clang  10.0.0git
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 (auto *TD = QTy->getAsTagDecl())
82  if (TD->hasAttr<MayAliasAttr>())
83  return true;
84 
85  // Also look for may_alias as a declaration attribute on a typedef.
86  // FIXME: We should follow GCC and model may_alias as a type attribute
87  // rather than as a declaration attribute.
88  while (auto *TT = QTy->getAs<TypedefType>()) {
89  if (TT->getDecl()->hasAttr<MayAliasAttr>())
90  return true;
91  QTy = TT->desugar();
92  }
93  return false;
94 }
95 
96 /// Check if the given type is a valid base type to be used in access tags.
97 static bool isValidBaseType(QualType QTy) {
98  if (QTy->isReferenceType())
99  return false;
100  if (const RecordType *TTy = QTy->getAs<RecordType>()) {
101  const RecordDecl *RD = TTy->getDecl()->getDefinition();
102  // Incomplete types are not valid base access types.
103  if (!RD)
104  return false;
105  if (RD->hasFlexibleArrayMember())
106  return false;
107  // RD can be struct, union, class, interface or enum.
108  // For now, we only handle struct and class.
109  if (RD->isStruct() || RD->isClass())
110  return true;
111  }
112  return false;
113 }
114 
115 llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
116  uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
117 
118  // Handle builtin types.
119  if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
120  switch (BTy->getKind()) {
121  // Character types are special and can alias anything.
122  // In C++, this technically only includes "char" and "unsigned char",
123  // and not "signed char". In C, it includes all three. For now,
124  // the risk of exploiting this detail in C++ seems likely to outweigh
125  // the benefit.
126  case BuiltinType::Char_U:
127  case BuiltinType::Char_S:
128  case BuiltinType::UChar:
129  case BuiltinType::SChar:
130  return getChar();
131 
132  // Unsigned types can alias their corresponding signed types.
133  case BuiltinType::UShort:
134  return getTypeInfo(Context.ShortTy);
135  case BuiltinType::UInt:
136  return getTypeInfo(Context.IntTy);
137  case BuiltinType::ULong:
138  return getTypeInfo(Context.LongTy);
139  case BuiltinType::ULongLong:
140  return getTypeInfo(Context.LongLongTy);
141  case BuiltinType::UInt128:
142  return getTypeInfo(Context.Int128Ty);
143 
144  // Treat all other builtin types as distinct types. This includes
145  // treating wchar_t, char16_t, and char32_t as distinct from their
146  // "underlying types".
147  default:
148  return createScalarTypeNode(BTy->getName(Features), getChar(), Size);
149  }
150  }
151 
152  // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
153  // an object through a glvalue of other than one of the following types the
154  // behavior is undefined: [...] a char, unsigned char, or std::byte type."
155  if (Ty->isStdByteType())
156  return getChar();
157 
158  // Handle pointers and references.
159  // TODO: Implement C++'s type "similarity" and consider dis-"similar"
160  // pointers distinct.
161  if (Ty->isPointerType() || Ty->isReferenceType())
162  return createScalarTypeNode("any pointer", getChar(), Size);
163 
164  // Accesses to arrays are accesses to objects of their element types.
165  if (CodeGenOpts.NewStructPathTBAA && Ty->isArrayType())
166  return getTypeInfo(cast<ArrayType>(Ty)->getElementType());
167 
168  // Enum types are distinct types. In C++ they have "underlying types",
169  // however they aren't related for TBAA.
170  if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
171  // In C++ mode, types have linkage, so we can rely on the ODR and
172  // on their mangled names, if they're external.
173  // TODO: Is there a way to get a program-wide unique name for a
174  // decl with local linkage or no linkage?
175  if (!Features.CPlusPlus || !ETy->getDecl()->isExternallyVisible())
176  return getChar();
177 
178  SmallString<256> OutName;
179  llvm::raw_svector_ostream Out(OutName);
180  MContext.mangleTypeName(QualType(ETy, 0), Out);
181  return createScalarTypeNode(OutName, getChar(), Size);
182  }
183 
184  // For now, handle any other kind of type conservatively.
185  return getChar();
186 }
187 
188 llvm::MDNode *CodeGenTBAA::getTypeInfo(QualType QTy) {
189  // At -O0 or relaxed aliasing, TBAA is not emitted for regular types.
190  if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
191  return nullptr;
192 
193  // If the type has the may_alias attribute (even on a typedef), it is
194  // effectively in the general char alias class.
195  if (TypeHasMayAlias(QTy))
196  return getChar();
197 
198  // We need this function to not fall back to returning the "omnipotent char"
199  // type node for aggregate and union types. Otherwise, any dereference of an
200  // aggregate will result into the may-alias access descriptor, meaning all
201  // subsequent accesses to direct and indirect members of that aggregate will
202  // be considered may-alias too.
203  // TODO: Combine getTypeInfo() and getBaseTypeInfo() into a single function.
