BasicAliasAnalysis.cpp revision 4cccb87b4d766719cd8cdf98bed1d433d245adb0
1//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===// 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 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/Analysis/Passes.h" 18#include "llvm/Constants.h" 19#include "llvm/DerivedTypes.h" 20#include "llvm/Function.h" 21#include "llvm/GlobalAlias.h" 22#include "llvm/GlobalVariable.h" 23#include "llvm/Instructions.h" 24#include "llvm/IntrinsicInst.h" 25#include "llvm/LLVMContext.h" 26#include "llvm/Operator.h" 27#include "llvm/Pass.h" 28#include "llvm/Analysis/CaptureTracking.h" 29#include "llvm/Analysis/MemoryBuiltins.h" 30#include "llvm/Analysis/ValueTracking.h" 31#include "llvm/Target/TargetData.h" 32#include "llvm/ADT/SmallPtrSet.h" 33#include "llvm/ADT/SmallVector.h" 34#include "llvm/Support/ErrorHandling.h" 35#include "llvm/Support/GetElementPtrTypeIterator.h" 36#include <algorithm> 37using namespace llvm; 38 39//===----------------------------------------------------------------------===// 40// Useful predicates 41//===----------------------------------------------------------------------===// 42 43/// isKnownNonNull - Return true if we know that the specified value is never 44/// null. 45static bool isKnownNonNull(const Value *V) { 46 // Alloca never returns null, malloc might. 47 if (isa<AllocaInst>(V)) return true; 48 49 // A byval argument is never null. 50 if (const Argument *A = dyn_cast<Argument>(V)) 51 return A->hasByValAttr(); 52 53 // Global values are not null unless extern weak. 54 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 55 return !GV->hasExternalWeakLinkage(); 56 return false; 57} 58 59/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 60/// object that never escapes from the function. 61static bool isNonEscapingLocalObject(const Value *V) { 62 // If this is a local allocation, check to see if it escapes. 63 if (isa<AllocaInst>(V) || isNoAliasCall(V)) 64 // Set StoreCaptures to True so that we can assume in our callers that the 65 // pointer is not the result of a load instruction. Currently 66 // PointerMayBeCaptured doesn't have any special analysis for the 67 // StoreCaptures=false case; if it did, our callers could be refined to be 68 // more precise. 69 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 70 71 // If this is an argument that corresponds to a byval or noalias argument, 72 // then it has not escaped before entering the function. Check if it escapes 73 // inside the function. 74 if (const Argument *A = dyn_cast<Argument>(V)) 75 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 76 // Don't bother analyzing arguments already known not to escape. 77 if (A->hasNoCaptureAttr()) 78 return true; 79 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 80 } 81 return false; 82} 83 84/// isEscapeSource - Return true if the pointer is one which would have 85/// been considered an escape by isNonEscapingLocalObject. 86static bool isEscapeSource(const Value *V) { 87 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V)) 88 return true; 89 90 // The load case works because isNonEscapingLocalObject considers all 91 // stores to be escapes (it passes true for the StoreCaptures argument 92 // to PointerMayBeCaptured). 93 if (isa<LoadInst>(V)) 94 return true; 95 96 return false; 97} 98 99/// isObjectSmallerThan - Return true if we can prove that the object specified 100/// by V is smaller than Size. 101static bool isObjectSmallerThan(const Value *V, uint64_t Size, 102 const TargetData &TD) { 103 const Type *AccessTy; 104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 105 AccessTy = GV->getType()->getElementType(); 106 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) { 107 if (!AI->isArrayAllocation()) 108 AccessTy = AI->getType()->getElementType(); 109 else 110 return false; 111 } else if (const CallInst* CI = extractMallocCall(V)) { 112 if (!isArrayMalloc(V, &TD)) 113 // The size is the argument to the malloc call. 114 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0))) 115 return (C->getZExtValue() < Size); 116 return false; 117 } else if (const Argument *A = dyn_cast<Argument>(V)) { 118 if (A->hasByValAttr()) 119 AccessTy = cast<PointerType>(A->getType())->getElementType(); 120 else 121 return false; 122 } else { 123 return false; 124 } 125 126 if (AccessTy->isSized()) 127 return TD.getTypeAllocSize(AccessTy) < Size; 128 return false; 129} 130 131//===----------------------------------------------------------------------===// 132// NoAA Pass 133//===----------------------------------------------------------------------===// 134 135namespace { 136 /// NoAA - This class implements the -no-aa pass, which always returns "I 137 /// don't know" for alias queries. NoAA is unlike other alias analysis 138 /// implementations, in that it does not chain to a previous analysis. As 139 /// such it doesn't follow many of the rules that other alias analyses must. 140 /// 141 struct NoAA : public ImmutablePass, public AliasAnalysis { 142 static char ID; // Class identification, replacement for typeinfo 143 NoAA() : ImmutablePass(ID) { 144 