BasicAliasAnalysis.cpp revision eed707b1e6097aac2bb6b3d47271f6300ace7f2e
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/Operator.h" 27#include "llvm/Pass.h" 28#include "llvm/Target/TargetData.h" 29#include "llvm/ADT/SmallVector.h" 30#include "llvm/ADT/STLExtras.h" 31#include "llvm/Support/Compiler.h" 32#include "llvm/Support/ErrorHandling.h" 33#include "llvm/Support/GetElementPtrTypeIterator.h" 34#include <algorithm> 35using namespace llvm; 36 37//===----------------------------------------------------------------------===// 38// Useful predicates 39//===----------------------------------------------------------------------===// 40 41static const GEPOperator *isGEP(const Value *V) { 42 return dyn_cast<GEPOperator>(V); 43} 44 45static const Value *GetGEPOperands(const Value *V, 46 SmallVector<Value*, 16> &GEPOps) { 47 assert(GEPOps.empty() && "Expect empty list to populate!"); 48 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 49 cast<User>(V)->op_end()); 50 51 // Accumulate all of the chained indexes into the operand array 52 V = cast<User>(V)->getOperand(0); 53 54 while (const User *G = isGEP(V)) { 55 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 56 !cast<Constant>(GEPOps[0])->isNullValue()) 57 break; // Don't handle folding arbitrary pointer offsets yet... 58 GEPOps.erase(GEPOps.begin()); // Drop the zero index 59 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 60 V = G->getOperand(0); 61 } 62 return V; 63} 64 65/// isKnownNonNull - Return true if we know that the specified value is never 66/// null. 67static bool isKnownNonNull(const Value *V) { 68 // Alloca never returns null, malloc might. 69 if (isa<AllocaInst>(V)) return true; 70 71 // A byval argument is never null. 72 if (const Argument *A = dyn_cast<Argument>(V)) 73 return A->hasByValAttr(); 74 75 // Global values are not null unless extern weak. 76 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 77 return !GV->hasExternalWeakLinkage(); 78 return false; 79} 80 81/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 82/// object that never escapes from the function. 83static bool isNonEscapingLocalObject(const Value *V) { 84 // If this is a local allocation, check to see if it escapes. 85 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 86 return !PointerMayBeCaptured(V, false); 87 88 // If this is an argument that corresponds to a byval or noalias argument, 89 // then it has not escaped before entering the function. Check if it escapes 90 // inside the function. 91 if (const Argument *A = dyn_cast<Argument>(V)) 92 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 93 // Don't bother analyzing arguments already known not to escape. 94 if (A->hasNoCaptureAttr()) 95 return true; 96 return !PointerMayBeCaptured(V, false); 97 } 98 return false; 99} 100 101 102/// isObjectSmallerThan - Return true if we can prove that the object specified 103/// by V is smaller than Size. 104static bool isObjectSmallerThan(const Value *V, unsigned Size, 105 const TargetData &TD) { 106 const Type *AccessTy; 107 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 108 AccessTy = GV->getType()->getElementType(); 109 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 110 if (!AI->isArrayAllocation()) 111 AccessTy = AI->getType()->getElementType(); 112 else 113 return false; 114 } else if (const Argument *A = dyn_cast<Argument>(V)) { 115 if (A->hasByValAttr()) 116 AccessTy = cast<PointerType>(A->getType())->getElementType(); 117 else 118 return false; 119 } else { 120 return false; 121 } 122 123 if (AccessTy->isSized()) 124 return TD.getTypeAllocSize(AccessTy) < Size; 125 return false; 126} 127 128//===----------------------------------------------------------------------===// 129// NoAA Pass 130//===----------------------------------------------------------------------===// 131 132namespace { 133 /// NoAA - This class implements the -no-aa pass, which always returns "I 134 /// don't know" for alias queries. NoAA is unlike other alias analysis 135 /// implementations, in that it does not chain to a previous analysis. As 136 /// such it doesn't follow many of the rules that other alias analyses must. 137 /// 138 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 139 static char ID; // Class identification, replacement for typeinfo 140 NoAA() : ImmutablePass(&ID) {} 141 explicit NoAA(void *PID) : ImmutablePass(PID) { } 142 143 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 144 AU.addRequired<TargetData>(); 145 } 146 147 virtual void initializePass() { 148 TD = &getAnalysis<TargetData>(); 149 } 150 151 