BasicAliasAnalysis.cpp revision ceb4d1aecb9deffe59b3dcdc9a783ffde8477be9
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/Passes.h" 18#include "llvm/Constants.h" 19#include "llvm/DerivedTypes.h" 20#include "llvm/Function.h" 21#include "llvm/GlobalVariable.h" 22#include "llvm/Instructions.h" 23#include "llvm/IntrinsicInst.h" 24#include "llvm/Pass.h" 25#include "llvm/Target/TargetData.h" 26#include "llvm/ADT/SmallVector.h" 27#include "llvm/ADT/STLExtras.h" 28#include "llvm/Support/Compiler.h" 29#include "llvm/Support/GetElementPtrTypeIterator.h" 30#include "llvm/Support/ManagedStatic.h" 31#include <algorithm> 32using namespace llvm; 33 34//===----------------------------------------------------------------------===// 35// Useful predicates 36//===----------------------------------------------------------------------===// 37 38// Determine if a value escapes from the function it is contained in (being 39// returned by the function does not count as escaping here). If a value local 40// to the function does not escape, there is no way another function can mod/ref 41// it. We do this by looking at its uses and determining if they can escape 42// (recursively). 43static bool AddressMightEscape(const Value *V) { 44 for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end(); 45 UI != E; ++UI) { 46 const Instruction *I = cast<Instruction>(*UI); 47 switch (I->getOpcode()) { 48 case Instruction::Load: 49 break; //next use. 50 case Instruction::Store: 51 if (I->getOperand(0) == V) 52 return true; // Escapes if the pointer is stored. 53 break; // next use. 54 case Instruction::GetElementPtr: 55 if (AddressMightEscape(I)) 56 return true; 57 break; // next use. 58 case Instruction::BitCast: 59 if (AddressMightEscape(I)) 60 return true; 61 break; // next use 62 case Instruction::Ret: 63 // If returned, the address will escape to calling functions, but no 64 // callees could modify it. 65 break; // next use 66 case Instruction::Call: 67 // If the argument to the call has the nocapture attribute, then the call 68 // may store or load to the pointer, but it cannot escape. 69 if (cast<CallInst>(I)->paramHasAttr(UI.getOperandNo(), 70 Attribute::NoCapture)) 71 continue; 72 return true; 73 case Instruction::Invoke: 74 // If the argument to the call has the nocapture attribute, then the call 75 // may store or load to the pointer, but it cannot escape. 76 if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2, 77 Attribute::NoCapture)) 78 continue; 79 return true; 80 default: 81 return true; 82 } 83 } 84 return false; 85} 86 87static const User *isGEP(const Value *V) { 88 if (isa<GetElementPtrInst>(V) || 89 (isa<ConstantExpr>(V) && 90 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr)) 91 return cast<User>(V); 92 return 0; 93} 94 95static const Value *GetGEPOperands(const Value *V, 96 SmallVector<Value*, 16> &GEPOps) { 97 assert(GEPOps.empty() && "Expect empty list to populate!"); 98 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1, 99 cast<User>(V)->op_end()); 100 101 // Accumulate all of the chained indexes into the operand array 102 V = cast<User>(V)->getOperand(0); 103 104 while (const User *G = isGEP(V)) { 105 if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) || 106 !cast<Constant>(GEPOps[0])->isNullValue()) 107 break; // Don't handle folding arbitrary pointer offsets yet... 108 GEPOps.erase(GEPOps.begin()); // Drop the zero index 109 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end()); 110 V = G->getOperand(0); 111 } 112 return V; 113} 114 115/// isNoAliasCall - Return true if this pointer is returned by a noalias 116/// function. 117static bool isNoAliasCall(const Value *V) { 118 if (isa<CallInst>(V) || isa<InvokeInst>(V)) 119 return CallSite(const_cast<Instruction*>(cast<Instruction>(V))) 120 .paramHasAttr(0, Attribute::NoAlias); 121 return false; 122} 123 124/// isIdentifiedObject - Return true if this pointer refers to a distinct and 125/// identifiable object. This returns true for: 126/// Global Variables and Functions 127/// Allocas and Mallocs 128/// ByVal and NoAlias Arguments 129/// NoAlias returns 130/// 131static bool isIdentifiedObject(const Value *V) { 132 if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V)) 133 return true; 134 if (const Argument *A = dyn_cast<Argument>(V)) 135 return A->hasNoAliasAttr() || A->hasByValAttr(); 136 return false; 137} 138 139/// isKnownNonNull - Return true if we know that the specified value is never 140/// null. 141static bool isKnownNonNull(const Value *V) { 142 // Alloca never returns null, malloc might. 