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 // The max limit of the search depth in DecomposeGEPExpression() and
51 // GetUnderlyingObject(), both functions need to use the same search
52 // depth otherwise the algorithm in aliasGEP will assert.
53 static const unsigned MaxLookupSearchDepth = 6;
55 //===----------------------------------------------------------------------===//
57 //===----------------------------------------------------------------------===//
59 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
60 /// object that never escapes from the function.
61 static bool isNonEscapingLocalObject(const Value *V) {
62 // If this is a local allocation, check to see if it escapes.
63 if (isa<AllocaInst>(V) || isNoAliasCall(V))
64 // Set StoreCaptures to True so that we can assume in our callers that the
65 // pointer is not the result of a load instruction. Currently
66 // PointerMayBeCaptured doesn't have any special analysis for the
67 // StoreCaptures=false case; if it did, our callers could be refined to be
69 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
71 // If this is an argument that corresponds to a byval or noalias argument,
72 // then it has not escaped before entering the function. Check if it escapes
73 // inside the function.
74 if (const Argument *A = dyn_cast<Argument>(V))
75 if (A->hasByValAttr() || A->hasNoAliasAttr())
76 // Note even if the argument is marked nocapture we still need to check
77 // for copies made inside the function. The nocapture attribute only
78 // specifies that there are no copies made that outlive the function.
79 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
84 /// isEscapeSource - Return true if the pointer is one which would have
85 /// been considered an escape by isNonEscapingLocalObject.
86 static bool isEscapeSource(const Value *V) {
87 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
90 // The load case works because isNonEscapingLocalObject considers all
91 // stores to be escapes (it passes true for the StoreCaptures argument
92 // to PointerMayBeCaptured).
99 /// getObjectSize - Return the size of the object specified by V, or
100 /// UnknownSize if unknown.
101 static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
102 const TargetLibraryInfo &TLI,
103 bool RoundToAlign = false) {
105 if (getObjectSize(V, Size, &DL, &TLI, RoundToAlign))
107 return AliasAnalysis::UnknownSize;
110 /// isObjectSmallerThan - Return true if we can prove that the object specified
111 /// by V is smaller than Size.
112 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
113 const DataLayout &DL,
114 const TargetLibraryInfo &TLI) {
115 // Note that the meanings of the "object" are slightly different in the
116 // following contexts:
117 // c1: llvm::getObjectSize()
118 // c2: llvm.objectsize() intrinsic
119 // c3: isObjectSmallerThan()
120 // c1 and c2 share the same meaning; however, the meaning of "object" in c3
121 // refers to the "entire object".
123 // Consider this example:
124 // char *p = (char*)malloc(100)
127 // In the context of c1 and c2, the "object" pointed by q refers to the
128 // stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
130 // However, in the context of c3, the "object" refers to the chunk of memory
131 // being allocated. So, the "object" has 100 bytes, and q points to the middle
132 // the "object". In case q is passed to isObjectSmallerThan() as the 1st
133 // parameter, before the llvm::getObjectSize() is called to get the size of
134 // entire object, we should:
135 // - either rewind the pointer q to the base-address of the object in
136 // question (in this case rewind to p), or
137 // - just give up. It is up to caller to make sure the pointer is pointing
138 // to the base address the object.
140 // We go for 2nd option for simplicity.
141 if (!isIdentifiedObject(V))
144 // This function needs to use the aligned object size because we allow
145 // reads a bit past the end given sufficient alignment.
146 uint64_t ObjectSize = getObjectSize(V, DL, TLI, /*RoundToAlign*/true);
148 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
151 /// isObjectSize - Return true if we can prove that the object specified
152 /// by V has size Size.
153 static bool isObjectSize(const Value *V, uint64_t Size,
154 const DataLayout &DL, const TargetLibraryInfo &TLI) {
155 uint64_t ObjectSize = getObjectSize(V, DL, TLI);
156 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
159 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
160 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
161 /// Further, an IdentifiedFunctionLocal can not alias with any function
162 /// arguments other than itself, which is not necessarily true for
163 /// IdentifiedObjects.
164 static bool isIdentifiedFunctionLocal(const Value *V)
166 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);
170 //===----------------------------------------------------------------------===//
171 // GetElementPtr Instruction Decomposition and Analysis
172 //===----------------------------------------------------------------------===//
181 struct VariableGEPIndex {
183 ExtensionKind Extension;
186 bool operator==(const VariableGEPIndex &Other) const {
187 return V == Other.V && Extension == Other.Extension &&
188 Scale == Other.Scale;
191 bool operator!=(const VariableGEPIndex &Other) const {
192 return !operator==(Other);
198 /// GetLinearExpression - Analyze the specified value as a linear expression:
199 /// "A*V + B", where A and B are constant integers. Return the scale and offset
200 /// values as APInts and return V as a Value*, and return whether we looked
201 /// through any sign or zero extends. The incoming Value is known to have
202 /// IntegerType and it may already be sign or zero extended.
204 /// Note that this looks through extends, so the high bits may not be
205 /// represented in the result.
