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/CaptureTracking.h"
18 #include "llvm/Analysis/MallocHelper.h"
19 #include "llvm/Analysis/Passes.h"
20 #include "llvm/Constants.h"
21 #include "llvm/DerivedTypes.h"
22 #include "llvm/Function.h"
23 #include "llvm/GlobalVariable.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/IntrinsicInst.h"
26 #include "llvm/LLVMContext.h"
27 #include "llvm/Operator.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/GetElementPtrTypeIterator.h"
38 //===----------------------------------------------------------------------===//
40 //===----------------------------------------------------------------------===//
42 static const Value *GetGEPOperands(const Value *V,
43 SmallVector<Value*, 16> &GEPOps) {
44 assert(GEPOps.empty() && "Expect empty list to populate!");
45 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
46 cast<User>(V)->op_end());
48 // Accumulate all of the chained indexes into the operand array
49 V = cast<User>(V)->getOperand(0);
51 while (const GEPOperator *G = dyn_cast<GEPOperator>(V)) {
52 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
53 !cast<Constant>(GEPOps[0])->isNullValue())
54 break; // Don't handle folding arbitrary pointer offsets yet...
55 GEPOps.erase(GEPOps.begin()); // Drop the zero index
56 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
62 /// isKnownNonNull - Return true if we know that the specified value is never
64 static bool isKnownNonNull(const Value *V) {
65 // Alloca never returns null, malloc might.
66 if (isa<AllocaInst>(V)) return true;
68 // A byval argument is never null.
69 if (const Argument *A = dyn_cast<Argument>(V))
70 return A->hasByValAttr();
72 // Global values are not null unless extern weak.
73 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
74 return !GV->hasExternalWeakLinkage();
78 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
79 /// object that never escapes from the function.
80 static bool isNonEscapingLocalObject(const Value *V) {
81 // If this is a local allocation, check to see if it escapes.
82 if (isa<AllocaInst>(V) || isNoAliasCall(V))
83 return !PointerMayBeCaptured(V, false);
85 // If this is an argument that corresponds to a byval or noalias argument,
86 // then it has not escaped before entering the function. Check if it escapes
87 // inside the function.
88 if (const Argument *A = dyn_cast<Argument>(V))
89 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
90 // Don't bother analyzing arguments already known not to escape.
91 if (A->hasNoCaptureAttr())
93 return !PointerMayBeCaptured(V, false);
99 /// isObjectSmallerThan - Return true if we can prove that the object specified
100 /// by V is smaller than Size.
101 static bool isObjectSmallerThan(const Value *V, unsigned Size,
102 LLVMContext &Context, const TargetData &TD) {
103 const Type *AccessTy;
104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
105 AccessTy = GV->getType()->getElementType();
106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
107 if (!AI->isArrayAllocation())
108 AccessTy = AI->getType()->getElementType();
111 } else if (const CallInst* CI = extractMallocCall(V)) {
112 if (!isArrayMalloc(V, Context, &TD))
113 // The size is the argument to the malloc call.
114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
115 return (C->getZExtValue() < Size);
117 } else if (const Argument *A = dyn_cast<Argument>(V)) {
118 if (A->hasByValAttr())
119 AccessTy = cast<PointerType>(A->getType())->getElementType();
126 if (AccessTy->isSized())
127 return TD.getTypeAllocSize(AccessTy) < Size;
131 //===----------------------------------------------------------------------===//
133 //===----------------------------------------------------------------------===//
136 /// NoAA - This class implements the -no-aa pass, which always returns "I
137 /// don't know" for alias queries. NoAA is unlike other alias analysis
138 /// implementations, in that it does not chain to a previous analysis. As
139 /// such it doesn't follow many of the rules that other alias analyses must.
141 struct NoAA : public ImmutablePass, public AliasAnalysis {
142 static char ID; // Class identification, replacement for typeinfo
143 NoAA() : ImmutablePass(&ID) {}
144 explicit NoAA(void *PID) : ImmutablePass(PID) { }
146 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
149 virtual void initializePass() {
150 TD = getAnalysisIfAvailable<TargetData>();
153 virtual AliasResult alias(const Value *V1, unsigned V1Size,
154 const Value *V2, unsigned V2Size) {
158 virtual void getArgumentAccesses(Function *F, CallSite CS,
159 std::vector<PointerAccessInfo> &Info) {
160 llvm_unreachable("This method may not be called on this function!");
163 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
164 virtual bool pointsToConstantMemory(const Value *P) { return false; }
165 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
168 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
171 virtual bool hasNoModRefInfoForCalls() const { return true; }
173 virtual void deleteValue(Value *V) {}
174 virtual void copyValue(Value *From, Value *To) {}
176 } // End of anonymous namespace
178 // Register this pass...
