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 GEPOperator *V,
42 SmallVector<Value*, 16> &GEPOps) {
43 assert(GEPOps.empty() && "Expect empty list to populate!");
44 GEPOps.insert(GEPOps.end(), V->op_begin()+1, V->op_end());
46 // Accumulate all of the chained indexes into the operand array.
47 Value *BasePtr = V->getOperand(0);
49 V = dyn_cast<GEPOperator>(BasePtr);
50 if (V == 0) return BasePtr;
52 // Don't handle folding arbitrary pointer offsets yet.
53 if (!isa<Constant>(GEPOps[0]) || !cast<Constant>(GEPOps[0])->isNullValue())
56 GEPOps.erase(GEPOps.begin()); // Drop the zero index
57 GEPOps.insert(GEPOps.begin(), V->op_begin()+1, V->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
221 // instruction against another.
222 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size,
223 const Value *V2, unsigned V2Size);
225 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI
226 // instruction against another.
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 // Note: this doesn't require GV to be "ODR" because it isn't legal for a
267 // global to be marked constant in some modules and non-constant in others.
268 // GV may even be a declaration, not a definition.
269 return GV->isConstant();
274 /// getModRefInfo - Check to see if the specified callsite can clobber the
275 /// specified memory object. Since we only look at local properties of this
276 /// function, we really can't say much about this query. We do, however, use
277 /// simple "address taken" analysis on local objects.
278 AliasAnalysis::ModRefResult
279 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
280 const Value *Object = P->getUnderlyingObject();
282 // If this is a tail call and P points to a stack location, we know that
283 // the tail call cannot access or modify the local stack.
284 // We cannot exclude byval arguments here; these belong to the caller of
285 // the current function not to the current function, and a tail callee
286 // may reference them.
287 if (isa<AllocaInst>(Object))
288 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
289 if (CI->isTailCall())
292 // If the pointer is to a locally allocated object that does not escape,
293 // then the call can not mod/ref the pointer unless the call takes the pointer
294 // as an argument, and itself doesn't capture it.
295 if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
296 isNonEscapingLocalObject(Object)) {
297 bool PassedAsArg = false;
299 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
300 CI != CE; ++CI, ++ArgNo) {
301 // Only look at the no-capture pointer arguments.
302 if (!isa<PointerType>((*CI)->getType()) ||
303 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture))
306 // If this is a no-capture pointer argument, see if we can tell that it
307 // is impossible to alias the pointer we're checking. If not, we have to
308 // assume that the call could touch the pointer, even though it doesn't
310 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) {
320 // Finally, handle specific knowledge of intrinsics.
321 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
323 return AliasAnalysis::getModRefInfo(CS, P, Size);
325 switch (II->getIntrinsicID()) {
327 case Intrinsic::memcpy:
328 case Intrinsic::memmove: {
330 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
331 Len = LenCI->getZExtValue();
332 Value *Dest = II->getOperand(1);
333 Value *Src = II->getOperand(2);
334 if (isNoAlias(Dest, Len, P, Size)) {
335 if (isNoAlias(Src, Len, P, Size))
341 case Intrinsic::memset:
342 // Since memset is 'accesses arguments' only, the AliasAnalysis base class
343 // will handle it for the variable length case.
344 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
345 unsigned Len = LenCI->getZExtValue();
346 Value *Dest = II->getOperand(1);
347 if (isNoAlias(Dest, Len, P, Size))
351 case Intrinsic::atomic_cmp_swap:
352 case Intrinsic::atomic_swap:
353 case Intrinsic::atomic_load_add:
354 case Intrinsic::atomic_load_sub:
355 case Intrinsic::atomic_load_and:
356 case Intrinsic::atomic_load_nand:
357 case Intrinsic::atomic_load_or:
358 case Intrinsic::atomic_load_xor:
359 case Intrinsic::atomic_load_max:
360 case Intrinsic::atomic_load_min:
361 case Intrinsic::atomic_load_umax:
362 case Intrinsic::atomic_load_umin:
364 Value *Op1 = II->getOperand(1);
365 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
366 if (isNoAlias(Op1, Op1Size, P, Size))
370 case Intrinsic::lifetime_start:
371 case Intrinsic::lifetime_end:
372 case Intrinsic::invariant_start: {
373 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
374 if (isNoAlias(II->getOperand(2), PtrSize, P, Size))
378 case Intrinsic::invariant_end: {
379 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
380 if (isNoAlias(II->getOperand(3), PtrSize, P, Size))
386 // The AliasAnalysis base class has some smarts, lets use them.
