1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/CaptureTracking.h"
18 #include "llvm/Analysis/MallocHelper.h"
19 #include "llvm/Analysis/Passes.h"
20 #include "llvm/Constants.h"
21 #include "llvm/DerivedTypes.h"
22 #include "llvm/Function.h"
23 #include "llvm/GlobalVariable.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/IntrinsicInst.h"
26 #include "llvm/LLVMContext.h"
27 #include "llvm/Operator.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/Support/Compiler.h"
34 #include "llvm/Support/ErrorHandling.h"
35 #include "llvm/Support/GetElementPtrTypeIterator.h"
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
43 static const GEPOperator *isGEP(const Value *V) {
44 return dyn_cast<GEPOperator>(V);
47 static const Value *GetGEPOperands(const Value *V,
48 SmallVector<Value*, 16> &GEPOps) {
49 assert(GEPOps.empty() && "Expect empty list to populate!");
50 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
51 cast<User>(V)->op_end());
53 // Accumulate all of the chained indexes into the operand array
54 V = cast<User>(V)->getOperand(0);
56 while (const User *G = isGEP(V)) {
57 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
58 !cast<Constant>(GEPOps[0])->isNullValue())
59 break; // Don't handle folding arbitrary pointer offsets yet...
60 GEPOps.erase(GEPOps.begin()); // Drop the zero index
61 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
67 /// isKnownNonNull - Return true if we know that the specified value is never
69 static bool isKnownNonNull(const Value *V) {
70 // Alloca never returns null, malloc might.
71 if (isa<AllocaInst>(V)) return true;
73 // A byval argument is never null.
74 if (const Argument *A = dyn_cast<Argument>(V))
75 return A->hasByValAttr();
77 // Global values are not null unless extern weak.
78 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
79 return !GV->hasExternalWeakLinkage();
83 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
84 /// object that never escapes from the function.
85 static bool isNonEscapingLocalObject(const Value *V) {
86 // If this is a local allocation, check to see if it escapes.
87 if (isa<AllocationInst>(V) || isNoAliasCall(V))
88 return !PointerMayBeCaptured(V, false);
90 // If this is an argument that corresponds to a byval or noalias argument,
91 // then it has not escaped before entering the function. Check if it escapes
92 // inside the function.
93 if (const Argument *A = dyn_cast<Argument>(V))
94 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
95 // Don't bother analyzing arguments already known not to escape.
96 if (A->hasNoCaptureAttr())
98 return !PointerMayBeCaptured(V, false);
104 /// isObjectSmallerThan - Return true if we can prove that the object specified
105 /// by V is smaller than Size.
106 static bool isObjectSmallerThan(const Value *V, unsigned Size,
107 LLVMContext &Context, const TargetData &TD) {
108 const Type *AccessTy;
109 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
110 AccessTy = GV->getType()->getElementType();
111 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
112 if (!AI->isArrayAllocation())
113 AccessTy = AI->getType()->getElementType();
116 } else if (const CallInst* CI = extractMallocCall(V)) {
117 if (!isArrayMalloc(V, Context, &TD))
118 // The size is the argument to the malloc call.
119 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
120 return (C->getZExtValue() < Size);
122 } else if (const Argument *A = dyn_cast<Argument>(V)) {
123 if (A->hasByValAttr())
124 AccessTy = cast<PointerType>(A->getType())->getElementType();
131 if (AccessTy->isSized())
132 return TD.getTypeAllocSize(AccessTy) < Size;
136 //===----------------------------------------------------------------------===//
138 //===----------------------------------------------------------------------===//
141 /// NoAA - This class implements the -no-aa pass, which always returns "I
142 /// don't know" for alias queries. NoAA is unlike other alias analysis
143 /// implementations, in that it does not chain to a previous analysis. As
144 /// such it doesn't follow many of the rules that other alias analyses must.
146 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
147 static char ID; // Class identification, replacement for typeinfo
148 NoAA() : ImmutablePass(&ID) {}
149 explicit NoAA(void *PID) : ImmutablePass(PID) { }
151 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
154 virtual void initializePass() {
155 TD = getAnalysisIfAvailable<TargetData>();
158 virtual AliasResult alias(const Value *V1, unsigned V1Size,
159 const Value *V2, unsigned V2Size) {
163 virtual void getArgumentAccesses(Function *F, CallSite CS,
164 std::vector<PointerAccessInfo> &Info) {
165 llvm_unreachable("This method may not be called on this function!");
168 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
169 virtual bool pointsToConstantMemory(const Value *P) { return false; }
170 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
173 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
176 virtual bool hasNoModRefInfoForCalls() const { return true; }
178 virtual void deleteValue(Value *V) {}
179 virtual void copyValue(Value *From, Value *To) {}
181 } // End of anonymous namespace
183 // Register this pass...
