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. Relevant standards
11 // 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 "Mangle.h"
20 #include "clang/AST/ASTContext.h"
21 #include "llvm/LLVMContext.h"
22 #include "llvm/Metadata.h"
23 using namespace clang;
24 using namespace CodeGen;
25 
26 CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, llvm::LLVMContext& VMContext,
27                          const LangOptions &Features, MangleContext &MContext)
28   : Context(Ctx), VMContext(VMContext), Features(Features), MContext(MContext),
29     Root(0), Char(0) {
30 }
31 
32 CodeGenTBAA::~CodeGenTBAA() {
33 }
34 
35 llvm::MDNode *CodeGenTBAA::getTBAAInfoForNamedType(llvm::StringRef NameStr,
36                                                    llvm::MDNode *Parent) {
37   llvm::Value *Ops[] = {
38     llvm::MDString::get(VMContext, NameStr),
39     Parent
40   };
41 
42   return llvm::MDNode::get(VMContext, Ops, llvm::array_lengthof(Ops));
43 }
44 
45 llvm::MDNode *
46 CodeGenTBAA::getTBAAInfo(QualType QTy) {
47   Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
48 
49   if (llvm::MDNode *N = MetadataCache[Ty])
50     return N;
51 
52   if (!Root) {
53     Root = getTBAAInfoForNamedType("Experimental TBAA", 0);
54     Char = getTBAAInfoForNamedType("omnipotent char", Root);
55   }
56 
57   // For now, just emit a very minimal tree.
58   if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
59     switch (BTy->getKind()) {
60     // Character types are special and can alias anything.
61     // In C++, this technically only includes "char" and "unsigned char",
62     // and not "signed char". In C, it includes all three. For now,
63     // the risk of exploiting this detail in C++ seems likely to outweigh
64     // the benefit.
65     case BuiltinType::Char_U:
66     case BuiltinType::Char_S:
67     case BuiltinType::UChar:
68     case BuiltinType::SChar:
69       return Char;
70 
71     // Unsigned types can alias their corresponding signed types.
72     case BuiltinType::UShort:
73       return getTBAAInfo(Context.ShortTy);
74     case BuiltinType::UInt:
75       return getTBAAInfo(Context.IntTy);
76     case BuiltinType::ULong:
77       return getTBAAInfo(Context.LongTy);
78     case BuiltinType::ULongLong:
79       return getTBAAInfo(Context.LongLongTy);
80     case BuiltinType::UInt128:
81       return getTBAAInfo(Context.Int128Ty);
82 
83     // Treat all other builtin types as distinct types. This includes
84     // treating wchar_t, char16_t, and char32_t as distinct from their
85     // "underlying types".
86     default:
87       return MetadataCache[Ty] =
88                getTBAAInfoForNamedType(BTy->getName(Features), Char);
89     }
90   }
91 
92   // TODO: Implement C++'s type "similarity" and consider dis-"similar"
93   // pointers distinct.
94   if (Ty->isPointerType())
95     return MetadataCache[Ty] = getTBAAInfoForNamedType("any pointer", Char);
96 
97   // Enum types are distinct types. In C++ they have "underlying types",
98   // however they aren't related for TBAA.
99   if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
100     // In C mode, two anonymous enums are compatible iff their members
101     // are the same -- see C99 6.2.7p1. For now, be conservative. We could
102     // theoretically implement this by combining information about all the
103     // members into a single identifying MDNode.
104     if (!Features.CPlusPlus &&
105         ETy->getDecl()->getTypedefForAnonDecl())
106       return MetadataCache[Ty] = Char;
107 
108     // In C++ mode, types have linkage, so we can rely on the ODR and
109     // on their mangled names, if they're external.
110     // TODO: Is there a way to get a program-wide unique name for a
111     // decl with local linkage or no linkage?
112     if (Features.CPlusPlus &&
113         ETy->getDecl()->getLinkage() != ExternalLinkage)
114       return MetadataCache[Ty] = Char;
115 
116     // TODO: This is using the RTTI name. Is there a better way to get
117     // a unique string for a type?
118     llvm::SmallString<256> OutName;
119     MContext.mangleCXXRTTIName(QualType(ETy, 0), OutName);
120     return MetadataCache[Ty] = getTBAAInfoForNamedType(OutName, Char);
121   }
122 
123   // For now, handle any other kind of type conservatively.
124   return MetadataCache[Ty] = Char;
125 }
126