IfConversion.cpp revision cc8fb46cdc47cac1d4b3b92ffcca75f3216a8698 
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> DisableTriangleR("disable-ifcvt-triangle-rev",
40                                 cl::init(false), cl::Hidden);
41  cl::opt<bool> DisableTriangleF("disable-ifcvt-triangle-false",
42                                 cl::init(false), cl::Hidden);
43  cl::opt<bool> DisableTriangleFR("disable-ifcvt-triangle-false-rev",
44                                  cl::init(false), cl::Hidden);
45  cl::opt<bool> DisableDiamond("disable-ifcvt-diamond",
46                               cl::init(false), cl::Hidden);
47}
48
49STATISTIC(NumSimple,       "Number of simple if-conversions performed");
50STATISTIC(NumSimpleFalse,  "Number of simple (F) if-conversions performed");
51STATISTIC(NumTriangle,     "Number of triangle if-conversions performed");
52STATISTIC(NumTriangleRev,  "Number of triangle (R) if-conversions performed");
53STATISTIC(NumTriangleFalse,"Number of triangle (F) if-conversions performed");
54STATISTIC(NumTriangleFRev, "Number of triangle (F/R) if-conversions performed");
55STATISTIC(NumDiamonds,     "Number of diamond if-conversions performed");
56STATISTIC(NumIfConvBBs,    "Number of if-converted blocks");
57
58namespace {
59  class IfConverter : public MachineFunctionPass {
60    enum BBICKind {
61      ICNotClassfied,  // BB data valid, but not classified.
62      ICSimple,        // BB is entry of an one split, no rejoin sub-CFG.
63      ICSimpleFalse,   // Same as ICSimple, but on the false path.
64      ICTriangle,      // BB is entry of a triangle sub-CFG.
65      ICTriangleRev,   // Same as ICTriangle, but true path rev condition.
66      ICTriangleFalse, // Same as ICTriangle, but on the false path.
67      ICTriangleFRev,  // Same as ICTriangleFalse, but false path rev condition.
68      ICDiamond        // BB is entry of a diamond sub-CFG.
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    /// IsDone          - True if BB is not to be considered for ifcvt.
80    /// IsBeingAnalyzed - True if BB is currently being analyzed.
81    /// IsAnalyzed      - True if BB has been analyzed (info is still valid).
82    /// IsEnqueued      - True if BB has been enqueued to be ifcvt'ed.
83    /// IsBrAnalyzable  - True if AnalyzeBranch() returns false.
84    /// HasFallThrough  - True if BB may fallthrough to the following BB.
85    /// IsUnpredicable  - True if BB is known to be unpredicable.
86    /// ClobbersPredicate- True if BB would modify the predicate (e.g. has
87    ///                   cmp, call, etc.)
88    /// NonPredSize     - Number of non-predicated instructions.
89    /// BB              - Corresponding MachineBasicBlock.
90    /// TrueBB / FalseBB- See AnalyzeBranch().
91    /// BrCond          - Conditions for end of block conditional branches.
92    /// Predicate       - Predicate used in the BB.
93    struct BBInfo {
94      BBICKind Kind;
95      bool IsDone          : 1;
96      bool IsBeingAnalyzed : 1;
97      bool IsAnalyzed      : 1;
98      bool IsEnqueued      : 1;
99      bool IsBrAnalyzable  : 1;
100      bool HasFallThrough  : 1;
101      bool IsUnpredicable  : 1;
102      bool ClobbersPred    : 1;
103      unsigned NonPredSize;
104      MachineBasicBlock *BB;
105      MachineBasicBlock *TrueBB;
106      MachineBasicBlock *FalseBB;
107      MachineBasicBlock *TailBB;
108      std::vector<MachineOperand> BrCond;
109      std::vector<MachineOperand> Predicate;
110      BBInfo() : Kind(ICNotClassfied), IsDone(false), IsBeingAnalyzed(false),
111                 IsAnalyzed(false), IsEnqueued(false), IsBrAnalyzable(false),
112                 HasFallThrough(false), IsUnpredicable(false),
113                 ClobbersPred(false), NonPredSize(0),
114                 BB(0), TrueBB(0), FalseBB(0), TailBB(0) {}
115    };
116
117    /// Roots - Basic blocks that do not have successors. These are the starting
118    /// points of Graph traversal.
119    std::vector<MachineBasicBlock*> Roots;
120
121    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
122    /// basic block number.
