BasicAliasAnalysis.cpp revision 194710
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/Pass.h" 26#include "llvm/Target/TargetData.h" 27#include "llvm/ADT/SmallVector.h" 28#include "llvm/ADT/STLExtras.h" 29#include "llvm/Support/Compiler.h" 30#include "llvm/Support/GetElementPtrTypeIterator.h" 31#include <algorithm> 32using namespace llvm; 33 34//===----------------------------------------------------------------------===// 35// Useful predicates 36//===----------------------------------------------------------------------===// 37 38static const User *isGEP(const Value *V) { 39 if (isa<GetElementPtrInst>(V) || 40 (isa<ConstantExpr>(V) && 41 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 42 return cast<User>(V); 43 return 0; 44} 45 46static const Value *GetGEPOperands(const Value *V, 47 SmallVector<Value*, 16> &GEPOps) { 48 assert(GEPOps.empty() && "Expect empty list to populate!"); 49 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 50 cast<User>(V)->op_end()); 51 52 // Accumulate all of the chained indexes into the operand array 53 V = cast<User>(V)->getOperand(0); 54 55 while (const User *G = isGEP(V)) { 56 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 57 !cast<Constant>(GEPOps[0])->isNullValue()) 58 break; // Don't handle folding arbitrary pointer offsets yet... 59 GEPOps.erase(GEPOps.begin()); // Drop the zero index 60 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 61 V = G->getOperand(0); 62 } 63 return V; 64} 65 66/// isKnownNonNull - Return true if we know that the specified value is never 67/// null. 68static bool isKnownNonNull(const Value *V) { 69 // Alloca never returns null, malloc might. 70 if (isa<AllocaInst>(V)) return true; 71 72 // A byval argument is never null. 73 if (const Argument *A = dyn_cast<Argument>(V)) 74 return A->hasByValAttr(); 75 76 // Global values are not null unless extern weak. 77 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 78 return !GV->hasExternalWeakLinkage(); 79 return false; 80} 81 82/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 83/// object that never escapes from the function. 84static bool isNonEscapingLocalObject(const Value *V) { 85 // If this is a local allocation, check to see if it escapes. 86 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 87 return !PointerMayBeCaptured(V, false); 88 89 // If this is an argument that corresponds to a byval or noalias argument, 90 // then it has not escaped before entering the function. Check if it escapes 91 // inside the function. 92 if (const Argument *A = dyn_cast<Argument>(V)) 93 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 94 // Don't bother analyzing arguments already known not to escape. 95 if (A->hasNoCaptureAttr()) 96 return true; 97 return !PointerMayBeCaptured(V, false); 98 } 99 return false; 100} 101 102 103/// isObjectSmallerThan - Return true if we can prove that the object specified 104/// by V is smaller than Size. 105static bool isObjectSmallerThan(const Value *V, unsigned Size, 106 const TargetData &TD) { 107 const Type *AccessTy; 108 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 109 AccessTy = GV->getType()->getElementType(); 110 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 111 if (!AI->isArrayAllocation()) 112 AccessTy = AI->getType()->getElementType(); 113 else 114 return false; 115 } else if (const Argument *A = dyn_cast<Argument>(V)) { 116 if (A->hasByValAttr()) 117 AccessTy = cast<PointerType>(A->getType())->getElementType(); 118 else 119 return false; 120 } else { 121 return false; 122 } 123 124 if (AccessTy->isSized()) 125 return TD.getTypeAllocSize(AccessTy) < Size; 126 return false; 127} 128 129//===----------------------------------------------------------------------===// 130// NoAA Pass 131//===----------------------------------------------------------------------===// 132 133namespace { 134 /// NoAA - This class implements the -no-aa pass, which always returns "I 135 /// don't know" for alias queries. NoAA is unlike other alias analysis 136 /// implementations, in that it does not chain to a previous analysis. As 137 /// such it doesn't follow many of the rules that other alias analyses must. 138 /// 139 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 140 static char ID; // Class identification, replacement for typeinfo 141 NoAA() : ImmutablePass(&ID) {} 142 explicit NoAA(void *PID) : ImmutablePass(PID) { } 143 144 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 145 AU.addRequired<TargetData>(); 146 } 147 148 virtual void initializePass() { 149 TD = &getAnalysis<TargetData>(); 150 } 151 152 virtual AliasResult alias(const Value *V1, unsigned V1Size, 153 const Value *V2, unsigned V2Size) { 154 return MayAlias; 155 } 156 157 virtual void getArgumentAccesses(Function *F, CallSite CS, 158 std::vector<PointerAccessInfo> &Info) { 159 assert(0 && "This method may not be called on this function!"); 160 } 161 162 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 163 virtual bool pointsToConstantMemory(const Value *P) { return false; } 164 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 165 return ModRef; 166 } 167 