143 if (isa<AllocaInst>(V)) return true; 144 145 // A byval argument is never null. 146 if (const Argument *A = dyn_cast<Argument>(V)) 147 return A->hasByValAttr(); 148 149 // Global values are not null unless extern weak. 150 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) 151 return !GV->hasExternalWeakLinkage(); 152 return false; 153} 154 155/// isNonEscapingLocalObject - Return true if the pointer is to a function-local 156/// object that never escapes from the function. 157static bool isNonEscapingLocalObject(const Value *V) { 158 // If this is a local allocation, check to see if it escapes. 159 if (isa<AllocationInst>(V) || isNoAliasCall(V)) 160 return !AddressMightEscape(V); 161 162 // If this is an argument that corresponds to a byval or noalias argument, 163 // then it has not escaped before entering the function. Check if it escapes 164 // inside the function. 165 if (const Argument *A = dyn_cast<Argument>(V)) 166 if (A->hasByValAttr() || A->hasNoAliasAttr()) { 167 // Don't bother analyzing arguments already known not to escape. 168 if (A->hasNoCaptureAttr()) 169 return true; 170 return !AddressMightEscape(V); 171 } 172 return false; 173} 174 175 176/// isObjectSmallerThan - Return true if we can prove that the object specified 177/// by V is smaller than Size. 178static bool isObjectSmallerThan(const Value *V, unsigned Size, 179 const TargetData &TD) { 180 const Type *AccessTy; 181 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 182 AccessTy = GV->getType()->getElementType(); 183 } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) { 184 if (!AI->isArrayAllocation()) 185 AccessTy = AI->getType()->getElementType(); 186 else 187 return false; 188 } else if (const Argument *A = dyn_cast<Argument>(V)) { 189 if (A->hasByValAttr()) 190 AccessTy = cast<PointerType>(A->getType())->getElementType(); 191 else 192 return false; 193 } else { 194 return false; 195 } 196 197 if (AccessTy->isSized()) 198 return TD.getTypePaddedSize(AccessTy) < Size; 199 return false; 200} 201 202//===----------------------------------------------------------------------===// 203// NoAA Pass 204//===----------------------------------------------------------------------===// 205 206namespace { 207 /// NoAA - This class implements the -no-aa pass, which always returns "I 208 /// don't know" for alias queries. NoAA is unlike other alias analysis 209 /// implementations, in that it does not chain to a previous analysis. As 210 /// such it doesn't follow many of the rules that other alias analyses must. 211 /// 212 struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis { 213 static char ID; // Class identification, replacement for typeinfo 214 NoAA() : ImmutablePass(&ID) {} 215 explicit NoAA(void *PID) : ImmutablePass(PID) { } 216 217 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 218 AU.addRequired<TargetData>(); 219 } 220 221 virtual void initializePass() { 222 TD = &getAnalysis<TargetData>(); 223 } 224 225 virtual AliasResult alias(const Value *V1, unsigned V1Size, 226 const Value *V2, unsigned V2Size) { 227 return MayAlias; 228 } 229 230 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS, 231 std::vector<PointerAccessInfo> *Info) { 232 return UnknownModRefBehavior; 233 } 234 235 virtual void getArgumentAccesses(Function *F, CallSite CS, 236 std::vector<PointerAccessInfo> &Info) { 237 assert(0 && "This method may not be called on this function!"); 238 } 239 240 virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { } 241 virtual bool pointsToConstantMemory(const Value *P) { return false; } 242 virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { 243 return ModRef; 244 } 245 virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { 246 return ModRef; 247 } 248 virtual bool hasNoModRefInfoForCalls() const { return true; } 249 250 