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 call is to a few known safe intrinsics, we know that it does
68 // TODO: Eventually just check the 'nocapture' attribute.
69 if (!isa<MemIntrinsic>(I))
79 static const User *isGEP(const Value *V) {
80 if (isa<GetElementPtrInst>(V) ||
81 (isa<ConstantExpr>(V) &&
82 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
87 static const Value *GetGEPOperands(const Value *V,
88 SmallVector<Value*, 16> &GEPOps){
89 assert(GEPOps.empty() && "Expect empty list to populate!");
90 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
91 cast<User>(V)->op_end());
93 // Accumulate all of the chained indexes into the operand array
94 V = cast<User>(V)->getOperand(0);
96 while (const User *G = isGEP(V)) {
97 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
98 !cast<Constant>(GEPOps[0])->isNullValue())
99 break; // Don't handle folding arbitrary pointer offsets yet...
100 GEPOps.erase(GEPOps.begin()); // Drop the zero index
101 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
102 V = G->getOperand(0);
107 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
108 /// identifiable object. This returns true for:
109 /// Global Variables and Functions
110 /// Allocas and Mallocs
111 /// ByVal and NoAlias Arguments
113 static bool isIdentifiedObject(const Value *V) {
114 if (isa<GlobalValue>(V) || isa<AllocationInst>(V))
116 if (const Argument *A = dyn_cast<Argument>(V))
117 return A->hasNoAliasAttr() || A->hasByValAttr();
121 /// isKnownNonNull - Return true if we know that the specified value is never
123 static bool isKnownNonNull(const Value *V) {
124 // Alloca never returns null, malloc might.
125 if (isa<AllocaInst>(V)) return true;
127 // A byval argument is never null.
128 if (const Argument *A = dyn_cast<Argument>(V))
129 return A->hasByValAttr();
131 // Global values are not null unless extern weak.
132 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
133 return !GV->hasExternalWeakLinkage();
137 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
138 /// object that never escapes from the function.
139 static bool isNonEscapingLocalObject(const Value *V) {
140 // If this is a local allocation, check to see if it escapes.
141 if (isa<AllocationInst>(V))
142 return !AddressMightEscape(V);
144 // If this is an argument that corresponds to a byval or noalias argument,
145 // it can't escape either.
146 if (const Argument *A = dyn_cast<Argument>(V))
147 if (A->hasByValAttr() || A->hasNoAliasAttr())
148 return !AddressMightEscape(V);
153 /// isObjectSmallerThan - Return true if we can prove that the object specified
154 /// by V is smaller than Size.
155 static bool isObjectSmallerThan(const Value *V, unsigned Size,
156 const TargetData &TD) {
157 const Type *AccessTy = 0;
158 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
159 AccessTy = GV->getType()->getElementType();
161 if (const AllocationInst *AI = dyn_cast<AllocationInst>(V))
162 if (!AI->isArrayAllocation())
163 AccessTy = AI->getType()->getElementType();
165 if (const Argument *A = dyn_cast<Argument>(V))
166 if (A->hasByValAttr())
167 AccessTy = cast<PointerType>(A->getType())->getElementType();
169 if (AccessTy && AccessTy->isSized())
170 return TD.getABITypeSize(AccessTy) < Size;
174 //===----------------------------------------------------------------------===//
176 //===----------------------------------------------------------------------===//
179 /// NoAA - This class implements the -no-aa pass, which always returns "I
180 /// don't know" for alias queries. NoAA is unlike other alias analysis
181 /// implementations, in that it does not chain to a previous analysis. As
182 /// such it doesn't follow many of the rules that other alias analyses must.
184 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
185 static char ID; // Class identification, replacement for typeinfo
186 NoAA() : ImmutablePass(&ID) {}
187 explicit NoAA(void *PID) : ImmutablePass(PID) { }
189 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
190 AU.addRequired<TargetData>();
193 virtual void initializePass() {
194 TD = &getAnalysis<TargetData>();
197 virtual AliasResult alias(const Value *V1, unsigned V1Size,
198 const Value *V2, unsigned V2Size) {
202 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
203 std::vector<PointerAccessInfo> *Info) {
204 return UnknownModRefBehavior;
207 virtual void getArgumentAccesses(Function *F, CallSite CS,
208 std::vector<PointerAccessInfo> &Info) {
209 assert(0 && "This method may not be called on this function!");
212 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
213 virtual bool pointsToConstantMemory(const Value *P) { return false; }
214 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
217 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
220 virtual bool hasNoModRefInfoForCalls() const { return true; }
222 virtual void deleteValue(Value *V) {}
223 virtual void copyValue(Value *From, Value *To) {}
225 } // End of anonymous namespace
227 // Register this pass...
