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/CaptureTracking.h"
18 #include "llvm/Analysis/MallocHelper.h"
19 #include "llvm/Analysis/Passes.h"
20 #include "llvm/Constants.h"
21 #include "llvm/DerivedTypes.h"
22 #include "llvm/Function.h"
23 #include "llvm/GlobalVariable.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/IntrinsicInst.h"
26 #include "llvm/LLVMContext.h"
27 #include "llvm/Operator.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/GetElementPtrTypeIterator.h"
38 //===----------------------------------------------------------------------===//
40 //===----------------------------------------------------------------------===//
42 static const Value *GetGEPOperands(const Value *V,
43 SmallVector<Value*, 16> &GEPOps) {
44 assert(GEPOps.empty() && "Expect empty list to populate!");
45 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
46 cast<User>(V)->op_end());
48 // Accumulate all of the chained indexes into the operand array
49 V = cast<User>(V)->getOperand(0);
51 while (const GEPOperator *G = dyn_cast<GEPOperator>(V)) {
52 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
53 !cast<Constant>(GEPOps[0])->isNullValue())
54 break; // Don't handle folding arbitrary pointer offsets yet...
55 GEPOps.erase(GEPOps.begin()); // Drop the zero index
56 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
62 /// isKnownNonNull - Return true if we know that the specified value is never
64 static bool isKnownNonNull(const Value *V) {
65 // Alloca never returns null, malloc might.
66 if (isa<AllocaInst>(V)) return true;
68 // A byval argument is never null.
69 if (const Argument *A = dyn_cast<Argument>(V))
70 return A->hasByValAttr();
72 // Global values are not null unless extern weak.
73 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
74 return !GV->hasExternalWeakLinkage();
78 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
79 /// object that never escapes from the function.
80 static bool isNonEscapingLocalObject(const Value *V) {
81 // If this is a local allocation, check to see if it escapes.
82 if (isa<AllocaInst>(V) || isNoAliasCall(V))
83 return !PointerMayBeCaptured(V, false);
85 // If this is an argument that corresponds to a byval or noalias argument,
86 // then it has not escaped before entering the function. Check if it escapes
87 // inside the function.
88 if (const Argument *A = dyn_cast<Argument>(V))
89 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
90 // Don't bother analyzing arguments already known not to escape.
91 if (A->hasNoCaptureAttr())
93 return !PointerMayBeCaptured(V, false);
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 LLVMContext &Context, 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, Context, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
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 void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
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) {
171 virtual bool hasNoModRefInfoForCalls() const { return true; }
173 virtual void deleteValue(Value *V) {}
174 virtual void copyValue(Value *From, Value *To) {}
176 } // End of anonymous namespace
178 // Register this pass...
180 static RegisterPass<NoAA>
181 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
183 // Declare that we implement the AliasAnalysis interface
184 static RegisterAnalysisGroup<AliasAnalysis> V(U);
186 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
188 //===----------------------------------------------------------------------===//
190 //===----------------------------------------------------------------------===//
193 /// BasicAliasAnalysis - This is the default alias analysis implementation.
194 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
195 /// derives from the NoAA class.
196 struct BasicAliasAnalysis : public NoAA {
197 static char ID; // Class identification, replacement for typeinfo
198 BasicAliasAnalysis() : NoAA(&ID) {}
199 AliasResult alias(const Value *V1, unsigned V1Size,
200 const Value *V2, unsigned V2Size) {
201 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!");
202 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
207 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
208 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
210 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
211 /// non-escaping allocations.
212 virtual bool hasNoModRefInfoForCalls() const { return false; }
214 /// pointsToConstantMemory - Chase pointers until we find a (constant
216 bool pointsToConstantMemory(const Value *P);
219 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call.
220 SmallPtrSet<const Value*, 16> VisitedPHIs;
222 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
224 AliasResult aliasGEP(const Value *V1, unsigned V1Size,
225 const Value *V2, unsigned V2Size);
227 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
229 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
230 const Value *V2, unsigned V2Size);
232 /// aliasSelect - Disambiguate a Select instruction against another value.
233 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
234 const Value *V2, unsigned V2Size);
236 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
237 const Value *V2, unsigned V2Size);
239 // CheckGEPInstructions - Check two GEP instructions with known
240 // must-aliasing base pointers. This checks to see if the index expressions
241 // preclude the pointers from aliasing...
243 CheckGEPInstructions(const Type* BasePtr1Ty,
244 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
245 const Type *BasePtr2Ty,
246 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
248 } // End of anonymous namespace
250 // Register this pass...
