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 DependenceResult getDependence(const Instruction *First,
175 DependenceQueryFlags FirstFlags,
176 const Instruction *Second,
177 DependenceQueryFlags SecondFlags) {
181 virtual void deleteValue(Value *V) {}
182 virtual void copyValue(Value *From, Value *To) {}
184 /// getAdjustedAnalysisPointer - This method is used when a pass implements
185 /// an analysis interface through multiple inheritance. If needed, it
186 /// should override this to adjust the this pointer as needed for the
187 /// specified pass info.
188 virtual void *getAdjustedAnalysisPointer(const void *ID) {
189 if (ID == &AliasAnalysis::ID)
190 return (AliasAnalysis*)this;
194 } // End of anonymous namespace
196 // Register this pass...
198 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
199 "No Alias Analysis (always returns 'may' alias)",
202 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
204 //===----------------------------------------------------------------------===//
205 // GetElementPtr Instruction Decomposition and Analysis
206 //===----------------------------------------------------------------------===//
215 struct VariableGEPIndex {
217 ExtensionKind Extension;
223 /// GetLinearExpression - Analyze the specified value as a linear expression:
224 /// "A*V + B", where A and B are constant integers. Return the scale and offset
225 /// values as APInts and return V as a Value*, and return whether we looked
226 /// through any sign or zero extends. The incoming Value is known to have
227 /// IntegerType and it may already be sign or zero extended.
229 /// Note that this looks through extends, so the high bits may not be
230 /// represented in the result.
231 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
232 ExtensionKind &Extension,
233 const TargetData &TD, unsigned Depth) {
234 assert(V->getType()->isIntegerTy() && "Not an integer value");
236 // Limit our recursion depth.
243 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
244 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
245 switch (BOp->getOpcode()) {
247 case Instruction::Or:
248 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
250 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
253 case Instruction::Add:
254 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
256 Offset += RHSC->getValue();
258 case Instruction::Mul:
259 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
261 Offset *= RHSC->getValue();
262 Scale *= RHSC->getValue();
264 case Instruction::Shl:
265 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
267 Offset <<= RHSC->getValue().getLimitedValue();
268 Scale <<= RHSC->getValue().getLimitedValue();
274 // Since GEP indices are sign extended anyway, we don't care about the high
275 // bits of a sign or zero extended value - just scales and offsets. The
276 // extensions have to be consistent though.
277 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
278 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
279 Value *CastOp = cast<CastInst>(V)->getOperand(0);
280 unsigned OldWidth = Scale.getBitWidth();
281 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
282 Scale.trunc(SmallWidth);
283 Offset.trunc(SmallWidth);
284 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
286 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
288 Scale.zext(OldWidth);
289 Offset.zext(OldWidth);
299 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
300 /// into a base pointer with a constant offset and a number of scaled symbolic
303 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
304 /// the VarIndices vector) are Value*'s that are known to be scaled by the
305 /// specified amount, but which may have other unrepresented high bits. As such,
306 /// the gep cannot necessarily be reconstructed from its decomposed form.
308 /// When TargetData is around, this function is capable of analyzing everything
309 /// that Value::getUnderlyingObject() can look through. When not, it just looks
310 /// through pointer casts.
313 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
314 SmallVectorImpl<VariableGEPIndex> &VarIndices,
315 const TargetData *TD) {
316 // Limit recursion depth to limit compile time in crazy cases.
317 unsigned MaxLookup = 6;
321 // See if this is a bitcast or GEP.
322 const Operator *Op = dyn_cast<Operator>(V);
324 // The only non-operator case we can handle are GlobalAliases.
325 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
326 if (!GA->mayBeOverridden()) {
327 V = GA->getAliasee();
334 if (Op->getOpcode() == Instruction::BitCast) {
335 V = Op->getOperand(0);
339 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
343 // Don't attempt to analyze GEPs over unsized objects.
344 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
345 ->getElementType()->isSized())
348 // If we are lacking TargetData information, we can't compute the offets of
349 // elements computed by GEPs. However, we can handle bitcast equivalent
352 if (!GEPOp->hasAllZeroIndices())
354 V = GEPOp->getOperand(0);
358 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
359 gep_type_iterator GTI = gep_type_begin(GEPOp);
360 for (User::const_op_iterator I = GEPOp->op_begin()+1,
361 E = GEPOp->op_end(); I != E; ++I) {
363 // Compute the (potentially symbolic) offset in bytes for this index.
