BasicAliasAnalysis.cpp revision f88380ba2cf472db6576a8534315449931e8cf50
1//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source 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, a 143// unique object or an argument, return it. 144static const Value *getUnderlyingObject(const Value *V) { 145 if (!isa<PointerType>(V->getType())) return 0; 146 147 // If we are at some type of object, return it. GlobalValues and Allocations 148 // have unique addresses. 149 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isa<Argument>(V)) 150 return V; 151 152 // Traverse through different addressing mechanisms... 153 if (const Instruction *I = dyn_cast<Instruction>(V)) { 154 if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I)) 155 return getUnderlyingObject(I->getOperand(0)); 156 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) { 157 if (CE->getOpcode() == Instruction::BitCast || 158 CE->getOpcode() == Instruction::GetElementPtr) 159 return getUnderlyingObject(CE->getOperand(0)); 160 } 161 return 0; 162} 163 164static const User *isGEP(const Value *V) { 165 if (isa<GetElementPtrInst>(V) || 166 (isa<ConstantExpr>(V) && 167 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 168 return cast<User>(V); 169 return 0; 170} 171 172static const Value *GetGEPOperands(const Value *V, 173 SmallVector<Value*, 16> &GEPOps){ 174 assert(GEPOps.empty() && "Expect empty list to populate!"); 175 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 176 cast<User>(V)->op_end()); 177 178 // Accumulate all of the chained indexes into the operand array 179 V = cast<User>(V)->getOperand(0); 180 181 while (const User *G = isGEP(V)) { 182 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 183 !cast<Constant>(GEPOps[0])->isNullValue()) 184 break; // Don't handle folding arbitrary pointer offsets yet... 185 GEPOps.erase(GEPOps.begin()); // Drop the zero index 186 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 187 V = G->getOperand(0); 188 } 189 return V; 190} 191 192/// pointsToConstantMemory - Chase pointers until we find a (constant 193/// global) or not. 194bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 195 if (const Value *V = getUnderlyingObject(P)) 196 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 197 return GV->isConstant(); 198 return false; 199} 200 201// Determine if an AllocationInst instruction escapes from the function it is 202// contained in. If it does not escape, there is no way for another function to 203// mod/ref it. We do this by looking at its uses and determining if the uses 204// can escape (recursively). 205static bool AddressMightEscape(const Value *V) { 206 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end(); 207 UI != E; ++UI) { 208 const Instruction *I = cast<Instruction>(*UI); 209 switch (I->getOpcode()) { 210 case Instruction::Load: 211 break; //next use. 212 case Instruction::Store: 213 if (I->getOperand(0) == V) 214 return true; // Escapes if the pointer is stored. 215 break; // next use. 216 case Instruction::GetElementPtr: 217 if (AddressMightEscape(I)) 218 return true; 219 break; // next use. 220 case Instruction::BitCast: 221 if (!isa<PointerType>(I->getType())) 222 return true; 223 if (AddressMightEscape(I)) 224 return true; 225 break; // next use 226 case Instruction::Ret: 227 // If returned, the address will escape to calling functions, but no 228 // callees could modify it. 229 break; // next use 230 default: 231 return true; 232 } 233 } 234 return false; 235} 236 237// getModRefInfo - Check to see if the specified callsite can clobber the 238// specified memory object. Since we only look at local properties of this 239// function, we really can't say much about this query. We do, however, use 240// simple "address taken" analysis on local objects. 241// 242AliasAnalysis::ModRefResult 243BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 244 if (!isa<Constant>(P)) 245 if (const AllocationInst *AI = 246 dyn_cast_or_null<AllocationInst>(getUnderlyingObject(P))) { 247 // Okay, the pointer is to a stack allocated object. If we can prove that 248 // the pointer never "escapes", then we know the call cannot clobber it, 249 // because it simply can't get its address. 250 if (!AddressMightEscape(AI)) 251 return NoModRef; 252 253 // If this is a tail call and P points to a stack location, we know that 254 // the tail call cannot access or modify the local stack. 255 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 256 if (CI->isTailCall() && isa<AllocaInst>(AI)) 257 return NoModRef; 258 } 259 260 // The AliasAnalysis base class has some smarts, lets use them. 261 return AliasAnalysis::getModRefInfo(CS, P, Size); 262} 263 264static bool isNoAliasArgument(const Argument *Arg) { 265 const Function *Func = Arg->getParent(); 266 const ParamAttrsList *Attr = Func->getParamAttrs(); 267 if (Attr) { 268 unsigned Idx = 1; 269 for (Function::const_arg_iterator I = Func->arg_begin(), 270 E = Func->arg_end(); I != E; ++I, ++Idx) { 271 if (&(*I) == Arg && 272 Attr->paramHasAttr(Idx, ParamAttr::NoAlias)) 273 return true; 274 } 275 } 276 return false; 277} 278 279// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 280// as array references. Note that this function is heavily tail recursive. 