JumpThreading.cpp revision 40d8c28b27377199b7465ba2c5a2c59c6fd12fa9
1//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the Jump Threading pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "jump-threading"
15#include "llvm/Transforms/Scalar.h"
16#include "llvm/IntrinsicInst.h"
17#include "llvm/LLVMContext.h"
18#include "llvm/Pass.h"
19#include "llvm/Analysis/InstructionSimplify.h"
20#include "llvm/Transforms/Utils/BasicBlockUtils.h"
21#include "llvm/Transforms/Utils/Local.h"
22#include "llvm/Transforms/Utils/SSAUpdater.h"
23#include "llvm/Target/TargetData.h"
24#include "llvm/ADT/DenseMap.h"
25#include "llvm/ADT/Statistic.h"
26#include "llvm/ADT/STLExtras.h"
27#include "llvm/ADT/SmallPtrSet.h"
28#include "llvm/ADT/SmallSet.h"
29#include "llvm/Support/CommandLine.h"
30#include "llvm/Support/Debug.h"
31#include "llvm/Support/raw_ostream.h"
32using namespace llvm;
33
34STATISTIC(NumThreads, "Number of jumps threaded");
35STATISTIC(NumFolds,   "Number of terminators folded");
36STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
37
38static cl::opt<unsigned>
39Threshold("jump-threading-threshold",
40          cl::desc("Max block size to duplicate for jump threading"),
41          cl::init(6), cl::Hidden);
42
43namespace {
44  /// This pass performs 'jump threading', which looks at blocks that have
45  /// multiple predecessors and multiple successors.  If one or more of the
46  /// predecessors of the block can be proven to always jump to one of the
47  /// successors, we forward the edge from the predecessor to the successor by
48  /// duplicating the contents of this block.
49  ///
50  /// An example of when this can occur is code like this:
51  ///
52  ///   if () { ...
53  ///     X = 4;
54  ///   }
55  ///   if (X < 3) {
56  ///
57  /// In this case, the unconditional branch at the end of the first if can be
58  /// revectored to the false side of the second if.
59  ///
60  class JumpThreading : public FunctionPass {
61    TargetData *TD;
62#ifdef NDEBUG
63    SmallPtrSet<BasicBlock*, 16> LoopHeaders;
64#else
65    SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
66#endif
67  public:
68    static char ID; // Pass identification
69    JumpThreading() : FunctionPass(&ID) {}
70
71    bool runOnFunction(Function &F);
72    void FindLoopHeaders(Function &F);
73
74    bool ProcessBlock(BasicBlock *BB);
75    bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
76                    BasicBlock *SuccBB);
77    bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
78                                          BasicBlock *PredBB);
79
80    typedef SmallVectorImpl<std::pair<ConstantInt*,
81                                      BasicBlock*> > PredValueInfo;
82
83    bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
84                                         PredValueInfo &Result);
85    bool ProcessThreadableEdges(Instruction *CondInst, BasicBlock *BB);
86
87
88    bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
89    bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
90
91    bool ProcessJumpOnPHI(PHINode *PN);
92
93    bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
94  };
95}
96
97char JumpThreading::ID = 0;
98static RegisterPass<JumpThreading>
99X("jump-threading", "Jump Threading");
100
101// Public interface to the Jump Threading pass
102FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
103
104/// runOnFunction - Top level algorithm.
105///
106bool JumpThreading::runOnFunction(Function &F) {
107  DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n");
108  TD = getAnalysisIfAvailable<TargetData>();
109
110  FindLoopHeaders(F);
111
112  bool AnotherIteration = true, EverChanged = false;
113  while (AnotherIteration) {
114    AnotherIteration = false;
115    bool Changed = false;
116    for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
117      BasicBlock *BB = I;
118      // Thread all of the branches we can over this block.
119      while (ProcessBlock(BB))
120        Changed = true;
121
122      ++I;
123
124      // If the block is trivially dead, zap it.  This eliminates the successor
125      // edges which simplifies the CFG.
126      if (pred_begin(BB) == pred_end(BB) &&
127          BB != &BB->getParent()->getEntryBlock()) {
128        DEBUG(errs() << "  JT: Deleting dead block '" << BB->getName()
129              << "' with terminator: " << *BB->getTerminator() << '\n');
130        LoopHeaders.erase(BB);
131        DeleteDeadBlock(BB);
132        Changed = true;
133      } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
134        // Can't thread an unconditional jump, but if the block is "almost
135        // empty", we can replace uses of it with uses of the successor and make
136        // this dead.
137        if (BI->isUnconditional() &&
138            BB != &BB->getParent()->getEntryBlock()) {
139          BasicBlock::iterator BBI = BB->getFirstNonPHI();
140          // Ignore dbg intrinsics.
141          while (isa<DbgInfoIntrinsic>(BBI))
142            ++BBI;
143          // If the terminator is the only non-phi instruction, try to nuke it.
144          if (BBI->isTerminator()) {
145            // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
146            // block, we have to make sure it isn't in the LoopHeaders set.  We
147            // reinsert afterward in the rare case when the block isn't deleted.
148            bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
149
150            if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
151              Changed = true;
152            else if (ErasedFromLoopHeaders)
153              LoopHeaders.insert(BB);
154          }
155        }
156      }
157    }
158    AnotherIteration = Changed;
159    EverChanged |= Changed;
160  }
161
162  LoopHeaders.clear();
163  return EverChanged;
164}
165
166/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
167/// thread across it.
168static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
169  /// Ignore PHI nodes, these will be flattened when duplication happens.
170  BasicBlock::const_iterator I = BB->getFirstNonPHI();
171
172  // Sum up the cost of each instruction until we get to the terminator.  Don't
173  // include the terminator because the copy won't include it.
