BasicAliasAnalysis.cpp revision 256281
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//===----------------------------------------------------------------------===// 146// GetElementPtr Instruction Decomposition and Analysis 147//===----------------------------------------------------------------------===// 148 149namespace { 150 enum ExtensionKind { 151 EK_NotExtended, 152 EK_SignExt, 153 EK_ZeroExt 154 }; 155 156 struct VariableGEPIndex { 157 const Value *V; 158 ExtensionKind Extension; 159 int64_t Scale; 160 161 bool operator==(const VariableGEPIndex &Other) const { 162 return V == Other.V && Extension == Other.Extension && 163 Scale == Other.Scale; 164 } 165 166 bool operator!=(const VariableGEPIndex &Other) const { 167 return !operator==(Other); 168 } 169 }; 170} 171 172 173/// GetLinearExpression - Analyze the specified value as a linear expression: 174/// "A*V + B", where A and B are constant integers. Return the scale and offset 175/// values as APInts and return V as a Value*, and return whether we looked 176/// through any sign or zero extends. The incoming Value is known to have 177/// IntegerType and it may already be sign or zero extended. 178/// 179/// Note that this looks through extends, so the high bits may not be 180/// represented in the result. 181static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset, 182 ExtensionKind &Extension, 183 const DataLayout &TD, unsigned Depth) { 184 assert(V->getType()->isIntegerTy() && "Not an integer value"); 185 186 // Limit our recursion depth. 187 if (Depth == 6) { 188 Scale = 1; 189 Offset = 0; 190 return V; 191 } 192 193 if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) { 194 if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) { 195 switch (BOp->getOpcode()) { 196 default: break; 197 case Instruction::Or: 198 // X|C == X+C if all the bits in C are unset in X. Otherwise we can't 199 // analyze it. 200 if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &TD)) 201 break; 202 // FALL THROUGH. 203 case Instruction::Add: 204 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 205 TD, Depth+1); 206 Offset += RHSC->getValue(); 207 return V; 208 case Instruction::Mul: 209 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 210 TD, Depth+1); 211 Offset *= RHSC->getValue(); 212 Scale *= RHSC->getValue(); 213 return V; 214 case Instruction::Shl: 215 V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension, 216 TD, Depth+1); 217 Offset <<= RHSC->getValue().getLimitedValue(); 218 Scale <<= RHSC->getValue().getLimitedValue(); 219 return V; 220 } 221 } 222 } 223 224 // Since GEP indices are sign extended anyway, we don't care about the high 225 // bits of a sign or zero extended value - just scales and offsets. The 226 // extensions have to be consistent though. 227 if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) || 228 (isa<ZExtInst>(V) && Extension != EK_SignExt)) { 229 Value *CastOp = cast<CastInst>(V)->getOperand(0); 230 unsigned OldWidth = Scale.getBitWidth(); 231 unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits(); 232 Scale = Scale.trunc(SmallWidth); 233 Offset = Offset.trunc(SmallWidth); 234 Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt; 235 236 Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, 237 TD, Depth+1); 238 Scale = Scale.zext(OldWidth); 239 Offset = Offset.zext(OldWidth); 240 241 return Result; 242 } 243 244 Scale = 1; 245 Offset = 0; 246 return V; 247} 248 249/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it 250/// into a base pointer with a constant offset and a number of scaled symbolic 251/// offsets. 252/// 253/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in 254/// the VarIndices vector) are Value*'s that are known to be scaled by the 255/// specified amount, but which may have other unrepresented high bits. As such, 256/// the gep cannot necessarily be reconstructed from its decomposed form. 257/// 258/// When DataLayout is around, this function is capable of analyzing everything 259/// that GetUnderlyingObject can look through. When not, it just looks 260/// through pointer casts. 261/// 262static const Value * 263DecomposeGEPExpression(const Value *V, int64_t &BaseOffs, 264 SmallVectorImpl<VariableGEPIndex> &VarIndices, 265 const DataLayout *TD) { 266 // Limit recursion depth to limit compile time in crazy cases. 267 unsigned MaxLookup = 6; 268 269 BaseOffs = 0; 270 do { 271 // See if this is a bitcast or GEP. 272 const Operator *Op = dyn_cast<Operator>(V); 273 if (Op == 0) { 274 // The only non-operator case we can handle are GlobalAliases. 275 if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 276 if (!GA->mayBeOverridden()) { 277 V = GA->getAliasee(); 278 continue; 279 } 280 } 281 return V; 282 } 283 284 if (Op->getOpcode() == Instruction::BitCast) { 285 V = Op->getOperand(0); 286 continue; 287 } 288 289 const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op); 290 if (GEPOp == 0) { 291 // If it's not a GEP, hand it off to SimplifyInstruction to see if it 292 // can come up with something. This matches what GetUnderlyingObject does. 293 if (const Instruction *I = dyn_cast<Instruction>(V)) 294 // TODO: Get a DominatorTree and use it here. 295 if (const Value *Simplified = 296 SimplifyInstruction(const_cast<Instruction *>(I), TD)) { 297 V = Simplified; 298 continue; 299 } 300 301 return V; 302 } 303 304 // Don't attempt to analyze GEPs over unsized objects. 