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/Compiler.h"
34 #include "llvm/Support/ErrorHandling.h"
35 #include "llvm/Support/GetElementPtrTypeIterator.h"
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
43 static const Value *GetGEPOperands(const Value *V,
44 SmallVector<Value*, 16> &GEPOps) {
45 assert(GEPOps.empty() && "Expect empty list to populate!");
46 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
47 cast<User>(V)->op_end());
49 // Accumulate all of the chained indexes into the operand array
50 V = cast<User>(V)->getOperand(0);
52 while (const GEPOperator *G = dyn_cast<GEPOperator>(V)) {
53 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
54 !cast<Constant>(GEPOps[0])->isNullValue())
55 break; // Don't handle folding arbitrary pointer offsets yet...
56 GEPOps.erase(GEPOps.begin()); // Drop the zero index
57 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
63 /// isKnownNonNull - Return true if we know that the specified value is never
65 static bool isKnownNonNull(const Value *V) {
66 // Alloca never returns null, malloc might.
67 if (isa<AllocaInst>(V)) return true;
69 // A byval argument is never null.
70 if (const Argument *A = dyn_cast<Argument>(V))
71 return A->hasByValAttr();
73 // Global values are not null unless extern weak.
74 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
75 return !GV->hasExternalWeakLinkage();
79 /// isNonEscapingLocalObject - Return true if the pointer is to a function-local
80 /// object that never escapes from the function.
81 static bool isNonEscapingLocalObject(const Value *V) {
82 // If this is a local allocation, check to see if it escapes.
83 if (isa<AllocaInst>(V) || isNoAliasCall(V))
84 return !PointerMayBeCaptured(V, false);
86 // If this is an argument that corresponds to a byval or noalias argument,
87 // then it has not escaped before entering the function. Check if it escapes
88 // inside the function.
89 if (const Argument *A = dyn_cast<Argument>(V))
90 if (A->hasByValAttr() || A->hasNoAliasAttr()) {
91 // Don't bother analyzing arguments already known not to escape.
92 if (A->hasNoCaptureAttr())
94 return !PointerMayBeCaptured(V, false);
100 /// isObjectSmallerThan - Return true if we can prove that the object specified
101 /// by V is smaller than Size.
102 static bool isObjectSmallerThan(const Value *V, unsigned Size,
103 LLVMContext &Context, const TargetData &TD) {
104 const Type *AccessTy;
105 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
106 AccessTy = GV->getType()->getElementType();
107 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
108 if (!AI->isArrayAllocation())
109 AccessTy = AI->getType()->getElementType();
112 } else if (const CallInst* CI = extractMallocCall(V)) {
113 if (!isArrayMalloc(V, Context, &TD))
114 // The size is the argument to the malloc call.
115 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
116 return (C->getZExtValue() < Size);
118 } else if (const Argument *A = dyn_cast<Argument>(V)) {
119 if (A->hasByValAttr())
120 AccessTy = cast<PointerType>(A->getType())->getElementType();
127 if (AccessTy->isSized())
128 return TD.getTypeAllocSize(AccessTy) < Size;
132 //===----------------------------------------------------------------------===//
134 //===----------------------------------------------------------------------===//
137 /// NoAA - This class implements the -no-aa pass, which always returns "I
138 /// don't know" for alias queries. NoAA is unlike other alias analysis
139 /// implementations, in that it does not chain to a previous analysis. As
140 /// such it doesn't follow many of the rules that other alias analyses must.
142 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
143 static char ID; // Class identification, replacement for typeinfo
144 NoAA() : ImmutablePass(&ID) {}
145 explicit NoAA(void *PID) : ImmutablePass(PID) { }
147 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
150 virtual void initializePass() {
151 TD = getAnalysisIfAvailable<TargetData>();
154 virtual AliasResult alias(const Value *V1, unsigned V1Size,
155 const Value *V2, unsigned V2Size) {
159 virtual void getArgumentAccesses(Function *F, CallSite CS,
160 std::vector<PointerAccessInfo> &Info) {
161 llvm_unreachable("This method may not be called on this function!");
164 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
165 virtual bool pointsToConstantMemory(const Value *P) { return false; }
166 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
169 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
172 virtual bool hasNoModRefInfoForCalls() const { return true; }
174 virtual void deleteValue(Value *V) {}
175 virtual void copyValue(Value *From, Value *To) {}
177 } // End of anonymous namespace
179 // Register this pass...
