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/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 //===----------------------------------------------------------------------===//
132 // GetElementPtr Instruction Decomposition and Analysis
133 //===----------------------------------------------------------------------===//
142 struct VariableGEPIndex {
144 ExtensionKind Extension;
150 /// GetLinearExpression - Analyze the specified value as a linear expression:
151 /// "A*V + B", where A and B are constant integers. Return the scale and offset
152 /// values as APInts and return V as a Value*, and return whether we looked
153 /// through any sign or zero extends. The incoming Value is known to have
154 /// IntegerType and it may already be sign or zero extended.
156 /// Note that this looks through extends, so the high bits may not be
157 /// represented in the result.
158 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
159 ExtensionKind &Extension,
160 const TargetData &TD, unsigned Depth) {
161 assert(V->getType()->isIntegerTy() && "Not an integer value");
163 // Limit our recursion depth.
170 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
171 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
172 switch (BOp->getOpcode()) {
174 case Instruction::Or:
175 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
177 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
180 case Instruction::Add:
181 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
183 Offset += RHSC->getValue();
185 case Instruction::Mul:
186 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
188 Offset *= RHSC->getValue();
189 Scale *= RHSC->getValue();
191 case Instruction::Shl:
192 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
194 Offset <<= RHSC->getValue().getLimitedValue();
195 Scale <<= RHSC->getValue().getLimitedValue();
201 // Since GEP indices are sign extended anyway, we don't care about the high
202 // bits of a sign or zero extended value - just scales and offsets. The
203 // extensions have to be consistent though.
204 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
205 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
206 Value *CastOp = cast<CastInst>(V)->getOperand(0);
207 unsigned OldWidth = Scale.getBitWidth();
208 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
209 Scale.trunc(SmallWidth);
210 Offset.trunc(SmallWidth);
211 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
213 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
215 Scale.zext(OldWidth);
216 Offset.zext(OldWidth);
226 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
227 /// into a base pointer with a constant offset and a number of scaled symbolic
230 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
231 /// the VarIndices vector) are Value*'s that are known to be scaled by the
232 /// specified amount, but which may have other unrepresented high bits. As such,
233 /// the gep cannot necessarily be reconstructed from its decomposed form.
235 /// When TargetData is around, this function is capable of analyzing everything
236 /// that Value::getUnderlyingObject() can look through. When not, it just looks
237 /// through pointer casts.
240 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
241 SmallVectorImpl<VariableGEPIndex> &VarIndices,
242 const TargetData *TD) {
243 // Limit recursion depth to limit compile time in crazy cases.
244 unsigned MaxLookup = 6;
248 // See if this is a bitcast or GEP.
249 const Operator *Op = dyn_cast<Operator>(V);
251 // The only non-operator case we can handle are GlobalAliases.
252 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
253 if (!GA->mayBeOverridden()) {
254 V = GA->getAliasee();
261 if (Op->getOpcode() == Instruction::BitCast) {
262 V = Op->getOperand(0);
266 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
270 // Don't attempt to analyze GEPs over unsized objects.
271 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
272 ->getElementType()->isSized())
275 // If we are lacking TargetData information, we can't compute the offets of
276 // elements computed by GEPs. However, we can handle bitcast equivalent
279 if (!GEPOp->hasAllZeroIndices())
281 V = GEPOp->getOperand(0);
285 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
286 gep_type_iterator GTI = gep_type_begin(GEPOp);
287 for (User::const_op_iterator I = GEPOp->op_begin()+1,
288 E = GEPOp->op_end(); I != E; ++I) {
290 // Compute the (potentially symbolic) offset in bytes for this index.
291 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
292 // For a struct, add the member offset.
293 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
294 if (FieldNo == 0) continue;
296 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
300 // For an array/pointer, add the element offset, explicitly scaled.
301 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
302 if (CIdx->isZero()) continue;
303 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
307 uint64_t Scale = TD->getTypeAllocSize(*GTI);
308 ExtensionKind Extension = EK_NotExtended;
310 // If the integer type is smaller than the pointer size, it is implicitly
311 // sign extended to pointer size.
