BasicAliasAnalysis.cpp revision 07cf79ef537caff6d39145f190a28a336e629b6f
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/CaptureTracking.h" 18#include "llvm/Analysis/Passes.h" 19#include "llvm/Constants.h" 20#include "llvm/DerivedTypes.h" 21#include "llvm/Function.h" 22#include "llvm/GlobalVariable.h" 23#include "llvm/Instructions.h" 24#include "llvm/IntrinsicInst.h" 25#include "llvm/LLVMContext.h" 26#include "llvm/Pass.h" 27#include "llvm/Target/TargetData.h" 28#include "llvm/ADT/SmallVector.h" 29#include "llvm/ADT/STLExtras.h" 30#include "llvm/Support/Compiler.h" 31#include "llvm/Support/GetElementPtrTypeIterator.h" 32#include <algorithm> 33using namespace llvm; 34 35//===----------------------------------------------------------------------===// 36// Useful predicates 37//===----------------------------------------------------------------------===// 38 39static const User *isGEP(const Value *V) { 40 if (isa<GetElementPtrInst>(V) || 41 (isa<ConstantExpr>(V) && 42 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 43 return cast<User>(V); 44 return 0; 45} 46 47static const Value *GetGEPOperands(const Value *V, 48 SmallVector<Value*, 16> &GEPOps) { 49 assert(GEPOps.empty() && "Expect empty list to populate!"); 50 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 51 cast<User>(V)->op_end()); 52 53 // Accumulate all of the chained indexes into the operand array 54 V = cast<User>(V)->getOperand(0); 55 56 while (const User *G = isGEP(V)) { 57 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 58 !cast<Constant>(GEPOps[0])->isNullValue()) 59 break; // Don't handle folding arbitrary pointer offsets yet... 60 GEPOps.erase(GEPOps.begin()); // Drop the zero index 61 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 62 V = G->getOperand(0); 63 } 64 return V; 65} 66 67/// isKnownNonNull - Return true if we know that the specified value is never 68/// null. 69static bool isKnownNonNull(const Value *V) { 70 // Alloca never returns null, malloc might. 71 if (isa<AllocaInst>(V)) return true; 72 73 // A byval argument is never null. 74 if (const Argument *A = dyn_cast<Argument>(V)) 75 return A->hasByValAttr(); 76 77 // Global values are not null unless extern weak. 78 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 79 return !GV->hasExternalWeakLinkage(); 80 return false; 81} 82 83/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 84/// object that never escapes from the function. 85static bool isNonEscapingLocalObject(const Value *V) { 86 // If this is a local allocation, check to see if it escapes. 87 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 88 return !PointerMayBeCaptured(V, false); 89 90 // If this is an argument that corresponds to a byval or noalias argument, 91 // then it has not escaped before entering the function. Check if it escapes 92 // inside the function. 93 if (const Argument *A = dyn_cast<Argument>(V)) 94 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 95 // Don't bother analyzing arguments already known not to escape. 96 if (A->hasNoCaptureAttr()) 97 return true; 98 return !PointerMayBeCaptured(V, false); 99 } 100 return false; 101} 102 103 104/// isObjectSmallerThan - Return true if we can prove that the object specified 105/// by V is smaller than Size. 106static bool isObjectSmallerThan(const Value *V, unsigned Size, 107 const TargetData &TD) { 108 const Type *AccessTy; 109 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 110 AccessTy = GV->getType()->getElementType(); 111 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 112 if (!AI->isArrayAllocation()) 113 AccessTy = AI->getType()->getElementType(); 114 else 115 return false; 116 } else if (const Argument *A = dyn_cast<Argument>(V)) { 117 if (A->hasByValAttr()) 118 AccessTy = cast<PointerType>(A->getType())->getElementType(); 119 else 120 return false; 121 } else { 122 return false; 123 } 124 125 if (AccessTy->isSized()) 126 return TD.getTypeAllocSize(AccessTy) < Size; 127 return false; 128} 129 130//===----------------------------------------------------------------------===// 131// NoAA Pass 132//===----------------------------------------------------------------------===// 133 134namespace { 135 /// NoAA - This class implements the -no-aa pass, which always returns "I 136 /// don't know" for alias queries. NoAA is unlike other alias analysis 137 /// implementations, in that it does not chain to a previous analysis. As 138 /// such it doesn't follow many of the rules that