BasicAliasAnalysis.cpp revision 74c996cbd12aa39e75990e3e694549bb1be90e4d
1//===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file defines the primary stateless implementation of the 11// Alias Analysis interface that implements identities (two different 12// globals cannot alias, etc), but does no stateful analysis. 13// 14//===----------------------------------------------------------------------===// 15 16#include "llvm/Analysis/Passes.h" 17#include "llvm/ADT/SmallPtrSet.h" 18#include "llvm/ADT/SmallVector.h" 19#include "llvm/Analysis/AliasAnalysis.h" 20#include "llvm/Analysis/CaptureTracking.h" 21#include "llvm/Analysis/InstructionSimplify.h" 22#include "llvm/Analysis/MemoryBuiltins.h" 23#include "llvm/Analysis/ValueTracking.h" 24#include "llvm/IR/Constants.h" 25#include "llvm/IR/DataLayout.h" 26#include "llvm/IR/DerivedTypes.h" 27#include "llvm/IR/Function.h" 28#include "llvm/IR/GlobalAlias.h" 29#include "llvm/IR/GlobalVariable.h" 30#include "llvm/IR/Instructions.h" 31#include "llvm/IR/IntrinsicInst.h" 32#include "llvm/IR/LLVMContext.h" 33#include "llvm/IR/Operator.h" 34#include "llvm/Pass.h" 35#include "llvm/Support/ErrorHandling.h" 36#include "llvm/Support/GetElementPtrTypeIterator.h" 37#include "llvm/Target/TargetLibraryInfo.h" 38#include <algorithm> 39using namespace llvm; 40 41//===----------------------------------------------------------------------===// 42// Useful predicates 43//===----------------------------------------------------------------------===// 44 45/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 46/// object that never escapes from the function. 47static bool isNonEscapingLocalObject(const Value *V) { 48 // If this is a local allocation, check to see if it escapes. 49 if (isa<AllocaInst>(V) || isNoAliasCall(V)) 50 // Set StoreCaptures to True so that we can assume in our callers that the 51 // pointer is not the result of a load instruction. Currently 52 // PointerMayBeCaptured doesn't have any special analysis for the 53 // StoreCaptures=false case; if it did, our callers could be refined to be 54 // more precise. 55 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 56 57 // If this is an argument that corresponds to a byval or noalias argument, 58 // then it has not escaped before entering the function. Check if it escapes 59 // inside the function. 60 if (const Argument *A = dyn_cast<Argument>(V)) 61 if (A->hasByValAttr() || A->hasNoAliasAttr()) 62 // Note even if the argument is marked nocapture we still need to check 63 // for copies made inside the function. The nocapture attribute only 64 // specifies that there are no copies made that outlive the function. 65 return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); 66 67 return false; 68} 69 70/// isEscapeSource - Return true if the pointer is one which would have 71/// been considered an escape by isNonEscapingLocalObject. 72static bool isEscapeSource(const Value *V) { 73 if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V)) 74 return true; 75 76 // The load case works because isNonEscapingLocalObject considers all 77 // stores to be escapes (it passes true for the StoreCaptures argument 78 // to PointerMayBeCaptured). 79 if (isa<LoadInst>(V)) 80 return true; 81 82 return false; 83} 84 85/// getObjectSize - Return the size of the object specified by V, or 86/// UnknownSize if unknown. 87static uint64_t getObjectSize(const Value *V, const DataLayout &TD, 88 const TargetLibraryInfo &TLI, 89 bool RoundToAlign = false) { 90 uint64_t Size; 91 if (getObjectSize(V, Size, &TD, &TLI, RoundToAlign)) 92 return Size; 93 return AliasAnalysis::UnknownSize; 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, uint64_t Size, 99 const DataLayout &TD, 100 const TargetLibraryInfo &TLI) { 101 // Note that the meanings of the "object" are slightly different in the 102 // following contexts: 103 // c1: llvm::getObjectSize() 104 // c2: llvm.objectsize() intrinsic 105 // c3: isObjectSmallerThan() 106 // c1 and c2 share the same meaning; however, the meaning of "object" in c3 107 // refers to the "entire object". 108 // 109 // Consider this example: 110 // char *p = (char*)malloc(100) 111 // char *q = p+80; 112 // 113 // In the context of c1 and c2, the "object" pointed by q refers to the 114 // stretch of memory of q[0:19]. So, getObjectSize(q) should return 20. 115 // 116 // However, in the context of c3, the "object" refers to the chunk of memory 117 // being allocated. So, the "object" has 100 bytes, and q points to the middle 118 // the "object". In case q is passed to isObjectSmallerThan() as the 1st 119 // parameter, before the llvm::getObjectSize() is called to get the size of 120 // entire object, we should: 121 // - either rewind the pointer q to the base-address of the object in 122 // question (in this case rewind to p), or 123 // - just give up. It is up to caller to make sure the pointer is pointing 124 // to the base address the object. 125 // 126 // We go for 2nd option for simplicity. 127 if (!isIdentifiedObject(V)) 128 return false; 129 130 // This function needs to use the aligned object size because we allow 131 // reads a bit past the end given sufficient alignment. 132 uint64_t ObjectSize = getObjectSize(V, TD, TLI, /*RoundToAlign*/true); 133 134 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size; 135} 136 137/// isObjectSize - Return true if we can prove that the object specified 138/// by V has size Size. 139static bool isObjectSize(const Value *V, uint64_t Size, 140 const DataLayout &TD, const TargetLibraryInfo &TLI) { 141 uint64_t ObjectSize = getObjectSize(V, TD, TLI); 142 return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size; 143} 144 145/// isIdentifiedFunctionLocal - Return true if V is umabigously identified 146/// at the function-level. Different IdentifiedFunctionLocals can't alias. 147/// Further, an IdentifiedFunctionLocal can not alias with any function 148/// arguments other than itself, which is not neccessarily true for 149/// IdentifiedObjects. 