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/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/InstructionSimplify.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/Target/TargetData.h"
33 #include "llvm/ADT/SmallPtrSet.h"
34 #include "llvm/ADT/SmallVector.h"
35 #include "llvm/Support/ErrorHandling.h"
36 #include "llvm/Support/GetElementPtrTypeIterator.h"
40 //===----------------------------------------------------------------------===//
42 //===----------------------------------------------------------------------===//
44 /// isKnownNonNull - Return true if we know that the specified value is never
46 static bool isKnownNonNull(const Value *V) {
47 // Alloca never returns null, malloc might.
48 if (isa<AllocaInst>(V)) return true;
50 // A byval argument is never null.
51 if (const Argument *A = dyn_cast<Argument>(V))
52 return A->hasByValAttr();
54 // Global values are not null unless extern weak.
55 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
56 return !GV->hasExternalWeakLinkage();
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 // Don't bother analyzing arguments already known not to escape.
78 if (A->hasNoCaptureAttr())
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 /// isObjectSmallerThan - Return true if we can prove that the object specified
101 /// by V is smaller than Size.
102 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
103 const TargetData &TD) {
104 const Type *AccessTy;
105 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
106 AccessTy = GV->getType()->getElementType();
107 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
108 if (!AI->isArrayAllocation())
109 AccessTy = AI->getType()->getElementType();
112 } else if (const CallInst* CI = extractMallocCall(V)) {
113 if (!isArrayMalloc(V, &TD))
114 // The size is the argument to the malloc call.
115 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
116 return (C->getZExtValue() < Size);
118 } else if (const Argument *A = dyn_cast<Argument>(V)) {
119 if (A->hasByValAttr())
120 AccessTy = cast<PointerType>(A->getType())->getElementType();
127 if (AccessTy->isSized())
128 return TD.getTypeAllocSize(AccessTy) < Size;
132 //===----------------------------------------------------------------------===//
133 // GetElementPtr Instruction Decomposition and Analysis
134 //===----------------------------------------------------------------------===//
143 struct VariableGEPIndex {
145 ExtensionKind Extension;
151 /// GetLinearExpression - Analyze the specified value as a linear expression:
152 /// "A*V + B", where A and B are constant integers. Return the scale and offset
153 /// values as APInts and return V as a Value*, and return whether we looked
154 /// through any sign or zero extends. The incoming Value is known to have
155 /// IntegerType and it may already be sign or zero extended.
157 /// Note that this looks through extends, so the high bits may not be
158 /// represented in the result.
159 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
160 ExtensionKind &Extension,
161 const TargetData &TD, unsigned Depth) {
162 assert(V->getType()->isIntegerTy() && "Not an integer value");
164 // Limit our recursion depth.
171 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
172 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
173 switch (BOp->getOpcode()) {
175 case Instruction::Or:
176 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
178 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
181 case Instruction::Add:
182 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
184 Offset += RHSC->getValue();
186 case Instruction::Mul:
187 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
189 Offset *= RHSC->getValue();
190 Scale *= RHSC->getValue();
192 case Instruction::Shl:
193 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
195 Offset <<= RHSC->getValue().getLimitedValue();
196 Scale <<= RHSC->getValue().getLimitedValue();
202 // Since GEP indices are sign extended anyway, we don't care about the high
203 // bits of a sign or zero extended value - just scales and offsets. The
204 // extensions have to be consistent though.
205 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
206 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
207 Value *CastOp = cast<CastInst>(V)->getOperand(0);
208 unsigned OldWidth = Scale.getBitWidth();
209 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
210 Scale = Scale.trunc(SmallWidth);
211 Offset = Offset.trunc(SmallWidth);
212 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
214 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
216 Scale = Scale.zext(OldWidth);
217 Offset = Offset.zext(OldWidth);
227 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
228 /// into a base pointer with a constant offset and a number of scaled symbolic
231 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
232 /// the VarIndices vector) are Value*'s that are known to be scaled by the
233 /// specified amount, but which may have other unrepresented high bits. As such,
234 /// the gep cannot necessarily be reconstructed from its decomposed form.
