1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/LLVMContext.h"
26 #include "llvm/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/Target/TargetData.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/SmallVector.h"
34 #include "llvm/Support/ErrorHandling.h"
35 #include "llvm/Support/GetElementPtrTypeIterator.h"
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
43 /// isKnownNonNull - Return true if we know that the specified value is never
45 static bool isKnownNonNull(const Value *V) {
46 // Alloca never returns null, malloc might.
47 if (isa<AllocaInst>(V)) return true;
49 // A byval argument is never null.
50 if (const Argument *A = dyn_cast<Argument>(V))
51 return A->hasByValAttr();
53 // Global values are not null unless extern weak.
54 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
55 return !GV->hasExternalWeakLinkage();
59 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
60 /// object that never escapes from the function.
61 static bool isNonEscapingLocalObject(const Value *V) {
62 // If this is a local allocation, check to see if it escapes.
63 if (isa<AllocaInst>(V) || isNoAliasCall(V))
64 // Set StoreCaptures to True so that we can assume in our callers that the
65 // pointer is not the result of a load instruction. Currently
66 // PointerMayBeCaptured doesn't have any special analysis for the
67 // StoreCaptures=false case; if it did, our callers could be refined to be
69 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
71 // If this is an argument that corresponds to a byval or noalias argument,
72 // then it has not escaped before entering the function. Check if it escapes
73 // inside the function.
74 if (const Argument *A = dyn_cast<Argument>(V))
75 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
76 // Don't bother analyzing arguments already known not to escape.
77 if (A->hasNoCaptureAttr())
79 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
84 /// isEscapeSource - Return true if the pointer is one which would have
85 /// been considered an escape by isNonEscapingLocalObject.
86 static bool isEscapeSource(const Value *V) {
87 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
90 // The load case works because isNonEscapingLocalObject considers all
91 // stores to be escapes (it passes true for the StoreCaptures argument
92 // to PointerMayBeCaptured).
99 /// isObjectSmallerThan - Return true if we can prove that the object specified
100 /// by V is smaller than Size.
101 static bool isObjectSmallerThan(const Value *V, unsigned Size,
102 const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 AccessTy = GV->getType()->getElementType();
106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
107 if (!AI->isArrayAllocation())
108 AccessTy = AI->getType()->getElementType();
111 } else if (const CallInst* CI = extractMallocCall(V)) {
112 if (!isArrayMalloc(V, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
115 return (C->getZExtValue() < Size);
117 } else if (const Argument *A = dyn_cast<Argument>(V)) {
118 if (A->hasByValAttr())
119 AccessTy = cast<PointerType>(A->getType())->getElementType();
126 if (AccessTy->isSized())
127 return TD.getTypeAllocSize(AccessTy) < Size;
131 //===----------------------------------------------------------------------===//
133 //===----------------------------------------------------------------------===//
136 /// NoAA - This class implements the -no-aa pass, which always returns "I
137 /// don't know" for alias queries. NoAA is unlike other alias analysis
138 /// implementations, in that it does not chain to a previous analysis. As
139 /// such it doesn't follow many of the rules that other alias analyses must.
141 struct NoAA : public ImmutablePass, public AliasAnalysis {
142 static char ID; // Class identification, replacement for typeinfo
143 NoAA() : ImmutablePass(ID) {}
144 explicit NoAA(char &PID) : ImmutablePass(PID) { }
146 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
149 virtual void initializePass() {
150 TD = getAnalysisIfAvailable<TargetData>();
153 virtual AliasResult alias(const Location &LocA, const Location &LocB) {
157 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
158 return UnknownModRefBehavior;
160 virtual ModRefBehavior getModRefBehavior(const Function *F) {
161 return UnknownModRefBehavior;
164 virtual bool pointsToConstantMemory(const Location &Loc) { return false; }
165 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
166 const Location &Loc) {
169 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
170 ImmutableCallSite CS2) {
174 virtual void deleteValue(Value *V) {}
175 virtual void copyValue(Value *From, Value *To) {}
177 /// getAdjustedAnalysisPointer - This method is used when a pass implements
178 /// an analysis interface through multiple inheritance. If needed, it
179 /// should override this to adjust the this pointer as needed for the
180 /// specified pass info.
