1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/LLVMContext.h"
26 #include "llvm/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/Target/TargetData.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/SmallVector.h"
34 #include "llvm/Support/ErrorHandling.h"
35 #include "llvm/Support/GetElementPtrTypeIterator.h"
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
43 /// isKnownNonNull - Return true if we know that the specified value is never
45 static bool isKnownNonNull(const Value *V) {
46 // Alloca never returns null, malloc might.
47 if (isa<AllocaInst>(V)) return true;
49 // A byval argument is never null.
50 if (const Argument *A = dyn_cast<Argument>(V))
51 return A->hasByValAttr();
53 // Global values are not null unless extern weak.
54 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
55 return !GV->hasExternalWeakLinkage();
59 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
60 /// object that never escapes from the function.
61 static bool isNonEscapingLocalObject(const Value *V) {
62 // If this is a local allocation, check to see if it escapes.
63 if (isa<AllocaInst>(V) || isNoAliasCall(V))
64 // Set StoreCaptures to True so that we can assume in our callers that the
65 // pointer is not the result of a load instruction. Currently
66 // PointerMayBeCaptured doesn't have any special analysis for the
67 // StoreCaptures=false case; if it did, our callers could be refined to be
69 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
71 // If this is an argument that corresponds to a byval or noalias argument,
72 // then it has not escaped before entering the function. Check if it escapes
73 // inside the function.
74 if (const Argument *A = dyn_cast<Argument>(V))
75 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
76 // Don't bother analyzing arguments already known not to escape.
77 if (A->hasNoCaptureAttr())
79 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
84 /// isEscapeSource - Return true if the pointer is one which would have
85 /// been considered an escape by isNonEscapingLocalObject.
86 static bool isEscapeSource(const Value *V) {
87 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
90 // The load case works because isNonEscapingLocalObject considers all
91 // stores to be escapes (it passes true for the StoreCaptures argument
92 // to PointerMayBeCaptured).
99 /// isObjectSmallerThan - Return true if we can prove that the object specified
100 /// by V is smaller than Size.
101 static bool isObjectSmallerThan(const Value *V, unsigned Size,
102 const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 AccessTy = GV->getType()->getElementType();
106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
107 if (!AI->isArrayAllocation())
108 AccessTy = AI->getType()->getElementType();
111 } else if (const CallInst* CI = extractMallocCall(V)) {
112 if (!isArrayMalloc(V, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
115 return (C->getZExtValue() < Size);
117 } else if (const Argument *A = dyn_cast<Argument>(V)) {
118 if (A->hasByValAttr())
119 AccessTy = cast<PointerType>(A->getType())->getElementType();
126 if (AccessTy->isSized())
127 return TD.getTypeAllocSize(AccessTy) < Size;
131 //===----------------------------------------------------------------------===//
133 //===----------------------------------------------------------------------===//
136 /// NoAA - This class implements the -no-aa pass, which always returns "I
137 /// don't know" for alias queries. NoAA is unlike other alias analysis
138 /// implementations, in that it does not chain to a previous analysis. As
139 /// such it doesn't follow many of the rules that other alias analyses must.
141 struct NoAA : public ImmutablePass, public AliasAnalysis {
142 static char ID; // Class identification, replacement for typeinfo
143 NoAA() : ImmutablePass(ID) {}
144 explicit NoAA(char &PID) : ImmutablePass(PID) { }
146 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
149 virtual void initializePass() {
150 TD = getAnalysisIfAvailable<TargetData>();
153 virtual AliasResult alias(const Location &LocA, const Location &LocB) {
157 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
158 return UnknownModRefBehavior;
160 virtual ModRefBehavior getModRefBehavior(const Function *F) {
161 return UnknownModRefBehavior;
164 virtual bool pointsToConstantMemory(const Location &Loc) { return false; }
165 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
166 const Location &Loc) {
169 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
170 ImmutableCallSite CS2) {
174 virtual void deleteValue(Value *V) {}
175 virtual void copyValue(Value *From, Value *To) {}
177 /// getAdjustedAnalysisPointer - This method is used when a pass implements
178 /// an analysis interface through multiple inheritance. If needed, it
179 /// should override this to adjust the this pointer as needed for the
180 /// specified pass info.
