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> 35 using namespace llvm; 36 37 //===----------------------------------------------------------------------===// 38 // Useful predicates 39 //===----------------------------------------------------------------------===// 40 41 static const GEPOperator *isGEP(const Value *V) { 42 return dyn_cast<GEPOperator>(V); 43 } 44 45 static 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. 67 static 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. 83 static 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. 104 static 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 //===----------------------------------------------------------------------===// 131 132 namespace { 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 } 145 146 virtual void initializePass() { 147 TD = getAnalysisIfAvailable<TargetData>(); 148 } 149 150 virtual AliasResult alias(const Value *V1, unsigned V1Size, 151 const Value *V2, unsigned V2Size) { 152 return MayAlias; 153 } 154 155 virtual void getArgumentAccesses(Function *F, CallSite CS, 156 std::vector<PointerAccessInfo> &Info) { 157 llvm_unreachable("This method may not be called on this function!"); 158 } 159 160 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 161 virtual bool pointsToConstantMemory(const Value *P) { return false; } 162 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 163 return ModRef; 164 } 165 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 166 return ModRef; 167 } 168 virtual bool hasNoModRefInfoForCalls() const { return true; } 169 170 virtual void deleteValue(Value *V) {} 171 virtual void copyValue(Value *From, Value *To) {} 172 }; 173 } // End of anonymous namespace 174 175 // Register this pass... 176 char NoAA::ID = 0; 177 static RegisterPass<NoAA> 178 U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 179 180 // Declare that we implement the AliasAnalysis interface 181 static RegisterAnalysisGroup<AliasAnalysis> V(U); 182 183 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 184 185 //===----------------------------------------------------------------------===// 186 // BasicAA Pass 187 //===----------------------------------------------------------------------===// 188 189 namespace { 190 /// BasicAliasAnalysis - This is the default alias analysis implementation. 191 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 192 /// derives from the NoAA class. 193 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 194 static char ID; // Class identification, replacement for typeinfo 195 BasicAliasAnalysis() : NoAA(&ID) {} 196 AliasResult alias(const Value *V1, unsigned V1Size, 197 const Value *V2, unsigned V2Size); 198 199 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 200 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 201 202 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 203 /// non-escaping allocations. 204 virtual bool hasNoModRefInfoForCalls() const { return false; } 205 206 /// pointsToConstantMemory - Chase pointers until we find a (constant 207 /// global) or not. 208 bool pointsToConstantMemory(const Value *P); 209 210 private: 211 // CheckGEPInstructions - Check two GEP instructions with known 212 // must-aliasing base pointers. This checks to see if the index expressions 213 // preclude the pointers from aliasing... 214 AliasResult 215 CheckGEPInstructions(const Type* BasePtr1Ty, 216 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 217 const Type *BasePtr2Ty, 218 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 219 }; 220 } // End of anonymous namespace 221 222 // Register this pass... 223 char BasicAliasAnalysis::ID = 0; 224 static RegisterPass<BasicAliasAnalysis> 225 X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 226 227 // Declare that we implement the AliasAnalysis interface 228 static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 229 230 ImmutablePass *llvm::createBasicAliasAnalysisPass() { 231 return new BasicAliasAnalysis(); 232 } 233 234 235 /// pointsToConstantMemory - Chase pointers until we find a (constant 236 /// global) or not. 237 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 238 if (const GlobalVariable *GV = 239 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 240 return GV->isConstant(); 241 return false; 242 } 243 244 245 // getModRefInfo - Check to see if the specified callsite can clobber the 246 // specified memory object. Since we only look at local properties of this 247 // function, we really can't say much about this query. We do, however, use 248 // simple "address taken" analysis on local objects. 249 // 250 AliasAnalysis::ModRefResult 251 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 252 if (!isa<Constant>(P)) { 253 const Value *Object = P->getUnderlyingObject(); 254 255 // If this is a tail call and P points to a stack location, we know that 256 // the tail call cannot access or modify the local stack. 