initializeNoAAPass(*PassRegistry::getPassRegistry()); 145 } 146 explicit NoAA(char &PID) : ImmutablePass(PID) {} 147 148 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 149 } 150 151 virtual void initializePass() { 152 TD = getAnalysisIfAvailable<TargetData>(); 153 } 154 155 virtual AliasResult alias(const Location &LocA, const Location &LocB) { 156 return MayAlias; 157 } 158 159 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS) { 160 return UnknownModRefBehavior; 161 } 162 virtual ModRefBehavior getModRefBehavior(const Function *F) { 163 return UnknownModRefBehavior; 164 } 165 166 virtual bool pointsToConstantMemory(const Location &Loc) { return false; } 167 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 168 const Location &Loc) { 169 return ModRef; 170 } 171 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 172 ImmutableCallSite CS2) { 173 return ModRef; 174 } 175 176 virtual void deleteValue(Value *V) {} 177 virtual void copyValue(Value *From, Value *To) {} 178 179 /// getAdjustedAnalysisPointer - This method is used when a pass implements 180 /// an analysis interface through multiple inheritance. If needed, it 181 /// should override this to adjust the this pointer as needed for the 182 /// specified pass info. 183 virtual void *getAdjustedAnalysisPointer(const void *ID) { 184 if (ID == &AliasAnalysis::ID) 185 return (AliasAnalysis*)this; 186 return this; 187 } 188 }; 189} // End of anonymous namespace 190 191// Register this pass... 192char NoAA::ID = 0; 193INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa", 194 "No Alias Analysis (always returns 'may' alias)", 195 true, true, true) 196 197ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 198 199//===----------------------------------------------------------------------===// 200// GetElementPtr Instruction Decomposition and Analysis 201//===----------------------------------------------------------------------===// 202 203namespace { 204 enum ExtensionKind { 205 EK_NotExtended, 206 EK_SignExt, 207 EK_ZeroExt 208 }; 209 210 struct VariableGEPIndex { 211 const Value *V; 212 ExtensionKind Extension; 213 int64_t Scale; 214 }; 215} 216 217 218/// GetLinearExpression - Analyze the specified value as a linear expression: 219/// "A*V + B", where A and B are constant integers. Return the scale and offset 220/// values as APInts and return V as a Value*, and return whether we looked 221/// through any sign or zero extends. The incoming Value is known to have 222/// IntegerType and it may already be sign or zero extended. 223/// 224/// Note that this looks through extends, so the high bits may not be 225/// represented in the result. 226static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset, 227 ExtensionKind &Extension, 228 const TargetData &TD, unsigned Depth) { 229 assert(V->getType()->isIntegerTy() && "Not an integer value"); 230 231 // Limit our recursion depth. 232 if (Depth == 6) { 233 Scale = 1; 234 Offset = 0; 235 return V; 236 } 237 238 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) { 239 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) { 240 switch (BOp->getOpcode()) { 241 default: break; 242 case Instruction::Or: 243 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't 244 // analyze it. 245 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD)) 246 break; 247 // FALL THROUGH. 248 case Instruction::Add: 249 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 250 TD, Depth+1); 251 Offset += RHSC->getValue(); 252 return V; 253 case Instruction::Mul: 254 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 255 TD, Depth+1); 256 Offset *= RHSC->getValue(); 257 Scale *= RHSC->getValue(); 258 return V; 259 case Instruction::Shl: 260 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 261 TD, Depth+1); 262 Offset <<= RHSC->getValue().getLimitedValue(); 263 Scale <<= RHSC->getValue().getLimitedValue(); 264 return V; 265 } 266 } 267 } 268 269 // Since GEP indices are sign extended anyway, we don't care about the high 270 // bits of a sign or zero extended value - just scales and offsets. The 271 // extensions have to be consistent though. 272 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) || 273 (isa<ZExtInst>(V) && Extension != EK_SignExt)) { 274 Value *CastOp = cast<CastInst>(V)->getOperand(0); 275 unsigned OldWidth = Scale.getBitWidth(); 276 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits(); 277 Scale.trunc(SmallWidth); 278 Offset.trunc(SmallWidth); 279 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt; 280 281 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, 282 TD, Depth+1); 283 Scale.zext(OldWidth); 284 Offset.zext(OldWidth); 285 286 return Result; 287 } 288 289 Scale = 1; 290 Offset = 0; 291 return V; 292} 293 294/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it 295/// into a base pointer with a constant offset and a number of scaled symbolic 296/// offsets. 297/// 298/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in 299/// the VarIndices vector) are Value*'s that are known to be scaled by the 300/// specified amount, but which may have other unrepresented high bits. As such, 301/// the gep cannot necessarily be reconstructed from its decomposed form. 302/// 303/// When TargetData is around, this function is capable of analyzing everything 304/// that Value::getUnderlyingObject() can look through. When not, it just looks 305/// through pointer casts. 