BasicAliasAnalysis.cpp revision 71339c965ca6268b9bff91213364783c3d06f666
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 82/// isObjectSmallerThan - Return true if we can prove that the object specified 83/// by V is smaller than Size. 84static bool isObjectSmallerThan(const Value *V, unsigned Size, 85 const TargetData &TD) { 86 const Type *AccessTy; 87 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 88 AccessTy = GV->getType()->getElementType(); 89 } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) { 90 if (!AI->isArrayAllocation()) 91 AccessTy = AI->getType()->getElementType(); 92 else 93 return false; 94 } else if (const CallInst* CI = extractMallocCall(V)) { 95 if (!isArrayMalloc(V, &TD)) 96 // The size is the argument to the malloc call. 97 if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0))) 98 return (C->getZExtValue() < Size); 99 return false; 100 } else if (const Argument *A = dyn_cast<Argument>(V)) { 101 if (A->hasByValAttr()) 102 AccessTy = cast<PointerType>(A->getType())->getElementType(); 103 else 104 return false; 105 } else { 106 return false; 107 } 108 109 if (AccessTy->isSized()) 110 return TD.getTypeAllocSize(AccessTy) < Size; 111 return false; 112} 113 114//===----------------------------------------------------------------------===// 115// NoAA Pass 116//===----------------------------------------------------------------------===// 117 118namespace { 119 /// NoAA - This class implements the -no-aa pass, which always returns "I 120 /// don't know" for alias queries. NoAA is unlike other alias analysis 121 /// implementations, in that it does not chain to a previous analysis. As 122 /// such it doesn't follow many of the rules that other alias analyses must. 123 /// 124 struct NoAA : public ImmutablePass, public AliasAnalysis { 125 static char ID; // Class identification, replacement for typeinfo 126 NoAA() : ImmutablePass(&ID) {} 127 explicit NoAA(void *PID) : ImmutablePass(PID) { } 128 129 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 130 } 131 132 virtual void initializePass() { 133 TD = getAnalysisIfAvailable<TargetData>(); 134 } 135 136 virtual AliasResult alias(const Value *V1, unsigned V1Size, 137 const Value *V2, unsigned V2Size) { 138 return MayAlias; 139 } 140 141 virtual void getArgumentAccesses(Function *F, CallSite CS, 142 std::vector<PointerAccessInfo> &Info) { 143 llvm_unreachable("This method may not be called on this function!"); 144 } 145 146 virtual bool pointsToConstantMemory(const Value *P) { return false; } 147 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 148 return ModRef; 149 } 150 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 151 return ModRef; 152 } 153 154 virtual void deleteValue(Value *V) {} 155 virtual void copyValue(Value *From, Value *To) {} 156 157 /// getAdjustedAnalysisPointer - This method is used when a pass implements 158 /// an analysis interface through multiple inheritance. If needed, it should 159 /// override this to adjust the this pointer as needed for the specified pass 160 /// info. 161 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 162 if (PI->isPassID(&AliasAnalysis::ID)) 163 return (AliasAnalysis*)this; 164 return this; 165 } 166 }; 167} // End of anonymous namespace 168 169// Register this pass... 170char NoAA::ID = 0; 171static RegisterPass<NoAA> 172U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 173 174// Declare that we implement the AliasAnalysis interface 175static RegisterAnalysisGroup<AliasAnalysis> V(U); 176 177ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 178 179//===----------------------------------------------------------------------===// 180// BasicAA Pass 181//===----------------------------------------------------------------------===// 182 183namespace { 184 /// BasicAliasAnalysis - This is the default alias analysis implementation. 185 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 186 /// derives from the NoAA class. 187 struct BasicAliasAnalysis : public NoAA { 188 static char ID; // Class identification, replacement for typeinfo 189 BasicAliasAnalysis() : NoAA(&ID) {} 190 AliasResult alias(const Value *V1, unsigned V1Size, 191 const Value *V2, unsigned V2Size) { 192 assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!"); 193 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size); 194 VisitedPHIs.clear(); 195 return Alias; 196 } 197 198 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 199 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 200 201 /// pointsToConstantMemory - Chase pointers until we find a (constant 202 /// global) or not. 203 bool pointsToConstantMemory(const Value *P); 204 205 /// getAdjustedAnalysisPointer - This method is used when a pass implements 206 /// an analysis interface through multiple inheritance. If needed, it should 207 /// override this to adjust the this pointer as needed for the specified pass 208 /// info. 209 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 210 if (PI->isPassID(&AliasAnalysis::ID)) 211 return (AliasAnalysis*)this; 212 return this; 213 } 214 215 private: 216 // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call. 217 SmallPtrSet<const Value*, 16> VisitedPHIs; 218 219 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 220 // instruction against another. 