IfConversion.cpp revision e705213b679fdfc27b74b12adf7f40c94b97f9b9 
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  // Do not ifcvt if either path is a back edge to the entry block.
241  if (BBI.TrueBB == BB || BBI.FalseBB == BB)
242    return;
243
244  StructuralAnalysis(BBI.TrueBB);
245  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
246
247  // No false branch. This BB must end with a conditional branch and a
248  // fallthrough.
249  if (!BBI.FalseBB)
250    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
251  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
252
253  StructuralAnalysis(BBI.FalseBB);
254  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
255
256  // Look for more opportunities to if-convert a triangle. Try to restructure
257  // the CFG to form a triangle with the 'false' path.
258  std::vector<MachineOperand> RevCond(BBI.BrCond);
259  bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
260  if (FalseBBI.FalseBB) {
261    if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB)
262      return;
263    std::vector<MachineOperand> Cond(BBI.BrCond);
264    if (CanRevCond &&
265        FalseBBI.TrueBB && FalseBBI.BB->pred_size() == 1 &&
266        FeasibilityAnalysis(FalseBBI, RevCond, true)) {
267      std::vector<MachineOperand> FalseCond(FalseBBI.BrCond);
268      if (FalseBBI.TrueBB == BBI.TrueBB &&
269          TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
270        // Reverse 'true' and 'false' paths.
271        ReverseBranchCondition(BBI);
272        BBI.Kind = ICTriangle;
273        FalseBBI.Kind = ICChild;
274      } else if (FalseBBI.FalseBB == BBI.TrueBB &&
275                 !TII->ReverseBranchCondition(FalseCond) &&
276                 TII->SubsumesPredicate(FalseCond, BBI.BrCond)) {
277        // Reverse 'false' block's 'true' and 'false' paths and then
278        // reverse 'true' and 'false' paths.
279        ReverseBranchCondition(FalseBBI);
280        ReverseBranchCondition(BBI);
281        BBI.Kind = ICTriangle;
282        FalseBBI.Kind = ICChild;
283      }
284    }
285  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB && CanRevCond &&
286             TrueBBI.BB->pred_size() == 1 &&
287             FalseBBI.BB->pred_size() == 1 &&
288             // Check the 'true' and 'false' blocks if either isn't ended with
289             // a branch. If the block does not fallthrough to another block
290             // then we need to add a branch to its successor.
291             !(TrueBBI.ModifyPredicate &&
292               !TrueBBI.TrueBB && TrueBBI.BB->succ_size()) &&
293             !(FalseBBI.ModifyPredicate &&
294               !FalseBBI.TrueBB && FalseBBI.BB->succ_size()) &&
295             FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
296             FeasibilityAnalysis(FalseBBI, RevCond)) {
297    // Diamond:
298    //   EBB
299    //   / \_
300    //  |   |
301    // TBB FBB
302    //   \ /
303    //  TailBB
304    // Note TailBB can be empty.
305    BBI.Kind = ICDiamond;
306    TrueBBI.Kind = FalseBBI.Kind = ICChild;
307    BBI.TailBB = TrueBBI.TrueBB;
308  } else {
309    // FIXME: Consider duplicating if BB is small.
310    bool TryTriangle = TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB &&
311                       TrueBBI.BB->pred_size() == 1;
312    bool TrySimple = TrueBBI.BrCond.size() == 0 && TrueBBI.BB->pred_size() == 1;
313    if ((TryTriangle || TrySimple) &&
314        FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
315      if (TryTriangle) {
316        // Triangle:
317        //   EBB
318        //   | \_
319        //   |  |
320        //   | TBB
321        //   |  /
322        //   FBB
323        BBI.Kind = ICTriangle;
324        TrueBBI.Kind = ICChild;
325      } else {
326        // Simple (split, no rejoin):
327        //   EBB
328        //   | \_
329        //   |  |
330        //   | TBB---> exit
331        //   |
332        //   FBB
333        BBI.Kind = ICSimple;
334        TrueBBI.Kind = ICChild;
335      }
336    } else if (FalseBBI.BrCond.size() == 0 && FalseBBI.BB->pred_size() == 1) {
337      // Try the other path...