306/// 307static const Value * 308DecomposeGEPExpression(const Value *V, int64_t &BaseOffs, 309 SmallVectorImpl<VariableGEPIndex> &VarIndices, 310 const TargetData *TD) { 311 // Limit recursion depth to limit compile time in crazy cases. 312 unsigned MaxLookup = 6; 313 314 BaseOffs = 0; 315 do { 316 // See if this is a bitcast or GEP. 317 const Operator *Op = dyn_cast<Operator>(V); 318 if (Op == 0) { 319 // The only non-operator case we can handle are GlobalAliases. 320 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 321 if (!GA->mayBeOverridden()) { 322 V = GA->getAliasee(); 323 continue; 324 } 325 } 326 return V; 327 } 328 329 if (Op->getOpcode() == Instruction::BitCast) { 330 V = Op->getOperand(0); 331 continue; 332 } 333 334 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op); 335 if (GEPOp == 0) 336 return V; 337 338 // Don't attempt to analyze GEPs over unsized objects. 339 if (!cast<PointerType>(GEPOp->getOperand(0)->getType()) 340 ->getElementType()->isSized()) 341 return V; 342 343 // If we are lacking TargetData information, we can't compute the offets of 344 // elements computed by GEPs. However, we can handle bitcast equivalent 345 // GEPs. 346 if (TD == 0) { 347 if (!GEPOp->hasAllZeroIndices()) 348 return V; 349 V = GEPOp->getOperand(0); 350 continue; 351 } 352 353 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices. 354 gep_type_iterator GTI = gep_type_begin(GEPOp); 355 for (User::const_op_iterator I = GEPOp->op_begin()+1, 356 E = GEPOp->op_end(); I != E; ++I) { 357 Value *Index = *I; 358 // Compute the (potentially symbolic) offset in bytes for this index. 359 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) { 360 // For a struct, add the member offset. 361 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue(); 362 if (FieldNo == 0) continue; 363 364 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo); 365 continue; 366 } 367 368 // For an array/pointer, add the element offset, explicitly scaled. 369 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) { 370 if (CIdx->isZero()) continue; 371 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue(); 372 continue; 373 } 374 375 uint64_t Scale = TD->getTypeAllocSize(*GTI); 376 ExtensionKind Extension = EK_NotExtended; 377 378 // If the integer type is smaller than the pointer size, it is implicitly 379 // sign extended to pointer size. 380 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth(); 381 if (TD->getPointerSizeInBits() > Width) 382 Extension = EK_SignExt; 383 384 // Use GetLinearExpression to decompose the index into a C1*V+C2 form. 385 APInt IndexScale(Width, 0), IndexOffset(Width, 0); 386 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, 387 *TD, 0); 388 389 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale. 390 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale. 391 BaseOffs += IndexOffset.getSExtValue()*Scale; 392 Scale *= IndexScale.getSExtValue(); 393 394 395 // If we already had an occurrance of this index variable, merge this 396 // scale into it. For example, we want to handle: 397 // A[x][x] -> x*16 + x*4 -> x*20 398 // This also ensures that 'x' only appears in the index list once. 399 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) { 400 if (VarIndices[i].V == Index && 401 VarIndices[i].Extension == Extension) { 402 Scale += VarIndices[i].Scale; 403 VarIndices.erase(VarIndices.begin()+i); 404 break; 405 } 406 } 407 408 // Make sure that we have a scale that makes sense for this target's 409 // pointer size. 410 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) { 411 Scale <<= ShiftBits; 412 Scale = (int64_t)Scale >> ShiftBits; 413 } 414 415 if (Scale) { 416 VariableGEPIndex Entry = {Index, Extension, Scale}; 417 VarIndices.push_back(Entry); 418 } 419 } 420 421 // Analyze the base pointer next. 422 V = GEPOp->getOperand(0); 423 } while (--MaxLookup); 424 425 // If the chain of expressions is too deep, just return early. 426 return V; 427} 428 429/// GetIndexDifference - Dest and Src are the variable indices from two 430/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 431/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 432/// difference between the two pointers. 433static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest, 434 const SmallVectorImpl<VariableGEPIndex> &Src) { 435 if (Src.empty()) return; 436 437 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 438 const Value *V = Src[i].V; 439 ExtensionKind Extension = Src[i].Extension; 440 int64_t Scale = Src[i].Scale; 441 442 // Find V in Dest. This is N^2, but pointer indices almost never have more 443 // than a few variable indexes. 444 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 445 if (Dest[j].V != V || Dest[j].Extension != Extension) continue; 446 447 // If we found it, subtract off Scale V's from the entry in Dest. If it 448 // goes to zero, remove the entry. 