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 168 return ModRef; 169 } 170 virtual bool hasNoModRefInfoForCalls() const { return true; } 171 172 virtual void deleteValue(Value *V) {} 173 virtual void copyValue(Value *From, Value *To) {} 174 }; 175} // End of anonymous namespace 176 177// Register this pass... 178char NoAA::ID = 0; 179static RegisterPass<NoAA> 180U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 181 182// Declare that we implement the AliasAnalysis interface 183static RegisterAnalysisGroup<AliasAnalysis> V(U); 184 185ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 186 187//===----------------------------------------------------------------------===// 188// BasicAA Pass 189//===----------------------------------------------------------------------===// 190 191namespace { 192 /// BasicAliasAnalysis - This is the default alias analysis implementation. 193 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 194 /// derives from the NoAA class. 195 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 196 static char ID; // Class identification, replacement for typeinfo 197 BasicAliasAnalysis() : NoAA(&ID) {} 198 AliasResult alias(const Value *V1, unsigned V1Size, 199 const Value *V2, unsigned V2Size); 200 201 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 202 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 203 204 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 205 /// non-escaping allocations. 206 virtual bool hasNoModRefInfoForCalls() const { return false; } 207 208 /// pointsToConstantMemory - Chase pointers until we find a (constant 209 /// global) or not. 210 bool pointsToConstantMemory(const Value *P); 211 212 private: 213 // CheckGEPInstructions - Check two GEP instructions with known 214 // must-aliasing base pointers. This checks to see if the index expressions 215 // preclude the pointers from aliasing... 216 AliasResult 217 CheckGEPInstructions(const Type* BasePtr1Ty, 218 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 219 const Type *BasePtr2Ty, 220 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 221 }; 222} // End of anonymous namespace 223 224// Register this pass... 225char BasicAliasAnalysis::ID = 0; 226static RegisterPass<BasicAliasAnalysis> 227X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 228 229// Declare that we implement the AliasAnalysis interface 230static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 231 232ImmutablePass *llvm::createBasicAliasAnalysisPass() { 233 return new BasicAliasAnalysis(); 234} 235 236 237/// pointsToConstantMemory - Chase pointers until we find a (constant 238/// global) or not. 239bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 240 if (const GlobalVariable *GV = 241 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 242 return GV->isConstant(); 243 return false; 244} 245 246 247// getModRefInfo - Check to see if the specified callsite can clobber the 248// specified memory object. Since we only look at local properties of this 249// function, we really can't say much about this query. We do, however, use 250// simple "address taken" analysis on local objects. 251// 252AliasAnalysis::ModRefResult 253BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 254 if (!isa<Constant>(P)) { 255 const Value *Object = P->getUnderlyingObject(); 256 257 // If this is a tail call and P points to a stack location, we know that 258 // the tail call cannot access or modify the local stack. 259 // We cannot exclude byval arguments here; these belong to the caller of 260 // the current function not to the current function, and a tail callee 261 // may reference them. 262 if (isa<AllocaInst>(Object)) 263 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 264 if (CI->isTailCall()) 265 return NoModRef; 266 267 // If the pointer is to a locally allocated object that does not escape, 268 // then the call can not mod/ref the pointer unless the call takes the 269 // argument without capturing it. 270 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) { 271 bool passedAsArg = false; 272 // TODO: Eventually only check 'nocapture' arguments. 273 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 274 CI != CE; ++CI) 275 if (isa<PointerType>((*CI)->getType()) && 276 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 277 passedAsArg = true; 278 279 if (!passedAsArg) 280 return NoModRef; 281 } 282 } 283 284 // The AliasAnalysis base class has some smarts, lets use them. 285 return AliasAnalysis::getModRefInfo(CS, P, Size); 286} 287 288 289AliasAnalysis::ModRefResult 290BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 291 // If CS1 or CS2 are readnone, they don't interact. 292 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 293 if (CS1B == DoesNotAccessMemory) return NoModRef; 294 295 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 296 if (CS2B == DoesNotAccessMemory) return NoModRef; 297 298 // If they both only read from memory, just return ref. 299 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 300 return Ref; 301 302 // Otherwise, fall back to NoAA (mod+ref). 303 return NoAA::getModRefInfo(CS1, CS2); 304} 305 306 307// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 308// as array references. 