1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalAlias.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/Operator.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Analysis/CaptureTracking.h"
28 #include "llvm/Analysis/MemoryBuiltins.h"
29 #include "llvm/Analysis/ValueTracking.h"
30 #include "llvm/Target/TargetData.h"
31 #include "llvm/ADT/SmallPtrSet.h"
32 #include "llvm/ADT/SmallVector.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/GetElementPtrTypeIterator.h"
38 //===----------------------------------------------------------------------===//
40 //===----------------------------------------------------------------------===//
42 /// isKnownNonNull - Return true if we know that the specified value is never
44 static bool isKnownNonNull(const Value *V) {
45 // Alloca never returns null, malloc might.
46 if (isa<AllocaInst>(V)) return true;
48 // A byval argument is never null.
49 if (const Argument *A = dyn_cast<Argument>(V))
50 return A->hasByValAttr();
52 // Global values are not null unless extern weak.
53 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
54 return !GV->hasExternalWeakLinkage();
58 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
59 /// object that never escapes from the function.
60 static bool isNonEscapingLocalObject(const Value *V) {
61 // If this is a local allocation, check to see if it escapes.
62 if (isa<AllocaInst>(V) || isNoAliasCall(V))
63 // Set StoreCaptures to True so that we can assume in our callers that the
64 // pointer is not the result of a load instruction. Currently
65 // PointerMayBeCaptured doesn't have any special analysis for the
66 // StoreCaptures=false case; if it did, our callers could be refined to be
68 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
70 // If this is an argument that corresponds to a byval or noalias argument,
71 // then it has not escaped before entering the function. Check if it escapes
72 // inside the function.
73 if (const Argument *A = dyn_cast<Argument>(V))
74 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
75 // Don't bother analyzing arguments already known not to escape.
76 if (A->hasNoCaptureAttr())
78 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
83 /// isEscapeSource - Return true if the pointer is one which would have
84 /// been considered an escape by isNonEscapingLocalObject.
85 static bool isEscapeSource(const Value *V) {
86 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
89 // The load case works because isNonEscapingLocalObject considers all
90 // stores to be escapes (it passes true for the StoreCaptures argument
91 // to PointerMayBeCaptured).
98 /// isObjectSmallerThan - Return true if we can prove that the object specified
99 /// by V is smaller than Size.
100 static bool isObjectSmallerThan(const Value *V, unsigned Size,
101 const TargetData &TD) {
102 const Type *AccessTy;
103 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
104 AccessTy = GV->getType()->getElementType();
105 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
106 if (!AI->isArrayAllocation())
107 AccessTy = AI->getType()->getElementType();
110 } else if (const CallInst* CI = extractMallocCall(V)) {
111 if (!isArrayMalloc(V, &TD))
112 // The size is the argument to the malloc call.
113 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
114 return (C->getZExtValue() < Size);
116 } else if (const Argument *A = dyn_cast<Argument>(V)) {
117 if (A->hasByValAttr())
118 AccessTy = cast<PointerType>(A->getType())->getElementType();
125 if (AccessTy->isSized())
126 return TD.getTypeAllocSize(AccessTy) < Size;
130 //===----------------------------------------------------------------------===//
132 //===----------------------------------------------------------------------===//
135 /// NoAA - This class implements the -no-aa pass, which always returns "I
136 /// don't know" for alias queries. NoAA is unlike other alias analysis
137 /// implementations, in that it does not chain to a previous analysis. As
138 /// such it doesn't follow many of the rules that other alias analyses must.
140 struct NoAA : public ImmutablePass, public AliasAnalysis {
141 static char ID; // Class identification, replacement for typeinfo
142 NoAA() : ImmutablePass(ID) {}
143 explicit NoAA(char &PID) : ImmutablePass(PID) { }
145 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
148 virtual void initializePass() {
149 TD = getAnalysisIfAvailable<TargetData>();
152 virtual AliasResult alias(const Value *V1, unsigned V1Size,
153 const Value *V2, unsigned V2Size) {
157 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
158 return UnknownModRefBehavior;
160 virtual ModRefBehavior getModRefBehavior(const Function *F) {
161 return UnknownModRefBehavior;
164 virtual bool pointsToConstantMemory(const Value *P) { return false; }
165 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
166 const Value *P, unsigned Size) {
169 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
170 ImmutableCallSite CS2) {
174 virtual void deleteValue(Value *V) {}
175 virtual void copyValue(Value *From, Value *To) {}
177 /// getAdjustedAnalysisPointer - This method is used when a pass implements
178 /// an analysis interface through multiple inheritance. If needed, it
179 /// should override this to adjust the this pointer as needed for the
180 /// specified pass info.
