BasicAliasAnalysis.cpp revision 1f74590e9d1b9cf0f1f81a156efea73f76546e05
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/GlobalVariable.h" 22#include "llvm/Instructions.h" 23#include "llvm/IntrinsicInst.h" 24#include "llvm/Operator.h" 25#include "llvm/Pass.h" 26#include "llvm/Analysis/CaptureTracking.h" 27#include "llvm/Analysis/MemoryBuiltins.h" 28#include "llvm/Analysis/ValueTracking.h" 29#include "llvm/Target/TargetData.h" 30#include "llvm/ADT/SmallPtrSet.h" 31#include "llvm/ADT/SmallVector.h" 32#include "llvm/Support/ErrorHandling.h" 33#include <algorithm> 34using namespace llvm; 35 36//===----------------------------------------------------------------------===// 37// Useful predicates 38//===----------------------------------------------------------------------===// 39 40/// isKnownNonNull - Return true if we know that the specified value is never 41/// null. 42static bool isKnownNonNull(const Value *V) { 43 // Alloca never returns null, malloc might. 44 if (isa<AllocaInst>(V)) return true; 45 46 // A byval argument is never null. 47 if (const Argument *A = dyn_cast<Argument>(V)) 48 return A->hasByValAttr(); 49 50 // Global values are not null unless extern weak. 51 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 52 return !GV->hasExternalWeakLinkage(); 53 return false; 54} 55 56/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 57/// object that never escapes from the function. 58static bool isNonEscapingLocalObject(const Value *V) { 59 // If this is a local allocation, check to see if it escapes. 60 if (isa<AllocaInst>(V) || isNoAliasCall(V)) 61 // Set StoreCaptures to True so that we can assume in our callers that the 62 // pointer is not the result of a load instruction. Currently 63 // PointerMayBeCaptured doesn't have any special analysis for the 64 // StoreCaptures=false case; if it did, our callers could be refined to be 65 // more precise. 66 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 67 68 // If this is an argument that corresponds to a byval or noalias argument, 69 // then it has not escaped before entering the function. Check if it escapes 70 // inside the function. 71 if (const Argument *A = dyn_cast<Argument>(V)) 72 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 73 // Don't bother analyzing arguments already known not to escape. 74 if (A->hasNoCaptureAttr()) 75 return true; 76 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 77 } 78 return false; 79} 80 81/// isEscapeSource - Return true if the pointer is one which would have 82/// been considered an escape by isNonEscapingLocalObject. 83static bool isEscapeSource(const Value *V) { 84 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V)) 85 return true; 86 87 // The load case works because isNonEscapingLocalObject considers all 88 // stores to be escapes (it passes true for the StoreCaptures argument 89 // to PointerMayBeCaptured). 90 if (isa<LoadInst>(V)) 91 return true; 92 93 return false; 94} 95 96/// isObjectSmallerThan - Return true if we can prove that the object specified 97/// by V is smaller than Size. 98static bool isObjectSmallerThan(const Value *V, unsigned Size, 99 const TargetData &TD) { 100 const Type *AccessTy; 101 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 102 AccessTy = GV->getType()->getElementType(); 103 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) { 104 if (!AI->isArrayAllocation()) 105 AccessTy = AI->getType()->getElementType(); 106 else 107 return false; 108 } else if (const CallInst* CI = extractMallocCall(V)) { 109 if (!isArrayMalloc(V, &TD)) 110 // The size is the argument to the malloc call. 111 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0))) 112 return (C->getZExtValue() < Size); 113 return false; 114 } else if (const Argument *A = dyn_cast<Argument>(V)) { 115 if (A->hasByValAttr()) 116 AccessTy = cast<PointerType>(A->getType())->getElementType(); 117 else 118 return false; 119 } else { 120 return false; 121 } 122 123 if (AccessTy->isSized()) 124 return TD.getTypeAllocSize(AccessTy) < Size; 125 return false; 126} 127 128//===----------------------------------------------------------------------===// 129// NoAA Pass 130//===----------------------------------------------------------------------===// 131 132namespace { 133 /// NoAA - This class implements the -no-aa pass, which always returns "I 134 /// don't know" for alias queries. NoAA is unlike other alias analysis 135 /// implementations, in that it does not chain to