other alias analyses must. 139 /// 140 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 141 static char ID; // Class identification, replacement for typeinfo 142 NoAA() : ImmutablePass(&ID) {} 143 explicit NoAA(void *PID) : ImmutablePass(PID) { } 144 145 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 146 AU.addRequired<TargetData>(); 147 } 148 149 virtual void initializePass() { 150 TD = &getAnalysis<TargetData>(); 151 } 152 153 virtual AliasResult alias(const Value *V1, unsigned V1Size, 154 const Value *V2, unsigned V2Size) { 155 return MayAlias; 156 } 157 158 virtual void getArgumentAccesses(Function *F, CallSite CS, 159 std::vector<PointerAccessInfo> &Info) { 160 assert(0 && "This method may not be called on this function!"); 161 } 162 163 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 164 virtual bool pointsToConstantMemory(const Value *P) { return false; } 165 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 166 return ModRef; 167 } 168 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 169 return ModRef; 170 } 171 virtual bool hasNoModRefInfoForCalls() const { return true; } 172 173 virtual void deleteValue(Value *V) {} 174 virtual void copyValue(Value *From, Value *To) {} 175 }; 176} // End of anonymous namespace 177 178// Register this pass... 179char NoAA::ID = 0; 180static RegisterPass<NoAA> 181U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 182 183// Declare that we implement the AliasAnalysis interface 184static RegisterAnalysisGroup<AliasAnalysis> V(U); 185 186ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 187 188//===----------------------------------------------------------------------===// 189// BasicAA Pass 190//===----------------------------------------------------------------------===// 191 192namespace { 193 /// BasicAliasAnalysis - This is the default alias analysis implementation. 194 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 195 /// derives from the NoAA class. 196 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 197 static char ID; // Class identification, replacement for typeinfo 198 BasicAliasAnalysis() : NoAA(&ID) {} 199 AliasResult alias(const Value *V1, unsigned V1Size, 200 const Value *V2, unsigned V2Size); 201 202 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 203 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 204 205 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 206 /// non-escaping allocations. 207 virtual bool hasNoModRefInfoForCalls() const { return false; } 208 209 /// pointsToConstantMemory - Chase pointers until we find a (constant 210 /// global) or not. 211 bool pointsToConstantMemory(const Value *P); 212 213 private: 214 // CheckGEPInstructions - Check two GEP instructions with known 215 // must-aliasing base pointers. This checks to see if the index expressions 216 // preclude the pointers from aliasing... 217 AliasResult 218 CheckGEPInstructions(const Type* BasePtr1Ty, 219 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 220 const Type *BasePtr2Ty, 221 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 222 }; 223} // End of anonymous namespace 224 225// Register this pass... 226char BasicAliasAnalysis::ID = 0; 227static RegisterPass<BasicAliasAnalysis> 228X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 229 230// Declare that we implement the AliasAnalysis interface 231static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 232 233ImmutablePass *llvm::createBasicAliasAnalysisPass() { 234 return new BasicAliasAnalysis(); 235} 236 237 238/// pointsToConstantMemory - Chase pointers until we find a (constant 239/// global) or not. 240bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 241 if (const GlobalVariable *GV = 242 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 243 return GV->isConstant(); 244 return false; 245} 246 247 248// getModRefInfo - Check to see if the specified callsite can clobber the 249// specified memory object. Since we only look at local properties of this 250// function, we really can't say much about this query. We do, however, use 251// simple "address taken" analysis on local objects. 252// 253AliasAnalysis::ModRefResult 254BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 255 if (!isa<Constant>(P)) { 256 const Value *Object = P->getUnderlyingObject(); 257 258 // If this is a tail call and P points to a stack location, we know that 259 // the tail call cannot access or modify the local stack. 