clang  9.0.0svn
CodeGenTBAA.cpp
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1 //===-- CodeGenTBAA.cpp - TBAA information for LLVM CodeGen ---------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This is the code that manages TBAA information and defines the TBAA policy
10 // for the optimizer to use. Relevant standards text includes:
11 //
12 // C99 6.5p7
13 // C++ [basic.lval] (p10 in n3126, p15 in some earlier versions)
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "CodeGenTBAA.h"
18 #include "clang/AST/ASTContext.h"
19 #include "clang/AST/Attr.h"
20 #include "clang/AST/Mangle.h"
21 #include "clang/AST/RecordLayout.h"
23 #include "llvm/ADT/SmallSet.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/LLVMContext.h"
26 #include "llvm/IR/Metadata.h"
27 #include "llvm/IR/Module.h"
28 #include "llvm/IR/Type.h"
29 using namespace clang;
30 using namespace CodeGen;
31 
32 CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, llvm::Module &M,
33  const CodeGenOptions &CGO,
34  const LangOptions &Features, MangleContext &MContext)
35  : Context(Ctx), Module(M), CodeGenOpts(CGO),
36  Features(Features), MContext(MContext), MDHelper(M.getContext()),
37  Root(nullptr), Char(nullptr)
38 {}
39 
41 }
42 
43 llvm::MDNode *CodeGenTBAA::getRoot() {
44  // Define the root of the tree. This identifies the tree, so that
45  // if our LLVM IR is linked with LLVM IR from a different front-end
46  // (or a different version of this front-end), their TBAA trees will
47  // remain distinct, and the optimizer will treat them conservatively.
48  if (!Root) {
49  if (Features.CPlusPlus)
50  Root = MDHelper.createTBAARoot("Simple C++ TBAA");
51  else
52  Root = MDHelper.createTBAARoot("Simple C/C++ TBAA");
53  }
54 
55  return Root;
56 }
57 
58 llvm::MDNode *CodeGenTBAA::createScalarTypeNode(StringRef Name,
59  llvm::MDNode *Parent,
60  uint64_t Size) {
61  if (CodeGenOpts.NewStructPathTBAA) {
62  llvm::Metadata *Id = MDHelper.createString(Name);
63  return MDHelper.createTBAATypeNode(Parent, Size, Id);
64  }
65  return MDHelper.createTBAAScalarTypeNode(Name, Parent);
66 }
67 
68 llvm::MDNode *CodeGenTBAA::getChar() {
69  // Define the root of the tree for user-accessible memory. C and C++
70  // give special powers to char and certain similar types. However,
71  // these special powers only cover user-accessible memory, and doesn't
72  // include things like vtables.
73  if (!Char)
74  Char = createScalarTypeNode("omnipotent char", getRoot(), /* Size= */ 1);
75 
76  return Char;
77 }
78 
79 static bool TypeHasMayAlias(QualType QTy) {
80  // Tagged types have declarations, and therefore may have attributes.
81  if (const TagType *TTy = dyn_cast<TagType>(QTy))
82  return TTy->getDecl()->hasAttr<MayAliasAttr>();
83 
84  // Typedef types have declarations, and therefore may have attributes.
85  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
86  if (TTy->getDecl()->hasAttr<MayAliasAttr>())
87  return true;
88  // Also, their underlying types may have relevant attributes.
89  return TypeHasMayAlias(TTy->desugar());
90  }
91 
92  return false;
93 }
94 
95 /// Check if the given type is a valid base type to be used in access tags.
96 static bool isValidBaseType(QualType QTy) {
97  if (QTy->isReferenceType())
98  return false;
99  if (const RecordType *TTy = QTy->getAs<RecordType>()) {
100  const RecordDecl *RD = TTy->getDecl()->getDefinition();
101  // Incomplete types are not valid base access types.
102  if (!RD)
103  return false;
104  if (RD->hasFlexibleArrayMember())
105  return false;
106  // RD can be struct, union, class, interface or enum.
107  // For now, we only handle struct and class.
108  if (RD->isStruct() || RD->isClass())
109  return true;
110  }
111  return false;
112 }
113 
114 llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
115  uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
116 
117  // Handle builtin types.
118  if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
119  switch (BTy->getKind()) {
120  // Character types are special and can alias anything.
121  // In C++, this technically only includes "char" and "unsigned char",
122  // and not "signed char". In C, it includes all three. For now,
123  // the risk of exploiting this detail in C++ seems likely to outweigh
124  // the benefit.
