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/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"
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;
47 //===----------------------------------------------------------------------===//
48 // Default chaining methods
49 //===----------------------------------------------------------------------===//
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);
57 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
59 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
60 return AA->pointsToConstantMemory(Loc, OrLocal);
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);
70 void AliasAnalysis::deleteValue(Value *V) {
71 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
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);
80 void AliasAnalysis::addEscapingUse(Use &U) {
81 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
82 AA->addEscapingUse(U);
86 AliasAnalysis::ModRefResult
87 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
88 const Location &Loc) {
89 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
91 ModRefBehavior MRB = getModRefBehavior(CS);
92 if (MRB == DoesNotAccessMemory)
95 ModRefResult Mask = ModRef;
96 if (onlyReadsMemory(MRB))
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();
105 const Value *Arg = *AI;
106 if (!Arg->getType()->isPointerTy())
108 ModRefResult ArgMask;
110 getArgLocation(CS, (unsigned) std::distance(CS.arg_begin(), AI),
112 if (!isNoAlias(CSLoc, Loc)) {
114 AllArgsMask = ModRefResult(AllArgsMask | ArgMask);
120 Mask = ModRefResult(Mask & AllArgsMask);
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);
128 // If this is the end of the chain, don't forward.
129 if (!AA) return Mask;
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);
136 AliasAnalysis::ModRefResult
137 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
138 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
140 // If CS1 or CS2 are readnone, they don't interact.
141 ModRefBehavior CS1B = getModRefBehavior(CS1);
142 if (CS1B == DoesNotAccessMemory) return NoModRef;
144 ModRefBehavior CS2B = getModRefBehavior(CS2);
145 if (CS2B == DoesNotAccessMemory) return NoModRef;
147 // If they both only read from memory, there is no dependence.
148 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
151 AliasAnalysis::ModRefResult Mask = ModRef;
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);
158 // If CS2 only access memory through arguments, accumulate the mod/ref
159 // information from CS1's references to the memory referenced by
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())
169 ModRefResult ArgMask;
171 getArgLocation(CS2, (unsigned) std::distance(CS2.arg_begin(), I),
173 // ArgMask indicates what CS2 might do to CS2Loc, and the dependence of
174 // CS1 on that location is the inverse.
177 else if (ArgMask == Ref)
180 R = ModRefResult((R | (getModRefInfo(CS1, CS2Loc) & ArgMask)) & Mask);
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())
198 ModRefResult ArgMask;
200 getArgLocation(CS1, (unsigned) std::distance(CS1.arg_begin(), I),
202 // ArgMask indicates what CS1 might do to CS1Loc; if CS1 might Mod
203 // CS1Loc, then we care about either a Mod or a Ref by CS2. If CS1
204 // might Ref, then we care only about a Mod by CS2.
205 ModRefResult ArgR = getModRefInfo(CS2, CS1Loc);
206 if (((ArgMask & Mod) != NoModRef && (ArgR & ModRef) != NoModRef) ||
207 ((ArgMask & Ref) != NoModRef && (ArgR & Mod) != NoModRef))
208 R = ModRefResult((R | ArgMask) & Mask);
217 // If this is the end of the chain, don't forward.
218 if (!AA) return Mask;
220 // Otherwise, fall back to the next AA in the chain. But we can merge
221 // in any mask we've managed to compute.
222 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
225 AliasAnalysis::ModRefBehavior
226 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
227 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
229 ModRefBehavior Min = UnknownModRefBehavior;
231 // Call back into the alias analysis with the other form of getModRefBehavior
232 // to see if it can give a better response.
233 if (const Function *F = CS.getCalledFunction())
234 Min = getModRefBehavior(F);
236 // If this is the end of the chain, don't forward.
239 // Otherwise, fall back to the next AA in the chain. But we can merge
240 // in any result we've managed to compute.
