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/Passes.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/GlobalVariable.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/IntrinsicInst.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/Support/Compiler.h"
29 #include "llvm/Support/GetElementPtrTypeIterator.h"
30 #include "llvm/Support/ManagedStatic.h"
34 //===----------------------------------------------------------------------===//
36 //===----------------------------------------------------------------------===//
38 // Determine if a value escapes from the function it is contained in (being
39 // returned by the function does not count as escaping here). If a value local
40 // to the function does not escape, there is no way another function can mod/ref
41 // it. We do this by looking at its uses and determining if they can escape
43 static bool AddressMightEscape(const Value *V) {
44 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end();
46 const Instruction *I = cast<Instruction>(*UI);
47 switch (I->getOpcode()) {
48 case Instruction::Load:
50 case Instruction::Store:
51 if (I->getOperand(0) == V)
52 return true; // Escapes if the pointer is stored.
54 case Instruction::GetElementPtr:
55 if (AddressMightEscape(I))
58 case Instruction::BitCast:
59 if (AddressMightEscape(I))
62 case Instruction::Ret:
63 // If returned, the address will escape to calling functions, but no
64 // callees could modify it.
66 case Instruction::Call:
67 // If the argument to the call has the nocapture attribute, then the call
68 // may store or load to the pointer, but it cannot escape.
69 if (cast<CallInst>(I)->paramHasAttr(UI.getOperandNo(),
70 Attribute::NoCapture))
73 case Instruction::Invoke:
74 // If the argument to the call has the nocapture attribute, then the call
75 // may store or load to the pointer, but it cannot escape.
76 if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
77 Attribute::NoCapture))
87 static const User *isGEP(const Value *V) {
88 if (isa<GetElementPtrInst>(V) ||
89 (isa<ConstantExpr>(V) &&
90 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
95 static const Value *GetGEPOperands(const Value *V,
96 SmallVector<Value*, 16> &GEPOps) {
97 assert(GEPOps.empty() && "Expect empty list to populate!");
98 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
99 cast<User>(V)->op_end());
101 // Accumulate all of the chained indexes into the operand array
102 V = cast<User>(V)->getOperand(0);
104 while (const User *G = isGEP(V)) {
105 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
106 !cast<Constant>(GEPOps[0])->isNullValue())
107 break; // Don't handle folding arbitrary pointer offsets yet...
108 GEPOps.erase(GEPOps.begin()); // Drop the zero index
109 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
110 V = G->getOperand(0);
115 /// isNoAliasCall - Return true if this pointer is returned by a noalias
117 static bool isNoAliasCall(const Value *V) {
118 if (isa<CallInst>(V) || isa<InvokeInst>(V))
119 return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
120 .paramHasAttr(0, Attribute::NoAlias);
124 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
125 /// identifiable object. This returns true for:
126 /// Global Variables and Functions
127 /// Allocas and Mallocs
128 /// ByVal and NoAlias Arguments
131 static bool isIdentifiedObject(const Value *V) {
132 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
134 if (const Argument *A = dyn_cast<Argument>(V))
135 return A->hasNoAliasAttr() || A->hasByValAttr();
139 /// isKnownNonNull - Return true if we know that the specified value is never
141 static bool isKnownNonNull(const Value *V) {
142 // Alloca never returns null, malloc might.
143 if (isa<AllocaInst>(V)) return true;
145 // A byval argument is never null.
146 if (const Argument *A = dyn_cast<Argument>(V))
147 return A->hasByValAttr();
149 // Global values are not null unless extern weak.
150 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
151 return !GV->hasExternalWeakLinkage();
155 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
156 /// object that never escapes from the function.
157 static bool isNonEscapingLocalObject(const Value *V) {
158 // If this is a local allocation, check to see if it escapes.
159 if (isa<AllocationInst>(V) || isNoAliasCall(V))
160 return !AddressMightEscape(V);
162 // If this is an argument that corresponds to a byval or noalias argument,
163 // then it has not escaped before entering the function. Check if it escapes
164 // inside the function.
