BasicAliasAnalysis.cpp revision 17f647d33237222bb8f79dad5046114a89a7efdf
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(CallSite CS, Value *P, unsigned Size) { 157 return ModRef; 158 } 159 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 160 return ModRef; 161 } 162 163 virtual void deleteValue(Value *V) {} 164 virtual void copyValue(Value *From, Value *To) {} 165 166 /// getAdjustedAnalysisPointer - This method is used when a pass implements 167 /// an analysis interface through multiple inheritance. If needed, it should 168 /// override this to adjust the this pointer as needed for the specified pass 169 /// info. 170 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 171 if (PI->isPassID(&AliasAnalysis::ID)) 172 return (AliasAnalysis*)this; 173 return this; 174 } 175 }; 176} // End of anonymous namespace 177 178// Register this pass... 179char NoAA::ID = 0; 180INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa", 181 "No Alias Analysis (always returns 'may' alias)", 182 true, true, false); 183 184ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 185 186//===----------------------------------------------------------------------===// 187// BasicAliasAnalysis Pass 188//===----------------------------------------------------------------------===// 189 190#ifndef NDEBUG 191static const Function *getParent(const Value *V) { 192 if (const Instruction *inst = dyn_cast<Instruction>(V)) 193 return inst->getParent()->getParent(); 194 195 if (const Argument *arg = dyn_cast<Argument>(V)) 196 return arg->getParent(); 197 198 return NULL; 199} 200 201static bool notDifferentParent(const Value *O1, const Value *O2) { 202 203 const Function *F1 = getParent(O1); 204 const Function *F2 = getParent(O2); 205 206 return !F1 || !F2 || F1 == F2; 207} 208#endif 209 210namespace { 211 /// BasicAliasAnalysis - This is the default alias analysis implementation. 212 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 213 /// derives from the NoAA class. 214 struct BasicAliasAnalysis : public NoAA { 215 static char ID; // Class identification, replacement for typeinfo 216 BasicAliasAnalysis() : NoAA(&ID) {} 217 218 AliasResult alias(const Value *V1, unsigned V1Size, 219 const Value *V2, unsigned V2Size) { 220 assert(Visited.empty() && "Visited must be cleared after use!"); 221 assert(notDifferentParent(V1, V2) && 222 "BasicAliasAnalysis doesn't support interprocedural queries."); 223 AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size); 224 Visited.clear(); 225 return Alias; 226 } 227 228 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 229 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 230 231 /// pointsToConstantMemory - Chase pointers until we find a (constant 232 /// global) or not. 233 bool pointsToConstantMemory(const Value *P); 234 235 /// getAdjustedAnalysisPointer - This method is used when a pass implements 236 /// an analysis interface through multiple inheritance. If needed, it should 237 /// override this to adjust the this pointer as needed for the specified pass 238 /// info. 239 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { 240 if (PI->isPassID(&AliasAnalysis::ID)) 241 return (AliasAnalysis*)this; 242 return this; 243 } 244 245 private: 246 // Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP(). 247 SmallPtrSet<const Value*, 16> Visited; 248 249 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 250 // instruction against another. 251 AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size, 252 const Value *V2, unsigned V2Size, 253 const Value *UnderlyingV1, const Value *UnderlyingV2); 254 255 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 256 // instruction against another. 257 AliasResult aliasPHI(const PHINode *PN, unsigned PNSize, 258 const Value *V2, unsigned V2Size); 259 260 /// aliasSelect - Disambiguate a Select instruction against another value. 261 AliasResult aliasSelect(const SelectInst *SI, unsigned SISize, 262 const Value *V2, unsigned V2Size); 263 264 AliasResult aliasCheck(const Value *V1, unsigned V1Size, 265 const Value *V2, unsigned V2Size); 266 }; 267} // End of anonymous namespace 268 269// Register this pass... 270char BasicAliasAnalysis::ID = 0; 271INITIALIZE_AG_PASS(BasicAliasAnalysis, AliasAnalysis, "basicaa", 272 "Basic Alias Analysis (default AA impl)", 273 false, true, true); 274 275ImmutablePass *llvm::createBasicAliasAnalysisPass() { 276 return new BasicAliasAnalysis(); 277} 278 279 280/// pointsToConstantMemory - Chase pointers until we find a (constant 281/// global) or not. 282bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 283 if (const GlobalVariable *GV = 284 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 285 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 286 // global to be marked constant in some modules and non-constant in others. 287 // GV may even be a declaration, not a definition. 