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/MemoryBuiltins.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/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Target/TargetData.h"
29 #include "llvm/ADT/SmallSet.h"
30 #include "llvm/ADT/SmallVector.h"
31 #include "llvm/ADT/STLExtras.h"
32 #include "llvm/Support/ErrorHandling.h"
33 #include "llvm/Support/GetElementPtrTypeIterator.h"
37 //===----------------------------------------------------------------------===//
39 //===----------------------------------------------------------------------===//
41 static const Value *GetGEPOperands(const Value *V,
42 SmallVector<Value*, 16> &GEPOps) {
43 assert(GEPOps.empty() && "Expect empty list to populate!");
44 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
45 cast<User>(V)->op_end());
47 // Accumulate all of the chained indexes into the operand array
48 V = cast<User>(V)->getOperand(0);
50 while (const GEPOperator *G = dyn_cast<GEPOperator>(V)) {
51 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
52 !cast<Constant>(GEPOps[0])->isNullValue())
53 break; // Don't handle folding arbitrary pointer offsets yet...
54 GEPOps.erase(GEPOps.begin()); // Drop the zero index
55 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
61 /// isKnownNonNull - Return true if we know that the specified value is never
63 static bool isKnownNonNull(const Value *V) {
64 // Alloca never returns null, malloc might.
65 if (isa<AllocaInst>(V)) return true;
67 // A byval argument is never null.
68 if (const Argument *A = dyn_cast<Argument>(V))
69 return A->hasByValAttr();
71 // Global values are not null unless extern weak.
72 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
73 return !GV->hasExternalWeakLinkage();
77 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
78 /// object that never escapes from the function.
79 static bool isNonEscapingLocalObject(const Value *V) {
80 // If this is a local allocation, check to see if it escapes.
81 if (isa<AllocaInst>(V) || isNoAliasCall(V))
82 // Set StoreCaptures to True so that we can assume in our callers that the
83 // pointer is not the result of a load instruction. Currently
84 // PointerMayBeCaptured doesn't have any special analysis for the
85 // StoreCaptures=false case; if it did, our callers could be refined to be
87 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
89 // If this is an argument that corresponds to a byval or noalias argument,
90 // then it has not escaped before entering the function. Check if it escapes
91 // inside the function.
92 if (const Argument *A = dyn_cast<Argument>(V))
93 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
94 // Don't bother analyzing arguments already known not to escape.
95 if (A->hasNoCaptureAttr())
97 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
103 /// isObjectSmallerThan - Return true if we can prove that the object specified
104 /// by V is smaller than Size.
105 static bool isObjectSmallerThan(const Value *V, unsigned Size,
106 const TargetData &TD) {
107 const Type *AccessTy;
108 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
109 AccessTy = GV->getType()->getElementType();
110 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
111 if (!AI->isArrayAllocation())
112 AccessTy = AI->getType()->getElementType();
115 } else if (const CallInst* CI = extractMallocCall(V)) {
116 if (!isArrayMalloc(V, &TD))
117 // The size is the argument to the malloc call.
118 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
119 return (C->getZExtValue() < Size);
121 } else if (const Argument *A = dyn_cast<Argument>(V)) {
122 if (A->hasByValAttr())
123 AccessTy = cast<PointerType>(A->getType())->getElementType();
130 if (AccessTy->isSized())
131 return TD.getTypeAllocSize(AccessTy) < Size;
135 //===----------------------------------------------------------------------===//
137 //===----------------------------------------------------------------------===//
140 /// NoAA - This class implements the -no-aa pass, which always returns "I
141 /// don't know" for alias queries. NoAA is unlike other alias analysis
142 /// implementations, in that it does not chain to a previous analysis. As
143 /// such it doesn't follow many of the rules that other alias analyses must.
145 struct NoAA : public ImmutablePass, public AliasAnalysis {
146 static char ID; // Class identification, replacement for typeinfo
147 NoAA() : ImmutablePass(&ID) {}
148 explicit NoAA(void *PID) : ImmutablePass(PID) { }
150 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
153 virtual void initializePass() {
154 TD = getAnalysisIfAvailable<TargetData>();
157 virtual AliasResult alias(const Value *V1, unsigned V1Size,
158 const Value *V2, unsigned V2Size) {
162 virtual void getArgumentAccesses(Function *F, CallSite CS,
163 std::vector<PointerAccessInfo> &Info) {
164 llvm_unreachable("This method may not be called on this function!");
167 virtual bool pointsToConstantMemory(const Value *P) { return false; }
168 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
171 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
175 virtual void deleteValue(Value *V) {}
176 virtual void copyValue(Value *From, Value *To) {}
178 } // End of anonymous namespace
180 // Register this pass...
