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, LocB.Ptr, LocB.Size);
513 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
514 const Location &Loc);
516 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
517 ImmutableCallSite CS2) {
518 // The AliasAnalysis base class has some smarts, lets use them.
519 return AliasAnalysis::getModRefInfo(CS1, CS2);
522 /// pointsToConstantMemory - Chase pointers until we find a (constant
524 virtual bool pointsToConstantMemory(const Location &Loc);
526 /// getModRefBehavior - Return the behavior when calling the given
528 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
530 /// getModRefBehavior - Return the behavior when calling the given function.
531 /// For use when the call site is not known.
532 virtual ModRefBehavior getModRefBehavior(const Function *F);
534 /// getAdjustedAnalysisPointer - This method is used when a pass implements
535 /// an analysis interface through multiple inheritance. If needed, it
536 /// should override this to adjust the this pointer as needed for the
537 /// specified pass info.
538 virtual void *getAdjustedAnalysisPointer(const void *ID) {
539 if (ID == &AliasAnalysis::ID)
540 return (AliasAnalysis*)this;
545 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
546 SmallPtrSet<const Value*, 16> Visited;
548 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
549 // instruction against another.
550 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
551 const Value *V2, unsigned V2Size,
552 const Value *UnderlyingV1, const Value *UnderlyingV2);
554 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
555 // instruction against another.
556 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
557 const Value *V2, unsigned V2Size);
559 /// aliasSelect - Disambiguate a Select instruction against another value.
560 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
561 const Value *V2, unsigned V2Size);
563 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
564 const Value *V2, unsigned V2Size);
566 } // End of anonymous namespace
568 // Register this pass...
569 char BasicAliasAnalysis::ID = 0;
570 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
571 "Basic Alias Analysis (default AA impl)",
574 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
575 return new BasicAliasAnalysis();
579 /// pointsToConstantMemory - Chase pointers until we find a (constant
581 bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
582 if (const GlobalVariable *GV =
583 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject()))
584 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
585 // global to be marked constant in some modules and non-constant in others.
586 // GV may even be a declaration, not a definition.
587 return GV->isConstant();
589 return AliasAnalysis::pointsToConstantMemory(Loc);
592 /// getModRefBehavior - Return the behavior when calling the given call site.
593 AliasAnalysis::ModRefBehavior
594 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
595 if (CS.doesNotAccessMemory())
596 // Can't do better than this.
597 return DoesNotAccessMemory;
599 ModRefBehavior Min = UnknownModRefBehavior;
601 // If the callsite knows it only reads memory, don't return worse
603 if (CS.onlyReadsMemory())
604 Min = OnlyReadsMemory;
606 // The AliasAnalysis base class has some smarts, lets use them.
607 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
610 /// getModRefBehavior - Return the behavior when calling the given function.
611 /// For use when the call site is not known.
612 AliasAnalysis::ModRefBehavior
613 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
614 if (F->doesNotAccessMemory())
615 // Can't do better than this.
616 return DoesNotAccessMemory;
617 if (F->onlyReadsMemory())
618 return OnlyReadsMemory;
619 if (unsigned id = F->getIntrinsicID())
620 return getIntrinsicModRefBehavior(id);
622 return AliasAnalysis::getModRefBehavior(F);
625 /// getModRefInfo - Check to see if the specified callsite can clobber the
626 /// specified memory object. Since we only look at local properties of this
627 /// function, we really can't say much about this query. We do, however, use
628 /// simple "address taken" analysis on local objects.
629 AliasAnalysis::ModRefResult
630 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
631 const Location &Loc) {
632 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
633 "AliasAnalysis query involving multiple functions!");
635 const Value *Object = Loc.Ptr->getUnderlyingObject();
637 // If this is a tail call and Loc.Ptr points to a stack location, we know that
638 // the tail call cannot access or modify the local stack.
639 // We cannot exclude byval arguments here; these belong to the caller of
640 // the current function not to the current function, and a tail callee
641 // may reference them.
642 if (isa<AllocaInst>(Object))
643 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
644 if (CI->isTailCall())
647 // If the pointer is to a locally allocated object that does not escape,
648 // then the call can not mod/ref the pointer unless the call takes the pointer
649 // as an argument, and itself doesn't capture it.
