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/Pass.h"
32 #include "llvm/BasicBlock.h"
33 #include "llvm/Function.h"
34 #include "llvm/IntrinsicInst.h"
35 #include "llvm/Instructions.h"
36 #include "llvm/LLVMContext.h"
37 #include "llvm/Type.h"
38 #include "llvm/Target/TargetData.h"
41 // Register the AliasAnalysis interface, providing a nice name to refer to.
42 INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
43 char AliasAnalysis::ID = 0;
45 //===----------------------------------------------------------------------===//
46 // Default chaining methods
47 //===----------------------------------------------------------------------===//
49 AliasAnalysis::AliasResult
50 AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
51 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
52 return AA->alias(LocA, LocB);
55 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
57 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
58 return AA->pointsToConstantMemory(Loc, OrLocal);
61 void AliasAnalysis::deleteValue(Value *V) {
62 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
66 void AliasAnalysis::copyValue(Value *From, Value *To) {
67 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
68 AA->copyValue(From, To);
71 void AliasAnalysis::addEscapingUse(Use &U) {
72 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
73 AA->addEscapingUse(U);
77 AliasAnalysis::ModRefResult
78 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
79 const Location &Loc) {
80 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
82 ModRefBehavior MRB = getModRefBehavior(CS);
83 if (MRB == DoesNotAccessMemory)
86 ModRefResult Mask = ModRef;
87 if (onlyReadsMemory(MRB))
90 if (onlyAccessesArgPointees(MRB)) {
91 bool doesAlias = false;
92 if (doesAccessArgPointees(MRB)) {
93 MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
94 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
96 const Value *Arg = *AI;
97 if (!Arg->getType()->isPointerTy())
99 Location CSLoc(Arg, UnknownSize, CSTag);
100 if (!isNoAlias(CSLoc, Loc)) {
110 // If Loc is a constant memory location, the call definitely could not
111 // modify the memory location.
112 if ((Mask & Mod) && pointsToConstantMemory(Loc))
113 Mask = ModRefResult(Mask & ~Mod);
115 // If this is the end of the chain, don't forward.
116 if (!AA) return Mask;
118 // Otherwise, fall back to the next AA in the chain. But we can merge
119 // in any mask we've managed to compute.
120 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
123 AliasAnalysis::ModRefResult
124 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
125 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
127 // If CS1 or CS2 are readnone, they don't interact.
128 ModRefBehavior CS1B = getModRefBehavior(CS1);
129 if (CS1B == DoesNotAccessMemory) return NoModRef;
131 ModRefBehavior CS2B = getModRefBehavior(CS2);
132 if (CS2B == DoesNotAccessMemory) return NoModRef;
134 // If they both only read from memory, there is no dependence.
135 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
138 AliasAnalysis::ModRefResult Mask = ModRef;
140 // If CS1 only reads memory, the only dependence on CS2 can be
141 // from CS1 reading memory written by CS2.
142 if (onlyReadsMemory(CS1B))
143 Mask = ModRefResult(Mask & Ref);
145 // If CS2 only access memory through arguments, accumulate the mod/ref
146 // information from CS1's references to the memory referenced by
148 if (onlyAccessesArgPointees(CS2B)) {
149 AliasAnalysis::ModRefResult R = NoModRef;
150 if (doesAccessArgPointees(CS2B)) {
151 MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
152 for (ImmutableCallSite::arg_iterator
153 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
154 const Value *Arg = *I;
155 if (!Arg->getType()->isPointerTy())
157 Location CS2Loc(Arg, UnknownSize, CS2Tag);
158 R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask);
166 // If CS1 only accesses memory through arguments, check if CS2 references
167 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
168 if (onlyAccessesArgPointees(CS1B)) {
169 AliasAnalysis::ModRefResult R = NoModRef;
170 if (doesAccessArgPointees(CS1B)) {
171 MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
172 for (ImmutableCallSite::arg_iterator
173 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
174 const Value *Arg = *I;
175 if (!Arg->getType()->isPointerTy())
177 Location CS1Loc(Arg, UnknownSize, CS1Tag);
178 if (getModRefInfo(CS2, CS1Loc) != NoModRef) {
188 // If this is the end of the chain, don't forward.
189 if (!AA) return Mask;
191 // Otherwise, fall back to the next AA in the chain. But we can merge
192 // in any mask we've managed to compute.
193 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
196 AliasAnalysis::ModRefBehavior
197 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
198 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
200 ModRefBehavior Min = UnknownModRefBehavior;
202 // Call back into the alias analysis with the other form of getModRefBehavior
203 // to see if it can give a better response.
204 if (const Function *F = CS.getCalledFunction())
205 Min = getModRefBehavior(F);
207 // If this is the end of the chain, don't forward.
