1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
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,
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
25 //===----------------------------------------------------------------------===//
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/Dominators.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/IR/BasicBlock.h"
32 #include "llvm/IR/DataLayout.h"
33 #include "llvm/IR/Function.h"
34 #include "llvm/IR/Instructions.h"
35 #include "llvm/IR/IntrinsicInst.h"
36 #include "llvm/IR/LLVMContext.h"
37 #include "llvm/IR/Type.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Target/TargetLibraryInfo.h"
42 // Register the AliasAnalysis interface, providing a nice name to refer to.
43 INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
44 char AliasAnalysis::ID = 0;
46 //===----------------------------------------------------------------------===//
47 // Default chaining methods
48 //===----------------------------------------------------------------------===//
50 AliasAnalysis::AliasResult
51 AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
52 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
53 return AA->alias(LocA, LocB);
56 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
58 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
59 return AA->pointsToConstantMemory(Loc, OrLocal);
62 void AliasAnalysis::deleteValue(Value *V) {
63 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
67 void AliasAnalysis::copyValue(Value *From, Value *To) {
68 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
69 AA->copyValue(From, To);
72 void AliasAnalysis::addEscapingUse(Use &U) {
73 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
74 AA->addEscapingUse(U);
78 AliasAnalysis::ModRefResult
79 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
80 const Location &Loc) {
81 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
83 ModRefBehavior MRB = getModRefBehavior(CS);
84 if (MRB == DoesNotAccessMemory)
87 ModRefResult Mask = ModRef;
88 if (onlyReadsMemory(MRB))
91 if (onlyAccessesArgPointees(MRB)) {
92 bool doesAlias = false;
93 if (doesAccessArgPointees(MRB)) {
94 MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
95 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
97 const Value *Arg = *AI;
98 if (!Arg->getType()->isPointerTy())
100 Location CSLoc(Arg, UnknownSize, CSTag);
101 if (!isNoAlias(CSLoc, Loc)) {
111 // If Loc is a constant memory location, the call definitely could not
112 // modify the memory location.
113 if ((Mask & Mod) && pointsToConstantMemory(Loc))
114 Mask = ModRefResult(Mask & ~Mod);
116 // If this is the end of the chain, don't forward.
117 if (!AA) return Mask;
119 // Otherwise, fall back to the next AA in the chain. But we can merge
120 // in any mask we've managed to compute.
121 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
124 AliasAnalysis::ModRefResult
125 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
126 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
128 // If CS1 or CS2 are readnone, they don't interact.
129 ModRefBehavior CS1B = getModRefBehavior(CS1);
130 if (CS1B == DoesNotAccessMemory) return NoModRef;
132 ModRefBehavior CS2B = getModRefBehavior(CS2);
133 if (CS2B == DoesNotAccessMemory) return NoModRef;
135 // If they both only read from memory, there is no dependence.
136 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
139 AliasAnalysis::ModRefResult Mask = ModRef;
141 // If CS1 only reads memory, the only dependence on CS2 can be
142 // from CS1 reading memory written by CS2.
143 if (onlyReadsMemory(CS1B))
144 Mask = ModRefResult(Mask & Ref);
146 // If CS2 only access memory through arguments, accumulate the mod/ref
147 // information from CS1's references to the memory referenced by
149 if (onlyAccessesArgPointees(CS2B)) {
150 AliasAnalysis::ModRefResult R = NoModRef;
151 if (doesAccessArgPointees(CS2B)) {
152 MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
153 for (ImmutableCallSite::arg_iterator
154 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
155 const Value *Arg = *I;
156 if (!Arg->getType()->isPointerTy())
158 Location CS2Loc(Arg, UnknownSize, CS2Tag);
159 R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask);
167 // If CS1 only accesses memory through arguments, check if CS2 references
168 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
169 if (onlyAccessesArgPointees(CS1B)) {
170 AliasAnalysis::ModRefResult R = NoModRef;
171 if (doesAccessArgPointees(CS1B)) {
172 MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
173 for (ImmutableCallSite::arg_iterator
174 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
175 const Value *Arg = *I;
176 if (!Arg->getType()->isPointerTy())
178 Location CS1Loc(Arg, UnknownSize, CS1Tag);
179 if (getModRefInfo(CS2, CS1Loc) != NoModRef) {
189 // If this is the end of the chain, don't forward.