123    std::vector<BBInfo> BBAnalysis;
124
125    const TargetLowering *TLI;
126    const TargetInstrInfo *TII;
127    bool MadeChange;
128  public:
129    static char ID;
130    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
131
132    virtual bool runOnMachineFunction(MachineFunction &MF);
133    virtual const char *getPassName() const { return "If converter"; }
134
135  private:
136    bool ReverseBranchCondition(BBInfo &BBI);
137    bool ValidSimple(BBInfo &TrueBBI) const;
138    bool ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
139                       bool FalseBranch = false) const;
140    bool ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const;
141    void ScanInstructions(BBInfo &BBI);
142    BBInfo &AnalyzeBlock(MachineBasicBlock *BB);
143    bool FeasibilityAnalysis(BBInfo &BBI, std::vector<MachineOperand> &Cond,
144                             bool isTriangle = false, bool RevBranch = false);
145    bool AttemptRestructuring(BBInfo &BBI);
146    bool AnalyzeBlocks(MachineFunction &MF,
147                       std::vector<BBInfo*> &Candidates);
148    void ReTryPreds(MachineBasicBlock *BB);
149    void RemoveExtraEdges(BBInfo &BBI);
150    bool IfConvertSimple(BBInfo &BBI);
151    bool IfConvertTriangle(BBInfo &BBI);
152    bool IfConvertDiamond(BBInfo &BBI);
153    void PredicateBlock(BBInfo &BBI,
154                        std::vector<MachineOperand> &Cond,
155                        bool IgnoreTerm = false);
156    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
157
158    // blockAlwaysFallThrough - Block ends without a terminator.
159    bool blockAlwaysFallThrough(BBInfo &BBI) const {
160      return BBI.IsBrAnalyzable && BBI.TrueBB == NULL;
161    }
162
163    // IfcvtCandidateCmp - Used to sort if-conversion candidates.
164    static bool IfcvtCandidateCmp(BBInfo* C1, BBInfo* C2){
165      // Favor diamond over triangle, etc.
166      return (unsigned)C1->Kind < (unsigned)C2->Kind;
167    }
168  };
169  char IfConverter::ID = 0;
170}
171
172FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
173
174bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
175  TLI = MF.getTarget().getTargetLowering();
176  TII = MF.getTarget().getInstrInfo();
177  if (!TII) return false;
178
179  static int FnNum = -1;
180  DOUT << "\nIfcvt: function (" << ++FnNum <<  ") \'"
181       << MF.getFunction()->getName() << "\'";
182
183  if (FnNum < IfCvtFnStart || (IfCvtFnStop != -1 && FnNum > IfCvtFnStop)) {
184    DOUT << " skipped\n";
185    return false;
186  }
187  DOUT << "\n";
188
189  MF.RenumberBlocks();
190  BBAnalysis.resize(MF.getNumBlockIDs());
191
192  // Look for root nodes, i.e. blocks without successors.
193  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
194    if (I->succ_size() == 0)
195      Roots.push_back(I);
196
197  std::vector<BBInfo*> Candidates;
198  MadeChange = false;
199  while (IfCvtLimit == -1 || (int)NumIfConvBBs < IfCvtLimit) {
200    // Do an intial analysis for each basic block and finding all the potential
201    // candidates to perform if-convesion.
202    bool Change = AnalyzeBlocks(MF, Candidates);
203    while (!Candidates.empty()) {
204      BBInfo &BBI = *Candidates.back();
205      Candidates.pop_back();
206
207      // If the block has been evicted out of the queue or it has already been
208      // marked dead (due to it being predicated), then skip it.
