1 //===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This family of functions perform manipulations on basic blocks, and
11 // instructions contained within basic blocks.
13 //===----------------------------------------------------------------------===//
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"
32 /// DeleteDeadBlock - Delete the specified block, which must have no
34 void 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();
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);
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).
54 I.replaceAllUsesWith(UndefValue::get(I.getType()));
55 BB->getInstList().pop_back();
59 BB->eraseFromParent();
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.
66 void 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));
71 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
72 PN->eraseFromParent();
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.
81 bool 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)
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);
97 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
98 /// if possible. The return value indicates success or failure.
99 bool 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
105 if (pred_begin(BB) == pred_end(BB) ||
106 BB->hasAddressTaken()) return false;
108 BasicBlock *PredBB = *PI++;
109 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
111 PredBB = 0; // There are multiple different predecessors...
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;
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!
130 // Can't merge if there are multiple successors.
131 if (!OnlySucc) return false;
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)
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...
149 // Delete the unconditional branch from the predecessor...
150 PredBB->getInstList().pop_back();
152 // Move all definitions in the successor to the predecessor...
153 PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
155 // Make all PHI nodes that referred to BB now refer to Pred as their
157 BB->replaceAllUsesWith(PredBB);
159 // Inherit predecessors name if it exists.
160 if (!PredBB->hasName())
161 PredBB->takeName(BB);
163 // Finally, erase the old block and update dominator info.
165 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) {
166 DomTreeNode* DTN = DT->getNode(BB);
167 DomTreeNode* PredDTN = DT->getNode(PredBB);
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);
180 BB->eraseFromParent();
186 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
187 /// with a value, then remove and delete the original instruction.
189 void 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);
195 // Make sure to propagate a name if there is one already.
196 if (I.hasName() && !V->hasName())
199 // Delete the unnecessary instruction now...
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.
208 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
209 BasicBlock::iterator &BI, Instruction *I) {
210 assert(I->getParent() == 0 &&
211 "ReplaceInstWithInst: Instruction already inserted into basic block!");
213 // Insert the new instruction into the basic block...
214 BasicBlock::iterator New = BIL.insert(BI, I);
216 // Replace all uses of the old instruction, and delete it.
217 ReplaceInstWithValue(BIL, BI, I);
219 // Move BI back to point to the newly inserted instruction
223 /// ReplaceInstWithInst - Replace the instruction specified by From with the
224 /// instruction specified by To.
226 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
227 BasicBlock::iterator BI(From);
228 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
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.
238 void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) {
239 assert(SuccNum < TI->getNumSuccessors() &&
240 "Trying to remove a nonexistant successor!");
242 // If our old successor block contains any PHI nodes, remove the entry in the
243 // PHI nodes that comes from this branch...
245 BasicBlock *BB = TI->getParent();
246 TI->getSuccessor(SuccNum)->removePredecessor(BB);
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...
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());
261 // Create the return...
262 NewTI = ReturnInst::Create(TI->getContext(), RetVal);
266 case Instruction::Invoke: // Should convert to call
267 case Instruction::Switch: // Should remove entry
269 case Instruction::Ret: // Cannot happen, has no successors!
270 llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!");
273 if (NewTI) // If it's a different instruction, replace.
274 ReplaceInstWithInst(TI, NewTI);
277 /// GetSuccessorNumber - Search for the specified successor of basic block BB
278 /// and return its position in the terminator instruction's list of
279 /// successors. It is an error to call this with a block that is not a
281 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
282 TerminatorInst *Term = BB->getTerminator();
284 unsigned e = Term->getNumSuccessors();
286 for (unsigned i = 0; ; ++i) {
287 assert(i != e && "Didn't find edge?");
288 if (Term->getSuccessor(i) == Succ)
294 /// SplitEdge - Split the edge connecting specified block. Pass P must
296 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
297 unsigned SuccNum = GetSuccessorNumber(BB, Succ);
299 // If this is a critical edge, let SplitCriticalEdge do it.
300 TerminatorInst *LatchTerm = BB->getTerminator();
301 if (SplitCriticalEdge(LatchTerm, SuccNum, P))
302 return LatchTerm->getSuccessor(SuccNum);
304 // If the edge isn't critical, then BB has a single successor or Succ has a
305 // single pred. Split the block.
306 BasicBlock::iterator SplitPoint;
307 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
308 // If the successor only has a single pred, split the top of the successor
310 assert(SP == BB && "CFG broken");
312 return SplitBlock(Succ, Succ->begin(), P);
314 // Otherwise, if BB has a single successor, split it at the bottom of the
316 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
317 "Should have a single succ!");
318 return SplitBlock(BB, BB->getTerminator(), P);
322 /// SplitBlock - Split the specified block at the specified instruction - every
323 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
324 /// to a new block. The two blocks are joined by an unconditional branch and
325 /// the loop info is updated.