190 if (!AA) return Mask;
192 // Otherwise, fall back to the next AA in the chain. But we can merge
193 // in any mask we've managed to compute.
194 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
197 AliasAnalysis::ModRefBehavior
198 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
199 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
201 ModRefBehavior Min = UnknownModRefBehavior;
203 // Call back into the alias analysis with the other form of getModRefBehavior
204 // to see if it can give a better response.
205 if (const Function *F = CS.getCalledFunction())
206 Min = getModRefBehavior(F);
208 // If this is the end of the chain, don't forward.
211 // Otherwise, fall back to the next AA in the chain. But we can merge
212 // in any result we've managed to compute.
213 return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
216 AliasAnalysis::ModRefBehavior
217 AliasAnalysis::getModRefBehavior(const Function *F) {
218 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
219 return AA->getModRefBehavior(F);
222 //===----------------------------------------------------------------------===//
223 // AliasAnalysis non-virtual helper method implementation
224 //===----------------------------------------------------------------------===//
226 AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
227 return Location(LI->getPointerOperand(),
228 getTypeStoreSize(LI->getType()),
229 LI->getMetadata(LLVMContext::MD_tbaa));
232 AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
233 return Location(SI->getPointerOperand(),
234 getTypeStoreSize(SI->getValueOperand()->getType()),
235 SI->getMetadata(LLVMContext::MD_tbaa));
238 AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
239 return Location(VI->getPointerOperand(),
241 VI->getMetadata(LLVMContext::MD_tbaa));
244 AliasAnalysis::Location
245 AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
246 return Location(CXI->getPointerOperand(),
247 getTypeStoreSize(CXI->getCompareOperand()->getType()),
248 CXI->getMetadata(LLVMContext::MD_tbaa));
251 AliasAnalysis::Location
252 AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
253 return Location(RMWI->getPointerOperand(),
254 getTypeStoreSize(RMWI->getValOperand()->getType()),
255 RMWI->getMetadata(LLVMContext::MD_tbaa));
258 AliasAnalysis::Location
259 AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
260 uint64_t Size = UnknownSize;
261 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
262 Size = C->getValue().getZExtValue();
264 // memcpy/memmove can have TBAA tags. For memcpy, they apply
265 // to both the source and the destination.
266 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
268 return Location(MTI->getRawSource(), Size, TBAATag);
271 AliasAnalysis::Location
272 AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
273 uint64_t Size = UnknownSize;
274 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
275 Size = C->getValue().getZExtValue();
277 // memcpy/memmove can have TBAA tags. For memcpy, they apply
278 // to both the source and the destination.
279 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
281 return Location(MTI->getRawDest(), Size, TBAATag);
286 AliasAnalysis::ModRefResult
287 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
288 // Be conservative in the face of volatile/atomic.
289 if (!L->isUnordered())
292 // If the load address doesn't alias the given address, it doesn't read
293 // or write the specified memory.
294 if (!alias(getLocation(L), Loc))
297 // Otherwise, a load just reads.
301 AliasAnalysis::ModRefResult
302 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
303 // Be conservative in the face of volatile/atomic.
304 if (!S->isUnordered())
307 // If the store address cannot alias the pointer in question, then the
308 // specified memory cannot be modified by the store.
309 if (!alias(getLocation(S), Loc))
312 // If the pointer is a pointer to constant memory, then it could not have been
313 // modified by this store.
314 if (pointsToConstantMemory(Loc))
317 // Otherwise, a store just writes.
