1 //===- llvm/Analysis/BasicAliasAnalysis.h - Alias Analysis Impl -*- C++ -*-===//
3 // This file defines the default implementation of the Alias Analysis interface
4 // that simply implements a few identities (two different globals cannot alias,
5 // etc), but otherwise does no analysis.
7 //===----------------------------------------------------------------------===//
9 #include "llvm/Analysis/AliasAnalysis.h"
10 #include "llvm/Pass.h"
11 #include "llvm/iMemory.h"
12 #include "llvm/iOther.h"
13 #include "llvm/ConstantHandling.h"
14 #include "llvm/GlobalValue.h"
15 #include "llvm/DerivedTypes.h"
16 #include "llvm/Target/TargetData.h"
18 // Make sure that anything that uses AliasAnalysis pulls in this file...
23 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
25 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
26 AliasAnalysis::getAnalysisUsage(AU);
29 virtual void initializePass();
31 // alias - This is the only method here that does anything interesting...
33 AliasResult alias(const Value *V1, unsigned V1Size,
34 const Value *V2, unsigned V2Size);
36 // CheckGEPInstructions - Check two GEP instructions of compatible types and
37 // equal number of arguments. This checks to see if the index expressions
38 // preclude the pointers from aliasing...
39 AliasResult CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1Size,
40 GetElementPtrInst *GEP2, unsigned G2Size);
43 // Register this pass...
44 RegisterOpt<BasicAliasAnalysis>
45 X("basicaa", "Basic Alias Analysis (default AA impl)");
47 // Declare that we implement the AliasAnalysis interface
48 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
49 } // End of anonymous namespace
51 void BasicAliasAnalysis::initializePass() {
52 InitializeAliasAnalysis(this);
57 // hasUniqueAddress - Return true if the
58 static inline bool hasUniqueAddress(const Value *V) {
59 return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
62 static const Value *getUnderlyingObject(const Value *V) {
63 if (!isa<PointerType>(V->getType())) return 0;
65 // If we are at some type of object... return it.
66 if (hasUniqueAddress(V)) return V;
68 // Traverse through different addressing mechanisms...
69 if (const Instruction *I = dyn_cast<Instruction>(V)) {
70 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
71 return getUnderlyingObject(I->getOperand(0));
72 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
73 if (CE->getOpcode() == Instruction::Cast ||
74 CE->getOpcode() == Instruction::GetElementPtr)
75 return getUnderlyingObject(CE->getOperand(0));
76 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
77 return CPR->getValue();
83 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
84 // as array references. Note that this function is heavily tail recursive.
85 // Hopefully we have a smart C++ compiler. :)
87 AliasAnalysis::AliasResult
88 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
89 const Value *V2, unsigned V2Size) {
90 // Strip off constant pointer refs if they exist
91 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
93 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
96 // Are we checking for alias of the same value?
97 if (V1 == V2) return MustAlias;
99 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
100 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
101 return NoAlias; // Scalars cannot alias each other
103 // Strip off cast instructions...
104 if (const Instruction *I = dyn_cast<CastInst>(V1))
105 return alias(I->getOperand(0), V1Size, V2, V2Size);
106 if (const Instruction *I = dyn_cast<CastInst>(V2))
107 return alias(V1, V1Size, I->getOperand(0), V2Size);
109 // Figure out what objects these things are pointing to if we can...
110 const Value *O1 = getUnderlyingObject(V1);
111 const Value *O2 = getUnderlyingObject(V2);
113 // Pointing at a discernable object?
115 // If they are two different objects, we know that we have no alias...
116 if (O1 != O2) return NoAlias;
118 // If they are the same object, they we can look at the indexes. If they
119 // index off of the object is the same for both pointers, they must alias.
120 // If they are provably different, they must not alias. Otherwise, we can't
122 } else if (O1 && isa<ConstantPointerNull>(V2)) {
123 return NoAlias; // Unique values don't alias null
124 } else if (O2 && isa<ConstantPointerNull>(V1)) {
125 return NoAlias; // Unique values don't alias null
128 // If we have two gep instructions with identical indices, return an alias
129 // result equal to the alias result of the original pointer...
131 if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
132 if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
133 if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
134 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
136 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size,
137 (GetElementPtrInst*)GEP2, V2Size);
138 if (GAlias != MayAlias)
142 // Check to see if these two pointers are related by a getelementptr
143 // instruction. If one pointer is a GEP with a non-zero index of the other
144 // pointer, we know they cannot alias.
146 if (isa<GetElementPtrInst>(V2)) {
148 std::swap(V1Size, V2Size);
151 if (V1Size != ~0U && V2Size != ~0U)
152 if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) {
153 AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size);
154 if (R == MustAlias) {
155 // If there is at least one non-zero constant index, we know they cannot
157 bool ConstantFound = false;
158 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
159 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
160 if (!C->isNullValue()) {
161 ConstantFound = true;
165 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
168 // Otherwise we have to check to see that the distance is more than
169 // the size of the argument... build an index vector that is equal to
170 // the arguments provided, except substitute 0's for any variable
171 // indexes we find...