281// Hopefully we have a smart C++ compiler. :) 282// 283AliasAnalysis::AliasResult 284BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 285 const Value *V2, unsigned V2Size) { 286 // Strip off any constant expression casts if they exist 287 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 288 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 289 V1 = CE->getOperand(0); 290 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 291 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 292 V2 = CE->getOperand(0); 293 294 // Are we checking for alias of the same value? 295 if (V1 == V2) return MustAlias; 296 297 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) && 298 V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty) 299 return NoAlias; // Scalars cannot alias each other 300 301 // Strip off cast instructions... 302 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 303 return alias(I->getOperand(0), V1Size, V2, V2Size); 304 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 305 return alias(V1, V1Size, I->getOperand(0), V2Size); 306 307 // Figure out what objects these things are pointing to if we can... 308 const Value *O1 = getUnderlyingObject(V1); 309 const Value *O2 = getUnderlyingObject(V2); 310 311 // Pointing at a discernible object? 312 if (O1) { 313 if (O2) { 314 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 315 // Incoming argument cannot alias locally allocated object! 316 if (isa<AllocationInst>(O2)) return NoAlias; 317 318 // If they are two different objects, and one is a noalias argument 319 // then they do not alias. 320 if (O1 != O2 && isNoAliasArgument(O1Arg)) 321 return NoAlias; 322 323 // Otherwise, nothing is known... 324 } 325 326 if (const Argument *O2Arg = dyn_cast<Argument>(O2)) { 327 // Incoming argument cannot alias locally allocated object! 328 if (isa<AllocationInst>(O1)) return NoAlias; 329 330 // If they are two different objects, and one is a noalias argument 331 // then they do not alias. 332 if (O1 != O2 && isNoAliasArgument(O2Arg)) 333 return NoAlias; 334 335 // Otherwise, nothing is known... 336 337 } else if (O1 != O2) { 338 if (!isa<Argument>(O1)) 339 // If they are two different objects, and neither is an argument, 340 // we know that we have no alias... 341 return NoAlias; 342 } 343 344 // If they are the same object, they we can look at the indexes. If they 345 // index off of the object is the same for both pointers, they must alias. 346 // If they are provably different, they must not alias. Otherwise, we 347 // can't tell anything. 348 } 349 350 351 if (!isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) 352 return NoAlias; // Unique values don't alias null 353 354 if (isa<GlobalVariable>(O1) || 355 (isa<AllocationInst>(O1) && 356 !cast<AllocationInst>(O1)->isArrayAllocation())) 357 if (cast<PointerType>(O1->getType())->getElementType()->isSized()) { 358 // If the size of the other access is larger than the total size of the 359 // global/alloca/malloc, it cannot be accessing the global (it's 360 // undefined to load or store bytes before or after an object). 361 const Type *ElTy = cast<PointerType>(O1->getType())->getElementType(); 362 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 363 if (GlobalSize < V2Size && V2Size != ~0U) 364 return NoAlias; 365 } 366 } 367 368 if (O2) { 369 if (!isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) 370 return NoAlias; // Unique values don't alias null 371 372 if (isa<GlobalVariable>(O2) || 373 (isa<AllocationInst>(O2) && 374 !cast<AllocationInst>(O2)->isArrayAllocation())) 375 if (cast<PointerType>(O2->getType())->getElementType()->isSized()) { 376 // If the size of the other access is larger than the total size of the 377 // global/alloca/malloc, it cannot be accessing the object (it's 378 // undefined to load or store bytes before or after an object). 379 const Type *ElTy = cast<PointerType>(O2->getType())->getElementType(); 380 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 381 if (GlobalSize < V1Size && V1Size != ~0U) 382 return NoAlias; 383 } 384 } 385 386 // If we have two gep instructions with must-alias'ing base pointers, figure 387 // out if the indexes to the GEP tell us anything about the derived pointer. 388 // Note that we also handle chains of getelementptr instructions as well as 389 // constant expression getelementptrs here. 390 // 391 if (isGEP(V1) && isGEP(V2)) { 392 // Drill down into the first non-gep value, to test for must-aliasing of 393 // the base pointers. 394 const Value *BasePtr1 = V1, *BasePtr2 = V2; 395 do { 396 BasePtr1 = cast<User>(BasePtr1)->getOperand(0); 397 } while (isGEP(BasePtr1) && 398 cast<User>(BasePtr1)->getOperand(1) == 399 Constant::getNullValue(cast<User>(BasePtr1)->getOperand(1)->getType())); 400 do { 401 BasePtr2 = cast<User>(BasePtr2)->getOperand(0); 402 } while (isGEP(BasePtr2) && 403 cast<User>(BasePtr2)->getOperand(1) == 404 Constant::getNullValue(cast<User>(BasePtr2)->getOperand(1)->getType())); 405 406 // Do the base pointers alias? 