BasicAliasAnalysis.cpp revision db4708cf86cece22539ff022cc0601612dd02ead
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/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// GetElementPtr Instruction Decomposition and Analysis 133//===----------------------------------------------------------------------===// 134 135namespace { 136 enum ExtensionKind { 137 EK_NotExtended, 138 EK_SignExt, 139 EK_ZeroExt 140 }; 141 142 struct VariableGEPIndex { 143 const Value *V; 144 ExtensionKind Extension; 145 int64_t Scale; 146 }; 147} 148 149 150/// GetLinearExpression - Analyze the specified value as a linear expression: 151/// "A*V + B", where A and B are constant integers. Return the scale and offset 152/// values as APInts and return V as a Value*, and return whether we looked 153/// through any sign or zero extends. The incoming Value is known to have 154/// IntegerType and it may already be sign or zero extended. 155/// 156/// Note that this looks through extends, so the high bits may not be 157/// represented in the result. 158static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset, 159 ExtensionKind &Extension, 160 const TargetData &TD, unsigned Depth) { 161 assert(V->getType()->isIntegerTy() && "Not an integer value"); 162 163 // Limit our recursion depth. 164 if (Depth == 6) { 165 Scale = 1; 166 Offset = 0; 167 return V; 168 } 169 170 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) { 171 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) { 172 switch (BOp->getOpcode()) { 173 default: break; 174 case Instruction::Or: 175 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't 176 // analyze it. 177 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD)) 178 break; 179 // FALL THROUGH. 180 case Instruction::Add: 181 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 182 TD, Depth+1); 183 Offset += RHSC->getValue(); 184 return V; 185 case Instruction::Mul: 186 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 187 TD, Depth+1); 188 Offset *= RHSC->getValue(); 189 Scale *= RHSC->getValue(); 190 return V; 191 case Instruction::Shl: 192 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 193 TD, Depth+1); 194 Offset <<= RHSC->getValue().getLimitedValue(); 195 Scale <<= RHSC->getValue().getLimitedValue(); 196 return V; 197 } 198 } 199 } 200 201 // Since GEP indices are sign extended anyway, we don't care about the high 202 // bits of a sign or zero extended value - just scales and offsets. The 203 // extensions have to be consistent though. 204 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) || 205 (isa<ZExtInst>(V) && Extension != EK_SignExt)) { 206 Value *CastOp = cast<CastInst>(V)->getOperand(0); 207 unsigned OldWidth = Scale.getBitWidth(); 208 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits(); 209 Scale.trunc(SmallWidth); 210 Offset.trunc(SmallWidth); 211 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt; 212 213 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, 214 TD, Depth+1); 215 Scale.zext(OldWidth); 216 Offset.zext(OldWidth); 217 218 return Result; 219 } 220 221 Scale = 1; 222 Offset = 0; 223 return V; 224} 225 226/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it 227/// into a base pointer with a constant offset and a number of scaled symbolic 228/// offsets. 229/// 230/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in 231/// the VarIndices vector) are Value*'s that are known to be scaled by the 232/// specified amount, but which may have other unrepresented high bits. As such, 233/// the gep cannot necessarily be reconstructed from its decomposed form. 234/// 235/// When TargetData is around, this function is capable of analyzing everything 236/// that Value::getUnderlyingObject() can look through. When not, it just looks 237/// through pointer casts. 238/// 239static const Value * 240DecomposeGEPExpression(const Value *V, int64_t &BaseOffs, 241 SmallVectorImpl<VariableGEPIndex> &VarIndices, 242 const TargetData *TD) { 243 // Limit recursion depth to limit compile time in crazy cases. 244 unsigned MaxLookup = 6; 245 246 BaseOffs = 0; 247 do { 248 // See if this is a bitcast or GEP. 249 const Operator *Op = dyn_cast<Operator>(V); 250 if (Op == 0) { 251 // The only non-operator case we can handle are GlobalAliases. 