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 AliasAnalysis::alias(const MemoryLocation &LocA,
52 const MemoryLocation &LocB) {
53 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
54 return AA->alias(LocA, LocB);
57 bool AliasAnalysis::pointsToConstantMemory(const MemoryLocation &Loc,
59 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
60 return AA->pointsToConstantMemory(Loc, OrLocal);
63 AliasAnalysis::ModRefResult
64 AliasAnalysis::getArgModRefInfo(ImmutableCallSite CS, unsigned ArgIdx) {
65 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
66 return AA->getArgModRefInfo(CS, ArgIdx);
69 void AliasAnalysis::deleteValue(Value *V) {
70 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
74 void AliasAnalysis::copyValue(Value *From, Value *To) {
75 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
76 AA->copyValue(From, To);
79 void AliasAnalysis::addEscapingUse(Use &U) {
80 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
81 AA->addEscapingUse(U);
84 AliasAnalysis::ModRefResult
85 AliasAnalysis::getModRefInfo(Instruction *I, ImmutableCallSite Call) {
86 // We may have two calls
87 if (auto CS = ImmutableCallSite(I)) {
88 // Check if the two calls modify the same memory
89 return getModRefInfo(Call, CS);
91 // Otherwise, check if the call modifies or references the
92 // location this memory access defines. The best we can say
93 // is that if the call references what this instruction
94 // defines, it must be clobbered by this location.
95 const MemoryLocation DefLoc = MemoryLocation::get(I);
96 if (getModRefInfo(Call, DefLoc) != AliasAnalysis::NoModRef)
97 return AliasAnalysis::ModRef;
99 return AliasAnalysis::NoModRef;
102 AliasAnalysis::ModRefResult
103 AliasAnalysis::getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc) {
104 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
106 ModRefBehavior MRB = getModRefBehavior(CS);
107 if (MRB == DoesNotAccessMemory)
110 ModRefResult Mask = ModRef;
111 if (onlyReadsMemory(MRB))
114 if (onlyAccessesArgPointees(MRB)) {
115 bool doesAlias = false;
116 ModRefResult AllArgsMask = NoModRef;
117 if (doesAccessArgPointees(MRB)) {
118 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
120 const Value *Arg = *AI;
121 if (!Arg->getType()->isPointerTy())
123 unsigned ArgIdx = std::distance(CS.arg_begin(), AI);
124 MemoryLocation ArgLoc =
125 MemoryLocation::getForArgument(CS, ArgIdx, *TLI);
126 if (!isNoAlias(ArgLoc, Loc)) {
127 ModRefResult ArgMask = getArgModRefInfo(CS, ArgIdx);
129 AllArgsMask = ModRefResult(AllArgsMask | ArgMask);
135 Mask = ModRefResult(Mask & AllArgsMask);
138 // If Loc is a constant memory location, the call definitely could not
139 // modify the memory location.
140 if ((Mask & Mod) && pointsToConstantMemory(Loc))
141 Mask = ModRefResult(Mask & ~Mod);
143 // If this is the end of the chain, don't forward.
144 if (!AA) return Mask;
146 // Otherwise, fall back to the next AA in the chain. But we can merge
147 // in any mask we've managed to compute.
148 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
151 AliasAnalysis::ModRefResult
152 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
153 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
155 // If CS1 or CS2 are readnone, they don't interact.
156 ModRefBehavior CS1B = getModRefBehavior(CS1);
157 if (CS1B == DoesNotAccessMemory) return NoModRef;
159 ModRefBehavior CS2B = getModRefBehavior(CS2);
160 if (CS2B == DoesNotAccessMemory) return NoModRef;
162 // If they both only read from memory, there is no dependence.
163 if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
166 AliasAnalysis::ModRefResult Mask = ModRef;
168 // If CS1 only reads memory, the only dependence on CS2 can be
169 // from CS1 reading memory written by CS2.
170 if (onlyReadsMemory(CS1B))
171 Mask = ModRefResult(Mask & Ref);
173 // If CS2 only access memory through arguments, accumulate the mod/ref
174 // information from CS1's references to the memory referenced by
176 if (onlyAccessesArgPointees(CS2B)) {
177 AliasAnalysis::ModRefResult R = NoModRef;
178 if (doesAccessArgPointees(CS2B)) {
179 for (ImmutableCallSite::arg_iterator
180 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
181 const Value *Arg = *I;
182 if (!Arg->getType()->isPointerTy())
184 unsigned CS2ArgIdx = std::distance(CS2.arg_begin(), I);
185 auto CS2ArgLoc = MemoryLocation::getForArgument(CS2, CS2ArgIdx, *TLI);
187 // ArgMask indicates what CS2 might do to CS2ArgLoc, and the dependence of
188 // CS1 on that location is the inverse.
