BasicAliasAnalysis.cpp revision ef562471cebd643288a19fca1ece8c52e70ceff0
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/IntrinsicInst.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 (AddressMightEscape(I)) 223 return true; 224 break; // next use 225 case Instruction::Ret: 226 // If returned, the address will escape to calling functions, but no 227 // callees could modify it. 228 break; // next use 229 case Instruction::Call: 230 // If the call is to a few known safe intrinsics, we know that it does 231 // not escape 232 if (!isa<MemIntrinsic>(I)) 233 return true; 234 break; // next use 235 default: 236 return true; 237 } 238 } 239 return false; 240} 241 242// getModRefInfo - Check to see if the specified callsite can clobber the 243// specified memory object. Since we only look at local properties of this 244// function, we really can't say much about this query. We do, however, use 245// simple "address taken" analysis on local objects. 246// 247AliasAnalysis::ModRefResult 248BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 249 if (!isa<Constant>(P)) { 250 const Value *Object = getUnderlyingObject(P); 251 // Allocations and byval arguments are "new" objects. 252 if (Object && 253 (isa<AllocationInst>(Object) || isa<Argument>(Object))) { 254 // Okay, the pointer is to a stack allocated (or effectively so, for 255 // for noalias parameters) object. If the address of this object doesn't 256 // escape from this function body to a callee, then we know that no 257 // callees can mod/ref it unless they are actually passed it. 258 if (isa<AllocationInst>(Object) || 259 cast<Argument>(Object)->hasByValAttr() || 260 cast<Argument>(Object)->hasNoAliasAttr()) 261 if (!AddressMightEscape(Object)) { 262 bool passedAsArg = false; 263 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 264 CI != CE; ++CI) 265 if (getUnderlyingObject(CI->get()) == P) 266 passedAsArg = true; 267 268 if (!passedAsArg) 269 return NoModRef; 270 } 271 272 // If this is a tail call and P points to a stack location, we know that 273 // the tail call cannot access or modify the local stack. 274 if (isa<AllocaInst>(Object) || 275 (isa<Argument>(Object) && cast<Argument>(Object)->hasByValAttr())) 276 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 277 if (CI->isTailCall()) 278 return NoModRef; 279 } 280 } 281 282 // The AliasAnalysis base class has some smarts, lets use them. 283 return AliasAnalysis::getModRefInfo(CS, P, Size); 284} 285 286// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 287// as array references. Note that this function is heavily tail recursive. 288// Hopefully we have a smart C++ compiler. :) 289// 290AliasAnalysis::AliasResult 291BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 292 const Value *V2, unsigned V2Size) { 293 // Strip off any constant expression casts if they exist 294 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 295 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 296 V1 = CE->getOperand(0); 297 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 298 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 299 V2 = CE->getOperand(0); 300 301 // Are we checking for alias of the same value? 302 if (V1 == V2) return MustAlias; 303 304 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) && 305 V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty) 306 return NoAlias; // Scalars cannot alias each other 307 308 // Strip off cast instructions... 309 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 310 return alias(I->getOperand(0), V1Size, V2, V2Size); 311 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 312 return alias(V1, V1Size, I->getOperand(0), V2Size); 313 314 // Figure out what objects these things are pointing to if we can... 315 const Value *O1 = getUnderlyingObject(V1); 316 const Value *O2 = getUnderlyingObject(V2); 317 318 // Pointing at a discernible object? 319 if (O1) { 320 if (O2) { 321 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 322 // Incoming argument cannot alias locally allocated object! 323 if (isa<AllocationInst>(O2)) return NoAlias; 324 325 // If they are two different objects, and one is a noalias argument 326 // then they do not alias. 327 if (O1 != O2 && O1Arg->hasNoAliasAttr()) 328 return NoAlias; 329 330 // Byval arguments can't alias globals or other arguments. 331 if (O1 != O2 && O1Arg->hasByValAttr()) return NoAlias; 332 333 // Otherwise, nothing is known... 334 } 335 336 if (const Argument *O2Arg = dyn_cast<Argument>(O2)) { 337 // Incoming argument cannot alias locally allocated object! 338 if (isa<AllocationInst>(O1)) return NoAlias; 339 340 // If they are two different objects, and one is a noalias argument 341 // then they do not alias. 