continue;
942    if (TII->isPredicated(I))
943      continue;
944    if (!TII->PredicateInstruction(I, Cond)) {
945      cerr << "Unable to predicate " << *I << "!\n";
946      abort();
947    }
948  }
949
950  BBI.NonPredSize = 0;
951  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
952
953  NumIfConvBBs++;
954}
955
956/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
957///
958void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
959  ToBBI.BB->splice(ToBBI.BB->end(),
960                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
961
962  // Redirect all branches to FromBB to ToBB.
963  std::vector<MachineBasicBlock *> Preds(FromBBI.BB->pred_begin(),
964                                         FromBBI.BB->pred_end());
965  for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
966    MachineBasicBlock *Pred = Preds[i];
967    if (Pred == ToBBI.BB)
968      continue;
969    Pred->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
970  }
971
972  std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
973                                         FromBBI.BB->succ_end());
974  MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
975  MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : NULL;
976
977  for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
978    MachineBasicBlock *Succ = Succs[i];
979    // Fallthrough edge can't be transferred.
980    if (Succ == FallThrough)
981      continue;
982    FromBBI.BB->removeSuccessor(Succ);
983    if (!ToBBI.BB->isSuccessor(Succ))
984      ToBBI.BB->addSuccessor(Succ);
985  }
986
987  // Now FromBBI always fall through to the next block!
988  if (NBB)
989    FromBBI.BB->addSuccessor(NBB);
990
991  ToBBI.NonPredSize += FromBBI.NonPredSize;
992  FromBBI.NonPredSize = 0;
993
994  ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
995  ToBBI.HasFallThrough = FromBBI.HasFallThrough;
996
997  std::copy(FromBBI.Predicate.begin(), FromBBI.Predicate.end(),
998            std::back_inserter(ToBBI.Predicate));
999  FromBBI.Predicate.clear();
1000}
1001