src/llvm/lib/Analysis/AliasAnalysis.cpp

   1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
   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 implements the generic AliasAnalysis interface which is used as the
  11 // common interface used by all clients and implementations of alias analysis.
  12 //
  13 // This file also implements the default version of the AliasAnalysis interface
  14 // that is to be used when no other implementation is specified.  This does some
  15 // simple tests that detect obvious cases: two different global pointers cannot
  16 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
  17 // etc.
  18 //
  19 // This alias analysis implementation really isn't very good for anything, but
  20 // it is very fast, and makes a nice clean default implementation.  Because it
  21 // handles lots of little corner cases, other, more complex, alias analysis
  22 // implementations may choose to rely on this pass to resolve these simple and
  23 // easy cases.
  24 //
  25 //===----------------------------------------------------------------------===//
  26
  27 #include "llvm/Analysis/AliasAnalysis.h"
  28 #include "llvm/Analysis/CFG.h"
  29 #include "llvm/Analysis/CaptureTracking.h"
  30 #include "llvm/Analysis/ValueTracking.h"
  31 #include "llvm/IR/BasicBlock.h"
  32 #include "llvm/IR/DataLayout.h"
  33 #include "llvm/IR/Dominators.h"
  34 #include "llvm/IR/Function.h"
  35 #include "llvm/IR/Instructions.h"
  36 #include "llvm/IR/IntrinsicInst.h"
  37 #include "llvm/IR/LLVMContext.h"
  38 #include "llvm/IR/Type.h"
  39 #include "llvm/Pass.h"
  40 #include "llvm/Target/TargetLibraryInfo.h"
  41 using namespace llvm;
  42
  43 // Register the AliasAnalysis interface, providing a nice name to refer to.
  44 INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
  45 char AliasAnalysis::ID = 0;
  46
  47 //===----------------------------------------------------------------------===//
  48 // Default chaining methods
  49 //===----------------------------------------------------------------------===//
  50
  51 AliasAnalysis::AliasResult
  52 AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
  53   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  54   return AA->alias(LocA, LocB);
  55 }
  56
  57 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
  58                                            bool OrLocal) {
  59   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  60   return AA->pointsToConstantMemory(Loc, OrLocal);
  61 }
  62
  63 AliasAnalysis::Location
  64 AliasAnalysis::getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
  65                               AliasAnalysis::ModRefResult &Mask) {
  66   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  67   return AA->getArgLocation(CS, ArgIdx, Mask);
  68 }
  69
  70 void AliasAnalysis::deleteValue(Value *V) {
  71   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  72   AA->deleteValue(V);
  73 }
  74
  75 void AliasAnalysis::copyValue(Value *From, Value *To) {
  76   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  77   AA->copyValue(From, To);
  78 }
  79
  80 void AliasAnalysis::addEscapingUse(Use &U) {
  81   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  82   AA->addEscapingUse(U);
  83 }
  84
  85
  86 AliasAnalysis::ModRefResult
  87 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
  88                              const Location &Loc) {
  89   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  90
  91   ModRefBehavior MRB = getModRefBehavior(CS);
  92   if (MRB == DoesNotAccessMemory)
  93     return NoModRef;
  94
  95   ModRefResult Mask = ModRef;
  96   if (onlyReadsMemory(MRB))
  97     Mask = Ref;
  98
  99   if (onlyAccessesArgPointees(MRB)) {
 100     bool doesAlias = false;
 101     ModRefResult AllArgsMask = NoModRef;
 102     if (doesAccessArgPointees(MRB)) {
 103       for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 104            AI != AE; ++AI) {
 105         const Value *Arg = *AI;
 106         if (!Arg->getType()->isPointerTy())
 107           continue;
 108         ModRefResult ArgMask;
 109         Location CSLoc =
 110           getArgLocation(CS, (unsigned) std::distance(CS.arg_begin(), AI),
 111                          ArgMask);
 112         if (!isNoAlias(CSLoc, Loc)) {
 113           doesAlias = true;
 114           AllArgsMask = ModRefResult(AllArgsMask | ArgMask);
 115         }
 116       }
 117     }
 118     if (!doesAlias)
 119       return NoModRef;
 120     Mask = ModRefResult(Mask & AllArgsMask);
 121   }
 122
 123   // If Loc is a constant memory location, the call definitely could not
 124   // modify the memory location.
