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/AssumptionTracker.h"
21 #include "llvm/Analysis/CFG.h"
22 #include "llvm/Analysis/CaptureTracking.h"
23 #include "llvm/Analysis/InstructionSimplify.h"
24 #include "llvm/Analysis/LoopInfo.h"
25 #include "llvm/Analysis/MemoryBuiltins.h"
26 #include "llvm/Analysis/ValueTracking.h"
27 #include "llvm/IR/Constants.h"
28 #include "llvm/IR/DataLayout.h"
29 #include "llvm/IR/DerivedTypes.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/GetElementPtrTypeIterator.h"
33 #include "llvm/IR/GlobalAlias.h"
34 #include "llvm/IR/GlobalVariable.h"
35 #include "llvm/IR/Instructions.h"
36 #include "llvm/IR/IntrinsicInst.h"
37 #include "llvm/IR/LLVMContext.h"
38 #include "llvm/IR/Operator.h"
39 #include "llvm/Pass.h"
40 #include "llvm/Support/ErrorHandling.h"
41 #include "llvm/Target/TargetLibraryInfo.h"
45 /// Cutoff after which to stop analysing a set of phi nodes potentially involved
46 /// in a cycle. Because we are analysing 'through' phi nodes we need to be
47 /// careful with value equivalence. We use reachability to make sure a value
48 /// cannot be involved in a cycle.
49 const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
51 // The max limit of the search depth in DecomposeGEPExpression() and
52 // GetUnderlyingObject(), both functions need to use the same search
53 // depth otherwise the algorithm in aliasGEP will assert.
54 static const unsigned MaxLookupSearchDepth = 6;
56 //===----------------------------------------------------------------------===//
58 //===----------------------------------------------------------------------===//
60 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
61 /// object that never escapes from the function.
62 static bool isNonEscapingLocalObject(const Value *V) {
63 // If this is a local allocation, check to see if it escapes.
64 if (isa<AllocaInst>(V) || isNoAliasCall(V))
65 // Set StoreCaptures to True so that we can assume in our callers that the
66 // pointer is not the result of a load instruction. Currently
67 // PointerMayBeCaptured doesn't have any special analysis for the
68 // StoreCaptures=false case; if it did, our callers could be refined to be
70 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
72 // If this is an argument that corresponds to a byval or noalias argument,
73 // then it has not escaped before entering the function. Check if it escapes
74 // inside the function.
75 if (const Argument *A = dyn_cast<Argument>(V))
76 if (A->hasByValAttr() || A->hasNoAliasAttr())
77 // Note even if the argument is marked nocapture we still need to check
78 // for copies made inside the function. The nocapture attribute only
79 // specifies that there are no copies made that outlive the function.
80 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
85 /// isEscapeSource - Return true if the pointer is one which would have
86 /// been considered an escape by isNonEscapingLocalObject.
87 static bool isEscapeSource(const Value *V) {
88 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
91 // The load case works because isNonEscapingLocalObject considers all
92 // stores to be escapes (it passes true for the StoreCaptures argument
93 // to PointerMayBeCaptured).
100 /// getObjectSize - Return the size of the object specified by V, or
101 /// UnknownSize if unknown.
102 static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
103 const TargetLibraryInfo &TLI,
104 bool RoundToAlign = false) {
106 if (getObjectSize(V, Size, &DL, &TLI, RoundToAlign))
108 return AliasAnalysis::UnknownSize;
111 /// isObjectSmallerThan - Return true if we can prove that the object specified
112 /// by V is smaller than Size.
113 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
114 const DataLayout &DL,
115 const TargetLibraryInfo &TLI) {
116 // Note that the meanings of the "object" are slightly different in the
117 // following contexts:
118 // c1: llvm::getObjectSize()
119 // c2: llvm.objectsize() intrinsic
120 // c3: isObjectSmallerThan()
121 // c1 and c2 share the same meaning; however, the meaning of "object" in c3
122 // refers to the "entire object".
124 // Consider this example:
125 // char *p = (char*)malloc(100)
128 // In the context of c1 and c2, the "object" pointed by q refers to the
129 // stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
131 // However, in the context of c3, the "object" refers to the chunk of memory
132 // being allocated. So, the "object" has 100 bytes, and q points to the middle
133 // the "object". In case q is passed to isObjectSmallerThan() as the 1st
134 // parameter, before the llvm::getObjectSize() is called to get the size of
135 // entire object, we should:
136 // - either rewind the pointer q to the base-address of the object in
137 // question (in this case rewind to p), or
138 // - just give up. It is up to caller to make sure the pointer is pointing
139 // to the base address the object.
141 // We go for 2nd option for simplicity.
142 if (!isIdentifiedObject(V))
145 // This function needs to use the aligned object size because we allow
146 // reads a bit past the end given sufficient alignment.
147 uint64_t ObjectSize = getObjectSize(V, DL, TLI, /*RoundToAlign*/true);
149 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
152 /// isObjectSize - Return true if we can prove that the object specified
153 /// by V has size Size.
154 static bool isObjectSize(const Value *V, uint64_t Size,
155 const DataLayout &DL, const TargetLibraryInfo &TLI) {
156 uint64_t ObjectSize = getObjectSize(V, DL, TLI);
157 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
160 //===----------------------------------------------------------------------===//
161 // GetElementPtr Instruction Decomposition and Analysis
162 //===----------------------------------------------------------------------===//
171 struct VariableGEPIndex {
173 ExtensionKind Extension;
176 bool operator==(const VariableGEPIndex &Other) const {
177 return V == Other.V && Extension == Other.Extension &&
178 Scale == Other.Scale;
181 bool operator!=(const VariableGEPIndex &Other) const {
182 return !operator==(Other);
188 /// GetLinearExpression - Analyze the specified value as a linear expression:
189 /// "A*V + B", where A and B are constant integers. Return the scale and offset
190 /// values as APInts and return V as a Value*, and return whether we looked
191 /// through any sign or zero extends. The incoming Value is known to have
192 /// IntegerType and it may already be sign or zero extended.