342 if (O1 != O2 && O2Arg->hasNoAliasAttr()) 343 return NoAlias; 344 345 // Byval arguments can't alias globals or other arguments. 346 if (O1 != O2 && O2Arg->hasByValAttr()) return NoAlias; 347 348 // Otherwise, nothing is known... 349 350 } else if (O1 != O2 && !isa<Argument>(O1)) { 351 // If they are two different objects, and neither is an argument, 352 // we know that we have no alias. 353 return NoAlias; 354 } 355 356 // If they are the same object, they we can look at the indexes. If they 357 // index off of the object is the same for both pointers, they must alias. 358 // If they are provably different, they must not alias. Otherwise, we 359 // can't tell anything. 360 } 361 362 // Unique values don't alias null, except non-byval arguments. 363 if (isa<ConstantPointerNull>(V2)) { 364 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 365 if (O1Arg->hasByValAttr()) 366 return NoAlias; 367 } else { 368 return NoAlias; 369 } 370 } 371 372 if (isa<GlobalVariable>(O1) || 373 (isa<AllocationInst>(O1) && 374 !cast<AllocationInst>(O1)->isArrayAllocation())) 375 if (cast<PointerType>(O1->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 global (it's 378 // undefined to load or store bytes before or after an object). 379 const Type *ElTy = cast<PointerType>(O1->getType())->getElementType(); 380 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 381 if (GlobalSize < V2Size && V2Size != ~0U) 382 return NoAlias; 383 } 384 } 385 386 if (O2) { 387 if (!isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) 388 return NoAlias; // Unique values don't alias null 389 390 if (isa<GlobalVariable>(O2) || 391 (isa<AllocationInst>(O2) && 392 !cast<AllocationInst>(O2)->isArrayAllocation())) 393 if (cast<PointerType>(O2->getType())->getElementType()->isSized()) { 394 // If the size of the other access is larger than the total size of the 395 // global/alloca/malloc, it cannot be accessing the object (it's 396 // undefined to load or store bytes before or after an object). 397 const Type *ElTy = cast<PointerType>(O2->getType())->getElementType(); 398 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 399 if (GlobalSize < V1Size && V1Size != ~0U) 400 return NoAlias; 401 } 402 } 403 404 // If we have two gep instructions with must-alias'ing base pointers, figure 405 // out if the indexes to the GEP tell us anything about the derived pointer. 406 // Note that we also handle chains of getelementptr instructions as well as 407 // constant expression getelementptrs here. 408 // 409 if (isGEP(V1) && isGEP(V2)) { 410 // Drill down into the first non-gep value, to test for must-aliasing of 411 // the base pointers. 412 const User *G = cast<User>(V1); 413 while (isGEP(G->getOperand(0)) && 414 G->getOperand(1) == 415 Constant::getNullValue(G->getOperand(1)->getType())) 416 G = cast<User>(G->getOperand(0)); 417 const Value *BasePtr1 = G->getOperand(0); 418 419 G = cast<User>(V2); 420 while (isGEP(G->getOperand(0)) && 421 G->getOperand(1) == 422 Constant::getNullValue(G->getOperand(1)->getType())) 423 G = cast<User>(G->getOperand(0)); 424 const Value *BasePtr2 = G->getOperand(0); 425 426 // Do the base pointers alias? 427 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 428 if (BaseAlias == NoAlias) return NoAlias; 429 if (BaseAlias == MustAlias) { 430 // If the base pointers alias each other exactly, check to see if we can 431 // figure out anything about the resultant pointers, to try to prove 432 // non-aliasing. 433 434 // Collect all of the chained GEP operands together into one simple place 435 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 436 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 437 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 438 439 // If GetGEPOperands were able to fold to the same must-aliased pointer, 440 // do the comparison. 441 if (BasePtr1 == BasePtr2) { 442 AliasResult GAlias = 443 CheckGEPInstructions(BasePtr1->getType(), 444 &GEP1Ops[0], GEP1Ops.size(), V1Size, 445 BasePtr2->getType(), 446 &GEP2Ops[0], GEP2Ops.size(), V2Size); 447 if (GAlias != MayAlias) 448 return GAlias; 449 } 450 } 451 } 452 453 // Check to see if these two pointers are related by a getelementptr 454 // instruction. If one pointer is a GEP with a non-zero index of the other 455 // pointer, we know they cannot alias. 456 // 457 if (isGEP(V2)) { 458 std::swap(V1, V2); 459 std::swap(V1Size, V2Size); 460 } 461 462 if (V1Size != ~0U && V2Size != ~0U) 463 if (isGEP(V1)) { 464 SmallVector<Value*, 16> GEPOperands; 465 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 466 467 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 468 if (R == MustAlias) { 469 // If there is at least one non-zero constant index, we know they cannot 470 // alias. 