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, bool Interprocedural) {
59 // If this is a local allocation, check to see if it escapes.
60 if (isa<AllocaInst>(V) ||
61 (!Interprocedural && isNoAliasCall(V)))
62 // Set StoreCaptures to True so that we can assume in our callers that the
63 // pointer is not the result of a load instruction. Currently
64 // PointerMayBeCaptured doesn't have any special analysis for the
65 // StoreCaptures=false case; if it did, our callers could be refined to be
67 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
69 // If this is an argument that corresponds to a byval or noalias argument,
70 // then it has not escaped before entering the function. Check if it escapes
71 // inside the function.
73 if (const Argument *A = dyn_cast<Argument>(V))
74 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
75 // Don't bother analyzing arguments already known not to escape.
76 if (A->hasNoCaptureAttr())
78 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
83 /// isEscapeSource - Return true if the pointer is one which would have
84 /// been considered an escape by isNonEscapingLocalObject.
85 static bool isEscapeSource(const Value *V, bool Interprocedural) {
87 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
90 // The load case works because isNonEscapingLocalObject considers all
91 // stores to be escapes (it passes true for the StoreCaptures argument
92 // to PointerMayBeCaptured).
99 /// isObjectSmallerThan - Return true if we can prove that the object specified
100 /// by V is smaller than Size.
101 static bool isObjectSmallerThan(const Value *V, unsigned Size,
102 const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 AccessTy = GV->getType()->getElementType();
106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
107 if (!AI->isArrayAllocation())
108 AccessTy = AI->getType()->getElementType();
111 } else if (const CallInst* CI = extractMallocCall(V)) {
112 if (!isArrayMalloc(V, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
115 return (C->getZExtValue() < Size);
117 } else if (const Argument *A = dyn_cast<Argument>(V)) {
118 if (A->hasByValAttr())
119 AccessTy = cast<PointerType>(A->getType())->getElementType();
126 if (AccessTy->isSized())
127 return TD.getTypeAllocSize(AccessTy) < Size;
131 //===----------------------------------------------------------------------===//
133 //===----------------------------------------------------------------------===//
136 /// NoAA - This class implements the -no-aa pass, which always returns "I
137 /// don't know" for alias queries. NoAA is unlike other alias analysis
138 /// implementations, in that it does not chain to a previous analysis. As
139 /// such it doesn't follow many of the rules that other alias analyses must.
141 struct NoAA : public ImmutablePass, public AliasAnalysis {
142 static char ID; // Class identification, replacement for typeinfo
143 NoAA() : ImmutablePass(&ID) {}
144 explicit NoAA(void *PID) : ImmutablePass(PID) { }
146 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
149 virtual void initializePass() {
150 TD = getAnalysisIfAvailable<TargetData>();
153 virtual AliasResult alias(const Value *V1, unsigned V1Size,
154 const Value *V2, unsigned V2Size) {
158 virtual void getArgumentAccesses(Function *F, CallSite CS,
159 std::vector<PointerAccessInfo> &Info) {
160 llvm_unreachable("This method may not be called on this function!");
163 virtual bool pointsToConstantMemory(const Value *P) { return false; }
164 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
167 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
171 virtual void deleteValue(Value *V) {}
172 virtual void copyValue(Value *From, Value *To) {}
174 /// getAdjustedAnalysisPointer - This method is used when a pass implements
175 /// an analysis interface through multiple inheritance. If needed, it should
176 /// override this to adjust the this pointer as needed for the specified pass
178 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
179 if (PI->isPassID(&AliasAnalysis::ID))
180 return (AliasAnalysis*)this;
184 } // End of anonymous namespace
186 // Register this pass...
188 static RegisterPass<NoAA>
189 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
191 // Declare that we implement the AliasAnalysis interface
192 static RegisterAnalysisGroup<AliasAnalysis> V(U);
194 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
196 //===----------------------------------------------------------------------===//
197 // BasicAliasAnalysis Pass
198 //===----------------------------------------------------------------------===//
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) {
213 const Function *F1 = getParent(O1);
214 const Function *F2 = getParent(O2);
216 return !F1 || !F2 || F1 == F2;
221 /// BasicAliasAnalysis - This is the default alias analysis implementation.
222 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
223 /// derives from the NoAA class.
224 struct BasicAliasAnalysis : public NoAA {
225 /// Interprocedural - Flag for "interprocedural" mode, where we must
226 /// support queries of values which live in different functions.