251 char BasicAliasAnalysis::ID = 0;
252 static RegisterPass<BasicAliasAnalysis>
253 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
255 // Declare that we implement the AliasAnalysis interface
256 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
258 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
259 return new BasicAliasAnalysis();
263 /// pointsToConstantMemory - Chase pointers until we find a (constant
265 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
266 if (const GlobalVariable *GV =
267 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
268 return GV->isConstant();
273 // getModRefInfo - Check to see if the specified callsite can clobber the
274 // specified memory object. Since we only look at local properties of this
275 // function, we really can't say much about this query. We do, however, use
276 // simple "address taken" analysis on local objects.
278 AliasAnalysis::ModRefResult
279 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
280 if (!isa<Constant>(P)) {
281 const Value *Object = P->getUnderlyingObject();
283 // If this is a tail call and P points to a stack location, we know that
284 // the tail call cannot access or modify the local stack.
285 // We cannot exclude byval arguments here; these belong to the caller of
286 // the current function not to the current function, and a tail callee
287 // may reference them.
288 if (isa<AllocaInst>(Object))
289 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
290 if (CI->isTailCall())
293 // If the pointer is to a locally allocated object that does not escape,
294 // then the call can not mod/ref the pointer unless the call takes the
295 // argument without capturing it.
296 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
297 bool passedAsArg = false;
298 // TODO: Eventually only check 'nocapture' arguments.
299 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
301 if (isa<PointerType>((*CI)->getType()) &&
302 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
309 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
310 switch (II->getIntrinsicID()) {
312 case Intrinsic::memcpy:
313 case Intrinsic::memmove: {
315 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
316 Len = LenCI->getZExtValue();
317 Value *Dest = II->getOperand(1);
318 Value *Src = II->getOperand(2);
319 if (alias(Dest, Len, P, Size) == NoAlias) {
320 if (alias(Src, Len, P, Size) == NoAlias)
326 case Intrinsic::memset:
327 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
328 unsigned Len = LenCI->getZExtValue();
329 Value *Dest = II->getOperand(1);
330 if (alias(Dest, Len, P, Size) == NoAlias)
334 case Intrinsic::atomic_cmp_swap:
335 case Intrinsic::atomic_swap:
336 case Intrinsic::atomic_load_add:
337 case Intrinsic::atomic_load_sub:
338 case Intrinsic::atomic_load_and:
339 case Intrinsic::atomic_load_nand:
340 case Intrinsic::atomic_load_or:
341 case Intrinsic::atomic_load_xor:
342 case Intrinsic::atomic_load_max:
343 case Intrinsic::atomic_load_min:
344 case Intrinsic::atomic_load_umax:
345 case Intrinsic::atomic_load_umin:
347 Value *Op1 = II->getOperand(1);
348 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
349 if (alias(Op1, Op1Size, P, Size) == NoAlias)
353 case Intrinsic::lifetime_start:
354 case Intrinsic::lifetime_end:
355 case Intrinsic::invariant_start: {
356 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
357 if (alias(II->getOperand(2), PtrSize, P, Size) == NoAlias)
361 case Intrinsic::invariant_end: {
362 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
363 if (alias(II->getOperand(3), PtrSize, P, Size) == NoAlias)
371 // The AliasAnalysis base class has some smarts, lets use them.
372 return AliasAnalysis::getModRefInfo(CS, P, Size);
376 AliasAnalysis::ModRefResult
377 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
378 // If CS1 or CS2 are readnone, they don't interact.
379 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
380 if (CS1B == DoesNotAccessMemory) return NoModRef;
382 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
383 if (CS2B == DoesNotAccessMemory) return NoModRef;
385 // If they both only read from memory, just return ref.
386 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
389 // Otherwise, fall back to NoAA (mod+ref).
390 return NoAA::getModRefInfo(CS1, CS2);
393 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
396 AliasAnalysis::AliasResult
397 BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size,
398 const Value *V2, unsigned V2Size) {
399 // If we have two gep instructions with must-alias'ing base pointers, figure
400 // out if the indexes to the GEP tell us anything about the derived pointer.
401 // Note that we also handle chains of getelementptr instructions as well as
402 // constant expression getelementptrs here.
404 if (isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
405 const User *GEP1 = cast<User>(V1);
406 const User *GEP2 = cast<User>(V2);
408 // If V1 and V2 are identical GEPs, just recurse down on both of them.