150static bool isIdentifiedFunctionLocal(const Value *V) 151{ 152 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V); 153} 154 155 156//===----------------------------------------------------------------------===// 157// GetElementPtr Instruction Decomposition and Analysis 158//===----------------------------------------------------------------------===// 159 160namespace { 161 enum ExtensionKind { 162 EK_NotExtended, 163 EK_SignExt, 164 EK_ZeroExt 165 }; 166 167 struct VariableGEPIndex { 168 const Value *V; 169 ExtensionKind Extension; 170 int64_t Scale; 171 172 bool operator==(const VariableGEPIndex &Other) const { 173 return V == Other.V && Extension == Other.Extension && 174 Scale == Other.Scale; 175 } 176 177 bool operator!=(const VariableGEPIndex &Other) const { 178 return !operator==(Other); 179 } 180 }; 181} 182 183 184/// GetLinearExpression - Analyze the specified value as a linear expression: 185/// "A*V + B", where A and B are constant integers. Return the scale and offset 186/// values as APInts and return V as a Value*, and return whether we looked 187/// through any sign or zero extends. The incoming Value is known to have 188/// IntegerType and it may already be sign or zero extended. 189/// 190/// Note that this looks through extends, so the high bits may not be 191/// represented in the result. 192static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset, 193 ExtensionKind &Extension, 194 const DataLayout &TD, unsigned Depth) { 195 assert(V->getType()->isIntegerTy() && "Not an integer value"); 196 197 // Limit our recursion depth. 198 if (Depth == 6) { 199 Scale = 1; 200 Offset = 0; 201 return V; 202 } 203 204 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) { 205 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) { 206 switch (BOp->getOpcode()) { 207 default: break; 208 case Instruction::Or: 209 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't 210 // analyze it. 211 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD)) 212 break; 213 // FALL THROUGH. 214 case Instruction::Add: 215 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 216 TD, Depth+1); 217 Offset += RHSC->getValue(); 218 return V; 219 case Instruction::Mul: 220 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 221 TD, Depth+1); 222 Offset *= RHSC->getValue(); 223 Scale *= RHSC->getValue(); 224 return V; 225 case Instruction::Shl: 226 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 227 TD, Depth+1); 228 Offset <<= RHSC->getValue().getLimitedValue(); 229 Scale <<= RHSC->getValue().getLimitedValue(); 230 return V; 231 } 232 } 233 } 234 235 // Since GEP indices are sign extended anyway, we don't care about the high 236 // bits of a sign or zero extended value - just scales and offsets. The 237 // extensions have to be consistent though. 238 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) || 239 (isa<ZExtInst>(V) && Extension != EK_SignExt)) { 240 Value *CastOp = cast<CastInst>(V)->getOperand(0); 241 unsigned OldWidth = Scale.getBitWidth(); 242 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits(); 243 Scale = Scale.trunc(SmallWidth); 244 Offset = Offset.trunc(SmallWidth); 245 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt; 246 247 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, 248 TD, Depth+1); 249 Scale = Scale.zext(OldWidth); 250 Offset = Offset.zext(OldWidth); 251 252 return Result; 253 } 254 255 Scale = 1; 256 Offset = 0; 257 return V; 258} 259 260/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it 261/// into a base pointer with a constant offset and a number of scaled symbolic 262/// offsets. 263/// 264/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in 265/// the VarIndices vector) are Value*'s that are known to be scaled by the 266/// specified amount, but which may have other unrepresented high bits. As such, 267/// the gep cannot necessarily be reconstructed from its decomposed form. 268/// 269/// When DataLayout is around, this function is capable of analyzing everything 270/// that GetUnderlyingObject can look through. When not, it just looks 271/// through pointer casts. 272/// 273static const Value * 274DecomposeGEPExpression(const Value *V, int64_t &BaseOffs, 275 SmallVectorImpl<VariableGEPIndex> &VarIndices, 276 const DataLayout *TD) { 277 // Limit recursion depth to limit compile time in crazy cases. 278 unsigned MaxLookup = 6; 279 280 BaseOffs = 0; 281 do { 282 // See if this is a bitcast or GEP. 283 const Operator *Op = dyn_cast<Operator>(V); 284 if (Op == 0) { 285 // The only non-operator case we can handle are GlobalAliases. 286 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 287 if (!GA->mayBeOverridden()) { 288 V = GA->getAliasee(); 289 continue; 290 } 291 } 292 return V; 293 } 294 295 if (Op->getOpcode() == Instruction::BitCast) { 296 V = Op->getOperand(0); 297 continue; 298 } 299 300 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op); 301 if (GEPOp == 0) { 302 // If it's not a GEP, hand it off to SimplifyInstruction to see if it 303 // can come up with something. This matches what GetUnderlyingObject does. 304 if (const Instruction *I = dyn_cast<Instruction>(V)) 305 // TODO: Get a DominatorTree and use it here. 306 if (const Value *Simplified = 307 SimplifyInstruction(const_cast<Instruction *>(I), TD)) { 308 V = Simplified; 309 continue; 310 } 311 312 return V; 313 } 314 315 // Don't attempt to analyze GEPs over unsized objects. 316 if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized()) 317 return V; 318 319 // If we are lacking DataLayout information, we can't compute the offets of 320 // elements computed by GEPs. However, we can handle bitcast equivalent 321 // GEPs. 322 if (TD == 0) { 323 if (!GEPOp->hasAllZeroIndices()) 324 return V; 325 V = GEPOp->getOperand(0); 326 continue; 327 } 328 329 unsigned AS = GEPOp->getPointerAddressSpace(); 330 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices. 331 gep_type_iterator GTI = gep_type_begin(GEPOp); 332 for (User::const_op_iterator I = GEPOp->op_begin()+1, 333 E = GEPOp->op_end(); I != E; ++I) { 334 Value *Index = *I; 335 // Compute the (potentially symbolic) offset in bytes for this index. 336 if (StructType *STy = dyn_cast<StructType>(*GTI++)) { 337 // For a struct, add the member offset. 