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.
36 // If the third field is present, it's an integer which if equal to 1
37 // indicates that the type is "constant" (meaning pointsToConstantMemory
38 // should return true; see
39 // http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
41 // TODO: The current metadata format doesn't support struct
42 // fields. For example:
47 // void foo(struct X *x, struct X *y, double *p) {
51 // Struct X has a double member, so the store to *x can alias the store to *p.
52 // Currently it's not possible to precisely describe all the things struct X
53 // aliases, so struct assignments must use conservative TBAA nodes. There's
54 // no scheme for attaching metadata to @llvm.memcpy yet either.
56 //===----------------------------------------------------------------------===//
58 #include "llvm/Analysis/AliasAnalysis.h"
59 #include "llvm/Analysis/Passes.h"
60 #include "llvm/Module.h"
61 #include "llvm/Metadata.h"
62 #include "llvm/Pass.h"
63 #include "llvm/Support/CommandLine.h"
66 // For testing purposes, enable TBAA only via a special option.
67 static cl::opt<bool> EnableTBAA("enable-tbaa");
70 /// TBAANode - This is a simple wrapper around an MDNode which provides a
71 /// higher-level interface by hiding the details of how alias analysis
72 /// information is encoded in its operands.
77 TBAANode() : Node(0) {}
78 explicit TBAANode(const MDNode *N) : Node(N) {}
80 /// getNode - Get the MDNode for this TBAANode.
81 const MDNode *getNode() const { return Node; }
83 /// getParent - Get this TBAANode's Alias tree parent.
84 TBAANode getParent() const {
85 if (Node->getNumOperands() < 2)
87 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
90 // Ok, this node has a valid parent. Return it.
94 /// TypeIsImmutable - Test if this TBAANode represents a type for objects
95 /// which are not modified (by any means) in the context where this
96 /// AliasAnalysis is relevant.
97 bool TypeIsImmutable() const {
98 if (Node->getNumOperands() < 3)
100 ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
103 // TODO: Think about the encoding.
110 /// TypeBasedAliasAnalysis - This is a simple alias analysis
111 /// implementation that uses TypeBased to answer queries.
112 class TypeBasedAliasAnalysis : public ImmutablePass,
113 public AliasAnalysis {
115 static char ID; // Class identification, replacement for typeinfo
116 TypeBasedAliasAnalysis() : ImmutablePass(ID) {
117 initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
120 virtual void initializePass() {
121 InitializeAliasAnalysis(this);
124 /// getAdjustedAnalysisPointer - This method is used when a pass implements
125 /// an analysis interface through multiple inheritance. If needed, it
126 /// should override this to adjust the this pointer as needed for the
127 /// specified pass info.
128 virtual void *getAdjustedAnalysisPointer(const void *PI) {
129 if (PI == &AliasAnalysis::ID)
130 return (AliasAnalysis*)this;
134 bool Aliases(const MDNode *A, const MDNode *B) const;
137 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
138 virtual AliasResult alias(const Location &LocA, const Location &LocB);
139 virtual bool pointsToConstantMemory(const Location &Loc);
141 } // End of anonymous namespace
143 // Register this pass...
144 char TypeBasedAliasAnalysis::ID = 0;
145 INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
146 "Type-Based Alias Analysis", false, true, false)
148 ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
149 return new TypeBasedAliasAnalysis();
153 TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
154 AU.setPreservesAll();
155 AliasAnalysis::getAnalysisUsage(AU);
158 /// Aliases - Test whether the type represented by A may alias the
159 /// type represented by B.
161 TypeBasedAliasAnalysis::Aliases(const MDNode *A,
162 const MDNode *B) const {
163 // Keep track of the root node for A and B.
164 TBAANode RootA, RootB;
166 // Climb the tree from A to see if we reach B.
167 for (TBAANode T(A); ; ) {
168 if (T.getNode() == B)
169 // B is an ancestor of A.
178 // Climb the tree from B to see if we reach A.
179 for (TBAANode T(B); ; ) {
180 if (T.getNode() == A)
181 // A is an ancestor of B.
190 // Neither node is an ancestor of the other.
192 // If they have different roots, they're part of different potentially
193 // unrelated type systems, so we must be conservative.
194 if (RootA.getNode() != RootB.getNode())
197 // If they have the same root, then we've proved there's no alias.
201 AliasAnalysis::AliasResult
202 TypeBasedAliasAnalysis::alias(const Location &LocA,
203 const Location &LocB) {
205 return AliasAnalysis::alias(LocA, LocB);
207 // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
209 const MDNode *AM = LocA.TBAATag;
210 if (!AM) return AliasAnalysis::alias(LocA, LocB);
211 const MDNode *BM = LocB.TBAATag;
212 if (!BM) return AliasAnalysis::alias(LocA, LocB);
214 // If they may alias, chain to the next AliasAnalysis.
216 return AliasAnalysis::alias(LocA, LocB);
218 // Otherwise return a definitive result.
222 bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
224 return AliasAnalysis::pointsToConstantMemory(Loc);
226 const MDNode *M = Loc.TBAATag;
227 if (!M) return false;
229 // If this is an "immutable" type, we can assume the pointer is pointing
230 // to constant memory.
231 if (TBAANode(M).TypeIsImmutable())
234 return AliasAnalysis::pointsToConstantMemory(Loc);