449 if (Dest[j].Scale != Scale) 450 Dest[j].Scale -= Scale; 451 else 452 Dest.erase(Dest.begin()+j); 453 Scale = 0; 454 break; 455 } 456 457 // If we didn't consume this entry, add it to the end of the Dest list. 458 if (Scale) { 459 VariableGEPIndex Entry = { V, Extension, -Scale }; 460 Dest.push_back(Entry); 461 } 462 } 463} 464 465//===----------------------------------------------------------------------===// 466// BasicAliasAnalysis Pass 467//===----------------------------------------------------------------------===// 468 469#ifndef NDEBUG 470static const Function *getParent(const Value *V) { 471 if (const Instruction *inst = dyn_cast<Instruction>(V)) 472 return inst->getParent()->getParent(); 473 474 if (const Argument *arg = dyn_cast<Argument>(V)) 475 return arg->getParent(); 476 477 return NULL; 478} 479 480static bool notDifferentParent(const Value *O1, const Value *O2) { 481 482 const Function *F1 = getParent(O1); 483 const Function *F2 = getParent(O2); 484 485 return !F1 || !F2 || F1 == F2; 486} 487#endif 488 489namespace { 490 /// BasicAliasAnalysis - This is the default alias analysis implementation. 491 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 492 /// derives from the NoAA class. 493 struct BasicAliasAnalysis : public NoAA { 494 static char ID; // Class identification, replacement for typeinfo 495 BasicAliasAnalysis() : NoAA(ID) { 496 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry()); 497 } 498 499 virtual void initializePass() { 500 InitializeAliasAnalysis(this); 501 } 502 503 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 504 AU.addRequired<AliasAnalysis>(); 505 } 506 507 virtual AliasResult alias(const Location &LocA, 508 const Location &LocB) { 509 assert(Visited.empty() && "Visited must be cleared after use!"); 510 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) && 511 "BasicAliasAnalysis doesn't support interprocedural queries."); 512 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag, 513 LocB.Ptr, LocB.Size, LocB.TBAATag); 514 Visited.clear(); 515 return Alias; 516 } 517 518 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 519 const Location &Loc); 520 521 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 522 ImmutableCallSite CS2) { 523 // The AliasAnalysis base class has some smarts, lets use them. 524 return AliasAnalysis::getModRefInfo(CS1, CS2); 525 } 526 527 /// pointsToConstantMemory - Chase pointers until we find a (constant 528 /// global) or not. 529 virtual bool pointsToConstantMemory(const Location &Loc); 530 531 /// getModRefBehavior - Return the behavior when calling the given 532 /// call site. 533 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); 534 535 /// getModRefBehavior - Return the behavior when calling the given function. 536 /// For use when the call site is not known. 537 virtual ModRefBehavior getModRefBehavior(const Function *F); 538 539 /// getAdjustedAnalysisPointer - This method is used when a pass implements 540 /// an analysis interface through multiple inheritance. If needed, it 541 /// should override this to adjust the this pointer as needed for the 542 /// specified pass info. 543 virtual void *getAdjustedAnalysisPointer(const void *ID) { 544 if (ID == &AliasAnalysis::ID) 545 return (AliasAnalysis*)this; 546 return this; 547 } 548 549 private: 550 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP(). 551 SmallPtrSet<const Value*, 16> Visited; 552 553 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 554 // instruction against another. 555 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size, 556 const Value *V2, uint64_t V2Size, 557 const MDNode *V2TBAAInfo, 558 const Value *UnderlyingV1, const Value *UnderlyingV2); 559 560 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 561 // instruction against another. 562 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize, 563 const MDNode *PNTBAAInfo, 564 const Value *V2, uint64_t V2Size, 565 const MDNode *V2TBAAInfo); 566 567 /// aliasSelect - Disambiguate a Select instruction against another value. 568 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize, 569 const MDNode *SITBAAInfo, 570 const Value *V2, uint64_t V2Size, 571 const MDNode *V2TBAAInfo); 572 573 AliasResult aliasCheck(const Value *V1, uint64_t V1Size, 574 const MDNode *V1TBAATag, 575 const Value *V2, uint64_t V2Size, 576 const MDNode *V2TBAATag); 577 }; 578} // End of anonymous namespace 579 580// Register this pass... 581char BasicAliasAnalysis::ID = 0; 582INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa", 583 "Basic Alias Analysis (default AA impl)", 584 false, true, false) 585 586ImmutablePass *llvm::createBasicAliasAnalysisPass() { 587 return new BasicAliasAnalysis(); 588} 589 590 591/// pointsToConstantMemory - Chase pointers until we find a (constant 592/// global) or not. 593bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) { 594 if (const GlobalVariable *GV = 595 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject())) 596 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 597 // global to be marked constant in some modules and non-constant in others. 