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/Compiler.h"
33 #include "llvm/Support/ErrorHandling.h"
37 //===----------------------------------------------------------------------===//
39 //===----------------------------------------------------------------------===//
41 /// isKnownNonNull - Return true if we know that the specified value is never
43 static bool isKnownNonNull(const Value *V) {
44 // Alloca never returns null, malloc might.
45 if (isa<AllocaInst>(V)) return true;
47 // A byval argument is never null.
48 if (const Argument *A = dyn_cast<Argument>(V))
49 return A->hasByValAttr();
51 // Global values are not null unless extern weak.
52 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
53 return !GV->hasExternalWeakLinkage();
57 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
58 /// object that never escapes from the function.
59 static bool isNonEscapingLocalObject(const Value *V, bool Interprocedural) {
60 // If this is a local allocation, check to see if it escapes.
61 if (isa<AllocaInst>(V) ||
62 (!Interprocedural && isNoAliasCall(V)))
63 // Set StoreCaptures to True so that we can assume in our callers that the
64 // pointer is not the result of a load instruction. Currently
65 // PointerMayBeCaptured doesn't have any special analysis for the
66 // StoreCaptures=false case; if it did, our callers could be refined to be
68 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
70 // If this is an argument that corresponds to a byval or noalias argument,
71 // then it has not escaped before entering the function. Check if it escapes
72 // inside the function.
74 if (const Argument *A = dyn_cast<Argument>(V))
75 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
76 // Don't bother analyzing arguments already known not to escape.
77 if (A->hasNoCaptureAttr())
79 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
84 /// isEscapeSource - Return true if the pointer is one which would have
85 /// been considered an escape by isNonEscapingLocalObject.
86 static bool isEscapeSource(const Value *V, bool Interprocedural) {
88 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
91 // The load case works because isNonEscapingLocalObject considers all
92 // stores to be escapes (it passes true for the StoreCaptures argument
93 // to PointerMayBeCaptured).
100 /// isObjectSmallerThan - Return true if we can prove that the object specified
101 /// by V is smaller than Size.
102 static bool isObjectSmallerThan(const Value *V, unsigned Size,
103 const TargetData &TD) {
104 const Type *AccessTy;
105 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
106 AccessTy = GV->getType()->getElementType();
107 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
108 if (!AI->isArrayAllocation())
109 AccessTy = AI->getType()->getElementType();
112 } else if (const CallInst* CI = extractMallocCall(V)) {
113 if (!isArrayMalloc(V, &TD))
114 // The size is the argument to the malloc call.
115 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
116 return (C->getZExtValue() < Size);
118 } else if (const Argument *A = dyn_cast<Argument>(V)) {
119 if (A->hasByValAttr())
120 AccessTy = cast<PointerType>(A->getType())->getElementType();
127 if (AccessTy->isSized())
128 return TD.getTypeAllocSize(AccessTy) < Size;
132 //===----------------------------------------------------------------------===//
134 //===----------------------------------------------------------------------===//
137 /// NoAA - This class implements the -no-aa pass, which always returns "I
138 /// don't know" for alias queries. NoAA is unlike other alias analysis
139 /// implementations, in that it does not chain to a previous analysis. As
140 /// such it doesn't follow many of the rules that other alias analyses must.
142 struct NoAA : public ImmutablePass, public AliasAnalysis {
143 static char ID; // Class identification, replacement for typeinfo
144 NoAA() : ImmutablePass(&ID) {}
145 explicit NoAA(void *PID) : ImmutablePass(PID) { }
147 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
150 virtual void initializePass() {
151 TD = getAnalysisIfAvailable<TargetData>();
154 virtual AliasResult alias(const Value *V1, unsigned V1Size,
155 const Value *V2, unsigned V2Size) {
159 virtual void getArgumentAccesses(Function *F, CallSite CS,
160 std::vector<PointerAccessInfo> &Info) {
161 llvm_unreachable("This method may not be called on this function!");
164 virtual bool pointsToConstantMemory(const Value *P) { return false; }
165 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
168 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite 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 should
177 /// override this to adjust the this pointer as needed for the specified pass
179 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
180 if (PI->isPassID(&AliasAnalysis::ID))
181 return (AliasAnalysis*)this;
185 } // End of anonymous namespace
187 // Register this pass...
