BasicAliasAnalysis.cpp revision d087480166d8f0ec2a44732c7ec087f4eb1ba527
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/ParameterAttributes.h" 22#include "llvm/GlobalVariable.h" 23#include "llvm/Instructions.h" 24#include "llvm/Intrinsics.h" 25#include "llvm/Pass.h" 26#include "llvm/Target/TargetData.h" 27#include "llvm/ADT/SmallVector.h" 28#include "llvm/ADT/STLExtras.h" 29#include "llvm/Support/Compiler.h" 30#include "llvm/Support/GetElementPtrTypeIterator.h" 31#include "llvm/Support/ManagedStatic.h" 32#include <algorithm> 33using namespace llvm; 34 35namespace { 36 /// NoAA - This class implements the -no-aa pass, which always returns "I 37 /// don't know" for alias queries. NoAA is unlike other alias analysis 38 /// implementations, in that it does not chain to a previous analysis. As 39 /// such it doesn't follow many of the rules that other alias analyses must. 40 /// 41 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 42 static char ID; // Class identification, replacement for typeinfo 43 NoAA() : ImmutablePass((intptr_t)&ID) {} 44 explicit NoAA(intptr_t PID) : ImmutablePass(PID) { } 45 46 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 47 AU.addRequired<TargetData>(); 48 } 49 50 virtual void initializePass() { 51 TD = &getAnalysis<TargetData>(); 52 } 53 54 virtual AliasResult alias(const Value *V1, unsigned V1Size, 55 const Value *V2, unsigned V2Size) { 56 return MayAlias; 57 } 58 59 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS, 60 std::vector<PointerAccessInfo> *Info) { 61 return UnknownModRefBehavior; 62 } 63 64 virtual void getArgumentAccesses(Function *F, CallSite CS, 65 std::vector<PointerAccessInfo> &Info) { 66 assert(0 && "This method may not be called on this function!"); 67 } 68 69 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 70 virtual bool pointsToConstantMemory(const Value *P) { return false; } 71 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 72 return ModRef; 73 } 74 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 75 return ModRef; 76 } 77 virtual bool hasNoModRefInfoForCalls() const { return true; } 78 79 virtual void deleteValue(Value *V) {} 80 virtual void copyValue(Value *From, Value *To) {} 81 }; 82 83 // Register this pass... 84 char NoAA::ID = 0; 85 RegisterPass<NoAA> 86 U("no-aa", "No Alias Analysis (always returns 'may' alias)"); 87 88 // Declare that we implement the AliasAnalysis interface 89 RegisterAnalysisGroup<AliasAnalysis> V(U); 90} // End of anonymous namespace 91 92ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 93 94namespace { 95 /// BasicAliasAnalysis - This is the default alias analysis implementation. 96 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 97 /// derives from the NoAA class. 98 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 99 static char ID; // Class identification, replacement for typeinfo 100 BasicAliasAnalysis() : NoAA((intptr_t)&ID) { } 101 AliasResult alias(const Value *V1, unsigned V1Size, 102 const Value *V2, unsigned V2Size); 103 104 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 105 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 106 return NoAA::getModRefInfo(CS1,CS2); 107 } 108 109 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 110 /// non-escaping allocations. 111 virtual bool hasNoModRefInfoForCalls() const { return false; } 112 113 /// pointsToConstantMemory - Chase pointers until we find a (constant 114 /// global) or not. 115 bool pointsToConstantMemory(const Value *P); 116 117 private: 118 // CheckGEPInstructions - Check two GEP instructions with known 119 // must-aliasing base pointers. This checks to see if the index expressions 120 // preclude the pointers from aliasing... 121 AliasResult 122 CheckGEPInstructions(const Type* BasePtr1Ty, 123 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 124 const Type *BasePtr2Ty, 125 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 126 }; 127 128 // Register this pass... 129 char BasicAliasAnalysis::ID = 0; 130 RegisterPass<BasicAliasAnalysis> 131 X("basicaa", "Basic Alias Analysis (default AA impl)"); 132 133 // Declare that we implement the AliasAnalysis interface 134 RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 135} // End of anonymous namespace 136 137ImmutablePass *llvm::createBasicAliasAnalysisPass() { 138 return new BasicAliasAnalysis(); 139} 140 141/// getUnderlyingObject - This traverses the use chain to figure out what object 142/// the specified value points to. If the value points to, or is derived from, 143/// a unique object or an argument, return it. This returns: 144/// Arguments, GlobalVariables, Functions, Allocas, Mallocs. 145static const Value *getUnderlyingObject(const Value *V) { 146 if (!isa<PointerType>(V->getType())) return 0; 147 148 // If we are at some type of object, return it. GlobalValues and Allocations 149 // have unique addresses. 150 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isa<Argument>(V)) 151 return V; 152 153 // Traverse through different addressing mechanisms... 