IfConversion.cpp revision 8ed680cce3d0cff361f352abce75a917093f0656 
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 'children' have been modified, abort!
157      case ICDead:
158        // Block has been already been if-converted, abort!
159        break;
160      case ICSimple:
161      case ICSimpleFalse:
162        DOUT << "Ifcvt (Simple" << (BBI.Kind == ICSimpleFalse ? " false" : "")
163             << "): BB#" << BBI.BB->getNumber() << " ";
164        RetVal = IfConvertSimple(BBI);
165        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
166        if (RetVal)
167          if (BBI.Kind == ICSimple) NumSimple++;
168          else                      NumSimpleRev++;
169       break;
170      case ICTriangle:
171        DOUT << "Ifcvt (Triangle): BB#" << BBI.BB->getNumber() << " ";
172        RetVal = IfConvertTriangle(BBI);
173        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
174        if (RetVal) NumTriangle++;
175        break;
176      case ICDiamond:
177        DOUT << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " ";
178        RetVal = IfConvertDiamond(BBI);
179        DOUT << (RetVal ? "succeeded!" : "failed!") << "\n";
180        if (RetVal) NumDiamonds++;
181        break;
182      }
183      Change |= RetVal;
184    }
185
186    if (!Change)
187      break;
188    MadeChange |= Change;
189  }
190
191  Roots.clear();
192  BBAnalysis.clear();
193
194  return MadeChange;
195}
196
197static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
198                                         MachineBasicBlock *TrueBB) {
199  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
200         E = BB->succ_end(); SI != E; ++SI) {
201    MachineBasicBlock *SuccBB = *SI;
202    if (SuccBB != TrueBB)
203      return SuccBB;
204  }
205  return NULL;
206}
207
208bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
209  if (!TII->ReverseBranchCondition(BBI.BrCond)) {
210    TII->RemoveBranch(*BBI.BB);
211    TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond);
212    std::swap(BBI.TrueBB, BBI.FalseBB);
213    return true;
214  }
215  return false;
216}
217
218/// StructuralAnalysis - Analyze the structure of the sub-CFG starting from
219/// the specified block. Record its successors and whether it looks like an
220/// if-conversion candidate.
221void IfConverter::StructuralAnalysis(MachineBasicBlock *BB) {
222  BBInfo &BBI = BBAnalysis[BB->getNumber()];
223
224  if (BBI.Kind == ICReAnalyze) {
225    BBI.BrCond.clear();
226    BBI.TrueBB = BBI.FalseBB = NULL;
227  } else {
228    if (BBI.Kind != ICNotAnalyzed)
229      return;  // Already analyzed.
230    BBI.BB = BB;
231    BBI.NonPredSize = std::distance(BB->begin(), BB->end());
232  }
233
234  // Look for 'root' of a simple (non-nested) triangle or diamond.
235  BBI.Kind = ICNotClassfied;
236  BBI.IsAnalyzable =
237    !TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
238  if (!BBI.IsAnalyzable || BBI.BrCond.size() == 0)
239    return;
240
241  StructuralAnalysis(BBI.TrueBB);
242  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
243
244  // No false branch. This BB must end with a conditional branch and a
245  // fallthrough.
246  if (!BBI.FalseBB)
247    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
248  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
249
250  StructuralAnalysis(BBI.FalseBB);
251  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
252
253  // Look for more opportunities to if-convert a triangle. Try to restructure
254  // the CFG to form a triangle with the 'false' path.
255  std::vector<MachineOperand> RevCond(BBI.BrCond);
256  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
257  if (FalseBBI.FalseBB) {
258    if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB)
259      return;
260    std::vector<MachineOperand> Cond(BBI.BrCond);
261    if (CanRevCond &&
262        FalseBBI.TrueBB && FalseBBI.BB->pred_size() == 1 &&
263        FeasibilityAnalysis(FalseBBI, RevCond, true)) {
264      std::vector<MachineOperand> FalseCond(FalseBBI.BrCond);
265      if (FalseBBI.TrueBB == BBI.TrueBB &&
266          TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
267        // Reverse 'true' and 'false' paths.
