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 //===----------------------------------------------------------------------===//
202 /// GetLinearExpression - Analyze the specified value as a linear expression:
203 /// "A*V + B", where A and B are constant integers. Return the scale and offset
204 /// values as APInts and return V as a Value*. The incoming Value is known to
205 /// have IntegerType. Note that this looks through extends, so the high bits
206 /// may not be represented in the result.
207 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
208 const TargetData *TD, unsigned Depth) {
209 assert(V->getType()->isIntegerTy() && "Not an integer value");
211 // Limit our recursion depth.
218 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
219 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
220 switch (BOp->getOpcode()) {
222 case Instruction::Or:
223 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
225 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), TD))
228 case Instruction::Add:
229 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
230 Offset += RHSC->getValue();
232 case Instruction::Mul:
233 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
234 Offset *= RHSC->getValue();
235 Scale *= RHSC->getValue();
237 case Instruction::Shl:
238 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, TD, Depth+1);
239 Offset <<= RHSC->getValue().getLimitedValue();
240 Scale <<= RHSC->getValue().getLimitedValue();
246 // Since GEP indices are sign extended anyway, we don't care about the high
247 // bits of a sign extended value - just scales and offsets.
248 if (isa<SExtInst>(V)) {
249 Value *CastOp = cast<CastInst>(V)->getOperand(0);
250 unsigned OldWidth = Scale.getBitWidth();
251 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
252 Scale.trunc(SmallWidth);
253 Offset.trunc(SmallWidth);
254 Value *Result = GetLinearExpression(CastOp, Scale, Offset, TD, Depth+1);
255 Scale.zext(OldWidth);
256 Offset.zext(OldWidth);
265 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
266 /// into a base pointer with a constant offset and a number of scaled symbolic
269 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
270 /// the VarIndices vector) are Value*'s that are known to be scaled by the
271 /// specified amount, but which may have other unrepresented high bits. As such,
272 /// the gep cannot necessarily be reconstructed from its decomposed form.
274 /// When TargetData is around, this function is capable of analyzing everything
275 /// that Value::getUnderlyingObject() can look through. When not, it just looks
276 /// through pointer casts.
279 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
280 SmallVectorImpl<std::pair<const Value*, int64_t> > &VarIndices,
281 const TargetData *TD) {
282 // Limit recursion depth to limit compile time in crazy cases.
283 unsigned MaxLookup = 6;
287 // See if this is a bitcast or GEP.
288 const Operator *Op = dyn_cast<Operator>(V);
290 // The only non-operator case we can handle are GlobalAliases.
291 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
292 if (!GA->mayBeOverridden()) {
293 V = GA->getAliasee();
300 if (Op->getOpcode() == Instruction::BitCast) {
301 V = Op->getOperand(0);
305 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
309 // Don't attempt to analyze GEPs over unsized objects.
310 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
311 ->getElementType()->isSized())
314 // If we are lacking TargetData information, we can't compute the offets of
315 // elements computed by GEPs. However, we can handle bitcast equivalent
318 if (!GEPOp->hasAllZeroIndices())
320 V = GEPOp->getOperand(0);
324 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
325 gep_type_iterator GTI = gep_type_begin(GEPOp);
326 for (User::const_op_iterator I = GEPOp->op_begin()+1,
327 E = GEPOp->op_end(); I != E; ++I) {
329 // Compute the (potentially symbolic) offset in bytes for this index.
330 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
331 // For a struct, add the member offset.
332 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
333 if (FieldNo == 0) continue;
335 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
339 // For an array/pointer, add the element offset, explicitly scaled.
340 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
341 if (CIdx->isZero()) continue;
342 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
346 uint64_t Scale = TD->getTypeAllocSize(*GTI);
348 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
349 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
350 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
351 Index = GetLinearExpression(Index, IndexScale, IndexOffset, TD, 0);
353 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
354 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
355 BaseOffs += IndexOffset.getZExtValue()*Scale;
356 Scale *= IndexScale.getZExtValue();
359 // If we already had an occurrance of this index variable, merge this
360 // scale into it. For example, we want to handle:
361 // A[x][x] -> x*16 + x*4 -> x*20
362 // This also ensures that 'x' only appears in the index list once.
363 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
364 if (VarIndices[i].first == Index) {
365 Scale += VarIndices[i].second;
366 VarIndices.erase(VarIndices.begin()+i);
371 // Make sure that we have a scale that makes sense for this target's
373 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
379 VarIndices.push_back(std::make_pair(Index, Scale));
382 // Analyze the base pointer next.