 125   if ((Mask & Mod) && pointsToConstantMemory(Loc))
 126     Mask = ModRefResult(Mask & ~Mod);
 127
 128   // If this is the end of the chain, don't forward.
 129   if (!AA) return Mask;
 130
 131   // Otherwise, fall back to the next AA in the chain. But we can merge
 132   // in any mask we've managed to compute.
 133   return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
 134 }
 135
 136 AliasAnalysis::ModRefResult
 137 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
 138   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 139
 140   // If CS1 or CS2 are readnone, they don't interact.
 141   ModRefBehavior CS1B = getModRefBehavior(CS1);
 142   if (CS1B == DoesNotAccessMemory) return NoModRef;
 143
 144   ModRefBehavior CS2B = getModRefBehavior(CS2);
 145   if (CS2B == DoesNotAccessMemory) return NoModRef;
 146
 147   // If they both only read from memory, there is no dependence.
 148   if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
 149     return NoModRef;
 150
 151   AliasAnalysis::ModRefResult Mask = ModRef;
 152
 153   // If CS1 only reads memory, the only dependence on CS2 can be
 154   // from CS1 reading memory written by CS2.
 155   if (onlyReadsMemory(CS1B))
 156     Mask = ModRefResult(Mask & Ref);
 157
 158   // If CS2 only access memory through arguments, accumulate the mod/ref
 159   // information from CS1's references to the memory referenced by
 160   // CS2's arguments.
 161   if (onlyAccessesArgPointees(CS2B)) {
 162     AliasAnalysis::ModRefResult R = NoModRef;
 163     if (doesAccessArgPointees(CS2B)) {
 164       for (ImmutableCallSite::arg_iterator
 165            I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
 166         const Value *Arg = *I;
 167         if (!Arg->getType()->isPointerTy())
 168           continue;
 169         ModRefResult ArgMask;
 170         Location CS2Loc =
 171           getArgLocation(CS2, (unsigned) std::distance(CS2.arg_begin(), I),
 172                          ArgMask);
 173         // ArgMask indicates what CS2 might do to CS2Loc, and the dependence of
 174         // CS1 on that location is the inverse.
 175         if (ArgMask == Mod)
 176           ArgMask = ModRef;
 177         else if (ArgMask == Ref)
 178           ArgMask = Mod;
 179
 180         R = ModRefResult((R | (getModRefInfo(CS1, CS2Loc) & ArgMask)) & Mask);
 181         if (R == Mask)
 182           break;
 183       }
 184     }
 185     return R;
 186   }
 187
 188   // If CS1 only accesses memory through arguments, check if CS2 references
 189   // any of the memory referenced by CS1's arguments. If not, return NoModRef.
 190   if (onlyAccessesArgPointees(CS1B)) {
 191     AliasAnalysis::ModRefResult R = NoModRef;
 192     if (doesAccessArgPointees(CS1B)) {
 193       for (ImmutableCallSite::arg_iterator
 194            I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
 195         const Value *Arg = *I;
 196         if (!Arg->getType()->isPointerTy())
 197           continue;
 198         ModRefResult ArgMask;
 199         Location CS1Loc = getArgLocation(
 200             CS1, (unsigned)std::distance(CS1.arg_begin(), I), ArgMask);
 201         // ArgMask indicates what CS1 might do to CS1Loc; if CS1 might Mod
 202         // CS1Loc, then we care about either a Mod or a Ref by CS2. If CS1
 203         // might Ref, then we care only about a Mod by CS2.
 204         ModRefResult ArgR = getModRefInfo(CS2, CS1Loc);
 205         if (((ArgMask & Mod) != NoModRef && (ArgR & ModRef) != NoModRef) ||
 206             ((ArgMask & Ref) != NoModRef && (ArgR & Mod)    != NoModRef))
 207           R = ModRefResult((R | ArgMask) & Mask);
 208
 209         if (R == Mask)
 210           break;
 211       }
 212     }
 213     return R;
 214   }
 215
 216   // If this is the end of the chain, don't forward.
