BasicAliasAnalysis.cpp revision 8556d2a7f155c7edfaf454a3acda8ce28863c5e4
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 "llvm/Support/ManagedStatic.h" 32#include <algorithm> 33using namespace llvm; 34 35//===----------------------------------------------------------------------===// 36// Useful predicates 37//===----------------------------------------------------------------------===// 38 39static const User *isGEP(const Value *V) { 40 if (isa<GetElementPtrInst>(V) || 41 (isa<ConstantExpr>(V) && 42 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 43 return cast<User>(V); 44 return 0; 45} 46 47static const Value *GetGEPOperands(const Value *V, 48 SmallVector<Value*, 16> &GEPOps) { 49 assert(GEPOps.empty() && "Expect empty list to populate!"); 50 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 51 cast<User>(V)->op_end()); 52 53 // Accumulate all of the chained indexes into the operand array 54 V = cast<User>(V)->getOperand(0); 55 56 while (const User *G = isGEP(V)) { 57 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 58 !cast<Constant>(GEPOps[0])->isNullValue()) 59 break; // Don't handle folding arbitrary pointer offsets yet... 60 GEPOps.erase(GEPOps.begin()); // Drop the zero index 61 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 62 V = G->getOperand(0); 63 } 64 return V; 65} 66 67/// isNoAliasCall - Return true if this pointer is returned by a noalias 68/// function. 69static bool isNoAliasCall(const Value *V) { 70 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 71 return CallSite(const_cast<Instruction*>(cast<Instruction>(V))) 72 .paramHasAttr(0, Attribute::NoAlias); 73 return false; 74} 75 76/// isIdentifiedObject - Return true if this pointer refers to a distinct and 77/// identifiable object. This returns true for: 78/// Global Variables and Functions 79/// Allocas and Mallocs 80/// ByVal and NoAlias Arguments 81/// NoAlias returns 82/// 83static bool isIdentifiedObject(const Value *V) { 84 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V)) 85 return true; 86 if (const Argument *A = dyn_cast<Argument>(V)) 87 return A->hasNoAliasAttr() || A->hasByValAttr(); 88 return false; 89} 90 91/// isKnownNonNull - Return true if we know that the specified value is never 92/// null. 93static bool isKnownNonNull(const Value *V) { 94 // Alloca never returns null, malloc might. 95 if (isa<AllocaInst>(V)) return true; 96 97 // A byval argument is never null. 98 if (const Argument *A = dyn_cast<Argument>(V)) 99 return A->hasByValAttr(); 100 101 // Global values are not null unless extern weak. 102 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 103 return !GV->hasExternalWeakLinkage(); 104 return false; 105} 106 107/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 108/// object that never escapes from the function. 109static bool isNonEscapingLocalObject(const Value *V) { 110 // If this is a local allocation, check to see if it escapes. 111 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 112 return !PointerMayBeCaptured(V, false); 113 114 // If this is an argument that corresponds to a byval or noalias argument, 115 // then it has not escaped before entering the function. Check if it escapes 116 // inside the function. 117 if (const Argument *A = dyn_cast<Argument>(V)) 118 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 119 // Don't bother analyzing arguments already known not to escape. 120 if (A->hasNoCaptureAttr()) 121 return true; 122 return !PointerMayBeCaptured(V, false); 123 } 124 return false; 125} 126 127 128/// isObjectSmallerThan - Return true if we can prove that the object specified 129/// by V is smaller than Size. 130static bool isObjectSmallerThan(const Value *V, unsigned Size, 131 const TargetData &TD) { 132 const Type *AccessTy; 133 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 134 AccessTy = GV->getType()->getElementType(); 135 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 136 if (!AI->isArrayAllocation()) 137 AccessTy = AI->getType()->getElementType(); 138 else 139 return false; 140 } else if (const Argument *A = dyn_cast<Argument>(V)) { 141 if (A->hasByValAttr()) 142 AccessTy = cast<PointerType>(A->getType())->getElementType(); 143 else 144 return false; 145 } else { 146 return false; 147 } 148 149 if (AccessTy->isSized()) 150 return TD.getTypePaddedSize(AccessTy) < Size; 151 return false; 152} 153 154//===----------------------------------------------------------------------===// 155// NoAA Pass 156//===----------------------------------------------------------------------===// 157 158namespace { 159 /// NoAA - This class implements the -no-aa pass, which always returns "I 160 /// don't know" for alias queries. NoAA is unlike other alias analysis 161 /// implementations, in that it does not chain to a previous analysis. As 162 /// such it doesn't follow many of the rules that other alias analyses must. 163 /// 164 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 165 static char ID; // Class identification, replacement for typeinfo 166 NoAA() : ImmutablePass(&ID) {} 167 explicit NoAA(void *PID) : ImmutablePass(PID) { } 168 169 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 