210 // Otherwise, fall back to the next AA in the chain. But we can merge
211 // in any result we've managed to compute.
212 return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
215 AliasAnalysis::ModRefBehavior
216 AliasAnalysis::getModRefBehavior(const Function *F) {
217 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
218 return AA->getModRefBehavior(F);
221 //===----------------------------------------------------------------------===//
222 // AliasAnalysis non-virtual helper method implementation
223 //===----------------------------------------------------------------------===//
225 AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
226 return Location(LI->getPointerOperand(),
227 getTypeStoreSize(LI->getType()),
228 LI->getMetadata(LLVMContext::MD_tbaa));
231 AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
232 return Location(SI->getPointerOperand(),
233 getTypeStoreSize(SI->getValueOperand()->getType()),
234 SI->getMetadata(LLVMContext::MD_tbaa));
237 AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
238 return Location(VI->getPointerOperand(),
240 VI->getMetadata(LLVMContext::MD_tbaa));
243 AliasAnalysis::Location
244 AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
245 return Location(CXI->getPointerOperand(),
246 getTypeStoreSize(CXI->getCompareOperand()->getType()),
247 CXI->getMetadata(LLVMContext::MD_tbaa));
250 AliasAnalysis::Location
251 AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
252 return Location(RMWI->getPointerOperand(),
253 getTypeStoreSize(RMWI->getValOperand()->getType()),
254 RMWI->getMetadata(LLVMContext::MD_tbaa));
257 AliasAnalysis::Location
258 AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
259 uint64_t Size = UnknownSize;
260 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
261 Size = C->getValue().getZExtValue();
263 // memcpy/memmove can have TBAA tags. For memcpy, they apply
264 // to both the source and the destination.
265 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
267 return Location(MTI->getRawSource(), Size, TBAATag);
270 AliasAnalysis::Location
271 AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
272 uint64_t Size = UnknownSize;
273 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
274 Size = C->getValue().getZExtValue();
276 // memcpy/memmove can have TBAA tags. For memcpy, they apply
277 // to both the source and the destination.
278 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
280 return Location(MTI->getRawDest(), Size, TBAATag);
285 AliasAnalysis::ModRefResult
286 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
287 // Be conservative in the face of volatile/atomic.
288 if (!L->isUnordered())
291 // If the load address doesn't alias the given address, it doesn't read
292 // or write the specified memory.
293 if (!alias(getLocation(L), Loc))
296 // Otherwise, a load just reads.
300 AliasAnalysis::ModRefResult
301 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
302 // Be conservative in the face of volatile/atomic.
303 if (!S->isUnordered())
306 // If the store address cannot alias the pointer in question, then the
307 // specified memory cannot be modified by the store.
308 if (!alias(getLocation(S), Loc))
311 // If the pointer is a pointer to constant memory, then it could not have been
312 // modified by this store.
313 if (pointsToConstantMemory(Loc))
316 // Otherwise, a store just writes.
320 AliasAnalysis::ModRefResult
321 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
322 // If the va_arg address cannot alias the pointer in question, then the
323 // specified memory cannot be accessed by the va_arg.
324 if (!alias(getLocation(V), Loc))
327 // If the pointer is a pointer to constant memory, then it could not have been
328 // modified by this va_arg.
329 if (pointsToConstantMemory(Loc))
332 // Otherwise, a va_arg reads and writes.
336 AliasAnalysis::ModRefResult
337 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
338 // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
339 if (CX->getOrdering() > Monotonic)
342 // If the cmpxchg address does not alias the location, it does not access it.
343 if (!alias(getLocation(CX), Loc))
349 AliasAnalysis::ModRefResult
350 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
351 // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
352 if (RMW->getOrdering() > Monotonic)
355 // If the atomicrmw address does not alias the location, it does not access it.
356 if (!alias(getLocation(RMW), Loc))
363 /// Only find pointer captures which happen before the given instruction. Uses
364 /// the dominator tree to determine whether one instruction is before another.
365 struct CapturesBefore : public CaptureTracker {
366 CapturesBefore(const Instruction *I, DominatorTree *DT)
367 : BeforeHere(I), DT(DT), Captured(false) {}
369 void tooManyUses() { Captured = true; }
371 bool shouldExplore(Use *U) {
372 Instruction *I = cast<Instruction>(U->getUser());
373 BasicBlock *BB = I->getParent();
374 if (BeforeHere != I &&
375 (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
380 bool captured(Use *U) {
381 Instruction *I = cast<Instruction>(U->getUser());
382 BasicBlock *BB = I->getParent();
383 if (BeforeHere != I &&
384 (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
390 const Instruction *BeforeHere;
397 // FIXME: this is really just shoring-up a deficiency in alias analysis.