185 static RegisterPass<NoAA>
186 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
188 // Declare that we implement the AliasAnalysis interface
189 static RegisterAnalysisGroup<AliasAnalysis> V(U);
191 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
193 //===----------------------------------------------------------------------===//
195 //===----------------------------------------------------------------------===//
198 /// BasicAliasAnalysis - This is the default alias analysis implementation.
199 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
200 /// derives from the NoAA class.
201 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
202 static char ID; // Class identification, replacement for typeinfo
203 BasicAliasAnalysis() : NoAA(&ID) {}
204 AliasResult alias(const Value *V1, unsigned V1Size,
205 const Value *V2, unsigned V2Size) {
206 SmallSet<const Value*, 16> VisitedPHIs;
207 return aliasCheck(V1, V1Size, V2, V2Size, VisitedPHIs);
210 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
211 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
213 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
214 /// non-escaping allocations.
215 virtual bool hasNoModRefInfoForCalls() const { return false; }
217 /// pointsToConstantMemory - Chase pointers until we find a (constant
219 bool pointsToConstantMemory(const Value *P);
222 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
224 AliasResult aliasGEP(const Value *V1, unsigned V1Size,
225 const Value *V2, unsigned V2Size,
226 SmallSet<const Value*, 16> &VisitedPHIs);
228 AliasResult aliasPHI(const Value *V1, unsigned V1Size,
229 const Value *V2, unsigned V2Size,
230 SmallSet<const Value*, 16> &VisitedPHIs);
232 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
233 const Value *V2, unsigned V2Size,
234 SmallSet<const Value*, 16> &VisitedPHIs);
236 // CheckGEPInstructions - Check two GEP instructions with known
237 // must-aliasing base pointers. This checks to see if the index expressions
238 // preclude the pointers from aliasing...
240 CheckGEPInstructions(const Type* BasePtr1Ty,
241 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
242 const Type *BasePtr2Ty,
243 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
245 } // End of anonymous namespace
247 // Register this pass...
248 char BasicAliasAnalysis::ID = 0;
249 static RegisterPass<BasicAliasAnalysis>
250 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
252 // Declare that we implement the AliasAnalysis interface
253 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
255 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
256 return new BasicAliasAnalysis();
260 /// pointsToConstantMemory - Chase pointers until we find a (constant
262 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
263 if (const GlobalVariable *GV =
264 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
265 return GV->isConstant();
270 // getModRefInfo - Check to see if the specified callsite can clobber the
271 // specified memory object. Since we only look at local properties of this
272 // function, we really can't say much about this query. We do, however, use
273 // simple "address taken" analysis on local objects.
275 AliasAnalysis::ModRefResult
276 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
277 if (!isa<Constant>(P)) {
278 const Value *Object = P->getUnderlyingObject();
280 // If this is a tail call and P points to a stack location, we know that
281 // the tail call cannot access or modify the local stack.
282 // We cannot exclude byval arguments here; these belong to the caller of
283 // the current function not to the current function, and a tail callee
284 // may reference them.
285 if (isa<AllocaInst>(Object))
286 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
287 if (CI->isTailCall())
290 // If the pointer is to a locally allocated object that does not escape,
291 // then the call can not mod/ref the pointer unless the call takes the
292 // argument without capturing it.
293 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
294 bool passedAsArg = false;
295 // TODO: Eventually only check 'nocapture' arguments.
296 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
298 if (isa<PointerType>((*CI)->getType()) &&
299 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
306 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
307 switch (II->getIntrinsicID()) {
309 case Intrinsic::atomic_cmp_swap:
310 case Intrinsic::atomic_swap:
311 case Intrinsic::atomic_load_add:
312 case Intrinsic::atomic_load_sub:
313 case Intrinsic::atomic_load_and:
314 case Intrinsic::atomic_load_nand:
315 case Intrinsic::atomic_load_or:
316 case Intrinsic::atomic_load_xor:
317 case Intrinsic::atomic_load_max:
318 case Intrinsic::atomic_load_min:
319 case Intrinsic::atomic_load_umax:
320 case Intrinsic::atomic_load_umin:
321 if (alias(II->getOperand(1), Size, P, Size) == NoAlias)
328 // The AliasAnalysis base class has some smarts, lets use them.
