BasicAliasAnalysis.cpp revision 20d6f0982ad33818cfa141f80157ac13e36d5550
1//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file defines the default implementation of the Alias Analysis interface 11// that simply implements a few identities (two different globals cannot alias, 12// etc), but otherwise does no analysis. 13// 14//===----------------------------------------------------------------------===// 15 16#include "llvm/Analysis/AliasAnalysis.h" 17#include "llvm/Analysis/Passes.h" 18#include "llvm/Constants.h" 19#include "llvm/DerivedTypes.h" 20#include "llvm/Function.h" 21#include "llvm/GlobalVariable.h" 22#include "llvm/Instructions.h" 23#include "llvm/IntrinsicInst.h" 24#include "llvm/Pass.h" 25#include "llvm/Target/TargetData.h" 26#include "llvm/ADT/SmallVector.h" 27#include "llvm/ADT/STLExtras.h" 28#include "llvm/Support/Compiler.h" 29#include "llvm/Support/GetElementPtrTypeIterator.h" 30#include "llvm/Support/ManagedStatic.h" 31#include <algorithm> 32using namespace llvm; 33 34//===----------------------------------------------------------------------===// 35// Useful predicates 36//===----------------------------------------------------------------------===// 37 38// Determine if an AllocationInst instruction escapes from the function it is 39// contained in. If it does not escape, there is no way for another function to 40// mod/ref it. We do this by looking at its uses and determining if the uses 41// can escape (recursively). 42static bool AddressMightEscape(const Value *V) { 43 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end(); 44 UI != E; ++UI) { 45 const Instruction *I = cast<Instruction>(*UI); 46 switch (I->getOpcode()) { 47 case Instruction::Load: 48 break; //next use. 49 case Instruction::Store: 50 if (I->getOperand(0) == V) 51 return true; // Escapes if the pointer is stored. 52 break; // next use. 53 case Instruction::GetElementPtr: 54 if (AddressMightEscape(I)) 55 return true; 56 break; // next use. 57 case Instruction::BitCast: 58 if (AddressMightEscape(I)) 59 return true; 60 break; // next use 61 case Instruction::Ret: 62 // If returned, the address will escape to calling functions, but no 63 // callees could modify it. 64 break; // next use 65 case Instruction::Call: 66 // If the call is to a few known safe intrinsics, we know that it does 67 // not escape. 68 // TODO: Eventually just check the 'nocapture' attribute. 69 if (!isa<MemIntrinsic>(I)) 70 return true; 71 break; // next use 72 default: 73 return true; 74 } 75 } 76 return false; 77} 78 79static const User *isGEP(const Value *V) { 80 if (isa<GetElementPtrInst>(V) || 81 (isa<ConstantExpr>(V) && 82 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 83 return cast<User>(V); 84 return 0; 85} 86 87static const Value *GetGEPOperands(const Value *V, 88 SmallVector<Value*, 16> &GEPOps){ 89 assert(GEPOps.empty() && "Expect empty list to populate!"); 90 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 91 cast<User>(V)->op_end()); 92 93 // Accumulate all of the chained indexes into the operand array 94 V = cast<User>(V)->getOperand(0); 95 96 while (const User *G = isGEP(V)) { 97 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 98 !cast<Constant>(GEPOps[0])->isNullValue()) 99 break; // Don't handle folding arbitrary pointer offsets yet... 100 GEPOps.erase(GEPOps.begin()); // Drop the zero index 101 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 102 V = G->getOperand(0); 103 } 104 return V; 105} 106 107/// isNoAliasCall - Return true if this pointer is returned by a noalias 108/// function. 109static bool isNoAliasCall(const Value *V) { 110 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 111 return CallSite(const_cast<Instruction*>(cast<Instruction>(V))) 112 .paramHasAttr(0, Attribute::NoAlias); 113 return false; 114} 115 116/// isIdentifiedObject - Return true if this pointer refers to a distinct and 117/// identifiable object. This returns true for: 118/// Global Variables and Functions 119/// Allocas and Mallocs 120/// ByVal and NoAlias Arguments 121/// NoAlias returns 122/// 123static bool isIdentifiedObject(const Value *V) { 124 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V)) 125 return true; 126 if (const Argument *A = dyn_cast<Argument>(V)) 127 return A->hasNoAliasAttr() || A->hasByValAttr(); 128 return false; 129} 130 131/// isKnownNonNull - Return true if we know that the specified value is never 132/// null. 133static bool isKnownNonNull(const Value *V) { 134 // Alloca never returns null, malloc might. 