AliasAnalysis.cpp revision 081c34b725980f995be9080eaec24cd3dfaaf065
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 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 54 return AA->pointsToConstantMemory(Loc); 55} 56 57void AliasAnalysis::deleteValue(Value *V) { 58 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 59 AA->deleteValue(V); 60} 61 62void AliasAnalysis::copyValue(Value *From, Value *To) { 63 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 64 AA->copyValue(From, To); 65} 66 67AliasAnalysis::ModRefResult 68AliasAnalysis::getModRefInfo(ImmutableCallSite CS, 69 const Location &Loc) { 70 // Don't assert AA because BasicAA calls us in order to make use of the 71 // logic here. 72 73 ModRefBehavior MRB = getModRefBehavior(CS); 74 if (MRB == DoesNotAccessMemory) 75 return NoModRef; 76 77 ModRefResult Mask = ModRef; 78 if (MRB == OnlyReadsMemory) 79 Mask = Ref; 80 else if (MRB == AliasAnalysis::AccessesArguments) { 81 bool doesAlias = false; 82 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end(); 83 AI != AE; ++AI) 84 if (!isNoAlias(Location(*AI), Loc)) { 85 doesAlias = true; 86 break; 87 } 88 89 if (!doesAlias) 90 return NoModRef; 91 } 92 93 // If Loc is a constant memory location, the call definitely could not 94 // modify the memory location. 95 if ((Mask & Mod) && pointsToConstantMemory(Loc)) 96 Mask = ModRefResult(Mask & ~Mod); 97 98 // If this is BasicAA, don't forward. 99 if (!AA) return Mask; 100 101 // Otherwise, fall back to the next AA in the chain. But we can merge 102 // in any mask we've managed to compute. 103 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask); 104} 105 106AliasAnalysis::ModRefResult 107AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) { 108 // Don't assert AA because BasicAA calls us in order to make use of the 109 // logic here. 110 111 // If CS1 or CS2 are readnone, they don't interact. 112 ModRefBehavior CS1B = getModRefBehavior(CS1); 113 if (CS1B == DoesNotAccessMemory) return NoModRef; 114 115 ModRefBehavior CS2B = getModRefBehavior(CS2); 116 if (CS2B == DoesNotAccessMemory) return NoModRef; 117 118 // If they both only read from memory, there is no dependence. 119 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 120 return NoModRef; 121 122 AliasAnalysis::ModRefResult Mask = ModRef; 123 124 // If CS1 only reads memory, the only dependence on CS2 can be 125 // from CS1 reading memory written by CS2. 126 if (CS1B == OnlyReadsMemory) 127 Mask = ModRefResult(Mask & Ref); 128 129 // If CS2 only access memory through arguments, accumulate the mod/ref 130 // information from CS1's references to the memory referenced by 131 // CS2's arguments. 132 if (CS2B == AccessesArguments) { 133 AliasAnalysis::ModRefResult R = NoModRef; 134 for (ImmutableCallSite::arg_iterator 135 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) { 136 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask); 137 if (R == Mask) 138 break; 139 } 140 return R; 141 } 142 143 // If CS1 only accesses memory through arguments, check if CS2 references 144 // any of the memory referenced by CS1's arguments. If not, return NoModRef. 145 if (CS1B == AccessesArguments) { 146 AliasAnalysis::ModRefResult R = NoModRef; 147 for (ImmutableCallSite::arg_iterator 148 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) 149 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) { 150 R = Mask; 151 break; 152 } 153 if (R == NoModRef) 154 return R; 155 } 156 157 // If this is BasicAA, don't forward. 158 if (!AA) return Mask; 159 160 // Otherwise, fall back to the next AA in the chain. But we can merge 161 // in any mask we've managed to compute. 162 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask); 163} 164 165AliasAnalysis::ModRefBehavior 166AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 167 // Don't assert AA because BasicAA calls us in order to make use of the 168 // logic here. 169 170 ModRefBehavior Min = UnknownModRefBehavior; 171 172 // Call back into the alias analysis with the other form of getModRefBehavior 173 // to see if it can give a better response. 174 if (const Function *F = CS.getCalledFunction()) 175 Min = getModRefBehavior(F); 176 177 // If this is BasicAA, don't forward. 178 if (!AA) return Min; 179 180 // Otherwise, fall back to the next AA in the chain. But we can merge 181 // in any result we've managed to compute. 182 return std::min(AA->getModRefBehavior(CS), Min); 183} 184 185AliasAnalysis::ModRefBehavior 186AliasAnalysis::getModRefBehavior(const Function *F) { 187 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 188 return AA->getModRefBehavior(F); 189} 190 191//===----------------------------------------------------------------------===// 192// AliasAnalysis non-virtual helper method implementation 193//===----------------------------------------------------------------------===// 194 195AliasAnalysis::ModRefResult 196AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) { 197 // Be conservative in the face of volatile. 198 if (L->isVolatile()) 199 return ModRef; 200 201 // If the load address doesn't alias the given address, it doesn't read 202 // or write the specified memory. 203 if (!alias(Location(L->getOperand(0), 204 getTypeStoreSize(L->getType()), 205 L->getMetadata(LLVMContext::MD_tbaa)), 206 Loc)) 207 return NoModRef; 208 209 // Otherwise, a load just reads. 