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) {
53 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
54 return AA->pointsToConstantMemory(Loc);
57 void AliasAnalysis::deleteValue(Value *V) {
58 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
62 void AliasAnalysis::copyValue(Value *From, Value *To) {
63 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
64 AA->copyValue(From, To);
67 AliasAnalysis::ModRefResult
68 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
69 const Location &Loc) {
70 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
72 ModRefBehavior MRB = getModRefBehavior(CS);
73 if (MRB == DoesNotAccessMemory)
76 ModRefResult Mask = ModRef;
77 if (MRB == OnlyReadsMemory)
79 else if (MRB == AliasAnalysis::AccessesArguments) {
80 bool doesAlias = false;
81 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
83 if (!isNoAlias(Location(*AI), Loc)) {
92 // If Loc is a constant memory location, the call definitely could not
93 // modify the memory location.
94 if ((Mask & Mod) && pointsToConstantMemory(Loc))
95 Mask = ModRefResult(Mask & ~Mod);
97 // If this is the end of the chain, don't forward.
100 // Otherwise, fall back to the next AA in the chain. But we can merge
101 // in any mask we've managed to compute.
102 return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
105 AliasAnalysis::ModRefResult
106 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
107 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
109 // If CS1 or CS2 are readnone, they don't interact.
110 ModRefBehavior CS1B = getModRefBehavior(CS1);
111 if (CS1B == DoesNotAccessMemory) return NoModRef;
113 ModRefBehavior CS2B = getModRefBehavior(CS2);
114 if (CS2B == DoesNotAccessMemory) return NoModRef;
116 // If they both only read from memory, there is no dependence.
117 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
120 AliasAnalysis::ModRefResult Mask = ModRef;
122 // If CS1 only reads memory, the only dependence on CS2 can be
123 // from CS1 reading memory written by CS2.
124 if (CS1B == OnlyReadsMemory)
125 Mask = ModRefResult(Mask & Ref);
127 // If CS2 only access memory through arguments, accumulate the mod/ref
128 // information from CS1's references to the memory referenced by
130 if (CS2B == AccessesArguments) {
131 AliasAnalysis::ModRefResult R = NoModRef;
132 for (ImmutableCallSite::arg_iterator
133 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
134 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask);
141 // If CS1 only accesses memory through arguments, check if CS2 references
142 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
143 if (CS1B == AccessesArguments) {
144 AliasAnalysis::ModRefResult R = NoModRef;
145 for (ImmutableCallSite::arg_iterator
146 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I)
147 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) {
155 // If this is the end of the chain, don't forward.
156 if (!AA) return Mask;
158 // Otherwise, fall back to the next AA in the chain. But we can merge
159 // in any mask we've managed to compute.
160 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
163 AliasAnalysis::ModRefBehavior
164 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
165 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
167 ModRefBehavior Min = UnknownModRefBehavior;
169 // Call back into the alias analysis with the other form of getModRefBehavior
170 // to see if it can give a better response.
171 if (const Function *F = CS.getCalledFunction())
172 Min = getModRefBehavior(F);
174 // If this is the end of the chain, don't forward.
177 // Otherwise, fall back to the next AA in the chain. But we can merge
178 // in any result we've managed to compute.
179 return std::min(AA->getModRefBehavior(CS), Min);
182 AliasAnalysis::ModRefBehavior
183 AliasAnalysis::getModRefBehavior(const Function *F) {
184 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
185 return AA->getModRefBehavior(F);
188 //===----------------------------------------------------------------------===//
189 // AliasAnalysis non-virtual helper method implementation
190 //===----------------------------------------------------------------------===//
192 AliasAnalysis::ModRefResult
193 AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
194 // Be conservative in the face of volatile.
198 // If the load address doesn't alias the given address, it doesn't read
199 // or write the specified memory.