338 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue(); 339 if (FieldNo == 0) continue; 340 341 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo); 342 continue; 343 } 344 345 // For an array/pointer, add the element offset, explicitly scaled. 346 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) { 347 if (CIdx->isZero()) continue; 348 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue(); 349 continue; 350 } 351 352 uint64_t Scale = TD->getTypeAllocSize(*GTI); 353 ExtensionKind Extension = EK_NotExtended; 354 355 // If the integer type is smaller than the pointer size, it is implicitly 356 // sign extended to pointer size. 357 unsigned Width = Index->getType()->getIntegerBitWidth(); 358 if (TD->getPointerSizeInBits(AS) > Width) 359 Extension = EK_SignExt; 360 361 // Use GetLinearExpression to decompose the index into a C1*V+C2 form. 362 APInt IndexScale(Width, 0), IndexOffset(Width, 0); 363 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, 364 *TD, 0); 365 366 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale. 367 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale. 368 BaseOffs += IndexOffset.getSExtValue()*Scale; 369 Scale *= IndexScale.getSExtValue(); 370 371 // If we already had an occurrence of this index variable, merge this 372 // scale into it. For example, we want to handle: 373 // A[x][x] -> x*16 + x*4 -> x*20 374 // This also ensures that 'x' only appears in the index list once. 375 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) { 376 if (VarIndices[i].V == Index && 377 VarIndices[i].Extension == Extension) { 378 Scale += VarIndices[i].Scale; 379 VarIndices.erase(VarIndices.begin()+i); 380 break; 381 } 382 } 383 384 // Make sure that we have a scale that makes sense for this target's 385 // pointer size. 386 if (unsigned ShiftBits = 64 - TD->getPointerSizeInBits(AS)) { 387 Scale <<= ShiftBits; 388 Scale = (int64_t)Scale >> ShiftBits; 389 } 390 391 if (Scale) { 392 VariableGEPIndex Entry = {Index, Extension, 393 static_cast<int64_t>(Scale)}; 394 VarIndices.push_back(Entry); 395 } 396 } 397 398 // Analyze the base pointer next. 399 V = GEPOp->getOperand(0); 400 } while (--MaxLookup); 401 402 // If the chain of expressions is too deep, just return early. 403 return V; 404} 405 406/// GetIndexDifference - Dest and Src are the variable indices from two 407/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 408/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 409/// difference between the two pointers. 410static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest, 411 const SmallVectorImpl<VariableGEPIndex> &Src) { 412 if (Src.empty()) return; 413 414 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 415 const Value *V = Src[i].V; 416 ExtensionKind Extension = Src[i].Extension; 417 int64_t Scale = Src[i].Scale; 418 419 // Find V in Dest. This is N^2, but pointer indices almost never have more 420 // than a few variable indexes. 421 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 422 if (Dest[j].V != V || Dest[j].Extension != Extension) continue; 423 424 // If we found it, subtract off Scale V's from the entry in Dest. If it 425 // goes to zero, remove the entry. 426 if (Dest[j].Scale != Scale) 427 Dest[j].Scale -= Scale; 428 else 429 Dest.erase(Dest.begin()+j); 430 Scale = 0; 431 break; 432 } 433 434 // If we didn't consume this entry, add it to the end of the Dest list. 435 if (Scale) { 436 VariableGEPIndex Entry = { V, Extension, -Scale }; 437 Dest.push_back(Entry); 438 } 439 } 440} 441 442//===----------------------------------------------------------------------===// 443// BasicAliasAnalysis Pass 444//===----------------------------------------------------------------------===// 445 446#ifndef NDEBUG 447static const Function *getParent(const Value *V) { 448 if (const Instruction *inst = dyn_cast<Instruction>(V)) 449 return inst->getParent()->getParent(); 450 451 if (const Argument *arg = dyn_cast<Argument>(V)) 452 return arg->getParent(); 453 454 return NULL; 455} 456 457static bool notDifferentParent(const Value *O1, const Value *O2) { 458 459 const Function *F1 = getParent(O1); 460 const Function *F2 = getParent(O2); 461 462 return !F1 || !F2 || F1 == F2; 463} 464#endif 465 466namespace { 467 /// BasicAliasAnalysis - This is the primary alias analysis implementation. 468 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis { 469 static char ID; // Class identification, replacement for typeinfo 470 BasicAliasAnalysis() : ImmutablePass(ID) { 471 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry()); 472 } 473 474 virtual void initializePass() { 475 InitializeAliasAnalysis(this); 476 } 477 478 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 479 AU.addRequired<AliasAnalysis>(); 480 AU.addRequired<TargetLibraryInfo>(); 481 } 482 483 virtual AliasResult alias(const Location &LocA, 484 const Location &LocB) { 485 assert(AliasCache.empty() && "AliasCache must be cleared after use!"); 486 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) && 487 "BasicAliasAnalysis doesn't support interprocedural queries."); 488 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag, 489 LocB.Ptr, LocB.Size, LocB.TBAATag); 490 // AliasCache rarely has more than 1 or 2 elements, always use 491 // shrink_and_clear so it quickly returns to the inline capacity of the 492 // SmallDenseMap if it ever grows larger. 493 // FIXME: This should really be shrink_to_inline_capacity_and_clear(). 494 AliasCache.shrink_and_clear(); 495 return Alias; 496 } 497 498 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 499 const Location &Loc); 500 501 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 502 ImmutableCallSite CS2) { 503 // The AliasAnalysis base class has some smarts, lets use them. 504 return AliasAnalysis::getModRefInfo(CS1, CS2); 505 } 506 507 /// pointsToConstantMemory - Chase pointers until we find a (constant 508 /// global) or not. 509 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal); 510 511 /// getModRefBehavior - Return the behavior when calling the given 512 /// call site. 513 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); 514 515 /// getModRefBehavior - Return the behavior when calling the given function. 