387 return AliasAnalysis::getModRefInfo(CS, P, Size);
391 AliasAnalysis::ModRefResult
392 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
393 // If CS1 or CS2 are readnone, they don't interact.
394 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
395 if (CS1B == DoesNotAccessMemory) return NoModRef;
397 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
398 if (CS2B == DoesNotAccessMemory) return NoModRef;
400 // If they both only read from memory, just return ref.
401 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
404 // Otherwise, fall back to NoAA (mod+ref).
405 return NoAA::getModRefInfo(CS1, CS2);
408 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
409 /// against another pointer. We know that V1 is a GEP, but we don't know
410 /// anything about V2.
412 AliasAnalysis::AliasResult
413 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size,
414 const Value *V2, unsigned V2Size) {
415 // If we have two gep instructions with must-alias'ing base pointers, figure
416 // out if the indexes to the GEP tell us anything about the derived pointer.
417 // Note that we also handle chains of getelementptr instructions as well as
418 // constant expression getelementptrs here.
420 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
421 // If V1 and V2 are identical GEPs, just recurse down on both of them.
422 // This allows us to analyze things like:
423 // P = gep A, 0, i, 1
424 // Q = gep B, 0, i, 1
425 // by just analyzing A and B. This is even safe for variable indices.
426 if (GEP1->getType() == GEP2->getType() &&
427 GEP1->getNumOperands() == GEP2->getNumOperands() &&
428 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
429 // All operands are the same, ignoring the base.
430 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
431 return aliasCheck(GEP1->getOperand(0), V1Size,
432 GEP2->getOperand(0), V2Size);
434 // Drill down into the first non-gep value, to test for must-aliasing of
435 // the base pointers.
436 while (isa<GEPOperator>(GEP1->getOperand(0)) &&
437 GEP1->getOperand(1) ==
438 Constant::getNullValue(GEP1->getOperand(1)->getType()))
439 GEP1 = cast<GEPOperator>(GEP1->getOperand(0));
440 const Value *BasePtr1 = GEP1->getOperand(0);
442 while (isa<GEPOperator>(GEP2->getOperand(0)) &&
443 GEP2->getOperand(1) ==
444 Constant::getNullValue(GEP2->getOperand(1)->getType()))
445 GEP2 = cast<GEPOperator>(GEP2->getOperand(0));
446 const Value *BasePtr2 = GEP2->getOperand(0);
448 // Do the base pointers alias?
449 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U);
450 if (BaseAlias == NoAlias) return NoAlias;
451 if (BaseAlias == MustAlias) {
452 // If the base pointers alias each other exactly, check to see if we can
453 // figure out anything about the resultant pointers, to try to prove
456 // Collect all of the chained GEP operands together into one simple place
457 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
458 BasePtr1 = GetGEPOperands(GEP1, GEP1Ops);
459 BasePtr2 = GetGEPOperands(GEP2, GEP2Ops);
461 // If GetGEPOperands were able to fold to the same must-aliased pointer,
462 // do the comparison.
463 if (BasePtr1 == BasePtr2) {
465 CheckGEPInstructions(BasePtr1->getType(),
466 &GEP1Ops[0], GEP1Ops.size(), V1Size,
468 &GEP2Ops[0], GEP2Ops.size(), V2Size);
469 if (GAlias != MayAlias)
475 // Check to see if these two pointers are related by a getelementptr
476 // instruction. If one pointer is a GEP with a non-zero index of the other
477 // pointer, we know they cannot alias.
