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/Operator.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Analysis/CaptureTracking.h"
28 #include "llvm/Analysis/MemoryBuiltins.h"
29 #include "llvm/Analysis/ValueTracking.h"
30 #include "llvm/Target/TargetData.h"
31 #include "llvm/ADT/SmallPtrSet.h"
32 #include "llvm/ADT/SmallVector.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/GetElementPtrTypeIterator.h"
38 //===----------------------------------------------------------------------===//
40 //===----------------------------------------------------------------------===//
42 /// isKnownNonNull - Return true if we know that the specified value is never
44 static bool isKnownNonNull(const Value *V) {
45 // Alloca never returns null, malloc might.
46 if (isa<AllocaInst>(V)) return true;
48 // A byval argument is never null.
49 if (const Argument *A = dyn_cast<Argument>(V))
50 return A->hasByValAttr();
52 // Global values are not null unless extern weak.
53 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
54 return !GV->hasExternalWeakLinkage();
58 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
59 /// object that never escapes from the function.
60 static bool isNonEscapingLocalObject(const Value *V) {
61 // If this is a local allocation, check to see if it escapes.
62 if (isa<AllocaInst>(V) || isNoAliasCall(V))
63 // Set StoreCaptures to True so that we can assume in our callers that the
64 // pointer is not the result of a load instruction. Currently
65 // PointerMayBeCaptured doesn't have any special analysis for the
66 // StoreCaptures=false case; if it did, our callers could be refined to be
68 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
70 // If this is an argument that corresponds to a byval or noalias argument,
71 // then it has not escaped before entering the function. Check if it escapes
72 // inside the function.
73 if (const Argument *A = dyn_cast<Argument>(V))
74 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
75 // Don't bother analyzing arguments already known not to escape.
76 if (A->hasNoCaptureAttr())
78 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
83 /// isEscapeSource - Return true if the pointer is one which would have
84 /// been considered an escape by isNonEscapingLocalObject.
85 static bool isEscapeSource(const Value *V) {
86 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
89 // The load case works because isNonEscapingLocalObject considers all
90 // stores to be escapes (it passes true for the StoreCaptures argument
91 // to PointerMayBeCaptured).
98 /// isObjectSmallerThan - Return true if we can prove that the object specified
99 /// by V is smaller than Size.
100 static bool isObjectSmallerThan(const Value *V, unsigned Size,
101 const TargetData &TD) {
102 const Type *AccessTy;
103 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
104 AccessTy = GV->getType()->getElementType();
105 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
106 if (!AI->isArrayAllocation())
107 AccessTy = AI->getType()->getElementType();
110 } else if (const CallInst* CI = extractMallocCall(V)) {
111 if (!isArrayMalloc(V, &TD))
112 // The size is the argument to the malloc call.
113 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
114 return (C->getZExtValue() < Size);
116 } else if (const Argument *A = dyn_cast<Argument>(V)) {
117 if (A->hasByValAttr())
118 AccessTy = cast<PointerType>(A->getType())->getElementType();
125 if (AccessTy->isSized())
126 return TD.getTypeAllocSize(AccessTy) < Size;
130 //===----------------------------------------------------------------------===//
132 //===----------------------------------------------------------------------===//
135 /// NoAA - This class implements the -no-aa pass, which always returns "I
136 /// don't know" for alias queries. NoAA is unlike other alias analysis
137 /// implementations, in that it does not chain to a previous analysis. As
138 /// such it doesn't follow many of the rules that other alias analyses must.
