BasicAliasAnalysis.cpp revision 223017
1//===- BasicAliasAnalysis.cpp - Stateless 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 primary stateless implementation of the 11// Alias Analysis interface that implements identities (two different 12// globals cannot alias, etc), but does no stateful 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/InstructionSimplify.h" 31#include "llvm/Analysis/ValueTracking.h" 32#include "llvm/Target/TargetData.h" 33#include "llvm/ADT/SmallPtrSet.h" 34#include "llvm/ADT/SmallVector.h" 35#include "llvm/Support/ErrorHandling.h" 36#include "llvm/Support/GetElementPtrTypeIterator.h" 37#include <algorithm> 38using namespace llvm; 39 40//===----------------------------------------------------------------------===// 41// Useful predicates 42//===----------------------------------------------------------------------===// 43 44/// isKnownNonNull - Return true if we know that the specified value is never 45/// null. 46static bool isKnownNonNull(const Value *V) { 47 // Alloca never returns null, malloc might. 48 if (isa<AllocaInst>(V)) return true; 49 50 // A byval argument is never null. 51 if (const Argument *A = dyn_cast<Argument>(V)) 52 return A->hasByValAttr(); 53 54 // Global values are not null unless extern weak. 55 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 56 return !GV->hasExternalWeakLinkage(); 57 return false; 58} 59 60/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 61/// object that never escapes from the function. 62static bool isNonEscapingLocalObject(const Value *V) { 63 // If this is a local allocation, check to see if it escapes. 64 if (isa<AllocaInst>(V) || isNoAliasCall(V)) 65 // Set StoreCaptures to True so that we can assume in our callers that the 66 // pointer is not the result of a load instruction. Currently 67 // PointerMayBeCaptured doesn't have any special analysis for the 68 // StoreCaptures=false case; if it did, our callers could be refined to be 69 // more precise. 70 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 71 72 // If this is an argument that corresponds to a byval or noalias argument, 73 // then it has not escaped before entering the function. Check if it escapes 74 // inside the function. 75 if (const Argument *A = dyn_cast<Argument>(V)) 76 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 77 // Don't bother analyzing arguments already known not to escape. 78 if (A->hasNoCaptureAttr()) 79 return true; 80 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 81 } 82 return false; 83} 84 85/// isEscapeSource - Return true if the pointer is one which would have 86/// been considered an escape by isNonEscapingLocalObject. 87static bool isEscapeSource(const Value *V) { 88 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V)) 89 return true; 90 91 // The load case works because isNonEscapingLocalObject considers all 92 // stores to be escapes (it passes true for the StoreCaptures argument 93 // to PointerMayBeCaptured). 94 if (isa<LoadInst>(V)) 95 return true; 96 97 return false; 98} 99 100/// getObjectSize - Return the size of the object specified by V, or 101/// UnknownSize if unknown. 102static uint64_t getObjectSize(const Value *V, const TargetData &TD) { 103 const Type *AccessTy; 104 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 105 if (!GV->hasDefinitiveInitializer()) 106 return AliasAnalysis::UnknownSize; 107 AccessTy = GV->getType()->getElementType(); 108 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) { 109 if (!AI->isArrayAllocation()) 110 AccessTy = AI->getType()->getElementType(); 111 else 112 return AliasAnalysis::UnknownSize; 113 } else if (const CallInst* CI = extractMallocCall(V)) { 114 if (!isArrayMalloc(V, &TD)) 115 // The size is the argument to the malloc call. 116 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0))) 117 return C->getZExtValue(); 118 return AliasAnalysis::UnknownSize; 119 } else if (const Argument *A = dyn_cast<Argument>(V)) { 120 if (A->hasByValAttr()) 121 AccessTy = cast<PointerType>(A->getType())->getElementType(); 122 else 123 return AliasAnalysis::UnknownSize; 124 } else { 125 return AliasAnalysis::UnknownSize; 126 } 127 128 if (AccessTy->isSized()) 129 return TD.getTypeAllocSize(AccessTy); 130 return AliasAnalysis::UnknownSize; 131} 132 133/// isObjectSmallerThan - Return true if we can prove that the object specified 134/// by V is smaller than Size. 135static bool isObjectSmallerThan(const Value *V, uint64_t Size, 136 const TargetData &TD) { 137 uint64_t ObjectSize = getObjectSize(V, TD); 138 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size; 139} 140 141/// isObjectSize - Return true if we can prove that the object specified 142/// by V has size Size. 143static bool isObjectSize(const Value *V, uint64_t Size, 144 const TargetData &TD) { 145 uint64_t ObjectSize = getObjectSize(V, TD); 146 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size; 147} 148 149//===----------------------------------------------------------------------===// 150// GetElementPtr Instruction Decomposition and Analysis 151//===----------------------------------------------------------------------===// 152 153namespace { 154 enum ExtensionKind { 155 EK_NotExtended, 156 EK_SignExt, 157 EK_ZeroExt 158 }; 159 160 struct VariableGEPIndex { 161 const Value *V; 162 ExtensionKind Extension; 163 int64_t Scale; 164 }; 165} 166 167 168/// GetLinearExpression - Analyze the specified value as a linear expression: 169/// "A*V + B", where A and B are constant integers. Return the scale and offset 170/// values as APInts and return V as a Value*, and return whether we looked 171/// through any sign or zero extends. The incoming Value is known to have 172/// IntegerType and it may already be sign or zero extended. 