174  unsigned Size = 0;
175  for (; !isa<TerminatorInst>(I); ++I) {
176    // Debugger intrinsics don't incur code size.
177    if (isa<DbgInfoIntrinsic>(I)) continue;
178
179    // If this is a pointer->pointer bitcast, it is free.
180    if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
181      continue;
182
183    // All other instructions count for at least one unit.
184    ++Size;
185
186    // Calls are more expensive.  If they are non-intrinsic calls, we model them
187    // as having cost of 4.  If they are a non-vector intrinsic, we model them
188    // as having cost of 2 total, and if they are a vector intrinsic, we model
189    // them as having cost 1.
190    if (const CallInst *CI = dyn_cast<CallInst>(I)) {
191      if (!isa<IntrinsicInst>(CI))
192        Size += 3;
193      else if (!isa<VectorType>(CI->getType()))
194        Size += 1;
195    }
196  }
197
198  // Threading through a switch statement is particularly profitable.  If this
199  // block ends in a switch, decrease its cost to make it more likely to happen.
200  if (isa<SwitchInst>(I))
201    Size = Size > 6 ? Size-6 : 0;
202
203  return Size;
204}
205
206/// FindLoopHeaders - We do not want jump threading to turn proper loop
207/// structures into irreducible loops.  Doing this breaks up the loop nesting
208/// hierarchy and pessimizes later transformations.  To prevent this from
209/// happening, we first have to find the loop headers.  Here we approximate this
210/// by finding targets of backedges in the CFG.
211///
212/// Note that there definitely are cases when we want to allow threading of
213/// edges across a loop header.  For example, threading a jump from outside the
214/// loop (the preheader) to an exit block of the loop is definitely profitable.
215/// It is also almost always profitable to thread backedges from within the loop
216/// to exit blocks, and is often profitable to thread backedges to other blocks
217/// within the loop (forming a nested loop).  This simple analysis is not rich
218/// enough to track all of these properties and keep it up-to-date as the CFG
219/// mutates, so we don't allow any of these transformations.
220///
221void JumpThreading::FindLoopHeaders(Function &F) {
222  SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
223  FindFunctionBackedges(F, Edges);
224
225  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
226    LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
227}
228
229/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
230/// if we can infer that the value is a known ConstantInt in any of our
231/// predecessors.  If so, return the known list of value and pred BB in the
232/// result vector.  If a value is known to be undef, it is returned as null.
233///
234/// The BB basic block is known to start with a PHI node.
235///
236/// This returns true if there were any known values.
237///
238///
239/// TODO: Per PR2563, we could infer value range information about a predecessor
240/// based on its terminator.
241bool JumpThreading::
242ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
243  PHINode *TheFirstPHI = cast<PHINode>(BB->begin());
244
245  // If V is a constantint, then it is known in all predecessors.
246  if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
247    ConstantInt *CI = dyn_cast<ConstantInt>(V);
248    Result.resize(TheFirstPHI->getNumIncomingValues());
249    for (unsigned i = 0, e = Result.size(); i != e; ++i)
250      Result[i] = std::make_pair(CI, TheFirstPHI->getIncomingBlock(i));
251    return true;
252  }
253
254  // If V is a non-instruction value, or an instruction in a different block,
255  // then it can't be derived from a PHI.
256  Instruction *I = dyn_cast<Instruction>(V);
257  if (I == 0 || I->getParent() != BB)
258    return false;
259
260  /// If I is a PHI node, then we know the incoming values for any constants.
261  if (PHINode *PN = dyn_cast<PHINode>(I)) {
262    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
263      Value *InVal = PN->getIncomingValue(i);
264      if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
265        ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
266        Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
267      }
268    }
269    return !Result.empty();
270  }
271
272  SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
273
274  // Handle some boolean conditions.
275  if (I->getType()->getPrimitiveSizeInBits() == 1) {
276    // X | true -> true
277    // X & false -> false
278    if (I->getOpcode() == Instruction::Or ||
279        I->getOpcode() == Instruction::And) {
280      ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
281      ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
282
283      if (LHSVals.empty() && RHSVals.empty())
284        return false;
285
286      ConstantInt *InterestingVal;
287      if (I->getOpcode() == Instruction::Or)
288        InterestingVal = ConstantInt::getTrue(I->getContext());
289      else
290        InterestingVal = ConstantInt::getFalse(I->getContext());
291
292      // Scan for the sentinel.
293      for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
294        if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0)
295          Result.push_back(LHSVals[i]);
296      for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
297        if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0)
298          Result.push_back(RHSVals[i]);
299      return !Result.empty();
300    }
301
302    // TODO: Should handle the NOT form of XOR.
303
304  }
305
306  // Handle compare with phi operand, where the PHI is defined in this block.
307  if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
308    PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
309    if (PN && PN->getParent() == BB) {
310      // We can do this simplification if any comparisons fold to true or false.
311      // See if any do.
312      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
313        BasicBlock *PredBB = PN->getIncomingBlock(i);
314        Value *LHS = PN->getIncomingValue(i);
315        Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
316
317        Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS);
318        if (Res == 0) continue;
319
320        if (isa<UndefValue>(Res))
321          Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
322        else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
323          Result.push_back(std::make_pair(CI, PredBB));
324      }
325
326      return !Result.empty();
327    }
328
329    // TODO: We could also recurse to see if we can determine constants another
330    // way.
331  }
332  return false;
333}
334
335
336
337/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
338/// in an undefined jump, decide which block is best to revector to.
339///
340/// Since we can pick an arbitrary destination, we pick the successor with the
341/// fewest predecessors.  This should reduce the in-degree of the others.
