1 //===- BasicAliasAnalysis.cpp - Local 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 default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/LLVMContext.h"
26 #include "llvm/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/Target/TargetData.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/SmallVector.h"
34 #include "llvm/Support/ErrorHandling.h"
35 #include "llvm/Support/GetElementPtrTypeIterator.h"
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
43 /// isKnownNonNull - Return true if we know that the specified value is never
45 static bool isKnownNonNull(const Value *V) {
46 // Alloca never returns null, malloc might.
47 if (isa<AllocaInst>(V)) return true;
49 // A byval argument is never null.
50 if (const Argument *A = dyn_cast<Argument>(V))
51 return A->hasByValAttr();
53 // Global values are not null unless extern weak.
54 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
55 return !GV->hasExternalWeakLinkage();
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 // Don't bother analyzing arguments already known not to escape.
77 if (A->hasNoCaptureAttr())
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 /// isObjectSmallerThan - Return true if we can prove that the object specified
100 /// by V is smaller than Size.
101 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
102 const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 AccessTy = GV->getType()->getElementType();
106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
107 if (!AI->isArrayAllocation())
108 AccessTy = AI->getType()->getElementType();
111 } else if (const CallInst* CI = extractMallocCall(V)) {
112 if (!isArrayMalloc(V, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
115 return (C->getZExtValue() < Size);
117 } else if (const Argument *A = dyn_cast<Argument>(V)) {
118 if (A->hasByValAttr())
119 AccessTy = cast<PointerType>(A->getType())->getElementType();
126 if (AccessTy->isSized())
127 return TD.getTypeAllocSize(AccessTy) < Size;
131 //===----------------------------------------------------------------------===//
133 //===----------------------------------------------------------------------===//
136 /// NoAA - This class implements the -no-aa pass, which always returns "I
137 /// don't know" for alias queries. NoAA is unlike other alias analysis
138 /// implementations, in that it does not chain to a previous analysis. As
139 /// such it doesn't follow many of the rules that other alias analyses must.
141 struct NoAA : public ImmutablePass, public AliasAnalysis {
142 static char ID; // Class identification, replacement for typeinfo
143 NoAA() : ImmutablePass(ID) {
144 initializeNoAAPass(*PassRegistry::getPassRegistry());
146 explicit NoAA(char &PID) : ImmutablePass(PID) {}
148 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
151 virtual void initializePass() {
152 TD = getAnalysisIfAvailable<TargetData>();
155 virtual AliasResult alias(const Location &LocA, const Location &LocB) {
159 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
160 return UnknownModRefBehavior;
162 virtual ModRefBehavior getModRefBehavior(const Function *F) {
163 return UnknownModRefBehavior;
166 virtual bool pointsToConstantMemory(const Location &Loc) { return false; }
167 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
168 const Location &Loc) {
171 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
172 ImmutableCallSite CS2) {
176 virtual void deleteValue(Value *V) {}
177 virtual void copyValue(Value *From, Value *To) {}
179 /// getAdjustedAnalysisPointer - This method is used when a pass implements
180 /// an analysis interface through multiple inheritance. If needed, it
181 /// should override this to adjust the this pointer as needed for the
182 /// specified pass info.
183 virtual void *getAdjustedAnalysisPointer(const void *ID) {
184 if (ID == &AliasAnalysis::ID)
185 return (AliasAnalysis*)this;
189 } // End of anonymous namespace
191 // Register this pass...
193 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
194 "No Alias Analysis (always returns 'may' alias)",
197 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
199 //===----------------------------------------------------------------------===//
200 // GetElementPtr Instruction Decomposition and Analysis
201 //===----------------------------------------------------------------------===//
210 struct VariableGEPIndex {
212 ExtensionKind Extension;
218 /// GetLinearExpression - Analyze the specified value as a linear expression:
219 /// "A*V + B", where A and B are constant integers. Return the scale and offset
220 /// values as APInts and return V as a Value*, and return whether we looked
221 /// through any sign or zero extends. The incoming Value is known to have
222 /// IntegerType and it may already be sign or zero extended.