650 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
651 isNonEscapingLocalObject(Object)) {
652 bool PassedAsArg = false;
654 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
655 CI != CE; ++CI, ++ArgNo) {
656 // Only look at the no-capture pointer arguments.
657 if (!(*CI)->getType()->isPointerTy() ||
658 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
661 // If this is a no-capture pointer argument, see if we can tell that it
662 // is impossible to alias the pointer we're checking. If not, we have to
663 // assume that the call could touch the pointer, even though it doesn't
665 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
675 // Finally, handle specific knowledge of intrinsics.
676 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
678 switch (II->getIntrinsicID()) {
680 case Intrinsic::memcpy:
681 case Intrinsic::memmove: {
682 unsigned Len = UnknownSize;
683 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
684 Len = LenCI->getZExtValue();
685 Value *Dest = II->getArgOperand(0);
686 Value *Src = II->getArgOperand(1);
687 if (isNoAlias(Location(Dest, Len), Loc)) {
688 if (isNoAlias(Location(Src, Len), Loc))
694 case Intrinsic::memset:
695 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
696 // will handle it for the variable length case.
697 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
698 unsigned Len = LenCI->getZExtValue();
699 Value *Dest = II->getArgOperand(0);
700 if (isNoAlias(Location(Dest, Len), Loc))
704 case Intrinsic::atomic_cmp_swap:
705 case Intrinsic::atomic_swap:
706 case Intrinsic::atomic_load_add:
707 case Intrinsic::atomic_load_sub:
708 case Intrinsic::atomic_load_and:
709 case Intrinsic::atomic_load_nand:
710 case Intrinsic::atomic_load_or:
711 case Intrinsic::atomic_load_xor:
712 case Intrinsic::atomic_load_max:
713 case Intrinsic::atomic_load_min:
714 case Intrinsic::atomic_load_umax:
715 case Intrinsic::atomic_load_umin:
717 Value *Op1 = II->getArgOperand(0);
718 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
719 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
720 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
724 case Intrinsic::lifetime_start:
725 case Intrinsic::lifetime_end:
726 case Intrinsic::invariant_start: {
728 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
729 if (isNoAlias(Location(II->getArgOperand(1),
731 II->getMetadata(LLVMContext::MD_tbaa)),
736 case Intrinsic::invariant_end: {
738 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
739 if (isNoAlias(Location(II->getArgOperand(2),
741 II->getMetadata(LLVMContext::MD_tbaa)),
748 // The AliasAnalysis base class has some smarts, lets use them.
749 return AliasAnalysis::getModRefInfo(CS, Loc);
752 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
753 /// against another pointer. We know that V1 is a GEP, but we don't know
754 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
755 /// UnderlyingV2 is the same for V2.
757 AliasAnalysis::AliasResult
758 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
759 const Value *V2, unsigned V2Size,
760 const Value *UnderlyingV1,
761 const Value *UnderlyingV2) {
762 // If this GEP has been visited before, we're on a use-def cycle.
763 // Such cycles are only valid when PHI nodes are involved or in unreachable
764 // code. The visitPHI function catches cycles containing PHIs, but there
765 // could still be a cycle without PHIs in unreachable code.
766 if (!Visited.insert(GEP1))
769 int64_t GEP1BaseOffset;
770 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
772 // If we have two gep instructions with must-alias'ing base pointers, figure
773 // out if the indexes to the GEP tell us anything about the derived pointer.
774 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
775 // Do the base pointers alias?
776 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
777 UnderlyingV2, UnknownSize);
779 // If we get a No or May, then return it immediately, no amount of analysis
780 // will improve this situation.
781 if (BaseAlias != MustAlias) return BaseAlias;
783 // Otherwise, we have a MustAlias. Since the base pointers alias each other
784 // exactly, see if the computed offset from the common pointer tells us
785 // about the relation of the resulting pointer.
786 const Value *GEP1BasePtr =
787 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
789 int64_t GEP2BaseOffset;
790 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
791 const Value *GEP2BasePtr =
792 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
794 // If DecomposeGEPExpression isn't able to look all the way through the
795 // addressing operation, we must not have TD and this is too complex for us
796 // to handle without it.
