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/GlobalVariable.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/IntrinsicInst.h"
24 #include "llvm/Operator.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Analysis/CaptureTracking.h"
27 #include "llvm/Analysis/MemoryBuiltins.h"
28 #include "llvm/Analysis/ValueTracking.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/Support/ErrorHandling.h"
36 //===----------------------------------------------------------------------===//
38 //===----------------------------------------------------------------------===//
40 /// isKnownNonNull - Return true if we know that the specified value is never
42 static bool isKnownNonNull(const Value *V) {
43 // Alloca never returns null, malloc might.
44 if (isa<AllocaInst>(V)) return true;
46 // A byval argument is never null.
47 if (const Argument *A = dyn_cast<Argument>(V))
48 return A->hasByValAttr();
50 // Global values are not null unless extern weak.
51 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
52 return !GV->hasExternalWeakLinkage();
56 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
57 /// object that never escapes from the function.
58 static bool isNonEscapingLocalObject(const Value *V) {
59 // If this is a local allocation, check to see if it escapes.
60 if (isa<AllocaInst>(V) || isNoAliasCall(V))
61 // Set StoreCaptures to True so that we can assume in our callers that the
62 // pointer is not the result of a load instruction. Currently
63 // PointerMayBeCaptured doesn't have any special analysis for the
64 // StoreCaptures=false case; if it did, our callers could be refined to be
66 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
68 // If this is an argument that corresponds to a byval or noalias argument,
69 // then it has not escaped before entering the function. Check if it escapes
70 // inside the function.
71 if (const Argument *A = dyn_cast<Argument>(V))
72 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
73 // Don't bother analyzing arguments already known not to escape.
74 if (A->hasNoCaptureAttr())
76 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
81 /// isEscapeSource - Return true if the pointer is one which would have
82 /// been considered an escape by isNonEscapingLocalObject.
83 static bool isEscapeSource(const Value *V) {
84 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
87 // The load case works because isNonEscapingLocalObject considers all
88 // stores to be escapes (it passes true for the StoreCaptures argument
89 // to PointerMayBeCaptured).
96 /// isObjectSmallerThan - Return true if we can prove that the object specified
97 /// by V is smaller than Size.
98 static bool isObjectSmallerThan(const Value *V, unsigned Size,
99 const TargetData &TD) {
100 const Type *AccessTy;
101 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
102 AccessTy = GV->getType()->getElementType();
103 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
104 if (!AI->isArrayAllocation())
105 AccessTy = AI->getType()->getElementType();
108 } else if (const CallInst* CI = extractMallocCall(V)) {
109 if (!isArrayMalloc(V, &TD))
110 // The size is the argument to the malloc call.
111 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
112 return (C->getZExtValue() < Size);
114 } else if (const Argument *A = dyn_cast<Argument>(V)) {
115 if (A->hasByValAttr())
116 AccessTy = cast<PointerType>(A->getType())->getElementType();
123 if (AccessTy->isSized())
124 return TD.getTypeAllocSize(AccessTy) < Size;
128 //===----------------------------------------------------------------------===//
130 //===----------------------------------------------------------------------===//
133 /// NoAA - This class implements the -no-aa pass, which always returns "I
134 /// don't know" for alias queries. NoAA is unlike other alias analysis
135 /// implementations, in that it does not chain to a previous analysis. As
136 /// such it doesn't follow many of the rules that other alias analyses must.
