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 of compatible types and
45 // equal number of arguments. This checks to see if the index expressions
46 // preclude the pointers from aliasing...
47 AliasResult CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1Size,
48 GetElementPtrInst *GEP2, unsigned G2Size);
51 // Register this pass...
52 RegisterOpt<BasicAliasAnalysis>
53 X("basicaa", "Basic Alias Analysis (default AA impl)");
55 // Declare that we implement the AliasAnalysis interface
56 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
57 } // End of anonymous namespace
59 void BasicAliasAnalysis::initializePass() {
60 InitializeAliasAnalysis(this);
63 // hasUniqueAddress - Return true if the specified value points to something
64 // with a unique, discernable, address.
65 static inline bool hasUniqueAddress(const Value *V) {
66 return isa<GlobalValue>(V) || isa<AllocationInst>(V);
69 // getUnderlyingObject - This traverses the use chain to figure out what object
70 // the specified value points to. If the value points to, or is derived from, a
71 // unique object or an argument, return it.
72 static const Value *getUnderlyingObject(const Value *V) {
73 if (!isa<PointerType>(V->getType())) return 0;
75 // If we are at some type of object... return it.
76 if (hasUniqueAddress(V) || isa<Argument>(V)) return V;
78 // Traverse through different addressing mechanisms...
79 if (const Instruction *I = dyn_cast<Instruction>(V)) {
80 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
81 return getUnderlyingObject(I->getOperand(0));
82 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
83 if (CE->getOpcode() == Instruction::Cast ||
84 CE->getOpcode() == Instruction::GetElementPtr)
85 return getUnderlyingObject(CE->getOperand(0));
86 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
87 return CPR->getValue();
93 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
94 // as array references. Note that this function is heavily tail recursive.
95 // Hopefully we have a smart C++ compiler. :)
97 AliasAnalysis::AliasResult
98 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
99 const Value *V2, unsigned V2Size) {
100 // Strip off constant pointer refs if they exist
101 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
102 V1 = CPR->getValue();
103 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
104 V2 = CPR->getValue();
106 // Are we checking for alias of the same value?
107 if (V1 == V2) return MustAlias;
109 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
110 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
111 return NoAlias; // Scalars cannot alias each other
113 // Strip off cast instructions...
114 if (const Instruction *I = dyn_cast<CastInst>(V1))
115 return alias(I->getOperand(0), V1Size, V2, V2Size);
116 if (const Instruction *I = dyn_cast<CastInst>(V2))
117 return alias(V1, V1Size, I->getOperand(0), V2Size);
119 // Figure out what objects these things are pointing to if we can...
120 const Value *O1 = getUnderlyingObject(V1);
121 const Value *O2 = getUnderlyingObject(V2);
123 // Pointing at a discernible object?
125 if (isa<Argument>(O1)) {
126 // Incoming argument cannot alias locally allocated object!
127 if (isa<AllocationInst>(O2)) return NoAlias;
128 // Otherwise, nothing is known...
129 } else if (isa<Argument>(O2)) {
130 // Incoming argument cannot alias locally allocated object!
131 if (isa<AllocationInst>(O1)) return NoAlias;
132 // Otherwise, nothing is known...
134 // If they are two different objects, we know that we have no alias...
135 if (O1 != O2) return NoAlias;
138 // If they are the same object, they we can look at the indexes. If they
139 // index off of the object is the same for both pointers, they must alias.
140 // If they are provably different, they must not alias. Otherwise, we can't
142 } else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) {
143 return NoAlias; // Unique values don't alias null
144 } else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) {
145 return NoAlias; // Unique values don't alias null
148 // If we have two gep instructions with identical indices, return an alias
149 // result equal to the alias result of the original pointer...
151 if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
152 if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
153 if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
154 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
156 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size,
157 (GetElementPtrInst*)GEP2, V2Size);
158 if (GAlias != MayAlias)
162 // Check to see if these two pointers are related by a getelementptr
163 // instruction. If one pointer is a GEP with a non-zero index of the other
164 // pointer, we know they cannot alias.
166 if (isa<GetElementPtrInst>(V2)) {
168 std::swap(V1Size, V2Size);
171 if (V1Size != ~0U && V2Size != ~0U)
172 if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) {
173 AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size);
174 if (R == MustAlias) {
175 // If there is at least one non-zero constant index, we know they cannot
177 bool ConstantFound = false;
178 bool AllZerosFound = true;
179 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
180 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i))) {
181 if (!C->isNullValue()) {
182 ConstantFound = true;
186 AllZerosFound = false;
189 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
190 // the ptr, the end result is a must alias also.
195 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
198 // Otherwise we have to check to see that the distance is more than
199 // the size of the argument... build an index vector that is equal to
200 // the arguments provided, except substitute 0's for any variable
201 // indexes we find...
203 std::vector<Value*> Indices;
204 Indices.reserve(GEP->getNumOperands()-1);
205 for (unsigned i = 1; i != GEP->getNumOperands(); ++i)
206 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
207 Indices.push_back((Value*)C);
209 Indices.push_back(Constant::getNullValue(Type::LongTy));
210 const Type *Ty = GEP->getOperand(0)->getType();
211 int Offset = getTargetData().getIndexedOffset(Ty, Indices);
212 if (Offset >= (int)V2Size || Offset <= -(int)V1Size)
221 static Value *CheckArrayIndicesForOverflow(const Type *PtrTy,
222 const std::vector<Value*> &Indices,
223 const ConstantInt *Idx) {
224 if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) {
225 if (IdxS->getValue() < 0) // Underflow on the array subscript?
