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 /// getObjectSize - Return the size of the object specified by V, or
101 /// UnknownSize if unknown.
102 static uint64_t getObjectSize(const Value *V, const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 if (!GV->hasDefinitiveInitializer())
106 return AliasAnalysis::UnknownSize;
107 AccessTy = GV->getType()->getElementType();
108 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
109 if (!AI->isArrayAllocation())
110 AccessTy = AI->getType()->getElementType();
112 return AliasAnalysis::UnknownSize;
113 } else if (const CallInst* CI = extractMallocCall(V)) {
114 if (!isArrayMalloc(V, &TD))
115 // The size is the argument to the malloc call.
116 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
117 return C->getZExtValue();
118 return AliasAnalysis::UnknownSize;
119 } else if (const Argument *A = dyn_cast<Argument>(V)) {
120 if (A->hasByValAttr())
121 AccessTy = cast<PointerType>(A->getType())->getElementType();
123 return AliasAnalysis::UnknownSize;
125 return AliasAnalysis::UnknownSize;
128 if (AccessTy->isSized())
129 return TD.getTypeAllocSize(AccessTy);
130 return AliasAnalysis::UnknownSize;
133 /// isObjectSmallerThan - Return true if we can prove that the object specified
134 /// by V is smaller than Size.
135 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
136 const TargetData &TD) {
137 uint64_t ObjectSize = getObjectSize(V, TD);
138 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
141 /// isObjectSize - Return true if we can prove that the object specified
142 /// by V has size Size.
143 static bool isObjectSize(const Value *V, uint64_t Size,
144 const TargetData &TD) {
145 uint64_t ObjectSize = getObjectSize(V, TD);
146 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
149 //===----------------------------------------------------------------------===//
150 // GetElementPtr Instruction Decomposition and Analysis
151 //===----------------------------------------------------------------------===//
160 struct VariableGEPIndex {
162 ExtensionKind Extension;
168 /// GetLinearExpression - Analyze the specified value as a linear expression:
169 /// "A*V + B", where A and B are constant integers. Return the scale and offset
170 /// values as APInts and return V as a Value*, and return whether we looked
171 /// through any sign or zero extends. The incoming Value is known to have
172 /// IntegerType and it may already be sign or zero extended.
174 /// Note that this looks through extends, so the high bits may not be
175 /// represented in the result.
176 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
177 ExtensionKind &Extension,
178 const TargetData &TD, unsigned Depth) {
179 assert(V->getType()->isIntegerTy() && "Not an integer value");
181 // Limit our recursion depth.
188 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
189 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
190 switch (BOp->getOpcode()) {
192 case Instruction::Or:
193 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
195 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
198 case Instruction::Add:
199 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
201 Offset += RHSC->getValue();
203 case Instruction::Mul:
204 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
206 Offset *= RHSC->getValue();
207 Scale *= RHSC->getValue();
209 case Instruction::Shl:
210 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
212 Offset <<= RHSC->getValue().getLimitedValue();
213 Scale <<= RHSC->getValue().getLimitedValue();
219 // Since GEP indices are sign extended anyway, we don't care about the high
220 // bits of a sign or zero extended value - just scales and offsets. The
221 // extensions have to be consistent though.
222 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
223 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
224 Value *CastOp = cast<CastInst>(V)->getOperand(0);
225 unsigned OldWidth = Scale.getBitWidth();
226 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
227 Scale = Scale.trunc(SmallWidth);
228 Offset = Offset.trunc(SmallWidth);
229 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
231 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
233 Scale = Scale.zext(OldWidth);
234 Offset = Offset.zext(OldWidth);
244 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
245 /// into a base pointer with a constant offset and a number of scaled symbolic
248 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
249 /// the VarIndices vector) are Value*'s that are known to be scaled by the
250 /// specified amount, but which may have other unrepresented high bits. As such,
251 /// the gep cannot necessarily be reconstructed from its decomposed form.
253 /// When TargetData is around, this function is capable of analyzing everything
254 /// that GetUnderlyingObject can look through. When not, it just looks
255 /// through pointer casts.