a previous analysis. As 136 /// such it doesn't follow many of the rules that other alias analyses must. 137 /// 138 struct NoAA : public ImmutablePass, public AliasAnalysis { 139 static char ID; // Class identification, replacement for typeinfo 140 NoAA() : ImmutablePass(&ID) {} 141 explicit NoAA(void *PID) : ImmutablePass(PID) { } 142 143 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 144 } 145 146 virtual void initializePass() { 147 TD = getAnalysisIfAvailable<TargetData>(); 148 } 149 150 virtual AliasResult alias(const Value *V1, unsigned V1Size, 151 const Value *V2, unsigned V2Size) { 152 return MayAlias; 153 } 154 155 virtual bool pointsToConstantMemory(const Value *P) { return false; } 156 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 157 const Value *P, unsigned Size) { 158 return ModRef; 159 } 160 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 161 ImmutableCallSite CS2) { 162 return ModRef; 163 } 164 165 virtual void deleteValue(Value *V) {} 166 virtual void copyValue(Value *From, Value *To) {} 167 168 /// getAdjustedAnalysisPointer - This method is used when a pass implements 169 /// an analysis interface through multiple inheritance. If needed, it 170 /// should override this to adjust the this pointer as needed for the 171 /// specified pass info. 172 virtual void *getAdjustedAnalysisPointer(AnalysisID PI) { 173 if (PI == &AliasAnalysis::ID) 174 return (AliasAnalysis*)this; 175 return this; 176 } 177 }; 178} // End of anonymous namespace 179 180// Register this pass... 181char NoAA::ID = 0; 182INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa", 183 "No Alias Analysis (always returns 'may' alias)", 184 true, true, false); 185 186ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 187 188//===----------------------------------------------------------------------===// 189// BasicAliasAnalysis Pass 190//===----------------------------------------------------------------------===// 191 192#ifndef NDEBUG 193static const Function *getParent(const Value *V) { 194 if (const Instruction *inst = dyn_cast<Instruction>(V)) 195 return inst->getParent()->getParent(); 196 197 if (const Argument *arg = dyn_cast<Argument>(V)) 198 return arg->getParent(); 199 200 return NULL; 201} 202 203static bool notDifferentParent(const Value *O1, const Value *O2) { 204 205 const Function *F1 = getParent(O1); 206 const Function *F2 = getParent(O2); 207 208 return !F1 || !F2 || F1 == F2; 209} 210#endif 211 212namespace { 213 /// BasicAliasAnalysis - This is the default alias analysis implementation. 214 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 215 /// derives from the NoAA class. 216 struct BasicAliasAnalysis : public NoAA { 217 static char ID; // Class identification, replacement for typeinfo 218 BasicAliasAnalysis() : NoAA(&ID) {} 219 220 AliasResult alias(const Value *V1, unsigned V1Size, 221 const Value *V2, unsigned V2Size) { 222 assert(Visited.empty() && "Visited must be cleared after use!"); 223 assert(notDifferentParent(V1, V2) && 224 "BasicAliasAnalysis doesn't support interprocedural queries."); 225 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size); 226 Visited.clear(); 227 return Alias; 228 } 229 230 ModRefResult getModRefInfo(ImmutableCallSite CS, 231 const Value *P, unsigned Size); 232 ModRefResult getModRefInfo(ImmutableCallSite CS1, 233 ImmutableCallSite CS2); 234 235 /// pointsToConstantMemory - Chase pointers until we find a (constant 236 /// global) or not. 237 bool pointsToConstantMemory(const Value *P); 238 239 /// getAdjustedAnalysisPointer - This method is used when a pass implements 240 /// an analysis interface through multiple inheritance. If needed, it 241 /// should override this to adjust the this pointer as needed for the 242 /// specified pass info. 243 virtual void *getAdjustedAnalysisPointer(AnalysisID PI) { 244 if (PI == &AliasAnalysis::ID) 245 return (AliasAnalysis*)this; 246 return this; 247 } 248 249 private: 250 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP(). 251 SmallPtrSet<const Value*, 16> Visited; 252 253 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 254 // instruction against another. 255 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size, 256 const Value *V2, unsigned V2Size, 257 const Value *UnderlyingV1, const Value *UnderlyingV2); 258 259 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 260 // instruction against another. 