170 AU.addRequired<TargetData>(); 171 } 172 173 virtual void initializePass() { 174 TD = &getAnalysis<TargetData>(); 175 } 176 177 virtual AliasResult alias(const Value *V1, unsigned V1Size, 178 const Value *V2, unsigned V2Size) { 179 return MayAlias; 180 } 181 182 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS, 183 std::vector<PointerAccessInfo> *Info) { 184 return UnknownModRefBehavior; 185 } 186 187 virtual void getArgumentAccesses(Function *F, CallSite CS, 188 std::vector<PointerAccessInfo> &Info) { 189 assert(0 && "This method may not be called on this function!"); 190 } 191 192 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 193 virtual bool pointsToConstantMemory(const Value *P) { return false; } 194 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 195 return ModRef; 196 } 197 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 198 return ModRef; 199 } 200 virtual bool hasNoModRefInfoForCalls() const { return true; } 201 202 virtual void deleteValue(Value *V) {} 203 virtual void copyValue(Value *From, Value *To) {} 204 }; 205} // End of anonymous namespace 206 207// Register this pass... 208char NoAA::ID = 0; 209static RegisterPass<NoAA> 210U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 211 212// Declare that we implement the AliasAnalysis interface 213static RegisterAnalysisGroup<AliasAnalysis> V(U); 214 215ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 216 217//===----------------------------------------------------------------------===// 218// BasicAA Pass 219//===----------------------------------------------------------------------===// 220 221namespace { 222 /// BasicAliasAnalysis - This is the default alias analysis implementation. 223 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 224 /// derives from the NoAA class. 225 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 226 static char ID; // Class identification, replacement for typeinfo 227 BasicAliasAnalysis() : NoAA(&ID) {} 228 AliasResult alias(const Value *V1, unsigned V1Size, 229 const Value *V2, unsigned V2Size); 230 231 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 232 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 233 234 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 235 /// non-escaping allocations. 236 virtual bool hasNoModRefInfoForCalls() const { return false; } 237 238 /// pointsToConstantMemory - Chase pointers until we find a (constant 239 /// global) or not. 240 bool pointsToConstantMemory(const Value *P); 241 242 private: 243 // CheckGEPInstructions - Check two GEP instructions with known 244 // must-aliasing base pointers. This checks to see if the index expressions 245 // preclude the pointers from aliasing... 246 AliasResult 247 CheckGEPInstructions(const Type* BasePtr1Ty, 248 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 249 const Type *BasePtr2Ty, 250 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 251 }; 252} // End of anonymous namespace 253 254// Register this pass... 255char BasicAliasAnalysis::ID = 0; 256static RegisterPass<BasicAliasAnalysis> 257X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 258 259// Declare that we implement the AliasAnalysis interface 260static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 261 262ImmutablePass *llvm::createBasicAliasAnalysisPass() { 263 return new BasicAliasAnalysis(); 264} 265 266 267/// pointsToConstantMemory - Chase pointers until we find a (constant 268/// global) or not. 269bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 270 if (const GlobalVariable *GV = 271 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 272 return GV->isConstant(); 273 return false; 274} 275 276// getModRefInfo - Check to see if the specified callsite can clobber the 277// specified memory object. Since we only look at local properties of this 278// function, we really can't say much about this query. We do, however, use 279// simple "address taken" analysis on local objects. 280// 281AliasAnalysis::ModRefResult 282BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 283 if (!isa<Constant>(P)) { 284 const Value *Object = P->getUnderlyingObject(); 285 286 // If this is a tail call and P points to a stack location, we know that 287 // the tail call cannot access or modify the local stack. 288 // We cannot exclude byval arguments here; these belong to the caller of 289 // the current function not to the current function, and a tail callee 290 // may reference them. 291 if (isa<AllocaInst>(Object)) 292 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 293 if (CI->isTailCall()) 294 return NoModRef; 295 296 // If the pointer is to a locally allocated object that does not escape, 297 // then the call can not mod/ref the pointer unless the call takes the 298 // argument without capturing it. 299 if (isNonEscapingLocalObject(Object)) { 300 bool passedAsArg = false; 301 // TODO: Eventually only check 'nocapture' arguments. 