516 /// For use when the call site is not known. 517 virtual ModRefBehavior getModRefBehavior(const Function *F); 518 519 /// getAdjustedAnalysisPointer - This method is used when a pass implements 520 /// an analysis interface through multiple inheritance. If needed, it 521 /// should override this to adjust the this pointer as needed for the 522 /// specified pass info. 523 virtual void *getAdjustedAnalysisPointer(const void *ID) { 524 if (ID == &AliasAnalysis::ID) 525 return (AliasAnalysis*)this; 526 return this; 527 } 528 529 private: 530 // AliasCache - Track alias queries to guard against recursion. 531 typedef std::pair<Location, Location> LocPair; 532 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy; 533 AliasCacheTy AliasCache; 534 535 // Visited - Track instructions visited by pointsToConstantMemory. 536 SmallPtrSet<const Value*, 16> Visited; 537 538 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 539 // instruction against another. 540 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size, 541 const MDNode *V1TBAAInfo, 542 const Value *V2, uint64_t V2Size, 543 const MDNode *V2TBAAInfo, 544 const Value *UnderlyingV1, const Value *UnderlyingV2); 545 546 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 547 // instruction against another. 548 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize, 549 const MDNode *PNTBAAInfo, 550 const Value *V2, uint64_t V2Size, 551 const MDNode *V2TBAAInfo); 552 553 /// aliasSelect - Disambiguate a Select instruction against another value. 554 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize, 555 const MDNode *SITBAAInfo, 556 const Value *V2, uint64_t V2Size, 557 const MDNode *V2TBAAInfo); 558 559 AliasResult aliasCheck(const Value *V1, uint64_t V1Size, 560 const MDNode *V1TBAATag, 561 const Value *V2, uint64_t V2Size, 562 const MDNode *V2TBAATag); 563 }; 564} // End of anonymous namespace 565 566// Register this pass... 567char BasicAliasAnalysis::ID = 0; 568INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa", 569 "Basic Alias Analysis (stateless AA impl)", 570 false, true, false) 571INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 572INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa", 573 "Basic Alias Analysis (stateless AA impl)", 574 false, true, false) 575 576 577ImmutablePass *llvm::createBasicAliasAnalysisPass() { 578 return new BasicAliasAnalysis(); 579} 580 581/// pointsToConstantMemory - Returns whether the given pointer value 582/// points to memory that is local to the function, with global constants being 583/// considered local to all functions. 584bool 585BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) { 586 assert(Visited.empty() && "Visited must be cleared after use!"); 587 588 unsigned MaxLookup = 8; 589 SmallVector<const Value *, 16> Worklist; 590 Worklist.push_back(Loc.Ptr); 591 do { 592 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), TD); 593 if (!Visited.insert(V)) { 594 Visited.clear(); 595 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 596 } 597 598 // An alloca instruction defines local memory. 599 if (OrLocal && isa<AllocaInst>(V)) 600 continue; 601 602 // A global constant counts as local memory for our purposes. 603 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 604 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 605 // global to be marked constant in some modules and non-constant in 606 // others. GV may even be a declaration, not a definition. 607 if (!GV->isConstant()) { 608 Visited.clear(); 609 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 610 } 611 continue; 612 } 613 614 // If both select values point to local memory, then so does the select. 615 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) { 616 Worklist.push_back(SI->getTrueValue()); 617 Worklist.push_back(SI->getFalseValue()); 618 continue; 619 } 620 621 // If all values incoming to a phi node point to local memory, then so does 622 // the phi. 623 if (const PHINode *PN = dyn_cast<PHINode>(V)) { 624 // Don't bother inspecting phi nodes with many operands. 625 if (PN->getNumIncomingValues() > MaxLookup) { 626 Visited.clear(); 627 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 628 } 629 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 630 Worklist.push_back(PN->getIncomingValue(i)); 631 continue; 632 } 633 634 // Otherwise be conservative. 635 Visited.clear(); 636 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 637 638 } while (!Worklist.empty() && --MaxLookup); 639 640 Visited.clear(); 641 return Worklist.empty(); 642} 643 644/// getModRefBehavior - Return the behavior when calling the given call site. 645AliasAnalysis::ModRefBehavior 646BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 647 if (CS.doesNotAccessMemory()) 648 // Can't do better than this. 649 return DoesNotAccessMemory; 650 651 ModRefBehavior Min = UnknownModRefBehavior; 652 653 // If the callsite knows it only reads memory, don't return worse 654 // than that. 655 if (CS.onlyReadsMemory()) 656 Min = OnlyReadsMemory; 657 658 // The AliasAnalysis base class has some smarts, lets use them. 659 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min); 660} 661 662/// getModRefBehavior - Return the behavior when calling the given function. 663/// For use when the call site is not known. 664AliasAnalysis::ModRefBehavior 665BasicAliasAnalysis::getModRefBehavior(const Function *F) { 666 // If the function declares it doesn't access memory, we can't do better. 667 if (F->doesNotAccessMemory()) 668 return DoesNotAccessMemory; 669 670 // For intrinsics, we can check the table. 671 if (unsigned iid = F->getIntrinsicID()) { 672#define GET_INTRINSIC_MODREF_BEHAVIOR 673#include "llvm/IR/Intrinsics.gen" 674#undef GET_INTRINSIC_MODREF_BEHAVIOR 675 } 676 677 ModRefBehavior Min = UnknownModRefBehavior; 678 679 // If the function declares it only reads memory, go with that. 680 if (F->onlyReadsMemory()) 681 Min = OnlyReadsMemory; 682 683 // Otherwise be conservative. 