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 // Do compensate for the two BB operands, i.e. Arg1 is at index 3!
77 if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
78 Attribute::NoCapture))
88 static const User *isGEP(const Value *V) {
89 if (isa<GetElementPtrInst>(V) ||
90 (isa<ConstantExpr>(V) &&
91 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
96 static const Value *GetGEPOperands(const Value *V,
97 SmallVector<Value*, 16> &GEPOps) {
98 assert(GEPOps.empty() && "Expect empty list to populate!");
99 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
100 cast<User>(V)->op_end());
102 // Accumulate all of the chained indexes into the operand array
103 V = cast<User>(V)->getOperand(0);
105 while (const User *G = isGEP(V)) {
106 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
107 !cast<Constant>(GEPOps[0])->isNullValue())
108 break; // Don't handle folding arbitrary pointer offsets yet...
109 GEPOps.erase(GEPOps.begin()); // Drop the zero index
110 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
111 V = G->getOperand(0);
116 /// isNoAliasCall - Return true if this pointer is returned by a noalias
118 static bool isNoAliasCall(const Value *V) {
119 if (isa<CallInst>(V) || isa<InvokeInst>(V))
120 return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
121 .paramHasAttr(0, Attribute::NoAlias);
125 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
126 /// identifiable object. This returns true for:
127 /// Global Variables and Functions
128 /// Allocas and Mallocs
129 /// ByVal and NoAlias Arguments
132 static bool isIdentifiedObject(const Value *V) {
133 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
135 if (const Argument *A = dyn_cast<Argument>(V))
136 return A->hasNoAliasAttr() || A->hasByValAttr();
140 /// isKnownNonNull - Return true if we know that the specified value is never
142 static bool isKnownNonNull(const Value *V) {
143 // Alloca never returns null, malloc might.
144 if (isa<AllocaInst>(V)) return true;
146 // A byval argument is never null.
147 if (const Argument *A = dyn_cast<Argument>(V))
148 return A->hasByValAttr();
150 // Global values are not null unless extern weak.
151 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
152 return !GV->hasExternalWeakLinkage();
156 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
157 /// object that never escapes from the function.
158 static bool isNonEscapingLocalObject(const Value *V) {
159 // If this is a local allocation, check to see if it escapes.
160 if (isa<AllocationInst>(V) || isNoAliasCall(V))
161 return !AddressMightEscape(V);
163 // If this is an argument that corresponds to a byval or noalias argument,
164 // then it has not escaped before entering the function. Check if it escapes
165 // inside the function.
166 if (const Argument *A = dyn_cast<Argument>(V))
167 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
168 // Don't bother analyzing arguments already known not to escape.
169 if (A->hasNoCaptureAttr())
171 return !AddressMightEscape(V);
177 /// isObjectSmallerThan - Return true if we can prove that the object specified
178 /// by V is smaller than Size.
179 static bool isObjectSmallerThan(const Value *V, unsigned Size,
180 const TargetData &TD) {
181 const Type *AccessTy;
182 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
183 AccessTy = GV->getType()->getElementType();
184 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
185 if (!AI->isArrayAllocation())
186 AccessTy = AI->getType()->getElementType();
189 } else if (const Argument *A = dyn_cast<Argument>(V)) {
190 if (A->hasByValAttr())
191 AccessTy = cast<PointerType>(A->getType())->getElementType();
198 if (AccessTy->isSized())
199 return TD.getTypePaddedSize(AccessTy) < Size;
203 //===----------------------------------------------------------------------===//
205 //===----------------------------------------------------------------------===//
208 /// NoAA - This class implements the -no-aa pass, which always returns "I
209 /// don't know" for alias queries. NoAA is unlike other alias analysis
210 /// implementations, in that it does not chain to a previous analysis. As
211 /// such it doesn't follow many of the rules that other alias analyses must.
213 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
214 static char ID; // Class identification, replacement for typeinfo
215 NoAA() : ImmutablePass(&ID) {}
216 explicit NoAA(void *PID) : ImmutablePass(PID) { }
218 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
219 AU.addRequired<TargetData>();
222 virtual void initializePass() {
223 TD = &getAnalysis<TargetData>();
226 virtual AliasResult alias(const Value *V1, unsigned V1Size,
227 const Value *V2, unsigned V2Size) {
231 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
232 std::vector<PointerAccessInfo> *Info) {
233 return UnknownModRefBehavior;
236 virtual void getArgumentAccesses(Function *F, CallSite CS,
237 std::vector<PointerAccessInfo> &Info) {
238 assert(0 && "This method may not be called on this function!");
241 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
242 virtual bool pointsToConstantMemory(const Value *P) { return false; }
243 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
246 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
249 virtual bool hasNoModRefInfoForCalls() const { return true; }
251 virtual void deleteValue(Value *V) {}
252 virtual void copyValue(Value *From, Value *To) {}
254 } // End of anonymous namespace
256 // Register this pass...