364 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
365 // For a struct, add the member offset.
366 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
367 if (FieldNo == 0) continue;
369 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
373 // For an array/pointer, add the element offset, explicitly scaled.
374 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
375 if (CIdx->isZero()) continue;
376 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
380 uint64_t Scale = TD->getTypeAllocSize(*GTI);
381 ExtensionKind Extension = EK_NotExtended;
383 // If the integer type is smaller than the pointer size, it is implicitly
384 // sign extended to pointer size.
385 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
386 if (TD->getPointerSizeInBits() > Width)
387 Extension = EK_SignExt;
389 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
390 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
391 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
394 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
395 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
396 BaseOffs += IndexOffset.getZExtValue()*Scale;
397 Scale *= IndexScale.getZExtValue();
400 // If we already had an occurrance of this index variable, merge this
401 // scale into it. For example, we want to handle:
402 // A[x][x] -> x*16 + x*4 -> x*20
403 // This also ensures that 'x' only appears in the index list once.
404 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
405 if (VarIndices[i].V == Index &&
406 VarIndices[i].Extension == Extension) {
407 Scale += VarIndices[i].Scale;
408 VarIndices.erase(VarIndices.begin()+i);
413 // Make sure that we have a scale that makes sense for this target's
415 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
421 VariableGEPIndex Entry = {Index, Extension, Scale};
422 VarIndices.push_back(Entry);
426 // Analyze the base pointer next.
427 V = GEPOp->getOperand(0);
428 } while (--MaxLookup);
430 // If the chain of expressions is too deep, just return early.
434 /// GetIndexDifference - Dest and Src are the variable indices from two
435 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
436 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
437 /// difference between the two pointers.
438 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
439 const SmallVectorImpl<VariableGEPIndex> &Src) {
440 if (Src.empty()) return;
442 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
443 const Value *V = Src[i].V;
444 ExtensionKind Extension = Src[i].Extension;
445 int64_t Scale = Src[i].Scale;
447 // Find V in Dest. This is N^2, but pointer indices almost never have more
448 // than a few variable indexes.
449 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
450 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
452 // If we found it, subtract off Scale V's from the entry in Dest. If it
453 // goes to zero, remove the entry.
454 if (Dest[j].Scale != Scale)
455 Dest[j].Scale -= Scale;
457 Dest.erase(Dest.begin()+j);
462 // If we didn't consume this entry, add it to the end of the Dest list.
464 VariableGEPIndex Entry = { V, Extension, -Scale };
465 Dest.push_back(Entry);
470 //===----------------------------------------------------------------------===//
471 // BasicAliasAnalysis Pass
472 //===----------------------------------------------------------------------===//
475 static const Function *getParent(const Value *V) {
476 if (const Instruction *inst = dyn_cast<Instruction>(V))
477 return inst->getParent()->getParent();
479 if (const Argument *arg = dyn_cast<Argument>(V))
480 return arg->getParent();
485 static bool notDifferentParent(const Value *O1, const Value *O2) {
487 const Function *F1 = getParent(O1);
488 const Function *F2 = getParent(O2);
490 return !F1 || !F2 || F1 == F2;
495 /// BasicAliasAnalysis - This is the default alias analysis implementation.
496 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
497 /// derives from the NoAA class.
498 struct BasicAliasAnalysis : public NoAA {
499 static char ID; // Class identification, replacement for typeinfo
500 BasicAliasAnalysis() : NoAA(ID) {}
502 virtual AliasResult alias(const Value *V1, unsigned V1Size,
503 const Value *V2, unsigned V2Size) {
504 assert(Visited.empty() && "Visited must be cleared after use!");
505 assert(notDifferentParent(V1, V2) &&
506 "BasicAliasAnalysis doesn't support interprocedural queries.");
507 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
512 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
513 const Value *P, unsigned Size);
515 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
516 ImmutableCallSite CS2) {
517 // The AliasAnalysis base class has some smarts, lets use them.