268        ReverseBranchCondition(BBI);
269        BBI.Kind = ICTriangle;
270        FalseBBI.Kind = ICChild;
271      } else if (FalseBBI.FalseBB == BBI.TrueBB &&
272                 !TII->ReverseBranchCondition(FalseCond) &&
273                 TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
274        // Reverse 'false' block's 'true' and 'false' paths and then
275        // reverse 'true' and 'false' paths.
276        ReverseBranchCondition(FalseBBI);
277        ReverseBranchCondition(BBI);
278        BBI.Kind = ICTriangle;
279        FalseBBI.Kind = ICChild;
280      }
281    }
282  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB && CanRevCond &&
283             TrueBBI.BB->pred_size() == 1 &&
284             TrueBBI.BB->pred_size() == 1 &&
285             // Check the 'true' and 'false' blocks if either isn't ended with
286             // a branch. If the block does not fallthrough to another block
287             // then we need to add a branch to its successor.
288             !(TrueBBI.ModifyPredicate &&
289               !TrueBBI.TrueBB && TrueBBI.BB->succ_size()) &&
290             !(FalseBBI.ModifyPredicate &&
291               !FalseBBI.TrueBB && FalseBBI.BB->succ_size()) &&
292             FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
293             FeasibilityAnalysis(FalseBBI, RevCond)) {
294    // Diamond:
295    //   EBB
296    //   / \_
297    //  |   |
298    // TBB FBB
299    //   \ /
300    //  TailBB
301    // Note MBB can be empty in case both TBB and FBB are return blocks.
302    BBI.Kind = ICDiamond;
303    TrueBBI.Kind = FalseBBI.Kind = ICChild;
304    BBI.TailBB = TrueBBI.TrueBB;
305  } else {
306    // FIXME: Consider duplicating if BB is small.
307    bool TryTriangle = TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB &&
308                       BBI.TrueBB->pred_size() == 1;
309    bool TrySimple = TrueBBI.BrCond.size() == 0 && BBI.TrueBB->pred_size() == 1;
310    if ((TryTriangle || TrySimple) &&
311        FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
312      if (TryTriangle) {
313        // Triangle:
314        //   EBB
315        //   | \_
316        //   |  |
317        //   | TBB
318        //   |  /
319        //   FBB
320        BBI.Kind = ICTriangle;
321        TrueBBI.Kind = ICChild;
322      } else {
323        // Simple (split, no rejoin):
324        //   EBB
325        //   | \_
326        //   |  |
327        //   | TBB---> exit
328        //   |
329        //   FBB
330        BBI.Kind = ICSimple;
331        TrueBBI.Kind = ICChild;
332      }
333    } else if (FalseBBI.BrCond.size() == 0 && BBI.FalseBB->pred_size() == 1) {
334      // Try 'simple' on the other path...
335      std::vector<MachineOperand> RevCond(BBI.BrCond);
336      if (TII->ReverseBranchCondition(RevCond))
337        assert(false && "Unable to reverse branch condition!");
338      if (FeasibilityAnalysis(FalseBBI, RevCond)) {
339        BBI.Kind = ICSimpleFalse;
340        FalseBBI.Kind = ICChild;
341      }
342    }
343  }
344  return;
345}
346
347/// FeasibilityAnalysis - Determine if the block is predicable. In most
348/// cases, that means all the instructions in the block has M_PREDICABLE flag.
349/// Also checks if the block contains any instruction which can clobber a
350/// predicate (e.g. condition code register). If so, the block is not
351/// predicable unless it's the last instruction. If IgnoreTerm is true then
352/// all the terminator instructions are skipped.
353bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
354                                      std::vector<MachineOperand> &Cond,
355                                      bool IgnoreTerm) {
356  // If the block is dead, or it is going to be the entry block of a sub-CFG
357  // that will be if-converted, then it cannot be predicated.
358  if (BBI.Kind != ICNotAnalyzed &&
359      BBI.Kind != ICNotClassfied &&
360      BBI.Kind != ICChild)
361    return false;
362
363  // Check predication threshold.
364  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
365    return false;
366
367  // If it is already predicated, check if its predicate subsumes the new
368  // predicate.