598 // GV may even be a declaration, not a definition. 599 return GV->isConstant(); 600 601 return AliasAnalysis::pointsToConstantMemory(Loc); 602} 603 604/// getModRefBehavior - Return the behavior when calling the given call site. 605AliasAnalysis::ModRefBehavior 606BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 607 if (CS.doesNotAccessMemory()) 608 // Can't do better than this. 609 return DoesNotAccessMemory; 610 611 ModRefBehavior Min = UnknownModRefBehavior; 612 613 // If the callsite knows it only reads memory, don't return worse 614 // than that. 615 if (CS.onlyReadsMemory()) 616 Min = OnlyReadsMemory; 617 618 // The AliasAnalysis base class has some smarts, lets use them. 619 return std::min(AliasAnalysis::getModRefBehavior(CS), Min); 620} 621 622/// getModRefBehavior - Return the behavior when calling the given function. 623/// For use when the call site is not known. 624AliasAnalysis::ModRefBehavior 625BasicAliasAnalysis::getModRefBehavior(const Function *F) { 626 if (F->doesNotAccessMemory()) 627 // Can't do better than this. 628 return DoesNotAccessMemory; 629 if (F->onlyReadsMemory()) 630 return OnlyReadsMemory; 631 if (unsigned id = F->getIntrinsicID()) 632 return getIntrinsicModRefBehavior(id); 633 634 return AliasAnalysis::getModRefBehavior(F); 635} 636 637/// getModRefInfo - Check to see if the specified callsite can clobber the 638/// specified memory object. Since we only look at local properties of this 639/// function, we really can't say much about this query. We do, however, use 640/// simple "address taken" analysis on local objects. 641AliasAnalysis::ModRefResult 642BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 643 const Location &Loc) { 644 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) && 645 "AliasAnalysis query involving multiple functions!"); 646 647 const Value *Object = Loc.Ptr->getUnderlyingObject(); 648 649 // If this is a tail call and Loc.Ptr points to a stack location, we know that 650 // the tail call cannot access or modify the local stack. 651 // We cannot exclude byval arguments here; these belong to the caller of 652 // the current function not to the current function, and a tail callee 653 // may reference them. 654 if (isa<AllocaInst>(Object)) 655 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 656 if (CI->isTailCall()) 657 return NoModRef; 658 659 // If the pointer is to a locally allocated object that does not escape, 660 // then the call can not mod/ref the pointer unless the call takes the pointer 661 // as an argument, and itself doesn't capture it. 662 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 663 isNonEscapingLocalObject(Object)) { 664 bool PassedAsArg = false; 665 unsigned ArgNo = 0; 666 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 667 CI != CE; ++CI, ++ArgNo) { 668 // Only look at the no-capture pointer arguments. 669 if (!(*CI)->getType()->isPointerTy() || 670 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) 671 continue; 672 673 // If this is a no-capture pointer argument, see if we can tell that it 674 // is impossible to alias the pointer we're checking. If not, we have to 675 // assume that the call could touch the pointer, even though it doesn't 676 // escape. 677 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) { 678 PassedAsArg = true; 679 break; 680 } 681 } 682 683 if (!PassedAsArg) 684 return NoModRef; 685 } 686 687 // Finally, handle specific knowledge of intrinsics. 688 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 689 if (II != 0) 690 switch (II->getIntrinsicID()) { 691 default: break; 692 case Intrinsic::memcpy: 693 case Intrinsic::memmove: { 694 uint64_t Len = UnknownSize; 695 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 696 Len = LenCI->getZExtValue(); 697 Value *Dest = II->getArgOperand(0); 698 Value *Src = II->getArgOperand(1); 699 if (isNoAlias(Location(Dest, Len), Loc)) { 700 if (isNoAlias(Location(Src, Len), Loc)) 701 return NoModRef; 702 return Ref; 703 } 704 break; 705 } 706 case Intrinsic::memset: 707 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 708 // will handle it for the variable length case. 709 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 710 uint64_t Len = LenCI->getZExtValue(); 711 Value *Dest = II->getArgOperand(0); 712 if (isNoAlias(Location(Dest, Len), Loc)) 713 return NoModRef; 714 } 715 break; 716 case Intrinsic::atomic_cmp_swap: 717 case Intrinsic::atomic_swap: 718 case Intrinsic::atomic_load_add: 719 case Intrinsic::atomic_load_sub: 720 case Intrinsic::atomic_load_and: 721 case Intrinsic::atomic_load_nand: 722 case Intrinsic::atomic_load_or: 723 case Intrinsic::atomic_load_xor: 724 case Intrinsic::atomic_load_max: 725 case Intrinsic::atomic_load_min: 726 case Intrinsic::atomic_load_umax: 727 case Intrinsic::atomic_load_umin: 728 if (TD) { 729 Value *Op1 = II->getArgOperand(0); 730 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType()); 731 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa); 732 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc)) 733 return NoModRef; 734 } 735 break; 736 case Intrinsic::lifetime_start: 737 case Intrinsic::lifetime_end: 738 