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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/Pass.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/Function.h"
31 #include "llvm/IntrinsicInst.h"
32 #include "llvm/Instructions.h"
33 #include "llvm/LLVMContext.h"
34 #include "llvm/Type.h"
35 #include "llvm/Target/TargetData.h"
36 using namespace llvm;
37 
38 // Register the AliasAnalysis interface, providing a nice name to refer to.
39 INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
40 char AliasAnalysis::ID = 0;
41 
42 //===----------------------------------------------------------------------===//
43 // Default chaining methods
44 //===----------------------------------------------------------------------===//
45 
46 AliasAnalysis::AliasResult
alias(const Location & LocA,const Location & LocB)47 AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
48   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
49   return AA->alias(LocA, LocB);
50 }
51 
pointsToConstantMemory(const Location & Loc,bool OrLocal)52 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
53                                            bool OrLocal) {
54   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
55   return AA->pointsToConstantMemory(Loc, OrLocal);
56 }
57 
deleteValue(Value * V)58 void AliasAnalysis::deleteValue(Value *V) {
59   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
60   AA->deleteValue(V);
61 }
62 
copyValue(Value * From,Value * To)63 void AliasAnalysis::copyValue(Value *From, Value *To) {
64   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
65   AA->copyValue(From, To);
66 }
67 
addEscapingUse(Use & U)68 void AliasAnalysis::addEscapingUse(Use &U) {
69   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
70   AA->addEscapingUse(U);
71 }
72 
73 
74 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS,const Location & Loc)75 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
76                              const Location &Loc) {
77   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
78 
79   ModRefBehavior MRB = getModRefBehavior(CS);
80   if (MRB == DoesNotAccessMemory)
81     return NoModRef;
82 
83   ModRefResult Mask = ModRef;
84   if (onlyReadsMemory(MRB))
85     Mask = Ref;
86 
87   if (onlyAccessesArgPointees(MRB)) {
88     bool doesAlias = false;
89     if (doesAccessArgPointees(MRB)) {
90       MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
91       for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
92            AI != AE; ++AI) {
93         const Value *Arg = *AI;
94         if (!Arg->getType()->isPointerTy())
95           continue;
96         Location CSLoc(Arg, UnknownSize, CSTag);
97         if (!isNoAlias(CSLoc, Loc)) {
98           doesAlias = true;
99           break;
100         }
101       }
102     }
103     if (!doesAlias)
104       return NoModRef;
105   }
106 
107   // If Loc is a constant memory location, the call definitely could not
108   // modify the memory location.
109   if ((Mask & Mod) && pointsToConstantMemory(Loc))
110     Mask = ModRefResult(Mask & ~Mod);
111 
112   // If this is the end of the chain, don't forward.
113   if (!AA) return Mask;
114 
115   // Otherwise, fall back to the next AA in the chain. But we can merge
116   // in any mask we've managed to compute.
117   return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
118 }
119 
120 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS1,ImmutableCallSite CS2)121 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
122   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
123 
124   // If CS1 or CS2 are readnone, they don't interact.
125   ModRefBehavior CS1B = getModRefBehavior(CS1);
126   if (CS1B == DoesNotAccessMemory) return NoModRef;
127 
128   ModRefBehavior CS2B = getModRefBehavior(CS2);
129   if (CS2B == DoesNotAccessMemory) return NoModRef;
130 
131   // If they both only read from memory, there is no dependence.
132   if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
133     return NoModRef;
134 
135   AliasAnalysis::ModRefResult Mask = ModRef;
136 
137   // If CS1 only reads memory, the only dependence on CS2 can be
138   // from CS1 reading memory written by CS2.
139   if (onlyReadsMemory(CS1B))
140     Mask = ModRefResult(Mask & Ref);
141 
142   // If CS2 only access memory through arguments, accumulate the mod/ref
143   // information from CS1's references to the memory referenced by
144   // CS2's arguments.
145   if (onlyAccessesArgPointees(CS2B)) {
146     AliasAnalysis::ModRefResult R = NoModRef;
147     if (doesAccessArgPointees(CS2B)) {
148       MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
149       for (ImmutableCallSite::arg_iterator
150            I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
151         const Value *Arg = *I;
152         if (!Arg->getType()->isPointerTy())
153           continue;
154         Location CS2Loc(Arg, UnknownSize, CS2Tag);
155         R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask);
156         if (R == Mask)
157           break;
158       }
159     }
160     return R;
161   }
162 
163   // If CS1 only accesses memory through arguments, check if CS2 references
164   // any of the memory referenced by CS1's arguments. If not, return NoModRef.