135 if (isa<AllocaInst>(V)) return true; 136 137 // A byval argument is never null. 138 if (const Argument *A = dyn_cast<Argument>(V)) 139 return A->hasByValAttr(); 140 141 // Global values are not null unless extern weak. 142 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 143 return !GV->hasExternalWeakLinkage(); 144 return false; 145} 146 147/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 148/// object that never escapes from the function. 149static bool isNonEscapingLocalObject(const Value *V) { 150 // If this is a local allocation, check to see if it escapes. 151 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 152 return !AddressMightEscape(V); 153 154 // If this is an argument that corresponds to a byval or noalias argument, 155 // it can't escape either. 156 if (const Argument *A = dyn_cast<Argument>(V)) 157 if (A->hasByValAttr() || A->hasNoAliasAttr()) 158 return !AddressMightEscape(V); 159 return false; 160} 161 162 163/// isObjectSmallerThan - Return true if we can prove that the object specified 164/// by V is smaller than Size. 165static bool isObjectSmallerThan(const Value *V, unsigned Size, 166 const TargetData &TD) { 167 const Type *AccessTy; 168 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 169 AccessTy = GV->getType()->getElementType(); 170 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 171 if (!AI->isArrayAllocation()) 172 AccessTy = AI->getType()->getElementType(); 173 else 174 return false; 175 } else if (const Argument *A = dyn_cast<Argument>(V)) { 176 if (A->hasByValAttr()) 177 AccessTy = cast<PointerType>(A->getType())->getElementType(); 178 else 179 return false; 180 } else { 181 return false; 182 } 183 184 if (AccessTy->isSized()) 185 return TD.getABITypeSize(AccessTy) < Size; 186 return false; 187} 188 189//===----------------------------------------------------------------------===// 190// NoAA Pass 191//===----------------------------------------------------------------------===// 192 193namespace { 194 /// NoAA - This class implements the -no-aa pass, which always returns "I 195 /// don't know" for alias queries. NoAA is unlike other alias analysis 196 /// implementations, in that it does not chain to a previous analysis. As 197 /// such it doesn't follow many of the rules that other alias analyses must. 198 /// 199 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 200 static char ID; // Class identification, replacement for typeinfo 201 NoAA() : ImmutablePass(&ID) {} 202 explicit NoAA(void *PID) : ImmutablePass(PID) { } 203 204 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 205 AU.addRequired<TargetData>(); 206 } 207 208 virtual void initializePass() { 209 TD = &getAnalysis<TargetData>(); 210 } 211 212 virtual AliasResult alias(const Value *V1, unsigned V1Size, 213 const Value *V2, unsigned V2Size) { 214 return MayAlias; 215 } 216 217 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS, 218 std::vector<PointerAccessInfo> *Info) { 219 return UnknownModRefBehavior; 220 } 221 222 virtual void getArgumentAccesses(Function *F, CallSite CS, 223 std::vector<PointerAccessInfo> &Info) { 224 assert(0 && "This method may not be called on this function!"); 225 } 226 227 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 228 virtual bool pointsToConstantMemory(const Value *P) { return false; } 229 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 230 return ModRef; 231 } 232 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 233 return ModRef; 234 } 235 virtual bool hasNoModRefInfoForCalls() const { return true; } 236 237 virtual void deleteValue(Value *V) {} 238 virtual void copyValue(Value *From, Value *To) {} 239 }; 240} // End of anonymous namespace 241 242// Register this pass... 243char NoAA::ID = 0; 244static RegisterPass<NoAA> 245U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 246 247// Declare that we implement the AliasAnalysis interface 248static RegisterAnalysisGroup<AliasAnalysis> V(U); 249 250ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 251 252//===----------------------------------------------------------------------===// 253// BasicAA Pass 254//===----------------------------------------------------------------------===// 255 256namespace { 257 /// BasicAliasAnalysis - This is the default alias analysis implementation. 258 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 259 /// derives from the NoAA class. 260 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 261 static char ID; // Class identification, replacement for typeinfo 262 BasicAliasAnalysis() : NoAA(&ID) {} 263 AliasResult alias(const Value *V1, unsigned V1Size, 264 const Value *V2, unsigned V2Size); 265 266 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 267 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 268 269 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 270 /// non-escaping allocations. 