206 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
207 ExtensionKind &Extension,
208 const DataLayout &DL, unsigned Depth) {
209 assert(V->getType()->isIntegerTy() && "Not an integer value");
211 // Limit our recursion depth.
218 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
219 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
220 switch (BOp->getOpcode()) {
222 case Instruction::Or:
223 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
225 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &DL))
228 case Instruction::Add:
229 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
231 Offset += RHSC->getValue();
233 case Instruction::Mul:
234 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
236 Offset *= RHSC->getValue();
237 Scale *= RHSC->getValue();
239 case Instruction::Shl:
240 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
242 Offset <<= RHSC->getValue().getLimitedValue();
243 Scale <<= RHSC->getValue().getLimitedValue();
249 // Since GEP indices are sign extended anyway, we don't care about the high
250 // bits of a sign or zero extended value - just scales and offsets. The
251 // extensions have to be consistent though.
252 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
253 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
254 Value *CastOp = cast<CastInst>(V)->getOperand(0);
255 unsigned OldWidth = Scale.getBitWidth();
256 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
257 Scale = Scale.trunc(SmallWidth);
258 Offset = Offset.trunc(SmallWidth);
259 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
261 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
263 Scale = Scale.zext(OldWidth);
264 Offset = Offset.zext(OldWidth);
274 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
275 /// into a base pointer with a constant offset and a number of scaled symbolic
278 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
279 /// the VarIndices vector) are Value*'s that are known to be scaled by the
280 /// specified amount, but which may have other unrepresented high bits. As such,
281 /// the gep cannot necessarily be reconstructed from its decomposed form.
283 /// When DataLayout is around, this function is capable of analyzing everything
284 /// that GetUnderlyingObject can look through. To be able to do that
285 /// GetUnderlyingObject and DecomposeGEPExpression must use the same search
286 /// depth (MaxLookupSearchDepth).
287 /// When DataLayout not is around, it just looks through pointer casts.
290 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
291 SmallVectorImpl<VariableGEPIndex> &VarIndices,
292 bool &MaxLookupReached, const DataLayout *DL) {
293 // Limit recursion depth to limit compile time in crazy cases.
294 unsigned MaxLookup = MaxLookupSearchDepth;
295 MaxLookupReached = false;
299 // See if this is a bitcast or GEP.
300 const Operator *Op = dyn_cast<Operator>(V);
302 // The only non-operator case we can handle are GlobalAliases.
303 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
304 if (!GA->mayBeOverridden()) {
305 V = GA->getAliasee();
312 if (Op->getOpcode() == Instruction::BitCast) {
313 V = Op->getOperand(0);
317 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
319 // If it's not a GEP, hand it off to SimplifyInstruction to see if it
320 // can come up with something. This matches what GetUnderlyingObject does.
321 if (const Instruction *I = dyn_cast<Instruction>(V))
322 // TODO: Get a DominatorTree and use it here.
323 if (const Value *Simplified =
324 SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
332 // Don't attempt to analyze GEPs over unsized objects.
333 if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized())
336 // If we are lacking DataLayout information, we can't compute the offets of
337 // elements computed by GEPs. However, we can handle bitcast equivalent
340 if (!GEPOp->hasAllZeroIndices())
342 V = GEPOp->getOperand(0);
346 unsigned AS = GEPOp->getPointerAddressSpace();
347 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
348 gep_type_iterator GTI = gep_type_begin(GEPOp);
349 for (User::const_op_iterator I = GEPOp->op_begin()+1,
350 E = GEPOp->op_end(); I != E; ++I) {
352 // Compute the (potentially symbolic) offset in bytes for this index.
353 if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
354 // For a struct, add the member offset.
355 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
356 if (FieldNo == 0) continue;
358 BaseOffs += DL->getStructLayout(STy)->getElementOffset(FieldNo);
362 // For an array/pointer, add the element offset, explicitly scaled.
363 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
364 if (CIdx->isZero()) continue;
365 BaseOffs += DL->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
369 uint64_t Scale = DL->getTypeAllocSize(*GTI);
370 ExtensionKind Extension = EK_NotExtended;
372 // If the integer type is smaller than the pointer size, it is implicitly
373 // sign extended to pointer size.
374 unsigned Width = Index->getType()->getIntegerBitWidth();
375 if (DL->getPointerSizeInBits(AS) > Width)
376 Extension = EK_SignExt;
378 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
379 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
380 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
383 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
384 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
385 BaseOffs += IndexOffset.getSExtValue()*Scale;
386 Scale *= IndexScale.getSExtValue();
388 // If we already had an occurrence of this index variable, merge this
389 // scale into it. For example, we want to handle:
390 // A[x][x] -> x*16 + x*4 -> x*20
391 // This also ensures that 'x' only appears in the index list once.
392 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
393 if (VarIndices[i].V == Index &&
394 VarIndices[i].Extension == Extension) {
395 Scale += VarIndices[i].Scale;
396 VarIndices.erase(VarIndices.begin()+i);
401 // Make sure that we have a scale that makes sense for this target's
403 if (unsigned ShiftBits = 64 - DL->getPointerSizeInBits(AS)) {
405 Scale = (int64_t)Scale >> ShiftBits;
409 VariableGEPIndex Entry = {Index, Extension,
410 static_cast<int64_t>(Scale)};
411 VarIndices.push_back(Entry);
415 // Analyze the base pointer next.
