IfConversion.cpp revision 96dd9a8b1b969ece5504559ec240930bcb1dddd9 
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/Debug.h"
23#include "llvm/ADT/DepthFirstIterator.h"
24#include "llvm/ADT/Statistic.h"
25using namespace llvm;
26
27STATISTIC(NumSimple,    "Number of simple if-conversions performed");
28STATISTIC(NumSimpleRev, "Number of simple (reversed) if-conversions performed");
29STATISTIC(NumTriangle,  "Number of triangle if-conversions performed");
30STATISTIC(NumDiamonds,  "Number of diamond if-conversions performed");
31STATISTIC(NumIfConvBBs, "Number of if-converted blocks");
32
33namespace {
34  class IfConverter : public MachineFunctionPass {
35    enum BBICKind {
36      ICNotAnalyzed,   // BB has not been analyzed.
37      ICReAnalyze,     // BB must be re-analyzed.
38      ICNotClassfied,  // BB data valid, but not classified.
39      ICSimple,        // BB is entry of an one split, no rejoin sub-CFG.
40      ICSimpleFalse,   // Same as ICSimple, but on the false path.
41      ICTriangle,      // BB is entry of a triangle sub-CFG.
42      ICDiamond,       // BB is entry of a diamond sub-CFG.
43      ICChild,         // BB is part of the sub-CFG that'll be predicated.
44      ICDead           // BB cannot be if-converted again.
45    };
46
47    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
48    /// if-conversion feasibility analysis. This includes results from
49    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
50    /// classification, and common tail block of its successors (if it's a
51    /// diamond shape), its size, whether it's predicable, and whether any
52    /// instruction can clobber the 'would-be' predicate.
53    ///
54    /// Kind            - Type of block. See BBICKind.
55    /// NonPredSize     - Number of non-predicated instructions.
56    /// IsAnalyzable    - True if AnalyzeBranch() returns false.
57    /// ModifyPredicate - True if BB would modify the predicate (e.g. has
58    ///                   cmp, call, etc.)
59    /// BB              - Corresponding MachineBasicBlock.
60    /// TrueBB / FalseBB- See AnalyzeBranch().
61    /// BrCond          - Conditions for end of block conditional branches.
62    /// Predicate       - Predicate used in the BB.
63    struct BBInfo {
64      BBICKind Kind;
65      unsigned NonPredSize;
66      bool IsAnalyzable;
67      bool ModifyPredicate;
68      MachineBasicBlock *BB;
69      MachineBasicBlock *TrueBB;
70      MachineBasicBlock *FalseBB;
71      MachineBasicBlock *TailBB;
72      std::vector<MachineOperand> BrCond;
73      std::vector<MachineOperand> Predicate;
74      BBInfo() : Kind(ICNotAnalyzed), NonPredSize(0),
75                 IsAnalyzable(false), ModifyPredicate(false),
76                 BB(0), TrueBB(0), FalseBB(0), TailBB(0) {}
77    };
78
79    /// Roots - Basic blocks that do not have successors. These are the starting
80    /// points of Graph traversal.
81    std::vector<MachineBasicBlock*> Roots;
82
83    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
84    /// basic block number.
85    std::vector<BBInfo> BBAnalysis;
86
87    const TargetLowering *TLI;
88    const TargetInstrInfo *TII;
89    bool MadeChange;
90  public:
91    static char ID;
92    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
93
94    virtual bool runOnMachineFunction(MachineFunction &MF);
95    virtual const char *getPassName() const { return "If converter"; }
96
97  private:
98    bool ReverseBranchCondition(BBInfo &BBI);
99    bool BlockModifyPredicate(MachineBasicBlock *BB) const;
100    void StructuralAnalysis(MachineBasicBlock *BB);
101    bool FeasibilityAnalysis(BBInfo &BBI,
102                             std::vector<MachineOperand> &Cond,
103                             bool IgnoreTerm = false);
104    bool AttemptRestructuring(BBInfo &BBI);
105    bool AnalyzeBlocks(MachineFunction &MF,
106                       std::vector<BBInfo*> &Candidates);
107    void ReTryPreds(MachineBasicBlock *BB);
108    bool IfConvertSimple(BBInfo &BBI);
109    bool IfConvertTriangle(BBInfo &BBI);
110    bool IfConvertDiamond(BBInfo &BBI);
111    void PredicateBlock(BBInfo &BBI,
112                        std::vector<MachineOperand> &Cond,
113                        bool IgnoreTerm = false);
114    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
115
116    // blockFallsThrough - Block ends without a terminator.