261 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize, 262 const Value *V2, unsigned V2Size); 263 264 /// aliasSelect - Disambiguate a Select instruction against another value. 265 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize, 266 const Value *V2, unsigned V2Size); 267 268 AliasResult aliasCheck(const Value *V1, unsigned V1Size, 269 const Value *V2, unsigned V2Size); 270 }; 271} // End of anonymous namespace 272 273// Register this pass... 274char BasicAliasAnalysis::ID = 0; 275INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa", 276 "Basic Alias Analysis (default AA impl)", 277 false, true, true); 278 279ImmutablePass *llvm::createBasicAliasAnalysisPass() { 280 return new BasicAliasAnalysis(); 281} 282 283 284/// pointsToConstantMemory - Chase pointers until we find a (constant 285/// global) or not. 286bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 287 if (const GlobalVariable *GV = 288 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 289 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 290 // global to be marked constant in some modules and non-constant in others. 291 // GV may even be a declaration, not a definition. 292 return GV->isConstant(); 293 return false; 294} 295 296 297/// getModRefInfo - Check to see if the specified callsite can clobber the 298/// specified memory object. Since we only look at local properties of this 299/// function, we really can't say much about this query. We do, however, use 300/// simple "address taken" analysis on local objects. 301AliasAnalysis::ModRefResult 302BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 303 const Value *P, unsigned Size) { 304 assert(notDifferentParent(CS.getInstruction(), P) && 305 "AliasAnalysis query involving multiple functions!"); 306 307 const Value *Object = P->getUnderlyingObject(); 308 309 // If this is a tail call and P points to a stack location, we know that 310 // the tail call cannot access or modify the local stack. 311 // We cannot exclude byval arguments here; these belong to the caller of 312 // the current function not to the current function, and a tail callee 313 // may reference them. 314 if (isa<AllocaInst>(Object)) 315 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 316 if (CI->isTailCall()) 317 return NoModRef; 318 319 // If the pointer is to a locally allocated object that does not escape, 320 // then the call can not mod/ref the pointer unless the call takes the pointer 321 // as an argument, and itself doesn't capture it. 322 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 323 isNonEscapingLocalObject(Object)) { 324 bool PassedAsArg = false; 325 unsigned ArgNo = 0; 326 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 327 CI != CE; ++CI, ++ArgNo) { 328 // Only look at the no-capture pointer arguments. 329 if (!(*CI)->getType()->isPointerTy() || 330 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) 331 continue; 332 333 // If this is a no-capture pointer argument, see if we can tell that it 334 // is impossible to alias the pointer we're checking. If not, we have to 335 // assume that the call could touch the pointer, even though it doesn't 336 // escape. 337 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) { 338 PassedAsArg = true; 339 break; 340 } 341 } 342 343 if (!PassedAsArg) 344 return NoModRef; 345 } 346 347 // Finally, handle specific knowledge of intrinsics. 348 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 349 if (II == 0) 350 return AliasAnalysis::getModRefInfo(CS, P, Size); 351 352 switch (II->getIntrinsicID()) { 353 default: break; 354 case Intrinsic::memcpy: 355 case Intrinsic::memmove: { 356 unsigned Len = UnknownSize; 357 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 358 Len = LenCI->getZExtValue(); 359 Value *Dest = II->getArgOperand(0); 360 Value *Src = II->getArgOperand(1); 361 if (isNoAlias(Dest, Len, P, Size)) { 362 if (isNoAlias(Src, Len, P, Size)) 363 return NoModRef; 364 return Ref; 365 } 366 break; 367 } 368 case Intrinsic::memset: 369 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 370 // will handle it for the variable length case. 371 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 372 unsigned Len = LenCI->getZExtValue(); 373 Value *Dest = II->getArgOperand(0); 374 if (isNoAlias(Dest, Len, P, Size)) 375 return NoModRef; 376 } 377 break; 378 case Intrinsic::atomic_cmp_swap: 379 case Intrinsic::atomic_swap: 380 case Intrinsic::atomic_load_add: 381 case Intrinsic::atomic_load_sub: 382 case Intrinsic::atomic_load_and: 383 case