1 //===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the primary stateless implementation of the
11 // Alias Analysis interface that implements identities (two different
12 // globals cannot alias, etc), but does no stateful analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/LLVMContext.h"
26 #include "llvm/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/InstructionSimplify.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/Target/TargetData.h"
33 #include "llvm/ADT/SmallPtrSet.h"
34 #include "llvm/ADT/SmallVector.h"
35 #include "llvm/Support/ErrorHandling.h"
36 #include "llvm/Support/GetElementPtrTypeIterator.h"
40 //===----------------------------------------------------------------------===//
42 //===----------------------------------------------------------------------===//
44 /// isKnownNonNull - Return true if we know that the specified value is never
46 static bool isKnownNonNull(const Value *V) {
47 // Alloca never returns null, malloc might.
48 if (isa<AllocaInst>(V)) return true;
50 // A byval argument is never null.
51 if (const Argument *A = dyn_cast<Argument>(V))
52 return A->hasByValAttr();
54 // Global values are not null unless extern weak.
55 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
56 return !GV->hasExternalWeakLinkage();
60 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
61 /// object that never escapes from the function.
62 static bool isNonEscapingLocalObject(const Value *V) {
63 // If this is a local allocation, check to see if it escapes.
64 if (isa<AllocaInst>(V) || isNoAliasCall(V))
65 // Set StoreCaptures to True so that we can assume in our callers that the
66 // pointer is not the result of a load instruction. Currently
67 // PointerMayBeCaptured doesn't have any special analysis for the
68 // StoreCaptures=false case; if it did, our callers could be refined to be
70 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
72 // If this is an argument that corresponds to a byval or noalias argument,
73 // then it has not escaped before entering the function. Check if it escapes
74 // inside the function.
75 if (const Argument *A = dyn_cast<Argument>(V))
76 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
77 // Don't bother analyzing arguments already known not to escape.
78 if (A->hasNoCaptureAttr())
80 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
85 /// isEscapeSource - Return true if the pointer is one which would have
86 /// been considered an escape by isNonEscapingLocalObject.
87 static bool isEscapeSource(const Value *V) {
88 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
91 // The load case works because isNonEscapingLocalObject considers all
92 // stores to be escapes (it passes true for the StoreCaptures argument
93 // to PointerMayBeCaptured).
100 /// isObjectSmallerThan - Return true if we can prove that the object specified
101 /// by V is smaller than Size.
102 static bool isObjectSmallerThan(const Value *V, uint64_t Size,
103 const TargetData &TD) {
104 const Type *AccessTy;
105 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
106 if (!GV->hasDefinitiveInitializer())
108 AccessTy = GV->getType()->getElementType();
109 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
110 if (!AI->isArrayAllocation())
111 AccessTy = AI->getType()->getElementType();
114 } else if (const CallInst* CI = extractMallocCall(V)) {
115 if (!isArrayMalloc(V, &TD))
116 // The size is the argument to the malloc call.
117 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
118 return (C->getZExtValue() < Size);
120 } else if (const Argument *A = dyn_cast<Argument>(V)) {
121 if (A->hasByValAttr())
122 AccessTy = cast<PointerType>(A->getType())->getElementType();
129 if (AccessTy->isSized())
130 return TD.getTypeAllocSize(AccessTy) < Size;
134 //===----------------------------------------------------------------------===//
135 // GetElementPtr Instruction Decomposition and Analysis
136 //===----------------------------------------------------------------------===//
145 struct VariableGEPIndex {
147 ExtensionKind Extension;
153 /// GetLinearExpression - Analyze the specified value as a linear expression:
154 /// "A*V + B", where A and B are constant integers. Return the scale and offset
155 /// values as APInts and return V as a Value*, and return whether we looked
156 /// through any sign or zero extends. The incoming Value is known to have
157 /// IntegerType and it may already be sign or zero extended.
159 /// Note that this looks through extends, so the high bits may not be
160 /// represented in the result.
161 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
162 ExtensionKind &Extension,
163 const TargetData &TD, unsigned Depth) {
164 assert(V->getType()->isIntegerTy() && "Not an integer value");
166 // Limit our recursion depth.