312 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
313 if (TD->getPointerSizeInBits() > Width)
314 Extension = EK_SignExt;
316 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
317 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
318 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
321 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
322 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
323 BaseOffs += IndexOffset.getSExtValue()*Scale;
324 Scale *= IndexScale.getSExtValue();
327 // If we already had an occurrance of this index variable, merge this
328 // scale into it. For example, we want to handle:
329 // A[x][x] -> x*16 + x*4 -> x*20
330 // This also ensures that 'x' only appears in the index list once.
331 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
332 if (VarIndices[i].V == Index &&
333 VarIndices[i].Extension == Extension) {
334 Scale += VarIndices[i].Scale;
335 VarIndices.erase(VarIndices.begin()+i);
340 // Make sure that we have a scale that makes sense for this target's
342 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
344 Scale = (int64_t)Scale >> ShiftBits;
348 VariableGEPIndex Entry = {Index, Extension, Scale};
349 VarIndices.push_back(Entry);
353 // Analyze the base pointer next.
354 V = GEPOp->getOperand(0);
355 } while (--MaxLookup);
357 // If the chain of expressions is too deep, just return early.
361 /// GetIndexDifference - Dest and Src are the variable indices from two
362 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
363 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
364 /// difference between the two pointers.
365 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
366 const SmallVectorImpl<VariableGEPIndex> &Src) {
367 if (Src.empty()) return;
369 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
370 const Value *V = Src[i].V;
371 ExtensionKind Extension = Src[i].Extension;
372 int64_t Scale = Src[i].Scale;
374 // Find V in Dest. This is N^2, but pointer indices almost never have more
375 // than a few variable indexes.
376 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
377 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
379 // If we found it, subtract off Scale V's from the entry in Dest. If it
380 // goes to zero, remove the entry.
381 if (Dest[j].Scale != Scale)
382 Dest[j].Scale -= Scale;
384 Dest.erase(Dest.begin()+j);
389 // If we didn't consume this entry, add it to the end of the Dest list.
391 VariableGEPIndex Entry = { V, Extension, -Scale };
392 Dest.push_back(Entry);
397 //===----------------------------------------------------------------------===//
398 // BasicAliasAnalysis Pass
399 //===----------------------------------------------------------------------===//
402 static const Function *getParent(const Value *V) {
403 if (const Instruction *inst = dyn_cast<Instruction>(V))
404 return inst->getParent()->getParent();
406 if (const Argument *arg = dyn_cast<Argument>(V))
407 return arg->getParent();
412 static bool notDifferentParent(const Value *O1, const Value *O2) {
414 const Function *F1 = getParent(O1);
415 const Function *F2 = getParent(O2);
417 return !F1 || !F2 || F1 == F2;
422 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
423 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
424 static char ID; // Class identification, replacement for typeinfo
425 BasicAliasAnalysis() : ImmutablePass(ID) {
426 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
429 virtual void initializePass() {
430 InitializeAliasAnalysis(this);
433 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
434 AU.addRequired<AliasAnalysis>();
437 virtual AliasResult alias(const Location &LocA,
438 const Location &LocB) {
439 assert(Visited.empty() && "Visited must be cleared after use!");
440 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
441 "BasicAliasAnalysis doesn't support interprocedural queries.");
442 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
443 LocB.Ptr, LocB.Size, LocB.TBAATag);
448 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
449 const Location &Loc);
451 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
452 ImmutableCallSite CS2) {
453 // The AliasAnalysis base class has some smarts, lets use them.
454 return AliasAnalysis::getModRefInfo(CS1, CS2);
457 /// pointsToConstantMemory - Chase pointers until we find a (constant
459 virtual bool pointsToConstantMemory(const Location &Loc);
461 /// getModRefBehavior - Return the behavior when calling the given
463 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
465 /// getModRefBehavior - Return the behavior when calling the given function.
466 /// For use when the call site is not known.
467 virtual ModRefBehavior getModRefBehavior(const Function *F);
469 /// getAdjustedAnalysisPointer - This method is used when a pass implements
470 /// an analysis interface through multiple inheritance. If needed, it
471 /// should override this to adjust the this pointer as needed for the
472 /// specified pass info.