479 if (V1Size == ~0U || V2Size == ~0U)
482 SmallVector<Value*, 16> GEPOperands;
483 const Value *BasePtr = GetGEPOperands(GEP1, GEPOperands);
485 AliasResult R = aliasCheck(BasePtr, ~0U, V2, V2Size);
487 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
488 // If V2 is known not to alias GEP base pointer, then the two values
489 // cannot alias per GEP semantics: "A pointer value formed from a
490 // getelementptr instruction is associated with the addresses associated
491 // with the first operand of the getelementptr".
494 // If there is at least one non-zero constant index, we know they cannot
496 bool ConstantFound = false;
497 bool AllZerosFound = true;
498 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
499 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
500 if (!C->isNullValue()) {
501 ConstantFound = true;
502 AllZerosFound = false;
506 AllZerosFound = false;
509 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
510 // the ptr, the end result is a must alias also.
515 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
518 // Otherwise we have to check to see that the distance is more than
519 // the size of the argument... build an index vector that is equal to
520 // the arguments provided, except substitute 0's for any variable
521 // indexes we find...
523 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
524 for (unsigned i = 0; i != GEPOperands.size(); ++i)
525 if (!isa<ConstantInt>(GEPOperands[i]))
526 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
527 int64_t Offset = TD->getIndexedOffset(BasePtr->getType(),
531 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
539 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
540 /// instruction against another.
541 AliasAnalysis::AliasResult
542 BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize,
543 const Value *V2, unsigned V2Size) {
544 // If the values are Selects with the same condition, we can do a more precise
545 // check: just check for aliases between the values on corresponding arms.
546 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
547 if (SI->getCondition() == SI2->getCondition()) {
549 aliasCheck(SI->getTrueValue(), SISize,
550 SI2->getTrueValue(), V2Size);
551 if (Alias == MayAlias)
553 AliasResult ThisAlias =
554 aliasCheck(SI->getFalseValue(), SISize,
555 SI2->getFalseValue(), V2Size);
556 if (ThisAlias != Alias)
561 // If both arms of the Select node NoAlias or MustAlias V2, then returns
562 // NoAlias / MustAlias. Otherwise, returns MayAlias.
564 aliasCheck(SI->getTrueValue(), SISize, V2, V2Size);
565 if (Alias == MayAlias)
567 AliasResult ThisAlias =
568 aliasCheck(SI->getFalseValue(), SISize, V2, V2Size);
569 if (ThisAlias != Alias)
574 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
576 AliasAnalysis::AliasResult
577 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
578 const Value *V2, unsigned V2Size) {
579 // The PHI node has already been visited, avoid recursion any further.
580 if (!VisitedPHIs.insert(PN))
583 // If the values are PHIs in the same block, we can do a more precise
584 // as well as efficient check: just check for aliases between the values
585 // on corresponding edges.
586 if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
587 if (PN2->getParent() == PN->getParent()) {
589 aliasCheck(PN->getIncomingValue(0), PNSize,
590 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
592 if (Alias == MayAlias)
594 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
595 AliasResult ThisAlias =
596 aliasCheck(PN->getIncomingValue(i), PNSize,
597 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
599 if (ThisAlias != Alias)
605 SmallPtrSet<Value*, 4> UniqueSrc;
606 SmallVector<Value*, 4> V1Srcs;
607 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
608 Value *PV1 = PN->getIncomingValue(i);
609 if (isa<PHINode>(PV1))
610 // If any of the source itself is a PHI, return MayAlias conservatively
611 // to avoid compile time explosion. The worst possible case is if both
612 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
613 // and 'n' are the number of PHI sources.
615 if (UniqueSrc.insert(PV1))
616 V1Srcs.push_back(PV1);
619 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize);
620 // Early exit if the check of the first PHI source against V2 is MayAlias.
621 // Other results are not possible.
622 if (Alias == MayAlias)
625 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
626 // NoAlias / MustAlias. Otherwise, returns MayAlias.
627 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
628 Value *V = V1Srcs[i];
630 // If V2 is a PHI, the recursive case will have been caught in the
631 // above aliasCheck call, so these subsequent calls to aliasCheck
632 // don't need to assume that V2 is being visited recursively.
633 VisitedPHIs.erase(V2);
635 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
636 if (ThisAlias != Alias || ThisAlias == MayAlias)
643 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
644 // such as array references.
