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(void *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 void getArgumentAccesses(Function *F, CallSite CS,
156 std::vector<PointerAccessInfo> &Info) {
157 llvm_unreachable("This method may not be called on this function!");
160 virtual bool pointsToConstantMemory(const Value *P) { return false; }
161 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
164 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
168 virtual void deleteValue(Value *V) {}
169 virtual void copyValue(Value *From, Value *To) {}
171 /// getAdjustedAnalysisPointer - This method is used when a pass implements
172 /// an analysis interface through multiple inheritance. If needed, it should
173 /// override this to adjust the this pointer as needed for the specified pass
175 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
176 if (PI->isPassID(&AliasAnalysis::ID))
177 return (AliasAnalysis*)this;
181 } // End of anonymous namespace
183 // Register this pass...
185 static RegisterPass<NoAA>
186 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
188 // Declare that we implement the AliasAnalysis interface
189 static RegisterAnalysisGroup<AliasAnalysis> V(U);
191 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
193 //===----------------------------------------------------------------------===//
194 // BasicAliasAnalysis Pass
195 //===----------------------------------------------------------------------===//
198 static const Function *getParent(const Value *V) {
199 if (const Instruction *inst = dyn_cast<Instruction>(V))
200 return inst->getParent()->getParent();
202 if (const Argument *arg = dyn_cast<Argument>(V))
203 return arg->getParent();
208 static bool notDifferentParent(const Value *O1, const Value *O2) {
210 const Function *F1 = getParent(O1);
211 const Function *F2 = getParent(O2);
213 return !F1 || !F2 || F1 == F2;
218 /// BasicAliasAnalysis - This is the default alias analysis implementation.
219 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
220 /// derives from the NoAA class.
221 struct BasicAliasAnalysis : public NoAA {
222 static char ID; // Class identification, replacement for typeinfo
223 BasicAliasAnalysis() : NoAA(&ID) {}
225 AliasResult alias(const Value *V1, unsigned V1Size,
226 const Value *V2, unsigned V2Size) {
227 assert(Visited.empty() && "Visited must be cleared after use!");
228 assert(notDifferentParent(V1, V2) &&
229 "BasicAliasAnalysis doesn't support interprocedural queries.");
230 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
235 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
236 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
238 /// pointsToConstantMemory - Chase pointers until we find a (constant
240 bool pointsToConstantMemory(const Value *P);
242 /// getAdjustedAnalysisPointer - This method is used when a pass implements
243 /// an analysis interface through multiple inheritance. If needed, it should
244 /// override this to adjust the this pointer as needed for the specified pass
246 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
247 if (PI->isPassID(&AliasAnalysis::ID))
248 return (AliasAnalysis*)this;
253 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
254 SmallPtrSet<const Value*, 16> Visited;
256 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
257 // instruction against another.
258 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
259 const Value *V2, unsigned V2Size,
260 const Value *UnderlyingV1, const Value *UnderlyingV2);
262 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
263 // instruction against another.
264 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
265 const Value *V2, unsigned V2Size);
267 /// aliasSelect - Disambiguate a Select instruction against another value.
268 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
269 const Value *V2, unsigned V2Size);
271 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
272 const Value *V2, unsigned V2Size);
274 } // End of anonymous namespace
276 // Register this pass...
277 char BasicAliasAnalysis::ID = 0;
278 static RegisterPass<BasicAliasAnalysis>
279 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
281 // Declare that we implement the AliasAnalysis interface
282 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
284 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
285 return new BasicAliasAnalysis();
289 /// pointsToConstantMemory - Chase pointers until we find a (constant
291 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
292 if (const GlobalVariable *GV =
293 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
294 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
295 // global to be marked constant in some modules and non-constant in others.
296 // GV may even be a declaration, not a definition.
297 return GV->isConstant();
302 /// getModRefInfo - Check to see if the specified callsite can clobber the
303 /// specified memory object. Since we only look at local properties of this
304 /// function, we really can't say much about this query. We do, however, use
305 /// simple "address taken" analysis on local objects.
306 AliasAnalysis::ModRefResult
307 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
308 assert(notDifferentParent(CS.getInstruction(), P) &&
309 "AliasAnalysis query involving multiple functions!");
311 const Value *Object = P->getUnderlyingObject();
313 // If this is a tail call and P points to a stack location, we know that
314 // the tail call cannot access or modify the local stack.
315 // We cannot exclude byval arguments here; these belong to the caller of
316 // the current function not to the current function, and a tail callee
317 // may reference them.