258 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
259 SmallVectorImpl<VariableGEPIndex> &VarIndices,
260 const TargetData *TD) {
261 // Limit recursion depth to limit compile time in crazy cases.
262 unsigned MaxLookup = 6;
266 // See if this is a bitcast or GEP.
267 const Operator *Op = dyn_cast<Operator>(V);
269 // The only non-operator case we can handle are GlobalAliases.
270 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
271 if (!GA->mayBeOverridden()) {
272 V = GA->getAliasee();
279 if (Op->getOpcode() == Instruction::BitCast) {
280 V = Op->getOperand(0);
284 if (const Instruction *I = dyn_cast<Instruction>(V))
285 // TODO: Get a DominatorTree and use it here.
286 if (const Value *Simplified =
287 SimplifyInstruction(const_cast<Instruction *>(I), TD)) {
292 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
296 // Don't attempt to analyze GEPs over unsized objects.
297 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
298 ->getElementType()->isSized())
301 // If we are lacking TargetData information, we can't compute the offets of
302 // elements computed by GEPs. However, we can handle bitcast equivalent
305 if (!GEPOp->hasAllZeroIndices())
307 V = GEPOp->getOperand(0);
311 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
312 gep_type_iterator GTI = gep_type_begin(GEPOp);
313 for (User::const_op_iterator I = GEPOp->op_begin()+1,
314 E = GEPOp->op_end(); I != E; ++I) {
316 // Compute the (potentially symbolic) offset in bytes for this index.
317 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
318 // For a struct, add the member offset.
319 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
320 if (FieldNo == 0) continue;
322 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
326 // For an array/pointer, add the element offset, explicitly scaled.
327 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
328 if (CIdx->isZero()) continue;
329 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
333 uint64_t Scale = TD->getTypeAllocSize(*GTI);
334 ExtensionKind Extension = EK_NotExtended;
336 // If the integer type is smaller than the pointer size, it is implicitly
337 // sign extended to pointer size.
338 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
339 if (TD->getPointerSizeInBits() > Width)
340 Extension = EK_SignExt;
342 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
343 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
344 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
347 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
348 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
349 BaseOffs += IndexOffset.getSExtValue()*Scale;
350 Scale *= IndexScale.getSExtValue();
353 // If we already had an occurrance of this index variable, merge this
354 // scale into it. For example, we want to handle:
355 // A[x][x] -> x*16 + x*4 -> x*20
356 // This also ensures that 'x' only appears in the index list once.
357 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
358 if (VarIndices[i].V == Index &&
359 VarIndices[i].Extension == Extension) {
360 Scale += VarIndices[i].Scale;
361 VarIndices.erase(VarIndices.begin()+i);
366 // Make sure that we have a scale that makes sense for this target's
368 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
370 Scale = (int64_t)Scale >> ShiftBits;
374 VariableGEPIndex Entry = {Index, Extension, Scale};
375 VarIndices.push_back(Entry);
379 // Analyze the base pointer next.
380 V = GEPOp->getOperand(0);
381 } while (--MaxLookup);
383 // If the chain of expressions is too deep, just return early.
387 /// GetIndexDifference - Dest and Src are the variable indices from two
388 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
389 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
390 /// difference between the two pointers.
391 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
392 const SmallVectorImpl<VariableGEPIndex> &Src) {
393 if (Src.empty()) return;
395 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
396 const Value *V = Src[i].V;
397 ExtensionKind Extension = Src[i].Extension;
398 int64_t Scale = Src[i].Scale;
400 // Find V in Dest. This is N^2, but pointer indices almost never have more
401 // than a few variable indexes.
402 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
403 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
405 // If we found it, subtract off Scale V's from the entry in Dest. If it
406 // goes to zero, remove the entry.
407 if (Dest[j].Scale != Scale)
408 Dest[j].Scale -= Scale;
410 Dest.erase(Dest.begin()+j);
415 // If we didn't consume this entry, add it to the end of the Dest list.
