IfConversion.cpp revision f5305f9cc767ac18c258b03425c2c26345003acc 
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  if (TrueBBI.FalseBB) {
531    std::vector<MachineOperand> RevCond(TrueBBI.BrCond);
532    if (TII->ReverseBranchCondition(RevCond))
533      assert(false && "Unable to reverse branch condition!");
534    TII->InsertBranch(*BBI.TrueBB, TrueBBI.FalseBB, NULL, RevCond);
535  }
536
537  // Join the 'true' and 'false' blocks if the 'false' block has no other
538  // predecessors. Otherwise, add a unconditional branch from 'true' to 'false'.
539  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
540  bool FalseBBDead = false;
541  if (FalseBBI.BB->pred_size() == 2) {
542    MergeBlocks(TrueBBI, FalseBBI);
543    FalseBBDead = true;
544  } else if (!isNextBlock(TrueBBI.BB, FalseBBI.BB))
545    InsertUncondBranch(TrueBBI.BB, FalseBBI.BB, TII);
546
547  // Now merge the entry of the triangle with the true block.
548  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
549  MergeBlocks(BBI, TrueBBI);
550  std::copy(BBI.BrCond.begin(), BBI.BrCond.end(),
551            std::back_inserter(BBI.Predicate));
552
553  // Update block info. BB can be iteratively if-converted.
554  BBI.Kind = ICReAnalyze;
555  ReTryPreds(BBI.BB);
556  TrueBBI.Kind = ICDead;
557  if (FalseBBDead)
558    FalseBBI.Kind = ICDead;
559
560  // FIXME: Must maintain LiveIns.
561  return true;
562}
563
564/// IfConvertDiamond - If convert a diamond sub-CFG.
565///
566bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
567  BBI.Kind = ICNotClassfied;
568
569  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
570  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
571
572  SmallVector<MachineInstr*, 2> Dups;
573  if (!BBI.TailBB) {
574    // No common merge block. Check if the terminators (e.g. return) are
575    // the same or predicable.
576    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
577    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
578    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
579      if (TT->isIdenticalTo(FT))
580        Dups.push_back(TT);  // Will erase these later.
581      else if (!TT->isPredicable() && !FT->isPredicable())
582        return false; // Can't if-convert. Abort!
583      ++TT;
584      ++FT;
585    }
586
587    // One of the two pathes have more terminators, make sure they are
588    // all predicable.
589    while (TT != BBI.TrueBB->end()) {
590      if (!TT->isPredicable()) {
591        return false; // Can't if-convert. Abort!
592      }
593      ++TT;
594    }
595    while (FT != BBI.FalseBB->end()) {
596      if (!FT->isPredicable()) {
597        return false; // Can't if-convert. Abort!
598      }
599      ++FT;
600    }
601  }
602
603  // Remove the duplicated instructions from the 'true' block.
604  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
605    Dups[i]->eraseFromParent();
606    --TrueBBI.NonPredSize;
607  }
608
609  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
610  // Either the block fallthrough to another block or it ends with a
611  // return. If it's the former, add a branch to its successor.
612  bool TrueNeedBr  = !TrueBBI.TrueBB && BBI.TrueBB->succ_size();
613  bool FalseNeedBr = !FalseBBI.TrueBB && BBI.FalseBB->succ_size();
614
615  // Merge the 'true' and 'false' blocks by copying the instructions
616  // from the 'false' block to the 'true' block. That is, unless the true
617  // block would clobber the predicate, in that case, do the opposite.
618  std::vector<MachineOperand> RevCond(BBI.BrCond);
619  TII->ReverseBranchCondition(RevCond);
620  BBInfo *CvtBBI;
621  if (!TrueBBI.ModifyPredicate) {
622    // Predicate the 'true' block after removing its branch.
623    TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
624    PredicateBlock(TrueBBI, BBI.BrCond);
625
626    // Predicate the 'false' block.
627    PredicateBlock(FalseBBI, RevCond, true);
628
629    if (TrueNeedBr)
630      TII->InsertBranch(*BBI.TrueBB, *BBI.TrueBB->succ_begin(), NULL,
631                        BBI.BrCond);
632    // Add an unconditional branch from 'false' to to 'false' successor if it
633    // will not be the fallthrough block.