case Intrinsic::invariant_start: { 739 uint64_t PtrSize = 740 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 741 if (isNoAlias(Location(II->getArgOperand(1), 742 PtrSize, 743 II->getMetadata(LLVMContext::MD_tbaa)), 744 Loc)) 745 return NoModRef; 746 break; 747 } 748 case Intrinsic::invariant_end: { 749 uint64_t PtrSize = 750 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 751 if (isNoAlias(Location(II->getArgOperand(2), 752 PtrSize, 753 II->getMetadata(LLVMContext::MD_tbaa)), 754 Loc)) 755 return NoModRef; 756 break; 757 } 758 } 759 760 // The AliasAnalysis base class has some smarts, lets use them. 761 return AliasAnalysis::getModRefInfo(CS, Loc); 762} 763 764/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 765/// against another pointer. We know that V1 is a GEP, but we don't know 766/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 767/// UnderlyingV2 is the same for V2. 768/// 769AliasAnalysis::AliasResult 770BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size, 771 const Value *V2, uint64_t V2Size, 772 const MDNode *V2TBAAInfo, 773 const Value *UnderlyingV1, 774 const Value *UnderlyingV2) { 775 // If this GEP has been visited before, we're on a use-def cycle. 776 // Such cycles are only valid when PHI nodes are involved or in unreachable 777 // code. The visitPHI function catches cycles containing PHIs, but there 778 // could still be a cycle without PHIs in unreachable code. 779 if (!Visited.insert(GEP1)) 780 return MayAlias; 781 782 int64_t GEP1BaseOffset; 783 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices; 784 785 // If we have two gep instructions with must-alias'ing base pointers, figure 786 // out if the indexes to the GEP tell us anything about the derived pointer. 787 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 788 // Do the base pointers alias? 789 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0, 790 UnderlyingV2, UnknownSize, 0); 791 792 // If we get a No or May, then return it immediately, no amount of analysis 793 // will improve this situation. 794 if (BaseAlias != MustAlias) return BaseAlias; 795 796 // Otherwise, we have a MustAlias. Since the base pointers alias each other 797 // exactly, see if the computed offset from the common pointer tells us 798 // about the relation of the resulting pointer. 799 const Value *GEP1BasePtr = 800 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 801 802 int64_t GEP2BaseOffset; 803 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 804 const Value *GEP2BasePtr = 805 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 806 807 // If DecomposeGEPExpression isn't able to look all the way through the 808 // addressing operation, we must not have TD and this is too complex for us 809 // to handle without it. 810 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 811 assert(TD == 0 && 812 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 813 return MayAlias; 814 } 815 816 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 817 // symbolic difference. 818 GEP1BaseOffset -= GEP2BaseOffset; 819 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 820 821 } else { 822 // Check to see if these two pointers are related by the getelementptr 823 // instruction. If one pointer is a GEP with a non-zero index of the other 824 // pointer, we know they cannot alias. 825 826 // If both accesses are unknown size, we can't do anything useful here. 827 if (V1Size == UnknownSize && V2Size == UnknownSize) 828 return MayAlias; 829 830 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0, 831 V2, V2Size, V2TBAAInfo); 832 if (R != MustAlias) 833 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 834 // If V2 is known not to alias GEP base pointer, then the two values 835 // cannot alias per GEP semantics: "A pointer value formed from a 836 // getelementptr instruction is associated with the addresses associated 837 // with the first operand of the getelementptr". 838 return R; 839 840 const Value *GEP1BasePtr = 841 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 842 843 // If DecomposeGEPExpression isn't able to look all the way through the 844 // addressing operation, we must not have TD and this is too complex for us 845 // to handle without it. 846 if (GEP1BasePtr != UnderlyingV1) { 847 assert(TD == 0 && 848 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 849 return MayAlias; 850 } 851 } 852 853 // In the two GEP Case, if there is no difference in the offsets of the 854 // computed pointers, the resultant pointers are a must alias. This 855 // hapens when we have two lexically identical GEP's (for example). 856 // 857 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 858 // must aliases the GEP, the end result is a must alias also. 859 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 860 return MustAlias; 861 862 // If we have a known constant offset, see if this offset is larger than the 863 // access size being queried. If so, and if no variable indices can remove 864 // pieces of this constant, then we know we have a no-alias. For example, 865 // &A[100] != &A. 866 867 // In order to handle cases like &A[100][i] where i is an out of range 868 // subscript, we have to ignore all constant offset pieces that are a multiple 869 // of a scaled index. Do this by removing constant offsets that are a 870 // multiple of any of our variable indices. This allows us to transform 871 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 872 // provides an offset of 4 bytes (assuming a <= 4 byte access). 