302 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 303 CI != CE; ++CI) 304 if (isa<PointerType>((*CI)->getType()) && 305 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 306 passedAsArg = true; 307 308 if (!passedAsArg) 309 return NoModRef; 310 } 311 } 312 313 // The AliasAnalysis base class has some smarts, lets use them. 314 return AliasAnalysis::getModRefInfo(CS, P, Size); 315} 316 317 318AliasAnalysis::ModRefResult 319BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 320 // If CS1 or CS2 are readnone, they don't interact. 321 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 322 if (CS1B == DoesNotAccessMemory) return NoModRef; 323 324 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 325 if (CS2B == DoesNotAccessMemory) return NoModRef; 326 327 // If they both only read from memory, just return ref. 328 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 329 return Ref; 330 331 // Otherwise, fall back to NoAA (mod+ref). 332 return NoAA::getModRefInfo(CS1, CS2); 333} 334 335 336// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 337// as array references. 338// 339AliasAnalysis::AliasResult 340BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 341 const Value *V2, unsigned V2Size) { 342 // Strip off any constant expression casts if they exist 343 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 344 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 345 V1 = CE->getOperand(0); 346 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 347 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 348 V2 = CE->getOperand(0); 349 350 // Are we checking for alias of the same value? 351 if (V1 == V2) return MustAlias; 352 353 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 354 return NoAlias; // Scalars cannot alias each other 355 356 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 357 // pointer<->pointer bitcasts. 358 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 359 return alias(I->getOperand(0), V1Size, V2, V2Size); 360 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 361 return alias(V1, V1Size, I->getOperand(0), V2Size); 362 363 // Figure out what objects these things are pointing to if we can. 364 const Value *O1 = V1->getUnderlyingObject(); 365 const Value *O2 = V2->getUnderlyingObject(); 366 367 if (O1 != O2) { 368 // If V1/V2 point to two different objects we know that we have no alias. 369 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 370 return NoAlias; 371 372 // Arguments can't alias with local allocations or noalias calls. 373 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 374 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 375 return NoAlias; 376 377 // Most objects can't alias null. 378 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 379 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 380 return NoAlias; 381 } 382 383 // If the size of one access is larger than the entire object on the other 384 // side, then we know such behavior is undefined and can assume no alias. 385 const TargetData &TD = getTargetData(); 386 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 387 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 388 return NoAlias; 389 390 // If one pointer is the result of a call/invoke and the other is a 391 // non-escaping local object, then we know the object couldn't escape to a 392 // point where the call could return it. 393 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 394 isNonEscapingLocalObject(O2)) 395 return NoAlias; 396 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 397 isNonEscapingLocalObject(O1)) 398 return NoAlias; 399 400 // If we have two gep instructions with must-alias'ing base pointers, figure 401 // out if the indexes to the GEP tell us anything about the derived pointer. 402 // Note that we also handle chains of getelementptr instructions as well as 403 // constant expression getelementptrs here. 404 // 405 if (isGEP(V1) && isGEP(V2)) { 406 const User *GEP1 = cast<User>(V1); 407 const User *GEP2 = cast<User>(V2); 408 409 // If V1 and V2 are identical GEPs, just recurse down on both of them. 410 // This allows us to analyze things like: 411 // P = gep A, 0, i, 1 412 // Q = gep B, 0, i, 1 413 // by just analyzing A and B. This is even safe for variable indices. 414 if (GEP1->getType() == GEP2->getType() && 415 GEP1->getNumOperands() == GEP2->getNumOperands() && 416 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 417 // All operands are the same, ignoring the base. 418 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 419 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 420 421 422 // Drill down into the first non-gep value, to test for must-aliasing of 423 // the base pointers. 424 while (isGEP(GEP1->getOperand(0)) && 425 GEP1->getOperand(1) == 426 Constant::getNullValue(GEP1->getOperand(1)->getType())) 427 GEP1 = cast<User>(GEP1->getOperand(0)); 428 const Value *BasePtr1 = GEP1->getOperand(0); 429 430 while (isGEP(GEP2->getOperand(0)) && 431 GEP2->getOperand(1) == 432 Constant::getNullValue(GEP2->getOperand(1)->getType())) 433 GEP2 = cast<User>(GEP2->getOperand(0)); 434 const Value *BasePtr2 = GEP2->getOperand(0); 435 436 // Do the base pointers alias? 