407 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 408 if (BaseAlias == NoAlias) return NoAlias; 409 if (BaseAlias == MustAlias) { 410 // If the base pointers alias each other exactly, check to see if we can 411 // figure out anything about the resultant pointers, to try to prove 412 // non-aliasing. 413 414 // Collect all of the chained GEP operands together into one simple place 415 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 416 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 417 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 418 419 // If GetGEPOperands were able to fold to the same must-aliased pointer, 420 // do the comparison. 421 if (BasePtr1 == BasePtr2) { 422 AliasResult GAlias = 423 CheckGEPInstructions(BasePtr1->getType(), 424 &GEP1Ops[0], GEP1Ops.size(), V1Size, 425 BasePtr2->getType(), 426 &GEP2Ops[0], GEP2Ops.size(), V2Size); 427 if (GAlias != MayAlias) 428 return GAlias; 429 } 430 } 431 } 432 433 // Check to see if these two pointers are related by a getelementptr 434 // instruction. If one pointer is a GEP with a non-zero index of the other 435 // pointer, we know they cannot alias. 436 // 437 if (isGEP(V2)) { 438 std::swap(V1, V2); 439 std::swap(V1Size, V2Size); 440 } 441 442 if (V1Size != ~0U && V2Size != ~0U) 443 if (isGEP(V1)) { 444 SmallVector<Value*, 16> GEPOperands; 445 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 446 447 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 448 if (R == MustAlias) { 449 // If there is at least one non-zero constant index, we know they cannot 450 // alias. 451 bool ConstantFound = false; 452 bool AllZerosFound = true; 453 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 454 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 455 if (!C->isNullValue()) { 456 ConstantFound = true; 457 AllZerosFound = false; 458 break; 459 } 460 } else { 461 AllZerosFound = false; 462 } 463 464 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 465 // the ptr, the end result is a must alias also. 466 if (AllZerosFound) 467 return MustAlias; 468 469 if (ConstantFound) { 470 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 471 return NoAlias; 472 473 // Otherwise we have to check to see that the distance is more than 474 // the size of the argument... build an index vector that is equal to 475 // the arguments provided, except substitute 0's for any variable 476 // indexes we find... 477 if (cast<PointerType>( 478 BasePtr->getType())->getElementType()->isSized()) { 479 for (unsigned i = 0; i != GEPOperands.size(); ++i) 480 if (!isa<ConstantInt>(GEPOperands[i])) 481 GEPOperands[i] = 482 Constant::getNullValue(GEPOperands[i]->getType()); 483 int64_t Offset = 484 getTargetData().getIndexedOffset(BasePtr->getType(), 485 &GEPOperands[0], 486 GEPOperands.size()); 487 488 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 489 return NoAlias; 490 } 491 } 492 } 493 } 494 495 return MayAlias; 496} 497 498// This function is used to determin if the indices of two GEP instructions are 499// equal. V1 and V2 are the indices. 500static bool IndexOperandsEqual(Value *V1, Value *V2) { 501 if (V1->getType() == V2->getType()) 502 return V1 == V2; 503 if (Constant *C1 = dyn_cast<Constant>(V1)) 504 if (Constant *C2 = dyn_cast<Constant>(V2)) { 505 // Sign extend the constants to long types, if necessary 506 if (C1->getType() != Type::Int64Ty) 507 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 508 if (C2->getType() != Type::Int64Ty) 509 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 510 return C1 == C2; 511 } 512 return false; 513} 514 515/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 516/// base pointers. This checks to see if the index expressions preclude the 517/// pointers from aliasing... 518AliasAnalysis::AliasResult 519BasicAliasAnalysis::CheckGEPInstructions( 520 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 521 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 522 // We currently can't handle the case when the base pointers have different 523 // primitive types. Since this is uncommon anyway, we are happy being 524 // extremely conservative. 525 if (BasePtr1Ty != BasePtr2Ty) 526 return MayAlias; 527 528 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 529 530 // Find the (possibly empty) initial sequence of equal values... which are not 531 // necessarily constants. 532 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 533 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 534 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 535 unsigned UnequalOper = 0; 536 while (UnequalOper != MinOperands && 537 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 538 // Advance through the type as we go... 539 ++UnequalOper; 540 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 541 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 542 else { 543 // If all operands equal each other, then the derived pointers must 544 // alias each other... 