209      if (!BBI.IsEnqueued || BBI.IsDone)
210        continue;
211
212      bool RetVal = false;
213      switch (BBI.Kind) {
214      default: assert(false && "Unexpected!");
215        break;
216      case ICSimple:
217      case ICSimpleFalse: {
218        bool isFalse = BBI.Kind == ICSimpleFalse;
219        if ((isFalse && DisableSimpleF) || (!isFalse && DisableSimple)) break;
220        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
221             << "): BB#" << BBI.BB->getNumber() << " ("
222             << ((BBI.Kind == ICSimpleFalse)
223                 ? BBI.FalseBB->getNumber() : BBI.TrueBB->getNumber()) << ") ";
224        RetVal = IfConvertSimple(BBI);
225        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
226        if (RetVal)
227          if (isFalse) NumSimpleFalse++;
228          else         NumSimple++;
229       break;
230      }
231      case ICTriangle:
232      case ICTriangleRev:
233      case ICTriangleFalse:
234      case ICTriangleFRev: {
235        bool isFalse    = BBI.Kind == ICTriangleFalse;
236        bool isRev      = (BBI.Kind == ICTriangleRev || BBI.Kind == ICTriangleFRev);
237        if (DisableTriangle && !isFalse && !isRev) break;
238        if (DisableTriangleR && !isFalse && isRev) break;
239        if (DisableTriangleF && isFalse && !isRev) break;
240        if (DisableTriangleFR && isFalse && isRev) break;
241        DOUT << "Ifcvt (Triangle";
242        if (isFalse)
243          DOUT << " false";
244        if (isRev)
245          DOUT << " rev";
246        DOUT << "): BB#" << BBI.BB->getNumber() << " (T:"
247             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber()
248             << ") ";
249        RetVal = IfConvertTriangle(BBI);
250        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
251        if (RetVal) {
252          if (isFalse) {
253            if (isRev) NumTriangleFRev++;
254            else       NumTriangleFalse++;
255          } else {
256            if (isRev) NumTriangleRev++;
257            else       NumTriangle++;
258          }
259        }
260        break;
261      }
262      case ICDiamond:
263        if (DisableDiamond) break;
264        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
265             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber();
266        if (BBI.TailBB)
267          DOUT << "," << BBI.TailBB->getNumber() ;
268        DOUT << ") ";
269        RetVal = IfConvertDiamond(BBI);
270        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
271        if (RetVal) NumDiamonds++;
272        break;
273      }
274      Change |= RetVal;
275
276      if (IfCvtLimit != -1 && (int)NumIfConvBBs >= IfCvtLimit)
277        break;
278    }
279
280    if (!Change)
281      break;
282    MadeChange |= Change;
283  }
284
285  Roots.clear();
286  BBAnalysis.clear();
287
288  return MadeChange;
289}
290
291static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
292                                         MachineBasicBlock *TrueBB) {
293  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
294         E = BB->succ_end(); SI != E; ++SI) {
295    MachineBasicBlock *SuccBB = *SI;
296    if (SuccBB != TrueBB)
297      return SuccBB;
298  }
299  return NULL;
300}
301
302bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
303  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
304    TII->RemoveBranch(*BBI.BB);
305    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
306    std::swap(BBI.TrueBB, BBI.FalseBB);
307    return true;
308  }
309  return false;
310}
311
312/// getNextBlock - Returns the next block in the function blocks ordering. If
313/// it is the end, returns NULL.
314static inline MachineBasicBlock *getNextBlock(MachineBasicBlock *BB) {
315  MachineFunction::iterator I = BB;
316  MachineFunction::iterator E = BB->getParent()->end();
317  if (++I == E)
318    return NULL;
319  return I;
320}
321
322/// ValidSimple - Returns true if the 'true' block (along with its
323/// predecessor) forms a valid simple shape for ifcvt.
324bool IfConverter::ValidSimple(BBInfo &TrueBBI) const {
325  if (TrueBBI.IsBeingAnalyzed)
326    return false;
327
328  return !blockAlwaysFallThrough(TrueBBI) &&
329    TrueBBI.BrCond.size() == 0 && TrueBBI.BB->pred_size() == 1;
330}
331
332/// ValidTriangle - Returns true if the 'true' and 'false' blocks (along
333/// with their common predecessor) forms a valid triangle shape for ifcvt.
334bool IfConverter::ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
335                                bool FalseBranch) const {
336  if (TrueBBI.IsBeingAnalyzed)
337    return false;
338
339  if (TrueBBI.BB->pred_size() != 1)
340    return false;
341
342  MachineBasicBlock *TExit = FalseBranch ? TrueBBI.FalseBB : TrueBBI.TrueBB;
343  if (!TExit && blockAlwaysFallThrough(TrueBBI)) {
344    MachineFunction::iterator I = TrueBBI.BB;
345    if (++I == TrueBBI.BB->getParent()->end())
346      return false;
347    TExit = I;
348  }
349  return TExit && TExit == FalseBBI.BB;
350}
351
352/// ValidDiamond - Returns true if the 'true' and 'false' blocks (along
353/// with their common predecessor) forms a valid diamond shape for ifcvt.
354bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI) const {
355  if (TrueBBI.IsBeingAnalyzed || FalseBBI.IsBeingAnalyzed)
356    return false;
357
358  MachineBasicBlock *TT = TrueBBI.TrueBB;
359  MachineBasicBlock *FT = FalseBBI.TrueBB;
360
361  if (!TT && blockAlwaysFallThrough(TrueBBI))
362    TT = getNextBlock(TrueBBI.BB);
363  if (!FT && blockAlwaysFallThrough(FalseBBI))
364    FT = getNextBlock(FalseBBI.BB);
365  if (TT != FT)
366    return false;
367  if (TT == NULL && (TrueBBI.IsBrAnalyzable || FalseBBI.IsBrAnalyzable))
368    return false;
369  // FIXME: Allow false block to have an early exit?