473 virtual void *getAdjustedAnalysisPointer(const void *ID) {
474 if (ID == &AliasAnalysis::ID)
475 return (AliasAnalysis*)this;
480 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
481 SmallPtrSet<const Value*, 16> Visited;
483 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
484 // instruction against another.
485 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
486 const Value *V2, uint64_t V2Size,
487 const MDNode *V2TBAAInfo,
488 const Value *UnderlyingV1, const Value *UnderlyingV2);
490 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
491 // instruction against another.
492 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
493 const MDNode *PNTBAAInfo,
494 const Value *V2, uint64_t V2Size,
495 const MDNode *V2TBAAInfo);
497 /// aliasSelect - Disambiguate a Select instruction against another value.
498 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
499 const MDNode *SITBAAInfo,
500 const Value *V2, uint64_t V2Size,
501 const MDNode *V2TBAAInfo);
503 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
504 const MDNode *V1TBAATag,
505 const Value *V2, uint64_t V2Size,
506 const MDNode *V2TBAATag);
508 } // End of anonymous namespace
510 // Register this pass...
511 char BasicAliasAnalysis::ID = 0;
512 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
513 "Basic Alias Analysis (stateless AA impl)",
516 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
517 return new BasicAliasAnalysis();
521 /// pointsToConstantMemory - Chase pointers until we find a (constant
523 bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
524 if (const GlobalVariable *GV =
525 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject()))
526 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
527 // global to be marked constant in some modules and non-constant in others.
528 // GV may even be a declaration, not a definition.
529 return GV->isConstant();
531 return AliasAnalysis::pointsToConstantMemory(Loc);
534 /// getModRefBehavior - Return the behavior when calling the given call site.
535 AliasAnalysis::ModRefBehavior
536 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
537 if (CS.doesNotAccessMemory())
538 // Can't do better than this.
539 return DoesNotAccessMemory;
541 ModRefBehavior Min = UnknownModRefBehavior;
543 // If the callsite knows it only reads memory, don't return worse
545 if (CS.onlyReadsMemory())
546 Min = OnlyReadsMemory;
548 // The AliasAnalysis base class has some smarts, lets use them.
549 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
552 /// getModRefBehavior - Return the behavior when calling the given function.
553 /// For use when the call site is not known.
554 AliasAnalysis::ModRefBehavior
555 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
556 // If the function declares it doesn't access memory, we can't do better.
557 if (F->doesNotAccessMemory())
558 return DoesNotAccessMemory;
560 // For intrinsics, we can check the table.
561 if (unsigned iid = F->getIntrinsicID()) {
562 #define GET_INTRINSIC_MODREF_BEHAVIOR
563 #include "llvm/Intrinsics.gen"
564 #undef GET_INTRINSIC_MODREF_BEHAVIOR
567 // If the function declares it only reads memory, go with that.
568 if (F->onlyReadsMemory())
569 return OnlyReadsMemory;
571 // Otherwise be conservative.
572 return AliasAnalysis::getModRefBehavior(F);
575 /// getModRefInfo - Check to see if the specified callsite can clobber the
576 /// specified memory object. Since we only look at local properties of this
577 /// function, we really can't say much about this query. We do, however, use
578 /// simple "address taken" analysis on local objects.
579 AliasAnalysis::ModRefResult
580 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
581 const Location &Loc) {
582 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
583 "AliasAnalysis query involving multiple functions!");
585 const Value *Object = Loc.Ptr->getUnderlyingObject();
587 // If this is a tail call and Loc.Ptr points to a stack location, we know that
588 // the tail call cannot access or modify the local stack.
589 // We cannot exclude byval arguments here; these belong to the caller of
590 // the current function not to the current function, and a tail callee
591 // may reference them.
592 if (isa<AllocaInst>(Object))
593 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
594 if (CI->isTailCall())
597 // If the pointer is to a locally allocated object that does not escape,
598 // then the call can not mod/ref the pointer unless the call takes the pointer
599 // as an argument, and itself doesn't capture it.
600 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
601 isNonEscapingLocalObject(Object)) {
602 bool PassedAsArg = false;
604 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
605 CI != CE; ++CI, ++ArgNo) {
606 // Only look at the no-capture pointer arguments.