173/// 174/// Note that this looks through extends, so the high bits may not be 175/// represented in the result. 176static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset, 177 ExtensionKind &Extension, 178 const TargetData &TD, unsigned Depth) { 179 assert(V->getType()->isIntegerTy() && "Not an integer value"); 180 181 // Limit our recursion depth. 182 if (Depth == 6) { 183 Scale = 1; 184 Offset = 0; 185 return V; 186 } 187 188 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) { 189 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) { 190 switch (BOp->getOpcode()) { 191 default: break; 192 case Instruction::Or: 193 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't 194 // analyze it. 195 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD)) 196 break; 197 // FALL THROUGH. 198 case Instruction::Add: 199 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 200 TD, Depth+1); 201 Offset += RHSC->getValue(); 202 return V; 203 case Instruction::Mul: 204 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 205 TD, Depth+1); 206 Offset *= RHSC->getValue(); 207 Scale *= RHSC->getValue(); 208 return V; 209 case Instruction::Shl: 210 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 211 TD, Depth+1); 212 Offset <<= RHSC->getValue().getLimitedValue(); 213 Scale <<= RHSC->getValue().getLimitedValue(); 214 return V; 215 } 216 } 217 } 218 219 // Since GEP indices are sign extended anyway, we don't care about the high 220 // bits of a sign or zero extended value - just scales and offsets. The 221 // extensions have to be consistent though. 222 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) || 223 (isa<ZExtInst>(V) && Extension != EK_SignExt)) { 224 Value *CastOp = cast<CastInst>(V)->getOperand(0); 225 unsigned OldWidth = Scale.getBitWidth(); 226 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits(); 227 Scale = Scale.trunc(SmallWidth); 228 Offset = Offset.trunc(SmallWidth); 229 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt; 230 231 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, 232 TD, Depth+1); 233 Scale = Scale.zext(OldWidth); 234 Offset = Offset.zext(OldWidth); 235 236 return Result; 237 } 238 239 Scale = 1; 240 Offset = 0; 241 return V; 242} 243 244/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it 245/// into a base pointer with a constant offset and a number of scaled symbolic 246/// offsets. 247/// 248/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in 249/// the VarIndices vector) are Value*'s that are known to be scaled by the 250/// specified amount, but which may have other unrepresented high bits. As such, 251/// the gep cannot necessarily be reconstructed from its decomposed form. 252/// 253/// When TargetData is around, this function is capable of analyzing everything 254/// that GetUnderlyingObject can look through. When not, it just looks 255/// through pointer casts. 256/// 257static const Value * 258DecomposeGEPExpression(const Value *V, int64_t &BaseOffs, 259 SmallVectorImpl<VariableGEPIndex> &VarIndices, 260 const TargetData *TD) { 261 // Limit recursion depth to limit compile time in crazy cases. 262 unsigned MaxLookup = 6; 263 264 BaseOffs = 0; 265 do { 266 // See if this is a bitcast or GEP. 267 const Operator *Op = dyn_cast<Operator>(V); 268 if (Op == 0) { 269 // The only non-operator case we can handle are GlobalAliases. 270 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 271 if (!GA->mayBeOverridden()) { 272 V = GA->getAliasee(); 273 continue; 274 } 275 } 276 return V; 277 } 278 279 if (Op->getOpcode() == Instruction::BitCast) { 280 V = Op->getOperand(0); 281 continue; 282 } 283 284 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op); 285 if (GEPOp == 0) { 286 // If it's not a GEP, hand it off to SimplifyInstruction to see if it 287 // can come up with something. This matches what GetUnderlyingObject does. 288 if (const Instruction *I = dyn_cast<Instruction>(V)) 289 // TODO: Get a DominatorTree and use it here. 290 if (const Value *Simplified = 291 SimplifyInstruction(const_cast<Instruction *>(I), TD)) { 292 V = Simplified; 293 continue; 294 } 295 296 return V; 297 } 298 299 // Don't attempt to analyze GEPs over unsized objects. 300 if (!cast<PointerType>(GEPOp->getOperand(0)->getType()) 301 ->getElementType()->isSized()) 302 return V; 303 304 // If we are lacking TargetData information, we can't compute the offets of 305 // elements computed by GEPs. However, we can handle bitcast equivalent 306 // GEPs. 307 if (TD == 0) { 308 if (!GEPOp->hasAllZeroIndices()) 309 return V; 310 V = GEPOp->getOperand(0); 311 continue; 312 } 313 314 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices. 315 gep_type_iterator GTI = gep_type_begin(GEPOp); 316 for (User::const_op_iterator I = GEPOp->op_begin()+1, 317 E = GEPOp->op_end(); I != E; ++I) { 318 Value *Index = *I; 319 // Compute the (potentially symbolic) offset in bytes for this index. 320 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) { 321 // For a struct, add the member offset. 322 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue(); 323 if (FieldNo == 0) continue; 324 325 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo); 326 continue; 327 } 328 329 // For an array/pointer, add the element offset, explicitly scaled. 