117    bool blockFallsThrough(BBInfo &BBI) const {
118      return BBI.IsAnalyzable && BBI.TrueBB == NULL;
119    }
120
121    // IfcvtCandidateCmp - Used to sort if-conversion candidates.
122    static bool IfcvtCandidateCmp(BBInfo* C1, BBInfo* C2){
123      // Favor diamond over triangle, etc.
124      return (unsigned)C1->Kind < (unsigned)C2->Kind;
125    }
126  };
127  char IfConverter::ID = 0;
128}
129
130FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
131
132bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
133  TLI = MF.getTarget().getTargetLowering();
134  TII = MF.getTarget().getInstrInfo();
135  if (!TII) return false;
136
137  DOUT << "\nIfcvt: function \'" << MF.getFunction()->getName() << "\'\n";
138
139  MF.RenumberBlocks();
140  BBAnalysis.resize(MF.getNumBlockIDs());
141
142  // Look for root nodes, i.e. blocks without successors.
143  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
144    if (I->succ_size() == 0)
145      Roots.push_back(I);
146
147  std::vector<BBInfo*> Candidates;
148  MadeChange = false;
149  while (true) {
150    // Do an intial analysis for each basic block and finding all the potential
151    // candidates to perform if-convesion.
152    bool Change = AnalyzeBlocks(MF, Candidates);
153    while (!Candidates.empty()) {
154      BBInfo &BBI = *Candidates.back();
155      Candidates.pop_back();
156
157      bool RetVal = false;
158      switch (BBI.Kind) {
159      default: assert(false && "Unexpected!");
160        break;
161      case ICReAnalyze:
162        // One or more of 'children' have been modified, abort!
163      case ICDead:
164        // Block has been already been if-converted, abort!
165        break;
166      case ICSimple:
167      case ICSimpleFalse: {
168        bool isRev = BBI.Kind == ICSimpleFalse;
169        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
170             << "): BB#" << BBI.BB->getNumber() << " ("
171             << ((BBI.Kind == ICSimpleFalse)
172                 ? BBI.FalseBB->getNumber() : BBI.TrueBB->getNumber()) << ") ";
173        RetVal = IfConvertSimple(BBI);
174        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
175        if (RetVal)
176          if (isRev) NumSimpleRev++;
177          else       NumSimple++;
178       break;
179      }
180      case ICTriangle:
181        DOUT << "Ifcvt (Triangle): BB#" << BBI.BB->getNumber() << " (T:"
182             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber()
183             << ") ";
184        RetVal = IfConvertTriangle(BBI);
185        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
186        if (RetVal) NumTriangle++;
187        break;
188      case ICDiamond:
189        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
190             << BBI.TrueBB->getNumber() << ",F:" << BBI.FalseBB->getNumber();
191        if (BBI.TailBB)
192          DOUT << "," << BBI.TailBB->getNumber() ;
193        DOUT << ") ";
194        RetVal = IfConvertDiamond(BBI);
195        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
196        if (RetVal) NumDiamonds++;
197        break;
198      }
199      Change |= RetVal;
200    }
201
202    if (!Change)
203      break;
204    MadeChange |= Change;
205  }
206
207  Roots.clear();
208  BBAnalysis.clear();
209
210  return MadeChange;
211}
212
213static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
214                                         MachineBasicBlock *TrueBB) {
215  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
216         E = BB->succ_end(); SI != E; ++SI) {
217    MachineBasicBlock *SuccBB = *SI;
218    if (SuccBB != TrueBB)
219      return SuccBB;
220  }
221  return NULL;
222}
223
224bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
225  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
226    TII->RemoveBranch(*BBI.BB);
227    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
228    std::swap(BBI.TrueBB, BBI.FalseBB);
229    return true;
230  }
231  return false;
232}
233
234/// BlockModifyPredicate - Returns true if any instruction in the block may
235/// clobber the condition code or register(s) used to predicate instructions,
236/// e.g. call, cmp.