virtual void deleteValue(Value *V) {} 251 virtual void copyValue(Value *From, Value *To) {} 252 }; 253} // End of anonymous namespace 254 255// Register this pass... 256char NoAA::ID = 0; 257static RegisterPass<NoAA> 258U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); 259 260// Declare that we implement the AliasAnalysis interface 261static RegisterAnalysisGroup<AliasAnalysis> V(U); 262 263ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } 264 265//===----------------------------------------------------------------------===// 266// BasicAA Pass 267//===----------------------------------------------------------------------===// 268 269namespace { 270 /// BasicAliasAnalysis - This is the default alias analysis implementation. 271 /// Because it doesn't chain to a previous alias analysis (like -no-aa), it 272 /// derives from the NoAA class. 273 struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA { 274 static char ID; // Class identification, replacement for typeinfo 275 BasicAliasAnalysis() : NoAA(&ID) {} 276 AliasResult alias(const Value *V1, unsigned V1Size, 277 const Value *V2, unsigned V2Size); 278 279 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); 280 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); 281 282 /// hasNoModRefInfoForCalls - We can provide mod/ref information against 283 /// non-escaping allocations. 284 virtual bool hasNoModRefInfoForCalls() const { return false; } 285 286 /// pointsToConstantMemory - Chase pointers until we find a (constant 287 /// global) or not. 288 bool pointsToConstantMemory(const Value *P); 289 290 private: 291 // CheckGEPInstructions - Check two GEP instructions with known 292 // must-aliasing base pointers. This checks to see if the index expressions 293 // preclude the pointers from aliasing... 294 AliasResult 295 CheckGEPInstructions(const Type* BasePtr1Ty, 296 Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size, 297 const Type *BasePtr2Ty, 298 Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size); 299 }; 300} // End of anonymous namespace 301 302// Register this pass... 303char BasicAliasAnalysis::ID = 0; 304static RegisterPass<BasicAliasAnalysis> 305X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); 306 307// Declare that we implement the AliasAnalysis interface 308static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); 309 310ImmutablePass *llvm::createBasicAliasAnalysisPass() { 311 return new BasicAliasAnalysis(); 312} 313 314 315/// pointsToConstantMemory - Chase pointers until we find a (constant 316/// global) or not. 317bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { 318 if (const GlobalVariable *GV = 319 dyn_cast<GlobalVariable>(P->getUnderlyingObject())) 320 return GV->isConstant(); 321 return false; 322} 323 324// getModRefInfo - Check to see if the specified callsite can clobber the 325// specified memory object. Since we only look at local properties of this 326// function, we really can't say much about this query. We do, however, use 327// simple "address taken" analysis on local objects. 328// 329AliasAnalysis::ModRefResult 330BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { 331 if (!isa<Constant>(P)) { 332 const Value *Object = P->getUnderlyingObject(); 333 334 // If this is a tail call and P points to a stack location, we know that 335 // the tail call cannot access or modify the local stack. 336 // We cannot exclude byval arguments here; these belong to the caller of 337 // the current function not to the current function, and a tail callee 338 // may reference them. 339 if (isa<AllocaInst>(Object)) 340 if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) 341 if (CI->isTailCall()) 342 return NoModRef; 343 344 // If the pointer is to a locally allocated object that does not escape, 345 // then the call can not mod/ref the pointer unless the call takes the 346 // argument without capturing it. 347 if (isNonEscapingLocalObject(Object)) { 348 bool passedAsArg = false; 349 // TODO: Eventually only check 'nocapture' arguments. 