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->getArgOperand(0)))
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(Visited.empty() && "Visited 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 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
217 SmallPtrSet<const Value*, 16> Visited;
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->getArgOperand(2)))
322 Len = LenCI->getZExtValue();
323 Value *Dest = II->getArgOperand(0);
324 Value *Src = II->getArgOperand(1);
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->getArgOperand(2))) {
336 unsigned Len = LenCI->getZExtValue();
337 Value *Dest = II->getArgOperand(0);
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->getArgOperand(0);
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->getArgOperand(0))->getZExtValue();
365 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
369 case Intrinsic::invariant_end: {
370 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
371 if (isNoAlias(II->getArgOperand(2), 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 // If this GEP has been visited before, we're on a use-def cycle.
444 // Such cycles are only valid when PHI nodes are involved or in unreachable
445 // code. The visitPHI function catches cycles containing PHIs, but there
446 // could still be a cycle without PHIs in unreachable code.
447 if (!Visited.insert(GEP1))
450 int64_t GEP1BaseOffset;
451 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
453 // If we have two gep instructions with must-alias'ing base pointers, figure
454 // out if the indexes to the GEP tell us anything about the derived pointer.
455 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
456 // Do the base pointers alias?
457 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U);
459 // If we get a No or May, then return it immediately, no amount of analysis
460 // will improve this situation.
461 if (BaseAlias != MustAlias) return BaseAlias;
463 // Otherwise, we have a MustAlias. Since the base pointers alias each other
464 // exactly, see if the computed offset from the common pointer tells us
465 // about the relation of the resulting pointer.
466 const Value *GEP1BasePtr =
467 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
469 int64_t GEP2BaseOffset;
470 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
471 const Value *GEP2BasePtr =
472 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
474 // If DecomposeGEPExpression isn't able to look all the way through the
475 // addressing operation, we must not have TD and this is too complex for us
476 // to handle without it.
477 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
479 "DecomposeGEPExpression and getUnderlyingObject disagree!");
483 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
484 // symbolic difference.
485 GEP1BaseOffset -= GEP2BaseOffset;
486 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices);
489 // Check to see if these two pointers are related by the getelementptr
490 // instruction. If one pointer is a GEP with a non-zero index of the other
491 // pointer, we know they cannot alias.
493 // If both accesses are unknown size, we can't do anything useful here.
494 if (V1Size == ~0U && V2Size == ~0U)
497 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size);
499 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
500 // If V2 is known not to alias GEP base pointer, then the two values
501 // cannot alias per GEP semantics: "A pointer value formed from a
502 // getelementptr instruction is associated with the addresses associated
503 // with the first operand of the getelementptr".
506 const Value *GEP1BasePtr =
507 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
509 // If DecomposeGEPExpression isn't able to look all the way through the
510 // addressing operation, we must not have TD and this is too complex for us
511 // to handle without it.
512 if (GEP1BasePtr != UnderlyingV1) {
514 "DecomposeGEPExpression and getUnderlyingObject disagree!");
519 // In the two GEP Case, if there is no difference in the offsets of the
520 // computed pointers, the resultant pointers are a must alias. This
521 // hapens when we have two lexically identical GEP's (for example).
523 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
524 // must aliases the GEP, the end result is a must alias also.
525 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
528 // If we have a known constant offset, see if this offset is larger than the
529 // access size being queried. If so, and if no variable indices can remove
530 // pieces of this constant, then we know we have a no-alias. For example,
533 // In order to handle cases like &A[100][i] where i is an out of range
534 // subscript, we have to ignore all constant offset pieces that are a multiple
535 // of a scaled index. Do this by removing constant offsets that are a
536 // multiple of any of our variable indices. This allows us to transform
537 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
538 // provides an offset of 4 bytes (assuming a <= 4 byte access).
539 for (unsigned i = 0, e = GEP1VariableIndices.size();
540 i != e && GEP1BaseOffset;++i)
541 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
542 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
544 // If our known offset is bigger than the access size, we know we don't have
546 if (GEP1BaseOffset) {
547 if (GEP1BaseOffset >= (int64_t)V2Size ||
548 GEP1BaseOffset <= -(int64_t)V1Size)
555 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
556 /// instruction against another.
557 AliasAnalysis::AliasResult
558 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
559 const Value *V2, unsigned V2Size) {
560 // If this select has been visited before, we're on a use-def cycle.
561 // Such cycles are only valid when PHI nodes are involved or in unreachable
562 // code. The visitPHI function catches cycles containing PHIs, but there
563 // could still be a cycle without PHIs in unreachable code.
564 if (!Visited.insert(SI))
567 // If the values are Selects with the same condition, we can do a more precise
568 // check: just check for aliases between the values on corresponding arms.