189 ModRefResult ArgMask = getArgModRefInfo(CS2, CS2ArgIdx);
192 else if (ArgMask == Ref)
195 R = ModRefResult((R | (getModRefInfo(CS1, CS2ArgLoc) & ArgMask)) & Mask);
203 // If CS1 only accesses memory through arguments, check if CS2 references
204 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
205 if (onlyAccessesArgPointees(CS1B)) {
206 AliasAnalysis::ModRefResult R = NoModRef;
207 if (doesAccessArgPointees(CS1B)) {
208 for (ImmutableCallSite::arg_iterator
209 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
210 const Value *Arg = *I;
211 if (!Arg->getType()->isPointerTy())
213 unsigned CS1ArgIdx = std::distance(CS1.arg_begin(), I);
214 auto CS1ArgLoc = MemoryLocation::getForArgument(CS1, CS1ArgIdx, *TLI);
216 // ArgMask indicates what CS1 might do to CS1ArgLoc; if CS1 might Mod
217 // CS1ArgLoc, then we care about either a Mod or a Ref by CS2. If CS1
218 // might Ref, then we care only about a Mod by CS2.
219 ModRefResult ArgMask = getArgModRefInfo(CS1, CS1ArgIdx);
220 ModRefResult ArgR = getModRefInfo(CS2, CS1ArgLoc);
221 if (((ArgMask & Mod) != NoModRef && (ArgR & ModRef) != NoModRef) ||
222 ((ArgMask & Ref) != NoModRef && (ArgR & Mod) != NoModRef))
223 R = ModRefResult((R | ArgMask) & Mask);
232 // If this is the end of the chain, don't forward.
233 if (!AA) return Mask;
235 // Otherwise, fall back to the next AA in the chain. But we can merge
236 // in any mask we've managed to compute.
237 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
240 AliasAnalysis::ModRefBehavior
241 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
242 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
244 ModRefBehavior Min = UnknownModRefBehavior;
246 // Call back into the alias analysis with the other form of getModRefBehavior
247 // to see if it can give a better response.
248 if (const Function *F = CS.getCalledFunction())
249 Min = getModRefBehavior(F);
251 // If this is the end of the chain, don't forward.
254 // Otherwise, fall back to the next AA in the chain. But we can merge
255 // in any result we've managed to compute.
256 return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
259 AliasAnalysis::ModRefBehavior
260 AliasAnalysis::getModRefBehavior(const Function *F) {
261 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
262 return AA->getModRefBehavior(F);
265 //===----------------------------------------------------------------------===//
266 // AliasAnalysis non-virtual helper method implementation
267 //===----------------------------------------------------------------------===//
269 AliasAnalysis::ModRefResult
270 AliasAnalysis::getModRefInfo(const LoadInst *L, const MemoryLocation &Loc) {
271 // Be conservative in the face of volatile/atomic.
272 if (!L->isUnordered())
275 // If the load address doesn't alias the given address, it doesn't read
276 // or write the specified memory.
277 if (Loc.Ptr && !alias(MemoryLocation::get(L), Loc))
280 // Otherwise, a load just reads.
284 AliasAnalysis::ModRefResult
285 AliasAnalysis::getModRefInfo(const StoreInst *S, const MemoryLocation &Loc) {
286 // Be conservative in the face of volatile/atomic.
287 if (!S->isUnordered())
291 // If the store address cannot alias the pointer in question, then the
292 // specified memory cannot be modified by the store.
293 if (!alias(MemoryLocation::get(S), Loc))
296 // If the pointer is a pointer to constant memory, then it could not have
297 // been modified by this store.
298 if (pointsToConstantMemory(Loc))
303 // Otherwise, a store just writes.
307 AliasAnalysis::ModRefResult
308 AliasAnalysis::getModRefInfo(const VAArgInst *V, const MemoryLocation &Loc) {
311 // If the va_arg address cannot alias the pointer in question, then the
312 // specified memory cannot be accessed by the va_arg.
313 if (!alias(MemoryLocation::get(V), Loc))
316 // If the pointer is a pointer to constant memory, then it could not have
317 // been modified by this va_arg.
318 if (pointsToConstantMemory(Loc))
322 // Otherwise, a va_arg reads and writes.
326 AliasAnalysis::ModRefResult
327 AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX,
328 const MemoryLocation &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,
342 const MemoryLocation &Loc) {
343 // Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
344 if (RMW->getOrdering() > Monotonic)
347 // If the atomicrmw address does not alias the location, it does not access it.
348 if (Loc.Ptr && !alias(MemoryLocation::get(RMW), Loc))
354 // FIXME: this is really just shoring-up a deficiency in alias analysis.
355 // BasicAA isn't willing to spend linear time determining whether an alloca
356 // was captured before or after this particular call, while we are. However,
357 // with a smarter AA in place, this test is just wasting compile time.