189 static RegisterPass<NoAA>
190 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
192 // Declare that we implement the AliasAnalysis interface
193 static RegisterAnalysisGroup<AliasAnalysis> V(U);
195 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
197 //===----------------------------------------------------------------------===//
198 // BasicAliasAnalysis Pass
199 //===----------------------------------------------------------------------===//
201 static const Function *getParent(const Value *V) {
202 if(const Instruction *inst = dyn_cast<Instruction>(V))
203 return inst->getParent()->getParent();
205 if(const Argument *arg = dyn_cast<Argument>(V))
206 return arg->getParent();
211 static bool sameParent(const Value *O1, const Value *O2) ATTRIBUTE_UNUSED;
213 static bool sameParent(const Value *O1, const Value *O2) {
215 const Function *F1 = getParent(O1);
216 const Function *F2 = getParent(O2);
218 return !F1 || !F2 || F1 == F2;
222 /// BasicAliasAnalysis - This is the default alias analysis implementation.
223 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
224 /// derives from the NoAA class.
225 struct BasicAliasAnalysis : public NoAA {
226 /// Interprocedural - Flag for "interprocedural" mode, where we must
227 /// support queries of values which live in different functions.
228 bool Interprocedural;
230 static char ID; // Class identification, replacement for typeinfo
232 : NoAA(&ID), Interprocedural(false) {}
233 BasicAliasAnalysis(void *PID, bool interprocedural)
234 : NoAA(PID), Interprocedural(interprocedural) {}
236 AliasResult alias(const Value *V1, unsigned V1Size,
237 const Value *V2, unsigned V2Size) {
238 assert(Visited.empty() && "Visited must be cleared after use!");
240 assert((Interprocedural || sameParent(V1, V2)) &&
241 "BasicAliasAnalysis (-basicaa) doesn't support interprocedural "
242 "queries; use InterproceduralAliasAnalysis "
243 "(-interprocedural-basic-aa) instead.");
245 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
250 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
251 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
253 /// pointsToConstantMemory - Chase pointers until we find a (constant
255 bool pointsToConstantMemory(const Value *P);
257 /// getAdjustedAnalysisPointer - This method is used when a pass implements
258 /// an analysis interface through multiple inheritance. If needed, it should
259 /// override this to adjust the this pointer as needed for the specified pass
261 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
262 if (PI->isPassID(&AliasAnalysis::ID))
263 return (AliasAnalysis*)this;
268 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
269 SmallPtrSet<const Value*, 16> Visited;
271 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
272 // instruction against another.
273 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
274 const Value *V2, unsigned V2Size,
275 const Value *UnderlyingV1, const Value *UnderlyingV2);
277 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
278 // instruction against another.
279 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
280 const Value *V2, unsigned V2Size);
282 /// aliasSelect - Disambiguate a Select instruction against another value.
283 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
284 const Value *V2, unsigned V2Size);
286 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
287 const Value *V2, unsigned V2Size);
289 } // End of anonymous namespace
291 // Register this pass...
292 char BasicAliasAnalysis::ID = 0;
293 static RegisterPass<BasicAliasAnalysis>
294 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
296 // Declare that we implement the AliasAnalysis interface
297 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
299 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
300 return new BasicAliasAnalysis();
304 /// pointsToConstantMemory - Chase pointers until we find a (constant
306 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
307 if (const GlobalVariable *GV =
308 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
309 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
310 // global to be marked constant in some modules and non-constant in others.
311 // GV may even be a declaration, not a definition.