140 struct NoAA : public ImmutablePass, public AliasAnalysis {
141 static char ID; // Class identification, replacement for typeinfo
142 NoAA() : ImmutablePass(ID) {}
143 explicit NoAA(char &PID) : ImmutablePass(PID) { }
145 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
148 virtual void initializePass() {
149 TD = getAnalysisIfAvailable<TargetData>();
152 virtual AliasResult alias(const Value *V1, unsigned V1Size,
153 const Value *V2, unsigned V2Size) {
157 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
158 return UnknownModRefBehavior;
160 virtual ModRefBehavior getModRefBehavior(const Function *F) {
161 return UnknownModRefBehavior;
164 virtual bool pointsToConstantMemory(const Value *P) { return false; }
165 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
166 const Value *P, unsigned Size) {
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*. The incoming Value is known to
219 /// have IntegerType. Note that this looks through extends, so the high bits
220 /// may not be represented in the result.
221 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
222 const TargetData *TD, unsigned Depth) {
223 assert(V->getType()->isIntegerTy() && "Not an integer value");
225 // Limit our recursion depth.
232 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
233 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
234 switch (BOp->getOpcode()) {
236 case Instruction::Or:
237 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
239 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), TD))
242 case Instruction::Add:
243 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
244 Offset += RHSC->getValue();
246 case Instruction::Mul:
247 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
248 Offset *= RHSC->getValue();
249 Scale *= RHSC->getValue();
251 case Instruction::Shl:
252 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
253 Offset <<= RHSC->getValue().getLimitedValue();
254 Scale <<= RHSC->getValue().getLimitedValue();
260 // Since GEP indices are sign extended anyway, we don't care about the high
261 // bits of a sign extended value - just scales and offsets.
262 if (isa<SExtInst>(V)) {
263 Value *CastOp = cast<CastInst>(V)->getOperand(0);
264 unsigned OldWidth = Scale.getBitWidth();
265 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
266 Scale.trunc(SmallWidth);
267 Offset.trunc(SmallWidth);
268 Value *Result = GetLinearExpression(CastOp, Scale, Offset, TD, Depth+1);
269 Scale.zext(OldWidth);
270 Offset.zext(OldWidth);
279 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
280 /// into a base pointer with a constant offset and a number of scaled symbolic
283 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
284 /// the VarIndices vector) are Value*'s that are known to be scaled by the
285 /// specified amount, but which may have other unrepresented high bits. As such,
286 /// the gep cannot necessarily be reconstructed from its decomposed form.
288 /// When TargetData is around, this function is capable of analyzing everything
289 /// that Value::getUnderlyingObject() can look through. When not, it just looks
290 /// through pointer casts.
293 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
294 SmallVectorImpl<VariableGEPIndex> &VarIndices,
295 const TargetData *TD) {
296 // Limit recursion depth to limit compile time in crazy cases.
297 unsigned MaxLookup = 6;
301 // See if this is a bitcast or GEP.
302 const Operator *Op = dyn_cast<Operator>(V);
304 // The only non-operator case we can handle are GlobalAliases.
305 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
306 if (!GA->mayBeOverridden()) {
307 V = GA->getAliasee();
314 if (Op->getOpcode() == Instruction::BitCast) {
315 V = Op->getOperand(0);
319 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
323 // Don't attempt to analyze GEPs over unsized objects.
324 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
325 ->getElementType()->isSized())
328 // If we are lacking TargetData information, we can't compute the offets of
329 // elements computed by GEPs. However, we can handle bitcast equivalent
332 if (!GEPOp->hasAllZeroIndices())
334 V = GEPOp->getOperand(0);
338 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
339 gep_type_iterator GTI = gep_type_begin(GEPOp);
340 for (User::const_op_iterator I = GEPOp->op_begin()+1,
341 E = GEPOp->op_end(); I != E; ++I) {
343 // Compute the (potentially symbolic) offset in bytes for this index.
344 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
345 // For a struct, add the member offset.
346 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
347 if (FieldNo == 0) continue;
349 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
353 // For an array/pointer, add the element offset, explicitly scaled.