241 return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
244 AliasAnalysis::ModRefBehavior
245 AliasAnalysis::getModRefBehavior(const Function *F) {
246 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
247 return AA->getModRefBehavior(F);
250 //===----------------------------------------------------------------------===//
251 // AliasAnalysis non-virtual helper method implementation
252 //===----------------------------------------------------------------------===//
254 AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
255 return Location(LI->getPointerOperand(),
256 getTypeStoreSize(LI->getType()),
257 LI->getMetadata(LLVMContext::MD_tbaa));
260 AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
261 return Location(SI->getPointerOperand(),
262 getTypeStoreSize(SI->getValueOperand()->getType()),
263 SI->getMetadata(LLVMContext::MD_tbaa));
266 AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
267 return Location(VI->getPointerOperand(),
269 VI->getMetadata(LLVMContext::MD_tbaa));
272 AliasAnalysis::Location
273 AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
274 return Location(CXI->getPointerOperand(),
275 getTypeStoreSize(CXI->getCompareOperand()->getType()),
276 CXI->getMetadata(LLVMContext::MD_tbaa));
279 AliasAnalysis::Location
280 AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
281 return Location(RMWI->getPointerOperand(),
282 getTypeStoreSize(RMWI->getValOperand()->getType()),
283 RMWI->getMetadata(LLVMContext::MD_tbaa));
286 AliasAnalysis::Location
287 AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
288 uint64_t Size = UnknownSize;
289 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
290 Size = C->getValue().getZExtValue();
292 // memcpy/memmove can have TBAA tags. For memcpy, they apply
293 // to both the source and the destination.
294 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
296 return Location(MTI->getRawSource(), Size, TBAATag);
299 AliasAnalysis::Location
300 AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
301 uint64_t Size = UnknownSize;
302 if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
303 Size = C->getValue().getZExtValue();
305 // memcpy/memmove can have TBAA tags. For memcpy, they apply
306 // to both the source and the destination.
307 MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
309 return Location(MTI->getRawDest(), Size, TBAATag);
314 AliasAnalysis::ModRefResult
315 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
316 // Be conservative in the face of volatile/atomic.
317 if (!L->isUnordered())
320 // If the load address doesn't alias the given address, it doesn't read
321 // or write the specified memory.
322 if (!alias(getLocation(L), Loc))
325 // Otherwise, a load just reads.
329 AliasAnalysis::ModRefResult
330 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
331 // Be conservative in the face of volatile/atomic.
332 if (!S->isUnordered())
335 // If the store address cannot alias the pointer in question, then the
336 // specified memory cannot be modified by the store.
337 if (!alias(getLocation(S), Loc))
340 // If the pointer is a pointer to constant memory, then it could not have been
341 // modified by this store.
342 if (pointsToConstantMemory(Loc))
345 // Otherwise, a store just writes.
349 AliasAnalysis::ModRefResult
350 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
351 // If the va_arg address cannot alias the pointer in question, then the
352 // specified memory cannot be accessed by the va_arg.
353 if (!alias(getLocation(V), Loc))
356 // If the pointer is a pointer to constant memory, then it could not have been
357 // modified by this va_arg.
358 if (pointsToConstantMemory(Loc))
361 // Otherwise, a va_arg reads and writes.
365 AliasAnalysis::ModRefResult
366 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
367 // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
368 if (CX->getSuccessOrdering() > Monotonic)
371 // If the cmpxchg address does not alias the location, it does not access it.
372 if (!alias(getLocation(CX), Loc))
378 AliasAnalysis::ModRefResult
379 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
380 // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
381 if (RMW->getOrdering() > Monotonic)
384 // If the atomicrmw address does not alias the location, it does not access it.
385 if (!alias(getLocation(RMW), Loc))
391 // FIXME: this is really just shoring-up a deficiency in alias analysis.
392 // BasicAA isn't willing to spend linear time determining whether an alloca
393 // was captured before or after this particular call, while we are. However,
394 // with a smarter AA in place, this test is just wasting compile time.
395 AliasAnalysis::ModRefResult
396 AliasAnalysis::callCapturesBefore(const Instruction *I,
397 const AliasAnalysis::Location &MemLoc,
399 if (!DT || !DL) return AliasAnalysis::ModRef;
401 const Value *Object = GetUnderlyingObject(MemLoc.Ptr, DL);
402 if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
403 isa<Constant>(Object))
404 return AliasAnalysis::ModRef;
406 ImmutableCallSite CS(I);
407 if (!CS.getInstruction() || CS.getInstruction() == Object)
408 return AliasAnalysis::ModRef;
410 if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
411 /* StoreCaptures */ true, I, DT,
412 /* include Object */ true))
413 return AliasAnalysis::ModRef;
416 AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
417 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
418 CI != CE; ++CI, ++ArgNo) {
419 // Only look at the no-capture or byval pointer arguments. If this
420 // pointer were passed to arguments that were neither of these, then it
421 // couldn't be no-capture.