236 /// When TargetData is around, this function is capable of analyzing everything
237 /// that GetUnderlyingObject can look through. When not, it just looks
238 /// through pointer casts.
241 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
242 SmallVectorImpl<VariableGEPIndex> &VarIndices,
243 const TargetData *TD) {
244 // Limit recursion depth to limit compile time in crazy cases.
245 unsigned MaxLookup = 6;
249 // See if this is a bitcast or GEP.
250 const Operator *Op = dyn_cast<Operator>(V);
252 // The only non-operator case we can handle are GlobalAliases.
253 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
254 if (!GA->mayBeOverridden()) {
255 V = GA->getAliasee();
262 if (Op->getOpcode() == Instruction::BitCast) {
263 V = Op->getOperand(0);
267 if (const Instruction *I = dyn_cast<Instruction>(V))
268 // TODO: Get a DominatorTree and use it here.
269 if (const Value *Simplified =
270 SimplifyInstruction(const_cast<Instruction *>(I), TD)) {
275 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
279 // Don't attempt to analyze GEPs over unsized objects.
280 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
281 ->getElementType()->isSized())
284 // If we are lacking TargetData information, we can't compute the offets of
285 // elements computed by GEPs. However, we can handle bitcast equivalent
288 if (!GEPOp->hasAllZeroIndices())
290 V = GEPOp->getOperand(0);
294 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
295 gep_type_iterator GTI = gep_type_begin(GEPOp);
296 for (User::const_op_iterator I = GEPOp->op_begin()+1,
297 E = GEPOp->op_end(); I != E; ++I) {
299 // Compute the (potentially symbolic) offset in bytes for this index.
300 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
301 // For a struct, add the member offset.
302 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
303 if (FieldNo == 0) continue;
305 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
309 // For an array/pointer, add the element offset, explicitly scaled.
310 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
311 if (CIdx->isZero()) continue;
312 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
316 uint64_t Scale = TD->getTypeAllocSize(*GTI);
317 ExtensionKind Extension = EK_NotExtended;
319 // If the integer type is smaller than the pointer size, it is implicitly
320 // sign extended to pointer size.
321 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
322 if (TD->getPointerSizeInBits() > Width)
323 Extension = EK_SignExt;
325 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
326 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
327 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
330 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
331 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
332 BaseOffs += IndexOffset.getSExtValue()*Scale;
333 Scale *= IndexScale.getSExtValue();
336 // If we already had an occurrance of this index variable, merge this
337 // scale into it. For example, we want to handle:
338 // A[x][x] -> x*16 + x*4 -> x*20
339 // This also ensures that 'x' only appears in the index list once.
340 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
341 if (VarIndices[i].V == Index &&
342 VarIndices[i].Extension == Extension) {
343 Scale += VarIndices[i].Scale;
344 VarIndices.erase(VarIndices.begin()+i);
349 // Make sure that we have a scale that makes sense for this target's
351 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
353 Scale = (int64_t)Scale >> ShiftBits;
357 VariableGEPIndex Entry = {Index, Extension, Scale};
358 VarIndices.push_back(Entry);
362 // Analyze the base pointer next.
363 V = GEPOp->getOperand(0);
364 } while (--MaxLookup);
366 // If the chain of expressions is too deep, just return early.
370 /// GetIndexDifference - Dest and Src are the variable indices from two
371 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
372 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
373 /// difference between the two pointers.
374 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
375 const SmallVectorImpl<VariableGEPIndex> &Src) {
376 if (Src.empty()) return;
378 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
379 const Value *V = Src[i].V;
380 ExtensionKind Extension = Src[i].Extension;
381 int64_t Scale = Src[i].Scale;
383 // Find V in Dest. This is N^2, but pointer indices almost never have more
384 // than a few variable indexes.
385 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
386 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
388 // If we found it, subtract off Scale V's from the entry in Dest. If it
389 // goes to zero, remove the entry.