260 // We cannot exclude byval arguments here; these belong to the caller of 261 // the current function not to the current function, and a tail callee 262 // may reference them. 263 if (isa<AllocaInst>(Object)) 264 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 265 if (CI->isTailCall()) 266 return NoModRef; 267 268 // If the pointer is to a locally allocated object that does not escape, 269 // then the call can not mod/ref the pointer unless the call takes the 270 // argument without capturing it. 271 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) { 272 bool passedAsArg = false; 273 // TODO: Eventually only check 'nocapture' arguments. 274 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 275 CI != CE; ++CI) 276 if (isa<PointerType>((*CI)->getType()) && 277 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 278 passedAsArg = true; 279 280 if (!passedAsArg) 281 return NoModRef; 282 } 283 } 284 285 // The AliasAnalysis base class has some smarts, lets use them. 286 return AliasAnalysis::getModRefInfo(CS, P, Size); 287} 288 289 290AliasAnalysis::ModRefResult 291BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 292 // If CS1 or CS2 are readnone, they don't interact. 293 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 294 if (CS1B == DoesNotAccessMemory) return NoModRef; 295 296 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 297 if (CS2B == DoesNotAccessMemory) return NoModRef; 298 299 // If they both only read from memory, just return ref. 300 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 301 return Ref; 302 303 // Otherwise, fall back to NoAA (mod+ref). 304 return NoAA::getModRefInfo(CS1, CS2); 305} 306 307 308// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 309// as array references. 310// 311AliasAnalysis::AliasResult 312BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 313 const Value *V2, unsigned V2Size) { 314 // Strip off any constant expression casts if they exist 315 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 316 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 317 V1 = CE->getOperand(0); 318 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 319 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 320 V2 = CE->getOperand(0); 321 322 // Are we checking for alias of the same value? 323 if (V1 == V2) return MustAlias; 324 325 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 326 return NoAlias; // Scalars cannot alias each other 327 328 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 329 // pointer<->pointer bitcasts. 330 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 331 return alias(I->getOperand(0), V1Size, V2, V2Size); 332 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 333 return alias(V1, V1Size, I->getOperand(0), V2Size); 334 335 // Figure out what objects these things are pointing to if we can. 336 const Value *O1 = V1->getUnderlyingObject(); 337 const Value *O2 = V2->getUnderlyingObject(); 338 339 if (O1 != O2) { 340 // If V1/V2 point to two different objects we know that we have no alias. 341 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 342 return NoAlias; 343 344 // Arguments can't alias with local allocations or noalias calls. 345 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 346 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 347 return NoAlias; 348 349 // Most objects can't alias null. 350 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 351 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 352 return NoAlias; 353 } 354 355 // If the size of one access is larger than the entire object on the other 356 // side, then we know such behavior is undefined and can assume no alias. 357 const TargetData &TD = getTargetData(); 358 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 359 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 360 return NoAlias; 361 362 // If one pointer is the result of a call/invoke and the other is a 363 // non-escaping local object, then we know the object couldn't escape to a 364 // point where the call could return it. 365 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 366 isNonEscapingLocalObject(O2) && O1 != O2) 367 return NoAlias; 368 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 369 isNonEscapingLocalObject(O1) && O1 != O2) 370 return NoAlias; 371 372 // If we have two gep instructions with must-alias'ing base pointers, figure 373 // out if the indexes to the GEP tell us anything about the derived pointer. 