BasicAliasAnalysis.cpp revision 001dbfebcbbded8c8e74b19e838b50da2b6c6fb5
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/ErrorHandling.h" 32#include "llvm/Support/GetElementPtrTypeIterator.h" 33#include <algorithm> 34using namespace llvm; 35 36//===----------------------------------------------------------------------===// 37// Useful predicates 38//===----------------------------------------------------------------------===// 39 40static const User *isGEP(const Value *V) { 41 if (isa<GetElementPtrInst>(V) || 42 (isa<ConstantExpr>(V) && 43 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 44 return cast<User>(V); 45 return 0; 46} 47 48static const Value *GetGEPOperands(const Value *V, 49 SmallVector<Value*, 16> &GEPOps) { 50 assert(GEPOps.empty() && "Expect empty list to populate!"); 51 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 52 cast<User>(V)->op_end()); 53 54 // Accumulate all of the chained indexes into the operand array 55 V = cast<User>(V)->getOperand(0); 56 57 while (const User *G = isGEP(V)) { 58 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 59 !cast<Constant>(GEPOps[0])->isNullValue()) 60 break; // Don't handle folding arbitrary pointer offsets yet... 61 GEPOps.erase(GEPOps.begin()); // Drop the zero index 62 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 63 V = G->getOperand(0); 64 } 65 return V; 66} 67 68/// isKnownNonNull - Return true if we know that the specified value is never 69/// null. 70static bool isKnownNonNull(const Value *V) { 71 // Alloca never returns null, malloc might. 72 if (isa<AllocaInst>(V)) return true; 73 74 // A byval argument is never null. 75 if (const Argument *A = dyn_cast<Argument>(V)) 76 return A->hasByValAttr(); 77 78 // Global values are not null unless extern weak. 79 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 80 return !GV->hasExternalWeakLinkage(); 81 return false; 82} 83 84/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 85/// object that never escapes from the function. 86static bool isNonEscapingLocalObject(const Value *V) { 87 // If this is a local allocation, check to see if it escapes. 88 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 89 return !PointerMayBeCaptured(V, false); 90 91 // If this is an argument that corresponds to a byval or noalias argument, 92 // then it has not escaped before entering the function. Check if it escapes 93 // inside the function. 94 if (const Argument *A = dyn_cast<Argument>(V)) 95 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 96 // Don't bother analyzing arguments already known not to escape. 97 if (A->hasNoCaptureAttr()) 98 return true; 99 return !PointerMayBeCaptured(V, false); 100 } 101 return false; 102} 103 104 105/// isObjectSmallerThan - Return true if we can prove that the object specified 106/// by V is smaller than Size. 107static bool isObjectSmallerThan(const Value *V, unsigned Size, 108 const TargetData &TD) { 109 const Type *AccessTy; 110 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 111 AccessTy = GV->getType()->getElementType(); 112 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 113 if (!AI->isArrayAllocation()) 114 AccessTy = AI->getType()->getElementType(); 115 else 116 return false; 117 } else if (const Argument *A = dyn_cast<Argument>(V)) { 118 if (A->hasByValAttr()) 119 AccessTy = cast<PointerType>(A->getType())->getElementType(); 120 else 121 return false; 122 } else { 123 return false; 124 } 125 126 if (AccessTy->isSized()) 127 return TD.getTypeAllocSize(AccessTy) < Size; 128 return false; 129} 130 131//===----------------------------------------------------------------------===// 132// NoAA Pass 133//===----------------------------------------------------------------------===// 134 135namespace { 136 /// NoAA - This class implements the -no-aa pass, which always returns "I 137 /// don't know" for alias queries. NoAA is unlike other alias analysis 138 /// implementations, in that it does not chain to a previous analysis. As 139 /// such it doesn't follow many of the rules that other alias analyses must. 140 /// 141 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 142 static char ID; // Class identification, replacement for typeinfo 143 NoAA() : ImmutablePass(&ID) {} 144 explicit NoAA(void *PID) : ImmutablePass(PID) { } 145 146 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 147 AU.addRequired<TargetData>(); 148 } 149 150 virtual void initializePass() { 151 TD = &getAnalysis<TargetData>(); 152 } 153 154 virtual AliasResult alias(const Value *V1, unsigned V1Size, 155 const Value *V2, unsigned V2Size) { 156 return MayAlias; 157 } 158 159 virtual void getArgumentAccesses(Function *F, CallSite CS, 160 std::vector<PointerAccessInfo> &Info) { 161 llvm_unreachable("This method may not be called on this function!"); 162 } 163 164 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 165 virtual bool pointsToConstantMemory(const Value *P) { return false; } 166 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 167 return ModRef; 168 } 169 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 170 return ModRef; 171 } 172 virtual bool hasNoModRefInfoForCalls() const { return true; } 173 174 virtual void deleteValue(Value *V) {} 175 virtual void copyValue(Value *From, Value *To) {} 176 }; 177} // End of anonymous namespace 178 179// Register this pass... 180char NoAA::ID = 0; 181static RegisterPass<NoAA> 182U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 183 184// Declare that we implement the AliasAnalysis interface 185static RegisterAnalysisGroup<AliasAnalysis> V(U); 186 187ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 188 189//===----------------------------------------------------------------------===// 190// BasicAA Pass 191//===----------------------------------------------------------------------===// 192 193namespace { 194 /// BasicAliasAnalysis - This is the default alias analysis implementation. 