684 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min); 685} 686 687/// getModRefInfo - Check to see if the specified callsite can clobber the 688/// specified memory object. Since we only look at local properties of this 689/// function, we really can't say much about this query. We do, however, use 690/// simple "address taken" analysis on local objects. 691AliasAnalysis::ModRefResult 692BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 693 const Location &Loc) { 694 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) && 695 "AliasAnalysis query involving multiple functions!"); 696 697 const Value *Object = GetUnderlyingObject(Loc.Ptr, TD); 698 699 // If this is a tail call and Loc.Ptr points to a stack location, we know that 700 // the tail call cannot access or modify the local stack. 701 // We cannot exclude byval arguments here; these belong to the caller of 702 // the current function not to the current function, and a tail callee 703 // may reference them. 704 if (isa<AllocaInst>(Object)) 705 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 706 if (CI->isTailCall()) 707 return NoModRef; 708 709 // If the pointer is to a locally allocated object that does not escape, 710 // then the call can not mod/ref the pointer unless the call takes the pointer 711 // as an argument, and itself doesn't capture it. 712 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 713 isNonEscapingLocalObject(Object)) { 714 bool PassedAsArg = false; 715 unsigned ArgNo = 0; 716 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 717 CI != CE; ++CI, ++ArgNo) { 718 // Only look at the no-capture or byval pointer arguments. If this 719 // pointer were passed to arguments that were neither of these, then it 720 // couldn't be no-capture. 721 if (!(*CI)->getType()->isPointerTy() || 722 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo))) 723 continue; 724 725 // If this is a no-capture pointer argument, see if we can tell that it 726 // is impossible to alias the pointer we're checking. If not, we have to 727 // assume that the call could touch the pointer, even though it doesn't 728 // escape. 729 if (!isNoAlias(Location(*CI), Location(Object))) { 730 PassedAsArg = true; 731 break; 732 } 733 } 734 735 if (!PassedAsArg) 736 return NoModRef; 737 } 738 739 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>(); 740 ModRefResult Min = ModRef; 741 742 // Finally, handle specific knowledge of intrinsics. 743 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 744 if (II != 0) 745 switch (II->getIntrinsicID()) { 746 default: break; 747 case Intrinsic::memcpy: 748 case Intrinsic::memmove: { 749 uint64_t Len = UnknownSize; 750 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 751 Len = LenCI->getZExtValue(); 752 Value *Dest = II->getArgOperand(0); 753 Value *Src = II->getArgOperand(1); 754 // If it can't overlap the source dest, then it doesn't modref the loc. 755 if (isNoAlias(Location(Dest, Len), Loc)) { 756 if (isNoAlias(Location(Src, Len), Loc)) 757 return NoModRef; 758 // If it can't overlap the dest, then worst case it reads the loc. 759 Min = Ref; 760 } else if (isNoAlias(Location(Src, Len), Loc)) { 761 // If it can't overlap the source, then worst case it mutates the loc. 762 Min = Mod; 763 } 764 break; 765 } 766 case Intrinsic::memset: 767 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 768 // will handle it for the variable length case. 769 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 770 uint64_t Len = LenCI->getZExtValue(); 771 Value *Dest = II->getArgOperand(0); 772 if (isNoAlias(Location(Dest, Len), Loc)) 773 return NoModRef; 774 } 775 // We know that memset doesn't load anything. 776 Min = Mod; 777 break; 778 case Intrinsic::lifetime_start: 779 case Intrinsic::lifetime_end: 780 case Intrinsic::invariant_start: { 781 uint64_t PtrSize = 782 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 783 if (isNoAlias(Location(II->getArgOperand(1), 784 PtrSize, 785 II->getMetadata(LLVMContext::MD_tbaa)), 786 Loc)) 787 return NoModRef; 788 break; 789 } 790 case Intrinsic::invariant_end: { 791 uint64_t PtrSize = 792 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 793 if (isNoAlias(Location(II->getArgOperand(2), 794 PtrSize, 795 II->getMetadata(LLVMContext::MD_tbaa)), 796 Loc)) 797 return NoModRef; 798 break; 799 } 800 case Intrinsic::arm_neon_vld1: { 801 // LLVM's vld1 and vst1 intrinsics currently only support a single 802 // vector register. 803 uint64_t Size = 804 TD ? TD->getTypeStoreSize(II->getType()) : UnknownSize; 805 if (isNoAlias(Location(II->getArgOperand(0), Size, 806 II->getMetadata(LLVMContext::MD_tbaa)), 807 Loc)) 808 return NoModRef; 809 break; 810 } 811 case Intrinsic::arm_neon_vst1: { 812 uint64_t Size = 813 TD ? TD->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize; 814 if (isNoAlias(Location(II->getArgOperand(0), Size, 815 II->getMetadata(LLVMContext::MD_tbaa)), 816 Loc)) 817 return NoModRef; 818 break; 819 } 820 } 821 822 // We can bound the aliasing properties of memset_pattern16 just as we can 823 // for memcpy/memset. This is particularly important because the 824 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16 825 // whenever possible. 826 else if (TLI.has(LibFunc::memset_pattern16) && 827 CS.getCalledFunction() && 828 CS.getCalledFunction()->getName() == "memset_pattern16") { 829 const Function *MS = CS.getCalledFunction(); 830 FunctionType *MemsetType = MS->getFunctionType(); 831 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 && 832 isa<PointerType>(MemsetType->getParamType(0)) && 833 isa<PointerType>(MemsetType->getParamType(1)) && 834 isa<IntegerType>(MemsetType->getParamType(2))) { 835 uint64_t Len = UnknownSize; 836 if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2))) 837 Len = LenCI->getZExtValue(); 838 const Value *Dest = CS.getArgument(0); 839 const Value *Src = CS.getArgument(1); 840 // If it can't overlap the source dest, then it doesn't modref the loc. 841 if (isNoAlias(Location(Dest, Len), Loc)) { 842 // Always reads 16 bytes of the source. 