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/TargetLibraryInfo.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/IR/BasicBlock.h"
33 #include "llvm/IR/DataLayout.h"
34 #include "llvm/IR/Dominators.h"
35 #include "llvm/IR/Function.h"
36 #include "llvm/IR/Instructions.h"
37 #include "llvm/IR/IntrinsicInst.h"
38 #include "llvm/IR/LLVMContext.h"
39 #include "llvm/IR/Type.h"
40 #include "llvm/Pass.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);
85 AliasAnalysis::ModRefResult
86 AliasAnalysis::getModRefInfo(Instruction *I, ImmutableCallSite Call) {
87 // We may have two calls
88 if (auto CS = ImmutableCallSite(I)) {
89 // Check if the two calls modify the same memory
90 return getModRefInfo(Call, CS);
92 // Otherwise, check if the call modifies or references the
93 // location this memory access defines. The best we can say
94 // is that if the call references what this instruction
95 // defines, it must be clobbered by this location.
96 const AliasAnalysis::Location DefLoc = MemoryLocation::get(I);
97 if (getModRefInfo(Call, DefLoc) != AliasAnalysis::NoModRef)
98 return AliasAnalysis::ModRef;
100 return AliasAnalysis::NoModRef;
103 AliasAnalysis::ModRefResult
104 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
105 const Location &Loc) {
106 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
108 ModRefBehavior MRB = getModRefBehavior(CS);
109 if (MRB == DoesNotAccessMemory)
112 ModRefResult Mask = ModRef;
113 if (onlyReadsMemory(MRB))
116 if (onlyAccessesArgPointees(MRB)) {
117 bool doesAlias = false;
118 ModRefResult AllArgsMask = NoModRef;
119 if (doesAccessArgPointees(MRB)) {
120 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
122 const Value *Arg = *AI;
123 if (!Arg->getType()->isPointerTy())
125 ModRefResult ArgMask;
127 getArgLocation(CS, (unsigned) std::distance(CS.arg_begin(), AI),
129 if (!isNoAlias(CSLoc, Loc)) {
131 AllArgsMask = ModRefResult(AllArgsMask | ArgMask);
137 Mask = ModRefResult(Mask & AllArgsMask);
140 // If Loc is a constant memory location, the call definitely could not
141 // modify the memory location.
142 if ((Mask & Mod) && pointsToConstantMemory(Loc))
143 Mask = ModRefResult(Mask & ~Mod);
145 // If this is the end of the chain, don't forward.
146 if (!AA) return Mask;
148 // Otherwise, fall back to the next AA in the chain. But we can merge
149 // in any mask we've managed to compute.
150 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
153 AliasAnalysis::ModRefResult
154 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
155 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
157 // If CS1 or CS2 are readnone, they don't interact.
158 ModRefBehavior CS1B = getModRefBehavior(CS1);
159 if (CS1B == DoesNotAccessMemory) return NoModRef;
161 ModRefBehavior CS2B = getModRefBehavior(CS2);
162 if (CS2B == DoesNotAccessMemory) return NoModRef;
164 // If they both only read from memory, there is no dependence.
165 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
168 AliasAnalysis::ModRefResult Mask = ModRef;
170 // If CS1 only reads memory, the only dependence on CS2 can be
171 // from CS1 reading memory written by CS2.
172 if (onlyReadsMemory(CS1B))
173 Mask = ModRefResult(Mask & Ref);
175 // If CS2 only access memory through arguments, accumulate the mod/ref
176 // information from CS1's references to the memory referenced by
178 if (onlyAccessesArgPointees(CS2B)) {
179 AliasAnalysis::ModRefResult R = NoModRef;
180 if (doesAccessArgPointees(CS2B)) {
181 for (ImmutableCallSite::arg_iterator
182 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
183 const Value *Arg = *I;
184 if (!Arg->getType()->isPointerTy())
186 ModRefResult ArgMask;
188 getArgLocation(CS2, (unsigned) std::distance(CS2.arg_begin(), I),
190 // ArgMask indicates what CS2 might do to CS2Loc, and the dependence of
191 // CS1 on that location is the inverse.
