BasicAliasAnalysis.cpp revision 851bfff50f7251e4d00b5fffbb5e140544cc4a63
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 (isa<PointerType>((*CI)->getType()) && 266 ( getUnderlyingObject(*CI) == P || 267 alias(cast<Value>(CI), ~0ULL, P, ~0ULL) != NoAlias) ) 268 passedAsArg = true; 269 270 if (!passedAsArg) 271 return NoModRef; 272 } 273 274 // If this is a tail call and P points to a stack location, we know that 275 // the tail call cannot access or modify the local stack. 276 if (isa<AllocaInst>(Object) || 277 (isa<Argument>(Object) && cast<Argument>(Object)->hasByValAttr())) 278 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 279 if (CI->isTailCall()) 280 return NoModRef; 281 } 282 } 283 284 // The AliasAnalysis base class has some smarts, lets use them. 285 return AliasAnalysis::getModRefInfo(CS, P, Size); 286} 287 288// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 289// as array references. Note that this function is heavily tail recursive. 290// Hopefully we have a smart C++ compiler. :) 291// 292AliasAnalysis::AliasResult 293BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 294 const Value *V2, unsigned V2Size) { 295 // Strip off any constant expression casts if they exist 296 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 297 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 298 V1 = CE->getOperand(0); 299 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 300 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 301 V2 = CE->getOperand(0); 302 303 // Are we checking for alias of the same value? 304 if (V1 == V2) return MustAlias; 305 306 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) && 307 V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty) 308 return NoAlias; // Scalars cannot alias each other 309 310 // Strip off cast instructions... 311 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 312 return alias(I->getOperand(0), V1Size, V2, V2Size); 313 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 314 return alias(V1, V1Size, I->getOperand(0), V2Size); 315 316 // Figure out what objects these things are pointing to if we can... 317 const Value *O1 = getUnderlyingObject(V1); 318 const Value *O2 = getUnderlyingObject(V2); 319 320 // Pointing at a discernible object? 321 if (O1) { 322 if (O2) { 323 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 324 // Incoming argument cannot alias locally allocated object! 325 if (isa<AllocationInst>(O2)) return NoAlias; 326 327 // If they are two different objects, and one is a noalias argument 328 // then they do not alias. 329 if (O1 != O2 && O1Arg->hasNoAliasAttr()) 330 return NoAlias; 331 332 // Byval arguments can't alias globals or other arguments. 333 if (O1 != O2 && O1Arg->hasByValAttr()) return NoAlias; 334 335 // Otherwise, nothing is known... 336 } 337 338 if (const Argument *O2Arg = dyn_cast<Argument>(O2)) { 339 // Incoming argument cannot alias locally allocated object! 340 if (isa<AllocationInst>(O1)) return NoAlias; 341 342 // If they are two different objects, and one is a noalias argument 343 // then they do not alias. 344 if (O1 != O2 && O2Arg->hasNoAliasAttr()) 345 return NoAlias; 346 347 // Byval arguments can't alias globals or other arguments. 348 if (O1 != O2 && O2Arg->hasByValAttr()) return NoAlias; 349 350 // Otherwise, nothing is known... 351 352 } else if (O1 != O2 && !isa<Argument>(O1)) { 353 // If they are two different objects, and neither is an argument, 354 // we know that we have no alias. 355 return NoAlias; 356 } 357 358 // If they are the same object, they we can look at the indexes. If they 359 // index off of the object is the same for both pointers, they must alias. 360 // If they are provably different, they must not alias. Otherwise, we 361 // can't tell anything. 362 } 363 364 // Unique values don't alias null, except non-byval arguments. 365 if (isa<ConstantPointerNull>(V2)) { 366 if (const Argument *O1Arg = dyn_cast<Argument>(O1)) { 367 if (O1Arg->hasByValAttr()) 368 return NoAlias; 369 } else { 370 return NoAlias; 371 } 372 } 373 374 if (isa<GlobalVariable>(O1) || 375 (isa<AllocationInst>(O1) && 376 !cast<AllocationInst>(O1)->isArrayAllocation())) 377 if (cast<PointerType>(O1->getType())->getElementType()->isSized()) { 378 // If the size of the other access is larger than the total size of the 379 // global/alloca/malloc, it cannot be accessing the global (it's 380 // undefined to load or store bytes before or after an object). 381 const Type *ElTy = cast<PointerType>(O1->getType())->getElementType(); 382 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 383 if (GlobalSize < V2Size && V2Size != ~0U) 384 return NoAlias; 385 } 386 } 387 388 if (O2) { 389 if (!isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) 390 return NoAlias; // Unique values don't alias null 391 392 if (isa<GlobalVariable>(O2) || 393 (isa<AllocationInst>(O2) && 394 !cast<AllocationInst>(O2)->isArrayAllocation())) 395 if (cast<PointerType>(O2->getType())->getElementType()->isSized()) { 396 // If the size of the other access is larger than the total size of the 397 // global/alloca/malloc, it cannot be accessing the object (it's 398 // undefined to load or store bytes before or after an object). 399 const Type *ElTy = cast<PointerType>(O2->getType())->getElementType(); 400 unsigned GlobalSize = getTargetData().getABITypeSize(ElTy); 401 if (GlobalSize < V1Size && V1Size != ~0U) 402 return NoAlias; 403 } 404 } 405 406 // If we have two gep instructions with must-alias'ing base pointers, figure 407 // out if the indexes to the GEP tell us anything about the derived pointer. 