virtual AliasResult alias(const Value *V1, unsigned V1Size, 152 const Value *V2, unsigned V2Size) { 153 return MayAlias; 154 } 155 156 virtual void getArgumentAccesses(Function *F, CallSite CS, 157 std::vector<PointerAccessInfo> &Info) { 158 llvm_unreachable("This method may not be called on this function!"); 159 } 160 161 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 162 virtual bool pointsToConstantMemory(const Value *P) { return false; } 163 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 164 return ModRef; 165 } 166 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 167 return ModRef; 168 } 169 virtual bool hasNoModRefInfoForCalls() const { return true; } 170 171 virtual void deleteValue(Value *V) {} 172 virtual void copyValue(Value *From, Value *To) {} 173 }; 174} // End of anonymous namespace 175 176// Register this pass... 177char NoAA::ID = 0; 178static RegisterPass<NoAA> 179U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 180 181// Declare that we implement the AliasAnalysis interface 182static RegisterAnalysisGroup<AliasAnalysis> V(U); 183 184ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 185 186//===----------------------------------------------------------------------===// 187// BasicAA Pass 188//===----------------------------------------------------------------------===// 189 190namespace { 191 /// BasicAliasAnalysis - This is the default alias analysis implementation. 192 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 193 /// derives from the NoAA class. 194 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 195 static char ID; // Class identification, replacement for typeinfo 196 BasicAliasAnalysis() : NoAA(&ID) {} 197 AliasResult alias(const Value *V1, unsigned V1Size, 198 const Value *V2, unsigned V2Size); 199 200 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 201 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 202 203 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 204 /// non-escaping allocations. 205 virtual bool hasNoModRefInfoForCalls() const { return false; } 206 207 /// pointsToConstantMemory - Chase pointers until we find a (constant 208 /// global) or not. 209 bool pointsToConstantMemory(const Value *P); 210 211 private: 212 // CheckGEPInstructions - Check two GEP instructions with known 213 // must-aliasing base pointers. This checks to see if the index expressions 214 // preclude the pointers from aliasing... 215 AliasResult 216 CheckGEPInstructions(const Type* BasePtr1Ty, 217 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 218 const Type *BasePtr2Ty, 219 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 220 }; 221} // End of anonymous namespace 222 223// Register this pass... 224char BasicAliasAnalysis::ID = 0; 225static RegisterPass<BasicAliasAnalysis> 226X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 227 228// Declare that we implement the AliasAnalysis interface 229static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 230 231ImmutablePass *llvm::createBasicAliasAnalysisPass() { 232 return new BasicAliasAnalysis(); 233} 234 235 236/// pointsToConstantMemory - Chase pointers until we find a (constant 237/// global) or not. 238bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 239 if (const GlobalVariable *GV = 240 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 241 return GV->isConstant(); 242 return false; 243} 244 245 246// getModRefInfo - Check to see if the specified callsite can clobber the 247// specified memory object. Since we only look at local properties of this 248// function, we really can't say much about this query. We do, however, use 249// simple "address taken" analysis on local objects. 250// 251AliasAnalysis::ModRefResult 252BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 253 if (!isa<Constant>(P)) { 254 const Value *Object = P->getUnderlyingObject(); 255 256 // If this is a tail call and P points to a stack location, we know that 257 // the tail call cannot access or modify the local stack. 258 // We cannot exclude byval arguments here; these belong to the caller of 259 // the current function not to the current function, and a tail callee 260 // may reference them. 261 if (isa<AllocaInst>(Object)) 262 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 263 if (CI->isTailCall()) 264 return NoModRef; 265 266 // If the pointer is to a locally allocated object that does not escape, 267 // then the call can not mod/ref the pointer unless the call takes the 268 // argument without capturing it. 269 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) { 270 bool passedAsArg = false; 271 // TODO: Eventually only check 'nocapture' arguments. 