181 virtual void *getAdjustedAnalysisPointer(const void *ID) {
182 if (ID == &AliasAnalysis::ID)
183 return (AliasAnalysis*)this;
187 } // End of anonymous namespace
189 // Register this pass...
191 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
192 "No Alias Analysis (always returns 'may' alias)",
195 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
197 //===----------------------------------------------------------------------===//
198 // GetElementPtr Instruction Decomposition and Analysis
199 //===----------------------------------------------------------------------===//
208 struct VariableGEPIndex {
210 ExtensionKind Extension;
216 /// GetLinearExpression - Analyze the specified value as a linear expression:
217 /// "A*V + B", where A and B are constant integers. Return the scale and offset
218 /// values as APInts and return V as a Value*, 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.getZExtValue()*Scale;
390 Scale *= IndexScale.getZExtValue();
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()) {
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 Value *V1, unsigned V1Size,
496 const Value *V2, unsigned V2Size) {
497 assert(Visited.empty() && "Visited must be cleared after use!");
498 assert(notDifferentParent(V1, V2) &&
499 "BasicAliasAnalysis doesn't support interprocedural queries.");
500 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
505 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
506 const Value *P, unsigned Size);
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 Value *P);
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 Value *P) {
574 if (const GlobalVariable *GV =
575 dyn_cast<GlobalVariable>(P->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(P);
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 Value *P, unsigned Size) {
624 assert(notDifferentParent(CS.getInstruction(), P) &&
625 "AliasAnalysis query involving multiple functions!");
627 const Value *Object = P->getUnderlyingObject();
629 // If this is a tail call and P 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(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
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(Dest, Len, P, Size)) {
680 if (isNoAlias(Src, Len, P, Size))
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(Dest, Len, P, Size))
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 if (isNoAlias(Op1, Op1Size, P, Size))
715 case Intrinsic::lifetime_start:
716 case Intrinsic::lifetime_end:
717 case Intrinsic::invariant_start: {
719 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
720 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
724 case Intrinsic::invariant_end: {
726 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
727 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
733 // The AliasAnalysis base class has some smarts, lets use them.
734 return AliasAnalysis::getModRefInfo(CS, P, Size);
738 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
739 /// against another pointer. We know that V1 is a GEP, but we don't know
740 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
741 /// UnderlyingV2 is the same for V2.
743 AliasAnalysis::AliasResult
744 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
745 const Value *V2, unsigned V2Size,
746 const Value *UnderlyingV1,
747 const Value *UnderlyingV2) {
748 // If this GEP has been visited before, we're on a use-def cycle.
749 // Such cycles are only valid when PHI nodes are involved or in unreachable
750 // code. The visitPHI function catches cycles containing PHIs, but there
751 // could still be a cycle without PHIs in unreachable code.
752 if (!Visited.insert(GEP1))
755 int64_t GEP1BaseOffset;
756 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
758 // If we have two gep instructions with must-alias'ing base pointers, figure
759 // out if the indexes to the GEP tell us anything about the derived pointer.
760 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
761 // Do the base pointers alias?
762 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
763 UnderlyingV2, UnknownSize);
765 // If we get a No or May, then return it immediately, no amount of analysis
766 // will improve this situation.
767 if (BaseAlias != MustAlias) return BaseAlias;
769 // Otherwise, we have a MustAlias. Since the base pointers alias each other
770 // exactly, see if the computed offset from the common pointer tells us
771 // about the relation of the resulting pointer.
772 const Value *GEP1BasePtr =
773 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
775 int64_t GEP2BaseOffset;
776 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
777 const Value *GEP2BasePtr =
778 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
780 // If DecomposeGEPExpression isn't able to look all the way through the
781 // addressing operation, we must not have TD and this is too complex for us
782 // to handle without it.