252 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 253 if (!GA->mayBeOverridden()) { 254 V = GA->getAliasee(); 255 continue; 256 } 257 } 258 return V; 259 } 260 261 if (Op->getOpcode() == Instruction::BitCast) { 262 V = Op->getOperand(0); 263 continue; 264 } 265 266 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op); 267 if (GEPOp == 0) 268 return V; 269 270 // Don't attempt to analyze GEPs over unsized objects. 271 if (!cast<PointerType>(GEPOp->getOperand(0)->getType()) 272 ->getElementType()->isSized()) 273 return V; 274 275 // If we are lacking TargetData information, we can't compute the offets of 276 // elements computed by GEPs. However, we can handle bitcast equivalent 277 // GEPs. 278 if (TD == 0) { 279 if (!GEPOp->hasAllZeroIndices()) 280 return V; 281 V = GEPOp->getOperand(0); 282 continue; 283 } 284 285 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices. 286 gep_type_iterator GTI = gep_type_begin(GEPOp); 287 for (User::const_op_iterator I = GEPOp->op_begin()+1, 288 E = GEPOp->op_end(); I != E; ++I) { 289 Value *Index = *I; 290 // Compute the (potentially symbolic) offset in bytes for this index. 291 if (const StructType *STy = dyn_cast<StructType>(*GTI++)) { 292 // For a struct, add the member offset. 293 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue(); 294 if (FieldNo == 0) continue; 295 296 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo); 297 continue; 298 } 299 300 // For an array/pointer, add the element offset, explicitly scaled. 301 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) { 302 if (CIdx->isZero()) continue; 303 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue(); 304 continue; 305 } 306 307 uint64_t Scale = TD->getTypeAllocSize(*GTI); 308 ExtensionKind Extension = EK_NotExtended; 309 310 // If the integer type is smaller than the pointer size, it is implicitly 311 // sign extended to pointer size. 312 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth(); 313 if (TD->getPointerSizeInBits() > Width) 314 Extension = EK_SignExt; 315 316 // Use GetLinearExpression to decompose the index into a C1*V+C2 form. 317 APInt IndexScale(Width, 0), IndexOffset(Width, 0); 318 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, 319 *TD, 0); 320 321 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale. 322 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale. 323 BaseOffs += IndexOffset.getSExtValue()*Scale; 324 Scale *= IndexScale.getSExtValue(); 325 326 327 // If we already had an occurrance of this index variable, merge this 328 // scale into it. For example, we want to handle: 329 // A[x][x] -> x*16 + x*4 -> x*20 330 // This also ensures that 'x' only appears in the index list once. 331 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) { 332 if (VarIndices[i].V == Index && 333 VarIndices[i].Extension == Extension) { 334 Scale += VarIndices[i].Scale; 335 VarIndices.erase(VarIndices.begin()+i); 336 break; 337 } 338 } 339 340 // Make sure that we have a scale that makes sense for this target's 341 // pointer size. 342 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) { 343 Scale <<= ShiftBits; 344 Scale = (int64_t)Scale >> ShiftBits; 345 } 346 347 if (Scale) { 348 VariableGEPIndex Entry = {Index, Extension, Scale}; 349 VarIndices.push_back(Entry); 350 } 351 } 352 353 // Analyze the base pointer next. 354 V = GEPOp->getOperand(0); 355 } while (--MaxLookup); 356 357 // If the chain of expressions is too deep, just return early. 358 return V; 359} 360 361/// GetIndexDifference - Dest and Src are the variable indices from two 362/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 363/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 364/// difference between the two pointers. 365static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest, 366 const SmallVectorImpl<VariableGEPIndex> &Src) { 367 if (Src.empty()) return; 368 369 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 370 const Value *V = Src[i].V; 371 ExtensionKind Extension = Src[i].Extension; 372 int64_t Scale = Src[i].Scale; 373 374 // Find V in Dest. This is N^2, but pointer indices almost never have more 375 // than a few variable indexes. 