271 virtual bool hasNoModRefInfoForCalls() const { return false; } 272 273 /// pointsToConstantMemory - Chase pointers until we find a (constant 274 /// global) or not. 275 bool pointsToConstantMemory(const Value *P); 276 277 private: 278 // CheckGEPInstructions - Check two GEP instructions with known 279 // must-aliasing base pointers. This checks to see if the index expressions 280 // preclude the pointers from aliasing... 281 AliasResult 282 CheckGEPInstructions(const Type* BasePtr1Ty, 283 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 284 const Type *BasePtr2Ty, 285 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 286 }; 287} // End of anonymous namespace 288 289// Register this pass... 290char BasicAliasAnalysis::ID = 0; 291static RegisterPass<BasicAliasAnalysis> 292X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 293 294// Declare that we implement the AliasAnalysis interface 295static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 296 297ImmutablePass *llvm::createBasicAliasAnalysisPass() { 298 return new BasicAliasAnalysis(); 299} 300 301 302/// pointsToConstantMemory - Chase pointers until we find a (constant 303/// global) or not. 304bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 305 if (const GlobalVariable *GV = 306 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 307 return GV->isConstant(); 308 return false; 309} 310 311// getModRefInfo - Check to see if the specified callsite can clobber the 312// specified memory object. Since we only look at local properties of this 313// function, we really can't say much about this query. We do, however, use 314// simple "address taken" analysis on local objects. 315// 316AliasAnalysis::ModRefResult 317BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 318 if (!isa<Constant>(P)) { 319 const Value *Object = P->getUnderlyingObject(); 320 321 // If this is a tail call and P points to a stack location, we know that 322 // the tail call cannot access or modify the local stack. 323 // We cannot exclude byval arguments here; these belong to the caller of 324 // the current function not to the current function, and a tail callee 325 // may reference them. 326 if (isa<AllocaInst>(Object)) 327 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 328 if (CI->isTailCall()) 329 return NoModRef; 330 331 // If the pointer is to a locally allocated object that does not escape, 332 // then the call can not mod/ref the pointer unless the call takes the 333 // argument without capturing it. 334 if (isNonEscapingLocalObject(Object)) { 335 bool passedAsArg = false; 336 // TODO: Eventually only check 'nocapture' arguments. 337 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 338 CI != CE; ++CI) 339 if (isa<PointerType>((*CI)->getType()) && 340 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 341 passedAsArg = true; 342 343 if (!passedAsArg) 344 return NoModRef; 345 } 346 } 347 348 // The AliasAnalysis base class has some smarts, lets use them. 349 return AliasAnalysis::getModRefInfo(CS, P, Size); 350} 351 352 353AliasAnalysis::ModRefResult 354BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 355 // If CS1 or CS2 are readnone, they don't interact. 356 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 357 if (CS1B == DoesNotAccessMemory) return NoModRef; 358 359 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 360 if (CS2B == DoesNotAccessMemory) return NoModRef; 361 362 // If they both only read from memory, just return ref. 363 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 364 return Ref; 365 366 // Otherwise, fall back to NoAA (mod+ref). 367 return NoAA::getModRefInfo(CS1, CS2); 368} 369 370 371// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 372// as array references. Note that this function is heavily tail recursive. 373// Hopefully we have a smart C++ compiler. :) 374// 375AliasAnalysis::AliasResult 376BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 377 const Value *V2, unsigned V2Size) { 378 // Strip off any constant expression casts if they exist 379 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 380 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 381 V1 = CE->getOperand(0); 382 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 383 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 384 V2 = CE->getOperand(0); 385 386 // Are we checking for alias of the same value? 