369  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Cond))
370    return false;
371
372  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
373       I != E; ++I) {
374    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
375      continue;
376    // TODO: check if instruction clobbers predicate.
377    if (!I->isPredicable())
378      return false;
379  }
380
381  return true;
382}
383
384/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
385/// expose more if-conversion opportunities. e.g.
386///
387///                cmp
388///                b le BB1
389///                /  \____
390///               /        |
391///             cmp        |
392///             b eq BB1   |
393///              /  \____  |
394///             /        \ |
395///                      BB1
396///  ==>
397///
398///                cmp
399///                b eq BB1
400///                /  \____
401///               /        |
402///             cmp        |
403///             b le BB1   |
404///              /  \____  |
405///             /        \ |
406///                      BB1
407bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
408  return false;
409}
410
411/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
412/// candidates. It returns true if any CFG restructuring is done to expose more
413/// if-conversion opportunities.
414bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
415                                std::vector<BBInfo*> &Candidates) {
416  bool Change = false;
417  std::set<MachineBasicBlock*> Visited;
418  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
419    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
420           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
421      MachineBasicBlock *BB = *I;
422      StructuralAnalysis(BB);
423      BBInfo &BBI = BBAnalysis[BB->getNumber()];
424      switch (BBI.Kind) {
425        case ICSimple:
426        case ICSimpleFalse:
427        case ICTriangle:
428        case ICDiamond:
429          Candidates.push_back(&BBI);
430          break;
431        default:
432          Change |= AttemptRestructuring(BBI);
433          break;
434      }
435    }
436  }
437
438  // Sort to favor more complex ifcvt scheme.
439  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
440
441  return Change;
442}
443
444/// isNextBlock - Returns true either if ToBB the next block after BB or
445/// that all the intervening blocks are empty.
446static bool isNextBlock(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
447  MachineFunction *MF = BB->getParent();
448  MachineFunction::iterator I = BB;
449  while (++I != MF->end() && I != MachineFunction::iterator(ToBB))
450    if (!I->empty())
451      return false;
452  return true;
453}
454
455/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
456/// to determine if it can be if-converted.
457void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
458  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
459         E = BB->pred_end(); PI != E; ++PI) {
460    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
461    PBBI.Kind = ICReAnalyze;
462  }
463}
464
465/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
466///
467static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
468                               const TargetInstrInfo *TII) {
469  std::vector<MachineOperand> NoCond;
470  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
471}
472
473/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
474///
475bool IfConverter::IfConvertSimple(BBInfo &BBI) {
476  bool ReverseCond = BBI.Kind == ICSimpleFalse;
477
478  BBI.Kind = ICNotClassfied;
479
480  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
481  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
482  BBInfo *CvtBBI = &TrueBBI;
483  BBInfo *NextBBI = &FalseBBI;
484
485  std::vector<MachineOperand> Cond(BBI.BrCond);
486  if (ReverseCond) {
487    std::swap(CvtBBI, NextBBI);
488    TII->ReverseBranchCondition(Cond);
489  }
490
491  PredicateBlock(*CvtBBI, Cond);
492  // If the 'true' block ends without a branch, add a conditional branch
493  // to its successor unless that happens to be the 'false' block.
494  if (CvtBBI->IsAnalyzable && CvtBBI->TrueBB == NULL) {
495    assert(CvtBBI->BB->succ_size() == 1 && "Unexpected!");
496    MachineBasicBlock *SuccBB = *CvtBBI->BB->succ_begin();
497    if (SuccBB != NextBBI->BB)
498      TII->InsertBranch(*CvtBBI->BB, SuccBB, NULL, Cond);
499  }
500
501  // Merge converted block into entry block. Also add an unconditional branch
502  // to the 'false' branch.
503  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
504  MergeBlocks(BBI, *CvtBBI);
505  if (!isNextBlock(BBI.BB, NextBBI->BB))
506    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
507  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
508
509  // Update block info. BB can be iteratively if-converted.
510  BBI.Kind = ICReAnalyze;
511  ReTryPreds(BBI.BB);
512  CvtBBI->Kind = ICDead;
513
514  // FIXME: Must maintain LiveIns.
515  return true;
516}
517
518/// IfConvertTriangle - If convert a triangle sub-CFG.