843 if (isNoAlias(Location(Src, 16), Loc)) 844 return NoModRef; 845 // If it can't overlap the dest, then worst case it reads the loc. 846 Min = Ref; 847 // Always reads 16 bytes of the source. 848 } else if (isNoAlias(Location(Src, 16), Loc)) { 849 // If it can't overlap the source, then worst case it mutates the loc. 850 Min = Mod; 851 } 852 } 853 } 854 855 // The AliasAnalysis base class has some smarts, lets use them. 856 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min); 857} 858 859static bool areVarIndicesEqual(SmallVectorImpl<VariableGEPIndex> &Indices1, 860 SmallVectorImpl<VariableGEPIndex> &Indices2) { 861 unsigned Size1 = Indices1.size(); 862 unsigned Size2 = Indices2.size(); 863 864 if (Size1 != Size2) 865 return false; 866 867 for (unsigned I = 0; I != Size1; ++I) 868 if (Indices1[I] != Indices2[I]) 869 return false; 870 871 return true; 872} 873 874/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 875/// against another pointer. We know that V1 is a GEP, but we don't know 876/// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, TD), 877/// UnderlyingV2 is the same for V2. 878/// 879AliasAnalysis::AliasResult 880BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size, 881 const MDNode *V1TBAAInfo, 882 const Value *V2, uint64_t V2Size, 883 const MDNode *V2TBAAInfo, 884 const Value *UnderlyingV1, 885 const Value *UnderlyingV2) { 886 int64_t GEP1BaseOffset; 887 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices; 888 889 // If we have two gep instructions with must-alias or not-alias'ing base 890 // pointers, figure out if the indexes to the GEP tell us anything about the 891 // derived pointer. 892 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 893 // Do the base pointers alias? 894 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0, 895 UnderlyingV2, UnknownSize, 0); 896 897 // Check for geps of non-aliasing underlying pointers where the offsets are 898 // identical. 899 if ((BaseAlias == MayAlias) && V1Size == V2Size) { 900 // Do the base pointers alias assuming type and size. 901 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size, 902 V1TBAAInfo, UnderlyingV2, 903 V2Size, V2TBAAInfo); 904 if (PreciseBaseAlias == NoAlias) { 905 // See if the computed offset from the common pointer tells us about the 906 // relation of the resulting pointer. 907 int64_t GEP2BaseOffset; 908 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 909 const Value *GEP2BasePtr = 910 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 911 const Value *GEP1BasePtr = 912 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 913 // DecomposeGEPExpression and GetUnderlyingObject should return the 914 // same result except when DecomposeGEPExpression has no DataLayout. 915 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 916 assert(TD == 0 && 917 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 918 return MayAlias; 919 } 920 // Same offsets. 921 if (GEP1BaseOffset == GEP2BaseOffset && 922 areVarIndicesEqual(GEP1VariableIndices, GEP2VariableIndices)) 923 return NoAlias; 924 GEP1VariableIndices.clear(); 925 } 926 } 927 928 // If we get a No or May, then return it immediately, no amount of analysis 929 // will improve this situation. 930 if (BaseAlias != MustAlias) return BaseAlias; 931 932 // Otherwise, we have a MustAlias. Since the base pointers alias each other 933 // exactly, see if the computed offset from the common pointer tells us 934 // about the relation of the resulting pointer. 935 const Value *GEP1BasePtr = 936 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 937 938 int64_t GEP2BaseOffset; 939 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 940 const Value *GEP2BasePtr = 941 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 942 943 // DecomposeGEPExpression and GetUnderlyingObject should return the 944 // same result except when DecomposeGEPExpression has no DataLayout. 945 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 946 assert(TD == 0 && 947 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 948 return MayAlias; 949 } 950 951 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 952 // symbolic difference. 953 GEP1BaseOffset -= GEP2BaseOffset; 954 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 955 956 } else { 957 // Check to see if these two pointers are related by the getelementptr 958 // instruction. If one pointer is a GEP with a non-zero index of the other 959 // pointer, we know they cannot alias. 960 961 // If both accesses are unknown size, we can't do anything useful here. 962 if (V1Size == UnknownSize && V2Size == UnknownSize) 963 return MayAlias; 964 965 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0, 966 V2, V2Size, V2TBAAInfo); 967 if (R != MustAlias) 968 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 969 // If V2 is known not to alias GEP base pointer, then the two values 970 // cannot alias per GEP semantics: "A pointer value formed from a 971 // getelementptr instruction is associated with the addresses associated 972 // with the first operand of the getelementptr". 973 return R; 974 975 const Value *GEP1BasePtr = 976 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 977 978 // DecomposeGEPExpression and GetUnderlyingObject should return the 979 // same result except when DecomposeGEPExpression has no DataLayout. 980 if (GEP1BasePtr != UnderlyingV1) { 981 assert(TD == 0 && 982 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 983 return MayAlias; 984 } 985 } 986 987 // In the two GEP Case, if there is no difference in the offsets of the 988 // computed pointers, the resultant pointers are a must alias. This 989 // hapens when we have two lexically identical GEP's (for example). 990 // 991 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 992 // must aliases the GEP, the end result is a must alias also. 993 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 994 return MustAlias; 995 996 // If there is a constant difference between the pointers, but the difference 997 // is less than the size of the associated memory object, then we know 998 // that the objects are partially overlapping. If the difference is 999 // greater, we know they do not overlap. 