309// 310AliasAnalysis::AliasResult 311BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 312 const Value *V2, unsigned V2Size) { 313 // Strip off any constant expression casts if they exist 314 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 315 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 316 V1 = CE->getOperand(0); 317 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 318 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 319 V2 = CE->getOperand(0); 320 321 // Are we checking for alias of the same value? 322 if (V1 == V2) return MustAlias; 323 324 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 325 return NoAlias; // Scalars cannot alias each other 326 327 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 328 // pointer<->pointer bitcasts. 329 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 330 return alias(I->getOperand(0), V1Size, V2, V2Size); 331 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 332 return alias(V1, V1Size, I->getOperand(0), V2Size); 333 334 // Figure out what objects these things are pointing to if we can. 335 const Value *O1 = V1->getUnderlyingObject(); 336 const Value *O2 = V2->getUnderlyingObject(); 337 338 if (O1 != O2) { 339 // If V1/V2 point to two different objects we know that we have no alias. 340 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 341 return NoAlias; 342 343 // Arguments can't alias with local allocations or noalias calls. 344 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 345 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 346 return NoAlias; 347 348 // Most objects can't alias null. 349 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 350 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 351 return NoAlias; 352 } 353 354 // If the size of one access is larger than the entire object on the other 355 // side, then we know such behavior is undefined and can assume no alias. 356 const TargetData &TD = getTargetData(); 357 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 358 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 359 return NoAlias; 360 361 // If one pointer is the result of a call/invoke and the other is a 362 // non-escaping local object, then we know the object couldn't escape to a 363 // point where the call could return it. 364 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 365 isNonEscapingLocalObject(O2) && O1 != O2) 366 return NoAlias; 367 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 368 isNonEscapingLocalObject(O1) && O1 != O2) 369 return NoAlias; 370 371 // If we have two gep instructions with must-alias'ing base pointers, figure 372 // out if the indexes to the GEP tell us anything about the derived pointer. 373 // Note that we also handle chains of getelementptr instructions as well as 374 // constant expression getelementptrs here. 375 // 376 if (isGEP(V1) && isGEP(V2)) { 377 const User *GEP1 = cast<User>(V1); 378 const User *GEP2 = cast<User>(V2); 379 380 // If V1 and V2 are identical GEPs, just recurse down on both of them. 381 // This allows us to analyze things like: 382 // P = gep A, 0, i, 1 383 // Q = gep B, 0, i, 1 384 // by just analyzing A and B. This is even safe for variable indices. 385 if (GEP1->getType() == GEP2->getType() && 386 GEP1->getNumOperands() == GEP2->getNumOperands() && 387 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 388 // All operands are the same, ignoring the base. 389 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 390 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 391 392 393 // Drill down into the first non-gep value, to test for must-aliasing of 394 // the base pointers. 395 while (isGEP(GEP1->getOperand(0)) && 396 GEP1->getOperand(1) == 397 Constant::getNullValue(GEP1->getOperand(1)->getType())) 398 GEP1 = cast<User>(GEP1->getOperand(0)); 399 const Value *BasePtr1 = GEP1->getOperand(0); 400 401 while (isGEP(GEP2->getOperand(0)) && 402 GEP2->getOperand(1) == 403 Constant::getNullValue(GEP2->getOperand(1)->getType())) 404 GEP2 = cast<User>(GEP2->getOperand(0)); 405 const Value *BasePtr2 = GEP2->getOperand(0); 406 407 // Do the base pointers alias? 408 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 409 if (BaseAlias == NoAlias) return NoAlias; 410 if (BaseAlias == MustAlias) { 411 // If the base pointers alias each other exactly, check to see if we can 412 // figure out anything about the resultant pointers, to try to prove 413 // non-aliasing. 414 415 // Collect all of the chained GEP operands together into one simple place 416 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 417 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 418 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 419 420 // If GetGEPOperands were able to fold to the same must-aliased pointer, 421 // do the comparison. 422 if (BasePtr1 == BasePtr2) { 423 AliasResult GAlias = 424 CheckGEPInstructions(BasePtr1->getType(), 425 &GEP1Ops[0], GEP1Ops.size(), V1Size, 426 BasePtr2->getType(), 427 &GEP2Ops[0], GEP2Ops.size(), V2Size); 428 if (GAlias != MayAlias) 429 return GAlias; 430 } 431 } 432 } 433 434 // Check to see if these two pointers are related by a getelementptr 435 // instruction. If one pointer is a GEP with a non-zero index of the other 436 // pointer, we know they cannot alias. 