327 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
328 BasicBlock::iterator SplitIt = SplitPt;
329 while (isa<PHINode>(SplitIt))
331 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
333 // The new block lives in whichever loop the old one did. This preserves
334 // LCSSA as well, because we force the split point to be after any PHI nodes.
335 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>())
336 if (Loop *L = LI->getLoopFor(Old))
337 L->addBasicBlockToLoop(New, LI->getBase());
339 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>())
341 // Old dominates New. New node domiantes all other nodes dominated by Old.
342 DomTreeNode *OldNode = DT->getNode(Old);
343 std::vector<DomTreeNode *> Children;
344 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
346 Children.push_back(*I);
348 DomTreeNode *NewNode = DT->addNewBlock(New,Old);
350 for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
351 E = Children.end(); I != E; ++I)
352 DT->changeImmediateDominator(*I, NewNode);
355 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>())
362 /// SplitBlockPredecessors - This method transforms BB by introducing a new
363 /// basic block into the function, and moving some of the predecessors of BB to
364 /// be predecessors of the new block. The new predecessors are indicated by the
365 /// Preds array, which has NumPreds elements in it. The new block is given a
366 /// suffix of 'Suffix'.
368 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
369 /// DominanceFrontier, LoopInfo, and LCCSA but no other analyses.
370 /// In particular, it does not preserve LoopSimplify (because it's
371 /// complicated to handle the case where one of the edges being split
372 /// is an exit of a loop with other exits).
374 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
375 BasicBlock *const *Preds,
376 unsigned NumPreds, const char *Suffix,
378 // Create new basic block, insert right before the original block.
379 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix,
380 BB->getParent(), BB);
382 // The new block unconditionally branches to the old block.
383 BranchInst *BI = BranchInst::Create(BB, NewBB);
385 LoopInfo *LI = P ? P->getAnalysisIfAvailable<LoopInfo>() : 0;
386 Loop *L = LI ? LI->getLoopFor(BB) : 0;
387 bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID);
389 // Move the edges from Preds to point to NewBB instead of BB.
390 // While here, if we need to preserve loop analyses, collect
391 // some information about how this split will affect loops.
392 bool HasLoopExit = false;
393 bool IsLoopEntry = !!L;
394 bool SplitMakesNewLoopHeader = false;
395 for (unsigned i = 0; i != NumPreds; ++i) {
396 // This is slightly more strict than necessary; the minimum requirement
397 // is that there be no more than one indirectbr branching to BB. And
398 // all BlockAddress uses would need to be updated.
399 assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
400 "Cannot split an edge from an IndirectBrInst");
402 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
405 // If we need to preserve LCSSA, determine if any of
406 // the preds is a loop exit.
408 if (Loop *PL = LI->getLoopFor(Preds[i]))
409 if (!PL->contains(BB))
411 // If we need to preserve LoopInfo, note whether any of the
412 // preds crosses an interesting loop boundary.
414 if (L->contains(Preds[i]))
417 SplitMakesNewLoopHeader = true;
422 // Update dominator tree and dominator frontier if available.
423 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0;
425 DT->splitBlock(NewBB);
426 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0)
427 DF->splitBlock(NewBB);
429 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
430 // node becomes an incoming value for BB's phi node. However, if the Preds
431 // list is empty, we need to insert dummy entries into the PHI nodes in BB to
432 // account for the newly created predecessor.
434 // Insert dummy values as the incoming value.
435 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
436 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
440 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0;
444 // Add the new block to the nearest enclosing loop (and not an
445 // adjacent loop). To find this, examine each of the predecessors and
446 // determine which loops enclose them, and select the most-nested loop
447 // which contains the loop containing the block being split.
448 Loop *InnermostPredLoop = 0;
449 for (unsigned i = 0; i != NumPreds; ++i)
450 if (Loop *PredLoop = LI->getLoopFor(Preds[i])) {
451 // Seek a loop which actually contains the block being split (to
452 // avoid adjacent loops).
453 while (PredLoop && !PredLoop->contains(BB))
454 PredLoop = PredLoop->getParentLoop();
455 // Select the most-nested of these loops which contains the block.
457 PredLoop->contains(BB) &&
458 (!InnermostPredLoop ||
459 InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
460 InnermostPredLoop = PredLoop;
462 if (InnermostPredLoop)
463 InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase());
465 L->addBasicBlockToLoop(NewBB, LI->getBase());
466 if (SplitMakesNewLoopHeader)
467 L->moveToHeader(NewBB);
471 // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
472 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
473 PHINode *PN = cast<PHINode>(I++);
475 // Check to see if all of the values coming in are the same. If so, we
476 // don't need to create a new PHI node, unless it's needed for LCSSA.
479 InVal = PN->getIncomingValueForBlock(Preds[0]);
480 for (unsigned i = 1; i != NumPreds; ++i)
481 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
488 // If all incoming values for the new PHI would be the same, just don't
489 // make a new PHI. Instead, just remove the incoming values from the old
491 for (unsigned i = 0; i != NumPreds; ++i)
492 PN->removeIncomingValue(Preds[i], false);
494 // If the values coming into the block are not the same, we need a PHI.