229 static RegisterPass<NoAA>
230 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
232 // Declare that we implement the AliasAnalysis interface
233 static RegisterAnalysisGroup<AliasAnalysis> V(U);
235 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
237 //===----------------------------------------------------------------------===//
239 //===----------------------------------------------------------------------===//
242 /// BasicAliasAnalysis - This is the default alias analysis implementation.
243 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
244 /// derives from the NoAA class.
245 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
246 static char ID; // Class identification, replacement for typeinfo
247 BasicAliasAnalysis() : NoAA(&ID) {}
248 AliasResult alias(const Value *V1, unsigned V1Size,
249 const Value *V2, unsigned V2Size);
251 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
252 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
253 return NoAA::getModRefInfo(CS1,CS2);
256 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
257 /// non-escaping allocations.
258 virtual bool hasNoModRefInfoForCalls() const { return false; }
260 /// pointsToConstantMemory - Chase pointers until we find a (constant
262 bool pointsToConstantMemory(const Value *P);
265 // CheckGEPInstructions - Check two GEP instructions with known
266 // must-aliasing base pointers. This checks to see if the index expressions
267 // preclude the pointers from aliasing...
269 CheckGEPInstructions(const Type* BasePtr1Ty,
270 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
271 const Type *BasePtr2Ty,
272 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
274 } // End of anonymous namespace
276 // Register this pass...
277 char BasicAliasAnalysis::ID = 0;
278 static RegisterPass<BasicAliasAnalysis>
279 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
281 // Declare that we implement the AliasAnalysis interface
282 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
284 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
285 return new BasicAliasAnalysis();
289 /// pointsToConstantMemory - Chase pointers until we find a (constant
291 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
292 if (const GlobalVariable *GV =
293 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
294 return GV->isConstant();
298 // getModRefInfo - Check to see if the specified callsite can clobber the
299 // specified memory object. Since we only look at local properties of this
300 // function, we really can't say much about this query. We do, however, use
301 // simple "address taken" analysis on local objects.
303 AliasAnalysis::ModRefResult
304 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
305 if (!isa<Constant>(P)) {
306 const Value *Object = P->getUnderlyingObject();
308 // If this is a tail call and P points to a stack location, we know that
309 // the tail call cannot access or modify the local stack.
310 // We cannot exclude byval arguments here; these belong to the caller of
311 // the current function not to the current function, and a tail callee
312 // may reference them.
313 if (isa<AllocaInst>(Object))
314 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
315 if (CI->isTailCall())
318 // If the pointer is to a locally allocated object that does not escape,
319 // then the call can not mod/ref the pointer unless the call takes the
320 // argument without capturing it.
321 if (isNonEscapingLocalObject(Object)) {
322 bool passedAsArg = false;
323 // TODO: Eventually only check 'nocapture' arguments.
324 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
326 if (isa<PointerType>((*CI)->getType()) &&
327 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
335 // The AliasAnalysis base class has some smarts, lets use them.
336 return AliasAnalysis::getModRefInfo(CS, P, Size);
340 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
341 // as array references. Note that this function is heavily tail recursive.
342 // Hopefully we have a smart C++ compiler. :)
344 AliasAnalysis::AliasResult
345 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
346 const Value *V2, unsigned V2Size) {
347 // Strip off any constant expression casts if they exist
348 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
349 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
350 V1 = CE->getOperand(0);
351 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
352 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
353 V2 = CE->getOperand(0);
355 // Are we checking for alias of the same value?
356 if (V1 == V2) return MustAlias;
358 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
359 V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty)
360 return NoAlias; // Scalars cannot alias each other
362 // Strip off cast instructions...
363 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
364 return alias(I->getOperand(0), V1Size, V2, V2Size);
365 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
366 return alias(V1, V1Size, I->getOperand(0), V2Size);
368 // Figure out what objects these things are pointing to if we can...
