BasicAliasAnalysis.cpp revision 1af55e169396dfa057f0a6067fa8b16eb4317909
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/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" 25using namespace llvm; 26 27// Make sure that anything that uses AliasAnalysis pulls in this file... 28namespace llvm { void BasicAAStub() {} } 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// hasUniqueAddress - Return true if the specified value points to something 64// with a unique, discernable, address. 65static inline bool hasUniqueAddress(const Value *V) { 66 return isa<GlobalValue>(V) || isa<AllocationInst>(V); 67} 68 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. 72static const Value *getUnderlyingObject(const Value *V) { 73 if (!isa<PointerType>(V->getType())) return 0; 74 75 // If we are at some type of object... return it. 76 if (hasUniqueAddress(V) || isa<Argument>(V)) return V; 77 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(); 88 } 89 return 0; 90} 91 92 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. :) 96// 97AliasAnalysis::AliasResult 98BasicAliasAnalysis::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(); 105 106 // Are we checking for alias of the same value? 107 if (V1 == V2) return MustAlias; 108 109 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 112 113 // 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); 118 119 // Figure out what objects these things are pointing to if we can... 120 const Value *O1 = getUnderlyingObject(V1); 121 const Value *O2 = getUnderlyingObject(V2); 122 123 // Pointing at a discernible object? 124 if (O1 && O2) { 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... 133 } else { 134 // If they are two different objects, we know that we have no alias... 135 if (O1 != O2) return NoAlias; 136 } 137 138 // 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 141 // tell anything. 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 146 } 147 148 // If we have two gep instructions with identical indices, return an alias 149 // result equal to the alias result of the original pointer... 150 // 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()) { 155 AliasResult GAlias = 156 CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size, 157 (GetElementPtrInst*)GEP2, V2Size); 158 if (GAlias != MayAlias) 159 return GAlias; 160 } 161 162 // 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. 165 // 166 if (isa<GetElementPtrInst>(V2)) { 167 std::swap(V1, V2); 168 std::swap(V1Size, V2Size); 169 } 170 171 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 176 // alias. 177 bool ConstantFound = false; 178 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 179 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i))) 180 if (!C->isNullValue()) { 181 ConstantFound = true; 182 break; 183 } 184 if (ConstantFound) { 185 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 186 return NoAlias; 187 188 // Otherwise we have to check to see that the distance is more than 189 // the size of the argument... build an index vector that is equal to 190 // the arguments provided, except substitute 0's for any variable 191 // indexes we find... 192 193 std::vector<Value*> Indices; 194 Indices.reserve(GEP->getNumOperands()-1); 195 for (unsigned i = 1; i != GEP->getNumOperands(); ++i) 196 if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i))) 197 Indices.push_back((Value*)C); 198 else 199 Indices.push_back(Constant::getNullValue(Type::LongTy)); 200 const Type *Ty = GEP->getOperand(0)->getType(); 201 int Offset = getTargetData().getIndexedOffset(Ty, Indices); 202 if (Offset >= (int)V2Size || Offset <= -(int)V1Size) 203 return NoAlias; 204 } 205 } 206 } 207 208 return MayAlias; 209} 210 211static Value *CheckArrayIndicesForOverflow(const Type *PtrTy, 212 const std::vector<Value*> &Indices, 213 const ConstantInt *Idx) { 214 if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) { 215 if (IdxS->getValue() < 0) // Underflow on the array subscript? 216 return Constant::getNullValue(Type::LongTy); 217 else { // Check for overflow 218 const ArrayType *ATy = 219 cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true)); 220 if (IdxS->getValue() >= (int64_t)ATy->getNumElements()) 221 return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1); 222 } 223 } 224 return (Value*)Idx; // Everything is acceptable. 225} 226 227// CheckGEPInstructions - Check two GEP instructions of compatible types and 228// equal number of arguments. This checks to see if the index expressions 229// preclude the pointers from aliasing... 230// 231AliasAnalysis::AliasResult 232BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S, 233 GetElementPtrInst *GEP2, unsigned G2S){ 234 // Do the base pointers alias? 235 AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S, 236 GEP2->getOperand(0), G2S); 237 if (BaseAlias != MustAlias) // No or May alias: We cannot add anything... 238 return BaseAlias; 239 240 // Find the (possibly empty) initial sequence of equal values... 