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 an AllocationInst instruction escapes from the function it is
39 // contained in. If it does not escape, there is no way for another function to
40 // mod/ref it. We do this by looking at its uses and determining if the uses
41 // can escape (recursively).
42 static bool AddressMightEscape(const Value *V) {
43 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end();
45 const Instruction *I = cast<Instruction>(*UI);
46 switch (I->getOpcode()) {
47 case Instruction::Load:
49 case Instruction::Store:
50 if (I->getOperand(0) == V)
51 return true; // Escapes if the pointer is stored.
53 case Instruction::GetElementPtr:
54 if (AddressMightEscape(I))
57 case Instruction::BitCast:
58 if (AddressMightEscape(I))
61 case Instruction::Ret:
62 // If returned, the address will escape to calling functions, but no
63 // callees could modify it.
65 case Instruction::Call:
66 // If the argument to the call has the nocapture attribute, then the call
67 // may store or load to the pointer, but it cannot escape.
68 if (cast<CallInst>(I)->paramHasAttr(UI.getOperandNo(),
69 Attribute::NoCapture))
72 // FIXME: MemIntrinsics should have their operands marked nocapture!
73 if (isa<MemIntrinsic>(I))
76 case Instruction::Invoke:
77 // If the argument to the call has the nocapture attribute, then the call
78 // may store or load to the pointer, but it cannot escape.
79 if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
80 Attribute::NoCapture))
90 static const User *isGEP(const Value *V) {
91 if (isa<GetElementPtrInst>(V) ||
92 (isa<ConstantExpr>(V) &&
93 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
98 static const Value *GetGEPOperands(const Value *V,
99 SmallVector<Value*, 16> &GEPOps) {
100 assert(GEPOps.empty() && "Expect empty list to populate!");
101 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
102 cast<User>(V)->op_end());
104 // Accumulate all of the chained indexes into the operand array
105 V = cast<User>(V)->getOperand(0);
107 while (const User *G = isGEP(V)) {
108 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
109 !cast<Constant>(GEPOps[0])->isNullValue())
110 break; // Don't handle folding arbitrary pointer offsets yet...
111 GEPOps.erase(GEPOps.begin()); // Drop the zero index
112 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
113 V = G->getOperand(0);
118 /// isNoAliasCall - Return true if this pointer is returned by a noalias
120 static bool isNoAliasCall(const Value *V) {
121 if (isa<CallInst>(V) || isa<InvokeInst>(V))
122 return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
123 .paramHasAttr(0, Attribute::NoAlias);
127 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
128 /// identifiable object. This returns true for:
129 /// Global Variables and Functions
130 /// Allocas and Mallocs
131 /// ByVal and NoAlias Arguments
134 static bool isIdentifiedObject(const Value *V) {
135 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
137 if (const Argument *A = dyn_cast<Argument>(V))
138 return A->hasNoAliasAttr() || A->hasByValAttr();
142 /// isKnownNonNull - Return true if we know that the specified value is never
144 static bool isKnownNonNull(const Value *V) {
145 // Alloca never returns null, malloc might.
146 if (isa<AllocaInst>(V)) return true;
148 // A byval argument is never null.
149 if (const Argument *A = dyn_cast<Argument>(V))
150 return A->hasByValAttr();
152 // Global values are not null unless extern weak.
153 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
154 return !GV->hasExternalWeakLinkage();
158 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
159 /// object that never escapes from the function.
160 static bool isNonEscapingLocalObject(const Value *V) {
161 // If this is a local allocation, check to see if it escapes.
162 if (isa<AllocationInst>(V) || isNoAliasCall(V))
163 return !AddressMightEscape(V);
165 // If this is an argument that corresponds to a byval or noalias argument,
166 // it can't escape either.
167 if (const Argument *A = dyn_cast<Argument>(V))
168 if (A->hasByValAttr() || A->hasNoAliasAttr())
169 return !AddressMightEscape(V);
174 /// isObjectSmallerThan - Return true if we can prove that the object specified
175 /// by V is smaller than Size.