237bool IfConverter::BlockModifyPredicate(MachineBasicBlock *BB) const {
238  for (MachineBasicBlock::const_reverse_iterator I = BB->rbegin(),
239         E = BB->rend(); I != E; ++I)
240    if (I->getInstrDescriptor()->Flags & M_CLOBBERS_PRED)
241      return true;
242  return false;
243}
244
245/// StructuralAnalysis - Analyze the structure of the sub-CFG starting from
246/// the specified block. Record its successors and whether it looks like an
247/// if-conversion candidate.
248void IfConverter::StructuralAnalysis(MachineBasicBlock *BB) {
249  BBInfo &BBI = BBAnalysis[BB->getNumber()];
250
251  if (BBI.Kind == ICReAnalyze) {
252    BBI.BrCond.clear();
253    BBI.TrueBB = BBI.FalseBB = NULL;
254  } else {
255    if (BBI.Kind != ICNotAnalyzed)
256      return;  // Already analyzed.
257    BBI.BB = BB;
258    BBI.NonPredSize = std::distance(BB->begin(), BB->end());
259    BBI.ModifyPredicate = BlockModifyPredicate(BB);
260  }
261
262  // Look for 'root' of a simple (non-nested) triangle or diamond.
263  BBI.Kind = ICNotClassfied;
264  BBI.IsAnalyzable =
265    !TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
266  if (!BBI.IsAnalyzable || BBI.BrCond.size() == 0)
267    return;
268  // Do not ifcvt if either path is a back edge to the entry block.
269  if (BBI.TrueBB == BB || BBI.FalseBB == BB)
270    return;
271
272  StructuralAnalysis(BBI.TrueBB);
273  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
274
275  // No false branch. This BB must end with a conditional branch and a
276  // fallthrough.
277  if (!BBI.FalseBB)
278    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
279  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
280
281  StructuralAnalysis(BBI.FalseBB);
282  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
283
284  // If both paths are dead, then forget about it.
285  if (TrueBBI.Kind == ICDead && FalseBBI.Kind == ICDead) {
286    BBI.Kind = ICDead;
287    return;
288  }
289
290  // Look for more opportunities to if-convert a triangle. Try to restructure
291  // the CFG to form a triangle with the 'false' path.
292  std::vector<MachineOperand> RevCond(BBI.BrCond);
293  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
294  if (FalseBBI.FalseBB) {
295    if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB)
296      return;
297    std::vector<MachineOperand> Cond(BBI.BrCond);
298    if (CanRevCond &&
299        FalseBBI.TrueBB && FalseBBI.BB->pred_size() == 1 &&
300        FeasibilityAnalysis(FalseBBI, RevCond, true)) {
301      std::vector<MachineOperand> FalseCond(FalseBBI.BrCond);
302      if (FalseBBI.TrueBB == BBI.TrueBB &&
303          TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
304        // Reverse 'true' and 'false' paths.
305        ReverseBranchCondition(BBI);
306        BBI.Kind = ICTriangle;
307        FalseBBI.Kind = ICChild;
308      } else if (FalseBBI.FalseBB == BBI.TrueBB &&
309                 !TII->ReverseBranchCondition(FalseCond) &&
310                 TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
311        // Reverse 'false' block's 'true' and 'false' paths and then
312        // reverse 'true' and 'false' paths.
313        ReverseBranchCondition(FalseBBI);
314        ReverseBranchCondition(BBI);
315        BBI.Kind = ICTriangle;
316        FalseBBI.Kind = ICChild;
317      }
318    }
319  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB && CanRevCond &&
320             TrueBBI.BB->pred_size() == 1 &&
321             FalseBBI.BB->pred_size() == 1 &&
322             // Check the 'true' and 'false' blocks if either isn't ended with
323             // a branch. If the block does not fallthrough to another block
324             // then we need to add a branch to its successor.
325             !(TrueBBI.ModifyPredicate &&
326               !TrueBBI.TrueBB && TrueBBI.BB->succ_size()) &&
327             !(FalseBBI.ModifyPredicate &&
328               !FalseBBI.TrueBB && FalseBBI.BB->succ_size()) &&
329             FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
330             FeasibilityAnalysis(FalseBBI, RevCond)) {
331    // Diamond:
332    //   EBB
333    //   / \_
334    //  |   |
335    // TBB FBB
336    //   \ /
337    //  TailBB
338    // Note TailBB can be empty.