409 // This allows us to analyze things like:
410 // P = gep A, 0, i, 1
411 // Q = gep B, 0, i, 1
412 // by just analyzing A and B. This is even safe for variable indices.
413 if (GEP1->getType() == GEP2->getType() &&
414 GEP1->getNumOperands() == GEP2->getNumOperands() &&
415 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
416 // All operands are the same, ignoring the base.
417 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
418 return aliasCheck(GEP1->getOperand(0), V1Size,
419 GEP2->getOperand(0), V2Size);
421 // Drill down into the first non-gep value, to test for must-aliasing of
422 // the base pointers.
423 while (isa<GEPOperator>(GEP1->getOperand(0)) &&
424 GEP1->getOperand(1) ==
425 Constant::getNullValue(GEP1->getOperand(1)->getType()))
426 GEP1 = cast<User>(GEP1->getOperand(0));
427 const Value *BasePtr1 = GEP1->getOperand(0);
429 while (isa<GEPOperator>(GEP2->getOperand(0)) &&
430 GEP2->getOperand(1) ==
431 Constant::getNullValue(GEP2->getOperand(1)->getType()))
432 GEP2 = cast<User>(GEP2->getOperand(0));
433 const Value *BasePtr2 = GEP2->getOperand(0);
435 // Do the base pointers alias?
436 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U);
437 if (BaseAlias == NoAlias) return NoAlias;
438 if (BaseAlias == MustAlias) {
439 // If the base pointers alias each other exactly, check to see if we can
440 // figure out anything about the resultant pointers, to try to prove
443 // Collect all of the chained GEP operands together into one simple place
444 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
445 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
446 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
448 // If GetGEPOperands were able to fold to the same must-aliased pointer,
449 // do the comparison.
450 if (BasePtr1 == BasePtr2) {
452 CheckGEPInstructions(BasePtr1->getType(),
453 &GEP1Ops[0], GEP1Ops.size(), V1Size,
455 &GEP2Ops[0], GEP2Ops.size(), V2Size);
456 if (GAlias != MayAlias)
462 // Check to see if these two pointers are related by a getelementptr
463 // instruction. If one pointer is a GEP with a non-zero index of the other
464 // pointer, we know they cannot alias.
466 if (V1Size == ~0U || V2Size == ~0U)
469 SmallVector<Value*, 16> GEPOperands;
470 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
472 AliasResult R = aliasCheck(BasePtr, ~0U, V2, V2Size);
474 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
475 // If V2 is known not to alias GEP base pointer, then the two values
476 // cannot alias per GEP semantics: "A pointer value formed from a
477 // getelementptr instruction is associated with the addresses associated
478 // with the first operand of the getelementptr".
481 // If there is at least one non-zero constant index, we know they cannot
483 bool ConstantFound = false;
484 bool AllZerosFound = true;
485 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
486 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
487 if (!C->isNullValue()) {
488 ConstantFound = true;
489 AllZerosFound = false;
493 AllZerosFound = false;
496 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
497 // the ptr, the end result is a must alias also.
502 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
505 // Otherwise we have to check to see that the distance is more than
506 // the size of the argument... build an index vector that is equal to
507 // the arguments provided, except substitute 0's for any variable
508 // indexes we find...
510 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
511 for (unsigned i = 0; i != GEPOperands.size(); ++i)
512 if (!isa<ConstantInt>(GEPOperands[i]))
513 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
514 int64_t Offset = TD->getIndexedOffset(BasePtr->getType(),
518 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
526 // aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select instruction
528 AliasAnalysis::AliasResult
529 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
530 const Value *V2, unsigned V2Size) {
531 // If the values are Selects with the same condition, we can do a more precise
532 // check: just check for aliases between the values on corresponding arms.
533 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
534 if (SI->getCondition() == SI2->getCondition()) {
536 aliasCheck(SI->getTrueValue(), SISize,
537 SI2->getTrueValue(), V2Size);
538 if (Alias == MayAlias)
540 AliasResult ThisAlias =
541 aliasCheck(SI->getFalseValue(), SISize,
542 SI2->getFalseValue(), V2Size);
543 if (ThisAlias != Alias)
548 // If both arms of the Select node NoAlias or MustAlias V2, then returns
549 // NoAlias / MustAlias. Otherwise, returns MayAlias.
551 aliasCheck(SI->getTrueValue(), SISize, V2, V2Size);
552 if (Alias == MayAlias)
554 AliasResult ThisAlias =
555 aliasCheck(SI->getFalseValue(), SISize, V2, V2Size);
556 if (ThisAlias != Alias)
561 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
563 AliasAnalysis::AliasResult
564 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
565 const Value *V2, unsigned V2Size) {
566 // The PHI node has already been visited, avoid recursion any further.