257 // We cannot exclude byval arguments here; these belong to the caller of 258 // the current function not to the current function, and a tail callee 259 // may reference them. 260 if (isa<AllocaInst>(Object)) 261 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 262 if (CI->isTailCall()) 263 return NoModRef; 264 265 // If the pointer is to a locally allocated object that does not escape, 266 // then the call can not mod/ref the pointer unless the call takes the 267 // argument without capturing it. 268 if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) { 269 bool passedAsArg = false; 270 // TODO: Eventually only check 'nocapture' arguments. 271 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 272 CI != CE; ++CI) 273 if (isa<PointerType>((*CI)->getType()) && 274 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 275 passedAsArg = true; 276 277 if (!passedAsArg) 278 return NoModRef; 279 } 280 } 281 282 // The AliasAnalysis base class has some smarts, lets use them. 283 return AliasAnalysis::getModRefInfo(CS, P, Size); 284 } 285 286 287 AliasAnalysis::ModRefResult 288 BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 289 // If CS1 or CS2 are readnone, they don't interact. 290 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 291 if (CS1B == DoesNotAccessMemory) return NoModRef; 292 293 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 294 if (CS2B == DoesNotAccessMemory) return NoModRef; 295 296 // If they both only read from memory, just return ref. 297 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 298 return Ref; 299 300 // Otherwise, fall back to NoAA (mod+ref). 301 return NoAA::getModRefInfo(CS1, CS2); 302 } 303 304 305 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 306 // as array references. 307 // 308 AliasAnalysis::AliasResult 309 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 310 const Value *V2, unsigned V2Size) { 311 // Strip off any constant expression casts if they exist 312 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 313 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 314 V1 = CE->getOperand(0); 315 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 316 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 317 V2 = CE->getOperand(0); 318 319 // Are we checking for alias of the same value? 320 if (V1 == V2) return MustAlias; 321 322 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 323 return NoAlias; // Scalars cannot alias each other 324 325 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 326 // pointer<->pointer bitcasts. 327 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 328 return alias(I->getOperand(0), V1Size, V2, V2Size); 329 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 330 return alias(V1, V1Size, I->getOperand(0), V2Size); 331 332 // Figure out what objects these things are pointing to if we can. 333 const Value *O1 = V1->getUnderlyingObject(); 334 const Value *O2 = V2->getUnderlyingObject(); 335 336 if (O1 != O2) { 337 // If V1/V2 point to two different objects we know that we have no alias. 338 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 339 return NoAlias; 340 341 // Arguments can't alias with local allocations or noalias calls. 342 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 343 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 344 return NoAlias; 345 346 // Most objects can't alias null. 347 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 348 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 349 return NoAlias; 350 } 351 352 // If the size of one access is larger than the entire object on the other 353 // side, then we know such behavior is undefined and can assume no alias. 354 if (TD) 355 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) || 356 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD))) 357 return NoAlias; 358 359 // If one pointer is the result of a call/invoke and the other is a 360 // non-escaping local object, then we know the object couldn't escape to a 361 // point where the call could return it. 362 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 363 isNonEscapingLocalObject(O2) && O1 != O2) 364 return NoAlias; 365 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 366 isNonEscapingLocalObject(O1) && O1 != O2) 367 return NoAlias; 368 369 // If we have two gep instructions with must-alias'ing base pointers, figure 370 // out if the indexes to the GEP tell us anything about the derived pointer. 