IfConversion.cpp revision b6665f62c5cb19a04c4bb0eecbc498d406ec85b3 
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 has been converted and merged, it's now dead.
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 - FIXME: Not used right now. True if BB would modify
58    ///                   the predicate (e.g. has 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    void StructuralAnalysis(MachineBasicBlock *BB);
100    bool FeasibilityAnalysis(BBInfo &BBI,
101                             std::vector<MachineOperand> &Cond,
102                             bool IgnoreTerm = false);
103    bool AttemptRestructuring(BBInfo &BBI);
104    bool AnalyzeBlocks(MachineFunction &MF,
105                       std::vector<BBInfo*> &Candidates);
106    void ReTryPreds(MachineBasicBlock *BB);
107    bool IfConvertSimple(BBInfo &BBI);
108    bool IfConvertTriangle(BBInfo &BBI);
109    bool IfConvertDiamond(BBInfo &BBI);
110    void PredicateBlock(BBInfo &BBI,
111                        std::vector<MachineOperand> &Cond,
112                        bool IgnoreTerm = false);
113    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
114
115    // IfcvtCandidateCmp - Used to sort if-conversion candidates.
116    static bool IfcvtCandidateCmp(BBInfo* C1, BBInfo* C2){
117      // Favor diamond over triangle, etc.
118      return (unsigned)C1->Kind < (unsigned)C2->Kind;
119    }
120  };
121  char IfConverter::ID = 0;
122}
123
124FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
125
126bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
127  TLI = MF.getTarget().getTargetLowering();
128  TII = MF.getTarget().getInstrInfo();
129  if (!TII) return false;
130
131  DOUT << "\nIfcvt: function \'" << MF.getFunction()->getName() << "\'\n";
132
133  MF.RenumberBlocks();
134  BBAnalysis.resize(MF.getNumBlockIDs());
135
136  // Look for root nodes, i.e. blocks without successors.
137  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
138    if (I->succ_size() == 0)
139      Roots.push_back(I);
140
141  std::vector<BBInfo*> Candidates;
142  MadeChange = false;
143  while (true) {
144    // Do an intial analysis for each basic block and finding all the potential
145    // candidates to perform if-convesion.
146    bool Change = AnalyzeBlocks(MF, Candidates);
147    while (!Candidates.empty()) {
148      BBInfo &BBI = *Candidates.back();
149      Candidates.pop_back();
150
151      bool RetVal = false;
152      switch (BBI.Kind) {
153      default: assert(false && "Unexpected!");
154        break;
155      case ICReAnalyze:
156        // One or more of 'childrean' have been modified, abort!
157        break;
158      case ICSimple:
159      case ICSimpleFalse:
160        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
161             << "): BB#" << BBI.BB->getNumber() << " ";
162        RetVal = IfConvertSimple(BBI);
163        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
164        if (RetVal)
165          if (BBI.Kind == ICSimple) NumSimple++;
166          else                      NumSimpleRev++;
167       break;
168      case ICTriangle:
169        DOUT << "Ifcvt (Triangle): BB#" << BBI.BB->getNumber() << " ";
170        RetVal = IfConvertTriangle(BBI);
171        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
172        if (RetVal) NumTriangle++;
173        break;
174      case ICDiamond:
175        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " ";
176        RetVal = IfConvertDiamond(BBI);
177        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
178        if (RetVal) NumDiamonds++;
179        break;
180      }
181      Change |= RetVal;
182    }
183
184    if (!Change)
185      break;
186    MadeChange |= Change;
187  }
188
189  Roots.clear();
190  BBAnalysis.clear();
191
192  return MadeChange;
193}
194
195static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
196                                         MachineBasicBlock *TrueBB) {
197  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
198         E = BB->succ_end(); SI != E; ++SI) {
199    MachineBasicBlock *SuccBB = *SI;
200    if (SuccBB != TrueBB)
201      return SuccBB;
202  }
203  return NULL;
204}
205
206bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
207  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
208    TII->RemoveBranch(*BBI.BB);
209    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
210    std::swap(BBI.TrueBB, BBI.FalseBB);
211    return true;
212  }
213  return false;
214}
215
216/// StructuralAnalysis - Analyze the structure of the sub-CFG starting from
217/// the specified block. Record its successors and whether it looks like an
218/// if-conversion candidate.