258 static RegisterPass<NoAA>
259 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
261 // Declare that we implement the AliasAnalysis interface
262 static RegisterAnalysisGroup<AliasAnalysis> V(U);
264 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
266 //===----------------------------------------------------------------------===//
268 //===----------------------------------------------------------------------===//
271 /// BasicAliasAnalysis - This is the default alias analysis implementation.
272 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
273 /// derives from the NoAA class.
274 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
275 static char ID; // Class identification, replacement for typeinfo
276 BasicAliasAnalysis() : NoAA(&ID) {}
277 AliasResult alias(const Value *V1, unsigned V1Size,
278 const Value *V2, unsigned V2Size);
280 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
281 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
283 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
284 /// non-escaping allocations.
285 virtual bool hasNoModRefInfoForCalls() const { return false; }
287 /// pointsToConstantMemory - Chase pointers until we find a (constant
289 bool pointsToConstantMemory(const Value *P);
292 // CheckGEPInstructions - Check two GEP instructions with known
293 // must-aliasing base pointers. This checks to see if the index expressions
294 // preclude the pointers from aliasing...
296 CheckGEPInstructions(const Type* BasePtr1Ty,
297 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
298 const Type *BasePtr2Ty,
299 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
301 } // End of anonymous namespace
303 // Register this pass...
304 char BasicAliasAnalysis::ID = 0;
305 static RegisterPass<BasicAliasAnalysis>
306 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
308 // Declare that we implement the AliasAnalysis interface
309 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
311 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
312 return new BasicAliasAnalysis();
316 /// pointsToConstantMemory - Chase pointers until we find a (constant
318 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
319 if (const GlobalVariable *GV =
320 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
321 return GV->isConstant();
325 // getModRefInfo - Check to see if the specified callsite can clobber the
326 // specified memory object. Since we only look at local properties of this
327 // function, we really can't say much about this query. We do, however, use
328 // simple "address taken" analysis on local objects.
330 AliasAnalysis::ModRefResult
331 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
332 if (!isa<Constant>(P)) {
333 const Value *Object = P->getUnderlyingObject();
335 // If this is a tail call and P points to a stack location, we know that
336 // the tail call cannot access or modify the local stack.
337 // We cannot exclude byval arguments here; these belong to the caller of
338 // the current function not to the current function, and a tail callee
339 // may reference them.
340 if (isa<AllocaInst>(Object))
341 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
342 if (CI->isTailCall())
345 // If the pointer is to a locally allocated object that does not escape,
346 // then the call can not mod/ref the pointer unless the call takes the
347 // argument without capturing it.
348 if (isNonEscapingLocalObject(Object)) {
349 bool passedAsArg = false;
350 // TODO: Eventually only check 'nocapture' arguments.
351 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
353 if (isa<PointerType>((*CI)->getType()) &&
354 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
362 // The AliasAnalysis base class has some smarts, lets use them.
363 return AliasAnalysis::getModRefInfo(CS, P, Size);
367 AliasAnalysis::ModRefResult
368 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
369 // If CS1 or CS2 are readnone, they don't interact.
370 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
371 if (CS1B == DoesNotAccessMemory) return NoModRef;
373 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
374 if (CS2B == DoesNotAccessMemory) return NoModRef;
376 // If they both only read from memory, just return ref.
377 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
380 // Otherwise, fall back to NoAA (mod+ref).
381 return NoAA::getModRefInfo(CS1, CS2);
385 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
386 // as array references.
388 AliasAnalysis::AliasResult
389 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
390 const Value *V2, unsigned V2Size) {
391 // Strip off any constant expression casts if they exist
392 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
393 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
394 V1 = CE->getOperand(0);
395 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
396 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
397 V2 = CE->getOperand(0);
399 // Are we checking for alias of the same value?
400 if (V1 == V2) return MustAlias;
402 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
403 return NoAlias; // Scalars cannot alias each other
405 // Strip off cast instructions. Since V1 and V2 are pointers, they must be
406 // pointer<->pointer bitcasts.