181 virtual void *getAdjustedAnalysisPointer(const void *ID) {
182 if (ID == &AliasAnalysis::ID)
183 return (AliasAnalysis*)this;
187 } // End of anonymous namespace
189 // Register this pass...
191 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
192 "No Alias Analysis (always returns 'may' alias)",
195 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
197 //===----------------------------------------------------------------------===//
198 // GetElementPtr Instruction Decomposition and Analysis
199 //===----------------------------------------------------------------------===//
208 struct VariableGEPIndex {
210 ExtensionKind Extension;
216 /// GetLinearExpression - Analyze the specified value as a linear expression:
217 /// "A*V + B", where A and B are constant integers. Return the scale and offset
218 /// values as APInts and return V as a Value*, and return whether we looked
219 /// through any sign or zero extends. The incoming Value is known to have
220 /// IntegerType and it may already be sign or zero extended.
222 /// Note that this looks through extends, so the high bits may not be
223 /// represented in the result.
224 static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
225 ExtensionKind &Extension,
226 const TargetData &TD, unsigned Depth) {
227 assert(V->getType()->isIntegerTy() && "Not an integer value");
229 // Limit our recursion depth.
236 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
237 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
238 switch (BOp->getOpcode()) {
240 case Instruction::Or:
241 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't
243 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD))
246 case Instruction::Add:
247 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
249 Offset += RHSC->getValue();
251 case Instruction::Mul:
252 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
254 Offset *= RHSC->getValue();
255 Scale *= RHSC->getValue();
257 case Instruction::Shl:
258 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
260 Offset <<= RHSC->getValue().getLimitedValue();
261 Scale <<= RHSC->getValue().getLimitedValue();
267 // Since GEP indices are sign extended anyway, we don't care about the high
268 // bits of a sign or zero extended value - just scales and offsets. The
269 // extensions have to be consistent though.
270 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
271 (isa<ZExtInst>(V) && Extension != EK_SignExt)) {
272 Value *CastOp = cast<CastInst>(V)->getOperand(0);
273 unsigned OldWidth = Scale.getBitWidth();
274 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
275 Scale.trunc(SmallWidth);
276 Offset.trunc(SmallWidth);
277 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
279 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
281 Scale.zext(OldWidth);
282 Offset.zext(OldWidth);
292 /// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
293 /// into a base pointer with a constant offset and a number of scaled symbolic
296 /// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
297 /// the VarIndices vector) are Value*'s that are known to be scaled by the
298 /// specified amount, but which may have other unrepresented high bits. As such,
299 /// the gep cannot necessarily be reconstructed from its decomposed form.
301 /// When TargetData is around, this function is capable of analyzing everything
302 /// that Value::getUnderlyingObject() can look through. When not, it just looks
303 /// through pointer casts.
306 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
307 SmallVectorImpl<VariableGEPIndex> &VarIndices,
308 const TargetData *TD) {
309 // Limit recursion depth to limit compile time in crazy cases.
310 unsigned MaxLookup = 6;
314 // See if this is a bitcast or GEP.
315 const Operator *Op = dyn_cast<Operator>(V);
317 // The only non-operator case we can handle are GlobalAliases.
318 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
319 if (!GA->mayBeOverridden()) {
320 V = GA->getAliasee();
327 if (Op->getOpcode() == Instruction::BitCast) {
328 V = Op->getOperand(0);
332 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
336 // Don't attempt to analyze GEPs over unsized objects.
337 if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
338 ->getElementType()->isSized())
341 // If we are lacking TargetData information, we can't compute the offets of
342 // elements computed by GEPs. However, we can handle bitcast equivalent
345 if (!GEPOp->hasAllZeroIndices())
347 V = GEPOp->getOperand(0);
351 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
352 gep_type_iterator GTI = gep_type_begin(GEPOp);
353 for (User::const_op_iterator I = GEPOp->op_begin()+1,
354 E = GEPOp->op_end(); I != E; ++I) {
356 // Compute the (potentially symbolic) offset in bytes for this index.