390 if (Dest[j].Scale != Scale)
391 Dest[j].Scale -= Scale;
393 Dest.erase(Dest.begin()+j);
398 // If we didn't consume this entry, add it to the end of the Dest list.
400 VariableGEPIndex Entry = { V, Extension, -Scale };
401 Dest.push_back(Entry);
406 //===----------------------------------------------------------------------===//
407 // BasicAliasAnalysis Pass
408 //===----------------------------------------------------------------------===//
411 static const Function *getParent(const Value *V) {
412 if (const Instruction *inst = dyn_cast<Instruction>(V))
413 return inst->getParent()->getParent();
415 if (const Argument *arg = dyn_cast<Argument>(V))
416 return arg->getParent();
421 static bool notDifferentParent(const Value *O1, const Value *O2) {
423 const Function *F1 = getParent(O1);
424 const Function *F2 = getParent(O2);
426 return !F1 || !F2 || F1 == F2;
431 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
432 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
433 static char ID; // Class identification, replacement for typeinfo
434 BasicAliasAnalysis() : ImmutablePass(ID) {
435 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
438 virtual void initializePass() {
439 InitializeAliasAnalysis(this);
442 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
443 AU.addRequired<AliasAnalysis>();
446 virtual AliasResult alias(const Location &LocA,
447 const Location &LocB) {
448 assert(Visited.empty() && "Visited must be cleared after use!");
449 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
450 "BasicAliasAnalysis doesn't support interprocedural queries.");
451 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
452 LocB.Ptr, LocB.Size, LocB.TBAATag);
457 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
458 const Location &Loc);
460 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
461 ImmutableCallSite CS2) {
462 // The AliasAnalysis base class has some smarts, lets use them.
463 return AliasAnalysis::getModRefInfo(CS1, CS2);
466 /// pointsToConstantMemory - Chase pointers until we find a (constant
468 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
470 /// getModRefBehavior - Return the behavior when calling the given
472 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
474 /// getModRefBehavior - Return the behavior when calling the given function.
475 /// For use when the call site is not known.
476 virtual ModRefBehavior getModRefBehavior(const Function *F);
478 /// getAdjustedAnalysisPointer - This method is used when a pass implements
479 /// an analysis interface through multiple inheritance. If needed, it
480 /// should override this to adjust the this pointer as needed for the
481 /// specified pass info.
482 virtual void *getAdjustedAnalysisPointer(const void *ID) {
483 if (ID == &AliasAnalysis::ID)
484 return (AliasAnalysis*)this;
489 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
490 SmallPtrSet<const Value*, 16> Visited;
492 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
493 // instruction against another.
494 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
495 const Value *V2, uint64_t V2Size,
496 const MDNode *V2TBAAInfo,
497 const Value *UnderlyingV1, const Value *UnderlyingV2);
499 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
500 // instruction against another.
501 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
502 const MDNode *PNTBAAInfo,
503 const Value *V2, uint64_t V2Size,
504 const MDNode *V2TBAAInfo);
506 /// aliasSelect - Disambiguate a Select instruction against another value.
507 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
508 const MDNode *SITBAAInfo,
509 const Value *V2, uint64_t V2Size,
510 const MDNode *V2TBAAInfo);
512 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
513 const MDNode *V1TBAATag,
514 const Value *V2, uint64_t V2Size,
515 const MDNode *V2TBAATag);
517 } // End of anonymous namespace
519 // Register this pass...
520 char BasicAliasAnalysis::ID = 0;
521 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
522 "Basic Alias Analysis (stateless AA impl)",
525 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
526 return new BasicAliasAnalysis();
529 /// pointsToConstantMemory - Returns whether the given pointer value
530 /// points to memory that is local to the function, with global constants being
531 /// considered local to all functions.
533 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
534 assert(Visited.empty() && "Visited must be cleared after use!");
536 unsigned MaxLookup = 8;
537 SmallVector<const Value *, 16> Worklist;
538 Worklist.push_back(Loc.Ptr);
540 const Value *V = GetUnderlyingObject(Worklist.pop_back_val());
541 if (!Visited.insert(V)) {
543 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
546 // An alloca instruction defines local memory.