371 // Note that we also handle chains of getelementptr instructions as well as 372 // constant expression getelementptrs here. 373 // 374 if (isGEP(V1) && isGEP(V2)) { 375 const User *GEP1 = cast<User>(V1); 376 const User *GEP2 = cast<User>(V2); 377 378 // If V1 and V2 are identical GEPs, just recurse down on both of them. 379 // This allows us to analyze things like: 380 // P = gep A, 0, i, 1 381 // Q = gep B, 0, i, 1 382 // by just analyzing A and B. This is even safe for variable indices. 383 if (GEP1->getType() == GEP2->getType() && 384 GEP1->getNumOperands() == GEP2->getNumOperands() && 385 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 386 // All operands are the same, ignoring the base. 387 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 388 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 389 390 391 // Drill down into the first non-gep value, to test for must-aliasing of 392 // the base pointers. 393 while (isGEP(GEP1->getOperand(0)) && 394 GEP1->getOperand(1) == 395 Constant::getNullValue(GEP1->getOperand(1)->getType())) 396 GEP1 = cast<User>(GEP1->getOperand(0)); 397 const Value *BasePtr1 = GEP1->getOperand(0); 398 399 while (isGEP(GEP2->getOperand(0)) && 400 GEP2->getOperand(1) == 401 Constant::getNullValue(GEP2->getOperand(1)->getType())) 402 GEP2 = cast<User>(GEP2->getOperand(0)); 403 const Value *BasePtr2 = GEP2->getOperand(0); 404 405 // Do the base pointers alias? 406 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 407 if (BaseAlias == NoAlias) return NoAlias; 408 if (BaseAlias == MustAlias) { 409 // If the base pointers alias each other exactly, check to see if we can 410 // figure out anything about the resultant pointers, to try to prove 411 // non-aliasing. 412 413 // Collect all of the chained GEP operands together into one simple place 414 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 415 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 416 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 417 418 // If GetGEPOperands were able to fold to the same must-aliased pointer, 419 // do the comparison. 420 if (BasePtr1 == BasePtr2) { 421 AliasResult GAlias = 422 CheckGEPInstructions(BasePtr1->getType(), 423 &GEP1Ops[0], GEP1Ops.size(), V1Size, 424 BasePtr2->getType(), 425 &GEP2Ops[0], GEP2Ops.size(), V2Size); 426 if (GAlias != MayAlias) 427 return GAlias; 428 } 429 } 430 } 431 432 // Check to see if these two pointers are related by a getelementptr 433 // instruction. If one pointer is a GEP with a non-zero index of the other 434 // pointer, we know they cannot alias. 435 // 436 if (isGEP(V2)) { 437 std::swap(V1, V2); 438 std::swap(V1Size, V2Size); 439 } 440 441 if (V1Size != ~0U && V2Size != ~0U) 442 if (isGEP(V1)) { 443 SmallVector<Value*, 16> GEPOperands; 444 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 445 446 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 447 if (R == MustAlias) { 448 // If there is at least one non-zero constant index, we know they cannot 449 // alias. 450 bool ConstantFound = false; 451 bool AllZerosFound = true; 452 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 453 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 454 if (!C->isNullValue()) { 455 ConstantFound = true; 456 AllZerosFound = false; 457 break; 458 } 459 } else { 460 AllZerosFound = false; 461 } 462 463 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 464 // the ptr, the end result is a must alias also. 465 if (AllZerosFound) 466 return MustAlias; 467 468 if (ConstantFound) { 469 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 470 return NoAlias; 471 472 // Otherwise we have to check to see that the distance is more than 473 // the size of the argument... build an index vector that is equal to 474 // the arguments provided, except substitute 0's for any variable 475 // indexes we find... 476 if (TD && cast<PointerType>( 477 BasePtr->getType())->getElementType()->isSized()) { 478 for (unsigned i = 0; i != GEPOperands.size(); ++i) 479 if (!isa<ConstantInt>(GEPOperands[i])) 480 GEPOperands[i] = 481 Constant::getNullValue(GEPOperands[i]->getType()); 482 int64_t Offset = 483 TD->getIndexedOffset(BasePtr->getType(), 484 &GEPOperands[0], 485 GEPOperands.size()); 486 487 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 488 return NoAlias; 489 } 490 } 491 } 492 } 493 494 return MayAlias; 495 } 496 497 // This function is used to determine if the indices of two GEP instructions are 498 // equal. V1 and V2 are the indices. 499 static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) { 500 if (V1->getType() == V2->getType()) 501 return V1 == V2; 502 if (Constant *C1 = dyn_cast<Constant>(V1)) 503 if (Constant *C2 = dyn_cast<Constant>(V2)) { 504 // Sign extend the constants to long types, if necessary 505 if (C1->getType() != Type::getInt64Ty(Context)) 506 C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context)); 507 if (C2->getType() != Type::getInt64Ty(Context)) 508 C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context)); 509 return C1 == C2; 510 } 511 return false; 512 } 513 514 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 515 /// base pointers. This checks to see if the index expressions preclude the 516 /// pointers from aliasing... 