567 if (!VisitedPHIs.insert(PN))
570 // If the values are PHIs in the same block, we can do a more precise
571 // as well as efficient check: just check for aliases between the values
572 // on corresponding edges.
573 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
574 if (PN2->getParent() == PN->getParent()) {
576 aliasCheck(PN->getIncomingValue(0), PNSize,
577 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
579 if (Alias == MayAlias)
581 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
582 AliasResult ThisAlias =
583 aliasCheck(PN->getIncomingValue(i), PNSize,
584 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
586 if (ThisAlias != Alias)
592 SmallPtrSet<Value*, 4> UniqueSrc;
593 SmallVector<Value*, 4> V1Srcs;
594 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
595 Value *PV1 = PN->getIncomingValue(i);
596 if (isa<PHINode>(PV1))
597 // If any of the source itself is a PHI, return MayAlias conservatively
598 // to avoid compile time explosion. The worst possible case is if both
599 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
600 // and 'n' are the number of PHI sources.
602 if (UniqueSrc.insert(PV1))
603 V1Srcs.push_back(PV1);
606 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
607 // Early exit if the check of the first PHI source against V2 is MayAlias.
608 // Other results are not possible.
609 if (Alias == MayAlias)
612 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
613 // NoAlias / MustAlias. Otherwise, returns MayAlias.
614 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
615 Value *V = V1Srcs[i];
617 // If V2 is a PHI, the recursive case will have been caught in the
618 // above aliasCheck call, so these subsequent calls to aliasCheck
619 // don't need to assume that V2 is being visited recursively.
620 VisitedPHIs.erase(V2);
622 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
623 if (ThisAlias != Alias || ThisAlias == MayAlias)
630 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
631 // such as array references.
633 AliasAnalysis::AliasResult
634 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
635 const Value *V2, unsigned V2Size) {
636 // Strip off any casts if they exist.
637 V1 = V1->stripPointerCasts();
638 V2 = V2->stripPointerCasts();
640 // Are we checking for alias of the same value?
641 if (V1 == V2) return MustAlias;
643 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
644 return NoAlias; // Scalars cannot alias each other
646 // Figure out what objects these things are pointing to if we can.
647 const Value *O1 = V1->getUnderlyingObject();
648 const Value *O2 = V2->getUnderlyingObject();
651 // If V1/V2 point to two different objects we know that we have no alias.
652 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
655 // Arguments can't alias with local allocations or noalias calls.
656 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
657 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
660 // Most objects can't alias null.
661 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
662 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
666 // If the size of one access is larger than the entire object on the other
667 // side, then we know such behavior is undefined and can assume no alias.
668 LLVMContext &Context = V1->getContext();
670 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, Context, *TD)) ||
671 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, Context, *TD)))
674 // If one pointer is the result of a call/invoke and the other is a
675 // non-escaping local object, then we know the object couldn't escape to a
676 // point where the call could return it.
677 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
678 isNonEscapingLocalObject(O2) && O1 != O2)
680 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
681 isNonEscapingLocalObject(O1) && O1 != O2)
684 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
686 std::swap(V1Size, V2Size);
688 if (isa<GEPOperator>(V1))
689 return aliasGEP(V1, V1Size, V2, V2Size);
691 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
693 std::swap(V1Size, V2Size);
695 if (const PHINode *PN = dyn_cast<PHINode>(V1))
696 return aliasPHI(PN, V1Size, V2, V2Size);
698 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
700 std::swap(V1Size, V2Size);
702 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
703 return aliasSelect(S1, V1Size, V2, V2Size);
708 // This function is used to determine if the indices of two GEP instructions are
709 // equal. V1 and V2 are the indices.
710 static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) {
711 if (V1->getType() == V2->getType())
713 if (Constant *C1 = dyn_cast<Constant>(V1))
714 if (Constant *C2 = dyn_cast<Constant>(V2)) {
715 // Sign extend the constants to long types, if necessary
716 if (C1->getType() != Type::getInt64Ty(Context))
717 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context));
718 if (C2->getType() != Type::getInt64Ty(Context))
719 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context));
725 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
726 /// base pointers. This checks to see if the index expressions preclude the
727 /// pointers from aliasing...