338      std::vector<MachineOperand> RevCond(BBI.BrCond);
339      if (!TII->ReverseBranchCondition(RevCond) &&
340          FeasibilityAnalysis(FalseBBI, RevCond)) {
341        if (FalseBBI.TrueBB && FalseBBI.TrueBB == BBI.TrueBB &&
342            FalseBBI.BB->pred_size() == 1) {
343          // Reverse 'true' and 'false' paths.
344          ReverseBranchCondition(BBI);
345          BBI.Kind = ICTriangle;
346          FalseBBI.Kind = ICChild;
347        } else {
348          BBI.Kind = ICSimpleFalse;
349          FalseBBI.Kind = ICChild;
350        }
351      }
352    }
353  }
354  return;
355}
356
357/// FeasibilityAnalysis - Determine if the block is predicable. In most
358/// cases, that means all the instructions in the block has M_PREDICABLE flag.
359/// Also checks if the block contains any instruction which can clobber a
360/// predicate (e.g. condition code register). If so, the block is not
361/// predicable unless it's the last instruction. If IgnoreTerm is true then
362/// all the terminator instructions are skipped.
363bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
364                                      std::vector<MachineOperand> &Cond,
365                                      bool IgnoreTerm) {
366  // If the block is dead, or it is going to be the entry block of a sub-CFG
367  // that will be if-converted, then it cannot be predicated.
368  if (BBI.Kind != ICNotAnalyzed &&
369      BBI.Kind != ICNotClassfied &&
370      BBI.Kind != ICChild)
371    return false;
372
373  // Check predication threshold.
374  if (BBI.NonPredSize == 0 || BBI.NonPredSize > TLI->getIfCvtBlockSizeLimit())
375    return false;
376
377  // If it is already predicated, check if its predicate subsumes the new
378  // predicate.
379  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Cond))
380    return false;
381
382  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
383       I != E; ++I) {
384    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
385      continue;
386    // TODO: check if instruction clobbers predicate.
387    if (!I->isPredicable())
388      return false;
389  }
390
391  return true;
392}
393
394/// AttemptRestructuring - Restructure the sub-CFG rooted in the given block to
395/// expose more if-conversion opportunities. e.g.
396///
397///                cmp
398///                b le BB1
399///                /  \____
400///               /        |
401///             cmp        |
402///             b eq BB1   |
403///              /  \____  |
404///             /        \ |
405///                      BB1
406///  ==>
407///
408///                cmp
409///                b eq BB1
410///                /  \____
411///               /        |
412///             cmp        |
413///             b le BB1   |
414///              /  \____  |
415///             /        \ |
416///                      BB1
417bool IfConverter::AttemptRestructuring(BBInfo &BBI) {
418  return false;
419}
420
421/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
422/// candidates. It returns true if any CFG restructuring is done to expose more
423/// if-conversion opportunities.
424bool IfConverter::AnalyzeBlocks(MachineFunction &MF,
425                                std::vector<BBInfo*> &Candidates) {
426  bool Change = false;
427  std::set<MachineBasicBlock*> Visited;
428  for (unsigned i = 0, e = Roots.size(); i != e; ++i) {
429    for (idf_ext_iterator<MachineBasicBlock*> I=idf_ext_begin(Roots[i],Visited),
430           E = idf_ext_end(Roots[i], Visited); I != E; ++I) {
431      MachineBasicBlock *BB = *I;
432      StructuralAnalysis(BB);
433      BBInfo &BBI = BBAnalysis[BB->getNumber()];
434      switch (BBI.Kind) {
435        case ICSimple:
436        case ICSimpleFalse:
437        case ICTriangle:
438        case ICDiamond:
439          Candidates.push_back(&BBI);
440          break;
441        default:
442          Change |= AttemptRestructuring(BBI);
443          break;
444      }
445    }
446  }
447
448  // Sort to favor more complex ifcvt scheme.