173 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
174 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
175 switch (BOp->getOpcode()) {
177 case Instruction::Or:
178 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
180 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
183 case Instruction::Add:
184 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
186 Offset += RHSC->getValue();
188 case Instruction::Mul:
189 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
191 Offset *= RHSC->getValue();
192 Scale *= RHSC->getValue();
194 case Instruction::Shl:
195 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
197 Offset <<= RHSC->getValue().getLimitedValue();
198 Scale <<= RHSC->getValue().getLimitedValue();
204 // Since GEP indices are sign extended anyway, we don't care about the high
205 // bits of a sign or zero extended value - just scales and offsets. The
206 // extensions have to be consistent though.
207 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
208 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
209 Value *CastOp = cast<CastInst>(V)->getOperand(0);
210 unsigned OldWidth = Scale.getBitWidth();
211 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
212 Scale = Scale.trunc(SmallWidth);
213 Offset = Offset.trunc(SmallWidth);
214 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
216 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
218 Scale = Scale.zext(OldWidth);
219 Offset = Offset.zext(OldWidth);
229 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
230 /// into a base pointer with a constant offset and a number of scaled symbolic
233 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
234 /// the VarIndices vector) are Value*'s that are known to be scaled by the
235 /// specified amount, but which may have other unrepresented high bits. As such,
236 /// the gep cannot necessarily be reconstructed from its decomposed form.
238 /// When TargetData is around, this function is capable of analyzing everything
239 /// that GetUnderlyingObject can look through. When not, it just looks
240 /// through pointer casts.
243 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
244 SmallVectorImpl<VariableGEPIndex> &VarIndices,
245 const TargetData *TD) {
246 // Limit recursion depth to limit compile time in crazy cases.
247 unsigned MaxLookup = 6;
251 // See if this is a bitcast or GEP.
252 const Operator *Op = dyn_cast<Operator>(V);
254 // The only non-operator case we can handle are GlobalAliases.
255 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
256 if (!GA->mayBeOverridden()) {
257 V = GA->getAliasee();
264 if (Op->getOpcode() == Instruction::BitCast) {
265 V = Op->getOperand(0);
269 if (const Instruction *I = dyn_cast<Instruction>(V))
270 // TODO: Get a DominatorTree and use it here.
271 if (const Value *Simplified =
272 SimplifyInstruction(const_cast<Instruction *>(I), TD)) {
277 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
281 // Don't attempt to analyze GEPs over unsized objects.
282 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
283 ->getElementType()->isSized())
286 // If we are lacking TargetData information, we can't compute the offets of
287 // elements computed by GEPs. However, we can handle bitcast equivalent
290 if (!GEPOp->hasAllZeroIndices())
292 V = GEPOp->getOperand(0);
296 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
297 gep_type_iterator GTI = gep_type_begin(GEPOp);
298 for (User::const_op_iterator I = GEPOp->op_begin()+1,
299 E = GEPOp->op_end(); I != E; ++I) {
301 // Compute the (potentially symbolic) offset in bytes for this index.
302 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
303 // For a struct, add the member offset.
304 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
305 if (FieldNo == 0) continue;
307 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
311 // For an array/pointer, add the element offset, explicitly scaled.
312 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
313 if (CIdx->isZero()) continue;
314 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
318 uint64_t Scale = TD->getTypeAllocSize(*GTI);
319 ExtensionKind Extension = EK_NotExtended;
321 // If the integer type is smaller than the pointer size, it is implicitly
322 // sign extended to pointer size.
323 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
324 if (TD->getPointerSizeInBits() > Width)
325 Extension = EK_SignExt;
327 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
328 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
329 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
332 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
333 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
334 BaseOffs += IndexOffset.getSExtValue()*Scale;
335 Scale *= IndexScale.getSExtValue();
338 // If we already had an occurrance of this index variable, merge this
339 // scale into it. For example, we want to handle:
340 // A[x][x] -> x*16 + x*4 -> x*20
341 // This also ensures that 'x' only appears in the index list once.
342 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
343 if (VarIndices[i].V == Index &&
344 VarIndices[i].Extension == Extension) {
345 Scale += VarIndices[i].Scale;
346 VarIndices.erase(VarIndices.begin()+i);
351 // Make sure that we have a scale that makes sense for this target's
353 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
355 Scale = (int64_t)Scale >> ShiftBits;
359 VariableGEPIndex Entry = {Index, Extension, Scale};
360 VarIndices.push_back(Entry);
364 // Analyze the base pointer next.
