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 // FIXME: MemIntrinsics should have their operands marked nocapture!
74 if (isa<MemIntrinsic>(I))
77 case Instruction::Invoke:
78 // If the argument to the call has the nocapture attribute, then the call
79 // may store or load to the pointer, but it cannot escape.
80 if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
81 Attribute::NoCapture))
91 static const User *isGEP(const Value *V) {
92 if (isa<GetElementPtrInst>(V) ||
93 (isa<ConstantExpr>(V) &&
94 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
99 static const Value *GetGEPOperands(const Value *V,
100 SmallVector<Value*, 16> &GEPOps) {
101 assert(GEPOps.empty() && "Expect empty list to populate!");
102 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
103 cast<User>(V)->op_end());
105 // Accumulate all of the chained indexes into the operand array
106 V = cast<User>(V)->getOperand(0);
108 while (const User *G = isGEP(V)) {
109 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
110 !cast<Constant>(GEPOps[0])->isNullValue())
111 break; // Don't handle folding arbitrary pointer offsets yet...
112 GEPOps.erase(GEPOps.begin()); // Drop the zero index
113 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
114 V = G->getOperand(0);
119 /// isNoAliasCall - Return true if this pointer is returned by a noalias
121 static bool isNoAliasCall(const Value *V) {
122 if (isa<CallInst>(V) || isa<InvokeInst>(V))
123 return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
124 .paramHasAttr(0, Attribute::NoAlias);
128 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
129 /// identifiable object. This returns true for:
130 /// Global Variables and Functions
131 /// Allocas and Mallocs
132 /// ByVal and NoAlias Arguments
135 static bool isIdentifiedObject(const Value *V) {
136 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
138 if (const Argument *A = dyn_cast<Argument>(V))
139 return A->hasNoAliasAttr() || A->hasByValAttr();
143 /// isKnownNonNull - Return true if we know that the specified value is never
145 static bool isKnownNonNull(const Value *V) {
146 // Alloca never returns null, malloc might.
147 if (isa<AllocaInst>(V)) return true;
149 // A byval argument is never null.
150 if (const Argument *A = dyn_cast<Argument>(V))
151 return A->hasByValAttr();
153 // Global values are not null unless extern weak.
154 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
155 return !GV->hasExternalWeakLinkage();
159 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
160 /// object that never escapes from the function.
161 static bool isNonEscapingLocalObject(const Value *V) {
162 // If this is a local allocation, check to see if it escapes.
163 if (isa<AllocationInst>(V) || isNoAliasCall(V))
164 return !AddressMightEscape(V);
166 // If this is an argument that corresponds to a byval or noalias argument,
167 // then it has not escaped before entering the function. Check if it escapes
168 // inside the function.
169 if (const Argument *A = dyn_cast<Argument>(V))
170 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
171 // Don't bother analyzing arguments already known not to escape.
172 if (A->hasNoCaptureAttr())
174 return !AddressMightEscape(V);
180 /// isObjectSmallerThan - Return true if we can prove that the object specified
181 /// by V is smaller than Size.
182 static bool isObjectSmallerThan(const Value *V, unsigned Size,
183 const TargetData &TD) {
184 const Type *AccessTy;
185 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
186 AccessTy = GV->getType()->getElementType();
187 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
188 if (!AI->isArrayAllocation())
189 AccessTy = AI->getType()->getElementType();
192 } else if (const Argument *A = dyn_cast<Argument>(V)) {
193 if (A->hasByValAttr())
194 AccessTy = cast<PointerType>(A->getType())->getElementType();
201 if (AccessTy->isSized())
202 return TD.getABITypeSize(AccessTy) < Size;
206 //===----------------------------------------------------------------------===//
208 //===----------------------------------------------------------------------===//
211 /// NoAA - This class implements the -no-aa pass, which always returns "I
212 /// don't know" for alias queries. NoAA is unlike other alias analysis
213 /// implementations, in that it does not chain to a previous analysis. As
214 /// such it doesn't follow many of the rules that other alias analyses must.
