1 //===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===//
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 defines the TypeBasedAliasAnalysis pass, which implements
11 // metadata-based TBAA.
13 // In LLVM IR, memory does not have types, so LLVM's own type system is not
14 // suitable for doing TBAA. Instead, metadata is added to the IR to describe
15 // a type system of a higher level language. This can be used to implement
16 // typical C/C++ TBAA, but it can also be used to implement custom alias
17 // analysis behavior for other languages.
19 // The current metadata format is very simple. TBAA MDNodes have up to
20 // three fields, e.g.:
21 // !0 = metadata !{ metadata !"an example type tree" }
22 // !1 = metadata !{ metadata !"int", metadata !0 }
23 // !2 = metadata !{ metadata !"float", metadata !0 }
24 // !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
26 // The first field is an identity field. It can be any value, usually
27 // an MDString, which uniquely identifies the type. The most important
28 // name in the tree is the name of the root node. Two trees with
29 // different root node names are entirely disjoint, even if they
30 // have leaves with common names.
32 // The second field identifies the type's parent node in the tree, or
33 // is null or omitted for a root node. A type is considered to alias
34 // all of its decendents and all of its ancestors in the tree. Also,
35 // a type is considered to alias all types in other trees, so that
36 // bitcode produced from multiple front-ends is handled conservatively.
38 // If the third field is present, it's an integer which if equal to 1
39 // indicates that the type is "constant" (meaning pointsToConstantMemory
40 // should return true; see
41 // http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
43 // TODO: The current metadata format doesn't support struct
44 // fields. For example:
49 // void foo(struct X *x, struct X *y, double *p) {
53 // Struct X has a double member, so the store to *x can alias the store to *p.
54 // Currently it's not possible to precisely describe all the things struct X
55 // aliases, so struct assignments must use conservative TBAA nodes. There's
56 // no scheme for attaching metadata to @llvm.memcpy yet either.
58 //===----------------------------------------------------------------------===//
60 #include "llvm/Analysis/AliasAnalysis.h"
61 #include "llvm/Analysis/Passes.h"
62 #include "llvm/LLVMContext.h"
63 #include "llvm/Module.h"
64 #include "llvm/Metadata.h"
65 #include "llvm/Pass.h"
66 #include "llvm/Support/CommandLine.h"
69 // For testing purposes, enable TBAA only via a special option.
70 static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(false));
73 /// TBAANode - This is a simple wrapper around an MDNode which provides a
74 /// higher-level interface by hiding the details of how alias analysis
75 /// information is encoded in its operands.
80 TBAANode() : Node(0) {}
81 explicit TBAANode(const MDNode *N) : Node(N) {}
83 /// getNode - Get the MDNode for this TBAANode.
84 const MDNode *getNode() const { return Node; }
86 /// getParent - Get this TBAANode's Alias tree parent.
87 TBAANode getParent() const {
88 if (Node->getNumOperands() < 2)
90 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
93 // Ok, this node has a valid parent. Return it.
97 /// TypeIsImmutable - Test if this TBAANode represents a type for objects
98 /// which are not modified (by any means) in the context where this
99 /// AliasAnalysis is relevant.
100 bool TypeIsImmutable() const {
101 if (Node->getNumOperands() < 3)
103 ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
106 return CI->getValue()[0];
112 /// TypeBasedAliasAnalysis - This is a simple alias analysis
113 /// implementation that uses TypeBased to answer queries.
114 class TypeBasedAliasAnalysis : public ImmutablePass,
115 public AliasAnalysis {
117 static char ID; // Class identification, replacement for typeinfo
118 TypeBasedAliasAnalysis() : ImmutablePass(ID) {
119 initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
122 virtual void initializePass() {
123 InitializeAliasAnalysis(this);
126 /// getAdjustedAnalysisPointer - This method is used when a pass implements
127 /// an analysis interface through multiple inheritance. If needed, it
128 /// should override this to adjust the this pointer as needed for the
129 /// specified pass info.