357 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) {
358 // For a struct, add the member offset.
359 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
360 if (FieldNo == 0) continue;
362 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo);
366 // For an array/pointer, add the element offset, explicitly scaled.
367 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
368 if (CIdx->isZero()) continue;
369 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
373 uint64_t Scale = TD->getTypeAllocSize(*GTI);
374 ExtensionKind Extension = EK_NotExtended;
376 // If the integer type is smaller than the pointer size, it is implicitly
377 // sign extended to pointer size.
378 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth();
379 if (TD->getPointerSizeInBits() > Width)
380 Extension = EK_SignExt;
382 // Use GetLinearExpression to decompose the index into a C1*V+C2 form.
383 APInt IndexScale(Width, 0), IndexOffset(Width, 0);
384 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
387 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
388 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
389 BaseOffs += IndexOffset.getSExtValue()*Scale;
390 Scale *= IndexScale.getSExtValue();
393 // If we already had an occurrance of this index variable, merge this
394 // scale into it. For example, we want to handle:
395 // A[x][x] -> x*16 + x*4 -> x*20
396 // This also ensures that 'x' only appears in the index list once.
397 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
398 if (VarIndices[i].V == Index &&
399 VarIndices[i].Extension == Extension) {
400 Scale += VarIndices[i].Scale;
401 VarIndices.erase(VarIndices.begin()+i);
406 // Make sure that we have a scale that makes sense for this target's
408 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) {
410 Scale = (int64_t)Scale >> ShiftBits;
414 VariableGEPIndex Entry = {Index, Extension, Scale};
415 VarIndices.push_back(Entry);
419 // Analyze the base pointer next.
420 V = GEPOp->getOperand(0);
421 } while (--MaxLookup);
423 // If the chain of expressions is too deep, just return early.
427 /// GetIndexDifference - Dest and Src are the variable indices from two
428 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
429 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
430 /// difference between the two pointers.
431 static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
432 const SmallVectorImpl<VariableGEPIndex> &Src) {
433 if (Src.empty()) return;
435 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
436 const Value *V = Src[i].V;
437 ExtensionKind Extension = Src[i].Extension;
438 int64_t Scale = Src[i].Scale;
440 // Find V in Dest. This is N^2, but pointer indices almost never have more
441 // than a few variable indexes.
442 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
443 if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
445 // If we found it, subtract off Scale V's from the entry in Dest. If it
446 // goes to zero, remove the entry.
447 if (Dest[j].Scale != Scale)
448 Dest[j].Scale -= Scale;
450 Dest.erase(Dest.begin()+j);
455 // If we didn't consume this entry, add it to the end of the Dest list.
457 VariableGEPIndex Entry = { V, Extension, -Scale };
458 Dest.push_back(Entry);
463 //===----------------------------------------------------------------------===//
464 // BasicAliasAnalysis Pass
465 //===----------------------------------------------------------------------===//
468 static const Function *getParent(const Value *V) {
469 if (const Instruction *inst = dyn_cast<Instruction>(V))
470 return inst->getParent()->getParent();
472 if (const Argument *arg = dyn_cast<Argument>(V))
473 return arg->getParent();
478 static bool notDifferentParent(const Value *O1, const Value *O2) {
480 const Function *F1 = getParent(O1);
481 const Function *F2 = getParent(O2);
483 return !F1 || !F2 || F1 == F2;
488 /// BasicAliasAnalysis - This is the default alias analysis implementation.
489 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
490 /// derives from the NoAA class.
491 struct BasicAliasAnalysis : public NoAA {
492 static char ID; // Class identification, replacement for typeinfo
493 BasicAliasAnalysis() : NoAA(ID) {}
495 virtual AliasResult alias(const Location &LocA,
496 const Location &LocB) {
497 assert(Visited.empty() && "Visited must be cleared after use!");
498 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
499 "BasicAliasAnalysis doesn't support interprocedural queries.");
500 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocB.Ptr, LocB.Size);
505 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
506 const Location &Loc);
508 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
509 ImmutableCallSite CS2) {
510 // The AliasAnalysis base class has some smarts, lets use them.