408 // Note that we also handle chains of getelementptr instructions as well as 409 // constant expression getelementptrs here. 410 // 411 if (isGEP(V1) && isGEP(V2)) { 412 // Drill down into the first non-gep value, to test for must-aliasing of 413 // the base pointers. 414 const User *G = cast<User>(V1); 415 while (isGEP(G->getOperand(0)) && 416 G->getOperand(1) == 417 Constant::getNullValue(G->getOperand(1)->getType())) 418 G = cast<User>(G->getOperand(0)); 419 const Value *BasePtr1 = G->getOperand(0); 420 421 G = cast<User>(V2); 422 while (isGEP(G->getOperand(0)) && 423 G->getOperand(1) == 424 Constant::getNullValue(G->getOperand(1)->getType())) 425 G = cast<User>(G->getOperand(0)); 426 const Value *BasePtr2 = G->getOperand(0); 427 428 // Do the base pointers alias? 429 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 430 if (BaseAlias == NoAlias) return NoAlias; 431 if (BaseAlias == MustAlias) { 432 // If the base pointers alias each other exactly, check to see if we can 433 // figure out anything about the resultant pointers, to try to prove 434 // non-aliasing. 435 436 // Collect all of the chained GEP operands together into one simple place 437 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 438 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 439 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 440 441 // If GetGEPOperands were able to fold to the same must-aliased pointer, 442 // do the comparison. 443 if (BasePtr1 == BasePtr2) { 444 AliasResult GAlias = 445 CheckGEPInstructions(BasePtr1->getType(), 446 &GEP1Ops[0], GEP1Ops.size(), V1Size, 447 BasePtr2->getType(), 448 &GEP2Ops[0], GEP2Ops.size(), V2Size); 449 if (GAlias != MayAlias) 450 return GAlias; 451 } 452 } 453 } 454 455 // Check to see if these two pointers are related by a getelementptr 456 // instruction. If one pointer is a GEP with a non-zero index of the other 457 // pointer, we know they cannot alias. 458 // 459 if (isGEP(V2)) { 460 std::swap(V1, V2); 461 std::swap(V1Size, V2Size); 462 } 463 464 if (V1Size != ~0U && V2Size != ~0U) 465 if (isGEP(V1)) { 466 SmallVector<Value*, 16> GEPOperands; 467 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 468 469 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 470 if (R == MustAlias) { 471 // If there is at least one non-zero constant index, we know they cannot 472 // alias. 473 bool ConstantFound = false; 474 bool AllZerosFound = true; 475 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 476 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 477 if (!C->isNullValue()) { 478 ConstantFound = true; 479 AllZerosFound = false; 480 break; 481 } 482 } else { 483 AllZerosFound = false; 484 } 485 486 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 487 // the ptr, the end result is a must alias also. 488 if (AllZerosFound) 489 return MustAlias; 490 491 if (ConstantFound) { 492 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 493 return NoAlias; 494 495 // Otherwise we have to check to see that the distance is more than 496 // the size of the argument... build an index vector that is equal to 497 // the arguments provided, except substitute 0's for any variable 498 // indexes we find... 499 if (cast<PointerType>( 500 BasePtr->getType())->getElementType()->isSized()) { 501 for (unsigned i = 0; i != GEPOperands.size(); ++i) 502 if (!isa<ConstantInt>(GEPOperands[i])) 503 GEPOperands[i] = 504 Constant::getNullValue(GEPOperands[i]->getType()); 505 int64_t Offset = 506 getTargetData().getIndexedOffset(BasePtr->getType(), 507 &GEPOperands[0], 508 GEPOperands.size()); 509 510 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 511 return NoAlias; 512 } 513 } 514 } 515 } 516 517 return MayAlias; 518} 519 520// This function is used to determin if the indices of two GEP instructions are 521// equal. V1 and V2 are the indices. 522static bool IndexOperandsEqual(Value *V1, Value *V2) { 523 if (V1->getType() == V2->getType()) 524 return V1 == V2; 525 if (Constant *C1 = dyn_cast<Constant>(V1)) 526 if (Constant *C2 = dyn_cast<Constant>(V2)) { 527 // Sign extend the constants to long types, if necessary 528 if (C1->getType() != Type::Int64Ty) 529 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 530 if (C2->getType() != Type::Int64Ty) 531 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 532 return C1 == C2; 533 } 534 return false; 535} 536 537/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 538/// base pointers. This checks to see if the index expressions preclude the 539/// pointers from aliasing... 540AliasAnalysis::AliasResult 541BasicAliasAnalysis::CheckGEPInstructions( 542 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 543 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 544 // We currently can't handle the case when the base pointers have different 545 // primitive types. Since this is uncommon anyway, we are happy being 546 // extremely conservative. 547 if (BasePtr1Ty != BasePtr2Ty) 548 return MayAlias; 549 550 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 551 552 // Find the (possibly empty) initial sequence of equal values... which are not 553 // necessarily constants. 