1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Pass.h"
18 #include "llvm/Argument.h"
19 #include "llvm/iOther.h"
20 #include "llvm/ConstantHandling.h"
21 #include "llvm/GlobalValue.h"
22 #include "llvm/DerivedTypes.h"
23 #include "llvm/Target/TargetData.h"
24 #include "llvm/Support/GetElementPtrTypeIterator.h"
27 // Make sure that anything that uses AliasAnalysis pulls in this file...
28 void llvm::BasicAAStub() {}
31 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
33 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
34 AliasAnalysis::getAnalysisUsage(AU);
37 virtual void initializePass();
39 // alias - This is the only method here that does anything interesting...
41 AliasResult alias(const Value *V1, unsigned V1Size,
42 const Value *V2, unsigned V2Size);
44 // CheckGEPInstructions - Check two GEP instructions with known
45 // must-aliasing base pointers. This checks to see if the index expressions
46 // preclude the pointers from aliasing...
48 CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
50 const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
54 // Register this pass...
55 RegisterOpt<BasicAliasAnalysis>
56 X("basicaa", "Basic Alias Analysis (default AA impl)");
58 // Declare that we implement the AliasAnalysis interface
59 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
60 } // End of anonymous namespace
62 void BasicAliasAnalysis::initializePass() {
63 InitializeAliasAnalysis(this);
66 // hasUniqueAddress - Return true if the specified value points to something
67 // with a unique, discernable, address.
68 static inline bool hasUniqueAddress(const Value *V) {
69 return isa<GlobalValue>(V) || isa<AllocationInst>(V);
72 // getUnderlyingObject - This traverses the use chain to figure out what object
73 // the specified value points to. If the value points to, or is derived from, a
74 // unique object or an argument, return it.
75 static const Value *getUnderlyingObject(const Value *V) {
76 if (!isa<PointerType>(V->getType())) return 0;
78 // If we are at some type of object... return it.
79 if (hasUniqueAddress(V) || isa<Argument>(V)) return V;
81 // Traverse through different addressing mechanisms...
82 if (const Instruction *I = dyn_cast<Instruction>(V)) {
83 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
84 return getUnderlyingObject(I->getOperand(0));
85 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
86 if (CE->getOpcode() == Instruction::Cast ||
87 CE->getOpcode() == Instruction::GetElementPtr)
88 return getUnderlyingObject(CE->getOperand(0));
89 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
90 return CPR->getValue();
95 static const User *isGEP(const Value *V) {
96 if (isa<GetElementPtrInst>(V) ||
97 (isa<ConstantExpr>(V) &&
98 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
103 static const Value *GetGEPOperands(const Value *V, std::vector<Value*> &GEPOps){
104 assert(GEPOps.empty() && "Expect empty list to populate!");
105 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
106 cast<User>(V)->op_end());
108 // Accumulate all of the chained indexes into the operand array
109 V = cast<User>(V)->getOperand(0);
111 while (const User *G = isGEP(V)) {
112 if (!isa<Constant>(GEPOps[0]) ||
113 !cast<Constant>(GEPOps[0])->isNullValue())
114 break; // Don't handle folding arbitrary pointer offsets yet...
115 GEPOps.erase(GEPOps.begin()); // Drop the zero index
116 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
117 V = G->getOperand(0);
123 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
124 // as array references. Note that this function is heavily tail recursive.
125 // Hopefully we have a smart C++ compiler. :)
127 AliasAnalysis::AliasResult
128 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
129 const Value *V2, unsigned V2Size) {
130 // Strip off any constant expression casts if they exist
131 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
132 if (CE->getOpcode() == Instruction::Cast)
133 V1 = CE->getOperand(0);
134 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
135 if (CE->getOpcode() == Instruction::Cast)
136 V2 = CE->getOperand(0);
138 // Strip off constant pointer refs if they exist
139 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
140 V1 = CPR->getValue();
141 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
142 V2 = CPR->getValue();
144 // Are we checking for alias of the same value?
145 if (V1 == V2) return MustAlias;
147 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
148 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
149 return NoAlias; // Scalars cannot alias each other
151 // Strip off cast instructions...
152 if (const Instruction *I = dyn_cast<CastInst>(V1))
153 return alias(I->getOperand(0), V1Size, V2, V2Size);
154 if (const Instruction *I = dyn_cast<CastInst>(V2))
155 return alias(V1, V1Size, I->getOperand(0), V2Size);
157 // Figure out what objects these things are pointing to if we can...
158 const Value *O1 = getUnderlyingObject(V1);
159 const Value *O2 = getUnderlyingObject(V2);
161 // Pointing at a discernible object?
163 if (isa<Argument>(O1)) {
164 // Incoming argument cannot alias locally allocated object!
165 if (isa<AllocationInst>(O2)) return NoAlias;
166 // Otherwise, nothing is known...
167 } else if (isa<Argument>(O2)) {
168 // Incoming argument cannot alias locally allocated object!
169 if (isa<AllocationInst>(O1)) return NoAlias;
170 // Otherwise, nothing is known...
172 // If they are two different objects, we know that we have no alias...
