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 bool pointsToConstantMemory(const Value *P) { return false; }
156 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
157 const Value *P, unsigned Size) {
160 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
161 ImmutableCallSite CS2) {
165 virtual void deleteValue(Value *V) {}
166 virtual void copyValue(Value *From, Value *To) {}
168 /// getAdjustedAnalysisPointer - This method is used when a pass implements
169 /// an analysis interface through multiple inheritance. If needed, it should
170 /// override this to adjust the this pointer as needed for the specified pass
172 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
173 if (PI->isPassID(&AliasAnalysis::ID))
174 return (AliasAnalysis*)this;
178 } // End of anonymous namespace
180 // Register this pass...
182 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
183 "No Alias Analysis (always returns 'may' alias)",
186 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
188 //===----------------------------------------------------------------------===//
189 // BasicAliasAnalysis Pass
190 //===----------------------------------------------------------------------===//
193 static const Function *getParent(const Value *V) {
194 if (const Instruction *inst = dyn_cast<Instruction>(V))
195 return inst->getParent()->getParent();
197 if (const Argument *arg = dyn_cast<Argument>(V))
198 return arg->getParent();
203 static bool notDifferentParent(const Value *O1, const Value *O2) {
205 const Function *F1 = getParent(O1);
206 const Function *F2 = getParent(O2);
208 return !F1 || !F2 || F1 == F2;
213 /// BasicAliasAnalysis - This is the default alias analysis implementation.
214 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
215 /// derives from the NoAA class.
216 struct BasicAliasAnalysis : public NoAA {
217 static char ID; // Class identification, replacement for typeinfo
218 BasicAliasAnalysis() : NoAA(&ID) {}
220 AliasResult alias(const Value *V1, unsigned V1Size,
221 const Value *V2, unsigned V2Size) {
222 assert(Visited.empty() && "Visited must be cleared after use!");
223 assert(notDifferentParent(V1, V2) &&
224 "BasicAliasAnalysis doesn't support interprocedural queries.");
225 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
230 ModRefResult getModRefInfo(ImmutableCallSite CS,
231 const Value *P, unsigned Size);
232 ModRefResult getModRefInfo(ImmutableCallSite CS1,
233 ImmutableCallSite CS2);
235 /// pointsToConstantMemory - Chase pointers until we find a (constant
237 bool pointsToConstantMemory(const Value *P);
239 /// getAdjustedAnalysisPointer - This method is used when a pass implements
240 /// an analysis interface through multiple inheritance. If needed, it should
241 /// override this to adjust the this pointer as needed for the specified pass
243 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
244 if (PI->isPassID(&AliasAnalysis::ID))
245 return (AliasAnalysis*)this;
250 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
251 SmallPtrSet<const Value*, 16> Visited;
253 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
254 // instruction against another.
255 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
256 const Value *V2, unsigned V2Size,
257 const Value *UnderlyingV1, const Value *UnderlyingV2);
259 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
260 // instruction against another.
261 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
262 const Value *V2, unsigned V2Size);
264 /// aliasSelect - Disambiguate a Select instruction against another value.
265 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
266 const Value *V2, unsigned V2Size);
268 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
269 const Value *V2, unsigned V2Size);
271 } // End of anonymous namespace
273 // Register this pass...
274 char BasicAliasAnalysis::ID = 0;
275 INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa",
276 "Basic Alias Analysis (default AA impl)",
279 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
280 return new BasicAliasAnalysis();
284 /// pointsToConstantMemory - Chase pointers until we find a (constant
286 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
287 if (const GlobalVariable *GV =
288 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
289 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
290 // global to be marked constant in some modules and non-constant in others.
291 // GV may even be a declaration, not a definition.
292 return GV->isConstant();
297 /// getModRefInfo - Check to see if the specified callsite can clobber the
298 /// specified memory object. Since we only look at local properties of this
299 /// function, we really can't say much about this query. We do, however, use
300 /// simple "address taken" analysis on local objects.
301 AliasAnalysis::ModRefResult
302 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
303 const Value *P, unsigned Size) {
304 assert(notDifferentParent(CS.getInstruction(), P) &&
305 "AliasAnalysis query involving multiple functions!");
307 const Value *Object = P->getUnderlyingObject();
309 // If this is a tail call and P points to a stack location, we know that
310 // the tail call cannot access or modify the local stack.
311 // We cannot exclude byval arguments here; these belong to the caller of
312 // the current function not to the current function, and a tail callee
313 // may reference them.