471 bool ConstantFound = false; 472 bool AllZerosFound = true; 473 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 474 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 475 if (!C->isNullValue()) { 476 ConstantFound = true; 477 AllZerosFound = false; 478 break; 479 } 480 } else { 481 AllZerosFound = false; 482 } 483 484 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 485 // the ptr, the end result is a must alias also. 486 if (AllZerosFound) 487 return MustAlias; 488 489 if (ConstantFound) { 490 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 491 return NoAlias; 492 493 // Otherwise we have to check to see that the distance is more than 494 // the size of the argument... build an index vector that is equal to 495 // the arguments provided, except substitute 0's for any variable 496 // indexes we find... 497 if (cast<PointerType>( 498 BasePtr->getType())->getElementType()->isSized()) { 499 for (unsigned i = 0; i != GEPOperands.size(); ++i) 500 if (!isa<ConstantInt>(GEPOperands[i])) 501 GEPOperands[i] = 502 Constant::getNullValue(GEPOperands[i]->getType()); 503 int64_t Offset = 504 getTargetData().getIndexedOffset(BasePtr->getType(), 505 &GEPOperands[0], 506 GEPOperands.size()); 507 508 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 509 return NoAlias; 510 } 511 } 512 } 513 } 514 515 return MayAlias; 516} 517 518// This function is used to determin if the indices of two GEP instructions are 519// equal. V1 and V2 are the indices. 520static bool IndexOperandsEqual(Value *V1, Value *V2) { 521 if (V1->getType() == V2->getType()) 522 return V1 == V2; 523 if (Constant *C1 = dyn_cast<Constant>(V1)) 524 if (Constant *C2 = dyn_cast<Constant>(V2)) { 525 // Sign extend the constants to long types, if necessary 526 if (C1->getType() != Type::Int64Ty) 527 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 528 if (C2->getType() != Type::Int64Ty) 529 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 530 return C1 == C2; 531 } 532 return false; 533} 534 535/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 536/// base pointers. This checks to see if the index expressions preclude the 537/// pointers from aliasing... 538AliasAnalysis::AliasResult 539BasicAliasAnalysis::CheckGEPInstructions( 540 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 541 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 542 // We currently can't handle the case when the base pointers have different 543 // primitive types. Since this is uncommon anyway, we are happy being 544 // extremely conservative. 545 if (BasePtr1Ty != BasePtr2Ty) 546 return MayAlias; 547 548 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 549 550 // Find the (possibly empty) initial sequence of equal values... which are not 551 // necessarily constants. 552 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 553 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 554 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 555 unsigned UnequalOper = 0; 556 while (UnequalOper != MinOperands && 557 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 558 // Advance through the type as we go... 559 ++UnequalOper; 560 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 561 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 562 else { 563 // If all operands equal each other, then the derived pointers must 564 // alias each other... 565 BasePtr1Ty = 0; 566 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 567 "Ran out of type nesting, but not out of operands?"); 568 return MustAlias; 569 } 570 } 571 572 // If we have seen all constant operands, and run out of indexes on one of the 573 // getelementptrs, check to see if the tail of the leftover one is all zeros. 574 // If so, return mustalias. 575 if (UnequalOper == MinOperands) { 576 if (NumGEP1Ops < NumGEP2Ops) { 577 std::swap(GEP1Ops, GEP2Ops); 578 std::swap(NumGEP1Ops, NumGEP2Ops); 579 } 580 581 bool AllAreZeros = true; 582 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 583 if (!isa<Constant>(GEP1Ops[i]) || 584 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 585 AllAreZeros = false; 586 break; 587 } 588 if (AllAreZeros) return MustAlias; 589 } 590 591 592 // So now we know that the indexes derived from the base pointers, 593 // which are known to alias, are different. We can still determine a 594 // no-alias result if there are differing constant pairs in the index 595 // chain. For example: 596 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 597 // 598 // We have to be careful here about array accesses. In particular, consider: 599 // A[1][0] vs A[0][i] 600 // In this case, we don't *know* that the array will be accessed in bounds: 601 // the index could even be negative. Because of this, we have to 602 // conservatively *give up* and return may alias. We disregard differing 603 // array subscripts that are followed by a variable index without going 604 // through a struct. 