339    BBI.Kind = ICDiamond;
340    TrueBBI.Kind = FalseBBI.Kind = ICChild;
341    BBI.TailBB = TrueBBI.TrueBB;
342  } else {
343    // FIXME: Consider duplicating if BB is small.
344    bool TryTriangle = TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB &&
345                       TrueBBI.BB->pred_size() == 1;
346    bool TrySimple = TrueBBI.BrCond.size() == 0 && TrueBBI.BB->pred_size() == 1;
347    if ((TryTriangle || TrySimple) &&
348        FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
349      if (TryTriangle) {
350        // Triangle:
351        //   EBB
352        //   | \_
353        //   |  |
354        //   | TBB
355        //   |  /
356        //   FBB
357        BBI.Kind = ICTriangle;
358        TrueBBI.Kind = ICChild;
359      } else {
360        // Simple (split, no rejoin):
361        //   EBB
362        //   | \_
363        //   |  |
364        //   | TBB---> exit
365        //   |
366        //   FBB
367        BBI.Kind = ICSimple;
368        TrueBBI.Kind = ICChild;
369      }
370    } else if (FalseBBI.BrCond.size() == 0 && FalseBBI.BB->pred_size() == 1) {
371      // Try the other path...
372      bool TryTriangle = FalseBBI.TrueBB && FalseBBI.TrueBB == BBI.TrueBB &&
373                         FalseBBI.BB->pred_size() == 1;
374      std::vector<MachineOperand> RevCond(BBI.BrCond);
375      if (!TII->ReverseBranchCondition(RevCond) &&
376          FeasibilityAnalysis(FalseBBI, RevCond)) {
377        if (TryTriangle) {
378          // Reverse 'true' and 'false' paths.
379          ReverseBranchCondition(BBI);
380          BBI.Kind = ICTriangle;
381          FalseBBI.Kind = ICChild;
382        } else {
383          BBI.Kind = ICSimpleFalse;
384          FalseBBI.Kind = ICChild;
385        }
386      }
387    }
388  }
389  return;
390}
391
392/// FeasibilityAnalysis - Determine if the block is predicable. In most
393/// cases, that means all the instructions in the block has M_PREDICABLE flag.
394/// Also checks if the block contains any instruction which can clobber a
395/// predicate (e.g. condition code register). If so, the block is not
396/// predicable unless it's the last instruction. If IgnoreTerm is true then
397/// all the terminator instructions are skipped.
398bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
399                                      std::vector<MachineOperand> &Cond,
400                                      bool IgnoreTerm) {
401  // If the block is dead, or it is going to be the entry block of a sub-CFG
402  // that will be if-converted, then it cannot be predicated.
403  if (BBI.Kind != ICNotAnalyzed &&
404      BBI.Kind != ICNotClassfied &&
405      BBI.Kind != ICChild)
406    return false;
407
408  // Check predication threshold.
409  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
410    return false;
411
412  // If it is already predicated, check if its predicate subsumes the new
413  // predicate.
414  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Cond))
415    return false;
416
417  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
418       I != E; ++I) {
419    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
420      continue;
421    // TODO: check if instruction clobbers predicate.
422    if (!I->isPredicable())
423      return false;
424  }
425
426  return true;
427}
428
429/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
430/// expose more if-conversion opportunities. e.g.
431///
432///                cmp
433///                b le BB1
434///                /  \____
435///               /        |
436///             cmp        |
437///             b eq BB1   |
438///              /  \____  |
439///             /        \ |
440///                      BB1
441///  ==>
442///
443///                cmp
444///                b eq BB1
445///                /  \____
446///               /        |
447///             cmp        |
448///             b le BB1   |
449///              /  \____  |
450///             /        \ |
451///                      BB1
452bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
453  return false;
454}
455
456/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
457/// candidates. It returns true if any CFG restructuring is done to expose more
458/// if-conversion opportunities.
459bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
460                                std::vector<BBInfo*> &Candidates) {
461  bool Change = false;
462  std::set<MachineBasicBlock*> Visited;
463  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
464    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
465           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
466      MachineBasicBlock *BB = *I;
467      StructuralAnalysis(BB);
468      BBInfo &BBI = BBAnalysis[BB->getNumber()];
469      switch (BBI.Kind) {
470        case ICSimple:
471        case ICSimpleFalse:
472        case ICTriangle:
473        case ICDiamond:
474          Candidates.push_back(&BBI);
475          break;
476        default:
477          Change |= AttemptRestructuring(BBI);
478          break;
479      }
480    }
481  }
482
483  // Sort to favor more complex ifcvt scheme.