547 if (OrLocal && isa<AllocaInst>(V))
550 // A global constant counts as local memory for our purposes.
551 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
552 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
553 // global to be marked constant in some modules and non-constant in
554 // others. GV may even be a declaration, not a definition.
555 if (!GV->isConstant()) {
557 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
562 // If both select values point to local memory, then so does the select.
563 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
564 Worklist.push_back(SI->getTrueValue());
565 Worklist.push_back(SI->getFalseValue());
569 // If all values incoming to a phi node point to local memory, then so does
571 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
572 // Don't bother inspecting phi nodes with many operands.
573 if (PN->getNumIncomingValues() > MaxLookup) {
575 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
577 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
578 Worklist.push_back(PN->getIncomingValue(i));
582 // Otherwise be conservative.
584 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
586 } while (!Worklist.empty() && --MaxLookup);
589 return Worklist.empty();
592 /// getModRefBehavior - Return the behavior when calling the given call site.
593 AliasAnalysis::ModRefBehavior
594 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
595 if (CS.doesNotAccessMemory())
596 // Can't do better than this.
597 return DoesNotAccessMemory;
599 ModRefBehavior Min = UnknownModRefBehavior;
601 // If the callsite knows it only reads memory, don't return worse
603 if (CS.onlyReadsMemory())
604 Min = OnlyReadsMemory;
606 // The AliasAnalysis base class has some smarts, lets use them.
607 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
610 /// getModRefBehavior - Return the behavior when calling the given function.
611 /// For use when the call site is not known.
612 AliasAnalysis::ModRefBehavior
613 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
614 // If the function declares it doesn't access memory, we can't do better.
615 if (F->doesNotAccessMemory())
616 return DoesNotAccessMemory;
618 // For intrinsics, we can check the table.
619 if (unsigned iid = F->getIntrinsicID()) {
620 #define GET_INTRINSIC_MODREF_BEHAVIOR
621 #include "llvm/Intrinsics.gen"
622 #undef GET_INTRINSIC_MODREF_BEHAVIOR
625 ModRefBehavior Min = UnknownModRefBehavior;
627 // If the function declares it only reads memory, go with that.
628 if (F->onlyReadsMemory())
629 Min = OnlyReadsMemory;
631 // Otherwise be conservative.
632 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
635 /// getModRefInfo - Check to see if the specified callsite can clobber the
636 /// specified memory object. Since we only look at local properties of this
637 /// function, we really can't say much about this query. We do, however, use
638 /// simple "address taken" analysis on local objects.
639 AliasAnalysis::ModRefResult
640 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
641 const Location &Loc) {
642 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
643 "AliasAnalysis query involving multiple functions!");
645 const Value *Object = GetUnderlyingObject(Loc.Ptr);
647 // If this is a tail call and Loc.Ptr points to a stack location, we know that
648 // the tail call cannot access or modify the local stack.
649 // We cannot exclude byval arguments here; these belong to the caller of
650 // the current function not to the current function, and a tail callee
651 // may reference them.
652 if (isa<AllocaInst>(Object))
653 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
654 if (CI->isTailCall())
657 // If the pointer is to a locally allocated object that does not escape,
658 // then the call can not mod/ref the pointer unless the call takes the pointer
659 // as an argument, and itself doesn't capture it.
660 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
661 isNonEscapingLocalObject(Object)) {
662 bool PassedAsArg = false;
664 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
665 CI != CE; ++CI, ++ArgNo) {
666 // Only look at the no-capture pointer arguments.
667 if (!(*CI)->getType()->isPointerTy() ||
668 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
671 // If this is a no-capture pointer argument, see if we can tell that it
672 // is impossible to alias the pointer we're checking. If not, we have to
673 // assume that the call could touch the pointer, even though it doesn't
675 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
685 ModRefResult Min = ModRef;
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 it can't overlap the source dest, then it doesn't modref the loc.
