/3rd_party/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/CaptureTracking.h"
 29#include "llvm/Analysis/CFG.h"
 30#include "llvm/Analysis/Dominators.h"
 31#include "llvm/Analysis/ValueTracking.h"
 32#include "llvm/IR/BasicBlock.h"
 33#include "llvm/IR/DataLayout.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"
 41using namespace llvm;
 42
 43// Register the AliasAnalysis interface, providing a nice name to refer to.
 44INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
 45char AliasAnalysis::ID = 0;
 46
 47//===----------------------------------------------------------------------===//
 48// Default chaining methods
 49//===----------------------------------------------------------------------===//
 50
 51AliasAnalysis::AliasResult
 52AliasAnalysis::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
 57bool 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
 63void AliasAnalysis::deleteValue(Value *V) {
 64  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 65  AA->deleteValue(V);
 66}
 67
 68void AliasAnalysis::copyValue(Value *From, Value *To) {
 69  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 70  AA->copyValue(From, To);
 71}
 72
 73void AliasAnalysis::addEscapingUse(Use &U) {
 74  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 75  AA->addEscapingUse(U);
 76}
 77
 78
 79AliasAnalysis::ModRefResult
 80AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
 81                             const Location &Loc) {
 82  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
 83
 84  ModRefBehavior MRB = getModRefBehavior(CS);
 85  if (MRB == DoesNotAccessMemory)
 86    return NoModRef;
 87
 88  ModRefResult Mask = ModRef;
 89  if (onlyReadsMemory(MRB))
 90    Mask = Ref;
 91
 92  if (onlyAccessesArgPointees(MRB)) {
 93    bool doesAlias = false;
 94    if (doesAccessArgPointees(MRB)) {
 95      MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
 96      for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 97           AI != AE; ++AI) {
 98        const Value *Arg = *AI;
 99        if (!Arg->getType()->isPointerTy())
100          continue;
101        Location CSLoc(Arg, UnknownSize, CSTag);
102        if (!isNoAlias(CSLoc, Loc)) {
103          doesAlias = true;
104          break;
105        }
106      }
107    }
108    if (!doesAlias)
109      return NoModRef;
110  }
111
112  // If Loc is a constant memory location, the call definitely could not
113  // modify the memory location.
114  if ((Mask & Mod) && pointsToConstantMemory(Loc))
115    Mask = ModRefResult(Mask & ~Mod);
116
117  // If this is the end of the chain, don't forward.
118  if (!AA) return Mask;
119
120  // Otherwise, fall back to the next AA in the chain. But we can merge
121  // in any mask we've managed to compute.
122  return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
123}
124
125AliasAnalysis::ModRefResult
126AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
127  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
128
129  // If CS1 or CS2 are readnone, they don't interact.
130  ModRefBehavior CS1B = getModRefBehavior(CS1);
131  if (CS1B == DoesNotAccessMemory) return NoModRef;
132
133  ModRefBehavior CS2B = getModRefBehavior(CS2);
134  if (CS2B == DoesNotAccessMemory) return NoModRef;
135
136  // If they both only read from memory, there is no dependence.
137  if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
138    return NoModRef;
139
140  AliasAnalysis::ModRefResult Mask = ModRef;
141
142  // If CS1 only reads memory, the only dependence on CS2 can be
143  // from CS1 reading memory written by CS2.
144  if (onlyReadsMemory(CS1B))
145    Mask = ModRefResult(Mask & Ref);
146
147  // If CS2 only access memory through arguments, accumulate the mod/ref
148  // information from CS1's references to the memory referenced by
149  // CS2's arguments.
150  if (onlyAccessesArgPointees(CS2B)) {
151    AliasAnalysis::ModRefResult R = NoModRef;
152    if (doesAccessArgPointees(CS2B)) {
153      MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
154      for (ImmutableCallSite::arg_iterator
155           I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
156        const Value *Arg = *I;
157        if (!Arg->getType()->isPointerTy())
158          continue;
159        Location CS2Loc(Arg, UnknownSize, CS2Tag);
160        R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask);
161        if (R == Mask)
162          break;
163      }
164    }
165    return R;
166  }
167
168  // If CS1 only accesses memory through arguments, check if CS2 references
169  // any of the memory referenced by CS1's arguments. If not, return NoModRef.