437 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 438 if (BaseAlias == NoAlias) return NoAlias; 439 if (BaseAlias == MustAlias) { 440 // If the base pointers alias each other exactly, check to see if we can 441 // figure out anything about the resultant pointers, to try to prove 442 // non-aliasing. 443 444 // Collect all of the chained GEP operands together into one simple place 445 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 446 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 447 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 448 449 // If GetGEPOperands were able to fold to the same must-aliased pointer, 450 // do the comparison. 451 if (BasePtr1 == BasePtr2) { 452 AliasResult GAlias = 453 CheckGEPInstructions(BasePtr1->getType(), 454 &GEP1Ops[0], GEP1Ops.size(), V1Size, 455 BasePtr2->getType(), 456 &GEP2Ops[0], GEP2Ops.size(), V2Size); 457 if (GAlias != MayAlias) 458 return GAlias; 459 } 460 } 461 } 462 463 // Check to see if these two pointers are related by a getelementptr 464 // instruction. If one pointer is a GEP with a non-zero index of the other 465 // pointer, we know they cannot alias. 466 // 467 if (isGEP(V2)) { 468 std::swap(V1, V2); 469 std::swap(V1Size, V2Size); 470 } 471 472 if (V1Size != ~0U && V2Size != ~0U) 473 if (isGEP(V1)) { 474 SmallVector<Value*, 16> GEPOperands; 475 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 476 477 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 478 if (R == MustAlias) { 479 // If there is at least one non-zero constant index, we know they cannot 480 // alias. 481 bool ConstantFound = false; 482 bool AllZerosFound = true; 483 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 484 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 485 if (!C->isNullValue()) { 486 ConstantFound = true; 487 AllZerosFound = false; 488 break; 489 } 490 } else { 491 AllZerosFound = false; 492 } 493 494 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 495 // the ptr, the end result is a must alias also. 496 if (AllZerosFound) 497 return MustAlias; 498 499 if (ConstantFound) { 500 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 501 return NoAlias; 502 503 // Otherwise we have to check to see that the distance is more than 504 // the size of the argument... build an index vector that is equal to 505 // the arguments provided, except substitute 0's for any variable 506 // indexes we find... 507 if (cast<PointerType>( 508 BasePtr->getType())->getElementType()->isSized()) { 509 for (unsigned i = 0; i != GEPOperands.size(); ++i) 510 if (!isa<ConstantInt>(GEPOperands[i])) 511 GEPOperands[i] = 512 Constant::getNullValue(GEPOperands[i]->getType()); 513 int64_t Offset = 514 getTargetData().getIndexedOffset(BasePtr->getType(), 515 &GEPOperands[0], 516 GEPOperands.size()); 517 518 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 519 return NoAlias; 520 } 521 } 522 } 523 } 524 525 return MayAlias; 526} 527 528// This function is used to determine if the indices of two GEP instructions are 529// equal. V1 and V2 are the indices. 530static bool IndexOperandsEqual(Value *V1, Value *V2) { 531 if (V1->getType() == V2->getType()) 532 return V1 == V2; 533 if (Constant *C1 = dyn_cast<Constant>(V1)) 534 if (Constant *C2 = dyn_cast<Constant>(V2)) { 535 // Sign extend the constants to long types, if necessary 536 if (C1->getType() != Type::Int64Ty) 537 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 538 if (C2->getType() != Type::Int64Ty) 539 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 540 return C1 == C2; 541 } 542 return false; 543} 544 545/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 546/// base pointers. This checks to see if the index expressions preclude the 547/// pointers from aliasing... 548AliasAnalysis::AliasResult 549BasicAliasAnalysis::CheckGEPInstructions( 550 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 551 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 552 // We currently can't handle the case when the base pointers have different 553 // primitive types. Since this is uncommon anyway, we are happy being 554 // extremely conservative. 555 if (BasePtr1Ty != BasePtr2Ty) 556 return MayAlias; 557 558 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 559 560 // Find the (possibly empty) initial sequence of equal values... which are not 561 // necessarily constants. 562 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 563 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 564 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 565 unsigned UnequalOper = 0; 566 while (UnequalOper != MinOperands && 567 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 568 // Advance through the type as we go... 569 ++UnequalOper; 570 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 571 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 572 else { 573 // If all operands equal each other, then the derived pointers must 574 // alias each other... 575 BasePtr1Ty = 0; 576 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 577 "Ran out of type nesting, but not out of operands?"); 578 return MustAlias; 579 } 580 } 581 582 // If we have seen all constant operands, and run out of indexes on one of the 583 // getelementptrs, check to see if the tail of the leftover one is all zeros. 