221 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size, 222 const Value *V2, unsigned V2Size, 223 const Value *UnderlyingV1, const Value *UnderlyingV2); 224 225 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 226 // instruction against another. 227 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize, 228 const Value *V2, unsigned V2Size); 229 230 /// aliasSelect - Disambiguate a Select instruction against another value. 231 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize, 232 const Value *V2, unsigned V2Size); 233 234 AliasResult aliasCheck(const Value *V1, unsigned V1Size, 235 const Value *V2, unsigned V2Size); 236 }; 237} // End of anonymous namespace 238 239// Register this pass... 240char BasicAliasAnalysis::ID = 0; 241static RegisterPass<BasicAliasAnalysis> 242X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 243 244// Declare that we implement the AliasAnalysis interface 245static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 246 247ImmutablePass *llvm::createBasicAliasAnalysisPass() { 248 return new BasicAliasAnalysis(); 249} 250 251 252/// pointsToConstantMemory - Chase pointers until we find a (constant 253/// global) or not. 254bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 255 if (const GlobalVariable *GV = 256 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 257 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 258 // global to be marked constant in some modules and non-constant in others. 259 // GV may even be a declaration, not a definition. 260 return GV->isConstant(); 261 return false; 262} 263 264 265/// getModRefInfo - Check to see if the specified callsite can clobber the 266/// specified memory object. Since we only look at local properties of this 267/// function, we really can't say much about this query. We do, however, use 268/// simple "address taken" analysis on local objects. 269AliasAnalysis::ModRefResult 270BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 271 const Value *Object = P->getUnderlyingObject(); 272 273 // If this is a tail call and P points to a stack location, we know that 274 // the tail call cannot access or modify the local stack. 275 // We cannot exclude byval arguments here; these belong to the caller of 276 // the current function not to the current function, and a tail callee 277 // may reference them. 278 if (isa<AllocaInst>(Object)) 279 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 280 if (CI->isTailCall()) 281 return NoModRef; 282 283 // If the pointer is to a locally allocated object that does not escape, 284 // then the call can not mod/ref the pointer unless the call takes the pointer 285 // as an argument, and itself doesn't capture it. 286 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 287 isNonEscapingLocalObject(Object)) { 288 bool PassedAsArg = false; 289 unsigned ArgNo = 0; 290 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 291 CI != CE; ++CI, ++ArgNo) { 292 // Only look at the no-capture pointer arguments. 293 if (!(*CI)->getType()->isPointerTy() || 294 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) 295 continue; 296 297 // If this is a no-capture pointer argument, see if we can tell that it 298 // is impossible to alias the pointer we're checking. If not, we have to 299 // assume that the call could touch the pointer, even though it doesn't 300 // escape. 301 if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) { 302 PassedAsArg = true; 303 break; 304 } 305 } 306 307 if (!PassedAsArg) 308 return NoModRef; 309 } 310 311 // Finally, handle specific knowledge of intrinsics. 312 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 313 if (II == 0) 314 return AliasAnalysis::getModRefInfo(CS, P, Size); 315 316 switch (II->getIntrinsicID()) { 317 default: break; 318 case Intrinsic::memcpy: 319 case Intrinsic::memmove: { 320 unsigned Len = ~0U; 321 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 322 Len = LenCI->getZExtValue(); 323 Value *Dest = II->getArgOperand(0); 324 Value *Src = II->getArgOperand(1); 325 if (isNoAlias(Dest, Len, P, Size)) { 326 if (isNoAlias(Src, Len, P, Size)) 327 return NoModRef; 328 return Ref; 329 } 330 break; 331 } 332 case Intrinsic::memset: 333 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 334 // will handle it for the variable length case. 335 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 336 unsigned Len = LenCI->getZExtValue(); 337 Value *Dest = II->getArgOperand(0); 338 if (isNoAlias(Dest, Len, P, Size)) 339 return NoModRef; 340 } 341 break; 342 case Intrinsic::atomic_cmp_swap: 343 case Intrinsic::atomic_swap: 344 case Intrinsic::atomic_load_add: 345 case Intrinsic::atomic_load_sub: 346 case Intrinsic::atomic_load_and: 347 case Intrinsic::atomic_load_nand: 348 case Intrinsic::atomic_load_or: 349 case Intrinsic::atomic_load_xor: 350 case Intrinsic::atomic_load_max: 351 case Intrinsic::atomic_load_min: 352 case Intrinsic::atomic_load_umax: 353 