305 if (!cast<PointerType>(GEPOp->getOperand(0)->getType()) 306 ->getElementType()->isSized()) 307 return V; 308 309 // If we are lacking DataLayout information, we can't compute the offets of 310 // elements computed by GEPs. However, we can handle bitcast equivalent 311 // GEPs. 312 if (TD == 0) { 313 if (!GEPOp->hasAllZeroIndices()) 314 return V; 315 V = GEPOp->getOperand(0); 316 continue; 317 } 318 319 // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices. 320 gep_type_iterator GTI = gep_type_begin(GEPOp); 321 for (User::const_op_iterator I = GEPOp->op_begin()+1, 322 E = GEPOp->op_end(); I != E; ++I) { 323 Value *Index = *I; 324 // Compute the (potentially symbolic) offset in bytes for this index. 325 if (StructType *STy = dyn_cast<StructType>(*GTI++)) { 326 // For a struct, add the member offset. 327 unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue(); 328 if (FieldNo == 0) continue; 329 330 BaseOffs += TD->getStructLayout(STy)->getElementOffset(FieldNo); 331 continue; 332 } 333 334 // For an array/pointer, add the element offset, explicitly scaled. 335 if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) { 336 if (CIdx->isZero()) continue; 337 BaseOffs += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue(); 338 continue; 339 } 340 341 uint64_t Scale = TD->getTypeAllocSize(*GTI); 342 ExtensionKind Extension = EK_NotExtended; 343 344 // If the integer type is smaller than the pointer size, it is implicitly 345 // sign extended to pointer size. 346 unsigned Width = cast<IntegerType>(Index->getType())->getBitWidth(); 347 if (TD->getPointerSizeInBits() > Width) 348 Extension = EK_SignExt; 349 350 // Use GetLinearExpression to decompose the index into a C1*V+C2 form. 351 APInt IndexScale(Width, 0), IndexOffset(Width, 0); 352 Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, 353 *TD, 0); 354 355 // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale. 356 // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale. 357 BaseOffs += IndexOffset.getSExtValue()*Scale; 358 Scale *= IndexScale.getSExtValue(); 359 360 361 // If we already had an occurrence of this index variable, merge this 362 // scale into it. For example, we want to handle: 363 // A[x][x] -> x*16 + x*4 -> x*20 364 // This also ensures that 'x' only appears in the index list once. 365 for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) { 366 if (VarIndices[i].V == Index && 367 VarIndices[i].Extension == Extension) { 368 Scale += VarIndices[i].Scale; 369 VarIndices.erase(VarIndices.begin()+i); 370 break; 371 } 372 } 373 374 // Make sure that we have a scale that makes sense for this target's 375 // pointer size. 376 if (unsigned ShiftBits = 64-TD->getPointerSizeInBits()) { 377 Scale <<= ShiftBits; 378 Scale = (int64_t)Scale >> ShiftBits; 379 } 380 381 if (Scale) { 382 VariableGEPIndex Entry = {Index, Extension, 383 static_cast<int64_t>(Scale)}; 384 VarIndices.push_back(Entry); 385 } 386 } 387 388 // Analyze the base pointer next. 389 V = GEPOp->getOperand(0); 390 } while (--MaxLookup); 391 392 // If the chain of expressions is too deep, just return early. 393 return V; 394} 395 396/// GetIndexDifference - Dest and Src are the variable indices from two 397/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base 398/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic 399/// difference between the two pointers. 400static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest, 401 const SmallVectorImpl<VariableGEPIndex> &Src) { 402 if (Src.empty()) return; 403 404 for (unsigned i = 0, e = Src.size(); i != e; ++i) { 405 const Value *V = Src[i].V; 406 ExtensionKind Extension = Src[i].Extension; 407 int64_t Scale = Src[i].Scale; 408 409 // Find V in Dest. This is N^2, but pointer indices almost never have more 410 // than a few variable indexes. 411 for (unsigned j = 0, e = Dest.size(); j != e; ++j) { 412 if (Dest[j].V != V || Dest[j].Extension != Extension) continue; 413 414 // If we found it, subtract off Scale V's from the entry in Dest. If it 415 // goes to zero, remove the entry. 416 if (Dest[j].Scale != Scale) 417 Dest[j].Scale -= Scale; 418 else 419 Dest.erase(Dest.begin()+j); 420 Scale = 0; 421 break; 422 } 423 424 // If we didn't consume this entry, add it to the end of the Dest list. 425 if (Scale) { 426 VariableGEPIndex Entry = { V, Extension, -Scale }; 427 Dest.push_back(Entry); 428 } 429 } 430} 431 432//===----------------------------------------------------------------------===// 433// BasicAliasAnalysis Pass 434//===----------------------------------------------------------------------===// 435 436#ifndef NDEBUG 437static const Function *getParent(const Value *V) { 438 if (const Instruction *inst = dyn_cast<Instruction>(V)) 439 return inst->getParent()->getParent(); 440 441 if (const Argument *arg = dyn_cast<Argument>(V)) 442 return arg->getParent(); 443 444 return NULL; 445} 446 447static bool notDifferentParent(const Value *O1, const Value *O2) { 448 449 const Function *F1 = getParent(O1); 450 const Function *F2 = getParent(O2); 451 452 return !F1 || !F2 || F1 == F2; 453} 454#endif 455 456namespace { 457 /// BasicAliasAnalysis - This is the primary alias analysis implementation. 