180 static RegisterPass<NoAA>
181 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
183 // Declare that we implement the AliasAnalysis interface
184 static RegisterAnalysisGroup<AliasAnalysis> V(U);
186 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
188 //===----------------------------------------------------------------------===//
190 //===----------------------------------------------------------------------===//
193 /// BasicAliasAnalysis - This is the default alias analysis implementation.
194 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
195 /// derives from the NoAA class.
196 struct BasicAliasAnalysis : public NoAA {
197 static char ID; // Class identification, replacement for typeinfo
198 BasicAliasAnalysis() : NoAA(&ID) {}
199 AliasResult alias(const Value *V1, unsigned V1Size,
200 const Value *V2, unsigned V2Size) {
201 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!");
202 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
207 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
208 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
210 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
211 /// non-escaping allocations.
212 virtual bool hasNoModRefInfoForCalls() const { return false; }
214 /// pointsToConstantMemory - Chase pointers until we find a (constant
216 bool pointsToConstantMemory(const Value *P);
219 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call.
220 SmallPtrSet<const PHINode*, 16> VisitedPHIs;
222 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
224 AliasResult aliasGEP(const Value *V1, unsigned V1Size,
225 const Value *V2, unsigned V2Size);
227 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
229 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
230 const Value *V2, unsigned V2Size);
232 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
233 const Value *V2, unsigned V2Size);
235 // CheckGEPInstructions - Check two GEP instructions with known
236 // must-aliasing base pointers. This checks to see if the index expressions
237 // preclude the pointers from aliasing...
239 CheckGEPInstructions(const Type* BasePtr1Ty,
240 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
241 const Type *BasePtr2Ty,
242 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
244 } // End of anonymous namespace
246 // Register this pass...
247 char BasicAliasAnalysis::ID = 0;
248 static RegisterPass<BasicAliasAnalysis>
249 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
251 // Declare that we implement the AliasAnalysis interface
252 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
254 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
255 return new BasicAliasAnalysis();
259 /// pointsToConstantMemory - Chase pointers until we find a (constant
261 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
262 if (const GlobalVariable *GV =
263 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
264 return GV->isConstant();
269 // getModRefInfo - Check to see if the specified callsite can clobber the
270 // specified memory object. Since we only look at local properties of this
271 // function, we really can't say much about this query. We do, however, use
272 // simple "address taken" analysis on local objects.
274 AliasAnalysis::ModRefResult
275 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
276 if (!isa<Constant>(P)) {
277 const Value *Object = P->getUnderlyingObject();
279 // If this is a tail call and P points to a stack location, we know that
280 // the tail call cannot access or modify the local stack.
281 // We cannot exclude byval arguments here; these belong to the caller of
282 // the current function not to the current function, and a tail callee
283 // may reference them.
284 if (isa<AllocaInst>(Object))
285 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
286 if (CI->isTailCall())
289 // If the pointer is to a locally allocated object that does not escape,
290 // then the call can not mod/ref the pointer unless the call takes the
291 // argument without capturing it.
292 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
293 bool passedAsArg = false;
294 // TODO: Eventually only check 'nocapture' arguments.
295 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
297 if (isa<PointerType>((*CI)->getType()) &&
298 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
305 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
306 switch (II->getIntrinsicID()) {
308 case Intrinsic::memcpy:
309 case Intrinsic::memmove: {
311 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
312 Len = LenCI->getZExtValue();
313 Value *Dest = II->getOperand(1);
314 Value *Src = II->getOperand(2);
315 if (alias(Dest, Len, P, Size) == NoAlias) {
316 if (alias(Src, Len, P, Size) == NoAlias)
322 case Intrinsic::memset:
323 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
324 unsigned Len = LenCI->getZExtValue();
325 Value *Dest = II->getOperand(1);
326 if (alias(Dest, Len, P, Size) == NoAlias)
330 case Intrinsic::atomic_cmp_swap:
331 case Intrinsic::atomic_swap:
332 case Intrinsic::atomic_load_add:
333 case Intrinsic::atomic_load_sub:
334 case Intrinsic::atomic_load_and:
335 case Intrinsic::atomic_load_nand:
336 case Intrinsic::atomic_load_or:
337 case Intrinsic::atomic_load_xor:
338 case Intrinsic::atomic_load_max:
339 case Intrinsic::atomic_load_min:
340 case Intrinsic::atomic_load_umax:
341 case Intrinsic::atomic_load_umin:
343 Value *Op1 = II->getOperand(1);
344 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
345 if (alias(Op1, Op1Size, P, Size) == NoAlias)
349 case Intrinsic::lifetime_start:
350 case Intrinsic::lifetime_end:
351 case Intrinsic::invariant_start: {
352 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
353 if (alias(II->getOperand(2), PtrSize, P, Size) == NoAlias)
357 case Intrinsic::invariant_end: {
358 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
359 if (alias(II->getOperand(3), PtrSize, P, Size) == NoAlias)
367 // The AliasAnalysis base class has some smarts, lets use them.