365 V = GEPOp->getOperand(0);
366 } while (--MaxLookup);
368 // If the chain of expressions is too deep, just return early.
372 /// GetIndexDifference - Dest and Src are the variable indices from two
373 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
374 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
375 /// difference between the two pointers.
376 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
377 const SmallVectorImpl<VariableGEPIndex> &Src) {
378 if (Src.empty()) return;
380 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
381 const Value *V = Src[i].V;
382 ExtensionKind Extension = Src[i].Extension;
383 int64_t Scale = Src[i].Scale;
385 // Find V in Dest. This is N^2, but pointer indices almost never have more
386 // than a few variable indexes.
387 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
388 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
390 // If we found it, subtract off Scale V's from the entry in Dest. If it
391 // goes to zero, remove the entry.
392 if (Dest[j].Scale != Scale)
393 Dest[j].Scale -= Scale;
395 Dest.erase(Dest.begin()+j);
400 // If we didn't consume this entry, add it to the end of the Dest list.
402 VariableGEPIndex Entry = { V, Extension, -Scale };
403 Dest.push_back(Entry);
408 //===----------------------------------------------------------------------===//
409 // BasicAliasAnalysis Pass
410 //===----------------------------------------------------------------------===//
413 static const Function *getParent(const Value *V) {
414 if (const Instruction *inst = dyn_cast<Instruction>(V))
415 return inst->getParent()->getParent();
417 if (const Argument *arg = dyn_cast<Argument>(V))
418 return arg->getParent();
423 static bool notDifferentParent(const Value *O1, const Value *O2) {
425 const Function *F1 = getParent(O1);
426 const Function *F2 = getParent(O2);
428 return !F1 || !F2 || F1 == F2;
433 /// BasicAliasAnalysis - This is the primary alias analysis implementation.
434 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
435 static char ID; // Class identification, replacement for typeinfo
436 BasicAliasAnalysis() : ImmutablePass(ID) {
437 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
440 virtual void initializePass() {
441 InitializeAliasAnalysis(this);
444 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
445 AU.addRequired<AliasAnalysis>();
448 virtual AliasResult alias(const Location &LocA,
449 const Location &LocB) {
450 assert(Visited.empty() && "Visited must be cleared after use!");
451 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
452 "BasicAliasAnalysis doesn't support interprocedural queries.");
453 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
454 LocB.Ptr, LocB.Size, LocB.TBAATag);
459 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
460 const Location &Loc);
462 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
463 ImmutableCallSite CS2) {
464 // The AliasAnalysis base class has some smarts, lets use them.
465 return AliasAnalysis::getModRefInfo(CS1, CS2);
468 /// pointsToConstantMemory - Chase pointers until we find a (constant
470 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
472 /// getModRefBehavior - Return the behavior when calling the given
474 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
476 /// getModRefBehavior - Return the behavior when calling the given function.
477 /// For use when the call site is not known.
478 virtual ModRefBehavior getModRefBehavior(const Function *F);
480 /// getAdjustedAnalysisPointer - This method is used when a pass implements
481 /// an analysis interface through multiple inheritance. If needed, it
482 /// should override this to adjust the this pointer as needed for the
483 /// specified pass info.
484 virtual void *getAdjustedAnalysisPointer(const void *ID) {
485 if (ID == &AliasAnalysis::ID)
486 return (AliasAnalysis*)this;
491 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
492 SmallPtrSet<const Value*, 16> Visited;
494 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
495 // instruction against another.
496 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
497 const Value *V2, uint64_t V2Size,
498 const MDNode *V2TBAAInfo,
499 const Value *UnderlyingV1, const Value *UnderlyingV2);
501 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
502 // instruction against another.
503 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
504 const MDNode *PNTBAAInfo,
505 const Value *V2, uint64_t V2Size,
506 const MDNode *V2TBAAInfo);
508 /// aliasSelect - Disambiguate a Select instruction against another value.
509 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
510 const MDNode *SITBAAInfo,
511 const Value *V2, uint64_t V2Size,
512 const MDNode *V2TBAAInfo);
514 AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
515 const MDNode *V1TBAATag,
516 const Value *V2, uint64_t V2Size,
517 const MDNode *V2TBAATag);
519 } // End of anonymous namespace
521 // Register this pass...
