1 //===- llvm/Analysis/BasicAliasAnalysis.h - Alias Analysis Impl -*- C++ -*-===//
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/iMemory.h"
20 #include "llvm/iOther.h"
21 #include "llvm/ConstantHandling.h"
22 #include "llvm/GlobalValue.h"
23 #include "llvm/DerivedTypes.h"
24 #include "llvm/Target/TargetData.h"
28 // Make sure that anything that uses AliasAnalysis pulls in this file...
32 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
34 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
35 AliasAnalysis::getAnalysisUsage(AU);
38 virtual void initializePass();
40 // alias - This is the only method here that does anything interesting...
42 AliasResult alias(const Value *V1, unsigned V1Size,
43 const Value *V2, unsigned V2Size);
45 // CheckGEPInstructions - Check two GEP instructions of compatible types and
46 // equal number of arguments. This checks to see if the index expressions
47 // preclude the pointers from aliasing...
48 AliasResult CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1Size,
49 GetElementPtrInst *GEP2, unsigned G2Size);
52 // Register this pass...
53 RegisterOpt<BasicAliasAnalysis>
54 X("basicaa", "Basic Alias Analysis (default AA impl)");
56 // Declare that we implement the AliasAnalysis interface
57 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
58 } // End of anonymous namespace
60 void BasicAliasAnalysis::initializePass() {
61 InitializeAliasAnalysis(this);
64 // hasUniqueAddress - Return true if the specified value points to something
65 // with a unique, discernable, address.
66 static inline bool hasUniqueAddress(const Value *V) {
67 return isa<GlobalValue>(V) || isa<AllocationInst>(V);
70 // getUnderlyingObject - This traverses the use chain to figure out what object
71 // the specified value points to. If the value points to, or is derived from, a
72 // unique object or an argument, return it.
73 static const Value *getUnderlyingObject(const Value *V) {
74 if (!isa<PointerType>(V->getType())) return 0;
76 // If we are at some type of object... return it.
77 if (hasUniqueAddress(V) || isa<Argument>(V)) return V;
79 // Traverse through different addressing mechanisms...
80 if (const Instruction *I = dyn_cast<Instruction>(V)) {
81 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
82 return getUnderlyingObject(I->getOperand(0));
83 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
84 if (CE->getOpcode() == Instruction::Cast ||
85 CE->getOpcode() == Instruction::GetElementPtr)
86 return getUnderlyingObject(CE->getOperand(0));
87 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
88 return CPR->getValue();
94 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
95 // as array references. Note that this function is heavily tail recursive.
96 // Hopefully we have a smart C++ compiler. :)
98 AliasAnalysis::AliasResult
99 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
100 const Value *V2, unsigned V2Size) {
101 // Strip off constant pointer refs if they exist
102 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
103 V1 = CPR->getValue();
104 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
105 V2 = CPR->getValue();
107 // Are we checking for alias of the same value?
108 if (V1 == V2) return MustAlias;
110 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
111 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
112 return NoAlias; // Scalars cannot alias each other
114 // Strip off cast instructions...
115 if (const Instruction *I = dyn_cast<CastInst>(V1))
116 return alias(I->getOperand(0), V1Size, V2, V2Size);
117 if (const Instruction *I = dyn_cast<CastInst>(V2))
118 return alias(V1, V1Size, I->getOperand(0), V2Size);
120 // Figure out what objects these things are pointing to if we can...
121 const Value *O1 = getUnderlyingObject(V1);
122 const Value *O2 = getUnderlyingObject(V2);
124 // Pointing at a discernible object?
126 if (isa<Argument>(O1)) {
127 // Incoming argument cannot alias locally allocated object!
128 if (isa<AllocationInst>(O2)) return NoAlias;
129 // Otherwise, nothing is known...
130 } else if (isa<Argument>(O2)) {
131 // Incoming argument cannot alias locally allocated object!
132 if (isa<AllocationInst>(O1)) return NoAlias;
133 // Otherwise, nothing is known...
135 // If they are two different objects, we know that we have no alias...
136 if (O1 != O2) return NoAlias;
139 // If they are the same object, they we can look at the indexes. If they
140 // index off of the object is the same for both pointers, they must alias.
141 // If they are provably different, they must not alias. Otherwise, we can't
143 } else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) {
144 return NoAlias; // Unique values don't alias null
145 } else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) {
146 return NoAlias; // Unique values don't alias null
149 // If we have two gep instructions with identical indices, return an alias
150 // result equal to the alias result of the original pointer...
152 if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
153 if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
154 if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
155 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
157 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size,
158 (GetElementPtrInst*)GEP2, V2Size);
159 if (GAlias != MayAlias)
163 // Check to see if these two pointers are related by a getelementptr
164 // instruction. If one pointer is a GEP with a non-zero index of the other
165 // pointer, we know they cannot alias.
167 if (isa<GetElementPtrInst>(V2)) {
169 std::swap(V1Size, V2Size);
172 if (V1Size != ~0U && V2Size != ~0U)
173 if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) {
174 AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size);
175 if (R == MustAlias) {
176 // If there is at least one non-zero constant index, we know they cannot
178 bool ConstantFound = false;
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 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
189 // Otherwise we have to check to see that the distance is more than
190 // the size of the argument... build an index vector that is equal to
191 // the arguments provided, except substitute 0's for any variable
192 // indexes we find...