873 for (unsigned i = 0, e = GEP1VariableIndices.size(); 874 i != e && GEP1BaseOffset;++i) 875 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale) 876 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale; 877 878 // If our known offset is bigger than the access size, we know we don't have 879 // an alias. 880 if (GEP1BaseOffset) { 881 if (GEP1BaseOffset >= 0 ? 882 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) : 883 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size && 884 GEP1BaseOffset != INT64_MIN)) 885 return NoAlias; 886 } 887 888 return MayAlias; 889} 890 891/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 892/// instruction against another. 893AliasAnalysis::AliasResult 894BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize, 895 const MDNode *SITBAAInfo, 896 const Value *V2, uint64_t V2Size, 897 const MDNode *V2TBAAInfo) { 898 // If this select has been visited before, we're on a use-def cycle. 899 // Such cycles are only valid when PHI nodes are involved or in unreachable 900 // code. The visitPHI function catches cycles containing PHIs, but there 901 // could still be a cycle without PHIs in unreachable code. 902 if (!Visited.insert(SI)) 903 return MayAlias; 904 905 // If the values are Selects with the same condition, we can do a more precise 906 // check: just check for aliases between the values on corresponding arms. 907 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 908 if (SI->getCondition() == SI2->getCondition()) { 909 AliasResult Alias = 910 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo, 911 SI2->getTrueValue(), V2Size, V2TBAAInfo); 912 if (Alias == MayAlias) 913 return MayAlias; 914 AliasResult ThisAlias = 915 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo, 916 SI2->getFalseValue(), V2Size, V2TBAAInfo); 917 if (ThisAlias != Alias) 918 return MayAlias; 919 return Alias; 920 } 921 922 // If both arms of the Select node NoAlias or MustAlias V2, then returns 923 // NoAlias / MustAlias. Otherwise, returns MayAlias. 924 AliasResult Alias = 925 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo); 926 if (Alias == MayAlias) 927 return MayAlias; 928 929 // If V2 is visited, the recursive case will have been caught in the 930 // above aliasCheck call, so these subsequent calls to aliasCheck 931 // don't need to assume that V2 is being visited recursively. 932 Visited.erase(V2); 933 934 AliasResult ThisAlias = 935 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo); 936 if (ThisAlias != Alias) 937 return MayAlias; 938 return Alias; 939} 940 941// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 942// against another. 943AliasAnalysis::AliasResult 944BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize, 945 const MDNode *PNTBAAInfo, 946 const Value *V2, uint64_t V2Size, 947 const MDNode *V2TBAAInfo) { 948 // The PHI node has already been visited, avoid recursion any further. 949 if (!Visited.insert(PN)) 950 return MayAlias; 951 952 // If the values are PHIs in the same block, we can do a more precise 953 // as well as efficient check: just check for aliases between the values 954 // on corresponding edges. 955 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 956 if (PN2->getParent() == PN->getParent()) { 957 AliasResult Alias = 958 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo, 959 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 960 V2Size, V2TBAAInfo); 961 if (Alias == MayAlias) 962 return MayAlias; 963 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 964 AliasResult ThisAlias = 965 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo, 966 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 967 V2Size, V2TBAAInfo); 968 if (ThisAlias != Alias) 969 return MayAlias; 970 } 971 return Alias; 972 } 973 974 SmallPtrSet<Value*, 4> UniqueSrc; 975 SmallVector<Value*, 4> V1Srcs; 976 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 977 Value *PV1 = PN->getIncomingValue(i); 978 if (isa<PHINode>(PV1)) 979 // If any of the source itself is a PHI, return MayAlias conservatively 980 // to avoid compile time explosion. The worst possible case is if both 981 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 982 // and 'n' are the number of PHI sources. 983 return MayAlias; 984 if (UniqueSrc.insert(PV1)) 985 V1Srcs.push_back(PV1); 986 } 987 988 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo, 989 V1Srcs[0], PNSize, PNTBAAInfo); 990 // Early exit if the check of the first PHI source against V2 is MayAlias. 991 // Other results are not possible. 992 if (Alias == MayAlias) 993 return MayAlias; 994 995 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 996 // NoAlias / MustAlias. Otherwise, returns MayAlias. 