 217   if (!AA) return Mask;
 218
 219   // Otherwise, fall back to the next AA in the chain. But we can merge
 220   // in any mask we've managed to compute.
 221   return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
 222 }
 223
 224 AliasAnalysis::ModRefBehavior
 225 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
 226   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 227
 228   ModRefBehavior Min = UnknownModRefBehavior;
 229
 230   // Call back into the alias analysis with the other form of getModRefBehavior
 231   // to see if it can give a better response.
 232   if (const Function *F = CS.getCalledFunction())
 233     Min = getModRefBehavior(F);
 234
 235   // If this is the end of the chain, don't forward.
 236   if (!AA) return Min;
 237
 238   // Otherwise, fall back to the next AA in the chain. But we can merge
 239   // in any result we've managed to compute.
 240   return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
 241 }
 242
 243 AliasAnalysis::ModRefBehavior
 244 AliasAnalysis::getModRefBehavior(const Function *F) {
 245   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 246   return AA->getModRefBehavior(F);
 247 }
 248
 249 //===----------------------------------------------------------------------===//
 250 // AliasAnalysis non-virtual helper method implementation
 251 //===----------------------------------------------------------------------===//
 252
 253 AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
 254   AAMDNodes AATags;
 255   LI->getAAMetadata(AATags);
 256
 257   return Location(LI->getPointerOperand(),
 258                   getTypeStoreSize(LI->getType()), AATags);
 259 }
 260
 261 AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
 262   AAMDNodes AATags;
 263   SI->getAAMetadata(AATags);
 264
 265   return Location(SI->getPointerOperand(),
 266                   getTypeStoreSize(SI->getValueOperand()->getType()), AATags);
 267 }
 268
 269 AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
 270   AAMDNodes AATags;
 271   VI->getAAMetadata(AATags);
 272
 273   return Location(VI->getPointerOperand(), UnknownSize, AATags);
 274 }
 275
 276 AliasAnalysis::Location
 277 AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
 278   AAMDNodes AATags;
 279   CXI->getAAMetadata(AATags);
 280
 281   return Location(CXI->getPointerOperand(),
 282                   getTypeStoreSize(CXI->getCompareOperand()->getType()),
 283                   AATags);
 284 }
 285
 286 AliasAnalysis::Location
 287 AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
 288   AAMDNodes AATags;
 289   RMWI->getAAMetadata(AATags);
 290
 291   return Location(RMWI->getPointerOperand(),
 292                   getTypeStoreSize(RMWI->getValOperand()->getType()), AATags);
 293 }
 294
 295 AliasAnalysis::Location
 296 AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
 297   uint64_t Size = UnknownSize;
 298   if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
 299     Size = C->getValue().getZExtValue();
 300
 301   // memcpy/memmove can have AA tags. For memcpy, they apply
 302   // to both the source and the destination.
 303   AAMDNodes AATags;
 304   MTI->getAAMetadata(AATags);
 305
 306   return Location(MTI->getRawSource(), Size, AATags);
 307 }
 308
 309 AliasAnalysis::Location
 310 AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
 311   uint64_t Size = UnknownSize;
 312   if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
 313     Size = C->getValue().getZExtValue();
 314
 315   // memcpy/memmove can have AA tags. For memcpy, they apply
 316   // to both the source and the destination.
 317   AAMDNodes AATags;
 318   MTI->getAAMetadata(AATags);
 319
 320   return Location(MTI->getRawDest(), Size, AATags);
 321 }
 322
 323
 324
 325 AliasAnalysis::ModRefResult
 326 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
 327   // Be conservative in the face of volatile/atomic.
 328   if (!L->isUnordered())
 329     return ModRef;
 330
 331   // If the load address doesn't alias the given address, it doesn't read
 332   // or write the specified memory.
 333   if (!alias(getLocation(L), Loc))
 334     return NoModRef;
 335
 336   // Otherwise, a load just reads.
 337   return Ref;
 338 }
 339
 340 AliasAnalysis::ModRefResult
 341 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
 342   // Be conservative in the face of volatile/atomic.