458 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis { 459 static char ID; // Class identification, replacement for typeinfo 460 BasicAliasAnalysis() : ImmutablePass(ID) { 461 initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry()); 462 } 463 464 virtual void initializePass() { 465 InitializeAliasAnalysis(this); 466 } 467 468 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 469 AU.addRequired<AliasAnalysis>(); 470 AU.addRequired<TargetLibraryInfo>(); 471 } 472 473 virtual AliasResult alias(const Location &LocA, 474 const Location &LocB) { 475 assert(AliasCache.empty() && "AliasCache must be cleared after use!"); 476 assert(notDifferentParent(LocA.Ptr, LocB.Ptr) && 477 "BasicAliasAnalysis doesn't support interprocedural queries."); 478 AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag, 479 LocB.Ptr, LocB.Size, LocB.TBAATag); 480 // AliasCache rarely has more than 1 or 2 elements, always use 481 // shrink_and_clear so it quickly returns to the inline capacity of the 482 // SmallDenseMap if it ever grows larger. 483 // FIXME: This should really be shrink_to_inline_capacity_and_clear(). 484 AliasCache.shrink_and_clear(); 485 return Alias; 486 } 487 488 virtual ModRefResult getModRefInfo(ImmutableCallSite CS, 489 const Location &Loc); 490 491 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, 492 ImmutableCallSite CS2) { 493 // The AliasAnalysis base class has some smarts, lets use them. 494 return AliasAnalysis::getModRefInfo(CS1, CS2); 495 } 496 497 /// pointsToConstantMemory - Chase pointers until we find a (constant 498 /// global) or not. 499 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal); 500 501 /// getModRefBehavior - Return the behavior when calling the given 502 /// call site. 503 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); 504 505 /// getModRefBehavior - Return the behavior when calling the given function. 506 /// For use when the call site is not known. 507 virtual ModRefBehavior getModRefBehavior(const Function *F); 508 509 /// getAdjustedAnalysisPointer - This method is used when a pass implements 510 /// an analysis interface through multiple inheritance. If needed, it 511 /// should override this to adjust the this pointer as needed for the 512 /// specified pass info. 513 virtual void *getAdjustedAnalysisPointer(const void *ID) { 514 if (ID == &AliasAnalysis::ID) 515 return (AliasAnalysis*)this; 516 return this; 517 } 518 519 private: 520 // AliasCache - Track alias queries to guard against recursion. 521 typedef std::pair<Location, Location> LocPair; 522 typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy; 523 AliasCacheTy AliasCache; 524 525 // Visited - Track instructions visited by pointsToConstantMemory. 526 SmallPtrSet<const Value*, 16> Visited; 527 528 // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP 529 // instruction against another. 530 AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size, 531 const MDNode *V1TBAAInfo, 532 const Value *V2, uint64_t V2Size, 533 const MDNode *V2TBAAInfo, 534 const Value *UnderlyingV1, const Value *UnderlyingV2); 535 536 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI 537 // instruction against another. 538 AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize, 539 const MDNode *PNTBAAInfo, 540 const Value *V2, uint64_t V2Size, 541 const MDNode *V2TBAAInfo); 542 543 /// aliasSelect - Disambiguate a Select instruction against another value. 544 AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize, 545 const MDNode *SITBAAInfo, 546 const Value *V2, uint64_t V2Size, 547 const MDNode *V2TBAAInfo); 548 549 AliasResult aliasCheck(const Value *V1, uint64_t V1Size, 550 const MDNode *V1TBAATag, 551 const Value *V2, uint64_t V2Size, 552 const MDNode *V2TBAATag); 553 }; 554} // End of anonymous namespace 555 556// Register this pass... 557char BasicAliasAnalysis::ID = 0; 558INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa", 559 "Basic Alias Analysis (stateless AA impl)", 560 false, true, false) 561INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 562INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa", 563 "Basic Alias Analysis (stateless AA impl)", 564 false, true, false) 565 566 567ImmutablePass *llvm::createBasicAliasAnalysisPass() { 568 return new BasicAliasAnalysis(); 569} 570 571/// pointsToConstantMemory - Returns whether the given pointer value 572/// points to memory that is local to the function, with global constants being 573/// considered local to all functions. 574bool 575BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) { 576 assert(Visited.empty() && "Visited must be cleared after use!"); 577 578 unsigned MaxLookup = 8; 579 SmallVector<const Value *, 16> Worklist; 580 Worklist.push_back(Loc.Ptr); 581 do { 582 const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), TD); 583 if (!Visited.insert(V)) { 584 Visited.clear(); 585 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 586 } 587 588 // An alloca instruction defines local memory. 589 if (OrLocal && isa<AllocaInst>(V)) 590 continue; 591 592 // A global constant counts as local memory for our purposes. 593 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 594 // Note: this doesn't require GV to be "ODR" because it isn't legal for a 595 // global to be marked constant in some modules and non-constant in 596 // others. GV may even be a declaration, not a definition. 597 if (!GV->isConstant()) { 598 Visited.clear(); 599 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 600 } 601 continue; 602 } 603 604 // If both select values point to local memory, then so does the select. 