Intrinsic::atomic_load_nand: 384 case Intrinsic::atomic_load_or: 385 case Intrinsic::atomic_load_xor: 386 case Intrinsic::atomic_load_max: 387 case Intrinsic::atomic_load_min: 388 case Intrinsic::atomic_load_umax: 389 case Intrinsic::atomic_load_umin: 390 if (TD) { 391 Value *Op1 = II->getArgOperand(0); 392 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType()); 393 if (isNoAlias(Op1, Op1Size, P, Size)) 394 return NoModRef; 395 } 396 break; 397 case Intrinsic::lifetime_start: 398 case Intrinsic::lifetime_end: 399 case Intrinsic::invariant_start: { 400 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 401 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size)) 402 return NoModRef; 403 break; 404 } 405 case Intrinsic::invariant_end: { 406 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 407 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size)) 408 return NoModRef; 409 break; 410 } 411 } 412 413 // The AliasAnalysis base class has some smarts, lets use them. 414 return AliasAnalysis::getModRefInfo(CS, P, Size); 415} 416 417 418AliasAnalysis::ModRefResult 419BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1, 420 ImmutableCallSite CS2) { 421 // If CS1 or CS2 are readnone, they don't interact. 422 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 423 if (CS1B == DoesNotAccessMemory) return NoModRef; 424 425 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 426 if (CS2B == DoesNotAccessMemory) return NoModRef; 427 428 // If they both only read from memory, there is no dependence. 429 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 430 return NoModRef; 431 432 AliasAnalysis::ModRefResult Mask = ModRef; 433 434 // If CS1 only reads memory, the only dependence on CS2 can be 435 // from CS1 reading memory written by CS2. 436 if (CS1B == OnlyReadsMemory) 437 Mask = ModRefResult(Mask & Ref); 438 439 // If CS2 only access memory through arguments, accumulate the mod/ref 440 // information from CS1's references to the memory referenced by 441 // CS2's arguments. 442 if (CS2B == AccessesArguments) { 443 AliasAnalysis::ModRefResult R = NoModRef; 444 for (ImmutableCallSite::arg_iterator 445 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) { 446 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask); 447 if (R == Mask) 448 break; 449 } 450 return R; 451 } 452 453 // If CS1 only accesses memory through arguments, check if CS2 references 454 // any of the memory referenced by CS1's arguments. If not, return NoModRef. 455 if (CS1B == AccessesArguments) { 456 AliasAnalysis::ModRefResult R = NoModRef; 457 for (ImmutableCallSite::arg_iterator 458 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) 459 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) { 460 R = Mask; 461 break; 462 } 463 if (R == NoModRef) 464 return R; 465 } 466 467 // Otherwise, fall back to NoAA (mod+ref). 468 return ModRefResult(NoAA::getModRefInfo(CS1, CS2) & Mask); 469} 470 471/// GetIndexDifference - Dest and Src are the variable indices from two 472/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 473/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 474/// difference between the two pointers. 475static void GetIndexDifference( 476 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest, 477 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) { 478 if (Src.empty()) return; 479 480 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 481 const Value *V = Src[i].first; 482 int64_t Scale = Src[i].second; 483 484 // Find V in Dest. This is N^2, but pointer indices almost never have more 485 // than a few variable indexes. 486 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 487 if (Dest[j].first != V) continue; 488 489 // If we found it, subtract off Scale V's from the entry in Dest. If it 490 // goes to zero, remove the entry. 491 if (Dest[j].second != Scale) 492 Dest[j].second -= Scale; 493 else 494 Dest.erase(Dest.begin()+j); 495 Scale = 0; 496 break; 497 } 498 499 // If we didn't consume this entry, add it to the end of the Dest list. 500 if (Scale) 501 Dest.push_back(std::make_pair(V, -Scale)); 502 } 503} 504 505/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 506/// against another pointer. We know that V1 is a GEP, but we don't know 507/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 508/// UnderlyingV2 is the same for V2. 