329 return AliasAnalysis::getModRefInfo(CS, P, Size);
333 AliasAnalysis::ModRefResult
334 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
335 // If CS1 or CS2 are readnone, they don't interact.
336 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
337 if (CS1B == DoesNotAccessMemory) return NoModRef;
339 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
340 if (CS2B == DoesNotAccessMemory) return NoModRef;
342 // If they both only read from memory, just return ref.
343 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
346 // Otherwise, fall back to NoAA (mod+ref).
347 return NoAA::getModRefInfo(CS1, CS2);
350 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
353 AliasAnalysis::AliasResult
354 BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size,
355 const Value *V2, unsigned V2Size,
356 SmallSet<const Value*, 16> &VisitedPHIs) {
357 // If we have two gep instructions with must-alias'ing base pointers, figure
358 // out if the indexes to the GEP tell us anything about the derived pointer.
359 // Note that we also handle chains of getelementptr instructions as well as
360 // constant expression getelementptrs here.
362 if (isGEP(V1) && isGEP(V2)) {
363 const User *GEP1 = cast<User>(V1);
364 const User *GEP2 = cast<User>(V2);
366 // If V1 and V2 are identical GEPs, just recurse down on both of them.
367 // This allows us to analyze things like:
368 // P = gep A, 0, i, 1
369 // Q = gep B, 0, i, 1
370 // by just analyzing A and B. This is even safe for variable indices.
371 if (GEP1->getType() == GEP2->getType() &&
372 GEP1->getNumOperands() == GEP2->getNumOperands() &&
373 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
374 // All operands are the same, ignoring the base.
375 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
376 return aliasCheck(GEP1->getOperand(0), V1Size,
377 GEP2->getOperand(0), V2Size, VisitedPHIs);
379 // Drill down into the first non-gep value, to test for must-aliasing of
380 // the base pointers.
381 while (isGEP(GEP1->getOperand(0)) &&
382 GEP1->getOperand(1) ==
383 Constant::getNullValue(GEP1->getOperand(1)->getType()))
384 GEP1 = cast<User>(GEP1->getOperand(0));
385 const Value *BasePtr1 = GEP1->getOperand(0);
387 while (isGEP(GEP2->getOperand(0)) &&
388 GEP2->getOperand(1) ==
389 Constant::getNullValue(GEP2->getOperand(1)->getType()))
390 GEP2 = cast<User>(GEP2->getOperand(0));
391 const Value *BasePtr2 = GEP2->getOperand(0);
393 // Do the base pointers alias?
394 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U,
396 if (BaseAlias == NoAlias) return NoAlias;
397 if (BaseAlias == MustAlias) {
398 // If the base pointers alias each other exactly, check to see if we can
399 // figure out anything about the resultant pointers, to try to prove
402 // Collect all of the chained GEP operands together into one simple place
403 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
404 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
405 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
407 // If GetGEPOperands were able to fold to the same must-aliased pointer,
408 // do the comparison.
409 if (BasePtr1 == BasePtr2) {
411 CheckGEPInstructions(BasePtr1->getType(),
412 &GEP1Ops[0], GEP1Ops.size(), V1Size,
414 &GEP2Ops[0], GEP2Ops.size(), V2Size);
415 if (GAlias != MayAlias)
421 // Check to see if these two pointers are related by a getelementptr
422 // instruction. If one pointer is a GEP with a non-zero index of the other
423 // pointer, we know they cannot alias.
425 if (V1Size == ~0U || V2Size == ~0U)
428 SmallVector<Value*, 16> GEPOperands;
429 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
431 AliasResult R = aliasCheck(BasePtr, V1Size, V2, V2Size, VisitedPHIs);
432 if (R == MustAlias) {
433 // If there is at least one non-zero constant index, we know they cannot
435 bool ConstantFound = false;
436 bool AllZerosFound = true;
437 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
438 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
439 if (!C->isNullValue()) {
440 ConstantFound = true;
441 AllZerosFound = false;
445 AllZerosFound = false;
448 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
449 // the ptr, the end result is a must alias also.