605 if (const SelectInst *SI = dyn_cast<SelectInst>(V)) { 606 Worklist.push_back(SI->getTrueValue()); 607 Worklist.push_back(SI->getFalseValue()); 608 continue; 609 } 610 611 // If all values incoming to a phi node point to local memory, then so does 612 // the phi. 613 if (const PHINode *PN = dyn_cast<PHINode>(V)) { 614 // Don't bother inspecting phi nodes with many operands. 615 if (PN->getNumIncomingValues() > MaxLookup) { 616 Visited.clear(); 617 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 618 } 619 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 620 Worklist.push_back(PN->getIncomingValue(i)); 621 continue; 622 } 623 624 // Otherwise be conservative. 625 Visited.clear(); 626 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); 627 628 } while (!Worklist.empty() && --MaxLookup); 629 630 Visited.clear(); 631 return Worklist.empty(); 632} 633 634/// getModRefBehavior - Return the behavior when calling the given call site. 635AliasAnalysis::ModRefBehavior 636BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { 637 if (CS.doesNotAccessMemory()) 638 // Can't do better than this. 639 return DoesNotAccessMemory; 640 641 ModRefBehavior Min = UnknownModRefBehavior; 642 643 // If the callsite knows it only reads memory, don't return worse 644 // than that. 645 if (CS.onlyReadsMemory()) 646 Min = OnlyReadsMemory; 647 648 // The AliasAnalysis base class has some smarts, lets use them. 649 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min); 650} 651 652/// getModRefBehavior - Return the behavior when calling the given function. 653/// For use when the call site is not known. 654AliasAnalysis::ModRefBehavior 655BasicAliasAnalysis::getModRefBehavior(const Function *F) { 656 // If the function declares it doesn't access memory, we can't do better. 657 if (F->doesNotAccessMemory()) 658 return DoesNotAccessMemory; 659 660 // For intrinsics, we can check the table. 661 if (unsigned iid = F->getIntrinsicID()) { 662#define GET_INTRINSIC_MODREF_BEHAVIOR 663#include "llvm/IR/Intrinsics.gen" 664#undef GET_INTRINSIC_MODREF_BEHAVIOR 665 } 666 667 ModRefBehavior Min = UnknownModRefBehavior; 668 669 // If the function declares it only reads memory, go with that. 670 if (F->onlyReadsMemory()) 671 Min = OnlyReadsMemory; 672 673 // Otherwise be conservative. 674 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min); 675} 676 677/// getModRefInfo - Check to see if the specified callsite can clobber the 678/// specified memory object. Since we only look at local properties of this 679/// function, we really can't say much about this query. We do, however, use 680/// simple "address taken" analysis on local objects. 681AliasAnalysis::ModRefResult 682BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS, 683 const Location &Loc) { 684 assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) && 685 "AliasAnalysis query involving multiple functions!"); 686 687 const Value *Object = GetUnderlyingObject(Loc.Ptr, TD); 688 689 // If this is a tail call and Loc.Ptr points to a stack location, we know that 690 // the tail call cannot access or modify the local stack. 691 // We cannot exclude byval arguments here; these belong to the caller of 692 // the current function not to the current function, and a tail callee 693 // may reference them. 694 if (isa<AllocaInst>(Object)) 695 if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 696 if (CI->isTailCall()) 697 return NoModRef; 698 699 // If the pointer is to a locally allocated object that does not escape, 700 // then the call can not mod/ref the pointer unless the call takes the pointer 701 // as an argument, and itself doesn't capture it. 702 if (!isa<Constant>(Object) && CS.getInstruction() != Object && 703 isNonEscapingLocalObject(Object)) { 704 bool PassedAsArg = false; 705 unsigned ArgNo = 0; 706 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 707 CI != CE; ++CI, ++ArgNo) { 708 // Only look at the no-capture or byval pointer arguments. If this 709 // pointer were passed to arguments that were neither of these, then it 710 // couldn't be no-capture. 711 if (!(*CI)->getType()->isPointerTy() || 712 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo))) 713 continue; 714 715 // If this is a no-capture pointer argument, see if we can tell that it 716 // is impossible to alias the pointer we're checking. If not, we have to 717 // assume that the call could touch the pointer, even though it doesn't 718 // escape. 719 if (!isNoAlias(Location(*CI), Location(Object))) { 720 PassedAsArg = true; 721 break; 722 } 723 } 724 725 if (!PassedAsArg) 726 return NoModRef; 727 } 728 729 const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>(); 730 ModRefResult Min = ModRef; 731 732 // Finally, handle specific knowledge of intrinsics. 