AliasAnalysis.cpp revision 218893
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 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end(); 91 AI != AE; ++AI) 92 if (!isNoAlias(Location(*AI), Loc)) { 93 doesAlias = true; 94 break; 95 } 96 97 if (!doesAlias) 98 return NoModRef; 99 } 100 101 // If Loc is a constant memory location, the call definitely could not 102 // modify the memory location. 103 if ((Mask & Mod) && pointsToConstantMemory(Loc)) 104 Mask = ModRefResult(Mask & ~Mod); 105 106 // If this is the end of the chain, don't forward. 107 if (!AA) return Mask; 108 109 // Otherwise, fall back to the next AA in the chain. But we can merge 110 // in any mask we've managed to compute. 111 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask); 112} 113 114AliasAnalysis::ModRefResult 115AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) { 116 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 117 118 // If CS1 or CS2 are readnone, they don't interact. 119 ModRefBehavior CS1B = getModRefBehavior(CS1); 120 if (CS1B == DoesNotAccessMemory) return NoModRef; 121 122 ModRefBehavior CS2B = getModRefBehavior(CS2); 123 if (CS2B == DoesNotAccessMemory) return NoModRef; 124 125 // If they both only read from memory, there is no dependence. 126 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B)) 127 return NoModRef; 128 129 AliasAnalysis::ModRefResult Mask = ModRef; 130 131 // If CS1 only reads memory, the only dependence on CS2 can be 132 // from CS1 reading memory written by CS2. 133 if (onlyReadsMemory(CS1B)) 134 Mask = ModRefResult(Mask & Ref); 135 136 // If CS2 only access memory through arguments, accumulate the mod/ref 137 // information from CS1's references to the memory referenced by 138 // CS2's arguments. 139 if (onlyAccessesArgPointees(CS2B)) { 140 AliasAnalysis::ModRefResult R = NoModRef; 141 if (doesAccessArgPointees(CS2B)) 142 for (ImmutableCallSite::arg_iterator 143 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) { 144 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask); 145 if (R == Mask) 146 break; 147 } 148 return R; 149 } 150 151 // If CS1 only accesses memory through arguments, check if CS2 references 152 // any of the memory referenced by CS1's arguments. If not, return NoModRef. 153 if (onlyAccessesArgPointees(CS1B)) { 154 AliasAnalysis::ModRefResult R = NoModRef; 155 if (doesAccessArgPointees(CS1B)) 156 for (ImmutableCallSite::arg_iterator 157 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) 158 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) { 159 R = Mask; 160 break; 161 } 162 if (R == NoModRef) 163 return R; 164 } 165 166 // If this is the end of the chain, don't forward. 167 if (!AA) return Mask; 168 169 // Otherwise, fall back to the next AA in the chain. But we can merge 170 // in any mask we've managed to compute. 171 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask); 172} 173 174AliasAnalysis::ModRefBehavior 175AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 176 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 177 178 ModRefBehavior Min = UnknownModRefBehavior; 179 180 // Call back into the alias analysis with the other form of getModRefBehavior 181 // to see if it can give a better response. 182 if (const Function *F = CS.getCalledFunction()) 183 Min = getModRefBehavior(F); 184 185 // If this is the end of the chain, don't forward. 186 if (!AA) return Min; 187 188 // Otherwise, fall back to the next AA in the chain. But we can merge 189 // in any result we've managed to compute. 190 return ModRefBehavior(AA->getModRefBehavior(CS) & Min); 191} 192 193AliasAnalysis::ModRefBehavior 194AliasAnalysis::getModRefBehavior(const Function *F) { 195 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 196 return AA->getModRefBehavior(F); 197} 198 199//===----------------------------------------------------------------------===// 200// AliasAnalysis non-virtual helper method implementation 201//===----------------------------------------------------------------------===// 202 203AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) { 204 return Location(LI->getPointerOperand(), 205 getTypeStoreSize(LI->getType()), 206 LI->getMetadata(LLVMContext::MD_tbaa)); 207} 208 209AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) { 210 return Location(SI->getPointerOperand(), 211 getTypeStoreSize(SI->getValueOperand()->getType()), 212 SI->getMetadata(LLVMContext::MD_tbaa)); 213} 214 215AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) { 216 return Location(VI->getPointerOperand(), 217 UnknownSize, 218 VI->getMetadata(LLVMContext::MD_tbaa)); 219} 220 221 222AliasAnalysis::Location 223AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) { 224 uint64_t Size = UnknownSize; 225 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength())) 226 Size = C->getValue().getZExtValue(); 227 228 // memcpy/memmove can have TBAA tags. For memcpy, they apply 229 // to both the source and the destination. 230 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa); 231 232 return Location(MTI->getRawSource(), Size, TBAATag); 233} 234 235AliasAnalysis::Location 236AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) { 237 uint64_t Size = UnknownSize; 238 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength())) 239 Size = C->getValue().getZExtValue(); 240 241 // memcpy/memmove can have TBAA tags. For memcpy, they apply 242 // to both the source and the destination. 