318 if (isa<AllocaInst>(Object))
319 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
320 if (CI->isTailCall())
323 // If the pointer is to a locally allocated object that does not escape,
324 // then the call can not mod/ref the pointer unless the call takes the pointer
325 // as an argument, and itself doesn't capture it.
326 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
327 isNonEscapingLocalObject(Object)) {
328 bool PassedAsArg = false;
330 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
331 CI != CE; ++CI, ++ArgNo) {
332 // Only look at the no-capture pointer arguments.
333 if (!(*CI)->getType()->isPointerTy() ||
334 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
337 // If this is a no-capture pointer argument, see if we can tell that it
338 // is impossible to alias the pointer we're checking. If not, we have to
339 // assume that the call could touch the pointer, even though it doesn't
341 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) {
351 // Finally, handle specific knowledge of intrinsics.
352 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
354 return AliasAnalysis::getModRefInfo(CS, P, Size);
356 switch (II->getIntrinsicID()) {
358 case Intrinsic::memcpy:
359 case Intrinsic::memmove: {
361 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
362 Len = LenCI->getZExtValue();
363 Value *Dest = II->getArgOperand(0);
364 Value *Src = II->getArgOperand(1);
365 if (isNoAlias(Dest, Len, P, Size)) {
366 if (isNoAlias(Src, Len, P, Size))
372 case Intrinsic::memset:
373 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
374 // will handle it for the variable length case.
375 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
376 unsigned Len = LenCI->getZExtValue();
377 Value *Dest = II->getArgOperand(0);
378 if (isNoAlias(Dest, Len, P, Size))
382 case Intrinsic::atomic_cmp_swap:
383 case Intrinsic::atomic_swap:
384 case Intrinsic::atomic_load_add:
385 case Intrinsic::atomic_load_sub:
386 case Intrinsic::atomic_load_and:
387 case Intrinsic::atomic_load_nand:
388 case Intrinsic::atomic_load_or:
389 case Intrinsic::atomic_load_xor:
390 case Intrinsic::atomic_load_max:
391 case Intrinsic::atomic_load_min:
392 case Intrinsic::atomic_load_umax:
393 case Intrinsic::atomic_load_umin:
395 Value *Op1 = II->getArgOperand(0);
396 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
397 if (isNoAlias(Op1, Op1Size, P, Size))
401 case Intrinsic::lifetime_start:
402 case Intrinsic::lifetime_end:
403 case Intrinsic::invariant_start: {
404 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
405 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
409 case Intrinsic::invariant_end: {
410 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
411 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
417 // The AliasAnalysis base class has some smarts, lets use them.
418 return AliasAnalysis::getModRefInfo(CS, P, Size);
422 AliasAnalysis::ModRefResult
423 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
424 // If CS1 or CS2 are readnone, they don't interact.
425 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
426 if (CS1B == DoesNotAccessMemory) return NoModRef;
428 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
429 if (CS2B == DoesNotAccessMemory) return NoModRef;
431 // If they both only read from memory, just return ref.
432 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
435 // Otherwise, fall back to NoAA (mod+ref).
436 return NoAA::getModRefInfo(CS1, CS2);
439 /// GetIndiceDifference - Dest and Src are the variable indices from two
440 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
441 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
442 /// difference between the two pointers.
443 static void GetIndiceDifference(
444 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
445 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
446 if (Src.empty()) return;
448 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
449 const Value *V = Src[i].first;
450 int64_t Scale = Src[i].second;
452 // Find V in Dest. This is N^2, but pointer indices almost never have more
453 // than a few variable indexes.
454 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
455 if (Dest[j].first != V) continue;
457 // If we found it, subtract off Scale V's from the entry in Dest. If it
458 // goes to zero, remove the entry.
459 if (Dest[j].second != Scale)
460 Dest[j].second -= Scale;
462 Dest.erase(Dest.begin()+j);
467 // If we didn't consume this entry, add it to the end of the Dest list.
469 Dest.push_back(std::make_pair(V, -Scale));
473 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
474 /// against another pointer. We know that V1 is a GEP, but we don't know
475 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
476 /// UnderlyingV2 is the same for V2.
478 AliasAnalysis::AliasResult
479 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
480 const Value *V2, unsigned V2Size,
481 const Value *UnderlyingV1,
482 const Value *UnderlyingV2) {
483 // If this GEP has been visited before, we're on a use-def cycle.
484 // Such cycles are only valid when PHI nodes are involved or in unreachable
485 // code. The visitPHI function catches cycles containing PHIs, but there
486 // could still be a cycle without PHIs in unreachable code.