376 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 377 if (Dest[j].V != V || Dest[j].Extension != Extension) continue; 378 379 // If we found it, subtract off Scale V's from the entry in Dest. If it 380 // goes to zero, remove the entry. 381 if (Dest[j].Scale != Scale) 382 Dest[j].Scale -= Scale; 383 else 384 Dest.erase(Dest.begin()+j); 385 Scale = 0; 386 break; 387 } 388 389 // If we didn't consume this entry, add it to the end of the Dest list. 390 if (Scale) { 391 VariableGEPIndex Entry = { V, Extension, -Scale }; 392 Dest.push_back(Entry); 393 } 394 } 395} 396 397//===----------------------------------------------------------------------===// 398// BasicAliasAnalysis Pass 399//===----------------------------------------------------------------------===// 400 401#ifndef NDEBUG 402static const Function *getParent(const Value *V) { 403 if (const Instruction *inst = dyn_cast<Instruction>(V)) 404 return inst->getParent()->getParent(); 405 406 if (const Argument *arg = dyn_cast<Argument>(V)) 407 return arg->getParent(); 408 409 return NULL; 410} 411 412static bool notDifferentParent(const Value *O1, const Value *O2) { 413 414 const Function *F1 = getParent(O1); 415 const Function *F2 = getParent(O2); 416 417 return !F1 || !F2 || F1 == F2; 418} 419#endif 420 421namespace { 422 /// BasicAliasAnalysis - This is the primary alias analysis implementation. 423 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis { 424 static char ID; // Class identification, replacement for typeinfo 425 BasicAliasAnalysis() : ImmutablePass(ID) { 426 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry()); 427 } 428 429 virtual void initializePass() { 430 InitializeAliasAnalysis(this); 431 } 432 433 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 434 AU.addRequired<AliasAnalysis>(); 435 } 436 437 virtual AliasResult alias(const Location &LocA, 438 const Location &LocB) { 439 assert(Visited.empty() && "Visited must be cleared after use!"); 440 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) && 441 "BasicAliasAnalysis doesn't support interprocedural queries."); 442 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag, 443 LocB.Ptr, LocB.Size, LocB.TBAATag); 444 Visited.clear(); 445 return Alias; 446 } 447 448 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 449 const Location &Loc); 450 451 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 452 ImmutableCallSite CS2) { 453 // The AliasAnalysis base class has some smarts, lets use them. 454 return AliasAnalysis::getModRefInfo(CS1, CS2); 455 } 456 457 /// pointsToConstantMemory - Chase pointers until we find a (constant 458 /// global) or not. 459 virtual bool pointsToConstantMemory(const Location &Loc); 460 461 /// getModRefBehavior - Return the behavior when calling the given 462 /// call site. 463 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); 464 465 /// getModRefBehavior - Return the behavior when calling the given function. 466 /// For use when the call site is not known. 467 virtual ModRefBehavior getModRefBehavior(const Function *F); 468 469 /// getAdjustedAnalysisPointer - This method is used when a pass implements 470 /// an analysis interface through multiple inheritance. If needed, it 471 /// should override this to adjust the this pointer as needed for the 472 /// specified pass info. 473 virtual void *getAdjustedAnalysisPointer(const void *ID) { 474 if (ID == &AliasAnalysis::ID) 475 return (AliasAnalysis*)this; 476 return this; 477 } 478 479 private: 480 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP(). 481 SmallPtrSet<const Value*, 16> Visited; 482 483 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 484 // instruction against another. 485 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size, 486 const Value *V2, uint64_t V2Size, 487 const MDNode *V2TBAAInfo, 488 const Value *UnderlyingV1, const Value *UnderlyingV2); 489 490 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 491 // instruction against another. 492 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize, 493 const MDNode *PNTBAAInfo, 494 const Value *V2, uint64_t V2Size, 495 const MDNode *V2TBAAInfo); 496 497 /// aliasSelect - Disambiguate a Select instruction against another value. 