330 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) { 331 if (CIdx->isZero()) continue; 332 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue(); 333 continue; 334 } 335 336 uint64_t Scale = TD->getTypeAllocSize(*GTI); 337 ExtensionKind Extension = EK_NotExtended; 338 339 // If the integer type is smaller than the pointer size, it is implicitly 340 // sign extended to pointer size. 341 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth(); 342 if (TD->getPointerSizeInBits() > Width) 343 Extension = EK_SignExt; 344 345 // Use GetLinearExpression to decompose the index into a C1*V+C2 form. 346 APInt IndexScale(Width, 0), IndexOffset(Width, 0); 347 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, 348 *TD, 0); 349 350 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale. 351 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale. 352 BaseOffs += IndexOffset.getSExtValue()*Scale; 353 Scale *= IndexScale.getSExtValue(); 354 355 356 // If we already had an occurrence of this index variable, merge this 357 // scale into it. For example, we want to handle: 358 // A[x][x] -> x*16 + x*4 -> x*20 359 // This also ensures that 'x' only appears in the index list once. 360 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) { 361 if (VarIndices[i].V == Index && 362 VarIndices[i].Extension == Extension) { 363 Scale += VarIndices[i].Scale; 364 VarIndices.erase(VarIndices.begin()+i); 365 break; 366 } 367 } 368 369 // Make sure that we have a scale that makes sense for this target's 370 // pointer size. 371 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) { 372 Scale <<= ShiftBits; 373 Scale = (int64_t)Scale >> ShiftBits; 374 } 375 376 if (Scale) { 377 VariableGEPIndex Entry = {Index, Extension, Scale}; 378 VarIndices.push_back(Entry); 379 } 380 } 381 382 // Analyze the base pointer next. 383 V = GEPOp->getOperand(0); 384 } while (--MaxLookup); 385 386 // If the chain of expressions is too deep, just return early. 387 return V; 388} 389 390/// GetIndexDifference - Dest and Src are the variable indices from two 391/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 392/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 393/// difference between the two pointers. 394static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest, 395 const SmallVectorImpl<VariableGEPIndex> &Src) { 396 if (Src.empty()) return; 397 398 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 399 const Value *V = Src[i].V; 400 ExtensionKind Extension = Src[i].Extension; 401 int64_t Scale = Src[i].Scale; 402 403 // Find V in Dest. This is N^2, but pointer indices almost never have more 404 // than a few variable indexes. 405 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 406 if (Dest[j].V != V || Dest[j].Extension != Extension) continue; 407 408 // If we found it, subtract off Scale V's from the entry in Dest. If it 409 // goes to zero, remove the entry. 410 if (Dest[j].Scale != Scale) 411 Dest[j].Scale -= Scale; 412 else 413 Dest.erase(Dest.begin()+j); 414 Scale = 0; 415 break; 416 } 417 418 // If we didn't consume this entry, add it to the end of the Dest list. 419 if (Scale) { 420 VariableGEPIndex Entry = { V, Extension, -Scale }; 421 Dest.push_back(Entry); 422 } 423 } 424} 425 426//===----------------------------------------------------------------------===// 427// BasicAliasAnalysis Pass 428//===----------------------------------------------------------------------===// 429 430#ifndef NDEBUG 431static const Function *getParent(const Value *V) { 432 if (const Instruction *inst = dyn_cast<Instruction>(V)) 433 return inst->getParent()->getParent(); 434 435 if (const Argument *arg = dyn_cast<Argument>(V)) 436 return arg->getParent(); 437 438 return NULL; 439} 440 441static bool notDifferentParent(const Value *O1, const Value *O2) { 442 443 const Function *F1 = getParent(O1); 444 const Function *F2 = getParent(O2); 445 446 return !F1 || !F2 || F1 == F2; 447} 448#endif 449 450namespace { 451 /// BasicAliasAnalysis - This is the primary alias analysis implementation. 452 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis { 453 static char ID; // Class identification, replacement for typeinfo 454 BasicAliasAnalysis() : ImmutablePass(ID), 455 // AliasCache rarely has more than 1 or 2 elements, 456 // so start it off fairly small so that clear() 457 // doesn't have to tromp through 64 (the default) 458 // elements on each alias query. This really wants 459 // something like a SmallDenseMap. 460 AliasCache(8) { 461 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry()); 462 } 463 464 virtual void initializePass() { 465 InitializeAliasAnalysis(this); 466 } 467 468 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 469 AU.addRequired<AliasAnalysis>(); 470 } 471 472 virtual AliasResult alias(const Location &LocA, 473 const Location &LocB) { 474 assert(AliasCache.empty() && "AliasCache must be cleared after use!"); 475 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) && 476 "BasicAliasAnalysis doesn't support interprocedural queries."); 477 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag, 478 LocB.Ptr, LocB.Size, LocB.TBAATag); 479 AliasCache.clear(); 480 return Alias; 481 } 482 483 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 484 const Location &Loc); 485 486 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 487 ImmutableCallSite CS2) { 488 // The AliasAnalysis base class has some smarts, lets use them. 489 return AliasAnalysis::getModRefInfo(CS1, CS2); 490 } 491 492 /// pointsToConstantMemory - Chase pointers until we find a (constant 493 /// global) or not. 494 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal); 495 496 /// getModRefBehavior - Return the behavior when calling the given 497 /// call site. 498 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); 499 500 /// getModRefBehavior - Return the behavior when calling the given function. 