181 static RegisterPass<NoAA>
182 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
184 // Declare that we implement the AliasAnalysis interface
185 static RegisterAnalysisGroup<AliasAnalysis> V(U);
187 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
189 //===----------------------------------------------------------------------===//
191 //===----------------------------------------------------------------------===//
194 /// BasicAliasAnalysis - This is the default alias analysis implementation.
195 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
196 /// derives from the NoAA class.
197 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
198 static char ID; // Class identification, replacement for typeinfo
199 BasicAliasAnalysis() : NoAA(&ID) {}
200 AliasResult alias(const Value *V1, unsigned V1Size,
201 const Value *V2, unsigned V2Size) {
202 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!");
203 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size);
208 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
209 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
211 /// hasNoModRefInfoForCalls - We can provide mod/ref information against
212 /// non-escaping allocations.
213 virtual bool hasNoModRefInfoForCalls() const { return false; }
215 /// pointsToConstantMemory - Chase pointers until we find a (constant
217 bool pointsToConstantMemory(const Value *P);
220 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call.
221 SmallPtrSet<const PHINode*, 16> VisitedPHIs;
223 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
225 AliasResult aliasGEP(const Value *V1, unsigned V1Size,
226 const Value *V2, unsigned V2Size);
228 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
230 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize,
231 const Value *V2, unsigned V2Size);
233 AliasResult aliasCheck(const Value *V1, unsigned V1Size,
234 const Value *V2, unsigned V2Size);
236 // CheckGEPInstructions - Check two GEP instructions with known
237 // must-aliasing base pointers. This checks to see if the index expressions
238 // preclude the pointers from aliasing...
240 CheckGEPInstructions(const Type* BasePtr1Ty,
241 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
242 const Type *BasePtr2Ty,
243 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
245 } // End of anonymous namespace
247 // Register this pass...
248 char BasicAliasAnalysis::ID = 0;
249 static RegisterPass<BasicAliasAnalysis>
250 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
252 // Declare that we implement the AliasAnalysis interface
253 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
255 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
256 return new BasicAliasAnalysis();
260 /// pointsToConstantMemory - Chase pointers until we find a (constant
262 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
263 if (const GlobalVariable *GV =
264 dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
265 return GV->isConstant();
270 // getModRefInfo - Check to see if the specified callsite can clobber the
271 // specified memory object. Since we only look at local properties of this
272 // function, we really can't say much about this query. We do, however, use
273 // simple "address taken" analysis on local objects.
275 AliasAnalysis::ModRefResult
276 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
277 if (!isa<Constant>(P)) {
278 const Value *Object = P->getUnderlyingObject();
280 // If this is a tail call and P points to a stack location, we know that
281 // the tail call cannot access or modify the local stack.
282 // We cannot exclude byval arguments here; these belong to the caller of
283 // the current function not to the current function, and a tail callee
284 // may reference them.
285 if (isa<AllocaInst>(Object))
286 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
287 if (CI->isTailCall())
290 // If the pointer is to a locally allocated object that does not escape,
291 // then the call can not mod/ref the pointer unless the call takes the
292 // argument without capturing it.
293 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
294 bool passedAsArg = false;
295 // TODO: Eventually only check 'nocapture' arguments.