398 // BasicAA isn't willing to spend linear time determining whether an alloca
399 // was captured before or after this particular call, while we are. However,
400 // with a smarter AA in place, this test is just wasting compile time.
401 AliasAnalysis::ModRefResult
402 AliasAnalysis::callCapturesBefore(const Instruction *I,
403 const AliasAnalysis::Location &MemLoc,
405 if (!DT || !TD) return AliasAnalysis::ModRef;
407 const Value *Object = GetUnderlyingObject(MemLoc.Ptr, TD);
408 if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
409 isa<Constant>(Object))
410 return AliasAnalysis::ModRef;
412 ImmutableCallSite CS(I);
413 if (!CS.getInstruction() || CS.getInstruction() == Object)
414 return AliasAnalysis::ModRef;
416 CapturesBefore CB(I, DT);
417 llvm::PointerMayBeCaptured(Object, &CB);
419 return AliasAnalysis::ModRef;
422 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
423 CI != CE; ++CI, ++ArgNo) {
424 // Only look at the no-capture or byval pointer arguments. If this
425 // pointer were passed to arguments that were neither of these, then it
426 // couldn't be no-capture.
427 if (!(*CI)->getType()->isPointerTy() ||
428 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
431 // If this is a no-capture pointer argument, see if we can tell that it
432 // is impossible to alias the pointer we're checking. If not, we have to
433 // assume that the call could touch the pointer, even though it doesn't
435 if (!isNoAlias(AliasAnalysis::Location(*CI),
436 AliasAnalysis::Location(Object))) {
437 return AliasAnalysis::ModRef;
440 return AliasAnalysis::NoModRef;
443 // AliasAnalysis destructor: DO NOT move this to the header file for
444 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
445 // the AliasAnalysis.o file in the current .a file, causing alias analysis
446 // support to not be included in the tool correctly!
448 AliasAnalysis::~AliasAnalysis() {}
450 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
451 /// AliasAnalysis interface before any other methods are called.
453 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
454 TD = P->getAnalysisIfAvailable<TargetData>();
455 AA = &P->getAnalysis<AliasAnalysis>();
458 // getAnalysisUsage - All alias analysis implementations should invoke this
459 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
460 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
461 AU.addRequired<AliasAnalysis>(); // All AA's chain
464 /// getTypeStoreSize - Return the TargetData store size for the given type,
465 /// if known, or a conservative value otherwise.
467 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
468 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
471 /// canBasicBlockModify - Return true if it is possible for execution of the
472 /// specified basic block to modify the value pointed to by Ptr.
474 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
475 const Location &Loc) {
476 return canInstructionRangeModify(BB.front(), BB.back(), Loc);
479 /// canInstructionRangeModify - Return true if it is possible for the execution
480 /// of the specified instructions to modify the value pointed to by Ptr. The
481 /// instructions to consider are all of the instructions in the range of [I1,I2]
482 /// INCLUSIVE. I1 and I2 must be in the same basic block.
484 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
485 const Instruction &I2,
486 const Location &Loc) {
487 assert(I1.getParent() == I2.getParent() &&
488 "Instructions not in same basic block!");
489 BasicBlock::const_iterator I = &I1;
490 BasicBlock::const_iterator E = &I2;
491 ++E; // Convert from inclusive to exclusive range.
493 for (; I != E; ++I) // Check every instruction in range
494 if (getModRefInfo(I, Loc) & Mod)
499 /// isNoAliasCall - Return true if this pointer is returned by a noalias
501 bool llvm::isNoAliasCall(const Value *V) {
502 if (isa<CallInst>(V) || isa<InvokeInst>(V))
503 return ImmutableCallSite(cast<Instruction>(V))
504 .paramHasAttr(0, Attribute::NoAlias);
508 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
509 /// identifiable object. This returns true for:
510 /// Global Variables and Functions (but not Global Aliases)
511 /// Allocas and Mallocs
512 /// ByVal and NoAlias Arguments
515 bool llvm::isIdentifiedObject(const Value *V) {
516 if (isa<AllocaInst>(V))
518 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
520 if (isNoAliasCall(V))
522 if (const Argument *A = dyn_cast<Argument>(V))
523 return A->hasNoAliasAttr() || A->hasByValAttr();
527 /// isKnownNonNull - Return true if we know that the specified value is never
529 bool llvm::isKnownNonNull(const Value *V) {
530 // Alloca never returns null, malloc might.
531 if (isa<AllocaInst>(V)) return true;
533 // A byval argument is never null.
534 if (const Argument *A = dyn_cast<Argument>(V))
535 return A->hasByValAttr();
537 // Global values are not null unless extern weak.
538 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
539 return !GV->hasExternalWeakLinkage();