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 pointer
293 // as an argument, and itself doesn't capture it.
294 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
295 bool PassedAsArg = false;
297 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
298 CI != CE; ++CI, ++ArgNo) {
299 // Only look at the no-capture pointer arguments.
300 if (!isa<PointerType>((*CI)->getType()) ||
301 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
304 // If this is a no-capture pointer argument, see if we can tell that it
305 // is impossible to alias the pointer we're checking. If not, we have to
306 // assume that the call could touch the pointer, even though it doesn't
308 if (alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) {
318 // Finally, handle specific knowledge of intrinsics.
319 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
321 return AliasAnalysis::getModRefInfo(CS, P, Size);
323 switch (II->getIntrinsicID()) {
325 case Intrinsic::memcpy:
326 case Intrinsic::memmove: {
328 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
329 Len = LenCI->getZExtValue();
330 Value *Dest = II->getOperand(1);
331 Value *Src = II->getOperand(2);
332 if (alias(Dest, Len, P, Size) == NoAlias) {
333 if (alias(Src, Len, P, Size) == NoAlias)
339 case Intrinsic::memset:
340 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
341 unsigned Len = LenCI->getZExtValue();
342 Value *Dest = II->getOperand(1);
343 if (alias(Dest, Len, P, Size) == NoAlias)
347 case Intrinsic::atomic_cmp_swap:
348 case Intrinsic::atomic_swap:
349 case Intrinsic::atomic_load_add:
350 case Intrinsic::atomic_load_sub:
351 case Intrinsic::atomic_load_and:
352 case Intrinsic::atomic_load_nand:
353 case Intrinsic::atomic_load_or:
354 case Intrinsic::atomic_load_xor:
355 case Intrinsic::atomic_load_max:
356 case Intrinsic::atomic_load_min:
357 case Intrinsic::atomic_load_umax:
358 case Intrinsic::atomic_load_umin:
360 Value *Op1 = II->getOperand(1);
361 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
362 if (alias(Op1, Op1Size, P, Size) == NoAlias)
366 case Intrinsic::lifetime_start:
367 case Intrinsic::lifetime_end:
368 case Intrinsic::invariant_start: {
369 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
370 if (alias(II->getOperand(2), PtrSize, P, Size) == NoAlias)
374 case Intrinsic::invariant_end: {
375 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
376 if (alias(II->getOperand(3), PtrSize, P, Size) == NoAlias)
382 // The AliasAnalysis base class has some smarts, lets use them.
383 return AliasAnalysis::getModRefInfo(CS, P, Size);
387 AliasAnalysis::ModRefResult
388 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
389 // If CS1 or CS2 are readnone, they don't interact.
390 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
391 if (CS1B == DoesNotAccessMemory) return NoModRef;
393 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
394 if (CS2B == DoesNotAccessMemory) return NoModRef;
396 // If they both only read from memory, just return ref.
397 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
400 // Otherwise, fall back to NoAA (mod+ref).
401 return NoAA::getModRefInfo(CS1, CS2);
404 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
407 AliasAnalysis::AliasResult
408 BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size,
409 const Value *V2, unsigned V2Size) {
410 // If we have two gep instructions with must-alias'ing base pointers, figure
411 // out if the indexes to the GEP tell us anything about the derived pointer.
412 // Note that we also handle chains of getelementptr instructions as well as
413 // constant expression getelementptrs here.
415 if (isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
416 const User *GEP1 = cast<User>(V1);
417 const User *GEP2 = cast<User>(V2);
419 // If V1 and V2 are identical GEPs, just recurse down on both of them.
420 // This allows us to analyze things like:
421 // P = gep A, 0, i, 1
422 // Q = gep B, 0, i, 1
423 // by just analyzing A and B. This is even safe for variable indices.
424 if (GEP1->getType() == GEP2->getType() &&
425 GEP1->getNumOperands() == GEP2->getNumOperands() &&
426 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
427 // All operands are the same, ignoring the base.
428 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
429 return aliasCheck(GEP1->getOperand(0), V1Size,
430 GEP2->getOperand(0), V2Size);
432 // Drill down into the first non-gep value, to test for must-aliasing of
433 // the base pointers.