417 VariableGEPIndex Entry = { V, Extension, -Scale };
418 Dest.push_back(Entry);
423 //===----------------------------------------------------------------------===//
424 // BasicAliasAnalysis Pass
425 //===----------------------------------------------------------------------===//
428 static const Function *getParent(const Value *V) {
429 if (const Instruction *inst = dyn_cast<Instruction>(V))
430 return inst->getParent()->getParent();
432 if (const Argument *arg = dyn_cast<Argument>(V))
433 return arg->getParent();
438 static bool notDifferentParent(const Value *O1, const Value *O2) {
440 const Function *F1 = getParent(O1);
441 const Function *F2 = getParent(O2);
443 return !F1 || !F2 || F1 == F2;
448 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
449 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
450 static char ID; // Class identification, replacement for typeinfo
451 BasicAliasAnalysis() : ImmutablePass(ID) {
452 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
455 virtual void initializePass() {
456 InitializeAliasAnalysis(this);
459 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
460 AU.addRequired<AliasAnalysis>();
463 virtual AliasResult alias(const Location &LocA,
464 const Location &LocB) {
465 assert(Visited.empty() && "Visited must be cleared after use!");
466 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
467 "BasicAliasAnalysis doesn't support interprocedural queries.");
468 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
469 LocB.Ptr, LocB.Size, LocB.TBAATag);
474 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
475 const Location &Loc);
477 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
478 ImmutableCallSite CS2) {
479 // The AliasAnalysis base class has some smarts, lets use them.
480 return AliasAnalysis::getModRefInfo(CS1, CS2);
483 /// pointsToConstantMemory - Chase pointers until we find a (constant
485 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
487 /// getModRefBehavior - Return the behavior when calling the given
489 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
491 /// getModRefBehavior - Return the behavior when calling the given function.
492 /// For use when the call site is not known.
493 virtual ModRefBehavior getModRefBehavior(const Function *F);
495 /// getAdjustedAnalysisPointer - This method is used when a pass implements
496 /// an analysis interface through multiple inheritance. If needed, it
497 /// should override this to adjust the this pointer as needed for the
498 /// specified pass info.
499 virtual void *getAdjustedAnalysisPointer(const void *ID) {
500 if (ID == &AliasAnalysis::ID)
501 return (AliasAnalysis*)this;
506 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
507 SmallPtrSet<const Value*, 16> Visited;
509 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
510 // instruction against another.
511 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
512 const Value *V2, uint64_t V2Size,
513 const MDNode *V2TBAAInfo,
514 const Value *UnderlyingV1, const Value *UnderlyingV2);
516 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
517 // instruction against another.
518 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
519 const MDNode *PNTBAAInfo,
520 const Value *V2, uint64_t V2Size,
521 const MDNode *V2TBAAInfo);
523 /// aliasSelect - Disambiguate a Select instruction against another value.
524 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
525 const MDNode *SITBAAInfo,
526 const Value *V2, uint64_t V2Size,
527 const MDNode *V2TBAAInfo);
529 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
530 const MDNode *V1TBAATag,
531 const Value *V2, uint64_t V2Size,
532 const MDNode *V2TBAATag);
534 } // End of anonymous namespace
536 // Register this pass...
537 char BasicAliasAnalysis::ID = 0;
538 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
539 "Basic Alias Analysis (stateless AA impl)",
542 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
543 return new BasicAliasAnalysis();
546 /// pointsToConstantMemory - Returns whether the given pointer value
547 /// points to memory that is local to the function, with global constants being
548 /// considered local to all functions.
550 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
551 assert(Visited.empty() && "Visited must be cleared after use!");
553 unsigned MaxLookup = 8;
554 SmallVector<const Value *, 16> Worklist;
555 Worklist.push_back(Loc.Ptr);
557 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), TD);
558 if (!Visited.insert(V)) {
560 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
563 // An alloca instruction defines local memory.
564 if (OrLocal && isa<AllocaInst>(V))
567 // A global constant counts as local memory for our purposes.
568 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
569 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
570 // global to be marked constant in some modules and non-constant in
571 // others. GV may even be a declaration, not a definition.
572 if (!GV->isConstant()) {
574 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
579 // If both select values point to local memory, then so does the select.