511 return AliasAnalysis::getModRefInfo(CS1, CS2);
514 /// pointsToConstantMemory - Chase pointers until we find a (constant
516 virtual bool pointsToConstantMemory(const Location &Loc);
518 /// getModRefBehavior - Return the behavior when calling the given
520 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
522 /// getModRefBehavior - Return the behavior when calling the given function.
523 /// For use when the call site is not known.
524 virtual ModRefBehavior getModRefBehavior(const Function *F);
526 /// getAdjustedAnalysisPointer - This method is used when a pass implements
527 /// an analysis interface through multiple inheritance. If needed, it
528 /// should override this to adjust the this pointer as needed for the
529 /// specified pass info.
530 virtual void *getAdjustedAnalysisPointer(const void *ID) {
531 if (ID == &AliasAnalysis::ID)
532 return (AliasAnalysis*)this;
537 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
538 SmallPtrSet<const Value*, 16> Visited;
540 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
541 // instruction against another.
542 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
543 const Value *V2, unsigned V2Size,
544 const Value *UnderlyingV1, const Value *UnderlyingV2);
546 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
547 // instruction against another.
548 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
549 const Value *V2, unsigned V2Size);
551 /// aliasSelect - Disambiguate a Select instruction against another value.
552 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
553 const Value *V2, unsigned V2Size);
555 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
556 const Value *V2, unsigned V2Size);
558 } // End of anonymous namespace
560 // Register this pass...
561 char BasicAliasAnalysis::ID = 0;
562 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
563 "Basic Alias Analysis (default AA impl)",
566 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
567 return new BasicAliasAnalysis();
571 /// pointsToConstantMemory - Chase pointers until we find a (constant
573 bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
574 if (const GlobalVariable *GV =
575 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject()))
576 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
577 // global to be marked constant in some modules and non-constant in others.
578 // GV may even be a declaration, not a definition.
579 return GV->isConstant();
581 return NoAA::pointsToConstantMemory(Loc);
584 /// getModRefBehavior - Return the behavior when calling the given call site.
585 AliasAnalysis::ModRefBehavior
586 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
587 if (CS.doesNotAccessMemory())
588 // Can't do better than this.
589 return DoesNotAccessMemory;
591 ModRefBehavior Min = UnknownModRefBehavior;
593 // If the callsite knows it only reads memory, don't return worse
595 if (CS.onlyReadsMemory())
596 Min = OnlyReadsMemory;
598 // The AliasAnalysis base class has some smarts, lets use them.
599 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
602 /// getModRefBehavior - Return the behavior when calling the given function.
603 /// For use when the call site is not known.
604 AliasAnalysis::ModRefBehavior
605 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
606 if (F->doesNotAccessMemory())
607 // Can't do better than this.
608 return DoesNotAccessMemory;
609 if (F->onlyReadsMemory())
610 return OnlyReadsMemory;
611 if (unsigned id = F->getIntrinsicID())
612 return getIntrinsicModRefBehavior(id);
614 return NoAA::getModRefBehavior(F);
617 /// getModRefInfo - Check to see if the specified callsite can clobber the
618 /// specified memory object. Since we only look at local properties of this
619 /// function, we really can't say much about this query. We do, however, use
620 /// simple "address taken" analysis on local objects.
621 AliasAnalysis::ModRefResult
622 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
623 const Location &Loc) {
624 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
625 "AliasAnalysis query involving multiple functions!");
627 const Value *Object = Loc.Ptr->getUnderlyingObject();
629 // If this is a tail call and Loc.Ptr points to a stack location, we know that
630 // the tail call cannot access or modify the local stack.
631 // We cannot exclude byval arguments here; these belong to the caller of
632 // the current function not to the current function, and a tail callee
633 // may reference them.
634 if (isa<AllocaInst>(Object))
635 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
636 if (CI->isTailCall())
639 // If the pointer is to a locally allocated object that does not escape,
640 // then the call can not mod/ref the pointer unless the call takes the pointer
641 // as an argument, and itself doesn't capture it.