154 if (const Instruction *I = dyn_cast<Instruction>(V)) { 155 if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I)) 156 return getUnderlyingObject(I->getOperand(0)); 157 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) { 158 if (CE->getOpcode() == Instruction::BitCast || 159 CE->getOpcode() == Instruction::GetElementPtr) 160 return getUnderlyingObject(CE->getOperand(0)); 161 } 162 return 0; 163} 164 165static const User *isGEP(const Value *V) { 166 if (isa<GetElementPtrInst>(V) || 167 (isa<ConstantExpr>(V) && 168 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 169 return cast<User>(V); 170 return 0; 171} 172 173static const Value *GetGEPOperands(const Value *V, 174 SmallVector<Value*, 16> &GEPOps){ 175 assert(GEPOps.empty() && "Expect empty list to populate!"); 176 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 177 cast<User>(V)->op_end()); 178 179 // Accumulate all of the chained indexes into the operand array 180 V = cast<User>(V)->getOperand(0); 181 182 while (const User *G = isGEP(V)) { 183 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 184 !cast<Constant>(GEPOps[0])->isNullValue()) 185 break; // Don't handle folding arbitrary pointer offsets yet... 186 GEPOps.erase(GEPOps.begin()); // Drop the zero index 187 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 188 V = G->getOperand(0); 189 } 190 return V; 191} 192 193/// pointsToConstantMemory - Chase pointers until we find a (constant 194/// global) or not. 195bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 196 if (const Value *V = getUnderlyingObject(P)) 197 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 198 return GV->isConstant(); 199 return false; 200} 201 202// Determine if an AllocationInst instruction escapes from the function it is 203// contained in. If it does not escape, there is no way for another function to 204// mod/ref it. We do this by looking at its uses and determining if the uses 205// can escape (recursively). 206static bool AddressMightEscape(const Value *V) { 207 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end(); 208 UI != E; ++UI) { 209 const Instruction *I = cast<Instruction>(*UI); 210 switch (I->getOpcode()) { 211 case Instruction::Load: 212 break; //next use. 213 case Instruction::Store: 214 if (I->getOperand(0) == V) 215 return true; // Escapes if the pointer is stored. 216 break; // next use. 217 case Instruction::GetElementPtr: 218 if (AddressMightEscape(I)) 219 return true; 220 break; // next use. 221 case Instruction::BitCast: 222 if (!isa<PointerType>(I->getType())) 223 return true; 224 if (AddressMightEscape(I)) 225 return true; 226 break; // next use 227 case Instruction::Ret: 228 // If returned, the address will escape to calling functions, but no 229 // callees could modify it. 230 break; // next use 231 default: 232 return true; 233 } 234 } 235 return false; 236} 237 238// getModRefInfo - Check to see if the specified callsite can clobber the 239// specified memory object. Since we only look at local properties of this 240// function, we really can't say much about this query. We do, however, use 241// simple "address taken" analysis on local objects. 242// 243AliasAnalysis::ModRefResult 244BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 245 if (!isa<Constant>(P)) { 246 const Value *Object = getUnderlyingObject(P); 247 // Allocations and byval arguments are "new" objects. 248 if (Object && 249 (isa<AllocationInst>(Object) || 250 (isa<Argument>(Object) && cast<Argument>(Object)->hasByValAttr()))) { 251 // Okay, the pointer is to a stack allocated object. If we can prove that 252 // the pointer never "escapes", then we know the call cannot clobber it, 253 // because it simply can't get its address. 254 if (!AddressMightEscape(Object)) 255 return NoModRef; 256 257 // If this is a tail call and P points to a stack location, we know that 258 // the tail call cannot access or modify the local stack. 259 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 260 if (CI->isTailCall() && !isa<MallocInst>(Object)) 261 return NoModRef; 262 } 263 } 264 265 // The AliasAnalysis base class has some smarts, lets use them. 266 return AliasAnalysis::getModRefInfo(CS, P, Size); 267} 268 269// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 270// as array references. Note that this function is heavily tail recursive. 271// Hopefully we have a smart C++ compiler. :) 272// 273AliasAnalysis::AliasResult 274BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 275 const Value *V2, unsigned V2Size) { 276 // Strip off any constant expression casts if they exist 277 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 278 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 279 V1 = CE->getOperand(0); 280 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 281 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 282 V2 = CE->getOperand(0); 283 284 // Are we checking for alias of the same value? 