383 V = GEPOp->getOperand(0);
384 } while (--MaxLookup);
386 // If the chain of expressions is too deep, just return early.
390 /// GetIndexDifference - Dest and Src are the variable indices from two
391 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
392 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
393 /// difference between the two pointers.
394 static void GetIndexDifference(
395 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
396 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
397 if (Src.empty()) return;
399 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
400 const Value *V = Src[i].first;
401 int64_t Scale = Src[i].second;
403 // Find V in Dest. This is N^2, but pointer indices almost never have more
404 // than a few variable indexes.
405 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
406 if (Dest[j].first != V) continue;
408 // If we found it, subtract off Scale V's from the entry in Dest. If it
409 // goes to zero, remove the entry.
410 if (Dest[j].second != Scale)
411 Dest[j].second -= Scale;
413 Dest.erase(Dest.begin()+j);
418 // If we didn't consume this entry, add it to the end of the Dest list.
420 Dest.push_back(std::make_pair(V, -Scale));
424 //===----------------------------------------------------------------------===//
425 // BasicAliasAnalysis Pass
426 //===----------------------------------------------------------------------===//
429 static const Function *getParent(const Value *V) {
430 if (const Instruction *inst = dyn_cast<Instruction>(V))
431 return inst->getParent()->getParent();
433 if (const Argument *arg = dyn_cast<Argument>(V))
434 return arg->getParent();
439 static bool notDifferentParent(const Value *O1, const Value *O2) {
441 const Function *F1 = getParent(O1);
442 const Function *F2 = getParent(O2);
444 return !F1 || !F2 || F1 == F2;
449 /// BasicAliasAnalysis - This is the default alias analysis implementation.
450 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
451 /// derives from the NoAA class.
452 struct BasicAliasAnalysis : public NoAA {
453 static char ID; // Class identification, replacement for typeinfo
454 BasicAliasAnalysis() : NoAA(ID) {}
456 virtual AliasResult alias(const Value *V1, unsigned V1Size,
457 const Value *V2, unsigned V2Size) {
458 assert(Visited.empty() && "Visited must be cleared after use!");
459 assert(notDifferentParent(V1, V2) &&
460 "BasicAliasAnalysis doesn't support interprocedural queries.");
461 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
466 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
467 const Value *P, unsigned Size);
469 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
470 ImmutableCallSite CS2) {
471 // The AliasAnalysis base class has some smarts, lets use them.
472 return AliasAnalysis::getModRefInfo(CS1, CS2);
475 /// pointsToConstantMemory - Chase pointers until we find a (constant
477 virtual bool pointsToConstantMemory(const Value *P);
479 /// getModRefBehavior - Return the behavior when calling the given
481 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
483 /// getModRefBehavior - Return the behavior when calling the given function.
484 /// For use when the call site is not known.
485 virtual ModRefBehavior getModRefBehavior(const Function *F);
487 /// getAdjustedAnalysisPointer - This method is used when a pass implements
488 /// an analysis interface through multiple inheritance. If needed, it
489 /// should override this to adjust the this pointer as needed for the
490 /// specified pass info.
491 virtual void *getAdjustedAnalysisPointer(const void *ID) {
492 if (ID == &AliasAnalysis::ID)
493 return (AliasAnalysis*)this;
498 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
499 SmallPtrSet<const Value*, 16> Visited;
501 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
502 // instruction against another.
503 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
504 const Value *V2, unsigned V2Size,
505 const Value *UnderlyingV1, const Value *UnderlyingV2);
507 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
508 // instruction against another.
509 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
510 const Value *V2, unsigned V2Size);
512 /// aliasSelect - Disambiguate a Select instruction against another value.
513 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
514 const Value *V2, unsigned V2Size);
516 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
517 const Value *V2, unsigned V2Size);
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 (default AA impl)",
527 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
528 return new BasicAliasAnalysis();
532 /// pointsToConstantMemory - Chase pointers until we find a (constant
534 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
535 if (const GlobalVariable *GV =
536 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
537 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
538 // global to be marked constant in some modules and non-constant in others.
539 // GV may even be a declaration, not a definition.
540 return GV->isConstant();
542 return NoAA::pointsToConstantMemory(P);
545 /// getModRefBehavior - Return the behavior when calling the given call site.
546 AliasAnalysis::ModRefBehavior
547 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
548 if (CS.doesNotAccessMemory())
549 // Can't do better than this.
550 return DoesNotAccessMemory;
552 ModRefBehavior Min = UnknownModRefBehavior;
554 // If the callsite knows it only reads memory, don't return worse
556 if (CS.onlyReadsMemory())
557 Min = OnlyReadsMemory;
559 // The AliasAnalysis base class has some smarts, lets use them.