case Intrinsic::atomic_load_umin: 354 if (TD) { 355 Value *Op1 = II->getArgOperand(0); 356 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType()); 357 if (isNoAlias(Op1, Op1Size, P, Size)) 358 return NoModRef; 359 } 360 break; 361 case Intrinsic::lifetime_start: 362 case Intrinsic::lifetime_end: 363 case Intrinsic::invariant_start: { 364 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 365 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size)) 366 return NoModRef; 367 break; 368 } 369 case Intrinsic::invariant_end: { 370 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 371 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size)) 372 return NoModRef; 373 break; 374 } 375 } 376 377 // The AliasAnalysis base class has some smarts, lets use them. 378 return AliasAnalysis::getModRefInfo(CS, P, Size); 379} 380 381 382AliasAnalysis::ModRefResult 383BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 384 // If CS1 or CS2 are readnone, they don't interact. 385 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 386 if (CS1B == DoesNotAccessMemory) return NoModRef; 387 388 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 389 if (CS2B == DoesNotAccessMemory) return NoModRef; 390 391 // If they both only read from memory, just return ref. 392 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 393 return Ref; 394 395 // Otherwise, fall back to NoAA (mod+ref). 396 return NoAA::getModRefInfo(CS1, CS2); 397} 398 399/// GetIndiceDifference - Dest and Src are the variable indices from two 400/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 401/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 402/// difference between the two pointers. 403static void GetIndiceDifference( 404 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest, 405 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) { 406 if (Src.empty()) return; 407 408 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 409 const Value *V = Src[i].first; 410 int64_t Scale = Src[i].second; 411 412 // Find V in Dest. This is N^2, but pointer indices almost never have more 413 // than a few variable indexes. 414 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 415 if (Dest[j].first != V) continue; 416 417 // If we found it, subtract off Scale V's from the entry in Dest. If it 418 // goes to zero, remove the entry. 419 if (Dest[j].second != Scale) 420 Dest[j].second -= Scale; 421 else 422 Dest.erase(Dest.begin()+j); 423 Scale = 0; 424 break; 425 } 426 427 // If we didn't consume this entry, add it to the end of the Dest list. 428 if (Scale) 429 Dest.push_back(std::make_pair(V, -Scale)); 430 } 431} 432 433/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 434/// against another pointer. We know that V1 is a GEP, but we don't know 435/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 436/// UnderlyingV2 is the same for V2. 437/// 438AliasAnalysis::AliasResult 439BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size, 440 const Value *V2, unsigned V2Size, 441 const Value *UnderlyingV1, 442 const Value *UnderlyingV2) { 443 int64_t GEP1BaseOffset; 444 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices; 445 446 // If we have two gep instructions with must-alias'ing base pointers, figure 447 // out if the indexes to the GEP tell us anything about the derived pointer. 448 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 449 // Do the base pointers alias? 450 AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U); 451 452 // If we get a No or May, then return it immediately, no amount of analysis 453 // will improve this situation. 454 if (BaseAlias != MustAlias) return BaseAlias; 455 456 // Otherwise, we have a MustAlias. Since the base pointers alias each other 457 // exactly, see if the computed offset from the common pointer tells us 458 // about the relation of the resulting pointer. 459 const Value *GEP1BasePtr = 460 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 461 462 int64_t GEP2BaseOffset; 463 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices; 464 const Value *GEP2BasePtr = 465 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 466 467 // If DecomposeGEPExpression isn't able to look all the way through the 468 // addressing operation, we must not have TD and this is too complex for us 469 // to handle without it. 470 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 471 assert(TD == 0 && 472 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 473 return MayAlias; 474 } 475 476 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 477 // symbolic difference. 478 GEP1BaseOffset -= GEP2BaseOffset; 479 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices); 480 481 } else { 482 // Check to see if these two pointers are related by the getelementptr 483 // instruction. If one pointer is a GEP with a non-zero index of the other 484 // pointer, we know they cannot alias. 