607 if (!(*CI)->getType()->isPointerTy() ||
608 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
611 // If this is a no-capture pointer argument, see if we can tell that it
612 // is impossible to alias the pointer we're checking. If not, we have to
613 // assume that the call could touch the pointer, even though it doesn't
615 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
625 // Finally, handle specific knowledge of intrinsics.
626 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
628 switch (II->getIntrinsicID()) {
630 case Intrinsic::memcpy:
631 case Intrinsic::memmove: {
632 uint64_t Len = UnknownSize;
633 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
634 Len = LenCI->getZExtValue();
635 Value *Dest = II->getArgOperand(0);
636 Value *Src = II->getArgOperand(1);
637 if (isNoAlias(Location(Dest, Len), Loc)) {
638 if (isNoAlias(Location(Src, Len), Loc))
644 case Intrinsic::memset:
645 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
646 // will handle it for the variable length case.
647 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
648 uint64_t Len = LenCI->getZExtValue();
649 Value *Dest = II->getArgOperand(0);
650 if (isNoAlias(Location(Dest, Len), Loc))
654 case Intrinsic::atomic_cmp_swap:
655 case Intrinsic::atomic_swap:
656 case Intrinsic::atomic_load_add:
657 case Intrinsic::atomic_load_sub:
658 case Intrinsic::atomic_load_and:
659 case Intrinsic::atomic_load_nand:
660 case Intrinsic::atomic_load_or:
661 case Intrinsic::atomic_load_xor:
662 case Intrinsic::atomic_load_max:
663 case Intrinsic::atomic_load_min:
664 case Intrinsic::atomic_load_umax:
665 case Intrinsic::atomic_load_umin:
667 Value *Op1 = II->getArgOperand(0);
668 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType());
669 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
670 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
674 case Intrinsic::lifetime_start:
675 case Intrinsic::lifetime_end:
676 case Intrinsic::invariant_start: {
678 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
679 if (isNoAlias(Location(II->getArgOperand(1),
681 II->getMetadata(LLVMContext::MD_tbaa)),
686 case Intrinsic::invariant_end: {
688 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
689 if (isNoAlias(Location(II->getArgOperand(2),
691 II->getMetadata(LLVMContext::MD_tbaa)),
698 // The AliasAnalysis base class has some smarts, lets use them.
699 return AliasAnalysis::getModRefInfo(CS, Loc);
702 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
703 /// against another pointer. We know that V1 is a GEP, but we don't know
704 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
705 /// UnderlyingV2 is the same for V2.
707 AliasAnalysis::AliasResult
708 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
709 const Value *V2, uint64_t V2Size,
710 const MDNode *V2TBAAInfo,
711 const Value *UnderlyingV1,
712 const Value *UnderlyingV2) {
713 // If this GEP has been visited before, we're on a use-def cycle.
714 // Such cycles are only valid when PHI nodes are involved or in unreachable
715 // code. The visitPHI function catches cycles containing PHIs, but there
716 // could still be a cycle without PHIs in unreachable code.
717 if (!Visited.insert(GEP1))
720 int64_t GEP1BaseOffset;
721 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
723 // If we have two gep instructions with must-alias'ing base pointers, figure
724 // out if the indexes to the GEP tell us anything about the derived pointer.
725 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
726 // Do the base pointers alias?
727 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
728 UnderlyingV2, UnknownSize, 0);
730 // If we get a No or May, then return it immediately, no amount of analysis
731 // will improve this situation.
732 if (BaseAlias != MustAlias) return BaseAlias;
734 // Otherwise, we have a MustAlias. Since the base pointers alias each other
735 // exactly, see if the computed offset from the common pointer tells us
736 // about the relation of the resulting pointer.
737 const Value *GEP1BasePtr =
738 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
740 int64_t GEP2BaseOffset;
741 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
742 const Value *GEP2BasePtr =
743 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
745 // If DecomposeGEPExpression isn't able to look all the way through the
746 // addressing operation, we must not have TD and this is too complex for us
747 // to handle without it.
748 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
750 "DecomposeGEPExpression and getUnderlyingObject disagree!");
754 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
755 // symbolic difference.
