BasicBlockUtils.cpp revision adb6f226714dcfae363f51b453c4590b0f42da5e
1//===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==//
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 family of functions perform manipulations on basic blocks, and
11// instructions contained within basic blocks.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Transforms/Utils/BasicBlockUtils.h"
16#include "llvm/Function.h"
17#include "llvm/Instructions.h"
18#include "llvm/IntrinsicInst.h"
19#include "llvm/Constant.h"
20#include "llvm/Type.h"
21#include "llvm/Analysis/AliasAnalysis.h"
22#include "llvm/Analysis/LoopInfo.h"
23#include "llvm/Analysis/Dominators.h"
24#include "llvm/Target/TargetData.h"
25#include "llvm/Transforms/Utils/Local.h"
26#include "llvm/Transforms/Scalar.h"
27#include "llvm/Support/ErrorHandling.h"
28#include "llvm/Support/ValueHandle.h"
29#include <algorithm>
30using namespace llvm;
31
32/// DeleteDeadBlock - Delete the specified block, which must have no
33/// predecessors.
34void llvm::DeleteDeadBlock(BasicBlock *BB) {
35  assert((pred_begin(BB) == pred_end(BB) ||
36         // Can delete self loop.
37         BB->getSinglePredecessor() == BB) && "Block is not dead!");
38  TerminatorInst *BBTerm = BB->getTerminator();
39
40  // Loop through all of our successors and make sure they know that one
41  // of their predecessors is going away.
42  for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
43    BBTerm->getSuccessor(i)->removePredecessor(BB);
44
45  // Zap all the instructions in the block.
46  while (!BB->empty()) {
47    Instruction &I = BB->back();
48    // If this instruction is used, replace uses with an arbitrary value.
49    // Because control flow can't get here, we don't care what we replace the
50    // value with.  Note that since this block is unreachable, and all values
51    // contained within it must dominate their uses, that all uses will
52    // eventually be removed (they are themselves dead).
53    if (!I.use_empty())
54      I.replaceAllUsesWith(UndefValue::get(I.getType()));
55    BB->getInstList().pop_back();
56  }
57
58  // Zap the block!
59  BB->eraseFromParent();
60}
61
62/// FoldSingleEntryPHINodes - We know that BB has one predecessor.  If there are
63/// any single-entry PHI nodes in it, fold them away.  This handles the case
64/// when all entries to the PHI nodes in a block are guaranteed equal, such as
65/// when the block has exactly one predecessor.
66void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) {
67  while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
68    if (PN->getIncomingValue(0) != PN)
69      PN->replaceAllUsesWith(PN->getIncomingValue(0));
70    else
71      PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
72    PN->eraseFromParent();
73  }
74}
75
76
77/// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it
78/// is dead. Also recursively delete any operands that become dead as
79/// a result. This includes tracing the def-use list from the PHI to see if
80/// it is ultimately unused or if it reaches an unused cycle.
81bool llvm::DeleteDeadPHIs(BasicBlock *BB) {
82  // Recursively deleting a PHI may cause multiple PHIs to be deleted
83  // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete.
84  SmallVector<WeakVH, 8> PHIs;
85  for (BasicBlock::iterator I = BB->begin();
86       PHINode *PN = dyn_cast<PHINode>(I); ++I)
87    PHIs.push_back(PN);
88
89  bool Changed = false;
90  for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
91    if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
92      Changed |= RecursivelyDeleteDeadPHINode(PN);
93
94  return Changed;
95}
96
97/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
98/// if possible.  The return value indicates success or failure.
99bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) {
100  pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
101  // Can't merge the entry block.  Don't merge away blocks who have their
102  // address taken: this is a bug if the predecessor block is the entry node
103  // (because we'd end up taking the address of the entry) and undesirable in
104  // any case.