312 return GV->isConstant();
317 /// getModRefInfo - Check to see if the specified callsite can clobber the
318 /// specified memory object. Since we only look at local properties of this
319 /// function, we really can't say much about this query. We do, however, use
320 /// simple "address taken" analysis on local objects.
321 AliasAnalysis::ModRefResult
322 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
323 const Value *Object = P->getUnderlyingObject();
325 // If this is a tail call and P points to a stack location, we know that
326 // the tail call cannot access or modify the local stack.
327 // We cannot exclude byval arguments here; these belong to the caller of
328 // the current function not to the current function, and a tail callee
329 // may reference them.
330 if (isa<AllocaInst>(Object))
331 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
332 if (CI->isTailCall())
335 // If the pointer is to a locally allocated object that does not escape,
336 // then the call can not mod/ref the pointer unless the call takes the pointer
337 // as an argument, and itself doesn't capture it.
338 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
339 isNonEscapingLocalObject(Object, Interprocedural)) {
340 bool PassedAsArg = false;
342 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
343 CI != CE; ++CI, ++ArgNo) {
344 // Only look at the no-capture pointer arguments.
345 if (!(*CI)->getType()->isPointerTy() ||
346 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
349 // If this is a no-capture pointer argument, see if we can tell that it
350 // is impossible to alias the pointer we're checking. If not, we have to
351 // assume that the call could touch the pointer, even though it doesn't
353 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) {
363 // Finally, handle specific knowledge of intrinsics.
364 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
366 return AliasAnalysis::getModRefInfo(CS, P, Size);
368 switch (II->getIntrinsicID()) {
370 case Intrinsic::memcpy:
371 case Intrinsic::memmove: {
373 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
374 Len = LenCI->getZExtValue();
375 Value *Dest = II->getArgOperand(0);
376 Value *Src = II->getArgOperand(1);
377 if (isNoAlias(Dest, Len, P, Size)) {
378 if (isNoAlias(Src, Len, P, Size))
384 case Intrinsic::memset:
385 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
386 // will handle it for the variable length case.
387 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
388 unsigned Len = LenCI->getZExtValue();
389 Value *Dest = II->getArgOperand(0);
390 if (isNoAlias(Dest, Len, P, Size))
394 case Intrinsic::atomic_cmp_swap:
395 case Intrinsic::atomic_swap:
396 case Intrinsic::atomic_load_add:
397 case Intrinsic::atomic_load_sub:
398 case Intrinsic::atomic_load_and:
399 case Intrinsic::atomic_load_nand:
400 case Intrinsic::atomic_load_or:
401 case Intrinsic::atomic_load_xor:
402 case Intrinsic::atomic_load_max:
403 case Intrinsic::atomic_load_min:
404 case Intrinsic::atomic_load_umax:
405 case Intrinsic::atomic_load_umin:
407 Value *Op1 = II->getArgOperand(0);
408 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
409 if (isNoAlias(Op1, Op1Size, P, Size))
413 case Intrinsic::lifetime_start:
414 case Intrinsic::lifetime_end:
415 case Intrinsic::invariant_start: {
416 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
417 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
421 case Intrinsic::invariant_end: {
422 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
423 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
429 // The AliasAnalysis base class has some smarts, lets use them.
430 return AliasAnalysis::getModRefInfo(CS, P, Size);
434 AliasAnalysis::ModRefResult
435 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
436 // If CS1 or CS2 are readnone, they don't interact.
437 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
438 if (CS1B == DoesNotAccessMemory) return NoModRef;
440 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
441 if (CS2B == DoesNotAccessMemory) return NoModRef;
443 // If they both only read from memory, just return ref.
444 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
447 // Otherwise, fall back to NoAA (mod+ref).
448 return NoAA::getModRefInfo(CS1, CS2);
451 /// GetIndiceDifference - Dest and Src are the variable indices from two
452 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
453 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
454 /// difference between the two pointers.
