1 //===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===//
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 file defines the primary stateless implementation of the
11 // Alias Analysis interface that implements identities (two different
12 // globals cannot alias, etc), but does no stateful analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/Passes.h"
17 #include "llvm/ADT/SmallPtrSet.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/Analysis/AliasAnalysis.h"
20 #include "llvm/Analysis/CFG.h"
21 #include "llvm/Analysis/CaptureTracking.h"
22 #include "llvm/Analysis/InstructionSimplify.h"
23 #include "llvm/Analysis/LoopInfo.h"
24 #include "llvm/Analysis/MemoryBuiltins.h"
25 #include "llvm/Analysis/ValueTracking.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/DataLayout.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/Dominators.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/GetElementPtrTypeIterator.h"
32 #include "llvm/IR/GlobalAlias.h"
33 #include "llvm/IR/GlobalVariable.h"
34 #include "llvm/IR/Instructions.h"
35 #include "llvm/IR/IntrinsicInst.h"
36 #include "llvm/IR/LLVMContext.h"
37 #include "llvm/IR/Operator.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Support/ErrorHandling.h"
40 #include "llvm/Target/TargetLibraryInfo.h"
44 /// Cutoff after which to stop analysing a set of phi nodes potentially involved
45 /// in a cycle. Because we are analysing 'through' phi nodes we need to be
46 /// careful with value equivalence. We use reachability to make sure a value
47 /// cannot be involved in a cycle.
48 const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
50 //===----------------------------------------------------------------------===//
52 //===----------------------------------------------------------------------===//
54 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
55 /// object that never escapes from the function.
56 static bool isNonEscapingLocalObject(const Value *V) {
57 // If this is a local allocation, check to see if it escapes.
58 if (isa<AllocaInst>(V) || isNoAliasCall(V))
59 // Set StoreCaptures to True so that we can assume in our callers that the
60 // pointer is not the result of a load instruction. Currently
61 // PointerMayBeCaptured doesn't have any special analysis for the
62 // StoreCaptures=false case; if it did, our callers could be refined to be
64 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
66 // If this is an argument that corresponds to a byval or noalias argument,
67 // then it has not escaped before entering the function. Check if it escapes
68 // inside the function.
69 if (const Argument *A = dyn_cast<Argument>(V))
70 if (A->hasByValAttr() || A->hasNoAliasAttr())
71 // Note even if the argument is marked nocapture we still need to check
72 // for copies made inside the function. The nocapture attribute only
73 // specifies that there are no copies made that outlive the function.
74 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
79 /// isEscapeSource - Return true if the pointer is one which would have
80 /// been considered an escape by isNonEscapingLocalObject.
81 static bool isEscapeSource(const Value *V) {
82 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
85 // The load case works because isNonEscapingLocalObject considers all
86 // stores to be escapes (it passes true for the StoreCaptures argument
87 // to PointerMayBeCaptured).
94 /// getObjectSize - Return the size of the object specified by V, or
95 /// UnknownSize if unknown.
96 static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
97 const TargetLibraryInfo &TLI,
98 bool RoundToAlign = false) {
100 if (getObjectSize(V, Size, &DL, &TLI, RoundToAlign))
102 return AliasAnalysis::UnknownSize;
105 /// isObjectSmallerThan - Return true if we can prove that the object specified
106 /// by V is smaller than Size.
107 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
108 const DataLayout &DL,
109 const TargetLibraryInfo &TLI) {
110 // Note that the meanings of the "object" are slightly different in the
111 // following contexts:
112 // c1: llvm::getObjectSize()
113 // c2: llvm.objectsize() intrinsic
114 // c3: isObjectSmallerThan()
115 // c1 and c2 share the same meaning; however, the meaning of "object" in c3
116 // refers to the "entire object".
118 // Consider this example:
119 // char *p = (char*)malloc(100)
122 // In the context of c1 and c2, the "object" pointed by q refers to the
123 // stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
125 // However, in the context of c3, the "object" refers to the chunk of memory
126 // being allocated. So, the "object" has 100 bytes, and q points to the middle
127 // the "object". In case q is passed to isObjectSmallerThan() as the 1st
128 // parameter, before the llvm::getObjectSize() is called to get the size of
129 // entire object, we should:
130 // - either rewind the pointer q to the base-address of the object in
131 // question (in this case rewind to p), or
132 // - just give up. It is up to caller to make sure the pointer is pointing
133 // to the base address the object.
135 // We go for 2nd option for simplicity.
136 if (!isIdentifiedObject(V))
139 // This function needs to use the aligned object size because we allow
140 // reads a bit past the end given sufficient alignment.
141 uint64_t ObjectSize = getObjectSize(V, DL, TLI, /*RoundToAlign*/true);
143 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
146 /// isObjectSize - Return true if we can prove that the object specified
147 /// by V has size Size.
148 static bool isObjectSize(const Value *V, uint64_t Size,
149 const DataLayout &DL, const TargetLibraryInfo &TLI) {
150 uint64_t ObjectSize = getObjectSize(V, DL, TLI);
151 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
154 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
155 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
156 /// Further, an IdentifiedFunctionLocal can not alias with any function
157 /// arguments other than itself, which is not necessarily true for
158 /// IdentifiedObjects.