194 /// Note that this looks through extends, so the high bits may not be
195 /// represented in the result.
196 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
197 ExtensionKind &Extension,
198 const DataLayout &DL, unsigned Depth,
199 AssumptionTracker *AT,
201 assert(V->getType()->isIntegerTy() && "Not an integer value");
203 // Limit our recursion depth.
210 if (ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
211 // if it's a constant, just convert it to an offset
212 // and remove the variable.
213 Offset += Const->getValue();
214 assert(Scale == 0 && "Constant values don't have a scale");
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, 0,
229 case Instruction::Add:
230 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
231 DL, Depth+1, AT, DT);
232 Offset += RHSC->getValue();
234 case Instruction::Mul:
235 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
236 DL, Depth+1, AT, DT);
237 Offset *= RHSC->getValue();
238 Scale *= RHSC->getValue();
240 case Instruction::Shl:
241 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
242 DL, Depth+1, AT, DT);
243 Offset <<= RHSC->getValue().getLimitedValue();
244 Scale <<= RHSC->getValue().getLimitedValue();
250 // Since GEP indices are sign extended anyway, we don't care about the high
251 // bits of a sign or zero extended value - just scales and offsets. The
252 // extensions have to be consistent though.
253 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
254 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
255 Value *CastOp = cast<CastInst>(V)->getOperand(0);
256 unsigned OldWidth = Scale.getBitWidth();
257 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
258 Scale = Scale.trunc(SmallWidth);
259 Offset = Offset.trunc(SmallWidth);
260 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
262 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
263 DL, Depth+1, AT, DT);
264 Scale = Scale.zext(OldWidth);
266 // We have to sign-extend even if Extension == EK_ZeroExt as we can't
267 // decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
268 Offset = Offset.sext(OldWidth);
278 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
279 /// into a base pointer with a constant offset and a number of scaled symbolic
282 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
283 /// the VarIndices vector) are Value*'s that are known to be scaled by the
284 /// specified amount, but which may have other unrepresented high bits. As such,
285 /// the gep cannot necessarily be reconstructed from its decomposed form.
287 /// When DataLayout is around, this function is capable of analyzing everything
288 /// that GetUnderlyingObject can look through. To be able to do that
289 /// GetUnderlyingObject and DecomposeGEPExpression must use the same search
290 /// depth (MaxLookupSearchDepth).
291 /// When DataLayout not is around, it just looks through pointer casts.
294 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
295 SmallVectorImpl<VariableGEPIndex> &VarIndices,
296 bool &MaxLookupReached, const DataLayout *DL,
297 AssumptionTracker *AT, DominatorTree *DT) {
298 // Limit recursion depth to limit compile time in crazy cases.
299 unsigned MaxLookup = MaxLookupSearchDepth;
300 MaxLookupReached = false;
304 // See if this is a bitcast or GEP.
305 const Operator *Op = dyn_cast<Operator>(V);
307 // The only non-operator case we can handle are GlobalAliases.
308 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
309 if (!GA->mayBeOverridden()) {
310 V = GA->getAliasee();
317 if (Op->getOpcode() == Instruction::BitCast ||
318 Op->getOpcode() == Instruction::AddrSpaceCast) {
319 V = Op->getOperand(0);
323 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
325 // If it's not a GEP, hand it off to SimplifyInstruction to see if it
326 // can come up with something. This matches what GetUnderlyingObject does.
327 if (const Instruction *I = dyn_cast<Instruction>(V))
328 // TODO: Get a DominatorTree and AssumptionTracker and use them here
329 // (these are both now available in this function, but this should be
330 // updated when GetUnderlyingObject is updated). TLI should be
332 if (const Value *Simplified =
333 SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
341 // Don't attempt to analyze GEPs over unsized objects.
342 if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized())
345 // If we are lacking DataLayout information, we can't compute the offets of
346 // elements computed by GEPs. However, we can handle bitcast equivalent
349 if (!GEPOp->hasAllZeroIndices())
351 V = GEPOp->getOperand(0);
355 unsigned AS = GEPOp->getPointerAddressSpace();
356 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
357 gep_type_iterator GTI = gep_type_begin(GEPOp);
358 for (User::const_op_iterator I = GEPOp->op_begin()+1,
359 E = GEPOp->op_end(); I != E; ++I) {
361 // Compute the (potentially symbolic) offset in bytes for this index.
362 if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
363 // For a struct, add the member offset.
364 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
365 if (FieldNo == 0) continue;
367 BaseOffs += DL->getStructLayout(STy)->getElementOffset(FieldNo);
371 // For an array/pointer, add the element offset, explicitly scaled.
372 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
373 if (CIdx->isZero()) continue;
374 BaseOffs += DL->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
378 uint64_t Scale = DL->getTypeAllocSize(*GTI);
379 ExtensionKind Extension = EK_NotExtended;
381 // If the integer type is smaller than the pointer size, it is implicitly
382 // sign extended to pointer size.