288 return GV->isConstant(); 289 return false; 290} 291 292 293/// getModRefInfo - Check to see if the specified callsite can clobber the 294/// specified memory object. Since we only look at local properties of this 295/// function, we really can't say much about this query. We do, however, use 296/// simple "address taken" analysis on local objects. 297AliasAnalysis::ModRefResult 298BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 299 assert(notDifferentParent(CS.getInstruction(), P) && 300 "AliasAnalysis query involving multiple functions!"); 301 302 const Value *Object = P->getUnderlyingObject(); 303 304 // If this is a tail call and P points to a stack location, we know that 305 // the tail call cannot access or modify the local stack. 306 // We cannot exclude byval arguments here; these belong to the caller of 307 // the current function not to the current function, and a tail callee 308 // may reference them. 309 if (isa<AllocaInst>(Object)) 310 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 311 if (CI->isTailCall()) 312 return NoModRef; 313 314 // If the pointer is to a locally allocated object that does not escape, 315 // then the call can not mod/ref the pointer unless the call takes the pointer 316 // as an argument, and itself doesn't capture it. 317 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 318 isNonEscapingLocalObject(Object)) { 319 bool PassedAsArg = false; 320 unsigned ArgNo = 0; 321 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 322 CI != CE; ++CI, ++ArgNo) { 323 // Only look at the no-capture pointer arguments. 324 if (!(*CI)->getType()->isPointerTy() || 325 !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) 326 continue; 327 328 // If this is a no-capture pointer argument, see if we can tell that it 329 // is impossible to alias the pointer we're checking. If not, we have to 330 // assume that the call could touch the pointer, even though it doesn't 331 // escape. 332 if (!isNoAlias(cast<Value>(CI), UnknownSize, P, UnknownSize)) { 333 PassedAsArg = true; 334 break; 335 } 336 } 337 338 if (!PassedAsArg) 339 return NoModRef; 340 } 341 342 // Finally, handle specific knowledge of intrinsics. 343 IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 344 if (II == 0) 345 return AliasAnalysis::getModRefInfo(CS, P, Size); 346 347 switch (II->getIntrinsicID()) { 348 default: break; 349 case Intrinsic::memcpy: 350 case Intrinsic::memmove: { 351 unsigned Len = UnknownSize; 352 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 353 Len = LenCI->getZExtValue(); 354 Value *Dest = II->getArgOperand(0); 355 Value *Src = II->getArgOperand(1); 356 if (isNoAlias(Dest, Len, P, Size)) { 357 if (isNoAlias(Src, Len, P, Size)) 358 return NoModRef; 359 return Ref; 360 } 361 break; 362 } 363 case Intrinsic::memset: 364 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 365 // will handle it for the variable length case. 366 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 367 unsigned Len = LenCI->getZExtValue(); 368 Value *Dest = II->getArgOperand(0); 369 if (isNoAlias(Dest, Len, P, Size)) 370 return NoModRef; 371 } 372 break; 373 case Intrinsic::atomic_cmp_swap: 374 case Intrinsic::atomic_swap: 375 case Intrinsic::atomic_load_add: 376 case Intrinsic::atomic_load_sub: 377 case Intrinsic::atomic_load_and: 378 case Intrinsic::atomic_load_nand: 379 case Intrinsic::atomic_load_or: 380 case Intrinsic::atomic_load_xor: 381 case Intrinsic::atomic_load_max: 382 case Intrinsic::atomic_load_min: 383 case Intrinsic::atomic_load_umax: 384 case Intrinsic::atomic_load_umin: 385 if (TD) { 386 Value *Op1 = II->getArgOperand(0); 387 unsigned Op1Size = TD->getTypeStoreSize(Op1->getType()); 388 if (isNoAlias(Op1, Op1Size, P, Size)) 389 return NoModRef; 390 } 391 break; 392 case Intrinsic::lifetime_start: 393 case Intrinsic::lifetime_end: 394 case Intrinsic::invariant_start: { 395 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 396 if (isNoAlias(II->getArgOperand(1), PtrSize, P, Size)) 397 return NoModRef; 398 break; 399 } 400 case Intrinsic::invariant_end: { 401 unsigned PtrSize = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 402 if (isNoAlias(II->getArgOperand(2), PtrSize, P, Size)) 403 return NoModRef; 404 break; 405 } 406 } 407 408 // The AliasAnalysis base class has some smarts, lets use them. 409 return AliasAnalysis::getModRefInfo(CS, P, Size); 410} 411 412 413AliasAnalysis::ModRefResult 414BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 415 // If CS1 or CS2 are readnone, they don't interact. 416 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 417 if (CS1B == DoesNotAccessMemory) return NoModRef; 418 419 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 420 if (CS2B == DoesNotAccessMemory) return NoModRef; 421 422 // If they both only read from memory, just return ref. 423 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 424 return Ref; 425 426 // Otherwise, fall back to NoAA (mod+ref). 