BasicAliasAnalysis.cpp revision b576c94c15af9a440f69d9d03c2afead7971118c
1//===- llvm/Analysis/BasicAliasAnalysis.h - Alias Analysis Impl -*- C++ -*-===// 2// 3// The LLVM Compiler Infrastructure 4// 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. 7// 8//===----------------------------------------------------------------------===// 9// 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. 13// 14//===----------------------------------------------------------------------===// 15 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" 25 26// Make sure that anything that uses AliasAnalysis pulls in this file... 27void BasicAAStub() {} 28 29 30namespace { 31 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis { 32 33 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 34 AliasAnalysis::getAnalysisUsage(AU); 35 } 36 37 virtual void initializePass(); 38 39 // alias - This is the only method here that does anything interesting... 40 // 41 AliasResult alias(const Value *V1, unsigned V1Size, 42 const Value *V2, unsigned V2Size); 43 private: 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); 49 }; 50 51 // Register this pass... 52 RegisterOpt<BasicAliasAnalysis> 53 X("basicaa", "Basic Alias Analysis (default AA impl)"); 54 55 // Declare that we implement the AliasAnalysis interface 56 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y; 57} // End of anonymous namespace 58 59void BasicAliasAnalysis::initializePass() { 60 InitializeAliasAnalysis(this); 61} 62 63 64 65// hasUniqueAddress - Return true if the specified value points to something 66// with a unique, discernable, address. 67static inline bool hasUniqueAddress(const Value *V) { 68 return isa<GlobalValue>(V) || isa<AllocationInst>(V); 69} 70 71// getUnderlyingObject - This traverses the use chain to figure out what object 72// the specified value points to. If the value points to, or is derived from, a 73// unique object or an argument, return it. 74static const Value *getUnderlyingObject(const Value *V) { 75 if (!isa<PointerType>(V->getType())) return 0; 76 77 // If we are at some type of object... return it. 78 if (hasUniqueAddress(V) || isa<Argument>(V)) return V; 79 80 // Traverse through different addressing mechanisms... 81 if (const Instruction *I = dyn_cast<Instruction>(V)) { 82 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I)) 83 return getUnderlyingObject(I->getOperand(0)); 84 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) { 85 if (CE->getOpcode() == Instruction::Cast || 86 CE->getOpcode() == Instruction::GetElementPtr) 87 return getUnderlyingObject(CE->getOperand(0)); 88 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) { 89 return CPR->getValue(); 90 } 91 return 0; 92} 93 94 95// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 96// as array references. Note that this function is heavily tail recursive. 97// Hopefully we have a smart C++ compiler. :) 98// 99AliasAnalysis::AliasResult 100BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 101 const Value *V2, unsigned V2Size) { 102 // Strip off constant pointer refs if they exist 103 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1)) 104 V1 = CPR->getValue(); 105 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2)) 106 V2 = CPR->getValue(); 107 108 // Are we checking for alias of the same value? 109 if (V1 == V2) return MustAlias; 110 111 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) && 112 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy) 113 return NoAlias; // Scalars cannot alias each other 114 115 // Strip off cast instructions... 116 if (const Instruction *I = dyn_cast<CastInst>(V1)) 117 return alias(I->getOperand(0), V1Size, V2, V2Size); 118 if (const Instruction *I = dyn_cast<CastInst>(V2)) 119 return alias(V1, V1Size, I->getOperand(0), V2Size); 120 121 // Figure out what objects these things are pointing to if we can... 122 const Value *O1 = getUnderlyingObject(V1); 123 const Value *O2 = getUnderlyingObject(V2); 124 125 // Pointing at a discernible object? 126 if (O1 && O2) { 127 if (isa<Argument>(O1)) { 128 // Incoming argument cannot alias locally allocated object! 129 if (isa<AllocationInst>(O2)) return NoAlias; 130 // Otherwise, nothing is known... 131 } else if (isa<Argument>(O2)) { 132 // Incoming argument cannot alias locally allocated object! 133 if (isa<AllocationInst>(O1)) return NoAlias; 134 // Otherwise, nothing is known... 135 } else { 136 // If they are two different objects, we know that we have no alias... 