1000 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) { 1001 if (GEP1BaseOffset >= 0) { 1002 if (V2Size != UnknownSize) { 1003 if ((uint64_t)GEP1BaseOffset < V2Size) 1004 return PartialAlias; 1005 return NoAlias; 1006 } 1007 } else { 1008 if (V1Size != UnknownSize) { 1009 if (-(uint64_t)GEP1BaseOffset < V1Size) 1010 return PartialAlias; 1011 return NoAlias; 1012 } 1013 } 1014 } 1015 1016 // Try to distinguish something like &A[i][1] against &A[42][0]. 1017 // Grab the least significant bit set in any of the scales. 1018 if (!GEP1VariableIndices.empty()) { 1019 uint64_t Modulo = 0; 1020 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) 1021 Modulo |= (uint64_t)GEP1VariableIndices[i].Scale; 1022 Modulo = Modulo ^ (Modulo & (Modulo - 1)); 1023 1024 // We can compute the difference between the two addresses 1025 // mod Modulo. Check whether that difference guarantees that the 1026 // two locations do not alias. 1027 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1); 1028 if (V1Size != UnknownSize && V2Size != UnknownSize && 1029 ModOffset >= V2Size && V1Size <= Modulo - ModOffset) 1030 return NoAlias; 1031 } 1032 1033 // Statically, we can see that the base objects are the same, but the 1034 // pointers have dynamic offsets which we can't resolve. And none of our 1035 // little tricks above worked. 1036 // 1037 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the 1038 // practical effect of this is protecting TBAA in the case of dynamic 1039 // indices into arrays of unions or malloc'd memory. 1040 return PartialAlias; 1041} 1042 1043static AliasAnalysis::AliasResult 1044MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) { 1045 // If the results agree, take it. 1046 if (A == B) 1047 return A; 1048 // A mix of PartialAlias and MustAlias is PartialAlias. 1049 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) || 1050 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias)) 1051 return AliasAnalysis::PartialAlias; 1052 // Otherwise, we don't know anything. 1053 return AliasAnalysis::MayAlias; 1054} 1055 1056/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 1057/// instruction against another. 1058AliasAnalysis::AliasResult 1059BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize, 1060 const MDNode *SITBAAInfo, 1061 const Value *V2, uint64_t V2Size, 1062 const MDNode *V2TBAAInfo) { 1063 // If the values are Selects with the same condition, we can do a more precise 1064 // check: just check for aliases between the values on corresponding arms. 1065 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 1066 if (SI->getCondition() == SI2->getCondition()) { 1067 AliasResult Alias = 1068 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo, 1069 SI2->getTrueValue(), V2Size, V2TBAAInfo); 1070 if (Alias == MayAlias) 1071 return MayAlias; 1072 AliasResult ThisAlias = 1073 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo, 1074 SI2->getFalseValue(), V2Size, V2TBAAInfo); 1075 return MergeAliasResults(ThisAlias, Alias); 1076 } 1077 1078 // If both arms of the Select node NoAlias or MustAlias V2, then returns 1079 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1080 AliasResult Alias = 1081 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo); 1082 if (Alias == MayAlias) 1083 return MayAlias; 1084 1085 AliasResult ThisAlias = 1086 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo); 1087 return MergeAliasResults(ThisAlias, Alias); 1088} 1089 1090// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 1091// against another. 1092AliasAnalysis::AliasResult 1093BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize, 1094 const MDNode *PNTBAAInfo, 1095 const Value *V2, uint64_t V2Size, 1096 const MDNode *V2TBAAInfo) { 1097 // If the values are PHIs in the same block, we can do a more precise 1098 // as well as efficient check: just check for aliases between the values 1099 // on corresponding edges. 1100 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 1101 if (PN2->getParent() == PN->getParent()) { 1102 LocPair Locs(Location(PN, PNSize, PNTBAAInfo), 1103 Location(V2, V2Size, V2TBAAInfo)); 1104 if (PN > V2) 1105 std::swap(Locs.first, Locs.second); 1106 // Analyse the PHIs' inputs under the assumption that the PHIs are 1107 // NoAlias. 1108 // If the PHIs are May/MustAlias there must be (recursively) an input 1109 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or 1110 // there must be an operation on the PHIs within the PHIs' value cycle 1111 // that causes a MayAlias. 1112 // Pretend the phis do not alias. 1113 AliasResult Alias = NoAlias; 1114 assert(AliasCache.count(Locs) && 1115 "There must exist an entry for the phi node"); 1116 AliasResult OrigAliasResult = AliasCache[Locs]; 1117 AliasCache[Locs] = NoAlias; 1118 1119 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1120 AliasResult ThisAlias = 1121 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo, 1122 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 1123 V2Size, V2TBAAInfo); 1124 Alias = MergeAliasResults(ThisAlias, Alias); 1125 if (Alias == MayAlias) 1126 break; 1127 } 1128 1129 // Reset if speculation failed. 1130 if (Alias != NoAlias) 1131 AliasCache[Locs] = OrigAliasResult; 1132 1133 return Alias; 1134 } 1135 1136 SmallPtrSet<Value*, 4> UniqueSrc; 1137 SmallVector<Value*, 4> V1Srcs; 1138 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1139 Value *PV1 = PN->getIncomingValue(i); 1140 if (isa<PHINode>(PV1)) 1141 // If any of the source itself is a PHI, return MayAlias conservatively 1142 // to avoid compile time explosion. The worst possible case is if both 1143 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 1144 // and 'n' are the number of PHI sources. 1145 return MayAlias; 1146 if (UniqueSrc.insert(PV1)) 1147 V1Srcs.push_back(PV1); 1148 } 1149 1150 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo, 1151 V1Srcs[0], PNSize, PNTBAAInfo); 1152 // Early exit if the check of the first PHI source against V2 is MayAlias. 1153 // Other results are not possible. 