422 if (!(*CI)->getType()->isPointerTy() ||
423 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
426 // If this is a no-capture pointer argument, see if we can tell that it
427 // is impossible to alias the pointer we're checking. If not, we have to
428 // assume that the call could touch the pointer, even though it doesn't
430 if (isNoAlias(AliasAnalysis::Location(*CI),
431 AliasAnalysis::Location(Object)))
433 if (CS.doesNotAccessMemory(ArgNo))
435 if (CS.onlyReadsMemory(ArgNo)) {
436 R = AliasAnalysis::Ref;
439 return AliasAnalysis::ModRef;
444 // AliasAnalysis destructor: DO NOT move this to the header file for
445 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
446 // the AliasAnalysis.o file in the current .a file, causing alias analysis
447 // support to not be included in the tool correctly!
449 AliasAnalysis::~AliasAnalysis() {}
451 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
452 /// AliasAnalysis interface before any other methods are called.
454 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
455 DataLayoutPass *DLP = P->getAnalysisIfAvailable<DataLayoutPass>();
456 DL = DLP ? &DLP->getDataLayout() : nullptr;
457 TLI = P->getAnalysisIfAvailable<TargetLibraryInfo>();
458 AA = &P->getAnalysis<AliasAnalysis>();
461 // getAnalysisUsage - All alias analysis implementations should invoke this
462 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
463 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
464 AU.addRequired<AliasAnalysis>(); // All AA's chain
467 /// getTypeStoreSize - Return the DataLayout store size for the given type,
468 /// if known, or a conservative value otherwise.
470 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
471 return DL ? DL->getTypeStoreSize(Ty) : UnknownSize;
474 /// canBasicBlockModify - Return true if it is possible for execution of the
475 /// specified basic block to modify the value pointed to by Ptr.
477 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
478 const Location &Loc) {
479 return canInstructionRangeModify(BB.front(), BB.back(), Loc);
482 /// canInstructionRangeModify - Return true if it is possible for the execution
483 /// of the specified instructions to modify the value pointed to by Ptr. The
484 /// instructions to consider are all of the instructions in the range of [I1,I2]
485 /// INCLUSIVE. I1 and I2 must be in the same basic block.
487 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
488 const Instruction &I2,
489 const Location &Loc) {
490 assert(I1.getParent() == I2.getParent() &&
491 "Instructions not in same basic block!");
492 BasicBlock::const_iterator I = &I1;
493 BasicBlock::const_iterator E = &I2;
494 ++E; // Convert from inclusive to exclusive range.
496 for (; I != E; ++I) // Check every instruction in range
497 if (getModRefInfo(I, Loc) & Mod)
502 /// isNoAliasCall - Return true if this pointer is returned by a noalias
504 bool llvm::isNoAliasCall(const Value *V) {
505 if (isa<CallInst>(V) || isa<InvokeInst>(V))
506 return ImmutableCallSite(cast<Instruction>(V))
507 .paramHasAttr(0, Attribute::NoAlias);
511 /// isNoAliasArgument - Return true if this is an argument with the noalias
513 bool llvm::isNoAliasArgument(const Value *V)
515 if (const Argument *A = dyn_cast<Argument>(V))
516 return A->hasNoAliasAttr();
520 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
521 /// identifiable object. This returns true for:
522 /// Global Variables and Functions (but not Global Aliases)
523 /// Allocas and Mallocs
524 /// ByVal and NoAlias Arguments
527 bool llvm::isIdentifiedObject(const Value *V) {
528 if (isa<AllocaInst>(V))
530 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
532 if (isNoAliasCall(V))
534 if (const Argument *A = dyn_cast<Argument>(V))
535 return A->hasNoAliasAttr() || A->hasByValAttr();
539 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
540 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
541 /// Further, an IdentifiedFunctionLocal can not alias with any function
542 /// arguments other than itself, which is not necessarily true for
543 /// IdentifiedObjects.
544 bool llvm::isIdentifiedFunctionLocal(const Value *V)
546 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);