176 static bool isObjectSmallerThan(const Value *V, unsigned Size,
177 const TargetData &TD) {
178 const Type *AccessTy;
179 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
180 AccessTy = GV->getType()->getElementType();
181 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
182 if (!AI->isArrayAllocation())
183 AccessTy = AI->getType()->getElementType();
186 } else if (const Argument *A = dyn_cast<Argument>(V)) {
187 if (A->hasByValAttr())
188 AccessTy = cast<PointerType>(A->getType())->getElementType();
195 if (AccessTy->isSized())
196 return TD.getABITypeSize(AccessTy) < Size;
200 //===----------------------------------------------------------------------===//
202 //===----------------------------------------------------------------------===//
205 /// NoAA - This class implements the -no-aa pass, which always returns "I
206 /// don't know" for alias queries. NoAA is unlike other alias analysis
207 /// implementations, in that it does not chain to a previous analysis. As
208 /// such it doesn't follow many of the rules that other alias analyses must.
210 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
211 static char ID; // Class identification, replacement for typeinfo
212 NoAA() : ImmutablePass(&ID) {}
213 explicit NoAA(void *PID) : ImmutablePass(PID) { }
215 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
216 AU.addRequired<TargetData>();
219 virtual void initializePass() {
220 TD = &getAnalysis<TargetData>();
223 virtual AliasResult alias(const Value *V1, unsigned V1Size,
224 const Value *V2, unsigned V2Size) {
228 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
229 std::vector<PointerAccessInfo> *Info) {
230 return UnknownModRefBehavior;
233 virtual void getArgumentAccesses(Function *F, CallSite CS,
234 std::vector<PointerAccessInfo> &Info) {
235 assert(0 && "This method may not be called on this function!");
238 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
239 virtual bool pointsToConstantMemory(const Value *P) { return false; }
240 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
243 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
246 virtual bool hasNoModRefInfoForCalls() const { return true; }
248 virtual void deleteValue(Value *V) {}
249 virtual void copyValue(Value *From, Value *To) {}
251 } // End of anonymous namespace
253 // Register this pass...
255 static RegisterPass<NoAA>
256 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
258 // Declare that we implement the AliasAnalysis interface
259 static RegisterAnalysisGroup<AliasAnalysis> V(U);
261 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
263 //===----------------------------------------------------------------------===//
265 //===----------------------------------------------------------------------===//
268 /// BasicAliasAnalysis - This is the default alias analysis implementation.
269 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
270 /// derives from the NoAA class.
271 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
272 static char ID; // Class identification, replacement for typeinfo
273 BasicAliasAnalysis() : NoAA(&ID) {}
274 AliasResult alias(const Value *V1, unsigned V1Size,
275 const Value *V2, unsigned V2Size);
277 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
278 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
280 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
281 /// non-escaping allocations.
282 virtual bool hasNoModRefInfoForCalls() const { return false; }
284 /// pointsToConstantMemory - Chase pointers until we find a (constant
286 bool pointsToConstantMemory(const Value *P);
289 // CheckGEPInstructions - Check two GEP instructions with known
290 // must-aliasing base pointers. This checks to see if the index expressions
291 // preclude the pointers from aliasing...
293 CheckGEPInstructions(const Type* BasePtr1Ty,
294 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
295 const Type *BasePtr2Ty,
296 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
298 } // End of anonymous namespace
300 // Register this pass...
301 char BasicAliasAnalysis::ID = 0;
302 static RegisterPass<BasicAliasAnalysis>
303 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
305 // Declare that we implement the AliasAnalysis interface
306 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
308 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
309 return new BasicAliasAnalysis();
313 /// pointsToConstantMemory - Chase pointers until we find a (constant
315 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
316 if (const GlobalVariable *GV =
317 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
318 return GV->isConstant();
322 // getModRefInfo - Check to see if the specified callsite can clobber the
323 // specified memory object. Since we only look at local properties of this
324 // function, we really can't say much about this query. We do, however, use
325 // simple "address taken" analysis on local objects.