646 AliasAnalysis::AliasResult
647 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
648 const Value *V2, unsigned V2Size) {
649 // Strip off any casts if they exist.
650 V1 = V1->stripPointerCasts();
651 V2 = V2->stripPointerCasts();
653 // Are we checking for alias of the same value?
654 if (V1 == V2) return MustAlias;
656 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
657 return NoAlias; // Scalars cannot alias each other
659 // Figure out what objects these things are pointing to if we can.
660 const Value *O1 = V1->getUnderlyingObject();
661 const Value *O2 = V2->getUnderlyingObject();
663 // Null values in the default address space don't point to any object, so they
664 // don't alias any other pointer.
665 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
666 if (CPN->getType()->getAddressSpace() == 0)
668 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
669 if (CPN->getType()->getAddressSpace() == 0)
673 // If V1/V2 point to two different objects we know that we have no alias.
674 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
677 // Constant pointers can't alias with non-const isIdentifiedObject objects.
678 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
679 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
682 // Arguments can't alias with local allocations or noalias calls.
683 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
684 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
687 // Most objects can't alias null.
688 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
689 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
693 // If the size of one access is larger than the entire object on the other
694 // side, then we know such behavior is undefined and can assume no alias.
696 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
697 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
700 // If one pointer is the result of a call/invoke or load and the other is a
701 // non-escaping local object, then we know the object couldn't escape to a
702 // point where the call could return it. The load case works because
703 // isNonEscapingLocalObject considers all stores to be escapes (it
704 // passes true for the StoreCaptures argument to PointerMayBeCaptured).
706 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1) ||
707 isa<Argument>(O1)) &&
708 isNonEscapingLocalObject(O2))
710 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2) ||
711 isa<Argument>(O2)) &&
712 isNonEscapingLocalObject(O1))
716 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
718 std::swap(V1Size, V2Size);
720 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1))
721 return aliasGEP(GV1, V1Size, V2, V2Size);
723 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
725 std::swap(V1Size, V2Size);
727 if (const PHINode *PN = dyn_cast<PHINode>(V1))
728 return aliasPHI(PN, V1Size, V2, V2Size);
730 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
732 std::swap(V1Size, V2Size);
734 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1))
735 return aliasSelect(S1, V1Size, V2, V2Size);
740 // This function is used to determine if the indices of two GEP instructions are
741 // equal. V1 and V2 are the indices.
742 static bool IndexOperandsEqual(Value *V1, Value *V2) {
743 if (V1->getType() == V2->getType())
745 if (Constant *C1 = dyn_cast<Constant>(V1))
746 if (Constant *C2 = dyn_cast<Constant>(V2)) {
747 // Sign extend the constants to long types, if necessary
748 if (C1->getType() != Type::getInt64Ty(C1->getContext()))
749 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(C1->getContext()));
750 if (C2->getType() != Type::getInt64Ty(C1->getContext()))
751 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(C1->getContext()));
757 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
758 /// base pointers. This checks to see if the index expressions preclude the
759 /// pointers from aliasing.
760 AliasAnalysis::AliasResult
761 BasicAliasAnalysis::CheckGEPInstructions(
762 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
763 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
764 // We currently can't handle the case when the base pointers have different
765 // primitive types. Since this is uncommon anyway, we are happy being
766 // extremely conservative.
767 if (BasePtr1Ty != BasePtr2Ty)
770 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
772 // Find the (possibly empty) initial sequence of equal values... which are not
773 // necessarily constants.
774 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
775 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
776 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
777 unsigned UnequalOper = 0;
778 while (UnequalOper != MinOperands &&
779 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
780 // Advance through the type as we go...
782 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
783 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
785 // If all operands equal each other, then the derived pointers must
786 // alias each other...
788 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
789 "Ran out of type nesting, but not out of operands?");
794 // If we have seen all constant operands, and run out of indexes on one of the
795 // getelementptrs, check to see if the tail of the leftover one is all zeros.
796 // If so, return mustalias.