138 struct NoAA : public ImmutablePass, public AliasAnalysis {
139 static char ID; // Class identification, replacement for typeinfo
140 NoAA() : ImmutablePass(ID) {}
141 explicit NoAA(char &PID) : ImmutablePass(PID) { }
143 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
146 virtual void initializePass() {
147 TD = getAnalysisIfAvailable<TargetData>();
150 virtual AliasResult alias(const Value *V1, unsigned V1Size,
151 const Value *V2, unsigned V2Size) {
155 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) {
156 return UnknownModRefBehavior;
158 virtual ModRefBehavior getModRefBehavior(const Function *F) {
159 return UnknownModRefBehavior;
162 virtual bool pointsToConstantMemory(const Value *P) { return false; }
163 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
164 const Value *P, unsigned Size) {
167 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
168 ImmutableCallSite CS2) {
172 virtual void deleteValue(Value *V) {}
173 virtual void copyValue(Value *From, Value *To) {}
175 /// getAdjustedAnalysisPointer - This method is used when a pass implements
176 /// an analysis interface through multiple inheritance. If needed, it
177 /// should override this to adjust the this pointer as needed for the
178 /// specified pass info.
179 virtual void *getAdjustedAnalysisPointer(const void *ID) {
180 if (ID == &AliasAnalysis::ID)
181 return (AliasAnalysis*)this;
185 } // End of anonymous namespace
187 // Register this pass...
189 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
190 "No Alias Analysis (always returns 'may' alias)",
193 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
195 //===----------------------------------------------------------------------===//
196 // BasicAliasAnalysis Pass
197 //===----------------------------------------------------------------------===//
200 static const Function *getParent(const Value *V) {
201 if (const Instruction *inst = dyn_cast<Instruction>(V))
202 return inst->getParent()->getParent();
204 if (const Argument *arg = dyn_cast<Argument>(V))
205 return arg->getParent();
210 static bool notDifferentParent(const Value *O1, const Value *O2) {
212 const Function *F1 = getParent(O1);
213 const Function *F2 = getParent(O2);
215 return !F1 || !F2 || F1 == F2;
220 /// BasicAliasAnalysis - This is the default alias analysis implementation.
221 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
222 /// derives from the NoAA class.
223 struct BasicAliasAnalysis : public NoAA {
224 static char ID; // Class identification, replacement for typeinfo
225 BasicAliasAnalysis() : NoAA(ID) {}
227 virtual AliasResult alias(const Value *V1, unsigned V1Size,
228 const Value *V2, unsigned V2Size) {
229 assert(Visited.empty() && "Visited must be cleared after use!");
230 assert(notDifferentParent(V1, V2) &&
231 "BasicAliasAnalysis doesn't support interprocedural queries.");
232 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
237 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
238 const Value *P, unsigned Size);
240 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
241 ImmutableCallSite CS2) {
242 // The AliasAnalysis base class has some smarts, lets use them.
243 return AliasAnalysis::getModRefInfo(CS1, CS2);
246 /// pointsToConstantMemory - Chase pointers until we find a (constant
248 virtual bool pointsToConstantMemory(const Value *P);
250 /// getModRefBehavior - Return the behavior when calling the given
252 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
254 /// getModRefBehavior - Return the behavior when calling the given function.
255 /// For use when the call site is not known.
256 virtual ModRefBehavior getModRefBehavior(const Function *F);
258 /// getAdjustedAnalysisPointer - This method is used when a pass implements
259 /// an analysis interface through multiple inheritance. If needed, it
260 /// should override this to adjust the this pointer as needed for the
261 /// specified pass info.
262 virtual void *getAdjustedAnalysisPointer(const void *ID) {
263 if (ID == &AliasAnalysis::ID)
264 return (AliasAnalysis*)this;
269 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
270 SmallPtrSet<const Value*, 16> Visited;
272 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
273 // instruction against another.
274 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
275 const Value *V2, unsigned V2Size,
276 const Value *UnderlyingV1, const Value *UnderlyingV2);
278 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
279 // instruction against another.
280 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
281 const Value *V2, unsigned V2Size);
283 /// aliasSelect - Disambiguate a Select instruction against another value.
284 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
285 const Value *V2, unsigned V2Size);
287 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
288 const Value *V2, unsigned V2Size);
290 } // End of anonymous namespace
292 // Register this pass...