383 unsigned Width = Index->getType()->getIntegerBitWidth();
384 if (DL->getPointerSizeInBits(AS) > Width)
385 Extension = EK_SignExt;
387 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
388 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
389 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
392 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
393 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
394 BaseOffs += IndexOffset.getSExtValue()*Scale;
395 Scale *= IndexScale.getSExtValue();
397 // If we already had an occurrence of this index variable, merge this
398 // scale into it. For example, we want to handle:
399 // A[x][x] -> x*16 + x*4 -> x*20
400 // This also ensures that 'x' only appears in the index list once.
401 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
402 if (VarIndices[i].V == Index &&
403 VarIndices[i].Extension == Extension) {
404 Scale += VarIndices[i].Scale;
405 VarIndices.erase(VarIndices.begin()+i);
410 // Make sure that we have a scale that makes sense for this target's
412 if (unsigned ShiftBits = 64 - DL->getPointerSizeInBits(AS)) {
414 Scale = (int64_t)Scale >> ShiftBits;
418 VariableGEPIndex Entry = {Index, Extension,
419 static_cast<int64_t>(Scale)};
420 VarIndices.push_back(Entry);
424 // Analyze the base pointer next.
425 V = GEPOp->getOperand(0);
426 } while (--MaxLookup);
428 // If the chain of expressions is too deep, just return early.
429 MaxLookupReached = true;
433 //===----------------------------------------------------------------------===//
434 // BasicAliasAnalysis Pass
435 //===----------------------------------------------------------------------===//
438 static const Function *getParent(const Value *V) {
439 if (const Instruction *inst = dyn_cast<Instruction>(V))
440 return inst->getParent()->getParent();
442 if (const Argument *arg = dyn_cast<Argument>(V))
443 return arg->getParent();
448 static bool notDifferentParent(const Value *O1, const Value *O2) {
450 const Function *F1 = getParent(O1);
451 const Function *F2 = getParent(O2);
453 return !F1 || !F2 || F1 == F2;
458 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
459 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
460 static char ID; // Class identification, replacement for typeinfo
461 BasicAliasAnalysis() : ImmutablePass(ID) {
462 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
465 void initializePass() override {
466 InitializeAliasAnalysis(this);
469 void getAnalysisUsage(AnalysisUsage &AU) const override {
470 AU.addRequired<AliasAnalysis>();
471 AU.addRequired<AssumptionTracker>();
472 AU.addRequired<TargetLibraryInfo>();
475 AliasResult alias(const Location &LocA, const Location &LocB) override {
476 assert(AliasCache.empty() && "AliasCache must be cleared after use!");
477 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
478 "BasicAliasAnalysis doesn't support interprocedural queries.");
479 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags,
480 LocB.Ptr, LocB.Size, LocB.AATags);
481 // AliasCache rarely has more than 1 or 2 elements, always use
482 // shrink_and_clear so it quickly returns to the inline capacity of the
483 // SmallDenseMap if it ever grows larger.
484 // FIXME: This should really be shrink_to_inline_capacity_and_clear().
485 AliasCache.shrink_and_clear();
486 VisitedPhiBBs.clear();
490 ModRefResult getModRefInfo(ImmutableCallSite CS,
491 const Location &Loc) override;
493 ModRefResult getModRefInfo(ImmutableCallSite CS1,
494 ImmutableCallSite CS2) override;
496 /// pointsToConstantMemory - Chase pointers until we find a (constant
498 bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
500 /// Get the location associated with a pointer argument of a callsite.
501 Location getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
502 ModRefResult &Mask) override;
504 /// getModRefBehavior - Return the behavior when calling the given
506 ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
508 /// getModRefBehavior - Return the behavior when calling the given function.
509 /// For use when the call site is not known.
510 ModRefBehavior getModRefBehavior(const Function *F) override;
512 /// getAdjustedAnalysisPointer - This method is used when a pass implements
513 /// an analysis interface through multiple inheritance. If needed, it
514 /// should override this to adjust the this pointer as needed for the
515 /// specified pass info.
516 void *getAdjustedAnalysisPointer(const void *ID) override {
517 if (ID == &AliasAnalysis::ID)
518 return (AliasAnalysis*)this;
523 // AliasCache - Track alias queries to guard against recursion.
524 typedef std::pair<Location, Location> LocPair;
525 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
526 AliasCacheTy AliasCache;
528 /// \brief Track phi nodes we have visited. When interpret "Value" pointer
529 /// equality as value equality we need to make sure that the "Value" is not
530 /// part of a cycle. Otherwise, two uses could come from different
531 /// "iterations" of a cycle and see different values for the same "Value"
533 /// The following example shows the problem:
534 /// %p = phi(%alloca1, %addr2)
536 /// %addr1 = gep, %alloca2, 0, %l
537 /// %addr2 = gep %alloca2, 0, (%l + 1)
538 /// alias(%p, %addr1) -> MayAlias !
540 SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
542 // Visited - Track instructions visited by pointsToConstantMemory.
543 SmallPtrSet<const Value*, 16> Visited;
545 /// \brief Check whether two Values can be considered equivalent.
547 /// In addition to pointer equivalence of \p V1 and \p V2 this checks
548 /// whether they can not be part of a cycle in the value graph by looking at
549 /// all visited phi nodes an making sure that the phis cannot reach the
550 /// value. We have to do this because we are looking through phi nodes (That
551 /// is we say noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
552 bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
554 /// \brief Dest and Src are the variable indices from two decomposed
555 /// GetElementPtr instructions GEP1 and GEP2 which have common base
556 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
557 /// difference between the two pointers.
558 void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
559 const SmallVectorImpl<VariableGEPIndex> &Src);
561 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
562 // instruction against another.