314 if (isa<AllocaInst>(Object))
315 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
316 if (CI->isTailCall())
319 // If the pointer is to a locally allocated object that does not escape,
320 // then the call can not mod/ref the pointer unless the call takes the pointer
321 // as an argument, and itself doesn't capture it.
322 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
323 isNonEscapingLocalObject(Object)) {
324 bool PassedAsArg = false;
326 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
327 CI != CE; ++CI, ++ArgNo) {
328 // Only look at the no-capture pointer arguments.
329 if (!(*CI)->getType()->isPointerTy() ||
330 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
333 // If this is a no-capture pointer argument, see if we can tell that it
334 // is impossible to alias the pointer we're checking. If not, we have to
335 // assume that the call could touch the pointer, even though it doesn't
337 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) {
347 // Finally, handle specific knowledge of intrinsics.
348 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
350 return AliasAnalysis::getModRefInfo(CS, P, Size);
352 switch (II->getIntrinsicID()) {
354 case Intrinsic::memcpy:
355 case Intrinsic::memmove: {
356 unsigned Len = UnknownSize;
357 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
358 Len = LenCI->getZExtValue();
359 Value *Dest = II->getArgOperand(0);
360 Value *Src = II->getArgOperand(1);
361 if (isNoAlias(Dest, Len, P, Size)) {
362 if (isNoAlias(Src, Len, P, Size))
368 case Intrinsic::memset:
369 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
370 // will handle it for the variable length case.
371 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
372 unsigned Len = LenCI->getZExtValue();
373 Value *Dest = II->getArgOperand(0);
374 if (isNoAlias(Dest, Len, P, Size))
378 case Intrinsic::atomic_cmp_swap:
379 case Intrinsic::atomic_swap:
380 case Intrinsic::atomic_load_add:
381 case Intrinsic::atomic_load_sub:
382 case Intrinsic::atomic_load_and:
383 case Intrinsic::atomic_load_nand:
384 case Intrinsic::atomic_load_or:
385 case Intrinsic::atomic_load_xor:
386 case Intrinsic::atomic_load_max:
387 case Intrinsic::atomic_load_min:
388 case Intrinsic::atomic_load_umax:
389 case Intrinsic::atomic_load_umin:
391 Value *Op1 = II->getArgOperand(0);
392 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
393 if (isNoAlias(Op1, Op1Size, P, Size))
397 case Intrinsic::lifetime_start:
398 case Intrinsic::lifetime_end:
399 case Intrinsic::invariant_start: {
400 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
401 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
405 case Intrinsic::invariant_end: {
406 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
407 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
413 // The AliasAnalysis base class has some smarts, lets use them.
414 return AliasAnalysis::getModRefInfo(CS, P, Size);
418 AliasAnalysis::ModRefResult
419 BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
420 ImmutableCallSite CS2) {
421 // If CS1 or CS2 are readnone, they don't interact.
422 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
423 if (CS1B == DoesNotAccessMemory) return NoModRef;
425 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
426 if (CS2B == DoesNotAccessMemory) return NoModRef;
428 // If CS1 only reads from memory, just return ref.
429 if (CS1B == OnlyReadsMemory)
432 // Otherwise, fall back to NoAA (mod+ref).
433 return NoAA::getModRefInfo(CS1, CS2);
436 /// GetIndexDifference - Dest and Src are the variable indices from two
437 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
438 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
439 /// difference between the two pointers.
440 static void GetIndexDifference(
441 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
442 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
443 if (Src.empty()) return;
445 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
446 const Value *V = Src[i].first;
447 int64_t Scale = Src[i].second;
449 // Find V in Dest. This is N^2, but pointer indices almost never have more
450 // than a few variable indexes.
451 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
452 if (Dest[j].first != V) continue;
454 // If we found it, subtract off Scale V's from the entry in Dest. If it
455 // goes to zero, remove the entry.
456 if (Dest[j].second != Scale)
457 Dest[j].second -= Scale;
459 Dest.erase(Dest.begin()+j);
464 // If we didn't consume this entry, add it to the end of the Dest list.
466 Dest.push_back(std::make_pair(V, -Scale));
470 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
471 /// against another pointer. We know that V1 is a GEP, but we don't know
472 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
473 /// UnderlyingV2 is the same for V2.
475 AliasAnalysis::AliasResult
476 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
477 const Value *V2, unsigned V2Size,
478 const Value *UnderlyingV1,
479 const Value *UnderlyingV2) {
480 // If this GEP has been visited before, we're on a use-def cycle.
481 // Such cycles are only valid when PHI nodes are involved or in unreachable
482 // code. The visitPHI function catches cycles containing PHIs, but there
483 // could still be a cycle without PHIs in unreachable code.