580 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
581 Worklist.push_back(SI->getTrueValue());
582 Worklist.push_back(SI->getFalseValue());
586 // If all values incoming to a phi node point to local memory, then so does
588 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
589 // Don't bother inspecting phi nodes with many operands.
590 if (PN->getNumIncomingValues() > MaxLookup) {
592 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
594 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
595 Worklist.push_back(PN->getIncomingValue(i));
599 // Otherwise be conservative.
601 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
603 } while (!Worklist.empty() && --MaxLookup);
606 return Worklist.empty();
609 /// getModRefBehavior - Return the behavior when calling the given call site.
610 AliasAnalysis::ModRefBehavior
611 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
612 if (CS.doesNotAccessMemory())
613 // Can't do better than this.
614 return DoesNotAccessMemory;
616 ModRefBehavior Min = UnknownModRefBehavior;
618 // If the callsite knows it only reads memory, don't return worse
620 if (CS.onlyReadsMemory())
621 Min = OnlyReadsMemory;
623 // The AliasAnalysis base class has some smarts, lets use them.
624 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
627 /// getModRefBehavior - Return the behavior when calling the given function.
628 /// For use when the call site is not known.
629 AliasAnalysis::ModRefBehavior
630 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
631 // If the function declares it doesn't access memory, we can't do better.
632 if (F->doesNotAccessMemory())
633 return DoesNotAccessMemory;
635 // For intrinsics, we can check the table.
636 if (unsigned iid = F->getIntrinsicID()) {
637 #define GET_INTRINSIC_MODREF_BEHAVIOR
638 #include "llvm/Intrinsics.gen"
639 #undef GET_INTRINSIC_MODREF_BEHAVIOR
642 ModRefBehavior Min = UnknownModRefBehavior;
644 // If the function declares it only reads memory, go with that.
645 if (F->onlyReadsMemory())
646 Min = OnlyReadsMemory;
648 // Otherwise be conservative.
649 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
652 /// getModRefInfo - Check to see if the specified callsite can clobber the
653 /// specified memory object. Since we only look at local properties of this
654 /// function, we really can't say much about this query. We do, however, use
655 /// simple "address taken" analysis on local objects.
656 AliasAnalysis::ModRefResult
657 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
658 const Location &Loc) {
659 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
660 "AliasAnalysis query involving multiple functions!");
662 const Value *Object = GetUnderlyingObject(Loc.Ptr, TD);
664 // If this is a tail call and Loc.Ptr points to a stack location, we know that
665 // the tail call cannot access or modify the local stack.
666 // We cannot exclude byval arguments here; these belong to the caller of
667 // the current function not to the current function, and a tail callee
668 // may reference them.
669 if (isa<AllocaInst>(Object))
670 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
671 if (CI->isTailCall())
674 // If the pointer is to a locally allocated object that does not escape,
675 // then the call can not mod/ref the pointer unless the call takes the pointer
676 // as an argument, and itself doesn't capture it.
677 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
678 isNonEscapingLocalObject(Object)) {
679 bool PassedAsArg = false;
681 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
682 CI != CE; ++CI, ++ArgNo) {
683 // Only look at the no-capture pointer arguments.
684 if (!(*CI)->getType()->isPointerTy() ||
685 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
688 // If this is a no-capture pointer argument, see if we can tell that it
689 // is impossible to alias the pointer we're checking. If not, we have to
690 // assume that the call could touch the pointer, even though it doesn't
692 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
702 ModRefResult Min = ModRef;
704 // Finally, handle specific knowledge of intrinsics.
705 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
707 switch (II->getIntrinsicID()) {
709 case Intrinsic::memcpy:
710 case Intrinsic::memmove: {
711 uint64_t Len = UnknownSize;
712 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
713 Len = LenCI->getZExtValue();
714 Value *Dest = II->getArgOperand(0);
715 Value *Src = II->getArgOperand(1);
716 // If it can't overlap the source dest, then it doesn't modref the loc.
717 if (isNoAlias(Location(Dest, Len), Loc)) {
718 if (isNoAlias(Location(Src, Len), Loc))
720 // If it can't overlap the dest, then worst case it reads the loc.