216 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
217 static char ID; // Class identification, replacement for typeinfo
218 NoAA() : ImmutablePass(&ID) {}
219 explicit NoAA(void *PID) : ImmutablePass(PID) { }
221 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
222 AU.addRequired<TargetData>();
225 virtual void initializePass() {
226 TD = &getAnalysis<TargetData>();
229 virtual AliasResult alias(const Value *V1, unsigned V1Size,
230 const Value *V2, unsigned V2Size) {
234 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
235 std::vector<PointerAccessInfo> *Info) {
236 return UnknownModRefBehavior;
239 virtual void getArgumentAccesses(Function *F, CallSite CS,
240 std::vector<PointerAccessInfo> &Info) {
241 assert(0 && "This method may not be called on this function!");
244 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
245 virtual bool pointsToConstantMemory(const Value *P) { return false; }
246 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
249 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
252 virtual bool hasNoModRefInfoForCalls() const { return true; }
254 virtual void deleteValue(Value *V) {}
255 virtual void copyValue(Value *From, Value *To) {}
257 } // End of anonymous namespace
259 // Register this pass...
261 static RegisterPass<NoAA>
262 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
264 // Declare that we implement the AliasAnalysis interface
265 static RegisterAnalysisGroup<AliasAnalysis> V(U);
267 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
269 //===----------------------------------------------------------------------===//
271 //===----------------------------------------------------------------------===//
274 /// BasicAliasAnalysis - This is the default alias analysis implementation.
275 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
276 /// derives from the NoAA class.
277 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
278 static char ID; // Class identification, replacement for typeinfo
279 BasicAliasAnalysis() : NoAA(&ID) {}
280 AliasResult alias(const Value *V1, unsigned V1Size,
281 const Value *V2, unsigned V2Size);
283 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
284 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
286 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
287 /// non-escaping allocations.
288 virtual bool hasNoModRefInfoForCalls() const { return false; }
290 /// pointsToConstantMemory - Chase pointers until we find a (constant
292 bool pointsToConstantMemory(const Value *P);
295 // CheckGEPInstructions - Check two GEP instructions with known
296 // must-aliasing base pointers. This checks to see if the index expressions
297 // preclude the pointers from aliasing...
299 CheckGEPInstructions(const Type* BasePtr1Ty,
300 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
301 const Type *BasePtr2Ty,
302 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
304 } // End of anonymous namespace
306 // Register this pass...
307 char BasicAliasAnalysis::ID = 0;
308 static RegisterPass<BasicAliasAnalysis>
309 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
311 // Declare that we implement the AliasAnalysis interface
312 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
314 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
315 return new BasicAliasAnalysis();
319 /// pointsToConstantMemory - Chase pointers until we find a (constant
321 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
322 if (const GlobalVariable *GV =
323 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
324 return GV->isConstant();
328 // getModRefInfo - Check to see if the specified callsite can clobber the
329 // specified memory object. Since we only look at local properties of this
330 // function, we really can't say much about this query. We do, however, use
331 // simple "address taken" analysis on local objects.
333 AliasAnalysis::ModRefResult
334 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
335 if (!isa<Constant>(P)) {
336 const Value *Object = P->getUnderlyingObject();
338 // If this is a tail call and P points to a stack location, we know that
339 // the tail call cannot access or modify the local stack.
340 // We cannot exclude byval arguments here; these belong to the caller of
341 // the current function not to the current function, and a tail callee
342 // may reference them.
343 if (isa<AllocaInst>(Object))
344 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
345 if (CI->isTailCall())
348 // If the pointer is to a locally allocated object that does not escape,
349 // then the call can not mod/ref the pointer unless the call takes the
350 // argument without capturing it.
351 if (isNonEscapingLocalObject(Object)) {
352 bool passedAsArg = false;
353 // TODO: Eventually only check 'nocapture' arguments.
354 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
356 if (isa<PointerType>((*CI)->getType()) &&
357 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
365 // The AliasAnalysis base class has some smarts, lets use them.
366 return AliasAnalysis::getModRefInfo(CS, P, Size);
370 AliasAnalysis::ModRefResult
371 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
372 // If CS1 or CS2 are readnone, they don't interact.
373 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
374 if (CS1B == DoesNotAccessMemory) return NoModRef;
376 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
377 if (CS2B == DoesNotAccessMemory) return NoModRef;
379 // If they both only read from memory, just return ref.
380 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
383 // Otherwise, fall back to NoAA (mod+ref).
384 return NoAA::getModRefInfo(CS1, CS2);
388 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
389 // as array references.
391 AliasAnalysis::AliasResult
392 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
393 const Value *V2, unsigned V2Size) {
394 // Strip off any constant expression casts if they exist
395 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
396 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
397 V1 = CE->getOperand(0);
398 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
399 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
400 V2 = CE->getOperand(0);
402 // Are we checking for alias of the same value?
403 if (V1 == V2) return MustAlias;
405 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
406 return NoAlias; // Scalars cannot alias each other
408 // Strip off cast instructions. Since V1 and V2 are pointers, they must be
409 // pointer<->pointer bitcasts.