194 else if (ArgMask == Ref)
197 R = ModRefResult((R | (getModRefInfo(CS1, CS2Loc) & ArgMask)) & Mask);
205 // If CS1 only accesses memory through arguments, check if CS2 references
206 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
207 if (onlyAccessesArgPointees(CS1B)) {
208 AliasAnalysis::ModRefResult R = NoModRef;
209 if (doesAccessArgPointees(CS1B)) {
210 for (ImmutableCallSite::arg_iterator
211 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
212 const Value *Arg = *I;
213 if (!Arg->getType()->isPointerTy())
215 ModRefResult ArgMask;
216 Location CS1Loc = getArgLocation(
217 CS1, (unsigned)std::distance(CS1.arg_begin(), I), ArgMask);
218 // ArgMask indicates what CS1 might do to CS1Loc; if CS1 might Mod
219 // CS1Loc, then we care about either a Mod or a Ref by CS2. If CS1
220 // might Ref, then we care only about a Mod by CS2.
221 ModRefResult ArgR = getModRefInfo(CS2, CS1Loc);
222 if (((ArgMask & Mod) != NoModRef && (ArgR & ModRef) != NoModRef) ||
223 ((ArgMask & Ref) != NoModRef && (ArgR & Mod) != NoModRef))
224 R = ModRefResult((R | ArgMask) & Mask);
233 // If this is the end of the chain, don't forward.
234 if (!AA) return Mask;
236 // Otherwise, fall back to the next AA in the chain. But we can merge
237 // in any mask we've managed to compute.
238 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
241 AliasAnalysis::ModRefBehavior
242 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
243 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
245 ModRefBehavior Min = UnknownModRefBehavior;
247 // Call back into the alias analysis with the other form of getModRefBehavior
248 // to see if it can give a better response.
249 if (const Function *F = CS.getCalledFunction())
250 Min = getModRefBehavior(F);
252 // If this is the end of the chain, don't forward.
255 // Otherwise, fall back to the next AA in the chain. But we can merge
256 // in any result we've managed to compute.
257 return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
260 AliasAnalysis::ModRefBehavior
261 AliasAnalysis::getModRefBehavior(const Function *F) {
262 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
263 return AA->getModRefBehavior(F);
266 //===----------------------------------------------------------------------===//
267 // AliasAnalysis non-virtual helper method implementation
268 //===----------------------------------------------------------------------===//
270 AliasAnalysis::ModRefResult
271 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
272 // Be conservative in the face of volatile/atomic.
273 if (!L->isUnordered())
276 // If the load address doesn't alias the given address, it doesn't read
277 // or write the specified memory.
278 if (Loc.Ptr && !alias(MemoryLocation::get(L), Loc))
281 // Otherwise, a load just reads.
285 AliasAnalysis::ModRefResult
286 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
287 // Be conservative in the face of volatile/atomic.
288 if (!S->isUnordered())
292 // If the store address cannot alias the pointer in question, then the
293 // specified memory cannot be modified by the store.
294 if (!alias(MemoryLocation::get(S), Loc))
297 // If the pointer is a pointer to constant memory, then it could not have
298 // been modified by this store.
299 if (pointsToConstantMemory(Loc))
304 // Otherwise, a store just writes.
308 AliasAnalysis::ModRefResult
309 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
312 // If the va_arg address cannot alias the pointer in question, then the
313 // specified memory cannot be accessed by the va_arg.
314 if (!alias(MemoryLocation::get(V), Loc))
317 // If the pointer is a pointer to constant memory, then it could not have
318 // been modified by this va_arg.
319 if (pointsToConstantMemory(Loc))
323 // Otherwise, a va_arg reads and writes.
327 AliasAnalysis::ModRefResult
328 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
329 // Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
330 if (CX->getSuccessOrdering() > Monotonic)
333 // If the cmpxchg address does not alias the location, it does not access it.
334 if (Loc.Ptr && !alias(MemoryLocation::get(CX), Loc))
340 AliasAnalysis::ModRefResult
341 AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
342 // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
343 if (RMW->getOrdering() > Monotonic)
346 // If the atomicrmw address does not alias the location, it does not access it.
347 if (Loc.Ptr && !alias(MemoryLocation::get(RMW), Loc))
353 // FIXME: this is really just shoring-up a deficiency in alias analysis.
354 // BasicAA isn't willing to spend linear time determining whether an alloca
355 // was captured before or after this particular call, while we are. However,
356 // with a smarter AA in place, this test is just wasting compile time.