407 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
408 return alias(I->getOperand(0), V1Size, V2, V2Size);
409 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
410 return alias(V1, V1Size, I->getOperand(0), V2Size);
412 // Figure out what objects these things are pointing to if we can.
413 const Value *O1 = V1->getUnderlyingObject();
414 const Value *O2 = V2->getUnderlyingObject();
417 // If V1/V2 point to two different objects we know that we have no alias.
418 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
421 // Arguments can't alias with local allocations or noalias calls.
422 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
423 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
426 // Most objects can't alias null.
427 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
428 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
432 // If the size of one access is larger than the entire object on the other
433 // side, then we know such behavior is undefined and can assume no alias.
434 const TargetData &TD = getTargetData();
435 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
436 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
439 // If one pointer is the result of a call/invoke and the other is a
440 // non-escaping local object, then we know the object couldn't escape to a
441 // point where the call could return it.
442 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
443 isNonEscapingLocalObject(O2))
445 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
446 isNonEscapingLocalObject(O1))
449 // If we have two gep instructions with must-alias'ing base pointers, figure
450 // out if the indexes to the GEP tell us anything about the derived pointer.
451 // Note that we also handle chains of getelementptr instructions as well as
452 // constant expression getelementptrs here.
454 if (isGEP(V1) && isGEP(V2)) {
455 const User *GEP1 = cast<User>(V1);
456 const User *GEP2 = cast<User>(V2);
458 // If V1 and V2 are identical GEPs, just recurse down on both of them.
459 // This allows us to analyze things like:
460 // P = gep A, 0, i, 1
461 // Q = gep B, 0, i, 1
462 // by just analyzing A and B. This is even safe for variable indices.
463 if (GEP1->getType() == GEP2->getType() &&
464 GEP1->getNumOperands() == GEP2->getNumOperands() &&
465 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
466 // All operands are the same, ignoring the base.
467 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
468 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
471 // Drill down into the first non-gep value, to test for must-aliasing of
472 // the base pointers.
473 while (isGEP(GEP1->getOperand(0)) &&
474 GEP1->getOperand(1) ==
475 Constant::getNullValue(GEP1->getOperand(1)->getType()))
476 GEP1 = cast<User>(GEP1->getOperand(0));
477 const Value *BasePtr1 = GEP1->getOperand(0);
479 while (isGEP(GEP2->getOperand(0)) &&
480 GEP2->getOperand(1) ==
481 Constant::getNullValue(GEP2->getOperand(1)->getType()))
482 GEP2 = cast<User>(GEP2->getOperand(0));
483 const Value *BasePtr2 = GEP2->getOperand(0);
485 // Do the base pointers alias?
486 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
487 if (BaseAlias == NoAlias) return NoAlias;
488 if (BaseAlias == MustAlias) {
489 // If the base pointers alias each other exactly, check to see if we can
490 // figure out anything about the resultant pointers, to try to prove
493 // Collect all of the chained GEP operands together into one simple place
494 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
495 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
496 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
498 // If GetGEPOperands were able to fold to the same must-aliased pointer,
499 // do the comparison.
500 if (BasePtr1 == BasePtr2) {
502 CheckGEPInstructions(BasePtr1->getType(),
503 &GEP1Ops[0], GEP1Ops.size(), V1Size,
505 &GEP2Ops[0], GEP2Ops.size(), V2Size);
506 if (GAlias != MayAlias)
512 // Check to see if these two pointers are related by a getelementptr
513 // instruction. If one pointer is a GEP with a non-zero index of the other
514 // pointer, we know they cannot alias.
518 std::swap(V1Size, V2Size);
521 if (V1Size != ~0U && V2Size != ~0U)
523 SmallVector<Value*, 16> GEPOperands;
524 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
526 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
527 if (R == MustAlias) {
528 // If there is at least one non-zero constant index, we know they cannot
530 bool ConstantFound = false;
531 bool AllZerosFound = true;
532 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
533 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
534 if (!C->isNullValue()) {
535 ConstantFound = true;
536 AllZerosFound = false;
540 AllZerosFound = false;
543 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
544 // the ptr, the end result is a must alias also.
549 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
552 // Otherwise we have to check to see that the distance is more than
553 // the size of the argument... build an index vector that is equal to
554 // the arguments provided, except substitute 0's for any variable
555 // indexes we find...