416 V = GEPOp->getOperand(0);
417 } while (--MaxLookup);
419 // If the chain of expressions is too deep, just return early.
420 MaxLookupReached = true;
424 //===----------------------------------------------------------------------===//
425 // BasicAliasAnalysis Pass
426 //===----------------------------------------------------------------------===//
429 static const Function *getParent(const Value *V) {
430 if (const Instruction *inst = dyn_cast<Instruction>(V))
431 return inst->getParent()->getParent();
433 if (const Argument *arg = dyn_cast<Argument>(V))
434 return arg->getParent();
439 static bool notDifferentParent(const Value *O1, const Value *O2) {
441 const Function *F1 = getParent(O1);
442 const Function *F2 = getParent(O2);
444 return !F1 || !F2 || F1 == F2;
449 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
450 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
451 static char ID; // Class identification, replacement for typeinfo
452 BasicAliasAnalysis() : ImmutablePass(ID) {
453 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
456 void initializePass() override {
457 InitializeAliasAnalysis(this);
460 void getAnalysisUsage(AnalysisUsage &AU) const override {
461 AU.addRequired<AliasAnalysis>();
462 AU.addRequired<TargetLibraryInfo>();
465 AliasResult alias(const Location &LocA, const Location &LocB) override {
466 assert(AliasCache.empty() && "AliasCache must be cleared after use!");
467 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
468 "BasicAliasAnalysis doesn't support interprocedural queries.");
469 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
470 LocB.Ptr, LocB.Size, LocB.TBAATag);
471 // AliasCache rarely has more than 1 or 2 elements, always use
472 // shrink_and_clear so it quickly returns to the inline capacity of the
473 // SmallDenseMap if it ever grows larger.
474 // FIXME: This should really be shrink_to_inline_capacity_and_clear().
475 AliasCache.shrink_and_clear();
476 VisitedPhiBBs.clear();
480 ModRefResult getModRefInfo(ImmutableCallSite CS,
481 const Location &Loc) override;
483 ModRefResult getModRefInfo(ImmutableCallSite CS1,
484 ImmutableCallSite CS2) override {
485 // The AliasAnalysis base class has some smarts, lets use them.
486 return AliasAnalysis::getModRefInfo(CS1, CS2);
489 /// pointsToConstantMemory - Chase pointers until we find a (constant
491 bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
493 /// getModRefBehavior - Return the behavior when calling the given
495 ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
497 /// getModRefBehavior - Return the behavior when calling the given function.
498 /// For use when the call site is not known.
499 ModRefBehavior getModRefBehavior(const Function *F) override;
501 /// getAdjustedAnalysisPointer - This method is used when a pass implements
502 /// an analysis interface through multiple inheritance. If needed, it
503 /// should override this to adjust the this pointer as needed for the
504 /// specified pass info.
505 void *getAdjustedAnalysisPointer(const void *ID) override {
506 if (ID == &AliasAnalysis::ID)
507 return (AliasAnalysis*)this;
512 // AliasCache - Track alias queries to guard against recursion.
513 typedef std::pair<Location, Location> LocPair;
514 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
515 AliasCacheTy AliasCache;
517 /// \brief Track phi nodes we have visited. When interpret "Value" pointer
518 /// equality as value equality we need to make sure that the "Value" is not
519 /// part of a cycle. Otherwise, two uses could come from different
520 /// "iterations" of a cycle and see different values for the same "Value"
522 /// The following example shows the problem:
523 /// %p = phi(%alloca1, %addr2)
525 /// %addr1 = gep, %alloca2, 0, %l
526 /// %addr2 = gep %alloca2, 0, (%l + 1)
527 /// alias(%p, %addr1) -> MayAlias !
529 SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
531 // Visited - Track instructions visited by pointsToConstantMemory.
532 SmallPtrSet<const Value*, 16> Visited;
534 /// \brief Check whether two Values can be considered equivalent.
536 /// In addition to pointer equivalence of \p V1 and \p V2 this checks
537 /// whether they can not be part of a cycle in the value graph by looking at
538 /// all visited phi nodes an making sure that the phis cannot reach the
539 /// value. We have to do this because we are looking through phi nodes (That
540 /// is we say noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
541 bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
543 /// \brief Dest and Src are the variable indices from two decomposed
544 /// GetElementPtr instructions GEP1 and GEP2 which have common base
545 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
546 /// difference between the two pointers.
547 void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
548 const SmallVectorImpl<VariableGEPIndex> &Src);
550 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
551 // instruction against another.
552 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
553 const MDNode *V1TBAAInfo,
554 const Value *V2, uint64_t V2Size,
555 const MDNode *V2TBAAInfo,
556 const Value *UnderlyingV1, const Value *UnderlyingV2);
558 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
559 // instruction against another.