700 if (isNoAlias(Location(Dest, Len), Loc)) {
701 if (isNoAlias(Location(Src, Len), Loc))
703 // If it can't overlap the dest, then worst case it reads the loc.
705 } else if (isNoAlias(Location(Src, Len), Loc)) {
706 // If it can't overlap the source, then worst case it mutates the loc.
711 case Intrinsic::memset:
712 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
713 // will handle it for the variable length case.
714 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
715 uint64_t Len = LenCI->getZExtValue();
716 Value *Dest = II->getArgOperand(0);
717 if (isNoAlias(Location(Dest, Len), Loc))
720 // We know that memset doesn't load anything.
723 case Intrinsic::atomic_cmp_swap:
724 case Intrinsic::atomic_swap:
725 case Intrinsic::atomic_load_add:
726 case Intrinsic::atomic_load_sub:
727 case Intrinsic::atomic_load_and:
728 case Intrinsic::atomic_load_nand:
729 case Intrinsic::atomic_load_or:
730 case Intrinsic::atomic_load_xor:
731 case Intrinsic::atomic_load_max:
732 case Intrinsic::atomic_load_min:
733 case Intrinsic::atomic_load_umax:
734 case Intrinsic::atomic_load_umin:
736 Value *Op1 = II->getArgOperand(0);
737 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType());
738 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
739 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
743 case Intrinsic::lifetime_start:
744 case Intrinsic::lifetime_end:
745 case Intrinsic::invariant_start: {
747 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
748 if (isNoAlias(Location(II->getArgOperand(1),
750 II->getMetadata(LLVMContext::MD_tbaa)),
755 case Intrinsic::invariant_end: {
757 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
758 if (isNoAlias(Location(II->getArgOperand(2),
760 II->getMetadata(LLVMContext::MD_tbaa)),
767 // The AliasAnalysis base class has some smarts, lets use them.
768 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
771 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
772 /// against another pointer. We know that V1 is a GEP, but we don't know
773 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1),
774 /// UnderlyingV2 is the same for V2.
776 AliasAnalysis::AliasResult
777 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
778 const Value *V2, uint64_t V2Size,
779 const MDNode *V2TBAAInfo,
780 const Value *UnderlyingV1,
781 const Value *UnderlyingV2) {
782 // If this GEP has been visited before, we're on a use-def cycle.
783 // Such cycles are only valid when PHI nodes are involved or in unreachable
784 // code. The visitPHI function catches cycles containing PHIs, but there
785 // could still be a cycle without PHIs in unreachable code.
786 if (!Visited.insert(GEP1))
789 int64_t GEP1BaseOffset;
790 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
792 // If we have two gep instructions with must-alias'ing base pointers, figure
793 // out if the indexes to the GEP tell us anything about the derived pointer.
794 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
795 // Do the base pointers alias?
796 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
797 UnderlyingV2, UnknownSize, 0);
799 // If we get a No or May, then return it immediately, no amount of analysis
800 // will improve this situation.
801 if (BaseAlias != MustAlias) return BaseAlias;
803 // Otherwise, we have a MustAlias. Since the base pointers alias each other
804 // exactly, see if the computed offset from the common pointer tells us
805 // about the relation of the resulting pointer.
806 const Value *GEP1BasePtr =
807 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
809 int64_t GEP2BaseOffset;
810 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
811 const Value *GEP2BasePtr =
812 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
814 // If DecomposeGEPExpression isn't able to look all the way through the
815 // addressing operation, we must not have TD and this is too complex for us
816 // to handle without it.
817 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
819 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
823 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
824 // symbolic difference.
825 GEP1BaseOffset -= GEP2BaseOffset;
826 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
829 // Check to see if these two pointers are related by the getelementptr
830 // instruction. If one pointer is a GEP with a non-zero index of the other
831 // pointer, we know they cannot alias.
833 // If both accesses are unknown size, we can't do anything useful here.