722 } else if (isNoAlias(Location(Src, Len), Loc)) {
723 // If it can't overlap the source, then worst case it mutates the loc.
728 case Intrinsic::memset:
729 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
730 // will handle it for the variable length case.
731 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
732 uint64_t Len = LenCI->getZExtValue();
733 Value *Dest = II->getArgOperand(0);
734 if (isNoAlias(Location(Dest, Len), Loc))
737 // We know that memset doesn't load anything.
740 case Intrinsic::atomic_cmp_swap:
741 case Intrinsic::atomic_swap:
742 case Intrinsic::atomic_load_add:
743 case Intrinsic::atomic_load_sub:
744 case Intrinsic::atomic_load_and:
745 case Intrinsic::atomic_load_nand:
746 case Intrinsic::atomic_load_or:
747 case Intrinsic::atomic_load_xor:
748 case Intrinsic::atomic_load_max:
749 case Intrinsic::atomic_load_min:
750 case Intrinsic::atomic_load_umax:
751 case Intrinsic::atomic_load_umin:
753 Value *Op1 = II->getArgOperand(0);
754 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType());
755 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
756 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
760 case Intrinsic::lifetime_start:
761 case Intrinsic::lifetime_end:
762 case Intrinsic::invariant_start: {
764 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
765 if (isNoAlias(Location(II->getArgOperand(1),
767 II->getMetadata(LLVMContext::MD_tbaa)),
772 case Intrinsic::invariant_end: {
774 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
775 if (isNoAlias(Location(II->getArgOperand(2),
777 II->getMetadata(LLVMContext::MD_tbaa)),
784 // The AliasAnalysis base class has some smarts, lets use them.
785 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
788 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
789 /// against another pointer. We know that V1 is a GEP, but we don't know
790 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, TD),
791 /// UnderlyingV2 is the same for V2.
793 AliasAnalysis::AliasResult
794 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
795 const Value *V2, uint64_t V2Size,
796 const MDNode *V2TBAAInfo,
797 const Value *UnderlyingV1,
798 const Value *UnderlyingV2) {
799 // If this GEP has been visited before, we're on a use-def cycle.
800 // Such cycles are only valid when PHI nodes are involved or in unreachable
801 // code. The visitPHI function catches cycles containing PHIs, but there
802 // could still be a cycle without PHIs in unreachable code.
803 if (!Visited.insert(GEP1))
806 int64_t GEP1BaseOffset;
807 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
809 // If we have two gep instructions with must-alias'ing base pointers, figure
810 // out if the indexes to the GEP tell us anything about the derived pointer.
811 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
812 // Do the base pointers alias?
813 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
814 UnderlyingV2, UnknownSize, 0);
816 // If we get a No or May, then return it immediately, no amount of analysis
817 // will improve this situation.
818 if (BaseAlias != MustAlias) return BaseAlias;
820 // Otherwise, we have a MustAlias. Since the base pointers alias each other
821 // exactly, see if the computed offset from the common pointer tells us
822 // about the relation of the resulting pointer.
823 const Value *GEP1BasePtr =
824 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
826 int64_t GEP2BaseOffset;
827 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
828 const Value *GEP2BasePtr =
829 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
831 // If DecomposeGEPExpression isn't able to look all the way through the
832 // addressing operation, we must not have TD and this is too complex for us
833 // to handle without it.
834 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
836 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
840 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
841 // symbolic difference.
842 GEP1BaseOffset -= GEP2BaseOffset;
843 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
846 // Check to see if these two pointers are related by the getelementptr
847 // instruction. If one pointer is a GEP with a non-zero index of the other
848 // pointer, we know they cannot alias.
850 // If both accesses are unknown size, we can't do anything useful here.
851 if (V1Size == UnknownSize && V2Size == UnknownSize)
854 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
855 V2, V2Size, V2TBAAInfo);
857 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
858 // If V2 is known not to alias GEP base pointer, then the two values
859 // cannot alias per GEP semantics: "A pointer value formed from a
860 // getelementptr instruction is associated with the addresses associated
861 // with the first operand of the getelementptr".