484 if (!Visited.insert(GEP1))
487 int64_t GEP1BaseOffset;
488 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
490 // If we have two gep instructions with must-alias'ing base pointers, figure
491 // out if the indexes to the GEP tell us anything about the derived pointer.
492 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
493 // Do the base pointers alias?
494 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize,
495 UnderlyingV2, UnknownSize);
497 // If we get a No or May, then return it immediately, no amount of analysis
498 // will improve this situation.
499 if (BaseAlias != MustAlias) return BaseAlias;
501 // Otherwise, we have a MustAlias. Since the base pointers alias each other
502 // exactly, see if the computed offset from the common pointer tells us
503 // about the relation of the resulting pointer.
504 const Value *GEP1BasePtr =
505 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
507 int64_t GEP2BaseOffset;
508 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
509 const Value *GEP2BasePtr =
510 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
512 // If DecomposeGEPExpression isn't able to look all the way through the
513 // addressing operation, we must not have TD and this is too complex for us
514 // to handle without it.
515 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
517 "DecomposeGEPExpression and getUnderlyingObject disagree!");
521 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
522 // symbolic difference.
523 GEP1BaseOffset -= GEP2BaseOffset;
524 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices);
527 // Check to see if these two pointers are related by the getelementptr
528 // instruction. If one pointer is a GEP with a non-zero index of the other
529 // pointer, we know they cannot alias.
531 // If both accesses are unknown size, we can't do anything useful here.
532 if (V1Size == UnknownSize && V2Size == UnknownSize)
535 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size);
537 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
538 // If V2 is known not to alias GEP base pointer, then the two values
539 // cannot alias per GEP semantics: "A pointer value formed from a
540 // getelementptr instruction is associated with the addresses associated
541 // with the first operand of the getelementptr".
544 const Value *GEP1BasePtr =
545 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
547 // If DecomposeGEPExpression isn't able to look all the way through the
548 // addressing operation, we must not have TD and this is too complex for us
549 // to handle without it.
550 if (GEP1BasePtr != UnderlyingV1) {
552 "DecomposeGEPExpression and getUnderlyingObject disagree!");
557 // In the two GEP Case, if there is no difference in the offsets of the
558 // computed pointers, the resultant pointers are a must alias. This
559 // hapens when we have two lexically identical GEP's (for example).
561 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
562 // must aliases the GEP, the end result is a must alias also.
563 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
566 // If we have a known constant offset, see if this offset is larger than the
567 // access size being queried. If so, and if no variable indices can remove
568 // pieces of this constant, then we know we have a no-alias. For example,
571 // In order to handle cases like &A[100][i] where i is an out of range
572 // subscript, we have to ignore all constant offset pieces that are a multiple
573 // of a scaled index. Do this by removing constant offsets that are a
574 // multiple of any of our variable indices. This allows us to transform
575 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
576 // provides an offset of 4 bytes (assuming a <= 4 byte access).
577 for (unsigned i = 0, e = GEP1VariableIndices.size();
578 i != e && GEP1BaseOffset;++i)
579 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
580 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
582 // If our known offset is bigger than the access size, we know we don't have
584 if (GEP1BaseOffset) {
585 if (GEP1BaseOffset >= (int64_t)V2Size ||
586 GEP1BaseOffset <= -(int64_t)V1Size)
593 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
594 /// instruction against another.
595 AliasAnalysis::AliasResult
596 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
597 const Value *V2, unsigned V2Size) {
598 // If this select has been visited before, we're on a use-def cycle.
599 // Such cycles are only valid when PHI nodes are involved or in unreachable
600 // code. The visitPHI function catches cycles containing PHIs, but there
601 // could still be a cycle without PHIs in unreachable code.
602 if (!Visited.insert(SI))
605 // If the values are Selects with the same condition, we can do a more precise
606 // check: just check for aliases between the values on corresponding arms.
607 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
608 if (SI->getCondition() == SI2->getCondition()) {
610 aliasCheck(SI->getTrueValue(), SISize,
611 SI2->getTrueValue(), V2Size);
612 if (Alias == MayAlias)
614 AliasResult ThisAlias =
615 aliasCheck(SI->getFalseValue(), SISize,
616 SI2->getFalseValue(), V2Size);
617 if (ThisAlias != Alias)
622 // If both arms of the Select node NoAlias or MustAlias V2, then returns
623 // NoAlias / MustAlias. Otherwise, returns MayAlias.