342///
343static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
344  TerminatorInst *BBTerm = BB->getTerminator();
345  unsigned MinSucc = 0;
346  BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
347  // Compute the successor with the minimum number of predecessors.
348  unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
349  for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
350    TestBB = BBTerm->getSuccessor(i);
351    unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
352    if (NumPreds < MinNumPreds)
353      MinSucc = i;
354  }
355
356  return MinSucc;
357}
358
359/// ProcessBlock - If there are any predecessors whose control can be threaded
360/// through to a successor, transform them now.
361bool JumpThreading::ProcessBlock(BasicBlock *BB) {
362  // If this block has a single predecessor, and if that pred has a single
363  // successor, merge the blocks.  This encourages recursive jump threading
364  // because now the condition in this block can be threaded through
365  // predecessors of our predecessor block.
366  if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
367    if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
368        SinglePred != BB) {
369      // If SinglePred was a loop header, BB becomes one.
370      if (LoopHeaders.erase(SinglePred))
371        LoopHeaders.insert(BB);
372
373      // Remember if SinglePred was the entry block of the function.  If so, we
374      // will need to move BB back to the entry position.
375      bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
376      MergeBasicBlockIntoOnlyPred(BB);
377
378      if (isEntry && BB != &BB->getParent()->getEntryBlock())
379        BB->moveBefore(&BB->getParent()->getEntryBlock());
380      return true;
381    }
382  }
383
384  // Look to see if the terminator is a branch of switch, if not we can't thread
385  // it.
386  Value *Condition;
387  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
388    // Can't thread an unconditional jump.
389    if (BI->isUnconditional()) return false;
390    Condition = BI->getCondition();
391  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
392    Condition = SI->getCondition();
393  else
394    return false; // Must be an invoke.
395
396  // If the terminator of this block is branching on a constant, simplify the
397  // terminator to an unconditional branch.  This can occur due to threading in
398  // other blocks.
399  if (isa<ConstantInt>(Condition)) {
400    DEBUG(errs() << "  In block '" << BB->getName()
401          << "' folding terminator: " << *BB->getTerminator() << '\n');
402    ++NumFolds;
403    ConstantFoldTerminator(BB);
404    return true;
405  }
406
407  // If the terminator is branching on an undef, we can pick any of the
408  // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
409  if (isa<UndefValue>(Condition)) {
410    unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
411
412    // Fold the branch/switch.
413    TerminatorInst *BBTerm = BB->getTerminator();
414    for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
415      if (i == BestSucc) continue;
416      RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
417    }
418
419    DEBUG(errs() << "  In block '" << BB->getName()
420          << "' folding undef terminator: " << *BBTerm << '\n');
421    BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
422    BBTerm->eraseFromParent();
423    return true;
424  }
425
426  Instruction *CondInst = dyn_cast<Instruction>(Condition);
427
428  // If the condition is an instruction defined in another block, see if a
429  // predecessor has the same condition:
430  //     br COND, BBX, BBY
431  //  BBX:
432  //     br COND, BBZ, BBW
433  if (!Condition->hasOneUse() && // Multiple uses.
434      (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
435    pred_iterator PI = pred_begin(BB), E = pred_end(BB);
436    if (isa<BranchInst>(BB->getTerminator())) {
437      for (; PI != E; ++PI)
438        if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
439          if (PBI->isConditional() && PBI->getCondition() == Condition &&
440              ProcessBranchOnDuplicateCond(*PI, BB))
441            return true;
442    } else {
443      assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
444      for (; PI != E; ++PI)
445        if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
446          if (PSI->getCondition() == Condition &&
447              ProcessSwitchOnDuplicateCond(*PI, BB))
448            return true;
449    }
450  }
451
452  // All the rest of our checks depend on the condition being an instruction.
453  if (CondInst == 0)
454    return false;
455
456  // See if this is a phi node in the current block.
457  if (PHINode *PN = dyn_cast<PHINode>(CondInst))
458    if (PN->getParent() == BB)
459      return ProcessJumpOnPHI(PN);
460
461  if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
462    if (!isa<PHINode>(CondCmp->getOperand(0)) ||
463        cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB) {
464      // If we have a comparison, loop over the predecessors to see if there is
465      // a condition with a lexically identical value.
466      pred_iterator PI = pred_begin(BB), E = pred_end(BB);
467      for (; PI != E; ++PI)
468        if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
469          if (PBI->isConditional() && *PI != BB) {
470            if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
471              if (CI->getOperand(0) == CondCmp->getOperand(0) &&
472                  CI->getOperand(1) == CondCmp->getOperand(1) &&
473                  CI->getPredicate() == CondCmp->getPredicate()) {
474                // TODO: Could handle things like (x != 4) --> (x == 17)
475                if (ProcessBranchOnDuplicateCond(*PI, BB))
476                  return true;
477              }
478            }
479          }
480    }
481  }
482
483  // Check for some cases that are worth simplifying.  Right now we want to look
484  // for loads that are used by a switch or by the condition for the branch.  If
485  // we see one, check to see if it's partially redundant.  If so, insert a PHI
486  // which can then be used to thread the values.
487  //
488  // This is particularly important because reg2mem inserts loads and stores all
489  // over the place, and this blocks jump threading if we don't zap them.
490  Value *SimplifyValue = CondInst;
491  if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
492    if (isa<Constant>(CondCmp->getOperand(1)))
493      SimplifyValue = CondCmp->getOperand(0);
494
495  if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
496    if (SimplifyPartiallyRedundantLoad(LI))
497      return true;
498
499
500  // Handle a variety of cases where we are branching on something derived from
501  // a PHI node in the current block.  If we can prove that any predecessors
502  // compute a predictable value based on a PHI node, thread those predecessors.