556 if (cast<PointerType>(
557 BasePtr->getType())->getElementType()->isSized()) {
558 for (unsigned i = 0; i != GEPOperands.size(); ++i)
559 if (!isa<ConstantInt>(GEPOperands[i]))
561 Constant::getNullValue(GEPOperands[i]->getType());
563 getTargetData().getIndexedOffset(BasePtr->getType(),
567 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
577 // This function is used to determine if the indices of two GEP instructions are
578 // equal. V1 and V2 are the indices.
579 static bool IndexOperandsEqual(Value *V1, Value *V2) {
580 if (V1->getType() == V2->getType())
582 if (Constant *C1 = dyn_cast<Constant>(V1))
583 if (Constant *C2 = dyn_cast<Constant>(V2)) {
584 // Sign extend the constants to long types, if necessary
585 if (C1->getType() != Type::Int64Ty)
586 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty);
587 if (C2->getType() != Type::Int64Ty)
588 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty);
594 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
595 /// base pointers. This checks to see if the index expressions preclude the
596 /// pointers from aliasing...
597 AliasAnalysis::AliasResult
598 BasicAliasAnalysis::CheckGEPInstructions(
599 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
600 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
601 // We currently can't handle the case when the base pointers have different
602 // primitive types. Since this is uncommon anyway, we are happy being
603 // extremely conservative.
604 if (BasePtr1Ty != BasePtr2Ty)
607 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
609 // Find the (possibly empty) initial sequence of equal values... which are not
610 // necessarily constants.
611 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
612 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
613 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
614 unsigned UnequalOper = 0;
615 while (UnequalOper != MinOperands &&
616 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
617 // Advance through the type as we go...
619 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
620 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
622 // If all operands equal each other, then the derived pointers must
623 // alias each other...
625 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
626 "Ran out of type nesting, but not out of operands?");
631 // If we have seen all constant operands, and run out of indexes on one of the
632 // getelementptrs, check to see if the tail of the leftover one is all zeros.
633 // If so, return mustalias.
634 if (UnequalOper == MinOperands) {
635 if (NumGEP1Ops < NumGEP2Ops) {
636 std::swap(GEP1Ops, GEP2Ops);
637 std::swap(NumGEP1Ops, NumGEP2Ops);
640 bool AllAreZeros = true;
641 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
642 if (!isa<Constant>(GEP1Ops[i]) ||
643 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
647 if (AllAreZeros) return MustAlias;
651 // So now we know that the indexes derived from the base pointers,
652 // which are known to alias, are different. We can still determine a
653 // no-alias result if there are differing constant pairs in the index
654 // chain. For example:
655 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
657 // We have to be careful here about array accesses. In particular, consider:
658 // A[1][0] vs A[0][i]
659 // In this case, we don't *know* that the array will be accessed in bounds:
660 // the index could even be negative. Because of this, we have to
661 // conservatively *give up* and return may alias. We disregard differing
662 // array subscripts that are followed by a variable index without going
665 unsigned SizeMax = std::max(G1S, G2S);
666 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
668 // Scan for the first operand that is constant and unequal in the
669 // two getelementptrs...
670 unsigned FirstConstantOper = UnequalOper;
671 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
672 const Value *G1Oper = GEP1Ops[FirstConstantOper];
673 const Value *G2Oper = GEP2Ops[FirstConstantOper];
675 if (G1Oper != G2Oper) // Found non-equal constant indexes...
676 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
677 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
678 if (G1OC->getType() != G2OC->getType()) {
679 // Sign extend both operands to long.
680 if (G1OC->getType() != Type::Int64Ty)
681 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty);
682 if (G2OC->getType() != Type::Int64Ty)
683 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty);
684 GEP1Ops[FirstConstantOper] = G1OC;
685 GEP2Ops[FirstConstantOper] = G2OC;
689 // Handle the "be careful" case above: if this is an array/vector
690 // subscript, scan for a subsequent variable array index.
691 if (isa<SequentialType>(BasePtr1Ty)) {
693 cast<SequentialType>(BasePtr1Ty)->getElementType();
694 bool isBadCase = false;
696 for (unsigned Idx = FirstConstantOper+1;
697 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
698 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
699 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
703 NextTy = cast<SequentialType>(NextTy)->getElementType();
706 if (isBadCase) G1OC = 0;
709 // Make sure they are comparable (ie, not constant expressions), and
710 // make sure the GEP with the smaller leading constant is GEP1.