272 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 273 CI != CE; ++CI) 274 if (isa<PointerType>((*CI)->getType()) && 275 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 276 passedAsArg = true; 277 278 if (!passedAsArg) 279 return NoModRef; 280 } 281 } 282 283 // The AliasAnalysis base class has some smarts, lets use them. 284 return AliasAnalysis::getModRefInfo(CS, P, Size); 285} 286 287 288AliasAnalysis::ModRefResult 289BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 290 // If CS1 or CS2 are readnone, they don't interact. 291 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 292 if (CS1B == DoesNotAccessMemory) return NoModRef; 293 294 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 295 if (CS2B == DoesNotAccessMemory) return NoModRef; 296 297 // If they both only read from memory, just return ref. 298 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 299 return Ref; 300 301 // Otherwise, fall back to NoAA (mod+ref). 302 return NoAA::getModRefInfo(CS1, CS2); 303} 304 305 306// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 307// as array references. 308// 309AliasAnalysis::AliasResult 310BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 311 const Value *V2, unsigned V2Size) { 312 LLVMContext &Context = V1->getType()->getContext(); 313 314 // 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 LLVMContext &Context = GEPPointerTy->getContext(); 533 534 // Find the (possibly empty) initial sequence of equal values... which are not 535 // necessarily constants. 536 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 537 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 538 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 539 unsigned UnequalOper = 0; 540 while (UnequalOper != MinOperands && 541 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper], 542 Context)) { 543 // Advance through the type as we go... 544 ++UnequalOper; 545 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 546 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 547 else { 548 // If all operands equal each other, then the derived pointers must 549 // alias each other... 550 BasePtr1Ty = 0; 551 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 552 "Ran out of type nesting, but not out of operands?"); 553 return MustAlias; 554 } 555 } 556 557 // If we have seen all constant operands, and run out of indexes on one of the 558 // getelementptrs, check to see if the tail of the leftover one is all zeros. 559 // If so, return mustalias. 560 if (UnequalOper == MinOperands) { 561 if (NumGEP1Ops < NumGEP2Ops) { 562 std::swap(GEP1Ops, GEP2Ops); 563 std::swap(NumGEP1Ops, NumGEP2Ops); 564 } 565 566 bool AllAreZeros = true; 567 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 568 if (!isa<Constant>(GEP1Ops[i]) || 569 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 570 AllAreZeros = false; 571 break; 572 } 573 if (AllAreZeros) return MustAlias; 574 } 575 576 577 // So now we know that the indexes derived from the base pointers, 578 // which are known to alias, are different. We can still determine a 579 // no-alias result if there are differing constant pairs in the index 580 // chain. For example: 581 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 582 // 583 // We have to be careful here about array accesses. In particular, consider: 584 // A[1][0] vs A[0][i] 585 // In this case, we don't *know* that the array will be accessed in bounds: 586 // the index could even be negative. Because of this, we have to 587 // conservatively *give up* and return may alias. We disregard differing 588 // array subscripts that are followed by a variable index without going 589 // through a struct. 590 // 591 unsigned SizeMax = std::max(G1S, G2S); 592 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 593 594 // Scan for the first operand that is constant and unequal in the 595 // two getelementptrs... 596 unsigned FirstConstantOper = UnequalOper; 597 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 598 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 599 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 600 601 if (G1Oper != G2Oper) // Found non-equal constant indexes... 