733 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); 734 if (II != 0) 735 switch (II->getIntrinsicID()) { 736 default: break; 737 case Intrinsic::memcpy: 738 case Intrinsic::memmove: { 739 uint64_t Len = UnknownSize; 740 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) 741 Len = LenCI->getZExtValue(); 742 Value *Dest = II->getArgOperand(0); 743 Value *Src = II->getArgOperand(1); 744 // If it can't overlap the source dest, then it doesn't modref the loc. 745 if (isNoAlias(Location(Dest, Len), Loc)) { 746 if (isNoAlias(Location(Src, Len), Loc)) 747 return NoModRef; 748 // If it can't overlap the dest, then worst case it reads the loc. 749 Min = Ref; 750 } else if (isNoAlias(Location(Src, Len), Loc)) { 751 // If it can't overlap the source, then worst case it mutates the loc. 752 Min = Mod; 753 } 754 break; 755 } 756 case Intrinsic::memset: 757 // Since memset is 'accesses arguments' only, the AliasAnalysis base class 758 // will handle it for the variable length case. 759 if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) { 760 uint64_t Len = LenCI->getZExtValue(); 761 Value *Dest = II->getArgOperand(0); 762 if (isNoAlias(Location(Dest, Len), Loc)) 763 return NoModRef; 764 } 765 // We know that memset doesn't load anything. 766 Min = Mod; 767 break; 768 case Intrinsic::lifetime_start: 769 case Intrinsic::lifetime_end: 770 case Intrinsic::invariant_start: { 771 uint64_t PtrSize = 772 cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 773 if (isNoAlias(Location(II->getArgOperand(1), 774 PtrSize, 775 II->getMetadata(LLVMContext::MD_tbaa)), 776 Loc)) 777 return NoModRef; 778 break; 779 } 780 case Intrinsic::invariant_end: { 781 uint64_t PtrSize = 782 cast<ConstantInt>(II->getArgOperand(1))->getZExtValue(); 783 if (isNoAlias(Location(II->getArgOperand(2), 784 PtrSize, 785 II->getMetadata(LLVMContext::MD_tbaa)), 786 Loc)) 787 return NoModRef; 788 break; 789 } 790 case Intrinsic::arm_neon_vld1: { 791 // LLVM's vld1 and vst1 intrinsics currently only support a single 792 // vector register. 793 uint64_t Size = 794 TD ? TD->getTypeStoreSize(II->getType()) : UnknownSize; 795 if (isNoAlias(Location(II->getArgOperand(0), Size, 796 II->getMetadata(LLVMContext::MD_tbaa)), 797 Loc)) 798 return NoModRef; 799 break; 800 } 801 case Intrinsic::arm_neon_vst1: { 802 uint64_t Size = 803 TD ? TD->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize; 804 if (isNoAlias(Location(II->getArgOperand(0), Size, 805 II->getMetadata(LLVMContext::MD_tbaa)), 806 Loc)) 807 return NoModRef; 808 break; 809 } 810 } 811 812 // We can bound the aliasing properties of memset_pattern16 just as we can 813 // for memcpy/memset. This is particularly important because the 814 // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16 815 // whenever possible. 816 else if (TLI.has(LibFunc::memset_pattern16) && 817 CS.getCalledFunction() && 818 CS.getCalledFunction()->getName() == "memset_pattern16") { 819 const Function *MS = CS.getCalledFunction(); 820 FunctionType *MemsetType = MS->getFunctionType(); 821 if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 && 822 isa<PointerType>(MemsetType->getParamType(0)) && 823 isa<PointerType>(MemsetType->getParamType(1)) && 824 isa<IntegerType>(MemsetType->getParamType(2))) { 825 uint64_t Len = UnknownSize; 826 if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2))) 827 Len = LenCI->getZExtValue(); 828 const Value *Dest = CS.getArgument(0); 829 const Value *Src = CS.getArgument(1); 830 // If it can't overlap the source dest, then it doesn't modref the loc. 831 if (isNoAlias(Location(Dest, Len), Loc)) { 832 // Always reads 16 bytes of the source. 833 if (isNoAlias(Location(Src, 16), Loc)) 834 return NoModRef; 835 // If it can't overlap the dest, then worst case it reads the loc. 836 Min = Ref; 837 // Always reads 16 bytes of the source. 838 } else if (isNoAlias(Location(Src, 16), Loc)) { 839 // If it can't overlap the source, then worst case it mutates the loc. 840 Min = Mod; 841 } 842 } 843 } 844 845 // The AliasAnalysis base class has some smarts, lets use them. 846 return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min); 847} 848 849static bool areVarIndicesEqual(SmallVector<VariableGEPIndex, 4> &Indices1, 850 SmallVector<VariableGEPIndex, 4> &Indices2) { 851 unsigned Size1 = Indices1.size(); 852 unsigned Size2 = Indices2.size(); 853 854 if (Size1 != Size2) 855 return false; 856 857 for (unsigned I = 0; I != Size1; ++I) 858 if (Indices1[I] != Indices2[I]) 859 return false; 860 861 return true; 862} 863 864/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction 865/// against another pointer. We know that V1 is a GEP, but we don't know 866/// anything about V2. UnderlyingV1 is GetUnderlyingObject(GEP1, TD), 867/// UnderlyingV2 is the same for V2. 868/// 869AliasAnalysis::AliasResult 870BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size, 871 const MDNode *V1TBAAInfo, 872 const Value *V2, uint64_t V2Size, 873 const MDNode *V2TBAAInfo, 874 const Value *UnderlyingV1, 875 const Value *UnderlyingV2) { 876 int64_t GEP1BaseOffset; 877 SmallVector<VariableGEPIndex, 4> GEP1VariableIndices; 878 879 // If we have two gep instructions with must-alias or not-alias'ing base 880 // pointers, figure out if the indexes to the GEP tell us anything about the 881 // derived pointer. 882 if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { 883 // Do the base pointers alias? 884 AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0, 885 UnderlyingV2, UnknownSize, 0); 886 887 // Check for geps of non-aliasing underlying pointers where the offsets are 888 // identical. 889 if ((BaseAlias == MayAlias) && V1Size == V2Size) { 890 // Do the base pointers alias assuming type and size. 891 AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size, 892 V1TBAAInfo, UnderlyingV2, 893 V2Size, V2TBAAInfo); 894 if (PreciseBaseAlias == NoAlias) { 895 // See if the computed offset from the common pointer tells us about the 896 // relation of the resulting pointer. 