165 if (const Argument *A = dyn_cast<Argument>(V))
166 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
167 // Don't bother analyzing arguments already known not to escape.
168 if (A->hasNoCaptureAttr())
170 return !AddressMightEscape(V);
176 /// isObjectSmallerThan - Return true if we can prove that the object specified
177 /// by V is smaller than Size.
178 static bool isObjectSmallerThan(const Value *V, unsigned Size,
179 const TargetData &TD) {
180 const Type *AccessTy;
181 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
182 AccessTy = GV->getType()->getElementType();
183 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
184 if (!AI->isArrayAllocation())
185 AccessTy = AI->getType()->getElementType();
188 } else if (const Argument *A = dyn_cast<Argument>(V)) {
189 if (A->hasByValAttr())
190 AccessTy = cast<PointerType>(A->getType())->getElementType();
197 if (AccessTy->isSized())
198 return TD.getTypePaddedSize(AccessTy) < Size;
202 //===----------------------------------------------------------------------===//
204 //===----------------------------------------------------------------------===//
207 /// NoAA - This class implements the -no-aa pass, which always returns "I
208 /// don't know" for alias queries. NoAA is unlike other alias analysis
209 /// implementations, in that it does not chain to a previous analysis. As
210 /// such it doesn't follow many of the rules that other alias analyses must.
212 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
213 static char ID; // Class identification, replacement for typeinfo
214 NoAA() : ImmutablePass(&ID) {}
215 explicit NoAA(void *PID) : ImmutablePass(PID) { }
217 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
218 AU.addRequired<TargetData>();
221 virtual void initializePass() {
222 TD = &getAnalysis<TargetData>();
225 virtual AliasResult alias(const Value *V1, unsigned V1Size,
226 const Value *V2, unsigned V2Size) {
230 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
231 std::vector<PointerAccessInfo> *Info) {
232 return UnknownModRefBehavior;
235 virtual void getArgumentAccesses(Function *F, CallSite CS,
236 std::vector<PointerAccessInfo> &Info) {
237 assert(0 && "This method may not be called on this function!");
240 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
241 virtual bool pointsToConstantMemory(const Value *P) { return false; }
242 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
245 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
248 virtual bool hasNoModRefInfoForCalls() const { return true; }
250 virtual void deleteValue(Value *V) {}
251 virtual void copyValue(Value *From, Value *To) {}
253 } // End of anonymous namespace
255 // Register this pass...
257 static RegisterPass<NoAA>
258 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
260 // Declare that we implement the AliasAnalysis interface
261 static RegisterAnalysisGroup<AliasAnalysis> V(U);
263 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
265 //===----------------------------------------------------------------------===//
267 //===----------------------------------------------------------------------===//
270 /// BasicAliasAnalysis - This is the default alias analysis implementation.
271 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
272 /// derives from the NoAA class.
273 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
274 static char ID; // Class identification, replacement for typeinfo
275 BasicAliasAnalysis() : NoAA(&ID) {}
276 AliasResult alias(const Value *V1, unsigned V1Size,
277 const Value *V2, unsigned V2Size);
279 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
280 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
282 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
283 /// non-escaping allocations.
284 virtual bool hasNoModRefInfoForCalls() const { return false; }
286 /// pointsToConstantMemory - Chase pointers until we find a (constant
288 bool pointsToConstantMemory(const Value *P);
291 // CheckGEPInstructions - Check two GEP instructions with known
292 // must-aliasing base pointers. This checks to see if the index expressions
293 // preclude the pointers from aliasing...
295 CheckGEPInstructions(const Type* BasePtr1Ty,
296 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
297 const Type *BasePtr2Ty,
298 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
300 } // End of anonymous namespace
302 // Register this pass...