563 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
564 const AAMDNodes &V1AAInfo,
565 const Value *V2, uint64_t V2Size,
566 const AAMDNodes &V2AAInfo,
567 const Value *UnderlyingV1, const Value *UnderlyingV2);
569 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
570 // instruction against another.
571 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
572 const AAMDNodes &PNAAInfo,
573 const Value *V2, uint64_t V2Size,
574 const AAMDNodes &V2AAInfo);
576 /// aliasSelect - Disambiguate a Select instruction against another value.
577 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
578 const AAMDNodes &SIAAInfo,
579 const Value *V2, uint64_t V2Size,
580 const AAMDNodes &V2AAInfo);
582 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
584 const Value *V2, uint64_t V2Size,
587 } // End of anonymous namespace
589 // Register this pass...
590 char BasicAliasAnalysis::ID = 0;
591 INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
592 "Basic Alias Analysis (stateless AA impl)",
594 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
595 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
596 INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
597 "Basic Alias Analysis (stateless AA impl)",
601 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
602 return new BasicAliasAnalysis();
605 /// pointsToConstantMemory - Returns whether the given pointer value
606 /// points to memory that is local to the function, with global constants being
607 /// considered local to all functions.
609 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
610 assert(Visited.empty() && "Visited must be cleared after use!");
612 unsigned MaxLookup = 8;
613 SmallVector<const Value *, 16> Worklist;
614 Worklist.push_back(Loc.Ptr);
616 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), DL);
617 if (!Visited.insert(V)) {
619 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
622 // An alloca instruction defines local memory.
623 if (OrLocal && isa<AllocaInst>(V))
626 // A global constant counts as local memory for our purposes.
627 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
628 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
629 // global to be marked constant in some modules and non-constant in
630 // others. GV may even be a declaration, not a definition.
631 if (!GV->isConstant()) {
633 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
638 // If both select values point to local memory, then so does the select.
639 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
640 Worklist.push_back(SI->getTrueValue());
641 Worklist.push_back(SI->getFalseValue());
645 // If all values incoming to a phi node point to local memory, then so does
647 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
648 // Don't bother inspecting phi nodes with many operands.
649 if (PN->getNumIncomingValues() > MaxLookup) {
651 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
653 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
654 Worklist.push_back(PN->getIncomingValue(i));
658 // Otherwise be conservative.
660 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
662 } while (!Worklist.empty() && --MaxLookup);
665 return Worklist.empty();
668 static bool isMemsetPattern16(const Function *MS,
669 const TargetLibraryInfo &TLI) {
670 if (TLI.has(LibFunc::memset_pattern16) &&
671 MS->getName() == "memset_pattern16") {
672 FunctionType *MemsetType = MS->getFunctionType();
673 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
674 isa<PointerType>(MemsetType->getParamType(0)) &&
675 isa<PointerType>(MemsetType->getParamType(1)) &&
676 isa<IntegerType>(MemsetType->getParamType(2)))
683 /// getModRefBehavior - Return the behavior when calling the given call site.
684 AliasAnalysis::ModRefBehavior
685 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
686 if (CS.doesNotAccessMemory())
687 // Can't do better than this.
688 return DoesNotAccessMemory;
690 ModRefBehavior Min = UnknownModRefBehavior;
692 // If the callsite knows it only reads memory, don't return worse
694 if (CS.onlyReadsMemory())
695 Min = OnlyReadsMemory;
697 // The AliasAnalysis base class has some smarts, lets use them.
698 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
701 /// getModRefBehavior - Return the behavior when calling the given function.
702 /// For use when the call site is not known.
703 AliasAnalysis::ModRefBehavior
704 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
705 // If the function declares it doesn't access memory, we can't do better.
706 if (F->doesNotAccessMemory())
707 return DoesNotAccessMemory;
709 // For intrinsics, we can check the table.
710 if (unsigned iid = F->getIntrinsicID()) {
711 #define GET_INTRINSIC_MODREF_BEHAVIOR
712 #include "llvm/IR/Intrinsics.gen"
713 #undef GET_INTRINSIC_MODREF_BEHAVIOR
716 ModRefBehavior Min = UnknownModRefBehavior;
718 // If the function declares it only reads memory, go with that.
719 if (F->onlyReadsMemory())
720 Min = OnlyReadsMemory;
722 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
723 if (isMemsetPattern16(F, TLI))
724 Min = OnlyAccessesArgumentPointees;
726 // Otherwise be conservative.
727 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
730 AliasAnalysis::Location
731 BasicAliasAnalysis::getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
732 ModRefResult &Mask) {
733 Location Loc = AliasAnalysis::getArgLocation(CS, ArgIdx, Mask);
734 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
735 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
737 switch (II->getIntrinsicID()) {
739 case Intrinsic::memset:
740 case Intrinsic::memcpy:
741 case Intrinsic::memmove: {
742 assert((ArgIdx == 0 || ArgIdx == 1) &&
743 "Invalid argument index for memory intrinsic");
744 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
745 Loc.Size = LenCI->getZExtValue();
746 assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
747 "Memory intrinsic location pointer not argument?");
748 Mask = ArgIdx ? Ref : Mod;
751 case Intrinsic::lifetime_start:
752 case Intrinsic::lifetime_end:
753 case Intrinsic::invariant_start: {
754 assert(ArgIdx == 1 && "Invalid argument index");
755 assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
756 "Intrinsic location pointer not argument?");
757 Loc.Size = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
760 case Intrinsic::invariant_end: {
761 assert(ArgIdx == 2 && "Invalid argument index");
762 assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
763 "Intrinsic location pointer not argument?");
764 Loc.Size = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
767 case Intrinsic::arm_neon_vld1: {
768 assert(ArgIdx == 0 && "Invalid argument index");
769 assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
770 "Intrinsic location pointer not argument?");
771 // LLVM's vld1 and vst1 intrinsics currently only support a single
774 Loc.Size = DL->getTypeStoreSize(II->getType());
777 case Intrinsic::arm_neon_vst1: {
778 assert(ArgIdx == 0 && "Invalid argument index");
779 assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
780 "Intrinsic location pointer not argument?");
782 Loc.Size = DL->getTypeStoreSize(II->getArgOperand(1)->getType());
787 // We can bound the aliasing properties of memset_pattern16 just as we can
788 // for memcpy/memset. This is particularly important because the
789 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
790 // whenever possible.