427 return NoAA::getModRefInfo(CS1, CS2); 428} 429 430/// GetIndiceDifference - Dest and Src are the variable indices from two 431/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 432/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 433/// difference between the two pointers. 434static void GetIndiceDifference( 435 SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest, 436 const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) { 437 if (Src.empty()) return; 438 439 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 440 const Value *V = Src[i].first; 441 int64_t Scale = Src[i].second; 442 443 // Find V in Dest. This is N^2, but pointer indices almost never have more 444 // than a few variable indexes. 445 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 446 if (Dest[j].first != V) continue; 447 448 // If we found it, subtract off Scale V's from the entry in Dest. If it 449 // goes to zero, remove the entry. 450 if (Dest[j].second != Scale) 451 Dest[j].second -= Scale; 452 else 453 Dest.erase(Dest.begin()+j); 454 Scale = 0; 455 break; 456 } 457 458 // If we didn't consume this entry, add it to the end of the Dest list. 459 if (Scale) 460 Dest.push_back(std::make_pair(V, -Scale)); 461 } 462} 463 464/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 465/// against another pointer. We know that V1 is a GEP, but we don't know 466/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), 467/// UnderlyingV2 is the same for V2. 468/// 469AliasAnalysis::AliasResult 470BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size, 471 const Value *V2, unsigned V2Size, 472 const Value *UnderlyingV1, 473 const Value *UnderlyingV2) { 474 // If this GEP has been visited before, we're on a use-def cycle. 475 // Such cycles are only valid when PHI nodes are involved or in unreachable 476 // code. The visitPHI function catches cycles containing PHIs, but there 477 // could still be a cycle without PHIs in unreachable code. 478 if (!Visited.insert(GEP1)) 479 return MayAlias; 480 481 int64_t GEP1BaseOffset; 482 SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices; 483 484 // If we have two gep instructions with must-alias'ing base pointers, figure 485 // out if the indexes to the GEP tell us anything about the derived pointer. 486 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 487 // Do the base pointers alias? 488 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 489 UnderlyingV2, UnknownSize); 490 491 // If we get a No or May, then return it immediately, no amount of analysis 492 // will improve this situation. 493 if (BaseAlias != MustAlias) return BaseAlias; 494 495 // Otherwise, we have a MustAlias. Since the base pointers alias each other 496 // exactly, see if the computed offset from the common pointer tells us 497 // about the relation of the resulting pointer. 498 const Value *GEP1BasePtr = 499 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 500 501 int64_t GEP2BaseOffset; 502 SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices; 503 const Value *GEP2BasePtr = 504 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 505 506 // If DecomposeGEPExpression isn't able to look all the way through the 507 // addressing operation, we must not have TD and this is too complex for us 508 // to handle without it. 509 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 510 assert(TD == 0 && 511 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 512 return MayAlias; 513 } 514 515 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 516 // symbolic difference. 517 GEP1BaseOffset -= GEP2BaseOffset; 518 GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices); 519 520 } else { 521 // Check to see if these two pointers are related by the getelementptr 522 // instruction. If one pointer is a GEP with a non-zero index of the other 523 // pointer, we know they cannot alias. 524 525 // If both accesses are unknown size, we can't do anything useful here. 526 if (V1Size == UnknownSize && V2Size == UnknownSize) 527 return MayAlias; 528 529 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, V2, V2Size); 530 if (R != MustAlias) 531 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 532 // If V2 is known not to alias GEP base pointer, then the two values 533 // cannot alias per GEP semantics: "A pointer value formed from a 534 // getelementptr instruction is associated with the addresses associated 535 // with the first operand of the getelementptr". 536 return R; 537 538 const Value *GEP1BasePtr = 539 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 540 541 // If DecomposeGEPExpression isn't able to look all the way through the 542 // addressing operation, we must not have TD and this is too complex for us 543 // to handle without it. 544 if (GEP1BasePtr != UnderlyingV1) { 545 assert(TD == 0 && 546 "DecomposeGEPExpression and getUnderlyingObject disagree!"); 547 return MayAlias; 548 } 549 } 550 551 // In the two GEP Case, if there is no difference in the offsets of the 552 // computed pointers, the resultant pointers are a must alias. This 553 // hapens when we have two lexically identical GEP's (for example). 