181 virtual void *getAdjustedAnalysisPointer(const void *ID) {
182 if (ID == &AliasAnalysis::ID)
183 return (AliasAnalysis*)this;
187 } // End of anonymous namespace
189 // Register this pass...
191 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
192 "No Alias Analysis (always returns 'may' alias)",
195 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
197 //===----------------------------------------------------------------------===//
198 // GetElementPtr Instruction Decomposition and Analysis
199 //===----------------------------------------------------------------------===//
208 struct VariableGEPIndex {
210 ExtensionKind Extension;
216 /// GetLinearExpression - Analyze the specified value as a linear expression:
217 /// "A*V + B", where A and B are constant integers. Return the scale and offset
218 /// values as APInts and return V as a Value*, and return whether we looked
219 /// through any sign or zero extends. The incoming Value is known to have
220 /// IntegerType and it may already be sign or zero extended.
222 /// Note that this looks through extends, so the high bits may not be
223 /// represented in the result.
224 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
225 ExtensionKind &Extension,
226 const TargetData &TD, unsigned Depth) {
227 assert(V->getType()->isIntegerTy() && "Not an integer value");
229 // Limit our recursion depth.
236 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
237 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
238 switch (BOp->getOpcode()) {
240 case Instruction::Or:
241 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
243 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
246 case Instruction::Add:
247 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
249 Offset += RHSC->getValue();
251 case Instruction::Mul:
252 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
254 Offset *= RHSC->getValue();
255 Scale *= RHSC->getValue();
257 case Instruction::Shl:
258 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
260 Offset <<= RHSC->getValue().getLimitedValue();
261 Scale <<= RHSC->getValue().getLimitedValue();
267 // Since GEP indices are sign extended anyway, we don't care about the high
268 // bits of a sign or zero extended value - just scales and offsets. The
269 // extensions have to be consistent though.
270 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
271 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
272 Value *CastOp = cast<CastInst>(V)->getOperand(0);
273 unsigned OldWidth = Scale.getBitWidth();
274 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
275 Scale.trunc(SmallWidth);
276 Offset.trunc(SmallWidth);
277 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
279 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
281 Scale.zext(OldWidth);
282 Offset.zext(OldWidth);
292 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
293 /// into a base pointer with a constant offset and a number of scaled symbolic
296 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
297 /// the VarIndices vector) are Value*'s that are known to be scaled by the
298 /// specified amount, but which may have other unrepresented high bits. As such,
299 /// the gep cannot necessarily be reconstructed from its decomposed form.
301 /// When TargetData is around, this function is capable of analyzing everything
302 /// that Value::getUnderlyingObject() can look through. When not, it just looks
303 /// through pointer casts.
306 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
307 SmallVectorImpl<VariableGEPIndex> &VarIndices,
308 const TargetData *TD) {
309 // Limit recursion depth to limit compile time in crazy cases.
310 unsigned MaxLookup = 6;
314 // See if this is a bitcast or GEP.
315 const Operator *Op = dyn_cast<Operator>(V);
317 // The only non-operator case we can handle are GlobalAliases.
318 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
319 if (!GA->mayBeOverridden()) {
320 V = GA->getAliasee();
327 if (Op->getOpcode() == Instruction::BitCast) {
328 V = Op->getOperand(0);
332 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
336 // Don't attempt to analyze GEPs over unsized objects.
337 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
338 ->getElementType()->isSized())
341 // If we are lacking TargetData information, we can't compute the offets of
342 // elements computed by GEPs. However, we can handle bitcast equivalent
345 if (!GEPOp->hasAllZeroIndices())
347 V = GEPOp->getOperand(0);
351 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
352 gep_type_iterator GTI = gep_type_begin(GEPOp);
353 for (User::const_op_iterator I = GEPOp->op_begin()+1,
354 E = GEPOp->op_end(); I != E; ++I) {
356 // Compute the (potentially symbolic) offset in bytes for this index.
357 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
358 // For a struct, add the member offset.
359 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
360 if (FieldNo == 0) continue;
362 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
366 // For an array/pointer, add the element offset, explicitly scaled.