897 int64_t GEP2BaseOffset; 898 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 899 const Value *GEP2BasePtr = 900 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 901 const Value *GEP1BasePtr = 902 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 903 // DecomposeGEPExpression and GetUnderlyingObject should return the 904 // same result except when DecomposeGEPExpression has no DataLayout. 905 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 906 assert(TD == 0 && 907 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 908 return MayAlias; 909 } 910 // Same offsets. 911 if (GEP1BaseOffset == GEP2BaseOffset && 912 areVarIndicesEqual(GEP1VariableIndices, GEP2VariableIndices)) 913 return NoAlias; 914 GEP1VariableIndices.clear(); 915 } 916 } 917 918 // If we get a No or May, then return it immediately, no amount of analysis 919 // will improve this situation. 920 if (BaseAlias != MustAlias) return BaseAlias; 921 922 // Otherwise, we have a MustAlias. Since the base pointers alias each other 923 // exactly, see if the computed offset from the common pointer tells us 924 // about the relation of the resulting pointer. 925 const Value *GEP1BasePtr = 926 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 927 928 int64_t GEP2BaseOffset; 929 SmallVector<VariableGEPIndex, 4> GEP2VariableIndices; 930 const Value *GEP2BasePtr = 931 DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); 932 933 // DecomposeGEPExpression and GetUnderlyingObject should return the 934 // same result except when DecomposeGEPExpression has no DataLayout. 935 if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { 936 assert(TD == 0 && 937 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 938 return MayAlias; 939 } 940 941 // Subtract the GEP2 pointer from the GEP1 pointer to find out their 942 // symbolic difference. 943 GEP1BaseOffset -= GEP2BaseOffset; 944 GetIndexDifference(GEP1VariableIndices, GEP2VariableIndices); 945 946 } else { 947 // Check to see if these two pointers are related by the getelementptr 948 // instruction. If one pointer is a GEP with a non-zero index of the other 949 // pointer, we know they cannot alias. 950 951 // If both accesses are unknown size, we can't do anything useful here. 952 if (V1Size == UnknownSize && V2Size == UnknownSize) 953 return MayAlias; 954 955 AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, 0, 956 V2, V2Size, V2TBAAInfo); 957 if (R != MustAlias) 958 // If V2 may alias GEP base pointer, conservatively returns MayAlias. 959 // If V2 is known not to alias GEP base pointer, then the two values 960 // cannot alias per GEP semantics: "A pointer value formed from a 961 // getelementptr instruction is associated with the addresses associated 962 // with the first operand of the getelementptr". 963 return R; 964 965 const Value *GEP1BasePtr = 966 DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); 967 968 // DecomposeGEPExpression and GetUnderlyingObject should return the 969 // same result except when DecomposeGEPExpression has no DataLayout. 970 if (GEP1BasePtr != UnderlyingV1) { 971 assert(TD == 0 && 972 "DecomposeGEPExpression and GetUnderlyingObject disagree!"); 973 return MayAlias; 974 } 975 } 976 977 // In the two GEP Case, if there is no difference in the offsets of the 978 // computed pointers, the resultant pointers are a must alias. This 979 // hapens when we have two lexically identical GEP's (for example). 980 // 981 // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 982 // must aliases the GEP, the end result is a must alias also. 983 if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) 984 return MustAlias; 985 986 // If there is a constant difference between the pointers, but the difference 987 // is less than the size of the associated memory object, then we know 988 // that the objects are partially overlapping. If the difference is 989 // greater, we know they do not overlap. 990 if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) { 991 if (GEP1BaseOffset >= 0) { 992 if (V2Size != UnknownSize) { 993 if ((uint64_t)GEP1BaseOffset < V2Size) 994 return PartialAlias; 995 return NoAlias; 996 } 997 } else { 998 if (V1Size != UnknownSize) { 999 if (-(uint64_t)GEP1BaseOffset < V1Size) 1000 return PartialAlias; 1001 return NoAlias; 1002 } 1003 } 1004 } 1005 1006 // Try to distinguish something like &A[i][1] against &A[42][0]. 1007 // Grab the least significant bit set in any of the scales. 1008 if (!GEP1VariableIndices.empty()) { 1009 uint64_t Modulo = 0; 1010 for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) 1011 Modulo |= (uint64_t)GEP1VariableIndices[i].Scale; 1012 Modulo = Modulo ^ (Modulo & (Modulo - 1)); 1013 1014 // We can compute the difference between the two addresses 1015 // mod Modulo. Check whether that difference guarantees that the 1016 // two locations do not alias. 1017 uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1); 1018 if (V1Size != UnknownSize && V2Size != UnknownSize && 1019 ModOffset >= V2Size && V1Size <= Modulo - ModOffset) 1020 return NoAlias; 1021 } 1022 1023 // Statically, we can see that the base objects are the same, but the 1024 // pointers have dynamic offsets which we can't resolve. And none of our 1025 // little tricks above worked. 1026 // 1027 // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the 1028 // practical effect of this is protecting TBAA in the case of dynamic 1029 // indices into arrays of unions or malloc'd memory. 