105  if (pred_begin(BB) == pred_end(BB) ||
106      BB->hasAddressTaken()) return false;
107
108  BasicBlock *PredBB = *PI++;
109  for (; PI != PE; ++PI)  // Search all predecessors, see if they are all same
110    if (*PI != PredBB) {
111      PredBB = 0;       // There are multiple different predecessors...
112      break;
113    }
114
115  // Can't merge if there are multiple predecessors.
116  if (!PredBB) return false;
117  // Don't break self-loops.
118  if (PredBB == BB) return false;
119  // Don't break invokes.
120  if (isa<InvokeInst>(PredBB->getTerminator())) return false;
121
122  succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
123  BasicBlock* OnlySucc = BB;
124  for (; SI != SE; ++SI)
125    if (*SI != OnlySucc) {
126      OnlySucc = 0;     // There are multiple distinct successors!
127      break;
128    }
129
130  // Can't merge if there are multiple successors.
131  if (!OnlySucc) return false;
132
133  // Can't merge if there is PHI loop.
134  for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
135    if (PHINode *PN = dyn_cast<PHINode>(BI)) {
136      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
137        if (PN->getIncomingValue(i) == PN)
138          return false;
139    } else
140      break;
141  }
142
143  // Begin by getting rid of unneeded PHIs.
144  while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
145    PN->replaceAllUsesWith(PN->getIncomingValue(0));
146    BB->getInstList().pop_front();  // Delete the phi node...
147  }
148
149  // Delete the unconditional branch from the predecessor...
150  PredBB->getInstList().pop_back();
151
152  // Move all definitions in the successor to the predecessor...
153  PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
154
155  // Make all PHI nodes that referred to BB now refer to Pred as their
156  // source...
157  BB->replaceAllUsesWith(PredBB);
158
159  // Inherit predecessors name if it exists.
160  if (!PredBB->hasName())
161    PredBB->takeName(BB);
162
163  // Finally, erase the old block and update dominator info.
164  if (P) {
165    if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) {
166      DomTreeNode* DTN = DT->getNode(BB);
167      DomTreeNode* PredDTN = DT->getNode(PredBB);
168
169      if (DTN) {
170        SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
171        for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
172             DE = Children.end(); DI != DE; ++DI)
173          DT->changeImmediateDominator(*DI, PredDTN);
174
175        DT->eraseNode(BB);
176      }
177    }
178  }
179
180  BB->eraseFromParent();
181
182
183  return true;
184}
185
186/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
187/// with a value, then remove and delete the original instruction.
188///
189void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
190                                BasicBlock::iterator &BI, Value *V) {
191  Instruction &I = *BI;
192  // Replaces all of the uses of the instruction with uses of the value
193  I.replaceAllUsesWith(V);
194
195  // Make sure to propagate a name if there is one already.
196  if (I.hasName() && !V->hasName())
197    V->takeName(&I);
198
199  // Delete the unnecessary instruction now...
200  BI = BIL.erase(BI);
201}
202
203
204/// ReplaceInstWithInst - Replace the instruction specified by BI with the
205/// instruction specified by I.  The original instruction is deleted and BI is
206/// updated to point to the new instruction.
207///
208void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
209                               BasicBlock::iterator &BI, Instruction *I) {
210  assert(I->getParent() == 0 &&
211         "ReplaceInstWithInst: Instruction already inserted into basic block!");
212
213  // Insert the new instruction into the basic block...
214  BasicBlock::iterator New = BIL.insert(BI, I);
215
216  // Replace all uses of the old instruction, and delete it.
217  ReplaceInstWithValue(BIL, BI, I);
218
219  // Move BI back to point to the newly inserted instruction
220  BI = New;
221}
222
223/// ReplaceInstWithInst - Replace the instruction specified by From with the
224/// instruction specified by To.
225///
226void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
227  BasicBlock::iterator BI(From);
228  ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
229}
230
231/// RemoveSuccessor - Change the specified terminator instruction such that its
232/// successor SuccNum no longer exists.  Because this reduces the outgoing
233/// degree of the current basic block, the actual terminator instruction itself
234/// may have to be changed.  In the case where the last successor of the block
235/// is deleted, a return instruction is inserted in its place which can cause a
236/// surprising change in program behavior if it is not expected.