518 return AliasAnalysis::getModRefInfo(CS1, CS2);
521 /// pointsToConstantMemory - Chase pointers until we find a (constant
523 virtual bool pointsToConstantMemory(const Value *P);
525 /// getModRefBehavior - Return the behavior when calling the given
527 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
529 /// getModRefBehavior - Return the behavior when calling the given function.
530 /// For use when the call site is not known.
531 virtual ModRefBehavior getModRefBehavior(const Function *F);
533 virtual DependenceResult getDependence(const Instruction *First,
534 DependenceQueryFlags FirstFlags,
535 const Instruction *Second,
536 DependenceQueryFlags SecondFlags);
538 /// getAdjustedAnalysisPointer - This method is used when a pass implements
539 /// an analysis interface through multiple inheritance. If needed, it
540 /// should override this to adjust the this pointer as needed for the
541 /// specified pass info.
542 virtual void *getAdjustedAnalysisPointer(const void *ID) {
543 if (ID == &AliasAnalysis::ID)
544 return (AliasAnalysis*)this;
549 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
550 SmallPtrSet<const Value*, 16> Visited;
552 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
553 // instruction against another.
554 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
555 const Value *V2, unsigned V2Size,
556 const Value *UnderlyingV1, const Value *UnderlyingV2);
558 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
559 // instruction against another.
560 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
561 const Value *V2, unsigned V2Size);
563 /// aliasSelect - Disambiguate a Select instruction against another value.
564 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
565 const Value *V2, unsigned V2Size);
567 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
568 const Value *V2, unsigned V2Size);
570 } // End of anonymous namespace
572 // Register this pass...
573 char BasicAliasAnalysis::ID = 0;
574 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
575 "Basic Alias Analysis (default AA impl)",
578 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
579 return new BasicAliasAnalysis();
583 /// pointsToConstantMemory - Chase pointers until we find a (constant
585 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
586 if (const GlobalVariable *GV =
587 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
588 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
589 // global to be marked constant in some modules and non-constant in others.
590 // GV may even be a declaration, not a definition.
591 return GV->isConstant();
593 return NoAA::pointsToConstantMemory(P);
596 /// getModRefBehavior - Return the behavior when calling the given call site.
597 AliasAnalysis::ModRefBehavior
598 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
599 if (CS.doesNotAccessMemory())
600 // Can't do better than this.
601 return DoesNotAccessMemory;
603 ModRefBehavior Min = UnknownModRefBehavior;
605 // If the callsite knows it only reads memory, don't return worse
607 if (CS.onlyReadsMemory())
608 Min = OnlyReadsMemory;
610 // The AliasAnalysis base class has some smarts, lets use them.
611 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
614 /// getModRefBehavior - Return the behavior when calling the given function.
615 /// For use when the call site is not known.
616 AliasAnalysis::ModRefBehavior
617 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
618 if (F->doesNotAccessMemory())
619 // Can't do better than this.
620 return DoesNotAccessMemory;
621 if (F->onlyReadsMemory())
622 return OnlyReadsMemory;
623 if (unsigned id = F->getIntrinsicID())
624 return getIntrinsicModRefBehavior(id);
626 return NoAA::getModRefBehavior(F);
629 /// getModRefInfo - Check to see if the specified callsite can clobber the
630 /// specified memory object. Since we only look at local properties of this
631 /// function, we really can't say much about this query. We do, however, use
632 /// simple "address taken" analysis on local objects.
633 AliasAnalysis::ModRefResult
634 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
635 const Value *P, unsigned Size) {
636 assert(notDifferentParent(CS.getInstruction(), P) &&
637 "AliasAnalysis query involving multiple functions!");
639 const Value *Object = P->getUnderlyingObject();
641 // If this is a tail call and P points to a stack location, we know that
642 // the tail call cannot access or modify the local stack.
643 // We cannot exclude byval arguments here; these belong to the caller of
644 // the current function not to the current function, and a tail callee
645 // may reference them.
646 if (isa<AllocaInst>(Object))
647 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
648 if (CI->isTailCall())
651 // If the pointer is to a locally allocated object that does not escape,
652 // then the call can not mod/ref the pointer unless the call takes the pointer
653 // as an argument, and itself doesn't capture it.