834 if (V1Size == UnknownSize && V2Size == UnknownSize)
837 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
838 V2, V2Size, V2TBAAInfo);
840 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
841 // If V2 is known not to alias GEP base pointer, then the two values
842 // cannot alias per GEP semantics: "A pointer value formed from a
843 // getelementptr instruction is associated with the addresses associated
844 // with the first operand of the getelementptr".
847 const Value *GEP1BasePtr =
848 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
850 // If DecomposeGEPExpression isn't able to look all the way through the
851 // addressing operation, we must not have TD and this is too complex for us
852 // to handle without it.
853 if (GEP1BasePtr != UnderlyingV1) {
855 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
860 // In the two GEP Case, if there is no difference in the offsets of the
861 // computed pointers, the resultant pointers are a must alias. This
862 // hapens when we have two lexically identical GEP's (for example).
864 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
865 // must aliases the GEP, the end result is a must alias also.
866 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
869 // If there is a difference betwen the pointers, but the difference is
870 // less than the size of the associated memory object, then we know
871 // that the objects are partially overlapping.
872 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
873 if (GEP1BaseOffset >= 0 ?
874 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) :
875 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size &&
876 GEP1BaseOffset != INT64_MIN))
880 // If we have a known constant offset, see if this offset is larger than the
881 // access size being queried. If so, and if no variable indices can remove
882 // pieces of this constant, then we know we have a no-alias. For example,
885 // In order to handle cases like &A[100][i] where i is an out of range
886 // subscript, we have to ignore all constant offset pieces that are a multiple
887 // of a scaled index. Do this by removing constant offsets that are a
888 // multiple of any of our variable indices. This allows us to transform
889 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
890 // provides an offset of 4 bytes (assuming a <= 4 byte access).
891 for (unsigned i = 0, e = GEP1VariableIndices.size();
892 i != e && GEP1BaseOffset;++i)
893 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
894 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
896 // If our known offset is bigger than the access size, we know we don't have
898 if (GEP1BaseOffset) {
899 if (GEP1BaseOffset >= 0 ?
900 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
901 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
902 GEP1BaseOffset != INT64_MIN))
909 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
910 /// instruction against another.
911 AliasAnalysis::AliasResult
912 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
913 const MDNode *SITBAAInfo,
914 const Value *V2, uint64_t V2Size,
915 const MDNode *V2TBAAInfo) {
916 // If this select has been visited before, we're on a use-def cycle.
917 // Such cycles are only valid when PHI nodes are involved or in unreachable
918 // code. The visitPHI function catches cycles containing PHIs, but there
919 // could still be a cycle without PHIs in unreachable code.
920 if (!Visited.insert(SI))
923 // If the values are Selects with the same condition, we can do a more precise
924 // check: just check for aliases between the values on corresponding arms.
925 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
926 if (SI->getCondition() == SI2->getCondition()) {
928 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
929 SI2->getTrueValue(), V2Size, V2TBAAInfo);
930 if (Alias == MayAlias)
932 AliasResult ThisAlias =
933 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
934 SI2->getFalseValue(), V2Size, V2TBAAInfo);
935 if (ThisAlias != Alias)
940 // If both arms of the Select node NoAlias or MustAlias V2, then returns
941 // NoAlias / MustAlias. Otherwise, returns MayAlias.
943 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
944 if (Alias == MayAlias)
947 // If V2 is visited, the recursive case will have been caught in the
948 // above aliasCheck call, so these subsequent calls to aliasCheck
949 // don't need to assume that V2 is being visited recursively.
952 AliasResult ThisAlias =
953 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
954 if (ThisAlias != Alias)
959 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
961 AliasAnalysis::AliasResult
962 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
963 const MDNode *PNTBAAInfo,
964 const Value *V2, uint64_t V2Size,
965 const MDNode *V2TBAAInfo) {
966 // The PHI node has already been visited, avoid recursion any further.
967 if (!Visited.insert(PN))
970 // If the values are PHIs in the same block, we can do a more precise
971 // as well as efficient check: just check for aliases between the values
972 // on corresponding edges.