387 if (V1 == V2) return MustAlias; 388 389 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 390 return NoAlias; // Scalars cannot alias each other 391 392 // Strip off cast instructions... 393 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 394 return alias(I->getOperand(0), V1Size, V2, V2Size); 395 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 396 return alias(V1, V1Size, I->getOperand(0), V2Size); 397 398 // Figure out what objects these things are pointing to if we can... 399 const Value *O1 = V1->getUnderlyingObject(); 400 const Value *O2 = V2->getUnderlyingObject(); 401 402 if (O1 != O2) { 403 // If V1/V2 point to two different objects we know that we have no alias. 404 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 405 return NoAlias; 406 407 // Arguments can't alias with local allocations or noalias calls. 408 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 409 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 410 return NoAlias; 411 412 // Most objects can't alias null. 413 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 414 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 415 return NoAlias; 416 } 417 418 // If the size of one access is larger than the entire object on the other 419 // side, then we know such behavior is undefined and can assume no alias. 420 const TargetData &TD = getTargetData(); 421 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 422 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 423 return NoAlias; 424 425 // If one pointer is the result of a call/invoke and the other is a 426 // non-escaping local object, then we know the object couldn't escape to a 427 // point where the call could return it. 428 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 429 isNonEscapingLocalObject(O2)) 430 return NoAlias; 431 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 432 isNonEscapingLocalObject(O1)) 433 return NoAlias; 434 435 // If we have two gep instructions with must-alias'ing base pointers, figure 436 // out if the indexes to the GEP tell us anything about the derived pointer. 437 // Note that we also handle chains of getelementptr instructions as well as 438 // constant expression getelementptrs here. 439 // 440 if (isGEP(V1) && isGEP(V2)) { 441 // Drill down into the first non-gep value, to test for must-aliasing of 442 // the base pointers. 443 const User *G = cast<User>(V1); 444 while (isGEP(G->getOperand(0)) && 445 G->getOperand(1) == 446 Constant::getNullValue(G->getOperand(1)->getType())) 447 G = cast<User>(G->getOperand(0)); 448 const Value *BasePtr1 = G->getOperand(0); 449 450 G = cast<User>(V2); 451 while (isGEP(G->getOperand(0)) && 452 G->getOperand(1) == 453 Constant::getNullValue(G->getOperand(1)->getType())) 454 G = cast<User>(G->getOperand(0)); 455 const Value *BasePtr2 = G->getOperand(0); 456 457 // Do the base pointers alias? 458 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 459 if (BaseAlias == NoAlias) return NoAlias; 460 if (BaseAlias == MustAlias) { 461 // If the base pointers alias each other exactly, check to see if we can 462 // figure out anything about the resultant pointers, to try to prove 463 // non-aliasing. 464 465 // Collect all of the chained GEP operands together into one simple place 466 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 467 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 468 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 469 470 // If GetGEPOperands were able to fold to the same must-aliased pointer, 471 // do the comparison. 472 if (BasePtr1 == BasePtr2) { 473 AliasResult GAlias = 474 CheckGEPInstructions(BasePtr1->getType(), 475 &GEP1Ops[0], GEP1Ops.size(), V1Size, 476 BasePtr2->getType(), 477 &GEP2Ops[0], GEP2Ops.size(), V2Size); 478 if (GAlias != MayAlias) 479 return GAlias; 480 } 481 } 482 } 483 484 // Check to see if these two pointers are related by a getelementptr 485 // instruction. If one pointer is a GEP with a non-zero index of the other 486 // pointer, we know they cannot alias. 487 // 488 if (isGEP(V2)) { 489 std::swap(V1, V2); 490 std::swap(V1Size, V2Size); 491 } 492 493 if (V1Size != ~0U && V2Size != ~0U) 494 if (isGEP(V1)) { 495 SmallVector<Value*, 16> GEPOperands; 496 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 497 498 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 499 if (R == MustAlias) { 500 // If there is at least one non-zero constant index, we know they cannot 501 // alias. 