182 static RegisterPass<NoAA>
183 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
185 // Declare that we implement the AliasAnalysis interface
186 static RegisterAnalysisGroup<AliasAnalysis> V(U);
188 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
190 //===----------------------------------------------------------------------===//
192 //===----------------------------------------------------------------------===//
195 /// BasicAliasAnalysis - This is the default alias analysis implementation.
196 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
197 /// derives from the NoAA class.
198 struct BasicAliasAnalysis : public NoAA {
199 static char ID; // Class identification, replacement for typeinfo
200 BasicAliasAnalysis() : NoAA(&ID) {}
201 AliasResult alias(const Value *V1, unsigned V1Size,
202 const Value *V2, unsigned V2Size) {
203 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!");
204 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
209 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
210 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
212 /// pointsToConstantMemory - Chase pointers until we find a (constant
214 bool pointsToConstantMemory(const Value *P);
217 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call.
218 SmallPtrSet<const Value*, 16> VisitedPHIs;
220 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
222 AliasResult aliasGEP(const Value *V1, unsigned V1Size,
223 const Value *V2, unsigned V2Size);
225 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
227 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
228 const Value *V2, unsigned V2Size);
230 /// aliasSelect - Disambiguate a Select instruction against another value.
231 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize,
232 const Value *V2, unsigned V2Size);
234 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
235 const Value *V2, unsigned V2Size);
237 // CheckGEPInstructions - Check two GEP instructions with known
238 // must-aliasing base pointers. This checks to see if the index expressions
239 // preclude the pointers from aliasing...
241 CheckGEPInstructions(const Type* BasePtr1Ty,
242 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
243 const Type *BasePtr2Ty,
244 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
246 } // End of anonymous namespace
248 // Register this pass...
249 char BasicAliasAnalysis::ID = 0;
250 static RegisterPass<BasicAliasAnalysis>
251 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
253 // Declare that we implement the AliasAnalysis interface
254 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
256 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
257 return new BasicAliasAnalysis();
261 /// pointsToConstantMemory - Chase pointers until we find a (constant
263 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
264 if (const GlobalVariable *GV =
265 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
266 // FIXME: shouldn't this require GV to be "ODR"?
267 return GV->isConstant();
272 // getModRefInfo - Check to see if the specified callsite can clobber the
273 // specified memory object. Since we only look at local properties of this
274 // function, we really can't say much about this query. We do, however, use
275 // simple "address taken" analysis on local objects.
277 AliasAnalysis::ModRefResult
278 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
279 const Value *Object = P->getUnderlyingObject();
281 // If this is a tail call and P points to a stack location, we know that
282 // the tail call cannot access or modify the local stack.
283 // We cannot exclude byval arguments here; these belong to the caller of
284 // the current function not to the current function, and a tail callee
285 // may reference them.
286 if (isa<AllocaInst>(Object))
287 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
288 if (CI->isTailCall())
291 // If the pointer is to a locally allocated object that does not escape,
292 // then the call can not mod/ref the pointer unless the call takes the
293 // argument without capturing it.
294 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
295 bool passedAsArg = false;
296 // TODO: Eventually only check 'nocapture' arguments.
297 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
299 if (isa<PointerType>((*CI)->getType()) &&
300 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
307 // Finally, handle specific knowledge of intrinsics.
