AliasAnalysis.cpp revision 221345
1//===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements the generic AliasAnalysis interface which is used as the 11// common interface used by all clients and implementations of alias analysis. 12// 13// This file also implements the default version of the AliasAnalysis interface 14// that is to be used when no other implementation is specified. This does some 15// simple tests that detect obvious cases: two different global pointers cannot 16// alias, a global cannot alias a malloc, two different mallocs cannot alias, 17// etc. 18// 19// This alias analysis implementation really isn't very good for anything, but 20// it is very fast, and makes a nice clean default implementation. Because it 21// handles lots of little corner cases, other, more complex, alias analysis 22// implementations may choose to rely on this pass to resolve these simple and 23// easy cases. 24// 25//===----------------------------------------------------------------------===// 26 27#include "llvm/Analysis/AliasAnalysis.h" 28#include "llvm/Pass.h" 29#include "llvm/BasicBlock.h" 30#include "llvm/Function.h" 31#include "llvm/IntrinsicInst.h" 32#include "llvm/Instructions.h" 33#include "llvm/LLVMContext.h" 34#include "llvm/Type.h" 35#include "llvm/Target/TargetData.h" 36using namespace llvm; 37 38// Register the AliasAnalysis interface, providing a nice name to refer to. 39INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA) 40char AliasAnalysis::ID = 0; 41 42//===----------------------------------------------------------------------===// 43// Default chaining methods 44//===----------------------------------------------------------------------===// 45 46AliasAnalysis::AliasResult 47AliasAnalysis::alias(const Location &LocA, const Location &LocB) { 48 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 49 return AA->alias(LocA, LocB); 50} 51 52bool AliasAnalysis::pointsToConstantMemory(const Location &Loc, 53 bool OrLocal) { 54 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 55 return AA->pointsToConstantMemory(Loc, OrLocal); 56} 57 58void AliasAnalysis::deleteValue(Value *V) { 59 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 60 AA->deleteValue(V); 61} 62 63void AliasAnalysis::copyValue(Value *From, Value *To) { 64 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 65 AA->copyValue(From, To); 66} 67 68void AliasAnalysis::addEscapingUse(Use &U) { 69 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 70 AA->addEscapingUse(U); 71} 72 73 74AliasAnalysis::ModRefResult 75AliasAnalysis::getModRefInfo(ImmutableCallSite CS, 76 const Location &Loc) { 77 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 78 79 ModRefBehavior MRB = getModRefBehavior(CS); 80 if (MRB == DoesNotAccessMemory) 81 return NoModRef; 82 83 ModRefResult Mask = ModRef; 84 if (onlyReadsMemory(MRB)) 85 Mask = Ref; 86 87 if (onlyAccessesArgPointees(MRB)) { 88 bool doesAlias = false; 89 if (doesAccessArgPointees(MRB)) { 90 MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa); 91 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end(); 92 AI != AE; ++AI) { 93 const Value *Arg = *AI; 94 if (!Arg->getType()->isPointerTy()) 95 continue; 96 Location CSLoc(Arg, UnknownSize, CSTag); 97 if (!isNoAlias(CSLoc, Loc)) { 98 doesAlias = true; 99 break; 100 } 101 } 102 } 103 if (!doesAlias) 104 return NoModRef; 105 } 106 107 // If Loc is a constant memory location, the call definitely could not 108 // modify the memory location. 109 if ((Mask & Mod) && pointsToConstantMemory(Loc)) 110 Mask = ModRefResult(Mask & ~Mod); 111 112 // If this is the end of the chain, don't forward. 113 if (!AA) return Mask; 114 115 // Otherwise, fall back to the next AA in the chain. But we can merge 116 // in any mask we've managed to compute. 117 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask); 118} 119 120AliasAnalysis::ModRefResult 121AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) { 122 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 123 124 // If CS1 or CS2 are readnone, they don't interact. 125 ModRefBehavior CS1B = getModRefBehavior(CS1); 126 if (CS1B == DoesNotAccessMemory) return NoModRef; 127 128 ModRefBehavior CS2B = getModRefBehavior(CS2); 129 if (CS2B == DoesNotAccessMemory) return NoModRef; 130 131 // If they both only read from memory, there is no dependence. 132 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B)) 133 return NoModRef; 134 135 AliasAnalysis::ModRefResult Mask = ModRef; 136 137 // If CS1 only reads memory, the only dependence on CS2 can be 138 // from CS1 reading memory written by CS2. 