560 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
563 /// getModRefBehavior - Return the behavior when calling the given function.
564 /// For use when the call site is not known.
565 AliasAnalysis::ModRefBehavior
566 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
567 if (F->doesNotAccessMemory())
568 // Can't do better than this.
569 return DoesNotAccessMemory;
570 if (F->onlyReadsMemory())
571 return OnlyReadsMemory;
572 if (unsigned id = F->getIntrinsicID())
573 return getIntrinsicModRefBehavior(id);
575 return NoAA::getModRefBehavior(F);
578 /// getModRefInfo - Check to see if the specified callsite can clobber the
579 /// specified memory object. Since we only look at local properties of this
580 /// function, we really can't say much about this query. We do, however, use
581 /// simple "address taken" analysis on local objects.
582 AliasAnalysis::ModRefResult
583 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
584 const Value *P, unsigned Size) {
585 assert(notDifferentParent(CS.getInstruction(), P) &&
586 "AliasAnalysis query involving multiple functions!");
588 const Value *Object = P->getUnderlyingObject();
590 // If this is a tail call and P points to a stack location, we know that
591 // the tail call cannot access or modify the local stack.
592 // We cannot exclude byval arguments here; these belong to the caller of
593 // the current function not to the current function, and a tail callee
594 // may reference them.
595 if (isa<AllocaInst>(Object))
596 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
597 if (CI->isTailCall())
600 // If the pointer is to a locally allocated object that does not escape,
601 // then the call can not mod/ref the pointer unless the call takes the pointer
602 // as an argument, and itself doesn't capture it.
603 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
604 isNonEscapingLocalObject(Object)) {
605 bool PassedAsArg = false;
607 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
608 CI != CE; ++CI, ++ArgNo) {
609 // Only look at the no-capture pointer arguments.
610 if (!(*CI)->getType()->isPointerTy() ||
611 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
614 // If this is a no-capture pointer argument, see if we can tell that it
615 // is impossible to alias the pointer we're checking. If not, we have to
616 // assume that the call could touch the pointer, even though it doesn't
618 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
628 // Finally, handle specific knowledge of intrinsics.
629 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
631 switch (II->getIntrinsicID()) {
633 case Intrinsic::memcpy:
634 case Intrinsic::memmove: {
635 unsigned Len = UnknownSize;
636 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
637 Len = LenCI->getZExtValue();
638 Value *Dest = II->getArgOperand(0);
639 Value *Src = II->getArgOperand(1);
640 if (isNoAlias(Dest, Len, P, Size)) {
641 if (isNoAlias(Src, Len, P, Size))
647 case Intrinsic::memset:
648 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
649 // will handle it for the variable length case.
650 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
651 unsigned Len = LenCI->getZExtValue();
652 Value *Dest = II->getArgOperand(0);
653 if (isNoAlias(Dest, Len, P, Size))
657 case Intrinsic::atomic_cmp_swap:
658 case Intrinsic::atomic_swap:
659 case Intrinsic::atomic_load_add:
660 case Intrinsic::atomic_load_sub:
661 case Intrinsic::atomic_load_and:
662 case Intrinsic::atomic_load_nand:
663 case Intrinsic::atomic_load_or:
664 case Intrinsic::atomic_load_xor:
665 case Intrinsic::atomic_load_max:
666 case Intrinsic::atomic_load_min:
667 case Intrinsic::atomic_load_umax:
668 case Intrinsic::atomic_load_umin:
670 Value *Op1 = II->getArgOperand(0);
671 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
672 if (isNoAlias(Op1, Op1Size, P, Size))
676 case Intrinsic::lifetime_start:
677 case Intrinsic::lifetime_end:
678 case Intrinsic::invariant_start: {
680 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
681 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
685 case Intrinsic::invariant_end: {
687 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
688 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
694 // The AliasAnalysis base class has some smarts, lets use them.
695 return AliasAnalysis::getModRefInfo(CS, P, Size);
699 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
700 /// against another pointer. We know that V1 is a GEP, but we don't know
701 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
702 /// UnderlyingV2 is the same for V2.
704 AliasAnalysis::AliasResult
705 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
706 const Value *V2, unsigned V2Size,
707 const Value *UnderlyingV1,
708 const Value *UnderlyingV2) {
709 // If this GEP has been visited before, we're on a use-def cycle.
710 // Such cycles are only valid when PHI nodes are involved or in unreachable
711 // code. The visitPHI function catches cycles containing PHIs, but there
712 // could still be a cycle without PHIs in unreachable code.