170  if (onlyAccessesArgPointees(CS1B)) {
171    AliasAnalysis::ModRefResult R = NoModRef;
172    if (doesAccessArgPointees(CS1B)) {
173      MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
174      for (ImmutableCallSite::arg_iterator
175           I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
176        const Value *Arg = *I;
177        if (!Arg->getType()->isPointerTy())
178          continue;
179        Location CS1Loc(Arg, UnknownSize, CS1Tag);
180        if (getModRefInfo(CS2, CS1Loc) != NoModRef) {
181          R = Mask;
182          break;
183        }
184      }
185    }
186    if (R == NoModRef)
187      return R;
188  }
189
190  // If this is the end of the chain, don't forward.
191  if (!AA) return Mask;
192
193  // Otherwise, fall back to the next AA in the chain. But we can merge
194  // in any mask we've managed to compute.
195  return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
196}
197
198AliasAnalysis::ModRefBehavior
199AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
200  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
201
202  ModRefBehavior Min = UnknownModRefBehavior;
203
204  // Call back into the alias analysis with the other form of getModRefBehavior
205  // to see if it can give a better response.
206  if (const Function *F = CS.getCalledFunction())
207    Min = getModRefBehavior(F);
208
209  // If this is the end of the chain, don't forward.
210  if (!AA) return Min;
211
212  // Otherwise, fall back to the next AA in the chain. But we can merge
213  // in any result we've managed to compute.
214  return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
215}
216
217AliasAnalysis::ModRefBehavior
218AliasAnalysis::getModRefBehavior(const Function *F) {
219  assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
220  return AA->getModRefBehavior(F);
221}
222
223//===----------------------------------------------------------------------===//
224// AliasAnalysis non-virtual helper method implementation
225//===----------------------------------------------------------------------===//
226
227AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
228  return Location(LI->getPointerOperand(),
229                  getTypeStoreSize(LI->getType()),
230                  LI->getMetadata(LLVMContext::MD_tbaa));
231}
232
233AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
234  return Location(SI->getPointerOperand(),
235                  getTypeStoreSize(SI->getValueOperand()->getType()),
236                  SI->getMetadata(LLVMContext::MD_tbaa));
237}
238
239AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
240  return Location(VI->getPointerOperand(),
241                  UnknownSize,
242                  VI->getMetadata(LLVMContext::MD_tbaa));
243}
244
245AliasAnalysis::Location
246AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
247  return Location(CXI->getPointerOperand(),
248                  getTypeStoreSize(CXI->getCompareOperand()->getType()),
249                  CXI->getMetadata(LLVMContext::MD_tbaa));
250}
251
252AliasAnalysis::Location
253AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
254  return Location(RMWI->getPointerOperand(),
255                  getTypeStoreSize(RMWI->getValOperand()->getType()),
256                  RMWI->getMetadata(LLVMContext::MD_tbaa));
257}
258
259AliasAnalysis::Location 
260AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
261  uint64_t Size = UnknownSize;
262  if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
263    Size = C->getValue().getZExtValue();
264
265  // memcpy/memmove can have TBAA tags. For memcpy, they apply
266  // to both the source and the destination.
267  MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
268
269  return Location(MTI->getRawSource(), Size, TBAATag);
270}
271
272AliasAnalysis::Location 
273AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
274  uint64_t Size = UnknownSize;
275  if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
276    Size = C->getValue().getZExtValue();
277
278  // memcpy/memmove can have TBAA tags. For memcpy, they apply
279  // to both the source and the destination.
280  MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
281  
282  return Location(MTI->getRawDest(), Size, TBAATag);
283}
284
285
286
287AliasAnalysis::ModRefResult
288AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
289  // Be conservative in the face of volatile/atomic.
290  if (!L->isUnordered())
291    return ModRef;
292
293  // If the load address doesn't alias the given address, it doesn't read
294  // or write the specified memory.
295  if (!alias(getLocation(L), Loc))
296    return NoModRef;
297
298  // Otherwise, a load just reads.
299  return Ref;
300}
301
302AliasAnalysis::ModRefResult
303AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
304  // Be conservative in the face of volatile/atomic.
305  if (!S->isUnordered())
306    return ModRef;
307
308  // If the store address cannot alias the pointer in question, then the
309  // specified memory cannot be modified by the store.
310  if (!alias(getLocation(S), Loc))
311    return NoModRef;
312
313  // If the pointer is a pointer to constant memory, then it could not have been
314  // modified by this store.