560 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
561 const MDNode *PNTBAAInfo,
562 const Value *V2, uint64_t V2Size,
563 const MDNode *V2TBAAInfo);
565 /// aliasSelect - Disambiguate a Select instruction against another value.
566 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
567 const MDNode *SITBAAInfo,
568 const Value *V2, uint64_t V2Size,
569 const MDNode *V2TBAAInfo);
571 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
572 const MDNode *V1TBAATag,
573 const Value *V2, uint64_t V2Size,
574 const MDNode *V2TBAATag);
576 } // End of anonymous namespace
578 // Register this pass...
579 char BasicAliasAnalysis::ID = 0;
580 INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
581 "Basic Alias Analysis (stateless AA impl)",
583 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
584 INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
585 "Basic Alias Analysis (stateless AA impl)",
589 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
590 return new BasicAliasAnalysis();
593 /// pointsToConstantMemory - Returns whether the given pointer value
594 /// points to memory that is local to the function, with global constants being
595 /// considered local to all functions.
597 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
598 assert(Visited.empty() && "Visited must be cleared after use!");
600 unsigned MaxLookup = 8;
601 SmallVector<const Value *, 16> Worklist;
602 Worklist.push_back(Loc.Ptr);
604 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), DL);
605 if (!Visited.insert(V)) {
607 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
610 // An alloca instruction defines local memory.
611 if (OrLocal && isa<AllocaInst>(V))
614 // A global constant counts as local memory for our purposes.
615 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
616 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
617 // global to be marked constant in some modules and non-constant in
618 // others. GV may even be a declaration, not a definition.
619 if (!GV->isConstant()) {
621 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
626 // If both select values point to local memory, then so does the select.
627 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
628 Worklist.push_back(SI->getTrueValue());
629 Worklist.push_back(SI->getFalseValue());
633 // If all values incoming to a phi node point to local memory, then so does
635 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
636 // Don't bother inspecting phi nodes with many operands.
637 if (PN->getNumIncomingValues() > MaxLookup) {
639 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
641 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
642 Worklist.push_back(PN->getIncomingValue(i));
646 // Otherwise be conservative.
648 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
650 } while (!Worklist.empty() && --MaxLookup);
653 return Worklist.empty();
656 /// getModRefBehavior - Return the behavior when calling the given call site.
657 AliasAnalysis::ModRefBehavior
658 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
659 if (CS.doesNotAccessMemory())
660 // Can't do better than this.
661 return DoesNotAccessMemory;
663 ModRefBehavior Min = UnknownModRefBehavior;
665 // If the callsite knows it only reads memory, don't return worse
667 if (CS.onlyReadsMemory())
668 Min = OnlyReadsMemory;
670 // The AliasAnalysis base class has some smarts, lets use them.
671 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
674 /// getModRefBehavior - Return the behavior when calling the given function.
675 /// For use when the call site is not known.
676 AliasAnalysis::ModRefBehavior
677 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
678 // If the function declares it doesn't access memory, we can't do better.
679 if (F->doesNotAccessMemory())
680 return DoesNotAccessMemory;
682 // For intrinsics, we can check the table.
683 if (unsigned iid = F->getIntrinsicID()) {
684 #define GET_INTRINSIC_MODREF_BEHAVIOR
685 #include "llvm/IR/Intrinsics.gen"
686 #undef GET_INTRINSIC_MODREF_BEHAVIOR
689 ModRefBehavior Min = UnknownModRefBehavior;
691 // If the function declares it only reads memory, go with that.
692 if (F->onlyReadsMemory())
693 Min = OnlyReadsMemory;
695 // Otherwise be conservative.
696 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
699 /// getModRefInfo - Check to see if the specified callsite can clobber the
700 /// specified memory object. Since we only look at local properties of this
701 /// function, we really can't say much about this query. We do, however, use
702 /// simple "address taken" analysis on local objects.
703 AliasAnalysis::ModRefResult
704 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
705 const Location &Loc) {
706 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
707 "AliasAnalysis query involving multiple functions!");
709 const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
711 // If this is a tail call and Loc.Ptr points to a stack location, we know that
712 // the tail call cannot access or modify the local stack.
713 // We cannot exclude byval arguments here; these belong to the caller of
714 // the current function not to the current function, and a tail callee
715 // may reference them.
716 if (isa<AllocaInst>(Object))
717 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
718 if (CI->isTailCall())
721 // If the pointer is to a locally allocated object that does not escape,
722 // then the call can not mod/ref the pointer unless the call takes the pointer
723 // as an argument, and itself doesn't capture it.
724 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
725 isNonEscapingLocalObject(Object)) {
726 bool PassedAsArg = false;
728 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
729 CI != CE; ++CI, ++ArgNo) {
730 // Only look at the no-capture or byval pointer arguments. If this
731 // pointer were passed to arguments that were neither of these, then it
732 // couldn't be no-capture.
733 if (!(*CI)->getType()->isPointerTy() ||
734 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
737 // If this is a no-capture pointer argument, see if we can tell that it
738 // is impossible to alias the pointer we're checking. If not, we have to
739 // assume that the call could touch the pointer, even though it doesn't
741 if (!isNoAlias(Location(*CI), Location(Object))) {
751 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
752 ModRefResult Min = ModRef;
754 // Finally, handle specific knowledge of intrinsics.