350 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 351 CI != CE; ++CI) 352 if (isa<PointerType>((*CI)->getType()) && 353 alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias) 354 passedAsArg = true; 355 356 if (!passedAsArg) 357 return NoModRef; 358 } 359 } 360 361 // The AliasAnalysis base class has some smarts, lets use them. 362 return AliasAnalysis::getModRefInfo(CS, P, Size); 363} 364 365 366AliasAnalysis::ModRefResult 367BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { 368 // If CS1 or CS2 are readnone, they don't interact. 369 ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); 370 if (CS1B == DoesNotAccessMemory) return NoModRef; 371 372 ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); 373 if (CS2B == DoesNotAccessMemory) return NoModRef; 374 375 // If they both only read from memory, just return ref. 376 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) 377 return Ref; 378 379 // Otherwise, fall back to NoAA (mod+ref). 380 return NoAA::getModRefInfo(CS1, CS2); 381} 382 383 384// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such 385// as array references. 386// 387AliasAnalysis::AliasResult 388BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size, 389 const Value *V2, unsigned V2Size) { 390 // Strip off any constant expression casts if they exist 391 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1)) 392 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 393 V1 = CE->getOperand(0); 394 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2)) 395 if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType())) 396 V2 = CE->getOperand(0); 397 398 // Are we checking for alias of the same value? 399 if (V1 == V2) return MustAlias; 400 401 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 402 return NoAlias; // Scalars cannot alias each other 403 404 // Strip off cast instructions. Since V1 and V2 are pointers, they must be 405 // pointer<->pointer bitcasts. 406 if (const BitCastInst *I = dyn_cast<BitCastInst>(V1)) 407 return alias(I->getOperand(0), V1Size, V2, V2Size); 408 if (const BitCastInst *I = dyn_cast<BitCastInst>(V2)) 409 return alias(V1, V1Size, I->getOperand(0), V2Size); 410 411 // Figure out what objects these things are pointing to if we can. 412 const Value *O1 = V1->getUnderlyingObject(); 413 const Value *O2 = V2->getUnderlyingObject(); 414 415 if (O1 != O2) { 416 // If V1/V2 point to two different objects we know that we have no alias. 417 if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) 418 return NoAlias; 419 420 // Arguments can't alias with local allocations or noalias calls. 421 if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) || 422 (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1)))) 423 return NoAlias; 424 425 // Most objects can't alias null. 426 if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || 427 (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) 428 return NoAlias; 429 } 430 431 // If the size of one access is larger than the entire object on the other 432 // side, then we know such behavior is undefined and can assume no alias. 433 const TargetData &TD = getTargetData(); 434 if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) || 435 (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD))) 436 return NoAlias; 437 438 // If one pointer is the result of a call/invoke and the other is a 439 // non-escaping local object, then we know the object couldn't escape to a 440 // point where the call could return it. 441 if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) && 442 isNonEscapingLocalObject(O2)) 443 return NoAlias; 444 if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) && 445 isNonEscapingLocalObject(O1)) 446 return NoAlias; 447 448 // If we have two gep instructions with must-alias'ing base pointers, figure 449 // out if the indexes to the GEP tell us anything about the derived pointer. 450 // Note that we also handle chains of getelementptr instructions as well as 451 // constant expression getelementptrs here. 452 // 453 if (isGEP(V1) && isGEP(V2)) { 454 const User *GEP1 = cast<User>(V1); 455 const User *GEP2 = cast<User>(V2); 456 457 // If V1 and V2 are identical GEPs, just recurse down on both of them. 458 // This allows us to analyze things like: 459 // P = gep A, 0, i, 1 460 // Q = gep B, 0, i, 1 461 // by just analyzing A and B. This is even safe for variable indices. 