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"
22#include "clang/Basic/CodeGenOptions.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"
29using namespace clang;
30using namespace CodeGen;
31
32CodeGenTBAA::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
40CodeGenTBAA::~CodeGenTBAA() {
41}
42
43llvm::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
58llvm::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
68llvm::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
79static 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.
97static 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
115llvm::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 case BuiltinType::UShortFract:
145 return getTypeInfo(Context.ShortFractTy);
146 case BuiltinType::UFract:
147 return getTypeInfo(Context.FractTy);
148 case BuiltinType::ULongFract:
149 return getTypeInfo(Context.LongFractTy);
150
151 case BuiltinType::SatUShortFract:
152 return getTypeInfo(Context.SatShortFractTy);
153 case BuiltinType::SatUFract:
154 return getTypeInfo(Context.SatFractTy);
155 case BuiltinType::SatULongFract:
156 return getTypeInfo(Context.SatLongFractTy);
157
158 case BuiltinType::UShortAccum:
159 return getTypeInfo(Context.ShortAccumTy);
160 case BuiltinType::UAccum:
161 return getTypeInfo(Context.AccumTy);
162 case BuiltinType::ULongAccum:
163 return getTypeInfo(Context.LongAccumTy);
164
165 case BuiltinType::SatUShortAccum:
166 return getTypeInfo(Context.SatShortAccumTy);
167 case BuiltinType::SatUAccum:
168 return getTypeInfo(Context.SatAccumTy);
169 case BuiltinType::SatULongAccum:
170 return getTypeInfo(Context.SatLongAccumTy);
171
172 // Treat all other builtin types as distinct types. This includes
173 // treating wchar_t, char16_t, and char32_t as distinct from their
174 // "underlying types".
175 default:
176 return createScalarTypeNode(BTy->getName(Features), getChar(), Size);
177 }
178 }
179
180 // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
181 // an object through a glvalue of other than one of the following types the
182 // behavior is undefined: [...] a char, unsigned char, or std::byte type."
183 if (Ty->isStdByteType())
184 return getChar();
185
186 // Handle pointers and references.
187 // TODO: Implement C++'s type "similarity" and consider dis-"similar"
188 // pointers distinct.
189 if (Ty->isPointerType() || Ty->isReferenceType())
190 return createScalarTypeNode("any pointer", getChar(), Size);
191
192 // Accesses to arrays are accesses to objects of their element types.
193 if (CodeGenOpts.NewStructPathTBAA && Ty->isArrayType())
194 return getTypeInfo(cast<ArrayType>(Ty)->getElementType());
195
196 // Enum types are distinct types. In C++ they have "underlying types",
197 // however they aren't related for TBAA.
198 if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
199 // In C++ mode, types have linkage, so we can rely on the ODR and
200 // on their mangled names, if they're external.
201 // TODO: Is there a way to get a program-wide unique name for a
202 // decl with local linkage or no linkage?
203 if (!Features.CPlusPlus || !ETy->getDecl()->isExternallyVisible())
204 return getChar();
205
206 SmallString<256> OutName;
207 llvm::raw_svector_ostream Out(OutName);
208 MContext.mangleTypeName(QualType(ETy, 0), Out);
209 return createScalarTypeNode(OutName, getChar(), Size);
210 }
211
212 if (const auto *EIT = dyn_cast<ExtIntType>(Ty)) {
213 SmallString<256> OutName;
214 llvm::raw_svector_ostream Out(OutName);
215 // Don't specify signed/unsigned since integer types can alias despite sign
216 // differences.
217 Out << "_ExtInt(" << EIT->getNumBits() << ')';
218 return createScalarTypeNode(OutName, getChar(), Size);
219 }
220
221 // For now, handle any other kind of type conservatively.
222 return getChar();
223}
224
225llvm::MDNode *CodeGenTBAA::getTypeInfo(QualType QTy) {
226 // At -O0 or relaxed aliasing, TBAA is not emitted for regular types.
227 if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
228 return nullptr;
229
230 // If the type has the may_alias attribute (even on a typedef), it is
231 // effectively in the general char alias class.
232 if (TypeHasMayAlias(QTy))
233 return getChar();
234
235 // We need this function to not fall back to returning the "omnipotent char"
236 // type node for aggregate and union types. Otherwise, any dereference of an
237 // aggregate will result into the may-alias access descriptor, meaning all
238 // subsequent accesses to direct and indirect members of that aggregate will
239 // be considered may-alias too.
240 // TODO: Combine getTypeInfo() and getBaseTypeInfo() into a single function.