454 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
457 // Otherwise we have to check to see that the distance is more than
458 // the size of the argument... build an index vector that is equal to
459 // the arguments provided, except substitute 0's for any variable
460 // indexes we find...
462 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
463 for (unsigned i = 0; i != GEPOperands.size(); ++i)
464 if (!isa<ConstantInt>(GEPOperands[i]))
465 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
467 TD->getIndexedOffset(BasePtr->getType(),
471 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
480 AliasAnalysis::AliasResult
481 BasicAliasAnalysis::aliasPHI(const Value *V1, unsigned V1Size,
482 const Value *V2, unsigned V2Size,
483 SmallSet<const Value*, 16> &VisitedPHIs) {
484 // The PHI node has already been visited, avoid recursion any further.
485 if (!VisitedPHIs.insert(V1))
488 SmallSet<Value*, 4> UniqueSrc;
489 SmallVector<Value*, 4> V1Srcs;
490 const PHINode *PN = cast<PHINode>(V1);
491 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
492 Value *PV1 = PN->getIncomingValue(i);
493 if (isa<PHINode>(PV1))
494 // If any of the source itself is a PHI, return MayAlias conservatively
495 // to avoid compile time explosion.
497 if (UniqueSrc.insert(PV1))
498 V1Srcs.push_back(PV1);
501 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
502 // NoAlias / MustAlias. Otherwise, returns MayAlias.
503 AliasResult Alias = aliasCheck(V1Srcs[0], V1Size, V2, V2Size, VisitedPHIs);
504 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
505 Value *V = V1Srcs[i];
506 AliasResult ThisAlias = aliasCheck(V, V1Size, V2, V2Size, VisitedPHIs);
507 if (ThisAlias != Alias)
514 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
515 // such as array references.
517 AliasAnalysis::AliasResult
518 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
519 const Value *V2, unsigned V2Size,
520 SmallSet<const Value*, 16> &VisitedPHIs) {
521 // Strip off any casts if they exist.
522 V1 = V1->stripPointerCasts();
523 V2 = V2->stripPointerCasts();
525 // Are we checking for alias of the same value?
526 if (V1 == V2) return MustAlias;
528 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
529 return NoAlias; // Scalars cannot alias each other
531 // Figure out what objects these things are pointing to if we can.
532 const Value *O1 = V1->getUnderlyingObject();
533 const Value *O2 = V2->getUnderlyingObject();
536 // If V1/V2 point to two different objects we know that we have no alias.
537 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
540 // Arguments can't alias with local allocations or noalias calls.
541 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
542 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
545 // Most objects can't alias null.
546 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
547 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
551 // If the size of one access is larger than the entire object on the other
552 // side, then we know such behavior is undefined and can assume no alias.
553 LLVMContext &Context = V1->getContext();
555 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, Context, *TD)) ||
556 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, Context, *TD)))
559 // If one pointer is the result of a call/invoke and the other is a
560 // non-escaping local object, then we know the object couldn't escape to a
561 // point where the call could return it.
562 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
563 isNonEscapingLocalObject(O2) && O1 != O2)
565 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
566 isNonEscapingLocalObject(O1) && O1 != O2)
569 if (!isGEP(V1) && isGEP(V2)) {
571 std::swap(V1Size, V2Size);
574 return aliasGEP(V1, V1Size, V2, V2Size, VisitedPHIs);
576 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
578 std::swap(V1Size, V2Size);
580 if (isa<PHINode>(V1))
581 return aliasPHI(V1, V1Size, V2, V2Size, VisitedPHIs);
586 // This function is used to determine if the indices of two GEP instructions are
587 // equal. V1 and V2 are the indices.
588 static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) {
589 if (V1->getType() == V2->getType())
591 if (Constant *C1 = dyn_cast<Constant>(V1))
592 if (Constant *C2 = dyn_cast<Constant>(V2)) {
593 // Sign extend the constants to long types, if necessary
594 if (C1->getType() != Type::getInt64Ty(Context))
595 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context));
596 if (C2->getType() != Type::getInt64Ty(Context))
597 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context));
603 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
604 /// base pointers. This checks to see if the index expressions preclude the
605 /// pointers from aliasing...
606 AliasAnalysis::AliasResult
607 BasicAliasAnalysis::CheckGEPInstructions(
608 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
609 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
610 // We currently can't handle the case when the base pointers have different
611 // primitive types. Since this is uncommon anyway, we are happy being
612 // extremely conservative.