449  std::stable_sort(Candidates.begin(), Candidates.end(), IfcvtCandidateCmp);
450
451  return Change;
452}
453
454/// isNextBlock - Returns true either if ToBB the next block after BB or
455/// that all the intervening blocks are empty.
456static bool isNextBlock(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
457  MachineFunction::iterator I = BB;
458  MachineFunction::iterator TI = ToBB;
459  MachineFunction::iterator E = BB->getParent()->end();
460  while (++I != TI)
461    if (I == E || !I->empty())
462      return false;
463  return true;
464}
465
466/// ReTryPreds - Invalidate predecessor BB info so it would be re-analyzed
467/// to determine if it can be if-converted.
468void IfConverter::ReTryPreds(MachineBasicBlock *BB) {
469  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
470         E = BB->pred_end(); PI != E; ++PI) {
471    BBInfo &PBBI = BBAnalysis[(*PI)->getNumber()];
472    PBBI.Kind = ICReAnalyze;
473  }
474}
475
476/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
477///
478static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
479                               const TargetInstrInfo *TII) {
480  std::vector<MachineOperand> NoCond;
481  TII->InsertBranch(*BB, ToBB, NULL, NoCond);
482}
483
484/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
485///
486bool IfConverter::IfConvertSimple(BBInfo &BBI) {
487  bool ReverseCond = BBI.Kind == ICSimpleFalse;
488
489  BBI.Kind = ICNotClassfied;
490
491  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
492  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
493  BBInfo *CvtBBI = &TrueBBI;
494  BBInfo *NextBBI = &FalseBBI;
495
496  std::vector<MachineOperand> Cond(BBI.BrCond);
497  if (ReverseCond) {
498    std::swap(CvtBBI, NextBBI);
499    TII->ReverseBranchCondition(Cond);
500  }
501
502  PredicateBlock(*CvtBBI, Cond);
503  // If the 'true' block ends without a branch, add a conditional branch
504  // to its successor unless that happens to be the 'false' block.
505  if (CvtBBI->IsAnalyzable && CvtBBI->TrueBB == NULL) {
506    assert(CvtBBI->BB->succ_size() == 1 && "Unexpected!");
507    MachineBasicBlock *SuccBB = *CvtBBI->BB->succ_begin();
508    if (SuccBB != NextBBI->BB)
509      TII->InsertBranch(*CvtBBI->BB, SuccBB, NULL, Cond);
510  }
511
512  // Merge converted block into entry block. Also add an unconditional branch
513  // to the 'false' branch.
514  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
515  MergeBlocks(BBI, *CvtBBI);
516  if (!isNextBlock(BBI.BB, NextBBI->BB))
517    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
518  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));
519
520  // Update block info. BB can be iteratively if-converted.
521  BBI.Kind = ICReAnalyze;
522  ReTryPreds(BBI.BB);
523  CvtBBI->Kind = ICDead;
524
525  // FIXME: Must maintain LiveIns.
526  return true;
527}
528
529/// IfConvertTriangle - If convert a triangle sub-CFG.
530///
531bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
532  BBI.Kind = ICNotClassfied;
533
534  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
535
536  // Predicate the 'true' block after removing its branch.
537  TrueBBI.NonPredSize -= TII->RemoveBranch(*BBI.TrueBB);
538  PredicateBlock(TrueBBI, BBI.BrCond);
539
540  // If 'true' block has a 'false' successor, add an exit branch to it.
541  bool HasEarlyExit = TrueBBI.FalseBB != NULL;
542  if (HasEarlyExit) {
543    std::vector<MachineOperand> RevCond(TrueBBI.BrCond);
544    if (TII->ReverseBranchCondition(RevCond))
545      assert(false && "Unable to reverse branch condition!");
546    TII->InsertBranch(*BBI.TrueBB, TrueBBI.FalseBB, NULL, RevCond);
547  }
548
549  // Join the 'true' and 'false' blocks if the 'false' block has no other
550  // predecessors. Otherwise, add a unconditional branch from 'true' to 'false'.
