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);
82 /// isObjectSmallerThan - Return true if we can prove that the object specified
83 /// by V is smaller than Size.
84 static bool isObjectSmallerThan(const Value *V, unsigned Size,
85 const TargetData &TD) {
87 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
88 AccessTy = GV->getType()->getElementType();
89 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
90 if (!AI->isArrayAllocation())
91 AccessTy = AI->getType()->getElementType();
94 } else if (const CallInst* CI = extractMallocCall(V)) {
95 if (!isArrayMalloc(V, &TD))
96 // The size is the argument to the malloc call.
97 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
98 return (C->getZExtValue() < Size);
100 } else if (const Argument *A = dyn_cast<Argument>(V)) {
101 if (A->hasByValAttr())
102 AccessTy = cast<PointerType>(A->getType())->getElementType();
109 if (AccessTy->isSized())
110 return TD.getTypeAllocSize(AccessTy) < Size;
114 //===----------------------------------------------------------------------===//
116 //===----------------------------------------------------------------------===//
119 /// NoAA - This class implements the -no-aa pass, which always returns "I
120 /// don't know" for alias queries. NoAA is unlike other alias analysis
121 /// implementations, in that it does not chain to a previous analysis. As
122 /// such it doesn't follow many of the rules that other alias analyses must.
124 struct NoAA : public ImmutablePass, public AliasAnalysis {
125 static char ID; // Class identification, replacement for typeinfo
126 NoAA() : ImmutablePass(&ID) {}
127 explicit NoAA(void *PID) : ImmutablePass(PID) { }
129 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
132 virtual void initializePass() {
133 TD = getAnalysisIfAvailable<TargetData>();
136 virtual AliasResult alias(const Value *V1, unsigned V1Size,
137 const Value *V2, unsigned V2Size) {
141 virtual void getArgumentAccesses(Function *F, CallSite CS,
142 std::vector<PointerAccessInfo> &Info) {
143 llvm_unreachable("This method may not be called on this function!");
146 virtual bool pointsToConstantMemory(const Value *P) { return false; }
147 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
150 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
154 virtual void deleteValue(Value *V) {}
155 virtual void copyValue(Value *From, Value *To) {}
157 /// getAdjustedAnalysisPointer - This method is used when a pass implements
158 /// an analysis interface through multiple inheritance. If needed, it should
159 /// override this to adjust the this pointer as needed for the specified pass
161 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
162 if (PI->isPassID(&AliasAnalysis::ID))
163 return (AliasAnalysis*)this;
167 } // End of anonymous namespace
169 // Register this pass...
171 static RegisterPass<NoAA>
172 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
174 // Declare that we implement the AliasAnalysis interface
175 static RegisterAnalysisGroup<AliasAnalysis> V(U);
177 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
179 //===----------------------------------------------------------------------===//
181 //===----------------------------------------------------------------------===//
184 /// BasicAliasAnalysis - This is the default alias analysis implementation.
185 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
186 /// derives from the NoAA class.
187 struct BasicAliasAnalysis : public NoAA {
188 static char ID; // Class identification, replacement for typeinfo
189 BasicAliasAnalysis() : NoAA(&ID) {}
190 AliasResult alias(const Value *V1, unsigned V1Size,
191 const Value *V2, unsigned V2Size) {
192 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!");
193 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
198 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
199 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
201 /// pointsToConstantMemory - Chase pointers until we find a (constant
203 bool pointsToConstantMemory(const Value *P);
205 /// getAdjustedAnalysisPointer - This method is used when a pass implements
206 /// an analysis interface through multiple inheritance. If needed, it should
207 /// override this to adjust the this pointer as needed for the specified pass
209 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
210 if (PI->isPassID(&AliasAnalysis::ID))
211 return (AliasAnalysis*)this;
216 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call.
217 SmallPtrSet<const Value*, 16> VisitedPHIs;
219 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
220 // instruction against another.
221 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
222 const Value *V2, unsigned V2Size,
223 const Value *UnderlyingV1, const Value *UnderlyingV2);
225 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
226 // instruction against another.
227 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
228 const Value *V2, unsigned V2Size);
230 /// aliasSelect - Disambiguate a Select instruction against another value.
231 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
232 const Value *V2, unsigned V2Size);
234 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
235 const Value *V2, unsigned V2Size);
237 } // End of anonymous namespace
239 // Register this pass...
240 char BasicAliasAnalysis::ID = 0;
241 static RegisterPass<BasicAliasAnalysis>
242 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
244 // Declare that we implement the AliasAnalysis interface
245 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
247 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
248 return new BasicAliasAnalysis();
252 /// pointsToConstantMemory - Chase pointers until we find a (constant
254 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
255 if (const GlobalVariable *GV =
256 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
257 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
258 // global to be marked constant in some modules and non-constant in others.