973 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
974 if (PN2->getParent() == PN->getParent()) {
976 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
977 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
979 if (Alias == MayAlias)
981 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
982 AliasResult ThisAlias =
983 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
984 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
986 if (ThisAlias != Alias)
992 SmallPtrSet<Value*, 4> UniqueSrc;
993 SmallVector<Value*, 4> V1Srcs;
994 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
995 Value *PV1 = PN->getIncomingValue(i);
996 if (isa<PHINode>(PV1))
997 // If any of the source itself is a PHI, return MayAlias conservatively
998 // to avoid compile time explosion. The worst possible case is if both
999 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1000 // and 'n' are the number of PHI sources.
1002 if (UniqueSrc.insert(PV1))
1003 V1Srcs.push_back(PV1);
1006 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1007 V1Srcs[0], PNSize, PNTBAAInfo);
1008 // Early exit if the check of the first PHI source against V2 is MayAlias.
1009 // Other results are not possible.
1010 if (Alias == MayAlias)
1013 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1014 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1015 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1016 Value *V = V1Srcs[i];
1018 // If V2 is visited, the recursive case will have been caught in the
1019 // above aliasCheck call, so these subsequent calls to aliasCheck
1020 // don't need to assume that V2 is being visited recursively.
1023 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1024 V, PNSize, PNTBAAInfo);
1025 if (ThisAlias != Alias || ThisAlias == MayAlias)
1032 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1033 // such as array references.
1035 AliasAnalysis::AliasResult
1036 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1037 const MDNode *V1TBAAInfo,
1038 const Value *V2, uint64_t V2Size,
1039 const MDNode *V2TBAAInfo) {
1040 // If either of the memory references is empty, it doesn't matter what the
1041 // pointer values are.
1042 if (V1Size == 0 || V2Size == 0)
1045 // Strip off any casts if they exist.
1046 V1 = V1->stripPointerCasts();
1047 V2 = V2->stripPointerCasts();
1049 // Are we checking for alias of the same value?
1050 if (V1 == V2) return MustAlias;
1052 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1053 return NoAlias; // Scalars cannot alias each other
1055 // Figure out what objects these things are pointing to if we can.
1056 const Value *O1 = GetUnderlyingObject(V1);
1057 const Value *O2 = GetUnderlyingObject(V2);
1059 // Null values in the default address space don't point to any object, so they
1060 // don't alias any other pointer.
1061 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1062 if (CPN->getType()->getAddressSpace() == 0)
1064 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1065 if (CPN->getType()->getAddressSpace() == 0)
1069 // If V1/V2 point to two different objects we know that we have no alias.
1070 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1073 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1074 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1075 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1078 // Arguments can't alias with local allocations or noalias calls
1079 // in the same function.
1080 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1081 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1084 // Most objects can't alias null.
1085 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1086 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1089 // If one pointer is the result of a call/invoke or load and the other is a
1090 // non-escaping local object within the same function, then we know the
1091 // object couldn't escape to a point where the call could return it.
1093 // Note that if the pointers are in different functions, there are a
1094 // variety of complications. A call with a nocapture argument may still
1095 // temporary store the nocapture argument's value in a temporary memory
1096 // location if that memory location doesn't escape. Or it may pass a
1097 // nocapture value to other functions as long as they don't capture it.
1098 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1100 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1104 // If the size of one access is larger than the entire object on the other
1105 // side, then we know such behavior is undefined and can assume no alias.
1107 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1108 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1111 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1112 // GEP can't simplify, we don't even look at the PHI cases.
1113 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1115 std::swap(V1Size, V2Size);
1118 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1119 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1120 if (Result != MayAlias) return Result;
1123 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1125 std::swap(V1Size, V2Size);
1127 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1128 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1129 V2, V2Size, V2TBAAInfo);
1130 if (Result != MayAlias) return Result;
1133 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1135 std::swap(V1Size, V2Size);
1137 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1138 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1139 V2, V2Size, V2TBAAInfo);
1140 if (Result != MayAlias) return Result;
1143 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1144 Location(V2, V2Size, V2TBAAInfo));