1030 return PartialAlias; 1031} 1032 1033static AliasAnalysis::AliasResult 1034MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) { 1035 // If the results agree, take it. 1036 if (A == B) 1037 return A; 1038 // A mix of PartialAlias and MustAlias is PartialAlias. 1039 if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) || 1040 (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias)) 1041 return AliasAnalysis::PartialAlias; 1042 // Otherwise, we don't know anything. 1043 return AliasAnalysis::MayAlias; 1044} 1045 1046/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select 1047/// instruction against another. 1048AliasAnalysis::AliasResult 1049BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize, 1050 const MDNode *SITBAAInfo, 1051 const Value *V2, uint64_t V2Size, 1052 const MDNode *V2TBAAInfo) { 1053 // If the values are Selects with the same condition, we can do a more precise 1054 // check: just check for aliases between the values on corresponding arms. 1055 if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) 1056 if (SI->getCondition() == SI2->getCondition()) { 1057 AliasResult Alias = 1058 aliasCheck(SI->getTrueValue(), SISize, SITBAAInfo, 1059 SI2->getTrueValue(), V2Size, V2TBAAInfo); 1060 if (Alias == MayAlias) 1061 return MayAlias; 1062 AliasResult ThisAlias = 1063 aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo, 1064 SI2->getFalseValue(), V2Size, V2TBAAInfo); 1065 return MergeAliasResults(ThisAlias, Alias); 1066 } 1067 1068 // If both arms of the Select node NoAlias or MustAlias V2, then returns 1069 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1070 AliasResult Alias = 1071 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo); 1072 if (Alias == MayAlias) 1073 return MayAlias; 1074 1075 AliasResult ThisAlias = 1076 aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo); 1077 return MergeAliasResults(ThisAlias, Alias); 1078} 1079 1080// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction 1081// against another. 1082AliasAnalysis::AliasResult 1083BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize, 1084 const MDNode *PNTBAAInfo, 1085 const Value *V2, uint64_t V2Size, 1086 const MDNode *V2TBAAInfo) { 1087 // If the values are PHIs in the same block, we can do a more precise 1088 // as well as efficient check: just check for aliases between the values 1089 // on corresponding edges. 1090 if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) 1091 if (PN2->getParent() == PN->getParent()) { 1092 LocPair Locs(Location(PN, PNSize, PNTBAAInfo), 1093 Location(V2, V2Size, V2TBAAInfo)); 1094 if (PN > V2) 1095 std::swap(Locs.first, Locs.second); 1096 // Analyse the PHIs' inputs under the assumption that the PHIs are 1097 // NoAlias. 1098 // If the PHIs are May/MustAlias there must be (recursively) an input 1099 // operand from outside the PHIs' cycle that is MayAlias/MustAlias or 1100 // there must be an operation on the PHIs within the PHIs' value cycle 1101 // that causes a MayAlias. 1102 // Pretend the phis do not alias. 1103 AliasResult Alias = NoAlias; 1104 assert(AliasCache.count(Locs) && 1105 "There must exist an entry for the phi node"); 1106 AliasResult OrigAliasResult = AliasCache[Locs]; 1107 AliasCache[Locs] = NoAlias; 1108 1109 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1110 AliasResult ThisAlias = 1111 aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo, 1112 PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), 1113 V2Size, V2TBAAInfo); 1114 Alias = MergeAliasResults(ThisAlias, Alias); 1115 if (Alias == MayAlias) 1116 break; 1117 } 1118 1119 // Reset if speculation failed. 1120 if (Alias != NoAlias) 1121 AliasCache[Locs] = OrigAliasResult; 1122 1123 return Alias; 1124 } 1125 1126 SmallPtrSet<Value*, 4> UniqueSrc; 1127 SmallVector<Value*, 4> V1Srcs; 1128 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1129 Value *PV1 = PN->getIncomingValue(i); 1130 if (isa<PHINode>(PV1)) 1131 // If any of the source itself is a PHI, return MayAlias conservatively 1132 // to avoid compile time explosion. The worst possible case is if both 1133 // sides are PHI nodes. In which case, this is O(m x n) time where 'm' 1134 // and 'n' are the number of PHI sources. 1135 return MayAlias; 1136 if (UniqueSrc.insert(PV1)) 1137 V1Srcs.push_back(PV1); 1138 } 1139 1140 AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo, 1141 V1Srcs[0], PNSize, PNTBAAInfo); 1142 // Early exit if the check of the first PHI source against V2 is MayAlias. 1143 // Other results are not possible. 1144 if (Alias == MayAlias) 1145 return MayAlias; 1146 1147 // If all sources of the PHI node NoAlias or MustAlias V2, then returns 1148 // NoAlias / MustAlias. Otherwise, returns MayAlias. 1149 for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { 1150 Value *V = V1Srcs[i]; 1151 1152 AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo, 1153 V, PNSize, PNTBAAInfo); 1154 Alias = MergeAliasResults(ThisAlias, Alias); 1155 if (Alias == MayAlias) 1156 break; 1157 } 1158 1159 return Alias; 1160} 1161 1162// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, 1163// such as array references. 1164// 1165AliasAnalysis::AliasResult 1166BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size, 1167 const MDNode *V1TBAAInfo, 1168 const Value *V2, uint64_t V2Size, 1169 const MDNode *V2TBAAInfo) { 1170 // If either of the memory references is empty, it doesn't matter what the 1171 // pointer values are. 1172 if (V1Size == 0 || V2Size == 0) 1173 return NoAlias; 1174 1175 // Strip off any casts if they exist. 