130 virtual void *getAdjustedAnalysisPointer(const void *PI) {
131 if (PI == &AliasAnalysis::ID)
132 return (AliasAnalysis*)this;
136 bool Aliases(const MDNode *A, const MDNode *B) const;
139 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
140 virtual AliasResult alias(const Location &LocA, const Location &LocB);
141 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
142 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
143 virtual ModRefBehavior getModRefBehavior(const Function *F);
144 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
145 const Location &Loc);
146 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
147 ImmutableCallSite CS2);
149 } // End of anonymous namespace
151 // Register this pass...
152 char TypeBasedAliasAnalysis::ID = 0;
153 INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
154 "Type-Based Alias Analysis", false, true, false)
156 ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
157 return new TypeBasedAliasAnalysis();
161 TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
162 AU.setPreservesAll();
163 AliasAnalysis::getAnalysisUsage(AU);
166 /// Aliases - Test whether the type represented by A may alias the
167 /// type represented by B.
169 TypeBasedAliasAnalysis::Aliases(const MDNode *A,
170 const MDNode *B) const {
171 // Keep track of the root node for A and B.
172 TBAANode RootA, RootB;
174 // Climb the tree from A to see if we reach B.
175 for (TBAANode T(A); ; ) {
176 if (T.getNode() == B)
177 // B is an ancestor of A.
186 // Climb the tree from B to see if we reach A.
187 for (TBAANode T(B); ; ) {
188 if (T.getNode() == A)
189 // A is an ancestor of B.
198 // Neither node is an ancestor of the other.
200 // If they have different roots, they're part of different potentially
201 // unrelated type systems, so we must be conservative.
202 if (RootA.getNode() != RootB.getNode())
205 // If they have the same root, then we've proved there's no alias.
209 AliasAnalysis::AliasResult
210 TypeBasedAliasAnalysis::alias(const Location &LocA,
211 const Location &LocB) {
213 return AliasAnalysis::alias(LocA, LocB);
215 // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
217 const MDNode *AM = LocA.TBAATag;
218 if (!AM) return AliasAnalysis::alias(LocA, LocB);
219 const MDNode *BM = LocB.TBAATag;
220 if (!BM) return AliasAnalysis::alias(LocA, LocB);
222 // If they may alias, chain to the next AliasAnalysis.
224 return AliasAnalysis::alias(LocA, LocB);
226 // Otherwise return a definitive result.
230 bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc,
233 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
235 const MDNode *M = Loc.TBAATag;
236 if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
238 // If this is an "immutable" type, we can assume the pointer is pointing
239 // to constant memory.
240 if (TBAANode(M).TypeIsImmutable())
243 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
246 AliasAnalysis::ModRefBehavior
247 TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
249 return AliasAnalysis::getModRefBehavior(CS);
251 ModRefBehavior Min = UnknownModRefBehavior;
253 // If this is an "immutable" type, we can assume the call doesn't write
255 if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
256 if (TBAANode(M).TypeIsImmutable())
257 Min = OnlyReadsMemory;
259 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
262 AliasAnalysis::ModRefBehavior
263 TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
264 // Functions don't have metadata. Just chain to the next implementation.
265 return AliasAnalysis::getModRefBehavior(F);
268 AliasAnalysis::ModRefResult
269 TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
270 const Location &Loc) {
272 return AliasAnalysis::getModRefInfo(CS, Loc);
274 if (const MDNode *L = Loc.TBAATag)
275 if (const MDNode *M =
276 CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
280 return AliasAnalysis::getModRefInfo(CS, Loc);
283 AliasAnalysis::ModRefResult
284 TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
285 ImmutableCallSite CS2) {
287 return AliasAnalysis::getModRefInfo(CS1, CS2);
289 if (const MDNode *M1 =
290 CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
291 if (const MDNode *M2 =
292 CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
293 if (!Aliases(M1, M2))
296 return AliasAnalysis::getModRefInfo(CS1, CS2);