327 AliasAnalysis::ModRefResult
328 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
329 if (!isa<Constant>(P)) {
330 const Value *Object = P->getUnderlyingObject();
332 // If this is a tail call and P points to a stack location, we know that
333 // the tail call cannot access or modify the local stack.
334 // We cannot exclude byval arguments here; these belong to the caller of
335 // the current function not to the current function, and a tail callee
336 // may reference them.
337 if (isa<AllocaInst>(Object))
338 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
339 if (CI->isTailCall())
342 // If the pointer is to a locally allocated object that does not escape,
343 // then the call can not mod/ref the pointer unless the call takes the
344 // argument without capturing it.
345 if (isNonEscapingLocalObject(Object)) {
346 bool passedAsArg = false;
347 // TODO: Eventually only check 'nocapture' arguments.
348 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
350 if (isa<PointerType>((*CI)->getType()) &&
351 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
359 // The AliasAnalysis base class has some smarts, lets use them.
360 return AliasAnalysis::getModRefInfo(CS, P, Size);
364 AliasAnalysis::ModRefResult
365 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
366 // If CS1 or CS2 are readnone, they don't interact.
367 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
368 if (CS1B == DoesNotAccessMemory) return NoModRef;
370 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
371 if (CS2B == DoesNotAccessMemory) return NoModRef;
373 // If they both only read from memory, just return ref.
374 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
377 // Otherwise, fall back to NoAA (mod+ref).
378 return NoAA::getModRefInfo(CS1, CS2);
382 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
383 // as array references.
385 AliasAnalysis::AliasResult
386 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
387 const Value *V2, unsigned V2Size) {
388 // Strip off any constant expression casts if they exist
389 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
390 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
391 V1 = CE->getOperand(0);
392 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
393 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
394 V2 = CE->getOperand(0);
396 // Are we checking for alias of the same value?
397 if (V1 == V2) return MustAlias;
399 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
400 return NoAlias; // Scalars cannot alias each other
402 // Strip off cast instructions. Since V1 and V2 are pointers, they must be
403 // pointer<->pointer bitcasts.
404 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
405 return alias(I->getOperand(0), V1Size, V2, V2Size);
406 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
407 return alias(V1, V1Size, I->getOperand(0), V2Size);
409 // Figure out what objects these things are pointing to if we can.
410 const Value *O1 = V1->getUnderlyingObject();
411 const Value *O2 = V2->getUnderlyingObject();
414 // If V1/V2 point to two different objects we know that we have no alias.
415 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
418 // Arguments can't alias with local allocations or noalias calls.
419 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
420 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
423 // Most objects can't alias null.
424 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
425 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
429 // If the size of one access is larger than the entire object on the other
430 // side, then we know such behavior is undefined and can assume no alias.
431 const TargetData &TD = getTargetData();
432 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
433 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
436 // If one pointer is the result of a call/invoke and the other is a
437 // non-escaping local object, then we know the object couldn't escape to a
438 // point where the call could return it.
439 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
440 isNonEscapingLocalObject(O2))
442 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
443 isNonEscapingLocalObject(O1))
446 // If we have two gep instructions with must-alias'ing base pointers, figure
447 // out if the indexes to the GEP tell us anything about the derived pointer.
448 // Note that we also handle chains of getelementptr instructions as well as
449 // constant expression getelementptrs here.
451 if (isGEP(V1) && isGEP(V2)) {
452 const User *GEP1 = cast<User>(V1);
453 const User *GEP2 = cast<User>(V2);
455 // If V1 and V2 are identical GEPs, just recurse down on both of them.
456 // This allows us to analyze things like:
457 // P = gep A, 0, i, 1
458 // Q = gep B, 0, i, 1
459 // by just analyzing A and B. This is even safe for variable indices.