137 if (O1 != O2) return NoAlias; 138 } 139 140 // If they are the same object, they we can look at the indexes. If they 141 // index off of the object is the same for both pointers, they must alias. 142 // If they are provably different, they must not alias. Otherwise, we can't 143 // tell anything. 144 } else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) { 145 return NoAlias; // Unique values don't alias null 146 } else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) { 147 return NoAlias; // Unique values don't alias null 148 } 149 150 // If we have two gep instructions with identical indices, return an alias 151 // result equal to the alias result of the original pointer... 152 // 153 if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1)) 154 if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2)) 155 if (GEP1->getNumOperands() == GEP2->getNumOperands() && 156 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) { 157 AliasResult GAlias = 158 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size, 159 (GetElementPtrInst*)GEP2, V2Size); 160 if (GAlias != MayAlias) 161 return GAlias; 162 } 163 164 // Check to see if these two pointers are related by a getelementptr 165 // instruction. If one pointer is a GEP with a non-zero index of the other 166 // pointer, we know they cannot alias. 167 // 168 if (isa<GetElementPtrInst>(V2)) { 169 std::swap(V1, V2); 170 std::swap(V1Size, V2Size); 171 } 172 173 if (V1Size != ~0U && V2Size != ~0U) 174 if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) { 175 AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size); 176 if (R == MustAlias) { 177 // If there is at least one non-zero constant index, we know they cannot 178 // alias. 179 bool ConstantFound = false; 180 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 181 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i))) 182 if (!C->isNullValue()) { 183 ConstantFound = true; 184 break; 185 } 186 if (ConstantFound) { 187 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 188 return NoAlias; 189 190 // Otherwise we have to check to see that the distance is more than 191 // the size of the argument... build an index vector that is equal to 192 // the arguments provided, except substitute 0's for any variable 193 // indexes we find... 194 195 std::vector<Value*> Indices; 196 Indices.reserve(GEP->getNumOperands()-1); 197 for (unsigned i = 1; i != GEP->getNumOperands(); ++i) 198 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i))) 199 Indices.push_back((Value*)C); 200 else 201 Indices.push_back(Constant::getNullValue(Type::LongTy)); 202 const Type *Ty = GEP->getOperand(0)->getType(); 203 int Offset = getTargetData().getIndexedOffset(Ty, Indices); 204 if (Offset >= (int)V2Size || Offset <= -(int)V1Size) 205 return NoAlias; 206 } 207 } 208 } 209 210 return MayAlias; 211} 212 213static Value *CheckArrayIndicesForOverflow(const Type *PtrTy, 214 const std::vector<Value*> &Indices, 215 const ConstantInt *Idx) { 216 if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) { 217 if (IdxS->getValue() < 0) // Underflow on the array subscript? 218 return Constant::getNullValue(Type::LongTy); 219 else { // Check for overflow 220 const ArrayType *ATy = 221 cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true)); 222 if (IdxS->getValue() >= (int64_t)ATy->getNumElements()) 223 return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1); 224 } 225 } 226 return (Value*)Idx; // Everything is acceptable. 227} 228 229// CheckGEPInstructions - Check two GEP instructions of compatible types and 230// equal number of arguments. This checks to see if the index expressions 231// preclude the pointers from aliasing... 232// 233AliasAnalysis::AliasResult 234BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S, 235 GetElementPtrInst *GEP2, unsigned G2S){ 236 // Do the base pointers alias? 237 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S, 238 GEP2->getOperand(0), G2S); 239 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything... 240 return BaseAlias; 241 242 // Find the (possibly empty) initial sequence of equal values... 243 unsigned NumGEPOperands = GEP1->getNumOperands(); 244 unsigned UnequalOper = 1; 245 while (UnequalOper != NumGEPOperands && 246 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper)) 247 ++UnequalOper; 248 249 // If all operands equal each other, then the derived pointers must 250 // alias each other... 