165   if (onlyAccessesArgPointees(CS1B)) {
166     AliasAnalysis::ModRefResult R = NoModRef;
167     if (doesAccessArgPointees(CS1B)) {
168       MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
169       for (ImmutableCallSite::arg_iterator
170            I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
171         const Value *Arg = *I;
172         if (!Arg->getType()->isPointerTy())
173           continue;
174         Location CS1Loc(Arg, UnknownSize, CS1Tag);
175         if (getModRefInfo(CS2, CS1Loc) != NoModRef) {
176           R = Mask;
177           break;
178         }
179       }
180     }
181     if (R == NoModRef)
182       return R;
183   }
184 
185   // If this is the end of the chain, don't forward.
186   if (!AA) return Mask;
187 
188   // Otherwise, fall back to the next AA in the chain. But we can merge
189   // in any mask we've managed to compute.
190   return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
191 }
192 
193 AliasAnalysis::ModRefBehavior
getModRefBehavior(ImmutableCallSite CS)194 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
195   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
196 
197   ModRefBehavior Min = UnknownModRefBehavior;
198 
199   // Call back into the alias analysis with the other form of getModRefBehavior
200   // to see if it can give a better response.
201   if (const Function *F = CS.getCalledFunction())
202     Min = getModRefBehavior(F);
203 
204   // If this is the end of the chain, don't forward.
205   if (!AA) return Min;
206 
207   // Otherwise, fall back to the next AA in the chain. But we can merge
208   // in any result we've managed to compute.
209   return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
210 }
211 
212 AliasAnalysis::ModRefBehavior
getModRefBehavior(const Function * F)213 AliasAnalysis::getModRefBehavior(const Function *F) {
214   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
215   return AA->getModRefBehavior(F);
216 }
217 
218 //===----------------------------------------------------------------------===//
219 // AliasAnalysis non-virtual helper method implementation
220 //===----------------------------------------------------------------------===//
221 
getLocation(const LoadInst * LI)222 AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
223   return Location(LI->getPointerOperand(),
224                   getTypeStoreSize(LI->getType()),
225                   LI->getMetadata(LLVMContext::MD_tbaa));
226 }
227 
getLocation(const StoreInst * SI)228 AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
229   return Location(SI->getPointerOperand(),
230                   getTypeStoreSize(SI->getValueOperand()->getType()),
231                   SI->getMetadata(LLVMContext::MD_tbaa));
232 }
233 
getLocation(const VAArgInst * VI)234 AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
235   return Location(VI->getPointerOperand(),
236                   UnknownSize,
237                   VI->getMetadata(LLVMContext::MD_tbaa));
238 }
239 
240 AliasAnalysis::Location
getLocation(const AtomicCmpXchgInst * CXI)241 AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
242   return Location(CXI->getPointerOperand(),
243                   getTypeStoreSize(CXI->getCompareOperand()->getType()),
244                   CXI->getMetadata(LLVMContext::MD_tbaa));
245 }
246 
247 AliasAnalysis::Location
getLocation(const AtomicRMWInst * RMWI)248 AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
249   return Location(RMWI->getPointerOperand(),
250                   getTypeStoreSize(RMWI->getValOperand()->getType()),
251                   RMWI->getMetadata(LLVMContext::MD_tbaa));
252 }
253 
254 AliasAnalysis::Location
getLocationForSource(const MemTransferInst * MTI)255 AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
256   uint64_t Size = UnknownSize;
257   if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
258     Size = C->getValue().getZExtValue();
259 
260   // memcpy/memmove can have TBAA tags. For memcpy, they apply
261   // to both the source and the destination.
262   MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
263 
264   return Location(MTI->getRawSource(), Size, TBAATag);
265 }
266 
267 AliasAnalysis::Location
getLocationForDest(const MemIntrinsic * MTI)268 AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
269   uint64_t Size = UnknownSize;
270   if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
271     Size = C->getValue().getZExtValue();
272 
273   // memcpy/memmove can have TBAA tags. For memcpy, they apply
274   // to both the source and the destination.
275   MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
276 
277   return Location(MTI->getRawDest(), Size, TBAATag);
278 }
279 
280 
281 
282 AliasAnalysis::ModRefResult
getModRefInfo(const LoadInst * L,const Location & Loc)283 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
284   // Be conservative in the face of volatile/atomic.
285   if (!L->isUnordered())
286     return ModRef;
287 
288   // If the load address doesn't alias the given address, it doesn't read
289   // or write the specified memory.
290   if (!alias(getLocation(L), Loc))
291     return NoModRef;
292 
293   // Otherwise, a load just reads.
294   return Ref;
295 }
296 
297 AliasAnalysis::ModRefResult
getModRefInfo(const StoreInst * S,const Location & Loc)298 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
299   // Be conservative in the face of volatile/atomic.
300   if (!S->isUnordered())
301     return ModRef;
302 
303   // If the store address cannot alias the pointer in question, then the
304   // specified memory cannot be modified by the store.
305   if (!alias(getLocation(S), Loc))
306     return NoModRef;
307 
308   // If the pointer is a pointer to constant memory, then it could not have been
309   // modified by this store.
310   if (pointsToConstantMemory(Loc))
311     return NoModRef;
312 
313   // Otherwise, a store just writes.
