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 return CI->getValue()[0];
111 /// TypeBasedAliasAnalysis - This is a simple alias analysis
112 /// implementation that uses TypeBased to answer queries.
113 class TypeBasedAliasAnalysis : public ImmutablePass,
114 public AliasAnalysis {
116 static char ID; // Class identification, replacement for typeinfo
117 TypeBasedAliasAnalysis() : ImmutablePass(ID) {
118 initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
121 virtual void initializePass() {
122 InitializeAliasAnalysis(this);
125 /// getAdjustedAnalysisPointer - This method is used when a pass implements
126 /// an analysis interface through multiple inheritance. If needed, it
127 /// should override this to adjust the this pointer as needed for the
128 /// specified pass info.
129 virtual void *getAdjustedAnalysisPointer(const void *PI) {
130 if (PI == &AliasAnalysis::ID)
131 return (AliasAnalysis*)this;
135 bool Aliases(const MDNode *A, const MDNode *B) const;
138 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
139 virtual AliasResult alias(const Location &LocA, const Location &LocB);
140 virtual bool pointsToConstantMemory(const Location &Loc);
142 } // End of anonymous namespace
144 // Register this pass...
145 char TypeBasedAliasAnalysis::ID = 0;
146 INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
147 "Type-Based Alias Analysis", false, true, false)
149 ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
150 return new TypeBasedAliasAnalysis();
154 TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
155 AU.setPreservesAll();
156 AliasAnalysis::getAnalysisUsage(AU);
159 /// Aliases - Test whether the type represented by A may alias the
160 /// type represented by B.
162 TypeBasedAliasAnalysis::Aliases(const MDNode *A,
163 const MDNode *B) const {
164 // Keep track of the root node for A and B.
165 TBAANode RootA, RootB;
167 // Climb the tree from A to see if we reach B.
168 for (TBAANode T(A); ; ) {
169 if (T.getNode() == B)
170 // B is an ancestor of A.
179 // Climb the tree from B to see if we reach A.
180 for (TBAANode T(B); ; ) {
181 if (T.getNode() == A)
182 // A is an ancestor of B.
191 // Neither node is an ancestor of the other.
193 // If they have different roots, they're part of different potentially
194 // unrelated type systems, so we must be conservative.
195 if (RootA.getNode() != RootB.getNode())
198 // If they have the same root, then we've proved there's no alias.
202 AliasAnalysis::AliasResult
203 TypeBasedAliasAnalysis::alias(const Location &LocA,
204 const Location &LocB) {
206 return AliasAnalysis::alias(LocA, LocB);
208 // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
210 const MDNode *AM = LocA.TBAATag;
211 if (!AM) return AliasAnalysis::alias(LocA, LocB);
212 const MDNode *BM = LocB.TBAATag;
213 if (!BM) return AliasAnalysis::alias(LocA, LocB);
215 // If they may alias, chain to the next AliasAnalysis.
217 return AliasAnalysis::alias(LocA, LocB);
219 // Otherwise return a definitive result.
223 bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
225 return AliasAnalysis::pointsToConstantMemory(Loc);
227 const MDNode *M = Loc.TBAATag;
228 if (!M) return false;
230 // If this is an "immutable" type, we can assume the pointer is pointing
231 // to constant memory.
232 if (TBAANode(M).TypeIsImmutable())
235 return AliasAnalysis::pointsToConstantMemory(Loc);