367 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
368 if (CIdx->isZero()) continue;
369 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
373 uint64_t Scale = TD->getTypeAllocSize(*GTI);
374 ExtensionKind Extension = EK_NotExtended;
376 // If the integer type is smaller than the pointer size, it is implicitly
377 // sign extended to pointer size.
378 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
379 if (TD->getPointerSizeInBits() > Width)
380 Extension = EK_SignExt;
382 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
383 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
384 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
387 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
388 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
389 BaseOffs += IndexOffset.getSExtValue()*Scale;
390 Scale *= IndexScale.getSExtValue();
393 // If we already had an occurrance of this index variable, merge this
394 // scale into it. For example, we want to handle:
395 // A[x][x] -> x*16 + x*4 -> x*20
396 // This also ensures that 'x' only appears in the index list once.
397 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
398 if (VarIndices[i].V == Index &&
399 VarIndices[i].Extension == Extension) {
400 Scale += VarIndices[i].Scale;
401 VarIndices.erase(VarIndices.begin()+i);
406 // Make sure that we have a scale that makes sense for this target's
408 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
410 Scale = (int64_t)Scale >> ShiftBits;
414 VariableGEPIndex Entry = {Index, Extension, Scale};
415 VarIndices.push_back(Entry);
419 // Analyze the base pointer next.
420 V = GEPOp->getOperand(0);
421 } while (--MaxLookup);
423 // If the chain of expressions is too deep, just return early.
427 /// GetIndexDifference - Dest and Src are the variable indices from two
428 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
429 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
430 /// difference between the two pointers.
431 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
432 const SmallVectorImpl<VariableGEPIndex> &Src) {
433 if (Src.empty()) return;
435 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
436 const Value *V = Src[i].V;
437 ExtensionKind Extension = Src[i].Extension;
438 int64_t Scale = Src[i].Scale;
440 // Find V in Dest. This is N^2, but pointer indices almost never have more
441 // than a few variable indexes.
442 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
443 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
445 // If we found it, subtract off Scale V's from the entry in Dest. If it
446 // goes to zero, remove the entry.
447 if (Dest[j].Scale != Scale)
448 Dest[j].Scale -= Scale;
450 Dest.erase(Dest.begin()+j);
455 // If we didn't consume this entry, add it to the end of the Dest list.
457 VariableGEPIndex Entry = { V, Extension, -Scale };
458 Dest.push_back(Entry);
463 //===----------------------------------------------------------------------===//
464 // BasicAliasAnalysis Pass
465 //===----------------------------------------------------------------------===//
468 static const Function *getParent(const Value *V) {
469 if (const Instruction *inst = dyn_cast<Instruction>(V))
470 return inst->getParent()->getParent();
472 if (const Argument *arg = dyn_cast<Argument>(V))
473 return arg->getParent();
478 static bool notDifferentParent(const Value *O1, const Value *O2) {
480 const Function *F1 = getParent(O1);
481 const Function *F2 = getParent(O2);
483 return !F1 || !F2 || F1 == F2;
488 /// BasicAliasAnalysis - This is the default alias analysis implementation.
489 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
490 /// derives from the NoAA class.
491 struct BasicAliasAnalysis : public NoAA {
492 static char ID; // Class identification, replacement for typeinfo
493 BasicAliasAnalysis() : NoAA(ID) {}
495 virtual void initializePass() {
496 InitializeAliasAnalysis(this);
499 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
500 AU.addRequired<AliasAnalysis>();
503 virtual AliasResult alias(const Location &LocA,
504 const Location &LocB) {
505 assert(Visited.empty() && "Visited must be cleared after use!");
506 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
507 "BasicAliasAnalysis doesn't support interprocedural queries.");
508 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
509 LocB.Ptr, LocB.Size, LocB.TBAATag);
514 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
515 const Location &Loc);
517 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
518 ImmutableCallSite CS2) {
519 // The AliasAnalysis base class has some smarts, lets use them.
520 return AliasAnalysis::getModRefInfo(CS1, CS2);
523 /// pointsToConstantMemory - Chase pointers until we find a (constant
525 virtual bool pointsToConstantMemory(const Location &Loc);
527 /// getModRefBehavior - Return the behavior when calling the given
529 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
531 /// getModRefBehavior - Return the behavior when calling the given function.