551  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
552  bool FalseBBDead = false;
553  if (!HasEarlyExit && FalseBBI.BB->pred_size() == 2) {
554    MergeBlocks(TrueBBI, FalseBBI);
555    FalseBBDead = true;
556  } else if (!isNextBlock(TrueBBI.BB, FalseBBI.BB))
557    InsertUncondBranch(TrueBBI.BB, FalseBBI.BB, TII);
558
559  // Now merge the entry of the triangle with the true block.
560  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
561  MergeBlocks(BBI, TrueBBI);
562  std::copy(BBI.BrCond.begin(), BBI.BrCond.end(),
563            std::back_inserter(BBI.Predicate));
564
565  // Update block info. BB can be iteratively if-converted.
566  BBI.Kind = ICReAnalyze;
567  ReTryPreds(BBI.BB);
568  TrueBBI.Kind = ICDead;
569  if (FalseBBDead)
570    FalseBBI.Kind = ICDead;
571
572  // FIXME: Must maintain LiveIns.
573  return true;
574}
575
576/// IfConvertDiamond - If convert a diamond sub-CFG.
577///
578bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
579  BBI.Kind = ICNotClassfied;
580
581  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
582  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
583
584  SmallVector<MachineInstr*, 2> Dups;
585  if (!BBI.TailBB) {
586    // No common merge block. Check if the terminators (e.g. return) are
587    // the same or predicable.
588    MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
589    MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
590    while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
591      if (TT->isIdenticalTo(FT))
592        Dups.push_back(TT);  // Will erase these later.
593      else if (!TT->isPredicable() && !FT->isPredicable())
594        return false; // Can't if-convert. Abort!
595      ++TT;
596      ++FT;
597    }
598
599    // One of the two pathes have more terminators, make sure they are
600    // all predicable.
601    while (TT != BBI.TrueBB->end()) {
602      if (!TT->isPredicable()) {
603        return false; // Can't if-convert. Abort!
604      }
605      ++TT;
606    }
607    while (FT != BBI.FalseBB->end()) {
608      if (!FT->isPredicable()) {
609        return false; // Can't if-convert. Abort!
610      }
611      ++FT;
612    }
613  }
614
615  // Remove the duplicated instructions from the 'true' block.
616  for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
617    Dups[i]->eraseFromParent();
618    --TrueBBI.NonPredSize;
619  }
620
621  // Merge the 'true' and 'false' blocks by copying the instructions
622  // from the 'false' block to the 'true' block. That is, unless the true
623  // block would clobber the predicate, in that case, do the opposite.
624  BBInfo *BBI1 = &TrueBBI;
625  BBInfo *BBI2 = &FalseBBI;
626  std::vector<MachineOperand> RevCond(BBI.BrCond);
627  TII->ReverseBranchCondition(RevCond);
628  std::vector<MachineOperand> *Cond1 = &BBI.BrCond;
629  std::vector<MachineOperand> *Cond2 = &RevCond;
630  // Check the 'true' and 'false' blocks if either isn't ended with a branch.
631  // Either the block fallthrough to another block or it ends with a
632  // return. If it's the former, add a branch to its successor.
633  bool NeedBr1 = !BBI1->TrueBB && BBI1->BB->succ_size();
634  bool NeedBr2 = !BBI2->TrueBB && BBI2->BB->succ_size();
635
636  if ((TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate) ||
637      (!TrueBBI.ModifyPredicate && !FalseBBI.ModifyPredicate &&
638       NeedBr1 && !NeedBr2)) {
639    std::swap(BBI1, BBI2);
640    std::swap(Cond1, Cond2);
641    std::swap(NeedBr1, NeedBr2);
642  }
643
644  // Predicate the 'true' block after removing its branch.
645  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
646  PredicateBlock(*BBI1, *Cond1);
647
648  // Add an early exit branch if needed.
649  if (NeedBr1)
650    TII->InsertBranch(*BBI1->BB, *BBI1->BB->succ_begin(), NULL, *Cond1);
651
652  // Predicate the 'false' block.
653  PredicateBlock(*BBI2, *Cond2, true);
654
655  // Add an unconditional branch from 'false' to to 'false' successor if it
656  // will not be the fallthrough block.