864 const Value *GEP1BasePtr =
865 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
867 // If DecomposeGEPExpression isn't able to look all the way through the
868 // addressing operation, we must not have TD and this is too complex for us
869 // to handle without it.
870 if (GEP1BasePtr != UnderlyingV1) {
872 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
877 // In the two GEP Case, if there is no difference in the offsets of the
878 // computed pointers, the resultant pointers are a must alias. This
879 // hapens when we have two lexically identical GEP's (for example).
881 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
882 // must aliases the GEP, the end result is a must alias also.
883 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
886 // If there is a difference betwen the pointers, but the difference is
887 // less than the size of the associated memory object, then we know
888 // that the objects are partially overlapping.
889 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
890 if (GEP1BaseOffset >= 0 ?
891 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) :
892 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size &&
893 GEP1BaseOffset != INT64_MIN))
897 // If we have a known constant offset, see if this offset is larger than the
898 // access size being queried. If so, and if no variable indices can remove
899 // pieces of this constant, then we know we have a no-alias. For example,
902 // In order to handle cases like &A[100][i] where i is an out of range
903 // subscript, we have to ignore all constant offset pieces that are a multiple
904 // of a scaled index. Do this by removing constant offsets that are a
905 // multiple of any of our variable indices. This allows us to transform
906 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
907 // provides an offset of 4 bytes (assuming a <= 4 byte access).
908 for (unsigned i = 0, e = GEP1VariableIndices.size();
909 i != e && GEP1BaseOffset;++i)
910 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
911 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
913 // If our known offset is bigger than the access size, we know we don't have
915 if (GEP1BaseOffset) {
916 if (GEP1BaseOffset >= 0 ?
917 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
918 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
919 GEP1BaseOffset != INT64_MIN))
926 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
927 /// instruction against another.
928 AliasAnalysis::AliasResult
929 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
930 const MDNode *SITBAAInfo,
931 const Value *V2, uint64_t V2Size,
932 const MDNode *V2TBAAInfo) {
933 // If this select has been visited before, we're on a use-def cycle.
934 // Such cycles are only valid when PHI nodes are involved or in unreachable
935 // code. The visitPHI function catches cycles containing PHIs, but there
936 // could still be a cycle without PHIs in unreachable code.
937 if (!Visited.insert(SI))
940 // If the values are Selects with the same condition, we can do a more precise
941 // check: just check for aliases between the values on corresponding arms.
942 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
943 if (SI->getCondition() == SI2->getCondition()) {
945 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
946 SI2->getTrueValue(), V2Size, V2TBAAInfo);
947 if (Alias == MayAlias)
949 AliasResult ThisAlias =
950 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
951 SI2->getFalseValue(), V2Size, V2TBAAInfo);
952 if (ThisAlias != Alias)
957 // If both arms of the Select node NoAlias or MustAlias V2, then returns
958 // NoAlias / MustAlias. Otherwise, returns MayAlias.
960 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
961 if (Alias == MayAlias)
964 // If V2 is visited, the recursive case will have been caught in the
965 // above aliasCheck call, so these subsequent calls to aliasCheck
966 // don't need to assume that V2 is being visited recursively.
969 AliasResult ThisAlias =
970 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
971 if (ThisAlias != Alias)
976 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
978 AliasAnalysis::AliasResult
979 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
980 const MDNode *PNTBAAInfo,
981 const Value *V2, uint64_t V2Size,
982 const MDNode *V2TBAAInfo) {
983 // The PHI node has already been visited, avoid recursion any further.
984 if (!Visited.insert(PN))
987 // If the values are PHIs in the same block, we can do a more precise
988 // as well as efficient check: just check for aliases between the values
989 // on corresponding edges.