224 /// Note that this looks through extends, so the high bits may not be
225 /// represented in the result.
226 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
227 ExtensionKind &Extension,
228 const TargetData &TD, unsigned Depth) {
229 assert(V->getType()->isIntegerTy() && "Not an integer value");
231 // Limit our recursion depth.
238 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
239 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
240 switch (BOp->getOpcode()) {
242 case Instruction::Or:
243 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
245 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
248 case Instruction::Add:
249 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
251 Offset += RHSC->getValue();
253 case Instruction::Mul:
254 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
256 Offset *= RHSC->getValue();
257 Scale *= RHSC->getValue();
259 case Instruction::Shl:
260 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
262 Offset <<= RHSC->getValue().getLimitedValue();
263 Scale <<= RHSC->getValue().getLimitedValue();
269 // Since GEP indices are sign extended anyway, we don't care about the high
270 // bits of a sign or zero extended value - just scales and offsets. The
271 // extensions have to be consistent though.
272 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
273 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
274 Value *CastOp = cast<CastInst>(V)->getOperand(0);
275 unsigned OldWidth = Scale.getBitWidth();
276 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
277 Scale.trunc(SmallWidth);
278 Offset.trunc(SmallWidth);
279 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
281 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
283 Scale.zext(OldWidth);
284 Offset.zext(OldWidth);
294 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
295 /// into a base pointer with a constant offset and a number of scaled symbolic
298 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
299 /// the VarIndices vector) are Value*'s that are known to be scaled by the
300 /// specified amount, but which may have other unrepresented high bits. As such,
301 /// the gep cannot necessarily be reconstructed from its decomposed form.
303 /// When TargetData is around, this function is capable of analyzing everything
304 /// that Value::getUnderlyingObject() can look through. When not, it just looks
305 /// through pointer casts.
308 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
309 SmallVectorImpl<VariableGEPIndex> &VarIndices,
310 const TargetData *TD) {
311 // Limit recursion depth to limit compile time in crazy cases.
312 unsigned MaxLookup = 6;
316 // See if this is a bitcast or GEP.
317 const Operator *Op = dyn_cast<Operator>(V);
319 // The only non-operator case we can handle are GlobalAliases.
320 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
321 if (!GA->mayBeOverridden()) {
322 V = GA->getAliasee();
329 if (Op->getOpcode() == Instruction::BitCast) {
330 V = Op->getOperand(0);
334 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
338 // Don't attempt to analyze GEPs over unsized objects.
339 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
340 ->getElementType()->isSized())
343 // If we are lacking TargetData information, we can't compute the offets of
344 // elements computed by GEPs. However, we can handle bitcast equivalent
347 if (!GEPOp->hasAllZeroIndices())
349 V = GEPOp->getOperand(0);
353 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
354 gep_type_iterator GTI = gep_type_begin(GEPOp);
355 for (User::const_op_iterator I = GEPOp->op_begin()+1,
356 E = GEPOp->op_end(); I != E; ++I) {
358 // Compute the (potentially symbolic) offset in bytes for this index.
359 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
360 // For a struct, add the member offset.
361 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
362 if (FieldNo == 0) continue;
364 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
368 // For an array/pointer, add the element offset, explicitly scaled.
369 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
370 if (CIdx->isZero()) continue;
371 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
375 uint64_t Scale = TD->getTypeAllocSize(*GTI);
376 ExtensionKind Extension = EK_NotExtended;
378 // If the integer type is smaller than the pointer size, it is implicitly
379 // sign extended to pointer size.
380 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
381 if (TD->getPointerSizeInBits() > Width)
382 Extension = EK_SignExt;
384 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
385 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
386 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
389 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
390 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
391 BaseOffs += IndexOffset.getSExtValue()*Scale;
392 Scale *= IndexScale.getSExtValue();
395 // If we already had an occurrance of this index variable, merge this
396 // scale into it. For example, we want to handle:
397 // A[x][x] -> x*16 + x*4 -> x*20
398 // This also ensures that 'x' only appears in the index list once.