783 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
785 "DecomposeGEPExpression and getUnderlyingObject disagree!");
789 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
790 // symbolic difference.
791 GEP1BaseOffset -= GEP2BaseOffset;
792 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
795 // Check to see if these two pointers are related by the getelementptr
796 // instruction. If one pointer is a GEP with a non-zero index of the other
797 // pointer, we know they cannot alias.
799 // If both accesses are unknown size, we can't do anything useful here.
800 if (V1Size == UnknownSize && V2Size == UnknownSize)
803 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
805 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
806 // If V2 is known not to alias GEP base pointer, then the two values
807 // cannot alias per GEP semantics: "A pointer value formed from a
808 // getelementptr instruction is associated with the addresses associated
809 // with the first operand of the getelementptr".
812 const Value *GEP1BasePtr =
813 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
815 // If DecomposeGEPExpression isn't able to look all the way through the
816 // addressing operation, we must not have TD and this is too complex for us
817 // to handle without it.
818 if (GEP1BasePtr != UnderlyingV1) {
820 "DecomposeGEPExpression and getUnderlyingObject disagree!");
825 // In the two GEP Case, if there is no difference in the offsets of the
826 // computed pointers, the resultant pointers are a must alias. This
827 // hapens when we have two lexically identical GEP's (for example).
829 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
830 // must aliases the GEP, the end result is a must alias also.
831 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
834 // If we have a known constant offset, see if this offset is larger than the
835 // access size being queried. If so, and if no variable indices can remove
836 // pieces of this constant, then we know we have a no-alias. For example,
839 // In order to handle cases like &A[100][i] where i is an out of range
840 // subscript, we have to ignore all constant offset pieces that are a multiple
841 // of a scaled index. Do this by removing constant offsets that are a
842 // multiple of any of our variable indices. This allows us to transform
843 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
844 // provides an offset of 4 bytes (assuming a <= 4 byte access).
845 for (unsigned i = 0, e = GEP1VariableIndices.size();
846 i != e && GEP1BaseOffset;++i)
847 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
848 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
850 // If our known offset is bigger than the access size, we know we don't have
852 if (GEP1BaseOffset) {
853 if (GEP1BaseOffset >= (int64_t)V2Size ||
854 GEP1BaseOffset <= -(int64_t)V1Size)
861 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
862 /// instruction against another.
863 AliasAnalysis::AliasResult
864 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
865 const Value *V2, unsigned V2Size) {
866 // If this select has been visited before, we're on a use-def cycle.
867 // Such cycles are only valid when PHI nodes are involved or in unreachable
868 // code. The visitPHI function catches cycles containing PHIs, but there
869 // could still be a cycle without PHIs in unreachable code.
870 if (!Visited.insert(SI))
873 // If the values are Selects with the same condition, we can do a more precise
874 // check: just check for aliases between the values on corresponding arms.
875 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
876 if (SI->getCondition() == SI2->getCondition()) {
878 aliasCheck(SI->getTrueValue(), SISize,
879 SI2->getTrueValue(), V2Size);
880 if (Alias == MayAlias)
882 AliasResult ThisAlias =
883 aliasCheck(SI->getFalseValue(), SISize,
884 SI2->getFalseValue(), V2Size);
885 if (ThisAlias != Alias)
890 // If both arms of the Select node NoAlias or MustAlias V2, then returns
891 // NoAlias / MustAlias. Otherwise, returns MayAlias.
893 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
894 if (Alias == MayAlias)
897 // If V2 is visited, the recursive case will have been caught in the
898 // above aliasCheck call, so these subsequent calls to aliasCheck
899 // don't need to assume that V2 is being visited recursively.
902 AliasResult ThisAlias =
903 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
904 if (ThisAlias != Alias)
909 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
911 AliasAnalysis::AliasResult
912 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
913 const Value *V2, unsigned V2Size) {
914 // The PHI node has already been visited, avoid recursion any further.
915 if (!Visited.insert(PN))
918 // If the values are PHIs in the same block, we can do a more precise
919 // as well as efficient check: just check for aliases between the values
920 // on corresponding edges.