159 static bool isIdentifiedFunctionLocal(const Value *V)
161 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);
165 //===----------------------------------------------------------------------===//
166 // GetElementPtr Instruction Decomposition and Analysis
167 //===----------------------------------------------------------------------===//
176 struct VariableGEPIndex {
178 ExtensionKind Extension;
181 bool operator==(const VariableGEPIndex &Other) const {
182 return V == Other.V && Extension == Other.Extension &&
183 Scale == Other.Scale;
186 bool operator!=(const VariableGEPIndex &Other) const {
187 return !operator==(Other);
193 /// GetLinearExpression - Analyze the specified value as a linear expression:
194 /// "A*V + B", where A and B are constant integers. Return the scale and offset
195 /// values as APInts and return V as a Value*, and return whether we looked
196 /// through any sign or zero extends. The incoming Value is known to have
197 /// IntegerType and it may already be sign or zero extended.
199 /// Note that this looks through extends, so the high bits may not be
200 /// represented in the result.
201 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
202 ExtensionKind &Extension,
203 const DataLayout &DL, unsigned Depth) {
204 assert(V->getType()->isIntegerTy() && "Not an integer value");
206 // Limit our recursion depth.
213 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
214 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
215 switch (BOp->getOpcode()) {
217 case Instruction::Or:
218 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
220 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &DL))
223 case Instruction::Add:
224 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
226 Offset += RHSC->getValue();
228 case Instruction::Mul:
229 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
231 Offset *= RHSC->getValue();
232 Scale *= RHSC->getValue();
234 case Instruction::Shl:
235 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
237 Offset <<= RHSC->getValue().getLimitedValue();
238 Scale <<= RHSC->getValue().getLimitedValue();
244 // Since GEP indices are sign extended anyway, we don't care about the high
245 // bits of a sign or zero extended value - just scales and offsets. The
246 // extensions have to be consistent though.
247 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
248 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
249 Value *CastOp = cast<CastInst>(V)->getOperand(0);
250 unsigned OldWidth = Scale.getBitWidth();
251 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
252 Scale = Scale.trunc(SmallWidth);
253 Offset = Offset.trunc(SmallWidth);
254 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
256 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
258 Scale = Scale.zext(OldWidth);
259 Offset = Offset.zext(OldWidth);
269 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
270 /// into a base pointer with a constant offset and a number of scaled symbolic
273 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
274 /// the VarIndices vector) are Value*'s that are known to be scaled by the
275 /// specified amount, but which may have other unrepresented high bits. As such,
276 /// the gep cannot necessarily be reconstructed from its decomposed form.
278 /// When DataLayout is around, this function is capable of analyzing everything
279 /// that GetUnderlyingObject can look through. When not, it just looks
280 /// through pointer casts.
283 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
284 SmallVectorImpl<VariableGEPIndex> &VarIndices,
285 const DataLayout *DL) {
286 // Limit recursion depth to limit compile time in crazy cases.
287 unsigned MaxLookup = 6;
291 // See if this is a bitcast or GEP.
292 const Operator *Op = dyn_cast<Operator>(V);
294 // The only non-operator case we can handle are GlobalAliases.
295 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
296 if (!GA->mayBeOverridden()) {
297 V = GA->getAliasee();
304 if (Op->getOpcode() == Instruction::BitCast) {
305 V = Op->getOperand(0);
309 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
311 // If it's not a GEP, hand it off to SimplifyInstruction to see if it
312 // can come up with something. This matches what GetUnderlyingObject does.
313 if (const Instruction *I = dyn_cast<Instruction>(V))
314 // TODO: Get a DominatorTree and use it here.
315 if (const Value *Simplified =
316 SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
324 // Don't attempt to analyze GEPs over unsized objects.
325 if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized())
328 // If we are lacking DataLayout information, we can't compute the offets of
329 // elements computed by GEPs. However, we can handle bitcast equivalent
332 if (!GEPOp->hasAllZeroIndices())
334 V = GEPOp->getOperand(0);
338 unsigned AS = GEPOp->getPointerAddressSpace();
339 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
340 gep_type_iterator GTI = gep_type_begin(GEPOp);
341 for (User::const_op_iterator I = GEPOp->op_begin()+1,
342 E = GEPOp->op_end(); I != E; ++I) {
344 // Compute the (potentially symbolic) offset in bytes for this index.
345 if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
346 // For a struct, add the member offset.
347 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
348 if (FieldNo == 0) continue;
350 BaseOffs += DL->getStructLayout(STy)->getElementOffset(FieldNo);
354 // For an array/pointer, add the element offset, explicitly scaled.
355 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
356 if (CIdx->isZero()) continue;
357 BaseOffs += DL->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
361 uint64_t Scale = DL->getTypeAllocSize(*GTI);
362 ExtensionKind Extension = EK_NotExtended;
364 // If the integer type is smaller than the pointer size, it is implicitly
365 // sign extended to pointer size.