243 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa); 244 245 return Location(MTI->getRawDest(), Size, TBAATag); 246} 247 248 249 250AliasAnalysis::ModRefResult 251AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) { 252 // Be conservative in the face of volatile. 253 if (L->isVolatile()) 254 return ModRef; 255 256 // If the load address doesn't alias the given address, it doesn't read 257 // or write the specified memory. 258 if (!alias(getLocation(L), Loc)) 259 return NoModRef; 260 261 // Otherwise, a load just reads. 262 return Ref; 263} 264 265AliasAnalysis::ModRefResult 266AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) { 267 // Be conservative in the face of volatile. 268 if (S->isVolatile()) 269 return ModRef; 270 271 // If the store address cannot alias the pointer in question, then the 272 // specified memory cannot be modified by the store. 273 if (!alias(getLocation(S), Loc)) 274 return NoModRef; 275 276 // If the pointer is a pointer to constant memory, then it could not have been 277 // modified by this store. 278 if (pointsToConstantMemory(Loc)) 279 return NoModRef; 280 281 // Otherwise, a store just writes. 282 return Mod; 283} 284 285AliasAnalysis::ModRefResult 286AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) { 287 // If the va_arg address cannot alias the pointer in question, then the 288 // specified memory cannot be accessed by the va_arg. 289 if (!alias(getLocation(V), Loc)) 290 return NoModRef; 291 292 // If the pointer is a pointer to constant memory, then it could not have been 293 // modified by this va_arg. 294 if (pointsToConstantMemory(Loc)) 295 return NoModRef; 296 297 // Otherwise, a va_arg reads and writes. 298 return ModRef; 299} 300 301// AliasAnalysis destructor: DO NOT move this to the header file for 302// AliasAnalysis or else clients of the AliasAnalysis class may not depend on 303// the AliasAnalysis.o file in the current .a file, causing alias analysis 304// support to not be included in the tool correctly! 305// 306AliasAnalysis::~AliasAnalysis() {} 307 308/// InitializeAliasAnalysis - Subclasses must call this method to initialize the 309/// AliasAnalysis interface before any other methods are called. 310/// 311void AliasAnalysis::InitializeAliasAnalysis(Pass *P) { 312 TD = P->getAnalysisIfAvailable<TargetData>(); 313 AA = &P->getAnalysis<AliasAnalysis>(); 314} 315 316// getAnalysisUsage - All alias analysis implementations should invoke this 317// directly (using AliasAnalysis::getAnalysisUsage(AU)). 318void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 319 AU.addRequired<AliasAnalysis>(); // All AA's chain 320} 321 322/// getTypeStoreSize - Return the TargetData store size for the given type, 323/// if known, or a conservative value otherwise. 324/// 325uint64_t AliasAnalysis::getTypeStoreSize(const Type *Ty) { 326 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize; 327} 328 329/// canBasicBlockModify - Return true if it is possible for execution of the 330/// specified basic block to modify the value pointed to by Ptr. 331/// 332bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB, 333 const Location &Loc) { 334 return canInstructionRangeModify(BB.front(), BB.back(), Loc); 335} 336 337/// canInstructionRangeModify - Return true if it is possible for the execution 338/// of the specified instructions to modify the value pointed to by Ptr. The 339/// instructions to consider are all of the instructions in the range of [I1,I2] 340/// INCLUSIVE. I1 and I2 must be in the same basic block. 341/// 342bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1, 343 const Instruction &I2, 344 const Location &Loc) { 345 assert(I1.getParent() == I2.getParent() && 346 "Instructions not in same basic block!"); 347 BasicBlock::const_iterator I = &I1; 348 BasicBlock::const_iterator E = &I2; 349 ++E; // Convert from inclusive to exclusive range. 350 351 for (; I != E; ++I) // Check every instruction in range 352 if (getModRefInfo(I, Loc) & Mod) 353 return true; 354 return false; 355} 356 357/// isNoAliasCall - Return true if this pointer is returned by a noalias 358/// function. 359bool llvm::isNoAliasCall(const Value *V) { 360 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 361 return ImmutableCallSite(cast<Instruction>(V)) 362 .paramHasAttr(0, Attribute::NoAlias); 363 return false; 364} 365 366/// isIdentifiedObject - Return true if this pointer refers to a distinct and 367/// identifiable object. This returns true for: 368/// Global Variables and Functions (but not Global Aliases) 369/// Allocas and Mallocs 370/// ByVal and NoAlias Arguments 371/// NoAlias returns 372/// 373bool llvm::isIdentifiedObject(const Value *V) { 374 if (isa<AllocaInst>(V)) 375 return true; 376 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V)) 377 return true; 378 if (isNoAliasCall(V)) 379 return true; 380 if (const Argument *A = dyn_cast<Argument>(V)) 381 return A->hasNoAliasAttr() || A->hasByValAttr(); 382 return false; 383} 384