501 /// For use when the call site is not known. 502 virtual ModRefBehavior getModRefBehavior(const Function *F); 503 504 /// getAdjustedAnalysisPointer - This method is used when a pass implements 505 /// an analysis interface through multiple inheritance. If needed, it 506 /// should override this to adjust the this pointer as needed for the 507 /// specified pass info. 508 virtual void *getAdjustedAnalysisPointer(const void *ID) { 509 if (ID == &AliasAnalysis::ID) 510 return (AliasAnalysis*)this; 511 return this; 512 } 513 514 private: 515 // AliasCache - Track alias queries to guard against recursion. 516 typedef std::pair<Location, Location> LocPair; 517 typedef DenseMap<LocPair, AliasResult> AliasCacheTy; 518 AliasCacheTy AliasCache; 519 520 // Visited - Track instructions visited by pointsToConstantMemory. 521 SmallPtrSet<const Value*, 16> Visited; 522 523 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 524 // instruction against another. 525 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size, 526 const Value *V2, uint64_t V2Size, 527 const MDNode *V2TBAAInfo, 528 const Value *UnderlyingV1, const Value *UnderlyingV2); 529 530 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 531 // instruction against another. 532 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize, 533 const MDNode *PNTBAAInfo, 534 const Value *V2, uint64_t V2Size, 535 const MDNode *V2TBAAInfo); 536 537 /// aliasSelect - Disambiguate a Select instruction against another value. 538 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize, 539 const MDNode *SITBAAInfo, 540 const Value *V2, uint64_t V2Size, 541 const MDNode *V2TBAAInfo); 542 543 AliasResult aliasCheck(const Value *V1, uint64_t V1Size, 544 const MDNode *V1TBAATag, 545 const Value *V2, uint64_t V2Size, 546 const MDNode *V2TBAATag); 547 }; 548} // End of anonymous namespace 549 550// Register this pass... 551char BasicAliasAnalysis::ID = 0; 552INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa", 553 "Basic Alias Analysis (stateless AA impl)", 554 false, true, false) 555 556ImmutablePass *llvm::createBasicAliasAnalysisPass() { 557 return new BasicAliasAnalysis(); 558} 559 560/// pointsToConstantMemory - Returns whether the given pointer value 561/// points to memory that is local to the function, with global constants being 562/// considered local to all functions. 563bool 564BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) { 565 assert(Visited.empty() && "Visited must be cleared after use!"); 566 567 unsigned MaxLookup = 8; 568 SmallVector<const Value *, 16> Worklist; 569 Worklist.push_back(Loc.Ptr); 570 do { 571 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), TD); 572 if (!Visited.insert(V)) { 573 Visited.clear(); 574 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 575 } 576 577 // An alloca instruction defines local memory. 578 if (OrLocal && isa<AllocaInst>(V)) 579 continue; 580 581 // A global constant counts as local memory for our purposes. 582 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 583 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 584 // global to be marked constant in some modules and non-constant in 585 // others. GV may even be a declaration, not a definition. 586 if (!GV->isConstant()) { 587 Visited.clear(); 588 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 589 } 590 continue; 591 } 592 593 // If both select values point to local memory, then so does the select. 594 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) { 595 Worklist.push_back(SI->getTrueValue()); 596 Worklist.push_back(SI->getFalseValue()); 597 continue; 598 } 599 600 // If all values incoming to a phi node point to local memory, then so does 601 // the phi. 602 if (const PHINode *PN = dyn_cast<PHINode>(V)) { 603 // Don't bother inspecting phi nodes with many operands. 604 if (PN->getNumIncomingValues() > MaxLookup) { 605 Visited.clear(); 606 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 607 } 608 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 609 Worklist.push_back(PN->getIncomingValue(i)); 610 continue; 611 } 612 613 // Otherwise be conservative. 614 Visited.clear(); 615 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 616 617 } while (!Worklist.empty() && --MaxLookup); 618 619 Visited.clear(); 620 return Worklist.empty(); 621} 622 623/// getModRefBehavior - Return the behavior when calling the given call site. 624AliasAnalysis::ModRefBehavior 625BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 626 if (CS.doesNotAccessMemory()) 627 // Can't do better than this. 628 return DoesNotAccessMemory; 629 630 ModRefBehavior Min = UnknownModRefBehavior; 631 632 // If the callsite knows it only reads memory, don't return worse 633 // than that. 634 if (CS.onlyReadsMemory()) 635 Min = OnlyReadsMemory; 636 637 // The AliasAnalysis base class has some smarts, lets use them. 638 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min); 639} 640 641/// getModRefBehavior - Return the behavior when calling the given function. 642/// For use when the call site is not known. 643AliasAnalysis::ModRefBehavior 644BasicAliasAnalysis::getModRefBehavior(const Function *F) { 645 // If the function declares it doesn't access memory, we can't do better. 