437 // 438 if (isGEP(V2)) { 439 std::swap(V1, V2); 440 std::swap(V1Size, V2Size); 441 } 442 443 if (V1Size != ~0U && V2Size != ~0U) 444 if (isGEP(V1)) { 445 SmallVector<Value*, 16> GEPOperands; 446 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 447 448 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 449 if (R == MustAlias) { 450 // If there is at least one non-zero constant index, we know they cannot 451 // alias. 452 bool ConstantFound = false; 453 bool AllZerosFound = true; 454 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 455 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 456 if (!C->isNullValue()) { 457 ConstantFound = true; 458 AllZerosFound = false; 459 break; 460 } 461 } else { 462 AllZerosFound = false; 463 } 464 465 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 466 // the ptr, the end result is a must alias also. 467 if (AllZerosFound) 468 return MustAlias; 469 470 if (ConstantFound) { 471 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 472 return NoAlias; 473 474 // Otherwise we have to check to see that the distance is more than 475 // the size of the argument... build an index vector that is equal to 476 // the arguments provided, except substitute 0's for any variable 477 // indexes we find... 478 if (cast<PointerType>( 479 BasePtr->getType())->getElementType()->isSized()) { 480 for (unsigned i = 0; i != GEPOperands.size(); ++i) 481 if (!isa<ConstantInt>(GEPOperands[i])) 482 GEPOperands[i] = 483 Constant::getNullValue(GEPOperands[i]->getType()); 484 int64_t Offset = 485 getTargetData().getIndexedOffset(BasePtr->getType(), 486 &GEPOperands[0], 487 GEPOperands.size()); 488 489 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 490 return NoAlias; 491 } 492 } 493 } 494 } 495 496 return MayAlias; 497} 498 499// This function is used to determine if the indices of two GEP instructions are 500// equal. V1 and V2 are the indices. 501static bool IndexOperandsEqual(Value *V1, Value *V2) { 502 if (V1->getType() == V2->getType()) 503 return V1 == V2; 504 if (Constant *C1 = dyn_cast<Constant>(V1)) 505 if (Constant *C2 = dyn_cast<Constant>(V2)) { 506 // Sign extend the constants to long types, if necessary 507 if (C1->getType() != Type::Int64Ty) 508 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 509 if (C2->getType() != Type::Int64Ty) 510 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 511 return C1 == C2; 512 } 513 return false; 514} 515 516/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 517/// base pointers. This checks to see if the index expressions preclude the 518/// pointers from aliasing... 519AliasAnalysis::AliasResult 520BasicAliasAnalysis::CheckGEPInstructions( 521 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 522 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 523 // We currently can't handle the case when the base pointers have different 524 // primitive types. Since this is uncommon anyway, we are happy being 525 // extremely conservative. 526 if (BasePtr1Ty != BasePtr2Ty) 527 return MayAlias; 528 529 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 530 531 // Find the (possibly empty) initial sequence of equal values... which are not 532 // necessarily constants. 533 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 534 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 535 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 536 unsigned UnequalOper = 0; 537 while (UnequalOper != MinOperands && 538 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 539 // Advance through the type as we go... 540 ++UnequalOper; 541 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 542 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 543 else { 544 // If all operands equal each other, then the derived pointers must 545 // alias each other... 546 BasePtr1Ty = 0; 547 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 548 "Ran out of type nesting, but not out of operands?"); 549 return MustAlias; 550 } 551 } 552 553 // If we have seen all constant operands, and run out of indexes on one of the 554 // getelementptrs, check to see if the tail of the leftover one is all zeros. 555 // If so, return mustalias. 556 if (UnequalOper == MinOperands) { 557 if (NumGEP1Ops < NumGEP2Ops) { 558 std::swap(GEP1Ops, GEP2Ops); 559 std::swap(NumGEP1Ops, NumGEP2Ops); 560 } 561 562 bool AllAreZeros = true; 563 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 564 if (!isa<Constant>(GEP1Ops[i]) || 565 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 566 AllAreZeros = false; 567 break; 568 } 569 if (AllAreZeros) return MustAlias; 570 } 571 572 573 // So now we know that the indexes derived from the base pointers, 574 // which are known to alias, are different. We can still determine a 575 // no-alias result if there are differing constant pairs in the index 576 // chain. For example: 577 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 578 // 579 // We have to be careful here about array accesses. In particular, consider: 580 // A[1][0] vs A[0][i] 581 // In this case, we don't *know* that the array will be accessed in bounds: 582 // the index could even be negative. Because of this, we have to 583 // conservatively *give up* and return may alias. We disregard differing 584 // array subscripts that are followed by a variable index without going 585 // through a struct. 