354 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
355 if (CIdx->isZero()) continue;
356 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
360 uint64_t Scale = TD->getTypeAllocSize(*GTI);
361 ExtensionKind Extension = EK_NotExtended;
363 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
364 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
365 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
366 Index = GetLinearExpression(Index, IndexScale, IndexOffset, TD, 0);
368 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
369 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
370 BaseOffs += IndexOffset.getZExtValue()*Scale;
371 Scale *= IndexScale.getZExtValue();
374 // If we already had an occurrance of this index variable, merge this
375 // scale into it. For example, we want to handle:
376 // A[x][x] -> x*16 + x*4 -> x*20
377 // This also ensures that 'x' only appears in the index list once.
378 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
379 if (VarIndices[i].V == Index &&
380 VarIndices[i].Extension == Extension) {
381 Scale += VarIndices[i].Scale;
382 VarIndices.erase(VarIndices.begin()+i);
387 // Make sure that we have a scale that makes sense for this target's
389 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
395 VariableGEPIndex Entry = {Index, Extension, Scale};
396 VarIndices.push_back(Entry);
400 // Analyze the base pointer next.
401 V = GEPOp->getOperand(0);
402 } while (--MaxLookup);
404 // If the chain of expressions is too deep, just return early.
408 /// GetIndexDifference - Dest and Src are the variable indices from two
409 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
410 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
411 /// difference between the two pointers.
412 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
413 const SmallVectorImpl<VariableGEPIndex> &Src) {
414 if (Src.empty()) return;
416 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
417 const Value *V = Src[i].V;
418 ExtensionKind Extension = Src[i].Extension;
419 int64_t Scale = Src[i].Scale;
421 // Find V in Dest. This is N^2, but pointer indices almost never have more
422 // than a few variable indexes.
423 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
424 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
426 // If we found it, subtract off Scale V's from the entry in Dest. If it
427 // goes to zero, remove the entry.
428 if (Dest[j].Scale != Scale)
429 Dest[j].Scale -= Scale;
431 Dest.erase(Dest.begin()+j);
436 // If we didn't consume this entry, add it to the end of the Dest list.
438 VariableGEPIndex Entry = { V, Extension, -Scale };
439 Dest.push_back(Entry);
444 //===----------------------------------------------------------------------===//
445 // BasicAliasAnalysis Pass
446 //===----------------------------------------------------------------------===//
449 static const Function *getParent(const Value *V) {
450 if (const Instruction *inst = dyn_cast<Instruction>(V))
451 return inst->getParent()->getParent();
453 if (const Argument *arg = dyn_cast<Argument>(V))
454 return arg->getParent();
459 static bool notDifferentParent(const Value *O1, const Value *O2) {
461 const Function *F1 = getParent(O1);
462 const Function *F2 = getParent(O2);
464 return !F1 || !F2 || F1 == F2;
469 /// BasicAliasAnalysis - This is the default alias analysis implementation.
470 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
471 /// derives from the NoAA class.
472 struct BasicAliasAnalysis : public NoAA {
473 static char ID; // Class identification, replacement for typeinfo
474 BasicAliasAnalysis() : NoAA(ID) {}
476 virtual AliasResult alias(const Value *V1, unsigned V1Size,
477 const Value *V2, unsigned V2Size) {
478 assert(Visited.empty() && "Visited must be cleared after use!");
479 assert(notDifferentParent(V1, V2) &&
480 "BasicAliasAnalysis doesn't support interprocedural queries.");
481 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
486 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
487 const Value *P, unsigned Size);
489 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
490 ImmutableCallSite CS2) {
491 // The AliasAnalysis base class has some smarts, lets use them.
492 return AliasAnalysis::getModRefInfo(CS1, CS2);
495 /// pointsToConstantMemory - Chase pointers until we find a (constant
497 virtual bool pointsToConstantMemory(const Value *P);
499 /// getModRefBehavior - Return the behavior when calling the given
501 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
503 /// getModRefBehavior - Return the behavior when calling the given function.
504 /// For use when the call site is not known.