293 char BasicAliasAnalysis::ID = 0;
294 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
295 "Basic Alias Analysis (default AA impl)",
298 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
299 return new BasicAliasAnalysis();
303 /// pointsToConstantMemory - Chase pointers until we find a (constant
305 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
306 if (const GlobalVariable *GV =
307 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
308 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
309 // global to be marked constant in some modules and non-constant in others.
310 // GV may even be a declaration, not a definition.
311 return GV->isConstant();
313 return NoAA::pointsToConstantMemory(P);
316 /// getModRefBehavior - Return the behavior when calling the given call site.
317 AliasAnalysis::ModRefBehavior
318 BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
319 if (CS.doesNotAccessMemory())
320 // Can't do better than this.
321 return DoesNotAccessMemory;
323 ModRefBehavior Min = UnknownModRefBehavior;
325 // If the callsite knows it only reads memory, don't return worse
327 if (CS.onlyReadsMemory())
328 Min = OnlyReadsMemory;
330 // The AliasAnalysis base class has some smarts, lets use them.
331 return std::min(AliasAnalysis::getModRefBehavior(CS), Min);
334 /// getModRefBehavior - Return the behavior when calling the given function.
335 /// For use when the call site is not known.
336 AliasAnalysis::ModRefBehavior
337 BasicAliasAnalysis::getModRefBehavior(const Function *F) {
338 if (F->doesNotAccessMemory())
339 // Can't do better than this.
340 return DoesNotAccessMemory;
341 if (F->onlyReadsMemory())
342 return OnlyReadsMemory;
343 if (unsigned id = F->getIntrinsicID())
344 return getIntrinsicModRefBehavior(id);
346 return NoAA::getModRefBehavior(F);
349 /// getModRefInfo - Check to see if the specified callsite can clobber the
350 /// specified memory object. Since we only look at local properties of this
351 /// function, we really can't say much about this query. We do, however, use
352 /// simple "address taken" analysis on local objects.
353 AliasAnalysis::ModRefResult
354 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
355 const Value *P, unsigned Size) {
356 assert(notDifferentParent(CS.getInstruction(), P) &&
357 "AliasAnalysis query involving multiple functions!");
359 const Value *Object = P->getUnderlyingObject();
361 // If this is a tail call and P points to a stack location, we know that
362 // the tail call cannot access or modify the local stack.
363 // We cannot exclude byval arguments here; these belong to the caller of
364 // the current function not to the current function, and a tail callee
365 // may reference them.
366 if (isa<AllocaInst>(Object))
367 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
368 if (CI->isTailCall())
371 // If the pointer is to a locally allocated object that does not escape,
372 // then the call can not mod/ref the pointer unless the call takes the pointer
373 // as an argument, and itself doesn't capture it.
374 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
375 isNonEscapingLocalObject(Object)) {
376 bool PassedAsArg = false;
378 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
379 CI != CE; ++CI, ++ArgNo) {
380 // Only look at the no-capture pointer arguments.
381 if (!(*CI)->getType()->isPointerTy() ||
382 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
385 // If this is a no-capture pointer argument, see if we can tell that it
386 // is impossible to alias the pointer we're checking. If not, we have to
387 // assume that the call could touch the pointer, even though it doesn't
389 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
399 // Finally, handle specific knowledge of intrinsics.
400 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
402 switch (II->getIntrinsicID()) {
404 case Intrinsic::memcpy:
405 case Intrinsic::memmove: {
406 unsigned Len = UnknownSize;
407 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
408 Len = LenCI->getZExtValue();
409 Value *Dest = II->getArgOperand(0);
410 Value *Src = II->getArgOperand(1);
411 if (isNoAlias(Dest, Len, P, Size)) {
412 if (isNoAlias(Src, Len, P, Size))
418 case Intrinsic::memset:
419 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
420 // will handle it for the variable length case.