462 if (GEP1->getType() == GEP2->getType() && 463 GEP1->getNumOperands() == GEP2->getNumOperands() && 464 GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() && 465 // All operands are the same, ignoring the base. 466 std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1)) 467 return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size); 468 469 470 // Drill down into the first non-gep value, to test for must-aliasing of 471 // the base pointers. 472 while (isGEP(GEP1->getOperand(0)) && 473 GEP1->getOperand(1) == 474 Constant::getNullValue(GEP1->getOperand(1)->getType())) 475 GEP1 = cast<User>(GEP1->getOperand(0)); 476 const Value *BasePtr1 = GEP1->getOperand(0); 477 478 while (isGEP(GEP2->getOperand(0)) && 479 GEP2->getOperand(1) == 480 Constant::getNullValue(GEP2->getOperand(1)->getType())) 481 GEP2 = cast<User>(GEP2->getOperand(0)); 482 const Value *BasePtr2 = GEP2->getOperand(0); 483 484 // Do the base pointers alias? 485 AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U); 486 if (BaseAlias == NoAlias) return NoAlias; 487 if (BaseAlias == MustAlias) { 488 // If the base pointers alias each other exactly, check to see if we can 489 // figure out anything about the resultant pointers, to try to prove 490 // non-aliasing. 491 492 // Collect all of the chained GEP operands together into one simple place 493 SmallVector<Value*, 16> GEP1Ops, GEP2Ops; 494 BasePtr1 = GetGEPOperands(V1, GEP1Ops); 495 BasePtr2 = GetGEPOperands(V2, GEP2Ops); 496 497 // If GetGEPOperands were able to fold to the same must-aliased pointer, 498 // do the comparison. 499 if (BasePtr1 == BasePtr2) { 500 AliasResult GAlias = 501 CheckGEPInstructions(BasePtr1->getType(), 502 &GEP1Ops[0], GEP1Ops.size(), V1Size, 503 BasePtr2->getType(), 504 &GEP2Ops[0], GEP2Ops.size(), V2Size); 505 if (GAlias != MayAlias) 506 return GAlias; 507 } 508 } 509 } 510 511 // Check to see if these two pointers are related by a getelementptr 512 // instruction. If one pointer is a GEP with a non-zero index of the other 513 // pointer, we know they cannot alias. 514 // 515 if (isGEP(V2)) { 516 std::swap(V1, V2); 517 std::swap(V1Size, V2Size); 518 } 519 520 if (V1Size != ~0U && V2Size != ~0U) 521 if (isGEP(V1)) { 522 SmallVector<Value*, 16> GEPOperands; 523 const Value *BasePtr = GetGEPOperands(V1, GEPOperands); 524 525 AliasResult R = alias(BasePtr, V1Size, V2, V2Size); 526 if (R == MustAlias) { 527 // If there is at least one non-zero constant index, we know they cannot 528 // alias. 529 bool ConstantFound = false; 530 bool AllZerosFound = true; 531 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i) 532 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) { 533 if (!C->isNullValue()) { 534 ConstantFound = true; 535 AllZerosFound = false; 536 break; 537 } 538 } else { 539 AllZerosFound = false; 540 } 541 542 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases 543 // the ptr, the end result is a must alias also. 544 if (AllZerosFound) 545 return MustAlias; 546 547 if (ConstantFound) { 548 if (V2Size <= 1 && V1Size <= 1) // Just pointer check? 549 return NoAlias; 550 551 // Otherwise we have to check to see that the distance is more than 552 // the size of the argument... build an index vector that is equal to 553 // the arguments provided, except substitute 0's for any variable 554 // indexes we find... 555 if (cast<PointerType>( 556 BasePtr->getType())->getElementType()->isSized()) { 557 for (unsigned i = 0; i != GEPOperands.size(); ++i) 558 if (!isa<ConstantInt>(GEPOperands[i])) 559 GEPOperands[i] = 560 Constant::getNullValue(GEPOperands[i]->getType()); 561 int64_t Offset = 562 getTargetData().getIndexedOffset(BasePtr->getType(), 563 &GEPOperands[0], 564 GEPOperands.size()); 565 566 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size) 567 return NoAlias; 568 } 569 } 570 } 571 } 572 573 return MayAlias; 574} 575 576// This function is used to determine if the indices of two GEP instructions are 577// equal. V1 and V2 are the indices. 