285 if (V1 == V2) return MustAlias; 286 287 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) && 288 V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty) 289 return NoAlias; // Scalars cannot alias each other 290 291 // Strip off cast instructions... 292 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 293 return alias(I->getOperand(0), V1Size, V2, V2Size); 294 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 295 return alias(V1, V1Size, I->getOperand(0), V2Size); 296 297 // Figure out what objects these things are pointing to if we can... 298 const Value *O1 = getUnderlyingObject(V1); 299 const Value *O2 = getUnderlyingObject(V2); 300 301 // Pointing at a discernible object? 302 if (O1) { 303 if (O2) { 304 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 305 // Incoming argument cannot alias locally allocated object! 306 if (isa<AllocationInst>(O2)) return NoAlias; 307 308 // If they are two different objects, and one is a noalias argument 309 // then they do not alias. 310 if (O1 != O2 && O1Arg->hasNoAliasAttr()) 311 return NoAlias; 312 313 // Byval arguments can't alias globals or other arguments. 314 if (O1 != O2 && O1Arg->hasByValAttr()) return NoAlias; 315 316 // Otherwise, nothing is known... 317 } 318 319 if (const Argument *O2Arg = dyn_cast<Argument>(O2)) { 320 // Incoming argument cannot alias locally allocated object! 321 if (isa<AllocationInst>(O1)) return NoAlias; 322 323 // If they are two different objects, and one is a noalias argument 324 // then they do not alias. 325 if (O1 != O2 && O2Arg->hasNoAliasAttr()) 326 return NoAlias; 327 328 // Byval arguments can't alias globals or other arguments. 329 if (O1 != O2 && O2Arg->hasByValAttr()) return NoAlias; 330 331 // Otherwise, nothing is known... 332 333 } else if (O1 != O2 && !isa<Argument>(O1)) { 334 // If they are two different objects, and neither is an argument, 335 // we know that we have no alias. 336 return NoAlias; 337 } 338 339 // If they are the same object, they we can look at the indexes. If they 340 // index off of the object is the same for both pointers, they must alias. 341 // If they are provably different, they must not alias. Otherwise, we 342 // can't tell anything. 343 } 344 345 // Unique values don't alias null, except non-byval arguments. 346 if (isa<ConstantPointerNull>(V2)) { 347 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 348 if (O1Arg->hasByValAttr()) 349 return NoAlias; 350 } else { 351 return NoAlias; 352 } 353 } 354 355 if (isa<GlobalVariable>(O1) || 356 (isa<AllocationInst>(O1) && 357 !cast<AllocationInst>(O1)->isArrayAllocation())) 358 if (cast<PointerType>(O1->getType())->getElementType()->isSized()) { 359 // If the size of the other access is larger than the total size of the 360 // global/alloca/malloc, it cannot be accessing the global (it's 361 // undefined to load or store bytes before or after an object). 362 const Type *ElTy = cast<PointerType>(O1->getType())->getElementType(); 363 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 364 if (GlobalSize < V2Size && V2Size != ~0U) 365 return NoAlias; 366 } 367 } 368 369 if (O2) { 370 if (!isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) 371 return NoAlias; // Unique values don't alias null 372 373 if (isa<GlobalVariable>(O2) || 374 (isa<AllocationInst>(O2) && 375 !cast<AllocationInst>(O2)->isArrayAllocation())) 376 if (cast<PointerType>(O2->getType())->getElementType()->isSized()) { 377 // If the size of the other access is larger than the total size of the 378 // global/alloca/malloc, it cannot be accessing the object (it's 379 // undefined to load or store bytes before or after an object). 380 const Type *ElTy = cast<PointerType>(O2->getType())->getElementType(); 381 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 382 if (GlobalSize < V1Size && V1Size != ~0U) 383 return NoAlias; 384 } 385 } 386 387 // If we have two gep instructions with must-alias'ing base pointers, figure 388 // out if the indexes to the GEP tell us anything about the derived pointer. 389 // Note that we also handle chains of getelementptr instructions as well as 390 // constant expression getelementptrs here. 391 // 392 if (isGEP(V1) && isGEP(V2)) { 393 // Drill down into the first non-gep value, to test for must-aliasing of 394 // the base pointers. 395 const User *G = cast<User>(V1); 396 while (isGEP(G->getOperand(0)) && 397 G->getOperand(1) == 398 Constant::getNullValue(G->getOperand(1)->getType())) 399 G = cast<User>(G->getOperand(0)); 400 const Value *BasePtr1 = G->getOperand(0); 401 402 G = cast<User>(V2); 403 while (isGEP(G->getOperand(0)) && 404 G->getOperand(1) == 405 Constant::getNullValue(G->getOperand(1)->getType())) 406 G = cast<User>(G->getOperand(0)); 407 const Value *BasePtr2 = G->getOperand(0); 408 409 // Do the base pointers alias? 410 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 411 if (BaseAlias == NoAlias) return NoAlias; 412 if (BaseAlias == MustAlias) { 413 // If the base pointers alias each other exactly, check to see if we can 414 // figure out anything about the resultant pointers, to try to prove 415 // non-aliasing. 