517 AliasAnalysis::AliasResult 518 BasicAliasAnalysis::CheckGEPInstructions( 519 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 520 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 521 // We currently can't handle the case when the base pointers have different 522 // primitive types. Since this is uncommon anyway, we are happy being 523 // extremely conservative. 524 if (BasePtr1Ty != BasePtr2Ty) 525 return MayAlias; 526 527 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 528 529 LLVMContext &Context = GEPPointerTy->getContext(); 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 Context)) { 540 // Advance through the type as we go... 541 ++UnequalOper; 542 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 543 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 544 else { 545 // If all operands equal each other, then the derived pointers must 546 // alias each other... 547 BasePtr1Ty = 0; 548 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 549 "Ran out of type nesting, but not out of operands?"); 550 return MustAlias; 551 } 552 } 553 554 // If we have seen all constant operands, and run out of indexes on one of the 555 // getelementptrs, check to see if the tail of the leftover one is all zeros. 556 // If so, return mustalias. 557 if (UnequalOper == MinOperands) { 558 if (NumGEP1Ops < NumGEP2Ops) { 559 std::swap(GEP1Ops, GEP2Ops); 560 std::swap(NumGEP1Ops, NumGEP2Ops); 561 } 562 563 bool AllAreZeros = true; 564 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 565 if (!isa<Constant>(GEP1Ops[i]) || 566 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 567 AllAreZeros = false; 568 break; 569 } 570 if (AllAreZeros) return MustAlias; 571 } 572 573 574 // So now we know that the indexes derived from the base pointers, 575 // which are known to alias, are different. We can still determine a 576 // no-alias result if there are differing constant pairs in the index 577 // chain. For example: 578 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 579 // 580 // We have to be careful here about array accesses. In particular, consider: 581 // A[1][0] vs A[0][i] 582 // In this case, we don't *know* that the array will be accessed in bounds: 583 // the index could even be negative. Because of this, we have to 584 // conservatively *give up* and return may alias. We disregard differing 585 // array subscripts that are followed by a variable index without going 586 // through a struct. 587 // 588 unsigned SizeMax = std::max(G1S, G2S); 589 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 590 591 // Scan for the first operand that is constant and unequal in the 592 // two getelementptrs... 593 unsigned FirstConstantOper = UnequalOper; 594 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 595 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 596 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 597 598 if (G1Oper != G2Oper) // Found non-equal constant indexes... 599 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 600 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 601 if (G1OC->getType() != G2OC->getType()) { 602 // Sign extend both operands to long. 603 if (G1OC->getType() != Type::getInt64Ty(Context)) 604 G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context)); 605 if (G2OC->getType() != Type::getInt64Ty(Context)) 606 G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context)); 607 GEP1Ops[FirstConstantOper] = G1OC; 608 GEP2Ops[FirstConstantOper] = G2OC; 609 } 610 611 if (G1OC != G2OC) { 612 // Handle the "be careful" case above: if this is an array/vector 613 // subscript, scan for a subsequent variable array index. 614 if (const SequentialType *STy = 615 dyn_cast<SequentialType>(BasePtr1Ty)) { 616 const Type *NextTy = STy; 617 bool isBadCase = false; 618 619 for (unsigned Idx = FirstConstantOper; 620 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 621 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 622 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 623 isBadCase = true; 624 break; 625 } 626 // If the array is indexed beyond the bounds of the static type 627 // at this level, it will also fall into the "be careful" case. 628 // It would theoretically be possible to analyze these cases, 629 // but for now just be conservatively correct. 