399 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
400 if (VarIndices[i].V == Index &&
401 VarIndices[i].Extension == Extension) {
402 Scale += VarIndices[i].Scale;
403 VarIndices.erase(VarIndices.begin()+i);
408 // Make sure that we have a scale that makes sense for this target's
410 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
412 Scale = (int64_t)Scale >> ShiftBits;
416 VariableGEPIndex Entry = {Index, Extension, Scale};
417 VarIndices.push_back(Entry);
421 // Analyze the base pointer next.
422 V = GEPOp->getOperand(0);
423 } while (--MaxLookup);
425 // If the chain of expressions is too deep, just return early.
429 /// GetIndexDifference - Dest and Src are the variable indices from two
430 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
431 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
432 /// difference between the two pointers.
433 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
434 const SmallVectorImpl<VariableGEPIndex> &Src) {
435 if (Src.empty()) return;
437 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
438 const Value *V = Src[i].V;
439 ExtensionKind Extension = Src[i].Extension;
440 int64_t Scale = Src[i].Scale;
442 // Find V in Dest. This is N^2, but pointer indices almost never have more
443 // than a few variable indexes.
444 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
445 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
447 // If we found it, subtract off Scale V's from the entry in Dest. If it
448 // goes to zero, remove the entry.
449 if (Dest[j].Scale != Scale)
450 Dest[j].Scale -= Scale;
452 Dest.erase(Dest.begin()+j);
457 // If we didn't consume this entry, add it to the end of the Dest list.
459 VariableGEPIndex Entry = { V, Extension, -Scale };
460 Dest.push_back(Entry);
465 //===----------------------------------------------------------------------===//
466 // BasicAliasAnalysis Pass
467 //===----------------------------------------------------------------------===//
470 static const Function *getParent(const Value *V) {
471 if (const Instruction *inst = dyn_cast<Instruction>(V))
472 return inst->getParent()->getParent();
474 if (const Argument *arg = dyn_cast<Argument>(V))
475 return arg->getParent();
480 static bool notDifferentParent(const Value *O1, const Value *O2) {
482 const Function *F1 = getParent(O1);
483 const Function *F2 = getParent(O2);
485 return !F1 || !F2 || F1 == F2;
490 /// BasicAliasAnalysis - This is the default alias analysis implementation.
491 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
492 /// derives from the NoAA class.
493 struct BasicAliasAnalysis : public NoAA {
494 static char ID; // Class identification, replacement for typeinfo
495 BasicAliasAnalysis() : NoAA(ID) {
496 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
499 virtual void initializePass() {
500 InitializeAliasAnalysis(this);
503 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
504 AU.addRequired<AliasAnalysis>();
507 virtual AliasResult alias(const Location &LocA,
508 const Location &LocB) {
509 assert(Visited.empty() && "Visited must be cleared after use!");
510 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
511 "BasicAliasAnalysis doesn't support interprocedural queries.");
512 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
513 LocB.Ptr, LocB.Size, LocB.TBAATag);
518 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
519 const Location &Loc);
521 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
522 ImmutableCallSite CS2) {
523 // The AliasAnalysis base class has some smarts, lets use them.
524 return AliasAnalysis::getModRefInfo(CS1, CS2);
527 /// pointsToConstantMemory - Chase pointers until we find a (constant
529 virtual bool pointsToConstantMemory(const Location &Loc);
531 /// getModRefBehavior - Return the behavior when calling the given
533 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
535 /// getModRefBehavior - Return the behavior when calling the given function.
536 /// For use when the call site is not known.
537 virtual ModRefBehavior getModRefBehavior(const Function *F);
539 /// getAdjustedAnalysisPointer - This method is used when a pass implements
540 /// an analysis interface through multiple inheritance. If needed, it
541 /// should override this to adjust the this pointer as needed for the
542 /// specified pass info.
543 virtual void *getAdjustedAnalysisPointer(const void *ID) {
544 if (ID == &AliasAnalysis::ID)
545 return (AliasAnalysis*)this;
550 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
551 SmallPtrSet<const Value*, 16> Visited;
553 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
554 // instruction against another.
