AliasAnalysis.cpp revision 212904
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/Type.h" 34#include "llvm/Target/TargetData.h" 35using namespace llvm; 36 37// Register the AliasAnalysis interface, providing a nice name to refer to. 38static RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis"); 39char AliasAnalysis::ID = 0; 40 41//===----------------------------------------------------------------------===// 42// Default chaining methods 43//===----------------------------------------------------------------------===// 44 45AliasAnalysis::AliasResult 46AliasAnalysis::alias(const Value *V1, unsigned V1Size, 47 const Value *V2, unsigned V2Size) { 48 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 49 return AA->alias(V1, V1Size, V2, V2Size); 50} 51 52bool AliasAnalysis::pointsToConstantMemory(const Value *P) { 53 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!"); 54 return AA->pointsToConstantMemory(P); 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 Value *P, unsigned Size) { 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(*AI, ~0U, P, Size)) { 85 doesAlias = true; 86 break; 87 } 88 89 if (!doesAlias) 90 return NoModRef; 91 } 92 93 // If P points to a constant memory location, the call definitely could not 94 // modify the memory location. 95 if ((Mask & Mod) && pointsToConstantMemory(P)) 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, P, Size) & 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//===----------------------------------------------------------------------===// 193// AliasAnalysis non-virtual helper method implementation 194//===----------------------------------------------------------------------===// 195 196AliasAnalysis::ModRefResult 197AliasAnalysis::getModRefInfo(const LoadInst *L, const Value *P, unsigned Size) { 198 // Be conservative in the face of volatile. 199 if (L->isVolatile()) 200 return ModRef; 201 202 // If the load address doesn't alias the given address, it doesn't read 203 // or write the specified memory. 204 if (!alias(L->getOperand(0), getTypeStoreSize(L->getType()), P, Size)) 205 return NoModRef; 206 207 // Otherwise, a load just reads. 208 return Ref; 209} 210 211AliasAnalysis::ModRefResult 212AliasAnalysis::getModRefInfo(const StoreInst *S, const Value *P, unsigned Size) { 213 // Be conservative in the face of volatile. 214 if (S->isVolatile()) 215 return ModRef; 216 217 // If the store address cannot alias the pointer in question, then the 218 // specified memory cannot be modified by the store. 219 if (!alias(S->getOperand(1), 220 getTypeStoreSize(S->getOperand(0)->getType()), P, Size)) 221 return NoModRef; 222 223 // If the pointer is a pointer to constant memory, then it could not have been 224 // modified by this store. 225 if (pointsToConstantMemory(P)) 226 return NoModRef; 227 228 // Otherwise, a store just writes. 229 return Mod; 230} 231 232AliasAnalysis::ModRefResult 233AliasAnalysis::getModRefInfo(const VAArgInst *V, const Value *P, unsigned Size) { 234 // If the va_arg address cannot alias the pointer in question, then the 235 // specified memory cannot be accessed by the va_arg. 236 if (!alias(V->getOperand(0), UnknownSize, P, Size)) 237 return NoModRef; 238 239 // If the pointer is a pointer to constant memory, then it could not have been 240 // modified by this va_arg. 241 if (pointsToConstantMemory(P)) 242 return NoModRef; 243 244 // Otherwise, a va_arg reads and writes. 245 return ModRef; 246} 247 248 249AliasAnalysis::ModRefBehavior 250AliasAnalysis::getIntrinsicModRefBehavior(unsigned iid) { 251#define GET_INTRINSIC_MODREF_BEHAVIOR 252#include "llvm/Intrinsics.gen" 253#undef GET_INTRINSIC_MODREF_BEHAVIOR 254} 255 256// AliasAnalysis destructor: DO NOT move this to the header file for 257// AliasAnalysis or else clients of the AliasAnalysis class may not depend on 258// the AliasAnalysis.o file in the current .a file, causing alias analysis 259// support to not be included in the tool correctly! 260// 261AliasAnalysis::~AliasAnalysis() {} 262 263/// InitializeAliasAnalysis - Subclasses must call this method to initialize the 264/// AliasAnalysis interface before any other methods are called. 265/// 266void AliasAnalysis::InitializeAliasAnalysis(Pass *P) { 267 TD = P->getAnalysisIfAvailable<TargetData>(); 268 AA = &P->getAnalysis<AliasAnalysis>(); 269} 270 271// getAnalysisUsage - All alias analysis implementations should invoke this 272// directly (using AliasAnalysis::getAnalysisUsage(AU)). 273void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 274 AU.addRequired<AliasAnalysis>(); // All AA's chain 275} 276 277/// getTypeStoreSize - Return the TargetData store size for the given type, 278/// if known, or a conservative value otherwise. 279/// 280unsigned AliasAnalysis::getTypeStoreSize(const Type *Ty) { 281 return TD ? TD->getTypeStoreSize(Ty) : ~0u; 282} 283 284/// canBasicBlockModify - Return true if it is possible for execution of the 285/// specified basic block to modify the value pointed to by Ptr. 286/// 287bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB, 288 const Value *Ptr, unsigned Size) { 289 return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size); 290} 291 292/// canInstructionRangeModify - Return true if it is possible for the execution 293/// of the specified instructions to modify the value pointed to by Ptr. The 294/// instructions to consider are all of the instructions in the range of [I1,I2] 295/// INCLUSIVE. I1 and I2 must be in the same basic block. 296/// 297bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1, 298 const Instruction &I2, 299 const Value *Ptr, unsigned Size) { 300 assert(I1.getParent() == I2.getParent() && 301 "Instructions not in same basic block!"); 302 BasicBlock::const_iterator I = &I1; 303 BasicBlock::const_iterator E = &I2; 304 ++E; // Convert from inclusive to exclusive range. 305 306 for (; I != E; ++I) // Check every instruction in range 307 if (getModRefInfo(I, Ptr, Size) & Mod) 308 return true; 309 return false; 310} 311 312/// isNoAliasCall - Return true if this pointer is returned by a noalias 313/// function. 314bool llvm::isNoAliasCall(const Value *V) { 315 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 316 return ImmutableCallSite(cast<Instruction>(V)) 317 .paramHasAttr(0, Attribute::NoAlias); 318 return false; 319} 320 321/// isIdentifiedObject - Return true if this pointer refers to a distinct and 322/// identifiable object. This returns true for: 323/// Global Variables and Functions (but not Global Aliases) 324/// Allocas and Mallocs 325/// ByVal and NoAlias Arguments 326/// NoAlias returns 327/// 328bool llvm::isIdentifiedObject(const Value *V) { 329 if (isa<AllocaInst>(V)) 330 return true; 331 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V)) 332 return true; 333 if (isNoAliasCall(V)) 334 return true; 335 if (const Argument *A = dyn_cast<Argument>(V)) 336 return A->hasNoAliasAttr() || A->hasByValAttr(); 337 return false; 338} 339 340// Because of the way .a files work, we must force the BasicAA implementation to 341// be pulled in if the AliasAnalysis classes are pulled in. Otherwise we run 342// the risk of AliasAnalysis being used, but the default implementation not 343// being linked into the tool that uses it. 344DEFINING_FILE_FOR(AliasAnalysis) 345