990 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
991 if (PN2->getParent() == PN->getParent()) {
993 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
994 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
996 if (Alias == MayAlias)
998 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
999 AliasResult ThisAlias =
1000 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
1001 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
1002 V2Size, V2TBAAInfo);
1003 if (ThisAlias != Alias)
1009 SmallPtrSet<Value*, 4> UniqueSrc;
1010 SmallVector<Value*, 4> V1Srcs;
1011 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1012 Value *PV1 = PN->getIncomingValue(i);
1013 if (isa<PHINode>(PV1))
1014 // If any of the source itself is a PHI, return MayAlias conservatively
1015 // to avoid compile time explosion. The worst possible case is if both
1016 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1017 // and 'n' are the number of PHI sources.
1019 if (UniqueSrc.insert(PV1))
1020 V1Srcs.push_back(PV1);
1023 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1024 V1Srcs[0], PNSize, PNTBAAInfo);
1025 // Early exit if the check of the first PHI source against V2 is MayAlias.
1026 // Other results are not possible.
1027 if (Alias == MayAlias)
1030 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1031 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1032 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1033 Value *V = V1Srcs[i];
1035 // If V2 is visited, the recursive case will have been caught in the
1036 // above aliasCheck call, so these subsequent calls to aliasCheck
1037 // don't need to assume that V2 is being visited recursively.
1040 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1041 V, PNSize, PNTBAAInfo);
1042 if (ThisAlias != Alias || ThisAlias == MayAlias)
1049 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1050 // such as array references.
1052 AliasAnalysis::AliasResult
1053 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1054 const MDNode *V1TBAAInfo,
1055 const Value *V2, uint64_t V2Size,
1056 const MDNode *V2TBAAInfo) {
1057 // If either of the memory references is empty, it doesn't matter what the
1058 // pointer values are.
1059 if (V1Size == 0 || V2Size == 0)
1062 // Strip off any casts if they exist.
1063 V1 = V1->stripPointerCasts();
1064 V2 = V2->stripPointerCasts();
1066 // Are we checking for alias of the same value?
1067 if (V1 == V2) return MustAlias;
1069 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1070 return NoAlias; // Scalars cannot alias each other
1072 // Figure out what objects these things are pointing to if we can.
1073 const Value *O1 = GetUnderlyingObject(V1, TD);
1074 const Value *O2 = GetUnderlyingObject(V2, TD);
1076 // Null values in the default address space don't point to any object, so they
1077 // don't alias any other pointer.
1078 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1079 if (CPN->getType()->getAddressSpace() == 0)
1081 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1082 if (CPN->getType()->getAddressSpace() == 0)
1086 // If V1/V2 point to two different objects we know that we have no alias.
1087 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1090 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1091 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1092 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1095 // Arguments can't alias with local allocations or noalias calls
1096 // in the same function.
1097 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1098 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1101 // Most objects can't alias null.
1102 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1103 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1106 // If one pointer is the result of a call/invoke or load and the other is a
1107 // non-escaping local object within the same function, then we know the
1108 // object couldn't escape to a point where the call could return it.
1110 // Note that if the pointers are in different functions, there are a
1111 // variety of complications. A call with a nocapture argument may still
1112 // temporary store the nocapture argument's value in a temporary memory
1113 // location if that memory location doesn't escape. Or it may pass a
1114 // nocapture value to other functions as long as they don't capture it.
1115 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1117 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1121 // If the size of one access is larger than the entire object on the other
1122 // side, then we know such behavior is undefined and can assume no alias.
1124 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1125 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1128 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1129 // GEP can't simplify, we don't even look at the PHI cases.
1130 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1132 std::swap(V1Size, V2Size);
1135 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1136 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1137 if (Result != MayAlias) return Result;
1140 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1142 std::swap(V1Size, V2Size);
1144 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1145 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1146 V2, V2Size, V2TBAAInfo);
1147 if (Result != MayAlias) return Result;
1150 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1152 std::swap(V1Size, V2Size);
1154 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1155 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1156 V2, V2Size, V2TBAAInfo);
1157 if (Result != MayAlias) return Result;
1160 // If both pointers are pointing into the same object and one of them
1161 // accesses is accessing the entire object, then the accesses must
1162 // overlap in some way.
1164 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) ||
1165 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD)))
1166 return PartialAlias;
1168 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1169 Location(V2, V2Size, V2TBAAInfo));