195 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 196 /// derives from the NoAA class. 197 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 198 static char ID; // Class identification, replacement for typeinfo 199 BasicAliasAnalysis() : NoAA(&ID) {} 200 AliasResult alias(const Value *V1, unsigned V1Size, 201 const Value *V2, unsigned V2Size); 202 203 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 204 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 205 206 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 207 /// non-escaping allocations. 208 virtual bool hasNoModRefInfoForCalls() const { return false; } 209 210 /// pointsToConstantMemory - Chase pointers until we find a (constant 211 /// global) or not. 212 bool pointsToConstantMemory(const Value *P); 213 214 private: 215 // CheckGEPInstructions - Check two GEP instructions with known 216 // must-aliasing base pointers. This checks to see if the index expressions 217 // preclude the pointers from aliasing... 218 AliasResult 219 CheckGEPInstructions(const Type* BasePtr1Ty, 220 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 221 const Type *BasePtr2Ty, 222 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 223 }; 224} // End of anonymous namespace 225 226// Register this pass... 227char BasicAliasAnalysis::ID = 0; 228static RegisterPass<BasicAliasAnalysis> 229X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 230 231// Declare that we implement the AliasAnalysis interface 232static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 233 234ImmutablePass *llvm::createBasicAliasAnalysisPass() { 235 return new BasicAliasAnalysis(); 236} 237 238 239/// pointsToConstantMemory - Chase pointers until we find a (constant 240/// global) or not. 241bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 242 if (const GlobalVariable *GV = 243 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 244 return GV->isConstant(); 245 return false; 246} 247 248 249// getModRefInfo - Check to see if the specified callsite can clobber the 250// specified memory object. Since we only look at local properties of this 251// function, we really can't say much about this query. We do, however, use 252// simple "address taken" analysis on local objects. 253// 254AliasAnalysis::ModRefResult 255BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 256 if (!isa<Constant>(P)) { 257 const Value *Object = P->getUnderlyingObject(); 258 259 // If this is a tail call and P points to a stack location, we know that 260 // the tail call cannot access or modify the local stack. 261 // We cannot exclude byval arguments here; these belong to the caller of 262 // the current function not to the current function, and a tail callee 263 // may reference them. 264 if (isa<AllocaInst>(Object)) 265 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 266 if (CI->isTailCall()) 267 return NoModRef; 268 269 // If the pointer is to a locally allocated object that does not escape, 270 // then the call can not mod/ref the pointer unless the call takes the 271 // argument without capturing it. 272 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) { 273 bool passedAsArg = false; 274 // TODO: Eventually only check 'nocapture' arguments. 275 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 276 CI != CE; ++CI) 277 if (isa<PointerType>((*CI)->getType()) && 278 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 279 passedAsArg = true; 280 281 if (!passedAsArg) 282 return NoModRef; 283 } 284 } 285 286 // The AliasAnalysis base class has some smarts, lets use them. 287 return AliasAnalysis::getModRefInfo(CS, P, Size); 288} 289 290 291AliasAnalysis::ModRefResult 292BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 293 // If CS1 or CS2 are readnone, they don't interact. 294 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 295 if (CS1B == DoesNotAccessMemory) return NoModRef; 296 297 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 298 if (CS2B == DoesNotAccessMemory) return NoModRef; 299 300 // If they both only read from memory, just return ref. 301 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 302 return Ref; 303 304 // Otherwise, fall back to NoAA (mod+ref). 305 return NoAA::getModRefInfo(CS1, CS2); 306} 307 308 309// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 310// as array references. 