368 return AliasAnalysis::getModRefInfo(CS, P, Size);
372 AliasAnalysis::ModRefResult
373 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
374 // If CS1 or CS2 are readnone, they don't interact.
375 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
376 if (CS1B == DoesNotAccessMemory) return NoModRef;
378 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
379 if (CS2B == DoesNotAccessMemory) return NoModRef;
381 // If they both only read from memory, just return ref.
382 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
385 // Otherwise, fall back to NoAA (mod+ref).
386 return NoAA::getModRefInfo(CS1, CS2);
389 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
392 AliasAnalysis::AliasResult
393 BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size,
394 const Value *V2, unsigned V2Size) {
395 // If we have two gep instructions with must-alias'ing base pointers, figure
396 // out if the indexes to the GEP tell us anything about the derived pointer.
397 // Note that we also handle chains of getelementptr instructions as well as
398 // constant expression getelementptrs here.
400 if (isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
401 const User *GEP1 = cast<User>(V1);
402 const User *GEP2 = cast<User>(V2);
404 // If V1 and V2 are identical GEPs, just recurse down on both of them.
405 // This allows us to analyze things like:
406 // P = gep A, 0, i, 1
407 // Q = gep B, 0, i, 1
408 // by just analyzing A and B. This is even safe for variable indices.
409 if (GEP1->getType() == GEP2->getType() &&
410 GEP1->getNumOperands() == GEP2->getNumOperands() &&
411 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
412 // All operands are the same, ignoring the base.
413 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
414 return aliasCheck(GEP1->getOperand(0), V1Size,
415 GEP2->getOperand(0), V2Size);
417 // Drill down into the first non-gep value, to test for must-aliasing of
418 // the base pointers.
419 while (isa<GEPOperator>(GEP1->getOperand(0)) &&
420 GEP1->getOperand(1) ==
421 Constant::getNullValue(GEP1->getOperand(1)->getType()))
422 GEP1 = cast<User>(GEP1->getOperand(0));
423 const Value *BasePtr1 = GEP1->getOperand(0);
425 while (isa<GEPOperator>(GEP2->getOperand(0)) &&
426 GEP2->getOperand(1) ==
427 Constant::getNullValue(GEP2->getOperand(1)->getType()))
428 GEP2 = cast<User>(GEP2->getOperand(0));
429 const Value *BasePtr2 = GEP2->getOperand(0);
431 // Do the base pointers alias?
432 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U);
433 if (BaseAlias == NoAlias) return NoAlias;
434 if (BaseAlias == MustAlias) {
435 // If the base pointers alias each other exactly, check to see if we can
436 // figure out anything about the resultant pointers, to try to prove
439 // Collect all of the chained GEP operands together into one simple place
440 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
441 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
442 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
444 // If GetGEPOperands were able to fold to the same must-aliased pointer,
445 // do the comparison.
446 if (BasePtr1 == BasePtr2) {
448 CheckGEPInstructions(BasePtr1->getType(),
449 &GEP1Ops[0], GEP1Ops.size(), V1Size,
451 &GEP2Ops[0], GEP2Ops.size(), V2Size);
452 if (GAlias != MayAlias)
458 // Check to see if these two pointers are related by a getelementptr
459 // instruction. If one pointer is a GEP with a non-zero index of the other
460 // pointer, we know they cannot alias.
462 if (V1Size == ~0U || V2Size == ~0U)
465 SmallVector<Value*, 16> GEPOperands;
466 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
468 AliasResult R = aliasCheck(BasePtr, ~0U, V2, V2Size);
470 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
471 // If V2 is known not to alias GEP base pointer, then the two values
472 // cannot alias per GEP semantics: "A pointer value formed from a
473 // getelementptr instruction is associated with the addresses associated
474 // with the first operand of the getelementptr".