498 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize, 499 const MDNode *SITBAAInfo, 500 const Value *V2, uint64_t V2Size, 501 const MDNode *V2TBAAInfo); 502 503 AliasResult aliasCheck(const Value *V1, uint64_t V1Size, 504 const MDNode *V1TBAATag, 505 const Value *V2, uint64_t V2Size, 506 const MDNode *V2TBAATag); 507 }; 508} // End of anonymous namespace 509 510// Register this pass... 511char BasicAliasAnalysis::ID = 0; 512INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa", 513 "Basic Alias Analysis (stateless AA impl)", 514 false, true, false) 515 516ImmutablePass *llvm::createBasicAliasAnalysisPass() { 517 return new BasicAliasAnalysis(); 518} 519 520 521/// pointsToConstantMemory - Chase pointers until we find a (constant 522/// global) or not. 523bool BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc) { 524 if (const GlobalVariable *GV = 525 dyn_cast<GlobalVariable>(Loc.Ptr->getUnderlyingObject())) 526 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 527 // global to be marked constant in some modules and non-constant in others. 528 // GV may even be a declaration, not a definition. 529 return GV->isConstant(); 530 531 return AliasAnalysis::pointsToConstantMemory(Loc); 532} 533 534/// getModRefBehavior - Return the behavior when calling the given call site. 535AliasAnalysis::ModRefBehavior 536BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 537 if (CS.doesNotAccessMemory()) 538 // Can't do better than this. 539 return DoesNotAccessMemory; 540 541 ModRefBehavior Min = UnknownModRefBehavior; 542 543 // If the callsite knows it only reads memory, don't return worse 544 // than that. 545 if (CS.onlyReadsMemory()) 546 Min = OnlyReadsMemory; 547 548 // The AliasAnalysis base class has some smarts, lets use them. 549 return std::min(AliasAnalysis::getModRefBehavior(CS), Min); 550} 551 552/// getModRefBehavior - Return the behavior when calling the given function. 553/// For use when the call site is not known. 554AliasAnalysis::ModRefBehavior 555BasicAliasAnalysis::getModRefBehavior(const Function *F) { 556 if (F->doesNotAccessMemory()) 557 // Can't do better than this. 558 return DoesNotAccessMemory; 559 if (F->onlyReadsMemory()) 560 return OnlyReadsMemory; 561 if (unsigned id = F->getIntrinsicID()) 562 return getIntrinsicModRefBehavior(id); 563 564 return AliasAnalysis::getModRefBehavior(F); 565} 566 567/// getModRefInfo - Check to see if the specified callsite can clobber the 568/// specified memory object. Since we only look at local properties of this 569/// function, we really can't say much about this query. We do, however, use 570/// simple "address taken" analysis on local objects. 571AliasAnalysis::ModRefResult 572BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 573 const Location &Loc) { 574 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) && 575 "AliasAnalysis query involving multiple functions!"); 576 577 const Value *Object = Loc.Ptr->getUnderlyingObject(); 578 579 // If this is a tail call and Loc.Ptr points to a stack location, we know that 580 // the tail call cannot access or modify the local stack. 581 // We cannot exclude byval arguments here; these belong to the caller of 582 // the current function not to the current function, and a tail callee 583 // may reference them. 584 if (isa<AllocaInst>(Object)) 585 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 586 if (CI->isTailCall()) 587 return NoModRef; 588 589 // If the pointer is to a locally allocated object that does not escape, 590 // then the call can not mod/ref the pointer unless the call takes the pointer 591 // as an argument, and itself doesn't capture it. 592 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 593 isNonEscapingLocalObject(Object)) { 594 bool PassedAsArg = false; 595 unsigned ArgNo = 0; 596 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 597 CI != CE; ++CI, ++ArgNo) { 598 // Only look at the no-capture pointer arguments. 599 if (!(*CI)->getType()->isPointerTy() || 600 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) 601 continue; 602 603 // If this is a no-capture pointer argument, see if we can tell that it 604 // is impossible to alias the pointer we're checking. If not, we have to 605 // assume that the call could touch the pointer, even though it doesn't 606 // escape. 