358 AliasAnalysis::ModRefResult AliasAnalysis::callCapturesBefore(
359 const Instruction *I, const MemoryLocation &MemLoc, DominatorTree *DT) {
361 return AliasAnalysis::ModRef;
363 const Value *Object = GetUnderlyingObject(MemLoc.Ptr, *DL);
364 if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
365 isa<Constant>(Object))
366 return AliasAnalysis::ModRef;
368 ImmutableCallSite CS(I);
369 if (!CS.getInstruction() || CS.getInstruction() == Object)
370 return AliasAnalysis::ModRef;
372 if (llvm::PointerMayBeCapturedBefore(Object, /* ReturnCaptures */ true,
373 /* StoreCaptures */ true, I, DT,
374 /* include Object */ true))
375 return AliasAnalysis::ModRef;
378 AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
379 for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
380 CI != CE; ++CI, ++ArgNo) {
381 // Only look at the no-capture or byval pointer arguments. If this
382 // pointer were passed to arguments that were neither of these, then it
383 // couldn't be no-capture.
384 if (!(*CI)->getType()->isPointerTy() ||
385 (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
388 // If this is a no-capture pointer argument, see if we can tell that it
389 // is impossible to alias the pointer we're checking. If not, we have to
390 // assume that the call could touch the pointer, even though it doesn't
392 if (isNoAlias(MemoryLocation(*CI), MemoryLocation(Object)))
394 if (CS.doesNotAccessMemory(ArgNo))
396 if (CS.onlyReadsMemory(ArgNo)) {
397 R = AliasAnalysis::Ref;
400 return AliasAnalysis::ModRef;
405 // AliasAnalysis destructor: DO NOT move this to the header file for
406 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
407 // the AliasAnalysis.o file in the current .a file, causing alias analysis
408 // support to not be included in the tool correctly!
410 AliasAnalysis::~AliasAnalysis() {}
412 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
413 /// AliasAnalysis interface before any other methods are called.
415 void AliasAnalysis::InitializeAliasAnalysis(Pass *P, const DataLayout *NewDL) {
417 auto *TLIP = P->getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
418 TLI = TLIP ? &TLIP->getTLI() : nullptr;
419 AA = &P->getAnalysis<AliasAnalysis>();
422 // getAnalysisUsage - All alias analysis implementations should invoke this
423 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
424 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
425 AU.addRequired<AliasAnalysis>(); // All AA's chain
428 /// getTypeStoreSize - Return the DataLayout store size for the given type,
429 /// if known, or a conservative value otherwise.
431 uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
432 return DL ? DL->getTypeStoreSize(Ty) : UnknownSize;
435 /// canBasicBlockModify - Return true if it is possible for execution of the
436 /// specified basic block to modify the location Loc.
438 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
439 const MemoryLocation &Loc) {
440 return canInstructionRangeModRef(BB.front(), BB.back(), Loc, Mod);
443 /// canInstructionRangeModRef - Return true if it is possible for the
444 /// execution of the specified instructions to mod\ref (according to the
445 /// mode) the location Loc. The instructions to consider are all
446 /// of the instructions in the range of [I1,I2] INCLUSIVE.
447 /// I1 and I2 must be in the same basic block.
448 bool AliasAnalysis::canInstructionRangeModRef(const Instruction &I1,
449 const Instruction &I2,
450 const MemoryLocation &Loc,
451 const ModRefResult Mode) {
452 assert(I1.getParent() == I2.getParent() &&
453 "Instructions not in same basic block!");
454 BasicBlock::const_iterator I = &I1;
455 BasicBlock::const_iterator E = &I2;
456 ++E; // Convert from inclusive to exclusive range.
458 for (; I != E; ++I) // Check every instruction in range
459 if (getModRefInfo(I, Loc) & Mode)
464 /// isNoAliasCall - Return true if this pointer is returned by a noalias
466 bool llvm::isNoAliasCall(const Value *V) {
467 if (isa<CallInst>(V) || isa<InvokeInst>(V))
468 return ImmutableCallSite(cast<Instruction>(V))
469 .paramHasAttr(0, Attribute::NoAlias);
473 /// isNoAliasArgument - Return true if this is an argument with the noalias
475 bool llvm::isNoAliasArgument(const Value *V)
477 if (const Argument *A = dyn_cast<Argument>(V))
478 return A->hasNoAliasAttr();
482 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
483 /// identifiable object. This returns true for:
484 /// Global Variables and Functions (but not Global Aliases)
485 /// Allocas and Mallocs
486 /// ByVal and NoAlias Arguments
489 bool llvm::isIdentifiedObject(const Value *V) {
490 if (isa<AllocaInst>(V))
492 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
494 if (isNoAliasCall(V))
496 if (const Argument *A = dyn_cast<Argument>(V))
497 return A->hasNoAliasAttr() || A->hasByValAttr();
501 /// isIdentifiedFunctionLocal - Return true if V is umabigously identified
502 /// at the function-level. Different IdentifiedFunctionLocals can't alias.
503 /// Further, an IdentifiedFunctionLocal can not alias with any function
504 /// arguments other than itself, which is not necessarily true for
505 /// IdentifiedObjects.
506 bool llvm::isIdentifiedFunctionLocal(const Value *V)
508 return isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V);