455 static void GetIndiceDifference(
456 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
457 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
458 if (Src.empty()) return;
460 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
461 const Value *V = Src[i].first;
462 int64_t Scale = Src[i].second;
464 // Find V in Dest. This is N^2, but pointer indices almost never have more
465 // than a few variable indexes.
466 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
467 if (Dest[j].first != V) continue;
469 // If we found it, subtract off Scale V's from the entry in Dest. If it
470 // goes to zero, remove the entry.
471 if (Dest[j].second != Scale)
472 Dest[j].second -= Scale;
474 Dest.erase(Dest.begin()+j);
479 // If we didn't consume this entry, add it to the end of the Dest list.
481 Dest.push_back(std::make_pair(V, -Scale));
485 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
486 /// against another pointer. We know that V1 is a GEP, but we don't know
487 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
488 /// UnderlyingV2 is the same for V2.
490 AliasAnalysis::AliasResult
491 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
492 const Value *V2, unsigned V2Size,
493 const Value *UnderlyingV1,
494 const Value *UnderlyingV2) {
495 // If this GEP has been visited before, we're on a use-def cycle.
496 // Such cycles are only valid when PHI nodes are involved or in unreachable
497 // code. The visitPHI function catches cycles containing PHIs, but there
498 // could still be a cycle without PHIs in unreachable code.
499 if (!Visited.insert(GEP1))
502 int64_t GEP1BaseOffset;
503 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
505 // If we have two gep instructions with must-alias'ing base pointers, figure
506 // out if the indexes to the GEP tell us anything about the derived pointer.
507 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
508 // Do the base pointers alias?
509 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U);
511 // If we get a No or May, then return it immediately, no amount of analysis
512 // will improve this situation.
513 if (BaseAlias != MustAlias) return BaseAlias;
515 // Otherwise, we have a MustAlias. Since the base pointers alias each other
516 // exactly, see if the computed offset from the common pointer tells us
517 // about the relation of the resulting pointer.
518 const Value *GEP1BasePtr =
519 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
521 int64_t GEP2BaseOffset;
522 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
523 const Value *GEP2BasePtr =
524 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
526 // If DecomposeGEPExpression isn't able to look all the way through the
527 // addressing operation, we must not have TD and this is too complex for us
528 // to handle without it.
529 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
531 "DecomposeGEPExpression and getUnderlyingObject disagree!");
535 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
536 // symbolic difference.
537 GEP1BaseOffset -= GEP2BaseOffset;
538 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices);
541 // Check to see if these two pointers are related by the getelementptr
542 // instruction. If one pointer is a GEP with a non-zero index of the other
543 // pointer, we know they cannot alias.
545 // If both accesses are unknown size, we can't do anything useful here.
546 if (V1Size == ~0U && V2Size == ~0U)
549 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size);
551 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
552 // If V2 is known not to alias GEP base pointer, then the two values
553 // cannot alias per GEP semantics: "A pointer value formed from a
554 // getelementptr instruction is associated with the addresses associated
555 // with the first operand of the getelementptr".
558 const Value *GEP1BasePtr =
559 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
561 // If DecomposeGEPExpression isn't able to look all the way through the
562 // addressing operation, we must not have TD and this is too complex for us
563 // to handle without it.
564 if (GEP1BasePtr != UnderlyingV1) {
566 "DecomposeGEPExpression and getUnderlyingObject disagree!");
571 // In the two GEP Case, if there is no difference in the offsets of the
572 // computed pointers, the resultant pointers are a must alias. This
573 // hapens when we have two lexically identical GEP's (for example).
575 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
576 // must aliases the GEP, the end result is a must alias also.
577 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
580 // If we have a known constant offset, see if this offset is larger than the
581 // access size being queried. If so, and if no variable indices can remove
582 // pieces of this constant, then we know we have a no-alias. For example,
585 // In order to handle cases like &A[100][i] where i is an out of range
586 // subscript, we have to ignore all constant offset pieces that are a multiple
587 // of a scaled index. Do this by removing constant offsets that are a
588 // multiple of any of our variable indices. This allows us to transform
589 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
590 // provides an offset of 4 bytes (assuming a <= 4 byte access).