584 // If so, return mustalias. 585 if (UnequalOper == MinOperands) { 586 if (NumGEP1Ops < NumGEP2Ops) { 587 std::swap(GEP1Ops, GEP2Ops); 588 std::swap(NumGEP1Ops, NumGEP2Ops); 589 } 590 591 bool AllAreZeros = true; 592 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 593 if (!isa<Constant>(GEP1Ops[i]) || 594 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 595 AllAreZeros = false; 596 break; 597 } 598 if (AllAreZeros) return MustAlias; 599 } 600 601 602 // So now we know that the indexes derived from the base pointers, 603 // which are known to alias, are different. We can still determine a 604 // no-alias result if there are differing constant pairs in the index 605 // chain. For example: 606 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 607 // 608 // We have to be careful here about array accesses. In particular, consider: 609 // A[1][0] vs A[0][i] 610 // In this case, we don't *know* that the array will be accessed in bounds: 611 // the index could even be negative. Because of this, we have to 612 // conservatively *give up* and return may alias. We disregard differing 613 // array subscripts that are followed by a variable index without going 614 // through a struct. 615 // 616 unsigned SizeMax = std::max(G1S, G2S); 617 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 618 619 // Scan for the first operand that is constant and unequal in the 620 // two getelementptrs... 621 unsigned FirstConstantOper = UnequalOper; 622 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 623 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 624 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 625 626 if (G1Oper != G2Oper) // Found non-equal constant indexes... 627 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 628 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 629 if (G1OC->getType() != G2OC->getType()) { 630 // Sign extend both operands to long. 631 if (G1OC->getType() != Type::Int64Ty) 632 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 633 if (G2OC->getType() != Type::Int64Ty) 634 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 635 GEP1Ops[FirstConstantOper] = G1OC; 636 GEP2Ops[FirstConstantOper] = G2OC; 637 } 638 639 if (G1OC != G2OC) { 640 // Handle the "be careful" case above: if this is an array/vector 641 // subscript, scan for a subsequent variable array index. 642 if (isa<SequentialType>(BasePtr1Ty)) { 643 const Type *NextTy = 644 cast<SequentialType>(BasePtr1Ty)->getElementType(); 645 bool isBadCase = false; 646 647 for (unsigned Idx = FirstConstantOper+1; 648 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 649 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 650 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 651 isBadCase = true; 652 break; 653 } 654 NextTy = cast<SequentialType>(NextTy)->getElementType(); 655 } 656 657 if (isBadCase) G1OC = 0; 658 } 659 660 // Make sure they are comparable (ie, not constant expressions), and 661 // make sure the GEP with the smaller leading constant is GEP1. 662 if (G1OC) { 663 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 664 G1OC, G2OC); 665 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 666 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 667 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 668 std::swap(NumGEP1Ops, NumGEP2Ops); 669 } 670 break; 671 } 672 } 673 } 674 } 675 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 676 } 677 678 // No shared constant operands, and we ran out of common operands. At this 679 // point, the GEP instructions have run through all of their operands, and we 680 // haven't found evidence that there are any deltas between the GEP's. 681 // However, one GEP may have more operands than the other. If this is the 682 // case, there may still be hope. Check this now. 683 if (FirstConstantOper == MinOperands) { 684 // Make GEP1Ops be the longer one if there is a longer one. 685 if (NumGEP1Ops < NumGEP2Ops) { 686 std::swap(GEP1Ops, GEP2Ops); 687 std::swap(NumGEP1Ops, NumGEP2Ops); 688 } 689 690 // Is there anything to check? 691 if (NumGEP1Ops > MinOperands) { 692 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 693 if (isa<ConstantInt>(GEP1Ops[i]) && 694 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 695 // Yup, there's a constant in the tail. Set all variables to 696 // constants in the GEP instruction to make it suitable for 697 // TargetData::getIndexedOffset. 698 for (i = 0; i != MaxOperands; ++i) 699 if (!isa<ConstantInt>(GEP1Ops[i])) 700 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 701 // Okay, now get the offset. This is the relative offset for the full 702 // instruction. 703 const TargetData &TD = getTargetData(); 704 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 705 NumGEP1Ops); 706 707 // Now check without any constants at the end. 708 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 709 MinOperands); 710 711 // Make sure we compare the absolute difference. 712 if (Offset1 > Offset2) 713 std::swap(Offset1, Offset2); 714 715 // If the tail provided a bit enough offset, return noalias! 