370  return (TrueBBI.BB->pred_size() == 1 &&
371          FalseBBI.BB->pred_size() == 1 &&
372          !TrueBBI.FalseBB && !FalseBBI.FalseBB);
373}
374
375/// ScanInstructions - Scan all the instructions in the block to determine if
376/// the block is predicable. In most cases, that means all the instructions
377/// in the block has M_PREDICABLE flag. Also checks if the block contains any
378/// instruction which can clobber a predicate (e.g. condition code register).
379/// If so, the block is not predicable unless it's the last instruction.
380void IfConverter::ScanInstructions(BBInfo &BBI) {
381  if (BBI.IsDone)
382    return;
383
384  // First analyze the end of BB branches.
385  BBI.TrueBB = BBI.FalseBB;
386  BBI.BrCond.clear();
387  BBI.IsBrAnalyzable =
388    !TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
389  BBI.HasFallThrough = BBI.IsBrAnalyzable && BBI.FalseBB == NULL;
390
391  if (BBI.BrCond.size()) {
392    // No false branch. This BB must end with a conditional branch and a
393    // fallthrough.
394    if (!BBI.FalseBB)
395      BBI.FalseBB = findFalseBlock(BBI.BB, BBI.TrueBB);
396    assert(BBI.FalseBB && "Expected to find the fallthrough block!");
397  }
398
399  // Then scan all the instructions.
400  BBI.NonPredSize = 0;
401  BBI.ClobbersPred = false;
402  bool SeenCondBr = false;
403  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
404       I != E; ++I) {
405    const TargetInstrDescriptor *TID = I->getInstrDescriptor();
406    bool isPredicated = TII->isPredicated(I);
407    bool isCondBr = BBI.IsBrAnalyzable &&
408      (TID->Flags & M_BRANCH_FLAG) != 0 && (TID->Flags & M_BARRIER_FLAG) == 0;
409
410    if (!isPredicated && !isCondBr)
411      BBI.NonPredSize++;
412
413    if (BBI.ClobbersPred && !isPredicated) {
414      // Predicate modification instruction should end the block (except for
415      // already predicated instructions and end of block branches).
416      if (isCondBr) {
417        SeenCondBr = true;
418
419        // Conditional branches is not predicable. But it may be eliminated.
420        continue;
421      }
422
423      // Predicate may have been modified, the subsequent (currently)
424      // unpredocated instructions cannot be correctly predicated.
425      BBI.IsUnpredicable = true;
426      return;
427    }
428
429    if (TID->Flags & M_CLOBBERS_PRED)
430      BBI.ClobbersPred = true;
431
432    if (!I->isPredicable()) {
433      BBI.IsUnpredicable = true;
434      return;
435    }
436  }
437}
438
439/// FeasibilityAnalysis - Determine if the block is a suitable candidate to be
440/// predicated by the specified predicate.
441bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
442                                      std::vector<MachineOperand> &Pred,
443                                      bool isTriangle, bool RevBranch) {
444  // Forget about it if it's unpredicable.
445  if (BBI.IsUnpredicable)
446    return false;
447
448  // If the block is dead, or it is going to be the entry block of a sub-CFG
449  // that will be if-converted, then it cannot be predicated.
450  if (BBI.IsDone || BBI.IsEnqueued)
451    return false;
452
453  // Check predication threshold.
454  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
455    return false;
456
457  // If it is already predicated, check if its predicate subsumes the new
458  // predicate.
459  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Pred))
460    return false;
461
462  if (BBI.BrCond.size()) {
463    if (!isTriangle)
464      return false;
465
466    // Test predicate subsumsion.
467    std::vector<MachineOperand> RevPred(Pred);
468    std::vector<MachineOperand> Cond(BBI.BrCond);
469    if (RevBranch) {
470      if (TII->ReverseBranchCondition(Cond))
471        return false;
472    }
473    if (TII->ReverseBranchCondition(RevPred) ||
474        !TII->SubsumesPredicate(Cond, RevPred))
475      return false;
476  }
477
478  return true;
479}
480
481/// AnalyzeBlock - Analyze the structure of the sub-CFG starting from
482/// the specified block. Record its successors and whether it looks like an
483/// if-conversion candidate.
484IfConverter::BBInfo &IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
485  BBInfo &BBI = BBAnalysis[BB->getNumber()];
486
487  if (BBI.IsAnalyzed || BBI.IsBeingAnalyzed)
488    return BBI;
489
490  BBI.BB = BB;
491  BBI.IsBeingAnalyzed = true;
492  BBI.Kind = ICNotClassfied;
493
494  ScanInstructions(BBI);
495
496  // Unanalyable or ends with fallthrough or unconditional branch.
497  if (!BBI.IsBrAnalyzable || BBI.BrCond.size() == 0) {
498    BBI.IsBeingAnalyzed = false;
499    BBI.IsAnalyzed = true;
500    return BBI;
501  }
502
503  // Do not ifcvt if either path is a back edge to the entry block.