369 const Value *O1 = V1->getUnderlyingObject();
370 const Value *O2 = V2->getUnderlyingObject();
373 // If V1/V2 point to two different objects we know that we have no alias.
374 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
377 // Incoming argument cannot alias locally allocated object!
378 if ((isa<Argument>(O1) && isa<AllocationInst>(O2)) ||
379 (isa<Argument>(O2) && isa<AllocationInst>(O1)))
382 // Most objects can't alias null.
383 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
384 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
388 // If the size of one access is larger than the entire object on the other
389 // side, then we know such behavior is undefined and can assume no alias.
390 const TargetData &TD = getTargetData();
391 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
392 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
395 // If one pointer is the result of a call/invoke and the other is a
396 // non-escaping local object, then we know the object couldn't escape to a
397 // point where the call could return it.
398 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
399 isNonEscapingLocalObject(O2))
401 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
402 isNonEscapingLocalObject(O1))
405 // If we have two gep instructions with must-alias'ing base pointers, figure
406 // out if the indexes to the GEP tell us anything about the derived pointer.
407 // Note that we also handle chains of getelementptr instructions as well as
408 // constant expression getelementptrs here.
410 if (isGEP(V1) && isGEP(V2)) {
411 // Drill down into the first non-gep value, to test for must-aliasing of
412 // the base pointers.
413 const User *G = cast<User>(V1);
414 while (isGEP(G->getOperand(0)) &&
416 Constant::getNullValue(G->getOperand(1)->getType()))
417 G = cast<User>(G->getOperand(0));
418 const Value *BasePtr1 = G->getOperand(0);
421 while (isGEP(G->getOperand(0)) &&
423 Constant::getNullValue(G->getOperand(1)->getType()))
424 G = cast<User>(G->getOperand(0));
425 const Value *BasePtr2 = G->getOperand(0);
427 // Do the base pointers alias?
428 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
429 if (BaseAlias == NoAlias) return NoAlias;
430 if (BaseAlias == MustAlias) {
431 // If the base pointers alias each other exactly, check to see if we can
432 // figure out anything about the resultant pointers, to try to prove
435 // Collect all of the chained GEP operands together into one simple place
436 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
437 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
438 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
440 // If GetGEPOperands were able to fold to the same must-aliased pointer,
441 // do the comparison.
442 if (BasePtr1 == BasePtr2) {
444 CheckGEPInstructions(BasePtr1->getType(),
445 &GEP1Ops[0], GEP1Ops.size(), V1Size,
447 &GEP2Ops[0], GEP2Ops.size(), V2Size);
448 if (GAlias != MayAlias)
454 // Check to see if these two pointers are related by a getelementptr
455 // instruction. If one pointer is a GEP with a non-zero index of the other
456 // pointer, we know they cannot alias.
460 std::swap(V1Size, V2Size);
463 if (V1Size != ~0U && V2Size != ~0U)
465 SmallVector<Value*, 16> GEPOperands;
466 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
468 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
469 if (R == MustAlias) {
470 // If there is at least one non-zero constant index, we know they cannot
472 bool ConstantFound = false;
473 bool AllZerosFound = true;
474 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
475 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
476 if (!C->isNullValue()) {
477 ConstantFound = true;
478 AllZerosFound = false;
482 AllZerosFound = false;
485 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
486 // the ptr, the end result is a must alias also.
491 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
494 // Otherwise we have to check to see that the distance is more than
495 // the size of the argument... build an index vector that is equal to
496 // the arguments provided, except substitute 0's for any variable
497 // indexes we find...
498 if (cast<PointerType>(
499 BasePtr->getType())->getElementType()->isSized()) {
500 for (unsigned i = 0; i != GEPOperands.size(); ++i)
501 if (!isa<ConstantInt>(GEPOperands[i]))
503 Constant::getNullValue(GEPOperands[i]->getType());
505 getTargetData().getIndexedOffset(BasePtr->getType(),
509 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
519 // This function is used to determin if the indices of two GEP instructions are
520 // equal. V1 and V2 are the indices.