125  case BuiltinType::Char_U:
126  case BuiltinType::Char_S:
127  case BuiltinType::UChar:
128  case BuiltinType::SChar:
129  return getChar();
130 
131  // Unsigned types can alias their corresponding signed types.
132  case BuiltinType::UShort:
133  return getTypeInfo(Context.ShortTy);
134  case BuiltinType::UInt:
135  return getTypeInfo(Context.IntTy);
136  case BuiltinType::ULong:
137  return getTypeInfo(Context.LongTy);
138  case BuiltinType::ULongLong:
139  return getTypeInfo(Context.LongLongTy);
140  case BuiltinType::UInt128:
141  return getTypeInfo(Context.Int128Ty);
142 
143  // Treat all other builtin types as distinct types. This includes
144  // treating wchar_t, char16_t, and char32_t as distinct from their
145  // "underlying types".
146  default:
147  return createScalarTypeNode(BTy->getName(Features), getChar(), Size);
148  }
149  }
150 
151  // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
152  // an object through a glvalue of other than one of the following types the
153  // behavior is undefined: [...] a char, unsigned char, or std::byte type."
154  if (Ty->isStdByteType())
155  return getChar();
156 
157  // Handle pointers and references.
158  // TODO: Implement C++'s type "similarity" and consider dis-"similar"
159  // pointers distinct.
160  if (Ty->isPointerType() || Ty->isReferenceType())
161  return createScalarTypeNode("any pointer", getChar(), Size);
162 
163  // Accesses to arrays are accesses to objects of their element types.
164  if (CodeGenOpts.NewStructPathTBAA && Ty->isArrayType())
165  return getTypeInfo(cast<ArrayType>(Ty)->getElementType());
166 
167  // Enum types are distinct types. In C++ they have "underlying types",
168  // however they aren't related for TBAA.
169  if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
170  // In C++ mode, types have linkage, so we can rely on the ODR and
171  // on their mangled names, if they're external.
172  // TODO: Is there a way to get a program-wide unique name for a
173  // decl with local linkage or no linkage?
174  if (!Features.CPlusPlus || !ETy->getDecl()->isExternallyVisible())
175  return getChar();
176 
177  SmallString<256> OutName;
178  llvm::raw_svector_ostream Out(OutName);
179  MContext.mangleTypeName(QualType(ETy, 0), Out);
180  return createScalarTypeNode(OutName, getChar(), Size);
181  }
182 
183  // For now, handle any other kind of type conservatively.
184  return getChar();
185 }
186 
187 llvm::MDNode *CodeGenTBAA::getTypeInfo(QualType QTy) {
188  // At -O0 or relaxed aliasing, TBAA is not emitted for regular types.
189  if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
190  return nullptr;
191 
192  // If the type has the may_alias attribute (even on a typedef), it is
193  // effectively in the general char alias class.
194  if (TypeHasMayAlias(QTy))
195  return getChar();
196 
197  // We need this function to not fall back to returning the "omnipotent char"
198  // type node for aggregate and union types. Otherwise, any dereference of an
199  // aggregate will result into the may-alias access descriptor, meaning all
200  // subsequent accesses to direct and indirect members of that aggregate will
201  // be considered may-alias too.
202  // TODO: Combine getTypeInfo() and getBaseTypeInfo() into a single function.
203  if (isValidBaseType(QTy))
204  return getBaseTypeInfo(QTy);
205 
206  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
207  if (llvm::MDNode *N = MetadataCache[Ty])
208  return N;
209 
210  // Note that the following helper call is allowed to add new nodes to the
211  // cache, which invalidates all its previously obtained iterators. So we
212  // first generate the node for the type and then add that node to the cache.
213  llvm::MDNode *TypeNode = getTypeInfoHelper(Ty);
214  return MetadataCache[Ty] = TypeNode;
215 }
216 
218  // Pointee values may have incomplete types, but they shall never be
219  // dereferenced.
220  if (AccessType->isIncompleteType())
222 
223  if (TypeHasMayAlias(AccessType))
225 
226  uint64_t Size = Context.getTypeSizeInChars(AccessType).getQuantity();
227  return TBAAAccessInfo(getTypeInfo(AccessType), Size);
228 }
229 
231  llvm::DataLayout DL(&Module);
232  unsigned Size = DL.getPointerTypeSize(VTablePtrType);
233  return TBAAAccessInfo(createScalarTypeNode("vtable pointer", getRoot(), Size),
234  Size);
235 }
236 
237 bool
238 CodeGenTBAA::CollectFields(uint64_t BaseOffset,
239  QualType QTy,
241  Fields,
242  bool MayAlias) {
243  /* Things not handled yet include: C++ base classes, bitfields, */
244 
245  if (const RecordType *TTy = QTy->getAs<RecordType>()) {
246  const RecordDecl *RD = TTy->getDecl()->getDefinition();
247  if (RD->hasFlexibleArrayMember())
248  return false;
249 
250  // TODO: Handle C++ base classes.