434 while (isa<GEPOperator>(GEP1->getOperand(0)) &&
435 GEP1->getOperand(1) ==
436 Constant::getNullValue(GEP1->getOperand(1)->getType()))
437 GEP1 = cast<User>(GEP1->getOperand(0));
438 const Value *BasePtr1 = GEP1->getOperand(0);
440 while (isa<GEPOperator>(GEP2->getOperand(0)) &&
441 GEP2->getOperand(1) ==
442 Constant::getNullValue(GEP2->getOperand(1)->getType()))
443 GEP2 = cast<User>(GEP2->getOperand(0));
444 const Value *BasePtr2 = GEP2->getOperand(0);
446 // Do the base pointers alias?
447 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U);
448 if (BaseAlias == NoAlias) return NoAlias;
449 if (BaseAlias == MustAlias) {
450 // If the base pointers alias each other exactly, check to see if we can
451 // figure out anything about the resultant pointers, to try to prove
454 // Collect all of the chained GEP operands together into one simple place
455 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
456 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
457 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
459 // If GetGEPOperands were able to fold to the same must-aliased pointer,
460 // do the comparison.
461 if (BasePtr1 == BasePtr2) {
463 CheckGEPInstructions(BasePtr1->getType(),
464 &GEP1Ops[0], GEP1Ops.size(), V1Size,
466 &GEP2Ops[0], GEP2Ops.size(), V2Size);
467 if (GAlias != MayAlias)
473 // Check to see if these two pointers are related by a getelementptr
474 // instruction. If one pointer is a GEP with a non-zero index of the other
475 // pointer, we know they cannot alias.
477 if (V1Size == ~0U || V2Size == ~0U)
480 SmallVector<Value*, 16> GEPOperands;
481 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
483 AliasResult R = aliasCheck(BasePtr, ~0U, V2, V2Size);
485 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
486 // If V2 is known not to alias GEP base pointer, then the two values
487 // cannot alias per GEP semantics: "A pointer value formed from a
488 // getelementptr instruction is associated with the addresses associated
489 // with the first operand of the getelementptr".
492 // If there is at least one non-zero constant index, we know they cannot
494 bool ConstantFound = false;
495 bool AllZerosFound = true;
496 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
497 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
498 if (!C->isNullValue()) {
499 ConstantFound = true;
500 AllZerosFound = false;
504 AllZerosFound = false;
507 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
508 // the ptr, the end result is a must alias also.
513 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
516 // Otherwise we have to check to see that the distance is more than
517 // the size of the argument... build an index vector that is equal to
518 // the arguments provided, except substitute 0's for any variable
519 // indexes we find...
521 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
522 for (unsigned i = 0; i != GEPOperands.size(); ++i)
523 if (!isa<ConstantInt>(GEPOperands[i]))
524 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
525 int64_t Offset = TD->getIndexedOffset(BasePtr->getType(),
529 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
537 // aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select instruction
539 AliasAnalysis::AliasResult
540 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
541 const Value *V2, unsigned V2Size) {
542 // If the values are Selects with the same condition, we can do a more precise
543 // check: just check for aliases between the values on corresponding arms.
544 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
545 if (SI->getCondition() == SI2->getCondition()) {
547 aliasCheck(SI->getTrueValue(), SISize,
548 SI2->getTrueValue(), V2Size);
549 if (Alias == MayAlias)
551 AliasResult ThisAlias =
552 aliasCheck(SI->getFalseValue(), SISize,
553 SI2->getFalseValue(), V2Size);
554 if (ThisAlias != Alias)
559 // If both arms of the Select node NoAlias or MustAlias V2, then returns
560 // NoAlias / MustAlias. Otherwise, returns MayAlias.
562 aliasCheck(SI->getTrueValue(), SISize, V2, V2Size);
563 if (Alias == MayAlias)
565 AliasResult ThisAlias =
566 aliasCheck(SI->getFalseValue(), SISize, V2, V2Size);
567 if (ThisAlias != Alias)
572 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
574 AliasAnalysis::AliasResult
575 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
576 const Value *V2, unsigned V2Size) {
577 // The PHI node has already been visited, avoid recursion any further.
578 if (!VisitedPHIs.insert(PN))
581 // If the values are PHIs in the same block, we can do a more precise
582 // as well as efficient check: just check for aliases between the values
583 // on corresponding edges.
584 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
585 if (PN2->getParent() == PN->getParent()) {
587 aliasCheck(PN->getIncomingValue(0), PNSize,
588 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
590 if (Alias == MayAlias)
592 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
593 AliasResult ThisAlias =
594 aliasCheck(PN->getIncomingValue(i), PNSize,
595 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
597 if (ThisAlias != Alias)
603 SmallPtrSet<Value*, 4> UniqueSrc;
604 SmallVector<Value*, 4> V1Srcs;
605 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
606 Value *PV1 = PN->getIncomingValue(i);
607 if (isa<PHINode>(PV1))
608 // If any of the source itself is a PHI, return MayAlias conservatively
609 // to avoid compile time explosion. The worst possible case is if both
610 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
611 // and 'n' are the number of PHI sources.