315  if (pointsToConstantMemory(Loc))
316    return NoModRef;
317
318  // Otherwise, a store just writes.
319  return Mod;
320}
321
322AliasAnalysis::ModRefResult
323AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
324  // If the va_arg address cannot alias the pointer in question, then the
325  // specified memory cannot be accessed by the va_arg.
326  if (!alias(getLocation(V), Loc))
327    return NoModRef;
328
329  // If the pointer is a pointer to constant memory, then it could not have been
330  // modified by this va_arg.
331  if (pointsToConstantMemory(Loc))
332    return NoModRef;
333
334  // Otherwise, a va_arg reads and writes.
335  return ModRef;
336}
337
338AliasAnalysis::ModRefResult
339AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
340  // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
341  if (CX->getOrdering() > Monotonic)
342    return ModRef;
343
344  // If the cmpxchg address does not alias the location, it does not access it.
345  if (!alias(getLocation(CX), Loc))
346    return NoModRef;
347
348  return ModRef;
349}
350
351AliasAnalysis::ModRefResult
352AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
353  // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
354  if (RMW->getOrdering() > Monotonic)
355    return ModRef;
356
357  // If the atomicrmw address does not alias the location, it does not access it.
358  if (!alias(getLocation(RMW), Loc))
359    return NoModRef;
360
361  return ModRef;
362}
363
364namespace {
365  /// Only find pointer captures which happen before the given instruction. Uses
366  /// the dominator tree to determine whether one instruction is before another.
367  /// Only support the case where the Value is defined in the same basic block
368  /// as the given instruction and the use.
369  struct CapturesBefore : public CaptureTracker {
370    CapturesBefore(const Instruction *I, DominatorTree *DT)
371      : BeforeHere(I), DT(DT), Captured(false) {}
372
373    void tooManyUses() { Captured = true; }
374
375    bool shouldExplore(Use *U) {
376      Instruction *I = cast<Instruction>(U->getUser());
377      BasicBlock *BB = I->getParent();
378      // We explore this usage only if the usage can reach "BeforeHere".
379      // If use is not reachable from entry, there is no need to explore.
380      if (BeforeHere != I && !DT->isReachableFromEntry(BB))
381        return false;
382      // If the value is defined in the same basic block as use and BeforeHere,
383      // there is no need to explore the use if BeforeHere dominates use.
384      // Check whether there is a path from I to BeforeHere.
385      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
386          !isPotentiallyReachable(I, BeforeHere, DT))
387        return false;
388      return true;
389    }
390
391    bool captured(Use *U) {
392      Instruction *I = cast<Instruction>(U->getUser());
393      BasicBlock *BB = I->getParent();
394      // Same logic as in shouldExplore.
395      if (BeforeHere != I && !DT->isReachableFromEntry(BB))
396        return false;
397      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
398          !isPotentiallyReachable(I, BeforeHere, DT))
399        return false;
400      Captured = true;
401      return true;
402    }
403
404    const Instruction *BeforeHere;
405    DominatorTree *DT;
406
407    bool Captured;
408  };
409}
410
411// FIXME: this is really just shoring-up a deficiency in alias analysis.
412// BasicAA isn't willing to spend linear time determining whether an alloca
413// was captured before or after this particular call, while we are. However,
414// with a smarter AA in place, this test is just wasting compile time.
415AliasAnalysis::ModRefResult
416AliasAnalysis::callCapturesBefore(const Instruction *I,
417                                  const AliasAnalysis::Location &MemLoc,
418                                  DominatorTree *DT) {
419  if (!DT || !TD) return AliasAnalysis::ModRef;
420
421  const Value *Object = GetUnderlyingObject(MemLoc.Ptr, TD);
422  if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
423      isa<Constant>(Object))
424    return AliasAnalysis::ModRef;
425
426  ImmutableCallSite CS(I);
427  if (!CS.getInstruction() || CS.getInstruction() == Object)
428    return AliasAnalysis::ModRef;
429
430  CapturesBefore CB(I, DT);
431  llvm::PointerMayBeCaptured(Object, &CB);
432  if (CB.Captured)
433    return AliasAnalysis::ModRef;
434
435  unsigned ArgNo = 0;
436  AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
437  for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
438       CI != CE; ++CI, ++ArgNo) {
439    // Only look at the no-capture or byval pointer arguments.  If this
440    // pointer were passed to arguments that were neither of these, then it
441    // couldn't be no-capture.