477 // If there is at least one non-zero constant index, we know they cannot
479 bool ConstantFound = false;
480 bool AllZerosFound = true;
481 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
482 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
483 if (!C->isNullValue()) {
484 ConstantFound = true;
485 AllZerosFound = false;
489 AllZerosFound = false;
492 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
493 // the ptr, the end result is a must alias also.
498 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
501 // Otherwise we have to check to see that the distance is more than
502 // the size of the argument... build an index vector that is equal to
503 // the arguments provided, except substitute 0's for any variable
504 // indexes we find...
506 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
507 for (unsigned i = 0; i != GEPOperands.size(); ++i)
508 if (!isa<ConstantInt>(GEPOperands[i]))
509 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
510 int64_t Offset = TD->getIndexedOffset(BasePtr->getType(),
514 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
522 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
524 AliasAnalysis::AliasResult
525 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
526 const Value *V2, unsigned V2Size) {
527 // The PHI node has already been visited, avoid recursion any further.
528 if (!VisitedPHIs.insert(PN))
531 SmallPtrSet<Value*, 4> UniqueSrc;
532 SmallVector<Value*, 4> V1Srcs;
533 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
534 Value *PV1 = PN->getIncomingValue(i);
535 if (isa<PHINode>(PV1))
536 // If any of the source itself is a PHI, return MayAlias conservatively
537 // to avoid compile time explosion. The worst possible case is if both
538 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
539 // and 'n' are the number of PHI sources.
541 if (UniqueSrc.insert(PV1))
542 V1Srcs.push_back(PV1);
545 AliasResult Alias = aliasCheck(V1Srcs[0], PNSize, V2, V2Size);
546 // Early exit if the check of the first PHI source against V2 is MayAlias.
547 // Other results are not possible.
548 if (Alias == MayAlias)
551 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
552 // NoAlias / MustAlias. Otherwise, returns MayAlias.
553 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
554 Value *V = V1Srcs[i];
555 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
556 if (ThisAlias != Alias || ThisAlias == MayAlias)
563 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
564 // such as array references.
566 AliasAnalysis::AliasResult
567 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
568 const Value *V2, unsigned V2Size) {
569 // Strip off any casts if they exist.
570 V1 = V1->stripPointerCasts();
571 V2 = V2->stripPointerCasts();
573 // Are we checking for alias of the same value?
574 if (V1 == V2) return MustAlias;
576 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
577 return NoAlias; // Scalars cannot alias each other
579 // Figure out what objects these things are pointing to if we can.
580 const Value *O1 = V1->getUnderlyingObject();
581 const Value *O2 = V2->getUnderlyingObject();
584 // If V1/V2 point to two different objects we know that we have no alias.
585 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
588 // Arguments can't alias with local allocations or noalias calls.
589 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
590 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
593 // Most objects can't alias null.
594 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
595 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
599 // If the size of one access is larger than the entire object on the other
600 // side, then we know such behavior is undefined and can assume no alias.
601 LLVMContext &Context = V1->getContext();
603 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, Context, *TD)) ||
604 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, Context, *TD)))
607 // If one pointer is the result of a call/invoke and the other is a
608 // non-escaping local object, then we know the object couldn't escape to a
609 // point where the call could return it.
610 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
611 isNonEscapingLocalObject(O2) && O1 != O2)
613 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
614 isNonEscapingLocalObject(O1) && O1 != O2)
617 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
619 std::swap(V1Size, V2Size);
621 if (isa<GEPOperator>(V1))
622 return aliasGEP(V1, V1Size, V2, V2Size);
624 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
626 std::swap(V1Size, V2Size);
628 if (const PHINode *PN = dyn_cast<PHINode>(V1))
629 return aliasPHI(PN, V1Size, V2, V2Size);
634 // This function is used to determine if the indices of two GEP instructions are
635 // equal. V1 and V2 are the indices.
636 static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) {
637 if (V1->getType() == V2->getType())
639 if (Constant *C1 = dyn_cast<Constant>(V1))
640 if (Constant *C2 = dyn_cast<Constant>(V2)) {
641 // Sign extend the constants to long types, if necessary
642 if (C1->getType() != Type::getInt64Ty(Context))
643 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context));
644 if (C2->getType() != Type::getInt64Ty(Context))
645 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context));
651 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
652 /// base pointers. This checks to see if the index expressions preclude the
653 /// pointers from aliasing...