485 486 // If both accesses are unknown size, we can't do anything useful here. 487 if (V1Size == ~0U && V2Size == ~0U) 488 return MayAlias; 489 490 AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size); 491 if (R != MustAlias) 492 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 493 // If V2 is known not to alias GEP base pointer, then the two values 494 // cannot alias per GEP semantics: "A pointer value formed from a 495 // getelementptr instruction is associated with the addresses associated 496 // with the first operand of the getelementptr". 497 return R; 498 499 const Value *GEP1BasePtr = 500 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 501 502 // If DecomposeGEPExpression isn't able to look all the way through the 503 // addressing operation, we must not have TD and this is too complex for us 504 // to handle without it. 505 if (GEP1BasePtr != UnderlyingV1) { 506 assert(TD == 0 && 507 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 508 return MayAlias; 509 } 510 } 511 512 // In the two GEP Case, if there is no difference in the offsets of the 513 // computed pointers, the resultant pointers are a must alias. This 514 // hapens when we have two lexically identical GEP's (for example). 515 // 516 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 517 // must aliases the GEP, the end result is a must alias also. 518 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 519 return MustAlias; 520 521 // If we have a known constant offset, see if this offset is larger than the 522 // access size being queried. If so, and if no variable indices can remove 523 // pieces of this constant, then we know we have a no-alias. For example, 524 // &A[100] != &A. 525 526 // In order to handle cases like &A[100][i] where i is an out of range 527 // subscript, we have to ignore all constant offset pieces that are a multiple 528 // of a scaled index. Do this by removing constant offsets that are a 529 // multiple of any of our variable indices. This allows us to transform 530 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 531 // provides an offset of 4 bytes (assuming a <= 4 byte access). 532 for (unsigned i = 0, e = GEP1VariableIndices.size(); 533 i != e && GEP1BaseOffset;++i) 534 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second) 535 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second; 536 537 // If our known offset is bigger than the access size, we know we don't have 538 // an alias. 539 if (GEP1BaseOffset) { 540 if (GEP1BaseOffset >= (int64_t)V2Size || 541 GEP1BaseOffset <= -(int64_t)V1Size) 542 return NoAlias; 543 } 544 545 return MayAlias; 546} 547 548/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 549/// instruction against another. 550AliasAnalysis::AliasResult 551BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, 552 const Value *V2, unsigned V2Size) { 553 // If the values are Selects with the same condition, we can do a more precise 554 // check: just check for aliases between the values on corresponding arms. 555 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 556 if (SI->getCondition() == SI2->getCondition()) { 557 AliasResult Alias = 558 aliasCheck(SI->getTrueValue(), SISize, 559 SI2->getTrueValue(), V2Size); 560 if (Alias == MayAlias) 561 return MayAlias; 562 AliasResult ThisAlias = 563 aliasCheck(SI->getFalseValue(), SISize, 564 SI2->getFalseValue(), V2Size); 565 if (ThisAlias != Alias) 566 return MayAlias; 567 return Alias; 568 } 569 570 // If both arms of the Select node NoAlias or MustAlias V2, then returns 571 // NoAlias / MustAlias. Otherwise, returns MayAlias. 572 AliasResult Alias = 573 aliasCheck(SI->getTrueValue(), SISize, V2, V2Size); 574 if (Alias == MayAlias) 575 return MayAlias; 576 AliasResult ThisAlias = 577 aliasCheck(SI->getFalseValue(), SISize, V2, V2Size); 578 if (ThisAlias != Alias) 579 return MayAlias; 580 return Alias; 581} 582 583// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 584// against another. 585AliasAnalysis::AliasResult 586BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, 587 const Value *V2, unsigned V2Size) { 588 // The PHI node has already been visited, avoid recursion any further. 589 if (!VisitedPHIs.insert(PN)) 590 return MayAlias; 591 592 // If the values are PHIs in the same block, we can do a more precise 593 // as well as efficient check: just check for aliases between the values 594 // on corresponding edges. 595 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 596 if (PN2->getParent() == PN->getParent()) { 597 AliasResult Alias = 598 aliasCheck(PN->getIncomingValue(0), PNSize, 599 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 600 V2Size); 601 if (Alias == MayAlias) 602 return MayAlias; 603 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 604 AliasResult ThisAlias = 605 aliasCheck(PN->getIncomingValue(i), PNSize, 606 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 607 V2Size); 608 if (ThisAlias != Alias) 609 return MayAlias; 610 } 611 return Alias; 612 } 613 614 SmallPtrSet<Value*, 4> UniqueSrc; 615 SmallVector<Value*, 4> V1Srcs; 616 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 617 Value *PV1 = PN->getIncomingValue(i); 618 if (isa<PHINode>(PV1)) 619 // If any of the source itself is a PHI, return MayAlias conservatively 620 // to avoid compile time explosion. The worst possible case is if both 621 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 622 // and 'n' are the number of PHI sources. 