797 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
799 "DecomposeGEPExpression and getUnderlyingObject disagree!");
803 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
804 // symbolic difference.
805 GEP1BaseOffset -= GEP2BaseOffset;
806 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
809 // Check to see if these two pointers are related by the getelementptr
810 // instruction. If one pointer is a GEP with a non-zero index of the other
811 // pointer, we know they cannot alias.
813 // If both accesses are unknown size, we can't do anything useful here.
814 if (V1Size == UnknownSize && V2Size == UnknownSize)
817 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
819 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
820 // If V2 is known not to alias GEP base pointer, then the two values
821 // cannot alias per GEP semantics: "A pointer value formed from a
822 // getelementptr instruction is associated with the addresses associated
823 // with the first operand of the getelementptr".
826 const Value *GEP1BasePtr =
827 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
829 // If DecomposeGEPExpression isn't able to look all the way through the
830 // addressing operation, we must not have TD and this is too complex for us
831 // to handle without it.
832 if (GEP1BasePtr != UnderlyingV1) {
834 "DecomposeGEPExpression and getUnderlyingObject disagree!");
839 // In the two GEP Case, if there is no difference in the offsets of the
840 // computed pointers, the resultant pointers are a must alias. This
841 // hapens when we have two lexically identical GEP's (for example).
843 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
844 // must aliases the GEP, the end result is a must alias also.
845 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
848 // If we have a known constant offset, see if this offset is larger than the
849 // access size being queried. If so, and if no variable indices can remove
850 // pieces of this constant, then we know we have a no-alias. For example,
853 // In order to handle cases like &A[100][i] where i is an out of range
854 // subscript, we have to ignore all constant offset pieces that are a multiple
855 // of a scaled index. Do this by removing constant offsets that are a
856 // multiple of any of our variable indices. This allows us to transform
857 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
858 // provides an offset of 4 bytes (assuming a <= 4 byte access).
859 for (unsigned i = 0, e = GEP1VariableIndices.size();
860 i != e && GEP1BaseOffset;++i)
861 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
862 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
864 // If our known offset is bigger than the access size, we know we don't have
866 if (GEP1BaseOffset) {
867 if (GEP1BaseOffset >= (int64_t)V2Size ||
868 GEP1BaseOffset <= -(int64_t)V1Size)
875 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
876 /// instruction against another.
877 AliasAnalysis::AliasResult
878 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
879 const Value *V2, unsigned V2Size) {
880 // If this select has been visited before, we're on a use-def cycle.
881 // Such cycles are only valid when PHI nodes are involved or in unreachable
882 // code. The visitPHI function catches cycles containing PHIs, but there
883 // could still be a cycle without PHIs in unreachable code.
884 if (!Visited.insert(SI))
887 // If the values are Selects with the same condition, we can do a more precise
888 // check: just check for aliases between the values on corresponding arms.
889 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
890 if (SI->getCondition() == SI2->getCondition()) {
892 aliasCheck(SI->getTrueValue(), SISize,
893 SI2->getTrueValue(), V2Size);
894 if (Alias == MayAlias)
896 AliasResult ThisAlias =
897 aliasCheck(SI->getFalseValue(), SISize,
898 SI2->getFalseValue(), V2Size);
899 if (ThisAlias != Alias)
904 // If both arms of the Select node NoAlias or MustAlias V2, then returns
905 // NoAlias / MustAlias. Otherwise, returns MayAlias.
907 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
908 if (Alias == MayAlias)
911 // If V2 is visited, the recursive case will have been caught in the
912 // above aliasCheck call, so these subsequent calls to aliasCheck
913 // don't need to assume that V2 is being visited recursively.
916 AliasResult ThisAlias =
917 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
918 if (ThisAlias != Alias)
923 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
925 AliasAnalysis::AliasResult
926 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
927 const Value *V2, unsigned V2Size) {
928 // The PHI node has already been visited, avoid recursion any further.
929 if (!Visited.insert(PN))
932 // If the values are PHIs in the same block, we can do a more precise
933 // as well as efficient check: just check for aliases between the values
934 // on corresponding edges.