303 char BasicAliasAnalysis::ID = 0;
304 static RegisterPass<BasicAliasAnalysis>
305 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
307 // Declare that we implement the AliasAnalysis interface
308 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
310 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
311 return new BasicAliasAnalysis();
315 /// pointsToConstantMemory - Chase pointers until we find a (constant
317 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
318 if (const GlobalVariable *GV =
319 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
320 return GV->isConstant();
324 // getModRefInfo - Check to see if the specified callsite can clobber the
325 // specified memory object. Since we only look at local properties of this
326 // function, we really can't say much about this query. We do, however, use
327 // simple "address taken" analysis on local objects.
329 AliasAnalysis::ModRefResult
330 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
331 if (!isa<Constant>(P)) {
332 const Value *Object = P->getUnderlyingObject();
334 // If this is a tail call and P points to a stack location, we know that
335 // the tail call cannot access or modify the local stack.
336 // We cannot exclude byval arguments here; these belong to the caller of
337 // the current function not to the current function, and a tail callee
338 // may reference them.
339 if (isa<AllocaInst>(Object))
340 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
341 if (CI->isTailCall())
344 // If the pointer is to a locally allocated object that does not escape,
345 // then the call can not mod/ref the pointer unless the call takes the
346 // argument without capturing it.
347 if (isNonEscapingLocalObject(Object)) {
348 bool passedAsArg = false;
349 // TODO: Eventually only check 'nocapture' arguments.
350 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
352 if (isa<PointerType>((*CI)->getType()) &&
353 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
361 // The AliasAnalysis base class has some smarts, lets use them.
362 return AliasAnalysis::getModRefInfo(CS, P, Size);
366 AliasAnalysis::ModRefResult
367 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
368 // If CS1 or CS2 are readnone, they don't interact.
369 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
370 if (CS1B == DoesNotAccessMemory) return NoModRef;
372 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
373 if (CS2B == DoesNotAccessMemory) return NoModRef;
375 // If they both only read from memory, just return ref.
376 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
379 // Otherwise, fall back to NoAA (mod+ref).
380 return NoAA::getModRefInfo(CS1, CS2);
384 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
385 // as array references.
387 AliasAnalysis::AliasResult
388 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
389 const Value *V2, unsigned V2Size) {
390 // Strip off any constant expression casts if they exist
391 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
392 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
393 V1 = CE->getOperand(0);
394 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
395 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
396 V2 = CE->getOperand(0);
398 // Are we checking for alias of the same value?
399 if (V1 == V2) return MustAlias;
401 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
402 return NoAlias; // Scalars cannot alias each other
404 // Strip off cast instructions. Since V1 and V2 are pointers, they must be
405 // pointer<->pointer bitcasts.
406 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
407 return alias(I->getOperand(0), V1Size, V2, V2Size);
408 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
409 return alias(V1, V1Size, I->getOperand(0), V2Size);
411 // Figure out what objects these things are pointing to if we can.
412 const Value *O1 = V1->getUnderlyingObject();
413 const Value *O2 = V2->getUnderlyingObject();
416 // If V1/V2 point to two different objects we know that we have no alias.
417 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
420 // Arguments can't alias with local allocations or noalias calls.
421 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
422 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
425 // Most objects can't alias null.
426 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
427 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
431 // If the size of one access is larger than the entire object on the other
432 // side, then we know such behavior is undefined and can assume no alias.
433 const TargetData &TD = getTargetData();
434 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
435 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
438 // If one pointer is the result of a call/invoke and the other is a
439 // non-escaping local object, then we know the object couldn't escape to a
440 // point where the call could return it.
441 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
442 isNonEscapingLocalObject(O2))
444 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
445 isNonEscapingLocalObject(O1))
448 // If we have two gep instructions with must-alias'ing base pointers, figure
449 // out if the indexes to the GEP tell us anything about the derived pointer.
450 // Note that we also handle chains of getelementptr instructions as well as
451 // constant expression getelementptrs here.
453 if (isGEP(V1) && isGEP(V2)) {
454 const User *GEP1 = cast<User>(V1);
455 const User *GEP2 = cast<User>(V2);
457 // If V1 and V2 are identical GEPs, just recurse down on both of them.