308 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
310 return AliasAnalysis::getModRefInfo(CS, P, Size);
312 switch (II->getIntrinsicID()) {
314 case Intrinsic::memcpy:
315 case Intrinsic::memmove: {
317 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
318 Len = LenCI->getZExtValue();
319 Value *Dest = II->getOperand(1);
320 Value *Src = II->getOperand(2);
321 if (alias(Dest, Len, P, Size) == NoAlias) {
322 if (alias(Src, Len, P, Size) == NoAlias)
328 case Intrinsic::memset:
329 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
330 unsigned Len = LenCI->getZExtValue();
331 Value *Dest = II->getOperand(1);
332 if (alias(Dest, Len, P, Size) == NoAlias)
336 case Intrinsic::atomic_cmp_swap:
337 case Intrinsic::atomic_swap:
338 case Intrinsic::atomic_load_add:
339 case Intrinsic::atomic_load_sub:
340 case Intrinsic::atomic_load_and:
341 case Intrinsic::atomic_load_nand:
342 case Intrinsic::atomic_load_or:
343 case Intrinsic::atomic_load_xor:
344 case Intrinsic::atomic_load_max:
345 case Intrinsic::atomic_load_min:
346 case Intrinsic::atomic_load_umax:
347 case Intrinsic::atomic_load_umin:
349 Value *Op1 = II->getOperand(1);
350 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
351 if (alias(Op1, Op1Size, P, Size) == NoAlias)
355 case Intrinsic::lifetime_start:
356 case Intrinsic::lifetime_end:
357 case Intrinsic::invariant_start: {
358 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
359 if (alias(II->getOperand(2), PtrSize, P, Size) == NoAlias)
363 case Intrinsic::invariant_end: {
364 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
365 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();
650 // Null values in the default address space don't point to any object, so they
651 // don't alias any other pointer.
652 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
653 if (CPN->getType()->getAddressSpace() == 0)
655 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
656 if (CPN->getType()->getAddressSpace() == 0)
660 // If V1/V2 point to two different objects we know that we have no alias.
661 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
664 // Constant pointers can't alias with non-const isIdentifiedObject objects.
665 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
666 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
669 // Arguments can't alias with local allocations or noalias calls.
670 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
671 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
674 // Most objects can't alias null.
675 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
676 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
680 // If the size of one access is larger than the entire object on the other
681 // side, then we know such behavior is undefined and can assume no alias.
683 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
684 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
687 // If one pointer is the result of a call/invoke or load and the other is a
688 // non-escaping local object, then we know the object couldn't escape to a
689 // point where the call could return it. The load case works because
690 // isNonEscapingLocalObject considers all stores to be escapes (it
691 // passes true for the StoreCaptures argument to PointerMayBeCaptured).
693 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1)) &&
694 isNonEscapingLocalObject(O2))
696 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2)) &&
697 isNonEscapingLocalObject(O1))
701 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
703 std::swap(V1Size, V2Size);
705 if (isa<GEPOperator>(V1))
706 return aliasGEP(V1, V1Size, V2, V2Size);
708 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
710 std::swap(V1Size, V2Size);
712 if (const PHINode *PN = dyn_cast<PHINode>(V1))
713 return aliasPHI(PN, V1Size, V2, V2Size);
715 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
717 std::swap(V1Size, V2Size);
719 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
720 return aliasSelect(S1, V1Size, V2, V2Size);
725 // This function is used to determine if the indices of two GEP instructions are
726 // equal. V1 and V2 are the indices.
727 static bool IndexOperandsEqual(Value *V1, Value *V2) {
728 if (V1->getType() == V2->getType())
730 if (Constant *C1 = dyn_cast<Constant>(V1))
731 if (Constant *C2 = dyn_cast<Constant>(V2)) {
732 // Sign extend the constants to long types, if necessary
733 if (C1->getType() != Type::getInt64Ty(C1->getContext()))
734 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(C1->getContext()));
735 if (C2->getType() != Type::getInt64Ty(C1->getContext()))
736 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(C1->getContext()));
742 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
743 /// base pointers. This checks to see if the index expressions preclude the
744 /// pointers from aliasing...
745 AliasAnalysis::AliasResult
746 BasicAliasAnalysis::CheckGEPInstructions(
747 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
748 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
749 // We currently can't handle the case when the base pointers have different
750 // primitive types. Since this is uncommon anyway, we are happy being
751 // extremely conservative.
752 if (BasePtr1Ty != BasePtr2Ty)
755 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
757 // Find the (possibly empty) initial sequence of equal values... which are not
758 // necessarily constants.
759 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
760 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
761 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
762 unsigned UnequalOper = 0;
763 while (UnequalOper != MinOperands &&
764 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
765 // Advance through the type as we go...
767 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
768 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
770 // If all operands equal each other, then the derived pointers must
771 // alias each other...