173 if (O1 != O2) return NoAlias;
176 // If they are the same object, they we can look at the indexes. If they
177 // index off of the object is the same for both pointers, they must alias.
178 // If they are provably different, they must not alias. Otherwise, we can't
180 } else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) {
181 return NoAlias; // Unique values don't alias null
182 } else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) {
183 return NoAlias; // Unique values don't alias null
186 // If we have two gep instructions with must-alias'ing base pointers, figure
187 // out if the indexes to the GEP tell us anything about the derived pointer.
188 // Note that we also handle chains of getelementptr instructions as well as
189 // constant expression getelementptrs here.
191 if (isGEP(V1) && isGEP(V2)) {
192 // Drill down into the first non-gep value, to test for must-aliasing of
193 // the base pointers.
194 const Value *BasePtr1 = V1, *BasePtr2 = V2;
196 BasePtr1 = cast<User>(BasePtr1)->getOperand(0);
197 } while (isGEP(BasePtr1) &&
198 cast<User>(BasePtr1)->getOperand(1) ==
199 Constant::getNullValue(cast<User>(BasePtr1)->getOperand(1)->getType()));
201 BasePtr2 = cast<User>(BasePtr2)->getOperand(0);
202 } while (isGEP(BasePtr2) &&
203 cast<User>(BasePtr2)->getOperand(1) ==
204 Constant::getNullValue(cast<User>(BasePtr2)->getOperand(1)->getType()));
206 // Do the base pointers alias?
207 AliasResult BaseAlias = alias(BasePtr1, V1Size, BasePtr2, V2Size);
208 if (BaseAlias == NoAlias) return NoAlias;
209 if (BaseAlias == MustAlias) {
210 // If the base pointers alias each other exactly, check to see if we can
211 // figure out anything about the resultant pointers, to try to prove
214 // Collect all of the chained GEP operands together into one simple place
215 std::vector<Value*> GEP1Ops, GEP2Ops;
216 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
217 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
220 CheckGEPInstructions(BasePtr1->getType(), GEP1Ops, V1Size,
221 BasePtr2->getType(), GEP2Ops, V2Size);
222 if (GAlias != MayAlias)
227 // Check to see if these two pointers are related by a getelementptr
228 // instruction. If one pointer is a GEP with a non-zero index of the other
229 // pointer, we know they cannot alias.
233 std::swap(V1Size, V2Size);
236 if (V1Size != ~0U && V2Size != ~0U)
237 if (const User *GEP = isGEP(V1)) {
238 std::vector<Value*> GEPOperands;
239 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
241 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
242 if (R == MustAlias) {
243 // If there is at least one non-zero constant index, we know they cannot
245 bool ConstantFound = false;
246 bool AllZerosFound = true;
247 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
248 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
249 if (!C->isNullValue()) {
250 ConstantFound = true;
251 AllZerosFound = false;
255 AllZerosFound = false;
258 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
259 // the ptr, the end result is a must alias also.
264 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
267 // Otherwise we have to check to see that the distance is more than
268 // the size of the argument... build an index vector that is equal to
269 // the arguments provided, except substitute 0's for any variable
270 // indexes we find...
271 for (unsigned i = 0; i != GEPOperands.size(); ++i)
272 if (!isa<Constant>(GEPOperands[i]) ||
273 isa<ConstantExpr>(GEPOperands[i]))
274 GEPOperands[i] =Constant::getNullValue(GEPOperands[i]->getType());
275 int64_t Offset = getTargetData().getIndexedOffset(BasePtr->getType(),
277 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
286 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
287 /// base pointers. This checks to see if the index expressions preclude the
288 /// pointers from aliasing...
289 AliasAnalysis::AliasResult BasicAliasAnalysis::
290 CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
292 const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
294 // We currently can't handle the case when the base pointers have different
295 // primitive types. Since this is uncommon anyway, we are happy being
296 // extremely conservative.
297 if (BasePtr1Ty != BasePtr2Ty)
300 const Type *GEPPointerTy = BasePtr1Ty;
302 // Find the (possibly empty) initial sequence of equal values... which are not
303 // necessarily constants.
304 unsigned NumGEP1Operands = GEP1Ops.size(), NumGEP2Operands = GEP2Ops.size();
305 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
306 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
307 unsigned UnequalOper = 0;
308 while (UnequalOper != MinOperands &&
309 GEP1Ops[UnequalOper] == GEP2Ops[UnequalOper]) {
310 // Advance through the type as we go...
312 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
313 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
315 // If all operands equal each other, then the derived pointers must
316 // alias each other...
318 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
319 "Ran out of type nesting, but not out of operands?");
324 // If we have seen all constant operands, and run out of indexes on one of the
325 // getelementptrs, check to see if the tail of the leftover one is all zeros.
326 // If so, return mustalias.