554 // 555 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 556 // must aliases the GEP, the end result is a must alias also. 557 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 558 return MustAlias; 559 560 // If we have a known constant offset, see if this offset is larger than the 561 // access size being queried. If so, and if no variable indices can remove 562 // pieces of this constant, then we know we have a no-alias. For example, 563 // &A[100] != &A. 564 565 // In order to handle cases like &A[100][i] where i is an out of range 566 // subscript, we have to ignore all constant offset pieces that are a multiple 567 // of a scaled index. Do this by removing constant offsets that are a 568 // multiple of any of our variable indices. This allows us to transform 569 // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 570 // provides an offset of 4 bytes (assuming a <= 4 byte access). 571 for (unsigned i = 0, e = GEP1VariableIndices.size(); 572 i != e && GEP1BaseOffset;++i) 573 if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second) 574 GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second; 575 576 // If our known offset is bigger than the access size, we know we don't have 577 // an alias. 578 if (GEP1BaseOffset) { 579 if (GEP1BaseOffset >= (int64_t)V2Size || 580 GEP1BaseOffset <= -(int64_t)V1Size) 581 return NoAlias; 582 } 583 584 return MayAlias; 585} 586 587/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 588/// instruction against another. 589AliasAnalysis::AliasResult 590BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, 591 const Value *V2, unsigned V2Size) { 592 // If this select has been visited before, we're on a use-def cycle. 593 // Such cycles are only valid when PHI nodes are involved or in unreachable 594 // code. The visitPHI function catches cycles containing PHIs, but there 595 // could still be a cycle without PHIs in unreachable code. 596 if (!Visited.insert(SI)) 597 return MayAlias; 598 599 // If the values are Selects with the same condition, we can do a more precise 600 // check: just check for aliases between the values on corresponding arms. 601 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 602 if (SI->getCondition() == SI2->getCondition()) { 603 AliasResult Alias = 604 aliasCheck(SI->getTrueValue(), SISize, 605 SI2->getTrueValue(), V2Size); 606 if (Alias == MayAlias) 607 return MayAlias; 608 AliasResult ThisAlias = 609 aliasCheck(SI->getFalseValue(), SISize, 610 SI2->getFalseValue(), V2Size); 611 if (ThisAlias != Alias) 612 return MayAlias; 613 return Alias; 614 } 615 616 // If both arms of the Select node NoAlias or MustAlias V2, then returns 617 // NoAlias / MustAlias. Otherwise, returns MayAlias. 618 AliasResult Alias = 619 aliasCheck(V2, V2Size, SI->getTrueValue(), SISize); 620 if (Alias == MayAlias) 621 return MayAlias; 622 623 // If V2 is visited, the recursive case will have been caught in the 624 // above aliasCheck call, so these subsequent calls to aliasCheck 625 // don't need to assume that V2 is being visited recursively. 626 Visited.erase(V2); 627 628 AliasResult ThisAlias = 629 aliasCheck(V2, V2Size, SI->getFalseValue(), SISize); 630 if (ThisAlias != Alias) 631 return MayAlias; 632 return Alias; 633} 634 635// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 636// against another. 637AliasAnalysis::AliasResult 638BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, 639 const Value *V2, unsigned V2Size) { 640 // The PHI node has already been visited, avoid recursion any further. 641 if (!Visited.insert(PN)) 642 return MayAlias; 643 644 // If the values are PHIs in the same block, we can do a more precise 645 // as well as efficient check: just check for aliases between the values 646 // on corresponding edges. 647 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 648 if (PN2->getParent() == PN->getParent()) { 649 AliasResult Alias = 650 aliasCheck(PN->getIncomingValue(0), PNSize, 651 PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), 652 V2Size); 653 if (Alias == MayAlias) 654 return MayAlias; 655 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { 656 AliasResult ThisAlias = 657 aliasCheck(PN->getIncomingValue(i), PNSize, 658 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 659 V2Size); 660 if (ThisAlias != Alias) 661 return MayAlias; 662 } 663 return Alias; 664 } 665 666 SmallPtrSet<Value*, 4> UniqueSrc; 667 SmallVector<Value*, 4> V1Srcs; 668 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 669 Value *PV1 = PN->getIncomingValue(i); 670 if (isa<PHINode>(PV1)) 671 // If any of the source itself is a PHI, return MayAlias conservatively 672 // to avoid compile time explosion. The worst possible case is if both 673 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 674 // and 'n' are the number of PHI sources. 