623 return MayAlias; 624 if (UniqueSrc.insert(PV1)) 625 V1Srcs.push_back(PV1); 626 } 627 628 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); 629 // Early exit if the check of the first PHI source against V2 is MayAlias. 630 // Other results are not possible. 631 if (Alias == MayAlias) 632 return MayAlias; 633 634 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 635 // NoAlias / MustAlias. Otherwise, returns MayAlias. 636 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 637 Value *V = V1Srcs[i]; 638 639 // If V2 is a PHI, the recursive case will have been caught in the 640 // above aliasCheck call, so these subsequent calls to aliasCheck 641 // don't need to assume that V2 is being visited recursively. 642 VisitedPHIs.erase(V2); 643 644 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); 645 if (ThisAlias != Alias || ThisAlias == MayAlias) 646 return MayAlias; 647 } 648 649 return Alias; 650} 651 652// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 653// such as array references. 654// 655AliasAnalysis::AliasResult 656BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, 657 const Value *V2, unsigned V2Size) { 658 // If either of the memory references is empty, it doesn't matter what the 659 // pointer values are. 660 if (V1Size == 0 || V2Size == 0) 661 return NoAlias; 662 663 // Strip off any casts if they exist. 664 V1 = V1->stripPointerCasts(); 665 V2 = V2->stripPointerCasts(); 666 667 // Are we checking for alias of the same value? 668 if (V1 == V2) return MustAlias; 669 670 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 671 return NoAlias; // Scalars cannot alias each other 672 673 // Figure out what objects these things are pointing to if we can. 674 const Value *O1 = V1->getUnderlyingObject(); 675 const Value *O2 = V2->getUnderlyingObject(); 676 677 // Null values in the default address space don't point to any object, so they 678 // don't alias any other pointer. 679 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 680 if (CPN->getType()->getAddressSpace() == 0) 681 return NoAlias; 682 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 683 if (CPN->getType()->getAddressSpace() == 0) 684 return NoAlias; 685 686 if (O1 != O2) { 687 // If V1/V2 point to two different objects we know that we have no alias. 688 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 689 return NoAlias; 690 691 // Constant pointers can't alias with non-const isIdentifiedObject objects. 692 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 693 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 694 return NoAlias; 695 696 // Arguments can't alias with local allocations or noalias calls. 697 if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 698 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))) 699 return NoAlias; 700 701 // Most objects can't alias null. 702 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 703 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 704 return NoAlias; 705 } 706 707 // If the size of one access is larger than the entire object on the other 708 // side, then we know such behavior is undefined and can assume no alias. 709 if (TD) 710 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) || 711 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD))) 712 return NoAlias; 713 714 // If one pointer is the result of a call/invoke or load and the other is a 715 // non-escaping local object, then we know the object couldn't escape to a 716 // point where the call could return it. The load case works because 717 // isNonEscapingLocalObject considers all stores to be escapes (it 718 // passes true for the StoreCaptures argument to PointerMayBeCaptured). 719 if (O1 != O2) { 720 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1) || 721 isa<Argument>(O1)) && 722 isNonEscapingLocalObject(O2)) 723 return NoAlias; 724 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2) || 725 isa<Argument>(O2)) && 726 isNonEscapingLocalObject(O1)) 727 return NoAlias; 728 } 729 730 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 731 // GEP can't simplify, we don't even look at the PHI cases. 732 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 733 std::swap(V1, V2); 734 std::swap(V1Size, V2Size); 735 std::swap(O1, O2); 736 } 737 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) 738 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2); 739 740 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 741 std::swap(V1, V2); 742 std::swap(V1Size, V2Size); 743 } 744 if (const PHINode *PN = dyn_cast<PHINode>(V1)) 745 return aliasPHI(PN, V1Size, V2, V2Size); 746 747 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 748 std::swap(V1, V2); 749 std::swap(V1Size, V2Size); 750 } 751 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) 752 return aliasSelect(S1, V1Size, V2, V2Size); 753 754 return MayAlias; 755} 756 757// Make sure that anything that uses AliasAnalysis pulls in this file. 758DEFINING_FILE_FOR(BasicAliasAnalysis) 759