755 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
757 switch (II->getIntrinsicID()) {
759 case Intrinsic::memcpy:
760 case Intrinsic::memmove: {
761 uint64_t Len = UnknownSize;
762 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
763 Len = LenCI->getZExtValue();
764 Value *Dest = II->getArgOperand(0);
765 Value *Src = II->getArgOperand(1);
766 // If it can't overlap the source dest, then it doesn't modref the loc.
767 if (isNoAlias(Location(Dest, Len), Loc)) {
768 if (isNoAlias(Location(Src, Len), Loc))
770 // If it can't overlap the dest, then worst case it reads the loc.
772 } else if (isNoAlias(Location(Src, Len), Loc)) {
773 // If it can't overlap the source, then worst case it mutates the loc.
778 case Intrinsic::memset:
779 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
780 // will handle it for the variable length case.
781 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
782 uint64_t Len = LenCI->getZExtValue();
783 Value *Dest = II->getArgOperand(0);
784 if (isNoAlias(Location(Dest, Len), Loc))
787 // We know that memset doesn't load anything.
790 case Intrinsic::lifetime_start:
791 case Intrinsic::lifetime_end:
792 case Intrinsic::invariant_start: {
794 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
795 if (isNoAlias(Location(II->getArgOperand(1),
797 II->getMetadata(LLVMContext::MD_tbaa)),
802 case Intrinsic::invariant_end: {
804 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
805 if (isNoAlias(Location(II->getArgOperand(2),
807 II->getMetadata(LLVMContext::MD_tbaa)),
812 case Intrinsic::arm_neon_vld1: {
813 // LLVM's vld1 and vst1 intrinsics currently only support a single
816 DL ? DL->getTypeStoreSize(II->getType()) : UnknownSize;
817 if (isNoAlias(Location(II->getArgOperand(0), Size,
818 II->getMetadata(LLVMContext::MD_tbaa)),
823 case Intrinsic::arm_neon_vst1: {
825 DL ? DL->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize;
826 if (isNoAlias(Location(II->getArgOperand(0), Size,
827 II->getMetadata(LLVMContext::MD_tbaa)),
834 // We can bound the aliasing properties of memset_pattern16 just as we can
835 // for memcpy/memset. This is particularly important because the
836 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
837 // whenever possible.
838 else if (TLI.has(LibFunc::memset_pattern16) &&
839 CS.getCalledFunction() &&
840 CS.getCalledFunction()->getName() == "memset_pattern16") {
841 const Function *MS = CS.getCalledFunction();
842 FunctionType *MemsetType = MS->getFunctionType();
843 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
844 isa<PointerType>(MemsetType->getParamType(0)) &&
845 isa<PointerType>(MemsetType->getParamType(1)) &&
846 isa<IntegerType>(MemsetType->getParamType(2))) {
847 uint64_t Len = UnknownSize;
848 if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2)))
849 Len = LenCI->getZExtValue();
850 const Value *Dest = CS.getArgument(0);
851 const Value *Src = CS.getArgument(1);
852 // If it can't overlap the source dest, then it doesn't modref the loc.
853 if (isNoAlias(Location(Dest, Len), Loc)) {
854 // Always reads 16 bytes of the source.
855 if (isNoAlias(Location(Src, 16), Loc))
857 // If it can't overlap the dest, then worst case it reads the loc.
859 // Always reads 16 bytes of the source.
860 } else if (isNoAlias(Location(Src, 16), Loc)) {
861 // If it can't overlap the source, then worst case it mutates the loc.
867 // The AliasAnalysis base class has some smarts, lets use them.
868 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
871 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
872 /// against another pointer. We know that V1 is a GEP, but we don't know
873 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, DL),
874 /// UnderlyingV2 is the same for V2.
876 AliasAnalysis::AliasResult
877 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
878 const MDNode *V1TBAAInfo,
879 const Value *V2, uint64_t V2Size,
880 const MDNode *V2TBAAInfo,
881 const Value *UnderlyingV1,
882 const Value *UnderlyingV2) {
883 int64_t GEP1BaseOffset;
884 bool GEP1MaxLookupReached;
885 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
887 // If we have two gep instructions with must-alias or not-alias'ing base
888 // pointers, figure out if the indexes to the GEP tell us anything about the
890 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
891 // Do the base pointers alias?
892 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, nullptr,
893 UnderlyingV2, UnknownSize, nullptr);
895 // Check for geps of non-aliasing underlying pointers where the offsets are
897 if ((BaseAlias == MayAlias) && V1Size == V2Size) {
898 // Do the base pointers alias assuming type and size.
899 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
900 V1TBAAInfo, UnderlyingV2,
902 if (PreciseBaseAlias == NoAlias) {
903 // See if the computed offset from the common pointer tells us about the
904 // relation of the resulting pointer.
