CodeGenTBAA.cpp revision 2d7cb069fe101da3971a07900ff583380bcac184 
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    MDHelper(VMContext), 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 = MDHelper.CreateTBAARoot("Simple C/C++ TBAA");
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 = MDHelper.CreateTBAANode("omnipotent char", getRoot());
55
56  return Char;
57}
58
59static bool TypeHasMayAlias(QualType QTy) {
60  // Tagged types have declarations, and therefore may have attributes.
61  if (const TagType *TTy = dyn_cast<TagType>(QTy))
62    return TTy->getDecl()->hasAttr<MayAliasAttr>();
63
64  // Typedef types have declarations, and therefore may have attributes.
65  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
66    if (TTy->getDecl()->hasAttr<MayAliasAttr>())
67      return true;
68    // Also, their underlying types may have relevant attributes.
69    return TypeHasMayAlias(TTy->desugar());
70  }
71
72  return false;
73}
74
75llvm::MDNode *
76CodeGenTBAA::getTBAAInfo(QualType QTy) {
77  // If the type has the may_alias attribute (even on a typedef), it is
78  // effectively in the general char alias class.
79  if (TypeHasMayAlias(QTy))
80    return getChar();
81
82  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
83
84  if (llvm::MDNode *N = MetadataCache[Ty])
85    return N;
86
87  // Handle builtin types.
88  if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
89    switch (BTy->getKind()) {
90    // Character types are special and can alias anything.
91    // In C++, this technically only includes "char" and "unsigned char",
92    // and not "signed char". In C, it includes all three. For now,
93    // the risk of exploiting this detail in C++ seems likely to outweigh
94    // the benefit.
95    case BuiltinType::Char_U:
96    case BuiltinType::Char_S:
97    case BuiltinType::UChar:
98    case BuiltinType::SChar:
99      return getChar();
100
101    // Unsigned types can alias their corresponding signed types.
102    case BuiltinType::UShort:
103      return getTBAAInfo(Context.ShortTy);
104    case BuiltinType::UInt:
105      return getTBAAInfo(Context.IntTy);
106    case BuiltinType::ULong:
107      return getTBAAInfo(Context.LongTy);
108    case BuiltinType::ULongLong:
109      return getTBAAInfo(Context.LongLongTy);
110    case BuiltinType::UInt128:
111      return getTBAAInfo(Context.Int128Ty);
112
113    // Treat all other builtin types as distinct types. This includes
114    // treating wchar_t, char16_t, and char32_t as distinct from their
115    // "underlying types".
116    default:
117      return MetadataCache[Ty] =
118        MDHelper.CreateTBAANode(BTy->getName(Features), getChar());
119    }
120  }
121
122  // Handle pointers.
123  // TODO: Implement C++'s type "similarity" and consider dis-"similar"
124  // pointers distinct.
125  if (Ty->isPointerType())
126    return MetadataCache[Ty] = MDHelper.CreateTBAANode("any pointer",
127                                                       getChar());
128
129  // Enum types are distinct types. In C++ they have "underlying types",
130  // however they aren't related for TBAA.
131  if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
132    // In C mode, two anonymous enums are compatible iff their members
133    // are the same -- see C99 6.2.7p1. For now, be conservative. We could
134    // theoretically implement this by combining information about all the
135    // members into a single identifying MDNode.
136    if (!Features.CPlusPlus &&
137        ETy->getDecl()->getTypedefNameForAnonDecl())
138      return MetadataCache[Ty] = getChar();
139
140    // In C++ mode, types have linkage, so we can rely on the ODR and
141    // on their mangled names, if they're external.
142    // TODO: Is there a way to get a program-wide unique name for a
143    // decl with local linkage or no linkage?
144    if (Features.CPlusPlus &&
145        ETy->getDecl()->getLinkage() != ExternalLinkage)
146      return MetadataCache[Ty] = getChar();
147
148    // TODO: This is using the RTTI name. Is there a better way to get
149    // a unique string for a type?
150    SmallString<256> OutName;
151    llvm::raw_svector_ostream Out(OutName);
152    MContext.mangleCXXRTTIName(QualType(ETy, 0), Out);
153    Out.flush();
154    return MetadataCache[Ty] = MDHelper.CreateTBAANode(OutName, getChar());
155  }
156
157  // For now, handle any other kind of type conservatively.
158  return MetadataCache[Ty] = getChar();
159}
160
161llvm::MDNode *CodeGenTBAA::getTBAAInfoForVTablePtr() {
162  return MDHelper.CreateTBAANode("vtable pointer", getRoot());
163}
164