522 char BasicAliasAnalysis::ID = 0;
523 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
524 "Basic Alias Analysis (stateless AA impl)",
527 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
528 return new BasicAliasAnalysis();
531 /// pointsToConstantMemory - Returns whether the given pointer value
532 /// points to memory that is local to the function, with global constants being
533 /// considered local to all functions.
535 BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
536 assert(Visited.empty() && "Visited must be cleared after use!");
538 unsigned MaxLookup = 8;
539 SmallVector<const Value *, 16> Worklist;
540 Worklist.push_back(Loc.Ptr);
542 const Value *V = GetUnderlyingObject(Worklist.pop_back_val());
543 if (!Visited.insert(V)) {
545 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
548 // An alloca instruction defines local memory.
549 if (OrLocal && isa<AllocaInst>(V))
552 // A global constant counts as local memory for our purposes.
553 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
554 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
555 // global to be marked constant in some modules and non-constant in
556 // others. GV may even be a declaration, not a definition.
557 if (!GV->isConstant()) {
559 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
564 // If both select values point to local memory, then so does the select.
565 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
566 Worklist.push_back(SI->getTrueValue());
567 Worklist.push_back(SI->getFalseValue());
571 // If all values incoming to a phi node point to local memory, then so does
573 if (const PHINode *PN = dyn_cast<PHINode>(V)) {
574 // Don't bother inspecting phi nodes with many operands.
575 if (PN->getNumIncomingValues() > MaxLookup) {
577 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
579 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
580 Worklist.push_back(PN->getIncomingValue(i));
584 // Otherwise be conservative.
586 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
588 } while (!Worklist.empty() && --MaxLookup);
591 return Worklist.empty();
594 /// getModRefBehavior - Return the behavior when calling the given call site.
595 AliasAnalysis::ModRefBehavior
596 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
597 if (CS.doesNotAccessMemory())
598 // Can't do better than this.
599 return DoesNotAccessMemory;
601 ModRefBehavior Min = UnknownModRefBehavior;
603 // If the callsite knows it only reads memory, don't return worse
605 if (CS.onlyReadsMemory())
606 Min = OnlyReadsMemory;
608 // The AliasAnalysis base class has some smarts, lets use them.
609 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
612 /// getModRefBehavior - Return the behavior when calling the given function.
613 /// For use when the call site is not known.
614 AliasAnalysis::ModRefBehavior
615 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
616 // If the function declares it doesn't access memory, we can't do better.
617 if (F->doesNotAccessMemory())
618 return DoesNotAccessMemory;
620 // For intrinsics, we can check the table.
621 if (unsigned iid = F->getIntrinsicID()) {
622 #define GET_INTRINSIC_MODREF_BEHAVIOR
623 #include "llvm/Intrinsics.gen"
624 #undef GET_INTRINSIC_MODREF_BEHAVIOR
627 ModRefBehavior Min = UnknownModRefBehavior;
629 // If the function declares it only reads memory, go with that.
630 if (F->onlyReadsMemory())
631 Min = OnlyReadsMemory;
633 // Otherwise be conservative.
634 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
637 /// getModRefInfo - Check to see if the specified callsite can clobber the
638 /// specified memory object. Since we only look at local properties of this
639 /// function, we really can't say much about this query. We do, however, use
640 /// simple "address taken" analysis on local objects.
641 AliasAnalysis::ModRefResult
642 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
643 const Location &Loc) {
644 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
645 "AliasAnalysis query involving multiple functions!");
647 const Value *Object = GetUnderlyingObject(Loc.Ptr);
649 // If this is a tail call and Loc.Ptr points to a stack location, we know that
650 // the tail call cannot access or modify the local stack.
651 // We cannot exclude byval arguments here; these belong to the caller of
652 // the current function not to the current function, and a tail callee
653 // may reference them.
654 if (isa<AllocaInst>(Object))
655 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
656 if (CI->isTailCall())
659 // If the pointer is to a locally allocated object that does not escape,
660 // then the call can not mod/ref the pointer unless the call takes the pointer
661 // as an argument, and itself doesn't capture it.
662 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
663 isNonEscapingLocalObject(Object)) {
664 bool PassedAsArg = false;
666 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
667 CI != CE; ++CI, ++ArgNo) {
668 // Only look at the no-capture pointer arguments.
