CodeGenTBAA.cpp revision 5f9e272e632e951b1efe824cd16acb4d96077930 
1//===--- CodeGenTypes.cpp - TBAA information for LLVM CodeGen -------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This is the code that manages TBAA information and defines the TBAA policy
11// for the optimizer to use. Relevant standards text includes:
12//
13//   C99 6.5p7
14//   C++ [basic.lval] (p10 in n3126, p15 in some earlier versions)
15//
16//===----------------------------------------------------------------------===//
17
18#include "CodeGenTBAA.h"
19#include "clang/AST/ASTContext.h"
20#include "clang/AST/Mangle.h"
21#include "llvm/LLVMContext.h"
22#include "llvm/Metadata.h"
23#include "llvm/Constants.h"
24#include "llvm/Type.h"
25using namespace clang;
26using namespace CodeGen;
27
28CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, llvm::LLVMContext& VMContext,
29                         const LangOptions &Features, MangleContext &MContext)
30  : Context(Ctx), VMContext(VMContext), Features(Features), MContext(MContext),
31    Root(0), Char(0) {
32}
33
34CodeGenTBAA::~CodeGenTBAA() {
35}
36
37llvm::MDNode *CodeGenTBAA::getRoot() {
38  // Define the root of the tree. This identifies the tree, so that
39  // if our LLVM IR is linked with LLVM IR from a different front-end
40  // (or a different version of this front-end), their TBAA trees will
41  // remain distinct, and the optimizer will treat them conservatively.
42  if (!Root)
43    Root = getTBAAInfoForNamedType("Simple C/C++ TBAA", 0);
44
45  return Root;
46}
47
48llvm::MDNode *CodeGenTBAA::getChar() {
49  // Define the root of the tree for user-accessible memory. C and C++
50  // give special powers to char and certain similar types. However,
51  // these special powers only cover user-accessible memory, and doesn't
52  // include things like vtables.
53  if (!Char)
54    Char = getTBAAInfoForNamedType("omnipotent char", getRoot());
55
56  return Char;
57}
58
59/// getTBAAInfoForNamedType - Create a TBAA tree node with the given string
60/// as its identifier, and the given Parent node as its tree parent.
61llvm::MDNode *CodeGenTBAA::getTBAAInfoForNamedType(StringRef NameStr,
62                                                   llvm::MDNode *Parent,
63                                                   bool Readonly) {
64  // Currently there is only one flag defined - the readonly flag.
65  llvm::Value *Flags = 0;
66  if (Readonly)
67    Flags = llvm::ConstantInt::get(llvm::Type::getInt64Ty(VMContext), true);
68
69  // Set up the mdnode operand list.
70  llvm::Value *Ops[] = {
71    llvm::MDString::get(VMContext, NameStr),
72    Parent,
73    Flags
74  };
75
76  // Create the mdnode.
77  unsigned Len = llvm::array_lengthof(Ops) - !Flags;
78  return llvm::MDNode::get(VMContext, llvm::makeArrayRef(Ops, Len));
79}
80
81static bool TypeHasMayAlias(QualType QTy) {
82  // Tagged types have declarations, and therefore may have attributes.
83  if (const TagType *TTy = dyn_cast<TagType>(QTy))
84    return TTy->getDecl()->hasAttr<MayAliasAttr>();
85
86  // Typedef types have declarations, and therefore may have attributes.
87  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
88    if (TTy->getDecl()->hasAttr<MayAliasAttr>())
89      return true;
90    // Also, their underlying types may have relevant attributes.
91    return TypeHasMayAlias(TTy->desugar());
92  }
93
94  return false;
95}
96
97llvm::MDNode *
98CodeGenTBAA::getTBAAInfo(QualType QTy) {
99  // If the type has the may_alias attribute (even on a typedef), it is
100  // effectively in the general char alias class.
101  if (TypeHasMayAlias(QTy))
102    return getChar();
103
104  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
105
106  if (llvm::MDNode *N = MetadataCache[Ty])
107    return N;
108
109  // Handle builtin types.
110  if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
111    switch (BTy->getKind()) {
112    // Character types are special and can alias anything.
113    // In C++, this technically only includes "char" and "unsigned char",
114    // and not "signed char". In C, it includes all three. For now,
115    // the risk of exploiting this detail in C++ seems likely to outweigh
116    // the benefit.
117    case BuiltinType::Char_U:
118    case BuiltinType::Char_S:
119    case BuiltinType::UChar:
120    case BuiltinType::SChar:
121      return getChar();
122
123    // Unsigned types can alias their corresponding signed types.
124    case BuiltinType::UShort:
125      return getTBAAInfo(Context.ShortTy);
126    case BuiltinType::UInt:
127      return getTBAAInfo(Context.IntTy);
128    case BuiltinType::ULong:
129      return getTBAAInfo(Context.LongTy);
130    case BuiltinType::ULongLong:
131      return getTBAAInfo(Context.LongLongTy);
132    case BuiltinType::UInt128:
133      return getTBAAInfo(Context.Int128Ty);
134
135    // Treat all other builtin types as distinct types. This includes
136    // treating wchar_t, char16_t, and char32_t as distinct from their
137    // "underlying types".
138    default:
139      return MetadataCache[Ty] =
140               getTBAAInfoForNamedType(BTy->getName(Features), getChar());
141    }
142  }
143
144  // Handle pointers.
145  // TODO: Implement C++'s type "similarity" and consider dis-"similar"
146  // pointers distinct.
147  if (Ty->isPointerType())
148    return MetadataCache[Ty] = getTBAAInfoForNamedType("any pointer",
149                                                       getChar());
150
151  // Enum types are distinct types. In C++ they have "underlying types",
152  // however they aren't related for TBAA.
153  if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
154    // In C mode, two anonymous enums are compatible iff their members
155    // are the same -- see C99 6.2.7p1. For now, be conservative. We could
156    // theoretically implement this by combining information about all the
157    // members into a single identifying MDNode.
158    if (!Features.CPlusPlus &&
159        ETy->getDecl()->getTypedefNameForAnonDecl())
160      return MetadataCache[Ty] = getChar();
161
162    // In C++ mode, types have linkage, so we can rely on the ODR and
163    // on their mangled names, if they're external.
164    // TODO: Is there a way to get a program-wide unique name for a
165    // decl with local linkage or no linkage?
166    if (Features.CPlusPlus &&
167        ETy->getDecl()->getLinkage() != ExternalLinkage)
168      return MetadataCache[Ty] = getChar();
169
170    // TODO: This is using the RTTI name. Is there a better way to get
171    // a unique string for a type?
172    llvm::SmallString<256> OutName;
173    llvm::raw_svector_ostream Out(OutName);
174    MContext.mangleCXXRTTIName(QualType(ETy, 0), Out);
175    Out.flush();
176    return MetadataCache[Ty] = getTBAAInfoForNamedType(OutName, getChar());
177  }
178
179  // For now, handle any other kind of type conservatively.
180  return MetadataCache[Ty] = getChar();
181}
182