311// 312AliasAnalysis::AliasResult 313BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 314 const Value *V2, unsigned V2Size) { 315 Context = &V1->getType()->getContext(); 316 317 // Strip off any constant expression casts if they exist 318 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 319 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 320 V1 = CE->getOperand(0); 321 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 322 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 323 V2 = CE->getOperand(0); 324 325 // Are we checking for alias of the same value? 326 if (V1 == V2) return MustAlias; 327 328 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 329 return NoAlias; // Scalars cannot alias each other 330 331 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 332 // pointer<->pointer bitcasts. 333 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 334 return alias(I->getOperand(0), V1Size, V2, V2Size); 335 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 336 return alias(V1, V1Size, I->getOperand(0), V2Size); 337 338 // Figure out what objects these things are pointing to if we can. 339 const Value *O1 = V1->getUnderlyingObject(); 340 const Value *O2 = V2->getUnderlyingObject(); 341 342 if (O1 != O2) { 343 // If V1/V2 point to two different objects we know that we have no alias. 344 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 345 return NoAlias; 346 347 // Arguments can't alias with local allocations or noalias calls. 348 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 349 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 350 return NoAlias; 351 352 // Most objects can't alias null. 353 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 354 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 355 return NoAlias; 356 } 357 358 // If the size of one access is larger than the entire object on the other 359 // side, then we know such behavior is undefined and can assume no alias. 360 const TargetData &TD = getTargetData(); 361 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 362 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 363 return NoAlias; 364 365 // If one pointer is the result of a call/invoke and the other is a 366 // non-escaping local object, then we know the object couldn't escape to a 367 // point where the call could return it. 368 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 369 isNonEscapingLocalObject(O2) && O1 != O2) 370 return NoAlias; 371 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 372 isNonEscapingLocalObject(O1) && O1 != O2) 373 return NoAlias; 374 375 // If we have two gep instructions with must-alias'ing base pointers, figure 376 // out if the indexes to the GEP tell us anything about the derived pointer. 377 // Note that we also handle chains of getelementptr instructions as well as 378 // constant expression getelementptrs here. 379 // 380 if (isGEP(V1) && isGEP(V2)) { 381 const User *GEP1 = cast<User>(V1); 382 const User *GEP2 = cast<User>(V2); 383 384 // If V1 and V2 are identical GEPs, just recurse down on both of them. 385 // This allows us to analyze things like: 386 // P = gep A, 0, i, 1 387 // Q = gep B, 0, i, 1 388 // by just analyzing A and B. This is even safe for variable indices. 389 if (GEP1->getType() == GEP2->getType() && 390 GEP1->getNumOperands() == GEP2->getNumOperands() && 391 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 392 // All operands are the same, ignoring the base. 393 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 394 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 395 396 397 // Drill down into the first non-gep value, to test for must-aliasing of 398 // the base pointers. 399 while (isGEP(GEP1->getOperand(0)) && 400 GEP1->getOperand(1) == 401 Context->getNullValue(GEP1->getOperand(1)->getType())) 402 GEP1 = cast<User>(GEP1->getOperand(0)); 403 const Value *BasePtr1 = GEP1->getOperand(0); 404 405 while (isGEP(GEP2->getOperand(0)) && 406 GEP2->getOperand(1) == 407 Context->getNullValue(GEP2->getOperand(1)->getType())) 408 GEP2 = cast<User>(GEP2->getOperand(0)); 409 const Value *BasePtr2 = GEP2->getOperand(0); 410 411 // Do the base pointers alias? 412 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 413 if (BaseAlias == NoAlias) return NoAlias; 414 if (BaseAlias == MustAlias) { 415 // If the base pointers alias each other exactly, check to see if we can 416 // figure out anything about the resultant pointers, to try to prove 417 // non-aliasing. 418 419 // Collect all of the chained GEP operands together into one simple place 420 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 421 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 422 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 423 424 // If GetGEPOperands were able to fold to the same must-aliased pointer, 425 // do the comparison. 426 if (BasePtr1 == BasePtr2) { 427 AliasResult GAlias = 428 CheckGEPInstructions(BasePtr1->getType(), 429 &GEP1Ops[0], GEP1Ops.size(), V1Size, 430 BasePtr2->getType(), 431 &GEP2Ops[0], GEP2Ops.size(), V2Size); 432 if (GAlias != MayAlias) 433 return GAlias; 434 } 435 } 436 } 437 438 // Check to see if these two pointers are related by a getelementptr 439 // instruction. If one pointer is a GEP with a non-zero index of the other 440 // pointer, we know they cannot alias. 