605 // 606 unsigned SizeMax = std::max(G1S, G2S); 607 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 608 609 // Scan for the first operand that is constant and unequal in the 610 // two getelementptrs... 611 unsigned FirstConstantOper = UnequalOper; 612 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 613 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 614 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 615 616 if (G1Oper != G2Oper) // Found non-equal constant indexes... 617 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 618 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 619 if (G1OC->getType() != G2OC->getType()) { 620 // Sign extend both operands to long. 621 if (G1OC->getType() != Type::Int64Ty) 622 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 623 if (G2OC->getType() != Type::Int64Ty) 624 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 625 GEP1Ops[FirstConstantOper] = G1OC; 626 GEP2Ops[FirstConstantOper] = G2OC; 627 } 628 629 if (G1OC != G2OC) { 630 // Handle the "be careful" case above: if this is an array/vector 631 // subscript, scan for a subsequent variable array index. 632 if (isa<SequentialType>(BasePtr1Ty)) { 633 const Type *NextTy = 634 cast<SequentialType>(BasePtr1Ty)->getElementType(); 635 bool isBadCase = false; 636 637 for (unsigned Idx = FirstConstantOper+1; 638 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 639 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 640 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 641 isBadCase = true; 642 break; 643 } 644 NextTy = cast<SequentialType>(NextTy)->getElementType(); 645 } 646 647 if (isBadCase) G1OC = 0; 648 } 649 650 // Make sure they are comparable (ie, not constant expressions), and 651 // make sure the GEP with the smaller leading constant is GEP1. 652 if (G1OC) { 653 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 654 G1OC, G2OC); 655 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 656 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 657 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 658 std::swap(NumGEP1Ops, NumGEP2Ops); 659 } 660 break; 661 } 662 } 663 } 664 } 665 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 666 } 667 668 // No shared constant operands, and we ran out of common operands. At this 669 // point, the GEP instructions have run through all of their operands, and we 670 // haven't found evidence that there are any deltas between the GEP's. 671 // However, one GEP may have more operands than the other. If this is the 672 // case, there may still be hope. Check this now. 673 if (FirstConstantOper == MinOperands) { 674 // Make GEP1Ops be the longer one if there is a longer one. 675 if (NumGEP1Ops < NumGEP2Ops) { 676 std::swap(GEP1Ops, GEP2Ops); 677 std::swap(NumGEP1Ops, NumGEP2Ops); 678 } 679 680 // Is there anything to check? 681 if (NumGEP1Ops > MinOperands) { 682 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 683 if (isa<ConstantInt>(GEP1Ops[i]) && 684 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 685 // Yup, there's a constant in the tail. Set all variables to 686 // constants in the GEP instruction to make it suiteable for 687 // TargetData::getIndexedOffset. 688 for (i = 0; i != MaxOperands; ++i) 689 if (!isa<ConstantInt>(GEP1Ops[i])) 690 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 691 // Okay, now get the offset. This is the relative offset for the full 692 // instruction. 693 const TargetData &TD = getTargetData(); 694 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 695 NumGEP1Ops); 696 697 // Now check without any constants at the end. 698 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 699 MinOperands); 700 701 // If the tail provided a bit enough offset, return noalias! 702 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 703 return NoAlias; 704 } 705 } 706 707 // Couldn't find anything useful. 708 return MayAlias; 709 } 710 711 // If there are non-equal constants arguments, then we can figure 712 // out a minimum known delta between the two index expressions... at 713 // this point we know that the first constant index of GEP1 is less 714 // than the first constant index of GEP2. 