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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/Attr.h"
21 #include "clang/AST/Mangle.h"
22 #include "clang/AST/RecordLayout.h"
23 #include "clang/Frontend/CodeGenOptions.h"
24 #include "llvm/ADT/SmallSet.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/LLVMContext.h"
27 #include "llvm/IR/Metadata.h"
28 #include "llvm/IR/Type.h"
29 using namespace clang;
30 using namespace CodeGen;
31 
CodeGenTBAA(ASTContext & Ctx,llvm::LLVMContext & VMContext,const CodeGenOptions & CGO,const LangOptions & Features,MangleContext & MContext)32 CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, llvm::LLVMContext& VMContext,
33                          const CodeGenOptions &CGO,
34                          const LangOptions &Features, MangleContext &MContext)
35   : Context(Ctx), CodeGenOpts(CGO), Features(Features), MContext(MContext),
36     MDHelper(VMContext), Root(nullptr), Char(nullptr) {
37 }
38 
~CodeGenTBAA()39 CodeGenTBAA::~CodeGenTBAA() {
40 }
41 
getRoot()42 llvm::MDNode *CodeGenTBAA::getRoot() {
43   // Define the root of the tree. This identifies the tree, so that
44   // if our LLVM IR is linked with LLVM IR from a different front-end
45   // (or a different version of this front-end), their TBAA trees will
46   // remain distinct, and the optimizer will treat them conservatively.
47   if (!Root) {
48     if (Features.CPlusPlus)
49       Root = MDHelper.createTBAARoot("Simple C++ TBAA");
50     else
51       Root = MDHelper.createTBAARoot("Simple C/C++ TBAA");
52   }
53 
54   return Root;
55 }
56 
57 // For both scalar TBAA and struct-path aware TBAA, the scalar type has the
58 // same format: name, parent node, and offset.
createTBAAScalarType(StringRef Name,llvm::MDNode * Parent)59 llvm::MDNode *CodeGenTBAA::createTBAAScalarType(StringRef Name,
60                                                 llvm::MDNode *Parent) {
61   return MDHelper.createTBAAScalarTypeNode(Name, Parent);
62 }
63 
getChar()64 llvm::MDNode *CodeGenTBAA::getChar() {
65   // Define the root of the tree for user-accessible memory. C and C++
66   // give special powers to char and certain similar types. However,
67   // these special powers only cover user-accessible memory, and doesn't
68   // include things like vtables.
69   if (!Char)
70     Char = createTBAAScalarType("omnipotent char", getRoot());
71 
72   return Char;
73 }
74 
TypeHasMayAlias(QualType QTy)75 static bool TypeHasMayAlias(QualType QTy) {
76   // Tagged types have declarations, and therefore may have attributes.
77   if (const TagType *TTy = dyn_cast<TagType>(QTy))
78     return TTy->getDecl()->hasAttr<MayAliasAttr>();
79 
80   // Typedef types have declarations, and therefore may have attributes.
81   if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
82     if (TTy->getDecl()->hasAttr<MayAliasAttr>())
83       return true;
84     // Also, their underlying types may have relevant attributes.
85     return TypeHasMayAlias(TTy->desugar());
86   }
87 
88   return false;
89 }
90 
91 llvm::MDNode *
getTBAAInfo(QualType QTy)92 CodeGenTBAA::getTBAAInfo(QualType QTy) {
93   // At -O0 or relaxed aliasing, TBAA is not emitted for regular types.
94   if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
95     return nullptr;
96 
97   // If the type has the may_alias attribute (even on a typedef), it is
98   // effectively in the general char alias class.
99   if (TypeHasMayAlias(QTy))
100     return getChar();
101 
102   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
103 
104   if (llvm::MDNode *N = MetadataCache[Ty])
105     return N;
106 
107   // Handle builtin types.
108   if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
109     switch (BTy->getKind()) {
110     // Character types are special and can alias anything.
111     // In C++, this technically only includes "char" and "unsigned char",
112     // and not "signed char". In C, it includes all three. For now,
113     // the risk of exploiting this detail in C++ seems likely to outweigh
114     // the benefit.
115     case BuiltinType::Char_U:
116     case BuiltinType::Char_S:
117     case BuiltinType::UChar:
118     case BuiltinType::SChar:
119       return getChar();
120 
121     // Unsigned types can alias their corresponding signed types.