505 virtual ModRefBehavior getModRefBehavior(const Function *F);
507 /// getAdjustedAnalysisPointer - This method is used when a pass implements
508 /// an analysis interface through multiple inheritance. If needed, it
509 /// should override this to adjust the this pointer as needed for the
510 /// specified pass info.
511 virtual void *getAdjustedAnalysisPointer(const void *ID) {
512 if (ID == &AliasAnalysis::ID)
513 return (AliasAnalysis*)this;
518 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
519 SmallPtrSet<const Value*, 16> Visited;
521 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
522 // instruction against another.
523 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
524 const Value *V2, unsigned V2Size,
525 const Value *UnderlyingV1, const Value *UnderlyingV2);
527 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
528 // instruction against another.
529 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
530 const Value *V2, unsigned V2Size);
532 /// aliasSelect - Disambiguate a Select instruction against another value.
533 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
534 const Value *V2, unsigned V2Size);
536 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
537 const Value *V2, unsigned V2Size);
539 } // End of anonymous namespace
541 // Register this pass...
542 char BasicAliasAnalysis::ID = 0;
543 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
544 "Basic Alias Analysis (default AA impl)",
547 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
548 return new BasicAliasAnalysis();
552 /// pointsToConstantMemory - Chase pointers until we find a (constant
554 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
555 if (const GlobalVariable *GV =
556 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
557 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
558 // global to be marked constant in some modules and non-constant in others.
559 // GV may even be a declaration, not a definition.
560 return GV->isConstant();
562 return NoAA::pointsToConstantMemory(P);
565 /// getModRefBehavior - Return the behavior when calling the given call site.
566 AliasAnalysis::ModRefBehavior
567 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
568 if (CS.doesNotAccessMemory())
569 // Can't do better than this.
570 return DoesNotAccessMemory;
572 ModRefBehavior Min = UnknownModRefBehavior;
574 // If the callsite knows it only reads memory, don't return worse
576 if (CS.onlyReadsMemory())
577 Min = OnlyReadsMemory;
579 // The AliasAnalysis base class has some smarts, lets use them.
580 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
583 /// getModRefBehavior - Return the behavior when calling the given function.
584 /// For use when the call site is not known.
585 AliasAnalysis::ModRefBehavior
586 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
587 if (F->doesNotAccessMemory())
588 // Can't do better than this.
589 return DoesNotAccessMemory;
590 if (F->onlyReadsMemory())
591 return OnlyReadsMemory;
592 if (unsigned id = F->getIntrinsicID())
593 return getIntrinsicModRefBehavior(id);
595 return NoAA::getModRefBehavior(F);
598 /// getModRefInfo - Check to see if the specified callsite can clobber the
599 /// specified memory object. Since we only look at local properties of this
600 /// function, we really can't say much about this query. We do, however, use
601 /// simple "address taken" analysis on local objects.
602 AliasAnalysis::ModRefResult
603 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
604 const Value *P, unsigned Size) {
605 assert(notDifferentParent(CS.getInstruction(), P) &&
606 "AliasAnalysis query involving multiple functions!");
608 const Value *Object = P->getUnderlyingObject();
610 // If this is a tail call and P points to a stack location, we know that
611 // the tail call cannot access or modify the local stack.
612 // We cannot exclude byval arguments here; these belong to the caller of
613 // the current function not to the current function, and a tail callee
614 // may reference them.
615 if (isa<AllocaInst>(Object))
616 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
617 if (CI->isTailCall())
620 // If the pointer is to a locally allocated object that does not escape,
621 // then the call can not mod/ref the pointer unless the call takes the pointer
622 // as an argument, and itself doesn't capture it.
623 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
624 isNonEscapingLocalObject(Object)) {
625 bool PassedAsArg = false;
627 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
628 CI != CE; ++CI, ++ArgNo) {
629 // Only look at the no-capture pointer arguments.