327 if (UnequalOper == MinOperands) {
328 if (GEP1Ops.size() < GEP2Ops.size()) std::swap(GEP1Ops, GEP2Ops);
330 bool AllAreZeros = true;
331 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
332 if (!isa<Constant>(GEP1Ops[i]) ||
333 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
337 if (AllAreZeros) return MustAlias;
341 // So now we know that the indexes derived from the base pointers,
342 // which are known to alias, are different. We can still determine a
343 // no-alias result if there are differing constant pairs in the index
344 // chain. For example:
345 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
347 unsigned SizeMax = std::max(G1S, G2S);
348 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
350 // Scan for the first operand that is constant and unequal in the
351 // two getelemenptrs...
352 unsigned FirstConstantOper = UnequalOper;
353 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
354 const Value *G1Oper = GEP1Ops[FirstConstantOper];
355 const Value *G2Oper = GEP2Ops[FirstConstantOper];
357 if (G1Oper != G2Oper && // Found non-equal constant indexes...
358 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
359 // Make sure they are comparable (ie, not constant expressions)... and
360 // make sure the GEP with the smaller leading constant is GEP1.
361 ConstantBool *Compare = *cast<Constant>(G1Oper) > *cast<Constant>(G2Oper);
362 if (Compare) { // If they are comparable...
363 if (Compare->getValue())
364 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
368 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
371 // No shared constant operands, and we ran out of common operands. At this
372 // point, the GEP instructions have run through all of their operands, and we
373 // haven't found evidence that there are any deltas between the GEP's.
374 // However, one GEP may have more operands than the other. If this is the
375 // case, there may still be hope. This this now.
376 if (FirstConstantOper == MinOperands) {
377 // Make GEP1Ops be the longer one if there is a longer one.
378 if (GEP1Ops.size() < GEP2Ops.size())
379 std::swap(GEP1Ops, GEP2Ops);
381 // Is there anything to check?
382 if (GEP1Ops.size() > MinOperands) {
383 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
384 if (isa<Constant>(GEP1Ops[i]) && !isa<ConstantExpr>(GEP1Ops[i]) &&
385 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
386 // Yup, there's a constant in the tail. Set all variables to
387 // constants in the GEP instruction to make it suiteable for
388 // TargetData::getIndexedOffset.
389 for (i = 0; i != MaxOperands; ++i)
390 if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]))
391 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
392 // Okay, now get the offset. This is the relative offset for the full
394 const TargetData &TD = getTargetData();
395 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
397 // Now crop off any constants from the end...
398 GEP1Ops.resize(MinOperands);
399 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
401 // If the tail provided a bit enough offset, return noalias!
402 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
407 // Couldn't find anything useful.
411 // If there are non-equal constants arguments, then we can figure
412 // out a minimum known delta between the two index expressions... at
413 // this point we know that the first constant index of GEP1 is less
414 // than the first constant index of GEP2.
416 // Advance BasePtr[12]Ty over this first differing constant operand.
417 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(GEP2Ops[FirstConstantOper]);
418 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(GEP1Ops[FirstConstantOper]);
420 // We are going to be using TargetData::getIndexedOffset to determine the
421 // offset that each of the GEP's is reaching. To do this, we have to convert
422 // all variable references to constant references. To do this, we convert the
423 // initial equal sequence of variables into constant zeros to start with.
424 for (unsigned i = 0; i != FirstConstantOper; ++i) {
425 if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]) ||
426 !isa<Constant>(GEP2Ops[i]) || isa<ConstantExpr>(GEP2Ops[i])) {
427 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
428 GEP2Ops[i] = Constant::getNullValue(GEP2Ops[i]->getType());
432 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
434 // Loop over the rest of the operands...
435 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
436 const Value *Op1 = i < GEP1Ops.size() ? GEP1Ops[i] : 0;
437 const Value *Op2 = i < GEP2Ops.size() ? GEP2Ops[i] : 0;
438 // If they are equal, use a zero index...
439 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
440 if (!isa<Constant>(Op1) || isa<ConstantExpr>(Op1))
441 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
442 // Otherwise, just keep the constants we have.
445 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
446 // If this is an array index, make sure the array element is in range.
447 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
448 if (Op1C->getRawValue() >= AT->getNumElements())
449 return MayAlias; // Be conservative with out-of-range accesses
452 // GEP1 is known to produce a value less than GEP2. To be
453 // conservatively correct, we must assume the largest possible
454 // constant is used in this position. This cannot be the initial
455 // index to the GEP instructions (because we know we have at least one
456 // element before this one with the different constant arguments), so
457 // we know that the current index must be into either a struct or
458 // array. Because we know it's not constant, this cannot be a
459 // structure index. Because of this, we can calculate the maximum
462 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
463 GEP1Ops[i] = ConstantSInt::get(Type::LongTy,AT->getNumElements()-1);
468 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
469 // If this is an array index, make sure the array element is in range.
470 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
471 if (Op2C->getRawValue() >= AT->getNumElements())
472 return MayAlias; // Be conservative with out-of-range accesses
473 } else { // Conservatively assume the minimum value for this index
474 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
479 if (BasePtr1Ty && Op1) {
480 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
481 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
486 if (BasePtr2Ty && Op2) {
487 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
488 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
494 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops);
495 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops);
496 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
498 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
499 //std::cerr << "Determined that these two GEP's don't alias ["
500 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;