460 if (GEP1->getType() == GEP2->getType() &&
461 GEP1->getNumOperands() == GEP2->getNumOperands() &&
462 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
463 // All operands are the same, ignoring the base.
464 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
465 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
468 // Drill down into the first non-gep value, to test for must-aliasing of
469 // the base pointers.
470 while (isGEP(GEP1->getOperand(0)) &&
471 GEP1->getOperand(1) ==
472 Constant::getNullValue(GEP1->getOperand(1)->getType()))
473 GEP1 = cast<User>(GEP1->getOperand(0));
474 const Value *BasePtr1 = GEP1->getOperand(0);
476 while (isGEP(GEP2->getOperand(0)) &&
477 GEP2->getOperand(1) ==
478 Constant::getNullValue(GEP2->getOperand(1)->getType()))
479 GEP2 = cast<User>(GEP2->getOperand(0));
480 const Value *BasePtr2 = GEP2->getOperand(0);
482 // Do the base pointers alias?
483 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
484 if (BaseAlias == NoAlias) return NoAlias;
485 if (BaseAlias == MustAlias) {
486 // If the base pointers alias each other exactly, check to see if we can
487 // figure out anything about the resultant pointers, to try to prove
490 // Collect all of the chained GEP operands together into one simple place
491 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
492 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
493 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
495 // If GetGEPOperands were able to fold to the same must-aliased pointer,
496 // do the comparison.
497 if (BasePtr1 == BasePtr2) {
499 CheckGEPInstructions(BasePtr1->getType(),
500 &GEP1Ops[0], GEP1Ops.size(), V1Size,
502 &GEP2Ops[0], GEP2Ops.size(), V2Size);
503 if (GAlias != MayAlias)
509 // Check to see if these two pointers are related by a getelementptr
510 // instruction. If one pointer is a GEP with a non-zero index of the other
511 // pointer, we know they cannot alias.
515 std::swap(V1Size, V2Size);
518 if (V1Size != ~0U && V2Size != ~0U)
520 SmallVector<Value*, 16> GEPOperands;
521 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
523 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
524 if (R == MustAlias) {
525 // If there is at least one non-zero constant index, we know they cannot
527 bool ConstantFound = false;
528 bool AllZerosFound = true;
529 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
530 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
531 if (!C->isNullValue()) {
532 ConstantFound = true;
533 AllZerosFound = false;
537 AllZerosFound = false;
540 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
541 // the ptr, the end result is a must alias also.
546 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
549 // Otherwise we have to check to see that the distance is more than
550 // the size of the argument... build an index vector that is equal to
551 // the arguments provided, except substitute 0's for any variable
552 // indexes we find...
553 if (cast<PointerType>(
554 BasePtr->getType())->getElementType()->isSized()) {
555 for (unsigned i = 0; i != GEPOperands.size(); ++i)
556 if (!isa<ConstantInt>(GEPOperands[i]))
558 Constant::getNullValue(GEPOperands[i]->getType());
560 getTargetData().getIndexedOffset(BasePtr->getType(),
564 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
574 // This function is used to determine if the indices of two GEP instructions are
575 // equal. V1 and V2 are the indices.
576 static bool IndexOperandsEqual(Value *V1, Value *V2) {
577 if (V1->getType() == V2->getType())
579 if (Constant *C1 = dyn_cast<Constant>(V1))
580 if (Constant *C2 = dyn_cast<Constant>(V2)) {
581 // Sign extend the constants to long types, if necessary
582 if (C1->getType() != Type::Int64Ty)
583 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty);
584 if (C2->getType() != Type::Int64Ty)
585 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty);
591 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
592 /// base pointers. This checks to see if the index expressions preclude the
593 /// pointers from aliasing...
594 AliasAnalysis::AliasResult
595 BasicAliasAnalysis::CheckGEPInstructions(
596 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
597 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
598 // We currently can't handle the case when the base pointers have different
599 // primitive types. Since this is uncommon anyway, we are happy being
600 // extremely conservative.