613 if (BasePtr1Ty != BasePtr2Ty)
616 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
618 LLVMContext &Context = GEPPointerTy->getContext();
620 // Find the (possibly empty) initial sequence of equal values... which are not
621 // necessarily constants.
622 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
623 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
624 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
625 unsigned UnequalOper = 0;
626 while (UnequalOper != MinOperands &&
627 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper],
629 // Advance through the type as we go...
631 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
632 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
634 // If all operands equal each other, then the derived pointers must
635 // alias each other...
637 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
638 "Ran out of type nesting, but not out of operands?");
643 // If we have seen all constant operands, and run out of indexes on one of the
644 // getelementptrs, check to see if the tail of the leftover one is all zeros.
645 // If so, return mustalias.
646 if (UnequalOper == MinOperands) {
647 if (NumGEP1Ops < NumGEP2Ops) {
648 std::swap(GEP1Ops, GEP2Ops);
649 std::swap(NumGEP1Ops, NumGEP2Ops);
652 bool AllAreZeros = true;
653 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
654 if (!isa<Constant>(GEP1Ops[i]) ||
655 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
659 if (AllAreZeros) return MustAlias;
663 // So now we know that the indexes derived from the base pointers,
664 // which are known to alias, are different. We can still determine a
665 // no-alias result if there are differing constant pairs in the index
666 // chain. For example:
667 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
669 // We have to be careful here about array accesses. In particular, consider:
670 // A[1][0] vs A[0][i]
671 // In this case, we don't *know* that the array will be accessed in bounds:
672 // the index could even be negative. Because of this, we have to
673 // conservatively *give up* and return may alias. We disregard differing
674 // array subscripts that are followed by a variable index without going
677 unsigned SizeMax = std::max(G1S, G2S);
678 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
680 // Scan for the first operand that is constant and unequal in the
681 // two getelementptrs...
682 unsigned FirstConstantOper = UnequalOper;
683 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
684 const Value *G1Oper = GEP1Ops[FirstConstantOper];
685 const Value *G2Oper = GEP2Ops[FirstConstantOper];
687 if (G1Oper != G2Oper) // Found non-equal constant indexes...
688 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
689 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
690 if (G1OC->getType() != G2OC->getType()) {
691 // Sign extend both operands to long.
692 if (G1OC->getType() != Type::getInt64Ty(Context))
693 G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context));
694 if (G2OC->getType() != Type::getInt64Ty(Context))
695 G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context));
696 GEP1Ops[FirstConstantOper] = G1OC;
697 GEP2Ops[FirstConstantOper] = G2OC;
701 // Handle the "be careful" case above: if this is an array/vector
702 // subscript, scan for a subsequent variable array index.
703 if (const SequentialType *STy =
704 dyn_cast<SequentialType>(BasePtr1Ty)) {
705 const Type *NextTy = STy;
706 bool isBadCase = false;
708 for (unsigned Idx = FirstConstantOper;
709 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
710 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
711 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
715 // If the array is indexed beyond the bounds of the static type
716 // at this level, it will also fall into the "be careful" case.
717 // It would theoretically be possible to analyze these cases,
718 // but for now just be conservatively correct.
719 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
720 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
721 ATy->getNumElements() ||
722 cast<ConstantInt>(G2OC)->getZExtValue() >=
723 ATy->getNumElements()) {
727 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
728 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
729 VTy->getNumElements() ||
730 cast<ConstantInt>(G2OC)->getZExtValue() >=
731 VTy->getNumElements()) {
735 STy = cast<SequentialType>(NextTy);
736 NextTy = cast<SequentialType>(NextTy)->getElementType();
739 if (isBadCase) G1OC = 0;
742 // Make sure they are comparable (ie, not constant expressions), and
743 // make sure the GEP with the smaller leading constant is GEP1.
745 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
747 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
748 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
749 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
750 std::swap(NumGEP1Ops, NumGEP2Ops);
757 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
760 // No shared constant operands, and we ran out of common operands. At this
761 // point, the GEP instructions have run through all of their operands, and we
762 // haven't found evidence that there are any deltas between the GEP's.
763 // However, one GEP may have more operands than the other. If this is the
764 // case, there may still be hope. Check this now.
765 if (FirstConstantOper == MinOperands) {
766 // Without TargetData, we won't know what the offsets are.