607 if (!isNoAlias(Location(cast<Value>(CI)), Loc)) { 608 PassedAsArg = true; 609 break; 610 } 611 } 612 613 if (!PassedAsArg) 614 return NoModRef; 615 } 616 617 // Finally, handle specific knowledge of intrinsics. 618 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 619 if (II != 0) 620 switch (II->getIntrinsicID()) { 621 default: break; 622 case Intrinsic::memcpy: 623 case Intrinsic::memmove: { 624 uint64_t Len = UnknownSize; 625 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 626 Len = LenCI->getZExtValue(); 627 Value *Dest = II->getArgOperand(0); 628 Value *Src = II->getArgOperand(1); 629 if (isNoAlias(Location(Dest, Len), Loc)) { 630 if (isNoAlias(Location(Src, Len), Loc)) 631 return NoModRef; 632 return Ref; 633 } 634 break; 635 } 636 case Intrinsic::memset: 637 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 638 // will handle it for the variable length case. 639 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 640 uint64_t Len = LenCI->getZExtValue(); 641 Value *Dest = II->getArgOperand(0); 642 if (isNoAlias(Location(Dest, Len), Loc)) 643 return NoModRef; 644 } 645 break; 646 case Intrinsic::atomic_cmp_swap: 647 case Intrinsic::atomic_swap: 648 case Intrinsic::atomic_load_add: 649 case Intrinsic::atomic_load_sub: 650 case Intrinsic::atomic_load_and: 651 case Intrinsic::atomic_load_nand: 652 case Intrinsic::atomic_load_or: 653 case Intrinsic::atomic_load_xor: 654 case Intrinsic::atomic_load_max: 655 case Intrinsic::atomic_load_min: 656 case Intrinsic::atomic_load_umax: 657 case Intrinsic::atomic_load_umin: 658 if (TD) { 659 Value *Op1 = II->getArgOperand(0); 660 uint64_t Op1Size = TD->getTypeStoreSize(Op1->getType()); 661 MDNode *Tag = II->getMetadata(LLVMContext::MD_tbaa); 662 if (isNoAlias(Location(Op1, Op1Size, Tag), Loc)) 663 return NoModRef; 664 } 665 break; 666 case Intrinsic::lifetime_start: 667 case Intrinsic::lifetime_end: 668 case Intrinsic::invariant_start: { 669 uint64_t PtrSize = 670 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 671 if (isNoAlias(Location(II->getArgOperand(1), 672 PtrSize, 673 II->getMetadata(LLVMContext::MD_tbaa)), 674 Loc)) 675 return NoModRef; 676 break; 677 } 678 case Intrinsic::invariant_end: { 679 uint64_t PtrSize = 680 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 681 if (isNoAlias(Location(II->getArgOperand(2), 682 PtrSize, 683 II->getMetadata(LLVMContext::MD_tbaa)), 684 Loc)) 685 return NoModRef; 686 break; 687 } 688 } 689 690 // The AliasAnalysis base class has some smarts, lets use them. 691 return AliasAnalysis::getModRefInfo(CS, Loc); 692} 693 694/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 695/// against another pointer. We know that V1 is a GEP, but we don't know 696/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 697/// UnderlyingV2 is the same for V2. 698/// 699AliasAnalysis::AliasResult 700BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size, 701 const Value *V2, uint64_t V2Size, 702 const MDNode *V2TBAAInfo, 703 const Value *UnderlyingV1, 704 const Value *UnderlyingV2) { 705 // If this GEP has been visited before, we're on a use-def cycle. 706 // Such cycles are only valid when PHI nodes are involved or in unreachable 707 // code. The visitPHI function catches cycles containing PHIs, but there 708 // could still be a cycle without PHIs in unreachable code. 709 if (!Visited.insert(GEP1)) 710 return MayAlias; 711 712 int64_t GEP1BaseOffset; 713 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices; 714 715 // If we have two gep instructions with must-alias'ing base pointers, figure 716 // out if the indexes to the GEP tell us anything about the derived pointer. 717 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 718 // Do the base pointers alias? 719 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0, 720 UnderlyingV2, UnknownSize, 0); 721 722 // If we get a No or May, then return it immediately, no amount of analysis 723 // will improve this situation. 724 if (BaseAlias != MustAlias) return BaseAlias; 725 726 // Otherwise, we have a MustAlias. Since the base pointers alias each other 727 // exactly, see if the computed offset from the common pointer tells us 728 // about the relation of the resulting pointer. 