204  if (isValidBaseType(QTy))
205  return getBaseTypeInfo(QTy);
206 
207  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
208  if (llvm::MDNode *N = MetadataCache[Ty])
209  return N;
210 
211  // Note that the following helper call is allowed to add new nodes to the
212  // cache, which invalidates all its previously obtained iterators. So we
213  // first generate the node for the type and then add that node to the cache.
214  llvm::MDNode *TypeNode = getTypeInfoHelper(Ty);
215  return MetadataCache[Ty] = TypeNode;
216 }
217 
219  // Pointee values may have incomplete types, but they shall never be
220  // dereferenced.
221  if (AccessType->isIncompleteType())
223 
224  if (TypeHasMayAlias(AccessType))
226 
227  uint64_t Size = Context.getTypeSizeInChars(AccessType).getQuantity();
228  return TBAAAccessInfo(getTypeInfo(AccessType), Size);
229 }
230 
232  llvm::DataLayout DL(&Module);
233  unsigned Size = DL.getPointerTypeSize(VTablePtrType);
234  return TBAAAccessInfo(createScalarTypeNode("vtable pointer", getRoot(), Size),
235  Size);
236 }
237 
238 bool
239 CodeGenTBAA::CollectFields(uint64_t BaseOffset,
240  QualType QTy,
242  Fields,
243  bool MayAlias) {
244  /* Things not handled yet include: C++ base classes, bitfields, */
245 
246  if (const RecordType *TTy = QTy->getAs<RecordType>()) {
247  const RecordDecl *RD = TTy->getDecl()->getDefinition();
248  if (RD->hasFlexibleArrayMember())
249  return false;
250 
251  // TODO: Handle C++ base classes.
252  if (const CXXRecordDecl *Decl = dyn_cast<CXXRecordDecl>(RD))
253  if (Decl->bases_begin() != Decl->bases_end())
254  return false;
255 
256  const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
257 
258  unsigned idx = 0;
260  e = RD->field_end(); i != e; ++i, ++idx) {
261  if ((*i)->isZeroSize(Context) || (*i)->isUnnamedBitfield())
262  continue;
263  uint64_t Offset = BaseOffset +
264  Layout.getFieldOffset(idx) / Context.getCharWidth();
265  QualType FieldQTy = i->getType();
266  if (!CollectFields(Offset, FieldQTy, Fields,
267  MayAlias || TypeHasMayAlias(FieldQTy)))
268  return false;
269  }
270  return true;
271  }
272 
273  /* Otherwise, treat whatever it is as a field. */
274  uint64_t Offset = BaseOffset;
275  uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
276  llvm::MDNode *TBAAType = MayAlias ? getChar() : getTypeInfo(QTy);
277  llvm::MDNode *TBAATag = getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
278  Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
279  return true;
280 }
281 
282 llvm::MDNode *
284  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
285 
286  if (llvm::MDNode *N = StructMetadataCache[Ty])
287  return N;
288 
290  if (CollectFields(0, QTy, Fields, TypeHasMayAlias(QTy)))
291  return MDHelper.createTBAAStructNode(Fields);
292 
293  // For now, handle any other kind of type conservatively.
294  return StructMetadataCache[Ty] = nullptr;
295 }
296 
297 llvm::MDNode *CodeGenTBAA::getBaseTypeInfoHelper(const Type *Ty) {
298  if (auto *TTy = dyn_cast<RecordType>(Ty)) {
299  const RecordDecl *RD = TTy->getDecl()->getDefinition();
300  const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
302  for (FieldDecl *Field : RD->fields()) {
303  if (Field->isZeroSize(Context) || Field->isUnnamedBitfield())
304  continue;
305  QualType FieldQTy = Field->getType();
306  llvm::MDNode *TypeNode = isValidBaseType(FieldQTy) ?
307  getBaseTypeInfo(FieldQTy) : getTypeInfo(FieldQTy);
308  if (!TypeNode)
309  return BaseTypeMetadataCache[Ty] = nullptr;
310 
311  uint64_t BitOffset = Layout.getFieldOffset(Field->getFieldIndex());
312  uint64_t Offset = Context.toCharUnitsFromBits(BitOffset).getQuantity();
313  uint64_t Size = Context.getTypeSizeInChars(FieldQTy).getQuantity();
314  Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size,
315  TypeNode));
316  }
317 
318  SmallString<256> OutName;
319  if (Features.CPlusPlus) {
320  // Don't use the mangler for C code.
321  llvm::raw_svector_ostream Out(OutName);
322  MContext.mangleTypeName(QualType(Ty, 0), Out);
323  } else {
324  OutName = RD->getName();
325  }
326 
327  if (CodeGenOpts.NewStructPathTBAA) {
328  llvm::MDNode *Parent = getChar();
329  uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
330  llvm::Metadata *Id = MDHelper.createString(OutName);
331  return MDHelper.createTBAATypeNode(Parent, Size, Id, Fields);
332  }
333 
334  // Create the struct type node with a vector of pairs (offset, type).