791 else if (CS.getCalledFunction() &&
792 isMemsetPattern16(CS.getCalledFunction(), TLI)) {
793 assert((ArgIdx == 0 || ArgIdx == 1) &&
794 "Invalid argument index for memset_pattern16");
797 else if (const ConstantInt *LenCI =
798 dyn_cast<ConstantInt>(CS.getArgument(2)))
799 Loc.Size = LenCI->getZExtValue();
800 assert(Loc.Ptr == CS.getArgument(ArgIdx) &&
801 "memset_pattern16 location pointer not argument?");
802 Mask = ArgIdx ? Ref : Mod;
804 // FIXME: Handle memset_pattern4 and memset_pattern8 also.
809 static bool isAssumeIntrinsic(ImmutableCallSite CS) {
810 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
811 if (II && II->getIntrinsicID() == Intrinsic::assume)
817 /// getModRefInfo - Check to see if the specified callsite can clobber the
818 /// specified memory object. Since we only look at local properties of this
819 /// function, we really can't say much about this query. We do, however, use
820 /// simple "address taken" analysis on local objects.
821 AliasAnalysis::ModRefResult
822 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
823 const Location &Loc) {
824 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
825 "AliasAnalysis query involving multiple functions!");
827 const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
829 // If this is a tail call and Loc.Ptr points to a stack location, we know that
830 // the tail call cannot access or modify the local stack.
831 // We cannot exclude byval arguments here; these belong to the caller of
832 // the current function not to the current function, and a tail callee
833 // may reference them.
834 if (isa<AllocaInst>(Object))
835 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
836 if (CI->isTailCall())
839 // If the pointer is to a locally allocated object that does not escape,
840 // then the call can not mod/ref the pointer unless the call takes the pointer
841 // as an argument, and itself doesn't capture it.
842 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
843 isNonEscapingLocalObject(Object)) {
844 bool PassedAsArg = false;
846 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
847 CI != CE; ++CI, ++ArgNo) {
848 // Only look at the no-capture or byval pointer arguments. If this
849 // pointer were passed to arguments that were neither of these, then it
850 // couldn't be no-capture.
851 if (!(*CI)->getType()->isPointerTy() ||
852 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
855 // If this is a no-capture pointer argument, see if we can tell that it
856 // is impossible to alias the pointer we're checking. If not, we have to
857 // assume that the call could touch the pointer, even though it doesn't
859 if (!isNoAlias(Location(*CI), Location(Object))) {
869 // While the assume intrinsic is marked as arbitrarily writing so that
870 // proper control dependencies will be maintained, it never aliases any
871 // particular memory location.
872 if (isAssumeIntrinsic(CS))
875 // The AliasAnalysis base class has some smarts, lets use them.
876 return AliasAnalysis::getModRefInfo(CS, Loc);
879 AliasAnalysis::ModRefResult
880 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
881 ImmutableCallSite CS2) {
882 // While the assume intrinsic is marked as arbitrarily writing so that
883 // proper control dependencies will be maintained, it never aliases any
884 // particular memory location.
885 if (isAssumeIntrinsic(CS1) || isAssumeIntrinsic(CS2))
888 // The AliasAnalysis base class has some smarts, lets use them.
889 return AliasAnalysis::getModRefInfo(CS1, CS2);
892 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
893 /// against another pointer. We know that V1 is a GEP, but we don't know
894 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, DL),
895 /// UnderlyingV2 is the same for V2.
897 AliasAnalysis::AliasResult
898 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
899 const AAMDNodes &V1AAInfo,
900 const Value *V2, uint64_t V2Size,
901 const AAMDNodes &V2AAInfo,
902 const Value *UnderlyingV1,
903 const Value *UnderlyingV2) {
904 int64_t GEP1BaseOffset;
905 bool GEP1MaxLookupReached;
906 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
908 AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
909 DominatorTreeWrapperPass *DTWP =
910 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
911 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
913 // If we have two gep instructions with must-alias or not-alias'ing base
914 // pointers, figure out if the indexes to the GEP tell us anything about the
916 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
917 // Do the base pointers alias?
918 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, AAMDNodes(),
919 UnderlyingV2, UnknownSize, AAMDNodes());
921 // Check for geps of non-aliasing underlying pointers where the offsets are
923 if ((BaseAlias == MayAlias) && V1Size == V2Size) {
924 // Do the base pointers alias assuming type and size.
925 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
926 V1AAInfo, UnderlyingV2,
928 if (PreciseBaseAlias == NoAlias) {
929 // See if the computed offset from the common pointer tells us about the
930 // relation of the resulting pointer.