410 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
411 return alias(I->getOperand(0), V1Size, V2, V2Size);
412 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
413 return alias(V1, V1Size, I->getOperand(0), V2Size);
415 // Figure out what objects these things are pointing to if we can.
416 const Value *O1 = V1->getUnderlyingObject();
417 const Value *O2 = V2->getUnderlyingObject();
420 // If V1/V2 point to two different objects we know that we have no alias.
421 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
424 // Arguments can't alias with local allocations or noalias calls.
425 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
426 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
429 // Most objects can't alias null.
430 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
431 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
435 // If the size of one access is larger than the entire object on the other
436 // side, then we know such behavior is undefined and can assume no alias.
437 const TargetData &TD = getTargetData();
438 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
439 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
442 // If one pointer is the result of a call/invoke and the other is a
443 // non-escaping local object, then we know the object couldn't escape to a
444 // point where the call could return it.
445 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
446 isNonEscapingLocalObject(O2))
448 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
449 isNonEscapingLocalObject(O1))
452 // If we have two gep instructions with must-alias'ing base pointers, figure
453 // out if the indexes to the GEP tell us anything about the derived pointer.
454 // Note that we also handle chains of getelementptr instructions as well as
455 // constant expression getelementptrs here.
457 if (isGEP(V1) && isGEP(V2)) {
458 const User *GEP1 = cast<User>(V1);
459 const User *GEP2 = cast<User>(V2);
461 // If V1 and V2 are identical GEPs, just recurse down on both of them.
462 // This allows us to analyze things like:
463 // P = gep A, 0, i, 1
464 // Q = gep B, 0, i, 1
465 // by just analyzing A and B. This is even safe for variable indices.
466 if (GEP1->getType() == GEP2->getType() &&
467 GEP1->getNumOperands() == GEP2->getNumOperands() &&
468 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
469 // All operands are the same, ignoring the base.
470 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
471 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
474 // Drill down into the first non-gep value, to test for must-aliasing of
475 // the base pointers.
476 while (isGEP(GEP1->getOperand(0)) &&
477 GEP1->getOperand(1) ==
478 Constant::getNullValue(GEP1->getOperand(1)->getType()))
479 GEP1 = cast<User>(GEP1->getOperand(0));
480 const Value *BasePtr1 = GEP1->getOperand(0);
482 while (isGEP(GEP2->getOperand(0)) &&
483 GEP2->getOperand(1) ==
484 Constant::getNullValue(GEP2->getOperand(1)->getType()))
485 GEP2 = cast<User>(GEP2->getOperand(0));
486 const Value *BasePtr2 = GEP2->getOperand(0);
488 // Do the base pointers alias?
489 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
490 if (BaseAlias == NoAlias) return NoAlias;
491 if (BaseAlias == MustAlias) {
492 // If the base pointers alias each other exactly, check to see if we can
493 // figure out anything about the resultant pointers, to try to prove
496 // Collect all of the chained GEP operands together into one simple place
497 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
498 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
499 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
501 // If GetGEPOperands were able to fold to the same must-aliased pointer,
502 // do the comparison.
503 if (BasePtr1 == BasePtr2) {
505 CheckGEPInstructions(BasePtr1->getType(),
506 &GEP1Ops[0], GEP1Ops.size(), V1Size,
508 &GEP2Ops[0], GEP2Ops.size(), V2Size);
509 if (GAlias != MayAlias)
515 // Check to see if these two pointers are related by a getelementptr
516 // instruction. If one pointer is a GEP with a non-zero index of the other
517 // pointer, we know they cannot alias.
521 std::swap(V1Size, V2Size);
524 if (V1Size != ~0U && V2Size != ~0U)
526 SmallVector<Value*, 16> GEPOperands;
527 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
529 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
530 if (R == MustAlias) {
531 // If there is at least one non-zero constant index, we know they cannot
533 bool ConstantFound = false;
534 bool AllZerosFound = true;
535 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
536 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
537 if (!C->isNullValue()) {
538 ConstantFound = true;
539 AllZerosFound = false;
543 AllZerosFound = false;
546 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
547 // the ptr, the end result is a must alias also.
552 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
555 // Otherwise we have to check to see that the distance is more than
556 // the size of the argument... build an index vector that is equal to
557 // the arguments provided, except substitute 0's for any variable
558 // indexes we find...