555 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
556 const Value *V2, uint64_t V2Size,
557 const MDNode *V2TBAAInfo,
558 const Value *UnderlyingV1, const Value *UnderlyingV2);
560 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
561 // instruction against another.
562 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
563 const MDNode *PNTBAAInfo,
564 const Value *V2, uint64_t V2Size,
565 const MDNode *V2TBAAInfo);
567 /// aliasSelect - Disambiguate a Select instruction against another value.
568 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
569 const MDNode *SITBAAInfo,
570 const Value *V2, uint64_t V2Size,
571 const MDNode *V2TBAAInfo);
573 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
574 const MDNode *V1TBAATag,
575 const Value *V2, uint64_t V2Size,
576 const MDNode *V2TBAATag);
578 } // End of anonymous namespace
580 // Register this pass...
581 char BasicAliasAnalysis::ID = 0;
582 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
583 "Basic Alias Analysis (default AA impl)",
586 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
587 return new BasicAliasAnalysis();
591 /// pointsToConstantMemory - Chase pointers until we find a (constant
593 bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
594 if (const GlobalVariable *GV =
595 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject()))
596 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
597 // global to be marked constant in some modules and non-constant in others.
598 // GV may even be a declaration, not a definition.
599 return GV->isConstant();
601 return AliasAnalysis::pointsToConstantMemory(Loc);
604 /// getModRefBehavior - Return the behavior when calling the given call site.
605 AliasAnalysis::ModRefBehavior
606 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
607 if (CS.doesNotAccessMemory())
608 // Can't do better than this.
609 return DoesNotAccessMemory;
611 ModRefBehavior Min = UnknownModRefBehavior;
613 // If the callsite knows it only reads memory, don't return worse
615 if (CS.onlyReadsMemory())
616 Min = OnlyReadsMemory;
618 // The AliasAnalysis base class has some smarts, lets use them.
619 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
622 /// getModRefBehavior - Return the behavior when calling the given function.
623 /// For use when the call site is not known.
624 AliasAnalysis::ModRefBehavior
625 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
626 if (F->doesNotAccessMemory())
627 // Can't do better than this.
628 return DoesNotAccessMemory;
629 if (F->onlyReadsMemory())
630 return OnlyReadsMemory;
631 if (unsigned id = F->getIntrinsicID())
632 return getIntrinsicModRefBehavior(id);
634 return AliasAnalysis::getModRefBehavior(F);
637 /// getModRefInfo - Check to see if the specified callsite can clobber the
638 /// specified memory object. Since we only look at local properties of this
639 /// function, we really can't say much about this query. We do, however, use
640 /// simple "address taken" analysis on local objects.
641 AliasAnalysis::ModRefResult
642 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
643 const Location &Loc) {
644 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
645 "AliasAnalysis query involving multiple functions!");
647 const Value *Object = Loc.Ptr->getUnderlyingObject();
649 // If this is a tail call and Loc.Ptr points to a stack location, we know that
650 // the tail call cannot access or modify the local stack.
651 // We cannot exclude byval arguments here; these belong to the caller of
652 // the current function not to the current function, and a tail callee
653 // may reference them.
654 if (isa<AllocaInst>(Object))
655 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
656 if (CI->isTailCall())
659 // If the pointer is to a locally allocated object that does not escape,
660 // then the call can not mod/ref the pointer unless the call takes the pointer
661 // as an argument, and itself doesn't capture it.
662 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
663 isNonEscapingLocalObject(Object)) {
664 bool PassedAsArg = false;
666 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
667 CI != CE; ++CI, ++ArgNo) {
668 // Only look at the no-capture pointer arguments.
669 if (!(*CI)->getType()->isPointerTy() ||
670 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
673 // If this is a no-capture pointer argument, see if we can tell that it
674 // is impossible to alias the pointer we're checking. If not, we have to
675 // assume that the call could touch the pointer, even though it doesn't
677 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
687 // Finally, handle specific knowledge of intrinsics.