416 417 // Collect all of the chained GEP operands together into one simple place 418 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 419 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 420 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 421 422 // If GetGEPOperands were able to fold to the same must-aliased pointer, 423 // do the comparison. 424 if (BasePtr1 == BasePtr2) { 425 AliasResult GAlias = 426 CheckGEPInstructions(BasePtr1->getType(), 427 &GEP1Ops[0], GEP1Ops.size(), V1Size, 428 BasePtr2->getType(), 429 &GEP2Ops[0], GEP2Ops.size(), V2Size); 430 if (GAlias != MayAlias) 431 return GAlias; 432 } 433 } 434 } 435 436 // Check to see if these two pointers are related by a getelementptr 437 // instruction. If one pointer is a GEP with a non-zero index of the other 438 // pointer, we know they cannot alias. 439 // 440 if (isGEP(V2)) { 441 std::swap(V1, V2); 442 std::swap(V1Size, V2Size); 443 } 444 445 if (V1Size != ~0U && V2Size != ~0U) 446 if (isGEP(V1)) { 447 SmallVector<Value*, 16> GEPOperands; 448 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 449 450 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 451 if (R == MustAlias) { 452 // If there is at least one non-zero constant index, we know they cannot 453 // alias. 454 bool ConstantFound = false; 455 bool AllZerosFound = true; 456 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 457 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 458 if (!C->isNullValue()) { 459 ConstantFound = true; 460 AllZerosFound = false; 461 break; 462 } 463 } else { 464 AllZerosFound = false; 465 } 466 467 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 468 // the ptr, the end result is a must alias also. 469 if (AllZerosFound) 470 return MustAlias; 471 472 if (ConstantFound) { 473 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 474 return NoAlias; 475 476 // Otherwise we have to check to see that the distance is more than 477 // the size of the argument... build an index vector that is equal to 478 // the arguments provided, except substitute 0's for any variable 479 // indexes we find... 480 if (cast<PointerType>( 481 BasePtr->getType())->getElementType()->isSized()) { 482 for (unsigned i = 0; i != GEPOperands.size(); ++i) 483 if (!isa<ConstantInt>(GEPOperands[i])) 484 GEPOperands[i] = 485 Constant::getNullValue(GEPOperands[i]->getType()); 486 int64_t Offset = 487 getTargetData().getIndexedOffset(BasePtr->getType(), 488 &GEPOperands[0], 489 GEPOperands.size()); 490 491 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 492 return NoAlias; 493 } 494 } 495 } 496 } 497 498 return MayAlias; 499} 500 501// This function is used to determin if the indices of two GEP instructions are 502// equal. V1 and V2 are the indices. 503static bool IndexOperandsEqual(Value *V1, Value *V2) { 504 if (V1->getType() == V2->getType()) 505 return V1 == V2; 506 if (Constant *C1 = dyn_cast<Constant>(V1)) 507 if (Constant *C2 = dyn_cast<Constant>(V2)) { 508 // Sign extend the constants to long types, if necessary 509 if (C1->getType() != Type::Int64Ty) 510 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 511 if (C2->getType() != Type::Int64Ty) 512 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 513 return C1 == C2; 514 } 515 return false; 516} 517 518/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 519/// base pointers. This checks to see if the index expressions preclude the 520/// pointers from aliasing... 521AliasAnalysis::AliasResult 522BasicAliasAnalysis::CheckGEPInstructions( 523 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 524 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 525 // We currently can't handle the case when the base pointers have different 526 // primitive types. Since this is uncommon anyway, we are happy being 527 // extremely conservative. 528 if (BasePtr1Ty != BasePtr2Ty) 529 return MayAlias; 530 531 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 532 533 // Find the (possibly empty) initial sequence of equal values... which are not 534 // necessarily constants. 535 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 536 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 537 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 538 unsigned UnequalOper = 0; 539 while (UnequalOper != MinOperands && 540 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 541 // Advance through the type as we go... 542 ++UnequalOper; 543 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 544 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 545 else { 546 // If all operands equal each other, then the derived pointers must 547 // alias each other... 548 BasePtr1Ty = 0; 549 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 550 "Ran out of type nesting, but not out of operands?"); 551 return MustAlias; 552 } 553 } 554 555 // If we have seen all constant operands, and run out of indexes on one of the 556 // getelementptrs, check to see if the tail of the leftover one is all zeros. 557 // If so, return mustalias. 