729 const Value *GEP1BasePtr = 730 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 731 732 int64_t GEP2BaseOffset; 733 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 734 const Value *GEP2BasePtr = 735 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 736 737 // If DecomposeGEPExpression isn't able to look all the way through the 738 // addressing operation, we must not have TD and this is too complex for us 739 // to handle without it. 740 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 741 assert(TD == 0 && 742 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 743 return MayAlias; 744 } 745 746 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 747 // symbolic difference. 748 GEP1BaseOffset -= GEP2BaseOffset; 749 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 750 751 } else { 752 // Check to see if these two pointers are related by the getelementptr 753 // instruction. If one pointer is a GEP with a non-zero index of the other 754 // pointer, we know they cannot alias. 755 756 // If both accesses are unknown size, we can't do anything useful here. 757 if (V1Size == UnknownSize && V2Size == UnknownSize) 758 return MayAlias; 759 760 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0, 761 V2, V2Size, V2TBAAInfo); 762 if (R != MustAlias) 763 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 764 // If V2 is known not to alias GEP base pointer, then the two values 765 // cannot alias per GEP semantics: "A pointer value formed from a 766 // getelementptr instruction is associated with the addresses associated 767 // with the first operand of the getelementptr". 768 return R; 769 770 const Value *GEP1BasePtr = 771 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 772 773 // If DecomposeGEPExpression isn't able to look all the way through the 774 // addressing operation, we must not have TD and this is too complex for us 775 // to handle without it. 776 if (GEP1BasePtr != UnderlyingV1) { 777 assert(TD == 0 && 778 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 779 return MayAlias; 780 } 781 } 782 783 // In the two GEP Case, if there is no difference in the offsets of the 784 // computed pointers, the resultant pointers are a must alias. This 785 // hapens when we have two lexically identical GEP's (for example). 786 // 787 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 788 // must aliases the GEP, the end result is a must alias also. 789 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 790 return MustAlias; 791 792 // If we have a known constant offset, see if this offset is larger than the 793 // access size being queried. If so, and if no variable indices can remove 794 // pieces of this constant, then we know we have a no-alias. For example, 795 // &A[100] != &A. 796 797 // In order to handle cases like &A[100][i] where i is an out of range 798 // subscript, we have to ignore all constant offset pieces that are a multiple 799 // of a scaled index. Do this by removing constant offsets that are a 800 // multiple of any of our variable indices. This allows us to transform 801 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 802 // provides an offset of 4 bytes (assuming a <= 4 byte access). 803 for (unsigned i = 0, e = GEP1VariableIndices.size(); 804 i != e && GEP1BaseOffset;++i) 805 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale) 806 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale; 807 808 // If our known offset is bigger than the access size, we know we don't have 809 // an alias. 810 if (GEP1BaseOffset) { 811 if (GEP1BaseOffset >= 0 ? 812 (V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) : 813 (V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size && 814 GEP1BaseOffset != INT64_MIN)) 815 return NoAlias; 816 } 817 818 return MayAlias; 819} 820 821/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 822/// instruction against another. 823AliasAnalysis::AliasResult 824BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize, 825 const MDNode *SITBAAInfo, 826 const Value *V2, uint64_t V2Size, 827 const MDNode *V2TBAAInfo) { 828 // If this select has been visited before, we're on a use-def cycle. 