296 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
298 if (isa<PointerType>((*CI)->getType()) &&
299 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
306 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
307 switch (II->getIntrinsicID()) {
309 case Intrinsic::memcpy:
310 case Intrinsic::memmove: {
312 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3)))
313 Len = LenCI->getZExtValue();
314 Value *Dest = II->getOperand(1);
315 Value *Src = II->getOperand(2);
316 if (alias(Dest, Len, P, Size) == NoAlias) {
317 if (alias(Src, Len, P, Size) == NoAlias)
323 case Intrinsic::memset:
324 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) {
325 unsigned Len = LenCI->getZExtValue();
326 Value *Dest = II->getOperand(1);
327 if (alias(Dest, Len, P, Size) == NoAlias)
331 case Intrinsic::atomic_cmp_swap:
332 case Intrinsic::atomic_swap:
333 case Intrinsic::atomic_load_add:
334 case Intrinsic::atomic_load_sub:
335 case Intrinsic::atomic_load_and:
336 case Intrinsic::atomic_load_nand:
337 case Intrinsic::atomic_load_or:
338 case Intrinsic::atomic_load_xor:
339 case Intrinsic::atomic_load_max:
340 case Intrinsic::atomic_load_min:
341 case Intrinsic::atomic_load_umax:
342 case Intrinsic::atomic_load_umin:
344 Value *Op1 = II->getOperand(1);
345 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType());
346 if (alias(Op1, Op1Size, P, Size) == NoAlias)
350 case Intrinsic::lifetime_start:
351 case Intrinsic::lifetime_end:
352 case Intrinsic::invariant_start: {
353 unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue();
354 if (alias(II->getOperand(2), PtrSize, P, Size) == NoAlias)
358 case Intrinsic::invariant_end: {
359 unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue();
360 if (alias(II->getOperand(3), PtrSize, P, Size) == NoAlias)
368 // The AliasAnalysis base class has some smarts, lets use them.
369 return AliasAnalysis::getModRefInfo(CS, P, Size);
373 AliasAnalysis::ModRefResult
374 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
375 // If CS1 or CS2 are readnone, they don't interact.
376 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
377 if (CS1B == DoesNotAccessMemory) return NoModRef;
379 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
380 if (CS2B == DoesNotAccessMemory) return NoModRef;
382 // If they both only read from memory, just return ref.
383 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
386 // Otherwise, fall back to NoAA (mod+ref).
387 return NoAA::getModRefInfo(CS1, CS2);
390 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
393 AliasAnalysis::AliasResult
394 BasicAliasAnalysis::aliasGEP(const Value *V1, unsigned V1Size,
395 const Value *V2, unsigned V2Size) {
396 // If we have two gep instructions with must-alias'ing base pointers, figure
397 // out if the indexes to the GEP tell us anything about the derived pointer.
398 // Note that we also handle chains of getelementptr instructions as well as
399 // constant expression getelementptrs here.
401 if (isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
402 const User *GEP1 = cast<User>(V1);
403 const User *GEP2 = cast<User>(V2);
405 // If V1 and V2 are identical GEPs, just recurse down on both of them.
406 // This allows us to analyze things like:
407 // P = gep A, 0, i, 1
408 // Q = gep B, 0, i, 1
409 // by just analyzing A and B. This is even safe for variable indices.
410 if (GEP1->getType() == GEP2->getType() &&
411 GEP1->getNumOperands() == GEP2->getNumOperands() &&
412 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
413 // All operands are the same, ignoring the base.
414 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
415 return aliasCheck(GEP1->getOperand(0), V1Size,
416 GEP2->getOperand(0), V2Size);
418 // Drill down into the first non-gep value, to test for must-aliasing of
419 // the base pointers.
420 while (isa<GEPOperator>(GEP1->getOperand(0)) &&
421 GEP1->getOperand(1) ==
422 Constant::getNullValue(GEP1->getOperand(1)->getType()))
423 GEP1 = cast<User>(GEP1->getOperand(0));
424 const Value *BasePtr1 = GEP1->getOperand(0);
426 while (isa<GEPOperator>(GEP2->getOperand(0)) &&
427 GEP2->getOperand(1) ==
428 Constant::getNullValue(GEP2->getOperand(1)->getType()))
429 GEP2 = cast<User>(GEP2->getOperand(0));
430 const Value *BasePtr2 = GEP2->getOperand(0);
432 // Do the base pointers alias?