642 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
643 isNonEscapingLocalObject(Object)) {
644 bool PassedAsArg = false;
646 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
647 CI != CE; ++CI, ++ArgNo) {
648 // Only look at the no-capture pointer arguments.
649 if (!(*CI)->getType()->isPointerTy() ||
650 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
653 // If this is a no-capture pointer argument, see if we can tell that it
654 // is impossible to alias the pointer we're checking. If not, we have to
655 // assume that the call could touch the pointer, even though it doesn't
657 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) {
667 // Finally, handle specific knowledge of intrinsics.
668 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
670 switch (II->getIntrinsicID()) {
672 case Intrinsic::memcpy:
673 case Intrinsic::memmove: {
674 unsigned Len = UnknownSize;
675 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
676 Len = LenCI->getZExtValue();
677 Value *Dest = II->getArgOperand(0);
678 Value *Src = II->getArgOperand(1);
679 if (isNoAlias(Location(Dest, Len), Loc)) {
680 if (isNoAlias(Location(Src, Len), Loc))
686 case Intrinsic::memset:
687 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
688 // will handle it for the variable length case.
689 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
690 unsigned Len = LenCI->getZExtValue();
691 Value *Dest = II->getArgOperand(0);
692 if (isNoAlias(Location(Dest, Len), Loc))
696 case Intrinsic::atomic_cmp_swap:
697 case Intrinsic::atomic_swap:
698 case Intrinsic::atomic_load_add:
699 case Intrinsic::atomic_load_sub:
700 case Intrinsic::atomic_load_and:
701 case Intrinsic::atomic_load_nand:
702 case Intrinsic::atomic_load_or:
703 case Intrinsic::atomic_load_xor:
704 case Intrinsic::atomic_load_max:
705 case Intrinsic::atomic_load_min:
706 case Intrinsic::atomic_load_umax:
707 case Intrinsic::atomic_load_umin:
709 Value *Op1 = II->getArgOperand(0);
710 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
711 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa);
712 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc))
716 case Intrinsic::lifetime_start:
717 case Intrinsic::lifetime_end:
718 case Intrinsic::invariant_start: {
720 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
721 if (isNoAlias(Location(II->getArgOperand(1),
723 II->getMetadata(LLVMContext::MD_tbaa)),
728 case Intrinsic::invariant_end: {
730 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
731 if (isNoAlias(Location(II->getArgOperand(2),
733 II->getMetadata(LLVMContext::MD_tbaa)),
740 // The AliasAnalysis base class has some smarts, lets use them.
741 return AliasAnalysis::getModRefInfo(CS, Loc);
744 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
745 /// against another pointer. We know that V1 is a GEP, but we don't know
746 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
747 /// UnderlyingV2 is the same for V2.
749 AliasAnalysis::AliasResult
750 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
751 const Value *V2, unsigned V2Size,
752 const Value *UnderlyingV1,
753 const Value *UnderlyingV2) {
754 // If this GEP has been visited before, we're on a use-def cycle.
755 // Such cycles are only valid when PHI nodes are involved or in unreachable
756 // code. The visitPHI function catches cycles containing PHIs, but there
757 // could still be a cycle without PHIs in unreachable code.
758 if (!Visited.insert(GEP1))
761 int64_t GEP1BaseOffset;
762 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
764 // If we have two gep instructions with must-alias'ing base pointers, figure
765 // out if the indexes to the GEP tell us anything about the derived pointer.
766 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
767 // Do the base pointers alias?
768 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
769 UnderlyingV2, UnknownSize);
771 // If we get a No or May, then return it immediately, no amount of analysis
772 // will improve this situation.
773 if (BaseAlias != MustAlias) return BaseAlias;
775 // Otherwise, we have a MustAlias. Since the base pointers alias each other
776 // exactly, see if the computed offset from the common pointer tells us
777 // about the relation of the resulting pointer.
778 const Value *GEP1BasePtr =
779 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
781 int64_t GEP2BaseOffset;
782 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
783 const Value *GEP2BasePtr =
784 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
786 // If DecomposeGEPExpression isn't able to look all the way through the
787 // addressing operation, we must not have TD and this is too complex for us
788 // to handle without it.