654 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
655 isNonEscapingLocalObject(Object)) {
656 bool PassedAsArg = false;
658 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
659 CI != CE; ++CI, ++ArgNo) {
660 // Only look at the no-capture pointer arguments.
661 if (!(*CI)->getType()->isPointerTy() ||
662 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
665 // If this is a no-capture pointer argument, see if we can tell that it
666 // is impossible to alias the pointer we're checking. If not, we have to
667 // assume that the call could touch the pointer, even though it doesn't
669 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
679 // Finally, handle specific knowledge of intrinsics.
680 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
682 switch (II->getIntrinsicID()) {
684 case Intrinsic::memcpy:
685 case Intrinsic::memmove: {
686 unsigned Len = UnknownSize;
687 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
688 Len = LenCI->getZExtValue();
689 Value *Dest = II->getArgOperand(0);
690 Value *Src = II->getArgOperand(1);
691 if (isNoAlias(Dest, Len, P, Size)) {
692 if (isNoAlias(Src, Len, P, Size))
698 case Intrinsic::memset:
699 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
700 // will handle it for the variable length case.
701 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
702 unsigned Len = LenCI->getZExtValue();
703 Value *Dest = II->getArgOperand(0);
704 if (isNoAlias(Dest, Len, P, Size))
708 case Intrinsic::atomic_cmp_swap:
709 case Intrinsic::atomic_swap:
710 case Intrinsic::atomic_load_add:
711 case Intrinsic::atomic_load_sub:
712 case Intrinsic::atomic_load_and:
713 case Intrinsic::atomic_load_nand:
714 case Intrinsic::atomic_load_or:
715 case Intrinsic::atomic_load_xor:
716 case Intrinsic::atomic_load_max:
717 case Intrinsic::atomic_load_min:
718 case Intrinsic::atomic_load_umax:
719 case Intrinsic::atomic_load_umin:
721 Value *Op1 = II->getArgOperand(0);
722 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
723 if (isNoAlias(Op1, Op1Size, P, Size))
727 case Intrinsic::lifetime_start:
728 case Intrinsic::lifetime_end:
729 case Intrinsic::invariant_start: {
731 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
732 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
736 case Intrinsic::invariant_end: {
738 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
739 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
745 // The AliasAnalysis base class has some smarts, lets use them.
746 return AliasAnalysis::getModRefInfo(CS, P, Size);
749 AliasAnalysis::DependenceResult
750 BasicAliasAnalysis::getDependence(const Instruction *First,
751 DependenceQueryFlags FirstFlags,
752 const Instruction *Second,
753 DependenceQueryFlags SecondFlags) {
754 // We don't have anything special to say yet.
755 return getDependenceViaModRefInfo(First, FirstFlags, Second, SecondFlags);
758 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
759 /// against another pointer. We know that V1 is a GEP, but we don't know
760 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
761 /// UnderlyingV2 is the same for V2.
763 AliasAnalysis::AliasResult
764 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
765 const Value *V2, unsigned V2Size,
766 const Value *UnderlyingV1,
767 const Value *UnderlyingV2) {
768 // If this GEP has been visited before, we're on a use-def cycle.
769 // Such cycles are only valid when PHI nodes are involved or in unreachable
770 // code. The visitPHI function catches cycles containing PHIs, but there
771 // could still be a cycle without PHIs in unreachable code.
772 if (!Visited.insert(GEP1))
775 int64_t GEP1BaseOffset;
776 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
778 // If we have two gep instructions with must-alias'ing base pointers, figure
779 // out if the indexes to the GEP tell us anything about the derived pointer.
780 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
781 // Do the base pointers alias?
782 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
783 UnderlyingV2, UnknownSize);
785 // If we get a No or May, then return it immediately, no amount of analysis
786 // will improve this situation.
787 if (BaseAlias != MustAlias) return BaseAlias;
789 // Otherwise, we have a MustAlias. Since the base pointers alias each other
790 // exactly, see if the computed offset from the common pointer tells us
791 // about the relation of the resulting pointer.