613 if (UniqueSrc.insert(PV1))
614 V1Srcs.push_back(PV1);
617 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
618 // Early exit if the check of the first PHI source against V2 is MayAlias.
619 // Other results are not possible.
620 if (Alias == MayAlias)
623 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
624 // NoAlias / MustAlias. Otherwise, returns MayAlias.
625 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
626 Value *V = V1Srcs[i];
628 // If V2 is a PHI, the recursive case will have been caught in the
629 // above aliasCheck call, so these subsequent calls to aliasCheck
630 // don't need to assume that V2 is being visited recursively.
631 VisitedPHIs.erase(V2);
633 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
634 if (ThisAlias != Alias || ThisAlias == MayAlias)
641 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
642 // such as array references.
644 AliasAnalysis::AliasResult
645 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
646 const Value *V2, unsigned V2Size) {
647 // Strip off any casts if they exist.
648 V1 = V1->stripPointerCasts();
649 V2 = V2->stripPointerCasts();
651 // Are we checking for alias of the same value?
652 if (V1 == V2) return MustAlias;
654 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
655 return NoAlias; // Scalars cannot alias each other
657 // Figure out what objects these things are pointing to if we can.
658 const Value *O1 = V1->getUnderlyingObject();
659 const Value *O2 = V2->getUnderlyingObject();
661 // Null values in the default address space don't point to any object, so they
662 // don't alias any other pointer.
663 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
664 if (CPN->getType()->getAddressSpace() == 0)
666 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
667 if (CPN->getType()->getAddressSpace() == 0)
671 // If V1/V2 point to two different objects we know that we have no alias.
672 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
675 // Constant pointers can't alias with non-const isIdentifiedObject objects.
676 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
677 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
680 // Arguments can't alias with local allocations or noalias calls.
681 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
682 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
685 // Most objects can't alias null.
686 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
687 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
691 // If the size of one access is larger than the entire object on the other
692 // side, then we know such behavior is undefined and can assume no alias.
694 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
695 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
698 // If one pointer is the result of a call/invoke or load and the other is a
699 // non-escaping local object, then we know the object couldn't escape to a
700 // point where the call could return it. The load case works because
701 // isNonEscapingLocalObject considers all stores to be escapes (it
702 // passes true for the StoreCaptures argument to PointerMayBeCaptured).
704 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1)) &&
705 isNonEscapingLocalObject(O2))
707 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2)) &&
708 isNonEscapingLocalObject(O1))
712 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
714 std::swap(V1Size, V2Size);
716 if (isa<GEPOperator>(V1))
717 return aliasGEP(V1, V1Size, V2, V2Size);
719 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
721 std::swap(V1Size, V2Size);
723 if (const PHINode *PN = dyn_cast<PHINode>(V1))
724 return aliasPHI(PN, V1Size, V2, V2Size);
726 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
728 std::swap(V1Size, V2Size);
730 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
731 return aliasSelect(S1, V1Size, V2, V2Size);
736 // This function is used to determine if the indices of two GEP instructions are
737 // equal. V1 and V2 are the indices.
738 static bool IndexOperandsEqual(Value *V1, Value *V2) {
739 if (V1->getType() == V2->getType())
741 if (Constant *C1 = dyn_cast<Constant>(V1))
742 if (Constant *C2 = dyn_cast<Constant>(V2)) {
743 // Sign extend the constants to long types, if necessary
744 if (C1->getType() != Type::getInt64Ty(C1->getContext()))
745 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(C1->getContext()));
746 if (C2->getType() != Type::getInt64Ty(C1->getContext()))
747 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(C1->getContext()));
753 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
754 /// base pointers. This checks to see if the index expressions preclude the
755 /// pointers from aliasing...
756 AliasAnalysis::AliasResult
757 BasicAliasAnalysis::CheckGEPInstructions(
758 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
759 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
760 // We currently can't handle the case when the base pointers have different
761 // primitive types. Since this is uncommon anyway, we are happy being
762 // extremely conservative.
763 if (BasePtr1Ty != BasePtr2Ty)
766 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
768 // Find the (possibly empty) initial sequence of equal values... which are not
769 // necessarily constants.