905 int64_t GEP2BaseOffset;
906 bool GEP2MaxLookupReached;
907 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
908 const Value *GEP2BasePtr =
909 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
910 GEP2MaxLookupReached, DL);
911 const Value *GEP1BasePtr =
912 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
913 GEP1MaxLookupReached, DL);
914 // DecomposeGEPExpression and GetUnderlyingObject should return the
915 // same result except when DecomposeGEPExpression has no DataLayout.
916 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
918 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
921 // If the max search depth is reached the result is undefined
922 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
926 if (GEP1BaseOffset == GEP2BaseOffset &&
927 GEP1VariableIndices == GEP2VariableIndices)
929 GEP1VariableIndices.clear();
933 // If we get a No or May, then return it immediately, no amount of analysis
934 // will improve this situation.
935 if (BaseAlias != MustAlias) return BaseAlias;
937 // Otherwise, we have a MustAlias. Since the base pointers alias each other
938 // exactly, see if the computed offset from the common pointer tells us
939 // about the relation of the resulting pointer.
940 const Value *GEP1BasePtr =
941 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
942 GEP1MaxLookupReached, DL);
944 int64_t GEP2BaseOffset;
945 bool GEP2MaxLookupReached;
946 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
947 const Value *GEP2BasePtr =
948 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
949 GEP2MaxLookupReached, DL);
951 // DecomposeGEPExpression and GetUnderlyingObject should return the
952 // same result except when DecomposeGEPExpression has no DataLayout.
953 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
955 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
958 // If the max search depth is reached the result is undefined
959 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
962 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
963 // symbolic difference.
964 GEP1BaseOffset -= GEP2BaseOffset;
965 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
968 // Check to see if these two pointers are related by the getelementptr
969 // instruction. If one pointer is a GEP with a non-zero index of the other
970 // pointer, we know they cannot alias.
972 // If both accesses are unknown size, we can't do anything useful here.
973 if (V1Size == UnknownSize && V2Size == UnknownSize)
976 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, nullptr,
977 V2, V2Size, V2TBAAInfo);
979 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
980 // If V2 is known not to alias GEP base pointer, then the two values
981 // cannot alias per GEP semantics: "A pointer value formed from a
982 // getelementptr instruction is associated with the addresses associated
983 // with the first operand of the getelementptr".
986 const Value *GEP1BasePtr =
987 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
988 GEP1MaxLookupReached, DL);
990 // DecomposeGEPExpression and GetUnderlyingObject should return the
991 // same result except when DecomposeGEPExpression has no DataLayout.
992 if (GEP1BasePtr != UnderlyingV1) {
994 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
997 // If the max search depth is reached the result is undefined
998 if (GEP1MaxLookupReached)
1002 // In the two GEP Case, if there is no difference in the offsets of the
1003 // computed pointers, the resultant pointers are a must alias. This
1004 // hapens when we have two lexically identical GEP's (for example).
1006 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
1007 // must aliases the GEP, the end result is a must alias also.
1008 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
1011 // If there is a constant difference between the pointers, but the difference
1012 // is less than the size of the associated memory object, then we know
1013 // that the objects are partially overlapping. If the difference is
1014 // greater, we know they do not overlap.
1015 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
1016 if (GEP1BaseOffset >= 0) {
1017 if (V2Size != UnknownSize) {
1018 if ((uint64_t)GEP1BaseOffset < V2Size)
1019 return PartialAlias;
1023 // We have the situation where:
1026 // ---------------->|
1027 // |-->V1Size |-------> V2Size
1029 // We need to know that V2Size is not unknown, otherwise we might have
1030 // stripped a gep with negative index ('gep <ptr>, -1, ...).
1031 if (V1Size != UnknownSize && V2Size != UnknownSize) {
1032 if (-(uint64_t)GEP1BaseOffset < V1Size)
1033 return PartialAlias;
1039 // Try to distinguish something like &A[i][1] against &A[42][0].
1040 // Grab the least significant bit set in any of the scales.
1041 if (!GEP1VariableIndices.empty()) {
1042 uint64_t Modulo = 0;
1043 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
1044 Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
1045 Modulo = Modulo ^ (Modulo & (Modulo - 1));
1047 // We can compute the difference between the two addresses
1048 // mod Modulo. Check whether that difference guarantees that the
1049 // two locations do not alias.
1050 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
1051 if (V1Size != UnknownSize && V2Size != UnknownSize &&
1052 ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
1056 // Statically, we can see that the base objects are the same, but the
1057 // pointers have dynamic offsets which we can't resolve. And none of our
1058 // little tricks above worked.
1060 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
1061 // practical effect of this is protecting TBAA in the case of dynamic
1062 // indices into arrays of unions or malloc'd memory.
1063 return PartialAlias;
1066 static AliasAnalysis::AliasResult
1067 MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
1068 // If the results agree, take it.
1071 // A mix of PartialAlias and MustAlias is PartialAlias.
1072 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
1073 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
1074 return AliasAnalysis::PartialAlias;
1075 // Otherwise, we don't know anything.
1076 return AliasAnalysis::MayAlias;
1079 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
1080 /// instruction against another.