503  //
504  // We only bother doing this if the current block has a PHI node and if the
505  // conditional instruction lives in the current block.  If either condition
506  // fails, this won't be a computable value anyway.
507  if (CondInst->getParent() == BB && isa<PHINode>(BB->front()))
508    if (ProcessThreadableEdges(CondInst, BB))
509      return true;
510
511
512  // TODO: If we have: "br (X > 0)"  and we have a predecessor where we know
513  // "(X == 4)" thread through this block.
514
515  return false;
516}
517
518/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
519/// block that jump on exactly the same condition.  This means that we almost
520/// always know the direction of the edge in the DESTBB:
521///  PREDBB:
522///     br COND, DESTBB, BBY
523///  DESTBB:
524///     br COND, BBZ, BBW
525///
526/// If DESTBB has multiple predecessors, we can't just constant fold the branch
527/// in DESTBB, we have to thread over it.
528bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
529                                                 BasicBlock *BB) {
530  BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
531
532  // If both successors of PredBB go to DESTBB, we don't know anything.  We can
533  // fold the branch to an unconditional one, which allows other recursive
534  // simplifications.
535  bool BranchDir;
536  if (PredBI->getSuccessor(1) != BB)
537    BranchDir = true;
538  else if (PredBI->getSuccessor(0) != BB)
539    BranchDir = false;
540  else {
541    DEBUG(errs() << "  In block '" << PredBB->getName()
542          << "' folding terminator: " << *PredBB->getTerminator() << '\n');
543    ++NumFolds;
544    ConstantFoldTerminator(PredBB);
545    return true;
546  }
547
548  BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
549
550  // If the dest block has one predecessor, just fix the branch condition to a
551  // constant and fold it.
552  if (BB->getSinglePredecessor()) {
553    DEBUG(errs() << "  In block '" << BB->getName()
554          << "' folding condition to '" << BranchDir << "': "
555          << *BB->getTerminator() << '\n');
556    ++NumFolds;
557    Value *OldCond = DestBI->getCondition();
558    DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
559                                          BranchDir));
560    ConstantFoldTerminator(BB);
561    RecursivelyDeleteTriviallyDeadInstructions(OldCond);
562    return true;
563  }
564
565
566  // Next, figure out which successor we are threading to.
567  BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
568
569  SmallVector<BasicBlock*, 2> Preds;
570  Preds.push_back(PredBB);
571
572  // Ok, try to thread it!
573  return ThreadEdge(BB, Preds, SuccBB);
574}
575
576/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
577/// block that switch on exactly the same condition.  This means that we almost
578/// always know the direction of the edge in the DESTBB:
579///  PREDBB:
580///     switch COND [... DESTBB, BBY ... ]
581///  DESTBB:
582///     switch COND [... BBZ, BBW ]
583///
584/// Optimizing switches like this is very important, because simplifycfg builds
585/// switches out of repeated 'if' conditions.
586bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
587                                                 BasicBlock *DestBB) {
588  // Can't thread edge to self.
589  if (PredBB == DestBB)
590    return false;
591
592  SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
593  SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
594
595  // There are a variety of optimizations that we can potentially do on these
596  // blocks: we order them from most to least preferable.
597
598  // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
599  // directly to their destination.  This does not introduce *any* code size
600  // growth.  Skip debug info first.
601  BasicBlock::iterator BBI = DestBB->begin();
602  while (isa<DbgInfoIntrinsic>(BBI))
603    BBI++;
604
605  // FIXME: Thread if it just contains a PHI.
606  if (isa<SwitchInst>(BBI)) {
607    bool MadeChange = false;
608    // Ignore the default edge for now.
609    for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
610      ConstantInt *DestVal = DestSI->getCaseValue(i);
611      BasicBlock *DestSucc = DestSI->getSuccessor(i);
612
613      // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'.  See if
614      // PredSI has an explicit case for it.  If so, forward.  If it is covered
615      // by the default case, we can't update PredSI.
616      unsigned PredCase = PredSI->findCaseValue(DestVal);
617      if (PredCase == 0) continue;
618
619      // If PredSI doesn't go to DestBB on this value, then it won't reach the
620      // case on this condition.
621      if (PredSI->getSuccessor(PredCase) != DestBB &&
622          DestSI->getSuccessor(i) != DestBB)
623        continue;
624
625      // Otherwise, we're safe to make the change.  Make sure that the edge from
626      // DestSI to DestSucc is not critical and has no PHI nodes.
627      DEBUG(errs() << "FORWARDING EDGE " << *DestVal << "   FROM: " << *PredSI);
628      DEBUG(errs() << "THROUGH: " << *DestSI);
629
630      // If the destination has PHI nodes, just split the edge for updating
631      // simplicity.
632      if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
633        SplitCriticalEdge(DestSI, i, this);
634        DestSucc = DestSI->getSuccessor(i);
635      }
636      FoldSingleEntryPHINodes(DestSucc);
637      PredSI->setSuccessor(PredCase, DestSucc);
638      MadeChange = true;
639    }
640
641    if (MadeChange)
642      return true;
643  }
644
645  return false;
646}
647
648
649/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
650/// load instruction, eliminate it by replacing it with a PHI node.  This is an
651/// important optimization that encourages jump threading, and needs to be run
652/// interlaced with other jump threading tasks.
653bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
654  // Don't hack volatile loads.
655  if (LI->isVolatile()) return false;
656
657  // If the load is defined in a block with exactly one predecessor, it can't be
658  // partially redundant.
659  BasicBlock *LoadBB = LI->getParent();
660  if (LoadBB->getSinglePredecessor())
661    return false;
662
663  Value *LoadedPtr = LI->getOperand(0);
664
665  // If the loaded operand is defined in the LoadBB, it can't be available.