366 unsigned Width = Index->getType()->getIntegerBitWidth();
367 if (DL->getPointerSizeInBits(AS) > Width)
368 Extension = EK_SignExt;
370 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
371 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
372 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
375 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
376 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
377 BaseOffs += IndexOffset.getSExtValue()*Scale;
378 Scale *= IndexScale.getSExtValue();
380 // If we already had an occurrence of this index variable, merge this
381 // scale into it. For example, we want to handle:
382 // A[x][x] -> x*16 + x*4 -> x*20
383 // This also ensures that 'x' only appears in the index list once.
384 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
385 if (VarIndices[i].V == Index &&
386 VarIndices[i].Extension == Extension) {
387 Scale += VarIndices[i].Scale;
388 VarIndices.erase(VarIndices.begin()+i);
393 // Make sure that we have a scale that makes sense for this target's
395 if (unsigned ShiftBits = 64 - DL->getPointerSizeInBits(AS)) {
397 Scale = (int64_t)Scale >> ShiftBits;
401 VariableGEPIndex Entry = {Index, Extension,
402 static_cast<int64_t>(Scale)};
403 VarIndices.push_back(Entry);
407 // Analyze the base pointer next.
408 V = GEPOp->getOperand(0);
409 } while (--MaxLookup);
411 // If the chain of expressions is too deep, just return early.
415 //===----------------------------------------------------------------------===//
416 // BasicAliasAnalysis Pass
417 //===----------------------------------------------------------------------===//
420 static const Function *getParent(const Value *V) {
421 if (const Instruction *inst = dyn_cast<Instruction>(V))
422 return inst->getParent()->getParent();
424 if (const Argument *arg = dyn_cast<Argument>(V))
425 return arg->getParent();
430 static bool notDifferentParent(const Value *O1, const Value *O2) {
432 const Function *F1 = getParent(O1);
433 const Function *F2 = getParent(O2);
435 return !F1 || !F2 || F1 == F2;
440 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
441 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
442 static char ID; // Class identification, replacement for typeinfo
443 BasicAliasAnalysis() : ImmutablePass(ID) {
444 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
447 void initializePass() override {
448 InitializeAliasAnalysis(this);
451 void getAnalysisUsage(AnalysisUsage &AU) const override {
452 AU.addRequired<AliasAnalysis>();
453 AU.addRequired<TargetLibraryInfo>();
456 AliasResult alias(const Location &LocA, const Location &LocB) override {
457 assert(AliasCache.empty() && "AliasCache must be cleared after use!");
458 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
459 "BasicAliasAnalysis doesn't support interprocedural queries.");
460 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
461 LocB.Ptr, LocB.Size, LocB.TBAATag);
462 // AliasCache rarely has more than 1 or 2 elements, always use
463 // shrink_and_clear so it quickly returns to the inline capacity of the
464 // SmallDenseMap if it ever grows larger.
465 // FIXME: This should really be shrink_to_inline_capacity_and_clear().
466 AliasCache.shrink_and_clear();
467 VisitedPhiBBs.clear();
471 ModRefResult getModRefInfo(ImmutableCallSite CS,
472 const Location &Loc) override;
474 ModRefResult getModRefInfo(ImmutableCallSite CS1,
475 ImmutableCallSite CS2) override {
476 // The AliasAnalysis base class has some smarts, lets use them.
477 return AliasAnalysis::getModRefInfo(CS1, CS2);
480 /// pointsToConstantMemory - Chase pointers until we find a (constant
482 bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
484 /// getModRefBehavior - Return the behavior when calling the given
486 ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
488 /// getModRefBehavior - Return the behavior when calling the given function.
489 /// For use when the call site is not known.
490 ModRefBehavior getModRefBehavior(const Function *F) override;
492 /// getAdjustedAnalysisPointer - This method is used when a pass implements
493 /// an analysis interface through multiple inheritance. If needed, it
494 /// should override this to adjust the this pointer as needed for the
495 /// specified pass info.
496 void *getAdjustedAnalysisPointer(const void *ID) override {
497 if (ID == &AliasAnalysis::ID)
498 return (AliasAnalysis*)this;
503 // AliasCache - Track alias queries to guard against recursion.
504 typedef std::pair<Location, Location> LocPair;
505 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
506 AliasCacheTy AliasCache;
508 /// \brief Track phi nodes we have visited. When interpret "Value" pointer
509 /// equality as value equality we need to make sure that the "Value" is not
510 /// part of a cycle. Otherwise, two uses could come from different
511 /// "iterations" of a cycle and see different values for the same "Value"
513 /// The following example shows the problem:
514 /// %p = phi(%alloca1, %addr2)
516 /// %addr1 = gep, %alloca2, 0, %l
517 /// %addr2 = gep %alloca2, 0, (%l + 1)
518 /// alias(%p, %addr1) -> MayAlias !
520 SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
522 // Visited - Track instructions visited by pointsToConstantMemory.
523 SmallPtrSet<const Value*, 16> Visited;
525 /// \brief Check whether two Values can be considered equivalent.
527 /// In addition to pointer equivalence of \p V1 and \p V2 this checks
528 /// whether they can not be part of a cycle in the value graph by looking at
529 /// all visited phi nodes an making sure that the phis cannot reach the
530 /// value. We have to do this because we are looking through phi nodes (That
531 /// is we say noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
532 bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
534 /// \brief Dest and Src are the variable indices from two decomposed
535 /// GetElementPtr instructions GEP1 and GEP2 which have common base
536 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
537 /// difference between the two pointers.