502 bool ConstantFound = false; 503 bool AllZerosFound = true; 504 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 505 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 506 if (!C->isNullValue()) { 507 ConstantFound = true; 508 AllZerosFound = false; 509 break; 510 } 511 } else { 512 AllZerosFound = false; 513 } 514 515 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 516 // the ptr, the end result is a must alias also. 517 if (AllZerosFound) 518 return MustAlias; 519 520 if (ConstantFound) { 521 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 522 return NoAlias; 523 524 // Otherwise we have to check to see that the distance is more than 525 // the size of the argument... build an index vector that is equal to 526 // the arguments provided, except substitute 0's for any variable 527 // indexes we find... 528 if (cast<PointerType>( 529 BasePtr->getType())->getElementType()->isSized()) { 530 for (unsigned i = 0; i != GEPOperands.size(); ++i) 531 if (!isa<ConstantInt>(GEPOperands[i])) 532 GEPOperands[i] = 533 Constant::getNullValue(GEPOperands[i]->getType()); 534 int64_t Offset = 535 getTargetData().getIndexedOffset(BasePtr->getType(), 536 &GEPOperands[0], 537 GEPOperands.size()); 538 539 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 540 return NoAlias; 541 } 542 } 543 } 544 } 545 546 return MayAlias; 547} 548 549// This function is used to determine if the indices of two GEP instructions are 550// equal. V1 and V2 are the indices. 551static bool IndexOperandsEqual(Value *V1, Value *V2) { 552 if (V1->getType() == V2->getType()) 553 return V1 == V2; 554 if (Constant *C1 = dyn_cast<Constant>(V1)) 555 if (Constant *C2 = dyn_cast<Constant>(V2)) { 556 // Sign extend the constants to long types, if necessary 557 if (C1->getType() != Type::Int64Ty) 558 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 559 if (C2->getType() != Type::Int64Ty) 560 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 561 return C1 == C2; 562 } 563 return false; 564} 565 566/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 567/// base pointers. This checks to see if the index expressions preclude the 568/// pointers from aliasing... 569AliasAnalysis::AliasResult 570BasicAliasAnalysis::CheckGEPInstructions( 571 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 572 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 573 // We currently can't handle the case when the base pointers have different 574 // primitive types. Since this is uncommon anyway, we are happy being 575 // extremely conservative. 576 if (BasePtr1Ty != BasePtr2Ty) 577 return MayAlias; 578 579 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 580 581 // Find the (possibly empty) initial sequence of equal values... which are not 582 // necessarily constants. 583 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 584 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 585 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 586 unsigned UnequalOper = 0; 587 while (UnequalOper != MinOperands && 588 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 589 // Advance through the type as we go... 590 ++UnequalOper; 591 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 592 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 593 else { 594 // If all operands equal each other, then the derived pointers must 595 // alias each other... 596 BasePtr1Ty = 0; 597 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 598 "Ran out of type nesting, but not out of operands?"); 599 return MustAlias; 600 } 601 } 602 603 // If we have seen all constant operands, and run out of indexes on one of the 604 // getelementptrs, check to see if the tail of the leftover one is all zeros. 605 // If so, return mustalias. 606 if (UnequalOper == MinOperands) { 607 if (NumGEP1Ops < NumGEP2Ops) { 608 std::swap(GEP1Ops, GEP2Ops); 609 std::swap(NumGEP1Ops, NumGEP2Ops); 610 } 611 612 bool AllAreZeros = true; 613 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 614 if (!isa<Constant>(GEP1Ops[i]) || 615 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 616 AllAreZeros = false; 617 break; 618 } 619 if (AllAreZeros) return MustAlias; 620 } 621 622 623 // So now we know that the indexes derived from the base pointers, 624 // which are known to alias, are different. We can still determine a 625 // no-alias result if there are differing constant pairs in the index 626 // chain. For example: 627 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 628 // 629 // We have to be careful here about array accesses. In particular, consider: 630 // A[1][0] vs A[0][i] 631 // In this case, we don't *know* that the array will be accessed in bounds: 632 // the index could even be negative. Because of this, we have to 633 // conservatively *give up* and return may alias. We disregard differing 634 // array subscripts that are followed by a variable index without going 635 // through a struct. 