519///
520bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
521  BBI.Kind = ICNotClassfied;
522
523  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
524
525  // Predicate the 'true' block after removing its branch.
526  TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
527  PredicateBlock(TrueBBI, BBI.BrCond);
528
529  // If 'true' block has a 'false' successor, add an exit branch to it.
530  bool HasEarlyExit = TrueBBI.FalseBB != NULL;
531  if (HasEarlyExit) {
532    std::vector<MachineOperand> RevCond(TrueBBI.BrCond);
533    if (TII->ReverseBranchCondition(RevCond))
534      assert(false && "Unable to reverse branch condition!");
535    TII->InsertBranch(*BBI.TrueBB, TrueBBI.FalseBB, NULL, RevCond);
536  }
537
538  // Join the 'true' and 'false' blocks if the 'false' block has no other
539  // predecessors. Otherwise, add a unconditional branch from 'true' to 'false'.
540  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
541  bool FalseBBDead = false;
542  if (!HasEarlyExit && FalseBBI.BB->pred_size() == 2) {
543    MergeBlocks(TrueBBI, FalseBBI);
544    FalseBBDead = true;
545  } else if (!isNextBlock(TrueBBI.BB, FalseBBI.BB))
546    InsertUncondBranch(TrueBBI.BB, FalseBBI.BB, TII);
547
548  // Now merge the entry of the triangle with the true block.
549  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
550  MergeBlocks(BBI, TrueBBI);
551  std::copy(BBI.BrCond.begin(), BBI.BrCond.end(),
552            std::back_inserter(BBI.Predicate));
553
554  // Update block info. BB can be iteratively if-converted.
555  BBI.Kind = ICReAnalyze;
556  ReTryPreds(BBI.BB);
557  TrueBBI.Kind = ICDead;
558  if (FalseBBDead)
559    FalseBBI.Kind = ICDead;
560
561  // FIXME: Must maintain LiveIns.
562  return true;
563}
564
565/// IfConvertDiamond - If convert a diamond sub-CFG.
566///
567bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
568  BBI.Kind = ICNotClassfied;
569
570  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
571  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
572
573  SmallVector<MachineInstr*, 2> Dups;
574  if (!BBI.TailBB) {
575    // No common merge block. Check if the terminators (e.g. return) are
576    // the same or predicable.
577    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
578    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
579    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
580      if (TT->isIdenticalTo(FT))
581        Dups.push_back(TT);  // Will erase these later.
582      else if (!TT->isPredicable() && !FT->isPredicable())
583        return false; // Can't if-convert. Abort!
584      ++TT;
585      ++FT;
586    }
587
588    // One of the two pathes have more terminators, make sure they are
589    // all predicable.
590    while (TT != BBI.TrueBB->end()) {
591      if (!TT->isPredicable()) {
592        return false; // Can't if-convert. Abort!
593      }
594      ++TT;
595    }
596    while (FT != BBI.FalseBB->end()) {
597      if (!FT->isPredicable()) {
598        return false; // Can't if-convert. Abort!
599      }
600      ++FT;
601    }
602  }
603
604  // Remove the duplicated instructions from the 'true' block.
605  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
606    Dups[i]->eraseFromParent();
607    --TrueBBI.NonPredSize;
608  }
609
610  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
611  // Either the block fallthrough to another block or it ends with a
612  // return. If it's the former, add a branch to its successor.
613  bool TrueNeedBr  = !TrueBBI.TrueBB && BBI.TrueBB->succ_size();
614  bool FalseNeedBr = !FalseBBI.TrueBB && BBI.FalseBB->succ_size();
615
616  // Merge the 'true' and 'false' blocks by copying the instructions
617  // from the 'false' block to the 'true' block. That is, unless the true
618  // block would clobber the predicate, in that case, do the opposite.
619  std::vector<MachineOperand> RevCond(BBI.BrCond);
620  TII->ReverseBranchCondition(RevCond);
621  BBInfo *CvtBBI;
622  if (!TrueBBI.ModifyPredicate) {
623    // Predicate the 'true' block after removing its branch.
624    TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
625    PredicateBlock(TrueBBI, BBI.BrCond);
626
627    // Predicate the 'false' block.