669 if (!(*CI)->getType()->isPointerTy() ||
670 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
673 // If this is a no-capture pointer argument, see if we can tell that it
674 // is impossible to alias the pointer we're checking. If not, we have to
675 // assume that the call could touch the pointer, even though it doesn't
677 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
687 ModRefResult Min = ModRef;
689 // Finally, handle specific knowledge of intrinsics.
690 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
692 switch (II->getIntrinsicID()) {
694 case Intrinsic::memcpy:
695 case Intrinsic::memmove: {
696 uint64_t Len = UnknownSize;
697 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
698 Len = LenCI->getZExtValue();
699 Value *Dest = II->getArgOperand(0);
700 Value *Src = II->getArgOperand(1);
701 // If it can't overlap the source dest, then it doesn't modref the loc.
702 if (isNoAlias(Location(Dest, Len), Loc)) {
703 if (isNoAlias(Location(Src, Len), Loc))
705 // If it can't overlap the dest, then worst case it reads the loc.
707 } else if (isNoAlias(Location(Src, Len), Loc)) {
708 // If it can't overlap the source, then worst case it mutates the loc.
713 case Intrinsic::memset:
714 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
715 // will handle it for the variable length case.
716 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
717 uint64_t Len = LenCI->getZExtValue();
718 Value *Dest = II->getArgOperand(0);
719 if (isNoAlias(Location(Dest, Len), Loc))
722 // We know that memset doesn't load anything.
725 case Intrinsic::atomic_cmp_swap:
726 case Intrinsic::atomic_swap:
727 case Intrinsic::atomic_load_add:
728 case Intrinsic::atomic_load_sub:
729 case Intrinsic::atomic_load_and:
730 case Intrinsic::atomic_load_nand:
731 case Intrinsic::atomic_load_or:
732 case Intrinsic::atomic_load_xor:
733 case Intrinsic::atomic_load_max:
734 case Intrinsic::atomic_load_min:
735 case Intrinsic::atomic_load_umax:
736 case Intrinsic::atomic_load_umin:
738 Value *Op1 = II->getArgOperand(0);
739 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType());
740 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
741 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
745 case Intrinsic::lifetime_start:
746 case Intrinsic::lifetime_end:
747 case Intrinsic::invariant_start: {
749 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
750 if (isNoAlias(Location(II->getArgOperand(1),
752 II->getMetadata(LLVMContext::MD_tbaa)),
757 case Intrinsic::invariant_end: {
759 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
760 if (isNoAlias(Location(II->getArgOperand(2),
762 II->getMetadata(LLVMContext::MD_tbaa)),
769 // The AliasAnalysis base class has some smarts, lets use them.
770 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
773 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
774 /// against another pointer. We know that V1 is a GEP, but we don't know
775 /// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1),
776 /// UnderlyingV2 is the same for V2.
778 AliasAnalysis::AliasResult
779 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
780 const Value *V2, uint64_t V2Size,
781 const MDNode *V2TBAAInfo,
782 const Value *UnderlyingV1,
783 const Value *UnderlyingV2) {
784 // If this GEP has been visited before, we're on a use-def cycle.
785 // Such cycles are only valid when PHI nodes are involved or in unreachable
786 // code. The visitPHI function catches cycles containing PHIs, but there
787 // could still be a cycle without PHIs in unreachable code.
788 if (!Visited.insert(GEP1))
791 int64_t GEP1BaseOffset;
792 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
794 // If we have two gep instructions with must-alias'ing base pointers, figure
795 // out if the indexes to the GEP tell us anything about the derived pointer.
796 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
797 // Do the base pointers alias?
798 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
799 UnderlyingV2, UnknownSize, 0);
801 // If we get a No or May, then return it immediately, no amount of analysis
802 // will improve this situation.
803 if (BaseAlias != MustAlias) return BaseAlias;
805 // Otherwise, we have a MustAlias. Since the base pointers alias each other
806 // exactly, see if the computed offset from the common pointer tells us
807 // about the relation of the resulting pointer.
808 const Value *GEP1BasePtr =
809 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
811 int64_t GEP2BaseOffset;
812 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
813 const Value *GEP2BasePtr =
814 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
816 // If DecomposeGEPExpression isn't able to look all the way through the
817 // addressing operation, we must not have TD and this is too complex for us
818 // to handle without it.
819 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
821 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
825 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
826 // symbolic difference.