646 if (F->doesNotAccessMemory()) 647 return DoesNotAccessMemory; 648 649 // For intrinsics, we can check the table. 650 if (unsigned iid = F->getIntrinsicID()) { 651#define GET_INTRINSIC_MODREF_BEHAVIOR 652#include "llvm/Intrinsics.gen" 653#undef GET_INTRINSIC_MODREF_BEHAVIOR 654 } 655 656 ModRefBehavior Min = UnknownModRefBehavior; 657 658 // If the function declares it only reads memory, go with that. 659 if (F->onlyReadsMemory()) 660 Min = OnlyReadsMemory; 661 662 // Otherwise be conservative. 663 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min); 664} 665 666/// getModRefInfo - Check to see if the specified callsite can clobber the 667/// specified memory object. Since we only look at local properties of this 668/// function, we really can't say much about this query. We do, however, use 669/// simple "address taken" analysis on local objects. 670AliasAnalysis::ModRefResult 671BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 672 const Location &Loc) { 673 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) && 674 "AliasAnalysis query involving multiple functions!"); 675 676 const Value *Object = GetUnderlyingObject(Loc.Ptr, TD); 677 678 // If this is a tail call and Loc.Ptr points to a stack location, we know that 679 // the tail call cannot access or modify the local stack. 680 // We cannot exclude byval arguments here; these belong to the caller of 681 // the current function not to the current function, and a tail callee 682 // may reference them. 683 if (isa<AllocaInst>(Object)) 684 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 685 if (CI->isTailCall()) 686 return NoModRef; 687 688 // If the pointer is to a locally allocated object that does not escape, 689 // then the call can not mod/ref the pointer unless the call takes the pointer 690 // as an argument, and itself doesn't capture it. 691 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 692 isNonEscapingLocalObject(Object)) { 693 bool PassedAsArg = false; 694 unsigned ArgNo = 0; 695 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 696 CI != CE; ++CI, ++ArgNo) { 697 // Only look at the no-capture or byval pointer arguments. If this 698 // pointer were passed to arguments that were neither of these, then it 699 // couldn't be no-capture. 700 if (!(*CI)->getType()->isPointerTy() || 701 (!CS.paramHasAttr(ArgNo+1, Attribute::NoCapture) && 702 !CS.paramHasAttr(ArgNo+1, Attribute::ByVal))) 703 continue; 704 705 // If this is a no-capture pointer argument, see if we can tell that it 706 // is impossible to alias the pointer we're checking. If not, we have to 707 // assume that the call could touch the pointer, even though it doesn't 708 // escape. 709 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) { 710 PassedAsArg = true; 711 break; 712 } 713 } 714 715 if (!PassedAsArg) 716 return NoModRef; 717 } 718 719 ModRefResult Min = ModRef; 720 721 // Finally, handle specific knowledge of intrinsics. 722 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 723 if (II != 0) 724 switch (II->getIntrinsicID()) { 725 default: break; 726 case Intrinsic::memcpy: 727 case Intrinsic::memmove: { 728 uint64_t Len = UnknownSize; 729 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 730 Len = LenCI->getZExtValue(); 731 Value *Dest = II->getArgOperand(0); 732 Value *Src = II->getArgOperand(1); 733 // If it can't overlap the source dest, then it doesn't modref the loc. 734 if (isNoAlias(Location(Dest, Len), Loc)) { 735 if (isNoAlias(Location(Src, Len), Loc)) 736 return NoModRef; 737 // If it can't overlap the dest, then worst case it reads the loc. 738 Min = Ref; 739 } else if (isNoAlias(Location(Src, Len), Loc)) { 740 // If it can't overlap the source, then worst case it mutates the loc. 741 Min = Mod; 742 } 743 break; 744 } 745 case Intrinsic::memset: 746 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 747 // will handle it for the variable length case. 748 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 749 uint64_t Len = LenCI->getZExtValue(); 750 Value *Dest = II->getArgOperand(0); 751 if (isNoAlias(Location(Dest, Len), Loc)) 752 return NoModRef; 753 } 754 // We know that memset doesn't load anything. 755 Min = Mod; 756 break; 757 case Intrinsic::atomic_cmp_swap: 758 case Intrinsic::atomic_swap: 759 case Intrinsic::atomic_load_add: 760 case Intrinsic::atomic_load_sub: 761 case Intrinsic::atomic_load_and: 762 case Intrinsic::atomic_load_nand: 763 case Intrinsic::atomic_load_or: 764 case Intrinsic::atomic_load_xor: 765 case Intrinsic::atomic_load_max: 766 case Intrinsic::atomic_load_min: 767 case Intrinsic::atomic_load_umax: 768 case Intrinsic::atomic_load_umin: 769 if (TD) { 770 Value *Op1 = II->getArgOperand(0); 771 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType()); 772 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa); 773 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc)) 774 return NoModRef; 775 } 776 break; 777 case Intrinsic::lifetime_start: 778 case Intrinsic::lifetime_end: 779 case Intrinsic::invariant_start: { 780 uint64_t PtrSize = 781 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 782 if (isNoAlias(Location(II->getArgOperand(1), 783 PtrSize, 784 II->getMetadata(LLVMContext::MD_tbaa)), 785 Loc)) 786 return NoModRef; 787 break; 788 } 789 case Intrinsic::invariant_end: { 790 uint64_t PtrSize = 791 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 792 if (isNoAlias(Location(II->getArgOperand(2), 793 PtrSize, 794 II->getMetadata(LLVMContext::MD_tbaa)), 795 Loc)) 796 return NoModRef; 797 break; 798 } 799 case Intrinsic::arm_neon_vld1: { 800 // LLVM's vld1 and vst1 intrinsics currently only support a single 801 // vector register. 