630 if (!(*CI)->getType()->isPointerTy() ||
631 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
634 // If this is a no-capture pointer argument, see if we can tell that it
635 // is impossible to alias the pointer we're checking. If not, we have to
636 // assume that the call could touch the pointer, even though it doesn't
638 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
648 // Finally, handle specific knowledge of intrinsics.
649 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
651 switch (II->getIntrinsicID()) {
653 case Intrinsic::memcpy:
654 case Intrinsic::memmove: {
655 unsigned Len = UnknownSize;
656 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
657 Len = LenCI->getZExtValue();
658 Value *Dest = II->getArgOperand(0);
659 Value *Src = II->getArgOperand(1);
660 if (isNoAlias(Dest, Len, P, Size)) {
661 if (isNoAlias(Src, Len, P, Size))
667 case Intrinsic::memset:
668 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
669 // will handle it for the variable length case.
670 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
671 unsigned Len = LenCI->getZExtValue();
672 Value *Dest = II->getArgOperand(0);
673 if (isNoAlias(Dest, Len, P, Size))
677 case Intrinsic::atomic_cmp_swap:
678 case Intrinsic::atomic_swap:
679 case Intrinsic::atomic_load_add:
680 case Intrinsic::atomic_load_sub:
681 case Intrinsic::atomic_load_and:
682 case Intrinsic::atomic_load_nand:
683 case Intrinsic::atomic_load_or:
684 case Intrinsic::atomic_load_xor:
685 case Intrinsic::atomic_load_max:
686 case Intrinsic::atomic_load_min:
687 case Intrinsic::atomic_load_umax:
688 case Intrinsic::atomic_load_umin:
690 Value *Op1 = II->getArgOperand(0);
691 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
692 if (isNoAlias(Op1, Op1Size, P, Size))
696 case Intrinsic::lifetime_start:
697 case Intrinsic::lifetime_end:
698 case Intrinsic::invariant_start: {
700 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
701 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
705 case Intrinsic::invariant_end: {
707 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
708 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
714 // The AliasAnalysis base class has some smarts, lets use them.
715 return AliasAnalysis::getModRefInfo(CS, P, Size);
719 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
720 /// against another pointer. We know that V1 is a GEP, but we don't know
721 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
722 /// UnderlyingV2 is the same for V2.
724 AliasAnalysis::AliasResult
725 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
726 const Value *V2, unsigned V2Size,
727 const Value *UnderlyingV1,
728 const Value *UnderlyingV2) {
729 // If this GEP has been visited before, we're on a use-def cycle.
730 // Such cycles are only valid when PHI nodes are involved or in unreachable
731 // code. The visitPHI function catches cycles containing PHIs, but there
732 // could still be a cycle without PHIs in unreachable code.
733 if (!Visited.insert(GEP1))
736 int64_t GEP1BaseOffset;
737 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
739 // If we have two gep instructions with must-alias'ing base pointers, figure
740 // out if the indexes to the GEP tell us anything about the derived pointer.
741 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
742 // Do the base pointers alias?
743 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
744 UnderlyingV2, UnknownSize);
746 // If we get a No or May, then return it immediately, no amount of analysis
747 // will improve this situation.
748 if (BaseAlias != MustAlias) return BaseAlias;
750 // Otherwise, we have a MustAlias. Since the base pointers alias each other
751 // exactly, see if the computed offset from the common pointer tells us
752 // about the relation of the resulting pointer.
753 const Value *GEP1BasePtr =
754 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
756 int64_t GEP2BaseOffset;
757 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
758 const Value *GEP2BasePtr =
759 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
761 // If DecomposeGEPExpression isn't able to look all the way through the
762 // addressing operation, we must not have TD and this is too complex for us
763 // to handle without it.
764 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
766 "DecomposeGEPExpression and getUnderlyingObject disagree!");
770 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
771 // symbolic difference.
772 GEP1BaseOffset -= GEP2BaseOffset;
773 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
776 // Check to see if these two pointers are related by the getelementptr
777 // instruction. If one pointer is a GEP with a non-zero index of the other
778 // pointer, we know they cannot alias.