675 return MayAlias; 676 if (UniqueSrc.insert(PV1)) 677 V1Srcs.push_back(PV1); 678 } 679 680 AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); 681 // Early exit if the check of the first PHI source against V2 is MayAlias. 682 // Other results are not possible. 683 if (Alias == MayAlias) 684 return MayAlias; 685 686 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 687 // NoAlias / MustAlias. Otherwise, returns MayAlias. 688 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 689 Value *V = V1Srcs[i]; 690 691 // If V2 is visited, the recursive case will have been caught in the 692 // above aliasCheck call, so these subsequent calls to aliasCheck 693 // don't need to assume that V2 is being visited recursively. 694 Visited.erase(V2); 695 696 AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); 697 if (ThisAlias != Alias || ThisAlias == MayAlias) 698 return MayAlias; 699 } 700 701 return Alias; 702} 703 704// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 705// such as array references. 706// 707AliasAnalysis::AliasResult 708BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, 709 const Value *V2, unsigned V2Size) { 710 // If either of the memory references is empty, it doesn't matter what the 711 // pointer values are. 712 if (V1Size == 0 || V2Size == 0) 713 return NoAlias; 714 715 // Strip off any casts if they exist. 716 V1 = V1->stripPointerCasts(); 717 V2 = V2->stripPointerCasts(); 718 719 // Are we checking for alias of the same value? 720 if (V1 == V2) return MustAlias; 721 722 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 723 return NoAlias; // Scalars cannot alias each other 724 725 // Figure out what objects these things are pointing to if we can. 726 const Value *O1 = V1->getUnderlyingObject(); 727 const Value *O2 = V2->getUnderlyingObject(); 728 729 // Null values in the default address space don't point to any object, so they 730 // don't alias any other pointer. 731 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 732 if (CPN->getType()->getAddressSpace() == 0) 733 return NoAlias; 734 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 735 if (CPN->getType()->getAddressSpace() == 0) 736 return NoAlias; 737 738 if (O1 != O2) { 739 // If V1/V2 point to two different objects we know that we have no alias. 740 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 741 return NoAlias; 742 743 // Constant pointers can't alias with non-const isIdentifiedObject objects. 744 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 745 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 746 return NoAlias; 747 748 // Arguments can't alias with local allocations or noalias calls 749 // in the same function. 750 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 751 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 752 return NoAlias; 753 754 // Most objects can't alias null. 755 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 756 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 757 return NoAlias; 758 759 // If one pointer is the result of a call/invoke or load and the other is a 760 // non-escaping local object within the same function, then we know the 761 // object couldn't escape to a point where the call could return it. 762 // 763 // Note that if the pointers are in different functions, there are a 764 // variety of complications. A call with a nocapture argument may still 765 // temporary store the nocapture argument's value in a temporary memory 766 // location if that memory location doesn't escape. Or it may pass a 767 // nocapture value to other functions as long as they don't capture it. 768 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 769 return NoAlias; 770 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 771 return NoAlias; 772 } 773 774 // If the size of one access is larger than the entire object on the other 775 // side, then we know such behavior is undefined and can assume no alias. 776 if (TD) 777 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) || 778 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD))) 779 return NoAlias; 780 781 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 782 // GEP can't simplify, we don't even look at the PHI cases. 783 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 784 std::swap(V1, V2); 785 std::swap(V1Size, V2Size); 786 std::swap(O1, O2); 787 } 788 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) 789 return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2); 790 791 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 792 std::swap(V1, V2); 793 std::swap(V1Size, V2Size); 794 } 795 if (const PHINode *PN = dyn_cast<PHINode>(V1)) 796 return aliasPHI(PN, V1Size, V2, V2Size); 797 798 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 799 std::swap(V1, V2); 800 std::swap(V1Size, V2Size); 801 } 802 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) 803 return aliasSelect(S1, V1Size, V2, V2Size); 804 805 return MayAlias; 806} 807 808// Make sure that anything that uses AliasAnalysis pulls in this file. 809DEFINING_FILE_FOR(BasicAliasAnalysis) 810