713 if (!Visited.insert(GEP1))
716 int64_t GEP1BaseOffset;
717 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
719 // If we have two gep instructions with must-alias'ing base pointers, figure
720 // out if the indexes to the GEP tell us anything about the derived pointer.
721 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
722 // Do the base pointers alias?
723 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
724 UnderlyingV2, UnknownSize);
726 // If we get a No or May, then return it immediately, no amount of analysis
727 // will improve this situation.
728 if (BaseAlias != MustAlias) return BaseAlias;
730 // Otherwise, we have a MustAlias. Since the base pointers alias each other
731 // exactly, see if the computed offset from the common pointer tells us
732 // about the relation of the resulting pointer.
733 const Value *GEP1BasePtr =
734 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
736 int64_t GEP2BaseOffset;
737 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
738 const Value *GEP2BasePtr =
739 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
741 // If DecomposeGEPExpression isn't able to look all the way through the
742 // addressing operation, we must not have TD and this is too complex for us
743 // to handle without it.
744 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
746 "DecomposeGEPExpression and getUnderlyingObject disagree!");
750 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
751 // symbolic difference.
752 GEP1BaseOffset -= GEP2BaseOffset;
753 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
756 // Check to see if these two pointers are related by the getelementptr
757 // instruction. If one pointer is a GEP with a non-zero index of the other
758 // pointer, we know they cannot alias.
760 // If both accesses are unknown size, we can't do anything useful here.
761 if (V1Size == UnknownSize && V2Size == UnknownSize)
764 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
766 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
767 // If V2 is known not to alias GEP base pointer, then the two values
768 // cannot alias per GEP semantics: "A pointer value formed from a
769 // getelementptr instruction is associated with the addresses associated
770 // with the first operand of the getelementptr".
773 const Value *GEP1BasePtr =
774 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
776 // If DecomposeGEPExpression isn't able to look all the way through the
777 // addressing operation, we must not have TD and this is too complex for us
778 // to handle without it.
779 if (GEP1BasePtr != UnderlyingV1) {
781 "DecomposeGEPExpression and getUnderlyingObject disagree!");
786 // In the two GEP Case, if there is no difference in the offsets of the
787 // computed pointers, the resultant pointers are a must alias. This
788 // hapens when we have two lexically identical GEP's (for example).
790 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
791 // must aliases the GEP, the end result is a must alias also.
792 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
795 // If we have a known constant offset, see if this offset is larger than the
796 // access size being queried. If so, and if no variable indices can remove
797 // pieces of this constant, then we know we have a no-alias. For example,
800 // In order to handle cases like &A[100][i] where i is an out of range
801 // subscript, we have to ignore all constant offset pieces that are a multiple
802 // of a scaled index. Do this by removing constant offsets that are a
803 // multiple of any of our variable indices. This allows us to transform
804 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
805 // provides an offset of 4 bytes (assuming a <= 4 byte access).
806 for (unsigned i = 0, e = GEP1VariableIndices.size();
807 i != e && GEP1BaseOffset;++i)
808 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
809 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
811 // If our known offset is bigger than the access size, we know we don't have
813 if (GEP1BaseOffset) {
814 if (GEP1BaseOffset >= (int64_t)V2Size ||
815 GEP1BaseOffset <= -(int64_t)V1Size)
822 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
823 /// instruction against another.
824 AliasAnalysis::AliasResult
825 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
826 const Value *V2, unsigned V2Size) {
827 // If this select has been visited before, we're on a use-def cycle.
828 // Such cycles are only valid when PHI nodes are involved or in unreachable
829 // code. The visitPHI function catches cycles containing PHIs, but there
830 // could still be a cycle without PHIs in unreachable code.
831 if (!Visited.insert(SI))
834 // If the values are Selects with the same condition, we can do a more precise
835 // check: just check for aliases between the values on corresponding arms.
836 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
837 if (SI->getCondition() == SI2->getCondition()) {
839 aliasCheck(SI->getTrueValue(), SISize,
840 SI2->getTrueValue(), V2Size);
841 if (Alias == MayAlias)
843 AliasResult ThisAlias =
844 aliasCheck(SI->getFalseValue(), SISize,
845 SI2->getFalseValue(), V2Size);
846 if (ThisAlias != Alias)
851 // If both arms of the Select node NoAlias or MustAlias V2, then returns
852 // NoAlias / MustAlias. Otherwise, returns MayAlias.
854 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
855 if (Alias == MayAlias)
858 // If V2 is visited, the recursive case will have been caught in the
859 // above aliasCheck call, so these subsequent calls to aliasCheck
860 // don't need to assume that V2 is being visited recursively.