636 // 637 unsigned SizeMax = std::max(G1S, G2S); 638 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 639 640 // Scan for the first operand that is constant and unequal in the 641 // two getelementptrs... 642 unsigned FirstConstantOper = UnequalOper; 643 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 644 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 645 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 646 647 if (G1Oper != G2Oper) // Found non-equal constant indexes... 648 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 649 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 650 if (G1OC->getType() != G2OC->getType()) { 651 // Sign extend both operands to long. 652 if (G1OC->getType() != Type::Int64Ty) 653 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 654 if (G2OC->getType() != Type::Int64Ty) 655 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 656 GEP1Ops[FirstConstantOper] = G1OC; 657 GEP2Ops[FirstConstantOper] = G2OC; 658 } 659 660 if (G1OC != G2OC) { 661 // Handle the "be careful" case above: if this is an array/vector 662 // subscript, scan for a subsequent variable array index. 663 if (isa<SequentialType>(BasePtr1Ty)) { 664 const Type *NextTy = 665 cast<SequentialType>(BasePtr1Ty)->getElementType(); 666 bool isBadCase = false; 667 668 for (unsigned Idx = FirstConstantOper+1; 669 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 670 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 671 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 672 isBadCase = true; 673 break; 674 } 675 NextTy = cast<SequentialType>(NextTy)->getElementType(); 676 } 677 678 if (isBadCase) G1OC = 0; 679 } 680 681 // Make sure they are comparable (ie, not constant expressions), and 682 // make sure the GEP with the smaller leading constant is GEP1. 683 if (G1OC) { 684 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 685 G1OC, G2OC); 686 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 687 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 688 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 689 std::swap(NumGEP1Ops, NumGEP2Ops); 690 } 691 break; 692 } 693 } 694 } 695 } 696 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 697 } 698 699 // No shared constant operands, and we ran out of common operands. At this 700 // point, the GEP instructions have run through all of their operands, and we 701 // haven't found evidence that there are any deltas between the GEP's. 702 // However, one GEP may have more operands than the other. If this is the 703 // case, there may still be hope. Check this now. 704 if (FirstConstantOper == MinOperands) { 705 // Make GEP1Ops be the longer one if there is a longer one. 706 if (NumGEP1Ops < NumGEP2Ops) { 707 std::swap(GEP1Ops, GEP2Ops); 708 std::swap(NumGEP1Ops, NumGEP2Ops); 709 } 710 711 // Is there anything to check? 712 if (NumGEP1Ops > MinOperands) { 713 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 714 if (isa<ConstantInt>(GEP1Ops[i]) && 715 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 716 // Yup, there's a constant in the tail. Set all variables to 717 // constants in the GEP instruction to make it suitable for 718 // TargetData::getIndexedOffset. 719 for (i = 0; i != MaxOperands; ++i) 720 if (!isa<ConstantInt>(GEP1Ops[i])) 721 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 722 // Okay, now get the offset. This is the relative offset for the full 723 // instruction. 724 const TargetData &TD = getTargetData(); 725 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 726 NumGEP1Ops); 727 728 // Now check without any constants at the end. 729 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 730 MinOperands); 731 732 // Make sure we compare the absolute difference. 733 if (Offset1 > Offset2) 734 std::swap(Offset1, Offset2); 735 736 // If the tail provided a bit enough offset, return noalias! 737 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 738 return NoAlias; 739 // Otherwise break - we don't look for another constant in the tail. 740 break; 741 } 742 } 743 744 // Couldn't find anything useful. 745 return MayAlias; 746 } 747 748 // If there are non-equal constants arguments, then we can figure 749 // out a minimum known delta between the two index expressions... at 750 // this point we know that the first constant index of GEP1 is less 751 // than the first constant index of GEP2. 752 753 // Advance BasePtr[12]Ty over this first differing constant operand. 