487 if (!Visited.insert(GEP1))
490 int64_t GEP1BaseOffset;
491 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
493 // If we have two gep instructions with must-alias'ing base pointers, figure
494 // out if the indexes to the GEP tell us anything about the derived pointer.
495 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
496 // Do the base pointers alias?
497 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U);
499 // If we get a No or May, then return it immediately, no amount of analysis
500 // will improve this situation.
501 if (BaseAlias != MustAlias) return BaseAlias;
503 // Otherwise, we have a MustAlias. Since the base pointers alias each other
504 // exactly, see if the computed offset from the common pointer tells us
505 // about the relation of the resulting pointer.
506 const Value *GEP1BasePtr =
507 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
509 int64_t GEP2BaseOffset;
510 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
511 const Value *GEP2BasePtr =
512 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
514 // If DecomposeGEPExpression isn't able to look all the way through the
515 // addressing operation, we must not have TD and this is too complex for us
516 // to handle without it.
517 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
519 "DecomposeGEPExpression and getUnderlyingObject disagree!");
523 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
524 // symbolic difference.
525 GEP1BaseOffset -= GEP2BaseOffset;
526 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices);
529 // Check to see if these two pointers are related by the getelementptr
530 // instruction. If one pointer is a GEP with a non-zero index of the other
531 // pointer, we know they cannot alias.
533 // If both accesses are unknown size, we can't do anything useful here.
534 if (V1Size == ~0U && V2Size == ~0U)
537 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size);
539 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
540 // If V2 is known not to alias GEP base pointer, then the two values
541 // cannot alias per GEP semantics: "A pointer value formed from a
542 // getelementptr instruction is associated with the addresses associated
543 // with the first operand of the getelementptr".
546 const Value *GEP1BasePtr =
547 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
549 // If DecomposeGEPExpression isn't able to look all the way through the
550 // addressing operation, we must not have TD and this is too complex for us
551 // to handle without it.
552 if (GEP1BasePtr != UnderlyingV1) {
554 "DecomposeGEPExpression and getUnderlyingObject disagree!");
559 // In the two GEP Case, if there is no difference in the offsets of the
560 // computed pointers, the resultant pointers are a must alias. This
561 // hapens when we have two lexically identical GEP's (for example).
563 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
564 // must aliases the GEP, the end result is a must alias also.
565 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
568 // If we have a known constant offset, see if this offset is larger than the
569 // access size being queried. If so, and if no variable indices can remove
570 // pieces of this constant, then we know we have a no-alias. For example,
573 // In order to handle cases like &A[100][i] where i is an out of range
574 // subscript, we have to ignore all constant offset pieces that are a multiple
575 // of a scaled index. Do this by removing constant offsets that are a
576 // multiple of any of our variable indices. This allows us to transform
577 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
578 // provides an offset of 4 bytes (assuming a <= 4 byte access).
579 for (unsigned i = 0, e = GEP1VariableIndices.size();
580 i != e && GEP1BaseOffset;++i)
581 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
582 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
584 // If our known offset is bigger than the access size, we know we don't have
586 if (GEP1BaseOffset) {
587 if (GEP1BaseOffset >= (int64_t)V2Size ||
588 GEP1BaseOffset <= -(int64_t)V1Size)
595 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
596 /// instruction against another.
597 AliasAnalysis::AliasResult
598 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
599 const Value *V2, unsigned V2Size) {
600 // If this select has been visited before, we're on a use-def cycle.
601 // Such cycles are only valid when PHI nodes are involved or in unreachable
602 // code. The visitPHI function catches cycles containing PHIs, but there
603 // could still be a cycle without PHIs in unreachable code.
604 if (!Visited.insert(SI))
607 // If the values are Selects with the same condition, we can do a more precise
608 // check: just check for aliases between the values on corresponding arms.
609 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
610 if (SI->getCondition() == SI2->getCondition()) {
612 aliasCheck(SI->getTrueValue(), SISize,
613 SI2->getTrueValue(), V2Size);
614 if (Alias == MayAlias)
616 AliasResult ThisAlias =
617 aliasCheck(SI->getFalseValue(), SISize,
618 SI2->getFalseValue(), V2Size);
619 if (ThisAlias != Alias)
624 // If both arms of the Select node NoAlias or MustAlias V2, then returns
625 // NoAlias / MustAlias. Otherwise, returns MayAlias.
627 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
628 if (Alias == MayAlias)
631 // If V2 is visited, the recursive case will have been caught in the
632 // above aliasCheck call, so these subsequent calls to aliasCheck
633 // don't need to assume that V2 is being visited recursively.