122     case BuiltinType::UShort:
123       return getTBAAInfo(Context.ShortTy);
124     case BuiltinType::UInt:
125       return getTBAAInfo(Context.IntTy);
126     case BuiltinType::ULong:
127       return getTBAAInfo(Context.LongTy);
128     case BuiltinType::ULongLong:
129       return getTBAAInfo(Context.LongLongTy);
130     case BuiltinType::UInt128:
131       return getTBAAInfo(Context.Int128Ty);
132 
133     // Treat all other builtin types as distinct types. This includes
134     // treating wchar_t, char16_t, and char32_t as distinct from their
135     // "underlying types".
136     default:
137       return MetadataCache[Ty] =
138         createTBAAScalarType(BTy->getName(Features), getChar());
139     }
140   }
141 
142   // Handle pointers.
143   // TODO: Implement C++'s type "similarity" and consider dis-"similar"
144   // pointers distinct.
145   if (Ty->isPointerType())
146     return MetadataCache[Ty] = createTBAAScalarType("any pointer",
147                                                     getChar());
148 
149   // Enum types are distinct types. In C++ they have "underlying types",
150   // however they aren't related for TBAA.
151   if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
152     // In C++ mode, types have linkage, so we can rely on the ODR and
153     // on their mangled names, if they're external.
154     // TODO: Is there a way to get a program-wide unique name for a
155     // decl with local linkage or no linkage?
156     if (!Features.CPlusPlus || !ETy->getDecl()->isExternallyVisible())
157       return MetadataCache[Ty] = getChar();
158 
159     SmallString<256> OutName;
160     llvm::raw_svector_ostream Out(OutName);
161     MContext.mangleTypeName(QualType(ETy, 0), Out);
162     return MetadataCache[Ty] = createTBAAScalarType(OutName, getChar());
163   }
164 
165   // For now, handle any other kind of type conservatively.
166   return MetadataCache[Ty] = getChar();
167 }
168 
getTBAAInfoForVTablePtr()169 llvm::MDNode *CodeGenTBAA::getTBAAInfoForVTablePtr() {
170   return createTBAAScalarType("vtable pointer", getRoot());
171 }
172 
173 bool
CollectFields(uint64_t BaseOffset,QualType QTy,SmallVectorImpl<llvm::MDBuilder::TBAAStructField> & Fields,bool MayAlias)174 CodeGenTBAA::CollectFields(uint64_t BaseOffset,
175                            QualType QTy,
176                            SmallVectorImpl<llvm::MDBuilder::TBAAStructField> &
177                              Fields,
178                            bool MayAlias) {
179   /* Things not handled yet include: C++ base classes, bitfields, */
180 
181   if (const RecordType *TTy = QTy->getAs<RecordType>()) {
182     const RecordDecl *RD = TTy->getDecl()->getDefinition();
183     if (RD->hasFlexibleArrayMember())
184       return false;
185 
186     // TODO: Handle C++ base classes.
187     if (const CXXRecordDecl *Decl = dyn_cast<CXXRecordDecl>(RD))
188       if (Decl->bases_begin() != Decl->bases_end())
189         return false;
190 
191     const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
192 
193     unsigned idx = 0;
194     for (RecordDecl::field_iterator i = RD->field_begin(),
195          e = RD->field_end(); i != e; ++i, ++idx) {
196       uint64_t Offset = BaseOffset +
197                         Layout.getFieldOffset(idx) / Context.getCharWidth();
198       QualType FieldQTy = i->getType();
199       if (!CollectFields(Offset, FieldQTy, Fields,
200                          MayAlias || TypeHasMayAlias(FieldQTy)))
201         return false;
202     }
203     return true;
204   }
205 
206   /* Otherwise, treat whatever it is as a field. */
207   uint64_t Offset = BaseOffset;
208   uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
209   llvm::MDNode *TBAAInfo = MayAlias ? getChar() : getTBAAInfo(QTy);
210   llvm::MDNode *TBAATag = getTBAAScalarTagInfo(TBAAInfo);
211   Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
212   return true;
213 }
214 
215 llvm::MDNode *
getTBAAStructInfo(QualType QTy)216 CodeGenTBAA::getTBAAStructInfo(QualType QTy) {
217   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
218 
219   if (llvm::MDNode *N = StructMetadataCache[Ty])
220     return N;
221 
222   SmallVector<llvm::MDBuilder::TBAAStructField, 4> Fields;
223   if (CollectFields(0, QTy, Fields, TypeHasMayAlias(QTy)))
224     return MDHelper.createTBAAStructNode(Fields);
225 
226   // For now, handle any other kind of type conservatively.