829 // Such cycles are only valid when PHI nodes are involved or in unreachable 830 // code. The visitPHI function catches cycles containing PHIs, but there 831 // could still be a cycle without PHIs in unreachable code. 832 if (!Visited.insert(SI)) 833 return MayAlias; 834 835 // If the values are Selects with the same condition, we can do a more precise 836 // check: just check for aliases between the values on corresponding arms. 837 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 838 if (SI->getCondition() == SI2->getCondition()) { 839 AliasResult Alias = 840 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo, 841 SI2->getTrueValue(), V2Size, V2TBAAInfo); 842 if (Alias == MayAlias) 843 return MayAlias; 844 AliasResult ThisAlias = 845 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo, 846 SI2->getFalseValue(), V2Size, V2TBAAInfo); 847 if (ThisAlias != Alias) 848 return MayAlias; 849 return Alias; 850 } 851 852 // If both arms of the Select node NoAlias or MustAlias V2, then returns 853 // NoAlias / MustAlias. Otherwise, returns MayAlias. 854 AliasResult Alias = 855 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo); 856 if (Alias == MayAlias) 857 return MayAlias; 858 859 // If V2 is visited, the recursive case will have been caught in the 860 // above aliasCheck call, so these subsequent calls to aliasCheck 861 // don't need to assume that V2 is being visited recursively. 862 Visited.erase(V2); 863 864 AliasResult ThisAlias = 865 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo); 866 if (ThisAlias != Alias) 867 return MayAlias; 868 return Alias; 869} 870 871// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 872// against another. 873AliasAnalysis::AliasResult 874BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize, 875 const MDNode *PNTBAAInfo, 876 const Value *V2, uint64_t V2Size, 877 const MDNode *V2TBAAInfo) { 878 // The PHI node has already been visited, avoid recursion any further. 879 if (!Visited.insert(PN)) 880 return MayAlias; 881 882 // If the values are PHIs in the same block, we can do a more precise 883 // as well as efficient check: just check for aliases between the values 884 // on corresponding edges. 885 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 886 if (PN2->getParent() == PN->getParent()) { 887 AliasResult Alias = 888 aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo, 889 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 890 V2Size, V2TBAAInfo); 891 if (Alias == MayAlias) 892 return MayAlias; 893 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 894 AliasResult ThisAlias = 895 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo, 896 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 897 V2Size, V2TBAAInfo); 898 if (ThisAlias != Alias) 899 return MayAlias; 900 } 901 return Alias; 902 } 903 904 SmallPtrSet<Value*, 4> UniqueSrc; 905 SmallVector<Value*, 4> V1Srcs; 906 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 907 Value *PV1 = PN->getIncomingValue(i); 908 if (isa<PHINode>(PV1)) 909 // If any of the source itself is a PHI, return MayAlias conservatively 910 // to avoid compile time explosion. The worst possible case is if both 911 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 912 // and 'n' are the number of PHI sources. 913 return MayAlias; 914 if (UniqueSrc.insert(PV1)) 915 V1Srcs.push_back(PV1); 916 } 917 918 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo, 919 V1Srcs[0], PNSize, PNTBAAInfo); 920 // Early exit if the check of the first PHI source against V2 is MayAlias. 921 // Other results are not possible. 922 if (Alias == MayAlias) 923 return MayAlias; 924 925 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 926 // NoAlias / MustAlias. Otherwise, returns MayAlias. 927 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 928 Value *V = V1Srcs[i]; 929 930 // If V2 is visited, the recursive case will have been caught in the 931 // above aliasCheck call, so these subsequent calls to aliasCheck 932 // don't need to assume that V2 is being visited recursively. 933 Visited.erase(V2); 934 935 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo, 936 V, PNSize, PNTBAAInfo); 937 if (ThisAlias != Alias || ThisAlias == MayAlias) 938 return MayAlias; 939 } 940 941 return Alias; 942} 943 944// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 945// such as array references. 