421 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
422 unsigned Len = LenCI->getZExtValue();
423 Value *Dest = II->getArgOperand(0);
424 if (isNoAlias(Dest, Len, P, Size))
428 case Intrinsic::atomic_cmp_swap:
429 case Intrinsic::atomic_swap:
430 case Intrinsic::atomic_load_add:
431 case Intrinsic::atomic_load_sub:
432 case Intrinsic::atomic_load_and:
433 case Intrinsic::atomic_load_nand:
434 case Intrinsic::atomic_load_or:
435 case Intrinsic::atomic_load_xor:
436 case Intrinsic::atomic_load_max:
437 case Intrinsic::atomic_load_min:
438 case Intrinsic::atomic_load_umax:
439 case Intrinsic::atomic_load_umin:
441 Value *Op1 = II->getArgOperand(0);
442 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
443 if (isNoAlias(Op1, Op1Size, P, Size))
447 case Intrinsic::lifetime_start:
448 case Intrinsic::lifetime_end:
449 case Intrinsic::invariant_start: {
451 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
452 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
456 case Intrinsic::invariant_end: {
458 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
459 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
465 // The AliasAnalysis base class has some smarts, lets use them.
466 return AliasAnalysis::getModRefInfo(CS, P, Size);
470 /// GetIndexDifference - Dest and Src are the variable indices from two
471 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
472 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
473 /// difference between the two pointers.
474 static void GetIndexDifference(
475 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
476 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
477 if (Src.empty()) return;
479 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
480 const Value *V = Src[i].first;
481 int64_t Scale = Src[i].second;
483 // Find V in Dest. This is N^2, but pointer indices almost never have more
484 // than a few variable indexes.
485 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
486 if (Dest[j].first != V) continue;
488 // If we found it, subtract off Scale V's from the entry in Dest. If it
489 // goes to zero, remove the entry.
490 if (Dest[j].second != Scale)
491 Dest[j].second -= Scale;
493 Dest.erase(Dest.begin()+j);
498 // If we didn't consume this entry, add it to the end of the Dest list.
500 Dest.push_back(std::make_pair(V, -Scale));
504 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
505 /// against another pointer. We know that V1 is a GEP, but we don't know
506 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
507 /// UnderlyingV2 is the same for V2.
509 AliasAnalysis::AliasResult
510 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
511 const Value *V2, unsigned V2Size,
512 const Value *UnderlyingV1,
513 const Value *UnderlyingV2) {
514 // If this GEP has been visited before, we're on a use-def cycle.
515 // Such cycles are only valid when PHI nodes are involved or in unreachable
516 // code. The visitPHI function catches cycles containing PHIs, but there
517 // could still be a cycle without PHIs in unreachable code.
518 if (!Visited.insert(GEP1))
521 int64_t GEP1BaseOffset;
522 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
524 // If we have two gep instructions with must-alias'ing base pointers, figure
525 // out if the indexes to the GEP tell us anything about the derived pointer.
526 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
527 // Do the base pointers alias?
528 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
529 UnderlyingV2, UnknownSize);
531 // If we get a No or May, then return it immediately, no amount of analysis
532 // will improve this situation.
533 if (BaseAlias != MustAlias) return BaseAlias;
535 // Otherwise, we have a MustAlias. Since the base pointers alias each other
536 // exactly, see if the computed offset from the common pointer tells us
537 // about the relation of the resulting pointer.
538 const Value *GEP1BasePtr =
539 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
541 int64_t GEP2BaseOffset;
542 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
543 const Value *GEP2BasePtr =
544 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
546 // If DecomposeGEPExpression isn't able to look all the way through the
547 // addressing operation, we must not have TD and this is too complex for us
548 // to handle without it.
549 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
551 "DecomposeGEPExpression and getUnderlyingObject disagree!");
555 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
556 // symbolic difference.
557 GEP1BaseOffset -= GEP2BaseOffset;
558 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
561 // Check to see if these two pointers are related by the getelementptr
562 // instruction. If one pointer is a GEP with a non-zero index of the other
563 // pointer, we know they cannot alias.