321 AliasAnalysis::ModRefResult
322 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
323 // If the va_arg address cannot alias the pointer in question, then the
324 // specified memory cannot be accessed by the va_arg.
325 if (!alias(getLocation(V), Loc))
328 // If the pointer is a pointer to constant memory, then it could not have been
329 // modified by this va_arg.
330 if (pointsToConstantMemory(Loc))
333 // Otherwise, a va_arg reads and writes.
337 AliasAnalysis::ModRefResult
338 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
339 // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
340 if (CX->getOrdering() > Monotonic)
343 // If the cmpxchg address does not alias the location, it does not access it.
344 if (!alias(getLocation(CX), Loc))
350 AliasAnalysis::ModRefResult
351 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
352 // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
353 if (RMW->getOrdering() > Monotonic)
356 // If the atomicrmw address does not alias the location, it does not access it.
357 if (!alias(getLocation(RMW), Loc))
364 /// Determine whether there is a path from From to To within a single
365 /// function. Returns false only if we can prove that once 'From' has been
366 /// executed then 'To' can not be executed. Conservatively returns true.
367 static bool isPotentiallyReachable(const BasicBlock *From,
368 const BasicBlock *To) {
369 const unsigned MaxCheck = 5;
370 const BasicBlock *Current = From;
371 for (unsigned I = 0; I < MaxCheck; I++) {
372 unsigned NumSuccs = Current->getTerminator()->getNumSuccessors();
377 Current = Current->getTerminator()->getSuccessor(0);
384 /// Only find pointer captures which happen before the given instruction. Uses
385 /// the dominator tree to determine whether one instruction is before another.
386 /// Only support the case where the Value is defined in the same basic block
387 /// as the given instruction and the use.
388 struct CapturesBefore : public CaptureTracker {
389 CapturesBefore(const Instruction *I, DominatorTree *DT)
390 : BeforeHere(I), DT(DT), Captured(false) {}
392 void tooManyUses() { Captured = true; }
394 bool shouldExplore(Use *U) {
395 Instruction *I = cast<Instruction>(U->getUser());
396 BasicBlock *BB = I->getParent();
397 // We explore this usage only if the usage can reach "BeforeHere".
398 // If use is not reachable from entry, there is no need to explore.
399 if (BeforeHere != I && !DT->isReachableFromEntry(BB))
401 // If the value is defined in the same basic block as use and BeforeHere,
402 // there is no need to explore the use if BeforeHere dominates use.
403 // Check whether there is a path from I to BeforeHere.
404 if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
405 !isPotentiallyReachable(BB, BeforeHere->getParent()))
410 bool captured(Use *U) {
411 Instruction *I = cast<Instruction>(U->getUser());
412 BasicBlock *BB = I->getParent();
413 // Same logic as in shouldExplore.
414 if (BeforeHere != I && !DT->isReachableFromEntry(BB))
416 if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
417 !isPotentiallyReachable(BB, BeforeHere->getParent()))
423 const Instruction *BeforeHere;
430 // FIXME: this is really just shoring-up a deficiency in alias analysis.
431 // BasicAA isn't willing to spend linear time determining whether an alloca
432 // was captured before or after this particular call, while we are. However,
433 // with a smarter AA in place, this test is just wasting compile time.
434 AliasAnalysis::ModRefResult
435 AliasAnalysis::callCapturesBefore(const Instruction *I,
436 const AliasAnalysis::Location &MemLoc,
438 if (!DT || !TD) return AliasAnalysis::ModRef;
440 const Value *Object = GetUnderlyingObject(MemLoc.Ptr, TD);
441 if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
442 isa<Constant>(Object))
443 return AliasAnalysis::ModRef;
445 ImmutableCallSite CS(I);
446 if (!CS.getInstruction() || CS.getInstruction() == Object)
447 return AliasAnalysis::ModRef;
449 CapturesBefore CB(I, DT);
450 llvm::PointerMayBeCaptured(Object, &CB);
452 return AliasAnalysis::ModRef;
455 AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
456 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
457 CI != CE; ++CI, ++ArgNo) {
458 // Only look at the no-capture or byval pointer arguments. If this
459 // pointer were passed to arguments that were neither of these, then it
460 // couldn't be no-capture.