1176 V1 = V1->stripPointerCasts(); 1177 V2 = V2->stripPointerCasts(); 1178 1179 // Are we checking for alias of the same value? 1180 if (V1 == V2) return MustAlias; 1181 1182 if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) 1183 return NoAlias; // Scalars cannot alias each other 1184 1185 // Figure out what objects these things are pointing to if we can. 1186 const Value *O1 = GetUnderlyingObject(V1, TD); 1187 const Value *O2 = GetUnderlyingObject(V2, TD); 1188 1189 // Null values in the default address space don't point to any object, so they 1190 // don't alias any other pointer. 1191 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) 1192 if (CPN->getType()->getAddressSpace() == 0) 1193 return NoAlias; 1194 if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) 1195 if (CPN->getType()->getAddressSpace() == 0) 1196 return NoAlias; 1197 1198 if (O1 != O2) { 1199 // If V1/V2 point to two different objects we know that we have no alias. 1200 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 1201 return NoAlias; 1202 1203 // Constant pointers can't alias with non-const isIdentifiedObject objects. 1204 if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || 1205 (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) 1206 return NoAlias; 1207 1208 // Arguments can't alias with local allocations or noalias calls 1209 // in the same function. 1210 if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || 1211 (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) 1212 return NoAlias; 1213 1214 // Most objects can't alias null. 1215 if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) || 1216 (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2))) 1217 return NoAlias; 1218 1219 // If one pointer is the result of a call/invoke or load and the other is a 1220 // non-escaping local object within the same function, then we know the 1221 // object couldn't escape to a point where the call could return it. 1222 // 1223 // Note that if the pointers are in different functions, there are a 1224 // variety of complications. A call with a nocapture argument may still 1225 // temporary store the nocapture argument's value in a temporary memory 1226 // location if that memory location doesn't escape. Or it may pass a 1227 // nocapture value to other functions as long as they don't capture it. 1228 if (isEscapeSource(O1) && isNonEscapingLocalObject(O2)) 1229 return NoAlias; 1230 if (isEscapeSource(O2) && isNonEscapingLocalObject(O1)) 1231 return NoAlias; 1232 } 1233 1234 // If the size of one access is larger than the entire object on the other 1235 // side, then we know such behavior is undefined and can assume no alias. 1236 if (TD) 1237 if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD, *TLI)) || 1238 (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD, *TLI))) 1239 return NoAlias; 1240 1241 // Check the cache before climbing up use-def chains. This also terminates 1242 // otherwise infinitely recursive queries. 1243 LocPair Locs(Location(V1, V1Size, V1TBAAInfo), 1244 Location(V2, V2Size, V2TBAAInfo)); 1245 if (V1 > V2) 1246 std::swap(Locs.first, Locs.second); 1247 std::pair<AliasCacheTy::iterator, bool> Pair = 1248 AliasCache.insert(std::make_pair(Locs, MayAlias)); 1249 if (!Pair.second) 1250 return Pair.first->second; 1251 1252 // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the 1253 // GEP can't simplify, we don't even look at the PHI cases. 1254 if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { 1255 std::swap(V1, V2); 1256 std::swap(V1Size, V2Size); 1257 std::swap(O1, O2); 1258 std::swap(V1TBAAInfo, V2TBAAInfo); 1259 } 1260 if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) { 1261 AliasResult Result = aliasGEP(GV1, V1Size, V1TBAAInfo, V2, V2Size, V2TBAAInfo, O1, O2); 1262 if (Result != MayAlias) return AliasCache[Locs] = Result; 1263 } 1264 1265 if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { 1266 std::swap(V1, V2); 1267 std::swap(V1Size, V2Size); 1268 std::swap(V1TBAAInfo, V2TBAAInfo); 1269 } 1270 if (const PHINode *PN = dyn_cast<PHINode>(V1)) { 1271 AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo, 1272 V2, V2Size, V2TBAAInfo); 1273 if (Result != MayAlias) return AliasCache[Locs] = Result; 1274 } 1275 1276 if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { 1277 std::swap(V1, V2); 1278 std::swap(V1Size, V2Size); 1279 std::swap(V1TBAAInfo, V2TBAAInfo); 1280 } 1281 if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) { 1282 AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo, 1283 V2, V2Size, V2TBAAInfo); 1284 if (Result != MayAlias) return AliasCache[Locs] = Result; 1285 } 1286 1287 // If both pointers are pointing into the same object and one of them 1288 // accesses is accessing the entire object, then the accesses must 1289 // overlap in some way. 1290 if (TD && O1 == O2) 1291 if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD, *TLI)) || 1292 (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD, *TLI))) 1293 return AliasCache[Locs] = PartialAlias; 1294 1295 AliasResult Result = 1296 AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo), 1297 Location(V2, V2Size, V2TBAAInfo)); 1298 return AliasCache[Locs] = Result; 1299} 1300