484  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
485
486  return Change;
487}
488
489/// isNextBlock - Returns true either if ToBB the next block after BB or
490/// that all the intervening blocks are empty.
491static bool isNextBlock(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
492  MachineFunction::iterator I = BB;
493  MachineFunction::iterator TI = ToBB;
494  MachineFunction::iterator E = BB->getParent()->end();
495  while (++I != TI)
496    if (I == E || !I->empty())
497      return false;
498  return true;
499}
500
501/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
502/// to determine if it can be if-converted.
503void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
504  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
505         E = BB->pred_end(); PI != E; ++PI) {
506    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
507    PBBI.Kind = ICReAnalyze;
508  }
509}
510
511/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
512///
513static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
514                               const TargetInstrInfo *TII) {
515  std::vector<MachineOperand> NoCond;
516  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
517}
518
519/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
520///
521bool IfConverter::IfConvertSimple(BBInfo &BBI) {
522  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
523  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
524  BBInfo *CvtBBI = &TrueBBI;
525  BBInfo *NextBBI = &FalseBBI;
526
527  std::vector<MachineOperand> Cond(BBI.BrCond);
528  if (BBI.Kind == ICSimpleFalse) {
529    std::swap(CvtBBI, NextBBI);
530    TII->ReverseBranchCondition(Cond);
531  }
532
533  PredicateBlock(*CvtBBI, Cond);
534  // If the 'true' block ends without a branch, add a conditional branch
535  // to its successor unless that happens to be the 'false' block.
536  if (CvtBBI->IsAnalyzable && CvtBBI->TrueBB == NULL) {
537    assert(CvtBBI->BB->succ_size() == 1 && "Unexpected!");
538    MachineBasicBlock *SuccBB = *CvtBBI->BB->succ_begin();
539    if (SuccBB != NextBBI->BB)
540      TII->InsertBranch(*CvtBBI->BB, SuccBB, NULL, Cond);
541  }
542
543  // Merge converted block into entry block. Also add an unconditional branch
544  // to the 'false' branch.
545  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
546  MergeBlocks(BBI, *CvtBBI);
547  bool IterIfcvt = true;
548  if (!isNextBlock(BBI.BB, NextBBI->BB)) {
549    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
550    if (BBI.ModifyPredicate)
551      // Now ifcvt'd block will look like this:
552      // BB:
553      // ...
554      // t, f = cmp
555      // if t op
556      // b BBf
557      //
558      // We cannot further ifcvt this block because the unconditional branch will
559      // have to be predicated on the new condition, that will not be available
560      // if cmp executes.
561      IterIfcvt = false;
562  }
563  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
564
565  // Update block info. BB can be iteratively if-converted.
566  if (IterIfcvt)
567    BBI.Kind = ICReAnalyze;
568  else
569    BBI.Kind = ICDead;
570  ReTryPreds(BBI.BB);
571  CvtBBI->Kind = ICDead;
572
573  // FIXME: Must maintain LiveIns.
574  return true;
575}
576
577/// IfConvertTriangle - If convert a triangle sub-CFG.
578///
579bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
580  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
581
582  // Predicate the 'true' block after removing its branch.
583  TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
584  PredicateBlock(TrueBBI, BBI.BrCond);
585
586  // If 'true' block has a 'false' successor, add an exit branch to it.
587  bool HasEarlyExit = TrueBBI.FalseBB != NULL;
588  if (HasEarlyExit) {
589    std::vector<MachineOperand> RevCond(TrueBBI.BrCond);
590    if (TII->ReverseBranchCondition(RevCond))
591      assert(false && "Unable to reverse branch condition!");
592    TII->InsertBranch(*BBI.TrueBB, TrueBBI.FalseBB, NULL, RevCond);
593  }
594
595  // Join the 'true' and 'false' blocks if the 'false' block has no other
596  // predecessors. Otherwise, add a unconditional branch from 'true' to 'false'.