666  // FIXME: Could do PHI translation, that would be fun :)
667  if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
668    if (PtrOp->getParent() == LoadBB)
669      return false;
670
671  // Scan a few instructions up from the load, to see if it is obviously live at
672  // the entry to its block.
673  BasicBlock::iterator BBIt = LI;
674
675  if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB,
676                                                     BBIt, 6)) {
677    // If the value if the load is locally available within the block, just use
678    // it.  This frequently occurs for reg2mem'd allocas.
679    //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
680
681    // If the returned value is the load itself, replace with an undef. This can
682    // only happen in dead loops.
683    if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
684    LI->replaceAllUsesWith(AvailableVal);
685    LI->eraseFromParent();
686    return true;
687  }
688
689  // Otherwise, if we scanned the whole block and got to the top of the block,
690  // we know the block is locally transparent to the load.  If not, something
691  // might clobber its value.
692  if (BBIt != LoadBB->begin())
693    return false;
694
695
696  SmallPtrSet<BasicBlock*, 8> PredsScanned;
697  typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
698  AvailablePredsTy AvailablePreds;
699  BasicBlock *OneUnavailablePred = 0;
700
701  // If we got here, the loaded value is transparent through to the start of the
702  // block.  Check to see if it is available in any of the predecessor blocks.
703  for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
704       PI != PE; ++PI) {
705    BasicBlock *PredBB = *PI;
706
707    // If we already scanned this predecessor, skip it.
708    if (!PredsScanned.insert(PredBB))
709      continue;
710
711    // Scan the predecessor to see if the value is available in the pred.
712    BBIt = PredBB->end();
713    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
714    if (!PredAvailable) {
715      OneUnavailablePred = PredBB;
716      continue;
717    }
718
719    // If so, this load is partially redundant.  Remember this info so that we
720    // can create a PHI node.
721    AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
722  }
723
724  // If the loaded value isn't available in any predecessor, it isn't partially
725  // redundant.
726  if (AvailablePreds.empty()) return false;
727
728  // Okay, the loaded value is available in at least one (and maybe all!)
729  // predecessors.  If the value is unavailable in more than one unique
730  // predecessor, we want to insert a merge block for those common predecessors.
731  // This ensures that we only have to insert one reload, thus not increasing
732  // code size.
733  BasicBlock *UnavailablePred = 0;
734
735  // If there is exactly one predecessor where the value is unavailable, the
736  // already computed 'OneUnavailablePred' block is it.  If it ends in an
737  // unconditional branch, we know that it isn't a critical edge.
738  if (PredsScanned.size() == AvailablePreds.size()+1 &&
739      OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
740    UnavailablePred = OneUnavailablePred;
741  } else if (PredsScanned.size() != AvailablePreds.size()) {
742    // Otherwise, we had multiple unavailable predecessors or we had a critical
743    // edge from the one.
744    SmallVector<BasicBlock*, 8> PredsToSplit;
745    SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
746
747    for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
748      AvailablePredSet.insert(AvailablePreds[i].first);
749
750    // Add all the unavailable predecessors to the PredsToSplit list.
751    for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
752         PI != PE; ++PI)
753      if (!AvailablePredSet.count(*PI))
754        PredsToSplit.push_back(*PI);
755
756    // Split them out to their own block.
757    UnavailablePred =
758      SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
759                             "thread-split", this);
760  }
761
762  // If the value isn't available in all predecessors, then there will be
763  // exactly one where it isn't available.  Insert a load on that edge and add
764  // it to the AvailablePreds list.
765  if (UnavailablePred) {
766    assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
767           "Can't handle critical edge here!");
768    Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr",
769                                 UnavailablePred->getTerminator());
770    AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
771  }
772
773  // Now we know that each predecessor of this block has a value in
774  // AvailablePreds, sort them for efficient access as we're walking the preds.
775  array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
776
777  // Create a PHI node at the start of the block for the PRE'd load value.
778  PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
779  PN->takeName(LI);
780
781  // Insert new entries into the PHI for each predecessor.  A single block may
782  // have multiple entries here.
783  for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
784       ++PI) {
785    AvailablePredsTy::iterator I =
786      std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
787                       std::make_pair(*PI, (Value*)0));
788
789    assert(I != AvailablePreds.end() && I->first == *PI &&
790           "Didn't find entry for predecessor!");
791
792    PN->addIncoming(I->second, I->first);
793  }
794
795  //cerr << "PRE: " << *LI << *PN << "\n";
796
797  LI->replaceAllUsesWith(PN);
798  LI->eraseFromParent();
799
800  return true;
801}
802
803/// FindMostPopularDest - The specified list contains multiple possible
804/// threadable destinations.  Pick the one that occurs the most frequently in
805/// the list.
806static BasicBlock *
807FindMostPopularDest(BasicBlock *BB,
808                    const SmallVectorImpl<std::pair<BasicBlock*,
809                                  BasicBlock*> > &PredToDestList) {
810  assert(!PredToDestList.empty());
811
812  // Determine popularity.  If there are multiple possible destinations, we
813  // explicitly choose to ignore 'undef' destinations.  We prefer to thread
814  // blocks with known and real destinations to threading undef.  We'll handle
815  // them later if interesting.
816  DenseMap<BasicBlock*, unsigned> DestPopularity;
817  for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
818    if (PredToDestList[i].second)
819      DestPopularity[PredToDestList[i].second]++;
820
821  // Find the most popular dest.