259 // GV may even be a declaration, not a definition.
260 return GV->isConstant();
265 /// getModRefInfo - Check to see if the specified callsite can clobber the
266 /// specified memory object. Since we only look at local properties of this
267 /// function, we really can't say much about this query. We do, however, use
268 /// simple "address taken" analysis on local objects.
269 AliasAnalysis::ModRefResult
270 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
271 const Value *Object = P->getUnderlyingObject();
273 // If this is a tail call and P points to a stack location, we know that
274 // the tail call cannot access or modify the local stack.
275 // We cannot exclude byval arguments here; these belong to the caller of
276 // the current function not to the current function, and a tail callee
277 // may reference them.
278 if (isa<AllocaInst>(Object))
279 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
280 if (CI->isTailCall())
283 // If the pointer is to a locally allocated object that does not escape,
284 // then the call can not mod/ref the pointer unless the call takes the pointer
285 // as an argument, and itself doesn't capture it.
286 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
287 isNonEscapingLocalObject(Object)) {
288 bool PassedAsArg = false;
290 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
291 CI != CE; ++CI, ++ArgNo) {
292 // Only look at the no-capture pointer arguments.
293 if (!(*CI)->getType()->isPointerTy() ||
294 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
297 // If this is a no-capture pointer argument, see if we can tell that it
298 // is impossible to alias the pointer we're checking. If not, we have to
299 // assume that the call could touch the pointer, even though it doesn't
301 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) {
311 // Finally, handle specific knowledge of intrinsics.
312 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
314 return AliasAnalysis::getModRefInfo(CS, P, Size);
316 switch (II->getIntrinsicID()) {
318 case Intrinsic::memcpy:
319 case Intrinsic::memmove: {
321 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
322 Len = LenCI->getZExtValue();
323 Value *Dest = II->getOperand(1);
324 Value *Src = II->getOperand(2);
325 if (isNoAlias(Dest, Len, P, Size)) {
326 if (isNoAlias(Src, Len, P, Size))
332 case Intrinsic::memset:
333 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
334 // will handle it for the variable length case.
335 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
336 unsigned Len = LenCI->getZExtValue();
337 Value *Dest = II->getOperand(1);
338 if (isNoAlias(Dest, Len, P, Size))
342 case Intrinsic::atomic_cmp_swap:
343 case Intrinsic::atomic_swap:
344 case Intrinsic::atomic_load_add:
345 case Intrinsic::atomic_load_sub:
346 case Intrinsic::atomic_load_and:
347 case Intrinsic::atomic_load_nand:
348 case Intrinsic::atomic_load_or:
349 case Intrinsic::atomic_load_xor:
350 case Intrinsic::atomic_load_max:
351 case Intrinsic::atomic_load_min:
352 case Intrinsic::atomic_load_umax:
353 case Intrinsic::atomic_load_umin:
355 Value *Op1 = II->getOperand(1);
356 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
357 if (isNoAlias(Op1, Op1Size, P, Size))
361 case Intrinsic::lifetime_start:
362 case Intrinsic::lifetime_end:
363 case Intrinsic::invariant_start: {
364 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
365 if (isNoAlias(II->getOperand(2), PtrSize, P, Size))
369 case Intrinsic::invariant_end: {
370 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
371 if (isNoAlias(II->getOperand(3), PtrSize, P, Size))
377 // The AliasAnalysis base class has some smarts, lets use them.
378 return AliasAnalysis::getModRefInfo(CS, P, Size);
382 AliasAnalysis::ModRefResult
383 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
384 // If CS1 or CS2 are readnone, they don't interact.
385 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
386 if (CS1B == DoesNotAccessMemory) return NoModRef;
388 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
389 if (CS2B == DoesNotAccessMemory) return NoModRef;
391 // If they both only read from memory, just return ref.
392 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
395 // Otherwise, fall back to NoAA (mod+ref).
396 return NoAA::getModRefInfo(CS1, CS2);
399 /// GetIndiceDifference - Dest and Src are the variable indices from two
400 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
401 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
402 /// difference between the two pointers.
403 static void GetIndiceDifference(
404 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
405 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
406 if (Src.empty()) return;
408 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
409 const Value *V = Src[i].first;
410 int64_t Scale = Src[i].second;
412 // Find V in Dest. This is N^2, but pointer indices almost never have more
413 // than a few variable indexes.
414 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
415 if (Dest[j].first != V) continue;
417 // If we found it, subtract off Scale V's from the entry in Dest. If it
418 // goes to zero, remove the entry.
