BasicAliasAnalysis.cpp revision 3dcc91ee8c48f210d302937ecbbf0d277f8b656e
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 should 170 /// override this to adjust the this pointer as needed for the specified pass 171 /// info. 172 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 173 if (PI->isPassID(&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 should 241 /// override this to adjust the this pointer as needed for the specified pass 242 /// info. 243 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 244 if (PI->isPassID(&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 CS1 only reads from memory, just return ref. 429 if (CS1B == OnlyReadsMemory) 430 return Ref; 431 432 // Otherwise, fall back to NoAA (mod+ref). 433 return NoAA::getModRefInfo(CS1, CS2); 434} 435 436/// GetIndexDifference - Dest and Src are the variable indices from two 437/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 438/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 439/// difference between the two pointers. 440static void GetIndexDifference( 441 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest, 442 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) { 443 if (Src.empty()) return; 444 445 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 446 const Value *V = Src[i].first; 447 int64_t Scale = Src[i].second; 448 449 // Find V in Dest. This is N^2, but pointer indices almost never have more 450 // than a few variable indexes. 451 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 452 if (Dest[j].first != V) continue; 453 454 // If we found it, subtract off Scale V's from the entry in Dest. If it 455 // goes to zero, remove the entry. 456 if (Dest[j].second != Scale) 457 Dest[j].second -= Scale; 458 else 459 Dest.erase(Dest.begin()+j); 460 Scale = 0; 461 break; 462 } 463 464 // If we didn't consume this entry, add it to the end of the Dest list. 465 if (Scale) 466 Dest.push_back(std::make_pair(V, -Scale)); 467 } 468} 469 470/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 471/// against another pointer. We know that V1 is a GEP, but we don't know 472/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 473/// UnderlyingV2 is the same for V2. 474/// 475AliasAnalysis::AliasResult 476BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size, 477 const Value *V2, unsigned V2Size, 478 const Value *UnderlyingV1, 479 const Value *UnderlyingV2) { 480 // If this GEP has been visited before, we're on a use-def cycle. 481 // Such cycles are only valid when PHI nodes are involved or in unreachable 482 // code. The visitPHI function catches cycles containing PHIs, but there 483 // could still be a cycle without PHIs in unreachable code. 484 if (!Visited.insert(GEP1)) 485 return MayAlias; 486 487 int64_t GEP1BaseOffset; 488 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices; 489 490 // If we have two gep instructions with must-alias'ing base pointers, figure 491 // out if the indexes to the GEP tell us anything about the derived pointer. 492 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 493 // Do the base pointers alias? 494 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 495 UnderlyingV2, UnknownSize); 496 497 // If we get a No or May, then return it immediately, no amount of analysis 498 // will improve this situation. 499 if (BaseAlias != MustAlias) return BaseAlias; 500 501 // Otherwise, we have a MustAlias. Since the base pointers alias each other 502 // exactly, see if the computed offset from the common pointer tells us 503 // about the relation of the resulting pointer. 504 const Value *GEP1BasePtr = 505 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 506 507 int64_t GEP2BaseOffset; 508 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices; 509 const Value *GEP2BasePtr = 510 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 511 512 // If DecomposeGEPExpression isn't able to look all the way through the 513 // addressing operation, we must not have TD and this is too complex for us 514 // to handle without it. 515 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 516 assert(TD == 0 && 517 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 518 return MayAlias; 519 } 520 521 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 522 // symbolic difference. 523 GEP1BaseOffset -= GEP2BaseOffset; 524 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 525 526 } else { 527 // Check to see if these two pointers are related by the getelementptr 528 // instruction. If one pointer is a GEP with a non-zero index of the other 529 // pointer, we know they cannot alias. 530 531 // If both accesses are unknown size, we can't do anything useful here. 532 if (V1Size == UnknownSize && V2Size == UnknownSize) 533 return MayAlias; 534 535 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size); 536 if (R != MustAlias) 537 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 538 // If V2 is known not to alias GEP base pointer, then the two values 539 // cannot alias per GEP semantics: "A pointer value formed from a 540 // getelementptr instruction is associated with the addresses associated 541 // with the first operand of the getelementptr". 