578static bool IndexOperandsEqual(Value *V1, Value *V2) { 579 if (V1->getType() == V2->getType()) 580 return V1 == V2; 581 if (Constant *C1 = dyn_cast<Constant>(V1)) 582 if (Constant *C2 = dyn_cast<Constant>(V2)) { 583 // Sign extend the constants to long types, if necessary 584 if (C1->getType() != Type::Int64Ty) 585 C1 = ConstantExpr::getSExt(C1, Type::Int64Ty); 586 if (C2->getType() != Type::Int64Ty) 587 C2 = ConstantExpr::getSExt(C2, Type::Int64Ty); 588 return C1 == C2; 589 } 590 return false; 591} 592 593/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing 594/// base pointers. This checks to see if the index expressions preclude the 595/// pointers from aliasing... 596AliasAnalysis::AliasResult 597BasicAliasAnalysis::CheckGEPInstructions( 598 const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S, 599 const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) { 600 // We currently can't handle the case when the base pointers have different 601 // primitive types. Since this is uncommon anyway, we are happy being 602 // extremely conservative. 603 if (BasePtr1Ty != BasePtr2Ty) 604 return MayAlias; 605 606 const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty); 607 608 // Find the (possibly empty) initial sequence of equal values... which are not 609 // necessarily constants. 610 unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops; 611 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands); 612 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands); 613 unsigned UnequalOper = 0; 614 while (UnequalOper != MinOperands && 615 IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper])) { 616 // Advance through the type as we go... 617 ++UnequalOper; 618 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 619 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]); 620 else { 621 // If all operands equal each other, then the derived pointers must 622 // alias each other... 623 BasePtr1Ty = 0; 624 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands && 625 "Ran out of type nesting, but not out of operands?"); 626 return MustAlias; 627 } 628 } 629 630 // If we have seen all constant operands, and run out of indexes on one of the 631 // getelementptrs, check to see if the tail of the leftover one is all zeros. 632 // If so, return mustalias. 633 if (UnequalOper == MinOperands) { 634 if (NumGEP1Ops < NumGEP2Ops) { 635 std::swap(GEP1Ops, GEP2Ops); 636 std::swap(NumGEP1Ops, NumGEP2Ops); 637 } 638 639 bool AllAreZeros = true; 640 for (unsigned i = UnequalOper; i != MaxOperands; ++i) 641 if (!isa<Constant>(GEP1Ops[i]) || 642 !cast<Constant>(GEP1Ops[i])->isNullValue()) { 643 AllAreZeros = false; 644 break; 645 } 646 if (AllAreZeros) return MustAlias; 647 } 648 649 650 // So now we know that the indexes derived from the base pointers, 651 // which are known to alias, are different. We can still determine a 652 // no-alias result if there are differing constant pairs in the index 653 // chain. For example: 654 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S)) 655 // 656 // We have to be careful here about array accesses. In particular, consider: 657 // A[1][0] vs A[0][i] 658 // In this case, we don't *know* that the array will be accessed in bounds: 659 // the index could even be negative. Because of this, we have to 660 // conservatively *give up* and return may alias. We disregard differing 661 // array subscripts that are followed by a variable index without going 662 // through a struct. 663 // 664 unsigned SizeMax = std::max(G1S, G2S); 665 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work. 666 667 // Scan for the first operand that is constant and unequal in the 668 // two getelementptrs... 669 unsigned FirstConstantOper = UnequalOper; 670 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) { 671 const Value *G1Oper = GEP1Ops[FirstConstantOper]; 672 const Value *G2Oper = GEP2Ops[FirstConstantOper]; 673 674 if (G1Oper != G2Oper) // Found non-equal constant indexes... 675 if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper))) 676 if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){ 677 if (G1OC->getType() != G2OC->getType()) { 678 // Sign extend both operands to long. 679 if (G1OC->getType() != Type::Int64Ty) 680 G1OC = ConstantExpr::getSExt(G1OC, Type::Int64Ty); 681 if (G2OC->getType() != Type::Int64Ty) 682 G2OC = ConstantExpr::getSExt(G2OC, Type::Int64Ty); 683 GEP1Ops[FirstConstantOper] = G1OC; 684 GEP2Ops[FirstConstantOper] = G2OC; 685 } 686 687 if (G1OC != G2OC) { 688 // Handle the "be careful" case above: if this is an array/vector 689 // subscript, scan for a subsequent variable array index. 