374 // Note that we also handle chains of getelementptr instructions as well as 375 // constant expression getelementptrs here. 376 // 377 if (isGEP(V1) && isGEP(V2)) { 378 const User *GEP1 = cast<User>(V1); 379 const User *GEP2 = cast<User>(V2); 380 381 // If V1 and V2 are identical GEPs, just recurse down on both of them. 382 // This allows us to analyze things like: 383 // P = gep A, 0, i, 1 384 // Q = gep B, 0, i, 1 385 // by just analyzing A and B. This is even safe for variable indices. 386 if (GEP1->getType() == GEP2->getType() && 387 GEP1->getNumOperands() == GEP2->getNumOperands() && 388 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 389 // All operands are the same, ignoring the base. 390 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 391 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 392 393 394 // Drill down into the first non-gep value, to test for must-aliasing of 395 // the base pointers. 396 while (isGEP(GEP1->getOperand(0)) && 397 GEP1->getOperand(1) == 398 Context->getNullValue(GEP1->getOperand(1)->getType())) 399 GEP1 = cast<User>(GEP1->getOperand(0)); 400 const Value *BasePtr1 = GEP1->getOperand(0); 401 402 while (isGEP(GEP2->getOperand(0)) && 403 GEP2->getOperand(1) == 404 Context->getNullValue(GEP2->getOperand(1)->getType())) 405 GEP2 = cast<User>(GEP2->getOperand(0)); 406 const Value *BasePtr2 = GEP2->getOperand(0); 407 408 // Do the base pointers alias? 409 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 410 if (BaseAlias == NoAlias) return NoAlias; 411 if (BaseAlias == MustAlias) { 412 // If the base pointers alias each other exactly, check to see if we can 413 // figure out anything about the resultant pointers, to try to prove 414 // non-aliasing. 415 416 // Collect all of the chained GEP operands together into one simple place 417 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 418 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 419 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 420 421 // If GetGEPOperands were able to fold to the same must-aliased pointer, 422 // do the comparison. 423 if (BasePtr1 == BasePtr2) { 424 AliasResult GAlias = 425 CheckGEPInstructions(BasePtr1->getType(), 426 &GEP1Ops[0], GEP1Ops.size(), V1Size, 427 BasePtr2->getType(), 428 &GEP2Ops[0], GEP2Ops.size(), V2Size); 429 if (GAlias != MayAlias) 430 return GAlias; 431 } 432 } 433 } 434 435 // Check to see if these two pointers are related by a getelementptr 436 // instruction. If one pointer is a GEP with a non-zero index of the other 437 // pointer, we know they cannot alias. 438 // 439 if (isGEP(V2)) { 440 std::swap(V1, V2); 441 std::swap(V1Size, V2Size); 442 } 443 444 if (V1Size != ~0U && V2Size != ~0U) 445 if (isGEP(V1)) { 446 SmallVector<Value*, 16> GEPOperands; 447 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 448 449 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 450 if (R == MustAlias) { 451 // If there is at least one non-zero constant index, we know they cannot 452 // alias. 453 bool ConstantFound = false; 454 bool AllZerosFound = true; 455 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 456 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 457 if (!C->isNullValue()) { 458 ConstantFound = true; 459 AllZerosFound = false; 460 break; 461 } 462 } else { 463 AllZerosFound = false; 464 } 465 466 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 467 // the ptr, the end result is a must alias also. 468 if (AllZerosFound) 469 return MustAlias; 470 471 if (ConstantFound) { 472 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 473 return NoAlias; 474 475 // Otherwise we have to check to see that the distance is more than 476 // the size of the argument... build an index vector that is equal to 477 // the arguments provided, except substitute 0's for any variable 478 // indexes we find... 479 if (cast<PointerType>( 480 BasePtr->getType())->getElementType()->isSized()) { 481 for (unsigned i = 0; i != GEPOperands.size(); ++i) 482 if (!isa<ConstantInt>(GEPOperands[i])) 483 GEPOperands[i] = 484 Context->getNullValue(GEPOperands[i]->getType()); 485 int64_t Offset = 486 getTargetData().getIndexedOffset(BasePtr->getType(), 487 &GEPOperands[0], 488 GEPOperands.size()); 489 490 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 491 return NoAlias; 492 } 493 } 494 } 495 } 496 497 return MayAlias; 498} 499 500// This function is used to determine if the indices of two GEP instructions are 501// equal. V1 and V2 are the indices. 