822  DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
823  BasicBlock *MostPopularDest = DPI->first;
824  unsigned Popularity = DPI->second;
825  SmallVector<BasicBlock*, 4> SamePopularity;
826
827  for (++DPI; DPI != DestPopularity.end(); ++DPI) {
828    // If the popularity of this entry isn't higher than the popularity we've
829    // seen so far, ignore it.
830    if (DPI->second < Popularity)
831      ; // ignore.
832    else if (DPI->second == Popularity) {
833      // If it is the same as what we've seen so far, keep track of it.
834      SamePopularity.push_back(DPI->first);
835    } else {
836      // If it is more popular, remember it.
837      SamePopularity.clear();
838      MostPopularDest = DPI->first;
839      Popularity = DPI->second;
840    }
841  }
842
843  // Okay, now we know the most popular destination.  If there is more than
844  // destination, we need to determine one.  This is arbitrary, but we need
845  // to make a deterministic decision.  Pick the first one that appears in the
846  // successor list.
847  if (!SamePopularity.empty()) {
848    SamePopularity.push_back(MostPopularDest);
849    TerminatorInst *TI = BB->getTerminator();
850    for (unsigned i = 0; ; ++i) {
851      assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
852
853      if (std::find(SamePopularity.begin(), SamePopularity.end(),
854                    TI->getSuccessor(i)) == SamePopularity.end())
855        continue;
856
857      MostPopularDest = TI->getSuccessor(i);
858      break;
859    }
860  }
861
862  // Okay, we have finally picked the most popular destination.
863  return MostPopularDest;
864}
865
866bool JumpThreading::ProcessThreadableEdges(Instruction *CondInst,
867                                           BasicBlock *BB) {
868  // If threading this would thread across a loop header, don't even try to
869  // thread the edge.
870  if (LoopHeaders.count(BB))
871    return false;
872
873  SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
874  if (!ComputeValueKnownInPredecessors(CondInst, BB, PredValues))
875    return false;
876  assert(!PredValues.empty() &&
877         "ComputeValueKnownInPredecessors returned true with no values");
878
879  DEBUG(errs() << "IN BB: " << *BB;
880        for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
881          errs() << "  BB '" << BB->getName() << "': FOUND condition = ";
882          if (PredValues[i].first)
883            errs() << *PredValues[i].first;
884          else
885            errs() << "UNDEF";
886          errs() << " for pred '" << PredValues[i].second->getName()
887          << "'.\n";
888        });
889
890  // Decide what we want to thread through.  Convert our list of known values to
891  // a list of known destinations for each pred.  This also discards duplicate
892  // predecessors and keeps track of the undefined inputs (which are represented
893  // as a null dest in the PredToDestList).
894  SmallPtrSet<BasicBlock*, 16> SeenPreds;
895  SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
896
897  BasicBlock *OnlyDest = 0;
898  BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
899
900  for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
901    BasicBlock *Pred = PredValues[i].second;
902    if (!SeenPreds.insert(Pred))
903      continue;  // Duplicate predecessor entry.
904
905    // If the predecessor ends with an indirect goto, we can't change its
906    // destination.
907    if (isa<IndirectBrInst>(Pred->getTerminator()))
908      continue;
909
910    ConstantInt *Val = PredValues[i].first;
911
912    BasicBlock *DestBB;
913    if (Val == 0)      // Undef.
914      DestBB = 0;
915    else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
916      DestBB = BI->getSuccessor(Val->isZero());
917    else {
918      SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
919      DestBB = SI->getSuccessor(SI->findCaseValue(Val));
920    }
921
922    // If we have exactly one destination, remember it for efficiency below.
923    if (i == 0)
924      OnlyDest = DestBB;
925    else if (OnlyDest != DestBB)
926      OnlyDest = MultipleDestSentinel;
927
928    PredToDestList.push_back(std::make_pair(Pred, DestBB));
929  }
930
931  // If all edges were unthreadable, we fail.
932  if (PredToDestList.empty())
933    return false;
934
935  // Determine which is the most common successor.  If we have many inputs and
936  // this block is a switch, we want to start by threading the batch that goes
937  // to the most popular destination first.  If we only know about one
938  // threadable destination (the common case) we can avoid this.
939  BasicBlock *MostPopularDest = OnlyDest;
940
941  if (MostPopularDest == MultipleDestSentinel)
942    MostPopularDest = FindMostPopularDest(BB, PredToDestList);
943
944  // Now that we know what the most popular destination is, factor all
945  // predecessors that will jump to it into a single predecessor.
946  SmallVector<BasicBlock*, 16> PredsToFactor;
947  for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
948    if (PredToDestList[i].second == MostPopularDest) {
949      BasicBlock *Pred = PredToDestList[i].first;
950
951      // This predecessor may be a switch or something else that has multiple
952      // edges to the block.  Factor each of these edges by listing them
953      // according to # occurrences in PredsToFactor.
954      TerminatorInst *PredTI = Pred->getTerminator();
955      for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
956        if (PredTI->getSuccessor(i) == BB)
957          PredsToFactor.push_back(Pred);
958    }
959
960  // If the threadable edges are branching on an undefined value, we get to pick
961  // the destination that these predecessors should get to.
962  if (MostPopularDest == 0)
963    MostPopularDest = BB->getTerminator()->
964                            getSuccessor(GetBestDestForJumpOnUndef(BB));
965
966  // Ok, try to thread it!
967  return ThreadEdge(BB, PredsToFactor, MostPopularDest);
968}
969
970/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in
971/// the current block.  See if there are any simplifications we can do based on
972/// inputs to the phi node.
973///
974bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
975  BasicBlock *BB = PN->getParent();
976
977  // If any of the predecessor blocks end in an unconditional branch, we can
978  // *duplicate* the jump into that block in order to further encourage jump
979  // threading and to eliminate cases where we have branch on a phi of an icmp
980  // (branch on icmp is much better).