532 /// For use when the call site is not known.
533 virtual ModRefBehavior getModRefBehavior(const Function *F);
535 /// getAdjustedAnalysisPointer - This method is used when a pass implements
536 /// an analysis interface through multiple inheritance. If needed, it
537 /// should override this to adjust the this pointer as needed for the
538 /// specified pass info.
539 virtual void *getAdjustedAnalysisPointer(const void *ID) {
540 if (ID == &AliasAnalysis::ID)
541 return (AliasAnalysis*)this;
546 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
547 SmallPtrSet<const Value*, 16> Visited;
549 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
550 // instruction against another.
551 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
552 const Value *V2, unsigned V2Size,
553 const MDNode *V2TBAAInfo,
554 const Value *UnderlyingV1, const Value *UnderlyingV2);
556 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
557 // instruction against another.
558 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
559 const MDNode *PNTBAAInfo,
560 const Value *V2, unsigned V2Size,
561 const MDNode *V2TBAAInfo);
563 /// aliasSelect - Disambiguate a Select instruction against another value.
564 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
565 const MDNode *SITBAAInfo,
566 const Value *V2, unsigned V2Size,
567 const MDNode *V2TBAAInfo);
569 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
570 const MDNode *V1TBAATag,
571 const Value *V2, unsigned V2Size,
572 const MDNode *V2TBAATag);
574 } // End of anonymous namespace
576 // Register this pass...
577 char BasicAliasAnalysis::ID = 0;
578 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
579 "Basic Alias Analysis (default AA impl)",
582 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
583 return new BasicAliasAnalysis();
587 /// pointsToConstantMemory - Chase pointers until we find a (constant
589 bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
590 if (const GlobalVariable *GV =
591 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject()))
592 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
593 // global to be marked constant in some modules and non-constant in others.
594 // GV may even be a declaration, not a definition.
595 return GV->isConstant();
597 return AliasAnalysis::pointsToConstantMemory(Loc);
600 /// getModRefBehavior - Return the behavior when calling the given call site.
601 AliasAnalysis::ModRefBehavior
602 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
603 if (CS.doesNotAccessMemory())
604 // Can't do better than this.
605 return DoesNotAccessMemory;
607 ModRefBehavior Min = UnknownModRefBehavior;
609 // If the callsite knows it only reads memory, don't return worse
611 if (CS.onlyReadsMemory())
612 Min = OnlyReadsMemory;
614 // The AliasAnalysis base class has some smarts, lets use them.
615 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
618 /// getModRefBehavior - Return the behavior when calling the given function.
619 /// For use when the call site is not known.
620 AliasAnalysis::ModRefBehavior
621 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
622 if (F->doesNotAccessMemory())
623 // Can't do better than this.
624 return DoesNotAccessMemory;
625 if (F->onlyReadsMemory())
626 return OnlyReadsMemory;
627 if (unsigned id = F->getIntrinsicID())
628 return getIntrinsicModRefBehavior(id);
630 return AliasAnalysis::getModRefBehavior(F);
633 /// getModRefInfo - Check to see if the specified callsite can clobber the
634 /// specified memory object. Since we only look at local properties of this
635 /// function, we really can't say much about this query. We do, however, use
636 /// simple "address taken" analysis on local objects.
637 AliasAnalysis::ModRefResult
638 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
639 const Location &Loc) {
640 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
641 "AliasAnalysis query involving multiple functions!");
643 const Value *Object = Loc.Ptr->getUnderlyingObject();
645 // If this is a tail call and Loc.Ptr points to a stack location, we know that
646 // the tail call cannot access or modify the local stack.
647 // We cannot exclude byval arguments here; these belong to the caller of
648 // the current function not to the current function, and a tail callee
649 // may reference them.
650 if (isa<AllocaInst>(Object))
651 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
652 if (CI->isTailCall())
655 // If the pointer is to a locally allocated object that does not escape,
656 // then the call can not mod/ref the pointer unless the call takes the pointer
657 // as an argument, and itself doesn't capture it.
658 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
659 isNonEscapingLocalObject(Object)) {
660 bool PassedAsArg = false;
662 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
663 CI != CE; ++CI, ++ArgNo) {
664 // Only look at the no-capture pointer arguments.