502static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext *Context) { 503 if (V1->getType() == V2->getType()) 504 return V1 == V2; 505 if (Constant *C1 = dyn_cast<Constant>(V1)) 506 if (Constant *C2 = dyn_cast<Constant>(V2)) { 507 // Sign extend the constants to long types, if necessary 508 if (C1->getType() != Type::Int64Ty) 509 C1 = Context->getConstantExprSExt(C1, Type::Int64Ty); 510 if (C2->getType() != Type::Int64Ty) 511 C2 = Context->getConstantExprSExt(C2, Type::Int64Ty); 512 return C1 == C2; 513 } 514 return false; 515} 516 517/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 518/// base pointers. This checks to see if the index expressions preclude the 519/// pointers from aliasing... 520AliasAnalysis::AliasResult 521BasicAliasAnalysis::CheckGEPInstructions( 522 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 523 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 524 // We currently can't handle the case when the base pointers have different 525 // primitive types. Since this is uncommon anyway, we are happy being 526 // extremely conservative. 527 if (BasePtr1Ty != BasePtr2Ty) 528 return MayAlias; 529 530 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 531 532 // Find the (possibly empty) initial sequence of equal values... which are not 533 // necessarily constants. 534 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 535 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 536 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 537 unsigned UnequalOper = 0; 538 while (UnequalOper != MinOperands && 539 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper], 540 Context)) { 541 // Advance through the type as we go... 542 ++UnequalOper; 543 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 544 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 545 else { 546 // If all operands equal each other, then the derived pointers must 547 // alias each other... 548 BasePtr1Ty = 0; 549 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 550 "Ran out of type nesting, but not out of operands?"); 551 return MustAlias; 552 } 553 } 554 555 // If we have seen all constant operands, and run out of indexes on one of the 556 // getelementptrs, check to see if the tail of the leftover one is all zeros. 557 // If so, return mustalias. 558 if (UnequalOper == MinOperands) { 559 if (NumGEP1Ops < NumGEP2Ops) { 560 std::swap(GEP1Ops, GEP2Ops); 561 std::swap(NumGEP1Ops, NumGEP2Ops); 562 } 563 564 bool AllAreZeros = true; 565 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 566 if (!isa<Constant>(GEP1Ops[i]) || 567 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 568 AllAreZeros = false; 569 break; 570 } 571 if (AllAreZeros) return MustAlias; 572 } 573 574 575 // So now we know that the indexes derived from the base pointers, 576 // which are known to alias, are different. We can still determine a 577 // no-alias result if there are differing constant pairs in the index 578 // chain. For example: 579 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 580 // 581 // We have to be careful here about array accesses. In particular, consider: 582 // A[1][0] vs A[0][i] 583 // In this case, we don't *know* that the array will be accessed in bounds: 584 // the index could even be negative. Because of this, we have to 585 // conservatively *give up* and return may alias. We disregard differing 586 // array subscripts that are followed by a variable index without going 587 // through a struct. 588 // 589 unsigned SizeMax = std::max(G1S, G2S); 590 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 591 592 // Scan for the first operand that is constant and unequal in the 593 // two getelementptrs... 594 unsigned FirstConstantOper = UnequalOper; 595 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 596 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 597 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 598 599 if (G1Oper != G2Oper) // Found non-equal constant indexes... 600 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 601 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 602 if (G1OC->getType() != G2OC->getType()) { 603 // Sign extend both operands to long. 604 if (G1OC->getType() != Type::Int64Ty) 605 G1OC = Context->getConstantExprSExt(G1OC, Type::Int64Ty); 606 if (G2OC->getType() != Type::Int64Ty) 607 G2OC = Context->getConstantExprSExt(G2OC, Type::Int64Ty); 608 GEP1Ops[FirstConstantOper] = G1OC; 609 GEP2Ops[FirstConstantOper] = G2OC; 610 } 611 612 if (G1OC != G2OC) { 613 // Handle the "be careful" case above: if this is an array/vector 614 // subscript, scan for a subsequent variable array index. 