509/// 510AliasAnalysis::AliasResult 511BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size, 512 const Value *V2, unsigned V2Size, 513 const Value *UnderlyingV1, 514 const Value *UnderlyingV2) { 515 // If this GEP has been visited before, we're on a use-def cycle. 516 // Such cycles are only valid when PHI nodes are involved or in unreachable 517 // code. The visitPHI function catches cycles containing PHIs, but there 518 // could still be a cycle without PHIs in unreachable code. 519 if (!Visited.insert(GEP1)) 520 return MayAlias; 521 522 int64_t GEP1BaseOffset; 523 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices; 524 525 // If we have two gep instructions with must-alias'ing base pointers, figure 526 // out if the indexes to the GEP tell us anything about the derived pointer. 527 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 528 // Do the base pointers alias? 529 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 530 UnderlyingV2, UnknownSize); 531 532 // If we get a No or May, then return it immediately, no amount of analysis 533 // will improve this situation. 534 if (BaseAlias != MustAlias) return BaseAlias; 535 536 // Otherwise, we have a MustAlias. Since the base pointers alias each other 537 // exactly, see if the computed offset from the common pointer tells us 538 // about the relation of the resulting pointer. 539 const Value *GEP1BasePtr = 540 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 541 542 int64_t GEP2BaseOffset; 543 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices; 544 const Value *GEP2BasePtr = 545 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 546 547 // If DecomposeGEPExpression isn't able to look all the way through the 548 // addressing operation, we must not have TD and this is too complex for us 549 // to handle without it. 550 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 551 assert(TD == 0 && 552 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 553 return MayAlias; 554 } 555 556 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 557 // symbolic difference. 558 GEP1BaseOffset -= GEP2BaseOffset; 559 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 560 561 } else { 562 // Check to see if these two pointers are related by the getelementptr 563 // instruction. If one pointer is a GEP with a non-zero index of the other 564 // pointer, we know they cannot alias. 565 566 // If both accesses are unknown size, we can't do anything useful here. 567 if (V1Size == UnknownSize && V2Size == UnknownSize) 568 return MayAlias; 569 570 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size); 571 if (R != MustAlias) 572 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 573 // If V2 is known not to alias GEP base pointer, then the two values 574 // cannot alias per GEP semantics: "A pointer value formed from a 575 // getelementptr instruction is associated with the addresses associated 576 // with the first operand of the getelementptr". 577 return R; 578 579 const Value *GEP1BasePtr = 580 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 581 582 // If DecomposeGEPExpression isn't able to look all the way through the 583 // addressing operation, we must not have TD and this is too complex for us 584 // to handle without it. 585 if (GEP1BasePtr != UnderlyingV1) { 586 assert(TD == 0 && 587 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 588 return MayAlias; 589 } 590 } 591 592 // In the two GEP Case, if there is no difference in the offsets of the 593 // computed pointers, the resultant pointers are a must alias. This 594 // hapens when we have two lexically identical GEP's (for example). 595 // 596 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 597 // must aliases the GEP, the end result is a must alias also. 598 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 599 return MustAlias; 600 601 // If we have a known constant offset, see if this offset is larger than the 602 // access size being queried. If so, and if no variable indices can remove 603 // pieces of this constant, then we know we have a no-alias. For example, 604 // &A[100] != &A. 605 606 // In order to handle cases like &A[100][i] where i is an out of range 607 // subscript, we have to ignore all constant offset pieces that are a multiple 608 // of a scaled index. Do this by removing constant offsets that are a 609 // multiple of any of our variable indices. This allows us to transform 610 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 611 // provides an offset of 4 bytes (assuming a <= 4 byte access). 