792 const Value *GEP1BasePtr =
793 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
795 int64_t GEP2BaseOffset;
796 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
797 const Value *GEP2BasePtr =
798 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
800 // If DecomposeGEPExpression isn't able to look all the way through the
801 // addressing operation, we must not have TD and this is too complex for us
802 // to handle without it.
803 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
805 "DecomposeGEPExpression and getUnderlyingObject disagree!");
809 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
810 // symbolic difference.
811 GEP1BaseOffset -= GEP2BaseOffset;
812 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
815 // Check to see if these two pointers are related by the getelementptr
816 // instruction. If one pointer is a GEP with a non-zero index of the other
817 // pointer, we know they cannot alias.
819 // If both accesses are unknown size, we can't do anything useful here.
820 if (V1Size == UnknownSize && V2Size == UnknownSize)
823 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
825 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
826 // If V2 is known not to alias GEP base pointer, then the two values
827 // cannot alias per GEP semantics: "A pointer value formed from a
828 // getelementptr instruction is associated with the addresses associated
829 // with the first operand of the getelementptr".
832 const Value *GEP1BasePtr =
833 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
835 // If DecomposeGEPExpression isn't able to look all the way through the
836 // addressing operation, we must not have TD and this is too complex for us
837 // to handle without it.
838 if (GEP1BasePtr != UnderlyingV1) {
840 "DecomposeGEPExpression and getUnderlyingObject disagree!");
845 // In the two GEP Case, if there is no difference in the offsets of the
846 // computed pointers, the resultant pointers are a must alias. This
847 // hapens when we have two lexically identical GEP's (for example).
849 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
850 // must aliases the GEP, the end result is a must alias also.
851 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
854 // If we have a known constant offset, see if this offset is larger than the
855 // access size being queried. If so, and if no variable indices can remove
856 // pieces of this constant, then we know we have a no-alias. For example,
859 // In order to handle cases like &A[100][i] where i is an out of range
860 // subscript, we have to ignore all constant offset pieces that are a multiple
861 // of a scaled index. Do this by removing constant offsets that are a
862 // multiple of any of our variable indices. This allows us to transform
863 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
864 // provides an offset of 4 bytes (assuming a <= 4 byte access).
865 for (unsigned i = 0, e = GEP1VariableIndices.size();
866 i != e && GEP1BaseOffset;++i)
867 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
868 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
870 // If our known offset is bigger than the access size, we know we don't have
872 if (GEP1BaseOffset) {
873 if (GEP1BaseOffset >= (int64_t)V2Size ||
874 GEP1BaseOffset <= -(int64_t)V1Size)
881 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
882 /// instruction against another.
883 AliasAnalysis::AliasResult
884 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
885 const Value *V2, unsigned V2Size) {
886 // If this select has been visited before, we're on a use-def cycle.
887 // Such cycles are only valid when PHI nodes are involved or in unreachable
888 // code. The visitPHI function catches cycles containing PHIs, but there
889 // could still be a cycle without PHIs in unreachable code.
890 if (!Visited.insert(SI))
893 // If the values are Selects with the same condition, we can do a more precise
894 // check: just check for aliases between the values on corresponding arms.
895 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
896 if (SI->getCondition() == SI2->getCondition()) {
898 aliasCheck(SI->getTrueValue(), SISize,
899 SI2->getTrueValue(), V2Size);
900 if (Alias == MayAlias)
902 AliasResult ThisAlias =
903 aliasCheck(SI->getFalseValue(), SISize,
904 SI2->getFalseValue(), V2Size);
905 if (ThisAlias != Alias)
910 // If both arms of the Select node NoAlias or MustAlias V2, then returns
911 // NoAlias / MustAlias. Otherwise, returns MayAlias.
913 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
914 if (Alias == MayAlias)
917 // If V2 is visited, the recursive case will have been caught in the
918 // above aliasCheck call, so these subsequent calls to aliasCheck
919 // don't need to assume that V2 is being visited recursively.
922 AliasResult ThisAlias =
923 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
924 if (ThisAlias != Alias)
929 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
931 AliasAnalysis::AliasResult
932 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
933 const Value *V2, unsigned V2Size) {
934 // The PHI node has already been visited, avoid recursion any further.