665 if (!(*CI)->getType()->isPointerTy() ||
666 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
669 // If this is a no-capture pointer argument, see if we can tell that it
670 // is impossible to alias the pointer we're checking. If not, we have to
671 // assume that the call could touch the pointer, even though it doesn't
673 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
683 // Finally, handle specific knowledge of intrinsics.
684 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
686 switch (II->getIntrinsicID()) {
688 case Intrinsic::memcpy:
689 case Intrinsic::memmove: {
690 unsigned Len = UnknownSize;
691 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
692 Len = LenCI->getZExtValue();
693 Value *Dest = II->getArgOperand(0);
694 Value *Src = II->getArgOperand(1);
695 if (isNoAlias(Location(Dest, Len), Loc)) {
696 if (isNoAlias(Location(Src, Len), Loc))
702 case Intrinsic::memset:
703 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
704 // will handle it for the variable length case.
705 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
706 unsigned Len = LenCI->getZExtValue();
707 Value *Dest = II->getArgOperand(0);
708 if (isNoAlias(Location(Dest, Len), Loc))
712 case Intrinsic::atomic_cmp_swap:
713 case Intrinsic::atomic_swap:
714 case Intrinsic::atomic_load_add:
715 case Intrinsic::atomic_load_sub:
716 case Intrinsic::atomic_load_and:
717 case Intrinsic::atomic_load_nand:
718 case Intrinsic::atomic_load_or:
719 case Intrinsic::atomic_load_xor:
720 case Intrinsic::atomic_load_max:
721 case Intrinsic::atomic_load_min:
722 case Intrinsic::atomic_load_umax:
723 case Intrinsic::atomic_load_umin:
725 Value *Op1 = II->getArgOperand(0);
726 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
727 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
728 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
732 case Intrinsic::lifetime_start:
733 case Intrinsic::lifetime_end:
734 case Intrinsic::invariant_start: {
736 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
737 if (isNoAlias(Location(II->getArgOperand(1),
739 II->getMetadata(LLVMContext::MD_tbaa)),
744 case Intrinsic::invariant_end: {
746 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
747 if (isNoAlias(Location(II->getArgOperand(2),
749 II->getMetadata(LLVMContext::MD_tbaa)),
756 // The AliasAnalysis base class has some smarts, lets use them.
757 return AliasAnalysis::getModRefInfo(CS, Loc);
760 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
761 /// against another pointer. We know that V1 is a GEP, but we don't know
762 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
763 /// UnderlyingV2 is the same for V2.
765 AliasAnalysis::AliasResult
766 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
767 const Value *V2, unsigned V2Size,
768 const MDNode *V2TBAAInfo,
769 const Value *UnderlyingV1,
770 const Value *UnderlyingV2) {
771 // If this GEP has been visited before, we're on a use-def cycle.
772 // Such cycles are only valid when PHI nodes are involved or in unreachable
773 // code. The visitPHI function catches cycles containing PHIs, but there
774 // could still be a cycle without PHIs in unreachable code.
775 if (!Visited.insert(GEP1))
778 int64_t GEP1BaseOffset;
779 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
781 // If we have two gep instructions with must-alias'ing base pointers, figure
782 // out if the indexes to the GEP tell us anything about the derived pointer.
783 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
784 // Do the base pointers alias?
785 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
786 UnderlyingV2, UnknownSize, 0);
788 // If we get a No or May, then return it immediately, no amount of analysis
789 // will improve this situation.
790 if (BaseAlias != MustAlias) return BaseAlias;
792 // Otherwise, we have a MustAlias. Since the base pointers alias each other
793 // exactly, see if the computed offset from the common pointer tells us
794 // about the relation of the resulting pointer.
795 const Value *GEP1BasePtr =
796 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
798 int64_t GEP2BaseOffset;
799 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
800 const Value *GEP2BasePtr =
801 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
803 // If DecomposeGEPExpression isn't able to look all the way through the
804 // addressing operation, we must not have TD and this is too complex for us
805 // to handle without it.
806 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
808 "DecomposeGEPExpression and getUnderlyingObject disagree!");
812 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
813 // symbolic difference.