194 std::vector<Value*> Indices;
195 Indices.reserve(GEP->getNumOperands()-1);
196 for (unsigned i = 1; i != GEP->getNumOperands(); ++i)
197 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
198 Indices.push_back((Value*)C);
200 Indices.push_back(Constant::getNullValue(Type::LongTy));
201 const Type *Ty = GEP->getOperand(0)->getType();
202 int Offset = getTargetData().getIndexedOffset(Ty, Indices);
203 if (Offset >= (int)V2Size || Offset <= -(int)V1Size)
212 static Value *CheckArrayIndicesForOverflow(const Type *PtrTy,
213 const std::vector<Value*> &Indices,
214 const ConstantInt *Idx) {
215 if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) {
216 if (IdxS->getValue() < 0) // Underflow on the array subscript?
217 return Constant::getNullValue(Type::LongTy);
218 else { // Check for overflow
219 const ArrayType *ATy =
220 cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true));
221 if (IdxS->getValue() >= (int64_t)ATy->getNumElements())
222 return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1);
225 return (Value*)Idx; // Everything is acceptable.
228 // CheckGEPInstructions - Check two GEP instructions of compatible types and
229 // equal number of arguments. This checks to see if the index expressions
230 // preclude the pointers from aliasing...
232 AliasAnalysis::AliasResult
233 BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S,
234 GetElementPtrInst *GEP2, unsigned G2S){
235 // Do the base pointers alias?
236 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S,
237 GEP2->getOperand(0), G2S);
238 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything...
241 // Find the (possibly empty) initial sequence of equal values...
242 unsigned NumGEPOperands = GEP1->getNumOperands();
243 unsigned UnequalOper = 1;
244 while (UnequalOper != NumGEPOperands &&
245 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper))
248 // If all operands equal each other, then the derived pointers must
249 // alias each other...
250 if (UnequalOper == NumGEPOperands) return MustAlias;
252 // So now we know that the indexes derived from the base pointers,
253 // which are known to alias, are different. We can still determine a
254 // no-alias result if there are differing constant pairs in the index
255 // chain. For example:
256 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
258 unsigned SizeMax = std::max(G1S, G2S);
259 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
261 // Scan for the first operand that is constant and unequal in the
262 // two getelemenptrs...
263 unsigned FirstConstantOper = UnequalOper;
264 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) {
265 const Value *G1Oper = GEP1->getOperand(FirstConstantOper);
266 const Value *G2Oper = GEP2->getOperand(FirstConstantOper);
267 if (G1Oper != G2Oper && // Found non-equal constant indexes...
268 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
269 // Make sure they are comparable... and make sure the GEP with
270 // the smaller leading constant is GEP1.
271 ConstantBool *Compare =
272 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) >
273 *cast<Constant>(GEP2->getOperand(FirstConstantOper));
274 if (Compare) { // If they are comparable...
275 if (Compare->getValue())
276 std::swap(GEP1, GEP2); // Make GEP1 < GEP2
282 // No constant operands, we cannot tell anything...
283 if (FirstConstantOper == NumGEPOperands) return MayAlias;
285 // If there are non-equal constants arguments, then we can figure
286 // out a minimum known delta between the two index expressions... at
287 // this point we know that the first constant index of GEP1 is less
288 // than the first constant index of GEP2.
290 std::vector<Value*> Indices1;
291 Indices1.reserve(NumGEPOperands-1);
292 for (unsigned i = 1; i != FirstConstantOper; ++i)
293 if (GEP1->getOperand(i)->getType() == Type::UByteTy)
294 Indices1.push_back(GEP1->getOperand(i));
296 Indices1.push_back(Constant::getNullValue(Type::LongTy));
297 std::vector<Value*> Indices2;
298 Indices2.reserve(NumGEPOperands-1);
299 Indices2 = Indices1; // Copy the zeros prefix...
301 // Add the two known constant operands...
302 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper));
303 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper));
305 const Type *GEPPointerTy = GEP1->getOperand(0)->getType();
307 // Loop over the rest of the operands...
308 for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) {
309 const Value *Op1 = GEP1->getOperand(i);
310 const Value *Op2 = GEP2->getOperand(i);
311 if (Op1 == Op2) { // If they are equal, use a zero index...
312 if (!isa<Constant>(Op1)) {
313 Indices1.push_back(Constant::getNullValue(Op1->getType()));
314 Indices2.push_back(Indices1.back());
316 Indices1.push_back((Value*)Op1);
317 Indices2.push_back((Value*)Op2);
320 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
321 // If this is an array index, make sure the array element is in range...
322 if (i != 1) // The pointer index can be "out of range"
323 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices1, Op1C);
325 Indices1.push_back((Value*)Op1);
327 // GEP1 is known to produce a value less than GEP2. To be
328 // conservatively correct, we must assume the largest possible constant
329 // is used in this position. This cannot be the initial index to the
330 // GEP instructions (because we know we have at least one element before
331 // this one with the different constant arguments), so we know that the
332 // current index must be into either a struct or array. Because we know
333 // it's not constant, this cannot be a structure index. Because of
334 // this, we can calculate the maximum value possible.
336 const ArrayType *ElTy =
337 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true));
338 Indices1.push_back(ConstantSInt::get(Type::LongTy,
339 ElTy->getNumElements()-1));
342 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) {
343 // If this is an array index, make sure the array element is in range...
344 if (i != 1) // The pointer index can be "out of range"
345 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C);
347 Indices2.push_back((Value*)Op2);
349 else // Conservatively assume the minimum value for this index
350 Indices2.push_back(Constant::getNullValue(Op2->getType()));
354 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1);
355 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2);
356 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
358 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
359 //std::cerr << "Determined that these two GEP's don't alias ["
360 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
366 } // End llvm namespace