827 GEP1BaseOffset -= GEP2BaseOffset;
828 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
831 // Check to see if these two pointers are related by the getelementptr
832 // instruction. If one pointer is a GEP with a non-zero index of the other
833 // pointer, we know they cannot alias.
835 // If both accesses are unknown size, we can't do anything useful here.
836 if (V1Size == UnknownSize && V2Size == UnknownSize)
839 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0,
840 V2, V2Size, V2TBAAInfo);
842 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
843 // If V2 is known not to alias GEP base pointer, then the two values
844 // cannot alias per GEP semantics: "A pointer value formed from a
845 // getelementptr instruction is associated with the addresses associated
846 // with the first operand of the getelementptr".
849 const Value *GEP1BasePtr =
850 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
852 // If DecomposeGEPExpression isn't able to look all the way through the
853 // addressing operation, we must not have TD and this is too complex for us
854 // to handle without it.
855 if (GEP1BasePtr != UnderlyingV1) {
857 "DecomposeGEPExpression and GetUnderlyingObject disagree!");
862 // In the two GEP Case, if there is no difference in the offsets of the
863 // computed pointers, the resultant pointers are a must alias. This
864 // hapens when we have two lexically identical GEP's (for example).
866 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
867 // must aliases the GEP, the end result is a must alias also.
868 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
871 // If there is a difference betwen the pointers, but the difference is
872 // less than the size of the associated memory object, then we know
873 // that the objects are partially overlapping.
874 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
875 if (GEP1BaseOffset >= 0 ?
876 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) :
877 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size &&
878 GEP1BaseOffset != INT64_MIN))
882 // If we have a known constant offset, see if this offset is larger than the
883 // access size being queried. If so, and if no variable indices can remove
884 // pieces of this constant, then we know we have a no-alias. For example,
887 // In order to handle cases like &A[100][i] where i is an out of range
888 // subscript, we have to ignore all constant offset pieces that are a multiple
889 // of a scaled index. Do this by removing constant offsets that are a
890 // multiple of any of our variable indices. This allows us to transform
891 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
892 // provides an offset of 4 bytes (assuming a <= 4 byte access).
893 for (unsigned i = 0, e = GEP1VariableIndices.size();
894 i != e && GEP1BaseOffset;++i)
895 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
896 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
898 // If our known offset is bigger than the access size, we know we don't have
900 if (GEP1BaseOffset) {
901 if (GEP1BaseOffset >= 0 ?
902 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
903 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
904 GEP1BaseOffset != INT64_MIN))
911 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
912 /// instruction against another.
913 AliasAnalysis::AliasResult
914 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
915 const MDNode *SITBAAInfo,
916 const Value *V2, uint64_t V2Size,
917 const MDNode *V2TBAAInfo) {
918 // If this select has been visited before, we're on a use-def cycle.
919 // Such cycles are only valid when PHI nodes are involved or in unreachable
920 // code. The visitPHI function catches cycles containing PHIs, but there
921 // could still be a cycle without PHIs in unreachable code.
922 if (!Visited.insert(SI))
925 // If the values are Selects with the same condition, we can do a more precise
926 // check: just check for aliases between the values on corresponding arms.
927 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
928 if (SI->getCondition() == SI2->getCondition()) {
930 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo,
931 SI2->getTrueValue(), V2Size, V2TBAAInfo);
932 if (Alias == MayAlias)
934 AliasResult ThisAlias =
935 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
936 SI2->getFalseValue(), V2Size, V2TBAAInfo);
937 if (ThisAlias != Alias)
942 // If both arms of the Select node NoAlias or MustAlias V2, then returns
943 // NoAlias / MustAlias. Otherwise, returns MayAlias.
945 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
946 if (Alias == MayAlias)
949 // If V2 is visited, the recursive case will have been caught in the
950 // above aliasCheck call, so these subsequent calls to aliasCheck
951 // don't need to assume that V2 is being visited recursively.
954 AliasResult ThisAlias =
955 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
956 if (ThisAlias != Alias)
961 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
963 AliasAnalysis::AliasResult
964 BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
965 const MDNode *PNTBAAInfo,
966 const Value *V2, uint64_t V2Size,
967 const MDNode *V2TBAAInfo) {
968 // The PHI node has already been visited, avoid recursion any further.