789 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
791 "DecomposeGEPExpression and getUnderlyingObject disagree!");
795 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
796 // symbolic difference.
797 GEP1BaseOffset -= GEP2BaseOffset;
798 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
801 // Check to see if these two pointers are related by the getelementptr
802 // instruction. If one pointer is a GEP with a non-zero index of the other
803 // pointer, we know they cannot alias.
805 // If both accesses are unknown size, we can't do anything useful here.
806 if (V1Size == UnknownSize && V2Size == UnknownSize)
809 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
811 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
812 // If V2 is known not to alias GEP base pointer, then the two values
813 // cannot alias per GEP semantics: "A pointer value formed from a
814 // getelementptr instruction is associated with the addresses associated
815 // with the first operand of the getelementptr".
818 const Value *GEP1BasePtr =
819 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
821 // If DecomposeGEPExpression isn't able to look all the way through the
822 // addressing operation, we must not have TD and this is too complex for us
823 // to handle without it.
824 if (GEP1BasePtr != UnderlyingV1) {
826 "DecomposeGEPExpression and getUnderlyingObject disagree!");
831 // In the two GEP Case, if there is no difference in the offsets of the
832 // computed pointers, the resultant pointers are a must alias. This
833 // hapens when we have two lexically identical GEP's (for example).
835 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
836 // must aliases the GEP, the end result is a must alias also.
837 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
840 // If we have a known constant offset, see if this offset is larger than the
841 // access size being queried. If so, and if no variable indices can remove
842 // pieces of this constant, then we know we have a no-alias. For example,
845 // In order to handle cases like &A[100][i] where i is an out of range
846 // subscript, we have to ignore all constant offset pieces that are a multiple
847 // of a scaled index. Do this by removing constant offsets that are a
848 // multiple of any of our variable indices. This allows us to transform
849 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
850 // provides an offset of 4 bytes (assuming a <= 4 byte access).
851 for (unsigned i = 0, e = GEP1VariableIndices.size();
852 i != e && GEP1BaseOffset;++i)
853 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
854 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
856 // If our known offset is bigger than the access size, we know we don't have
858 if (GEP1BaseOffset) {
859 if (GEP1BaseOffset >= (int64_t)V2Size ||
860 GEP1BaseOffset <= -(int64_t)V1Size)
867 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
868 /// instruction against another.
869 AliasAnalysis::AliasResult
870 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
871 const Value *V2, unsigned V2Size) {
872 // If this select has been visited before, we're on a use-def cycle.
873 // Such cycles are only valid when PHI nodes are involved or in unreachable
874 // code. The visitPHI function catches cycles containing PHIs, but there
875 // could still be a cycle without PHIs in unreachable code.
876 if (!Visited.insert(SI))
879 // If the values are Selects with the same condition, we can do a more precise
880 // check: just check for aliases between the values on corresponding arms.
881 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
882 if (SI->getCondition() == SI2->getCondition()) {
884 aliasCheck(SI->getTrueValue(), SISize,
885 SI2->getTrueValue(), V2Size);
886 if (Alias == MayAlias)
888 AliasResult ThisAlias =
889 aliasCheck(SI->getFalseValue(), SISize,
890 SI2->getFalseValue(), V2Size);
891 if (ThisAlias != Alias)
896 // If both arms of the Select node NoAlias or MustAlias V2, then returns
897 // NoAlias / MustAlias. Otherwise, returns MayAlias.
899 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
900 if (Alias == MayAlias)
903 // If V2 is visited, the recursive case will have been caught in the
904 // above aliasCheck call, so these subsequent calls to aliasCheck
905 // don't need to assume that V2 is being visited recursively.
908 AliasResult ThisAlias =
909 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
910 if (ThisAlias != Alias)
915 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
917 AliasAnalysis::AliasResult
918 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
919 const Value *V2, unsigned V2Size) {
920 // The PHI node has already been visited, avoid recursion any further.
921 if (!Visited.insert(PN))
924 // If the values are PHIs in the same block, we can do a more precise
925 // as well as efficient check: just check for aliases between the values
926 // on corresponding edges.