802 uint64_t Size = 803 TD ? TD->getTypeStoreSize(II->getType()) : UnknownSize; 804 if (isNoAlias(Location(II->getArgOperand(0), Size, 805 II->getMetadata(LLVMContext::MD_tbaa)), 806 Loc)) 807 return NoModRef; 808 break; 809 } 810 case Intrinsic::arm_neon_vst1: { 811 uint64_t Size = 812 TD ? TD->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize; 813 if (isNoAlias(Location(II->getArgOperand(0), Size, 814 II->getMetadata(LLVMContext::MD_tbaa)), 815 Loc)) 816 return NoModRef; 817 break; 818 } 819 } 820 821 // The AliasAnalysis base class has some smarts, lets use them. 822 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min); 823} 824 825/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 826/// against another pointer. We know that V1 is a GEP, but we don't know 827/// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, TD), 828/// UnderlyingV2 is the same for V2. 829/// 830AliasAnalysis::AliasResult 831BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size, 832 const Value *V2, uint64_t V2Size, 833 const MDNode *V2TBAAInfo, 834 const Value *UnderlyingV1, 835 const Value *UnderlyingV2) { 836 int64_t GEP1BaseOffset; 837 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices; 838 839 // If we have two gep instructions with must-alias'ing base pointers, figure 840 // out if the indexes to the GEP tell us anything about the derived pointer. 841 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 842 // Do the base pointers alias? 843 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0, 844 UnderlyingV2, UnknownSize, 0); 845 846 // If we get a No or May, then return it immediately, no amount of analysis 847 // will improve this situation. 848 if (BaseAlias != MustAlias) return BaseAlias; 849 850 // Otherwise, we have a MustAlias. Since the base pointers alias each other 851 // exactly, see if the computed offset from the common pointer tells us 852 // about the relation of the resulting pointer. 853 const Value *GEP1BasePtr = 854 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 855 856 int64_t GEP2BaseOffset; 857 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 858 const Value *GEP2BasePtr = 859 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 860 861 // If DecomposeGEPExpression isn't able to look all the way through the 862 // addressing operation, we must not have TD and this is too complex for us 863 // to handle without it. 864 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 865 assert(TD == 0 && 866 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 867 return MayAlias; 868 } 869 870 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 871 // symbolic difference. 872 GEP1BaseOffset -= GEP2BaseOffset; 873 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 874 875 } else { 876 // Check to see if these two pointers are related by the getelementptr 877 // instruction. If one pointer is a GEP with a non-zero index of the other 878 // pointer, we know they cannot alias. 879 880 // If both accesses are unknown size, we can't do anything useful here. 881 if (V1Size == UnknownSize && V2Size == UnknownSize) 882 return MayAlias; 883 884 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0, 885 V2, V2Size, V2TBAAInfo); 886 if (R != MustAlias) 887 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 888 // If V2 is known not to alias GEP base pointer, then the two values 889 // cannot alias per GEP semantics: "A pointer value formed from a 890 // getelementptr instruction is associated with the addresses associated 891 // with the first operand of the getelementptr". 892 return R; 893 894 const Value *GEP1BasePtr = 895 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 896 897 // If DecomposeGEPExpression isn't able to look all the way through the 898 // addressing operation, we must not have TD and this is too complex for us 899 // to handle without it. 900 if (GEP1BasePtr != UnderlyingV1) { 901 assert(TD == 0 && 902 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 903 return MayAlias; 904 } 905 } 906 907 // In the two GEP Case, if there is no difference in the offsets of the 908 // computed pointers, the resultant pointers are a must alias. This 909 // hapens when we have two lexically identical GEP's (for example). 910 // 911 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 912 // must aliases the GEP, the end result is a must alias also. 913 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 914 return MustAlias; 915 916 // If there is a difference between the pointers, but the difference is 917 // less than the size of the associated memory object, then we know 918 // that the objects are partially overlapping. 919 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) { 920 if (GEP1BaseOffset >= 0 ? 921 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) : 922 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size && 923 GEP1BaseOffset != INT64_MIN)) 924 return PartialAlias; 925 } 926 927 // If we have a known constant offset, see if this offset is larger than the 928 // access size being queried. If so, and if no variable indices can remove 929 // pieces of this constant, then we know we have a no-alias. For example, 930 // &A[100] != &A. 931 932 // In order to handle cases like &A[100][i] where i is an out of range 933 // subscript, we have to ignore all constant offset pieces that are a multiple 934 // of a scaled index. Do this by removing constant offsets that are a 935 // multiple of any of our variable indices. This allows us to transform 936 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 937 // provides an offset of 4 bytes (assuming a <= 4 byte access). 