226 return Constant::getNullValue(Type::LongTy);
227 else { // Check for overflow
228 const ArrayType *ATy =
229 cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true));
230 if (IdxS->getValue() >= (int64_t)ATy->getNumElements())
231 return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1);
234 return (Value*)Idx; // Everything is acceptable.
237 // CheckGEPInstructions - Check two GEP instructions of compatible types and
238 // equal number of arguments. This checks to see if the index expressions
239 // preclude the pointers from aliasing...
241 AliasAnalysis::AliasResult
242 BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S,
243 GetElementPtrInst *GEP2, unsigned G2S){
244 // Do the base pointers alias?
245 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S,
246 GEP2->getOperand(0), G2S);
247 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything...
250 // Find the (possibly empty) initial sequence of equal values...
251 unsigned NumGEPOperands = GEP1->getNumOperands();
252 unsigned UnequalOper = 1;
253 while (UnequalOper != NumGEPOperands &&
254 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper))
257 // If all operands equal each other, then the derived pointers must
258 // alias each other...
259 if (UnequalOper == NumGEPOperands) return MustAlias;
261 // So now we know that the indexes derived from the base pointers,
262 // which are known to alias, are different. We can still determine a
263 // no-alias result if there are differing constant pairs in the index
264 // chain. For example:
265 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
267 unsigned SizeMax = std::max(G1S, G2S);
268 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
270 // Scan for the first operand that is constant and unequal in the
271 // two getelemenptrs...
272 unsigned FirstConstantOper = UnequalOper;
273 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) {
274 const Value *G1Oper = GEP1->getOperand(FirstConstantOper);
275 const Value *G2Oper = GEP2->getOperand(FirstConstantOper);
276 if (G1Oper != G2Oper && // Found non-equal constant indexes...
277 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
278 // Make sure they are comparable... and make sure the GEP with
279 // the smaller leading constant is GEP1.
280 ConstantBool *Compare =
281 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) >
282 *cast<Constant>(GEP2->getOperand(FirstConstantOper));
283 if (Compare) { // If they are comparable...
284 if (Compare->getValue())
285 std::swap(GEP1, GEP2); // Make GEP1 < GEP2
291 // No constant operands, we cannot tell anything...
292 if (FirstConstantOper == NumGEPOperands) return MayAlias;
294 // If there are non-equal constants arguments, then we can figure
295 // out a minimum known delta between the two index expressions... at
296 // this point we know that the first constant index of GEP1 is less
297 // than the first constant index of GEP2.
299 std::vector<Value*> Indices1;
300 Indices1.reserve(NumGEPOperands-1);
302 for (gep_type_iterator I = gep_type_begin(GEP1);
303 I.getOperandNum() != FirstConstantOper; ++I)
304 if (isa<StructType>(*I))
305 Indices1.push_back(I.getOperand());
307 Indices1.push_back(Constant::getNullValue(Type::LongTy));
309 std::vector<Value*> Indices2;
310 Indices2.reserve(NumGEPOperands-1);
311 Indices2 = Indices1; // Copy the zeros prefix...
313 // Add the two known constant operands...
314 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper));
315 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper));
317 const Type *GEPPointerTy = GEP1->getOperand(0)->getType();
319 // Loop over the rest of the operands...
320 for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) {
321 const Value *Op1 = GEP1->getOperand(i);
322 const Value *Op2 = GEP2->getOperand(i);
323 if (Op1 == Op2) { // If they are equal, use a zero index...
324 if (!isa<Constant>(Op1)) {
325 Indices1.push_back(Constant::getNullValue(Op1->getType()));
326 Indices2.push_back(Indices1.back());
328 Indices1.push_back((Value*)Op1);
329 Indices2.push_back((Value*)Op2);
332 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
333 // If this is an array index, make sure the array element is in range...
334 if (i != 1) // The pointer index can be "out of range"
335 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices1, Op1C);
337 Indices1.push_back((Value*)Op1);
339 // GEP1 is known to produce a value less than GEP2. To be
340 // conservatively correct, we must assume the largest possible constant
341 // is used in this position. This cannot be the initial index to the
342 // GEP instructions (because we know we have at least one element before
343 // this one with the different constant arguments), so we know that the
344 // current index must be into either a struct or array. Because we know
345 // it's not constant, this cannot be a structure index. Because of
346 // this, we can calculate the maximum value possible.
348 const ArrayType *ElTy =
349 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true));
350 Indices1.push_back(ConstantSInt::get(Type::LongTy,
351 ElTy->getNumElements()-1));
354 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) {
355 // If this is an array index, make sure the array element is in range...
356 if (i != 1) // The pointer index can be "out of range"
357 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C);
359 Indices2.push_back((Value*)Op2);
361 else // Conservatively assume the minimum value for this index
362 Indices2.push_back(Constant::getNullValue(Op2->getType()));
366 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1);
367 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2);
368 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
370 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
371 //std::cerr << "Determined that these two GEP's don't alias ["
372 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;