219void IfConverter::StructuralAnalysis(MachineBasicBlock *BB) {
220  BBInfo &BBI = BBAnalysis[BB->getNumber()];
221
222  if (BBI.Kind == ICReAnalyze)
223    BBI.BrCond.clear();
224  else {
225    if (BBI.Kind != ICNotAnalyzed)
226      return;  // Already analyzed.
227    BBI.BB = BB;
228    BBI.NonPredSize = std::distance(BB->begin(), BB->end());
229  }
230
231  // Look for 'root' of a simple (non-nested) triangle or diamond.
232  BBI.Kind = ICNotClassfied;
233  BBI.IsAnalyzable =
234    !TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
235  if (!BBI.IsAnalyzable || BBI.BrCond.size() == 0)
236    return;
237
238  StructuralAnalysis(BBI.TrueBB);
239  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
240
241  // No false branch. This BB must end with a conditional branch and a
242  // fallthrough.
243  if (!BBI.FalseBB)
244    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
245  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
246
247  StructuralAnalysis(BBI.FalseBB);
248  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
249
250  // Look for more opportunities to if-convert a triangle. Try to restructure
251  // the CFG to form a triangle with the 'false' path.
252  std::vector<MachineOperand> RevCond(BBI.BrCond);
253  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
254  if (FalseBBI.FalseBB) {
255    if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB)
256      return;
257    std::vector<MachineOperand> Cond(BBI.BrCond);
258    if (CanRevCond &&
259        FalseBBI.TrueBB && FalseBBI.BB->pred_size() == 1 &&
260        FeasibilityAnalysis(FalseBBI, RevCond, true)) {
261      std::vector<MachineOperand> FalseCond(FalseBBI.BrCond);
262      if (FalseBBI.TrueBB == BBI.TrueBB &&
263          TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
264        // Reverse 'true' and 'false' paths.
265        ReverseBranchCondition(BBI);
266        BBI.Kind = ICTriangle;
267        FalseBBI.Kind = ICChild;
268      } else if (FalseBBI.FalseBB == BBI.TrueBB &&
269                 !TII->ReverseBranchCondition(FalseCond) &&
270                 TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
271        // Reverse 'false' block's 'true' and 'false' paths and then
272        // reverse 'true' and 'false' paths.
273        ReverseBranchCondition(FalseBBI);
274        ReverseBranchCondition(BBI);
275        BBI.Kind = ICTriangle;
276        FalseBBI.Kind = ICChild;
277      }
278    }
279  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB && CanRevCond &&
280             TrueBBI.BB->pred_size() == 1 &&
281             TrueBBI.BB->pred_size() == 1 &&
282             // Check the 'true' and 'false' blocks if either isn't ended with
283             // a branch. If the block does not fallthrough to another block
284             // then we need to add a branch to its successor.
285             !(TrueBBI.ModifyPredicate &&
286               !TrueBBI.TrueBB && TrueBBI.BB->succ_size()) &&
287             !(FalseBBI.ModifyPredicate &&
288               !FalseBBI.TrueBB && FalseBBI.BB->succ_size()) &&
289             FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
290             FeasibilityAnalysis(FalseBBI, RevCond)) {
291    // Diamond:
292    //   EBB
293    //   / \_
294    //  |   |
295    // TBB FBB
296    //   \ /
297    //  TailBB
298    // Note MBB can be empty in case both TBB and FBB are return blocks.
299    BBI.Kind = ICDiamond;
300    TrueBBI.Kind = FalseBBI.Kind = ICChild;
301    BBI.TailBB = TrueBBI.TrueBB;
302  } else {
303    // FIXME: Consider duplicating if BB is small.
