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/Pass.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/Function.h"
31 #include "llvm/IntrinsicInst.h"
32 #include "llvm/Instructions.h"
33 #include "llvm/LLVMContext.h"
34 #include "llvm/Type.h"
35 #include "llvm/Target/TargetData.h"
38 // Register the AliasAnalysis interface, providing a nice name to refer to.
39 INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
40 char AliasAnalysis::ID = 0;
42 //===----------------------------------------------------------------------===//
43 // Default chaining methods
44 //===----------------------------------------------------------------------===//
46 AliasAnalysis::AliasResult
47 AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
48 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
49 return AA->alias(LocA, LocB);
52 bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
54 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
55 return AA->pointsToConstantMemory(Loc, OrLocal);
58 void AliasAnalysis::deleteValue(Value *V) {
59 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
63 void AliasAnalysis::copyValue(Value *From, Value *To) {
64 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
65 AA->copyValue(From, To);
68 AliasAnalysis::ModRefResult
69 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
70 const Location &Loc) {
71 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
73 ModRefBehavior MRB = getModRefBehavior(CS);
74 if (MRB == DoesNotAccessMemory)
77 ModRefResult Mask = ModRef;
78 if (MRB == OnlyReadsMemory)
80 else if (MRB == AliasAnalysis::AccessesArguments) {
81 bool doesAlias = false;
82 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
84 if (!isNoAlias(Location(*AI), Loc)) {
93 // If Loc is a constant memory location, the call definitely could not
94 // modify the memory location.
95 if ((Mask & Mod) && pointsToConstantMemory(Loc))
96 Mask = ModRefResult(Mask & ~Mod);
98 // If this is the end of the chain, don't forward.
101 // Otherwise, fall back to the next AA in the chain. But we can merge
102 // in any mask we've managed to compute.
103 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
106 AliasAnalysis::ModRefResult
107 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
108 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
110 // If CS1 or CS2 are readnone, they don't interact.
111 ModRefBehavior CS1B = getModRefBehavior(CS1);
112 if (CS1B == DoesNotAccessMemory) return NoModRef;
114 ModRefBehavior CS2B = getModRefBehavior(CS2);
115 if (CS2B == DoesNotAccessMemory) return NoModRef;
117 // If they both only read from memory, there is no dependence.
118 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
121 AliasAnalysis::ModRefResult Mask = ModRef;
123 // If CS1 only reads memory, the only dependence on CS2 can be
124 // from CS1 reading memory written by CS2.
125 if (CS1B == OnlyReadsMemory)
126 Mask = ModRefResult(Mask & Ref);
128 // If CS2 only access memory through arguments, accumulate the mod/ref
129 // information from CS1's references to the memory referenced by
131 if (CS2B == AccessesArguments) {
132 AliasAnalysis::ModRefResult R = NoModRef;
133 for (ImmutableCallSite::arg_iterator
134 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
135 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask);
142 // If CS1 only accesses memory through arguments, check if CS2 references
143 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
144 if (CS1B == AccessesArguments) {
145 AliasAnalysis::ModRefResult R = NoModRef;
146 for (ImmutableCallSite::arg_iterator
147 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I)
148 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) {
156 // If this is the end of the chain, don't forward.
157 if (!AA) return Mask;
159 // Otherwise, fall back to the next AA in the chain. But we can merge
160 // in any mask we've managed to compute.
161 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
164 AliasAnalysis::ModRefBehavior
165 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
166 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
168 ModRefBehavior Min = UnknownModRefBehavior;
170 // Call back into the alias analysis with the other form of getModRefBehavior
171 // to see if it can give a better response.
172 if (const Function *F = CS.getCalledFunction())
173 Min = getModRefBehavior(F);
175 // If this is the end of the chain, don't forward.
178 // Otherwise, fall back to the next AA in the chain. But we can merge
179 // in any result we've managed to compute.
180 return std::min(AA->getModRefBehavior(CS), Min);
183 AliasAnalysis::ModRefBehavior
184 AliasAnalysis::getModRefBehavior(const Function *F) {
185 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
186 return AA->getModRefBehavior(F);
189 //===----------------------------------------------------------------------===//
190 // AliasAnalysis non-virtual helper method implementation
191 //===----------------------------------------------------------------------===//
193 AliasAnalysis::ModRefResult
194 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
195 // Be conservative in the face of volatile.