602 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 603 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 604 if (G1OC->getType() != G2OC->getType()) { 605 // Sign extend both operands to long. 606 if (G1OC->getType() != Type::Int64Ty) 607 G1OC = Context.getConstantExprSExt(G1OC, Type::Int64Ty); 608 if (G2OC->getType() != Type::Int64Ty) 609 G2OC = Context.getConstantExprSExt(G2OC, Type::Int64Ty); 610 GEP1Ops[FirstConstantOper] = G1OC; 611 GEP2Ops[FirstConstantOper] = G2OC; 612 } 613 614 if (G1OC != G2OC) { 615 // Handle the "be careful" case above: if this is an array/vector 616 // subscript, scan for a subsequent variable array index. 617 if (const SequentialType *STy = 618 dyn_cast<SequentialType>(BasePtr1Ty)) { 619 const Type *NextTy = STy; 620 bool isBadCase = false; 621 622 for (unsigned Idx = FirstConstantOper; 623 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 624 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 625 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 626 isBadCase = true; 627 break; 628 } 629 // If the array is indexed beyond the bounds of the static type 630 // at this level, it will also fall into the "be careful" case. 631 // It would theoretically be possible to analyze these cases, 632 // but for now just be conservatively correct. 633 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy)) 634 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 635 ATy->getNumElements() || 636 cast<ConstantInt>(G2OC)->getZExtValue() >= 637 ATy->getNumElements()) { 638 isBadCase = true; 639 break; 640 } 641 if (const VectorType *VTy = dyn_cast<VectorType>(STy)) 642 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 643 VTy->getNumElements() || 644 cast<ConstantInt>(G2OC)->getZExtValue() >= 645 VTy->getNumElements()) { 646 isBadCase = true; 647 break; 648 } 649 STy = cast<SequentialType>(NextTy); 650 NextTy = cast<SequentialType>(NextTy)->getElementType(); 651 } 652 653 if (isBadCase) G1OC = 0; 654 } 655 656 // Make sure they are comparable (ie, not constant expressions), and 657 // make sure the GEP with the smaller leading constant is GEP1. 658 if (G1OC) { 659 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 660 G1OC, G2OC); 661 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 662 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 663 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 664 std::swap(NumGEP1Ops, NumGEP2Ops); 665 } 666 break; 667 } 668 } 669 } 670 } 671 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 672 } 673 674 // No shared constant operands, and we ran out of common operands. At this 675 // point, the GEP instructions have run through all of their operands, and we 676 // haven't found evidence that there are any deltas between the GEP's. 677 // However, one GEP may have more operands than the other. If this is the 678 // case, there may still be hope. Check this now. 679 if (FirstConstantOper == MinOperands) { 680 // Make GEP1Ops be the longer one if there is a longer one. 681 if (NumGEP1Ops < NumGEP2Ops) { 682 std::swap(GEP1Ops, GEP2Ops); 683 std::swap(NumGEP1Ops, NumGEP2Ops); 684 } 685 686 // Is there anything to check? 687 if (NumGEP1Ops > MinOperands) { 688 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 689 if (isa<ConstantInt>(GEP1Ops[i]) && 690 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 691 // Yup, there's a constant in the tail. Set all variables to 692 // constants in the GEP instruction to make it suitable for 693 // TargetData::getIndexedOffset. 694 for (i = 0; i != MaxOperands; ++i) 695 if (!isa<ConstantInt>(GEP1Ops[i])) 696 GEP1Ops[i] = Context.getNullValue(GEP1Ops[i]->getType()); 697 // Okay, now get the offset. This is the relative offset for the full 698 // instruction. 699 const TargetData &TD = getTargetData(); 700 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 701 NumGEP1Ops); 702 703 // Now check without any constants at the end. 704 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 705 MinOperands); 706 707 // Make sure we compare the absolute difference. 708 if (Offset1 > Offset2) 709 std::swap(Offset1, Offset2); 710 711 // If the tail provided a bit enough offset, return noalias! 712 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 713 return NoAlias; 714 // Otherwise break - we don't look for another constant in the tail. 715 break; 716 } 717 } 718 719 // Couldn't find anything useful. 720 return MayAlias; 721 } 722 723 // If there are non-equal constants arguments, then we can figure 724 // out a minimum known delta between the two index expressions... at 725 // this point we know that the first constant index of GEP1 is less 726 // than the first constant index of GEP2. 727 728 // Advance BasePtr[12]Ty over this first differing constant operand. 