433 AliasResult BaseAlias = aliasCheck(BasePtr1, ~0U, BasePtr2, ~0U);
434 if (BaseAlias == NoAlias) return NoAlias;
435 if (BaseAlias == MustAlias) {
436 // If the base pointers alias each other exactly, check to see if we can
437 // figure out anything about the resultant pointers, to try to prove
440 // Collect all of the chained GEP operands together into one simple place
441 SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
442 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
443 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
445 // If GetGEPOperands were able to fold to the same must-aliased pointer,
446 // do the comparison.
447 if (BasePtr1 == BasePtr2) {
449 CheckGEPInstructions(BasePtr1->getType(),
450 &GEP1Ops[0], GEP1Ops.size(), V1Size,
452 &GEP2Ops[0], GEP2Ops.size(), V2Size);
453 if (GAlias != MayAlias)
459 // Check to see if these two pointers are related by a getelementptr
460 // instruction. If one pointer is a GEP with a non-zero index of the other
461 // pointer, we know they cannot alias.
463 if (V1Size == ~0U || V2Size == ~0U)
466 SmallVector<Value*, 16> GEPOperands;
467 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
469 AliasResult R = aliasCheck(BasePtr, ~0U, V2, V2Size);
471 // If V2 may alias GEP base pointer, conservatively returns MayAlias.
472 // If V2 is known not to alias GEP base pointer, then the two values
473 // cannot alias per GEP semantics: "A pointer value formed from a
474 // getelementptr instruction is associated with the addresses associated
475 // with the first operand of the getelementptr".
478 // If there is at least one non-zero constant index, we know they cannot
480 bool ConstantFound = false;
481 bool AllZerosFound = true;
482 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
483 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
484 if (!C->isNullValue()) {
485 ConstantFound = true;
486 AllZerosFound = false;
490 AllZerosFound = false;
493 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
494 // the ptr, the end result is a must alias also.
499 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
502 // Otherwise we have to check to see that the distance is more than
503 // the size of the argument... build an index vector that is equal to
504 // the arguments provided, except substitute 0's for any variable
505 // indexes we find...
507 cast<PointerType>(BasePtr->getType())->getElementType()->isSized()) {
508 for (unsigned i = 0; i != GEPOperands.size(); ++i)
509 if (!isa<ConstantInt>(GEPOperands[i]))
510 GEPOperands[i] = Constant::getNullValue(GEPOperands[i]->getType());
511 int64_t Offset = TD->getIndexedOffset(BasePtr->getType(),
515 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
523 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
525 AliasAnalysis::AliasResult
526 BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize,
527 const Value *V2, unsigned V2Size) {
528 // The PHI node has already been visited, avoid recursion any further.
529 if (!VisitedPHIs.insert(PN))
532 SmallPtrSet<Value*, 4> UniqueSrc;
533 SmallVector<Value*, 4> V1Srcs;
534 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
535 Value *PV1 = PN->getIncomingValue(i);
536 if (isa<PHINode>(PV1))
537 // If any of the source itself is a PHI, return MayAlias conservatively
538 // to avoid compile time explosion. The worst possible case is if both
539 // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
540 // and 'n' are the number of PHI sources.
542 if (UniqueSrc.insert(PV1))
543 V1Srcs.push_back(PV1);
546 AliasResult Alias = aliasCheck(V1Srcs[0], PNSize, V2, V2Size);
547 // Early exit if the check of the first PHI source against V2 is MayAlias.
548 // Other results are not possible.
549 if (Alias == MayAlias)
552 // If all sources of the PHI node NoAlias or MustAlias V2, then returns
553 // NoAlias / MustAlias. Otherwise, returns MayAlias.