728 AliasAnalysis::AliasResult
729 BasicAliasAnalysis::CheckGEPInstructions(
730 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
731 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
732 // We currently can't handle the case when the base pointers have different
733 // primitive types. Since this is uncommon anyway, we are happy being
734 // extremely conservative.
735 if (BasePtr1Ty != BasePtr2Ty)
738 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
740 LLVMContext &Context = GEPPointerTy->getContext();
742 // Find the (possibly empty) initial sequence of equal values... which are not
743 // necessarily constants.
744 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
745 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
746 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
747 unsigned UnequalOper = 0;
748 while (UnequalOper != MinOperands &&
749 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper],
751 // Advance through the type as we go...
753 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
754 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
756 // If all operands equal each other, then the derived pointers must
757 // alias each other...
759 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
760 "Ran out of type nesting, but not out of operands?");
765 // If we have seen all constant operands, and run out of indexes on one of the
766 // getelementptrs, check to see if the tail of the leftover one is all zeros.
767 // If so, return mustalias.
768 if (UnequalOper == MinOperands) {
769 if (NumGEP1Ops < NumGEP2Ops) {
770 std::swap(GEP1Ops, GEP2Ops);
771 std::swap(NumGEP1Ops, NumGEP2Ops);
774 bool AllAreZeros = true;
775 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
776 if (!isa<Constant>(GEP1Ops[i]) ||
777 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
781 if (AllAreZeros) return MustAlias;
785 // So now we know that the indexes derived from the base pointers,
786 // which are known to alias, are different. We can still determine a
787 // no-alias result if there are differing constant pairs in the index
788 // chain. For example:
789 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
791 // We have to be careful here about array accesses. In particular, consider:
792 // A[1][0] vs A[0][i]
793 // In this case, we don't *know* that the array will be accessed in bounds:
794 // the index could even be negative. Because of this, we have to
795 // conservatively *give up* and return may alias. We disregard differing
796 // array subscripts that are followed by a variable index without going
799 unsigned SizeMax = std::max(G1S, G2S);
800 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
802 // Scan for the first operand that is constant and unequal in the
803 // two getelementptrs...
804 unsigned FirstConstantOper = UnequalOper;
805 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
806 const Value *G1Oper = GEP1Ops[FirstConstantOper];
807 const Value *G2Oper = GEP2Ops[FirstConstantOper];
809 if (G1Oper != G2Oper) // Found non-equal constant indexes...
810 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
811 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
812 if (G1OC->getType() != G2OC->getType()) {
813 // Sign extend both operands to long.
814 if (G1OC->getType() != Type::getInt64Ty(Context))
815 G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context));
816 if (G2OC->getType() != Type::getInt64Ty(Context))
817 G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context));
818 GEP1Ops[FirstConstantOper] = G1OC;
819 GEP2Ops[FirstConstantOper] = G2OC;
823 // Handle the "be careful" case above: if this is an array/vector
824 // subscript, scan for a subsequent variable array index.
825 if (const SequentialType *STy =
826 dyn_cast<SequentialType>(BasePtr1Ty)) {
827 const Type *NextTy = STy;
828 bool isBadCase = false;
830 for (unsigned Idx = FirstConstantOper;
831 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
832 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
833 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
837 // If the array is indexed beyond the bounds of the static type
838 // at this level, it will also fall into the "be careful" case.
839 // It would theoretically be possible to analyze these cases,
840 // but for now just be conservatively correct.
841 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
842 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
843 ATy->getNumElements() ||
844 cast<ConstantInt>(G2OC)->getZExtValue() >=
845 ATy->getNumElements()) {
849 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
850 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
851 VTy->getNumElements() ||
852 cast<ConstantInt>(G2OC)->getZExtValue() >=
853 VTy->getNumElements()) {
857 STy = cast<SequentialType>(NextTy);
858 NextTy = cast<SequentialType>(NextTy)->getElementType();
861 if (isBadCase) G1OC = 0;
864 // Make sure they are comparable (ie, not constant expressions), and
865 // make sure the GEP with the smaller leading constant is GEP1.
867 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
869 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
870 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
871 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
872 std::swap(NumGEP1Ops, NumGEP2Ops);
879 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
882 // No shared constant operands, and we ran out of common operands. At this
883 // point, the GEP instructions have run through all of their operands, and we
884 // haven't found evidence that there are any deltas between the GEP's.
885 // However, one GEP may have more operands than the other. If this is the
886 // case, there may still be hope. Check this now.
887 if (FirstConstantOper == MinOperands) {
888 // Without TargetData, we won't know what the offsets are.