357 AliasAnalysis::ModRefResult
358 AliasAnalysis::callCapturesBefore(const Instruction *I,
359 const AliasAnalysis::Location &MemLoc,
362 return AliasAnalysis::ModRef;
364 const Value *Object = GetUnderlyingObject(MemLoc.Ptr, *DL);
365 if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
366 isa<Constant>(Object))
367 return AliasAnalysis::ModRef;
369 ImmutableCallSite CS(I);
370 if (!CS.getInstruction() || CS.getInstruction() == Object)
371 return AliasAnalysis::ModRef;
373 if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
374 /* StoreCaptures */ true, I, DT,
375 /* include Object */ true))
376 return AliasAnalysis::ModRef;
379 AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
380 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
381 CI != CE; ++CI, ++ArgNo) {
382 // Only look at the no-capture or byval pointer arguments. If this
383 // pointer were passed to arguments that were neither of these, then it
384 // couldn't be no-capture.
385 if (!(*CI)->getType()->isPointerTy() ||
386 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
389 // If this is a no-capture pointer argument, see if we can tell that it
390 // is impossible to alias the pointer we're checking. If not, we have to
391 // assume that the call could touch the pointer, even though it doesn't
393 if (isNoAlias(AliasAnalysis::Location(*CI),
394 AliasAnalysis::Location(Object)))
396 if (CS.doesNotAccessMemory(ArgNo))
398 if (CS.onlyReadsMemory(ArgNo)) {
399 R = AliasAnalysis::Ref;
402 return AliasAnalysis::ModRef;
407 // AliasAnalysis destructor: DO NOT move this to the header file for
408 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
409 // the AliasAnalysis.o file in the current .a file, causing alias analysis
410 // support to not be included in the tool correctly!
412 AliasAnalysis::~AliasAnalysis() {}
414 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
415 /// AliasAnalysis interface before any other methods are called.
417 void AliasAnalysis::InitializeAliasAnalysis(Pass *P, const DataLayout *NewDL) {
419 auto *TLIP = P->getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
420 TLI = TLIP ? &TLIP->getTLI() : nullptr;
421 AA = &P->getAnalysis<AliasAnalysis>();
424 // getAnalysisUsage - All alias analysis implementations should invoke this
425 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
426 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
427 AU.addRequired<AliasAnalysis>(); // All AA's chain
430 /// getTypeStoreSize - Return the DataLayout store size for the given type,
431 /// if known, or a conservative value otherwise.
433 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
434 return DL ? DL->getTypeStoreSize(Ty) : UnknownSize;
437 /// canBasicBlockModify - Return true if it is possible for execution of the
438 /// specified basic block to modify the location Loc.
440 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
441 const Location &Loc) {
442 return canInstructionRangeModRef(BB.front(), BB.back(), Loc, Mod);
445 /// canInstructionRangeModRef - Return true if it is possible for the
446 /// execution of the specified instructions to mod\ref (according to the
447 /// mode) the location Loc. The instructions to consider are all
448 /// of the instructions in the range of [I1,I2] INCLUSIVE.
449 /// I1 and I2 must be in the same basic block.
450 bool AliasAnalysis::canInstructionRangeModRef(const Instruction &I1,
451 const Instruction &I2,
453 const ModRefResult Mode) {
454 assert(I1.getParent() == I2.getParent() &&
455 "Instructions not in same basic block!");
456 BasicBlock::const_iterator I = &I1;
457 BasicBlock::const_iterator E = &I2;
458 ++E; // Convert from inclusive to exclusive range.
460 for (; I != E; ++I) // Check every instruction in range
461 if (getModRefInfo(I, Loc) & Mode)
466 /// isNoAliasCall - Return true if this pointer is returned by a noalias
468 bool llvm::isNoAliasCall(const Value *V) {
469 if (isa<CallInst>(V) || isa<InvokeInst>(V))
470 return ImmutableCallSite(cast<Instruction>(V))
471 .paramHasAttr(0, Attribute::NoAlias);
475 /// isNoAliasArgument - Return true if this is an argument with the noalias
477 bool llvm::isNoAliasArgument(const Value *V)
479 if (const Argument *A = dyn_cast<Argument>(V))
480 return A->hasNoAliasAttr();
484 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
485 /// identifiable object. This returns true for:
486 /// Global Variables and Functions (but not Global Aliases)
487 /// Allocas and Mallocs
488 /// ByVal and NoAlias Arguments
491 bool llvm::isIdentifiedObject(const Value *V) {
492 if (isa<AllocaInst>(V))
494 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
496 if (isNoAliasCall(V))
498 if (const Argument *A = dyn_cast<Argument>(V))
499 return A->hasNoAliasAttr() || A->hasByValAttr();
503 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
504 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
505 /// Further, an IdentifiedFunctionLocal can not alias with any function
506 /// arguments other than itself, which is not necessarily true for
507 /// IdentifiedObjects.
508 bool llvm::isIdentifiedFunctionLocal(const Value *V)
510 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);