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/Module.h"
63 #include "llvm/Metadata.h"
64 #include "llvm/Pass.h"
65 #include "llvm/Support/CommandLine.h"
68 // For testing purposes, enable TBAA only via a special option.
69 static cl::opt<bool> EnableTBAA("enable-tbaa");
72 /// TBAANode - This is a simple wrapper around an MDNode which provides a
73 /// higher-level interface by hiding the details of how alias analysis
74 /// information is encoded in its operands.
79 TBAANode() : Node(0) {}
80 explicit TBAANode(const MDNode *N) : Node(N) {}
82 /// getNode - Get the MDNode for this TBAANode.
83 const MDNode *getNode() const { return Node; }
85 /// getParent - Get this TBAANode's Alias tree parent.
86 TBAANode getParent() const {
87 if (Node->getNumOperands() < 2)
89 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
92 // Ok, this node has a valid parent. Return it.
96 /// TypeIsImmutable - Test if this TBAANode represents a type for objects
97 /// which are not modified (by any means) in the context where this
98 /// AliasAnalysis is relevant.
99 bool TypeIsImmutable() const {
100 if (Node->getNumOperands() < 3)
102 ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
105 // TODO: Think about the encoding.
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);
143 } // End of anonymous namespace
145 // Register this pass...
146 char TypeBasedAliasAnalysis::ID = 0;
147 INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
148 "Type-Based Alias Analysis", false, true, false)
150 ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
151 return new TypeBasedAliasAnalysis();
155 TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
156 AU.setPreservesAll();
157 AliasAnalysis::getAnalysisUsage(AU);
160 /// Aliases - Test whether the type represented by A may alias the
161 /// type represented by B.
163 TypeBasedAliasAnalysis::Aliases(const MDNode *A,
164 const MDNode *B) const {
165 // Keep track of the root node for A and B.
166 TBAANode RootA, RootB;
168 // Climb the tree from A to see if we reach B.
169 for (TBAANode T(A); ; ) {
170 if (T.getNode() == B)
171 // B is an ancestor of A.
180 // Climb the tree from B to see if we reach A.
181 for (TBAANode T(B); ; ) {
182 if (T.getNode() == A)
183 // A is an ancestor of B.
192 // Neither node is an ancestor of the other.
194 // If they have different roots, they're part of different potentially
195 // unrelated type systems, so we must be conservative.
196 if (RootA.getNode() != RootB.getNode())
199 // If they have the same root, then we've proved there's no alias.
203 AliasAnalysis::AliasResult
204 TypeBasedAliasAnalysis::alias(const Location &LocA,
205 const Location &LocB) {
207 return AliasAnalysis::alias(LocA, LocB);
209 // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
211 const MDNode *AM = LocA.TBAATag;
212 if (!AM) return AliasAnalysis::alias(LocA, LocB);
213 const MDNode *BM = LocB.TBAATag;
214 if (!BM) return AliasAnalysis::alias(LocA, LocB);
216 // If they may alias, chain to the next AliasAnalysis.
218 return AliasAnalysis::alias(LocA, LocB);
220 // Otherwise return a definitive result.
224 bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
226 return AliasAnalysis::pointsToConstantMemory(Loc);
228 const MDNode *M = Loc.TBAATag;
229 if (!M) return false;
231 // If this is an "immutable" type, we can assume the pointer is pointing
232 // to constant memory.
233 if (TBAANode(M).TypeIsImmutable())
236 return AliasAnalysis::pointsToConstantMemory(Loc);