304    bool TryTriangle = TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB &&
305                       BBI.TrueBB->pred_size() == 1;
306    bool TrySimple = TrueBBI.BrCond.size() == 0 && BBI.TrueBB->pred_size() == 1;
307    if ((TryTriangle || TrySimple) &&
308        FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
309      if (TryTriangle) {
310        // Triangle:
311        //   EBB
312        //   | \_
313        //   |  |
314        //   | TBB
315        //   |  /
316        //   FBB
317        BBI.Kind = ICTriangle;
318        TrueBBI.Kind = ICChild;
319      } else {
320        // Simple (split, no rejoin):
321        //   EBB
322        //   | \_
323        //   |  |
324        //   | TBB---> exit
325        //   |
326        //   FBB
327        BBI.Kind = ICSimple;
328        TrueBBI.Kind = ICChild;
329      }
330    } else if (FalseBBI.BrCond.size() == 0 && BBI.FalseBB->pred_size() == 1) {
331      // Try 'simple' on the other path...
332      std::vector<MachineOperand> RevCond(BBI.BrCond);
333      if (TII->ReverseBranchCondition(RevCond))
334        assert(false && "Unable to reverse branch condition!");
335      if (FeasibilityAnalysis(FalseBBI, RevCond)) {
336        BBI.Kind = ICSimpleFalse;
337        FalseBBI.Kind = ICChild;
338      }
339    }
340  }
341  return;
342}
343
344/// FeasibilityAnalysis - Determine if the block is predicable. In most
345/// cases, that means all the instructions in the block has M_PREDICABLE flag.
346/// Also checks if the block contains any instruction which can clobber a
347/// predicate (e.g. condition code register). If so, the block is not
348/// predicable unless it's the last instruction. If IgnoreTerm is true then
349/// all the terminator instructions are skipped.
350bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
351                                      std::vector<MachineOperand> &Cond,
352                                      bool IgnoreTerm) {
353  // If the block is dead, or it is going to be the entry block of a sub-CFG
354  // that will be if-converted, then it cannot be predicated.
355  if (BBI.Kind != ICNotAnalyzed &&
356      BBI.Kind != ICNotClassfied &&
357      BBI.Kind != ICChild)
358    return false;
359
360  // Check predication threshold.
361  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
362    return false;
363
364  // If it is already predicated, check if its predicate subsumes the new
365  // predicate.
366  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Cond))
367    return false;
368
369  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
370       I != E; ++I) {
371    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
372      continue;
373    // TODO: check if instruction clobbers predicate.
374    if (!I->isPredicable())
375      return false;
376  }
377
378  return true;
379}
380
381/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
382/// expose more if-conversion opportunities. e.g.
383///
384///                cmp
385///                b le BB1
386///                /  \____
387///               /        |
388///             cmp        |
389///             b eq BB1   |
390///              /  \____  |
391///             /        \ |
392///                      BB1
393///  ==>
394///
395///                cmp
396///                b eq BB1
397///                /  \____
398///               /        |
399///             cmp        |
400///             b le BB1   |
401///              /  \____  |
402///             /        \ |
403///                      BB1
404bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
405  return false;
406}
407
408/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
409/// candidates. It returns true if any CFG restructuring is done to expose more
410/// if-conversion opportunities.
411bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
412                                std::vector<BBInfo*> &Candidates) {
413  bool Change = false;
414  std::set<MachineBasicBlock*> Visited;
415  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
416    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
417           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
418      MachineBasicBlock *BB = *I;
419      StructuralAnalysis(BB);
420      BBInfo &BBI = BBAnalysis[BB->getNumber()];
421      switch (BBI.Kind) {
422        case ICSimple:
423        case ICSimpleFalse:
424        case ICTriangle:
425        case ICDiamond:
426          Candidates.push_back(&BBI);
427          break;
428        default:
429          Change |= AttemptRestructuring(BBI);
430          break;
431      }
432    }
433  }
434
435  // Sort to favor more complex ifcvt scheme.
436  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
437
438  return Change;
439}
440
441/// isNextBlock - Returns true either if ToBB the next block after BB or
442/// that all the intervening blocks are empty.