558 if (UnequalOper == MinOperands) { 559 if (NumGEP1Ops < NumGEP2Ops) { 560 std::swap(GEP1Ops, GEP2Ops); 561 std::swap(NumGEP1Ops, NumGEP2Ops); 562 } 563 564 bool AllAreZeros = true; 565 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 566 if (!isa<Constant>(GEP1Ops[i]) || 567 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 568 AllAreZeros = false; 569 break; 570 } 571 if (AllAreZeros) return MustAlias; 572 } 573 574 575 // So now we know that the indexes derived from the base pointers, 576 // which are known to alias, are different. We can still determine a 577 // no-alias result if there are differing constant pairs in the index 578 // chain. For example: 579 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 580 // 581 // We have to be careful here about array accesses. In particular, consider: 582 // A[1][0] vs A[0][i] 583 // In this case, we don't *know* that the array will be accessed in bounds: 584 // the index could even be negative. Because of this, we have to 585 // conservatively *give up* and return may alias. We disregard differing 586 // array subscripts that are followed by a variable index without going 587 // through a struct. 588 // 589 unsigned SizeMax = std::max(G1S, G2S); 590 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 591 592 // Scan for the first operand that is constant and unequal in the 593 // two getelementptrs... 594 unsigned FirstConstantOper = UnequalOper; 595 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 596 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 597 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 598 599 if (G1Oper != G2Oper) // Found non-equal constant indexes... 600 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 601 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 602 if (G1OC->getType() != G2OC->getType()) { 603 // Sign extend both operands to long. 604 if (G1OC->getType() != Type::Int64Ty) 605 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 606 if (G2OC->getType() != Type::Int64Ty) 607 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 608 GEP1Ops[FirstConstantOper] = G1OC; 609 GEP2Ops[FirstConstantOper] = G2OC; 610 } 611 612 if (G1OC != G2OC) { 613 // Handle the "be careful" case above: if this is an array/vector 614 // subscript, scan for a subsequent variable array index. 615 if (isa<SequentialType>(BasePtr1Ty)) { 616 const Type *NextTy = 617 cast<SequentialType>(BasePtr1Ty)->getElementType(); 618 bool isBadCase = false; 619 620 for (unsigned Idx = FirstConstantOper+1; 621 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 622 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 623 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 624 isBadCase = true; 625 break; 626 } 627 NextTy = cast<SequentialType>(NextTy)->getElementType(); 628 } 629 630 if (isBadCase) G1OC = 0; 631 } 632 633 // Make sure they are comparable (ie, not constant expressions), and 634 // make sure the GEP with the smaller leading constant is GEP1. 635 if (G1OC) { 636 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 637 G1OC, G2OC); 638 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 639 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 640 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 641 std::swap(NumGEP1Ops, NumGEP2Ops); 642 } 643 break; 644 } 645 } 646 } 647 } 648 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 649 } 650 651 // No shared constant operands, and we ran out of common operands. At this 652 // point, the GEP instructions have run through all of their operands, and we 653 // haven't found evidence that there are any deltas between the GEP's. 654 // However, one GEP may have more operands than the other. If this is the 655 // case, there may still be hope. Check this now. 656 if (FirstConstantOper == MinOperands) { 657 // Make GEP1Ops be the longer one if there is a longer one. 658 if (NumGEP1Ops < NumGEP2Ops) { 659 std::swap(GEP1Ops, GEP2Ops); 660 std::swap(NumGEP1Ops, NumGEP2Ops); 661 } 662 663 // Is there anything to check? 664 if (NumGEP1Ops > MinOperands) { 665 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 666 if (isa<ConstantInt>(GEP1Ops[i]) && 667 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 668 // Yup, there's a constant in the tail. Set all variables to 669 // constants in the GEP instruction to make it suiteable for 670 // TargetData::getIndexedOffset. 671 for (i = 0; i != MaxOperands; ++i) 672 if (!isa<ConstantInt>(GEP1Ops[i])) 673 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 674 // Okay, now get the offset. This is the relative offset for the full 675 // instruction. 676 const TargetData &TD = getTargetData(); 677 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 678 NumGEP1Ops); 679 680 // Now check without any constants at the end. 681 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 682 MinOperands); 683 684 // If the tail provided a bit enough offset, return noalias! 685 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 686 return NoAlias; 687 } 688 } 689 690 // Couldn't find anything useful. 