554 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
555 Value *V = V1Srcs[i];
556 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize);
557 if (ThisAlias != Alias || ThisAlias == MayAlias)
564 // aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases,
565 // such as array references.
567 AliasAnalysis::AliasResult
568 BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size,
569 const Value *V2, unsigned V2Size) {
570 // Strip off any casts if they exist.
571 V1 = V1->stripPointerCasts();
572 V2 = V2->stripPointerCasts();
574 // Are we checking for alias of the same value?
575 if (V1 == V2) return MustAlias;
577 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
578 return NoAlias; // Scalars cannot alias each other
580 // Figure out what objects these things are pointing to if we can.
581 const Value *O1 = V1->getUnderlyingObject();
582 const Value *O2 = V2->getUnderlyingObject();
585 // If V1/V2 point to two different objects we know that we have no alias.
586 if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
589 // Arguments can't alias with local allocations or noalias calls.
590 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
591 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
594 // Most objects can't alias null.
595 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
596 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
600 // If the size of one access is larger than the entire object on the other
601 // side, then we know such behavior is undefined and can assume no alias.
602 LLVMContext &Context = V1->getContext();
604 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, Context, *TD)) ||
605 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, Context, *TD)))
608 // If one pointer is the result of a call/invoke and the other is a
609 // non-escaping local object, then we know the object couldn't escape to a
610 // point where the call could return it.
611 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
612 isNonEscapingLocalObject(O2) && O1 != O2)
614 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
615 isNonEscapingLocalObject(O1) && O1 != O2)
618 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
620 std::swap(V1Size, V2Size);
622 if (isa<GEPOperator>(V1))
623 return aliasGEP(V1, V1Size, V2, V2Size);
625 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
627 std::swap(V1Size, V2Size);
629 if (const PHINode *PN = dyn_cast<PHINode>(V1))
630 return aliasPHI(PN, V1Size, V2, V2Size);
635 // This function is used to determine if the indices of two GEP instructions are
636 // equal. V1 and V2 are the indices.
637 static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) {
638 if (V1->getType() == V2->getType())
640 if (Constant *C1 = dyn_cast<Constant>(V1))
641 if (Constant *C2 = dyn_cast<Constant>(V2)) {
642 // Sign extend the constants to long types, if necessary
643 if (C1->getType() != Type::getInt64Ty(Context))
644 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context));
645 if (C2->getType() != Type::getInt64Ty(Context))
646 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context));
652 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
653 /// base pointers. This checks to see if the index expressions preclude the
654 /// pointers from aliasing...
655 AliasAnalysis::AliasResult
656 BasicAliasAnalysis::CheckGEPInstructions(
657 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
658 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
659 // We currently can't handle the case when the base pointers have different
660 // primitive types. Since this is uncommon anyway, we are happy being
661 // extremely conservative.
662 if (BasePtr1Ty != BasePtr2Ty)
665 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
667 LLVMContext &Context = GEPPointerTy->getContext();
669 // Find the (possibly empty) initial sequence of equal values... which are not
670 // necessarily constants.
671 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
672 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
673 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
674 unsigned UnequalOper = 0;
675 while (UnequalOper != MinOperands &&
676 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper],
678 // Advance through the type as we go...
680 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
681 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
683 // If all operands equal each other, then the derived pointers must
684 // alias each other...
686 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
687 "Ran out of type nesting, but not out of operands?");
692 // If we have seen all constant operands, and run out of indexes on one of the
693 // getelementptrs, check to see if the tail of the leftover one is all zeros.
694 // If so, return mustalias.