227 bool Interprocedural;
229 static char ID; // Class identification, replacement for typeinfo
231 : NoAA(&ID), Interprocedural(false) {}
232 BasicAliasAnalysis(void *PID, bool interprocedural)
233 : NoAA(PID), Interprocedural(interprocedural) {}
235 AliasResult alias(const Value *V1, unsigned V1Size,
236 const Value *V2, unsigned V2Size) {
237 assert(Visited.empty() && "Visited must be cleared after use!");
239 assert((Interprocedural || sameParent(V1, V2)) &&
240 "BasicAliasAnalysis (-basicaa) doesn't support interprocedural "
241 "queries; use InterproceduralAliasAnalysis "
242 "(-interprocedural-basic-aa) instead.");
244 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
249 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
250 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
252 /// pointsToConstantMemory - Chase pointers until we find a (constant
254 bool pointsToConstantMemory(const Value *P);
256 /// getAdjustedAnalysisPointer - This method is used when a pass implements
257 /// an analysis interface through multiple inheritance. If needed, it should
258 /// override this to adjust the this pointer as needed for the specified pass
260 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
261 if (PI->isPassID(&AliasAnalysis::ID))
262 return (AliasAnalysis*)this;
267 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
268 SmallPtrSet<const Value*, 16> Visited;
270 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
271 // instruction against another.
272 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
273 const Value *V2, unsigned V2Size,
274 const Value *UnderlyingV1, const Value *UnderlyingV2);
276 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
277 // instruction against another.
278 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
279 const Value *V2, unsigned V2Size);
281 /// aliasSelect - Disambiguate a Select instruction against another value.
282 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
283 const Value *V2, unsigned V2Size);
285 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
286 const Value *V2, unsigned V2Size);
288 } // End of anonymous namespace
290 // Register this pass...
291 char BasicAliasAnalysis::ID = 0;
292 static RegisterPass<BasicAliasAnalysis>
293 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
295 // Declare that we implement the AliasAnalysis interface
296 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
298 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
299 return new BasicAliasAnalysis();
303 /// pointsToConstantMemory - Chase pointers until we find a (constant
305 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
306 if (const GlobalVariable *GV =
307 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
308 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
309 // global to be marked constant in some modules and non-constant in others.
310 // GV may even be a declaration, not a definition.
311 return GV->isConstant();
316 /// getModRefInfo - Check to see if the specified callsite can clobber the
317 /// specified memory object. Since we only look at local properties of this
318 /// function, we really can't say much about this query. We do, however, use
319 /// simple "address taken" analysis on local objects.
320 AliasAnalysis::ModRefResult
321 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
322 const Value *Object = P->getUnderlyingObject();
324 // If this is a tail call and P points to a stack location, we know that
325 // the tail call cannot access or modify the local stack.
326 // We cannot exclude byval arguments here; these belong to the caller of
327 // the current function not to the current function, and a tail callee
328 // may reference them.
329 if (isa<AllocaInst>(Object))
330 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
331 if (CI->isTailCall())
334 // If the pointer is to a locally allocated object that does not escape,
335 // then the call can not mod/ref the pointer unless the call takes the pointer
336 // as an argument, and itself doesn't capture it.
337 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
338 isNonEscapingLocalObject(Object, Interprocedural)) {
339 bool PassedAsArg = false;
341 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
342 CI != CE; ++CI, ++ArgNo) {
343 // Only look at the no-capture pointer arguments.
344 if (!(*CI)->getType()->isPointerTy() ||
345 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
348 // If this is a no-capture pointer argument, see if we can tell that it
349 // is impossible to alias the pointer we're checking. If not, we have to
350 // assume that the call could touch the pointer, even though it doesn't
352 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) {
362 // Finally, handle specific knowledge of intrinsics.
363 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
365 return AliasAnalysis::getModRefInfo(CS, P, Size);
367 switch (II->getIntrinsicID()) {
369 case Intrinsic::memcpy:
370 case Intrinsic::memmove: {
372 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
373 Len = LenCI->getZExtValue();
374 Value *Dest = II->getArgOperand(0);
375 Value *Src = II->getArgOperand(1);
376 if (isNoAlias(Dest, Len, P, Size)) {
377 if (isNoAlias(Src, Len, P, Size))
383 case Intrinsic::memset:
384 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
385 // will handle it for the variable length case.