458 // This allows us to analyze things like:
459 // P = gep A, 0, i, 1
460 // Q = gep B, 0, i, 1
461 // by just analyzing A and B. This is even safe for variable indices.
462 if (GEP1->getType() == GEP2->getType() &&
463 GEP1->getNumOperands() == GEP2->getNumOperands() &&
464 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
465 // All operands are the same, ignoring the base.
466 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
467 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
470 // Drill down into the first non-gep value, to test for must-aliasing of
471 // the base pointers.
472 while (isGEP(GEP1->getOperand(0)) &&
473 GEP1->getOperand(1) ==
474 Constant::getNullValue(GEP1->getOperand(1)->getType()))
475 GEP1 = cast<User>(GEP1->getOperand(0));
476 const Value *BasePtr1 = GEP1->getOperand(0);
478 while (isGEP(GEP2->getOperand(0)) &&
479 GEP2->getOperand(1) ==
480 Constant::getNullValue(GEP2->getOperand(1)->getType()))
481 GEP2 = cast<User>(GEP2->getOperand(0));
482 const Value *BasePtr2 = GEP2->getOperand(0);
484 // Do the base pointers alias?
485 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
486 if (BaseAlias == NoAlias) return NoAlias;
487 if (BaseAlias == MustAlias) {
488 // If the base pointers alias each other exactly, check to see if we can
489 // figure out anything about the resultant pointers, to try to prove
492 // Collect all of the chained GEP operands together into one simple place
493 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
494 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
495 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
497 // If GetGEPOperands were able to fold to the same must-aliased pointer,
498 // do the comparison.
499 if (BasePtr1 == BasePtr2) {
501 CheckGEPInstructions(BasePtr1->getType(),
502 &GEP1Ops[0], GEP1Ops.size(), V1Size,
504 &GEP2Ops[0], GEP2Ops.size(), V2Size);
505 if (GAlias != MayAlias)
511 // Check to see if these two pointers are related by a getelementptr
512 // instruction. If one pointer is a GEP with a non-zero index of the other
513 // pointer, we know they cannot alias.
517 std::swap(V1Size, V2Size);
520 if (V1Size != ~0U && V2Size != ~0U)
522 SmallVector<Value*, 16> GEPOperands;
523 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
525 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
526 if (R == MustAlias) {
527 // If there is at least one non-zero constant index, we know they cannot
529 bool ConstantFound = false;
530 bool AllZerosFound = true;
531 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
532 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
533 if (!C->isNullValue()) {
534 ConstantFound = true;
535 AllZerosFound = false;
539 AllZerosFound = false;
542 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
543 // the ptr, the end result is a must alias also.
548 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
551 // Otherwise we have to check to see that the distance is more than
552 // the size of the argument... build an index vector that is equal to
553 // the arguments provided, except substitute 0's for any variable
554 // indexes we find...
555 if (cast<PointerType>(
556 BasePtr->getType())->getElementType()->isSized()) {
557 for (unsigned i = 0; i != GEPOperands.size(); ++i)
558 if (!isa<ConstantInt>(GEPOperands[i]))
560 Constant::getNullValue(GEPOperands[i]->getType());
562 getTargetData().getIndexedOffset(BasePtr->getType(),
566 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
576 // This function is used to determine if the indices of two GEP instructions are
577 // equal. V1 and V2 are the indices.
578 static bool IndexOperandsEqual(Value *V1, Value *V2) {
579 if (V1->getType() == V2->getType())
581 if (Constant *C1 = dyn_cast<Constant>(V1))
582 if (Constant *C2 = dyn_cast<Constant>(V2)) {
583 // Sign extend the constants to long types, if necessary
584 if (C1->getType() != Type::Int64Ty)
585 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty);
586 if (C2->getType() != Type::Int64Ty)
587 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty);
593 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
594 /// base pointers. This checks to see if the index expressions preclude the
595 /// pointers from aliasing...