441 // 442 if (isGEP(V2)) { 443 std::swap(V1, V2); 444 std::swap(V1Size, V2Size); 445 } 446 447 if (V1Size != ~0U && V2Size != ~0U) 448 if (isGEP(V1)) { 449 SmallVector<Value*, 16> GEPOperands; 450 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 451 452 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 453 if (R == MustAlias) { 454 // If there is at least one non-zero constant index, we know they cannot 455 // alias. 456 bool ConstantFound = false; 457 bool AllZerosFound = true; 458 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 459 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 460 if (!C->isNullValue()) { 461 ConstantFound = true; 462 AllZerosFound = false; 463 break; 464 } 465 } else { 466 AllZerosFound = false; 467 } 468 469 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 470 // the ptr, the end result is a must alias also. 471 if (AllZerosFound) 472 return MustAlias; 473 474 if (ConstantFound) { 475 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 476 return NoAlias; 477 478 // Otherwise we have to check to see that the distance is more than 479 // the size of the argument... build an index vector that is equal to 480 // the arguments provided, except substitute 0's for any variable 481 // indexes we find... 482 if (cast<PointerType>( 483 BasePtr->getType())->getElementType()->isSized()) { 484 for (unsigned i = 0; i != GEPOperands.size(); ++i) 485 if (!isa<ConstantInt>(GEPOperands[i])) 486 GEPOperands[i] = 487 Context->getNullValue(GEPOperands[i]->getType()); 488 int64_t Offset = 489 getTargetData().getIndexedOffset(BasePtr->getType(), 490 &GEPOperands[0], 491 GEPOperands.size()); 492 493 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 494 return NoAlias; 495 } 496 } 497 } 498 } 499 500 return MayAlias; 501} 502 503// This function is used to determine if the indices of two GEP instructions are 504// equal. V1 and V2 are the indices. 505static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext *Context) { 506 if (V1->getType() == V2->getType()) 507 return V1 == V2; 508 if (Constant *C1 = dyn_cast<Constant>(V1)) 509 if (Constant *C2 = dyn_cast<Constant>(V2)) { 510 // Sign extend the constants to long types, if necessary 511 if (C1->getType() != Type::Int64Ty) 512 C1 = Context->getConstantExprSExt(C1, Type::Int64Ty); 513 if (C2->getType() != Type::Int64Ty) 514 C2 = Context->getConstantExprSExt(C2, Type::Int64Ty); 515 return C1 == C2; 516 } 517 return false; 518} 519 520/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 521/// base pointers. This checks to see if the index expressions preclude the 522/// pointers from aliasing... 523AliasAnalysis::AliasResult 524BasicAliasAnalysis::CheckGEPInstructions( 525 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 526 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 527 // We currently can't handle the case when the base pointers have different 528 // primitive types. Since this is uncommon anyway, we are happy being 529 // extremely conservative. 530 if (BasePtr1Ty != BasePtr2Ty) 531 return MayAlias; 532 533 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 534 535 Context = &GEPPointerTy->getContext(); 536 537 // Find the (possibly empty) initial sequence of equal values... which are not 538 // necessarily constants. 539 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 540 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 541 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 542 unsigned UnequalOper = 0; 543 while (UnequalOper != MinOperands && 544 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper], 545 Context)) { 546 // Advance through the type as we go... 547 ++UnequalOper; 548 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 549 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 550 else { 551 // If all operands equal each other, then the derived pointers must 552 // alias each other... 553 BasePtr1Ty = 0; 554 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 555 "Ran out of type nesting, but not out of operands?"); 556 return MustAlias; 557 } 558 } 559 560 // If we have seen all constant operands, and run out of indexes on one of the 561 // getelementptrs, check to see if the tail of the leftover one is all zeros. 562 // If so, return mustalias. 563 if (UnequalOper == MinOperands) { 564 if (NumGEP1Ops < NumGEP2Ops) { 565 std::swap(GEP1Ops, GEP2Ops); 566 std::swap(NumGEP1Ops, NumGEP2Ops); 567 } 568 569 bool AllAreZeros = true; 570 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 571 if (!isa<Constant>(GEP1Ops[i]) || 572 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 573 AllAreZeros = false; 574 break; 575 } 576 if (AllAreZeros) return MustAlias; 577 } 578 579 580 // So now we know that the indexes derived from the base pointers, 581 // which are known to alias, are different. We can still determine a 582 // no-alias result if there are differing constant pairs in the index 583 // chain. For example: 584 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 585 // 586 // We have to be careful here about array accesses. In particular, consider: 587 // A[1][0] vs A[0][i] 588 // In this case, we don't *know* that the array will be accessed in bounds: 589 // the index could even be negative. Because of this, we have to 590 // conservatively *give up* and return may alias. We disregard differing 591 // array subscripts that are followed by a variable index without going 592 // through a struct. 