770 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
771 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
772 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
773 unsigned UnequalOper = 0;
774 while (UnequalOper != MinOperands &&
775 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
776 // Advance through the type as we go...
778 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
779 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
781 // If all operands equal each other, then the derived pointers must
782 // alias each other...
784 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
785 "Ran out of type nesting, but not out of operands?");
790 // If we have seen all constant operands, and run out of indexes on one of the
791 // getelementptrs, check to see if the tail of the leftover one is all zeros.
792 // If so, return mustalias.
793 if (UnequalOper == MinOperands) {
794 if (NumGEP1Ops < NumGEP2Ops) {
795 std::swap(GEP1Ops, GEP2Ops);
796 std::swap(NumGEP1Ops, NumGEP2Ops);
799 bool AllAreZeros = true;
800 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
801 if (!isa<Constant>(GEP1Ops[i]) ||
802 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
806 if (AllAreZeros) return MustAlias;
810 // So now we know that the indexes derived from the base pointers,
811 // which are known to alias, are different. We can still determine a
812 // no-alias result if there are differing constant pairs in the index
813 // chain. For example:
814 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
816 // We have to be careful here about array accesses. In particular, consider:
817 // A[1][0] vs A[0][i]
818 // In this case, we don't *know* that the array will be accessed in bounds:
819 // the index could even be negative. Because of this, we have to
820 // conservatively *give up* and return may alias. We disregard differing
821 // array subscripts that are followed by a variable index without going
824 unsigned SizeMax = std::max(G1S, G2S);
825 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
827 // Scan for the first operand that is constant and unequal in the
828 // two getelementptrs...
829 unsigned FirstConstantOper = UnequalOper;
830 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
831 const Value *G1Oper = GEP1Ops[FirstConstantOper];
832 const Value *G2Oper = GEP2Ops[FirstConstantOper];
834 if (G1Oper != G2Oper) // Found non-equal constant indexes...
835 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
836 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
837 if (G1OC->getType() != G2OC->getType()) {
838 // Sign extend both operands to long.
839 const Type *Int64Ty = Type::getInt64Ty(G1OC->getContext());
840 if (G1OC->getType() != Int64Ty)
841 G1OC = ConstantExpr::getSExt(G1OC, Int64Ty);
842 if (G2OC->getType() != Int64Ty)
843 G2OC = ConstantExpr::getSExt(G2OC, Int64Ty);
844 GEP1Ops[FirstConstantOper] = G1OC;
845 GEP2Ops[FirstConstantOper] = G2OC;
849 // Handle the "be careful" case above: if this is an array/vector
850 // subscript, scan for a subsequent variable array index.
851 if (const SequentialType *STy =
852 dyn_cast<SequentialType>(BasePtr1Ty)) {
853 const Type *NextTy = STy;
854 bool isBadCase = false;
856 for (unsigned Idx = FirstConstantOper;
857 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
858 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
859 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
863 // If the array is indexed beyond the bounds of the static type
864 // at this level, it will also fall into the "be careful" case.
865 // It would theoretically be possible to analyze these cases,
866 // but for now just be conservatively correct.
867 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
868 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
869 ATy->getNumElements() ||
870 cast<ConstantInt>(G2OC)->getZExtValue() >=
871 ATy->getNumElements()) {
875 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
876 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
877 VTy->getNumElements() ||
878 cast<ConstantInt>(G2OC)->getZExtValue() >=
879 VTy->getNumElements()) {
883 STy = cast<SequentialType>(NextTy);
884 NextTy = cast<SequentialType>(NextTy)->getElementType();
887 if (isBadCase) G1OC = 0;
890 // Make sure they are comparable (ie, not constant expressions), and
891 // make sure the GEP with the smaller leading constant is GEP1.
893 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
895 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
896 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
897 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
898 std::swap(NumGEP1Ops, NumGEP2Ops);
905 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
908 // No shared constant operands, and we ran out of common operands. At this
909 // point, the GEP instructions have run through all of their operands, and we
910 // haven't found evidence that there are any deltas between the GEP's.
911 // However, one GEP may have more operands than the other. If this is the
912 // case, there may still be hope. Check this now.
913 if (FirstConstantOper == MinOperands) {
914 // Without TargetData, we won't know what the offsets are.
918 // Make GEP1Ops be the longer one if there is a longer one.