931 int64_t GEP2BaseOffset;
932 bool GEP2MaxLookupReached;
933 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
934 const Value *GEP2BasePtr =
935 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
936 GEP2MaxLookupReached, DL, AT, DT);
937 const Value *GEP1BasePtr =
938 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
939 GEP1MaxLookupReached, DL, AT, DT);
940 // DecomposeGEPExpression and GetUnderlyingObject should return the
941 // same result except when DecomposeGEPExpression has no DataLayout.
942 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
944 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
947 // If the max search depth is reached the result is undefined
948 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
952 if (GEP1BaseOffset == GEP2BaseOffset &&
953 GEP1VariableIndices == GEP2VariableIndices)
955 GEP1VariableIndices.clear();
959 // If we get a No or May, then return it immediately, no amount of analysis
960 // will improve this situation.
961 if (BaseAlias != MustAlias) return BaseAlias;
963 // Otherwise, we have a MustAlias. Since the base pointers alias each other
964 // exactly, see if the computed offset from the common pointer tells us
965 // about the relation of the resulting pointer.
966 const Value *GEP1BasePtr =
967 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
968 GEP1MaxLookupReached, DL, AT, DT);
970 int64_t GEP2BaseOffset;
971 bool GEP2MaxLookupReached;
972 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
973 const Value *GEP2BasePtr =
974 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
975 GEP2MaxLookupReached, DL, AT, DT);
977 // DecomposeGEPExpression and GetUnderlyingObject should return the
978 // same result except when DecomposeGEPExpression has no DataLayout.
979 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
981 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
984 // If the max search depth is reached the result is undefined
985 if (GEP2MaxLookupReached || GEP1MaxLookupReached)
988 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
989 // symbolic difference.
990 GEP1BaseOffset -= GEP2BaseOffset;
991 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
994 // Check to see if these two pointers are related by the getelementptr
995 // instruction. If one pointer is a GEP with a non-zero index of the other
996 // pointer, we know they cannot alias.
998 // If both accesses are unknown size, we can't do anything useful here.
999 if (V1Size == UnknownSize && V2Size == UnknownSize)
1002 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, AAMDNodes(),
1003 V2, V2Size, V2AAInfo);
1005 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
1006 // If V2 is known not to alias GEP base pointer, then the two values
1007 // cannot alias per GEP semantics: "A pointer value formed from a
1008 // getelementptr instruction is associated with the addresses associated
1009 // with the first operand of the getelementptr".
1012 const Value *GEP1BasePtr =
1013 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
1014 GEP1MaxLookupReached, DL, AT, DT);
1016 // DecomposeGEPExpression and GetUnderlyingObject should return the
1017 // same result except when DecomposeGEPExpression has no DataLayout.
1018 if (GEP1BasePtr != UnderlyingV1) {
1020 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
1023 // If the max search depth is reached the result is undefined
1024 if (GEP1MaxLookupReached)
1028 // In the two GEP Case, if there is no difference in the offsets of the
1029 // computed pointers, the resultant pointers are a must alias. This
1030 // hapens when we have two lexically identical GEP's (for example).
1032 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
1033 // must aliases the GEP, the end result is a must alias also.
1034 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
1037 // If there is a constant difference between the pointers, but the difference
1038 // is less than the size of the associated memory object, then we know
1039 // that the objects are partially overlapping. If the difference is
1040 // greater, we know they do not overlap.
1041 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
1042 if (GEP1BaseOffset >= 0) {
1043 if (V2Size != UnknownSize) {
1044 if ((uint64_t)GEP1BaseOffset < V2Size)
1045 return PartialAlias;
1049 // We have the situation where:
1052 // ---------------->|
1053 // |-->V1Size |-------> V2Size
1055 // We need to know that V2Size is not unknown, otherwise we might have
1056 // stripped a gep with negative index ('gep <ptr>, -1, ...).
1057 if (V1Size != UnknownSize && V2Size != UnknownSize) {
1058 if (-(uint64_t)GEP1BaseOffset < V1Size)
1059 return PartialAlias;
1065 if (!GEP1VariableIndices.empty()) {
1066 uint64_t Modulo = 0;
1067 bool AllPositive = true;
1068 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) {
1070 // Try to distinguish something like &A[i][1] against &A[42][0].
1071 // Grab the least significant bit set in any of the scales. We
1072 // don't need std::abs here (even if the scale's negative) as we'll
1073 // be ^'ing Modulo with itself later.
1074 Modulo |= (uint64_t) GEP1VariableIndices[i].Scale;
1077 // If the Value could change between cycles, then any reasoning about
1078 // the Value this cycle may not hold in the next cycle. We'll just
1079 // give up if we can't determine conditions that hold for every cycle:
1080 const Value *V = GEP1VariableIndices[i].V;
1082 bool SignKnownZero, SignKnownOne;
1084 const_cast<Value *>(V),
1085 SignKnownZero, SignKnownOne,
1086 DL, 0, AT, nullptr, DT);
1088 // Zero-extension widens the variable, and so forces the sign
1090 bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
1091 SignKnownZero |= IsZExt;
1092 SignKnownOne &= !IsZExt;
1094 // If the variable begins with a zero then we know it's
1095 // positive, regardless of whether the value is signed or
1097 int64_t Scale = GEP1VariableIndices[i].Scale;
1099 (SignKnownZero && Scale >= 0) ||
1100 (SignKnownOne && Scale < 0);
1104 Modulo = Modulo ^ (Modulo & (Modulo - 1));
1106 // We can compute the difference between the two addresses
1107 // mod Modulo. Check whether that difference guarantees that the
1108 // two locations do not alias.