688 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
690 switch (II->getIntrinsicID()) {
692 case Intrinsic::memcpy:
693 case Intrinsic::memmove: {
694 uint64_t Len = UnknownSize;
695 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
696 Len = LenCI->getZExtValue();
697 Value *Dest = II->getArgOperand(0);
698 Value *Src = II->getArgOperand(1);
699 if (isNoAlias(Location(Dest, Len), Loc)) {
700 if (isNoAlias(Location(Src, Len), Loc))
706 case Intrinsic::memset:
707 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
708 // will handle it for the variable length case.
709 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
710 uint64_t Len = LenCI->getZExtValue();
711 Value *Dest = II->getArgOperand(0);
712 if (isNoAlias(Location(Dest, Len), Loc))
716 case Intrinsic::atomic_cmp_swap:
717 case Intrinsic::atomic_swap:
718 case Intrinsic::atomic_load_add:
719 case Intrinsic::atomic_load_sub:
720 case Intrinsic::atomic_load_and:
721 case Intrinsic::atomic_load_nand:
722 case Intrinsic::atomic_load_or:
723 case Intrinsic::atomic_load_xor:
724 case Intrinsic::atomic_load_max:
725 case Intrinsic::atomic_load_min:
726 case Intrinsic::atomic_load_umax:
727 case Intrinsic::atomic_load_umin:
729 Value *Op1 = II->getArgOperand(0);
730 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType());
731 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
732 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
736 case Intrinsic::lifetime_start:
737 case Intrinsic::lifetime_end:
738 case Intrinsic::invariant_start: {
740 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
741 if (isNoAlias(Location(II->getArgOperand(1),
743 II->getMetadata(LLVMContext::MD_tbaa)),
748 case Intrinsic::invariant_end: {
750 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
751 if (isNoAlias(Location(II->getArgOperand(2),
753 II->getMetadata(LLVMContext::MD_tbaa)),
760 // The AliasAnalysis base class has some smarts, lets use them.
761 return AliasAnalysis::getModRefInfo(CS, Loc);
764 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
765 /// against another pointer. We know that V1 is a GEP, but we don't know
766 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
767 /// UnderlyingV2 is the same for V2.
769 AliasAnalysis::AliasResult
770 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
771 const Value *V2, uint64_t V2Size,
772 const MDNode *V2TBAAInfo,
773 const Value *UnderlyingV1,
774 const Value *UnderlyingV2) {
775 // If this GEP has been visited before, we're on a use-def cycle.
776 // Such cycles are only valid when PHI nodes are involved or in unreachable
777 // code. The visitPHI function catches cycles containing PHIs, but there
778 // could still be a cycle without PHIs in unreachable code.
779 if (!Visited.insert(GEP1))
782 int64_t GEP1BaseOffset;
783 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
785 // If we have two gep instructions with must-alias'ing base pointers, figure
786 // out if the indexes to the GEP tell us anything about the derived pointer.
787 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
788 // Do the base pointers alias?
789 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
790 UnderlyingV2, UnknownSize, 0);
792 // If we get a No or May, then return it immediately, no amount of analysis
793 // will improve this situation.
794 if (BaseAlias != MustAlias) return BaseAlias;
796 // Otherwise, we have a MustAlias. Since the base pointers alias each other
797 // exactly, see if the computed offset from the common pointer tells us
798 // about the relation of the resulting pointer.
799 const Value *GEP1BasePtr =
800 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
802 int64_t GEP2BaseOffset;
803 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
804 const Value *GEP2BasePtr =
805 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
807 // If DecomposeGEPExpression isn't able to look all the way through the
808 // addressing operation, we must not have TD and this is too complex for us
809 // to handle without it.
810 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
812 "DecomposeGEPExpression and getUnderlyingObject disagree!");
816 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
817 // symbolic difference.
818 GEP1BaseOffset -= GEP2BaseOffset;
819 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
822 // Check to see if these two pointers are related by the getelementptr
823 // instruction. If one pointer is a GEP with a non-zero index of the other
824 // pointer, we know they cannot alias.
826 // If both accesses are unknown size, we can't do anything useful here.