773 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
774 "Ran out of type nesting, but not out of operands?");
779 // If we have seen all constant operands, and run out of indexes on one of the
780 // getelementptrs, check to see if the tail of the leftover one is all zeros.
781 // If so, return mustalias.
782 if (UnequalOper == MinOperands) {
783 if (NumGEP1Ops < NumGEP2Ops) {
784 std::swap(GEP1Ops, GEP2Ops);
785 std::swap(NumGEP1Ops, NumGEP2Ops);
788 bool AllAreZeros = true;
789 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
790 if (!isa<Constant>(GEP1Ops[i]) ||
791 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
795 if (AllAreZeros) return MustAlias;
799 // So now we know that the indexes derived from the base pointers,
800 // which are known to alias, are different. We can still determine a
801 // no-alias result if there are differing constant pairs in the index
802 // chain. For example:
803 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
805 // We have to be careful here about array accesses. In particular, consider:
806 // A[1][0] vs A[0][i]
807 // In this case, we don't *know* that the array will be accessed in bounds:
808 // the index could even be negative. Because of this, we have to
809 // conservatively *give up* and return may alias. We disregard differing
810 // array subscripts that are followed by a variable index without going
813 unsigned SizeMax = std::max(G1S, G2S);
814 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
816 // Scan for the first operand that is constant and unequal in the
817 // two getelementptrs...
818 unsigned FirstConstantOper = UnequalOper;
819 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
820 const Value *G1Oper = GEP1Ops[FirstConstantOper];
821 const Value *G2Oper = GEP2Ops[FirstConstantOper];
823 if (G1Oper != G2Oper) // Found non-equal constant indexes...
824 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
825 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
826 if (G1OC->getType() != G2OC->getType()) {
827 // Sign extend both operands to long.
828 const Type *Int64Ty = Type::getInt64Ty(G1OC->getContext());
829 if (G1OC->getType() != Int64Ty)
830 G1OC = ConstantExpr::getSExt(G1OC, Int64Ty);
831 if (G2OC->getType() != Int64Ty)
832 G2OC = ConstantExpr::getSExt(G2OC, Int64Ty);
833 GEP1Ops[FirstConstantOper] = G1OC;
834 GEP2Ops[FirstConstantOper] = G2OC;
838 // Handle the "be careful" case above: if this is an array/vector
839 // subscript, scan for a subsequent variable array index.
840 if (const SequentialType *STy =
841 dyn_cast<SequentialType>(BasePtr1Ty)) {
842 const Type *NextTy = STy;
843 bool isBadCase = false;
845 for (unsigned Idx = FirstConstantOper;
846 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
847 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
848 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
852 // If the array is indexed beyond the bounds of the static type
853 // at this level, it will also fall into the "be careful" case.
854 // It would theoretically be possible to analyze these cases,
855 // but for now just be conservatively correct.
856 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
857 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
858 ATy->getNumElements() ||
859 cast<ConstantInt>(G2OC)->getZExtValue() >=
860 ATy->getNumElements()) {
864 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
865 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
866 VTy->getNumElements() ||
867 cast<ConstantInt>(G2OC)->getZExtValue() >=
868 VTy->getNumElements()) {
872 STy = cast<SequentialType>(NextTy);
873 NextTy = cast<SequentialType>(NextTy)->getElementType();
876 if (isBadCase) G1OC = 0;
879 // Make sure they are comparable (ie, not constant expressions), and
880 // make sure the GEP with the smaller leading constant is GEP1.
882 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
884 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
885 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
886 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
887 std::swap(NumGEP1Ops, NumGEP2Ops);
894 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
897 // No shared constant operands, and we ran out of common operands. At this
898 // point, the GEP instructions have run through all of their operands, and we
899 // haven't found evidence that there are any deltas between the GEP's.
900 // However, one GEP may have more operands than the other. If this is the
901 // case, there may still be hope. Check this now.
902 if (FirstConstantOper == MinOperands) {
903 // Without TargetData, we won't know what the offsets are.
907 // Make GEP1Ops be the longer one if there is a longer one.
908 if (NumGEP1Ops < NumGEP2Ops) {
909 std::swap(GEP1Ops, GEP2Ops);
910 std::swap(NumGEP1Ops, NumGEP2Ops);
913 // Is there anything to check?