241 unsigned NumGEPOperands = GEP1->getNumOperands(); 242 unsigned UnequalOper = 1; 243 while (UnequalOper != NumGEPOperands && 244 GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper)) 245 ++UnequalOper; 246 247 // If all operands equal each other, then the derived pointers must 248 // alias each other... 249 if (UnequalOper == NumGEPOperands) return MustAlias; 250 251 // So now we know that the indexes derived from the base pointers, 252 // which are known to alias, are different. We can still determine a 253 // no-alias result if there are differing constant pairs in the index 254 // chain. For example: 255 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 256 // 257 unsigned SizeMax = std::max(G1S, G2S); 258 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work... 259 260 // Scan for the first operand that is constant and unequal in the 261 // two getelemenptrs... 262 unsigned FirstConstantOper = UnequalOper; 263 for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) { 264 const Value *G1Oper = GEP1->getOperand(FirstConstantOper); 265 const Value *G2Oper = GEP2->getOperand(FirstConstantOper); 266 if (G1Oper != G2Oper && // Found non-equal constant indexes... 267 isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) { 268 // Make sure they are comparable... and make sure the GEP with 269 // the smaller leading constant is GEP1. 270 ConstantBool *Compare = 271 *cast<Constant>(GEP1->getOperand(FirstConstantOper)) > 272 *cast<Constant>(GEP2->getOperand(FirstConstantOper)); 273 if (Compare) { // If they are comparable... 274 if (Compare->getValue()) 275 std::swap(GEP1, GEP2); // Make GEP1 < GEP2 276 break; 277 } 278 } 279 } 280 281 // No constant operands, we cannot tell anything... 282 if (FirstConstantOper == NumGEPOperands) return MayAlias; 283 284 // If there are non-equal constants arguments, then we can figure 285 // out a minimum known delta between the two index expressions... at 286 // this point we know that the first constant index of GEP1 is less 287 // than the first constant index of GEP2. 288 // 289 std::vector<Value*> Indices1; 290 Indices1.reserve(NumGEPOperands-1); 291 292 for (gep_type_iterator I = gep_type_begin(GEP1); 293 I.getOperandNum() != FirstConstantOper; ++I) 294 if (isa<StructType>(*I)) 295 Indices1.push_back(I.getOperand()); 296 else 297 Indices1.push_back(Constant::getNullValue(Type::LongTy)); 298 299 std::vector<Value*> Indices2; 300 Indices2.reserve(NumGEPOperands-1); 301 Indices2 = Indices1; // Copy the zeros prefix... 302 303 // Add the two known constant operands... 304 Indices1.push_back((Value*)GEP1->getOperand(FirstConstantOper)); 305 Indices2.push_back((Value*)GEP2->getOperand(FirstConstantOper)); 306 307 const Type *GEPPointerTy = GEP1->getOperand(0)->getType(); 308 309 // Loop over the rest of the operands... 310 for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) { 311 const Value *Op1 = GEP1->getOperand(i); 312 const Value *Op2 = GEP2->getOperand(i); 313 if (Op1 == Op2) { // If they are equal, use a zero index... 314 if (!isa<Constant>(Op1)) { 315 Indices1.push_back(Constant::getNullValue(Op1->getType())); 316 Indices2.push_back(Indices1.back()); 317 } else { 318 Indices1.push_back((Value*)Op1); 319 Indices2.push_back((Value*)Op2); 320 } 321 } else { 322 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 323 // If this is an array index, make sure the array element is in range... 324 if (i != 1) // The pointer index can be "out of range" 325 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices1, Op1C); 326 327 Indices1.push_back((Value*)Op1); 328 } else { 329 // GEP1 is known to produce a value less than GEP2. To be 330 // conservatively correct, we must assume the largest possible constant 331 // is used in this position. This cannot be the initial index to the 332 // GEP instructions (because we know we have at least one element before 333 // this one with the different constant arguments), so we know that the 334 // current index must be into either a struct or array. Because we know 335 // it's not constant, this cannot be a structure index. Because of 336 // this, we can calculate the maximum value possible. 337 // 338 const ArrayType *ElTy = 339 cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true)); 340 Indices1.push_back(ConstantSInt::get(Type::LongTy, 341 ElTy->getNumElements()-1)); 342 } 343 344 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) { 345 // If this is an array index, make sure the array element is in range... 346 if (i != 1) // The pointer index can be "out of range" 347 Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C); 348 349 Indices2.push_back((Value*)Op2); 350 } 351 else // Conservatively assume the minimum value for this index 352 Indices2.push_back(Constant::getNullValue(Op2->getType())); 353 } 354 } 355 356 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1); 357 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2); 358 assert(Offset1 < Offset2 &&"There is at least one different constant here!"); 359 360 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 361 //std::cerr << "Determined that these two GEP's don't alias [" 362 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 363 return NoAlias; 364 } 365 return MayAlias; 366} 367 368