 343   if (!S->isUnordered())
 344     return ModRef;
 345
 346   // If the store address cannot alias the pointer in question, then the
 347   // specified memory cannot be modified by the store.
 348   if (!alias(getLocation(S), Loc))
 349     return NoModRef;
 350
 351   // If the pointer is a pointer to constant memory, then it could not have been
 352   // modified by this store.
 353   if (pointsToConstantMemory(Loc))
 354     return NoModRef;
 355
 356   // Otherwise, a store just writes.
 357   return Mod;
 358 }
 359
 360 AliasAnalysis::ModRefResult
 361 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
 362   // If the va_arg address cannot alias the pointer in question, then the
 363   // specified memory cannot be accessed by the va_arg.
 364   if (!alias(getLocation(V), Loc))
 365     return NoModRef;
 366
 367   // If the pointer is a pointer to constant memory, then it could not have been
 368   // modified by this va_arg.
 369   if (pointsToConstantMemory(Loc))
 370     return NoModRef;
 371
 372   // Otherwise, a va_arg reads and writes.
 373   return ModRef;
 374 }
 375
 376 AliasAnalysis::ModRefResult
 377 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
 378   // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
 379   if (CX->getSuccessOrdering() > Monotonic)
 380     return ModRef;
 381
 382   // If the cmpxchg address does not alias the location, it does not access it.
 383   if (!alias(getLocation(CX), Loc))
 384     return NoModRef;
 385
 386   return ModRef;
 387 }
 388
 389 AliasAnalysis::ModRefResult
 390 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
 391   // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
 392   if (RMW->getOrdering() > Monotonic)
 393     return ModRef;
 394
 395   // If the atomicrmw address does not alias the location, it does not access it.
 396   if (!alias(getLocation(RMW), Loc))
 397     return NoModRef;
 398
 399   return ModRef;
 400 }
 401
 402 // FIXME: this is really just shoring-up a deficiency in alias analysis.
 403 // BasicAA isn't willing to spend linear time determining whether an alloca
 404 // was captured before or after this particular call, while we are. However,
 405 // with a smarter AA in place, this test is just wasting compile time.
 406 AliasAnalysis::ModRefResult
 407 AliasAnalysis::callCapturesBefore(const Instruction *I,
 408                                   const AliasAnalysis::Location &MemLoc,
 409                                   DominatorTree *DT) {
 410   if (!DT || !DL) return AliasAnalysis::ModRef;
 411
 412   const Value *Object = GetUnderlyingObject(MemLoc.Ptr, DL);
 413   if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
 414       isa<Constant>(Object))
 415     return AliasAnalysis::ModRef;
 416
 417   ImmutableCallSite CS(I);
 418   if (!CS.getInstruction() || CS.getInstruction() == Object)
 419     return AliasAnalysis::ModRef;
 420
 421   if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
 422                                        /* StoreCaptures */ true, I, DT,
 423                                        /* include Object */ true))
 424     return AliasAnalysis::ModRef;
 425
 426   unsigned ArgNo = 0;
 427   AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
 428   for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
 429        CI != CE; ++CI, ++ArgNo) {
 430     // Only look at the no-capture or byval pointer arguments.  If this
 431     // pointer were passed to arguments that were neither of these, then it
 432     // couldn't be no-capture.
 433     if (!(*CI)->getType()->isPointerTy() ||
 434         (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
 435       continue;
 436
 437     // If this is a no-capture pointer argument, see if we can tell that it
 438     // is impossible to alias the pointer we're checking.  If not, we have to
 439     // assume that the call could touch the pointer, even though it doesn't
 440     // escape.
 441     if (isNoAlias(AliasAnalysis::Location(*CI),
 442                   AliasAnalysis::Location(Object)))
 443       continue;
 444     if (CS.doesNotAccessMemory(ArgNo))
 445       continue;
 446     if (CS.onlyReadsMemory(ArgNo)) {
 447       R = AliasAnalysis::Ref;
 448       continue;
 449     }
 450     return AliasAnalysis::ModRef;
 451   }
 452   return R;
 453 }
 454
 455 // AliasAnalysis destructor: DO NOT move this to the header file for
 456 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
 457 // the AliasAnalysis.o file in the current .a file, causing alias analysis
 458 // support to not be included in the tool correctly!