657  if (NeedBr2 && !isNextBlock(BBI2->BB, *BBI2->BB->succ_begin()))
658    InsertUncondBranch(BBI2->BB, *BBI2->BB->succ_begin(), TII);
659
660  // Keep them as two separate blocks if there is an early exit.
661  if (!NeedBr1)
662    MergeBlocks(*BBI1, *BBI2);
663
664  // Remove the conditional branch from entry to the blocks.
665  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
666
667  // Merge the combined block into the entry of the diamond.
668  MergeBlocks(BBI, *BBI1);
669
670  // 'True' and 'false' aren't combined, see if we need to add a unconditional
671  // branch to the 'false' block.
672  if (NeedBr1 && !isNextBlock(BBI.BB, BBI2->BB))
673    InsertUncondBranch(BBI1->BB, BBI2->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  BBInfo *CvtBBI = NeedBr1 ? BBI2 : &BBI;
679  if (BBI.TailBB &&
680      BBI.TailBB->pred_size() == 1 && CvtBBI->BB->succ_size() == 1) {
681    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
682    BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
683    MergeBlocks(*CvtBBI, TailBBI);
684    TailBBI.Kind = ICDead;
685  }
686
687  // Update block info.
688  TrueBBI.Kind = ICDead;
689  FalseBBI.Kind = ICDead;
690
691  // FIXME: Must maintain LiveIns.
692  return true;
693}
694
695/// PredicateBlock - Predicate every instruction in the block with the specified
696/// condition. If IgnoreTerm is true, skip over all terminator instructions.
697void IfConverter::PredicateBlock(BBInfo &BBI,
698                                 std::vector<MachineOperand> &Cond,
699                                 bool IgnoreTerm) {
700  for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
701       I != E; ++I) {
702    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
703      continue;
704    if (TII->isPredicated(I))
705      continue;
706    if (!TII->PredicateInstruction(I, Cond)) {
707      cerr << "Unable to predicate " << *I << "!\n";
708      abort();
709    }
710  }
711
712  BBI.NonPredSize = 0;
713  NumIfConvBBs++;
714}
715
716/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
717///
718static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
719  for (MachineBasicBlock::pred_iterator I = FromBB->pred_begin(),
720         E = FromBB->pred_end(); I != E; ++I) {
721    MachineBasicBlock *Pred = *I;
722    Pred->removeSuccessor(FromBB);
723    if (!Pred->isSuccessor(ToBB))
724      Pred->addSuccessor(ToBB);
725  }
726}
727
728/// TransferSuccs - Transfer all the successors of FromBB to ToBB.
729///
730static void TransferSuccs(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
731  for (MachineBasicBlock::succ_iterator I = FromBB->succ_begin(),
732         E = FromBB->succ_end(); I != E; ++I) {
733    MachineBasicBlock *Succ = *I;
734    FromBB->removeSuccessor(Succ);
735    if (!ToBB->isSuccessor(Succ))
736      ToBB->addSuccessor(Succ);
737  }
738}
739
740/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
741///
742void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
743  ToBBI.BB->splice(ToBBI.BB->end(),
744                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
745
746  // If FromBBI is previously a successor, remove it from ToBBI's successor
747  // list and update its TrueBB / FalseBB field if needed.
748  if (ToBBI.BB->isSuccessor(FromBBI.BB))
749    ToBBI.BB->removeSuccessor(FromBBI.BB);
750
751  // Redirect all branches to FromBB to ToBB.
752  std::vector<MachineBasicBlock *> Preds(FromBBI.BB->pred_begin(),
753                                         FromBBI.BB->pred_end());
754  for (unsigned i = 0, e = Preds.size(); i != e; ++i)
755    Preds[i]->ReplaceUsesOfBlockWith(FromBBI.BB, ToBBI.BB);
756
757  // Transfer preds / succs and update size.
758  TransferPreds(ToBBI.BB, FromBBI.BB);
759  TransferSuccs(ToBBI.BB, FromBBI.BB);
760  ToBBI.NonPredSize += FromBBI.NonPredSize;
761  FromBBI.NonPredSize = 0;
762}
763