442    if (!(*CI)->getType()->isPointerTy() ||
443        (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
444      continue;
445
446    // If this is a no-capture pointer argument, see if we can tell that it
447    // is impossible to alias the pointer we're checking.  If not, we have to
448    // assume that the call could touch the pointer, even though it doesn't
449    // escape.
450    if (isNoAlias(AliasAnalysis::Location(*CI),
451		  AliasAnalysis::Location(Object)))
452      continue;
453    if (CS.doesNotAccessMemory(ArgNo))
454      continue;
455    if (CS.onlyReadsMemory(ArgNo)) {
456      R = AliasAnalysis::Ref;
457      continue;
458    }
459    return AliasAnalysis::ModRef;
460  }
461  return R;
462}
463
464// AliasAnalysis destructor: DO NOT move this to the header file for
465// AliasAnalysis or else clients of the AliasAnalysis class may not depend on
466// the AliasAnalysis.o file in the current .a file, causing alias analysis
467// support to not be included in the tool correctly!
468//
469AliasAnalysis::~AliasAnalysis() {}
470
471/// InitializeAliasAnalysis - Subclasses must call this method to initialize the
472/// AliasAnalysis interface before any other methods are called.
473///
474void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
475  TD = P->getAnalysisIfAvailable<DataLayout>();
476  TLI = P->getAnalysisIfAvailable<TargetLibraryInfo>();
477  AA = &P->getAnalysis<AliasAnalysis>();
478}
479
480// getAnalysisUsage - All alias analysis implementations should invoke this
481// directly (using AliasAnalysis::getAnalysisUsage(AU)).
482void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
483  AU.addRequired<AliasAnalysis>();         // All AA's chain
484}
485
486/// getTypeStoreSize - Return the DataLayout store size for the given type,
487/// if known, or a conservative value otherwise.
488///
489uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
490  return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
491}
492
493/// canBasicBlockModify - Return true if it is possible for execution of the
494/// specified basic block to modify the value pointed to by Ptr.
495///
496bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
497                                        const Location &Loc) {
498  return canInstructionRangeModify(BB.front(), BB.back(), Loc);
499}
500
501/// canInstructionRangeModify - Return true if it is possible for the execution
502/// of the specified instructions to modify the value pointed to by Ptr.  The
503/// instructions to consider are all of the instructions in the range of [I1,I2]
504/// INCLUSIVE.  I1 and I2 must be in the same basic block.
505///
506bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
507                                              const Instruction &I2,
508                                              const Location &Loc) {
509  assert(I1.getParent() == I2.getParent() &&
510         "Instructions not in same basic block!");
511  BasicBlock::const_iterator I = &I1;
512  BasicBlock::const_iterator E = &I2;
513  ++E;  // Convert from inclusive to exclusive range.
514
515  for (; I != E; ++I) // Check every instruction in range
516    if (getModRefInfo(I, Loc) & Mod)
517      return true;
518  return false;
519}
520
521/// isNoAliasCall - Return true if this pointer is returned by a noalias
522/// function.
523bool llvm::isNoAliasCall(const Value *V) {
524  if (isa<CallInst>(V) || isa<InvokeInst>(V))
525    return ImmutableCallSite(cast<Instruction>(V))
526      .paramHasAttr(0, Attribute::NoAlias);
527  return false;
528}
529
530/// isNoAliasArgument - Return true if this is an argument with the noalias
531/// attribute.
532bool llvm::isNoAliasArgument(const Value *V)
533{
534  if (const Argument *A = dyn_cast<Argument>(V))
535    return A->hasNoAliasAttr();
536  return false;
537}
538
539/// isIdentifiedObject - Return true if this pointer refers to a distinct and
540/// identifiable object.  This returns true for:
541///    Global Variables and Functions (but not Global Aliases)
542///    Allocas and Mallocs
543///    ByVal and NoAlias Arguments
544///    NoAlias returns
545///
546bool llvm::isIdentifiedObject(const Value *V) {
547  if (isa<AllocaInst>(V))
548    return true;
549  if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
550    return true;
551  if (isNoAliasCall(V))
552    return true;
553  if (const Argument *A = dyn_cast<Argument>(V))
554    return A->hasNoAliasAttr() || A->hasByValAttr();
555  return false;
556}