139 if (onlyReadsMemory(CS1B)) 140 Mask = ModRefResult(Mask & Ref); 141 142 // If CS2 only access memory through arguments, accumulate the mod/ref 143 // information from CS1's references to the memory referenced by 144 // CS2's arguments. 145 if (onlyAccessesArgPointees(CS2B)) { 146 AliasAnalysis::ModRefResult R = NoModRef; 147 if (doesAccessArgPointees(CS2B)) { 148 MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa); 149 for (ImmutableCallSite::arg_iterator 150 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) { 151 const Value *Arg = *I; 152 if (!Arg->getType()->isPointerTy()) 153 continue; 154 Location CS2Loc(Arg, UnknownSize, CS2Tag); 155 R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask); 156 if (R == Mask) 157 break; 158 } 159 } 160 return R; 161 } 162 163 // If CS1 only accesses memory through arguments, check if CS2 references 164 // any of the memory referenced by CS1's arguments. If not, return NoModRef. 165 if (onlyAccessesArgPointees(CS1B)) { 166 AliasAnalysis::ModRefResult R = NoModRef; 167 if (doesAccessArgPointees(CS1B)) { 168 MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa); 169 for (ImmutableCallSite::arg_iterator 170 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) { 171 const Value *Arg = *I; 172 if (!Arg->getType()->isPointerTy()) 173 continue; 174 Location CS1Loc(Arg, UnknownSize, CS1Tag); 175 if (getModRefInfo(CS2, CS1Loc) != NoModRef) { 176 R = Mask; 177 break; 178 } 179 } 180 } 181 if (R == NoModRef) 182 return R; 183 } 184 185 // If this is the end of the chain, don't forward. 186 if (!AA) return Mask; 187 188 // Otherwise, fall back to the next AA in the chain. But we can merge 189 // in any mask we've managed to compute. 190 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask); 191} 192 193AliasAnalysis::ModRefBehavior 194AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 195 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 196 197 ModRefBehavior Min = UnknownModRefBehavior; 198 199 // Call back into the alias analysis with the other form of getModRefBehavior 200 // to see if it can give a better response. 201 if (const Function *F = CS.getCalledFunction()) 202 Min = getModRefBehavior(F); 203 204 // If this is the end of the chain, don't forward. 205 if (!AA) return Min; 206 207 // Otherwise, fall back to the next AA in the chain. But we can merge 208 // in any result we've managed to compute. 209 return ModRefBehavior(AA->getModRefBehavior(CS) & Min); 210} 211 212AliasAnalysis::ModRefBehavior 213AliasAnalysis::getModRefBehavior(const Function *F) { 214 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 215 return AA->getModRefBehavior(F); 216} 217 218//===----------------------------------------------------------------------===// 219// AliasAnalysis non-virtual helper method implementation 220//===----------------------------------------------------------------------===// 221 222AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) { 223 return Location(LI->getPointerOperand(), 224 getTypeStoreSize(LI->getType()), 225 LI->getMetadata(LLVMContext::MD_tbaa)); 226} 227 228AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) { 229 return Location(SI->getPointerOperand(), 230 getTypeStoreSize(SI->getValueOperand()->getType()), 231 SI->getMetadata(LLVMContext::MD_tbaa)); 232} 233 234AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) { 235 return Location(VI->getPointerOperand(), 236 UnknownSize, 237 VI->getMetadata(LLVMContext::MD_tbaa)); 238} 239 240 241AliasAnalysis::Location 242AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) { 243 uint64_t Size = UnknownSize; 244 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength())) 245 Size = C->getValue().getZExtValue(); 246 247 // memcpy/memmove can have TBAA tags. For memcpy, they apply 248 // to both the source and the destination. 249 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa); 250 251 return Location(MTI->getRawSource(), Size, TBAATag); 252} 253 254AliasAnalysis::Location 255AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) { 256 uint64_t Size = UnknownSize; 257 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength())) 258 Size = C->getValue().getZExtValue(); 259 260 // memcpy/memmove can have TBAA tags. For memcpy, they apply 261 // to both the source and the destination. 262 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa); 263 264 return Location(MTI->getRawDest(), Size, TBAATag); 265} 266 267 268 269AliasAnalysis::ModRefResult 270AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) { 271 // Be conservative in the face of volatile. 