814 GEP1BaseOffset -= GEP2BaseOffset;
815 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
818 // Check to see if these two pointers are related by the getelementptr
819 // instruction. If one pointer is a GEP with a non-zero index of the other
820 // pointer, we know they cannot alias.
822 // If both accesses are unknown size, we can't do anything useful here.
823 if (V1Size == UnknownSize && V2Size == UnknownSize)
826 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
827 V2, V2Size, V2TBAAInfo);
829 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
830 // If V2 is known not to alias GEP base pointer, then the two values
831 // cannot alias per GEP semantics: "A pointer value formed from a
832 // getelementptr instruction is associated with the addresses associated
833 // with the first operand of the getelementptr".
836 const Value *GEP1BasePtr =
837 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
839 // If DecomposeGEPExpression isn't able to look all the way through the
840 // addressing operation, we must not have TD and this is too complex for us
841 // to handle without it.
842 if (GEP1BasePtr != UnderlyingV1) {
844 "DecomposeGEPExpression and getUnderlyingObject disagree!");
849 // In the two GEP Case, if there is no difference in the offsets of the
850 // computed pointers, the resultant pointers are a must alias. This
851 // hapens when we have two lexically identical GEP's (for example).
853 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
854 // must aliases the GEP, the end result is a must alias also.
855 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
858 // If we have a known constant offset, see if this offset is larger than the
859 // access size being queried. If so, and if no variable indices can remove
860 // pieces of this constant, then we know we have a no-alias. For example,
863 // In order to handle cases like &A[100][i] where i is an out of range
864 // subscript, we have to ignore all constant offset pieces that are a multiple
865 // of a scaled index. Do this by removing constant offsets that are a
866 // multiple of any of our variable indices. This allows us to transform
867 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
868 // provides an offset of 4 bytes (assuming a <= 4 byte access).
869 for (unsigned i = 0, e = GEP1VariableIndices.size();
870 i != e && GEP1BaseOffset;++i)
871 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
872 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
874 // If our known offset is bigger than the access size, we know we don't have
876 if (GEP1BaseOffset) {
877 if (GEP1BaseOffset >= (int64_t)V2Size ||
878 GEP1BaseOffset <= -(int64_t)V1Size)
885 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
886 /// instruction against another.
887 AliasAnalysis::AliasResult
888 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
889 const MDNode *SITBAAInfo,
890 const Value *V2, unsigned V2Size,
891 const MDNode *V2TBAAInfo) {
892 // If this select has been visited before, we're on a use-def cycle.
893 // Such cycles are only valid when PHI nodes are involved or in unreachable
894 // code. The visitPHI function catches cycles containing PHIs, but there
895 // could still be a cycle without PHIs in unreachable code.
896 if (!Visited.insert(SI))
899 // If the values are Selects with the same condition, we can do a more precise
900 // check: just check for aliases between the values on corresponding arms.
901 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
902 if (SI->getCondition() == SI2->getCondition()) {
904 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
905 SI2->getTrueValue(), V2Size, V2TBAAInfo);
906 if (Alias == MayAlias)
908 AliasResult ThisAlias =
909 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
910 SI2->getFalseValue(), V2Size, V2TBAAInfo);
911 if (ThisAlias != Alias)
916 // If both arms of the Select node NoAlias or MustAlias V2, then returns
917 // NoAlias / MustAlias. Otherwise, returns MayAlias.
919 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
920 if (Alias == MayAlias)
923 // If V2 is visited, the recursive case will have been caught in the
924 // above aliasCheck call, so these subsequent calls to aliasCheck
925 // don't need to assume that V2 is being visited recursively.
928 AliasResult ThisAlias =
929 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
930 if (ThisAlias != Alias)
935 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
937 AliasAnalysis::AliasResult
938 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
939 const MDNode *PNTBAAInfo,
940 const Value *V2, unsigned V2Size,
941 const MDNode *V2TBAAInfo) {
942 // The PHI node has already been visited, avoid recursion any further.
943 if (!Visited.insert(PN))
946 // If the values are PHIs in the same block, we can do a more precise
947 // as well as efficient check: just check for aliases between the values
948 // on corresponding edges.