981
982  // We don't want to do this tranformation for switches, because we don't
983  // really want to duplicate a switch.
984  if (isa<SwitchInst>(BB->getTerminator()))
985    return false;
986
987  // Look for unconditional branch predecessors.
988  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
989    BasicBlock *PredBB = PN->getIncomingBlock(i);
990    if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
991      if (PredBr->isUnconditional() &&
992          // Try to duplicate BB into PredBB.
993          DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
994        return true;
995  }
996
997  return false;
998}
999
1000
1001/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1002/// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
1003/// NewPred using the entries from OldPred (suitably mapped).
1004static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1005                                            BasicBlock *OldPred,
1006                                            BasicBlock *NewPred,
1007                                     DenseMap<Instruction*, Value*> &ValueMap) {
1008  for (BasicBlock::iterator PNI = PHIBB->begin();
1009       PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1010    // Ok, we have a PHI node.  Figure out what the incoming value was for the
1011    // DestBlock.
1012    Value *IV = PN->getIncomingValueForBlock(OldPred);
1013
1014    // Remap the value if necessary.
1015    if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1016      DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1017      if (I != ValueMap.end())
1018        IV = I->second;
1019    }
1020
1021    PN->addIncoming(IV, NewPred);
1022  }
1023}
1024
1025/// ThreadEdge - We have decided that it is safe and profitable to factor the
1026/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1027/// across BB.  Transform the IR to reflect this change.
1028bool JumpThreading::ThreadEdge(BasicBlock *BB,
1029                               const SmallVectorImpl<BasicBlock*> &PredBBs,
1030                               BasicBlock *SuccBB) {
1031  // If threading to the same block as we come from, we would infinite loop.
1032  if (SuccBB == BB) {
1033    DEBUG(errs() << "  Not threading across BB '" << BB->getName()
1034          << "' - would thread to self!\n");
1035    return false;
1036  }
1037
1038  // If threading this would thread across a loop header, don't thread the edge.
1039  // See the comments above FindLoopHeaders for justifications and caveats.
1040  if (LoopHeaders.count(BB)) {
1041    DEBUG(errs() << "  Not threading across loop header BB '" << BB->getName()
1042          << "' to dest BB '" << SuccBB->getName()
1043          << "' - it might create an irreducible loop!\n");
1044    return false;
1045  }
1046
1047  unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1048  if (JumpThreadCost > Threshold) {
1049    DEBUG(errs() << "  Not threading BB '" << BB->getName()
1050          << "' - Cost is too high: " << JumpThreadCost << "\n");
1051    return false;
1052  }
1053
1054  // And finally, do it!  Start by factoring the predecessors is needed.
1055  BasicBlock *PredBB;
1056  if (PredBBs.size() == 1)
1057    PredBB = PredBBs[0];
1058  else {
1059    DEBUG(errs() << "  Factoring out " << PredBBs.size()
1060          << " common predecessors.\n");
1061    PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1062                                    ".thr_comm", this);
1063  }
1064
1065  // And finally, do it!
1066  DEBUG(errs() << "  Threading edge from '" << PredBB->getName() << "' to '"
1067        << SuccBB->getName() << "' with cost: " << JumpThreadCost
1068        << ", across block:\n    "
1069        << *BB << "\n");
1070
1071  // We are going to have to map operands from the original BB block to the new
1072  // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
1073  // account for entry from PredBB.
1074  DenseMap<Instruction*, Value*> ValueMapping;
1075
1076  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1077                                         BB->getName()+".thread",
1078                                         BB->getParent(), BB);
1079  NewBB->moveAfter(PredBB);
1080
1081  BasicBlock::iterator BI = BB->begin();
1082  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1083    ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1084
1085  // Clone the non-phi instructions of BB into NewBB, keeping track of the
1086  // mapping and using it to remap operands in the cloned instructions.
1087  for (; !isa<TerminatorInst>(BI); ++BI) {
1088    Instruction *New = BI->clone();
1089    New->setName(BI->getName());
1090    NewBB->getInstList().push_back(New);
1091    ValueMapping[BI] = New;
1092
1093    // Remap operands to patch up intra-block references.
1094    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1095      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1096        DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1097        if (I != ValueMapping.end())
1098          New->setOperand(i, I->second);
1099      }
1100  }
1101
1102  // We didn't copy the terminator from BB over to NewBB, because there is now
1103  // an unconditional jump to SuccBB.  Insert the unconditional jump.
1104  BranchInst::Create(SuccBB, NewBB);
1105
1106  // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1107  // PHI nodes for NewBB now.
1108  AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1109
1110  // If there were values defined in BB that are used outside the block, then we
1111  // now have to update all uses of the value to use either the original value,
1112  // the cloned value, or some PHI derived value.  This can require arbitrary
1113  // PHI insertion, of which we are prepared to do, clean these up now.
1114  SSAUpdater SSAUpdate;
1115  SmallVector<Use*, 16> UsesToRename;
1116  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1117    // Scan all uses of this instruction to see if it is used outside of its
1118    // block, and if so, record them in UsesToRename.
1119    for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1120         ++UI) {
1121      Instruction *User = cast<Instruction>(*UI);
1122      if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1123        if (UserPN->getIncomingBlock(UI) == BB)
1124          continue;
1125      } else if (User->getParent() == BB)
1126        continue;
1127
1128      UsesToRename.push_back(&UI.getUse());
1129    }
1130
1131    // If there are no uses outside the block, we're done with this instruction.
1132    if (UsesToRename.empty())
1133      continue;
1134
1135    DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1136
1137    // We found a use of I outside of BB.  Rename all uses of I that are outside
1138    // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1139    // with the two values we know.