542 return R; 543 544 const Value *GEP1BasePtr = 545 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, 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) { 551 assert(TD == 0 && 552 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 553 return MayAlias; 554 } 555 } 556 557 // In the two GEP Case, if there is no difference in the offsets of the 558 // computed pointers, the resultant pointers are a must alias. This 559 // hapens when we have two lexically identical GEP's (for example). 560 // 561 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 562 // must aliases the GEP, the end result is a must alias also. 563 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 564 return MustAlias; 565 566 // If we have a known constant offset, see if this offset is larger than the 567 // access size being queried. If so, and if no variable indices can remove 568 // pieces of this constant, then we know we have a no-alias. For example, 569 // &A[100] != &A. 570 571 // In order to handle cases like &A[100][i] where i is an out of range 572 // subscript, we have to ignore all constant offset pieces that are a multiple 573 // of a scaled index. Do this by removing constant offsets that are a 574 // multiple of any of our variable indices. This allows us to transform 575 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 576 // provides an offset of 4 bytes (assuming a <= 4 byte access). 577 for (unsigned i = 0, e = GEP1VariableIndices.size(); 578 i != e && GEP1BaseOffset;++i) 579 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second) 580 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second; 581 582 // If our known offset is bigger than the access size, we know we don't have 583 // an alias. 584 if (GEP1BaseOffset) { 585 if (GEP1BaseOffset >= (int64_t)V2Size || 586 GEP1BaseOffset <= -(int64_t)V1Size) 587 return NoAlias; 588 } 589 590 return MayAlias; 591} 592 593/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 594/// instruction against another. 595AliasAnalysis::AliasResult 596BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, 597 const Value *V2, unsigned V2Size) { 598 // If this select has been visited before, we're on a use-def cycle. 599 // Such cycles are only valid when PHI nodes are involved or in unreachable 600 // code. The visitPHI function catches cycles containing PHIs, but there 601 // could still be a cycle without PHIs in unreachable code. 602 if (!Visited.insert(SI)) 603 return MayAlias; 604 605 // If the values are Selects with the same condition, we can do a more precise 606 // check: just check for aliases between the values on corresponding arms. 607 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 608 if (SI->getCondition() == SI2->getCondition()) { 609 AliasResult Alias = 610 aliasCheck(SI->getTrueValue(), SISize, 611 SI2->getTrueValue(), V2Size); 612 if (Alias == MayAlias) 613 return MayAlias; 614 AliasResult ThisAlias = 615 aliasCheck(SI->getFalseValue(), SISize, 616 SI2->getFalseValue(), V2Size); 617 if (ThisAlias != Alias) 618 return MayAlias; 619 return Alias; 620 } 621 622 // If both arms of the Select node NoAlias or MustAlias V2, then returns 623 // NoAlias / MustAlias. Otherwise, returns MayAlias. 624 AliasResult Alias = 625 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize); 626 if (Alias == MayAlias) 627 return MayAlias; 628 629 // If V2 is visited, the recursive case will have been caught in the 630 // above aliasCheck call, so these subsequent calls to aliasCheck 631 // don't need to assume that V2 is being visited recursively. 632 Visited.erase(V2); 633 634 AliasResult ThisAlias = 635 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize); 636 if (ThisAlias != Alias) 637 return MayAlias; 638 return Alias; 639} 640 641// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 642// against another. 643AliasAnalysis::AliasResult 644BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, 645 const Value *V2, unsigned V2Size) { 646 // The PHI node has already been visited, avoid recursion any further. 647 if (!Visited.insert(PN)) 648 return MayAlias; 649 650 // If the values are PHIs in the same block, we can do a more precise 651 // as well as efficient check: just check for aliases between the values 652 // on corresponding edges. 653 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 654 if (PN2->getParent() == PN->getParent()) { 655 AliasResult Alias = 656 aliasCheck(PN->getIncomingValue(0), PNSize, 657 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 658 V2Size); 659 if (Alias == MayAlias) 660 return MayAlias; 661 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 662 AliasResult ThisAlias = 663 aliasCheck(PN->getIncomingValue(i), PNSize, 664 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 665 V2Size); 666 if (ThisAlias != Alias) 667 return MayAlias; 668 } 669 return Alias; 670 } 671 672 SmallPtrSet<Value*, 4> UniqueSrc; 673 SmallVector<Value*, 4> V1Srcs; 674 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 675 Value *PV1 = PN->getIncomingValue(i); 676 if (isa<PHINode>(PV1)) 677 // If any of the source itself is a PHI, return MayAlias conservatively 678 // to avoid compile time explosion. The worst possible case is if both 679 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 680 // and 'n' are the number of PHI sources. 