495 // Create the new PHI node, insert it into NewBB at the end of the block
497 PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
498 if (AA) AA->copyValue(PN, NewPHI);
500 // Move all of the PHI values for 'Preds' to the new PHI.
501 for (unsigned i = 0; i != NumPreds; ++i) {
502 Value *V = PN->removeIncomingValue(Preds[i], false);
503 NewPHI->addIncoming(V, Preds[i]);
508 // Add an incoming value to the PHI node in the loop for the preheader
510 PN->addIncoming(InVal, NewBB);
516 /// FindFunctionBackedges - Analyze the specified function to find all of the
517 /// loop backedges in the function and return them. This is a relatively cheap
518 /// (compared to computing dominators and loop info) analysis.
520 /// The output is added to Result, as pairs of <from,to> edge info.
521 void llvm::FindFunctionBackedges(const Function &F,
522 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
523 const BasicBlock *BB = &F.getEntryBlock();
524 if (succ_begin(BB) == succ_end(BB))
527 SmallPtrSet<const BasicBlock*, 8> Visited;
528 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
529 SmallPtrSet<const BasicBlock*, 8> InStack;
532 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
535 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
536 const BasicBlock *ParentBB = Top.first;
537 succ_const_iterator &I = Top.second;
539 bool FoundNew = false;
540 while (I != succ_end(ParentBB)) {
542 if (Visited.insert(BB)) {
546 // Successor is in VisitStack, it's a back edge.
547 if (InStack.count(BB))
548 Result.push_back(std::make_pair(ParentBB, BB));
552 // Go down one level if there is a unvisited successor.
554 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
557 InStack.erase(VisitStack.pop_back_val().first);
559 } while (!VisitStack.empty());
566 /// AreEquivalentAddressValues - Test if A and B will obviously have the same
567 /// value. This includes recognizing that %t0 and %t1 will have the same
568 /// value in code like this:
569 /// %t0 = getelementptr \@a, 0, 3
570 /// store i32 0, i32* %t0
571 /// %t1 = getelementptr \@a, 0, 3
572 /// %t2 = load i32* %t1
574 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
575 // Test if the values are trivially equivalent.
576 if (A == B) return true;
578 // Test if the values come from identical arithmetic instructions.
579 // Use isIdenticalToWhenDefined instead of isIdenticalTo because
580 // this function is only used when one address use dominates the
581 // other, which means that they'll always either have the same
582 // value or one of them will have an undefined value.
583 if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
584 isa<PHINode>(A) || isa<GetElementPtrInst>(A))
585 if (const Instruction *BI = dyn_cast<Instruction>(B))
586 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
589 // Otherwise they may not be equivalent.
593 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
594 /// instruction before ScanFrom) checking to see if we have the value at the
595 /// memory address *Ptr locally available within a small number of instructions.
596 /// If the value is available, return it.
598 /// If not, return the iterator for the last validated instruction that the
599 /// value would be live through. If we scanned the entire block and didn't find
600 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
601 /// begin() and this returns null. ScanFrom could also be left
603 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If
604 /// it is set to 0, it will scan the whole block. You can also optionally
605 /// specify an alias analysis implementation, which makes this more precise.
606 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
607 BasicBlock::iterator &ScanFrom,
608 unsigned MaxInstsToScan,
610 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
612 // If we're using alias analysis to disambiguate get the size of *Ptr.
613 unsigned AccessSize = 0;
615 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
616 AccessSize = AA->getTypeStoreSize(AccessTy);
619 while (ScanFrom != ScanBB->begin()) {
620 // We must ignore debug info directives when counting (otherwise they
621 // would affect codegen).
622 Instruction *Inst = --ScanFrom;
623 if (isa<DbgInfoIntrinsic>(Inst))
626 // Restore ScanFrom to expected value in case next test succeeds
629 // Don't scan huge blocks.
630 if (MaxInstsToScan-- == 0) return 0;
633 // If this is a load of Ptr, the loaded value is available.
634 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
635 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr))
638 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
639 // If this is a store through Ptr, the value is available!
640 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr))
641 return SI->getOperand(0);
643 // If Ptr is an alloca and this is a store to a different alloca, ignore
644 // the store. This is a trivial form of alias analysis that is important
645 // for reg2mem'd code.
646 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
647 (isa<AllocaInst>(SI->getOperand(1)) ||
648 isa<GlobalVariable>(SI->getOperand(1))))
651 // If we have alias analysis and it says the store won't modify the loaded
652 // value, ignore the store.
654 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
657 // Otherwise the store that may or may not alias the pointer, bail out.
662 // If this is some other instruction that may clobber Ptr, bail out.
663 if (Inst->mayWriteToMemory()) {
664 // If alias analysis claims that it really won't modify the load,
667 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
670 // May modify the pointer, bail out.
676 // Got to the start of the block, we didn't find it, but are done for this