634    if (FalseNeedBr &&
635        !isNextBlock(BBI.BB, *BBI.FalseBB->succ_begin()))
636      InsertUncondBranch(BBI.FalseBB, *BBI.FalseBB->succ_begin(), TII);
637    MergeBlocks(TrueBBI, FalseBBI);
638    CvtBBI = &TrueBBI;
639  } else {
640    // Predicate the 'false' block after removing its branch.
641    FalseBBI.NonPredSize -= TII->RemoveBranch(*BBI.FalseBB);
642    PredicateBlock(FalseBBI, RevCond);
643
644    // Predicate the 'false' block.
645    PredicateBlock(TrueBBI, BBI.BrCond, true);
646
647    // Add a conditional branch from 'false' to 'false' successor if needed.
648    if (FalseNeedBr)
649      TII->InsertBranch(*BBI.FalseBB, *BBI.FalseBB->succ_begin(), NULL,
650                        RevCond);
651    // Add an unconditional branch from 'true' to to 'true' successor if it
652    // will not be the fallthrough block.
653    if (TrueNeedBr &&
654        !isNextBlock(BBI.BB, *BBI.TrueBB->succ_begin()))
655      InsertUncondBranch(BBI.TrueBB, *BBI.TrueBB->succ_begin(), TII);
656    MergeBlocks(FalseBBI, TrueBBI);
657    CvtBBI = &FalseBBI;
658  }
659
660  // Remove the conditional branch from entry to the blocks.
661  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
662
663  bool OkToIfcvt = true;
664  // Merge the combined block into the entry of the diamond if the entry
665  // block is its only predecessor. Otherwise, insert an unconditional
666  // branch from entry to the if-converted block.
667  if (CvtBBI->BB->pred_size() == 1) {
668    MergeBlocks(BBI, *CvtBBI);
669    CvtBBI = &BBI;
670    OkToIfcvt = false;
671  } else if (!isNextBlock(BBI.BB, CvtBBI->BB))
672    InsertUncondBranch(BBI.BB, CvtBBI->BB, TII);
673
674  // If the if-converted block fallthrough or unconditionally branch into the
675  // tail block, and the tail block does not have other predecessors, then
676  // fold the tail block in as well.
677  if (BBI.TailBB &&
678      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
679    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
680    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
681    MergeBlocks(*CvtBBI, TailBBI);
682    TailBBI.Kind = ICDead;
683  }
684
685  // Update block info. BB may be iteratively if-converted.
686  if (OkToIfcvt) {
687    BBI.Kind = ICReAnalyze;
688    ReTryPreds(BBI.BB);
689  }
690  TrueBBI.Kind = ICDead;
691  FalseBBI.Kind = ICDead;
692
693  // FIXME: Must maintain LiveIns.
694  return true;
695}
696
697/// PredicateBlock - Predicate every instruction in the block with the specified
698/// condition. If IgnoreTerm is true, skip over all terminator instructions.
699void IfConverter::PredicateBlock(BBInfo &BBI,
700                                 std::vector<MachineOperand> &Cond,
701                                 bool IgnoreTerm) {
702  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
703       I != E; ++I) {
704    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
705      continue;
706    if (TII->isPredicated(I))
707      continue;
708    if (!TII->PredicateInstruction(I, Cond)) {
709      cerr << "Unable to predicate " << *I << "!\n";
710      abort();
711    }
712  }
713
714  BBI.NonPredSize = 0;
715  NumIfConvBBs++;
716}
717
718/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
719///
720static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
721  for (MachineBasicBlock::pred_iterator I = FromBB->pred_begin(),
722         E = FromBB->pred_end(); I != E; ++I) {
723    MachineBasicBlock *Pred = *I;
724    Pred->removeSuccessor(FromBB);
725    if (!Pred->isSuccessor(ToBB))
726      Pred->addSuccessor(ToBB);
727  }
728}
729
730/// TransferSuccs - Transfer all the successors of FromBB to ToBB.
731///
732static void TransferSuccs(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
733  for (MachineBasicBlock::succ_iterator I = FromBB->succ_begin(),
734         E = FromBB->succ_end(); I != E; ++I) {
735    MachineBasicBlock *Succ = *I;
736    FromBB->removeSuccessor(Succ);
737    if (!ToBB->isSuccessor(Succ))
738      ToBB->addSuccessor(Succ);
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