314   return Mod;
315 }
316 
317 AliasAnalysis::ModRefResult
getModRefInfo(const VAArgInst * V,const Location & Loc)318 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
319   // If the va_arg address cannot alias the pointer in question, then the
320   // specified memory cannot be accessed by the va_arg.
321   if (!alias(getLocation(V), Loc))
322     return NoModRef;
323 
324   // If the pointer is a pointer to constant memory, then it could not have been
325   // modified by this va_arg.
326   if (pointsToConstantMemory(Loc))
327     return NoModRef;
328 
329   // Otherwise, a va_arg reads and writes.
330   return ModRef;
331 }
332 
333 AliasAnalysis::ModRefResult
getModRefInfo(const AtomicCmpXchgInst * CX,const Location & Loc)334 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
335   // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
336   if (CX->getOrdering() > Monotonic)
337     return ModRef;
338 
339   // If the cmpxchg address does not alias the location, it does not access it.
340   if (!alias(getLocation(CX), Loc))
341     return NoModRef;
342 
343   return ModRef;
344 }
345 
346 AliasAnalysis::ModRefResult
getModRefInfo(const AtomicRMWInst * RMW,const Location & Loc)347 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
348   // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
349   if (RMW->getOrdering() > Monotonic)
350     return ModRef;
351 
352   // If the atomicrmw address does not alias the location, it does not access it.
353   if (!alias(getLocation(RMW), Loc))
354     return NoModRef;
355 
356   return ModRef;
357 }
358 
359 
360 // AliasAnalysis destructor: DO NOT move this to the header file for
361 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
362 // the AliasAnalysis.o file in the current .a file, causing alias analysis
363 // support to not be included in the tool correctly!
364 //
~AliasAnalysis()365 AliasAnalysis::~AliasAnalysis() {}
366 
367 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
368 /// AliasAnalysis interface before any other methods are called.
369 ///
InitializeAliasAnalysis(Pass * P)370 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
371   TD = P->getAnalysisIfAvailable<TargetData>();
372   AA = &P->getAnalysis<AliasAnalysis>();
373 }
374 
375 // getAnalysisUsage - All alias analysis implementations should invoke this
376 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
getAnalysisUsage(AnalysisUsage & AU) const377 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
378   AU.addRequired<AliasAnalysis>();         // All AA's chain
379 }
380 
381 /// getTypeStoreSize - Return the TargetData store size for the given type,
382 /// if known, or a conservative value otherwise.
383 ///
getTypeStoreSize(Type * Ty)384 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
385   return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
386 }
387 
388 /// canBasicBlockModify - Return true if it is possible for execution of the
389 /// specified basic block to modify the value pointed to by Ptr.
390 ///
canBasicBlockModify(const BasicBlock & BB,const Location & Loc)391 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
392                                         const Location &Loc) {
393   return canInstructionRangeModify(BB.front(), BB.back(), Loc);
394 }
395 
396 /// canInstructionRangeModify - Return true if it is possible for the execution
397 /// of the specified instructions to modify the value pointed to by Ptr.  The
398 /// instructions to consider are all of the instructions in the range of [I1,I2]
399 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
400 ///
canInstructionRangeModify(const Instruction & I1,const Instruction & I2,const Location & Loc)401 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
402                                               const Instruction &I2,
403                                               const Location &Loc) {
404   assert(I1.getParent() == I2.getParent() &&
405          "Instructions not in same basic block!");
406   BasicBlock::const_iterator I = &I1;
407   BasicBlock::const_iterator E = &I2;
408   ++E;  // Convert from inclusive to exclusive range.
409 
410   for (; I != E; ++I) // Check every instruction in range
411     if (getModRefInfo(I, Loc) & Mod)
412       return true;
413   return false;
414 }
415 
416 /// isNoAliasCall - Return true if this pointer is returned by a noalias
417 /// function.
isNoAliasCall(const Value * V)418 bool llvm::isNoAliasCall(const Value *V) {
419   if (isa<CallInst>(V) || isa<InvokeInst>(V))
420     return ImmutableCallSite(cast<Instruction>(V))
421       .paramHasAttr(0, Attribute::NoAlias);
422   return false;
423 }
424 
425 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
426 /// identifiable object.  This returns true for:
427 ///    Global Variables and Functions (but not Global Aliases)
428 ///    Allocas and Mallocs
429 ///    ByVal and NoAlias Arguments
430 ///    NoAlias returns
431 ///
isIdentifiedObject(const Value * V)432 bool llvm::isIdentifiedObject(const Value *V) {
433   if (isa<AllocaInst>(V))
434     return true;
435   if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
436     return true;
437   if (isNoAliasCall(V))
438     return true;
439   if (const Argument *A = dyn_cast<Argument>(V))
440     return A->hasNoAliasAttr() || A->hasByValAttr();
441   return false;
442 }
443