419 if (Dest[j].second != Scale)
420 Dest[j].second -= Scale;
422 Dest.erase(Dest.begin()+j);
427 // If we didn't consume this entry, add it to the end of the Dest list.
429 Dest.push_back(std::make_pair(V, -Scale));
433 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
434 /// against another pointer. We know that V1 is a GEP, but we don't know
435 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
436 /// UnderlyingV2 is the same for V2.
438 AliasAnalysis::AliasResult
439 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
440 const Value *V2, unsigned V2Size,
441 const Value *UnderlyingV1,
442 const Value *UnderlyingV2) {
443 int64_t GEP1BaseOffset;
444 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
446 // If we have two gep instructions with must-alias'ing base pointers, figure
447 // out if the indexes to the GEP tell us anything about the derived pointer.
448 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
449 // Do the base pointers alias?
450 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U);
452 // If we get a No or May, then return it immediately, no amount of analysis
453 // will improve this situation.
454 if (BaseAlias != MustAlias) return BaseAlias;
456 // Otherwise, we have a MustAlias. Since the base pointers alias each other
457 // exactly, see if the computed offset from the common pointer tells us
458 // about the relation of the resulting pointer.
459 const Value *GEP1BasePtr =
460 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
462 int64_t GEP2BaseOffset;
463 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
464 const Value *GEP2BasePtr =
465 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
467 // If DecomposeGEPExpression isn't able to look all the way through the
468 // addressing operation, we must not have TD and this is too complex for us
469 // to handle without it.
470 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
472 "DecomposeGEPExpression and getUnderlyingObject disagree!");
476 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
477 // symbolic difference.
478 GEP1BaseOffset -= GEP2BaseOffset;
479 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices);
482 // Check to see if these two pointers are related by the getelementptr
483 // instruction. If one pointer is a GEP with a non-zero index of the other
484 // pointer, we know they cannot alias.
486 // If both accesses are unknown size, we can't do anything useful here.
487 if (V1Size == ~0U && V2Size == ~0U)
490 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size);
492 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
493 // If V2 is known not to alias GEP base pointer, then the two values
494 // cannot alias per GEP semantics: "A pointer value formed from a
495 // getelementptr instruction is associated with the addresses associated
496 // with the first operand of the getelementptr".
499 const Value *GEP1BasePtr =
500 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
502 // If DecomposeGEPExpression isn't able to look all the way through the
503 // addressing operation, we must not have TD and this is too complex for us
504 // to handle without it.
505 if (GEP1BasePtr != UnderlyingV1) {
507 "DecomposeGEPExpression and getUnderlyingObject disagree!");
512 // In the two GEP Case, if there is no difference in the offsets of the
513 // computed pointers, the resultant pointers are a must alias. This
514 // hapens when we have two lexically identical GEP's (for example).
516 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
517 // must aliases the GEP, the end result is a must alias also.
518 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
521 // If we have a known constant offset, see if this offset is larger than the
522 // access size being queried. If so, and if no variable indices can remove
523 // pieces of this constant, then we know we have a no-alias. For example,
526 // In order to handle cases like &A[100][i] where i is an out of range
527 // subscript, we have to ignore all constant offset pieces that are a multiple
528 // of a scaled index. Do this by removing constant offsets that are a
529 // multiple of any of our variable indices. This allows us to transform
530 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
531 // provides an offset of 4 bytes (assuming a <= 4 byte access).
532 for (unsigned i = 0, e = GEP1VariableIndices.size();
533 i != e && GEP1BaseOffset;++i)
534 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
535 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
537 // If our known offset is bigger than the access size, we know we don't have
539 if (GEP1BaseOffset) {
540 if (GEP1BaseOffset >= (int64_t)V2Size ||
541 GEP1BaseOffset <= -(int64_t)V1Size)
548 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
549 /// instruction against another.
550 AliasAnalysis::AliasResult
551 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
552 const Value *V2, unsigned V2Size) {
553 // If the values are Selects with the same condition, we can do a more precise
554 // check: just check for aliases between the values on corresponding arms.
555 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
556 if (SI->getCondition() == SI2->getCondition()) {
558 aliasCheck(SI->getTrueValue(), SISize,
559 SI2->getTrueValue(), V2Size);
560 if (Alias == MayAlias)
562 AliasResult ThisAlias =
563 aliasCheck(SI->getFalseValue(), SISize,
564 SI2->getFalseValue(), V2Size);
565 if (ThisAlias != Alias)
570 // If both arms of the Select node NoAlias or MustAlias V2, then returns
571 // NoAlias / MustAlias. Otherwise, returns MayAlias.