946// 947AliasAnalysis::AliasResult 948BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size, 949 const MDNode *V1TBAAInfo, 950 const Value *V2, uint64_t V2Size, 951 const MDNode *V2TBAAInfo) { 952 // If either of the memory references is empty, it doesn't matter what the 953 // pointer values are. 954 if (V1Size == 0 || V2Size == 0) 955 return NoAlias; 956 957 // Strip off any casts if they exist. 958 V1 = V1->stripPointerCasts(); 959 V2 = V2->stripPointerCasts(); 960 961 // Are we checking for alias of the same value? 962 if (V1 == V2) return MustAlias; 963 964 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 965 return NoAlias; // Scalars cannot alias each other 966 967 // Figure out what objects these things are pointing to if we can. 968 const Value *O1 = V1->getUnderlyingObject(); 969 const Value *O2 = V2->getUnderlyingObject(); 970 971 // Null values in the default address space don't point to any object, so they 972 // don't alias any other pointer. 973 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 974 if (CPN->getType()->getAddressSpace() == 0) 975 return NoAlias; 976 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 977 if (CPN->getType()->getAddressSpace() == 0) 978 return NoAlias; 979 980 if (O1 != O2) { 981 // If V1/V2 point to two different objects we know that we have no alias. 982 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 983 return NoAlias; 984 985 // Constant pointers can't alias with non-const isIdentifiedObject objects. 986 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 987 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 988 return NoAlias; 989 990 // Arguments can't alias with local allocations or noalias calls 991 // in the same function. 992 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 993 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 994 return NoAlias; 995 996 // Most objects can't alias null. 997 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 998 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 999 return NoAlias; 1000 1001 // If one pointer is the result of a call/invoke or load and the other is a 1002 // non-escaping local object within the same function, then we know the 1003 // object couldn't escape to a point where the call could return it. 1004 // 1005 // Note that if the pointers are in different functions, there are a 1006 // variety of complications. A call with a nocapture argument may still 1007 // temporary store the nocapture argument's value in a temporary memory 1008 // location if that memory location doesn't escape. Or it may pass a 1009 // nocapture value to other functions as long as they don't capture it. 1010 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 1011 return NoAlias; 1012 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 1013 return NoAlias; 1014 } 1015 1016 // If the size of one access is larger than the entire object on the other 1017 // side, then we know such behavior is undefined and can assume no alias. 1018 if (TD) 1019 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 1020 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 1021 return NoAlias; 1022 1023 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 1024 // GEP can't simplify, we don't even look at the PHI cases. 1025 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 1026 std::swap(V1, V2); 1027 std::swap(V1Size, V2Size); 1028 std::swap(O1, O2); 1029 } 1030 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) { 1031 AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2); 1032 if (Result != MayAlias) return Result; 1033 } 1034 1035 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 1036 std::swap(V1, V2); 1037 std::swap(V1Size, V2Size); 1038 } 1039 if (const PHINode *PN = dyn_cast<PHINode>(V1)) { 1040 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo, 1041 V2, V2Size, V2TBAAInfo); 1042 if (Result != MayAlias) return Result; 1043 } 1044 1045 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 1046 std::swap(V1, V2); 1047 std::swap(V1Size, V2Size); 1048 } 1049 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) { 1050 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo, 1051 V2, V2Size, V2TBAAInfo); 1052 if (Result != MayAlias) return Result; 1053 } 1054 1055 return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo), 1056 Location(V2, V2Size, V2TBAAInfo)); 1057} 1058