336  for (const auto &Field : Fields)
337  OffsetsAndTypes.push_back(std::make_pair(Field.Type, Field.Offset));
338  return MDHelper.createTBAAStructTypeNode(OutName, OffsetsAndTypes);
339  }
340 
341  return nullptr;
342 }
343 
345  if (!isValidBaseType(QTy))
346  return nullptr;
347 
348  const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
349  if (llvm::MDNode *N = BaseTypeMetadataCache[Ty])
350  return N;
351 
352  // Note that the following helper call is allowed to add new nodes to the
353  // cache, which invalidates all its previously obtained iterators. So we
354  // first generate the node for the type and then add that node to the cache.
355  llvm::MDNode *TypeNode = getBaseTypeInfoHelper(Ty);
356  return BaseTypeMetadataCache[Ty] = TypeNode;
357 }
358 
360  assert(!Info.isIncomplete() && "Access to an object of an incomplete type!");
361 
362  if (Info.isMayAlias())
363  Info = TBAAAccessInfo(getChar(), Info.Size);
364 
365  if (!Info.AccessType)
366  return nullptr;
367 
368  if (!CodeGenOpts.StructPathTBAA)
369  Info = TBAAAccessInfo(Info.AccessType, Info.Size);
370 
371  llvm::MDNode *&N = AccessTagMetadataCache[Info];
372  if (N)
373  return N;
374 
375  if (!Info.BaseType) {
376  Info.BaseType = Info.AccessType;
377  assert(!Info.Offset && "Nonzero offset for an access with no base type!");
378  }
379  if (CodeGenOpts.NewStructPathTBAA) {
380  return N = MDHelper.createTBAAAccessTag(Info.BaseType, Info.AccessType,
381  Info.Offset, Info.Size);
382  }
383  return N = MDHelper.createTBAAStructTagNode(Info.BaseType, Info.AccessType,
384  Info.Offset);
385 }
386 
389  if (SourceInfo.isMayAlias() || TargetInfo.isMayAlias())
391  return TargetInfo;
392 }
393 
396  TBAAAccessInfo InfoB) {
397  if (InfoA == InfoB)
398  return InfoA;
399 
400  if (!InfoA || !InfoB)
401  return TBAAAccessInfo();
402 
403  if (InfoA.isMayAlias() || InfoB.isMayAlias())
405 
406  // TODO: Implement the rest of the logic here. For example, two accesses
407  // with same final access types result in an access to an object of that final
408  // access type regardless of their base types.
410 }
411 
414  TBAAAccessInfo SrcInfo) {
415  if (DestInfo == SrcInfo)
416  return DestInfo;
417 
418  if (!DestInfo || !SrcInfo)
419  return TBAAAccessInfo();
420 
421  if (DestInfo.isMayAlias() || SrcInfo.isMayAlias())
423 
424  // TODO: Implement the rest of the logic here. For example, two accesses
425  // with same final access types result in an access to an object of that final
426  // access type regardless of their base types.
428 }
Defines the clang::ASTContext interface.
bool isStruct() const
Definition: Decl.h:3404
CanQualType LongLongTy
Definition: ASTContext.h:1025
A (possibly-)qualified type.
Definition: Type.h:654
bool isArrayType() const
Definition: Type.h:6570
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:88
llvm::MDNode * AccessType
AccessType - The final access type.
Definition: CodeGenTBAA.h:105
The base class of the type hierarchy.
Definition: Type.h:1450
CanQualType LongTy
Definition: ASTContext.h:1025
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:7002
bool isClass() const
Definition: Decl.h:3406
Represents a struct/union/class.
Definition: Decl.h:3748
Holds long-lived AST nodes (such as types and decls) that can be referred to throughout the semantic ...
Definition: ASTContext.h:168
bool isStdByteType() const
Definition: Type.cpp:2551
RecordDecl * getDefinition() const
Returns the RecordDecl that actually defines this struct/union/class.
Definition: Decl.h:3953
field_range fields() const
Definition: Decl.h:3963
Represents a member of a struct/union/class.
Definition: Decl.h:2729
bool isReferenceType() const
Definition: Type.h:6516
Keeps track of the various options that can be enabled, which controls the dialect of C or C++ that i...
Definition: LangOptions.h:53
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:97
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:4425
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:179
unsigned Offset
Definition: Format.cpp:1827
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:164
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:3966
CanQualType ShortTy
Definition: ASTContext.h:1025
TagDecl * getAsTagDecl() const
Retrieves the TagDecl that this type refers to, either because the type is a TagType or because it is...
Definition: Type.cpp:1698
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:4521
CanQualType Int128Ty
Definition: ASTContext.h:1025
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:3802
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:2053
A helper class that allows the use of isa/cast/dyncast to detect TagType objects of structs/unions/cl...
Definition: Type.h:4495
uint64_t getCharWidth() const
Return the size of the character type, in bits.
Definition: ASTContext.h:2108
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:2115
CanQualType getCanonicalType(QualType T) const
Return the canonical (structural) type corresponding to the specified potentially non-canonical type ...
Definition: ASTContext.h:2305
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:253
This class is used for builtin types like &#39;int&#39;.
Definition: Type.h:2465
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:250
CanQualType IntTy
Definition: ASTContext.h:1025
bool isPointerType() const
Definition: Type.h:6504
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