612 for (unsigned i = 0, e = GEP1VariableIndices.size(); 613 i != e && GEP1BaseOffset;++i) 614 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second) 615 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second; 616 617 // If our known offset is bigger than the access size, we know we don't have 618 // an alias. 619 if (GEP1BaseOffset) { 620 if (GEP1BaseOffset >= (int64_t)V2Size || 621 GEP1BaseOffset <= -(int64_t)V1Size) 622 return NoAlias; 623 } 624 625 return MayAlias; 626} 627 628/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 629/// instruction against another. 630AliasAnalysis::AliasResult 631BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, 632 const Value *V2, unsigned V2Size) { 633 // If this select has been visited before, we're on a use-def cycle. 634 // Such cycles are only valid when PHI nodes are involved or in unreachable 635 // code. The visitPHI function catches cycles containing PHIs, but there 636 // could still be a cycle without PHIs in unreachable code. 637 if (!Visited.insert(SI)) 638 return MayAlias; 639 640 // If the values are Selects with the same condition, we can do a more precise 641 // check: just check for aliases between the values on corresponding arms. 642 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 643 if (SI->getCondition() == SI2->getCondition()) { 644 AliasResult Alias = 645 aliasCheck(SI->getTrueValue(), SISize, 646 SI2->getTrueValue(), V2Size); 647 if (Alias == MayAlias) 648 return MayAlias; 649 AliasResult ThisAlias = 650 aliasCheck(SI->getFalseValue(), SISize, 651 SI2->getFalseValue(), V2Size); 652 if (ThisAlias != Alias) 653 return MayAlias; 654 return Alias; 655 } 656 657 // If both arms of the Select node NoAlias or MustAlias V2, then returns 658 // NoAlias / MustAlias. Otherwise, returns MayAlias. 659 AliasResult Alias = 660 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize); 661 if (Alias == MayAlias) 662 return MayAlias; 663 664 // If V2 is visited, the recursive case will have been caught in the 665 // above aliasCheck call, so these subsequent calls to aliasCheck 666 // don't need to assume that V2 is being visited recursively. 667 Visited.erase(V2); 668 669 AliasResult ThisAlias = 670 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize); 671 if (ThisAlias != Alias) 672 return MayAlias; 673 return Alias; 674} 675 676// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 677// against another. 678AliasAnalysis::AliasResult 679BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, 680 const Value *V2, unsigned V2Size) { 681 // The PHI node has already been visited, avoid recursion any further. 682 if (!Visited.insert(PN)) 683 return MayAlias; 684 685 // If the values are PHIs in the same block, we can do a more precise 686 // as well as efficient check: just check for aliases between the values 687 // on corresponding edges. 688 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 689 if (PN2->getParent() == PN->getParent()) { 690 AliasResult Alias = 691 aliasCheck(PN->getIncomingValue(0), PNSize, 692 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 693 V2Size); 694 if (Alias == MayAlias) 695 return MayAlias; 696 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 697 AliasResult ThisAlias = 698 aliasCheck(PN->getIncomingValue(i), PNSize, 699 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 700 V2Size); 701 if (ThisAlias != Alias) 702 return MayAlias; 703 } 704 return Alias; 705 } 706 707 SmallPtrSet<Value*, 4> UniqueSrc; 708 SmallVector<Value*, 4> V1Srcs; 709 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 710 Value *PV1 = PN->getIncomingValue(i); 711 if (isa<PHINode>(PV1)) 712 // If any of the source itself is a PHI, return MayAlias conservatively 713 // to avoid compile time explosion. The worst possible case is if both 714 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 715 // and 'n' are the number of PHI sources. 716 return MayAlias; 717 if (UniqueSrc.insert(PV1)) 718 V1Srcs.push_back(PV1); 719 } 720 721 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); 722 // Early exit if the check of the first PHI source against V2 is MayAlias. 723 // Other results are not possible. 724 if (Alias == MayAlias) 725 return MayAlias; 726 727 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 728 // NoAlias / MustAlias. Otherwise, returns MayAlias. 729 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 730 Value *V = V1Srcs[i]; 731 732 // If V2 is visited, the recursive case will have been caught in the 733 // above aliasCheck call, so these subsequent calls to aliasCheck 734 // don't need to assume that V2 is being visited recursively. 