914 if (NumGEP1Ops > MinOperands) {
915 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
916 if (isa<ConstantInt>(GEP1Ops[i]) &&
917 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
918 // Yup, there's a constant in the tail. Set all variables to
919 // constants in the GEP instruction to make it suitable for
920 // TargetData::getIndexedOffset.
921 for (i = 0; i != MaxOperands; ++i)
922 if (!isa<ConstantInt>(GEP1Ops[i]))
923 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
924 // Okay, now get the offset. This is the relative offset for the full
926 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
929 // Now check without any constants at the end.
930 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
933 // Make sure we compare the absolute difference.
934 if (Offset1 > Offset2)
935 std::swap(Offset1, Offset2);
937 // If the tail provided a bit enough offset, return noalias!
938 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
940 // Otherwise break - we don't look for another constant in the tail.
945 // Couldn't find anything useful.
949 // If there are non-equal constants arguments, then we can figure
950 // out a minimum known delta between the two index expressions... at
951 // this point we know that the first constant index of GEP1 is less
952 // than the first constant index of GEP2.
954 // Advance BasePtr[12]Ty over this first differing constant operand.
955 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
956 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
957 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
958 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
960 // We are going to be using TargetData::getIndexedOffset to determine the
961 // offset that each of the GEP's is reaching. To do this, we have to convert
962 // all variable references to constant references. To do this, we convert the
963 // initial sequence of array subscripts into constant zeros to start with.
964 const Type *ZeroIdxTy = GEPPointerTy;
965 for (unsigned i = 0; i != FirstConstantOper; ++i) {
966 if (!isa<StructType>(ZeroIdxTy))
967 GEP1Ops[i] = GEP2Ops[i] =
968 Constant::getNullValue(Type::getInt32Ty(ZeroIdxTy->getContext()));
970 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
971 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
974 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
976 // Loop over the rest of the operands...
977 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
978 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
979 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
980 // If they are equal, use a zero index...
981 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
982 if (!isa<ConstantInt>(Op1))
983 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
984 // Otherwise, just keep the constants we have.
987 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
988 // If this is an array index, make sure the array element is in range.
989 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
990 if (Op1C->getZExtValue() >= AT->getNumElements())
991 return MayAlias; // Be conservative with out-of-range accesses
992 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
993 if (Op1C->getZExtValue() >= VT->getNumElements())
994 return MayAlias; // Be conservative with out-of-range accesses
998 // GEP1 is known to produce a value less than GEP2. To be
999 // conservatively correct, we must assume the largest possible
1000 // constant is used in this position. This cannot be the initial
1001 // index to the GEP instructions (because we know we have at least one
1002 // element before this one with the different constant arguments), so
1003 // we know that the current index must be into either a struct or
1004 // array. Because we know it's not constant, this cannot be a
1005 // structure index. Because of this, we can calculate the maximum
1008 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
1010 ConstantInt::get(Type::getInt64Ty(AT->getContext()),
1011 AT->getNumElements()-1);
1012 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
1014 ConstantInt::get(Type::getInt64Ty(VT->getContext()),
1015 VT->getNumElements()-1);
1020 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
1021 // If this is an array index, make sure the array element is in range.
1022 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
1023 if (Op2C->getZExtValue() >= AT->getNumElements())
1024 return MayAlias; // Be conservative with out-of-range accesses
1025 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
1026 if (Op2C->getZExtValue() >= VT->getNumElements())
1027 return MayAlias; // Be conservative with out-of-range accesses
1029 } else { // Conservatively assume the minimum value for this index
1030 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
1035 if (BasePtr1Ty && Op1) {
1036 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
1037 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
1042 if (BasePtr2Ty && Op2) {
1043 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
1044 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
1050 if (TD && GEPPointerTy->getElementType()->isSized()) {
1052 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
1054 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
1055 assert(Offset1 != Offset2 &&
1056 "There is at least one different constant here!");
1058 // Make sure we compare the absolute difference.
1059 if (Offset1 > Offset2)
1060 std::swap(Offset1, Offset2);
1062 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
1063 //cerr << "Determined that these two GEP's don't alias ["
1064 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
1071 // Make sure that anything that uses AliasAnalysis pulls in this file...
1072 DEFINING_FILE_FOR(BasicAliasAnalysis)