780 // If both accesses are unknown size, we can't do anything useful here.
781 if (V1Size == UnknownSize && V2Size == UnknownSize)
784 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
786 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
787 // If V2 is known not to alias GEP base pointer, then the two values
788 // cannot alias per GEP semantics: "A pointer value formed from a
789 // getelementptr instruction is associated with the addresses associated
790 // with the first operand of the getelementptr".
793 const Value *GEP1BasePtr =
794 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
796 // If DecomposeGEPExpression isn't able to look all the way through the
797 // addressing operation, we must not have TD and this is too complex for us
798 // to handle without it.
799 if (GEP1BasePtr != UnderlyingV1) {
801 "DecomposeGEPExpression and getUnderlyingObject disagree!");
806 // In the two GEP Case, if there is no difference in the offsets of the
807 // computed pointers, the resultant pointers are a must alias. This
808 // hapens when we have two lexically identical GEP's (for example).
810 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
811 // must aliases the GEP, the end result is a must alias also.
812 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
815 // If we have a known constant offset, see if this offset is larger than the
816 // access size being queried. If so, and if no variable indices can remove
817 // pieces of this constant, then we know we have a no-alias. For example,
820 // In order to handle cases like &A[100][i] where i is an out of range
821 // subscript, we have to ignore all constant offset pieces that are a multiple
822 // of a scaled index. Do this by removing constant offsets that are a
823 // multiple of any of our variable indices. This allows us to transform
824 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
825 // provides an offset of 4 bytes (assuming a <= 4 byte access).
826 for (unsigned i = 0, e = GEP1VariableIndices.size();
827 i != e && GEP1BaseOffset;++i)
828 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
829 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
831 // If our known offset is bigger than the access size, we know we don't have
833 if (GEP1BaseOffset) {
834 if (GEP1BaseOffset >= (int64_t)V2Size ||
835 GEP1BaseOffset <= -(int64_t)V1Size)
842 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
843 /// instruction against another.
844 AliasAnalysis::AliasResult
845 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
846 const Value *V2, unsigned V2Size) {
847 // If this select has been visited before, we're on a use-def cycle.
848 // Such cycles are only valid when PHI nodes are involved or in unreachable
849 // code. The visitPHI function catches cycles containing PHIs, but there
850 // could still be a cycle without PHIs in unreachable code.
851 if (!Visited.insert(SI))
854 // If the values are Selects with the same condition, we can do a more precise
855 // check: just check for aliases between the values on corresponding arms.
856 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
857 if (SI->getCondition() == SI2->getCondition()) {
859 aliasCheck(SI->getTrueValue(), SISize,
860 SI2->getTrueValue(), V2Size);
861 if (Alias == MayAlias)
863 AliasResult ThisAlias =
864 aliasCheck(SI->getFalseValue(), SISize,
865 SI2->getFalseValue(), V2Size);
866 if (ThisAlias != Alias)
871 // If both arms of the Select node NoAlias or MustAlias V2, then returns
872 // NoAlias / MustAlias. Otherwise, returns MayAlias.
874 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
875 if (Alias == MayAlias)
878 // If V2 is visited, the recursive case will have been caught in the
879 // above aliasCheck call, so these subsequent calls to aliasCheck
880 // don't need to assume that V2 is being visited recursively.
883 AliasResult ThisAlias =
884 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
885 if (ThisAlias != Alias)
890 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
892 AliasAnalysis::AliasResult
893 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
894 const Value *V2, unsigned V2Size) {
895 // The PHI node has already been visited, avoid recursion any further.
896 if (!Visited.insert(PN))
899 // If the values are PHIs in the same block, we can do a more precise
900 // as well as efficient check: just check for aliases between the values
901 // on corresponding edges.