695 if (UnequalOper == MinOperands) {
696 if (NumGEP1Ops < NumGEP2Ops) {
697 std::swap(GEP1Ops, GEP2Ops);
698 std::swap(NumGEP1Ops, NumGEP2Ops);
701 bool AllAreZeros = true;
702 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
703 if (!isa<Constant>(GEP1Ops[i]) ||
704 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
708 if (AllAreZeros) return MustAlias;
712 // So now we know that the indexes derived from the base pointers,
713 // which are known to alias, are different. We can still determine a
714 // no-alias result if there are differing constant pairs in the index
715 // chain. For example:
716 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
718 // We have to be careful here about array accesses. In particular, consider:
719 // A[1][0] vs A[0][i]
720 // In this case, we don't *know* that the array will be accessed in bounds:
721 // the index could even be negative. Because of this, we have to
722 // conservatively *give up* and return may alias. We disregard differing
723 // array subscripts that are followed by a variable index without going
726 unsigned SizeMax = std::max(G1S, G2S);
727 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
729 // Scan for the first operand that is constant and unequal in the
730 // two getelementptrs...
731 unsigned FirstConstantOper = UnequalOper;
732 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
733 const Value *G1Oper = GEP1Ops[FirstConstantOper];
734 const Value *G2Oper = GEP2Ops[FirstConstantOper];
736 if (G1Oper != G2Oper) // Found non-equal constant indexes...
737 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
738 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
739 if (G1OC->getType() != G2OC->getType()) {
740 // Sign extend both operands to long.
741 if (G1OC->getType() != Type::getInt64Ty(Context))
742 G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context));
743 if (G2OC->getType() != Type::getInt64Ty(Context))
744 G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context));
745 GEP1Ops[FirstConstantOper] = G1OC;
746 GEP2Ops[FirstConstantOper] = G2OC;
750 // Handle the "be careful" case above: if this is an array/vector
751 // subscript, scan for a subsequent variable array index.
752 if (const SequentialType *STy =
753 dyn_cast<SequentialType>(BasePtr1Ty)) {
754 const Type *NextTy = STy;
755 bool isBadCase = false;
757 for (unsigned Idx = FirstConstantOper;
758 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
759 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
760 if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
764 // If the array is indexed beyond the bounds of the static type
765 // at this level, it will also fall into the "be careful" case.
766 // It would theoretically be possible to analyze these cases,
767 // but for now just be conservatively correct.
768 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
769 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
770 ATy->getNumElements() ||
771 cast<ConstantInt>(G2OC)->getZExtValue() >=
772 ATy->getNumElements()) {
776 if (const VectorType *VTy = dyn_cast<VectorType>(STy))
777 if (cast<ConstantInt>(G1OC)->getZExtValue() >=
778 VTy->getNumElements() ||
779 cast<ConstantInt>(G2OC)->getZExtValue() >=
780 VTy->getNumElements()) {
784 STy = cast<SequentialType>(NextTy);
785 NextTy = cast<SequentialType>(NextTy)->getElementType();
788 if (isBadCase) G1OC = 0;
791 // Make sure they are comparable (ie, not constant expressions), and
792 // make sure the GEP with the smaller leading constant is GEP1.
794 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
796 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
797 if (CV->getZExtValue()) { // If they are comparable and G2 > G1
798 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
799 std::swap(NumGEP1Ops, NumGEP2Ops);
806 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
809 // No shared constant operands, and we ran out of common operands. At this
810 // point, the GEP instructions have run through all of their operands, and we
811 // haven't found evidence that there are any deltas between the GEP's.
812 // However, one GEP may have more operands than the other. If this is the
813 // case, there may still be hope. Check this now.
814 if (FirstConstantOper == MinOperands) {
815 // Without TargetData, we won't know what the offsets are.
819 // Make GEP1Ops be the longer one if there is a longer one.
820 if (NumGEP1Ops < NumGEP2Ops) {
821 std::swap(GEP1Ops, GEP2Ops);
822 std::swap(NumGEP1Ops, NumGEP2Ops);
825 // Is there anything to check?
826 if (NumGEP1Ops > MinOperands) {
827 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
828 if (isa<ConstantInt>(GEP1Ops[i]) &&
829 !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
830 // Yup, there's a constant in the tail. Set all variables to
831 // constants in the GEP instruction to make it suitable for
832 // TargetData::getIndexedOffset.