538 void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
539 const SmallVectorImpl<VariableGEPIndex> &Src);
541 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
542 // instruction against another.
543 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
544 const MDNode *V1TBAAInfo,
545 const Value *V2, uint64_t V2Size,
546 const MDNode *V2TBAAInfo,
547 const Value *UnderlyingV1, const Value *UnderlyingV2);
549 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
550 // instruction against another.
551 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
552 const MDNode *PNTBAAInfo,
553 const Value *V2, uint64_t V2Size,
554 const MDNode *V2TBAAInfo);
556 /// aliasSelect - Disambiguate a Select instruction against another value.
557 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
558 const MDNode *SITBAAInfo,
559 const Value *V2, uint64_t V2Size,
560 const MDNode *V2TBAAInfo);
562 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
563 const MDNode *V1TBAATag,
564 const Value *V2, uint64_t V2Size,
565 const MDNode *V2TBAATag);
567 } // End of anonymous namespace
569 // Register this pass...
570 char BasicAliasAnalysis::ID = 0;
571 INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
572 "Basic Alias Analysis (stateless AA impl)",
574 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
575 INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
576 "Basic Alias Analysis (stateless AA impl)",
580 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
581 return new BasicAliasAnalysis();
584 /// pointsToConstantMemory - Returns whether the given pointer value
585 /// points to memory that is local to the function, with global constants being
586 /// considered local to all functions.
588 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
589 assert(Visited.empty() && "Visited must be cleared after use!");
591 unsigned MaxLookup = 8;
592 SmallVector<const Value *, 16> Worklist;
593 Worklist.push_back(Loc.Ptr);
595 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), DL);
596 if (!Visited.insert(V)) {
598 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
601 // An alloca instruction defines local memory.
602 if (OrLocal && isa<AllocaInst>(V))
605 // A global constant counts as local memory for our purposes.
606 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
607 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
608 // global to be marked constant in some modules and non-constant in
609 // others. GV may even be a declaration, not a definition.
610 if (!GV->isConstant()) {
612 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
617 // If both select values point to local memory, then so does the select.
618 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
619 Worklist.push_back(SI->getTrueValue());
620 Worklist.push_back(SI->getFalseValue());
624 // If all values incoming to a phi node point to local memory, then so does
626 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
627 // Don't bother inspecting phi nodes with many operands.
628 if (PN->getNumIncomingValues() > MaxLookup) {
630 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
632 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
633 Worklist.push_back(PN->getIncomingValue(i));
637 // Otherwise be conservative.
639 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
641 } while (!Worklist.empty() && --MaxLookup);
644 return Worklist.empty();
647 /// getModRefBehavior - Return the behavior when calling the given call site.
648 AliasAnalysis::ModRefBehavior
649 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
650 if (CS.doesNotAccessMemory())
651 // Can't do better than this.
652 return DoesNotAccessMemory;
654 ModRefBehavior Min = UnknownModRefBehavior;
656 // If the callsite knows it only reads memory, don't return worse
658 if (CS.onlyReadsMemory())
659 Min = OnlyReadsMemory;
661 // The AliasAnalysis base class has some smarts, lets use them.
662 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
665 /// getModRefBehavior - Return the behavior when calling the given function.
666 /// For use when the call site is not known.
667 AliasAnalysis::ModRefBehavior
668 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
669 // If the function declares it doesn't access memory, we can't do better.
670 if (F->doesNotAccessMemory())
671 return DoesNotAccessMemory;
673 // For intrinsics, we can check the table.
674 if (unsigned iid = F->getIntrinsicID()) {
675 #define GET_INTRINSIC_MODREF_BEHAVIOR
676 #include "llvm/IR/Intrinsics.gen"
677 #undef GET_INTRINSIC_MODREF_BEHAVIOR
680 ModRefBehavior Min = UnknownModRefBehavior;
682 // If the function declares it only reads memory, go with that.
683 if (F->onlyReadsMemory())
684 Min = OnlyReadsMemory;
686 // Otherwise be conservative.
687 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
690 /// getModRefInfo - Check to see if the specified callsite can clobber the
691 /// specified memory object. Since we only look at local properties of this
692 /// function, we really can't say much about this query. We do, however, use
693 /// simple "address taken" analysis on local objects.
694 AliasAnalysis::ModRefResult
695 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
696 const Location &Loc) {
697 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
698 "AliasAnalysis query involving multiple functions!");
700 const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
702 // If this is a tail call and Loc.Ptr points to a stack location, we know that
703 // the tail call cannot access or modify the local stack.
704 // We cannot exclude byval arguments here; these belong to the caller of
705 // the current function not to the current function, and a tail callee
706 // may reference them.
707 if (isa<AllocaInst>(Object))
708 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
709 if (CI->isTailCall())
712 // If the pointer is to a locally allocated object that does not escape,
713 // then the call can not mod/ref the pointer unless the call takes the pointer
714 // as an argument, and itself doesn't capture it.