827 if (V1Size == UnknownSize && V2Size == UnknownSize)
830 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
831 V2, V2Size, V2TBAAInfo);
833 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
834 // If V2 is known not to alias GEP base pointer, then the two values
835 // cannot alias per GEP semantics: "A pointer value formed from a
836 // getelementptr instruction is associated with the addresses associated
837 // with the first operand of the getelementptr".
840 const Value *GEP1BasePtr =
841 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
843 // If DecomposeGEPExpression isn't able to look all the way through the
844 // addressing operation, we must not have TD and this is too complex for us
845 // to handle without it.
846 if (GEP1BasePtr != UnderlyingV1) {
848 "DecomposeGEPExpression and getUnderlyingObject disagree!");
853 // In the two GEP Case, if there is no difference in the offsets of the
854 // computed pointers, the resultant pointers are a must alias. This
855 // hapens when we have two lexically identical GEP's (for example).
857 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
858 // must aliases the GEP, the end result is a must alias also.
859 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
862 // If we have a known constant offset, see if this offset is larger than the
863 // access size being queried. If so, and if no variable indices can remove
864 // pieces of this constant, then we know we have a no-alias. For example,
867 // In order to handle cases like &A[100][i] where i is an out of range
868 // subscript, we have to ignore all constant offset pieces that are a multiple
869 // of a scaled index. Do this by removing constant offsets that are a
870 // multiple of any of our variable indices. This allows us to transform
871 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
872 // provides an offset of 4 bytes (assuming a <= 4 byte access).
873 for (unsigned i = 0, e = GEP1VariableIndices.size();
874 i != e && GEP1BaseOffset;++i)
875 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
876 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
878 // If our known offset is bigger than the access size, we know we don't have
880 if (GEP1BaseOffset) {
881 if (GEP1BaseOffset >= 0 ?
882 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
883 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
884 GEP1BaseOffset != INT64_MIN))
891 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
892 /// instruction against another.
893 AliasAnalysis::AliasResult
894 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
895 const MDNode *SITBAAInfo,
896 const Value *V2, uint64_t V2Size,
897 const MDNode *V2TBAAInfo) {
898 // If this select has been visited before, we're on a use-def cycle.
899 // Such cycles are only valid when PHI nodes are involved or in unreachable
900 // code. The visitPHI function catches cycles containing PHIs, but there
901 // could still be a cycle without PHIs in unreachable code.
902 if (!Visited.insert(SI))
905 // If the values are Selects with the same condition, we can do a more precise
906 // check: just check for aliases between the values on corresponding arms.
907 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
908 if (SI->getCondition() == SI2->getCondition()) {
910 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
911 SI2->getTrueValue(), V2Size, V2TBAAInfo);
912 if (Alias == MayAlias)
914 AliasResult ThisAlias =
915 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
916 SI2->getFalseValue(), V2Size, V2TBAAInfo);
917 if (ThisAlias != Alias)
922 // If both arms of the Select node NoAlias or MustAlias V2, then returns
923 // NoAlias / MustAlias. Otherwise, returns MayAlias.
925 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
926 if (Alias == MayAlias)
929 // If V2 is visited, the recursive case will have been caught in the
930 // above aliasCheck call, so these subsequent calls to aliasCheck
931 // don't need to assume that V2 is being visited recursively.
934 AliasResult ThisAlias =
935 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
936 if (ThisAlias != Alias)
941 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
943 AliasAnalysis::AliasResult
944 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
945 const MDNode *PNTBAAInfo,
946 const Value *V2, uint64_t V2Size,
947 const MDNode *V2TBAAInfo) {
948 // The PHI node has already been visited, avoid recursion any further.
949 if (!Visited.insert(PN))
952 // If the values are PHIs in the same block, we can do a more precise
953 // as well as efficient check: just check for aliases between the values
954 // on corresponding edges.