919 if (NumGEP1Ops < NumGEP2Ops) {
920 std::swap(GEP1Ops, GEP2Ops);
921 std::swap(NumGEP1Ops, NumGEP2Ops);
924 // Is there anything to check?
925 if (NumGEP1Ops > MinOperands) {
926 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
927 if (isa<ConstantInt>(GEP1Ops[i]) &&
928 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
929 // Yup, there's a constant in the tail. Set all variables to
930 // constants in the GEP instruction to make it suitable for
931 // TargetData::getIndexedOffset.
932 for (i = 0; i != MaxOperands; ++i)
933 if (!isa<ConstantInt>(GEP1Ops[i]))
934 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
935 // Okay, now get the offset. This is the relative offset for the full
937 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
940 // Now check without any constants at the end.
941 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
944 // Make sure we compare the absolute difference.
945 if (Offset1 > Offset2)
946 std::swap(Offset1, Offset2);
948 // If the tail provided a bit enough offset, return noalias!
949 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
951 // Otherwise break - we don't look for another constant in the tail.
956 // Couldn't find anything useful.
960 // If there are non-equal constants arguments, then we can figure
961 // out a minimum known delta between the two index expressions... at
962 // this point we know that the first constant index of GEP1 is less
963 // than the first constant index of GEP2.
965 // Advance BasePtr[12]Ty over this first differing constant operand.
966 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
967 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
968 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
969 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
971 // We are going to be using TargetData::getIndexedOffset to determine the
972 // offset that each of the GEP's is reaching. To do this, we have to convert
973 // all variable references to constant references. To do this, we convert the
974 // initial sequence of array subscripts into constant zeros to start with.
975 const Type *ZeroIdxTy = GEPPointerTy;
976 for (unsigned i = 0; i != FirstConstantOper; ++i) {
977 if (!isa<StructType>(ZeroIdxTy))
978 GEP1Ops[i] = GEP2Ops[i] =
979 Constant::getNullValue(Type::getInt32Ty(ZeroIdxTy->getContext()));
981 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
982 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
985 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
987 // Loop over the rest of the operands...
988 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
989 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
990 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
991 // If they are equal, use a zero index...
992 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
993 if (!isa<ConstantInt>(Op1))
994 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
995 // Otherwise, just keep the constants we have.
998 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
999 // If this is an array index, make sure the array element is in range.
1000 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
1001 if (Op1C->getZExtValue() >= AT->getNumElements())
1002 return MayAlias; // Be conservative with out-of-range accesses
1003 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
1004 if (Op1C->getZExtValue() >= VT->getNumElements())
1005 return MayAlias; // Be conservative with out-of-range accesses
1009 // GEP1 is known to produce a value less than GEP2. To be
1010 // conservatively correct, we must assume the largest possible
1011 // constant is used in this position. This cannot be the initial
1012 // index to the GEP instructions (because we know we have at least one
1013 // element before this one with the different constant arguments), so
1014 // we know that the current index must be into either a struct or
1015 // array. Because we know it's not constant, this cannot be a
1016 // structure index. Because of this, we can calculate the maximum
1019 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
1021 ConstantInt::get(Type::getInt64Ty(AT->getContext()),
1022 AT->getNumElements()-1);
1023 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
1025 ConstantInt::get(Type::getInt64Ty(VT->getContext()),
1026 VT->getNumElements()-1);
1031 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
1032 // If this is an array index, make sure the array element is in range.
1033 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
1034 if (Op2C->getZExtValue() >= AT->getNumElements())
1035 return MayAlias; // Be conservative with out-of-range accesses
1036 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
1037 if (Op2C->getZExtValue() >= VT->getNumElements())
1038 return MayAlias; // Be conservative with out-of-range accesses
1040 } else { // Conservatively assume the minimum value for this index
1041 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
1046 if (BasePtr1Ty && Op1) {
1047 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
1048 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
1053 if (BasePtr2Ty && Op2) {
1054 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
1055 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
1061 if (TD && GEPPointerTy->getElementType()->isSized()) {
1063 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
1065 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
1066 assert(Offset1 != Offset2 &&
1067 "There is at least one different constant here!");
1069 // Make sure we compare the absolute difference.
1070 if (Offset1 > Offset2)
1071 std::swap(Offset1, Offset2);
1073 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
1074 //cerr << "Determined that these two GEP's don't alias ["
1075 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
1082 // Make sure that anything that uses AliasAnalysis pulls in this file...
1083 DEFINING_FILE_FOR(BasicAliasAnalysis)