630 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy)) 631 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 632 ATy->getNumElements() || 633 cast<ConstantInt>(G2OC)->getZExtValue() >= 634 ATy->getNumElements()) { 635 isBadCase = true; 636 break; 637 } 638 if (const VectorType *VTy = dyn_cast<VectorType>(STy)) 639 if (cast<ConstantInt>(G1OC)->getZExtValue() >= 640 VTy->getNumElements() || 641 cast<ConstantInt>(G2OC)->getZExtValue() >= 642 VTy->getNumElements()) { 643 isBadCase = true; 644 break; 645 } 646 STy = cast<SequentialType>(NextTy); 647 NextTy = cast<SequentialType>(NextTy)->getElementType(); 648 } 649 650 if (isBadCase) G1OC = 0; 651 } 652 653 // Make sure they are comparable (ie, not constant expressions), and 654 // make sure the GEP with the smaller leading constant is GEP1. 655 if (G1OC) { 656 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 657 G1OC, G2OC); 658 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 659 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 660 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 661 std::swap(NumGEP1Ops, NumGEP2Ops); 662 } 663 break; 664 } 665 } 666 } 667 } 668 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 669 } 670 671 // No shared constant operands, and we ran out of common operands. At this 672 // point, the GEP instructions have run through all of their operands, and we 673 // haven't found evidence that there are any deltas between the GEP's. 674 // However, one GEP may have more operands than the other. If this is the 675 // case, there may still be hope. Check this now. 676 if (FirstConstantOper == MinOperands) { 677 // Without TargetData, we won't know what the offsets are. 678 if (!TD) 679 return MayAlias; 680 681 // Make GEP1Ops be the longer one if there is a longer one. 682 if (NumGEP1Ops < NumGEP2Ops) { 683 std::swap(GEP1Ops, GEP2Ops); 684 std::swap(NumGEP1Ops, NumGEP2Ops); 685 } 686 687 // Is there anything to check? 688 if (NumGEP1Ops > MinOperands) { 689 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 690 if (isa<ConstantInt>(GEP1Ops[i]) && 691 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 692 // Yup, there's a constant in the tail. Set all variables to 693 // constants in the GEP instruction to make it suitable for 694 // TargetData::getIndexedOffset. 695 for (i = 0; i != MaxOperands; ++i) 696 if (!isa<ConstantInt>(GEP1Ops[i])) 697 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 698 // Okay, now get the offset. This is the relative offset for the full 699 // instruction. 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] = 742 Constant::getNullValue(Type::getInt32Ty(Context)); 743 744 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 745 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 746 } 747 748 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 749 750 // Loop over the rest of the operands... 751 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 752 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 753 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 754 // If they are equal, use a zero index... 755 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 756 if (!isa<ConstantInt>(Op1)) 757 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 758 // Otherwise, just keep the constants we have. 759 } else { 760 if (Op1) { 761 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 762 // If this is an array index, make sure the array element is in range. 763 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 764 if (Op1C->getZExtValue() >= AT->getNumElements()) 765 return MayAlias; // Be conservative with out-of-range accesses 766 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 767 if (Op1C->getZExtValue() >= VT->getNumElements()) 768 return MayAlias; // Be conservative with out-of-range accesses 769 } 770 771 } else { 772 // GEP1 is known to produce a value less than GEP2. To be 773 // conservatively correct, we must assume the largest possible 774 // constant is used in this position. This cannot be the initial 775 // index to the GEP instructions (because we know we have at least one 776 // element before this one with the different constant arguments), so 777 // we know that the current index must be into either a struct or 778 // array. Because we know it's not constant, this cannot be a 779 // structure index. Because of this, we can calculate the maximum 780 // value possible. 781 // 782 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 783 GEP1Ops[i] = 784 ConstantInt::get(Type::getInt64Ty(Context), 785 AT->getNumElements()-1); 786 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 787 GEP1Ops[i] = 788 ConstantInt::get(Type::getInt64Ty(Context), 789 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] = Constant::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 (TD && GEPPointerTy->getElementType()->isSized()) { 825 int64_t Offset1 = 826 TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 827 int64_t Offset2 = 828 TD->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... 846 DEFINING_FILE_FOR(BasicAliasAnalysis) 847