504  if (BBI.TrueBB == BB || BBI.FalseBB == BB) {
505    BBI.IsBeingAnalyzed = false;
506    BBI.IsAnalyzed = true;
507    return BBI;
508  }
509
510  BBInfo &TrueBBI  = AnalyzeBlock(BBI.TrueBB);
511  BBInfo &FalseBBI = AnalyzeBlock(BBI.FalseBB);
512
513  if (TrueBBI.IsDone && FalseBBI.IsDone) {
514    BBI.IsBeingAnalyzed = false;
515    BBI.IsAnalyzed = true;
516    return BBI;
517  }
518
519  std::vector<MachineOperand> RevCond(BBI.BrCond);
520  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
521
522  if (CanRevCond && ValidDiamond(TrueBBI, FalseBBI) &&
523      !(TrueBBI.ClobbersPred && FalseBBI.ClobbersPred) &&
524      FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
525      FeasibilityAnalysis(FalseBBI, RevCond)) {
526    // Diamond:
527    //   EBB
528    //   / \_
529    //  |   |
530    // TBB FBB
531    //   \ /
532    //  TailBB
533    // Note TailBB can be empty.
534    BBI.Kind = ICDiamond;
535    BBI.TailBB = TrueBBI.TrueBB;
536  } else {
537    // FIXME: Consider duplicating if BB is small.
538    if (ValidTriangle(TrueBBI, FalseBBI) &&
539        FeasibilityAnalysis(TrueBBI, BBI.BrCond, true)) {
540      // Triangle:
541      //   EBB
542      //   | \_
543      //   |  |
544      //   | TBB
545      //   |  /
546      //   FBB
547      BBI.Kind = ICTriangle;
548    } else if (ValidTriangle(TrueBBI, FalseBBI, true) &&
549               FeasibilityAnalysis(TrueBBI, BBI.BrCond, true, true)) {
550      BBI.Kind = ICTriangleRev;
551    } else if (ValidSimple(TrueBBI) &&
552               FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
553      // Simple (split, no rejoin):
554      //   EBB
555      //   | \_
556      //   |  |
557      //   | TBB---> exit
558      //   |
559      //   FBB
560      BBI.Kind = ICSimple;
561    } else if (CanRevCond) {
562      // Try the other path...
563      if (ValidTriangle(FalseBBI, TrueBBI) &&
564          FeasibilityAnalysis(FalseBBI, RevCond, true)) {
565        BBI.Kind = ICTriangleFalse;
566      } else if (ValidTriangle(FalseBBI, TrueBBI, true) &&
567                 FeasibilityAnalysis(FalseBBI, RevCond, true, true)) {
568        BBI.Kind = ICTriangleFRev;
569      } else if (ValidSimple(FalseBBI) &&
570                 FeasibilityAnalysis(FalseBBI, RevCond)) {
571        BBI.Kind = ICSimpleFalse;
572      }
573    }
574  }
575
576  BBI.IsBeingAnalyzed = false;
577  BBI.IsAnalyzed = true;
578  return BBI;
579}
580
581/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
582/// expose more if-conversion opportunities. e.g.
583///
584///                cmp
585///                b le BB1
586///                /  \____
587///               /        |
588///             cmp        |
589///             b eq BB1   |
590///              /  \____  |
591///             /        \ |
592///                      BB1
593///  ==>
594///
595///                cmp
596///                b eq BB1
597///                /  \____
598///               /        |
599///             cmp        |
600///             b le BB1   |
601///              /  \____  |
602///             /        \ |
603///                      BB1
604bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
605  return false;
606}
607
608/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
609/// candidates. It returns true if any CFG restructuring is done to expose more
610/// if-conversion opportunities.
611bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
612                                std::vector<BBInfo*> &Candidates) {
613  bool Change = false;
614  std::set<MachineBasicBlock*> Visited;
615  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
616    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
617           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
618      MachineBasicBlock *BB = *I;
619      BBInfo &BBI = AnalyzeBlock(BB);
620      switch (BBI.Kind) {
621        case ICSimple:
622        case ICSimpleFalse:
623        case ICTriangle:
624        case ICTriangleRev:
625        case ICTriangleFalse:
626        case ICTriangleFRev:
627        case ICDiamond:
628          Candidates.push_back(&BBI);
629          break;
630        default:
631          Change |= AttemptRestructuring(BBI);
632          break;
633      }
634    }
635  }
636
637  // Sort to favor more complex ifcvt scheme.