625 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
626 if (Alias == MayAlias)
629 // If V2 is visited, the recursive case will have been caught in the
630 // above aliasCheck call, so these subsequent calls to aliasCheck
631 // don't need to assume that V2 is being visited recursively.
634 AliasResult ThisAlias =
635 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
636 if (ThisAlias != Alias)
641 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
643 AliasAnalysis::AliasResult
644 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
645 const Value *V2, unsigned V2Size) {
646 // The PHI node has already been visited, avoid recursion any further.
647 if (!Visited.insert(PN))
650 // If the values are PHIs in the same block, we can do a more precise
651 // as well as efficient check: just check for aliases between the values
652 // on corresponding edges.
653 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
654 if (PN2->getParent() == PN->getParent()) {
656 aliasCheck(PN->getIncomingValue(0), PNSize,
657 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
659 if (Alias == MayAlias)
661 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
662 AliasResult ThisAlias =
663 aliasCheck(PN->getIncomingValue(i), PNSize,
664 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
666 if (ThisAlias != Alias)
672 SmallPtrSet<Value*, 4> UniqueSrc;
673 SmallVector<Value*, 4> V1Srcs;
674 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
675 Value *PV1 = PN->getIncomingValue(i);
676 if (isa<PHINode>(PV1))
677 // If any of the source itself is a PHI, return MayAlias conservatively
678 // to avoid compile time explosion. The worst possible case is if both
679 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
680 // and 'n' are the number of PHI sources.
682 if (UniqueSrc.insert(PV1))
683 V1Srcs.push_back(PV1);
686 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
687 // Early exit if the check of the first PHI source against V2 is MayAlias.
688 // Other results are not possible.
689 if (Alias == MayAlias)
692 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
693 // NoAlias / MustAlias. Otherwise, returns MayAlias.
694 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
695 Value *V = V1Srcs[i];
697 // If V2 is visited, the recursive case will have been caught in the
698 // above aliasCheck call, so these subsequent calls to aliasCheck
699 // don't need to assume that V2 is being visited recursively.
702 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
703 if (ThisAlias != Alias || ThisAlias == MayAlias)
710 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
711 // such as array references.
713 AliasAnalysis::AliasResult
714 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
715 const Value *V2, unsigned V2Size) {
716 // If either of the memory references is empty, it doesn't matter what the
717 // pointer values are.
718 if (V1Size == 0 || V2Size == 0)
721 // Strip off any casts if they exist.
722 V1 = V1->stripPointerCasts();
723 V2 = V2->stripPointerCasts();
725 // Are we checking for alias of the same value?
726 if (V1 == V2) return MustAlias;
728 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
729 return NoAlias; // Scalars cannot alias each other
731 // Figure out what objects these things are pointing to if we can.
732 const Value *O1 = V1->getUnderlyingObject();
733 const Value *O2 = V2->getUnderlyingObject();
735 // Null values in the default address space don't point to any object, so they
736 // don't alias any other pointer.
737 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
738 if (CPN->getType()->getAddressSpace() == 0)
740 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
741 if (CPN->getType()->getAddressSpace() == 0)
745 // If V1/V2 point to two different objects we know that we have no alias.
746 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
749 // Constant pointers can't alias with non-const isIdentifiedObject objects.
750 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
751 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
754 // Arguments can't alias with local allocations or noalias calls
755 // in the same function.
756 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
757 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
760 // Most objects can't alias null.
761 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
762 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
765 // If one pointer is the result of a call/invoke or load and the other is a
766 // non-escaping local object within the same function, then we know the
767 // object couldn't escape to a point where the call could return it.
769 // Note that if the pointers are in different functions, there are a
770 // variety of complications. A call with a nocapture argument may still
771 // temporary store the nocapture argument's value in a temporary memory
772 // location if that memory location doesn't escape. Or it may pass a
773 // nocapture value to other functions as long as they don't capture it.
774 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
776 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
780 // If the size of one access is larger than the entire object on the other
781 // side, then we know such behavior is undefined and can assume no alias.
783 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
784 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
787 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
788 // GEP can't simplify, we don't even look at the PHI cases.
789 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
791 std::swap(V1Size, V2Size);
794 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
795 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
797 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
799 std::swap(V1Size, V2Size);
801 if (const PHINode *PN = dyn_cast<PHINode>(V1))
802 return aliasPHI(PN, V1Size, V2, V2Size);
804 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
806 std::swap(V1Size, V2Size);
808 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
809 return aliasSelect(S1, V1Size, V2, V2Size);
814 // Make sure that anything that uses AliasAnalysis pulls in this file.
815 DEFINING_FILE_FOR(BasicAliasAnalysis)