735 Visited.erase(V2); 736 737 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); 738 if (ThisAlias != Alias || ThisAlias == MayAlias) 739 return MayAlias; 740 } 741 742 return Alias; 743} 744 745// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 746// such as array references. 747// 748AliasAnalysis::AliasResult 749BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, 750 const Value *V2, unsigned V2Size) { 751 // If either of the memory references is empty, it doesn't matter what the 752 // pointer values are. 753 if (V1Size == 0 || V2Size == 0) 754 return NoAlias; 755 756 // Strip off any casts if they exist. 757 V1 = V1->stripPointerCasts(); 758 V2 = V2->stripPointerCasts(); 759 760 // Are we checking for alias of the same value? 761 if (V1 == V2) return MustAlias; 762 763 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 764 return NoAlias; // Scalars cannot alias each other 765 766 // Figure out what objects these things are pointing to if we can. 767 const Value *O1 = V1->getUnderlyingObject(); 768 const Value *O2 = V2->getUnderlyingObject(); 769 770 // Null values in the default address space don't point to any object, so they 771 // don't alias any other pointer. 772 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 773 if (CPN->getType()->getAddressSpace() == 0) 774 return NoAlias; 775 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 776 if (CPN->getType()->getAddressSpace() == 0) 777 return NoAlias; 778 779 if (O1 != O2) { 780 // If V1/V2 point to two different objects we know that we have no alias. 781 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 782 return NoAlias; 783 784 // Constant pointers can't alias with non-const isIdentifiedObject objects. 785 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 786 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 787 return NoAlias; 788 789 // Arguments can't alias with local allocations or noalias calls 790 // in the same function. 791 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 792 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 793 return NoAlias; 794 795 // Most objects can't alias null. 796 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 797 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 798 return NoAlias; 799 800 // If one pointer is the result of a call/invoke or load and the other is a 801 // non-escaping local object within the same function, then we know the 802 // object couldn't escape to a point where the call could return it. 803 // 804 // Note that if the pointers are in different functions, there are a 805 // variety of complications. A call with a nocapture argument may still 806 // temporary store the nocapture argument's value in a temporary memory 807 // location if that memory location doesn't escape. Or it may pass a 808 // nocapture value to other functions as long as they don't capture it. 809 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 810 return NoAlias; 811 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 812 return NoAlias; 813 } 814 815 // If the size of one access is larger than the entire object on the other 816 // side, then we know such behavior is undefined and can assume no alias. 817 if (TD) 818 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 819 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 820 return NoAlias; 821 822 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 823 // GEP can't simplify, we don't even look at the PHI cases. 824 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 825 std::swap(V1, V2); 826 std::swap(V1Size, V2Size); 827 std::swap(O1, O2); 828 } 829 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) 830 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2); 831 832 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 833 std::swap(V1, V2); 834 std::swap(V1Size, V2Size); 835 } 836 if (const PHINode *PN = dyn_cast<PHINode>(V1)) 837 return aliasPHI(PN, V1Size, V2, V2Size); 838 839 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 840 std::swap(V1, V2); 841 std::swap(V1Size, V2Size); 842 } 843 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) 844 return aliasSelect(S1, V1Size, V2, V2Size); 845 846 return MayAlias; 847} 848 849// Make sure that anything that uses AliasAnalysis pulls in this file. 850DEFINING_FILE_FOR(BasicAliasAnalysis) 851