237///
238void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) {
239  assert(SuccNum < TI->getNumSuccessors() &&
240         "Trying to remove a nonexistant successor!");
241
242  // If our old successor block contains any PHI nodes, remove the entry in the
243  // PHI nodes that comes from this branch...
244  //
245  BasicBlock *BB = TI->getParent();
246  TI->getSuccessor(SuccNum)->removePredecessor(BB);
247
248  TerminatorInst *NewTI = 0;
249  switch (TI->getOpcode()) {
250  case Instruction::Br:
251    // If this is a conditional branch... convert to unconditional branch.
252    if (TI->getNumSuccessors() == 2) {
253      cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum));
254    } else {                    // Otherwise convert to a return instruction...
255      Value *RetVal = 0;
256
257      // Create a value to return... if the function doesn't return null...
258      if (!BB->getParent()->getReturnType()->isVoidTy())
259        RetVal = Constant::getNullValue(BB->getParent()->getReturnType());
260
261      // Create the return...
262      NewTI = ReturnInst::Create(TI->getContext(), RetVal);
263    }
264    break;
265
266  case Instruction::Invoke:    // Should convert to call
267  case Instruction::Switch:    // Should remove entry
268  default:
269  case Instruction::Ret:       // Cannot happen, has no successors!
270    llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!");
271  }
272
273  if (NewTI)   // If it's a different instruction, replace.
274    ReplaceInstWithInst(TI, NewTI);
275}
276
277/// SuccessorNumber - Search for the specified successor of basic block BB and
278/// return its position in the terminator instruction's list of successors.
279/// It is an error to call this with a block that is not a successor.
280unsigned llvm::SuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
281  TerminatorInst *Term = BB->getTerminator();
282#ifndef NDEBUG
283  unsigned e = Term->getNumSuccessors();
284#endif
285  for (unsigned i = 0; ; ++i) {
286    assert(i != e && "Didn't find edge?");
287    if (Term->getSuccessor(i) == Succ)
288      return i;
289  }
290  return 0;
291}
292
293/// SplitEdge -  Split the edge connecting specified block. Pass P must
294/// not be NULL.
295BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
296  unsigned SuccNum = SuccessorNumber(BB, Succ);
297
298  // If this is a critical edge, let SplitCriticalEdge do it.
299  TerminatorInst *LatchTerm = BB->getTerminator();
300  if (SplitCriticalEdge(LatchTerm, SuccNum, P))
301    return LatchTerm->getSuccessor(SuccNum);
302
303  // If the edge isn't critical, then BB has a single successor or Succ has a
304  // single pred.  Split the block.
305  BasicBlock::iterator SplitPoint;
306  if (BasicBlock *SP = Succ->getSinglePredecessor()) {
307    // If the successor only has a single pred, split the top of the successor
308    // block.
309    assert(SP == BB && "CFG broken");
310    SP = NULL;
311    return SplitBlock(Succ, Succ->begin(), P);
312  } else {
313    // Otherwise, if BB has a single successor, split it at the bottom of the
314    // block.
315    assert(BB->getTerminator()->getNumSuccessors() == 1 &&
316           "Should have a single succ!");
317    return SplitBlock(BB, BB->getTerminator(), P);
318  }
319}
320
321/// SplitBlock - Split the specified block at the specified instruction - every
322/// thing before SplitPt stays in Old and everything starting with SplitPt moves
323/// to a new block.  The two blocks are joined by an unconditional branch and
324/// the loop info is updated.
325///
326BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
327  BasicBlock::iterator SplitIt = SplitPt;
328  while (isa<PHINode>(SplitIt))
329    ++SplitIt;
330  BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
331
332  // The new block lives in whichever loop the old one did. This preserves
333  // LCSSA as well, because we force the split point to be after any PHI nodes.