615 if (const SequentialType *STy = 616 dyn_cast<SequentialType>(BasePtr1Ty)) { 617 const Type *NextTy = STy; 618 bool isBadCase = false; 619 620 for (unsigned Idx = FirstConstantOper; 621 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 622 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 623 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 624 isBadCase = true; 625 break; 626 } 627 // If the array is indexed beyond the bounds of the static type 628 // at this level, it will also fall into the "be careful" case. 629 // It would theoretically be possible to analyze these cases, 630 // but for now just be conservatively correct. 631 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy)) 632 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 633 ATy->getNumElements() || 634 cast<ConstantInt>(G2OC)->getZExtValue() >= 635 ATy->getNumElements()) { 636 isBadCase = true; 637 break; 638 } 639 if (const VectorType *VTy = dyn_cast<VectorType>(STy)) 640 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 641 VTy->getNumElements() || 642 cast<ConstantInt>(G2OC)->getZExtValue() >= 643 VTy->getNumElements()) { 644 isBadCase = true; 645 break; 646 } 647 STy = cast<SequentialType>(NextTy); 648 NextTy = cast<SequentialType>(NextTy)->getElementType(); 649 } 650 651 if (isBadCase) G1OC = 0; 652 } 653 654 // Make sure they are comparable (ie, not constant expressions), and 655 // make sure the GEP with the smaller leading constant is GEP1. 656 if (G1OC) { 657 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 658 G1OC, G2OC); 659 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 660 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 661 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 662 std::swap(NumGEP1Ops, NumGEP2Ops); 663 } 664 break; 665 } 666 } 667 } 668 } 669 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 670 } 671 672 // No shared constant operands, and we ran out of common operands. At this 673 // point, the GEP instructions have run through all of their operands, and we 674 // haven't found evidence that there are any deltas between the GEP's. 675 // However, one GEP may have more operands than the other. If this is the 676 // case, there may still be hope. Check this now. 677 if (FirstConstantOper == MinOperands) { 678 // Make GEP1Ops be the longer one if there is a longer one. 679 if (NumGEP1Ops < NumGEP2Ops) { 680 std::swap(GEP1Ops, GEP2Ops); 681 std::swap(NumGEP1Ops, NumGEP2Ops); 682 } 683 684 // Is there anything to check? 685 if (NumGEP1Ops > MinOperands) { 686 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 687 if (isa<ConstantInt>(GEP1Ops[i]) && 688 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 689 // Yup, there's a constant in the tail. Set all variables to 690 // constants in the GEP instruction to make it suitable for 691 // TargetData::getIndexedOffset. 692 for (i = 0; i != MaxOperands; ++i) 693 if (!isa<ConstantInt>(GEP1Ops[i])) 694 GEP1Ops[i] = Context->getNullValue(GEP1Ops[i]->getType()); 695 // Okay, now get the offset. This is the relative offset for the full 696 // instruction. 697 const TargetData &TD = getTargetData(); 698 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 699 NumGEP1Ops); 700 701 // Now check without any constants at the end. 702 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 703 MinOperands); 704 705 // Make sure we compare the absolute difference. 706 if (Offset1 > Offset2) 707 std::swap(Offset1, Offset2); 708 709 // If the tail provided a bit enough offset, return noalias! 710 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 711 return NoAlias; 712 // Otherwise break - we don't look for another constant in the tail. 713 break; 714 } 715 } 716 717 // Couldn't find anything useful. 718 return MayAlias; 719 } 720 721 // If there are non-equal constants arguments, then we can figure 722 // out a minimum known delta between the two index expressions... at 723 // this point we know that the first constant index of GEP1 is less 724 // than the first constant index of GEP2. 725 726 // Advance BasePtr[12]Ty over this first differing constant operand. 