593 // 594 unsigned SizeMax = std::max(G1S, G2S); 595 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 596 597 // Scan for the first operand that is constant and unequal in the 598 // two getelementptrs... 599 unsigned FirstConstantOper = UnequalOper; 600 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 601 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 602 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 603 604 if (G1Oper != G2Oper) // Found non-equal constant indexes... 605 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 606 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 607 if (G1OC->getType() != G2OC->getType()) { 608 // Sign extend both operands to long. 609 if (G1OC->getType() != Type::Int64Ty) 610 G1OC = Context->getConstantExprSExt(G1OC, Type::Int64Ty); 611 if (G2OC->getType() != Type::Int64Ty) 612 G2OC = Context->getConstantExprSExt(G2OC, Type::Int64Ty); 613 GEP1Ops[FirstConstantOper] = G1OC; 614 GEP2Ops[FirstConstantOper] = G2OC; 615 } 616 617 if (G1OC != G2OC) { 618 // Handle the "be careful" case above: if this is an array/vector 619 // subscript, scan for a subsequent variable array index. 620 if (const SequentialType *STy = 621 dyn_cast<SequentialType>(BasePtr1Ty)) { 622 const Type *NextTy = STy; 623 bool isBadCase = false; 624 625 for (unsigned Idx = FirstConstantOper; 626 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 627 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 628 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 629 isBadCase = true; 630 break; 631 } 632 // If the array is indexed beyond the bounds of the static type 633 // at this level, it will also fall into the "be careful" case. 634 // It would theoretically be possible to analyze these cases, 635 // but for now just be conservatively correct. 636 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy)) 637 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 638 ATy->getNumElements() || 639 cast<ConstantInt>(G2OC)->getZExtValue() >= 640 ATy->getNumElements()) { 641 isBadCase = true; 642 break; 643 } 644 if (const VectorType *VTy = dyn_cast<VectorType>(STy)) 645 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 646 VTy->getNumElements() || 647 cast<ConstantInt>(G2OC)->getZExtValue() >= 648 VTy->getNumElements()) { 649 isBadCase = true; 650 break; 651 } 652 STy = cast<SequentialType>(NextTy); 653 NextTy = cast<SequentialType>(NextTy)->getElementType(); 654 } 655 656 if (isBadCase) G1OC = 0; 657 } 658 659 // Make sure they are comparable (ie, not constant expressions), and 660 // make sure the GEP with the smaller leading constant is GEP1. 661 if (G1OC) { 662 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 663 G1OC, G2OC); 664 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 665 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 666 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 667 std::swap(NumGEP1Ops, NumGEP2Ops); 668 } 669 break; 670 } 671 } 672 } 673 } 674 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 675 } 676 677 // No shared constant operands, and we ran out of common operands. At this 678 // point, the GEP instructions have run through all of their operands, and we 679 // haven't found evidence that there are any deltas between the GEP's. 680 // However, one GEP may have more operands than the other. If this is the 681 // case, there may still be hope. Check this now. 682 if (FirstConstantOper == MinOperands) { 683 // Make GEP1Ops be the longer one if there is a longer one. 684 if (NumGEP1Ops < NumGEP2Ops) { 685 std::swap(GEP1Ops, GEP2Ops); 686 std::swap(NumGEP1Ops, NumGEP2Ops); 687 } 688 689 // Is there anything to check? 690 if (NumGEP1Ops > MinOperands) { 691 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 692 if (isa<ConstantInt>(GEP1Ops[i]) && 693 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 694 // Yup, there's a constant in the tail. Set all variables to 695 // constants in the GEP instruction to make it suitable for 696 // TargetData::getIndexedOffset. 697 for (i = 0; i != MaxOperands; ++i) 698 if (!isa<ConstantInt>(GEP1Ops[i])) 699 GEP1Ops[i] = Context->getNullValue(GEP1Ops[i]->getType()); 700 // Okay, now get the offset. This is the relative offset for the full 701 // instruction. 702 const TargetData &TD = getTargetData(); 703 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 704 NumGEP1Ops); 705 706 // Now check without any constants at the end. 707 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 708 MinOperands); 709 710 // Make sure we compare the absolute difference. 711 if (Offset1 > Offset2) 712 std::swap(Offset1, Offset2); 713 714 // If the tail provided a bit enough offset, return noalias! 