 459 //
 460 AliasAnalysis::~AliasAnalysis() {}
 461
 462 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
 463 /// AliasAnalysis interface before any other methods are called.
 464 ///
 465 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
 466   DataLayoutPass *DLP = P->getAnalysisIfAvailable<DataLayoutPass>();
 467   DL = DLP ? &DLP->getDataLayout() : nullptr;
 468   TLI = P->getAnalysisIfAvailable<TargetLibraryInfo>();
 469   AA = &P->getAnalysis<AliasAnalysis>();
 470 }
 471
 472 // getAnalysisUsage - All alias analysis implementations should invoke this
 473 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
 474 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 475   AU.addRequired<AliasAnalysis>();         // All AA's chain
 476 }
 477
 478 /// getTypeStoreSize - Return the DataLayout store size for the given type,
 479 /// if known, or a conservative value otherwise.
 480 ///
 481 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
 482   return DL ? DL->getTypeStoreSize(Ty) : UnknownSize;
 483 }
 484
 485 /// canBasicBlockModify - Return true if it is possible for execution of the
 486 /// specified basic block to modify the location Loc.
 487 ///
 488 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
 489                                         const Location &Loc) {
 490   return canInstructionRangeModRef(BB.front(), BB.back(), Loc, Mod);
 491 }
 492
 493 /// canInstructionRangeModRef - Return true if it is possible for the
 494 /// execution of the specified instructions to mod\ref (according to the
 495 /// mode) the location Loc. The instructions to consider are all
 496 /// of the instructions in the range of [I1,I2] INCLUSIVE.
 497 /// I1 and I2 must be in the same basic block.
 498 bool AliasAnalysis::canInstructionRangeModRef(const Instruction &I1,
 499                                               const Instruction &I2,
 500                                               const Location &Loc,
 501                                               const ModRefResult Mode) {
 502   assert(I1.getParent() == I2.getParent() &&
 503          "Instructions not in same basic block!");
 504   BasicBlock::const_iterator I = &I1;
 505   BasicBlock::const_iterator E = &I2;
 506   ++E;  // Convert from inclusive to exclusive range.
 507
 508   for (; I != E; ++I) // Check every instruction in range
 509     if (getModRefInfo(I, Loc) & Mode)
 510       return true;
 511   return false;
 512 }
 513
 514 /// isNoAliasCall - Return true if this pointer is returned by a noalias
 515 /// function.
 516 bool llvm::isNoAliasCall(const Value *V) {
 517   if (isa<CallInst>(V) || isa<InvokeInst>(V))
 518     return ImmutableCallSite(cast<Instruction>(V))
 519       .paramHasAttr(0, Attribute::NoAlias);
 520   return false;
 521 }
 522
 523 /// isNoAliasArgument - Return true if this is an argument with the noalias
 524 /// attribute.
 525 bool llvm::isNoAliasArgument(const Value *V)
 526 {
 527   if (const Argument *A = dyn_cast<Argument>(V))
 528     return A->hasNoAliasAttr();
 529   return false;
 530 }
 531
 532 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
 533 /// identifiable object.  This returns true for:
 534 ///    Global Variables and Functions (but not Global Aliases)
 535 ///    Allocas and Mallocs
 536 ///    ByVal and NoAlias Arguments
 537 ///    NoAlias returns
 538 ///
 539 bool llvm::isIdentifiedObject(const Value *V) {
 540   if (isa<AllocaInst>(V))
 541     return true;
 542   if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
 543     return true;
 544   if (isNoAliasCall(V))
 545     return true;
 546   if (const Argument *A = dyn_cast<Argument>(V))
 547     return A->hasNoAliasAttr() || A->hasByValAttr();
 548   return false;
 549 }
 550
 551 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
 552 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
 553 /// Further, an IdentifiedFunctionLocal can not alias with any function
 554 /// arguments other than itself, which is not necessarily true for
 555 /// IdentifiedObjects.
 556 bool llvm::isIdentifiedFunctionLocal(const Value *V)
 557 {
 558   return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);
 559 }