334  if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>())
335    if (Loop *L = LI->getLoopFor(Old))
336      L->addBasicBlockToLoop(New, LI->getBase());
337
338  if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>())
339    {
340      // Old dominates New. New node domiantes all other nodes dominated by Old.
341      DomTreeNode *OldNode = DT->getNode(Old);
342      std::vector<DomTreeNode *> Children;
343      for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
344           I != E; ++I)
345        Children.push_back(*I);
346
347      DomTreeNode *NewNode =   DT->addNewBlock(New,Old);
348
349      for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
350             E = Children.end(); I != E; ++I)
351        DT->changeImmediateDominator(*I, NewNode);
352    }
353
354  if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>())
355    DF->splitBlock(Old);
356
357  return New;
358}
359
360
361/// SplitBlockPredecessors - This method transforms BB by introducing a new
362/// basic block into the function, and moving some of the predecessors of BB to
363/// be predecessors of the new block.  The new predecessors are indicated by the
364/// Preds array, which has NumPreds elements in it.  The new block is given a
365/// suffix of 'Suffix'.
366///
367/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
368/// DominanceFrontier, LoopInfo, and LCCSA but no other analyses.
369/// In particular, it does not preserve LoopSimplify (because it's
370/// complicated to handle the case where one of the edges being split
371/// is an exit of a loop with other exits).
372///
373BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
374                                         BasicBlock *const *Preds,
375                                         unsigned NumPreds, const char *Suffix,
376                                         Pass *P) {
377  // Create new basic block, insert right before the original block.
378  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix,
379                                         BB->getParent(), BB);
380
381  // The new block unconditionally branches to the old block.
382  BranchInst *BI = BranchInst::Create(BB, NewBB);
383
384  LoopInfo *LI = P ? P->getAnalysisIfAvailable<LoopInfo>() : 0;
385  Loop *L = LI ? LI->getLoopFor(BB) : 0;
386  bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID);
387
388  // Move the edges from Preds to point to NewBB instead of BB.
389  // While here, if we need to preserve loop analyses, collect
390  // some information about how this split will affect loops.
391  bool HasLoopExit = false;
392  bool IsLoopEntry = !!L;
393  bool SplitMakesNewLoopHeader = false;
394  for (unsigned i = 0; i != NumPreds; ++i) {
395    // This is slightly more strict than necessary; the minimum requirement
396    // is that there be no more than one indirectbr branching to BB. And
397    // all BlockAddress uses would need to be updated.
398    assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
399           "Cannot split an edge from an IndirectBrInst");
400
401    Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
402
403    if (LI) {
404      // If we need to preserve LCSSA, determine if any of
405      // the preds is a loop exit.
406      if (PreserveLCSSA)
407        if (Loop *PL = LI->getLoopFor(Preds[i]))
408          if (!PL->contains(BB))
409            HasLoopExit = true;
410      // If we need to preserve LoopInfo, note whether any of the
411      // preds crosses an interesting loop boundary.
412      if (L) {
413        if (L->contains(Preds[i]))
414          IsLoopEntry = false;
415        else
416          SplitMakesNewLoopHeader = true;
417      }
418    }
419  }
420
421  // Update dominator tree and dominator frontier if available.
422  DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0;
423  if (DT)
424    DT->splitBlock(NewBB);
425  if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0)
426    DF->splitBlock(NewBB);
427
428  // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
429  // node becomes an incoming value for BB's phi node.  However, if the Preds
430  // list is empty, we need to insert dummy entries into the PHI nodes in BB to
431  // account for the newly created predecessor.
432  if (NumPreds == 0) {
433    // Insert dummy values as the incoming value.
434    for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
435      cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
436    return NewBB;
437  }
438
439  AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0;
440
441  if (L) {
442    if (IsLoopEntry) {
443      // Add the new block to the nearest enclosing loop (and not an
444      // adjacent loop). To find this, examine each of the predecessors and
445      // determine which loops enclose them, and select the most-nested loop
446      // which contains the loop containing the block being split.