636 AliasResult ThisAlias =
637 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
638 if (ThisAlias != Alias)
643 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
645 AliasAnalysis::AliasResult
646 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
647 const Value *V2, unsigned V2Size) {
648 // The PHI node has already been visited, avoid recursion any further.
649 if (!Visited.insert(PN))
652 // If the values are PHIs in the same block, we can do a more precise
653 // as well as efficient check: just check for aliases between the values
654 // on corresponding edges.
655 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
656 if (PN2->getParent() == PN->getParent()) {
658 aliasCheck(PN->getIncomingValue(0), PNSize,
659 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
661 if (Alias == MayAlias)
663 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
664 AliasResult ThisAlias =
665 aliasCheck(PN->getIncomingValue(i), PNSize,
666 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
668 if (ThisAlias != Alias)
674 SmallPtrSet<Value*, 4> UniqueSrc;
675 SmallVector<Value*, 4> V1Srcs;
676 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
677 Value *PV1 = PN->getIncomingValue(i);
678 if (isa<PHINode>(PV1))
679 // If any of the source itself is a PHI, return MayAlias conservatively
680 // to avoid compile time explosion. The worst possible case is if both
681 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
682 // and 'n' are the number of PHI sources.
684 if (UniqueSrc.insert(PV1))
685 V1Srcs.push_back(PV1);
688 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
689 // Early exit if the check of the first PHI source against V2 is MayAlias.
690 // Other results are not possible.
691 if (Alias == MayAlias)
694 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
695 // NoAlias / MustAlias. Otherwise, returns MayAlias.
696 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
697 Value *V = V1Srcs[i];
699 // If V2 is visited, the recursive case will have been caught in the
700 // above aliasCheck call, so these subsequent calls to aliasCheck
701 // don't need to assume that V2 is being visited recursively.
704 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
705 if (ThisAlias != Alias || ThisAlias == MayAlias)
712 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
713 // such as array references.
715 AliasAnalysis::AliasResult
716 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
717 const Value *V2, unsigned V2Size) {
718 // If either of the memory references is empty, it doesn't matter what the
719 // pointer values are.
720 if (V1Size == 0 || V2Size == 0)
723 // Strip off any casts if they exist.
724 V1 = V1->stripPointerCasts();
725 V2 = V2->stripPointerCasts();
727 // Are we checking for alias of the same value?
728 if (V1 == V2) return MustAlias;
730 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
731 return NoAlias; // Scalars cannot alias each other
733 // Figure out what objects these things are pointing to if we can.
734 const Value *O1 = V1->getUnderlyingObject();
735 const Value *O2 = V2->getUnderlyingObject();
737 // Null values in the default address space don't point to any object, so they
738 // don't alias any other pointer.
739 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
740 if (CPN->getType()->getAddressSpace() == 0)
742 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
743 if (CPN->getType()->getAddressSpace() == 0)
747 // If V1/V2 point to two different objects we know that we have no alias.
748 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
751 // Constant pointers can't alias with non-const isIdentifiedObject objects.
752 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
753 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
756 // Arguments can't alias with local allocations or noalias calls
757 // in the same function.
758 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
759 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
762 // Most objects can't alias null.
763 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
764 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
767 // If one pointer is the result of a call/invoke or load and the other is a
768 // non-escaping local object within the same function, then we know the
769 // object couldn't escape to a point where the call could return it.
771 // Note that if the pointers are in different functions, there are a
772 // variety of complications. A call with a nocapture argument may still
773 // temporary store the nocapture argument's value in a temporary memory
774 // location if that memory location doesn't escape. Or it may pass a
775 // nocapture value to other functions as long as they don't capture it.
776 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
778 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
782 // If the size of one access is larger than the entire object on the other
783 // side, then we know such behavior is undefined and can assume no alias.
785 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
786 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
789 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
790 // GEP can't simplify, we don't even look at the PHI cases.
791 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
793 std::swap(V1Size, V2Size);
796 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
797 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
799 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
801 std::swap(V1Size, V2Size);
803 if (const PHINode *PN = dyn_cast<PHINode>(V1))
804 return aliasPHI(PN, V1Size, V2, V2Size);
806 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
808 std::swap(V1Size, V2Size);
810 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
811 return aliasSelect(S1, V1Size, V2, V2Size);
816 // Make sure that anything that uses AliasAnalysis pulls in this file.
817 DEFINING_FILE_FOR(BasicAliasAnalysis)