545 BasePtr1Ty = 0; 546 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 547 "Ran out of type nesting, but not out of operands?"); 548 return MustAlias; 549 } 550 } 551 552 // If we have seen all constant operands, and run out of indexes on one of the 553 // getelementptrs, check to see if the tail of the leftover one is all zeros. 554 // If so, return mustalias. 555 if (UnequalOper == MinOperands) { 556 if (NumGEP1Ops < NumGEP2Ops) { 557 std::swap(GEP1Ops, GEP2Ops); 558 std::swap(NumGEP1Ops, NumGEP2Ops); 559 } 560 561 bool AllAreZeros = true; 562 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 563 if (!isa<Constant>(GEP1Ops[i]) || 564 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 565 AllAreZeros = false; 566 break; 567 } 568 if (AllAreZeros) return MustAlias; 569 } 570 571 572 // So now we know that the indexes derived from the base pointers, 573 // which are known to alias, are different. We can still determine a 574 // no-alias result if there are differing constant pairs in the index 575 // chain. For example: 576 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 577 // 578 // We have to be careful here about array accesses. In particular, consider: 579 // A[1][0] vs A[0][i] 580 // In this case, we don't *know* that the array will be accessed in bounds: 581 // the index could even be negative. Because of this, we have to 582 // conservatively *give up* and return may alias. We disregard differing 583 // array subscripts that are followed by a variable index without going 584 // through a struct. 585 // 586 unsigned SizeMax = std::max(G1S, G2S); 587 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 588 589 // Scan for the first operand that is constant and unequal in the 590 // two getelementptrs... 591 unsigned FirstConstantOper = UnequalOper; 592 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 593 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 594 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 595 596 if (G1Oper != G2Oper) // Found non-equal constant indexes... 597 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 598 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 599 if (G1OC->getType() != G2OC->getType()) { 600 // Sign extend both operands to long. 601 if (G1OC->getType() != Type::Int64Ty) 602 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 603 if (G2OC->getType() != Type::Int64Ty) 604 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 605 GEP1Ops[FirstConstantOper] = G1OC; 606 GEP2Ops[FirstConstantOper] = G2OC; 607 } 608 609 if (G1OC != G2OC) { 610 // Handle the "be careful" case above: if this is an array/vector 611 // subscript, scan for a subsequent variable array index. 612 if (isa<SequentialType>(BasePtr1Ty)) { 613 const Type *NextTy = 614 cast<SequentialType>(BasePtr1Ty)->getElementType(); 615 bool isBadCase = false; 616 617 for (unsigned Idx = FirstConstantOper+1; 618 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 619 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 620 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 621 isBadCase = true; 622 break; 623 } 624 NextTy = cast<SequentialType>(NextTy)->getElementType(); 625 } 626 627 if (isBadCase) G1OC = 0; 628 } 629 630 // Make sure they are comparable (ie, not constant expressions), and 631 // make sure the GEP with the smaller leading constant is GEP1. 632 if (G1OC) { 633 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 634 G1OC, G2OC); 635 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 636 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 637 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 638 std::swap(NumGEP1Ops, NumGEP2Ops); 639 } 640 break; 641 } 642 } 643 } 644 } 645 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 646 } 647 648 // No shared constant operands, and we ran out of common operands. At this 649 // point, the GEP instructions have run through all of their operands, and we 650 // haven't found evidence that there are any deltas between the GEP's. 651 // However, one GEP may have more operands than the other. If this is the 652 // case, there may still be hope. Check this now. 653 if (FirstConstantOper == MinOperands) { 654 // Make GEP1Ops be the longer one if there is a longer one. 655 if (NumGEP1Ops < NumGEP2Ops) { 656 std::swap(GEP1Ops, GEP2Ops); 657 std::swap(NumGEP1Ops, NumGEP2Ops); 658 } 659 660 // Is there anything to check? 661 if (NumGEP1Ops > MinOperands) { 662 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 663 if (isa<ConstantInt>(GEP1Ops[i]) && 664 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 665 // Yup, there's a constant in the tail. Set all variables to 666 // constants in the GEP instruction to make it suiteable for 667 // TargetData::getIndexedOffset. 668 for (i = 0; i != MaxOperands; ++i) 669 if (!isa<ConstantInt>(GEP1Ops[i])) 670 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 671 // Okay, now get the offset. This is the relative offset for the full 672 // instruction. 673 const TargetData &TD = getTargetData(); 674 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 675 NumGEP1Ops); 676 677 // Now check without any constants at the end. 678 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 679 MinOperands); 680 681 // If the tail provided a bit enough offset, return noalias! 