461 if (!(*CI)->getType()->isPointerTy() ||
462 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
465 // If this is a no-capture pointer argument, see if we can tell that it
466 // is impossible to alias the pointer we're checking. If not, we have to
467 // assume that the call could touch the pointer, even though it doesn't
469 if (isNoAlias(AliasAnalysis::Location(*CI),
470 AliasAnalysis::Location(Object)))
472 if (CS.doesNotAccessMemory(ArgNo))
474 if (CS.onlyReadsMemory(ArgNo)) {
475 R = AliasAnalysis::Ref;
478 return AliasAnalysis::ModRef;
483 // AliasAnalysis destructor: DO NOT move this to the header file for
484 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
485 // the AliasAnalysis.o file in the current .a file, causing alias analysis
486 // support to not be included in the tool correctly!
488 AliasAnalysis::~AliasAnalysis() {}
490 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
491 /// AliasAnalysis interface before any other methods are called.
493 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
494 TD = P->getAnalysisIfAvailable<DataLayout>();
495 TLI = P->getAnalysisIfAvailable<TargetLibraryInfo>();
496 AA = &P->getAnalysis<AliasAnalysis>();
499 // getAnalysisUsage - All alias analysis implementations should invoke this
500 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
501 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
502 AU.addRequired<AliasAnalysis>(); // All AA's chain
505 /// getTypeStoreSize - Return the DataLayout store size for the given type,
506 /// if known, or a conservative value otherwise.
508 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
509 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
512 /// canBasicBlockModify - Return true if it is possible for execution of the
513 /// specified basic block to modify the value pointed to by Ptr.
515 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
516 const Location &Loc) {
517 return canInstructionRangeModify(BB.front(), BB.back(), Loc);
520 /// canInstructionRangeModify - Return true if it is possible for the execution
521 /// of the specified instructions to modify the value pointed to by Ptr. The
522 /// instructions to consider are all of the instructions in the range of [I1,I2]
523 /// INCLUSIVE. I1 and I2 must be in the same basic block.
525 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
526 const Instruction &I2,
527 const Location &Loc) {
528 assert(I1.getParent() == I2.getParent() &&
529 "Instructions not in same basic block!");
530 BasicBlock::const_iterator I = &I1;
531 BasicBlock::const_iterator E = &I2;
532 ++E; // Convert from inclusive to exclusive range.
534 for (; I != E; ++I) // Check every instruction in range
535 if (getModRefInfo(I, Loc) & Mod)
540 /// isNoAliasCall - Return true if this pointer is returned by a noalias
542 bool llvm::isNoAliasCall(const Value *V) {
543 if (isa<CallInst>(V) || isa<InvokeInst>(V))
544 return ImmutableCallSite(cast<Instruction>(V))
545 .paramHasAttr(0, Attribute::NoAlias);
549 /// isNoAliasArgument - Return true if this is an argument with the noalias
551 bool llvm::isNoAliasArgument(const Value *V)
553 if (const Argument *A = dyn_cast<Argument>(V))
554 return A->hasNoAliasAttr();
558 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
559 /// identifiable object. This returns true for:
560 /// Global Variables and Functions (but not Global Aliases)
561 /// Allocas and Mallocs
562 /// ByVal and NoAlias Arguments
565 bool llvm::isIdentifiedObject(const Value *V) {
566 if (isa<AllocaInst>(V))
568 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
570 if (isNoAliasCall(V))
572 if (const Argument *A = dyn_cast<Argument>(V))
573 return A->hasNoAliasAttr() || A->hasByValAttr();