559 if (cast<PointerType>(
560 BasePtr->getType())->getElementType()->isSized()) {
561 for (unsigned i = 0; i != GEPOperands.size(); ++i)
562 if (!isa<ConstantInt>(GEPOperands[i]))
564 Constant::getNullValue(GEPOperands[i]->getType());
566 getTargetData().getIndexedOffset(BasePtr->getType(),
570 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
580 // This function is used to determine if the indices of two GEP instructions are
581 // equal. V1 and V2 are the indices.
582 static bool IndexOperandsEqual(Value *V1, Value *V2) {
583 if (V1->getType() == V2->getType())
585 if (Constant *C1 = dyn_cast<Constant>(V1))
586 if (Constant *C2 = dyn_cast<Constant>(V2)) {
587 // Sign extend the constants to long types, if necessary
588 if (C1->getType() != Type::Int64Ty)
589 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty);
590 if (C2->getType() != Type::Int64Ty)
591 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty);
597 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
598 /// base pointers. This checks to see if the index expressions preclude the
599 /// pointers from aliasing...
600 AliasAnalysis::AliasResult
601 BasicAliasAnalysis::CheckGEPInstructions(
602 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
603 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
604 // We currently can't handle the case when the base pointers have different
605 // primitive types. Since this is uncommon anyway, we are happy being
606 // extremely conservative.
607 if (BasePtr1Ty != BasePtr2Ty)
610 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
612 // Find the (possibly empty) initial sequence of equal values... which are not
613 // necessarily constants.
614 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
615 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
616 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
617 unsigned UnequalOper = 0;
618 while (UnequalOper != MinOperands &&
619 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
620 // Advance through the type as we go...
622 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
623 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
625 // If all operands equal each other, then the derived pointers must
626 // alias each other...
628 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
629 "Ran out of type nesting, but not out of operands?");
634 // If we have seen all constant operands, and run out of indexes on one of the
635 // getelementptrs, check to see if the tail of the leftover one is all zeros.
636 // If so, return mustalias.
637 if (UnequalOper == MinOperands) {
638 if (NumGEP1Ops < NumGEP2Ops) {
639 std::swap(GEP1Ops, GEP2Ops);
640 std::swap(NumGEP1Ops, NumGEP2Ops);
643 bool AllAreZeros = true;
644 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
645 if (!isa<Constant>(GEP1Ops[i]) ||
646 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
650 if (AllAreZeros) return MustAlias;
654 // So now we know that the indexes derived from the base pointers,
655 // which are known to alias, are different. We can still determine a
656 // no-alias result if there are differing constant pairs in the index
657 // chain. For example:
658 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
660 // We have to be careful here about array accesses. In particular, consider:
661 // A[1][0] vs A[0][i]
662 // In this case, we don't *know* that the array will be accessed in bounds:
663 // the index could even be negative. Because of this, we have to
664 // conservatively *give up* and return may alias. We disregard differing
665 // array subscripts that are followed by a variable index without going
668 unsigned SizeMax = std::max(G1S, G2S);
669 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
671 // Scan for the first operand that is constant and unequal in the
672 // two getelementptrs...
673 unsigned FirstConstantOper = UnequalOper;
674 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
675 const Value *G1Oper = GEP1Ops[FirstConstantOper];
676 const Value *G2Oper = GEP2Ops[FirstConstantOper];
678 if (G1Oper != G2Oper) // Found non-equal constant indexes...
679 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
680 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
681 if (G1OC->getType() != G2OC->getType()) {
682 // Sign extend both operands to long.
683 if (G1OC->getType() != Type::Int64Ty)
684 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty);
685 if (G2OC->getType() != Type::Int64Ty)
686 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty);
687 GEP1Ops[FirstConstantOper] = G1OC;
688 GEP2Ops[FirstConstantOper] = G2OC;
692 // Handle the "be careful" case above: if this is an array/vector
693 // subscript, scan for a subsequent variable array index.
694 if (isa<SequentialType>(BasePtr1Ty)) {
696 cast<SequentialType>(BasePtr1Ty)->getElementType();
697 bool isBadCase = false;
699 for (unsigned Idx = FirstConstantOper+1;
700 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
701 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
702 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
706 NextTy = cast<SequentialType>(NextTy)->getElementType();
709 if (isBadCase) G1OC = 0;
712 // Make sure they are comparable (ie, not constant expressions), and
713 // make sure the GEP with the smaller leading constant is GEP1.