712 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
714 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
715 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
716 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
717 std::swap(NumGEP1Ops, NumGEP2Ops);
724 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
727 // No shared constant operands, and we ran out of common operands. At this
728 // point, the GEP instructions have run through all of their operands, and we
729 // haven't found evidence that there are any deltas between the GEP's.
730 // However, one GEP may have more operands than the other. If this is the
731 // case, there may still be hope. Check this now.
732 if (FirstConstantOper == MinOperands) {
733 // Make GEP1Ops be the longer one if there is a longer one.
734 if (NumGEP1Ops < NumGEP2Ops) {
735 std::swap(GEP1Ops, GEP2Ops);
736 std::swap(NumGEP1Ops, NumGEP2Ops);
739 // Is there anything to check?
740 if (NumGEP1Ops > MinOperands) {
741 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
742 if (isa<ConstantInt>(GEP1Ops[i]) &&
743 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
744 // Yup, there's a constant in the tail. Set all variables to
745 // constants in the GEP instruction to make it suitable for
746 // TargetData::getIndexedOffset.
747 for (i = 0; i != MaxOperands; ++i)
748 if (!isa<ConstantInt>(GEP1Ops[i]))
749 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
750 // Okay, now get the offset. This is the relative offset for the full
752 const TargetData &TD = getTargetData();
753 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
756 // Now check without any constants at the end.
757 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
760 // Make sure we compare the absolute difference.
761 if (Offset1 > Offset2)
762 std::swap(Offset1, Offset2);
764 // If the tail provided a bit enough offset, return noalias!
765 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
767 // Otherwise break - we don't look for another constant in the tail.
772 // Couldn't find anything useful.
776 // If there are non-equal constants arguments, then we can figure
777 // out a minimum known delta between the two index expressions... at
778 // this point we know that the first constant index of GEP1 is less
779 // than the first constant index of GEP2.
781 // Advance BasePtr[12]Ty over this first differing constant operand.
782 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
783 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
784 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
785 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
787 // We are going to be using TargetData::getIndexedOffset to determine the
788 // offset that each of the GEP's is reaching. To do this, we have to convert
789 // all variable references to constant references. To do this, we convert the
790 // initial sequence of array subscripts into constant zeros to start with.
791 const Type *ZeroIdxTy = GEPPointerTy;
792 for (unsigned i = 0; i != FirstConstantOper; ++i) {
793 if (!isa<StructType>(ZeroIdxTy))
794 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty);
796 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
797 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
800 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
802 // Loop over the rest of the operands...
803 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
804 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
805 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
806 // If they are equal, use a zero index...
807 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
808 if (!isa<ConstantInt>(Op1))
809 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
810 // Otherwise, just keep the constants we have.
813 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
814 // If this is an array index, make sure the array element is in range.
815 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
816 if (Op1C->getZExtValue() >= AT->getNumElements())
817 return MayAlias; // Be conservative with out-of-range accesses
818 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
819 if (Op1C->getZExtValue() >= VT->getNumElements())
820 return MayAlias; // Be conservative with out-of-range accesses
824 // GEP1 is known to produce a value less than GEP2. To be
825 // conservatively correct, we must assume the largest possible
826 // constant is used in this position. This cannot be the initial
827 // index to the GEP instructions (because we know we have at least one
828 // element before this one with the different constant arguments), so
829 // we know that the current index must be into either a struct or
830 // array. Because we know it's not constant, this cannot be a
831 // structure index. Because of this, we can calculate the maximum
834 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
835 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1);
836 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
837 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1);
842 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
843 // If this is an array index, make sure the array element is in range.
844 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
845 if (Op2C->getZExtValue() >= AT->getNumElements())
846 return MayAlias; // Be conservative with out-of-range accesses
847 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
848 if (Op2C->getZExtValue() >= VT->getNumElements())
849 return MayAlias; // Be conservative with out-of-range accesses
851 } else { // Conservatively assume the minimum value for this index
852 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
857 if (BasePtr1Ty && Op1) {
858 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
859 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
864 if (BasePtr2Ty && Op2) {
865 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
866 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
872 if (GEPPointerTy->getElementType()->isSized()) {
874 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
876 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
877 assert(Offset1 != Offset2 &&
878 "There is at least one different constant here!");
880 // Make sure we compare the absolute difference.
881 if (Offset1 > Offset2)
882 std::swap(Offset1, Offset2);
884 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
885 //cerr << "Determined that these two GEP's don't alias ["
886 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
893 // Make sure that anything that uses AliasAnalysis pulls in this file...
894 DEFINING_FILE_FOR(BasicAliasAnalysis)