770 // Make GEP1Ops be the longer one if there is a longer one.
771 if (NumGEP1Ops < NumGEP2Ops) {
772 std::swap(GEP1Ops, GEP2Ops);
773 std::swap(NumGEP1Ops, NumGEP2Ops);
776 // Is there anything to check?
777 if (NumGEP1Ops > MinOperands) {
778 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
779 if (isa<ConstantInt>(GEP1Ops[i]) &&
780 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
781 // Yup, there's a constant in the tail. Set all variables to
782 // constants in the GEP instruction to make it suitable for
783 // TargetData::getIndexedOffset.
784 for (i = 0; i != MaxOperands; ++i)
785 if (!isa<ConstantInt>(GEP1Ops[i]))
786 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
787 // Okay, now get the offset. This is the relative offset for the full
789 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
792 // Now check without any constants at the end.
793 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
796 // Make sure we compare the absolute difference.
797 if (Offset1 > Offset2)
798 std::swap(Offset1, Offset2);
800 // If the tail provided a bit enough offset, return noalias!
801 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
803 // Otherwise break - we don't look for another constant in the tail.
808 // Couldn't find anything useful.
812 // If there are non-equal constants arguments, then we can figure
813 // out a minimum known delta between the two index expressions... at
814 // this point we know that the first constant index of GEP1 is less
815 // than the first constant index of GEP2.
817 // Advance BasePtr[12]Ty over this first differing constant operand.
818 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
819 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
820 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
821 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
823 // We are going to be using TargetData::getIndexedOffset to determine the
824 // offset that each of the GEP's is reaching. To do this, we have to convert
825 // all variable references to constant references. To do this, we convert the
826 // initial sequence of array subscripts into constant zeros to start with.
827 const Type *ZeroIdxTy = GEPPointerTy;
828 for (unsigned i = 0; i != FirstConstantOper; ++i) {
829 if (!isa<StructType>(ZeroIdxTy))
830 GEP1Ops[i] = GEP2Ops[i] =
831 Constant::getNullValue(Type::getInt32Ty(Context));
833 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
834 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
837 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
839 // Loop over the rest of the operands...
840 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
841 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
842 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
843 // If they are equal, use a zero index...
844 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
845 if (!isa<ConstantInt>(Op1))
846 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
847 // Otherwise, just keep the constants we have.
850 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
851 // If this is an array index, make sure the array element is in range.
852 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
853 if (Op1C->getZExtValue() >= AT->getNumElements())
854 return MayAlias; // Be conservative with out-of-range accesses
855 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
856 if (Op1C->getZExtValue() >= VT->getNumElements())
857 return MayAlias; // Be conservative with out-of-range accesses
861 // GEP1 is known to produce a value less than GEP2. To be
862 // conservatively correct, we must assume the largest possible
863 // constant is used in this position. This cannot be the initial
864 // index to the GEP instructions (because we know we have at least one
865 // element before this one with the different constant arguments), so
866 // we know that the current index must be into either a struct or
867 // array. Because we know it's not constant, this cannot be a
868 // structure index. Because of this, we can calculate the maximum
871 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
873 ConstantInt::get(Type::getInt64Ty(Context),
874 AT->getNumElements()-1);
875 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
877 ConstantInt::get(Type::getInt64Ty(Context),
878 VT->getNumElements()-1);
883 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
884 // If this is an array index, make sure the array element is in range.
885 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
886 if (Op2C->getZExtValue() >= AT->getNumElements())
887 return MayAlias; // Be conservative with out-of-range accesses
888 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
889 if (Op2C->getZExtValue() >= VT->getNumElements())
890 return MayAlias; // Be conservative with out-of-range accesses
892 } else { // Conservatively assume the minimum value for this index
893 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
898 if (BasePtr1Ty && Op1) {
899 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
900 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
905 if (BasePtr2Ty && Op2) {
906 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
907 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
913 if (TD && GEPPointerTy->getElementType()->isSized()) {
915 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
917 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
918 assert(Offset1 != Offset2 &&
919 "There is at least one different constant here!");
921 // Make sure we compare the absolute difference.
922 if (Offset1 > Offset2)
923 std::swap(Offset1, Offset2);
925 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
926 //cerr << "Determined that these two GEP's don't alias ["
927 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
934 // Make sure that anything that uses AliasAnalysis pulls in this file...
935 DEFINING_FILE_FOR(BasicAliasAnalysis)