863 AliasResult ThisAlias =
864 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
865 if (ThisAlias != Alias)
870 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
872 AliasAnalysis::AliasResult
873 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
874 const Value *V2, unsigned V2Size) {
875 // The PHI node has already been visited, avoid recursion any further.
876 if (!Visited.insert(PN))
879 // If the values are PHIs in the same block, we can do a more precise
880 // as well as efficient check: just check for aliases between the values
881 // on corresponding edges.
882 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
883 if (PN2->getParent() == PN->getParent()) {
885 aliasCheck(PN->getIncomingValue(0), PNSize,
886 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
888 if (Alias == MayAlias)
890 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
891 AliasResult ThisAlias =
892 aliasCheck(PN->getIncomingValue(i), PNSize,
893 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
895 if (ThisAlias != Alias)
901 SmallPtrSet<Value*, 4> UniqueSrc;
902 SmallVector<Value*, 4> V1Srcs;
903 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
904 Value *PV1 = PN->getIncomingValue(i);
905 if (isa<PHINode>(PV1))
906 // If any of the source itself is a PHI, return MayAlias conservatively
907 // to avoid compile time explosion. The worst possible case is if both
908 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
909 // and 'n' are the number of PHI sources.
911 if (UniqueSrc.insert(PV1))
912 V1Srcs.push_back(PV1);
915 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
916 // Early exit if the check of the first PHI source against V2 is MayAlias.
917 // Other results are not possible.
918 if (Alias == MayAlias)
921 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
922 // NoAlias / MustAlias. Otherwise, returns MayAlias.
923 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
924 Value *V = V1Srcs[i];
926 // If V2 is visited, the recursive case will have been caught in the
927 // above aliasCheck call, so these subsequent calls to aliasCheck
928 // don't need to assume that V2 is being visited recursively.
931 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
932 if (ThisAlias != Alias || ThisAlias == MayAlias)
939 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
940 // such as array references.
942 AliasAnalysis::AliasResult
943 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
944 const Value *V2, unsigned V2Size) {
945 // If either of the memory references is empty, it doesn't matter what the
946 // pointer values are.
947 if (V1Size == 0 || V2Size == 0)
950 // Strip off any casts if they exist.
951 V1 = V1->stripPointerCasts();
952 V2 = V2->stripPointerCasts();
954 // Are we checking for alias of the same value?
955 if (V1 == V2) return MustAlias;
957 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
958 return NoAlias; // Scalars cannot alias each other
960 // Figure out what objects these things are pointing to if we can.
961 const Value *O1 = V1->getUnderlyingObject();
962 const Value *O2 = V2->getUnderlyingObject();
964 // Null values in the default address space don't point to any object, so they
965 // don't alias any other pointer.
966 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
967 if (CPN->getType()->getAddressSpace() == 0)
969 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
970 if (CPN->getType()->getAddressSpace() == 0)
974 // If V1/V2 point to two different objects we know that we have no alias.
975 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
978 // Constant pointers can't alias with non-const isIdentifiedObject objects.
979 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
980 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
983 // Arguments can't alias with local allocations or noalias calls
984 // in the same function.
985 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
986 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
989 // Most objects can't alias null.
990 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
991 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
994 // If one pointer is the result of a call/invoke or load and the other is a
995 // non-escaping local object within the same function, then we know the
996 // object couldn't escape to a point where the call could return it.
998 // Note that if the pointers are in different functions, there are a
999 // variety of complications. A call with a nocapture argument may still
1000 // temporary store the nocapture argument's value in a temporary memory
1001 // location if that memory location doesn't escape. Or it may pass a
1002 // nocapture value to other functions as long as they don't capture it.
1003 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1005 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1009 // If the size of one access is larger than the entire object on the other
1010 // side, then we know such behavior is undefined and can assume no alias.
1012 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1013 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1016 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1017 // GEP can't simplify, we don't even look at the PHI cases.
1018 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1020 std::swap(V1Size, V2Size);
1023 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1024 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1026 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1028 std::swap(V1Size, V2Size);
1030 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1031 return aliasPHI(PN, V1Size, V2, V2Size);
1033 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1035 std::swap(V1Size, V2Size);
1037 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1038 return aliasSelect(S1, V1Size, V2, V2Size);
1040 return NoAA::alias(V1, V1Size, V2, V2Size);
1043 // Make sure that anything that uses AliasAnalysis pulls in this file.
1044 DEFINING_FILE_FOR(BasicAliasAnalysis)