715 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
716 isNonEscapingLocalObject(Object)) {
717 bool PassedAsArg = false;
719 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
720 CI != CE; ++CI, ++ArgNo) {
721 // Only look at the no-capture or byval pointer arguments. If this
722 // pointer were passed to arguments that were neither of these, then it
723 // couldn't be no-capture.
724 if (!(*CI)->getType()->isPointerTy() ||
725 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
728 // If this is a no-capture pointer argument, see if we can tell that it
729 // is impossible to alias the pointer we're checking. If not, we have to
730 // assume that the call could touch the pointer, even though it doesn't
732 if (!isNoAlias(Location(*CI), Location(Object))) {
742 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
743 ModRefResult Min = ModRef;
745 // Finally, handle specific knowledge of intrinsics.
746 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
748 switch (II->getIntrinsicID()) {
750 case Intrinsic::memcpy:
751 case Intrinsic::memmove: {
752 uint64_t Len = UnknownSize;
753 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
754 Len = LenCI->getZExtValue();
755 Value *Dest = II->getArgOperand(0);
756 Value *Src = II->getArgOperand(1);
757 // If it can't overlap the source dest, then it doesn't modref the loc.
758 if (isNoAlias(Location(Dest, Len), Loc)) {
759 if (isNoAlias(Location(Src, Len), Loc))
761 // If it can't overlap the dest, then worst case it reads the loc.
763 } else if (isNoAlias(Location(Src, Len), Loc)) {
764 // If it can't overlap the source, then worst case it mutates the loc.
769 case Intrinsic::memset:
770 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
771 // will handle it for the variable length case.
772 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
773 uint64_t Len = LenCI->getZExtValue();
774 Value *Dest = II->getArgOperand(0);
775 if (isNoAlias(Location(Dest, Len), Loc))
778 // We know that memset doesn't load anything.
781 case Intrinsic::lifetime_start:
782 case Intrinsic::lifetime_end:
783 case Intrinsic::invariant_start: {
785 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
786 if (isNoAlias(Location(II->getArgOperand(1),
788 II->getMetadata(LLVMContext::MD_tbaa)),
793 case Intrinsic::invariant_end: {
795 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
796 if (isNoAlias(Location(II->getArgOperand(2),
798 II->getMetadata(LLVMContext::MD_tbaa)),
803 case Intrinsic::arm_neon_vld1: {
804 // LLVM's vld1 and vst1 intrinsics currently only support a single
807 DL ? DL->getTypeStoreSize(II->getType()) : UnknownSize;
808 if (isNoAlias(Location(II->getArgOperand(0), Size,
809 II->getMetadata(LLVMContext::MD_tbaa)),
814 case Intrinsic::arm_neon_vst1: {
816 DL ? DL->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize;
817 if (isNoAlias(Location(II->getArgOperand(0), Size,
818 II->getMetadata(LLVMContext::MD_tbaa)),
825 // We can bound the aliasing properties of memset_pattern16 just as we can
826 // for memcpy/memset. This is particularly important because the
827 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
828 // whenever possible.
829 else if (TLI.has(LibFunc::memset_pattern16) &&
830 CS.getCalledFunction() &&
831 CS.getCalledFunction()->getName() == "memset_pattern16") {
832 const Function *MS = CS.getCalledFunction();
833 FunctionType *MemsetType = MS->getFunctionType();
834 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
835 isa<PointerType>(MemsetType->getParamType(0)) &&
836 isa<PointerType>(MemsetType->getParamType(1)) &&
837 isa<IntegerType>(MemsetType->getParamType(2))) {
838 uint64_t Len = UnknownSize;
839 if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2)))
840 Len = LenCI->getZExtValue();
841 const Value *Dest = CS.getArgument(0);
842 const Value *Src = CS.getArgument(1);
843 // If it can't overlap the source dest, then it doesn't modref the loc.
844 if (isNoAlias(Location(Dest, Len), Loc)) {
845 // Always reads 16 bytes of the source.
846 if (isNoAlias(Location(Src, 16), Loc))
848 // If it can't overlap the dest, then worst case it reads the loc.
850 // Always reads 16 bytes of the source.
851 } else if (isNoAlias(Location(Src, 16), Loc)) {
852 // If it can't overlap the source, then worst case it mutates the loc.
858 // The AliasAnalysis base class has some smarts, lets use them.
859 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
862 static bool areVarIndicesEqual(SmallVectorImpl<VariableGEPIndex> &Indices1,
863 SmallVectorImpl<VariableGEPIndex> &Indices2) {
864 unsigned Size1 = Indices1.size();
865 unsigned Size2 = Indices2.size();
870 for (unsigned I = 0; I != Size1; ++I)
871 if (Indices1[I] != Indices2[I])
877 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
878 /// against another pointer. We know that V1 is a GEP, but we don't know
879 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, DL),
880 /// UnderlyingV2 is the same for V2.
882 AliasAnalysis::AliasResult
883 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
884 const MDNode *V1TBAAInfo,
885 const Value *V2, uint64_t V2Size,
886 const MDNode *V2TBAAInfo,
887 const Value *UnderlyingV1,
888 const Value *UnderlyingV2) {
889 int64_t GEP1BaseOffset;
890 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
892 // If we have two gep instructions with must-alias or not-alias'ing base
893 // pointers, figure out if the indexes to the GEP tell us anything about the
895 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
896 // Do the base pointers alias?