949 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
950 if (PN2->getParent() == PN->getParent()) {
952 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
953 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
955 if (Alias == MayAlias)
957 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
958 AliasResult ThisAlias =
959 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
960 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
962 if (ThisAlias != Alias)
968 SmallPtrSet<Value*, 4> UniqueSrc;
969 SmallVector<Value*, 4> V1Srcs;
970 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
971 Value *PV1 = PN->getIncomingValue(i);
972 if (isa<PHINode>(PV1))
973 // If any of the source itself is a PHI, return MayAlias conservatively
974 // to avoid compile time explosion. The worst possible case is if both
975 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
976 // and 'n' are the number of PHI sources.
978 if (UniqueSrc.insert(PV1))
979 V1Srcs.push_back(PV1);
982 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
983 V1Srcs[0], PNSize, PNTBAAInfo);
984 // Early exit if the check of the first PHI source against V2 is MayAlias.
985 // Other results are not possible.
986 if (Alias == MayAlias)
989 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
990 // NoAlias / MustAlias. Otherwise, returns MayAlias.
991 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
992 Value *V = V1Srcs[i];
994 // If V2 is visited, the recursive case will have been caught in the
995 // above aliasCheck call, so these subsequent calls to aliasCheck
996 // don't need to assume that V2 is being visited recursively.
999 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1000 V, PNSize, PNTBAAInfo);
1001 if (ThisAlias != Alias || ThisAlias == MayAlias)
1008 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1009 // such as array references.
1011 AliasAnalysis::AliasResult
1012 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
1013 const MDNode *V1TBAAInfo,
1014 const Value *V2, unsigned V2Size,
1015 const MDNode *V2TBAAInfo) {
1016 // If either of the memory references is empty, it doesn't matter what the
1017 // pointer values are.
1018 if (V1Size == 0 || V2Size == 0)
1021 // Strip off any casts if they exist.
1022 V1 = V1->stripPointerCasts();
1023 V2 = V2->stripPointerCasts();
1025 // Are we checking for alias of the same value?
1026 if (V1 == V2) return MustAlias;
1028 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1029 return NoAlias; // Scalars cannot alias each other
1031 // Figure out what objects these things are pointing to if we can.
1032 const Value *O1 = V1->getUnderlyingObject();
1033 const Value *O2 = V2->getUnderlyingObject();
1035 // Null values in the default address space don't point to any object, so they
1036 // don't alias any other pointer.
1037 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1038 if (CPN->getType()->getAddressSpace() == 0)
1040 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1041 if (CPN->getType()->getAddressSpace() == 0)
1045 // If V1/V2 point to two different objects we know that we have no alias.
1046 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1049 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1050 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1051 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1054 // Arguments can't alias with local allocations or noalias calls
1055 // in the same function.
1056 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1057 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1060 // Most objects can't alias null.
1061 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1062 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1065 // If one pointer is the result of a call/invoke or load and the other is a
1066 // non-escaping local object within the same function, then we know the
1067 // object couldn't escape to a point where the call could return it.
1069 // Note that if the pointers are in different functions, there are a
1070 // variety of complications. A call with a nocapture argument may still
1071 // temporary store the nocapture argument's value in a temporary memory
1072 // location if that memory location doesn't escape. Or it may pass a
1073 // nocapture value to other functions as long as they don't capture it.
1074 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1076 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1080 // If the size of one access is larger than the entire object on the other
1081 // side, then we know such behavior is undefined and can assume no alias.
1083 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1084 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1087 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1088 // GEP can't simplify, we don't even look at the PHI cases.
1089 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1091 std::swap(V1Size, V2Size);
1094 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1095 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1096 if (Result != MayAlias) return Result;
1099 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1101 std::swap(V1Size, V2Size);
1103 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1104 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1105 V2, V2Size, V2TBAAInfo);
1106 if (Result != MayAlias) return Result;
1109 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1111 std::swap(V1Size, V2Size);
1113 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1114 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1115 V2, V2Size, V2TBAAInfo);
1116 if (Result != MayAlias) return Result;
1119 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1120 Location(V2, V2Size, V2TBAAInfo));
1123 // Make sure that anything that uses AliasAnalysis pulls in this file.
1124 DEFINING_FILE_FOR(BasicAliasAnalysis)