690 if (isa<SequentialType>(BasePtr1Ty)) { 691 const Type *NextTy = 692 cast<SequentialType>(BasePtr1Ty)->getElementType(); 693 bool isBadCase = false; 694 695 for (unsigned Idx = FirstConstantOper+1; 696 Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) { 697 const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx]; 698 if (!isa<Constant>(V1) || !isa<Constant>(V2)) { 699 isBadCase = true; 700 break; 701 } 702 NextTy = cast<SequentialType>(NextTy)->getElementType(); 703 } 704 705 if (isBadCase) G1OC = 0; 706 } 707 708 // Make sure they are comparable (ie, not constant expressions), and 709 // make sure the GEP with the smaller leading constant is GEP1. 710 if (G1OC) { 711 Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT, 712 G1OC, G2OC); 713 if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) { 714 if (CV->getZExtValue()) { // If they are comparable and G2 > G1 715 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2 716 std::swap(NumGEP1Ops, NumGEP2Ops); 717 } 718 break; 719 } 720 } 721 } 722 } 723 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper); 724 } 725 726 // No shared constant operands, and we ran out of common operands. At this 727 // point, the GEP instructions have run through all of their operands, and we 728 // haven't found evidence that there are any deltas between the GEP's. 729 // However, one GEP may have more operands than the other. If this is the 730 // case, there may still be hope. Check this now. 731 if (FirstConstantOper == MinOperands) { 732 // Make GEP1Ops be the longer one if there is a longer one. 733 if (NumGEP1Ops < NumGEP2Ops) { 734 std::swap(GEP1Ops, GEP2Ops); 735 std::swap(NumGEP1Ops, NumGEP2Ops); 736 } 737 738 // Is there anything to check? 739 if (NumGEP1Ops > MinOperands) { 740 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i) 741 if (isa<ConstantInt>(GEP1Ops[i]) && 742 !cast<ConstantInt>(GEP1Ops[i])->isZero()) { 743 // Yup, there's a constant in the tail. Set all variables to 744 // constants in the GEP instruction to make it suitable for 745 // TargetData::getIndexedOffset. 746 for (i = 0; i != MaxOperands; ++i) 747 if (!isa<ConstantInt>(GEP1Ops[i])) 748 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType()); 749 // Okay, now get the offset. This is the relative offset for the full 750 // instruction. 751 const TargetData &TD = getTargetData(); 752 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 753 NumGEP1Ops); 754 755 // Now check without any constants at the end. 756 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops, 757 MinOperands); 758 759 // Make sure we compare the absolute difference. 760 if (Offset1 > Offset2) 761 std::swap(Offset1, Offset2); 762 763 // If the tail provided a bit enough offset, return noalias! 764 if ((uint64_t)(Offset2-Offset1) >= SizeMax) 765 return NoAlias; 766 // Otherwise break - we don't look for another constant in the tail. 767 break; 768 } 769 } 770 771 // Couldn't find anything useful. 772 return MayAlias; 773 } 774 775 // If there are non-equal constants arguments, then we can figure 776 // out a minimum known delta between the two index expressions... at 777 // this point we know that the first constant index of GEP1 is less 778 // than the first constant index of GEP2. 779 780 // Advance BasePtr[12]Ty over this first differing constant operand. 781 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)-> 782 getTypeAtIndex(GEP2Ops[FirstConstantOper]); 783 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)-> 784 getTypeAtIndex(GEP1Ops[FirstConstantOper]); 785 786 // We are going to be using TargetData::getIndexedOffset to determine the 787 // offset that each of the GEP's is reaching. To do this, we have to convert 788 // all variable references to constant references. To do this, we convert the 789 // initial sequence of array subscripts into constant zeros to start with. 790 const Type *ZeroIdxTy = GEPPointerTy; 791 for (unsigned i = 0; i != FirstConstantOper; ++i) { 792 if (!isa<StructType>(ZeroIdxTy)) 793 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Type::Int32Ty); 794 795 if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy)) 796 ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]); 797 } 798 799 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok 800 801 // Loop over the rest of the operands... 