638  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
639
640  return Change;
641}
642
643/// canFallThroughTo - Returns true either if ToBB is the next block after BB or
644/// that all the intervening blocks are empty (given BB can fall through to its
645/// next block).
646static bool canFallThroughTo(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
647  MachineFunction::iterator I = BB;
648  MachineFunction::iterator TI = ToBB;
649  MachineFunction::iterator E = BB->getParent()->end();
650  while (++I != TI)
651    if (I == E || !I->empty())
652      return false;
653  return true;
654}
655
656/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
657/// to determine if it can be if-converted.
658void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
659  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
660         E = BB->pred_end(); PI != E; ++PI) {
661    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
662    if (!PBBI.IsDone && PBBI.Kind == ICNotClassfied) {
663      assert(PBBI.IsEnqueued && "Unexpected");
664      PBBI.IsAnalyzed = false;
665    }
666  }
667}
668
669/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
670///
671static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
672                               const TargetInstrInfo *TII) {
673  std::vector<MachineOperand> NoCond;
674  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
675}
676
677/// RemoveExtraEdges - Remove true / false edges if either / both are no longer
678/// successors.
679void IfConverter::RemoveExtraEdges(BBInfo &BBI) {
680  MachineBasicBlock *TBB = NULL, *FBB = NULL;
681  std::vector<MachineOperand> Cond;
682  bool isAnalyzable = !TII->AnalyzeBranch(*BBI.BB, TBB, FBB, Cond);
683  bool CanFallthrough = isAnalyzable && (TBB == NULL || FBB == NULL);
684  if (BBI.TrueBB && BBI.BB->isSuccessor(BBI.TrueBB))
685    if (!(BBI.TrueBB == TBB || BBI.TrueBB == FBB ||
686          (CanFallthrough && getNextBlock(BBI.BB) == BBI.TrueBB)))
687      BBI.BB->removeSuccessor(BBI.TrueBB);
688  if (BBI.FalseBB && BBI.BB->isSuccessor(BBI.FalseBB))
689    if (!(BBI.FalseBB == TBB || BBI.FalseBB == FBB ||
690          (CanFallthrough && getNextBlock(BBI.BB) == BBI.FalseBB)))
691      BBI.BB->removeSuccessor(BBI.FalseBB);
692}
693
694/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
695///
696bool IfConverter::IfConvertSimple(BBInfo &BBI) {
697  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
698  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
699  BBInfo *CvtBBI = &TrueBBI;
700  BBInfo *NextBBI = &FalseBBI;
701
702  std::vector<MachineOperand> Cond(BBI.BrCond);
703  if (BBI.Kind == ICSimpleFalse) {
704    std::swap(CvtBBI, NextBBI);
705    TII->ReverseBranchCondition(Cond);
706  }
707
708  PredicateBlock(*CvtBBI, Cond);
709
710  // Merge converted block into entry block.
711  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
712  MergeBlocks(BBI, *CvtBBI);
713
714  bool IterIfcvt = true;
715  if (!canFallThroughTo(BBI.BB, NextBBI->BB)) {
716    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
717    BBI.HasFallThrough = false;
718    // Now ifcvt'd block will look like this:
719    // BB:
720    // ...
721    // t, f = cmp
722    // if t op
723    // b BBf
724    //
725    // We cannot further ifcvt this block because the unconditional branch
726    // will have to be predicated on the new condition, that will not be
727    // available if cmp executes.
728    IterIfcvt = false;
729  }
730
731  RemoveExtraEdges(BBI);
732
733  // Update block info. BB can be iteratively if-converted.
734  if (!IterIfcvt)
735    BBI.IsDone = true;
736  ReTryPreds(BBI.BB);
737  CvtBBI->IsDone = true;
738
739  // FIXME: Must maintain LiveIns.
740  return true;
741}
742
743/// IfConvertTriangle - If convert a triangle sub-CFG.
744///
745bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
746  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
747  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
748  BBInfo *CvtBBI = &TrueBBI;
749  BBInfo *NextBBI = &FalseBBI;
750
751  std::vector<MachineOperand> Cond(BBI.BrCond);
752  if (BBI.Kind == ICTriangleFalse || BBI.Kind == ICTriangleFRev) {
753    std::swap(CvtBBI, NextBBI);
754    TII->ReverseBranchCondition(Cond);
755  }
756  if (BBI.Kind == ICTriangleRev || BBI.Kind == ICTriangleFRev) {
757    ReverseBranchCondition(*CvtBBI);
758    // BB has been changed, modify its predecessors (except for this
759    // one) so they don't get ifcvt'ed based on bad intel.