173 std::vector<Value*> Indices;
174 Indices.reserve(GEP->getNumOperands()-1);
175 for (unsigned i = 1; i != GEP->getNumOperands(); ++i)
176 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
177 Indices.push_back((Value*)C);
179 Indices.push_back(Constant::getNullValue(Type::LongTy));
180 const Type *Ty = GEP->getOperand(0)->getType();
181 int Offset = getTargetData().getIndexedOffset(Ty, Indices);
182 if (Offset >= (int)V2Size || Offset <= -(int)V1Size)
191 static Value *CheckArrayIndicesForOverflow(const Type *PtrTy,
192 const std::vector<Value*> &Indices,
193 const ConstantInt *Idx) {
194 if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) {
195 if (IdxS->getValue() < 0) // Underflow on the array subscript?
196 return Constant::getNullValue(Type::LongTy);
197 else { // Check for overflow
198 const ArrayType *ATy =
199 cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true));
200 if (IdxS->getValue() >= (int64_t)ATy->getNumElements())
201 return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1);
204 return (Value*)Idx; // Everything is acceptable.
207 // CheckGEPInstructions - Check two GEP instructions of compatible types and
208 // equal number of arguments. This checks to see if the index expressions
209 // preclude the pointers from aliasing...
211 AliasAnalysis::AliasResult
212 BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S,
213 GetElementPtrInst *GEP2, unsigned G2S){
214 // Do the base pointers alias?
215 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S,
216 GEP2->getOperand(0), G2S);
217 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything...
220 // Find the (possibly empty) initial sequence of equal values...
221 unsigned NumGEPOperands = GEP1->getNumOperands();
222 unsigned UnequalOper = 1;
223 while (UnequalOper != NumGEPOperands &&
224 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper))
227 // If all operands equal each other, then the derived pointers must
228 // alias each other...
229 if (UnequalOper == NumGEPOperands) return MustAlias;
231 // So now we know that the indexes derived from the base pointers,
232 // which are known to alias, are different. We can still determine a
233 // no-alias result if there are differing constant pairs in the index
234 // chain. For example:
235 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
237 unsigned SizeMax = std::max(G1S, G2S);
238 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
240 // Scan for the first operand that is constant and unequal in the
241 // two getelemenptrs...
242 unsigned FirstConstantOper = UnequalOper;
243 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) {
244 const Value *G1Oper = GEP1->getOperand(FirstConstantOper);
245 const Value *G2Oper = GEP2->getOperand(FirstConstantOper);
246 if (G1Oper != G2Oper && // Found non-equal constant indexes...
247 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
248 // Make sure they are comparable... and make sure the GEP with
249 // the smaller leading constant is GEP1.
250 ConstantBool *Compare =
251 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) >
252 *cast<Constant>(GEP2->getOperand(FirstConstantOper));
253 if (Compare) { // If they are comparable...
254 if (Compare->getValue())
255 std::swap(GEP1, GEP2); // Make GEP1 < GEP2
261 // No constant operands, we cannot tell anything...
262 if (FirstConstantOper == NumGEPOperands) return MayAlias;
264 // If there are non-equal constants arguments, then we can figure
265 // out a minimum known delta between the two index expressions... at
266 // this point we know that the first constant index of GEP1 is less
267 // than the first constant index of GEP2.
269 std::vector<Value*> Indices1;
270 Indices1.reserve(NumGEPOperands-1);
271 for (unsigned i = 1; i != FirstConstantOper; ++i)
272 if (GEP1->getOperand(i)->getType() == Type::UByteTy)
273 Indices1.push_back(GEP1->getOperand(i));
275 Indices1.push_back(Constant::getNullValue(Type::LongTy));
276 std::vector<Value*> Indices2;
277 Indices2.reserve(NumGEPOperands-1);
278 Indices2 = Indices1; // Copy the zeros prefix...
280 // Add the two known constant operands...
281 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper));
282 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper));
284 const Type *GEPPointerTy = GEP1->getOperand(0)->getType();
286 // Loop over the rest of the operands...
287 for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) {
288 const Value *Op1 = GEP1->getOperand(i);
289 const Value *Op2 = GEP2->getOperand(i);
290 if (Op1 == Op2) { // If they are equal, use a zero index...
291 if (!isa<Constant>(Op1)) {
292 Indices1.push_back(Constant::getNullValue(Op1->getType()));
293 Indices2.push_back(Indices1.back());
295 Indices1.push_back((Value*)Op1);
296 Indices2.push_back((Value*)Op2);
299 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
300 // If this is an array index, make sure the array element is in range...
301 if (i != 1) // The pointer index can be "out of range"
302 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices1, Op1C);
304 Indices1.push_back((Value*)Op1);
306 // GEP1 is known to produce a value less than GEP2. To be
307 // conservatively correct, we must assume the largest possible constant
308 // is used in this position. This cannot be the initial index to the
309 // GEP instructions (because we know we have at least one element before
310 // this one with the different constant arguments), so we know that the
311 // current index must be into either a struct or array. Because we know
312 // it's not constant, this cannot be a structure index. Because of
313 // this, we can calculate the maximum value possible.
315 const ArrayType *ElTy =
316 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true));
317 Indices1.push_back(ConstantSInt::get(Type::LongTy,
318 ElTy->getNumElements()-1));
321 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) {
322 // If this is an array index, make sure the array element is in range...
323 if (i != 1) // The pointer index can be "out of range"
324 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C);
326 Indices2.push_back((Value*)Op2);
328 else // Conservatively assume the minimum value for this index
329 Indices2.push_back(Constant::getNullValue(Op2->getType()));
333 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1);
334 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2);
335 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
337 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
338 //std::cerr << "Determined that these two GEP's don't alias ["
339 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;