210 return Ref; 211} 212 213AliasAnalysis::ModRefResult 214AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) { 215 // Be conservative in the face of volatile. 216 if (S->isVolatile()) 217 return ModRef; 218 219 // If the store address cannot alias the pointer in question, then the 220 // specified memory cannot be modified by the store. 221 if (!alias(Location(S->getOperand(1), 222 getTypeStoreSize(S->getOperand(0)->getType()), 223 S->getMetadata(LLVMContext::MD_tbaa)), 224 Loc)) 225 return NoModRef; 226 227 // If the pointer is a pointer to constant memory, then it could not have been 228 // modified by this store. 229 if (pointsToConstantMemory(Loc)) 230 return NoModRef; 231 232 // Otherwise, a store just writes. 233 return Mod; 234} 235 236AliasAnalysis::ModRefResult 237AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) { 238 // If the va_arg address cannot alias the pointer in question, then the 239 // specified memory cannot be accessed by the va_arg. 240 if (!alias(Location(V->getOperand(0), 241 UnknownSize, 242 V->getMetadata(LLVMContext::MD_tbaa)), 243 Loc)) 244 return NoModRef; 245 246 // If the pointer is a pointer to constant memory, then it could not have been 247 // modified by this va_arg. 248 if (pointsToConstantMemory(Loc)) 249 return NoModRef; 250 251 // Otherwise, a va_arg reads and writes. 252 return ModRef; 253} 254 255AliasAnalysis::ModRefBehavior 256AliasAnalysis::getIntrinsicModRefBehavior(unsigned iid) { 257#define GET_INTRINSIC_MODREF_BEHAVIOR 258#include "llvm/Intrinsics.gen" 259#undef GET_INTRINSIC_MODREF_BEHAVIOR 260} 261 262// AliasAnalysis destructor: DO NOT move this to the header file for 263// AliasAnalysis or else clients of the AliasAnalysis class may not depend on 264// the AliasAnalysis.o file in the current .a file, causing alias analysis 265// support to not be included in the tool correctly! 266// 267AliasAnalysis::~AliasAnalysis() {} 268 269/// InitializeAliasAnalysis - Subclasses must call this method to initialize the 270/// AliasAnalysis interface before any other methods are called. 271/// 272void AliasAnalysis::InitializeAliasAnalysis(Pass *P) { 273 TD = P->getAnalysisIfAvailable<TargetData>(); 274 AA = &P->getAnalysis<AliasAnalysis>(); 275} 276 277// getAnalysisUsage - All alias analysis implementations should invoke this 278// directly (using AliasAnalysis::getAnalysisUsage(AU)). 279void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 280 AU.addRequired<AliasAnalysis>(); // All AA's chain 281} 282 283/// getTypeStoreSize - Return the TargetData store size for the given type, 284/// if known, or a conservative value otherwise. 285/// 286unsigned AliasAnalysis::getTypeStoreSize(const Type *Ty) { 287 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize; 288} 289 290/// canBasicBlockModify - Return true if it is possible for execution of the 291/// specified basic block to modify the value pointed to by Ptr. 292/// 293bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB, 294 const Location &Loc) { 295 return canInstructionRangeModify(BB.front(), BB.back(), Loc); 296} 297 298/// canInstructionRangeModify - Return true if it is possible for the execution 299/// of the specified instructions to modify the value pointed to by Ptr. The 300/// instructions to consider are all of the instructions in the range of [I1,I2] 301/// INCLUSIVE. I1 and I2 must be in the same basic block. 302/// 303bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1, 304 const Instruction &I2, 305 const Location &Loc) { 306 assert(I1.getParent() == I2.getParent() && 307 "Instructions not in same basic block!"); 308 BasicBlock::const_iterator I = &I1; 309 BasicBlock::const_iterator E = &I2; 310 ++E; // Convert from inclusive to exclusive range. 311 312 for (; I != E; ++I) // Check every instruction in range 313 if (getModRefInfo(I, Loc) & Mod) 314 return true; 315 return false; 316} 317 318/// isNoAliasCall - Return true if this pointer is returned by a noalias 319/// function. 320bool llvm::isNoAliasCall(const Value *V) { 321 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 322 return ImmutableCallSite(cast<Instruction>(V)) 323 .paramHasAttr(0, Attribute::NoAlias); 324 return false; 325} 326 327/// isIdentifiedObject - Return true if this pointer refers to a distinct and 328/// identifiable object. This returns true for: 329/// Global Variables and Functions (but not Global Aliases) 330/// Allocas and Mallocs 331/// ByVal and NoAlias Arguments 332/// NoAlias returns 333/// 334bool llvm::isIdentifiedObject(const Value *V) { 335 if (isa<AllocaInst>(V)) 336 return true; 337 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V)) 338 return true; 339 if (isNoAliasCall(V)) 340 return true; 341 if (const Argument *A = dyn_cast<Argument>(V)) 342 return A->hasNoAliasAttr() || A->hasByValAttr(); 343 return false; 344} 345 346// Because of the way .a files work, we must force the BasicAA implementation to 347// be pulled in if the AliasAnalysis classes are pulled in. Otherwise we run 348// the risk of AliasAnalysis being used, but the default implementation not 349// being linked into the tool that uses it. 350DEFINING_FILE_FOR(AliasAnalysis) 351