251  if (const CXXRecordDecl *Decl = dyn_cast<CXXRecordDecl>(RD))
252  if (Decl->bases_begin() != Decl->bases_end())
253  return false;
254 
255  const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
256 
257  unsigned idx = 0;
259  e = RD->field_end(); i != e; ++i, ++idx) {
260  uint64_t Offset = BaseOffset +
261  Layout.getFieldOffset(idx) / Context.getCharWidth();
262  QualType FieldQTy = i->getType();
263  if (!CollectFields(Offset, FieldQTy, Fields,
264  MayAlias || TypeHasMayAlias(FieldQTy)))
265  return false;
266  }
267  return true;
268  }
269 
270  /* Otherwise, treat whatever it is as a field. */
271  uint64_t Offset = BaseOffset;
272  uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
273  llvm::MDNode *TBAAType = MayAlias ? getChar() : getTypeInfo(QTy);
274  llvm::MDNode *TBAATag = getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
275  Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
276  return true;
277 }
278 
279 llvm::MDNode *
281  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
282 
283  if (llvm::MDNode *N = StructMetadataCache[Ty])
284  return N;
285 
287  if (CollectFields(0, QTy, Fields, TypeHasMayAlias(QTy)))
288  return MDHelper.createTBAAStructNode(Fields);
289 
290  // For now, handle any other kind of type conservatively.
291  return StructMetadataCache[Ty] = nullptr;
292 }
293 
294 llvm::MDNode *CodeGenTBAA::getBaseTypeInfoHelper(const Type *Ty) {
295  if (auto *TTy = dyn_cast<RecordType>(Ty)) {
296  const RecordDecl *RD = TTy->getDecl()->getDefinition();
297  const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
299  for (FieldDecl *Field : RD->fields()) {
300  QualType FieldQTy = Field->getType();
301  llvm::MDNode *TypeNode = isValidBaseType(FieldQTy) ?
302  getBaseTypeInfo(FieldQTy) : getTypeInfo(FieldQTy);
303  if (!TypeNode)
304  return BaseTypeMetadataCache[Ty] = nullptr;
305 
306  uint64_t BitOffset = Layout.getFieldOffset(Field->getFieldIndex());
307  uint64_t Offset = Context.toCharUnitsFromBits(BitOffset).getQuantity();
308  uint64_t Size = Context.getTypeSizeInChars(FieldQTy).getQuantity();
309  Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size,
310  TypeNode));
311  }
312 
313  SmallString<256> OutName;
314  if (Features.CPlusPlus) {
315  // Don't use the mangler for C code.
316  llvm::raw_svector_ostream Out(OutName);
317  MContext.mangleTypeName(QualType(Ty, 0), Out);
318  } else {
319  OutName = RD->getName();
320  }
321 
322  if (CodeGenOpts.NewStructPathTBAA) {
323  llvm::MDNode *Parent = getChar();
324  uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
325  llvm::Metadata *Id = MDHelper.createString(OutName);
326  return MDHelper.createTBAATypeNode(Parent, Size, Id, Fields);
327  }
328 
329  // Create the struct type node with a vector of pairs (offset, type).
331  for (const auto &Field : Fields)
332  OffsetsAndTypes.push_back(std::make_pair(Field.Type, Field.Offset));
333  return MDHelper.createTBAAStructTypeNode(OutName, OffsetsAndTypes);
334  }
335 
336  return nullptr;
337 }
338 
340  if (!isValidBaseType(QTy))
341  return nullptr;
342 
343  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
344  if (llvm::MDNode *N = BaseTypeMetadataCache[Ty])
345  return N;
346 
347  // Note that the following helper call is allowed to add new nodes to the
348  // cache, which invalidates all its previously obtained iterators. So we
349  // first generate the node for the type and then add that node to the cache.
