CodeGenTBAA.cpp revision 94ff8e1f57c6382d91d0de981a4f311509d83e37 
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 "clang/Frontend/CodeGenOptions.h"
22#include "llvm/LLVMContext.h"
23#include "llvm/Metadata.h"
24#include "llvm/Constants.h"
25#include "llvm/Type.h"
26using namespace clang;
27using namespace CodeGen;
28
29CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, llvm::LLVMContext& VMContext,
30                         const CodeGenOptions &CGO,
31                         const LangOptions &Features, MangleContext &MContext)
32  : Context(Ctx), CodeGenOpts(CGO), Features(Features), MContext(MContext),
33    MDHelper(VMContext), Root(0), Char(0) {
34}
35
36CodeGenTBAA::~CodeGenTBAA() {
37}
38
39llvm::MDNode *CodeGenTBAA::getRoot() {
40  // Define the root of the tree. This identifies the tree, so that
41  // if our LLVM IR is linked with LLVM IR from a different front-end
42  // (or a different version of this front-end), their TBAA trees will
43  // remain distinct, and the optimizer will treat them conservatively.
44  if (!Root)
45    Root = MDHelper.createTBAARoot("Simple C/C++ TBAA");
46
47  return Root;
48}
49
50llvm::MDNode *CodeGenTBAA::getChar() {
51  // Define the root of the tree for user-accessible memory. C and C++
52  // give special powers to char and certain similar types. However,
53  // these special powers only cover user-accessible memory, and doesn't
54  // include things like vtables.
55  if (!Char)
56    Char = MDHelper.createTBAANode("omnipotent char", getRoot());
57
58  return Char;
59}
60
61static bool TypeHasMayAlias(QualType QTy) {
62  // Tagged types have declarations, and therefore may have attributes.
63  if (const TagType *TTy = dyn_cast<TagType>(QTy))
64    return TTy->getDecl()->hasAttr<MayAliasAttr>();
65
66  // Typedef types have declarations, and therefore may have attributes.
67  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
68    if (TTy->getDecl()->hasAttr<MayAliasAttr>())
69      return true;
70    // Also, their underlying types may have relevant attributes.
71    return TypeHasMayAlias(TTy->desugar());
72  }
73
74  return false;
75}
76
77llvm::MDNode *
78CodeGenTBAA::getTBAAInfo(QualType QTy) {
79  // At -O0 TBAA is not emitted for regular types.
80  if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
81    return NULL;
82
83  // If the type has the may_alias attribute (even on a typedef), it is
84  // effectively in the general char alias class.
85  if (TypeHasMayAlias(QTy))
86    return getChar();
87
88  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
89
90  if (llvm::MDNode *N = MetadataCache[Ty])
91    return N;
92
93  // Handle builtin types.
94  if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
95    switch (BTy->getKind()) {
96    // Character types are special and can alias anything.
97    // In C++, this technically only includes "char" and "unsigned char",
98    // and not "signed char". In C, it includes all three. For now,
99    // the risk of exploiting this detail in C++ seems likely to outweigh
100    // the benefit.
101    case BuiltinType::Char_U:
102    case BuiltinType::Char_S:
103    case BuiltinType::UChar:
104    case BuiltinType::SChar:
105      return getChar();
106
107    // Unsigned types can alias their corresponding signed types.
108    case BuiltinType::UShort:
109      return getTBAAInfo(Context.ShortTy);
110    case BuiltinType::UInt:
111      return getTBAAInfo(Context.IntTy);
112    case BuiltinType::ULong:
113      return getTBAAInfo(Context.LongTy);
114    case BuiltinType::ULongLong:
115      return getTBAAInfo(Context.LongLongTy);
116    case BuiltinType::UInt128:
117      return getTBAAInfo(Context.Int128Ty);
118
119    // Treat all other builtin types as distinct types. This includes
120    // treating wchar_t, char16_t, and char32_t as distinct from their
121    // "underlying types".
122    default:
123      return MetadataCache[Ty] =
124        MDHelper.createTBAANode(BTy->getName(Features), getChar());
125    }
126  }
127
128  // Handle pointers.
129  // TODO: Implement C++'s type "similarity" and consider dis-"similar"
130  // pointers distinct.
131  if (Ty->isPointerType())
132    return MetadataCache[Ty] = MDHelper.createTBAANode("any pointer",
133                                                       getChar());
134
135  // Enum types are distinct types. In C++ they have "underlying types",
136  // however they aren't related for TBAA.
137  if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
138    // In C mode, two anonymous enums are compatible if their members
139    // are the same -- see C99 6.2.7p1. For now, be conservative. We could
140    // theoretically implement this by combining information about all the
141    // members into a single identifying MDNode.
142    if (!Features.CPlusPlus &&
143        ETy->getDecl()->getTypedefNameForAnonDecl())
144      return MetadataCache[Ty] = getChar();
145
146    // In C++ mode, types have linkage, so we can rely on the ODR and
147    // on their mangled names, if they're external.
148    // TODO: Is there a way to get a program-wide unique name for a
149    // decl with local linkage or no linkage?
150    if (Features.CPlusPlus &&
151        ETy->getDecl()->getLinkage() != ExternalLinkage)
152      return MetadataCache[Ty] = getChar();
153
154    // TODO: This is using the RTTI name. Is there a better way to get
155    // a unique string for a type?
156    SmallString<256> OutName;
157    llvm::raw_svector_ostream Out(OutName);
158    MContext.mangleCXXRTTIName(QualType(ETy, 0), Out);
159    Out.flush();
160    return MetadataCache[Ty] = MDHelper.createTBAANode(OutName, getChar());
161  }
162
163  // For now, handle any other kind of type conservatively.
164  return MetadataCache[Ty] = getChar();
165}
166
167llvm::MDNode *CodeGenTBAA::getTBAAInfoForVTablePtr() {
168  return MDHelper.createTBAANode("vtable pointer", getRoot());
169}
170