955 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
956 if (PN2->getParent() == PN->getParent()) {
958 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
959 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
961 if (Alias == MayAlias)
963 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
964 AliasResult ThisAlias =
965 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
966 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
968 if (ThisAlias != Alias)
974 SmallPtrSet<Value*, 4> UniqueSrc;
975 SmallVector<Value*, 4> V1Srcs;
976 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
977 Value *PV1 = PN->getIncomingValue(i);
978 if (isa<PHINode>(PV1))
979 // If any of the source itself is a PHI, return MayAlias conservatively
980 // to avoid compile time explosion. The worst possible case is if both
981 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
982 // and 'n' are the number of PHI sources.
984 if (UniqueSrc.insert(PV1))
985 V1Srcs.push_back(PV1);
988 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
989 V1Srcs[0], PNSize, PNTBAAInfo);
990 // Early exit if the check of the first PHI source against V2 is MayAlias.
991 // Other results are not possible.
992 if (Alias == MayAlias)
995 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
996 // NoAlias / MustAlias. Otherwise, returns MayAlias.
997 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
998 Value *V = V1Srcs[i];
1000 // If V2 is visited, the recursive case will have been caught in the
1001 // above aliasCheck call, so these subsequent calls to aliasCheck
1002 // don't need to assume that V2 is being visited recursively.
1005 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1006 V, PNSize, PNTBAAInfo);
1007 if (ThisAlias != Alias || ThisAlias == MayAlias)
1014 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1015 // such as array references.
1017 AliasAnalysis::AliasResult
1018 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1019 const MDNode *V1TBAAInfo,
1020 const Value *V2, uint64_t V2Size,
1021 const MDNode *V2TBAAInfo) {
1022 // If either of the memory references is empty, it doesn't matter what the
1023 // pointer values are.
1024 if (V1Size == 0 || V2Size == 0)
1027 // Strip off any casts if they exist.
1028 V1 = V1->stripPointerCasts();
1029 V2 = V2->stripPointerCasts();
1031 // Are we checking for alias of the same value?
1032 if (V1 == V2) return MustAlias;
1034 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1035 return NoAlias; // Scalars cannot alias each other
1037 // Figure out what objects these things are pointing to if we can.
1038 const Value *O1 = V1->getUnderlyingObject();
1039 const Value *O2 = V2->getUnderlyingObject();
1041 // Null values in the default address space don't point to any object, so they
1042 // don't alias any other pointer.
1043 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1044 if (CPN->getType()->getAddressSpace() == 0)
1046 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1047 if (CPN->getType()->getAddressSpace() == 0)
1051 // If V1/V2 point to two different objects we know that we have no alias.
1052 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1055 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1056 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1057 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1060 // Arguments can't alias with local allocations or noalias calls
1061 // in the same function.
1062 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1063 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1066 // Most objects can't alias null.
1067 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1068 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1071 // If one pointer is the result of a call/invoke or load and the other is a
1072 // non-escaping local object within the same function, then we know the
1073 // object couldn't escape to a point where the call could return it.
1075 // Note that if the pointers are in different functions, there are a
1076 // variety of complications. A call with a nocapture argument may still
1077 // temporary store the nocapture argument's value in a temporary memory
1078 // location if that memory location doesn't escape. Or it may pass a
1079 // nocapture value to other functions as long as they don't capture it.
1080 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1082 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1086 // If the size of one access is larger than the entire object on the other
1087 // side, then we know such behavior is undefined and can assume no alias.
1089 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1090 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1093 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1094 // GEP can't simplify, we don't even look at the PHI cases.
1095 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1097 std::swap(V1Size, V2Size);
1100 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1101 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1102 if (Result != MayAlias) return Result;
1105 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1107 std::swap(V1Size, V2Size);
1109 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1110 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1111 V2, V2Size, V2TBAAInfo);
1112 if (Result != MayAlias) return Result;
1115 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1117 std::swap(V1Size, V2Size);
1119 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1120 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1121 V2, V2Size, V2TBAAInfo);
1122 if (Result != MayAlias) return Result;
1125 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1126 Location(V2, V2Size, V2TBAAInfo));
1129 // Make sure that anything that uses AliasAnalysis pulls in this file.
1130 DEFINING_FILE_FOR(BasicAliasAnalysis)