443static bool isNextBlock(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
444  MachineFunction::iterator I = BB;
445  while (++I != MachineFunction::iterator(ToBB))
446    if (!I->empty())
447      return false;
448  return true;
449}
450
451/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
452/// to determine if it can be if-converted.
453void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
454  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
455         E = BB->pred_end(); PI != E; ++PI) {
456    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
457    PBBI.Kind = ICReAnalyze;
458  }
459}
460
461/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
462///
463static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
464                               const TargetInstrInfo *TII) {
465  std::vector<MachineOperand> NoCond;
466  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
467}
468
469/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
470///
471bool IfConverter::IfConvertSimple(BBInfo &BBI) {
472  bool ReverseCond = BBI.Kind == ICSimpleFalse;
473
474  BBI.Kind = ICNotClassfied;
475
476  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
477  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
478  BBInfo *CvtBBI = &TrueBBI;
479  BBInfo *NextBBI = &FalseBBI;
480
481  std::vector<MachineOperand> Cond(BBI.BrCond);
482  if (ReverseCond) {
483    std::swap(CvtBBI, NextBBI);
484    TII->ReverseBranchCondition(Cond);
485  }
486
487  PredicateBlock(*CvtBBI, Cond);
488  // If the 'true' block ends without a branch, add a conditional branch
489  // to its successor unless that happens to be the 'false' block.
490  if (CvtBBI->IsAnalyzable && CvtBBI->TrueBB == NULL) {
491    assert(CvtBBI->BB->succ_size() == 1 && "Unexpected!");
492    MachineBasicBlock *SuccBB = *CvtBBI->BB->succ_begin();
493    if (SuccBB != NextBBI->BB)
494      TII->InsertBranch(*CvtBBI->BB, SuccBB, NULL, Cond);
495  }
496
497  // Merge converted block into entry block. Also add an unconditional branch
498  // to the 'false' branch.
499  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
500  MergeBlocks(BBI, *CvtBBI);
501  if (!isNextBlock(BBI.BB, NextBBI->BB))
502    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
503  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
504
505  // Update block info. BB can be iteratively if-converted.
506  BBI.Kind = ICNotAnalyzed;
507  BBI.TrueBB = BBI.FalseBB = NULL;
508  BBI.BrCond.clear();
509  TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
510  ReTryPreds(BBI.BB);
511  CvtBBI->Kind = ICDead;
512
513  // FIXME: Must maintain LiveIns.
514  return true;
515}
516
517/// IfConvertTriangle - If convert a triangle sub-CFG.
518///
519bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
520  BBI.Kind = ICNotClassfied;
521
522  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
523
524  // Predicate the 'true' block after removing its branch.
525  TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
526  PredicateBlock(TrueBBI, BBI.BrCond);
527
528  // If 'true' block has a 'false' successor, add an exit branch to it.
529  if (TrueBBI.FalseBB) {
530    std::vector<MachineOperand> RevCond(TrueBBI.BrCond);
531    if (TII->ReverseBranchCondition(RevCond))
532      assert(false && "Unable to reverse branch condition!");
533    TII->InsertBranch(*BBI.TrueBB, TrueBBI.FalseBB, NULL, RevCond);
534  }
535
536  // Join the 'true' and 'false' blocks if the 'false' block has no other
537  // predecessors. Otherwise, add a unconditional branch from 'true' to 'false'.
538  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
539  if (FalseBBI.BB->pred_size() == 2)
540    MergeBlocks(TrueBBI, FalseBBI);
541  else if (!isNextBlock(TrueBBI.BB, FalseBBI.BB))
542    InsertUncondBranch(TrueBBI.BB, FalseBBI.BB, TII);
543
544  // Now merge the entry of the triangle with the true block.
545  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
546  MergeBlocks(BBI, TrueBBI);
547  std::copy(BBI.BrCond.begin(), BBI.BrCond.end(),
548            std::back_inserter(BBI.Predicate));
549
550  // Update block info. BB can be iteratively if-converted.
551  BBI.Kind = ICNotClassfied;
552  BBI.TrueBB = BBI.FalseBB = NULL;
553  BBI.BrCond.clear();
554  TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
555  ReTryPreds(BBI.BB);
556  TrueBBI.Kind = ICDead;
557
558  // FIXME: Must maintain LiveIns.