1081 AliasAnalysis::AliasResult
1082 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
1083 const MDNode *SITBAAInfo,
1084 const Value *V2, uint64_t V2Size,
1085 const MDNode *V2TBAAInfo) {
1086 // If the values are Selects with the same condition, we can do a more precise
1087 // check: just check for aliases between the values on corresponding arms.
1088 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
1089 if (SI->getCondition() == SI2->getCondition()) {
1091 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
1092 SI2->getTrueValue(), V2Size, V2TBAAInfo);
1093 if (Alias == MayAlias)
1095 AliasResult ThisAlias =
1096 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
1097 SI2->getFalseValue(), V2Size, V2TBAAInfo);
1098 return MergeAliasResults(ThisAlias, Alias);
1101 // If both arms of the Select node NoAlias or MustAlias V2, then returns
1102 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1104 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
1105 if (Alias == MayAlias)
1108 AliasResult ThisAlias =
1109 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
1110 return MergeAliasResults(ThisAlias, Alias);
1113 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
1115 AliasAnalysis::AliasResult
1116 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
1117 const MDNode *PNTBAAInfo,
1118 const Value *V2, uint64_t V2Size,
1119 const MDNode *V2TBAAInfo) {
1120 // Track phi nodes we have visited. We use this information when we determine
1121 // value equivalence.
1122 VisitedPhiBBs.insert(PN->getParent());
1124 // If the values are PHIs in the same block, we can do a more precise
1125 // as well as efficient check: just check for aliases between the values
1126 // on corresponding edges.
1127 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
1128 if (PN2->getParent() == PN->getParent()) {
1129 LocPair Locs(Location(PN, PNSize, PNTBAAInfo),
1130 Location(V2, V2Size, V2TBAAInfo));
1132 std::swap(Locs.first, Locs.second);
1133 // Analyse the PHIs' inputs under the assumption that the PHIs are
1135 // If the PHIs are May/MustAlias there must be (recursively) an input
1136 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or
1137 // there must be an operation on the PHIs within the PHIs' value cycle
1138 // that causes a MayAlias.
1139 // Pretend the phis do not alias.
1140 AliasResult Alias = NoAlias;
1141 assert(AliasCache.count(Locs) &&
1142 "There must exist an entry for the phi node");
1143 AliasResult OrigAliasResult = AliasCache[Locs];
1144 AliasCache[Locs] = NoAlias;
1146 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1147 AliasResult ThisAlias =
1148 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
1149 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1150 V2Size, V2TBAAInfo);
1151 Alias = MergeAliasResults(ThisAlias, Alias);
1152 if (Alias == MayAlias)
1156 // Reset if speculation failed.
1157 if (Alias != NoAlias)
1158 AliasCache[Locs] = OrigAliasResult;
1163 SmallPtrSet<Value*, 4> UniqueSrc;
1164 SmallVector<Value*, 4> V1Srcs;
1165 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1166 Value *PV1 = PN->getIncomingValue(i);
1167 if (isa<PHINode>(PV1))
1168 // If any of the source itself is a PHI, return MayAlias conservatively
1169 // to avoid compile time explosion. The worst possible case is if both
1170 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1171 // and 'n' are the number of PHI sources.
1173 if (UniqueSrc.insert(PV1))
1174 V1Srcs.push_back(PV1);
1177 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1178 V1Srcs[0], PNSize, PNTBAAInfo);
1179 // Early exit if the check of the first PHI source against V2 is MayAlias.
1180 // Other results are not possible.
1181 if (Alias == MayAlias)
1184 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1185 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1186 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1187 Value *V = V1Srcs[i];
1189 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1190 V, PNSize, PNTBAAInfo);
1191 Alias = MergeAliasResults(ThisAlias, Alias);
1192 if (Alias == MayAlias)
1199 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1200 // such as array references.
1202 AliasAnalysis::AliasResult
1203 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1204 const MDNode *V1TBAAInfo,
1205 const Value *V2, uint64_t V2Size,
1206 const MDNode *V2TBAAInfo) {
1207 // If either of the memory references is empty, it doesn't matter what the
1208 // pointer values are.
1209 if (V1Size == 0 || V2Size == 0)
1212 // Strip off any casts if they exist.
1213 V1 = V1->stripPointerCasts();
1214 V2 = V2->stripPointerCasts();
1216 // Are we checking for alias of the same value?
1217 // Because we look 'through' phi nodes we could look at "Value" pointers from
1218 // different iterations. We must therefore make sure that this is not the
1219 // case. The function isValueEqualInPotentialCycles ensures that this cannot
1220 // happen by looking at the visited phi nodes and making sure they cannot
1222 if (isValueEqualInPotentialCycles(V1, V2))
1225 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1226 return NoAlias; // Scalars cannot alias each other
1228 // Figure out what objects these things are pointing to if we can.
1229 const Value *O1 = GetUnderlyingObject(V1, DL, MaxLookupSearchDepth);
1230 const Value *O2 = GetUnderlyingObject(V2, DL, MaxLookupSearchDepth);
1232 // Null values in the default address space don't point to any object, so they
1233 // don't alias any other pointer.