760    for (MachineBasicBlock::pred_iterator PI = CvtBBI->BB->pred_begin(),
761           E = CvtBBI->BB->pred_end(); PI != E; ++PI) {
762      MachineBasicBlock *PBB = *PI;
763      if (PBB == BBI.BB)
764        continue;
765      BBInfo &PBBI = BBAnalysis[PBB->getNumber()];
766      if (PBBI.IsEnqueued)
767        PBBI.IsEnqueued = false;
768    }
769  }
770
771  // Predicate the 'true' block after removing its branch.
772  TrueBBI.NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
773  PredicateBlock(*CvtBBI, Cond);
774
775  // If 'true' block has a 'false' successor, add an exit branch to it.
776  bool HasEarlyExit = CvtBBI->FalseBB != NULL;
777  if (HasEarlyExit) {
778    std::vector<MachineOperand> RevCond(CvtBBI->BrCond);
779    if (TII->ReverseBranchCondition(RevCond))
780      assert(false && "Unable to reverse branch condition!");
781    TII->InsertBranch(*CvtBBI->BB, CvtBBI->FalseBB, NULL, RevCond);
782  }
783
784  // Now merge the entry of the triangle with the true block.
785  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
786  MergeBlocks(BBI, *CvtBBI);
787
788  // Merge in the 'false' block if the 'false' block has no other
789  // predecessors. Otherwise, add a unconditional branch from to 'false'.
790  bool FalseBBDead = false;
791  bool IterIfcvt = true;
792  bool isFallThrough = canFallThroughTo(BBI.BB, NextBBI->BB);
793  if (!isFallThrough) {
794    // Only merge them if the true block does not fallthrough to the false
795    // block. By not merging them, we make it possible to iteratively
796    // ifcvt the blocks.
797    if (!HasEarlyExit && NextBBI->BB->pred_size() == 1) {
798      MergeBlocks(BBI, *NextBBI);
799      FalseBBDead = true;
800    } else {
801      InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
802      BBI.HasFallThrough = false;
803    }
804    // Mixed predicated and unpredicated code. This cannot be iteratively
805    // predicated.
806    IterIfcvt = false;
807  }
808
809  RemoveExtraEdges(BBI);
810
811  // Update block info. BB can be iteratively if-converted.
812  if (!IterIfcvt)
813    BBI.IsDone = true;
814  ReTryPreds(BBI.BB);
815  CvtBBI->IsDone = true;
816  if (FalseBBDead)
817    NextBBI->IsDone = true;
818
819  // FIXME: Must maintain LiveIns.
820  return true;
821}
822
823/// IfConvertDiamond - If convert a diamond sub-CFG.
824///
825bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
826  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
827  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
828
829  SmallVector<MachineInstr*, 2> Dups;
830  if (!BBI.TailBB) {
831    // No common merge block. Check if the terminators (e.g. return) are
832    // the same or predicable.
833    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
834    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
835    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
836      if (TT->isIdenticalTo(FT))
837        Dups.push_back(TT);  // Will erase these later.
838      else if (!TT->isPredicable() && !FT->isPredicable())
839        return false; // Can't if-convert. Abort!
840      ++TT;
841      ++FT;
842    }
843
844    // One of the two pathes have more terminators, make sure they are
845    // all predicable.
846    while (TT != BBI.TrueBB->end()) {
847      if (!TT->isPredicable()) {
848        return false; // Can't if-convert. Abort!
849      }
850      ++TT;
851    }
852    while (FT != BBI.FalseBB->end()) {
853      if (!FT->isPredicable()) {
854        return false; // Can't if-convert. Abort!
855      }
856      ++FT;
857    }
858  }
859
860  // Remove the duplicated instructions from the 'true' block.
861  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
862    Dups[i]->eraseFromParent();
863    --TrueBBI.NonPredSize;
864  }
865
866  // Merge the 'true' and 'false' blocks by copying the instructions
867  // from the 'false' block to the 'true' block. That is, unless the true
868  // block would clobber the predicate, in that case, do the opposite.
869  BBInfo *BBI1 = &TrueBBI;
870  BBInfo *BBI2 = &FalseBBI;
871  std::vector<MachineOperand> RevCond(BBI.BrCond);
872  TII->ReverseBranchCondition(RevCond);
873  std::vector<MachineOperand> *Cond1 = &BBI.BrCond;
874  std::vector<MachineOperand> *Cond2 = &RevCond;
875  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
876  // Either the block fallthrough to another block or it ends with a
877  // return. If it's the former, add a branch to its successor.
878  bool NeedBr1 = !BBI1->TrueBB && BBI1->BB->succ_size();
879  bool NeedBr2 = !BBI2->TrueBB && BBI2->BB->succ_size();
880
881  if ((TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred) ||
882      (!TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred &&
883       NeedBr1 && !NeedBr2)) {
884    std::swap(BBI1, BBI2);
885    std::swap(Cond1, Cond2);
886    std::swap(NeedBr1, NeedBr2);
887  }
888
889  // Predicate the 'true' block after removing its branch.