729 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 730 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 731 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 732 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 733 734 // We are going to be using TargetData::getIndexedOffset to determine the 735 // offset that each of the GEP's is reaching. To do this, we have to convert 736 // all variable references to constant references. To do this, we convert the 737 // initial sequence of array subscripts into constant zeros to start with. 738 const Type *ZeroIdxTy = GEPPointerTy; 739 for (unsigned i = 0; i != FirstConstantOper; ++i) { 740 if (!isa<StructType>(ZeroIdxTy)) 741 GEP1Ops[i] = GEP2Ops[i] = Context.getNullValue(Type::Int32Ty); 742 743 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 744 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 745 } 746 747 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 748 749 // Loop over the rest of the operands... 750 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 751 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 752 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 753 // If they are equal, use a zero index... 754 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 755 if (!isa<ConstantInt>(Op1)) 756 GEP1Ops[i] = GEP2Ops[i] = Context.getNullValue(Op1->getType()); 757 // Otherwise, just keep the constants we have. 758 } else { 759 if (Op1) { 760 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 761 // If this is an array index, make sure the array element is in range. 762 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 763 if (Op1C->getZExtValue() >= AT->getNumElements()) 764 return MayAlias; // Be conservative with out-of-range accesses 765 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 766 if (Op1C->getZExtValue() >= VT->getNumElements()) 767 return MayAlias; // Be conservative with out-of-range accesses 768 } 769 770 } else { 771 // GEP1 is known to produce a value less than GEP2. To be 772 // conservatively correct, we must assume the largest possible 773 // constant is used in this position. This cannot be the initial 774 // index to the GEP instructions (because we know we have at least one 775 // element before this one with the different constant arguments), so 776 // we know that the current index must be into either a struct or 777 // array. Because we know it's not constant, this cannot be a 778 // structure index. Because of this, we can calculate the maximum 779 // value possible. 780 // 781 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 782 GEP1Ops[i] = 783 ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 784 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 785 GEP1Ops[i] = 786 ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 787 } 788 } 789 790 if (Op2) { 791 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 792 // If this is an array index, make sure the array element is in range. 793 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 794 if (Op2C->getZExtValue() >= AT->getNumElements()) 795 return MayAlias; // Be conservative with out-of-range accesses 796 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 797 if (Op2C->getZExtValue() >= VT->getNumElements()) 798 return MayAlias; // Be conservative with out-of-range accesses 799 } 800 } else { // Conservatively assume the minimum value for this index 801 GEP2Ops[i] = Context.getNullValue(Op2->getType()); 802 } 803 } 804 } 805 806 if (BasePtr1Ty && Op1) { 807 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 808 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 809 else 810 BasePtr1Ty = 0; 811 } 812 813 if (BasePtr2Ty && Op2) { 814 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 815 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 816 else 817 BasePtr2Ty = 0; 818 } 819 } 820 821 if (GEPPointerTy->getElementType()->isSized()) { 822 int64_t Offset1 = 823 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 824 int64_t Offset2 = 825 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 826 assert(Offset1 != Offset2 && 827 "There is at least one different constant here!"); 828 829 // Make sure we compare the absolute difference. 830 if (Offset1 > Offset2) 831 std::swap(Offset1, Offset2); 832 833 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 834 //cerr << "Determined that these two GEP's don't alias [" 835 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 836 return NoAlias; 837 } 838 } 839 return MayAlias; 840} 841 842// Make sure that anything that uses AliasAnalysis pulls in this file... 843DEFINING_FILE_FOR(BasicAliasAnalysis) 844