601 if (BasePtr1Ty != BasePtr2Ty)
604 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
606 // Find the (possibly empty) initial sequence of equal values... which are not
607 // necessarily constants.
608 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
609 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
610 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
611 unsigned UnequalOper = 0;
612 while (UnequalOper != MinOperands &&
613 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
614 // Advance through the type as we go...
616 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
617 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
619 // If all operands equal each other, then the derived pointers must
620 // alias each other...
622 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
623 "Ran out of type nesting, but not out of operands?");
628 // If we have seen all constant operands, and run out of indexes on one of the
629 // getelementptrs, check to see if the tail of the leftover one is all zeros.
630 // If so, return mustalias.
631 if (UnequalOper == MinOperands) {
632 if (NumGEP1Ops < NumGEP2Ops) {
633 std::swap(GEP1Ops, GEP2Ops);
634 std::swap(NumGEP1Ops, NumGEP2Ops);
637 bool AllAreZeros = true;
638 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
639 if (!isa<Constant>(GEP1Ops[i]) ||
640 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
644 if (AllAreZeros) return MustAlias;
648 // So now we know that the indexes derived from the base pointers,
649 // which are known to alias, are different. We can still determine a
650 // no-alias result if there are differing constant pairs in the index
651 // chain. For example:
652 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
654 // We have to be careful here about array accesses. In particular, consider:
655 // A[1][0] vs A[0][i]
656 // In this case, we don't *know* that the array will be accessed in bounds:
657 // the index could even be negative. Because of this, we have to
658 // conservatively *give up* and return may alias. We disregard differing
659 // array subscripts that are followed by a variable index without going
662 unsigned SizeMax = std::max(G1S, G2S);
663 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
665 // Scan for the first operand that is constant and unequal in the
666 // two getelementptrs...
667 unsigned FirstConstantOper = UnequalOper;
668 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
669 const Value *G1Oper = GEP1Ops[FirstConstantOper];
670 const Value *G2Oper = GEP2Ops[FirstConstantOper];
672 if (G1Oper != G2Oper) // Found non-equal constant indexes...
673 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
674 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
675 if (G1OC->getType() != G2OC->getType()) {
676 // Sign extend both operands to long.
677 if (G1OC->getType() != Type::Int64Ty)
678 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty);
679 if (G2OC->getType() != Type::Int64Ty)
680 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty);
681 GEP1Ops[FirstConstantOper] = G1OC;
682 GEP2Ops[FirstConstantOper] = G2OC;
686 // Handle the "be careful" case above: if this is an array/vector
687 // subscript, scan for a subsequent variable array index.
688 if (isa<SequentialType>(BasePtr1Ty)) {
690 cast<SequentialType>(BasePtr1Ty)->getElementType();
691 bool isBadCase = false;
693 for (unsigned Idx = FirstConstantOper+1;
694 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
695 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
696 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
700 NextTy = cast<SequentialType>(NextTy)->getElementType();
703 if (isBadCase) G1OC = 0;
706 // Make sure they are comparable (ie, not constant expressions), and
707 // make sure the GEP with the smaller leading constant is GEP1.
709 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
711 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
712 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
713 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
714 std::swap(NumGEP1Ops, NumGEP2Ops);
721 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
724 // No shared constant operands, and we ran out of common operands. At this
725 // point, the GEP instructions have run through all of their operands, and we
726 // haven't found evidence that there are any deltas between the GEP's.
727 // However, one GEP may have more operands than the other. If this is the
728 // case, there may still be hope. Check this now.
729 if (FirstConstantOper == MinOperands) {
730 // Make GEP1Ops be the longer one if there is a longer one.
731 if (NumGEP1Ops < NumGEP2Ops) {
732 std::swap(GEP1Ops, GEP2Ops);
733 std::swap(NumGEP1Ops, NumGEP2Ops);
736 // Is there anything to check?