386 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
387 unsigned Len = LenCI->getZExtValue();
388 Value *Dest = II->getArgOperand(0);
389 if (isNoAlias(Dest, Len, P, Size))
393 case Intrinsic::atomic_cmp_swap:
394 case Intrinsic::atomic_swap:
395 case Intrinsic::atomic_load_add:
396 case Intrinsic::atomic_load_sub:
397 case Intrinsic::atomic_load_and:
398 case Intrinsic::atomic_load_nand:
399 case Intrinsic::atomic_load_or:
400 case Intrinsic::atomic_load_xor:
401 case Intrinsic::atomic_load_max:
402 case Intrinsic::atomic_load_min:
403 case Intrinsic::atomic_load_umax:
404 case Intrinsic::atomic_load_umin:
406 Value *Op1 = II->getArgOperand(0);
407 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
408 if (isNoAlias(Op1, Op1Size, P, Size))
412 case Intrinsic::lifetime_start:
413 case Intrinsic::lifetime_end:
414 case Intrinsic::invariant_start: {
415 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
416 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size))
420 case Intrinsic::invariant_end: {
421 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
422 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size))
428 // The AliasAnalysis base class has some smarts, lets use them.
429 return AliasAnalysis::getModRefInfo(CS, P, Size);
433 AliasAnalysis::ModRefResult
434 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
435 // If CS1 or CS2 are readnone, they don't interact.
436 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
437 if (CS1B == DoesNotAccessMemory) return NoModRef;
439 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
440 if (CS2B == DoesNotAccessMemory) return NoModRef;
442 // If they both only read from memory, just return ref.
443 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
446 // Otherwise, fall back to NoAA (mod+ref).
447 return NoAA::getModRefInfo(CS1, CS2);
450 /// GetIndiceDifference - Dest and Src are the variable indices from two
451 /// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
452 /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
453 /// difference between the two pointers.
454 static void GetIndiceDifference(
455 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest,
456 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) {
457 if (Src.empty()) return;
459 for (unsigned i = 0, e = Src.size(); i != e; ++i) {
460 const Value *V = Src[i].first;
461 int64_t Scale = Src[i].second;
463 // Find V in Dest. This is N^2, but pointer indices almost never have more
464 // than a few variable indexes.
465 for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
466 if (Dest[j].first != V) continue;
468 // If we found it, subtract off Scale V's from the entry in Dest. If it
469 // goes to zero, remove the entry.
470 if (Dest[j].second != Scale)
471 Dest[j].second -= Scale;
473 Dest.erase(Dest.begin()+j);
478 // If we didn't consume this entry, add it to the end of the Dest list.
480 Dest.push_back(std::make_pair(V, -Scale));
484 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
485 /// against another pointer. We know that V1 is a GEP, but we don't know
486 /// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(),
487 /// UnderlyingV2 is the same for V2.
489 AliasAnalysis::AliasResult
490 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
491 const Value *V2, unsigned V2Size,
492 const Value *UnderlyingV1,
493 const Value *UnderlyingV2) {
494 // If this GEP has been visited before, we're on a use-def cycle.
495 // Such cycles are only valid when PHI nodes are involved or in unreachable
496 // code. The visitPHI function catches cycles containing PHIs, but there
497 // could still be a cycle without PHIs in unreachable code.
498 if (!Visited.insert(GEP1))
501 int64_t GEP1BaseOffset;
502 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices;
504 // If we have two gep instructions with must-alias'ing base pointers, figure
505 // out if the indexes to the GEP tell us anything about the derived pointer.
506 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
507 // Do the base pointers alias?
508 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U);
510 // If we get a No or May, then return it immediately, no amount of analysis
511 // will improve this situation.
512 if (BaseAlias != MustAlias) return BaseAlias;
514 // Otherwise, we have a MustAlias. Since the base pointers alias each other
515 // exactly, see if the computed offset from the common pointer tells us
516 // about the relation of the resulting pointer.
517 const Value *GEP1BasePtr =
518 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
520 int64_t GEP2BaseOffset;
521 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices;
522 const Value *GEP2BasePtr =
523 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
525 // If DecomposeGEPExpression isn't able to look all the way through the
526 // addressing operation, we must not have TD and this is too complex for us
527 // to handle without it.
528 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
530 "DecomposeGEPExpression and getUnderlyingObject disagree!");
534 // Subtract the GEP2 pointer from the GEP1 pointer to find out their
535 // symbolic difference.
536 GEP1BaseOffset -= GEP2BaseOffset;
537 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices);
540 // Check to see if these two pointers are related by the getelementptr
541 // instruction. If one pointer is a GEP with a non-zero index of the other
542 // pointer, we know they cannot alias.
544 // If both accesses are unknown size, we can't do anything useful here.