227   return StructMetadataCache[Ty] = nullptr;
228 }
229 
230 /// Check if the given type can be handled by path-aware TBAA.
isTBAAPathStruct(QualType QTy)231 static bool isTBAAPathStruct(QualType QTy) {
232   if (const RecordType *TTy = QTy->getAs<RecordType>()) {
233     const RecordDecl *RD = TTy->getDecl()->getDefinition();
234     if (RD->hasFlexibleArrayMember())
235       return false;
236     // RD can be struct, union, class, interface or enum.
237     // For now, we only handle struct and class.
238     if (RD->isStruct() || RD->isClass())
239       return true;
240   }
241   return false;
242 }
243 
244 llvm::MDNode *
getTBAAStructTypeInfo(QualType QTy)245 CodeGenTBAA::getTBAAStructTypeInfo(QualType QTy) {
246   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
247   assert(isTBAAPathStruct(QTy));
248 
249   if (llvm::MDNode *N = StructTypeMetadataCache[Ty])
250     return N;
251 
252   if (const RecordType *TTy = QTy->getAs<RecordType>()) {
253     const RecordDecl *RD = TTy->getDecl()->getDefinition();
254 
255     const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
256     SmallVector <std::pair<llvm::MDNode*, uint64_t>, 4> Fields;
257     unsigned idx = 0;
258     for (RecordDecl::field_iterator i = RD->field_begin(),
259          e = RD->field_end(); i != e; ++i, ++idx) {
260       QualType FieldQTy = i->getType();
261       llvm::MDNode *FieldNode;
262       if (isTBAAPathStruct(FieldQTy))
263         FieldNode = getTBAAStructTypeInfo(FieldQTy);
264       else
265         FieldNode = getTBAAInfo(FieldQTy);
266       if (!FieldNode)
267         return StructTypeMetadataCache[Ty] = nullptr;
268       Fields.push_back(std::make_pair(
269           FieldNode, Layout.getFieldOffset(idx) / Context.getCharWidth()));
270     }
271 
272     SmallString<256> OutName;
273     if (Features.CPlusPlus) {
274       // Don't use the mangler for C code.
275       llvm::raw_svector_ostream Out(OutName);
276       MContext.mangleTypeName(QualType(Ty, 0), Out);
277     } else {
278       OutName = RD->getName();
279     }
280     // Create the struct type node with a vector of pairs (offset, type).
281     return StructTypeMetadataCache[Ty] =
282       MDHelper.createTBAAStructTypeNode(OutName, Fields);
283   }
284 
285   return StructMetadataCache[Ty] = nullptr;
286 }
287 
288 /// Return a TBAA tag node for both scalar TBAA and struct-path aware TBAA.
289 llvm::MDNode *
getTBAAStructTagInfo(QualType BaseQTy,llvm::MDNode * AccessNode,uint64_t Offset)290 CodeGenTBAA::getTBAAStructTagInfo(QualType BaseQTy, llvm::MDNode *AccessNode,
291                                   uint64_t Offset) {
292   if (!AccessNode)
293     return nullptr;
294 
295   if (!CodeGenOpts.StructPathTBAA)
296     return getTBAAScalarTagInfo(AccessNode);
297 
298   const Type *BTy = Context.getCanonicalType(BaseQTy).getTypePtr();
299   TBAAPathTag PathTag = TBAAPathTag(BTy, AccessNode, Offset);
300   if (llvm::MDNode *N = StructTagMetadataCache[PathTag])
301     return N;
302 
303   llvm::MDNode *BNode = nullptr;
304   if (isTBAAPathStruct(BaseQTy))
305     BNode  = getTBAAStructTypeInfo(BaseQTy);
306   if (!BNode)
307     return StructTagMetadataCache[PathTag] =
308        MDHelper.createTBAAStructTagNode(AccessNode, AccessNode, 0);
309 
310   return StructTagMetadataCache[PathTag] =
311     MDHelper.createTBAAStructTagNode(BNode, AccessNode, Offset);
312 }
313 
314 llvm::MDNode *
getTBAAScalarTagInfo(llvm::MDNode * AccessNode)315 CodeGenTBAA::getTBAAScalarTagInfo(llvm::MDNode *AccessNode) {
316   if (!AccessNode)
317     return nullptr;
318   if (llvm::MDNode *N = ScalarTagMetadataCache[AccessNode])
319     return N;
320 
321   return ScalarTagMetadataCache[AccessNode] =
322     MDHelper.createTBAAStructTagNode(AccessNode, AccessNode, 0);
323 }
324