737 if (NumGEP1Ops > MinOperands) {
738 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
739 if (isa<ConstantInt>(GEP1Ops[i]) &&
740 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
741 // Yup, there's a constant in the tail. Set all variables to
742 // constants in the GEP instruction to make it suitable for
743 // TargetData::getIndexedOffset.
744 for (i = 0; i != MaxOperands; ++i)
745 if (!isa<ConstantInt>(GEP1Ops[i]))
746 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
747 // Okay, now get the offset. This is the relative offset for the full
749 const TargetData &TD = getTargetData();
750 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
753 // Now check without any constants at the end.
754 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
757 // Make sure we compare the absolute difference.
758 if (Offset1 > Offset2)
759 std::swap(Offset1, Offset2);
761 // If the tail provided a bit enough offset, return noalias!
762 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
764 // Otherwise break - we don't look for another constant in the tail.
769 // Couldn't find anything useful.
773 // If there are non-equal constants arguments, then we can figure
774 // out a minimum known delta between the two index expressions... at
775 // this point we know that the first constant index of GEP1 is less
776 // than the first constant index of GEP2.
778 // Advance BasePtr[12]Ty over this first differing constant operand.
779 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
780 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
781 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
782 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
784 // We are going to be using TargetData::getIndexedOffset to determine the
785 // offset that each of the GEP's is reaching. To do this, we have to convert
786 // all variable references to constant references. To do this, we convert the
787 // initial sequence of array subscripts into constant zeros to start with.
788 const Type *ZeroIdxTy = GEPPointerTy;
789 for (unsigned i = 0; i != FirstConstantOper; ++i) {
790 if (!isa<StructType>(ZeroIdxTy))
791 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty);
793 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
794 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
797 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
799 // Loop over the rest of the operands...
800 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
801 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
802 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
803 // If they are equal, use a zero index...
804 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
805 if (!isa<ConstantInt>(Op1))
806 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
807 // Otherwise, just keep the constants we have.
810 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
811 // If this is an array index, make sure the array element is in range.
812 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
813 if (Op1C->getZExtValue() >= AT->getNumElements())
814 return MayAlias; // Be conservative with out-of-range accesses
815 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
816 if (Op1C->getZExtValue() >= VT->getNumElements())
817 return MayAlias; // Be conservative with out-of-range accesses
821 // GEP1 is known to produce a value less than GEP2. To be
822 // conservatively correct, we must assume the largest possible
823 // constant is used in this position. This cannot be the initial
824 // index to the GEP instructions (because we know we have at least one
825 // element before this one with the different constant arguments), so
826 // we know that the current index must be into either a struct or
827 // array. Because we know it's not constant, this cannot be a
828 // structure index. Because of this, we can calculate the maximum
831 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
832 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1);
833 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
834 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1);
839 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
840 // If this is an array index, make sure the array element is in range.
841 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
842 if (Op2C->getZExtValue() >= AT->getNumElements())
843 return MayAlias; // Be conservative with out-of-range accesses
844 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
845 if (Op2C->getZExtValue() >= VT->getNumElements())
846 return MayAlias; // Be conservative with out-of-range accesses
848 } else { // Conservatively assume the minimum value for this index
849 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
854 if (BasePtr1Ty && Op1) {
855 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
856 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
861 if (BasePtr2Ty && Op2) {
862 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
863 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
869 if (GEPPointerTy->getElementType()->isSized()) {
871 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
873 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
874 assert(Offset1 != Offset2 &&
875 "There is at least one different constant here!");
877 // Make sure we compare the absolute difference.
878 if (Offset1 > Offset2)
879 std::swap(Offset1, Offset2);
881 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
882 //cerr << "Determined that these two GEP's don't alias ["
883 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
890 // Make sure that anything that uses AliasAnalysis pulls in this file...
891 DEFINING_FILE_FOR(BasicAliasAnalysis)