756 GEP1BaseOffset -= GEP2BaseOffset;
757 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
760 // Check to see if these two pointers are related by the getelementptr
761 // instruction. If one pointer is a GEP with a non-zero index of the other
762 // pointer, we know they cannot alias.
764 // If both accesses are unknown size, we can't do anything useful here.
765 if (V1Size == UnknownSize && V2Size == UnknownSize)
768 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
769 V2, V2Size, V2TBAAInfo);
771 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
772 // If V2 is known not to alias GEP base pointer, then the two values
773 // cannot alias per GEP semantics: "A pointer value formed from a
774 // getelementptr instruction is associated with the addresses associated
775 // with the first operand of the getelementptr".
778 const Value *GEP1BasePtr =
779 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
781 // If DecomposeGEPExpression isn't able to look all the way through the
782 // addressing operation, we must not have TD and this is too complex for us
783 // to handle without it.
784 if (GEP1BasePtr != UnderlyingV1) {
786 "DecomposeGEPExpression and getUnderlyingObject disagree!");
791 // In the two GEP Case, if there is no difference in the offsets of the
792 // computed pointers, the resultant pointers are a must alias. This
793 // hapens when we have two lexically identical GEP's (for example).
795 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
796 // must aliases the GEP, the end result is a must alias also.
797 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
800 // If we have a known constant offset, see if this offset is larger than the
801 // access size being queried. If so, and if no variable indices can remove
802 // pieces of this constant, then we know we have a no-alias. For example,
805 // In order to handle cases like &A[100][i] where i is an out of range
806 // subscript, we have to ignore all constant offset pieces that are a multiple
807 // of a scaled index. Do this by removing constant offsets that are a
808 // multiple of any of our variable indices. This allows us to transform
809 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
810 // provides an offset of 4 bytes (assuming a <= 4 byte access).
811 for (unsigned i = 0, e = GEP1VariableIndices.size();
812 i != e && GEP1BaseOffset;++i)
813 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
814 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
816 // If our known offset is bigger than the access size, we know we don't have
818 if (GEP1BaseOffset) {
819 if (GEP1BaseOffset >= 0 ?
820 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
821 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
822 GEP1BaseOffset != INT64_MIN))
829 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
830 /// instruction against another.
831 AliasAnalysis::AliasResult
832 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
833 const MDNode *SITBAAInfo,
834 const Value *V2, uint64_t V2Size,
835 const MDNode *V2TBAAInfo) {
836 // If this select has been visited before, we're on a use-def cycle.
837 // Such cycles are only valid when PHI nodes are involved or in unreachable
838 // code. The visitPHI function catches cycles containing PHIs, but there
839 // could still be a cycle without PHIs in unreachable code.
840 if (!Visited.insert(SI))
843 // If the values are Selects with the same condition, we can do a more precise
844 // check: just check for aliases between the values on corresponding arms.
845 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
846 if (SI->getCondition() == SI2->getCondition()) {
848 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
849 SI2->getTrueValue(), V2Size, V2TBAAInfo);
850 if (Alias == MayAlias)
852 AliasResult ThisAlias =
853 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
854 SI2->getFalseValue(), V2Size, V2TBAAInfo);
855 if (ThisAlias != Alias)
860 // If both arms of the Select node NoAlias or MustAlias V2, then returns
861 // NoAlias / MustAlias. Otherwise, returns MayAlias.
863 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
864 if (Alias == MayAlias)
867 // If V2 is visited, the recursive case will have been caught in the
868 // above aliasCheck call, so these subsequent calls to aliasCheck
869 // don't need to assume that V2 is being visited recursively.
872 AliasResult ThisAlias =
873 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
874 if (ThisAlias != Alias)
879 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
881 AliasAnalysis::AliasResult
882 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
883 const MDNode *PNTBAAInfo,
884 const Value *V2, uint64_t V2Size,
885 const MDNode *V2TBAAInfo) {
886 // The PHI node has already been visited, avoid recursion any further.
887 if (!Visited.insert(PN))
890 // If the values are PHIs in the same block, we can do a more precise
891 // as well as efficient check: just check for aliases between the values
892 // on corresponding edges.