597  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
598  bool FalseBBDead = false;
599  bool IterIfcvt = true;
600  if (!HasEarlyExit && FalseBBI.BB->pred_size() == 2) {
601    MergeBlocks(TrueBBI, FalseBBI);
602    FalseBBDead = true;
603  } else if (!isNextBlock(TrueBBI.BB, FalseBBI.BB)) {
604    InsertUncondBranch(TrueBBI.BB, FalseBBI.BB, TII);
605    if (BBI.ModifyPredicate || TrueBBI.ModifyPredicate)
606      // Now ifcvt'd block will look like this:
607      // BB:
608      // ...
609      // t, f = cmp
610      // if t op
611      // b BBf
612      //
613      // We cannot further ifcvt this block because the unconditional branch will
614      // have to be predicated on the new condition, that will not be available
615      // if cmp executes.
616      IterIfcvt = false;
617  }
618
619  // Now merge the entry of the triangle with the true block.
620  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
621  MergeBlocks(BBI, TrueBBI);
622  std::copy(BBI.BrCond.begin(), BBI.BrCond.end(),
623            std::back_inserter(BBI.Predicate));
624
625  // Update block info. BB can be iteratively if-converted.
626  if (IterIfcvt)
627    BBI.Kind = ICReAnalyze;
628  else
629    BBI.Kind = ICDead;
630  ReTryPreds(BBI.BB);
631  TrueBBI.Kind = ICDead;
632  if (FalseBBDead)
633    FalseBBI.Kind = ICDead;
634
635  // FIXME: Must maintain LiveIns.
636  return true;
637}
638
639/// IfConvertDiamond - If convert a diamond sub-CFG.
640///
641bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
642  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
643  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
644
645  SmallVector<MachineInstr*, 2> Dups;
646  if (!BBI.TailBB) {
647    // No common merge block. Check if the terminators (e.g. return) are
648    // the same or predicable.
649    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
650    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
651    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
652      if (TT->isIdenticalTo(FT))
653        Dups.push_back(TT);  // Will erase these later.
654      else if (!TT->isPredicable() && !FT->isPredicable())
655        return false; // Can't if-convert. Abort!
656      ++TT;
657      ++FT;
658    }
659
660    // One of the two pathes have more terminators, make sure they are
661    // all predicable.
662    while (TT != BBI.TrueBB->end()) {
663      if (!TT->isPredicable()) {
664        return false; // Can't if-convert. Abort!
665      }
666      ++TT;
667    }
668    while (FT != BBI.FalseBB->end()) {
669      if (!FT->isPredicable()) {
670        return false; // Can't if-convert. Abort!
671      }
672      ++FT;
673    }
674  }
675
676  // Remove the duplicated instructions from the 'true' block.
677  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
678    Dups[i]->eraseFromParent();
679    --TrueBBI.NonPredSize;
680  }
681
682  // Merge the 'true' and 'false' blocks by copying the instructions
683  // from the 'false' block to the 'true' block. That is, unless the true
684  // block would clobber the predicate, in that case, do the opposite.
685  BBInfo *BBI1 = &TrueBBI;
686  BBInfo *BBI2 = &FalseBBI;
687  std::vector<MachineOperand> RevCond(BBI.BrCond);
688  TII->ReverseBranchCondition(RevCond);
689  std::vector<MachineOperand> *Cond1 = &BBI.BrCond;
690  std::vector<MachineOperand> *Cond2 = &RevCond;
691  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
692  // Either the block fallthrough to another block or it ends with a
693  // return. If it's the former, add a branch to its successor.
694  bool NeedBr1 = !BBI1->TrueBB && BBI1->BB->succ_size();
695  bool NeedBr2 = !BBI2->TrueBB && BBI2->BB->succ_size();
696
697  if ((TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate) ||
698      (!TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate &&
699       NeedBr1 && !NeedBr2)) {
700    std::swap(BBI1, BBI2);
701    std::swap(Cond1, Cond2);
702    std::swap(NeedBr1, NeedBr2);
703  }
704
705  // Predicate the 'true' block after removing its branch.
706  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
707  PredicateBlock(*BBI1, *Cond1);
708
709  // Add an early exit branch if needed.
710  if (NeedBr1)
711    TII->InsertBranch(*BBI1->BB, *BBI1->BB->succ_begin(), NULL, *Cond1);
712
713  // Predicate the 'false' block.
714  PredicateBlock(*BBI2, *Cond2, true);
715
716  // Add an unconditional branch from 'false' to to 'false' successor if it
717  // will not be the fallthrough block.