802 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) { 803 const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0; 804 const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0; 805 // If they are equal, use a zero index... 806 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) { 807 if (!isa<ConstantInt>(Op1)) 808 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType()); 809 // Otherwise, just keep the constants we have. 810 } else { 811 if (Op1) { 812 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) { 813 // If this is an array index, make sure the array element is in range. 814 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) { 815 if (Op1C->getZExtValue() >= AT->getNumElements()) 816 return MayAlias; // Be conservative with out-of-range accesses 817 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) { 818 if (Op1C->getZExtValue() >= VT->getNumElements()) 819 return MayAlias; // Be conservative with out-of-range accesses 820 } 821 822 } else { 823 // GEP1 is known to produce a value less than GEP2. To be 824 // conservatively correct, we must assume the largest possible 825 // constant is used in this position. This cannot be the initial 826 // index to the GEP instructions (because we know we have at least one 827 // element before this one with the different constant arguments), so 828 // we know that the current index must be into either a struct or 829 // array. Because we know it's not constant, this cannot be a 830 // structure index. Because of this, we can calculate the maximum 831 // value possible. 832 // 833 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) 834 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,AT->getNumElements()-1); 835 else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) 836 GEP1Ops[i] = ConstantInt::get(Type::Int64Ty,VT->getNumElements()-1); 837 } 838 } 839 840 if (Op2) { 841 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) { 842 // If this is an array index, make sure the array element is in range. 843 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) { 844 if (Op2C->getZExtValue() >= AT->getNumElements()) 845 return MayAlias; // Be conservative with out-of-range accesses 846 } else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) { 847 if (Op2C->getZExtValue() >= VT->getNumElements()) 848 return MayAlias; // Be conservative with out-of-range accesses 849 } 850 } else { // Conservatively assume the minimum value for this index 851 GEP2Ops[i] = Constant::getNullValue(Op2->getType()); 852 } 853 } 854 } 855 856 if (BasePtr1Ty && Op1) { 857 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty)) 858 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]); 859 else 860 BasePtr1Ty = 0; 861 } 862 863 if (BasePtr2Ty && Op2) { 864 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty)) 865 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]); 866 else 867 BasePtr2Ty = 0; 868 } 869 } 870 871 if (GEPPointerTy->getElementType()->isSized()) { 872 int64_t Offset1 = 873 getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops); 874 int64_t Offset2 = 875 getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops); 876 assert(Offset1 != Offset2 && 877 "There is at least one different constant here!"); 878 879 // Make sure we compare the absolute difference. 880 if (Offset1 > Offset2) 881 std::swap(Offset1, Offset2); 882 883 if ((uint64_t)(Offset2-Offset1) >= SizeMax) { 884 //cerr << "Determined that these two GEP's don't alias [" 885 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2; 886 return NoAlias; 887 } 888 } 889 return MayAlias; 890} 891 892// Make sure that anything that uses AliasAnalysis pulls in this file... 893DEFINING_FILE_FOR(BasicAliasAnalysis) 894