1109 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
1110 if (V1Size != UnknownSize && V2Size != UnknownSize &&
1111 ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
1114 // If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
1115 // If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
1116 // don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
1117 if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
1121 // Statically, we can see that the base objects are the same, but the
1122 // pointers have dynamic offsets which we can't resolve. And none of our
1123 // little tricks above worked.
1125 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
1126 // practical effect of this is protecting TBAA in the case of dynamic
1127 // indices into arrays of unions or malloc'd memory.
1128 return PartialAlias;
1131 static AliasAnalysis::AliasResult
1132 MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
1133 // If the results agree, take it.
1136 // A mix of PartialAlias and MustAlias is PartialAlias.
1137 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
1138 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
1139 return AliasAnalysis::PartialAlias;
1140 // Otherwise, we don't know anything.
1141 return AliasAnalysis::MayAlias;
1144 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
1145 /// instruction against another.
1146 AliasAnalysis::AliasResult
1147 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
1148 const AAMDNodes &SIAAInfo,
1149 const Value *V2, uint64_t V2Size,
1150 const AAMDNodes &V2AAInfo) {
1151 // If the values are Selects with the same condition, we can do a more precise
1152 // check: just check for aliases between the values on corresponding arms.
1153 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
1154 if (SI->getCondition() == SI2->getCondition()) {
1156 aliasCheck(SI->getTrueValue(), SISize, SIAAInfo,
1157 SI2->getTrueValue(), V2Size, V2AAInfo);
1158 if (Alias == MayAlias)
1160 AliasResult ThisAlias =
1161 aliasCheck(SI->getFalseValue(), SISize, SIAAInfo,
1162 SI2->getFalseValue(), V2Size, V2AAInfo);
1163 return MergeAliasResults(ThisAlias, Alias);
1166 // If both arms of the Select node NoAlias or MustAlias V2, then returns
1167 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1169 aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(), SISize, SIAAInfo);
1170 if (Alias == MayAlias)
1173 AliasResult ThisAlias =
1174 aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo);
1175 return MergeAliasResults(ThisAlias, Alias);
1178 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
1180 AliasAnalysis::AliasResult
1181 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
1182 const AAMDNodes &PNAAInfo,
1183 const Value *V2, uint64_t V2Size,
1184 const AAMDNodes &V2AAInfo) {
1185 // Track phi nodes we have visited. We use this information when we determine
1186 // value equivalence.
1187 VisitedPhiBBs.insert(PN->getParent());
1189 // If the values are PHIs in the same block, we can do a more precise
1190 // as well as efficient check: just check for aliases between the values
1191 // on corresponding edges.
1192 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
1193 if (PN2->getParent() == PN->getParent()) {
1194 LocPair Locs(Location(PN, PNSize, PNAAInfo),
1195 Location(V2, V2Size, V2AAInfo));
1197 std::swap(Locs.first, Locs.second);
1198 // Analyse the PHIs' inputs under the assumption that the PHIs are
1200 // If the PHIs are May/MustAlias there must be (recursively) an input
1201 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or
1202 // there must be an operation on the PHIs within the PHIs' value cycle
1203 // that causes a MayAlias.
1204 // Pretend the phis do not alias.
1205 AliasResult Alias = NoAlias;
1206 assert(AliasCache.count(Locs) &&
1207 "There must exist an entry for the phi node");
1208 AliasResult OrigAliasResult = AliasCache[Locs];
1209 AliasCache[Locs] = NoAlias;
1211 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1212 AliasResult ThisAlias =
1213 aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
1214 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1216 Alias = MergeAliasResults(ThisAlias, Alias);
1217 if (Alias == MayAlias)
1221 // Reset if speculation failed.
1222 if (Alias != NoAlias)
1223 AliasCache[Locs] = OrigAliasResult;
1228 SmallPtrSet<Value*, 4> UniqueSrc;
1229 SmallVector<Value*, 4> V1Srcs;
1230 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1231 Value *PV1 = PN->getIncomingValue(i);
1232 if (isa<PHINode>(PV1))
1233 // If any of the source itself is a PHI, return MayAlias conservatively
1234 // to avoid compile time explosion. The worst possible case is if both
1235 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1236 // and 'n' are the number of PHI sources.
1238 if (UniqueSrc.insert(PV1))
1239 V1Srcs.push_back(PV1);
1242 AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo,
1243 V1Srcs[0], PNSize, PNAAInfo);
1244 // Early exit if the check of the first PHI source against V2 is MayAlias.
1245 // Other results are not possible.
1246 if (Alias == MayAlias)
1249 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1250 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1251 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1252 Value *V = V1Srcs[i];
1254 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2AAInfo,
1255 V, PNSize, PNAAInfo);
1256 Alias = MergeAliasResults(ThisAlias, Alias);
1257 if (Alias == MayAlias)
1264 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1265 // such as array references.
1267 AliasAnalysis::AliasResult
1268 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1270 const Value *V2, uint64_t V2Size,
1271 AAMDNodes V2AAInfo) {
1272 // If either of the memory references is empty, it doesn't matter what the
1273 // pointer values are.
1274 if (V1Size == 0 || V2Size == 0)
1277 // Strip off any casts if they exist.
1278 V1 = V1->stripPointerCasts();
1279 V2 = V2->stripPointerCasts();
1281 // Are we checking for alias of the same value?
1282 // Because we look 'through' phi nodes we could look at "Value" pointers from
1283 // different iterations. We must therefore make sure that this is not the
1284 // case. The function isValueEqualInPotentialCycles ensures that this cannot
1285 // happen by looking at the visited phi nodes and making sure they cannot
1287 if (isValueEqualInPotentialCycles(V1, V2))
1290 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1291 return NoAlias; // Scalars cannot alias each other
1293 // Figure out what objects these things are pointing to if we can.