935 if (!Visited.insert(PN))
938 // If the values are PHIs in the same block, we can do a more precise
939 // as well as efficient check: just check for aliases between the values
940 // on corresponding edges.
941 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
942 if (PN2->getParent() == PN->getParent()) {
944 aliasCheck(PN->getIncomingValue(0), PNSize,
945 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
947 if (Alias == MayAlias)
949 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
950 AliasResult ThisAlias =
951 aliasCheck(PN->getIncomingValue(i), PNSize,
952 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
954 if (ThisAlias != Alias)
960 SmallPtrSet<Value*, 4> UniqueSrc;
961 SmallVector<Value*, 4> V1Srcs;
962 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
963 Value *PV1 = PN->getIncomingValue(i);
964 if (isa<PHINode>(PV1))
965 // If any of the source itself is a PHI, return MayAlias conservatively
966 // to avoid compile time explosion. The worst possible case is if both
967 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
968 // and 'n' are the number of PHI sources.
970 if (UniqueSrc.insert(PV1))
971 V1Srcs.push_back(PV1);
974 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
975 // Early exit if the check of the first PHI source against V2 is MayAlias.
976 // Other results are not possible.
977 if (Alias == MayAlias)
980 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
981 // NoAlias / MustAlias. Otherwise, returns MayAlias.
982 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
983 Value *V = V1Srcs[i];
985 // If V2 is visited, the recursive case will have been caught in the
986 // above aliasCheck call, so these subsequent calls to aliasCheck
987 // don't need to assume that V2 is being visited recursively.
990 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
991 if (ThisAlias != Alias || ThisAlias == MayAlias)
998 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
999 // such as array references.
1001 AliasAnalysis::AliasResult
1002 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
1003 const Value *V2, unsigned V2Size) {
1004 // If either of the memory references is empty, it doesn't matter what the
1005 // pointer values are.
1006 if (V1Size == 0 || V2Size == 0)
1009 // Strip off any casts if they exist.
1010 V1 = V1->stripPointerCasts();
1011 V2 = V2->stripPointerCasts();
1013 // Are we checking for alias of the same value?
1014 if (V1 == V2) return MustAlias;
1016 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1017 return NoAlias; // Scalars cannot alias each other
1019 // Figure out what objects these things are pointing to if we can.
1020 const Value *O1 = V1->getUnderlyingObject();
1021 const Value *O2 = V2->getUnderlyingObject();
1023 // Null values in the default address space don't point to any object, so they
1024 // don't alias any other pointer.
1025 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1026 if (CPN->getType()->getAddressSpace() == 0)
1028 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1029 if (CPN->getType()->getAddressSpace() == 0)
1033 // If V1/V2 point to two different objects we know that we have no alias.
1034 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1037 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1038 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1039 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1042 // Arguments can't alias with local allocations or noalias calls
1043 // in the same function.
1044 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1045 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1048 // Most objects can't alias null.
1049 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1050 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1053 // If one pointer is the result of a call/invoke or load and the other is a
1054 // non-escaping local object within the same function, then we know the
1055 // object couldn't escape to a point where the call could return it.
1057 // Note that if the pointers are in different functions, there are a
1058 // variety of complications. A call with a nocapture argument may still
1059 // temporary store the nocapture argument's value in a temporary memory
1060 // location if that memory location doesn't escape. Or it may pass a
1061 // nocapture value to other functions as long as they don't capture it.
1062 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1064 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1068 // If the size of one access is larger than the entire object on the other
1069 // side, then we know such behavior is undefined and can assume no alias.
1071 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1072 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1075 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1076 // GEP can't simplify, we don't even look at the PHI cases.
1077 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1079 std::swap(V1Size, V2Size);
1082 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1083 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1085 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1087 std::swap(V1Size, V2Size);
1089 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1090 return aliasPHI(PN, V1Size, V2, V2Size);
1092 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1094 std::swap(V1Size, V2Size);
1096 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1097 return aliasSelect(S1, V1Size, V2, V2Size);
1099 return NoAA::alias(V1, V1Size, V2, V2Size);
1102 // Make sure that anything that uses AliasAnalysis pulls in this file.
1103 DEFINING_FILE_FOR(BasicAliasAnalysis)