545 if (V1Size == ~0U && V2Size == ~0U)
548 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size);
550 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
551 // If V2 is known not to alias GEP base pointer, then the two values
552 // cannot alias per GEP semantics: "A pointer value formed from a
553 // getelementptr instruction is associated with the addresses associated
554 // with the first operand of the getelementptr".
557 const Value *GEP1BasePtr =
558 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
560 // If DecomposeGEPExpression isn't able to look all the way through the
561 // addressing operation, we must not have TD and this is too complex for us
562 // to handle without it.
563 if (GEP1BasePtr != UnderlyingV1) {
565 "DecomposeGEPExpression and getUnderlyingObject disagree!");
570 // In the two GEP Case, if there is no difference in the offsets of the
571 // computed pointers, the resultant pointers are a must alias. This
572 // hapens when we have two lexically identical GEP's (for example).
574 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
575 // must aliases the GEP, the end result is a must alias also.
576 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
579 // If we have a known constant offset, see if this offset is larger than the
580 // access size being queried. If so, and if no variable indices can remove
581 // pieces of this constant, then we know we have a no-alias. For example,
584 // In order to handle cases like &A[100][i] where i is an out of range
585 // subscript, we have to ignore all constant offset pieces that are a multiple
586 // of a scaled index. Do this by removing constant offsets that are a
587 // multiple of any of our variable indices. This allows us to transform
588 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
589 // provides an offset of 4 bytes (assuming a <= 4 byte access).
590 for (unsigned i = 0, e = GEP1VariableIndices.size();
591 i != e && GEP1BaseOffset;++i)
592 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second)
593 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second;
595 // If our known offset is bigger than the access size, we know we don't have
597 if (GEP1BaseOffset) {
598 if (GEP1BaseOffset >= (int64_t)V2Size ||
599 GEP1BaseOffset <= -(int64_t)V1Size)
606 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
607 /// instruction against another.
608 AliasAnalysis::AliasResult
609 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
610 const Value *V2, unsigned V2Size) {
611 // If this select has been visited before, we're on a use-def cycle.
612 // Such cycles are only valid when PHI nodes are involved or in unreachable
613 // code. The visitPHI function catches cycles containing PHIs, but there
614 // could still be a cycle without PHIs in unreachable code.
615 if (!Visited.insert(SI))
618 // If the values are Selects with the same condition, we can do a more precise
619 // check: just check for aliases between the values on corresponding arms.
620 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
621 if (SI->getCondition() == SI2->getCondition()) {
623 aliasCheck(SI->getTrueValue(), SISize,
624 SI2->getTrueValue(), V2Size);
625 if (Alias == MayAlias)
627 AliasResult ThisAlias =
628 aliasCheck(SI->getFalseValue(), SISize,
629 SI2->getFalseValue(), V2Size);
630 if (ThisAlias != Alias)
635 // If both arms of the Select node NoAlias or MustAlias V2, then returns
636 // NoAlias / MustAlias. Otherwise, returns MayAlias.
638 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize);
639 if (Alias == MayAlias)
642 // If V2 is visited, the recursive case will have been caught in the
643 // above aliasCheck call, so these subsequent calls to aliasCheck
644 // don't need to assume that V2 is being visited recursively.
647 AliasResult ThisAlias =
648 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize);
649 if (ThisAlias != Alias)
654 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
656 AliasAnalysis::AliasResult
657 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
658 const Value *V2, unsigned V2Size) {
659 // The PHI node has already been visited, avoid recursion any further.
660 if (!Visited.insert(PN))
663 // If the values are PHIs in the same block, we can do a more precise
664 // as well as efficient check: just check for aliases between the values
665 // on corresponding edges.
666 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
667 if (PN2->getParent() == PN->getParent()) {
669 aliasCheck(PN->getIncomingValue(0), PNSize,
670 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
672 if (Alias == MayAlias)
674 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
675 AliasResult ThisAlias =
676 aliasCheck(PN->getIncomingValue(i), PNSize,
677 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
679 if (ThisAlias != Alias)
685 SmallPtrSet<Value*, 4> UniqueSrc;
686 SmallVector<Value*, 4> V1Srcs;
687 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
688 Value *PV1 = PN->getIncomingValue(i);
689 if (isa<PHINode>(PV1))
690 // If any of the source itself is a PHI, return MayAlias conservatively
691 // to avoid compile time explosion. The worst possible case is if both
692 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
693 // and 'n' are the number of PHI sources.
695 if (UniqueSrc.insert(PV1))
696 V1Srcs.push_back(PV1);
699 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
700 // Early exit if the check of the first PHI source against V2 is MayAlias.
701 // Other results are not possible.