554 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 555 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 556 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 557 unsigned UnequalOper = 0; 558 while (UnequalOper != MinOperands && 559 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 560 // Advance through the type as we go... 561 ++UnequalOper; 562 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 563 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 564 else { 565 // If all operands equal each other, then the derived pointers must 566 // alias each other... 567 BasePtr1Ty = 0; 568 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 569 "Ran out of type nesting, but not out of operands?"); 570 return MustAlias; 571 } 572 } 573 574 // If we have seen all constant operands, and run out of indexes on one of the 575 // getelementptrs, check to see if the tail of the leftover one is all zeros. 576 // If so, return mustalias. 577 if (UnequalOper == MinOperands) { 578 if (NumGEP1Ops < NumGEP2Ops) { 579 std::swap(GEP1Ops, GEP2Ops); 580 std::swap(NumGEP1Ops, NumGEP2Ops); 581 } 582 583 bool AllAreZeros = true; 584 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 585 if (!isa<Constant>(GEP1Ops[i]) || 586 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 587 AllAreZeros = false; 588 break; 589 } 590 if (AllAreZeros) return MustAlias; 591 } 592 593 594 // So now we know that the indexes derived from the base pointers, 595 // which are known to alias, are different. We can still determine a 596 // no-alias result if there are differing constant pairs in the index 597 // chain. For example: 598 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 599 // 600 // We have to be careful here about array accesses. In particular, consider: 601 // A[1][0] vs A[0][i] 602 // In this case, we don't *know* that the array will be accessed in bounds: 603 // the index could even be negative. Because of this, we have to 604 // conservatively *give up* and return may alias. We disregard differing 605 // array subscripts that are followed by a variable index without going 606 // through a struct. 607 // 608 unsigned SizeMax = std::max(G1S, G2S); 609 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 610 611 // Scan for the first operand that is constant and unequal in the 612 // two getelementptrs... 613 unsigned FirstConstantOper = UnequalOper; 614 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 615 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 616 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 617 618 if (G1Oper != G2Oper) // Found non-equal constant indexes... 619 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 620 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 621 if (G1OC->getType() != G2OC->getType()) { 622 // Sign extend both operands to long. 623 if (G1OC->getType() != Type::Int64Ty) 624 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 625 if (G2OC->getType() != Type::Int64Ty) 626 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 627 GEP1Ops[FirstConstantOper] = G1OC; 628 GEP2Ops[FirstConstantOper] = G2OC; 629 } 630 631 if (G1OC != G2OC) { 632 // Handle the "be careful" case above: if this is an array/vector 633 // subscript, scan for a subsequent variable array index. 634 if (isa<SequentialType>(BasePtr1Ty)) { 635 const Type *NextTy = 636 cast<SequentialType>(BasePtr1Ty)->getElementType(); 637 bool isBadCase = false; 638 639 for (unsigned Idx = FirstConstantOper+1; 640 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 641 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 642 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 643 isBadCase = true; 644 break; 645 } 646 NextTy = cast<SequentialType>(NextTy)->getElementType(); 647 } 648 649 if (isBadCase) G1OC = 0; 650 } 651 652 // Make sure they are comparable (ie, not constant expressions), and 653 // make sure the GEP with the smaller leading constant is GEP1. 654 if (G1OC) { 655 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 656 G1OC, G2OC); 657 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 658 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 659 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 660 std::swap(NumGEP1Ops, NumGEP2Ops); 661 } 662 break; 663 } 664 } 665 } 666 } 667 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 668 } 669 670 // No shared constant operands, and we ran out of common operands. At this 671 // point, the GEP instructions have run through all of their operands, and we 672 // haven't found evidence that there are any deltas between the GEP's. 673 // However, one GEP may have more operands than the other. If this is the 674 // case, there may still be hope. Check this now. 675 if (FirstConstantOper == MinOperands) { 676 // Make GEP1Ops be the longer one if there is a longer one. 677 if (NumGEP1Ops < NumGEP2Ops) { 678 std::swap(GEP1Ops, GEP2Ops); 679 std::swap(NumGEP1Ops, NumGEP2Ops); 680 } 681 682 // Is there anything to check? 683 if (NumGEP1Ops > MinOperands) { 684 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 685 if (isa<ConstantInt>(GEP1Ops[i]) && 686 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 687 // Yup, there's a constant in the tail. Set all variables to 688 // constants in the GEP instruction to make it suiteable for 689 // TargetData::getIndexedOffset. 690 for (i = 0; i != MaxOperands; ++i) 691 if (!isa<ConstantInt>(GEP1Ops[i])) 692 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 693 // Okay, now get the offset. This is the relative offset for the full 694 // instruction. 