565 // If both accesses are unknown size, we can't do anything useful here.
566 if (V1Size == UnknownSize && V2Size == UnknownSize)
569 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
571 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
572 // If V2 is known not to alias GEP base pointer, then the two values
573 // cannot alias per GEP semantics: "A pointer value formed from a
574 // getelementptr instruction is associated with the addresses associated
575 // with the first operand of the getelementptr".
578 const Value *GEP1BasePtr =
579 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
581 // If DecomposeGEPExpression isn't able to look all the way through the
582 // addressing operation, we must not have TD and this is too complex for us
583 // to handle without it.
584 if (GEP1BasePtr != UnderlyingV1) {
586 "DecomposeGEPExpression and getUnderlyingObject disagree!");
591 // In the two GEP Case, if there is no difference in the offsets of the
592 // computed pointers, the resultant pointers are a must alias. This
593 // hapens when we have two lexically identical GEP's (for example).
595 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
596 // must aliases the GEP, the end result is a must alias also.
597 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
600 // If we have a known constant offset, see if this offset is larger than the
601 // access size being queried. If so, and if no variable indices can remove
602 // pieces of this constant, then we know we have a no-alias. For example,
605 // In order to handle cases like &A[100][i] where i is an out of range
606 // subscript, we have to ignore all constant offset pieces that are a multiple
607 // of a scaled index. Do this by removing constant offsets that are a
608 // multiple of any of our variable indices. This allows us to transform
609 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
610 // provides an offset of 4 bytes (assuming a <= 4 byte access).
611 for (unsigned i = 0, e = GEP1VariableIndices.size();
612 i != e && GEP1BaseOffset;++i)
613 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
614 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
616 // If our known offset is bigger than the access size, we know we don't have
618 if (GEP1BaseOffset) {
619 if (GEP1BaseOffset >= (int64_t)V2Size ||
620 GEP1BaseOffset <= -(int64_t)V1Size)
627 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
628 /// instruction against another.
629 AliasAnalysis::AliasResult
630 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
631 const Value *V2, unsigned V2Size) {
632 // If this select has been visited before, we're on a use-def cycle.
633 // Such cycles are only valid when PHI nodes are involved or in unreachable
634 // code. The visitPHI function catches cycles containing PHIs, but there
635 // could still be a cycle without PHIs in unreachable code.
636 if (!Visited.insert(SI))
639 // If the values are Selects with the same condition, we can do a more precise
640 // check: just check for aliases between the values on corresponding arms.
641 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
642 if (SI->getCondition() == SI2->getCondition()) {
644 aliasCheck(SI->getTrueValue(), SISize,
645 SI2->getTrueValue(), V2Size);
646 if (Alias == MayAlias)
648 AliasResult ThisAlias =
649 aliasCheck(SI->getFalseValue(), SISize,
650 SI2->getFalseValue(), V2Size);
651 if (ThisAlias != Alias)
656 // If both arms of the Select node NoAlias or MustAlias V2, then returns
657 // NoAlias / MustAlias. Otherwise, returns MayAlias.
659 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
660 if (Alias == MayAlias)
663 // If V2 is visited, the recursive case will have been caught in the
664 // above aliasCheck call, so these subsequent calls to aliasCheck
665 // don't need to assume that V2 is being visited recursively.
668 AliasResult ThisAlias =
669 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
670 if (ThisAlias != Alias)
675 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
677 AliasAnalysis::AliasResult
678 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
679 const Value *V2, unsigned V2Size) {
680 // The PHI node has already been visited, avoid recursion any further.
681 if (!Visited.insert(PN))
684 // If the values are PHIs in the same block, we can do a more precise
685 // as well as efficient check: just check for aliases between the values
686 // on corresponding edges.