447      Loop *InnermostPredLoop = 0;
448      for (unsigned i = 0; i != NumPreds; ++i)
449        if (Loop *PredLoop = LI->getLoopFor(Preds[i])) {
450          // Seek a loop which actually contains the block being split (to
451          // avoid adjacent loops).
452          while (PredLoop && !PredLoop->contains(BB))
453            PredLoop = PredLoop->getParentLoop();
454          // Select the most-nested of these loops which contains the block.
455          if (PredLoop &&
456              PredLoop->contains(BB) &&
457              (!InnermostPredLoop ||
458               InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
459            InnermostPredLoop = PredLoop;
460        }
461      if (InnermostPredLoop)
462        InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase());
463    } else {
464      L->addBasicBlockToLoop(NewBB, LI->getBase());
465      if (SplitMakesNewLoopHeader)
466        L->moveToHeader(NewBB);
467    }
468  }
469
470  // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
471  for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
472    PHINode *PN = cast<PHINode>(I++);
473
474    // Check to see if all of the values coming in are the same.  If so, we
475    // don't need to create a new PHI node, unless it's needed for LCSSA.
476    Value *InVal = 0;
477    if (!HasLoopExit) {
478      InVal = PN->getIncomingValueForBlock(Preds[0]);
479      for (unsigned i = 1; i != NumPreds; ++i)
480        if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
481          InVal = 0;
482          break;
483        }
484    }
485
486    if (InVal) {
487      // If all incoming values for the new PHI would be the same, just don't
488      // make a new PHI.  Instead, just remove the incoming values from the old
489      // PHI.
490      for (unsigned i = 0; i != NumPreds; ++i)
491        PN->removeIncomingValue(Preds[i], false);
492    } else {
493      // If the values coming into the block are not the same, we need a PHI.
494      // Create the new PHI node, insert it into NewBB at the end of the block
495      PHINode *NewPHI =
496        PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
497      if (AA) AA->copyValue(PN, NewPHI);
498
499      // Move all of the PHI values for 'Preds' to the new PHI.
500      for (unsigned i = 0; i != NumPreds; ++i) {
501        Value *V = PN->removeIncomingValue(Preds[i], false);
502        NewPHI->addIncoming(V, Preds[i]);
503      }
504      InVal = NewPHI;
505    }
506
507    // Add an incoming value to the PHI node in the loop for the preheader
508    // edge.
509    PN->addIncoming(InVal, NewBB);
510  }
511
512  return NewBB;
513}
514
515/// FindFunctionBackedges - Analyze the specified function to find all of the
516/// loop backedges in the function and return them.  This is a relatively cheap
517/// (compared to computing dominators and loop info) analysis.
518///
519/// The output is added to Result, as pairs of <from,to> edge info.
520void llvm::FindFunctionBackedges(const Function &F,
521     SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
522  const BasicBlock *BB = &F.getEntryBlock();
523  if (succ_begin(BB) == succ_end(BB))
524    return;
525
526  SmallPtrSet<const BasicBlock*, 8> Visited;
527  SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
528  SmallPtrSet<const BasicBlock*, 8> InStack;
529
530  Visited.insert(BB);
531  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
532  InStack.insert(BB);
533  do {
534    std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
535    const BasicBlock *ParentBB = Top.first;
536    succ_const_iterator &I = Top.second;
537
538    bool FoundNew = false;
539    while (I != succ_end(ParentBB)) {
540      BB = *I++;
541      if (Visited.insert(BB)) {
542        FoundNew = true;
543        break;
544      }
545      // Successor is in VisitStack, it's a back edge.
546      if (InStack.count(BB))
547        Result.push_back(std::make_pair(ParentBB, BB));
548    }
549
550    if (FoundNew) {
551      // Go down one level if there is a unvisited successor.
552      InStack.insert(BB);
553      VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
554    } else {
555      // Go up one level.