569 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
570 if (SI->getCondition() == SI2->getCondition()) {
572 aliasCheck(SI->getTrueValue(), SISize,
573 SI2->getTrueValue(), V2Size);
574 if (Alias == MayAlias)
576 AliasResult ThisAlias =
577 aliasCheck(SI->getFalseValue(), SISize,
578 SI2->getFalseValue(), V2Size);
579 if (ThisAlias != Alias)
584 // If both arms of the Select node NoAlias or MustAlias V2, then returns
585 // NoAlias / MustAlias. Otherwise, returns MayAlias.
587 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
588 if (Alias == MayAlias)
591 // If V2 is visited, the recursive case will have been caught in the
592 // above aliasCheck call, so these subsequent calls to aliasCheck
593 // don't need to assume that V2 is being visited recursively.
596 AliasResult ThisAlias =
597 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
598 if (ThisAlias != Alias)
603 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
605 AliasAnalysis::AliasResult
606 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
607 const Value *V2, unsigned V2Size) {
608 // The PHI node has already been visited, avoid recursion any further.
609 if (!Visited.insert(PN))
612 // If the values are PHIs in the same block, we can do a more precise
613 // as well as efficient check: just check for aliases between the values
614 // on corresponding edges.
615 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
616 if (PN2->getParent() == PN->getParent()) {
618 aliasCheck(PN->getIncomingValue(0), PNSize,
619 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
621 if (Alias == MayAlias)
623 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
624 AliasResult ThisAlias =
625 aliasCheck(PN->getIncomingValue(i), PNSize,
626 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
628 if (ThisAlias != Alias)
634 SmallPtrSet<Value*, 4> UniqueSrc;
635 SmallVector<Value*, 4> V1Srcs;
636 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
637 Value *PV1 = PN->getIncomingValue(i);
638 if (isa<PHINode>(PV1))
639 // If any of the source itself is a PHI, return MayAlias conservatively
640 // to avoid compile time explosion. The worst possible case is if both
641 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
642 // and 'n' are the number of PHI sources.
644 if (UniqueSrc.insert(PV1))
645 V1Srcs.push_back(PV1);
648 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
649 // Early exit if the check of the first PHI source against V2 is MayAlias.
650 // Other results are not possible.
651 if (Alias == MayAlias)
654 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
655 // NoAlias / MustAlias. Otherwise, returns MayAlias.
656 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
657 Value *V = V1Srcs[i];
659 // If V2 is visited, the recursive case will have been caught in the
660 // above aliasCheck call, so these subsequent calls to aliasCheck
661 // don't need to assume that V2 is being visited recursively.
664 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
665 if (ThisAlias != Alias || ThisAlias == MayAlias)
672 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
673 // such as array references.
675 AliasAnalysis::AliasResult
676 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
677 const Value *V2, unsigned V2Size) {
678 // If either of the memory references is empty, it doesn't matter what the
679 // pointer values are.
680 if (V1Size == 0 || V2Size == 0)
683 // Strip off any casts if they exist.
684 V1 = V1->stripPointerCasts();
685 V2 = V2->stripPointerCasts();
687 // Are we checking for alias of the same value?
688 if (V1 == V2) return MustAlias;
690 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
691 return NoAlias; // Scalars cannot alias each other
693 // Figure out what objects these things are pointing to if we can.
694 const Value *O1 = V1->getUnderlyingObject();
695 const Value *O2 = V2->getUnderlyingObject();
697 // Null values in the default address space don't point to any object, so they
698 // don't alias any other pointer.
699 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
700 if (CPN->getType()->getAddressSpace() == 0)
702 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
703 if (CPN->getType()->getAddressSpace() == 0)
707 // If V1/V2 point to two different objects we know that we have no alias.
708 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
711 // Constant pointers can't alias with non-const isIdentifiedObject objects.
712 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
713 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
716 // Arguments can't alias with local allocations or noalias calls.
717 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
718 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
721 // Most objects can't alias null.
722 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
723 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
727 // If the size of one access is larger than the entire object on the other
728 // side, then we know such behavior is undefined and can assume no alias.
730 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
731 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
734 // If one pointer is the result of a call/invoke or load and the other is a
735 // non-escaping local object, then we know the object couldn't escape to a
736 // point where the call could return it. The load case works because
737 // isNonEscapingLocalObject considers all stores to be escapes (it
738 // passes true for the StoreCaptures argument to PointerMayBeCaptured).
740 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1) ||
741 isa<Argument>(O1)) &&
742 isNonEscapingLocalObject(O2))
744 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2) ||
745 isa<Argument>(O2)) &&
746 isNonEscapingLocalObject(O1))
750 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
751 // GEP can't simplify, we don't even look at the PHI cases.
752 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
754 std::swap(V1Size, V2Size);
757 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
758 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
760 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
762 std::swap(V1Size, V2Size);
764 if (const PHINode *PN = dyn_cast<PHINode>(V1))
765 return aliasPHI(PN, V1Size, V2, V2Size);
767 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
769 std::swap(V1Size, V2Size);
771 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
772 return aliasSelect(S1, V1Size, V2, V2Size);
777 // Make sure that anything that uses AliasAnalysis pulls in this file.
778 DEFINING_FILE_FOR(BasicAliasAnalysis)