687 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
688 if (PN2->getParent() == PN->getParent()) {
690 aliasCheck(PN->getIncomingValue(0), PNSize,
691 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
693 if (Alias == MayAlias)
695 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
696 AliasResult ThisAlias =
697 aliasCheck(PN->getIncomingValue(i), PNSize,
698 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
700 if (ThisAlias != Alias)
706 SmallPtrSet<Value*, 4> UniqueSrc;
707 SmallVector<Value*, 4> V1Srcs;
708 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
709 Value *PV1 = PN->getIncomingValue(i);
710 if (isa<PHINode>(PV1))
711 // If any of the source itself is a PHI, return MayAlias conservatively
712 // to avoid compile time explosion. The worst possible case is if both
713 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
714 // and 'n' are the number of PHI sources.
716 if (UniqueSrc.insert(PV1))
717 V1Srcs.push_back(PV1);
720 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
721 // Early exit if the check of the first PHI source against V2 is MayAlias.
722 // Other results are not possible.
723 if (Alias == MayAlias)
726 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
727 // NoAlias / MustAlias. Otherwise, returns MayAlias.
728 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
729 Value *V = V1Srcs[i];
731 // If V2 is visited, the recursive case will have been caught in the
732 // above aliasCheck call, so these subsequent calls to aliasCheck
733 // don't need to assume that V2 is being visited recursively.
736 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
737 if (ThisAlias != Alias || ThisAlias == MayAlias)
744 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
745 // such as array references.
747 AliasAnalysis::AliasResult
748 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
749 const Value *V2, unsigned V2Size) {
750 // If either of the memory references is empty, it doesn't matter what the
751 // pointer values are.
752 if (V1Size == 0 || V2Size == 0)
755 // Strip off any casts if they exist.
756 V1 = V1->stripPointerCasts();
757 V2 = V2->stripPointerCasts();
759 // Are we checking for alias of the same value?
760 if (V1 == V2) return MustAlias;
762 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
763 return NoAlias; // Scalars cannot alias each other
765 // Figure out what objects these things are pointing to if we can.
766 const Value *O1 = V1->getUnderlyingObject();
767 const Value *O2 = V2->getUnderlyingObject();
769 // Null values in the default address space don't point to any object, so they
770 // don't alias any other pointer.
771 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
772 if (CPN->getType()->getAddressSpace() == 0)
774 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
775 if (CPN->getType()->getAddressSpace() == 0)
779 // If V1/V2 point to two different objects we know that we have no alias.
780 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
783 // Constant pointers can't alias with non-const isIdentifiedObject objects.
784 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
785 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
788 // Arguments can't alias with local allocations or noalias calls
789 // in the same function.
790 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
791 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
794 // Most objects can't alias null.
795 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
796 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
799 // If one pointer is the result of a call/invoke or load and the other is a
800 // non-escaping local object within the same function, then we know the
801 // object couldn't escape to a point where the call could return it.
803 // Note that if the pointers are in different functions, there are a
804 // variety of complications. A call with a nocapture argument may still
805 // temporary store the nocapture argument's value in a temporary memory
806 // location if that memory location doesn't escape. Or it may pass a
807 // nocapture value to other functions as long as they don't capture it.
808 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
810 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
814 // If the size of one access is larger than the entire object on the other
815 // side, then we know such behavior is undefined and can assume no alias.
817 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
818 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
821 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
822 // GEP can't simplify, we don't even look at the PHI cases.
823 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
825 std::swap(V1Size, V2Size);
828 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
829 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
831 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
833 std::swap(V1Size, V2Size);
835 if (const PHINode *PN = dyn_cast<PHINode>(V1))
836 return aliasPHI(PN, V1Size, V2, V2Size);
838 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
840 std::swap(V1Size, V2Size);
842 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
843 return aliasSelect(S1, V1Size, V2, V2Size);
845 return NoAA::alias(V1, V1Size, V2, V2Size);
848 // Make sure that anything that uses AliasAnalysis pulls in this file.
849 DEFINING_FILE_FOR(BasicAliasAnalysis)