681 return MayAlias; 682 if (UniqueSrc.insert(PV1)) 683 V1Srcs.push_back(PV1); 684 } 685 686 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); 687 // Early exit if the check of the first PHI source against V2 is MayAlias. 688 // Other results are not possible. 689 if (Alias == MayAlias) 690 return MayAlias; 691 692 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 693 // NoAlias / MustAlias. Otherwise, returns MayAlias. 694 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 695 Value *V = V1Srcs[i]; 696 697 // If V2 is visited, the recursive case will have been caught in the 698 // above aliasCheck call, so these subsequent calls to aliasCheck 699 // don't need to assume that V2 is being visited recursively. 700 Visited.erase(V2); 701 702 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); 703 if (ThisAlias != Alias || ThisAlias == MayAlias) 704 return MayAlias; 705 } 706 707 return Alias; 708} 709 710// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 711// such as array references. 712// 713AliasAnalysis::AliasResult 714BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, 715 const Value *V2, unsigned V2Size) { 716 // If either of the memory references is empty, it doesn't matter what the 717 // pointer values are. 718 if (V1Size == 0 || V2Size == 0) 719 return NoAlias; 720 721 // Strip off any casts if they exist. 722 V1 = V1->stripPointerCasts(); 723 V2 = V2->stripPointerCasts(); 724 725 // Are we checking for alias of the same value? 726 if (V1 == V2) return MustAlias; 727 728 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 729 return NoAlias; // Scalars cannot alias each other 730 731 // Figure out what objects these things are pointing to if we can. 732 const Value *O1 = V1->getUnderlyingObject(); 733 const Value *O2 = V2->getUnderlyingObject(); 734 735 // Null values in the default address space don't point to any object, so they 736 // don't alias any other pointer. 737 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 738 if (CPN->getType()->getAddressSpace() == 0) 739 return NoAlias; 740 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 741 if (CPN->getType()->getAddressSpace() == 0) 742 return NoAlias; 743 744 if (O1 != O2) { 745 // If V1/V2 point to two different objects we know that we have no alias. 746 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 747 return NoAlias; 748 749 // Constant pointers can't alias with non-const isIdentifiedObject objects. 750 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 751 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 752 return NoAlias; 753 754 // Arguments can't alias with local allocations or noalias calls 755 // in the same function. 756 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 757 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 758 return NoAlias; 759 760 // Most objects can't alias null. 761 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 762 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 763 return NoAlias; 764 765 // If one pointer is the result of a call/invoke or load and the other is a 766 // non-escaping local object within the same function, then we know the 767 // object couldn't escape to a point where the call could return it. 768 // 769 // Note that if the pointers are in different functions, there are a 770 // variety of complications. A call with a nocapture argument may still 771 // temporary store the nocapture argument's value in a temporary memory 772 // location if that memory location doesn't escape. Or it may pass a 773 // nocapture value to other functions as long as they don't capture it. 774 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 775 return NoAlias; 776 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 777 return NoAlias; 778 } 779 780 // If the size of one access is larger than the entire object on the other 781 // side, then we know such behavior is undefined and can assume no alias. 782 if (TD) 783 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 784 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 785 return NoAlias; 786 787 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 788 // GEP can't simplify, we don't even look at the PHI cases. 789 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 790 std::swap(V1, V2); 791 std::swap(V1Size, V2Size); 792 std::swap(O1, O2); 793 } 794 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) 795 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2); 796 797 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 798 std::swap(V1, V2); 799 std::swap(V1Size, V2Size); 800 } 801 if (const PHINode *PN = dyn_cast<PHINode>(V1)) 802 return aliasPHI(PN, V1Size, V2, V2Size); 803 804 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 805 std::swap(V1, V2); 806 std::swap(V1Size, V2Size); 807 } 808 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) 809 return aliasSelect(S1, V1Size, V2, V2Size); 810 811 return MayAlias; 812} 813 814// Make sure that anything that uses AliasAnalysis pulls in this file. 815DEFINING_FILE_FOR(BasicAliasAnalysis) 816