921 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
922 if (PN2->getParent() == PN->getParent()) {
924 aliasCheck(PN->getIncomingValue(0), PNSize,
925 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
927 if (Alias == MayAlias)
929 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
930 AliasResult ThisAlias =
931 aliasCheck(PN->getIncomingValue(i), PNSize,
932 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
934 if (ThisAlias != Alias)
940 SmallPtrSet<Value*, 4> UniqueSrc;
941 SmallVector<Value*, 4> V1Srcs;
942 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
943 Value *PV1 = PN->getIncomingValue(i);
944 if (isa<PHINode>(PV1))
945 // If any of the source itself is a PHI, return MayAlias conservatively
946 // to avoid compile time explosion. The worst possible case is if both
947 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
948 // and 'n' are the number of PHI sources.
950 if (UniqueSrc.insert(PV1))
951 V1Srcs.push_back(PV1);
954 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
955 // Early exit if the check of the first PHI source against V2 is MayAlias.
956 // Other results are not possible.
957 if (Alias == MayAlias)
960 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
961 // NoAlias / MustAlias. Otherwise, returns MayAlias.
962 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
963 Value *V = V1Srcs[i];
965 // If V2 is visited, the recursive case will have been caught in the
966 // above aliasCheck call, so these subsequent calls to aliasCheck
967 // don't need to assume that V2 is being visited recursively.
970 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
971 if (ThisAlias != Alias || ThisAlias == MayAlias)
978 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
979 // such as array references.
981 AliasAnalysis::AliasResult
982 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
983 const Value *V2, unsigned V2Size) {
984 // If either of the memory references is empty, it doesn't matter what the
985 // pointer values are.
986 if (V1Size == 0 || V2Size == 0)
989 // Strip off any casts if they exist.
990 V1 = V1->stripPointerCasts();
991 V2 = V2->stripPointerCasts();
993 // Are we checking for alias of the same value?
994 if (V1 == V2) return MustAlias;
996 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
997 return NoAlias; // Scalars cannot alias each other
999 // Figure out what objects these things are pointing to if we can.
1000 const Value *O1 = V1->getUnderlyingObject();
1001 const Value *O2 = V2->getUnderlyingObject();
1003 // Null values in the default address space don't point to any object, so they
1004 // don't alias any other pointer.
1005 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
1006 if (CPN->getType()->getAddressSpace() == 0)
1008 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
1009 if (CPN->getType()->getAddressSpace() == 0)
1013 // If V1/V2 point to two different objects we know that we have no alias.
1014 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
1017 // Constant pointers can't alias with non-const isIdentifiedObject objects.
1018 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
1019 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
1022 // Arguments can't alias with local allocations or noalias calls
1023 // in the same function.
1024 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
1025 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
1028 // Most objects can't alias null.
1029 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
1030 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
1033 // If one pointer is the result of a call/invoke or load and the other is a
1034 // non-escaping local object within the same function, then we know the
1035 // object couldn't escape to a point where the call could return it.
1037 // Note that if the pointers are in different functions, there are a
1038 // variety of complications. A call with a nocapture argument may still
1039 // temporary store the nocapture argument's value in a temporary memory
1040 // location if that memory location doesn't escape. Or it may pass a
1041 // nocapture value to other functions as long as they don't capture it.
1042 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
1044 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
1048 // If the size of one access is larger than the entire object on the other
1049 // side, then we know such behavior is undefined and can assume no alias.
1051 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
1052 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
1055 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
1056 // GEP can't simplify, we don't even look at the PHI cases.
1057 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
1059 std::swap(V1Size, V2Size);
1062 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
1063 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
1065 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
1067 std::swap(V1Size, V2Size);
1069 if (const PHINode *PN = dyn_cast<PHINode>(V1))
1070 return aliasPHI(PN, V1Size, V2, V2Size);
1072 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
1074 std::swap(V1Size, V2Size);
1076 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
1077 return aliasSelect(S1, V1Size, V2, V2Size);
1079 return NoAA::alias(V1, V1Size, V2, V2Size);
1082 // Make sure that anything that uses AliasAnalysis pulls in this file.
1083 DEFINING_FILE_FOR(BasicAliasAnalysis)