897 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
898 UnderlyingV2, UnknownSize, 0);
900 // Check for geps of non-aliasing underlying pointers where the offsets are
902 if ((BaseAlias == MayAlias) && V1Size == V2Size) {
903 // Do the base pointers alias assuming type and size.
904 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
905 V1TBAAInfo, UnderlyingV2,
907 if (PreciseBaseAlias == NoAlias) {
908 // See if the computed offset from the common pointer tells us about the
909 // relation of the resulting pointer.
910 int64_t GEP2BaseOffset;
911 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
912 const Value *GEP2BasePtr =
913 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, DL);
914 const Value *GEP1BasePtr =
915 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, DL);
916 // DecomposeGEPExpression and GetUnderlyingObject should return the
917 // same result except when DecomposeGEPExpression has no DataLayout.
918 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
920 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
924 if (GEP1BaseOffset == GEP2BaseOffset &&
925 areVarIndicesEqual(GEP1VariableIndices, GEP2VariableIndices))
927 GEP1VariableIndices.clear();
931 // If we get a No or May, then return it immediately, no amount of analysis
932 // will improve this situation.
933 if (BaseAlias != MustAlias) return BaseAlias;
935 // Otherwise, we have a MustAlias. Since the base pointers alias each other
936 // exactly, see if the computed offset from the common pointer tells us
937 // about the relation of the resulting pointer.
938 const Value *GEP1BasePtr =
939 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, DL);
941 int64_t GEP2BaseOffset;
942 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
943 const Value *GEP2BasePtr =
944 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, DL);
946 // DecomposeGEPExpression and GetUnderlyingObject should return the
947 // same result except when DecomposeGEPExpression has no DataLayout.
948 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
950 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
954 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
955 // symbolic difference.
956 GEP1BaseOffset -= GEP2BaseOffset;
957 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
960 // Check to see if these two pointers are related by the getelementptr
961 // instruction. If one pointer is a GEP with a non-zero index of the other
962 // pointer, we know they cannot alias.
964 // If both accesses are unknown size, we can't do anything useful here.
965 if (V1Size == UnknownSize && V2Size == UnknownSize)
968 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
969 V2, V2Size, V2TBAAInfo);
971 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
972 // If V2 is known not to alias GEP base pointer, then the two values
973 // cannot alias per GEP semantics: "A pointer value formed from a
974 // getelementptr instruction is associated with the addresses associated
975 // with the first operand of the getelementptr".
978 const Value *GEP1BasePtr =
979 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, DL);
981 // DecomposeGEPExpression and GetUnderlyingObject should return the
982 // same result except when DecomposeGEPExpression has no DataLayout.
983 if (GEP1BasePtr != UnderlyingV1) {
985 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
990 // In the two GEP Case, if there is no difference in the offsets of the
991 // computed pointers, the resultant pointers are a must alias. This
992 // hapens when we have two lexically identical GEP's (for example).
994 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
995 // must aliases the GEP, the end result is a must alias also.
996 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
999 // If there is a constant difference between the pointers, but the difference
1000 // is less than the size of the associated memory object, then we know
1001 // that the objects are partially overlapping. If the difference is
1002 // greater, we know they do not overlap.
1003 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
1004 if (GEP1BaseOffset >= 0) {
1005 if (V2Size != UnknownSize) {
1006 if ((uint64_t)GEP1BaseOffset < V2Size)
1007 return PartialAlias;
1011 // We have the situation where:
1014 // ---------------->|
1015 // |-->V1Size |-------> V2Size
1017 // We need to know that V2Size is not unknown, otherwise we might have
1018 // stripped a gep with negative index ('gep <ptr>, -1, ...).
1019 if (V1Size != UnknownSize && V2Size != UnknownSize) {
1020 if (-(uint64_t)GEP1BaseOffset < V1Size)
1021 return PartialAlias;
1027 // Try to distinguish something like &A[i][1] against &A[42][0].
1028 // Grab the least significant bit set in any of the scales.
1029 if (!GEP1VariableIndices.empty()) {
1030 uint64_t Modulo = 0;
1031 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
1032 Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
1033 Modulo = Modulo ^ (Modulo & (Modulo - 1));
1035 // We can compute the difference between the two addresses
1036 // mod Modulo. Check whether that difference guarantees that the
1037 // two locations do not alias.
1038 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
1039 if (V1Size != UnknownSize && V2Size != UnknownSize &&
1040 ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
1044 // Statically, we can see that the base objects are the same, but the
1045 // pointers have dynamic offsets which we can't resolve. And none of our
1046 // little tricks above worked.
1048 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
1049 // practical effect of this is protecting TBAA in the case of dynamic
1050 // indices into arrays of unions or malloc'd memory.
1051 return PartialAlias;
1054 static AliasAnalysis::AliasResult
1055 MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
1056 // If the results agree, take it.
1059 // A mix of PartialAlias and MustAlias is PartialAlias.