927 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
928 if (PN2->getParent() == PN->getParent()) {
930 aliasCheck(PN->getIncomingValue(0), PNSize,
931 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
933 if (Alias == MayAlias)
935 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
936 AliasResult ThisAlias =
937 aliasCheck(PN->getIncomingValue(i), PNSize,
938 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
940 if (ThisAlias != Alias)
946 SmallPtrSet<Value*, 4> UniqueSrc;
947 SmallVector<Value*, 4> V1Srcs;
948 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
949 Value *PV1 = PN->getIncomingValue(i);
950 if (isa<PHINode>(PV1))
951 // If any of the source itself is a PHI, return MayAlias conservatively
952 // to avoid compile time explosion. The worst possible case is if both
953 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
954 // and 'n' are the number of PHI sources.
956 if (UniqueSrc.insert(PV1))
957 V1Srcs.push_back(PV1);
960 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
961 // Early exit if the check of the first PHI source against V2 is MayAlias.
962 // Other results are not possible.
963 if (Alias == MayAlias)
966 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
967 // NoAlias / MustAlias. Otherwise, returns MayAlias.
968 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
969 Value *V = V1Srcs[i];
971 // If V2 is visited, the recursive case will have been caught in the
972 // above aliasCheck call, so these subsequent calls to aliasCheck
973 // don't need to assume that V2 is being visited recursively.
976 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
977 if (ThisAlias != Alias || ThisAlias == MayAlias)
984 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
985 // such as array references.
987 AliasAnalysis::AliasResult
988 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
989 const Value *V2, unsigned V2Size) {
990 // If either of the memory references is empty, it doesn't matter what the
991 // pointer values are.
992 if (V1Size == 0 || V2Size == 0)
995 // Strip off any casts if they exist.
996 V1 = V1->stripPointerCasts();
997 V2 = V2->stripPointerCasts();
999 // Are we checking for alias of the same value?
1000 if (V1 == V2) return MustAlias;
1002 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
1003 return NoAlias; // Scalars cannot alias each other
1005 // Figure out what objects these things are pointing to if we can.
1006 const Value *O1 = V1->getUnderlyingObject();
1007 const Value *O2 = V2->getUnderlyingObject();
1009 // Null values in the default address space don't point to any object, so they
1010 // don't alias any other pointer.
1011 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1012 if (CPN->getType()->getAddressSpace() == 0)
1014 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1015 if (CPN->getType()->getAddressSpace() == 0)
1019 // If V1/V2 point to two different objects we know that we have no alias.
1020 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1023 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1024 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1025 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1028 // Arguments can't alias with local allocations or noalias calls
1029 // in the same function.
1030 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1031 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1034 // Most objects can't alias null.
1035 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1036 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1039 // If one pointer is the result of a call/invoke or load and the other is a
1040 // non-escaping local object within the same function, then we know the
1041 // object couldn't escape to a point where the call could return it.
1043 // Note that if the pointers are in different functions, there are a
1044 // variety of complications. A call with a nocapture argument may still
1045 // temporary store the nocapture argument's value in a temporary memory
1046 // location if that memory location doesn't escape. Or it may pass a
1047 // nocapture value to other functions as long as they don't capture it.
1048 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1050 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1054 // If the size of one access is larger than the entire object on the other
1055 // side, then we know such behavior is undefined and can assume no alias.
1057 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1058 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1061 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1062 // GEP can't simplify, we don't even look at the PHI cases.
1063 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1065 std::swap(V1Size, V2Size);
1068 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1069 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1071 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1073 std::swap(V1Size, V2Size);
1075 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1076 return aliasPHI(PN, V1Size, V2, V2Size);
1078 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1080 std::swap(V1Size, V2Size);
1082 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1083 return aliasSelect(S1, V1Size, V2, V2Size);
1085 return NoAA::alias(Location(V1, V1Size), Location(V2, V2Size));
1088 // Make sure that anything that uses AliasAnalysis pulls in this file.
1089 DEFINING_FILE_FOR(BasicAliasAnalysis)