892 // Make GEP1Ops be the longer one if there is a longer one.
893 if (NumGEP1Ops < NumGEP2Ops) {
894 std::swap(GEP1Ops, GEP2Ops);
895 std::swap(NumGEP1Ops, NumGEP2Ops);
898 // Is there anything to check?
899 if (NumGEP1Ops > MinOperands) {
900 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
901 if (isa<ConstantInt>(GEP1Ops[i]) &&
902 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
903 // Yup, there's a constant in the tail. Set all variables to
904 // constants in the GEP instruction to make it suitable for
905 // TargetData::getIndexedOffset.
906 for (i = 0; i != MaxOperands; ++i)
907 if (!isa<ConstantInt>(GEP1Ops[i]))
908 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
909 // Okay, now get the offset. This is the relative offset for the full
911 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
914 // Now check without any constants at the end.
915 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
918 // Make sure we compare the absolute difference.
919 if (Offset1 > Offset2)
920 std::swap(Offset1, Offset2);
922 // If the tail provided a bit enough offset, return noalias!
923 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
925 // Otherwise break - we don't look for another constant in the tail.
930 // Couldn't find anything useful.
934 // If there are non-equal constants arguments, then we can figure
935 // out a minimum known delta between the two index expressions... at
936 // this point we know that the first constant index of GEP1 is less
937 // than the first constant index of GEP2.
939 // Advance BasePtr[12]Ty over this first differing constant operand.
940 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
941 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
942 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
943 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
945 // We are going to be using TargetData::getIndexedOffset to determine the
946 // offset that each of the GEP's is reaching. To do this, we have to convert
947 // all variable references to constant references. To do this, we convert the
948 // initial sequence of array subscripts into constant zeros to start with.
949 const Type *ZeroIdxTy = GEPPointerTy;
950 for (unsigned i = 0; i != FirstConstantOper; ++i) {
951 if (!isa<StructType>(ZeroIdxTy))
952 GEP1Ops[i] = GEP2Ops[i] =
953 Constant::getNullValue(Type::getInt32Ty(Context));
955 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
956 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
959 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
961 // Loop over the rest of the operands...
962 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
963 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
964 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
965 // If they are equal, use a zero index...
966 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
967 if (!isa<ConstantInt>(Op1))
968 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
969 // Otherwise, just keep the constants we have.
972 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
973 // If this is an array index, make sure the array element is in range.
974 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
975 if (Op1C->getZExtValue() >= AT->getNumElements())
976 return MayAlias; // Be conservative with out-of-range accesses
977 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
978 if (Op1C->getZExtValue() >= VT->getNumElements())
979 return MayAlias; // Be conservative with out-of-range accesses
983 // GEP1 is known to produce a value less than GEP2. To be
984 // conservatively correct, we must assume the largest possible
985 // constant is used in this position. This cannot be the initial
986 // index to the GEP instructions (because we know we have at least one
987 // element before this one with the different constant arguments), so
988 // we know that the current index must be into either a struct or
989 // array. Because we know it's not constant, this cannot be a
990 // structure index. Because of this, we can calculate the maximum
993 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
995 ConstantInt::get(Type::getInt64Ty(Context),
996 AT->getNumElements()-1);
997 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
999 ConstantInt::get(Type::getInt64Ty(Context),
1000 VT->getNumElements()-1);
1005 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
1006 // If this is an array index, make sure the array element is in range.
1007 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
1008 if (Op2C->getZExtValue() >= AT->getNumElements())
1009 return MayAlias; // Be conservative with out-of-range accesses
1010 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
1011 if (Op2C->getZExtValue() >= VT->getNumElements())
1012 return MayAlias; // Be conservative with out-of-range accesses
1014 } else { // Conservatively assume the minimum value for this index
1015 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
1020 if (BasePtr1Ty && Op1) {
1021 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
1022 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
1027 if (BasePtr2Ty && Op2) {
1028 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
1029 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
1035 if (TD && GEPPointerTy->getElementType()->isSized()) {
1037 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
1039 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
1040 assert(Offset1 != Offset2 &&
1041 "There is at least one different constant here!");
1043 // Make sure we compare the absolute difference.
1044 if (Offset1 > Offset2)
1045 std::swap(Offset1, Offset2);
1047 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
1048 //cerr << "Determined that these two GEP's don't alias ["
1049 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
1056 // Make sure that anything that uses AliasAnalysis pulls in this file...
1057 DEFINING_FILE_FOR(BasicAliasAnalysis)