902 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
903 if (PN2->getParent() == PN->getParent()) {
905 aliasCheck(PN->getIncomingValue(0), PNSize,
906 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
908 if (Alias == MayAlias)
910 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
911 AliasResult ThisAlias =
912 aliasCheck(PN->getIncomingValue(i), PNSize,
913 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
915 if (ThisAlias != Alias)
921 SmallPtrSet<Value*, 4> UniqueSrc;
922 SmallVector<Value*, 4> V1Srcs;
923 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
924 Value *PV1 = PN->getIncomingValue(i);
925 if (isa<PHINode>(PV1))
926 // If any of the source itself is a PHI, return MayAlias conservatively
927 // to avoid compile time explosion. The worst possible case is if both
928 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
929 // and 'n' are the number of PHI sources.
931 if (UniqueSrc.insert(PV1))
932 V1Srcs.push_back(PV1);
935 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
936 // Early exit if the check of the first PHI source against V2 is MayAlias.
937 // Other results are not possible.
938 if (Alias == MayAlias)
941 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
942 // NoAlias / MustAlias. Otherwise, returns MayAlias.
943 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
944 Value *V = V1Srcs[i];
946 // If V2 is visited, the recursive case will have been caught in the
947 // above aliasCheck call, so these subsequent calls to aliasCheck
948 // don't need to assume that V2 is being visited recursively.
951 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
952 if (ThisAlias != Alias || ThisAlias == MayAlias)
959 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
960 // such as array references.
962 AliasAnalysis::AliasResult
963 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
964 const Value *V2, unsigned V2Size) {
965 // If either of the memory references is empty, it doesn't matter what the
966 // pointer values are.
967 if (V1Size == 0 || V2Size == 0)
970 // Strip off any casts if they exist.
971 V1 = V1->stripPointerCasts();
972 V2 = V2->stripPointerCasts();
974 // Are we checking for alias of the same value?
975 if (V1 == V2) return MustAlias;
977 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
978 return NoAlias; // Scalars cannot alias each other
980 // Figure out what objects these things are pointing to if we can.
981 const Value *O1 = V1->getUnderlyingObject();
982 const Value *O2 = V2->getUnderlyingObject();
984 // Null values in the default address space don't point to any object, so they
985 // don't alias any other pointer.
986 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
987 if (CPN->getType()->getAddressSpace() == 0)
989 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
990 if (CPN->getType()->getAddressSpace() == 0)
994 // If V1/V2 point to two different objects we know that we have no alias.
995 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
998 // Constant pointers can't alias with non-const isIdentifiedObject objects.
999 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1000 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1003 // Arguments can't alias with local allocations or noalias calls
1004 // in the same function.
1005 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1006 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1009 // Most objects can't alias null.
1010 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1011 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1014 // If one pointer is the result of a call/invoke or load and the other is a
1015 // non-escaping local object within the same function, then we know the
1016 // object couldn't escape to a point where the call could return it.
1018 // Note that if the pointers are in different functions, there are a
1019 // variety of complications. A call with a nocapture argument may still
1020 // temporary store the nocapture argument's value in a temporary memory
1021 // location if that memory location doesn't escape. Or it may pass a
1022 // nocapture value to other functions as long as they don't capture it.
1023 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1025 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1029 // If the size of one access is larger than the entire object on the other
1030 // side, then we know such behavior is undefined and can assume no alias.
1032 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1033 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1036 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1037 // GEP can't simplify, we don't even look at the PHI cases.
1038 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1040 std::swap(V1Size, V2Size);
1043 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1044 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1046 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1048 std::swap(V1Size, V2Size);
1050 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1051 return aliasPHI(PN, V1Size, V2, V2Size);
1053 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1055 std::swap(V1Size, V2Size);
1057 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1058 return aliasSelect(S1, V1Size, V2, V2Size);
1060 return NoAA::alias(V1, V1Size, V2, V2Size);
1063 // Make sure that anything that uses AliasAnalysis pulls in this file.
1064 DEFINING_FILE_FOR(BasicAliasAnalysis)