691 return MayAlias; 692 } 693 694 // If there are non-equal constants arguments, then we can figure 695 // out a minimum known delta between the two index expressions... at 696 // this point we know that the first constant index of GEP1 is less 697 // than the first constant index of GEP2. 698 699 // Advance BasePtr[12]Ty over this first differing constant operand. 700 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 701 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 702 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 703 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 704 705 // We are going to be using TargetData::getIndexedOffset to determine the 706 // offset that each of the GEP's is reaching. To do this, we have to convert 707 // all variable references to constant references. To do this, we convert the 708 // initial sequence of array subscripts into constant zeros to start with. 709 const Type *ZeroIdxTy = GEPPointerTy; 710 for (unsigned i = 0; i != FirstConstantOper; ++i) { 711 if (!isa<StructType>(ZeroIdxTy)) 712 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 713 714 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 715 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 716 } 717 718 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 719 720 // Loop over the rest of the operands... 721 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 722 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 723 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 724 // If they are equal, use a zero index... 725 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 726 if (!isa<ConstantInt>(Op1)) 727 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 728 // Otherwise, just keep the constants we have. 729 } else { 730 if (Op1) { 731 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 732 // If this is an array index, make sure the array element is in range. 733 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 734 if (Op1C->getZExtValue() >= AT->getNumElements()) 735 return MayAlias; // Be conservative with out-of-range accesses 736 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 737 if (Op1C->getZExtValue() >= VT->getNumElements()) 738 return MayAlias; // Be conservative with out-of-range accesses 739 } 740 741 } else { 742 // GEP1 is known to produce a value less than GEP2. To be 743 // conservatively correct, we must assume the largest possible 744 // constant is used in this position. This cannot be the initial 745 // index to the GEP instructions (because we know we have at least one 746 // element before this one with the different constant arguments), so 747 // we know that the current index must be into either a struct or 748 // array. Because we know it's not constant, this cannot be a 749 // structure index. Because of this, we can calculate the maximum 750 // value possible. 751 // 752 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 753 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 754 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 755 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 756 } 757 } 758 759 if (Op2) { 760 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 761 // If this is an array index, make sure the array element is in range. 762 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 763 if (Op2C->getZExtValue() >= AT->getNumElements()) 764 return MayAlias; // Be conservative with out-of-range accesses 765 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 766 if (Op2C->getZExtValue() >= VT->getNumElements()) 767 return MayAlias; // Be conservative with out-of-range accesses 768 } 769 } else { // Conservatively assume the minimum value for this index 770 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 771 } 772 } 773 } 774 775 if (BasePtr1Ty && Op1) { 776 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 777 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 778 else 779 BasePtr1Ty = 0; 780 } 781 782 if (BasePtr2Ty && Op2) { 783 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 784 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 785 else 786 BasePtr2Ty = 0; 787 } 788 } 789 790 if (GEPPointerTy->getElementType()->isSized()) { 791 int64_t Offset1 = 792 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 793 int64_t Offset2 = 794 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 795 assert(Offset1 != Offset2 && 796 "There is at least one different constant here!"); 797 798 // Make sure we compare the absolute difference. 799 if (Offset1 > Offset2) 800 std::swap(Offset1, Offset2); 801 802 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 803 //cerr << "Determined that these two GEP's don't alias [" 804 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 805 return NoAlias; 806 } 807 } 808 return MayAlias; 809} 810 811// Make sure that anything that uses AliasAnalysis pulls in this file... 812DEFINING_FILE_FOR(BasicAliasAnalysis) 813