1060 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
1061 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
1062 return AliasAnalysis::PartialAlias;
1063 // Otherwise, we don't know anything.
1064 return AliasAnalysis::MayAlias;
1067 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
1068 /// instruction against another.
1069 AliasAnalysis::AliasResult
1070 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
1071 const MDNode *SITBAAInfo,
1072 const Value *V2, uint64_t V2Size,
1073 const MDNode *V2TBAAInfo) {
1074 // If the values are Selects with the same condition, we can do a more precise
1075 // check: just check for aliases between the values on corresponding arms.
1076 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
1077 if (SI->getCondition() == SI2->getCondition()) {
1079 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
1080 SI2->getTrueValue(), V2Size, V2TBAAInfo);
1081 if (Alias == MayAlias)
1083 AliasResult ThisAlias =
1084 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
1085 SI2->getFalseValue(), V2Size, V2TBAAInfo);
1086 return MergeAliasResults(ThisAlias, Alias);
1089 // If both arms of the Select node NoAlias or MustAlias V2, then returns
1090 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1092 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
1093 if (Alias == MayAlias)
1096 AliasResult ThisAlias =
1097 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
1098 return MergeAliasResults(ThisAlias, Alias);
1101 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
1103 AliasAnalysis::AliasResult
1104 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
1105 const MDNode *PNTBAAInfo,
1106 const Value *V2, uint64_t V2Size,
1107 const MDNode *V2TBAAInfo) {
1108 // Track phi nodes we have visited. We use this information when we determine
1109 // value equivalence.
1110 VisitedPhiBBs.insert(PN->getParent());
1112 // If the values are PHIs in the same block, we can do a more precise
1113 // as well as efficient check: just check for aliases between the values
1114 // on corresponding edges.
1115 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
1116 if (PN2->getParent() == PN->getParent()) {
1117 LocPair Locs(Location(PN, PNSize, PNTBAAInfo),
1118 Location(V2, V2Size, V2TBAAInfo));
1120 std::swap(Locs.first, Locs.second);
1121 // Analyse the PHIs' inputs under the assumption that the PHIs are
1123 // If the PHIs are May/MustAlias there must be (recursively) an input
1124 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or
1125 // there must be an operation on the PHIs within the PHIs' value cycle
1126 // that causes a MayAlias.
1127 // Pretend the phis do not alias.
1128 AliasResult Alias = NoAlias;
1129 assert(AliasCache.count(Locs) &&
1130 "There must exist an entry for the phi node");
1131 AliasResult OrigAliasResult = AliasCache[Locs];
1132 AliasCache[Locs] = NoAlias;
1134 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1135 AliasResult ThisAlias =
1136 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
1137 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1138 V2Size, V2TBAAInfo);
1139 Alias = MergeAliasResults(ThisAlias, Alias);
1140 if (Alias == MayAlias)
1144 // Reset if speculation failed.
1145 if (Alias != NoAlias)
1146 AliasCache[Locs] = OrigAliasResult;
1151 SmallPtrSet<Value*, 4> UniqueSrc;
1152 SmallVector<Value*, 4> V1Srcs;
1153 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1154 Value *PV1 = PN->getIncomingValue(i);
1155 if (isa<PHINode>(PV1))
1156 // If any of the source itself is a PHI, return MayAlias conservatively
1157 // to avoid compile time explosion. The worst possible case is if both
1158 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1159 // and 'n' are the number of PHI sources.
1161 if (UniqueSrc.insert(PV1))
1162 V1Srcs.push_back(PV1);
1165 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1166 V1Srcs[0], PNSize, PNTBAAInfo);
1167 // Early exit if the check of the first PHI source against V2 is MayAlias.
1168 // Other results are not possible.
1169 if (Alias == MayAlias)
1172 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1173 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1174 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1175 Value *V = V1Srcs[i];
1177 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1178 V, PNSize, PNTBAAInfo);
1179 Alias = MergeAliasResults(ThisAlias, Alias);
1180 if (Alias == MayAlias)
1187 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1188 // such as array references.
1190 AliasAnalysis::AliasResult
1191 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1192 const MDNode *V1TBAAInfo,
1193 const Value *V2, uint64_t V2Size,
1194 const MDNode *V2TBAAInfo) {
1195 // If either of the memory references is empty, it doesn't matter what the
1196 // pointer values are.
1197 if (V1Size == 0 || V2Size == 0)
1200 // Strip off any casts if they exist.
1201 V1 = V1->stripPointerCasts();
1202 V2 = V2->stripPointerCasts();
1204 // Are we checking for alias of the same value?
1205 // Because we look 'through' phi nodes we could look at "Value" pointers from
1206 // different iterations. We must therefore make sure that this is not the
1207 // case. The function isValueEqualInPotentialCycles ensures that this cannot
1208 // happen by looking at the visited phi nodes and making sure they cannot
1210 if (isValueEqualInPotentialCycles(V1, V2))
1213 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1214 return NoAlias; // Scalars cannot alias each other
1216 // Figure out what objects these things are pointing to if we can.
1217 const Value *O1 = GetUnderlyingObject(V1, DL);
1218 const Value *O2 = GetUnderlyingObject(V2, DL);
1220 // Null values in the default address space don't point to any object, so they
1221 // don't alias any other pointer.