682 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 683 return NoAlias; 684 } 685 } 686 687 // Couldn't find anything useful. 688 return MayAlias; 689 } 690 691 // If there are non-equal constants arguments, then we can figure 692 // out a minimum known delta between the two index expressions... at 693 // this point we know that the first constant index of GEP1 is less 694 // than the first constant index of GEP2. 695 696 // Advance BasePtr[12]Ty over this first differing constant operand. 697 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 698 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 699 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 700 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 701 702 // We are going to be using TargetData::getIndexedOffset to determine the 703 // offset that each of the GEP's is reaching. To do this, we have to convert 704 // all variable references to constant references. To do this, we convert the 705 // initial sequence of array subscripts into constant zeros to start with. 706 const Type *ZeroIdxTy = GEPPointerTy; 707 for (unsigned i = 0; i != FirstConstantOper; ++i) { 708 if (!isa<StructType>(ZeroIdxTy)) 709 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 710 711 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 712 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 713 } 714 715 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 716 717 // Loop over the rest of the operands... 718 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 719 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 720 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 721 // If they are equal, use a zero index... 722 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 723 if (!isa<ConstantInt>(Op1)) 724 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 725 // Otherwise, just keep the constants we have. 726 } else { 727 if (Op1) { 728 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 729 // If this is an array index, make sure the array element is in range. 730 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 731 if (Op1C->getZExtValue() >= AT->getNumElements()) 732 return MayAlias; // Be conservative with out-of-range accesses 733 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 734 if (Op1C->getZExtValue() >= VT->getNumElements()) 735 return MayAlias; // Be conservative with out-of-range accesses 736 } 737 738 } else { 739 // GEP1 is known to produce a value less than GEP2. To be 740 // conservatively correct, we must assume the largest possible 741 // constant is used in this position. This cannot be the initial 742 // index to the GEP instructions (because we know we have at least one 743 // element before this one with the different constant arguments), so 744 // we know that the current index must be into either a struct or 745 // array. Because we know it's not constant, this cannot be a 746 // structure index. Because of this, we can calculate the maximum 747 // value possible. 748 // 749 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 750 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 751 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 752 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 753 } 754 } 755 756 if (Op2) { 757 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 758 // If this is an array index, make sure the array element is in range. 759 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 760 if (Op2C->getZExtValue() >= AT->getNumElements()) 761 return MayAlias; // Be conservative with out-of-range accesses 762 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 763 if (Op2C->getZExtValue() >= VT->getNumElements()) 764 return MayAlias; // Be conservative with out-of-range accesses 765 } 766 } else { // Conservatively assume the minimum value for this index 767 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 768 } 769 } 770 } 771 772 if (BasePtr1Ty && Op1) { 773 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 774 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 775 else 776 BasePtr1Ty = 0; 777 } 778 779 if (BasePtr2Ty && Op2) { 780 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 781 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 782 else 783 BasePtr2Ty = 0; 784 } 785 } 786 787 if (GEPPointerTy->getElementType()->isSized()) { 788 int64_t Offset1 = 789 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 790 int64_t Offset2 = 791 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 792 assert(Offset1 != Offset2 && 793 "There is at least one different constant here!"); 794 795 // Make sure we compare the absolute difference. 796 if (Offset1 > Offset2) 797 std::swap(Offset1, Offset2); 798 799 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 800 //cerr << "Determined that these two GEP's don't alias [" 801 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 802 return NoAlias; 803 } 804 } 805 return MayAlias; 806} 807 808// Make sure that anything that uses AliasAnalysis pulls in this file... 809DEFINING_FILE_FOR(BasicAliasAnalysis) 810