702 if (Alias == MayAlias)
705 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
706 // NoAlias / MustAlias. Otherwise, returns MayAlias.
707 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
708 Value *V = V1Srcs[i];
710 // If V2 is visited, the recursive case will have been caught in the
711 // above aliasCheck call, so these subsequent calls to aliasCheck
712 // don't need to assume that V2 is being visited recursively.
715 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
716 if (ThisAlias != Alias || ThisAlias == MayAlias)
723 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
724 // such as array references.
726 AliasAnalysis::AliasResult
727 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
728 const Value *V2, unsigned V2Size) {
729 // If either of the memory references is empty, it doesn't matter what the
730 // pointer values are.
731 if (V1Size == 0 || V2Size == 0)
734 // Strip off any casts if they exist.
735 V1 = V1->stripPointerCasts();
736 V2 = V2->stripPointerCasts();
738 // Are we checking for alias of the same value?
739 if (V1 == V2) return MustAlias;
741 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
742 return NoAlias; // Scalars cannot alias each other
744 // Figure out what objects these things are pointing to if we can.
745 const Value *O1 = V1->getUnderlyingObject();
746 const Value *O2 = V2->getUnderlyingObject();
748 // Null values in the default address space don't point to any object, so they
749 // don't alias any other pointer.
750 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
751 if (CPN->getType()->getAddressSpace() == 0)
753 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
754 if (CPN->getType()->getAddressSpace() == 0)
758 // If V1/V2 point to two different objects we know that we have no alias.
759 if (isIdentifiedObject(O1, Interprocedural) &&
760 isIdentifiedObject(O2, Interprocedural))
763 // Constant pointers can't alias with non-const isIdentifiedObject objects.
764 if ((isa<Constant>(O1) &&
765 isIdentifiedObject(O2, Interprocedural) &&
766 !isa<Constant>(O2)) ||
767 (isa<Constant>(O2) &&
768 isIdentifiedObject(O1, Interprocedural) &&
772 // Arguments can't alias with local allocations or noalias calls, unless
773 // we have to consider interprocedural aliasing.
774 if (!Interprocedural)
775 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
776 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
779 // Most objects can't alias null.
780 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
781 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
785 // If the size of one access is larger than the entire object on the other
786 // side, then we know such behavior is undefined and can assume no alias.
788 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
789 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
792 // If one pointer is the result of a call/invoke or load and the other is a
793 // non-escaping local object, then we know the object couldn't escape to a
794 // point where the call could return it.
796 if (isEscapeSource(O1, Interprocedural) &&
797 isNonEscapingLocalObject(O2, Interprocedural))
799 if (isEscapeSource(O2, Interprocedural) &&
800 isNonEscapingLocalObject(O1, Interprocedural))
804 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
805 // GEP can't simplify, we don't even look at the PHI cases.
806 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
808 std::swap(V1Size, V2Size);
811 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
812 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2);
814 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
816 std::swap(V1Size, V2Size);
818 if (const PHINode *PN = dyn_cast<PHINode>(V1))
819 return aliasPHI(PN, V1Size, V2, V2Size);
821 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
823 std::swap(V1Size, V2Size);
825 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
826 return aliasSelect(S1, V1Size, V2, V2Size);
831 // Make sure that anything that uses AliasAnalysis pulls in this file.
832 DEFINING_FILE_FOR(BasicAliasAnalysis)
834 //===----------------------------------------------------------------------===//
835 // InterproceduralBasicAliasAnalysis Pass
836 //===----------------------------------------------------------------------===//
839 /// InterproceduralBasicAliasAnalysis - This is similar to basicaa, except
840 /// that it properly supports queries to values which live in different
843 /// Note that we don't currently take this to the extreme, analyzing all
844 /// call sites of a function to answer a query about an Argument.
846 struct InterproceduralBasicAliasAnalysis : public BasicAliasAnalysis {
847 static char ID; // Class identification, replacement for typeinfo
848 InterproceduralBasicAliasAnalysis() : BasicAliasAnalysis(&ID, true) {}
852 // Register this pass...
853 char InterproceduralBasicAliasAnalysis::ID = 0;
854 static RegisterPass<InterproceduralBasicAliasAnalysis>
855 W("interprocedural-basic-aa", "Interprocedural Basic Alias Analysis", false, true);
857 // Declare that we implement the AliasAnalysis interface
858 static RegisterAnalysisGroup<AliasAnalysis> Z(W);
860 ImmutablePass *llvm::createInterproceduralBasicAliasAnalysisPass() {
861 return new InterproceduralBasicAliasAnalysis();