591 for (unsigned i = 0, e = GEP1VariableIndices.size();
592 i != e && GEP1BaseOffset;++i)
593 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
594 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
596 // If our known offset is bigger than the access size, we know we don't have
598 if (GEP1BaseOffset) {
599 if (GEP1BaseOffset >= (int64_t)V2Size ||
600 GEP1BaseOffset <= -(int64_t)V1Size)
607 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
608 /// instruction against another.
609 AliasAnalysis::AliasResult
610 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
611 const Value *V2, unsigned V2Size) {
612 // If this select has been visited before, we're on a use-def cycle.
613 // Such cycles are only valid when PHI nodes are involved or in unreachable
614 // code. The visitPHI function catches cycles containing PHIs, but there
615 // could still be a cycle without PHIs in unreachable code.
616 if (!Visited.insert(SI))
619 // If the values are Selects with the same condition, we can do a more precise
620 // check: just check for aliases between the values on corresponding arms.
621 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
622 if (SI->getCondition() == SI2->getCondition()) {
624 aliasCheck(SI->getTrueValue(), SISize,
625 SI2->getTrueValue(), V2Size);
626 if (Alias == MayAlias)
628 AliasResult ThisAlias =
629 aliasCheck(SI->getFalseValue(), SISize,
630 SI2->getFalseValue(), V2Size);
631 if (ThisAlias != Alias)
636 // If both arms of the Select node NoAlias or MustAlias V2, then returns
637 // NoAlias / MustAlias. Otherwise, returns MayAlias.
639 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
640 if (Alias == MayAlias)
643 // If V2 is visited, the recursive case will have been caught in the
644 // above aliasCheck call, so these subsequent calls to aliasCheck
645 // don't need to assume that V2 is being visited recursively.
648 AliasResult ThisAlias =
649 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
650 if (ThisAlias != Alias)
655 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
657 AliasAnalysis::AliasResult
658 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
659 const Value *V2, unsigned V2Size) {
660 // The PHI node has already been visited, avoid recursion any further.
661 if (!Visited.insert(PN))
664 // If the values are PHIs in the same block, we can do a more precise
665 // as well as efficient check: just check for aliases between the values
666 // on corresponding edges.
667 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
668 if (PN2->getParent() == PN->getParent()) {
670 aliasCheck(PN->getIncomingValue(0), PNSize,
671 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
673 if (Alias == MayAlias)
675 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
676 AliasResult ThisAlias =
677 aliasCheck(PN->getIncomingValue(i), PNSize,
678 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
680 if (ThisAlias != Alias)
686 SmallPtrSet<Value*, 4> UniqueSrc;
687 SmallVector<Value*, 4> V1Srcs;
688 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
689 Value *PV1 = PN->getIncomingValue(i);
690 if (isa<PHINode>(PV1))
691 // If any of the source itself is a PHI, return MayAlias conservatively
692 // to avoid compile time explosion. The worst possible case is if both
693 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
694 // and 'n' are the number of PHI sources.
696 if (UniqueSrc.insert(PV1))
697 V1Srcs.push_back(PV1);
700 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
701 // Early exit if the check of the first PHI source against V2 is MayAlias.
702 // Other results are not possible.
703 if (Alias == MayAlias)
706 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
707 // NoAlias / MustAlias. Otherwise, returns MayAlias.
708 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
709 Value *V = V1Srcs[i];
711 // If V2 is visited, the recursive case will have been caught in the
712 // above aliasCheck call, so these subsequent calls to aliasCheck
713 // don't need to assume that V2 is being visited recursively.
716 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
717 if (ThisAlias != Alias || ThisAlias == MayAlias)
724 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
725 // such as array references.
727 AliasAnalysis::AliasResult
728 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
729 const Value *V2, unsigned V2Size) {
730 // If either of the memory references is empty, it doesn't matter what the
731 // pointer values are.