628    PredicateBlock(FalseBBI, RevCond, true);
629
630    if (TrueNeedBr)
631      TII->InsertBranch(*BBI.TrueBB, *BBI.TrueBB->succ_begin(), NULL,
632                        BBI.BrCond);
633    // Add an unconditional branch from 'false' to to 'false' successor if it
634    // will not be the fallthrough block.
635    if (FalseNeedBr &&
636        !isNextBlock(BBI.BB, *BBI.FalseBB->succ_begin()))
637      InsertUncondBranch(BBI.FalseBB, *BBI.FalseBB->succ_begin(), TII);
638    MergeBlocks(TrueBBI, FalseBBI);
639    CvtBBI = &TrueBBI;
640  } else {
641    // Predicate the 'false' block after removing its branch.
642    FalseBBI.NonPredSize -= TII->RemoveBranch(*BBI.FalseBB);
643    PredicateBlock(FalseBBI, RevCond);
644
645    // Predicate the 'false' block.
646    PredicateBlock(TrueBBI, BBI.BrCond, true);
647
648    // Add a conditional branch from 'false' to 'false' successor if needed.
649    if (FalseNeedBr)
650      TII->InsertBranch(*BBI.FalseBB, *BBI.FalseBB->succ_begin(), NULL,
651                        RevCond);
652    // Add an unconditional branch from 'true' to to 'true' successor if it
653    // will not be the fallthrough block.
654    if (TrueNeedBr &&
655        !isNextBlock(BBI.BB, *BBI.TrueBB->succ_begin()))
656      InsertUncondBranch(BBI.TrueBB, *BBI.TrueBB->succ_begin(), TII);
657    MergeBlocks(FalseBBI, TrueBBI);
658    CvtBBI = &FalseBBI;
659  }
660
661  // Remove the conditional branch from entry to the blocks.
662  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
663
664  bool OkToIfcvt = true;
665  // Merge the combined block into the entry of the diamond if the entry
666  // block is its only predecessor. Otherwise, insert an unconditional
667  // branch from entry to the if-converted block.
668  if (CvtBBI->BB->pred_size() == 1) {
669    MergeBlocks(BBI, *CvtBBI);
670    CvtBBI = &BBI;
671    OkToIfcvt = false;
672  } else if (!isNextBlock(BBI.BB, CvtBBI->BB))
673    InsertUncondBranch(BBI.BB, CvtBBI->BB, TII);
674
675  // If the if-converted block fallthrough or unconditionally branch into the
676  // tail block, and the tail block does not have other predecessors, then
677  // fold the tail block in as well.
678  if (BBI.TailBB &&
679      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
680    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
681    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
682    MergeBlocks(*CvtBBI, TailBBI);
683    TailBBI.Kind = ICDead;
684  }
685
686  // Update block info. BB may be iteratively if-converted.
687  if (OkToIfcvt) {
688    BBI.Kind = ICReAnalyze;
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    MachineBasicBlock *Succ = *I;
737    FromBB->removeSuccessor(Succ);
738    if (!ToBB->isSuccessor(Succ))
739      ToBB->addSuccessor(Succ);
740  }
741}
742
743/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
744///
745void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
746  ToBBI.BB->splice(ToBBI.BB->end(),
747                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
748
749  // If FromBBI is previously a successor, remove it from ToBBI's successor
750  // list and update its TrueBB / FalseBB field if needed.
751  if (ToBBI.BB->isSuccessor(FromBBI.BB))
752    ToBBI.BB->removeSuccessor(FromBBI.BB);
753
754  // Redirect all branches to FromBB to ToBB.
755  for (MachineBasicBlock::pred_iterator I = FromBBI.BB->pred_begin(),
756         E = FromBBI.BB->pred_end(); I != E; ++I)
757    (*I)->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
758
759  // Transfer preds / succs and update size.
760  TransferPreds(ToBBI.BB, FromBBI.BB);
761  TransferSuccs(ToBBI.BB, FromBBI.BB);
762  ToBBI.NonPredSize += FromBBI.NonPredSize;
763  FromBBI.NonPredSize = 0;
764}
765