938 for (unsigned i = 0, e = GEP1VariableIndices.size(); 939 i != e && GEP1BaseOffset;++i) 940 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale) 941 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale; 942 943 // If our known offset is bigger than the access size, we know we don't have 944 // an alias. 945 if (GEP1BaseOffset) { 946 if (GEP1BaseOffset >= 0 ? 947 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) : 948 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size && 949 GEP1BaseOffset != INT64_MIN)) 950 return NoAlias; 951 } 952 953 // Statically, we can see that the base objects are the same, but the 954 // pointers have dynamic offsets which we can't resolve. And none of our 955 // little tricks above worked. 956 // 957 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the 958 // practical effect of this is protecting TBAA in the case of dynamic 959 // indices into arrays of unions. An alternative way to solve this would 960 // be to have clang emit extra metadata for unions and/or union accesses. 961 // A union-specific solution wouldn't handle the problem for malloc'd 962 // memory however. 963 return PartialAlias; 964} 965 966static AliasAnalysis::AliasResult 967MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) { 968 // If the results agree, take it. 969 if (A == B) 970 return A; 971 // A mix of PartialAlias and MustAlias is PartialAlias. 972 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) || 973 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias)) 974 return AliasAnalysis::PartialAlias; 975 // Otherwise, we don't know anything. 976 return AliasAnalysis::MayAlias; 977} 978 979/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 980/// instruction against another. 981AliasAnalysis::AliasResult 982BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize, 983 const MDNode *SITBAAInfo, 984 const Value *V2, uint64_t V2Size, 985 const MDNode *V2TBAAInfo) { 986 // If the values are Selects with the same condition, we can do a more precise 987 // check: just check for aliases between the values on corresponding arms. 988 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 989 if (SI->getCondition() == SI2->getCondition()) { 990 AliasResult Alias = 991 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo, 992 SI2->getTrueValue(), V2Size, V2TBAAInfo); 993 if (Alias == MayAlias) 994 return MayAlias; 995 AliasResult ThisAlias = 996 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo, 997 SI2->getFalseValue(), V2Size, V2TBAAInfo); 998 return MergeAliasResults(ThisAlias, Alias); 999 } 1000 1001 // If both arms of the Select node NoAlias or MustAlias V2, then returns 1002 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1003 AliasResult Alias = 1004 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo); 1005 if (Alias == MayAlias) 1006 return MayAlias; 1007 1008 AliasResult ThisAlias = 1009 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo); 1010 return MergeAliasResults(ThisAlias, Alias); 1011} 1012 1013// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 1014// against another. 1015AliasAnalysis::AliasResult 1016BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize, 1017 const MDNode *PNTBAAInfo, 1018 const Value *V2, uint64_t V2Size, 1019 const MDNode *V2TBAAInfo) { 1020 // If the values are PHIs in the same block, we can do a more precise 1021 // as well as efficient check: just check for aliases between the values 1022 // on corresponding edges. 1023 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 1024 if (PN2->getParent() == PN->getParent()) { 1025 AliasResult Alias = 1026 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo, 1027 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 1028 V2Size, V2TBAAInfo); 1029 if (Alias == MayAlias) 1030 return MayAlias; 1031 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 1032 AliasResult ThisAlias = 1033 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo, 1034 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 1035 V2Size, V2TBAAInfo); 1036 Alias = MergeAliasResults(ThisAlias, Alias); 1037 if (Alias == MayAlias) 1038 break; 1039 } 1040 return Alias; 1041 } 1042 1043 SmallPtrSet<Value*, 4> UniqueSrc; 1044 SmallVector<Value*, 4> V1Srcs; 1045 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1046 Value *PV1 = PN->getIncomingValue(i); 1047 if (isa<PHINode>(PV1)) 1048 // If any of the source itself is a PHI, return MayAlias conservatively 1049 // to avoid compile time explosion. The worst possible case is if both 1050 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 1051 // and 'n' are the number of PHI sources. 1052 return MayAlias; 1053 if (UniqueSrc.insert(PV1)) 1054 V1Srcs.push_back(PV1); 1055 } 1056 1057 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo, 1058 V1Srcs[0], PNSize, PNTBAAInfo); 1059 // Early exit if the check of the first PHI source against V2 is MayAlias. 1060 // Other results are not possible. 1061 if (Alias == MayAlias) 1062 return MayAlias; 1063 1064 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 1065 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1066 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 1067 Value *V = V1Srcs[i]; 1068 1069 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo, 1070 V, PNSize, PNTBAAInfo); 1071 Alias = MergeAliasResults(ThisAlias, Alias); 1072 if (Alias == MayAlias) 1073 break; 1074 } 1075 1076 return Alias; 1077} 1078 1079// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 1080// such as array references. 