573 aliasCheck(SI->getTrueValue(), SISize, V2, V2Size);
574 if (Alias == MayAlias)
576 AliasResult ThisAlias =
577 aliasCheck(SI->getFalseValue(), SISize, V2, V2Size);
578 if (ThisAlias != Alias)
583 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
585 AliasAnalysis::AliasResult
586 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
587 const Value *V2, unsigned V2Size) {
588 // The PHI node has already been visited, avoid recursion any further.
589 if (!VisitedPHIs.insert(PN))
592 // If the values are PHIs in the same block, we can do a more precise
593 // as well as efficient check: just check for aliases between the values
594 // on corresponding edges.
595 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
596 if (PN2->getParent() == PN->getParent()) {
598 aliasCheck(PN->getIncomingValue(0), PNSize,
599 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
601 if (Alias == MayAlias)
603 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
604 AliasResult ThisAlias =
605 aliasCheck(PN->getIncomingValue(i), PNSize,
606 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
608 if (ThisAlias != Alias)
614 SmallPtrSet<Value*, 4> UniqueSrc;
615 SmallVector<Value*, 4> V1Srcs;
616 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
617 Value *PV1 = PN->getIncomingValue(i);
618 if (isa<PHINode>(PV1))
619 // If any of the source itself is a PHI, return MayAlias conservatively
620 // to avoid compile time explosion. The worst possible case is if both
621 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
622 // and 'n' are the number of PHI sources.
624 if (UniqueSrc.insert(PV1))
625 V1Srcs.push_back(PV1);
628 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
629 // Early exit if the check of the first PHI source against V2 is MayAlias.
630 // Other results are not possible.
631 if (Alias == MayAlias)
634 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
635 // NoAlias / MustAlias. Otherwise, returns MayAlias.
636 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
637 Value *V = V1Srcs[i];
639 // If V2 is a PHI, the recursive case will have been caught in the
640 // above aliasCheck call, so these subsequent calls to aliasCheck
641 // don't need to assume that V2 is being visited recursively.
642 VisitedPHIs.erase(V2);
644 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
645 if (ThisAlias != Alias || ThisAlias == MayAlias)
652 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
653 // such as array references.
655 AliasAnalysis::AliasResult
656 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
657 const Value *V2, unsigned V2Size) {
658 // If either of the memory references is empty, it doesn't matter what the
659 // pointer values are.
660 if (V1Size == 0 || V2Size == 0)
663 // Strip off any casts if they exist.
664 V1 = V1->stripPointerCasts();
665 V2 = V2->stripPointerCasts();
667 // Are we checking for alias of the same value?
668 if (V1 == V2) return MustAlias;
670 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
671 return NoAlias; // Scalars cannot alias each other
673 // Figure out what objects these things are pointing to if we can.
674 const Value *O1 = V1->getUnderlyingObject();
675 const Value *O2 = V2->getUnderlyingObject();
677 // Null values in the default address space don't point to any object, so they
678 // don't alias any other pointer.
679 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
680 if (CPN->getType()->getAddressSpace() == 0)
682 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
683 if (CPN->getType()->getAddressSpace() == 0)
687 // If V1/V2 point to two different objects we know that we have no alias.
688 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
691 // Constant pointers can't alias with non-const isIdentifiedObject objects.
692 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
693 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
696 // Arguments can't alias with local allocations or noalias calls.
697 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
698 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
701 // Most objects can't alias null.
702 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
703 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
707 // If the size of one access is larger than the entire object on the other
708 // side, then we know such behavior is undefined and can assume no alias.
710 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
711 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
714 // If one pointer is the result of a call/invoke or load and the other is a
715 // non-escaping local object, then we know the object couldn't escape to a
716 // point where the call could return it. The load case works because
717 // isNonEscapingLocalObject considers all stores to be escapes (it
718 // passes true for the StoreCaptures argument to PointerMayBeCaptured).
720 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1) ||
721 isa<Argument>(O1)) &&
722 isNonEscapingLocalObject(O2))
724 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2) ||
725 isa<Argument>(O2)) &&
726 isNonEscapingLocalObject(O1))
730 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
731 // GEP can't simplify, we don't even look at the PHI cases.
732 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
734 std::swap(V1Size, V2Size);
737 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
738 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
740 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
742 std::swap(V1Size, V2Size);
744 if (const PHINode *PN = dyn_cast<PHINode>(V1))
745 return aliasPHI(PN, V1Size, V2, V2Size);
747 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
749 std::swap(V1Size, V2Size);
751 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
752 return aliasSelect(S1, V1Size, V2, V2Size);
757 // Make sure that anything that uses AliasAnalysis pulls in this file.
758 DEFINING_FILE_FOR(BasicAliasAnalysis)