715 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 716 return NoAlias; 717 // Otherwise break - we don't look for another constant in the tail. 718 break; 719 } 720 } 721 722 // Couldn't find anything useful. 723 return MayAlias; 724 } 725 726 // If there are non-equal constants arguments, then we can figure 727 // out a minimum known delta between the two index expressions... at 728 // this point we know that the first constant index of GEP1 is less 729 // than the first constant index of GEP2. 730 731 // Advance BasePtr[12]Ty over this first differing constant operand. 732 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 733 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 734 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 735 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 736 737 // We are going to be using TargetData::getIndexedOffset to determine the 738 // offset that each of the GEP's is reaching. To do this, we have to convert 739 // all variable references to constant references. To do this, we convert the 740 // initial sequence of array subscripts into constant zeros to start with. 741 const Type *ZeroIdxTy = GEPPointerTy; 742 for (unsigned i = 0; i != FirstConstantOper; ++i) { 743 if (!isa<StructType>(ZeroIdxTy)) 744 GEP1Ops[i] = GEP2Ops[i] = Context->getNullValue(Type::Int32Ty); 745 746 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 747 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 748 } 749 750 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 751 752 // Loop over the rest of the operands... 753 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 754 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 755 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 756 // If they are equal, use a zero index... 757 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 758 if (!isa<ConstantInt>(Op1)) 759 GEP1Ops[i] = GEP2Ops[i] = Context->getNullValue(Op1->getType()); 760 // Otherwise, just keep the constants we have. 761 } else { 762 if (Op1) { 763 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 764 // If this is an array index, make sure the array element is in range. 765 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 766 if (Op1C->getZExtValue() >= AT->getNumElements()) 767 return MayAlias; // Be conservative with out-of-range accesses 768 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 769 if (Op1C->getZExtValue() >= VT->getNumElements()) 770 return MayAlias; // Be conservative with out-of-range accesses 771 } 772 773 } else { 774 // GEP1 is known to produce a value less than GEP2. To be 775 // conservatively correct, we must assume the largest possible 776 // constant is used in this position. This cannot be the initial 777 // index to the GEP instructions (because we know we have at least one 778 // element before this one with the different constant arguments), so 779 // we know that the current index must be into either a struct or 780 // array. Because we know it's not constant, this cannot be a 781 // structure index. Because of this, we can calculate the maximum 782 // value possible. 783 // 784 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 785 GEP1Ops[i] = 786 Context->getConstantInt(Type::Int64Ty,AT->getNumElements()-1); 787 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 788 GEP1Ops[i] = 789 Context->getConstantInt(Type::Int64Ty,VT->getNumElements()-1); 790 } 791 } 792 793 if (Op2) { 794 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 795 // If this is an array index, make sure the array element is in range. 796 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 797 if (Op2C->getZExtValue() >= AT->getNumElements()) 798 return MayAlias; // Be conservative with out-of-range accesses 799 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 800 if (Op2C->getZExtValue() >= VT->getNumElements()) 801 return MayAlias; // Be conservative with out-of-range accesses 802 } 803 } else { // Conservatively assume the minimum value for this index 804 GEP2Ops[i] = Context->getNullValue(Op2->getType()); 805 } 806 } 807 } 808 809 if (BasePtr1Ty && Op1) { 810 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 811 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 812 else 813 BasePtr1Ty = 0; 814 } 815 816 if (BasePtr2Ty && Op2) { 817 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 818 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 819 else 820 BasePtr2Ty = 0; 821 } 822 } 823 824 if (GEPPointerTy->getElementType()->isSized()) { 825 int64_t Offset1 = 826 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 827 int64_t Offset2 = 828 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 829 assert(Offset1 != Offset2 && 830 "There is at least one different constant here!"); 831 832 // Make sure we compare the absolute difference. 833 if (Offset1 > Offset2) 834 std::swap(Offset1, Offset2); 835 836 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 837 //cerr << "Determined that these two GEP's don't alias [" 838 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 839 return NoAlias; 840 } 841 } 842 return MayAlias; 843} 844 845// Make sure that anything that uses AliasAnalysis pulls in this file... 846DEFINING_FILE_FOR(BasicAliasAnalysis) 847