1154 if (Alias == MayAlias) 1155 return MayAlias; 1156 1157 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 1158 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1159 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 1160 Value *V = V1Srcs[i]; 1161 1162 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo, 1163 V, PNSize, PNTBAAInfo); 1164 Alias = MergeAliasResults(ThisAlias, Alias); 1165 if (Alias == MayAlias) 1166 break; 1167 } 1168 1169 return Alias; 1170} 1171 1172// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 1173// such as array references. 1174// 1175AliasAnalysis::AliasResult 1176BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size, 1177 const MDNode *V1TBAAInfo, 1178 const Value *V2, uint64_t V2Size, 1179 const MDNode *V2TBAAInfo) { 1180 // If either of the memory references is empty, it doesn't matter what the 1181 // pointer values are. 1182 if (V1Size == 0 || V2Size == 0) 1183 return NoAlias; 1184 1185 // Strip off any casts if they exist. 1186 V1 = V1->stripPointerCasts(); 1187 V2 = V2->stripPointerCasts(); 1188 1189 // Are we checking for alias of the same value? 1190 if (V1 == V2) return MustAlias; 1191 1192 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 1193 return NoAlias; // Scalars cannot alias each other 1194 1195 // Figure out what objects these things are pointing to if we can. 1196 const Value *O1 = GetUnderlyingObject(V1, TD); 1197 const Value *O2 = GetUnderlyingObject(V2, TD); 1198 1199 // Null values in the default address space don't point to any object, so they 1200 // don't alias any other pointer. 1201 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 1202 if (CPN->getType()->getAddressSpace() == 0) 1203 return NoAlias; 1204 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 1205 if (CPN->getType()->getAddressSpace() == 0) 1206 return NoAlias; 1207 1208 if (O1 != O2) { 1209 // If V1/V2 point to two different objects we know that we have no alias. 1210 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 1211 return NoAlias; 1212 1213 // Constant pointers can't alias with non-const isIdentifiedObject objects. 1214 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 1215 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 1216 return NoAlias; 1217 1218 // Function arguments can't alias with things that are known to be 1219 // unambigously identified at the function level. 1220 if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) || 1221 (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1))) 1222 return NoAlias; 1223 1224 // Most objects can't alias null. 1225 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 1226 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 1227 return NoAlias; 1228 1229 // If one pointer is the result of a call/invoke or load and the other is a 1230 // non-escaping local object within the same function, then we know the 1231 // object couldn't escape to a point where the call could return it. 1232 // 1233 // Note that if the pointers are in different functions, there are a 1234 // variety of complications. A call with a nocapture argument may still 1235 // temporary store the nocapture argument's value in a temporary memory 1236 // location if that memory location doesn't escape. Or it may pass a 1237 // nocapture value to other functions as long as they don't capture it. 1238 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 1239 return NoAlias; 1240 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 1241 return NoAlias; 1242 } 1243 1244 // If the size of one access is larger than the entire object on the other 1245 // side, then we know such behavior is undefined and can assume no alias. 1246 if (TD) 1247 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD, *TLI)) || 1248 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD, *TLI))) 1249 return NoAlias; 1250 1251 // Check the cache before climbing up use-def chains. This also terminates 1252 // otherwise infinitely recursive queries. 1253 LocPair Locs(Location(V1, V1Size, V1TBAAInfo), 1254 Location(V2, V2Size, V2TBAAInfo)); 1255 if (V1 > V2) 1256 std::swap(Locs.first, Locs.second); 1257 std::pair<AliasCacheTy::iterator, bool> Pair = 1258 AliasCache.insert(std::make_pair(Locs, MayAlias)); 1259 if (!Pair.second) 1260 return Pair.first->second; 1261 1262 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 1263 // GEP can't simplify, we don't even look at the PHI cases. 1264 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 1265 std::swap(V1, V2); 1266 std::swap(V1Size, V2Size); 1267 std::swap(O1, O2); 1268 std::swap(V1TBAAInfo, V2TBAAInfo); 1269 } 1270 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) { 1271 AliasResult Result = aliasGEP(GV1, V1Size, V1TBAAInfo, V2, V2Size, V2TBAAInfo, O1, O2); 1272 if (Result != MayAlias) return AliasCache[Locs] = Result; 1273 } 1274 1275 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 1276 std::swap(V1, V2); 1277 std::swap(V1Size, V2Size); 1278 std::swap(V1TBAAInfo, V2TBAAInfo); 1279 } 1280 if (const PHINode *PN = dyn_cast<PHINode>(V1)) { 1281 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo, 1282 V2, V2Size, V2TBAAInfo); 1283 if (Result != MayAlias) return AliasCache[Locs] = Result; 1284 } 1285 1286 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 1287 std::swap(V1, V2); 1288 std::swap(V1Size, V2Size); 1289 std::swap(V1TBAAInfo, V2TBAAInfo); 1290 } 1291 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) { 1292 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo, 1293 V2, V2Size, V2TBAAInfo); 1294 if (Result != MayAlias) return AliasCache[Locs] = Result; 1295 } 1296 1297 // If both pointers are pointing into the same object and one of them 1298 // accesses is accessing the entire object, then the accesses must 1299 // overlap in some way. 1300 if (TD && O1 == O2) 1301 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD, *TLI)) || 1302 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD, *TLI))) 1303 return AliasCache[Locs] = PartialAlias; 1304 1305 AliasResult Result = 1306 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo), 1307 Location(V2, V2Size, V2TBAAInfo)); 1308 return AliasCache[Locs] = Result; 1309} 1310