833 for (i = 0; i != MaxOperands; ++i)
834 if (!isa<ConstantInt>(GEP1Ops[i]))
835 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
836 // Okay, now get the offset. This is the relative offset for the full
838 int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
841 // Now check without any constants at the end.
842 int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
845 // Make sure we compare the absolute difference.
846 if (Offset1 > Offset2)
847 std::swap(Offset1, Offset2);
849 // If the tail provided a bit enough offset, return noalias!
850 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
852 // Otherwise break - we don't look for another constant in the tail.
857 // Couldn't find anything useful.
861 // If there are non-equal constants arguments, then we can figure
862 // out a minimum known delta between the two index expressions... at
863 // this point we know that the first constant index of GEP1 is less
864 // than the first constant index of GEP2.
866 // Advance BasePtr[12]Ty over this first differing constant operand.
867 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
868 getTypeAtIndex(GEP2Ops[FirstConstantOper]);
869 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
870 getTypeAtIndex(GEP1Ops[FirstConstantOper]);
872 // We are going to be using TargetData::getIndexedOffset to determine the
873 // offset that each of the GEP's is reaching. To do this, we have to convert
874 // all variable references to constant references. To do this, we convert the
875 // initial sequence of array subscripts into constant zeros to start with.
876 const Type *ZeroIdxTy = GEPPointerTy;
877 for (unsigned i = 0; i != FirstConstantOper; ++i) {
878 if (!isa<StructType>(ZeroIdxTy))
879 GEP1Ops[i] = GEP2Ops[i] =
880 Constant::getNullValue(Type::getInt32Ty(Context));
882 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
883 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
886 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
888 // Loop over the rest of the operands...
889 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
890 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
891 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
892 // If they are equal, use a zero index...
893 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
894 if (!isa<ConstantInt>(Op1))
895 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
896 // Otherwise, just keep the constants we have.
899 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
900 // If this is an array index, make sure the array element is in range.
901 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
902 if (Op1C->getZExtValue() >= AT->getNumElements())
903 return MayAlias; // Be conservative with out-of-range accesses
904 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
905 if (Op1C->getZExtValue() >= VT->getNumElements())
906 return MayAlias; // Be conservative with out-of-range accesses
910 // GEP1 is known to produce a value less than GEP2. To be
911 // conservatively correct, we must assume the largest possible
912 // constant is used in this position. This cannot be the initial
913 // index to the GEP instructions (because we know we have at least one
914 // element before this one with the different constant arguments), so
915 // we know that the current index must be into either a struct or
916 // array. Because we know it's not constant, this cannot be a
917 // structure index. Because of this, we can calculate the maximum
920 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
922 ConstantInt::get(Type::getInt64Ty(Context),
923 AT->getNumElements()-1);
924 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
926 ConstantInt::get(Type::getInt64Ty(Context),
927 VT->getNumElements()-1);
932 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
933 // If this is an array index, make sure the array element is in range.
934 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
935 if (Op2C->getZExtValue() >= AT->getNumElements())
936 return MayAlias; // Be conservative with out-of-range accesses
937 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
938 if (Op2C->getZExtValue() >= VT->getNumElements())
939 return MayAlias; // Be conservative with out-of-range accesses
941 } else { // Conservatively assume the minimum value for this index
942 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
947 if (BasePtr1Ty && Op1) {
948 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
949 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
954 if (BasePtr2Ty && Op2) {
955 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
956 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
962 if (TD && GEPPointerTy->getElementType()->isSized()) {
964 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
966 TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
967 assert(Offset1 != Offset2 &&
968 "There is at least one different constant here!");
970 // Make sure we compare the absolute difference.
971 if (Offset1 > Offset2)
972 std::swap(Offset1, Offset2);
974 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
975 //cerr << "Determined that these two GEP's don't alias ["
976 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
983 // Make sure that anything that uses AliasAnalysis pulls in this file...
984 DEFINING_FILE_FOR(BasicAliasAnalysis)