559  return true;
560}
561
562/// IfConvertDiamond - If convert a diamond sub-CFG.
563///
564bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
565  BBI.Kind = ICNotClassfied;
566
567  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
568  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
569
570  SmallVector<MachineInstr*, 2> Dups;
571  if (!BBI.TailBB) {
572    // No common merge block. Check if the terminators (e.g. return) are
573    // the same or predicable.
574    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
575    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
576    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
577      if (TT->isIdenticalTo(FT))
578        Dups.push_back(TT);  // Will erase these later.
579      else if (!TT->isPredicable() && !FT->isPredicable())
580        return false; // Can't if-convert. Abort!
581      ++TT;
582      ++FT;
583    }
584
585    // One of the two pathes have more terminators, make sure they are
586    // all predicable.
587    while (TT != BBI.TrueBB->end()) {
588      if (!TT->isPredicable()) {
589        return false; // Can't if-convert. Abort!
590      }
591      ++TT;
592    }
593    while (FT != BBI.FalseBB->end()) {
594      if (!FT->isPredicable()) {
595        return false; // Can't if-convert. Abort!
596      }
597      ++FT;
598    }
599  }
600
601  // Remove the duplicated instructions from the 'true' block.
602  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
603    Dups[i]->eraseFromParent();
604    --TrueBBI.NonPredSize;
605  }
606
607  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
608  // Either the block fallthrough to another block or it ends with a
609  // return. If it's the former, add a branch to its successor.
610  bool TrueNeedBr  = !TrueBBI.TrueBB && BBI.TrueBB->succ_size();
611  bool FalseNeedBr = !FalseBBI.TrueBB && BBI.FalseBB->succ_size();
612
613  // Merge the 'true' and 'false' blocks by copying the instructions
614  // from the 'false' block to the 'true' block. That is, unless the true
615  // block would clobber the predicate, in that case, do the opposite.
616  std::vector<MachineOperand> RevCond(BBI.BrCond);
617  TII->ReverseBranchCondition(RevCond);
618  BBInfo *CvtBBI;
619  if (!TrueBBI.ModifyPredicate) {
620    // Predicate the 'true' block after removing its branch.
621    TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
622    PredicateBlock(TrueBBI, BBI.BrCond);
623
624    // Predicate the 'false' block.
625    PredicateBlock(FalseBBI, RevCond, true);
626
627    if (TrueNeedBr)
628      TII->InsertBranch(*BBI.TrueBB, *BBI.TrueBB->succ_begin(), NULL,
629                        BBI.BrCond);
630    // Add an unconditional branch from 'false' to to 'false' successor if it
631    // will not be the fallthrough block.
632    if (FalseNeedBr &&
633        !isNextBlock(BBI.BB, *BBI.FalseBB->succ_begin()))
634      InsertUncondBranch(BBI.FalseBB, *BBI.FalseBB->succ_begin(), TII);
635    MergeBlocks(TrueBBI, FalseBBI);
636    CvtBBI = &TrueBBI;
637  } else {
638    // Predicate the 'false' block after removing its branch.
639    FalseBBI.NonPredSize -= TII->RemoveBranch(*BBI.FalseBB);
640    PredicateBlock(FalseBBI, RevCond);
641
642    // Predicate the 'false' block.
643    PredicateBlock(TrueBBI, BBI.BrCond, true);
644
645    // Add a conditional branch from 'false' to 'false' successor if needed.
646    if (FalseNeedBr)
647      TII->InsertBranch(*BBI.FalseBB, *BBI.FalseBB->succ_begin(), NULL,
648                        RevCond);
649    // Add an unconditional branch from 'true' to to 'true' successor if it
650    // will not be the fallthrough block.
651    if (TrueNeedBr &&
652        !isNextBlock(BBI.BB, *BBI.TrueBB->succ_begin()))
653      InsertUncondBranch(BBI.TrueBB, *BBI.TrueBB->succ_begin(), TII);
654    MergeBlocks(FalseBBI, TrueBBI);
655    CvtBBI = &FalseBBI;
656  }
657
658  // Remove the conditional branch from entry to the blocks.