350  llvm::MDNode *TypeNode = getBaseTypeInfoHelper(Ty);
351  return BaseTypeMetadataCache[Ty] = TypeNode;
352 }
353 
355  assert(!Info.isIncomplete() && "Access to an object of an incomplete type!");
356 
357  if (Info.isMayAlias())
358  Info = TBAAAccessInfo(getChar(), Info.Size);
359 
360  if (!Info.AccessType)
361  return nullptr;
362 
363  if (!CodeGenOpts.StructPathTBAA)
364  Info = TBAAAccessInfo(Info.AccessType, Info.Size);
365 
366  llvm::MDNode *&N = AccessTagMetadataCache[Info];
367  if (N)
368  return N;
369 
370  if (!Info.BaseType) {
371  Info.BaseType = Info.AccessType;
372  assert(!Info.Offset && "Nonzero offset for an access with no base type!");
373  }
374  if (CodeGenOpts.NewStructPathTBAA) {
375  return N = MDHelper.createTBAAAccessTag(Info.BaseType, Info.AccessType,
376  Info.Offset, Info.Size);
377  }
378  return N = MDHelper.createTBAAStructTagNode(Info.BaseType, Info.AccessType,
379  Info.Offset);
380 }
381 
384  if (SourceInfo.isMayAlias() || TargetInfo.isMayAlias())
386  return TargetInfo;
387 }
388 
391  TBAAAccessInfo InfoB) {
392  if (InfoA == InfoB)
393  return InfoA;
394 
395  if (!InfoA || !InfoB)
396  return TBAAAccessInfo();
397 
398  if (InfoA.isMayAlias() || InfoB.isMayAlias())
400 
401  // TODO: Implement the rest of the logic here. For example, two accesses
402  // with same final access types result in an access to an object of that final
403  // access type regardless of their base types.
405 }
406 
409  TBAAAccessInfo SrcInfo) {
410  if (DestInfo == SrcInfo)
411  return DestInfo;
412 
413  if (!DestInfo || !SrcInfo)
414  return TBAAAccessInfo();
415 
416  if (DestInfo.isMayAlias() || SrcInfo.isMayAlias())
418 
419  // TODO: Implement the rest of the logic here. For example, two accesses
420  // with same final access types result in an access to an object of that final
421  // access type regardless of their base types.
423 }
Defines the clang::ASTContext interface.
bool isStruct() const
Definition: Decl.h:3248
CanQualType LongLongTy
Definition: ASTContext.h:1024
A (possibly-)qualified type.
Definition: Type.h:639
bool isArrayType() const
Definition: Type.h:6355
const ASTRecordLayout & getASTRecordLayout(const RecordDecl *D) const
Get or compute information about the layout of the specified record (struct/union/class) D...
Decl - This represents one declaration (or definition), e.g.
Definition: DeclBase.h:86
llvm::MDNode * AccessType
AccessType - The final access type.
Definition: CodeGenTBAA.h:105
The base class of the type hierarchy.
Definition: Type.h:1414
CanQualType LongTy
Definition: ASTContext.h:1024
TBAAAccessInfo getAccessInfo(QualType AccessType)
getAccessInfo - Get TBAA information that describes an access to an object of the given type...
CodeGenTBAA(ASTContext &Ctx, llvm::Module &M, const CodeGenOptions &CGO, const LangOptions &Features, MangleContext &MContext)
Definition: CodeGenTBAA.cpp:32
const T * getAs() const
Member-template getAs<specific type>&#39;.
Definition: Type.h:6766
bool isClass() const
Definition: Decl.h:3250
Represents a struct/union/class.
Definition: Decl.h:3592
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:154
bool isStdByteType() const
Definition: Type.cpp:2524
RecordDecl * getDefinition() const
Returns the RecordDecl that actually defines this struct/union/class.
Definition: Decl.h:3773
field_range fields() const
Definition: Decl.h:3783
Represents a member of a struct/union/class.
Definition: Decl.h:2578
bool isReferenceType() const
Definition: Type.h:6318
Keeps track of the various options that can be enabled, which controls the dialect of C or C++ that i...
Definition: LangOptions.h:49
Describes a module or submodule.
Definition: Module.h:64
static bool isValidBaseType(QualType QTy)
Check if the given type is a valid base type to be used in access tags.
Definition: CodeGenTBAA.cpp:96
TBAAAccessInfo getVTablePtrAccessInfo(llvm::Type *VTablePtrType)
getVTablePtrAccessInfo - Get the TBAA information that describes an access to a virtual table pointer...
field_iterator field_begin() const
Definition: Decl.cpp:4150
static TBAAAccessInfo getIncompleteInfo()
Definition: CodeGenTBAA.h:71
const T * getTypePtr() const
Retrieve the underlying type pointer, which refers to a canonical type.
Definition: CanonicalType.h:83
NodeId Parent
Definition: ASTDiff.cpp:191
QuantityType getQuantity() const
getQuantity - Get the raw integer representation of this quantity.