890  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
891  PredicateBlock(*BBI1, *Cond1);
892
893  // Add an early exit branch if needed.
894  if (NeedBr1)
895    TII->InsertBranch(*BBI1->BB, *BBI1->BB->succ_begin(), NULL, *Cond1);
896
897  // Predicate the 'false' block.
898  PredicateBlock(*BBI2, *Cond2, true);
899
900  // Add an unconditional branch from 'false' to to 'false' successor if it
901  // will not be the fallthrough block.
902  if (NeedBr2 && !NeedBr1) {
903    // If BBI2 isn't going to be merged in, then the existing fallthrough
904    // or branch is fine.
905    if (!canFallThroughTo(BBI.BB, *BBI2->BB->succ_begin())) {
906      InsertUncondBranch(BBI2->BB, *BBI2->BB->succ_begin(), TII);
907      BBI2->HasFallThrough = false;
908    }
909  }
910
911  // Keep them as two separate blocks if there is an early exit.
912  if (!NeedBr1)
913    MergeBlocks(*BBI1, *BBI2);
914
915  // Remove the conditional branch from entry to the blocks.
916  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
917
918  // Merge the combined block into the entry of the diamond.
919  MergeBlocks(BBI, *BBI1);
920
921  // 'True' and 'false' aren't combined, see if we need to add a unconditional
922  // branch to the 'false' block.
923  if (NeedBr1 && !canFallThroughTo(BBI.BB, BBI2->BB)) {
924    InsertUncondBranch(BBI.BB, BBI2->BB, TII);
925    BBI1->HasFallThrough = false;
926  }
927
928  // If the if-converted block fallthrough or unconditionally branch into the
929  // tail block, and the tail block does not have other predecessors, then
930  // fold the tail block in as well.
931  BBInfo *CvtBBI = NeedBr1 ? BBI2 : &BBI;
932  if (BBI.TailBB &&
933      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
934    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
935    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
936    MergeBlocks(*CvtBBI, TailBBI);
937    TailBBI.IsDone = true;
938  }
939
940  RemoveExtraEdges(BBI);
941
942  // Update block info.
943  BBI.IsDone = TrueBBI.IsDone = FalseBBI.IsDone = true;
944
945  // FIXME: Must maintain LiveIns.
946  return true;
947}
948
949/// PredicateBlock - Predicate every instruction in the block with the specified
950/// condition. If IgnoreTerm is true, skip over all terminator instructions.
951void IfConverter::PredicateBlock(BBInfo &BBI,
952                                 std::vector<MachineOperand> &Cond,
953                                 bool IgnoreTerm) {
954  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
955       I != E; ++I) {
956    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
957      continue;
958    if (TII->isPredicated(I))
959      continue;
960    if (!TII->PredicateInstruction(I, Cond)) {
961      cerr << "Unable to predicate " << *I << "!\n";
962      abort();
963    }
964  }
965
966  BBI.NonPredSize = 0;
967  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
968
969  NumIfConvBBs++;
970}
971
972/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
973///
974void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
975  ToBBI.BB->splice(ToBBI.BB->end(),
976                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
977
978  // Redirect all branches to FromBB to ToBB.
979  std::vector<MachineBasicBlock *> Preds(FromBBI.BB->pred_begin(),
980                                         FromBBI.BB->pred_end());
981  for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
982    MachineBasicBlock *Pred = Preds[i];
983    if (Pred == ToBBI.BB)
984      continue;
985    Pred->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
986  }
987
988  std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
989                                         FromBBI.BB->succ_end());
990  MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
991  MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : NULL;
992
993  for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
994    MachineBasicBlock *Succ = Succs[i];
995    // Fallthrough edge can't be transferred.
996    if (Succ == FallThrough)
997      continue;
998    FromBBI.BB->removeSuccessor(Succ);
999    if (!ToBBI.BB->isSuccessor(Succ))
1000      ToBBI.BB->addSuccessor(Succ);
1001  }
1002
1003  // Now FromBBI always fall through to the next block!
1004  if (NBB)
1005    FromBBI.BB->addSuccessor(NBB);
1006
1007  ToBBI.NonPredSize += FromBBI.NonPredSize;
1008  FromBBI.NonPredSize = 0;
1009
1010  ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
1011  ToBBI.HasFallThrough = FromBBI.HasFallThrough;
1012
1013  std::copy(FromBBI.Predicate.begin(), FromBBI.Predicate.end(),
1014            std::back_inserter(ToBBI.Predicate));
1015  FromBBI.Predicate.clear();
1016}
1017