1081// 1082AliasAnalysis::AliasResult 1083BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size, 1084 const MDNode *V1TBAAInfo, 1085 const Value *V2, uint64_t V2Size, 1086 const MDNode *V2TBAAInfo) { 1087 // If either of the memory references is empty, it doesn't matter what the 1088 // pointer values are. 1089 if (V1Size == 0 || V2Size == 0) 1090 return NoAlias; 1091 1092 // Strip off any casts if they exist. 1093 V1 = V1->stripPointerCasts(); 1094 V2 = V2->stripPointerCasts(); 1095 1096 // Are we checking for alias of the same value? 1097 if (V1 == V2) return MustAlias; 1098 1099 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 1100 return NoAlias; // Scalars cannot alias each other 1101 1102 // Figure out what objects these things are pointing to if we can. 1103 const Value *O1 = GetUnderlyingObject(V1, TD); 1104 const Value *O2 = GetUnderlyingObject(V2, TD); 1105 1106 // Null values in the default address space don't point to any object, so they 1107 // don't alias any other pointer. 1108 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 1109 if (CPN->getType()->getAddressSpace() == 0) 1110 return NoAlias; 1111 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 1112 if (CPN->getType()->getAddressSpace() == 0) 1113 return NoAlias; 1114 1115 if (O1 != O2) { 1116 // If V1/V2 point to two different objects we know that we have no alias. 1117 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 1118 return NoAlias; 1119 1120 // Constant pointers can't alias with non-const isIdentifiedObject objects. 1121 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 1122 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 1123 return NoAlias; 1124 1125 // Arguments can't alias with local allocations or noalias calls 1126 // in the same function. 1127 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 1128 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 1129 return NoAlias; 1130 1131 // Most objects can't alias null. 1132 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 1133 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 1134 return NoAlias; 1135 1136 // If one pointer is the result of a call/invoke or load and the other is a 1137 // non-escaping local object within the same function, then we know the 1138 // object couldn't escape to a point where the call could return it. 1139 // 1140 // Note that if the pointers are in different functions, there are a 1141 // variety of complications. A call with a nocapture argument may still 1142 // temporary store the nocapture argument's value in a temporary memory 1143 // location if that memory location doesn't escape. Or it may pass a 1144 // nocapture value to other functions as long as they don't capture it. 1145 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 1146 return NoAlias; 1147 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 1148 return NoAlias; 1149 } 1150 1151 // If the size of one access is larger than the entire object on the other 1152 // side, then we know such behavior is undefined and can assume no alias. 1153 if (TD) 1154 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 1155 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 1156 return NoAlias; 1157 1158 // Check the cache before climbing up use-def chains. This also terminates 1159 // otherwise infinitely recursive queries. 1160 LocPair Locs(Location(V1, V1Size, V1TBAAInfo), 1161 Location(V2, V2Size, V2TBAAInfo)); 1162 if (V1 > V2) 1163 std::swap(Locs.first, Locs.second); 1164 std::pair<AliasCacheTy::iterator, bool> Pair = 1165 AliasCache.insert(std::make_pair(Locs, MayAlias)); 1166 if (!Pair.second) 1167 return Pair.first->second; 1168 1169 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 1170 // GEP can't simplify, we don't even look at the PHI cases. 1171 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 1172 std::swap(V1, V2); 1173 std::swap(V1Size, V2Size); 1174 std::swap(O1, O2); 1175 } 1176 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) { 1177 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2); 1178 if (Result != MayAlias) return AliasCache[Locs] = Result; 1179 } 1180 1181 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 1182 std::swap(V1, V2); 1183 std::swap(V1Size, V2Size); 1184 } 1185 if (const PHINode *PN = dyn_cast<PHINode>(V1)) { 1186 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo, 1187 V2, V2Size, V2TBAAInfo); 1188 if (Result != MayAlias) return AliasCache[Locs] = Result; 1189 } 1190 1191 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 1192 std::swap(V1, V2); 1193 std::swap(V1Size, V2Size); 1194 } 1195 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) { 1196 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo, 1197 V2, V2Size, V2TBAAInfo); 1198 if (Result != MayAlias) return AliasCache[Locs] = Result; 1199 } 1200 1201 // If both pointers are pointing into the same object and one of them 1202 // accesses is accessing the entire object, then the accesses must 1203 // overlap in some way. 1204 if (TD && O1 == O2) 1205 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) || 1206 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD))) 1207 return AliasCache[Locs] = PartialAlias; 1208 1209 AliasResult Result = 1210 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo), 1211 Location(V2, V2Size, V2TBAAInfo)); 1212 return AliasCache[Locs] = Result; 1213} 1214