1234 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1235 if (CPN->getType()->getAddressSpace() == 0)
1237 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1238 if (CPN->getType()->getAddressSpace() == 0)
1242 // If V1/V2 point to two different objects we know that we have no alias.
1243 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1246 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1247 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1248 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1251 // Function arguments can't alias with things that are known to be
1252 // unambigously identified at the function level.
1253 if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
1254 (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
1257 // Most objects can't alias null.
1258 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1259 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1262 // If one pointer is the result of a call/invoke or load and the other is a
1263 // non-escaping local object within the same function, then we know the
1264 // object couldn't escape to a point where the call could return it.
1266 // Note that if the pointers are in different functions, there are a
1267 // variety of complications. A call with a nocapture argument may still
1268 // temporary store the nocapture argument's value in a temporary memory
1269 // location if that memory location doesn't escape. Or it may pass a
1270 // nocapture value to other functions as long as they don't capture it.
1271 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1273 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1277 // If the size of one access is larger than the entire object on the other
1278 // side, then we know such behavior is undefined and can assume no alias.
1280 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *DL, *TLI)) ||
1281 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *DL, *TLI)))
1284 // Check the cache before climbing up use-def chains. This also terminates
1285 // otherwise infinitely recursive queries.
1286 LocPair Locs(Location(V1, V1Size, V1TBAAInfo),
1287 Location(V2, V2Size, V2TBAAInfo));
1289 std::swap(Locs.first, Locs.second);
1290 std::pair<AliasCacheTy::iterator, bool> Pair =
1291 AliasCache.insert(std::make_pair(Locs, MayAlias));
1293 return Pair.first->second;
1295 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1296 // GEP can't simplify, we don't even look at the PHI cases.
1297 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1299 std::swap(V1Size, V2Size);
1301 std::swap(V1TBAAInfo, V2TBAAInfo);
1303 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1304 AliasResult Result = aliasGEP(GV1, V1Size, V1TBAAInfo, V2, V2Size, V2TBAAInfo, O1, O2);
1305 if (Result != MayAlias) return AliasCache[Locs] = Result;
1308 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1310 std::swap(V1Size, V2Size);
1311 std::swap(V1TBAAInfo, V2TBAAInfo);
1313 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1314 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1315 V2, V2Size, V2TBAAInfo);
1316 if (Result != MayAlias) return AliasCache[Locs] = Result;
1319 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1321 std::swap(V1Size, V2Size);
1322 std::swap(V1TBAAInfo, V2TBAAInfo);
1324 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1325 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1326 V2, V2Size, V2TBAAInfo);
1327 if (Result != MayAlias) return AliasCache[Locs] = Result;
1330 // If both pointers are pointing into the same object and one of them
1331 // accesses is accessing the entire object, then the accesses must
1332 // overlap in some way.
1334 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *DL, *TLI)) ||
1335 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *DL, *TLI)))
1336 return AliasCache[Locs] = PartialAlias;
1338 AliasResult Result =
1339 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1340 Location(V2, V2Size, V2TBAAInfo));
1341 return AliasCache[Locs] = Result;
1344 bool BasicAliasAnalysis::isValueEqualInPotentialCycles(const Value *V,
1349 const Instruction *Inst = dyn_cast<Instruction>(V);
1353 if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
1356 // Use dominance or loop info if available.
1357 DominatorTreeWrapperPass *DTWP =
1358 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
1359 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
1360 LoopInfo *LI = getAnalysisIfAvailable<LoopInfo>();
1362 // Make sure that the visited phis cannot reach the Value. This ensures that
1363 // the Values cannot come from different iterations of a potential cycle the
1364 // phi nodes could be involved in.
1365 for (SmallPtrSet<const BasicBlock *, 8>::iterator PI = VisitedPhiBBs.begin(),
1366 PE = VisitedPhiBBs.end();
1368 if (isPotentiallyReachable((*PI)->begin(), Inst, DT, LI))
1374 /// GetIndexDifference - Dest and Src are the variable indices from two
1375 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
1376 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
1377 /// difference between the two pointers.
1378 void BasicAliasAnalysis::GetIndexDifference(
1379 SmallVectorImpl<VariableGEPIndex> &Dest,
1380 const SmallVectorImpl<VariableGEPIndex> &Src) {
1384 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
1385 const Value *V = Src[i].V;
1386 ExtensionKind Extension = Src[i].Extension;
1387 int64_t Scale = Src[i].Scale;
1389 // Find V in Dest. This is N^2, but pointer indices almost never have more
1390 // than a few variable indexes.
1391 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
1392 if (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
1393 Dest[j].Extension != Extension)
1396 // If we found it, subtract off Scale V's from the entry in Dest. If it
1397 // goes to zero, remove the entry.
1398 if (Dest[j].Scale != Scale)
1399 Dest[j].Scale -= Scale;
1401 Dest.erase(Dest.begin() + j);
1406 // If we didn't consume this entry, add it to the end of the Dest list.
1408 VariableGEPIndex Entry = { V, Extension, -Scale };
1409 Dest.push_back(Entry);