715 716 // Advance BasePtr[12]Ty over this first differing constant operand. 717 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 718 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 719 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 720 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 721 722 // We are going to be using TargetData::getIndexedOffset to determine the 723 // offset that each of the GEP's is reaching. To do this, we have to convert 724 // all variable references to constant references. To do this, we convert the 725 // initial sequence of array subscripts into constant zeros to start with. 726 const Type *ZeroIdxTy = GEPPointerTy; 727 for (unsigned i = 0; i != FirstConstantOper; ++i) { 728 if (!isa<StructType>(ZeroIdxTy)) 729 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 730 731 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 732 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 733 } 734 735 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 736 737 // Loop over the rest of the operands... 738 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 739 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 740 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 741 // If they are equal, use a zero index... 742 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 743 if (!isa<ConstantInt>(Op1)) 744 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 745 // Otherwise, just keep the constants we have. 746 } else { 747 if (Op1) { 748 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 749 // If this is an array index, make sure the array element is in range. 750 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 751 if (Op1C->getZExtValue() >= AT->getNumElements()) 752 return MayAlias; // Be conservative with out-of-range accesses 753 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 754 if (Op1C->getZExtValue() >= VT->getNumElements()) 755 return MayAlias; // Be conservative with out-of-range accesses 756 } 757 758 } else { 759 // GEP1 is known to produce a value less than GEP2. To be 760 // conservatively correct, we must assume the largest possible 761 // constant is used in this position. This cannot be the initial 762 // index to the GEP instructions (because we know we have at least one 763 // element before this one with the different constant arguments), so 764 // we know that the current index must be into either a struct or 765 // array. Because we know it's not constant, this cannot be a 766 // structure index. Because of this, we can calculate the maximum 767 // value possible. 768 // 769 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 770 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 771 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 772 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 773 } 774 } 775 776 if (Op2) { 777 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 778 // If this is an array index, make sure the array element is in range. 779 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 780 if (Op2C->getZExtValue() >= AT->getNumElements()) 781 return MayAlias; // Be conservative with out-of-range accesses 782 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 783 if (Op2C->getZExtValue() >= VT->getNumElements()) 784 return MayAlias; // Be conservative with out-of-range accesses 785 } 786 } else { // Conservatively assume the minimum value for this index 787 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 788 } 789 } 790 } 791 792 if (BasePtr1Ty && Op1) { 793 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 794 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 795 else 796 BasePtr1Ty = 0; 797 } 798 799 if (BasePtr2Ty && Op2) { 800 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 801 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 802 else 803 BasePtr2Ty = 0; 804 } 805 } 806 807 if (GEPPointerTy->getElementType()->isSized()) { 808 int64_t Offset1 = 809 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 810 int64_t Offset2 = 811 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 812 assert(Offset1 != Offset2 && 813 "There is at least one different constant here!"); 814 815 // Make sure we compare the absolute difference. 816 if (Offset1 > Offset2) 817 std::swap(Offset1, Offset2); 818 819 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 820 //cerr << "Determined that these two GEP's don't alias [" 821 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 822 return NoAlias; 823 } 824 } 825 return MayAlias; 826} 827 828// Make sure that anything that uses AliasAnalysis pulls in this file... 829DEFINING_FILE_FOR(BasicAliasAnalysis) 830