654 AliasAnalysis::AliasResult
655 BasicAliasAnalysis::CheckGEPInstructions(
656 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
657 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
658 // We currently can't handle the case when the base pointers have different
659 // primitive types. Since this is uncommon anyway, we are happy being
660 // extremely conservative.
661 if (BasePtr1Ty != BasePtr2Ty)
664 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
666 LLVMContext &Context = GEPPointerTy->getContext();
668 // Find the (possibly empty) initial sequence of equal values... which are not
669 // necessarily constants.
670 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
671 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
672 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
673 unsigned UnequalOper = 0;
674 while (UnequalOper != MinOperands &&
675 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper],
677 // Advance through the type as we go...
679 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
680 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
682 // If all operands equal each other, then the derived pointers must
683 // alias each other...
685 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
686 "Ran out of type nesting, but not out of operands?");
691 // If we have seen all constant operands, and run out of indexes on one of the
692 // getelementptrs, check to see if the tail of the leftover one is all zeros.
693 // If so, return mustalias.
694 if (UnequalOper == MinOperands) {
695 if (NumGEP1Ops < NumGEP2Ops) {
696 std::swap(GEP1Ops, GEP2Ops);
697 std::swap(NumGEP1Ops, NumGEP2Ops);
700 bool AllAreZeros = true;
701 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
702 if (!isa<Constant>(GEP1Ops[i]) ||
703 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
707 if (AllAreZeros) return MustAlias;
711 // So now we know that the indexes derived from the base pointers,
712 // which are known to alias, are different. We can still determine a
713 // no-alias result if there are differing constant pairs in the index
714 // chain. For example:
715 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
717 // We have to be careful here about array accesses. In particular, consider:
718 // A[1][0] vs A[0][i]
719 // In this case, we don't *know* that the array will be accessed in bounds:
720 // the index could even be negative. Because of this, we have to
721 // conservatively *give up* and return may alias. We disregard differing
722 // array subscripts that are followed by a variable index without going
725 unsigned SizeMax = std::max(G1S, G2S);
726 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
728 // Scan for the first operand that is constant and unequal in the
729 // two getelementptrs...
730 unsigned FirstConstantOper = UnequalOper;
731 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
732 const Value *G1Oper = GEP1Ops[FirstConstantOper];
733 const Value *G2Oper = GEP2Ops[FirstConstantOper];
735 if (G1Oper != G2Oper) // Found non-equal constant indexes...
736 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
737 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
738 if (G1OC->getType() != G2OC->getType()) {
739 // Sign extend both operands to long.
740 if (G1OC->getType() != Type::getInt64Ty(Context))
741 G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context));
742 if (G2OC->getType() != Type::getInt64Ty(Context))
743 G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context));
744 GEP1Ops[FirstConstantOper] = G1OC;
745 GEP2Ops[FirstConstantOper] = G2OC;
749 // Handle the "be careful" case above: if this is an array/vector
750 // subscript, scan for a subsequent variable array index.
751 if (const SequentialType *STy =
752 dyn_cast<SequentialType>(BasePtr1Ty)) {
753 const Type *NextTy = STy;
754 bool isBadCase = false;
756 for (unsigned Idx = FirstConstantOper;
757 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
758 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
759 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
763 // If the array is indexed beyond the bounds of the static type
764 // at this level, it will also fall into the "be careful" case.
765 // It would theoretically be possible to analyze these cases,
766 // but for now just be conservatively correct.
767 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
768 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
769 ATy->getNumElements() ||
770 cast<ConstantInt>(G2OC)->getZExtValue() >=
771 ATy->getNumElements()) {
775 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
776 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
777 VTy->getNumElements() ||
778 cast<ConstantInt>(G2OC)->getZExtValue() >=
779 VTy->getNumElements()) {
783 STy = cast<SequentialType>(NextTy);
784 NextTy = cast<SequentialType>(NextTy)->getElementType();
787 if (isBadCase) G1OC = 0;
790 // Make sure they are comparable (ie, not constant expressions), and
791 // make sure the GEP with the smaller leading constant is GEP1.
793 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
795 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
796 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
797 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
798 std::swap(NumGEP1Ops, NumGEP2Ops);
805 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
808 // No shared constant operands, and we ran out of common operands. At this
809 // point, the GEP instructions have run through all of their operands, and we
810 // haven't found evidence that there are any deltas between the GEP's.
811 // However, one GEP may have more operands than the other. If this is the
812 // case, there may still be hope. Check this now.