521 static bool IndexOperandsEqual(Value *V1, Value *V2) {
522 if (V1->getType() == V2->getType())
524 if (Constant *C1 = dyn_cast<Constant>(V1))
525 if (Constant *C2 = dyn_cast<Constant>(V2)) {
526 // Sign extend the constants to long types, if necessary
527 if (C1->getType() != Type::Int64Ty)
528 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty);
529 if (C2->getType() != Type::Int64Ty)
530 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty);
536 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
537 /// base pointers. This checks to see if the index expressions preclude the
538 /// pointers from aliasing...
539 AliasAnalysis::AliasResult
540 BasicAliasAnalysis::CheckGEPInstructions(
541 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
542 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
543 // We currently can't handle the case when the base pointers have different
544 // primitive types. Since this is uncommon anyway, we are happy being
545 // extremely conservative.
546 if (BasePtr1Ty != BasePtr2Ty)
549 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
551 // Find the (possibly empty) initial sequence of equal values... which are not
552 // necessarily constants.
553 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
554 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
555 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
556 unsigned UnequalOper = 0;
557 while (UnequalOper != MinOperands &&
558 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) {
559 // Advance through the type as we go...
561 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
562 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
564 // If all operands equal each other, then the derived pointers must
565 // alias each other...
567 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
568 "Ran out of type nesting, but not out of operands?");
573 // If we have seen all constant operands, and run out of indexes on one of the
574 // getelementptrs, check to see if the tail of the leftover one is all zeros.
575 // If so, return mustalias.
576 if (UnequalOper == MinOperands) {
577 if (NumGEP1Ops < NumGEP2Ops) {
578 std::swap(GEP1Ops, GEP2Ops);
579 std::swap(NumGEP1Ops, NumGEP2Ops);
582 bool AllAreZeros = true;
583 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
584 if (!isa<Constant>(GEP1Ops[i]) ||
585 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
589 if (AllAreZeros) return MustAlias;
593 // So now we know that the indexes derived from the base pointers,
594 // which are known to alias, are different. We can still determine a
595 // no-alias result if there are differing constant pairs in the index
596 // chain. For example:
597 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
599 // We have to be careful here about array accesses. In particular, consider:
600 // A[1][0] vs A[0][i]
601 // In this case, we don't *know* that the array will be accessed in bounds:
602 // the index could even be negative. Because of this, we have to
603 // conservatively *give up* and return may alias. We disregard differing
604 // array subscripts that are followed by a variable index without going
607 unsigned SizeMax = std::max(G1S, G2S);
608 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
610 // Scan for the first operand that is constant and unequal in the
611 // two getelementptrs...
612 unsigned FirstConstantOper = UnequalOper;
613 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
614 const Value *G1Oper = GEP1Ops[FirstConstantOper];
615 const Value *G2Oper = GEP2Ops[FirstConstantOper];
617 if (G1Oper != G2Oper) // Found non-equal constant indexes...
618 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
619 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
620 if (G1OC->getType() != G2OC->getType()) {
621 // Sign extend both operands to long.
622 if (G1OC->getType() != Type::Int64Ty)
623 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty);
624 if (G2OC->getType() != Type::Int64Ty)
625 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty);
626 GEP1Ops[FirstConstantOper] = G1OC;
627 GEP2Ops[FirstConstantOper] = G2OC;
631 // Handle the "be careful" case above: if this is an array/vector
632 // subscript, scan for a subsequent variable array index.
633 if (isa<SequentialType>(BasePtr1Ty)) {
635 cast<SequentialType>(BasePtr1Ty)->getElementType();
636 bool isBadCase = false;
638 for (unsigned Idx = FirstConstantOper+1;
639 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
640 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
641 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
645 NextTy = cast<SequentialType>(NextTy)->getElementType();
648 if (isBadCase) G1OC = 0;
651 // Make sure they are comparable (ie, not constant expressions), and
652 // make sure the GEP with the smaller leading constant is GEP1.
654 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
656 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
657 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
658 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
659 std::swap(NumGEP1Ops, NumGEP2Ops);
666 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
669 // No shared constant operands, and we ran out of common operands. At this
670 // point, the GEP instructions have run through all of their operands, and we
671 // haven't found evidence that there are any deltas between the GEP's.
672 // However, one GEP may have more operands than the other. If this is the
673 // case, there may still be hope. Check this now.