997 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 998 Value *V = V1Srcs[i]; 999 1000 // If V2 is visited, the recursive case will have been caught in the 1001 // above aliasCheck call, so these subsequent calls to aliasCheck 1002 // don't need to assume that V2 is being visited recursively. 1003 Visited.erase(V2); 1004 1005 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo, 1006 V, PNSize, PNTBAAInfo); 1007 if (ThisAlias != Alias || ThisAlias == MayAlias) 1008 return MayAlias; 1009 } 1010 1011 return Alias; 1012} 1013 1014// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 1015// such as array references. 1016// 1017AliasAnalysis::AliasResult 1018BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size, 1019 const MDNode *V1TBAAInfo, 1020 const Value *V2, uint64_t V2Size, 1021 const MDNode *V2TBAAInfo) { 1022 // If either of the memory references is empty, it doesn't matter what the 1023 // pointer values are. 1024 if (V1Size == 0 || V2Size == 0) 1025 return NoAlias; 1026 1027 // Strip off any casts if they exist. 1028 V1 = V1->stripPointerCasts(); 1029 V2 = V2->stripPointerCasts(); 1030 1031 // Are we checking for alias of the same value? 1032 if (V1 == V2) return MustAlias; 1033 1034 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 1035 return NoAlias; // Scalars cannot alias each other 1036 1037 // Figure out what objects these things are pointing to if we can. 1038 const Value *O1 = V1->getUnderlyingObject(); 1039 const Value *O2 = V2->getUnderlyingObject(); 1040 1041 // Null values in the default address space don't point to any object, so they 1042 // don't alias any other pointer. 1043 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 1044 if (CPN->getType()->getAddressSpace() == 0) 1045 return NoAlias; 1046 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 1047 if (CPN->getType()->getAddressSpace() == 0) 1048 return NoAlias; 1049 1050 if (O1 != O2) { 1051 // If V1/V2 point to two different objects we know that we have no alias. 1052 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 1053 return NoAlias; 1054 1055 // Constant pointers can't alias with non-const isIdentifiedObject objects. 1056 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 1057 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 1058 return NoAlias; 1059 1060 // Arguments can't alias with local allocations or noalias calls 1061 // in the same function. 1062 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 1063 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 1064 return NoAlias; 1065 1066 // Most objects can't alias null. 1067 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 1068 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 1069 return NoAlias; 1070 1071 // If one pointer is the result of a call/invoke or load and the other is a 1072 // non-escaping local object within the same function, then we know the 1073 // object couldn't escape to a point where the call could return it. 1074 // 1075 // Note that if the pointers are in different functions, there are a 1076 // variety of complications. A call with a nocapture argument may still 1077 // temporary store the nocapture argument's value in a temporary memory 1078 // location if that memory location doesn't escape. Or it may pass a 1079 // nocapture value to other functions as long as they don't capture it. 1080 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 1081 return NoAlias; 1082 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 1083 return NoAlias; 1084 } 1085 1086 // If the size of one access is larger than the entire object on the other 1087 // side, then we know such behavior is undefined and can assume no alias. 1088 if (TD) 1089 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 1090 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 1091 return NoAlias; 1092 1093 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 1094 // GEP can't simplify, we don't even look at the PHI cases. 1095 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 1096 std::swap(V1, V2); 1097 std::swap(V1Size, V2Size); 1098 std::swap(O1, O2); 1099 } 1100 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) { 1101 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2); 1102 if (Result != MayAlias) return Result; 1103 } 1104 1105 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 1106 std::swap(V1, V2); 1107 std::swap(V1Size, V2Size); 1108 } 1109 if (const PHINode *PN = dyn_cast<PHINode>(V1)) { 1110 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo, 1111 V2, V2Size, V2TBAAInfo); 1112 if (Result != MayAlias) return Result; 1113 } 1114 1115 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 1116 std::swap(V1, V2); 1117 std::swap(V1Size, V2Size); 1118 } 1119 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) { 1120 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo, 1121 V2, V2Size, V2TBAAInfo); 1122 if (Result != MayAlias) return Result; 1123 } 1124 1125 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo), 1126 Location(V2, V2Size, V2TBAAInfo)); 1127} 1128 1129// Make sure that anything that uses AliasAnalysis pulls in this file. 1130DEFINING_FILE_FOR(BasicAliasAnalysis) 1131