969 if (!Visited.insert(PN))
972 // If the values are PHIs in the same block, we can do a more precise
973 // as well as efficient check: just check for aliases between the values
974 // on corresponding edges.
975 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
976 if (PN2->getParent() == PN->getParent()) {
978 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
979 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
981 if (Alias == MayAlias)
983 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
984 AliasResult ThisAlias =
985 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
986 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
988 if (ThisAlias != Alias)
994 SmallPtrSet<Value*, 4> UniqueSrc;
995 SmallVector<Value*, 4> V1Srcs;
996 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
997 Value *PV1 = PN->getIncomingValue(i);
998 if (isa<PHINode>(PV1))
999 // If any of the source itself is a PHI, return MayAlias conservatively
1000 // to avoid compile time explosion. The worst possible case is if both
1001 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
1002 // and 'n' are the number of PHI sources.
1004 if (UniqueSrc.insert(PV1))
1005 V1Srcs.push_back(PV1);
1008 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
1009 V1Srcs[0], PNSize, PNTBAAInfo);
1010 // Early exit if the check of the first PHI source against V2 is MayAlias.
1011 // Other results are not possible.
1012 if (Alias == MayAlias)
1015 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
1016 // NoAlias / MustAlias. Otherwise, returns MayAlias.
1017 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
1018 Value *V = V1Srcs[i];
1020 // If V2 is visited, the recursive case will have been caught in the
1021 // above aliasCheck call, so these subsequent calls to aliasCheck
1022 // don't need to assume that V2 is being visited recursively.
1025 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
1026 V, PNSize, PNTBAAInfo);
1027 if (ThisAlias != Alias || ThisAlias == MayAlias)
1034 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
1035 // such as array references.
1037 AliasAnalysis::AliasResult
1038 BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
1039 const MDNode *V1TBAAInfo,
1040 const Value *V2, uint64_t V2Size,
1041 const MDNode *V2TBAAInfo) {
1042 // If either of the memory references is empty, it doesn't matter what the
1043 // pointer values are.
1044 if (V1Size == 0 || V2Size == 0)
1047 // Strip off any casts if they exist.
1048 V1 = V1->stripPointerCasts();
1049 V2 = V2->stripPointerCasts();
1051 // Are we checking for alias of the same value?
1052 if (V1 == V2) return MustAlias;
1054 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1055 return NoAlias; // Scalars cannot alias each other
1057 // Figure out what objects these things are pointing to if we can.
1058 const Value *O1 = GetUnderlyingObject(V1);
1059 const Value *O2 = GetUnderlyingObject(V2);
1061 // Null values in the default address space don't point to any object, so they
1062 // don't alias any other pointer.
1063 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1064 if (CPN->getType()->getAddressSpace() == 0)
1066 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1067 if (CPN->getType()->getAddressSpace() == 0)
1071 // If V1/V2 point to two different objects we know that we have no alias.
1072 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1075 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1076 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1077 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1080 // Arguments can't alias with local allocations or noalias calls
1081 // in the same function.
1082 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1083 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1086 // Most objects can't alias null.
1087 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1088 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1091 // If one pointer is the result of a call/invoke or load and the other is a
1092 // non-escaping local object within the same function, then we know the
1093 // object couldn't escape to a point where the call could return it.
1095 // Note that if the pointers are in different functions, there are a
1096 // variety of complications. A call with a nocapture argument may still
1097 // temporary store the nocapture argument's value in a temporary memory
1098 // location if that memory location doesn't escape. Or it may pass a
1099 // nocapture value to other functions as long as they don't capture it.
1100 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1102 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1106 // If the size of one access is larger than the entire object on the other
1107 // side, then we know such behavior is undefined and can assume no alias.
1109 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1110 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1113 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1114 // GEP can't simplify, we don't even look at the PHI cases.
1115 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1117 std::swap(V1Size, V2Size);
1120 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1121 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
1122 if (Result != MayAlias) return Result;
1125 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1127 std::swap(V1Size, V2Size);
1129 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1130 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
1131 V2, V2Size, V2TBAAInfo);
1132 if (Result != MayAlias) return Result;
1135 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1137 std::swap(V1Size, V2Size);
1139 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1140 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
1141 V2, V2Size, V2TBAAInfo);
1142 if (Result != MayAlias) return Result;
1145 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
1146 Location(V2, V2Size, V2TBAAInfo));