1222 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1223 if (CPN->getType()->getAddressSpace() == 0)
1225 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1226 if (CPN->getType()->getAddressSpace() == 0)
1230 // If V1/V2 point to two different objects we know that we have no alias.
1231 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1234 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1235 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1236 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1239 // Function arguments can't alias with things that are known to be
1240 // unambigously identified at the function level.
1241 if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
1242 (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
1245 // Most objects can't alias null.
1246 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1247 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1250 // If one pointer is the result of a call/invoke or load and the other is a
1251 // non-escaping local object within the same function, then we know the
1252 // object couldn't escape to a point where the call could return it.
1254 // Note that if the pointers are in different functions, there are a
1255 // variety of complications. A call with a nocapture argument may still
1256 // temporary store the nocapture argument's value in a temporary memory
1257 // location if that memory location doesn't escape. Or it may pass a
1258 // nocapture value to other functions as long as they don't capture it.
1259 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1261 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1265 // If the size of one access is larger than the entire object on the other
1266 // side, then we know such behavior is undefined and can assume no alias.
1268 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *DL, *TLI)) ||
1269 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *DL, *TLI)))
1272 // Check the cache before climbing up use-def chains. This also terminates
1273 // otherwise infinitely recursive queries.
1274 LocPair Locs(Location(V1, V1Size, V1TBAAInfo),
1275 Location(V2, V2Size, V2TBAAInfo));
1277 std::swap(Locs.first, Locs.second);
1278 std::pair<AliasCacheTy::iterator, bool> Pair =
1279 AliasCache.insert(std::make_pair(Locs, MayAlias));
1281 return Pair.first->second;
1283 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1284 // GEP can't simplify, we don't even look at the PHI cases.
1285 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1287 std::swap(V1Size, V2Size);
1289 std::swap(V1TBAAInfo, V2TBAAInfo);
1291 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1292 AliasResult Result = aliasGEP(GV1, V1Size, V1TBAAInfo, V2, V2Size, V2TBAAInfo, O1, O2);
1293 if (Result != MayAlias) return AliasCache[Locs] = Result;
1296 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1298 std::swap(V1Size, V2Size);
1299 std::swap(V1TBAAInfo, V2TBAAInfo);
1301 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1302 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1303 V2, V2Size, V2TBAAInfo);
1304 if (Result != MayAlias) return AliasCache[Locs] = Result;
1307 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1309 std::swap(V1Size, V2Size);
1310 std::swap(V1TBAAInfo, V2TBAAInfo);
1312 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1313 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1314 V2, V2Size, V2TBAAInfo);
1315 if (Result != MayAlias) return AliasCache[Locs] = Result;
1318 // If both pointers are pointing into the same object and one of them
1319 // accesses is accessing the entire object, then the accesses must
1320 // overlap in some way.
1322 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *DL, *TLI)) ||
1323 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *DL, *TLI)))
1324 return AliasCache[Locs] = PartialAlias;
1326 AliasResult Result =
1327 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1328 Location(V2, V2Size, V2TBAAInfo));
1329 return AliasCache[Locs] = Result;
1332 bool BasicAliasAnalysis::isValueEqualInPotentialCycles(const Value *V,
1337 const Instruction *Inst = dyn_cast<Instruction>(V);
1341 if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
1344 // Use dominance or loop info if available.
1345 DominatorTreeWrapperPass *DTWP =
1346 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
1347 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : 0;
1348 LoopInfo *LI = getAnalysisIfAvailable<LoopInfo>();
1350 // Make sure that the visited phis cannot reach the Value. This ensures that
1351 // the Values cannot come from different iterations of a potential cycle the
1352 // phi nodes could be involved in.
1353 for (SmallPtrSet<const BasicBlock *, 8>::iterator PI = VisitedPhiBBs.begin(),
1354 PE = VisitedPhiBBs.end();
1356 if (isPotentiallyReachable((*PI)->begin(), Inst, DT, LI))
1362 /// GetIndexDifference - Dest and Src are the variable indices from two
1363 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
1364 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
1365 /// difference between the two pointers.
1366 void BasicAliasAnalysis::GetIndexDifference(
1367 SmallVectorImpl<VariableGEPIndex> &Dest,
1368 const SmallVectorImpl<VariableGEPIndex> &Src) {
1372 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
1373 const Value *V = Src[i].V;
1374 ExtensionKind Extension = Src[i].Extension;
1375 int64_t Scale = Src[i].Scale;
1377 // Find V in Dest. This is N^2, but pointer indices almost never have more
1378 // than a few variable indexes.
1379 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
1380 if (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
1381 Dest[j].Extension != Extension)
1384 // If we found it, subtract off Scale V's from the entry in Dest. If it
1385 // goes to zero, remove the entry.
1386 if (Dest[j].Scale != Scale)
1387 Dest[j].Scale -= Scale;
1389 Dest.erase(Dest.begin() + j);
1394 // If we didn't consume this entry, add it to the end of the Dest list.
1396 VariableGEPIndex Entry = { V, Extension, -Scale };
1397 Dest.push_back(Entry);