556      InStack.erase(VisitStack.pop_back_val().first);
557    }
558  } while (!VisitStack.empty());
559
560
561}
562
563
564
565/// AreEquivalentAddressValues - Test if A and B will obviously have the same
566/// value. This includes recognizing that %t0 and %t1 will have the same
567/// value in code like this:
568///   %t0 = getelementptr \@a, 0, 3
569///   store i32 0, i32* %t0
570///   %t1 = getelementptr \@a, 0, 3
571///   %t2 = load i32* %t1
572///
573static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
574  // Test if the values are trivially equivalent.
575  if (A == B) return true;
576
577  // Test if the values come from identical arithmetic instructions.
578  // Use isIdenticalToWhenDefined instead of isIdenticalTo because
579  // this function is only used when one address use dominates the
580  // other, which means that they'll always either have the same
581  // value or one of them will have an undefined value.
582  if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
583      isa<PHINode>(A) || isa<GetElementPtrInst>(A))
584    if (const Instruction *BI = dyn_cast<Instruction>(B))
585      if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
586        return true;
587
588  // Otherwise they may not be equivalent.
589  return false;
590}
591
592/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
593/// instruction before ScanFrom) checking to see if we have the value at the
594/// memory address *Ptr locally available within a small number of instructions.
595/// If the value is available, return it.
596///
597/// If not, return the iterator for the last validated instruction that the
598/// value would be live through.  If we scanned the entire block and didn't find
599/// something that invalidates *Ptr or provides it, ScanFrom would be left at
600/// begin() and this returns null.  ScanFrom could also be left
601///
602/// MaxInstsToScan specifies the maximum instructions to scan in the block.  If
603/// it is set to 0, it will scan the whole block. You can also optionally
604/// specify an alias analysis implementation, which makes this more precise.
605Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
606                                      BasicBlock::iterator &ScanFrom,
607                                      unsigned MaxInstsToScan,
608                                      AliasAnalysis *AA) {
609  if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
610
611  // If we're using alias analysis to disambiguate get the size of *Ptr.
612  unsigned AccessSize = 0;
613  if (AA) {
614    const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
615    AccessSize = AA->getTypeStoreSize(AccessTy);
616  }
617
618  while (ScanFrom != ScanBB->begin()) {
619    // We must ignore debug info directives when counting (otherwise they
620    // would affect codegen).
621    Instruction *Inst = --ScanFrom;
622    if (isa<DbgInfoIntrinsic>(Inst))
623      continue;
624
625    // Restore ScanFrom to expected value in case next test succeeds
626    ScanFrom++;
627
628    // Don't scan huge blocks.
629    if (MaxInstsToScan-- == 0) return 0;
630
631    --ScanFrom;
632    // If this is a load of Ptr, the loaded value is available.
633    if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
634      if (AreEquivalentAddressValues(LI->getOperand(0), Ptr))
635        return LI;
636
637    if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
638      // If this is a store through Ptr, the value is available!
639      if (AreEquivalentAddressValues(SI->getOperand(1), Ptr))
640        return SI->getOperand(0);
641
642      // If Ptr is an alloca and this is a store to a different alloca, ignore
643      // the store.  This is a trivial form of alias analysis that is important
644      // for reg2mem'd code.
645      if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
646          (isa<AllocaInst>(SI->getOperand(1)) ||
647           isa<GlobalVariable>(SI->getOperand(1))))
648        continue;
649
650      // If we have alias analysis and it says the store won't modify the loaded
651      // value, ignore the store.
652      if (AA &&
653          (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
654        continue;
655
656      // Otherwise the store that may or may not alias the pointer, bail out.
657      ++ScanFrom;
658      return 0;
659    }
660
661    // If this is some other instruction that may clobber Ptr, bail out.
662    if (Inst->mayWriteToMemory()) {
663      // If alias analysis claims that it really won't modify the load,
664      // ignore it.
665      if (AA &&
666          (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
667        continue;
668
669      // May modify the pointer, bail out.
670      ++ScanFrom;
671      return 0;
672    }
673  }
674
675  // Got to the start of the block, we didn't find it, but are done for this
676  // block.
677  return 0;
678}
679
680