1140    SSAUpdate.Initialize(I);
1141    SSAUpdate.AddAvailableValue(BB, I);
1142    SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1143
1144    while (!UsesToRename.empty())
1145      SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1146    DEBUG(errs() << "\n");
1147  }
1148
1149
1150  // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
1151  // NewBB instead of BB.  This eliminates predecessors from BB, which requires
1152  // us to simplify any PHI nodes in BB.
1153  TerminatorInst *PredTerm = PredBB->getTerminator();
1154  for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1155    if (PredTerm->getSuccessor(i) == BB) {
1156      RemovePredecessorAndSimplify(BB, PredBB, TD);
1157      PredTerm->setSuccessor(i, NewBB);
1158    }
1159
1160  // At this point, the IR is fully up to date and consistent.  Do a quick scan
1161  // over the new instructions and zap any that are constants or dead.  This
1162  // frequently happens because of phi translation.
1163  BI = NewBB->begin();
1164  for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
1165    Instruction *Inst = BI++;
1166
1167    if (Value *V = SimplifyInstruction(Inst, TD)) {
1168      WeakVH BIHandle(BI);
1169      ReplaceAndSimplifyAllUses(Inst, V, TD);
1170      if (BIHandle == 0)
1171        BI = NewBB->begin();
1172      continue;
1173    }
1174
1175    RecursivelyDeleteTriviallyDeadInstructions(Inst);
1176  }
1177
1178  // Threaded an edge!
1179  ++NumThreads;
1180  return true;
1181}
1182
1183/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1184/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1185/// If we can duplicate the contents of BB up into PredBB do so now, this
1186/// improves the odds that the branch will be on an analyzable instruction like
1187/// a compare.
1188bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1189                                                     BasicBlock *PredBB) {
1190  // If BB is a loop header, then duplicating this block outside the loop would
1191  // cause us to transform this into an irreducible loop, don't do this.
1192  // See the comments above FindLoopHeaders for justifications and caveats.
1193  if (LoopHeaders.count(BB)) {
1194    DEBUG(errs() << "  Not duplicating loop header '" << BB->getName()
1195          << "' into predecessor block '" << PredBB->getName()
1196          << "' - it might create an irreducible loop!\n");
1197    return false;
1198  }
1199
1200  unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1201  if (DuplicationCost > Threshold) {
1202    DEBUG(errs() << "  Not duplicating BB '" << BB->getName()
1203          << "' - Cost is too high: " << DuplicationCost << "\n");
1204    return false;
1205  }
1206
1207  // Okay, we decided to do this!  Clone all the instructions in BB onto the end
1208  // of PredBB.
1209  DEBUG(errs() << "  Duplicating block '" << BB->getName() << "' into end of '"
1210        << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
1211        << DuplicationCost << " block is:" << *BB << "\n");
1212
1213  // We are going to have to map operands from the original BB block into the
1214  // PredBB block.  Evaluate PHI nodes in BB.
1215  DenseMap<Instruction*, Value*> ValueMapping;
1216
1217  BasicBlock::iterator BI = BB->begin();
1218  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1219    ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1220
1221  BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1222
1223  // Clone the non-phi instructions of BB into PredBB, keeping track of the
1224  // mapping and using it to remap operands in the cloned instructions.
1225  for (; BI != BB->end(); ++BI) {
1226    Instruction *New = BI->clone();
1227    New->setName(BI->getName());
1228    PredBB->getInstList().insert(OldPredBranch, New);
1229    ValueMapping[BI] = New;
1230
1231    // Remap operands to patch up intra-block references.
1232    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1233      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1234        DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1235        if (I != ValueMapping.end())
1236          New->setOperand(i, I->second);
1237      }
1238  }
1239
1240  // Check to see if the targets of the branch had PHI nodes. If so, we need to
1241  // add entries to the PHI nodes for branch from PredBB now.
1242  BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1243  AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1244                                  ValueMapping);
1245  AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1246                                  ValueMapping);
1247
1248  // If there were values defined in BB that are used outside the block, then we
1249  // now have to update all uses of the value to use either the original value,
1250  // the cloned value, or some PHI derived value.  This can require arbitrary
1251  // PHI insertion, of which we are prepared to do, clean these up now.
1252  SSAUpdater SSAUpdate;
1253  SmallVector<Use*, 16> UsesToRename;
1254  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1255    // Scan all uses of this instruction to see if it is used outside of its
1256    // block, and if so, record them in UsesToRename.
1257    for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1258         ++UI) {
1259      Instruction *User = cast<Instruction>(*UI);
1260      if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1261        if (UserPN->getIncomingBlock(UI) == BB)
1262          continue;
1263      } else if (User->getParent() == BB)
1264        continue;
1265
1266      UsesToRename.push_back(&UI.getUse());
1267    }
1268
1269    // If there are no uses outside the block, we're done with this instruction.
1270    if (UsesToRename.empty())
1271      continue;
1272
1273    DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1274
1275    // We found a use of I outside of BB.  Rename all uses of I that are outside
1276    // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1277    // with the two values we know.
1278    SSAUpdate.Initialize(I);
1279    SSAUpdate.AddAvailableValue(BB, I);
1280    SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1281
1282    while (!UsesToRename.empty())
1283      SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1284    DEBUG(errs() << "\n");
1285  }
1286
1287  // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1288  // that we nuked.
1289  RemovePredecessorAndSimplify(BB, PredBB, TD);
1290
1291  // Remove the unconditional branch at the end of the PredBB block.
1292  OldPredBranch->eraseFromParent();
1293
1294  ++NumDupes;
1295  return true;
1296}
1297
1298
1299