718  if (NeedBr2 && !isNextBlock(BBI2->BB, *BBI2->BB->succ_begin()))
719    InsertUncondBranch(BBI2->BB, *BBI2->BB->succ_begin(), TII);
720
721  // Keep them as two separate blocks if there is an early exit.
722  if (!NeedBr1)
723    MergeBlocks(*BBI1, *BBI2);
724
725  // Remove the conditional branch from entry to the blocks.
726  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
727
728  // Merge the combined block into the entry of the diamond.
729  MergeBlocks(BBI, *BBI1);
730
731  // 'True' and 'false' aren't combined, see if we need to add a unconditional
732  // branch to the 'false' block.
733  if (NeedBr1 && !isNextBlock(BBI.BB, BBI2->BB))
734    InsertUncondBranch(BBI1->BB, BBI2->BB, TII);
735
736  // If the if-converted block fallthrough or unconditionally branch into the
737  // tail block, and the tail block does not have other predecessors, then
738  // fold the tail block in as well.
739  BBInfo *CvtBBI = NeedBr1 ? BBI2 : &BBI;
740  if (BBI.TailBB &&
741      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
742    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
743    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
744    MergeBlocks(*CvtBBI, TailBBI);
745    TailBBI.Kind = ICDead;
746  }
747
748  // Update block info.
749  BBI.Kind = ICDead;
750  TrueBBI.Kind = ICDead;
751  FalseBBI.Kind = ICDead;
752
753  // FIXME: Must maintain LiveIns.
754  return true;
755}
756
757/// PredicateBlock - Predicate every instruction in the block with the specified
758/// condition. If IgnoreTerm is true, skip over all terminator instructions.
759void IfConverter::PredicateBlock(BBInfo &BBI,
760                                 std::vector<MachineOperand> &Cond,
761                                 bool IgnoreTerm) {
762  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
763       I != E; ++I) {
764    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
765      continue;
766    if (TII->isPredicated(I))
767      continue;
768    if (!TII->PredicateInstruction(I, Cond)) {
769      cerr << "Unable to predicate " << *I << "!\n";
770      abort();
771    }
772  }
773
774  BBI.NonPredSize = 0;
775  NumIfConvBBs++;
776}
777
778/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
779///
780static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
781  for (MachineBasicBlock::pred_iterator I = FromBB->pred_begin(),
782         E = FromBB->pred_end(); I != E; ++I) {
783    MachineBasicBlock *Pred = *I;
784    Pred->removeSuccessor(FromBB);
785    if (!Pred->isSuccessor(ToBB))
786      Pred->addSuccessor(ToBB);
787  }
788}
789
790/// TransferSuccs - Transfer all the successors of FromBB to ToBB.
791///
792static void TransferSuccs(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
793  for (MachineBasicBlock::succ_iterator I = FromBB->succ_begin(),
794         E = FromBB->succ_end(); I != E; ++I) {
795    MachineBasicBlock *Succ = *I;
796    FromBB->removeSuccessor(Succ);
797    if (!ToBB->isSuccessor(Succ))
798      ToBB->addSuccessor(Succ);
799  }
800}
801
802/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
803///
804void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
805  ToBBI.BB->splice(ToBBI.BB->end(),
806                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
807
808  // If FromBBI is previously a successor, remove it from ToBBI's successor
809  // list and update its TrueBB / FalseBB field if needed.
810  if (ToBBI.BB->isSuccessor(FromBBI.BB))
811    ToBBI.BB->removeSuccessor(FromBBI.BB);
812
813  // Redirect all branches to FromBB to ToBB.
814  std::vector<MachineBasicBlock *> Preds(FromBBI.BB->pred_begin(),
815                                         FromBBI.BB->pred_end());
816  for (unsigned i = 0, e = Preds.size(); i != e; ++i)
817    Preds[i]->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
818
819  // Transfer preds / succs and update size.
820  TransferPreds(ToBBI.BB, FromBBI.BB);
821  if (!blockFallsThrough(FromBBI))
822    TransferSuccs(ToBBI.BB, FromBBI.BB);
823  ToBBI.NonPredSize += FromBBI.NonPredSize;
824  FromBBI.NonPredSize = 0;
825
826  ToBBI.ModifyPredicate |= FromBBI.ModifyPredicate;
827}
828