754 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 755 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 756 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 757 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 758 759 // We are going to be using TargetData::getIndexedOffset to determine the 760 // offset that each of the GEP's is reaching. To do this, we have to convert 761 // all variable references to constant references. To do this, we convert the 762 // initial sequence of array subscripts into constant zeros to start with. 763 const Type *ZeroIdxTy = GEPPointerTy; 764 for (unsigned i = 0; i != FirstConstantOper; ++i) { 765 if (!isa<StructType>(ZeroIdxTy)) 766 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 767 768 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 769 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 770 } 771 772 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 773 774 // Loop over the rest of the operands... 775 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 776 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 777 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 778 // If they are equal, use a zero index... 779 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 780 if (!isa<ConstantInt>(Op1)) 781 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 782 // Otherwise, just keep the constants we have. 783 } else { 784 if (Op1) { 785 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 786 // If this is an array index, make sure the array element is in range. 787 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 788 if (Op1C->getZExtValue() >= AT->getNumElements()) 789 return MayAlias; // Be conservative with out-of-range accesses 790 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 791 if (Op1C->getZExtValue() >= VT->getNumElements()) 792 return MayAlias; // Be conservative with out-of-range accesses 793 } 794 795 } else { 796 // GEP1 is known to produce a value less than GEP2. To be 797 // conservatively correct, we must assume the largest possible 798 // constant is used in this position. This cannot be the initial 799 // index to the GEP instructions (because we know we have at least one 800 // element before this one with the different constant arguments), so 801 // we know that the current index must be into either a struct or 802 // array. Because we know it's not constant, this cannot be a 803 // structure index. Because of this, we can calculate the maximum 804 // value possible. 805 // 806 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 807 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 808 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 809 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 810 } 811 } 812 813 if (Op2) { 814 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 815 // If this is an array index, make sure the array element is in range. 816 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 817 if (Op2C->getZExtValue() >= AT->getNumElements()) 818 return MayAlias; // Be conservative with out-of-range accesses 819 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 820 if (Op2C->getZExtValue() >= VT->getNumElements()) 821 return MayAlias; // Be conservative with out-of-range accesses 822 } 823 } else { // Conservatively assume the minimum value for this index 824 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 825 } 826 } 827 } 828 829 if (BasePtr1Ty && Op1) { 830 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 831 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 832 else 833 BasePtr1Ty = 0; 834 } 835 836 if (BasePtr2Ty && Op2) { 837 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 838 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 839 else 840 BasePtr2Ty = 0; 841 } 842 } 843 844 if (GEPPointerTy->getElementType()->isSized()) { 845 int64_t Offset1 = 846 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 847 int64_t Offset2 = 848 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 849 assert(Offset1 != Offset2 && 850 "There is at least one different constant here!"); 851 852 // Make sure we compare the absolute difference. 853 if (Offset1 > Offset2) 854 std::swap(Offset1, Offset2); 855 856 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 857 //cerr << "Determined that these two GEP's don't alias [" 858 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 859 return NoAlias; 860 } 861 } 862 return MayAlias; 863} 864 865// Make sure that anything that uses AliasAnalysis pulls in this file... 866DEFINING_FILE_FOR(BasicAliasAnalysis) 867