797 if (UnequalOper == MinOperands) {
798 if (NumGEP1Ops < NumGEP2Ops) {
799 std::swap(GEP1Ops, GEP2Ops);
800 std::swap(NumGEP1Ops, NumGEP2Ops);
803 bool AllAreZeros = true;
804 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
805 if (!isa<Constant>(GEP1Ops[i]) ||
806 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
810 if (AllAreZeros) return MustAlias;
814 // So now we know that the indexes derived from the base pointers,
815 // which are known to alias, are different. We can still determine a
816 // no-alias result if there are differing constant pairs in the index
817 // chain. For example:
818 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
820 // We have to be careful here about array accesses. In particular, consider:
821 // A[1][0] vs A[0][i]
822 // In this case, we don't *know* that the array will be accessed in bounds:
823 // the index could even be negative. Because of this, we have to
824 // conservatively *give up* and return may alias. We disregard differing
825 // array subscripts that are followed by a variable index without going
828 unsigned SizeMax = std::max(G1S, G2S);
829 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
831 // Scan for the first operand that is constant and unequal in the
832 // two getelementptrs...
833 unsigned FirstConstantOper = UnequalOper;
834 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
835 const Value *G1Oper = GEP1Ops[FirstConstantOper];
836 const Value *G2Oper = GEP2Ops[FirstConstantOper];
838 if (G1Oper != G2Oper) // Found non-equal constant indexes...
839 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
840 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
841 if (G1OC->getType() != G2OC->getType()) {
842 // Sign extend both operands to long.
843 const Type *Int64Ty = Type::getInt64Ty(G1OC->getContext());
844 if (G1OC->getType() != Int64Ty)
845 G1OC = ConstantExpr::getSExt(G1OC, Int64Ty);
846 if (G2OC->getType() != Int64Ty)
847 G2OC = ConstantExpr::getSExt(G2OC, Int64Ty);
848 GEP1Ops[FirstConstantOper] = G1OC;
849 GEP2Ops[FirstConstantOper] = G2OC;
853 // Handle the "be careful" case above: if this is an array/vector
854 // subscript, scan for a subsequent variable array index.
855 if (const SequentialType *STy =
856 dyn_cast<SequentialType>(BasePtr1Ty)) {
857 const Type *NextTy = STy;
858 bool isBadCase = false;
860 for (unsigned Idx = FirstConstantOper;
861 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
862 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
863 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
867 // If the array is indexed beyond the bounds of the static type
868 // at this level, it will also fall into the "be careful" case.
869 // It would theoretically be possible to analyze these cases,
870 // but for now just be conservatively correct.
871 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
872 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
873 ATy->getNumElements() ||
874 cast<ConstantInt>(G2OC)->getZExtValue() >=
875 ATy->getNumElements()) {
879 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
880 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
881 VTy->getNumElements() ||
882 cast<ConstantInt>(G2OC)->getZExtValue() >=
883 VTy->getNumElements()) {
887 STy = cast<SequentialType>(NextTy);
888 NextTy = cast<SequentialType>(NextTy)->getElementType();
891 if (isBadCase) G1OC = 0;
894 // Make sure they are comparable (ie, not constant expressions), and
895 // make sure the GEP with the smaller leading constant is GEP1.
897 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
899 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
900 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
901 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
902 std::swap(NumGEP1Ops, NumGEP2Ops);
909 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
912 // No shared constant operands, and we ran out of common operands. At this
913 // point, the GEP instructions have run through all of their operands, and we
914 // haven't found evidence that there are any deltas between the GEP's.
915 // However, one GEP may have more operands than the other. If this is the
916 // case, there may still be hope. Check this now.
917 if (FirstConstantOper == MinOperands) {
918 // Without TargetData, we won't know what the offsets are.
922 // Make GEP1Ops be the longer one if there is a longer one.
923 if (NumGEP1Ops < NumGEP2Ops) {
924 std::swap(GEP1Ops, GEP2Ops);
925 std::swap(NumGEP1Ops, NumGEP2Ops);
928 // Is there anything to check?
929 if (NumGEP1Ops > MinOperands) {
930 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
931 if (isa<ConstantInt>(GEP1Ops[i]) &&
932 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
933 // Yup, there's a constant in the tail. Set all variables to
934 // constants in the GEP instruction to make it suitable for
935 // TargetData::getIndexedOffset.