596 AliasAnalysis::AliasResult
597 BasicAliasAnalysis::CheckGEPInstructions(
598 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
599 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
600 // We currently can't handle the case when the base pointers have different
601 // primitive types. Since this is uncommon anyway, we are happy being
602 // extremely conservative.
603 if (BasePtr1Ty != BasePtr2Ty)
606 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
608 // Find the (possibly empty) initial sequence of equal values... which are not
609 // necessarily constants.
610 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
611 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
612 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
613 unsigned UnequalOper = 0;
614 while (UnequalOper != MinOperands &&
615 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
616 // Advance through the type as we go...
618 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
619 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
621 // If all operands equal each other, then the derived pointers must
622 // alias each other...
624 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
625 "Ran out of type nesting, but not out of operands?");
630 // If we have seen all constant operands, and run out of indexes on one of the
631 // getelementptrs, check to see if the tail of the leftover one is all zeros.
632 // If so, return mustalias.
633 if (UnequalOper == MinOperands) {
634 if (NumGEP1Ops < NumGEP2Ops) {
635 std::swap(GEP1Ops, GEP2Ops);
636 std::swap(NumGEP1Ops, NumGEP2Ops);
639 bool AllAreZeros = true;
640 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
641 if (!isa<Constant>(GEP1Ops[i]) ||
642 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
646 if (AllAreZeros) return MustAlias;
650 // So now we know that the indexes derived from the base pointers,
651 // which are known to alias, are different. We can still determine a
652 // no-alias result if there are differing constant pairs in the index
653 // chain. For example:
654 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
656 // We have to be careful here about array accesses. In particular, consider:
657 // A[1][0] vs A[0][i]
658 // In this case, we don't *know* that the array will be accessed in bounds:
659 // the index could even be negative. Because of this, we have to
660 // conservatively *give up* and return may alias. We disregard differing
661 // array subscripts that are followed by a variable index without going
664 unsigned SizeMax = std::max(G1S, G2S);
665 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
667 // Scan for the first operand that is constant and unequal in the
668 // two getelementptrs...
669 unsigned FirstConstantOper = UnequalOper;
670 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
671 const Value *G1Oper = GEP1Ops[FirstConstantOper];
672 const Value *G2Oper = GEP2Ops[FirstConstantOper];
674 if (G1Oper != G2Oper) // Found non-equal constant indexes...
675 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
676 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
677 if (G1OC->getType() != G2OC->getType()) {
678 // Sign extend both operands to long.
679 if (G1OC->getType() != Type::Int64Ty)
680 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty);
681 if (G2OC->getType() != Type::Int64Ty)
682 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty);
683 GEP1Ops[FirstConstantOper] = G1OC;
684 GEP2Ops[FirstConstantOper] = G2OC;
688 // Handle the "be careful" case above: if this is an array/vector
689 // subscript, scan for a subsequent variable array index.
690 if (isa<SequentialType>(BasePtr1Ty)) {
692 cast<SequentialType>(BasePtr1Ty)->getElementType();
693 bool isBadCase = false;
695 for (unsigned Idx = FirstConstantOper+1;
696 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
697 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
698 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
702 NextTy = cast<SequentialType>(NextTy)->getElementType();
705 if (isBadCase) G1OC = 0;
708 // Make sure they are comparable (ie, not constant expressions), and
709 // make sure the GEP with the smaller leading constant is GEP1.
711 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
713 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
714 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
715 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
716 std::swap(NumGEP1Ops, NumGEP2Ops);
723 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
726 // No shared constant operands, and we ran out of common operands. At this
727 // point, the GEP instructions have run through all of their operands, and we
728 // haven't found evidence that there are any deltas between the GEP's.
729 // However, one GEP may have more operands than the other. If this is the
730 // case, there may still be hope. Check this now.
731 if (FirstConstantOper == MinOperands) {
732 // Make GEP1Ops be the longer one if there is a longer one.
733 if (NumGEP1Ops < NumGEP2Ops) {
734 std::swap(GEP1Ops, GEP2Ops);
735 std::swap(NumGEP1Ops, NumGEP2Ops);
738 // Is there anything to check?