1294 const Value *O1 = GetUnderlyingObject(V1, DL, MaxLookupSearchDepth);
1295 const Value *O2 = GetUnderlyingObject(V2, DL, MaxLookupSearchDepth);
1297 // Null values in the default address space don't point to any object, so they
1298 // don't alias any other pointer.
1299 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1300 if (CPN->getType()->getAddressSpace() == 0)
1302 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1303 if (CPN->getType()->getAddressSpace() == 0)
1307 // If V1/V2 point to two different objects we know that we have no alias.
1308 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1311 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1312 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1313 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1316 // Function arguments can't alias with things that are known to be
1317 // unambigously identified at the function level.
1318 if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
1319 (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
1322 // Most objects can't alias null.
1323 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1324 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1327 // If one pointer is the result of a call/invoke or load and the other is a
1328 // non-escaping local object within the same function, then we know the
1329 // object couldn't escape to a point where the call could return it.
1331 // Note that if the pointers are in different functions, there are a
1332 // variety of complications. A call with a nocapture argument may still
1333 // temporary store the nocapture argument's value in a temporary memory
1334 // location if that memory location doesn't escape. Or it may pass a
1335 // nocapture value to other functions as long as they don't capture it.
1336 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1338 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1342 // If the size of one access is larger than the entire object on the other
1343 // side, then we know such behavior is undefined and can assume no alias.
1345 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *DL, *TLI)) ||
1346 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *DL, *TLI)))
1349 // Check the cache before climbing up use-def chains. This also terminates
1350 // otherwise infinitely recursive queries.
1351 LocPair Locs(Location(V1, V1Size, V1AAInfo),
1352 Location(V2, V2Size, V2AAInfo));
1354 std::swap(Locs.first, Locs.second);
1355 std::pair<AliasCacheTy::iterator, bool> Pair =
1356 AliasCache.insert(std::make_pair(Locs, MayAlias));
1358 return Pair.first->second;
1360 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1361 // GEP can't simplify, we don't even look at the PHI cases.
1362 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1364 std::swap(V1Size, V2Size);
1366 std::swap(V1AAInfo, V2AAInfo);
1368 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1369 AliasResult Result = aliasGEP(GV1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O1, O2);
1370 if (Result != MayAlias) return AliasCache[Locs] = Result;
1373 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1375 std::swap(V1Size, V2Size);
1376 std::swap(V1AAInfo, V2AAInfo);
1378 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1379 AliasResult Result = aliasPHI(PN, V1Size, V1AAInfo,
1380 V2, V2Size, V2AAInfo);
1381 if (Result != MayAlias) return AliasCache[Locs] = Result;
1384 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1386 std::swap(V1Size, V2Size);
1387 std::swap(V1AAInfo, V2AAInfo);
1389 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1390 AliasResult Result = aliasSelect(S1, V1Size, V1AAInfo,
1391 V2, V2Size, V2AAInfo);
1392 if (Result != MayAlias) return AliasCache[Locs] = Result;
1395 // If both pointers are pointing into the same object and one of them
1396 // accesses is accessing the entire object, then the accesses must
1397 // overlap in some way.
1399 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *DL, *TLI)) ||
1400 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *DL, *TLI)))
1401 return AliasCache[Locs] = PartialAlias;
1403 AliasResult Result =
1404 AliasAnalysis::alias(Location(V1, V1Size, V1AAInfo),
1405 Location(V2, V2Size, V2AAInfo));
1406 return AliasCache[Locs] = Result;
1409 bool BasicAliasAnalysis::isValueEqualInPotentialCycles(const Value *V,
1414 const Instruction *Inst = dyn_cast<Instruction>(V);
1418 if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
1421 // Use dominance or loop info if available.
1422 DominatorTreeWrapperPass *DTWP =
1423 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
1424 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
1425 LoopInfo *LI = getAnalysisIfAvailable<LoopInfo>();
1427 // Make sure that the visited phis cannot reach the Value. This ensures that
1428 // the Values cannot come from different iterations of a potential cycle the
1429 // phi nodes could be involved in.
1430 for (auto *P : VisitedPhiBBs)
1431 if (isPotentiallyReachable(P->begin(), Inst, DT, LI))
1437 /// GetIndexDifference - Dest and Src are the variable indices from two
1438 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
1439 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
1440 /// difference between the two pointers.
1441 void BasicAliasAnalysis::GetIndexDifference(
1442 SmallVectorImpl<VariableGEPIndex> &Dest,
1443 const SmallVectorImpl<VariableGEPIndex> &Src) {
1447 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
1448 const Value *V = Src[i].V;
1449 ExtensionKind Extension = Src[i].Extension;
1450 int64_t Scale = Src[i].Scale;
1452 // Find V in Dest. This is N^2, but pointer indices almost never have more
1453 // than a few variable indexes.
1454 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
1455 if (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
1456 Dest[j].Extension != Extension)
1459 // If we found it, subtract off Scale V's from the entry in Dest. If it
1460 // goes to zero, remove the entry.
1461 if (Dest[j].Scale != Scale)
1462 Dest[j].Scale -= Scale;
1464 Dest.erase(Dest.begin() + j);
1469 // If we didn't consume this entry, add it to the end of the Dest list.
1471 VariableGEPIndex Entry = { V, Extension, -Scale };
1472 Dest.push_back(Entry);