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));
77 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
78 // as array references. Note that this function is heavily tail recursive.
79 // Hopefully we have a smart C++ compiler. :)
81 AliasAnalysis::AliasResult
82 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
83 const Value *V2, unsigned V2Size) {
84 // Strip off constant pointer refs if they exist
85 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
87 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
90 // Are we checking for alias of the same value?
91 if (V1 == V2) return MustAlias;
93 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
94 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
95 return NoAlias; // Scalars cannot alias each other
97 // Strip off cast instructions...
98 if (const Instruction *I = dyn_cast<CastInst>(V1))
99 return alias(I->getOperand(0), V1Size, V2, V2Size);
100 if (const Instruction *I = dyn_cast<CastInst>(V2))
101 return alias(V1, V1Size, I->getOperand(0), V2Size);
103 // Figure out what objects these things are pointing to if we can...
104 const Value *O1 = getUnderlyingObject(V1);
105 const Value *O2 = getUnderlyingObject(V2);
107 // Pointing at a discernable object?
109 // If they are two different objects, we know that we have no alias...
110 if (O1 != O2) return NoAlias;
112 // If they are the same object, they we can look at the indexes. If they
113 // index off of the object is the same for both pointers, they must alias.
114 // If they are provably different, they must not alias. Otherwise, we can't
116 } else if (O1 && isa<ConstantPointerNull>(V2)) {
117 return NoAlias; // Unique values don't alias null
118 } else if (O2 && isa<ConstantPointerNull>(V1)) {
119 return NoAlias; // Unique values don't alias null
122 // If we have two gep instructions with identical indices, return an alias
123 // result equal to the alias result of the original pointer...
125 if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
126 if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
127 if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
128 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
130 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size,
131 (GetElementPtrInst*)GEP2, V2Size);
132 if (GAlias != MayAlias)
136 // Check to see if these two pointers are related by a getelementptr
137 // instruction. If one pointer is a GEP with a non-zero index of the other
138 // pointer, we know they cannot alias.
140 if (isa<GetElementPtrInst>(V2)) {
142 std::swap(V1Size, V2Size);
145 if (V1Size != ~0U && V2Size != ~0U)
146 if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) {
147 AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size);
148 if (R == MustAlias) {
149 // If there is at least one non-zero constant index, we know they cannot
151 bool ConstantFound = false;
152 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
153 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
154 if (!C->isNullValue()) {
155 ConstantFound = true;
159 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
162 // Otherwise we have to check to see that the distance is more than
163 // the size of the argument... build an index vector that is equal to
164 // the arguments provided, except substitute 0's for any variable
165 // indexes we find...
167 std::vector<Value*> Indices;
168 Indices.reserve(GEP->getNumOperands()-1);
169 for (unsigned i = 1; i != GEP->getNumOperands(); ++i)
170 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
171 Indices.push_back((Value*)C);
173 Indices.push_back(Constant::getNullValue(Type::LongTy));
174 const Type *Ty = GEP->getOperand(0)->getType();
175 int Offset = getTargetData().getIndexedOffset(Ty, Indices);
176 if (Offset >= (int)V2Size || Offset <= -(int)V1Size)
185 // CheckGEPInstructions - Check two GEP instructions of compatible types and
186 // equal number of arguments. This checks to see if the index expressions
187 // preclude the pointers from aliasing...
189 AliasAnalysis::AliasResult
190 BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S,
191 GetElementPtrInst *GEP2, unsigned G2S){
192 // Do the base pointers alias?
193 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S,
194 GEP2->getOperand(0), G2S);
195 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything...
198 // Find the (possibly empty) initial sequence of equal values...
199 unsigned NumGEPOperands = GEP1->getNumOperands();
200 unsigned UnequalOper = 1;
201 while (UnequalOper != NumGEPOperands &&
202 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper))
205 // If all operands equal each other, then the derived pointers must
206 // alias each other...
207 if (UnequalOper == NumGEPOperands) return MustAlias;
209 // So now we know that the indexes derived from the base pointers,
210 // which are known to alias, are different. We can still determine a
211 // no-alias result if there are differing constant pairs in the index
212 // chain. For example:
213 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
215 unsigned SizeMax = std::max(G1S, G2S);
216 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
218 // Scan for the first operand that is constant and unequal in the
219 // two getelemenptrs...
220 unsigned FirstConstantOper = UnequalOper;
221 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) {
222 const Value *G1Oper = GEP1->getOperand(FirstConstantOper);
223 const Value *G2Oper = GEP2->getOperand(FirstConstantOper);
224 if (G1Oper != G2Oper && // Found non-equal constant indexes...
225 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
226 // Make sure they are comparable... and make sure the GEP with
227 // the smaller leading constant is GEP1.
228 ConstantBool *Compare =
229 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) >
230 *cast<Constant>(GEP2->getOperand(FirstConstantOper));
231 if (Compare) { // If they are comparable...
232 if (Compare->getValue())
233 std::swap(GEP1, GEP2); // Make GEP1 < GEP2
239 // No constant operands, we cannot tell anything...
240 if (FirstConstantOper == NumGEPOperands) return MayAlias;
242 // If there are non-equal constants arguments, then we can figure
243 // out a minimum known delta between the two index expressions... at
244 // this point we know that the first constant index of GEP1 is less
245 // than the first constant index of GEP2.
247 std::vector<Value*> Indices1;
248 Indices1.reserve(NumGEPOperands-1);
249 for (unsigned i = 1; i != FirstConstantOper; ++i)
250 if (GEP1->getOperand(i)->getType() == Type::UByteTy)
251 Indices1.push_back(GEP1->getOperand(i));
253 Indices1.push_back(Constant::getNullValue(Type::LongTy));
254 std::vector<Value*> Indices2;
255 Indices2.reserve(NumGEPOperands-1);
256 Indices2 = Indices1; // Copy the zeros prefix...
258 // Add the two known constant operands...
259 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper));
260 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper));
262 const Type *GEPPointerTy = GEP1->getOperand(0)->getType();
264 // Loop over the rest of the operands...
265 for (unsigned i = FirstConstantOper+1; i!=NumGEPOperands; ++i){
266 const Value *Op1 = GEP1->getOperand(i);
267 const Value *Op2 = GEP2->getOperand(i);
268 if (Op1 == Op2) { // If they are equal, use a zero index...
269 Indices1.push_back(Constant::getNullValue(Op1->getType()));
270 Indices2.push_back(Indices1.back());
272 if (isa<Constant>(Op1))
273 Indices1.push_back((Value*)Op1);
275 // GEP1 is known to produce a value less than GEP2. To be
276 // conservatively correct, we must assume the largest possible constant
277 // is used in this position. This cannot be the initial index to the
278 // GEP instructions (because we know we have at least one element before
279 // this one with the different constant arguments), so we know that the
280 // current index must be into either a struct or array. Because we know
281 // it's not constant, this cannot be a structure index. Because of
282 // this, we can calculate the maximum value possible.
284 const ArrayType *ElTy =
285 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true));
286 Indices1.push_back(ConstantSInt::get(Type::LongTy,
287 ElTy->getNumElements()-1));
290 if (isa<Constant>(Op2))
291 Indices2.push_back((Value*)Op2);
292 else // Conservatively assume the minimum value for this index
293 Indices2.push_back(Constant::getNullValue(Op1->getType()));
297 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1);
298 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2);
299 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
301 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
302 //std::cerr << "Determined that these two GEP's don't alias ["
303 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;