716 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 717 return NoAlias; 718 // Otherwise break - we don't look for another constant in the tail. 719 break; 720 } 721 } 722 723 // Couldn't find anything useful. 724 return MayAlias; 725 } 726 727 // If there are non-equal constants arguments, then we can figure 728 // out a minimum known delta between the two index expressions... at 729 // this point we know that the first constant index of GEP1 is less 730 // than the first constant index of GEP2. 731 732 // Advance BasePtr[12]Ty over this first differing constant operand. 733 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 734 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 735 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 736 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 737 738 // We are going to be using TargetData::getIndexedOffset to determine the 739 // offset that each of the GEP's is reaching. To do this, we have to convert 740 // all variable references to constant references. To do this, we convert the 741 // initial sequence of array subscripts into constant zeros to start with. 742 const Type *ZeroIdxTy = GEPPointerTy; 743 for (unsigned i = 0; i != FirstConstantOper; ++i) { 744 if (!isa<StructType>(ZeroIdxTy)) 745 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 746 747 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 748 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 749 } 750 751 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 752 753 // Loop over the rest of the operands... 754 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 755 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 756 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 757 // If they are equal, use a zero index... 758 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 759 if (!isa<ConstantInt>(Op1)) 760 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 761 // Otherwise, just keep the constants we have. 762 } else { 763 if (Op1) { 764 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 765 // If this is an array index, make sure the array element is in range. 766 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 767 if (Op1C->getZExtValue() >= AT->getNumElements()) 768 return MayAlias; // Be conservative with out-of-range accesses 769 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 770 if (Op1C->getZExtValue() >= VT->getNumElements()) 771 return MayAlias; // Be conservative with out-of-range accesses 772 } 773 774 } else { 775 // GEP1 is known to produce a value less than GEP2. To be 776 // conservatively correct, we must assume the largest possible 777 // constant is used in this position. This cannot be the initial 778 // index to the GEP instructions (because we know we have at least one 779 // element before this one with the different constant arguments), so 780 // we know that the current index must be into either a struct or 781 // array. Because we know it's not constant, this cannot be a 782 // structure index. Because of this, we can calculate the maximum 783 // value possible. 784 // 785 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 786 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 787 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 788 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 789 } 790 } 791 792 if (Op2) { 793 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 794 // If this is an array index, make sure the array element is in range. 795 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 796 if (Op2C->getZExtValue() >= AT->getNumElements()) 797 return MayAlias; // Be conservative with out-of-range accesses 798 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 799 if (Op2C->getZExtValue() >= VT->getNumElements()) 800 return MayAlias; // Be conservative with out-of-range accesses 801 } 802 } else { // Conservatively assume the minimum value for this index 803 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 804 } 805 } 806 } 807 808 if (BasePtr1Ty && Op1) { 809 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 810 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 811 else 812 BasePtr1Ty = 0; 813 } 814 815 if (BasePtr2Ty && Op2) { 816 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 817 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 818 else 819 BasePtr2Ty = 0; 820 } 821 } 822 823 if (GEPPointerTy->getElementType()->isSized()) { 824 int64_t Offset1 = 825 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 826 int64_t Offset2 = 827 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 828 assert(Offset1 != Offset2 && 829 "There is at least one different constant here!"); 830 831 // Make sure we compare the absolute difference. 832 if (Offset1 > Offset2) 833 std::swap(Offset1, Offset2); 834 835 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 836 //cerr << "Determined that these two GEP's don't alias [" 837 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 838 return NoAlias; 839 } 840 } 841 return MayAlias; 842} 843 844// Make sure that anything that uses AliasAnalysis pulls in this file... 845DEFINING_FILE_FOR(BasicAliasAnalysis) 846