739 if (NumGEP1Ops > MinOperands) {
740 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
741 if (isa<ConstantInt>(GEP1Ops[i]) &&
742 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
743 // Yup, there's a constant in the tail. Set all variables to
744 // constants in the GEP instruction to make it suitable for
745 // TargetData::getIndexedOffset.
746 for (i = 0; i != MaxOperands; ++i)
747 if (!isa<ConstantInt>(GEP1Ops[i]))
748 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
749 // Okay, now get the offset. This is the relative offset for the full
751 const TargetData &TD = getTargetData();
752 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
755 // Now check without any constants at the end.
756 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
759 // Make sure we compare the absolute difference.
760 if (Offset1 > Offset2)
761 std::swap(Offset1, Offset2);
763 // If the tail provided a bit enough offset, return noalias!
764 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
766 // Otherwise break - we don't look for another constant in the tail.
771 // Couldn't find anything useful.
775 // If there are non-equal constants arguments, then we can figure
776 // out a minimum known delta between the two index expressions... at
777 // this point we know that the first constant index of GEP1 is less
778 // than the first constant index of GEP2.
780 // Advance BasePtr[12]Ty over this first differing constant operand.
781 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
782 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
783 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
784 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
786 // We are going to be using TargetData::getIndexedOffset to determine the
787 // offset that each of the GEP's is reaching. To do this, we have to convert
788 // all variable references to constant references. To do this, we convert the
789 // initial sequence of array subscripts into constant zeros to start with.
790 const Type *ZeroIdxTy = GEPPointerTy;
791 for (unsigned i = 0; i != FirstConstantOper; ++i) {
792 if (!isa<StructType>(ZeroIdxTy))
793 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty);
795 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
796 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
799 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
801 // Loop over the rest of the operands...
802 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
803 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
804 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
805 // If they are equal, use a zero index...
806 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
807 if (!isa<ConstantInt>(Op1))
808 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
809 // Otherwise, just keep the constants we have.
812 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
813 // If this is an array index, make sure the array element is in range.
814 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
815 if (Op1C->getZExtValue() >= AT->getNumElements())
816 return MayAlias; // Be conservative with out-of-range accesses
817 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
818 if (Op1C->getZExtValue() >= VT->getNumElements())
819 return MayAlias; // Be conservative with out-of-range accesses
823 // GEP1 is known to produce a value less than GEP2. To be
824 // conservatively correct, we must assume the largest possible
825 // constant is used in this position. This cannot be the initial
826 // index to the GEP instructions (because we know we have at least one
827 // element before this one with the different constant arguments), so
828 // we know that the current index must be into either a struct or
829 // array. Because we know it's not constant, this cannot be a
830 // structure index. Because of this, we can calculate the maximum
833 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
834 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1);
835 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
836 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1);
841 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
842 // If this is an array index, make sure the array element is in range.
843 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
844 if (Op2C->getZExtValue() >= AT->getNumElements())
845 return MayAlias; // Be conservative with out-of-range accesses
846 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
847 if (Op2C->getZExtValue() >= VT->getNumElements())
848 return MayAlias; // Be conservative with out-of-range accesses
850 } else { // Conservatively assume the minimum value for this index
851 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
856 if (BasePtr1Ty && Op1) {
857 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
858 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
863 if (BasePtr2Ty && Op2) {
864 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
865 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
871 if (GEPPointerTy->getElementType()->isSized()) {
873 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
875 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
876 assert(Offset1 != Offset2 &&
877 "There is at least one different constant here!");
879 // Make sure we compare the absolute difference.
880 if (Offset1 > Offset2)
881 std::swap(Offset1, Offset2);
883 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
884 //cerr << "Determined that these two GEP's don't alias ["
885 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
892 // Make sure that anything that uses AliasAnalysis pulls in this file...
893 DEFINING_FILE_FOR(BasicAliasAnalysis)