695 const TargetData &TD = getTargetData(); 696 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 697 NumGEP1Ops); 698 699 // Now check without any constants at the end. 700 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 701 MinOperands); 702 703 // If the tail provided a bit enough offset, return noalias! 704 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 705 return NoAlias; 706 } 707 } 708 709 // Couldn't find anything useful. 710 return MayAlias; 711 } 712 713 // If there are non-equal constants arguments, then we can figure 714 // out a minimum known delta between the two index expressions... at 715 // this point we know that the first constant index of GEP1 is less 716 // than the first constant index of GEP2. 717 718 // Advance BasePtr[12]Ty over this first differing constant operand. 719 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 720 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 721 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 722 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 723 724 // We are going to be using TargetData::getIndexedOffset to determine the 725 // offset that each of the GEP's is reaching. To do this, we have to convert 726 // all variable references to constant references. To do this, we convert the 727 // initial sequence of array subscripts into constant zeros to start with. 728 const Type *ZeroIdxTy = GEPPointerTy; 729 for (unsigned i = 0; i != FirstConstantOper; ++i) { 730 if (!isa<StructType>(ZeroIdxTy)) 731 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 732 733 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 734 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 735 } 736 737 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 738 739 // Loop over the rest of the operands... 740 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 741 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 742 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 743 // If they are equal, use a zero index... 744 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 745 if (!isa<ConstantInt>(Op1)) 746 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 747 // Otherwise, just keep the constants we have. 748 } else { 749 if (Op1) { 750 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 751 // If this is an array index, make sure the array element is in range. 752 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 753 if (Op1C->getZExtValue() >= AT->getNumElements()) 754 return MayAlias; // Be conservative with out-of-range accesses 755 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 756 if (Op1C->getZExtValue() >= VT->getNumElements()) 757 return MayAlias; // Be conservative with out-of-range accesses 758 } 759 760 } else { 761 // GEP1 is known to produce a value less than GEP2. To be 762 // conservatively correct, we must assume the largest possible 763 // constant is used in this position. This cannot be the initial 764 // index to the GEP instructions (because we know we have at least one 765 // element before this one with the different constant arguments), so 766 // we know that the current index must be into either a struct or 767 // array. Because we know it's not constant, this cannot be a 768 // structure index. Because of this, we can calculate the maximum 769 // value possible. 770 // 771 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 772 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 773 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 774 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 775 } 776 } 777 778 if (Op2) { 779 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 780 // If this is an array index, make sure the array element is in range. 781 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 782 if (Op2C->getZExtValue() >= AT->getNumElements()) 783 return MayAlias; // Be conservative with out-of-range accesses 784 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 785 if (Op2C->getZExtValue() >= VT->getNumElements()) 786 return MayAlias; // Be conservative with out-of-range accesses 787 } 788 } else { // Conservatively assume the minimum value for this index 789 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 790 } 791 } 792 } 793 794 if (BasePtr1Ty && Op1) { 795 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 796 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 797 else 798 BasePtr1Ty = 0; 799 } 800 801 if (BasePtr2Ty && Op2) { 802 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 803 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 804 else 805 BasePtr2Ty = 0; 806 } 807 } 808 809 if (GEPPointerTy->getElementType()->isSized()) { 810 int64_t Offset1 = 811 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 812 int64_t Offset2 = 813 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 814 assert(Offset1 != Offset2 && 815 "There is at least one different constant here!"); 816 817 // Make sure we compare the absolute difference. 818 if (Offset1 > Offset2) 819 std::swap(Offset1, Offset2); 820 821 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 822 //cerr << "Determined that these two GEP's don't alias [" 823 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 824 return NoAlias; 825 } 826 } 827 return MayAlias; 828} 829 830// Make sure that anything that uses AliasAnalysis pulls in this file... 831DEFINING_FILE_FOR(BasicAliasAnalysis) 832