251 if (UnequalOper == NumGEPOperands) return MustAlias; 252 253 // So now we know that the indexes derived from the base pointers, 254 // which are known to alias, are different. We can still determine a 255 // no-alias result if there are differing constant pairs in the index 256 // chain. For example: 257 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 258 // 259 unsigned SizeMax = std::max(G1S, G2S); 260 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work... 261 262 // Scan for the first operand that is constant and unequal in the 263 // two getelemenptrs... 264 unsigned FirstConstantOper = UnequalOper; 265 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) { 266 const Value *G1Oper = GEP1->getOperand(FirstConstantOper); 267 const Value *G2Oper = GEP2->getOperand(FirstConstantOper); 268 if (G1Oper != G2Oper && // Found non-equal constant indexes... 269 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) { 270 // Make sure they are comparable... and make sure the GEP with 271 // the smaller leading constant is GEP1. 272 ConstantBool *Compare = 273 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) > 274 *cast<Constant>(GEP2->getOperand(FirstConstantOper)); 275 if (Compare) { // If they are comparable... 276 if (Compare->getValue()) 277 std::swap(GEP1, GEP2); // Make GEP1 < GEP2 278 break; 279 } 280 } 281 } 282 283 // No constant operands, we cannot tell anything... 284 if (FirstConstantOper == NumGEPOperands) return MayAlias; 285 286 // If there are non-equal constants arguments, then we can figure 287 // out a minimum known delta between the two index expressions... at 288 // this point we know that the first constant index of GEP1 is less 289 // than the first constant index of GEP2. 290 // 291 std::vector<Value*> Indices1; 292 Indices1.reserve(NumGEPOperands-1); 293 for (unsigned i = 1; i != FirstConstantOper; ++i) 294 if (GEP1->getOperand(i)->getType() == Type::UByteTy) 295 Indices1.push_back(GEP1->getOperand(i)); 296 else 297 Indices1.push_back(Constant::getNullValue(Type::LongTy)); 298 std::vector<Value*> Indices2; 299 Indices2.reserve(NumGEPOperands-1); 300 Indices2 = Indices1; // Copy the zeros prefix... 301 302 // Add the two known constant operands... 303 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper)); 304 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper)); 305 306 const Type *GEPPointerTy = GEP1->getOperand(0)->getType(); 307 308 // Loop over the rest of the operands... 309 for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) { 310 const Value *Op1 = GEP1->getOperand(i); 311 const Value *Op2 = GEP2->getOperand(i); 312 if (Op1 == Op2) { // If they are equal, use a zero index... 313 if (!isa<Constant>(Op1)) { 314 Indices1.push_back(Constant::getNullValue(Op1->getType())); 315 Indices2.push_back(Indices1.back()); 316 } else { 317 Indices1.push_back((Value*)Op1); 318 Indices2.push_back((Value*)Op2); 319 } 320 } else { 321 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 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, Indices1, Op1C); 325 326 Indices1.push_back((Value*)Op1); 327 } else { 328 // GEP1 is known to produce a value less than GEP2. To be 329 // conservatively correct, we must assume the largest possible constant 330 // is used in this position. This cannot be the initial index to the 331 // GEP instructions (because we know we have at least one element before 332 // this one with the different constant arguments), so we know that the 333 // current index must be into either a struct or array. Because we know 334 // it's not constant, this cannot be a structure index. Because of 335 // this, we can calculate the maximum value possible. 336 // 337 const ArrayType *ElTy = 338 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true)); 339 Indices1.push_back(ConstantSInt::get(Type::LongTy, 340 ElTy->getNumElements()-1)); 341 } 342 343 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) { 344 // If this is an array index, make sure the array element is in range... 345 if (i != 1) // The pointer index can be "out of range" 346 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C); 347 348 Indices2.push_back((Value*)Op2); 349 } 350 else // Conservatively assume the minimum value for this index 351 Indices2.push_back(Constant::getNullValue(Op2->getType())); 352 } 353 } 354 355 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1); 356 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2); 357 assert(Offset1 < Offset2 &&"There is at least one different constant here!"); 358 359 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 360 //std::cerr << "Determined that these two GEP's don't alias [" 361 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 362 return NoAlias; 363 } 364 return MayAlias; 365} 366 367