586 // 587 unsigned SizeMax = std::max(G1S, G2S); 588 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 589 590 // Scan for the first operand that is constant and unequal in the 591 // two getelementptrs... 592 unsigned FirstConstantOper = UnequalOper; 593 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 594 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 595 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 596 597 if (G1Oper != G2Oper) // Found non-equal constant indexes... 598 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 599 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 600 if (G1OC->getType() != G2OC->getType()) { 601 // Sign extend both operands to long. 602 if (G1OC->getType() != Type::Int64Ty) 603 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 604 if (G2OC->getType() != Type::Int64Ty) 605 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 606 GEP1Ops[FirstConstantOper] = G1OC; 607 GEP2Ops[FirstConstantOper] = G2OC; 608 } 609 610 if (G1OC != G2OC) { 611 // Handle the "be careful" case above: if this is an array/vector 612 // subscript, scan for a subsequent variable array index. 613 if (const SequentialType *STy = 614 dyn_cast<SequentialType>(BasePtr1Ty)) { 615 const Type *NextTy = STy; 616 bool isBadCase = false; 617 618 for (unsigned Idx = FirstConstantOper; 619 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 620 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 621 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 622 isBadCase = true; 623 break; 624 } 625 // If the array is indexed beyond the bounds of the static type 626 // at this level, it will also fall into the "be careful" case. 627 // It would theoretically be possible to analyze these cases, 628 // but for now just be conservatively correct. 629 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy)) 630 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 631 ATy->getNumElements() || 632 cast<ConstantInt>(G2OC)->getZExtValue() >= 633 ATy->getNumElements()) { 634 isBadCase = true; 635 break; 636 } 637 if (const VectorType *VTy = dyn_cast<VectorType>(STy)) 638 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 639 VTy->getNumElements() || 640 cast<ConstantInt>(G2OC)->getZExtValue() >= 641 VTy->getNumElements()) { 642 isBadCase = true; 643 break; 644 } 645 STy = cast<SequentialType>(NextTy); 646 NextTy = cast<SequentialType>(NextTy)->getElementType(); 647 } 648 649 if (isBadCase) G1OC = 0; 650 } 651 652 // Make sure they are comparable (ie, not constant expressions), and 653 // make sure the GEP with the smaller leading constant is GEP1. 654 if (G1OC) { 655 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 656 G1OC, G2OC); 657 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 658 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 659 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 660 std::swap(NumGEP1Ops, NumGEP2Ops); 661 } 662 break; 663 } 664 } 665 } 666 } 667 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 668 } 669 670 // No shared constant operands, and we ran out of common operands. At this 671 // point, the GEP instructions have run through all of their operands, and we 672 // haven't found evidence that there are any deltas between the GEP's. 673 // However, one GEP may have more operands than the other. If this is the 674 // case, there may still be hope. Check this now. 675 if (FirstConstantOper == MinOperands) { 676 // Make GEP1Ops be the longer one if there is a longer one. 677 if (NumGEP1Ops < NumGEP2Ops) { 678 std::swap(GEP1Ops, GEP2Ops); 679 std::swap(NumGEP1Ops, NumGEP2Ops); 680 } 681 682 // Is there anything to check? 683 if (NumGEP1Ops > MinOperands) { 684 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 685 if (isa<ConstantInt>(GEP1Ops[i]) && 686 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 687 // Yup, there's a constant in the tail. Set all variables to 688 // constants in the GEP instruction to make it suitable for 689 // TargetData::getIndexedOffset. 690 for (i = 0; i != MaxOperands; ++i) 691 if (!isa<ConstantInt>(GEP1Ops[i])) 692 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 693 // Okay, now get the offset. This is the relative offset for the full 694 // instruction. 695 const TargetData &TD = getTargetData(); 696 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 697 NumGEP1Ops); 698 699 // Now check without any constants at the end. 700 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 701 MinOperands); 702 703 // Make sure we compare the absolute difference. 704 if (Offset1 > Offset2) 705 std::swap(Offset1, Offset2); 706 707 // If the tail provided a bit enough offset, return noalias! 