272 if (L->isVolatile()) 273 return ModRef; 274 275 // If the load address doesn't alias the given address, it doesn't read 276 // or write the specified memory. 277 if (!alias(getLocation(L), Loc)) 278 return NoModRef; 279 280 // Otherwise, a load just reads. 281 return Ref; 282} 283 284AliasAnalysis::ModRefResult 285AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) { 286 // Be conservative in the face of volatile. 287 if (S->isVolatile()) 288 return ModRef; 289 290 // If the store address cannot alias the pointer in question, then the 291 // specified memory cannot be modified by the store. 292 if (!alias(getLocation(S), Loc)) 293 return NoModRef; 294 295 // If the pointer is a pointer to constant memory, then it could not have been 296 // modified by this store. 297 if (pointsToConstantMemory(Loc)) 298 return NoModRef; 299 300 // Otherwise, a store just writes. 301 return Mod; 302} 303 304AliasAnalysis::ModRefResult 305AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) { 306 // If the va_arg address cannot alias the pointer in question, then the 307 // specified memory cannot be accessed by the va_arg. 308 if (!alias(getLocation(V), Loc)) 309 return NoModRef; 310 311 // If the pointer is a pointer to constant memory, then it could not have been 312 // modified by this va_arg. 313 if (pointsToConstantMemory(Loc)) 314 return NoModRef; 315 316 // Otherwise, a va_arg reads and writes. 317 return ModRef; 318} 319 320// AliasAnalysis destructor: DO NOT move this to the header file for 321// AliasAnalysis or else clients of the AliasAnalysis class may not depend on 322// the AliasAnalysis.o file in the current .a file, causing alias analysis 323// support to not be included in the tool correctly! 324// 325AliasAnalysis::~AliasAnalysis() {} 326 327/// InitializeAliasAnalysis - Subclasses must call this method to initialize the 328/// AliasAnalysis interface before any other methods are called. 329/// 330void AliasAnalysis::InitializeAliasAnalysis(Pass *P) { 331 TD = P->getAnalysisIfAvailable<TargetData>(); 332 AA = &P->getAnalysis<AliasAnalysis>(); 333} 334 335// getAnalysisUsage - All alias analysis implementations should invoke this 336// directly (using AliasAnalysis::getAnalysisUsage(AU)). 337void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 338 AU.addRequired<AliasAnalysis>(); // All AA's chain 339} 340 341/// getTypeStoreSize - Return the TargetData store size for the given type, 342/// if known, or a conservative value otherwise. 343/// 344uint64_t AliasAnalysis::getTypeStoreSize(const Type *Ty) { 345 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize; 346} 347 348/// canBasicBlockModify - Return true if it is possible for execution of the 349/// specified basic block to modify the value pointed to by Ptr. 350/// 351bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB, 352 const Location &Loc) { 353 return canInstructionRangeModify(BB.front(), BB.back(), Loc); 354} 355 356/// canInstructionRangeModify - Return true if it is possible for the execution 357/// of the specified instructions to modify the value pointed to by Ptr. The 358/// instructions to consider are all of the instructions in the range of [I1,I2] 359/// INCLUSIVE. I1 and I2 must be in the same basic block. 360/// 361bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1, 362 const Instruction &I2, 363 const Location &Loc) { 364 assert(I1.getParent() == I2.getParent() && 365 "Instructions not in same basic block!"); 366 BasicBlock::const_iterator I = &I1; 367 BasicBlock::const_iterator E = &I2; 368 ++E; // Convert from inclusive to exclusive range. 369 370 for (; I != E; ++I) // Check every instruction in range 371 if (getModRefInfo(I, Loc) & Mod) 372 return true; 373 return false; 374} 375 376/// isNoAliasCall - Return true if this pointer is returned by a noalias 377/// function. 378bool llvm::isNoAliasCall(const Value *V) { 379 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 380 return ImmutableCallSite(cast<Instruction>(V)) 381 .paramHasAttr(0, Attribute::NoAlias); 382 return false; 383} 384 385/// isIdentifiedObject - Return true if this pointer refers to a distinct and 386/// identifiable object. This returns true for: 387/// Global Variables and Functions (but not Global Aliases) 388/// Allocas and Mallocs 389/// ByVal and NoAlias Arguments 390/// NoAlias returns 391/// 392bool llvm::isIdentifiedObject(const Value *V) { 393 if (isa<AllocaInst>(V)) 394 return true; 395 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V)) 396 return true; 397 if (isNoAliasCall(V)) 398 return true; 399 if (const Argument *A = dyn_cast<Argument>(V)) 400 return A->hasNoAliasAttr() || A->hasByValAttr(); 401 return false; 402} 403