935 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
936 if (PN2->getParent() == PN->getParent()) {
938 aliasCheck(PN->getIncomingValue(0), PNSize,
939 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
941 if (Alias == MayAlias)
943 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
944 AliasResult ThisAlias =
945 aliasCheck(PN->getIncomingValue(i), PNSize,
946 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
948 if (ThisAlias != Alias)
954 SmallPtrSet<Value*, 4> UniqueSrc;
955 SmallVector<Value*, 4> V1Srcs;
956 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
957 Value *PV1 = PN->getIncomingValue(i);
958 if (isa<PHINode>(PV1))
959 // If any of the source itself is a PHI, return MayAlias conservatively
960 // to avoid compile time explosion. The worst possible case is if both
961 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
962 // and 'n' are the number of PHI sources.
964 if (UniqueSrc.insert(PV1))
965 V1Srcs.push_back(PV1);
968 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
969 // Early exit if the check of the first PHI source against V2 is MayAlias.
970 // Other results are not possible.
971 if (Alias == MayAlias)
974 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
975 // NoAlias / MustAlias. Otherwise, returns MayAlias.
976 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
977 Value *V = V1Srcs[i];
979 // If V2 is visited, the recursive case will have been caught in the
980 // above aliasCheck call, so these subsequent calls to aliasCheck
981 // don't need to assume that V2 is being visited recursively.
984 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
985 if (ThisAlias != Alias || ThisAlias == MayAlias)
992 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
993 // such as array references.
995 AliasAnalysis::AliasResult
996 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
997 const Value *V2, unsigned V2Size) {
998 // If either of the memory references is empty, it doesn't matter what the
999 // pointer values are.
1000 if (V1Size == 0 || V2Size == 0)
1003 // Strip off any casts if they exist.
1004 V1 = V1->stripPointerCasts();
1005 V2 = V2->stripPointerCasts();
1007 // Are we checking for alias of the same value?
1008 if (V1 == V2) return MustAlias;
1010 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1011 return NoAlias; // Scalars cannot alias each other
1013 // Figure out what objects these things are pointing to if we can.
1014 const Value *O1 = V1->getUnderlyingObject();
1015 const Value *O2 = V2->getUnderlyingObject();
1017 // Null values in the default address space don't point to any object, so they
1018 // don't alias any other pointer.
1019 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1020 if (CPN->getType()->getAddressSpace() == 0)
1022 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1023 if (CPN->getType()->getAddressSpace() == 0)
1027 // If V1/V2 point to two different objects we know that we have no alias.
1028 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1031 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1032 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1033 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1036 // Arguments can't alias with local allocations or noalias calls
1037 // in the same function.
1038 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1039 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1042 // Most objects can't alias null.
1043 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1044 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1047 // If one pointer is the result of a call/invoke or load and the other is a
1048 // non-escaping local object within the same function, then we know the
1049 // object couldn't escape to a point where the call could return it.
1051 // Note that if the pointers are in different functions, there are a
1052 // variety of complications. A call with a nocapture argument may still
1053 // temporary store the nocapture argument's value in a temporary memory
1054 // location if that memory location doesn't escape. Or it may pass a
1055 // nocapture value to other functions as long as they don't capture it.
1056 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1058 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1062 // If the size of one access is larger than the entire object on the other
1063 // side, then we know such behavior is undefined and can assume no alias.
1065 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1066 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1069 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1070 // GEP can't simplify, we don't even look at the PHI cases.
1071 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1073 std::swap(V1Size, V2Size);
1076 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
1077 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1078 if (Result != MayAlias) return Result;
1081 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1083 std::swap(V1Size, V2Size);
1085 if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
1086 AliasResult Result = aliasPHI(PN, V1Size, V2, V2Size);
1087 if (Result != MayAlias) return Result;
1090 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1092 std::swap(V1Size, V2Size);
1094 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
1095 AliasResult Result = aliasSelect(S1, V1Size, V2, V2Size);
1096 if (Result != MayAlias) return Result;
1099 return AliasAnalysis::alias(Location(V1, V1Size), Location(V2, V2Size));
1102 // Make sure that anything that uses AliasAnalysis pulls in this file.
1103 DEFINING_FILE_FOR(BasicAliasAnalysis)