659  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
660
661  bool OkToIfcvt = true;
662  // Merge the combined block into the entry of the diamond if the entry
663  // block is its only predecessor. Otherwise, insert an unconditional
664  // branch from entry to the if-converted block.
665  if (CvtBBI->BB->pred_size() == 1) {
666    MergeBlocks(BBI, *CvtBBI);
667    CvtBBI = &BBI;
668    OkToIfcvt = false;
669  } else if (!isNextBlock(BBI.BB, CvtBBI->BB))
670    InsertUncondBranch(BBI.BB, CvtBBI->BB, TII);
671
672  // If the if-converted block fallthrough or unconditionally branch into the
673  // tail block, and the tail block does not have other predecessors, then
674  // fold the tail block in as well.
675  if (BBI.TailBB &&
676      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
677    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
678    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
679    MergeBlocks(*CvtBBI, TailBBI);
680    TailBBI.Kind = ICDead;
681  }
682
683  // Update block info. BB may be iteratively if-converted.
684  if (OkToIfcvt) {
685    BBI.Kind = ICNotClassfied;
686    BBI.TrueBB = BBI.FalseBB = NULL;
687    BBI.BrCond.clear();
688    TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
689    ReTryPreds(BBI.BB);
690  }
691  TrueBBI.Kind = ICDead;
692  FalseBBI.Kind = ICDead;
693
694  // FIXME: Must maintain LiveIns.
695  return true;
696}
697
698/// PredicateBlock - Predicate every instruction in the block with the specified
699/// condition. If IgnoreTerm is true, skip over all terminator instructions.
700void IfConverter::PredicateBlock(BBInfo &BBI,
701                                 std::vector<MachineOperand> &Cond,
702                                 bool IgnoreTerm) {
703  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
704       I != E; ++I) {
705    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
706      continue;
707    if (TII->isPredicated(I))
708      continue;
709    if (!TII->PredicateInstruction(I, Cond)) {
710      cerr << "Unable to predicate " << *I << "!\n";
711      abort();
712    }
713  }
714
715  BBI.NonPredSize = 0;
716  NumIfConvBBs++;
717}
718
719/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
720///
721static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
722  for (MachineBasicBlock::pred_iterator I = FromBB->pred_begin(),
723         E = FromBB->pred_end(); I != E; ++I) {
724    MachineBasicBlock *Pred = *I;
725    Pred->removeSuccessor(FromBB);
726    if (!Pred->isSuccessor(ToBB))
727      Pred->addSuccessor(ToBB);
728  }
729}
730
731/// TransferSuccs - Transfer all the successors of FromBB to ToBB.
732///
733static void TransferSuccs(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
734  for (MachineBasicBlock::succ_iterator I = FromBB->succ_begin(),
735         E = FromBB->succ_end(); I != E; ++I) {
736    FromBB->removeSuccessor(*I);
737    if (!ToBB->isSuccessor(*I))
738      ToBB->addSuccessor(*I);
739  }
740}
741
742/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
743///
744void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
745  ToBBI.BB->splice(ToBBI.BB->end(),
746                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
747
748  // If FromBBI is previously a successor, remove it from ToBBI's successor
749  // list and update its TrueBB / FalseBB field if needed.
750  if (ToBBI.BB->isSuccessor(FromBBI.BB))
751    ToBBI.BB->removeSuccessor(FromBBI.BB);
752
753  // Redirect all branches to FromBB to ToBB.
754  for (MachineBasicBlock::pred_iterator I = FromBBI.BB->pred_begin(),
755         E = FromBBI.BB->pred_end(); I != E; ++I)
756    (*I)->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
757
758  // Transfer preds / succs and update size.
759  TransferPreds(ToBBI.BB, FromBBI.BB);
760  TransferSuccs(ToBBI.BB, FromBBI.BB);
761  ToBBI.NonPredSize += FromBBI.NonPredSize;
762  FromBBI.NonPredSize = 0;
763}
764