727 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 728 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 729 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 730 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 731 732 // We are going to be using TargetData::getIndexedOffset to determine the 733 // offset that each of the GEP's is reaching. To do this, we have to convert 734 // all variable references to constant references. To do this, we convert the 735 // initial sequence of array subscripts into constant zeros to start with. 736 const Type *ZeroIdxTy = GEPPointerTy; 737 for (unsigned i = 0; i != FirstConstantOper; ++i) { 738 if (!isa<StructType>(ZeroIdxTy)) 739 GEP1Ops[i] = GEP2Ops[i] = Context->getNullValue(Type::Int32Ty); 740 741 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 742 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 743 } 744 745 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 746 747 // Loop over the rest of the operands... 748 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 749 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 750 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 751 // If they are equal, use a zero index... 752 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 753 if (!isa<ConstantInt>(Op1)) 754 GEP1Ops[i] = GEP2Ops[i] = Context->getNullValue(Op1->getType()); 755 // Otherwise, just keep the constants we have. 756 } else { 757 if (Op1) { 758 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 759 // If this is an array index, make sure the array element is in range. 760 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 761 if (Op1C->getZExtValue() >= AT->getNumElements()) 762 return MayAlias; // Be conservative with out-of-range accesses 763 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 764 if (Op1C->getZExtValue() >= VT->getNumElements()) 765 return MayAlias; // Be conservative with out-of-range accesses 766 } 767 768 } else { 769 // GEP1 is known to produce a value less than GEP2. To be 770 // conservatively correct, we must assume the largest possible 771 // constant is used in this position. This cannot be the initial 772 // index to the GEP instructions (because we know we have at least one 773 // element before this one with the different constant arguments), so 774 // we know that the current index must be into either a struct or 775 // array. Because we know it's not constant, this cannot be a 776 // structure index. Because of this, we can calculate the maximum 777 // value possible. 778 // 779 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 780 GEP1Ops[i] = 781 Context->getConstantInt(Type::Int64Ty,AT->getNumElements()-1); 782 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 783 GEP1Ops[i] = 784 Context->getConstantInt(Type::Int64Ty,VT->getNumElements()-1); 785 } 786 } 787 788 if (Op2) { 789 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 790 // If this is an array index, make sure the array element is in range. 791 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 792 if (Op2C->getZExtValue() >= AT->getNumElements()) 793 return MayAlias; // Be conservative with out-of-range accesses 794 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 795 if (Op2C->getZExtValue() >= VT->getNumElements()) 796 return MayAlias; // Be conservative with out-of-range accesses 797 } 798 } else { // Conservatively assume the minimum value for this index 799 GEP2Ops[i] = Context->getNullValue(Op2->getType()); 800 } 801 } 802 } 803 804 if (BasePtr1Ty && Op1) { 805 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 806 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 807 else 808 BasePtr1Ty = 0; 809 } 810 811 if (BasePtr2Ty && Op2) { 812 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 813 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 814 else 815 BasePtr2Ty = 0; 816 } 817 } 818 819 if (GEPPointerTy->getElementType()->isSized()) { 820 int64_t Offset1 = 821 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 822 int64_t Offset2 = 823 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 824 assert(Offset1 != Offset2 && 825 "There is at least one different constant here!"); 826 827 // Make sure we compare the absolute difference. 828 if (Offset1 > Offset2) 829 std::swap(Offset1, Offset2); 830 831 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 832 //cerr << "Determined that these two GEP's don't alias [" 833 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 834 return NoAlias; 835 } 836 } 837 return MayAlias; 838} 839 840// Make sure that anything that uses AliasAnalysis pulls in this file... 841DEFINING_FILE_FOR(BasicAliasAnalysis) 842