893 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
894 if (PN2->getParent() == PN->getParent()) {
896 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
897 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
899 if (Alias == MayAlias)
901 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
902 AliasResult ThisAlias =
903 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
904 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
906 if (ThisAlias != Alias)
912 SmallPtrSet<Value*, 4> UniqueSrc;
913 SmallVector<Value*, 4> V1Srcs;
914 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
915 Value *PV1 = PN->getIncomingValue(i);
916 if (isa<PHINode>(PV1))
917 // If any of the source itself is a PHI, return MayAlias conservatively
918 // to avoid compile time explosion. The worst possible case is if both
919 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
920 // and 'n' are the number of PHI sources.
922 if (UniqueSrc.insert(PV1))
923 V1Srcs.push_back(PV1);
926 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
927 V1Srcs[0], PNSize, PNTBAAInfo);
928 // Early exit if the check of the first PHI source against V2 is MayAlias.
929 // Other results are not possible.
930 if (Alias == MayAlias)
933 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
934 // NoAlias / MustAlias. Otherwise, returns MayAlias.
935 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
936 Value *V = V1Srcs[i];
938 // If V2 is visited, the recursive case will have been caught in the
939 // above aliasCheck call, so these subsequent calls to aliasCheck
940 // don't need to assume that V2 is being visited recursively.
943 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
944 V, PNSize, PNTBAAInfo);
945 if (ThisAlias != Alias || ThisAlias == MayAlias)
952 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
953 // such as array references.
955 AliasAnalysis::AliasResult
956 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
957 const MDNode *V1TBAAInfo,
958 const Value *V2, uint64_t V2Size,
959 const MDNode *V2TBAAInfo) {
960 // If either of the memory references is empty, it doesn't matter what the
961 // pointer values are.
962 if (V1Size == 0 || V2Size == 0)
965 // Strip off any casts if they exist.
966 V1 = V1->stripPointerCasts();
967 V2 = V2->stripPointerCasts();
969 // Are we checking for alias of the same value?
970 if (V1 == V2) return MustAlias;
972 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
973 return NoAlias; // Scalars cannot alias each other
975 // Figure out what objects these things are pointing to if we can.
976 const Value *O1 = V1->getUnderlyingObject();
977 const Value *O2 = V2->getUnderlyingObject();
979 // Null values in the default address space don't point to any object, so they
980 // don't alias any other pointer.
981 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
982 if (CPN->getType()->getAddressSpace() == 0)
984 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
985 if (CPN->getType()->getAddressSpace() == 0)
989 // If V1/V2 point to two different objects we know that we have no alias.
990 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
993 // Constant pointers can't alias with non-const isIdentifiedObject objects.
994 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
995 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
998 // Arguments can't alias with local allocations or noalias calls
999 // in the same function.
1000 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1001 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1004 // Most objects can't alias null.
1005 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1006 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1009 // If one pointer is the result of a call/invoke or load and the other is a
1010 // non-escaping local object within the same function, then we know the
1011 // object couldn't escape to a point where the call could return it.
1013 // Note that if the pointers are in different functions, there are a
1014 // variety of complications. A call with a nocapture argument may still
1015 // temporary store the nocapture argument's value in a temporary memory
1016 // location if that memory location doesn't escape. Or it may pass a
1017 // nocapture value to other functions as long as they don't capture it.
1018 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1020 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1024 // If the size of one access is larger than the entire object on the other
1025 // side, then we know such behavior is undefined and can assume no alias.
1027 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1028 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1031 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1032 // GEP can't simplify, we don't even look at the PHI cases.
1033 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1035 std::swap(V1Size, V2Size);
1038 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1039 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1040 if (Result != MayAlias) return Result;
1043 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1045 std::swap(V1Size, V2Size);
1047 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1048 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1049 V2, V2Size, V2TBAAInfo);
1050 if (Result != MayAlias) return Result;
1053 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1055 std::swap(V1Size, V2Size);
1057 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1058 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1059 V2, V2Size, V2TBAAInfo);
1060 if (Result != MayAlias) return Result;
1063 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1064 Location(V2, V2Size, V2TBAAInfo));