708 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 709 return NoAlias; 710 // Otherwise break - we don't look for another constant in the tail. 711 break; 712 } 713 } 714 715 // Couldn't find anything useful. 716 return MayAlias; 717 } 718 719 // If there are non-equal constants arguments, then we can figure 720 // out a minimum known delta between the two index expressions... at 721 // this point we know that the first constant index of GEP1 is less 722 // than the first constant index of GEP2. 723 724 // Advance BasePtr[12]Ty over this first differing constant operand. 725 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 726 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 727 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 728 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 729 730 // We are going to be using TargetData::getIndexedOffset to determine the 731 // offset that each of the GEP's is reaching. To do this, we have to convert 732 // all variable references to constant references. To do this, we convert the 733 // initial sequence of array subscripts into constant zeros to start with. 734 const Type *ZeroIdxTy = GEPPointerTy; 735 for (unsigned i = 0; i != FirstConstantOper; ++i) { 736 if (!isa<StructType>(ZeroIdxTy)) 737 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 738 739 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 740 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 741 } 742 743 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 744 745 // Loop over the rest of the operands... 746 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 747 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 748 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 749 // If they are equal, use a zero index... 750 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 751 if (!isa<ConstantInt>(Op1)) 752 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 753 // Otherwise, just keep the constants we have. 754 } else { 755 if (Op1) { 756 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 757 // If this is an array index, make sure the array element is in range. 758 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 759 if (Op1C->getZExtValue() >= AT->getNumElements()) 760 return MayAlias; // Be conservative with out-of-range accesses 761 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 762 if (Op1C->getZExtValue() >= VT->getNumElements()) 763 return MayAlias; // Be conservative with out-of-range accesses 764 } 765 766 } else { 767 // GEP1 is known to produce a value less than GEP2. To be 768 // conservatively correct, we must assume the largest possible 769 // constant is used in this position. This cannot be the initial 770 // index to the GEP instructions (because we know we have at least one 771 // element before this one with the different constant arguments), so 772 // we know that the current index must be into either a struct or 773 // array. Because we know it's not constant, this cannot be a 774 // structure index. Because of this, we can calculate the maximum 775 // value possible. 776 // 777 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 778 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 779 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 780 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 781 } 782 } 783 784 if (Op2) { 785 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 786 // If this is an array index, make sure the array element is in range. 787 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 788 if (Op2C->getZExtValue() >= AT->getNumElements()) 789 return MayAlias; // Be conservative with out-of-range accesses 790 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 791 if (Op2C->getZExtValue() >= VT->getNumElements()) 792 return MayAlias; // Be conservative with out-of-range accesses 793 } 794 } else { // Conservatively assume the minimum value for this index 795 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 796 } 797 } 798 } 799 800 if (BasePtr1Ty && Op1) { 801 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 802 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 803 else 804 BasePtr1Ty = 0; 805 } 806 807 if (BasePtr2Ty && Op2) { 808 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 809 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 810 else 811 BasePtr2Ty = 0; 812 } 813 } 814 815 if (GEPPointerTy->getElementType()->isSized()) { 816 int64_t Offset1 = 817 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 818 int64_t Offset2 = 819 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 820 assert(Offset1 != Offset2 && 821 "There is at least one different constant here!"); 822 823 // Make sure we compare the absolute difference. 824 if (Offset1 > Offset2) 825 std::swap(Offset1, Offset2); 826 827 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 828 //cerr << "Determined that these two GEP's don't alias [" 829 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 830 return NoAlias; 831 } 832 } 833 return MayAlias; 834} 835 836// Make sure that anything that uses AliasAnalysis pulls in this file... 837DEFINING_FILE_FOR(BasicAliasAnalysis) 838