Definition: CharUnits.h:178
unsigned Offset
Definition: Format.cpp:1639
ASTRecordLayout - This class contains layout information for one RecordDecl, which is a struct/union/...
Definition: RecordLayout.h:38
Exposes information about the current target.
Definition: TargetInfo.h:161
int Id
Definition: ASTDiff.cpp:190
TBAAAccessInfo mergeTBAAInfoForMemoryTransfer(TBAAAccessInfo DestInfo, TBAAAccessInfo SrcInfo)
mergeTBAAInfoForMemoryTransfer - Get merged TBAA information for the purpose of memory transfer calls...
static bool TypeHasMayAlias(QualType QTy)
Definition: CodeGenTBAA.cpp:79
field_iterator field_end() const
Definition: Decl.h:3786
CanQualType ShortTy
Definition: ASTContext.h:1024
llvm::MDNode * getBaseTypeInfo(QualType QTy)
getBaseTypeInfo - Get metadata that describes the given base access type.
TBAAAccessInfo mergeTBAAInfoForCast(TBAAAccessInfo SourceInfo, TBAAAccessInfo TargetInfo)
mergeTBAAInfoForCast - Get merged TBAA information for the purpose of type casts. ...
The l-value was considered opaque, so the alignment was determined from a type.
uint64_t getFieldOffset(unsigned FieldNo) const
getFieldOffset - Get the offset of the given field index, in bits.
Definition: RecordLayout.h:190
TBAAAccessInfo mergeTBAAInfoForConditionalOperator(TBAAAccessInfo InfoA, TBAAAccessInfo InfoB)
mergeTBAAInfoForConditionalOperator - Get merged TBAA information for the purpose of conditional oper...
A helper class that allows the use of isa/cast/dyncast to detect TagType objects of enums...
Definition: Type.h:4406
CanQualType Int128Ty
Definition: ASTContext.h:1024
llvm::MDNode * BaseType
BaseType - The base/leading access type.
Definition: CodeGenTBAA.h:101
MangleContext - Context for tracking state which persists across multiple calls to the C++ name mangl...
Definition: Mangle.h:43
uint64_t Size
Size - The size of access, in bytes.
Definition: CodeGenTBAA.h:112
llvm::MDNode * getTBAAStructInfo(QualType QTy)
getTBAAStructInfo - Get the TBAAStruct MDNode to be used for a memcpy of the given type...
bool hasFlexibleArrayMember() const
Definition: Decl.h:3646
virtual void mangleTypeName(QualType T, raw_ostream &)=0
Generates a unique string for an externally visible type for use with TBAA or type uniquing...
llvm::MDNode * getTypeInfo(QualType QTy)
getTypeInfo - Get metadata used to describe accesses to objects of the given type.
Dataflow Directional Tag Classes.
specific_decl_iterator - Iterates over a subrange of declarations stored in a DeclContext, providing only those that are of type SpecificDecl (or a class derived from it).
Definition: DeclBase.h:2025
A helper class that allows the use of isa/cast/dyncast to detect TagType objects of structs/unions/cl...
Definition: Type.h:4380
uint64_t getCharWidth() const
Return the size of the character type, in bits.
Definition: ASTContext.h:2076
CodeGenOptions - Track various options which control how the code is optimized and passed to the back...
bool isIncompleteType(NamedDecl **Def=nullptr) const
Types are partitioned into 3 broad categories (C99 6.2.5p1): object types, function types...
Definition: Type.cpp:2053
CanQualType getCanonicalType(QualType T) const
Return the canonical (structural) type corresponding to the specified potentially non-canonical type ...
Definition: ASTContext.h:2266
CharUnits toCharUnitsFromBits(int64_t BitSize) const
Convert a size in bits to a size in characters.
Represents a C++ struct/union/class.
Definition: DeclCXX.h:299
This class is used for builtin types like &#39;int&#39;.
Definition: Type.h:2401
llvm::MDNode * getAccessTagInfo(TBAAAccessInfo Info)
getAccessTagInfo - Get TBAA tag for a given memory access.
StringRef getName() const
Get the name of identifier for this declaration as a StringRef.
Definition: Decl.h:275
CanQualType IntTy
Definition: ASTContext.h:1024
bool isPointerType() const
Definition: Type.h:6306
CharUnits getTypeSizeInChars(QualType T) const
Return the size of the specified (complete) type T, in characters.
uint64_t Offset
Offset - The byte offset of the final access within the base one.
Definition: CodeGenTBAA.h:109
static TBAAAccessInfo getMayAliasInfo()
Definition: CodeGenTBAA.h:63