199 // If the load address doesn't alias the given address, it doesn't read
200 // or write the specified memory.
201 if (!alias(Location(L->getOperand(0),
202 getTypeStoreSize(L->getType()),
203 L->getMetadata(LLVMContext::MD_tbaa)),
207 // Otherwise, a load just reads.
211 AliasAnalysis::ModRefResult
212 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
213 // Be conservative in the face of volatile.
217 // If the store address cannot alias the pointer in question, then the
218 // specified memory cannot be modified by the store.
219 if (!alias(Location(S->getOperand(1),
220 getTypeStoreSize(S->getOperand(0)->getType()),
221 S->getMetadata(LLVMContext::MD_tbaa)),
225 // If the pointer is a pointer to constant memory, then it could not have been
226 // modified by this store.
227 if (pointsToConstantMemory(Loc))
230 // Otherwise, a store just writes.
234 AliasAnalysis::ModRefResult
235 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
236 // If the va_arg address cannot alias the pointer in question, then the
237 // specified memory cannot be accessed by the va_arg.
238 if (!alias(Location(V->getOperand(0),
240 V->getMetadata(LLVMContext::MD_tbaa)),
244 // If the pointer is a pointer to constant memory, then it could not have been
245 // modified by this va_arg.
246 if (pointsToConstantMemory(Loc))
249 // Otherwise, a va_arg reads and writes.
253 // AliasAnalysis destructor: DO NOT move this to the header file for
254 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
255 // the AliasAnalysis.o file in the current .a file, causing alias analysis
256 // support to not be included in the tool correctly!
258 AliasAnalysis::~AliasAnalysis() {}
260 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
261 /// AliasAnalysis interface before any other methods are called.
263 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
264 TD = P->getAnalysisIfAvailable<TargetData>();
265 AA = &P->getAnalysis<AliasAnalysis>();
268 // getAnalysisUsage - All alias analysis implementations should invoke this
269 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
270 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
271 AU.addRequired<AliasAnalysis>(); // All AA's chain
274 /// getTypeStoreSize - Return the TargetData store size for the given type,
275 /// if known, or a conservative value otherwise.
277 uint64_t AliasAnalysis::getTypeStoreSize(const Type *Ty) {
278 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
281 /// canBasicBlockModify - Return true if it is possible for execution of the
282 /// specified basic block to modify the value pointed to by Ptr.
284 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
285 const Location &Loc) {
286 return canInstructionRangeModify(BB.front(), BB.back(), Loc);
289 /// canInstructionRangeModify - Return true if it is possible for the execution
290 /// of the specified instructions to modify the value pointed to by Ptr. The
291 /// instructions to consider are all of the instructions in the range of [I1,I2]
292 /// INCLUSIVE. I1 and I2 must be in the same basic block.
294 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
295 const Instruction &I2,
296 const Location &Loc) {
297 assert(I1.getParent() == I2.getParent() &&
298 "Instructions not in same basic block!");
299 BasicBlock::const_iterator I = &I1;
300 BasicBlock::const_iterator E = &I2;
301 ++E; // Convert from inclusive to exclusive range.
303 for (; I != E; ++I) // Check every instruction in range
304 if (getModRefInfo(I, Loc) & Mod)
309 /// isNoAliasCall - Return true if this pointer is returned by a noalias
311 bool llvm::isNoAliasCall(const Value *V) {
312 if (isa<CallInst>(V) || isa<InvokeInst>(V))
313 return ImmutableCallSite(cast<Instruction>(V))
314 .paramHasAttr(0, Attribute::NoAlias);
318 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
319 /// identifiable object. This returns true for:
320 /// Global Variables and Functions (but not Global Aliases)
321 /// Allocas and Mallocs
322 /// ByVal and NoAlias Arguments
325 bool llvm::isIdentifiedObject(const Value *V) {
326 if (isa<AllocaInst>(V))
328 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
330 if (isNoAliasCall(V))
332 if (const Argument *A = dyn_cast<Argument>(V))
333 return A->hasNoAliasAttr() || A->hasByValAttr();