715 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
717 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
718 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
719 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
720 std::swap(NumGEP1Ops, NumGEP2Ops);
727 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
730 // No shared constant operands, and we ran out of common operands. At this
731 // point, the GEP instructions have run through all of their operands, and we
732 // haven't found evidence that there are any deltas between the GEP's.
733 // However, one GEP may have more operands than the other. If this is the
734 // case, there may still be hope. Check this now.
735 if (FirstConstantOper == MinOperands) {
736 // Make GEP1Ops be the longer one if there is a longer one.
737 if (NumGEP1Ops < NumGEP2Ops) {
738 std::swap(GEP1Ops, GEP2Ops);
739 std::swap(NumGEP1Ops, NumGEP2Ops);
742 // Is there anything to check?
743 if (NumGEP1Ops > MinOperands) {
744 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
745 if (isa<ConstantInt>(GEP1Ops[i]) &&
746 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
747 // Yup, there's a constant in the tail. Set all variables to
748 // constants in the GEP instruction to make it suitable for
749 // TargetData::getIndexedOffset.
750 for (i = 0; i != MaxOperands; ++i)
751 if (!isa<ConstantInt>(GEP1Ops[i]))
752 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
753 // Okay, now get the offset. This is the relative offset for the full
755 const TargetData &TD = getTargetData();
756 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
759 // Now check without any constants at the end.
760 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
763 // Make sure we compare the absolute difference.
764 if (Offset1 > Offset2)
765 std::swap(Offset1, Offset2);
767 // If the tail provided a bit enough offset, return noalias!
768 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
770 // Otherwise break - we don't look for another constant in the tail.
775 // Couldn't find anything useful.
779 // If there are non-equal constants arguments, then we can figure
780 // out a minimum known delta between the two index expressions... at
781 // this point we know that the first constant index of GEP1 is less
782 // than the first constant index of GEP2.
784 // Advance BasePtr[12]Ty over this first differing constant operand.
785 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
786 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
787 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
788 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
790 // We are going to be using TargetData::getIndexedOffset to determine the
791 // offset that each of the GEP's is reaching. To do this, we have to convert
792 // all variable references to constant references. To do this, we convert the
793 // initial sequence of array subscripts into constant zeros to start with.
794 const Type *ZeroIdxTy = GEPPointerTy;
795 for (unsigned i = 0; i != FirstConstantOper; ++i) {
796 if (!isa<StructType>(ZeroIdxTy))
797 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty);
799 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
800 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
803 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
805 // Loop over the rest of the operands...
806 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
807 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
808 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
809 // If they are equal, use a zero index...
810 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
811 if (!isa<ConstantInt>(Op1))
812 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
813 // Otherwise, just keep the constants we have.
816 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
817 // If this is an array index, make sure the array element is in range.
818 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
819 if (Op1C->getZExtValue() >= AT->getNumElements())
820 return MayAlias; // Be conservative with out-of-range accesses
821 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
822 if (Op1C->getZExtValue() >= VT->getNumElements())
823 return MayAlias; // Be conservative with out-of-range accesses
827 // GEP1 is known to produce a value less than GEP2. To be
828 // conservatively correct, we must assume the largest possible
829 // constant is used in this position. This cannot be the initial
830 // index to the GEP instructions (because we know we have at least one
831 // element before this one with the different constant arguments), so
832 // we know that the current index must be into either a struct or
833 // array. Because we know it's not constant, this cannot be a
834 // structure index. Because of this, we can calculate the maximum
837 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
838 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1);
839 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
840 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1);
845 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
846 // If this is an array index, make sure the array element is in range.
847 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
848 if (Op2C->getZExtValue() >= AT->getNumElements())
849 return MayAlias; // Be conservative with out-of-range accesses
850 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
851 if (Op2C->getZExtValue() >= VT->getNumElements())
852 return MayAlias; // Be conservative with out-of-range accesses
854 } else { // Conservatively assume the minimum value for this index
855 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
860 if (BasePtr1Ty && Op1) {
861 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
862 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
867 if (BasePtr2Ty && Op2) {
868 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
869 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
875 if (GEPPointerTy->getElementType()->isSized()) {
877 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
879 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
880 assert(Offset1 != Offset2 &&
881 "There is at least one different constant here!");
883 // Make sure we compare the absolute difference.
884 if (Offset1 > Offset2)
885 std::swap(Offset1, Offset2);
887 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
888 //cerr << "Determined that these two GEP's don't alias ["
889 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
896 // Make sure that anything that uses AliasAnalysis pulls in this file...
897 DEFINING_FILE_FOR(BasicAliasAnalysis)