200 if (!alias(Location(L->getOperand(0),
201 getTypeStoreSize(L->getType()),
202 L->getMetadata(LLVMContext::MD_tbaa)),
206 // Otherwise, a load just reads.
210 AliasAnalysis::ModRefResult
211 AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
212 // Be conservative in the face of volatile.
216 // If the store address cannot alias the pointer in question, then the
217 // specified memory cannot be modified by the store.
218 if (!alias(Location(S->getOperand(1),
219 getTypeStoreSize(S->getOperand(0)->getType()),
220 S->getMetadata(LLVMContext::MD_tbaa)),
224 // If the pointer is a pointer to constant memory, then it could not have been
225 // modified by this store.
226 if (pointsToConstantMemory(Loc))
229 // Otherwise, a store just writes.
233 AliasAnalysis::ModRefResult
234 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
235 // If the va_arg address cannot alias the pointer in question, then the
236 // specified memory cannot be accessed by the va_arg.
237 if (!alias(Location(V->getOperand(0),
239 V->getMetadata(LLVMContext::MD_tbaa)),
243 // If the pointer is a pointer to constant memory, then it could not have been
244 // modified by this va_arg.
245 if (pointsToConstantMemory(Loc))
248 // Otherwise, a va_arg reads and writes.
252 // AliasAnalysis destructor: DO NOT move this to the header file for
253 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
254 // the AliasAnalysis.o file in the current .a file, causing alias analysis
255 // support to not be included in the tool correctly!
257 AliasAnalysis::~AliasAnalysis() {}
259 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
260 /// AliasAnalysis interface before any other methods are called.
262 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
263 TD = P->getAnalysisIfAvailable<TargetData>();
264 AA = &P->getAnalysis<AliasAnalysis>();
267 // getAnalysisUsage - All alias analysis implementations should invoke this
268 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
269 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
270 AU.addRequired<AliasAnalysis>(); // All AA's chain
273 /// getTypeStoreSize - Return the TargetData store size for the given type,
274 /// if known, or a conservative value otherwise.
276 uint64_t AliasAnalysis::getTypeStoreSize(const Type *Ty) {
277 return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
280 /// canBasicBlockModify - Return true if it is possible for execution of the
281 /// specified basic block to modify the value pointed to by Ptr.
283 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
284 const Location &Loc) {
285 return canInstructionRangeModify(BB.front(), BB.back(), Loc);
288 /// canInstructionRangeModify - Return true if it is possible for the execution
289 /// of the specified instructions to modify the value pointed to by Ptr. The
290 /// instructions to consider are all of the instructions in the range of [I1,I2]
291 /// INCLUSIVE. I1 and I2 must be in the same basic block.
293 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
294 const Instruction &I2,
295 const Location &Loc) {
296 assert(I1.getParent() == I2.getParent() &&
297 "Instructions not in same basic block!");
298 BasicBlock::const_iterator I = &I1;
299 BasicBlock::const_iterator E = &I2;
300 ++E; // Convert from inclusive to exclusive range.
302 for (; I != E; ++I) // Check every instruction in range
303 if (getModRefInfo(I, Loc) & Mod)
308 /// isNoAliasCall - Return true if this pointer is returned by a noalias
310 bool llvm::isNoAliasCall(const Value *V) {
311 if (isa<CallInst>(V) || isa<InvokeInst>(V))
312 return ImmutableCallSite(cast<Instruction>(V))
313 .paramHasAttr(0, Attribute::NoAlias);
317 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
318 /// identifiable object. This returns true for:
319 /// Global Variables and Functions (but not Global Aliases)
320 /// Allocas and Mallocs
321 /// ByVal and NoAlias Arguments
324 bool llvm::isIdentifiedObject(const Value *V) {
325 if (isa<AllocaInst>(V))
327 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
329 if (isNoAliasCall(V))
331 if (const Argument *A = dyn_cast<Argument>(V))
332 return A->hasNoAliasAttr() || A->hasByValAttr();