936 for (i = 0; i != MaxOperands; ++i)
937 if (!isa<ConstantInt>(GEP1Ops[i]))
938 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
939 // Okay, now get the offset. This is the relative offset for the full
941 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
944 // Now check without any constants at the end.
945 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
948 // Make sure we compare the absolute difference.
949 if (Offset1 > Offset2)
950 std::swap(Offset1, Offset2);
952 // If the tail provided a bit enough offset, return noalias!
953 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
955 // Otherwise break - we don't look for another constant in the tail.
960 // Couldn't find anything useful.
964 // If there are non-equal constants arguments, then we can figure
965 // out a minimum known delta between the two index expressions... at
966 // this point we know that the first constant index of GEP1 is less
967 // than the first constant index of GEP2.
969 // Advance BasePtr[12]Ty over this first differing constant operand.
970 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
971 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
972 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
973 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
975 // We are going to be using TargetData::getIndexedOffset to determine the
976 // offset that each of the GEP's is reaching. To do this, we have to convert
977 // all variable references to constant references. To do this, we convert the
978 // initial sequence of array subscripts into constant zeros to start with.
979 const Type *ZeroIdxTy = GEPPointerTy;
980 for (unsigned i = 0; i != FirstConstantOper; ++i) {
981 if (!isa<StructType>(ZeroIdxTy))
982 GEP1Ops[i] = GEP2Ops[i] =
983 Constant::getNullValue(Type::getInt32Ty(ZeroIdxTy->getContext()));
985 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
986 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
989 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
991 // Loop over the rest of the operands...
992 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
993 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
994 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
995 // If they are equal, use a zero index...
996 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
997 if (!isa<ConstantInt>(Op1))
998 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
999 // Otherwise, just keep the constants we have.
1002 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
1003 // If this is an array index, make sure the array element is in range.
1004 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
1005 if (Op1C->getZExtValue() >= AT->getNumElements())
1006 return MayAlias; // Be conservative with out-of-range accesses
1007 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
1008 if (Op1C->getZExtValue() >= VT->getNumElements())
1009 return MayAlias; // Be conservative with out-of-range accesses
1013 // GEP1 is known to produce a value less than GEP2. To be
1014 // conservatively correct, we must assume the largest possible
1015 // constant is used in this position. This cannot be the initial
1016 // index to the GEP instructions (because we know we have at least one
1017 // element before this one with the different constant arguments), so
1018 // we know that the current index must be into either a struct or
1019 // array. Because we know it's not constant, this cannot be a
1020 // structure index. Because of this, we can calculate the maximum
1023 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
1025 ConstantInt::get(Type::getInt64Ty(AT->getContext()),
1026 AT->getNumElements()-1);
1027 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
1029 ConstantInt::get(Type::getInt64Ty(VT->getContext()),
1030 VT->getNumElements()-1);
1035 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
1036 // If this is an array index, make sure the array element is in range.
1037 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
1038 if (Op2C->getZExtValue() >= AT->getNumElements())
1039 return MayAlias; // Be conservative with out-of-range accesses
1040 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
1041 if (Op2C->getZExtValue() >= VT->getNumElements())
1042 return MayAlias; // Be conservative with out-of-range accesses
1044 } else { // Conservatively assume the minimum value for this index
1045 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
1050 if (BasePtr1Ty && Op1) {
1051 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
1052 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
1057 if (BasePtr2Ty && Op2) {
1058 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
1059 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
1065 if (TD && GEPPointerTy->getElementType()->isSized()) {
1067 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
1069 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
1070 assert(Offset1 != Offset2 &&
1071 "There is at least one different constant here!");
1073 // Make sure we compare the absolute difference.
1074 if (Offset1 > Offset2)
1075 std::swap(Offset1, Offset2);
1077 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
1078 //cerr << "Determined that these two GEP's don't alias ["
1079 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
1086 // Make sure that anything that uses AliasAnalysis pulls in this file.
1087 DEFINING_FILE_FOR(BasicAliasAnalysis)