732 if (V1Size == 0 || V2Size == 0)
735 // Strip off any casts if they exist.
736 V1 = V1->stripPointerCasts();
737 V2 = V2->stripPointerCasts();
739 // Are we checking for alias of the same value?
740 if (V1 == V2) return MustAlias;
742 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
743 return NoAlias; // Scalars cannot alias each other
745 // Figure out what objects these things are pointing to if we can.
746 const Value *O1 = V1->getUnderlyingObject();
747 const Value *O2 = V2->getUnderlyingObject();
749 // Null values in the default address space don't point to any object, so they
750 // don't alias any other pointer.
751 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
752 if (CPN->getType()->getAddressSpace() == 0)
754 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
755 if (CPN->getType()->getAddressSpace() == 0)
759 // If V1/V2 point to two different objects we know that we have no alias.
760 if (isIdentifiedObject(O1, Interprocedural) &&
761 isIdentifiedObject(O2, Interprocedural))
764 // Constant pointers can't alias with non-const isIdentifiedObject objects.
765 if ((isa<Constant>(O1) &&
766 isIdentifiedObject(O2, Interprocedural) &&
767 !isa<Constant>(O2)) ||
768 (isa<Constant>(O2) &&
769 isIdentifiedObject(O1, Interprocedural) &&
773 // Arguments can't alias with local allocations or noalias calls, unless
774 // we have to consider interprocedural aliasing.
775 if (!Interprocedural)
776 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
777 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
780 // Most objects can't alias null.
781 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
782 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
786 // If the size of one access is larger than the entire object on the other
787 // side, then we know such behavior is undefined and can assume no alias.
789 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
790 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
793 // If one pointer is the result of a call/invoke or load and the other is a
794 // non-escaping local object, then we know the object couldn't escape to a
795 // point where the call could return it.
797 if (isEscapeSource(O1, Interprocedural) &&
798 isNonEscapingLocalObject(O2, Interprocedural))
800 if (isEscapeSource(O2, Interprocedural) &&
801 isNonEscapingLocalObject(O1, Interprocedural))
805 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
806 // GEP can't simplify, we don't even look at the PHI cases.
807 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
809 std::swap(V1Size, V2Size);
812 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
813 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
815 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
817 std::swap(V1Size, V2Size);
819 if (const PHINode *PN = dyn_cast<PHINode>(V1))
820 return aliasPHI(PN, V1Size, V2, V2Size);
822 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
824 std::swap(V1Size, V2Size);
826 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
827 return aliasSelect(S1, V1Size, V2, V2Size);
832 // Make sure that anything that uses AliasAnalysis pulls in this file.
833 DEFINING_FILE_FOR(BasicAliasAnalysis)
835 //===----------------------------------------------------------------------===//
836 // InterproceduralBasicAliasAnalysis Pass
837 //===----------------------------------------------------------------------===//
840 /// InterproceduralBasicAliasAnalysis - This is similar to basicaa, except
841 /// that it properly supports queries to values which live in different
844 /// Note that we don't currently take this to the extreme, analyzing all
845 /// call sites of a function to answer a query about an Argument.
847 struct InterproceduralBasicAliasAnalysis : public BasicAliasAnalysis {
848 static char ID; // Class identification, replacement for typeinfo
849 InterproceduralBasicAliasAnalysis() : BasicAliasAnalysis(&ID, true) {}
853 // Register this pass...
854 char InterproceduralBasicAliasAnalysis::ID = 0;
855 static RegisterPass<InterproceduralBasicAliasAnalysis>
856 W("interprocedural-basic-aa", "Interprocedural Basic Alias Analysis", false, true);
858 // Declare that we implement the AliasAnalysis interface
859 static RegisterAnalysisGroup<AliasAnalysis> Z(W);
861 ImmutablePass *llvm::createInterproceduralBasicAliasAnalysisPass() {
862 return new InterproceduralBasicAliasAnalysis();