813 if (FirstConstantOper == MinOperands) {
814 // Without TargetData, we won't know what the offsets are.
818 // Make GEP1Ops be the longer one if there is a longer one.
819 if (NumGEP1Ops < NumGEP2Ops) {
820 std::swap(GEP1Ops, GEP2Ops);
821 std::swap(NumGEP1Ops, NumGEP2Ops);
824 // Is there anything to check?
825 if (NumGEP1Ops > MinOperands) {
826 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
827 if (isa<ConstantInt>(GEP1Ops[i]) &&
828 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
829 // Yup, there's a constant in the tail. Set all variables to
830 // constants in the GEP instruction to make it suitable for
831 // TargetData::getIndexedOffset.
832 for (i = 0; i != MaxOperands; ++i)
833 if (!isa<ConstantInt>(GEP1Ops[i]))
834 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
835 // Okay, now get the offset. This is the relative offset for the full
837 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
840 // Now check without any constants at the end.
841 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
844 // Make sure we compare the absolute difference.
845 if (Offset1 > Offset2)
846 std::swap(Offset1, Offset2);
848 // If the tail provided a bit enough offset, return noalias!
849 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
851 // Otherwise break - we don't look for another constant in the tail.
856 // Couldn't find anything useful.
860 // If there are non-equal constants arguments, then we can figure
861 // out a minimum known delta between the two index expressions... at
862 // this point we know that the first constant index of GEP1 is less
863 // than the first constant index of GEP2.
865 // Advance BasePtr[12]Ty over this first differing constant operand.
866 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
867 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
868 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
869 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
871 // We are going to be using TargetData::getIndexedOffset to determine the
872 // offset that each of the GEP's is reaching. To do this, we have to convert
873 // all variable references to constant references. To do this, we convert the
874 // initial sequence of array subscripts into constant zeros to start with.
875 const Type *ZeroIdxTy = GEPPointerTy;
876 for (unsigned i = 0; i != FirstConstantOper; ++i) {
877 if (!isa<StructType>(ZeroIdxTy))
878 GEP1Ops[i] = GEP2Ops[i] =
879 Constant::getNullValue(Type::getInt32Ty(Context));
881 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
882 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
885 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
887 // Loop over the rest of the operands...
888 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
889 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
890 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
891 // If they are equal, use a zero index...
892 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
893 if (!isa<ConstantInt>(Op1))
894 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
895 // Otherwise, just keep the constants we have.
898 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
899 // If this is an array index, make sure the array element is in range.
900 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
901 if (Op1C->getZExtValue() >= AT->getNumElements())
902 return MayAlias; // Be conservative with out-of-range accesses
903 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
904 if (Op1C->getZExtValue() >= VT->getNumElements())
905 return MayAlias; // Be conservative with out-of-range accesses
909 // GEP1 is known to produce a value less than GEP2. To be
910 // conservatively correct, we must assume the largest possible
911 // constant is used in this position. This cannot be the initial
912 // index to the GEP instructions (because we know we have at least one
913 // element before this one with the different constant arguments), so
914 // we know that the current index must be into either a struct or
915 // array. Because we know it's not constant, this cannot be a
916 // structure index. Because of this, we can calculate the maximum
919 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
921 ConstantInt::get(Type::getInt64Ty(Context),
922 AT->getNumElements()-1);
923 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
925 ConstantInt::get(Type::getInt64Ty(Context),
926 VT->getNumElements()-1);
931 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
932 // If this is an array index, make sure the array element is in range.
933 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
934 if (Op2C->getZExtValue() >= AT->getNumElements())
935 return MayAlias; // Be conservative with out-of-range accesses
936 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
937 if (Op2C->getZExtValue() >= VT->getNumElements())
938 return MayAlias; // Be conservative with out-of-range accesses
940 } else { // Conservatively assume the minimum value for this index
941 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
946 if (BasePtr1Ty && Op1) {
947 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
948 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
953 if (BasePtr2Ty && Op2) {
954 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
955 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
961 if (TD && GEPPointerTy->getElementType()->isSized()) {
963 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
965 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
966 assert(Offset1 != Offset2 &&
967 "There is at least one different constant here!");
969 // Make sure we compare the absolute difference.
970 if (Offset1 > Offset2)
971 std::swap(Offset1, Offset2);
973 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
974 //cerr << "Determined that these two GEP's don't alias ["
975 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
982 // Make sure that anything that uses AliasAnalysis pulls in this file...
983 DEFINING_FILE_FOR(BasicAliasAnalysis)