674 if (FirstConstantOper == MinOperands) {
675 // Make GEP1Ops be the longer one if there is a longer one.
676 if (NumGEP1Ops < NumGEP2Ops) {
677 std::swap(GEP1Ops, GEP2Ops);
678 std::swap(NumGEP1Ops, NumGEP2Ops);
681 // Is there anything to check?
682 if (NumGEP1Ops > MinOperands) {
683 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
684 if (isa<ConstantInt>(GEP1Ops[i]) &&
685 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
686 // Yup, there's a constant in the tail. Set all variables to
687 // constants in the GEP instruction to make it suitable for
688 // TargetData::getIndexedOffset.
689 for (i = 0; i != MaxOperands; ++i)
690 if (!isa<ConstantInt>(GEP1Ops[i]))
691 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
692 // Okay, now get the offset. This is the relative offset for the full
694 const TargetData &TD = getTargetData();
695 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
698 // Now check without any constants at the end.
699 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
702 // Make sure we compare the absolute difference.
703 if (Offset1 > Offset2)
704 std::swap(Offset1, Offset2);
706 // If the tail provided a bit enough offset, return noalias!
707 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
709 // Otherwise break - we don't look for another constant in the tail.
714 // Couldn't find anything useful.
718 // If there are non-equal constants arguments, then we can figure
719 // out a minimum known delta between the two index expressions... at
720 // this point we know that the first constant index of GEP1 is less
721 // than the first constant index of GEP2.
723 // Advance BasePtr[12]Ty over this first differing constant operand.
724 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
725 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
726 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
727 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
729 // We are going to be using TargetData::getIndexedOffset to determine the
730 // offset that each of the GEP's is reaching. To do this, we have to convert
731 // all variable references to constant references. To do this, we convert the
732 // initial sequence of array subscripts into constant zeros to start with.
733 const Type *ZeroIdxTy = GEPPointerTy;
734 for (unsigned i = 0; i != FirstConstantOper; ++i) {
735 if (!isa<StructType>(ZeroIdxTy))
736 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty);
738 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
739 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
742 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
744 // Loop over the rest of the operands...
745 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
746 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
747 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
748 // If they are equal, use a zero index...
749 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
750 if (!isa<ConstantInt>(Op1))
751 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
752 // Otherwise, just keep the constants we have.
755 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
756 // If this is an array index, make sure the array element is in range.
757 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
758 if (Op1C->getZExtValue() >= AT->getNumElements())
759 return MayAlias; // Be conservative with out-of-range accesses
760 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
761 if (Op1C->getZExtValue() >= VT->getNumElements())
762 return MayAlias; // Be conservative with out-of-range accesses
766 // GEP1 is known to produce a value less than GEP2. To be
767 // conservatively correct, we must assume the largest possible
768 // constant is used in this position. This cannot be the initial
769 // index to the GEP instructions (because we know we have at least one
770 // element before this one with the different constant arguments), so
771 // we know that the current index must be into either a struct or
772 // array. Because we know it's not constant, this cannot be a
773 // structure index. Because of this, we can calculate the maximum
776 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
777 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1);
778 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
779 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1);
784 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
785 // If this is an array index, make sure the array element is in range.
786 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
787 if (Op2C->getZExtValue() >= AT->getNumElements())
788 return MayAlias; // Be conservative with out-of-range accesses
789 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
790 if (Op2C->getZExtValue() >= VT->getNumElements())
791 return MayAlias; // Be conservative with out-of-range accesses
793 } else { // Conservatively assume the minimum value for this index
794 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
799 if (BasePtr1Ty && Op1) {
800 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
801 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
806 if (BasePtr2Ty && Op2) {
807 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
808 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
814 if (GEPPointerTy->getElementType()->isSized()) {
816 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
818 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
819 assert(Offset1 != Offset2 &&
820 "There is at least one different constant here!");
822 // Make sure we compare the absolute difference.
823 if (Offset1 > Offset2)
824 std::swap(Offset1, Offset2);
826 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
827 //cerr << "Determined that these two GEP's don't alias ["
828 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
835 // Make sure that anything that uses AliasAnalysis pulls in this file...
836 DEFINING_FILE_FOR(BasicAliasAnalysis)