241 if (isValidBaseType(QTy))
242 return getBaseTypeInfo(QTy);
243
244 const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
245 if (llvm::MDNode *N = MetadataCache[Ty])
246 return N;
247
248 // Note that the following helper call is allowed to add new nodes to the
249 // cache, which invalidates all its previously obtained iterators. So we
250 // first generate the node for the type and then add that node to the cache.
251 llvm::MDNode *TypeNode = getTypeInfoHelper(Ty);
252 return MetadataCache[Ty] = TypeNode;
253}
254
255TBAAAccessInfo CodeGenTBAA::getAccessInfo(QualType AccessType) {
256 // Pointee values may have incomplete types, but they shall never be
257 // dereferenced.
258 if (AccessType->isIncompleteType())
259 return TBAAAccessInfo::getIncompleteInfo();
260
261 if (TypeHasMayAlias(AccessType))
262 return TBAAAccessInfo::getMayAliasInfo();
263
264 uint64_t Size = Context.getTypeSizeInChars(AccessType).getQuantity();
265 return TBAAAccessInfo(getTypeInfo(AccessType), Size);
266}
267
268TBAAAccessInfo CodeGenTBAA::getVTablePtrAccessInfo(llvm::Type *VTablePtrType) {
269 llvm::DataLayout DL(&Module);
270 unsigned Size = DL.getPointerTypeSize(VTablePtrType);
271 return TBAAAccessInfo(createScalarTypeNode("vtable pointer", getRoot(), Size),
272 Size);
273}
274
275bool
276CodeGenTBAA::CollectFields(uint64_t BaseOffset,
277 QualType QTy,
278 SmallVectorImpl<llvm::MDBuilder::TBAAStructField> &
279 Fields,
280 bool MayAlias) {
281 /* Things not handled yet include: C++ base classes, bitfields, */
282
283 if (const RecordType *TTy = QTy->getAs<RecordType>()) {
284 const RecordDecl *RD = TTy->getDecl()->getDefinition();
285 if (RD->hasFlexibleArrayMember())
286 return false;
287
288 // TODO: Handle C++ base classes.
289 if (const CXXRecordDecl *Decl = dyn_cast<CXXRecordDecl>(RD))
290 if (Decl->bases_begin() != Decl->bases_end())
291 return false;
292
293 const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
294
295 unsigned idx = 0;
296 for (RecordDecl::field_iterator i = RD->field_begin(),
297 e = RD->field_end(); i != e; ++i, ++idx) {
298 if ((*i)->isZeroSize(Context) || (*i)->isUnnamedBitfield())
299 continue;
300 uint64_t Offset = BaseOffset +
301 Layout.getFieldOffset(idx) / Context.getCharWidth();
302 QualType FieldQTy = i->getType();
303 if (!CollectFields(Offset, FieldQTy, Fields,
304 MayAlias || TypeHasMayAlias(FieldQTy)))
305 return false;
306 }
307 return true;
308 }
309
310 /* Otherwise, treat whatever it is as a field. */
311 uint64_t Offset = BaseOffset;
312 uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
313 llvm::MDNode *TBAAType = MayAlias ? getChar() : getTypeInfo(QTy);
314 llvm::MDNode *TBAATag = getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
315 Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
316 return true;
317}
318
319llvm::MDNode *
320CodeGenTBAA::getTBAAStructInfo(QualType QTy) {
321 const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
322
323 if (llvm::MDNode *N = StructMetadataCache[Ty])
324 return N;
325
326 SmallVector<llvm::MDBuilder::TBAAStructField, 4> Fields;
327 if (CollectFields(0, QTy, Fields, TypeHasMayAlias(QTy)))
328 return MDHelper.createTBAAStructNode(Fields);
329
330 // For now, handle any other kind of type conservatively.
331 return StructMetadataCache[Ty] = nullptr;
332}
333
334llvm::MDNode *CodeGenTBAA::getBaseTypeInfoHelper(const Type *Ty) {
335 if (auto *TTy = dyn_cast<RecordType>(Ty)) {
336 const RecordDecl *RD = TTy->getDecl()->getDefinition();
337 const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
338 SmallVector<llvm::MDBuilder::TBAAStructField, 4> Fields;
339 for (FieldDecl *Field : RD->fields()) {
340 if (Field->isZeroSize(Context) || Field->isUnnamedBitfield())
341 continue;
342 QualType FieldQTy = Field->getType();
343 llvm::MDNode *TypeNode = isValidBaseType(FieldQTy) ?
344 getBaseTypeInfo(FieldQTy) : getTypeInfo(FieldQTy);
345 if (!TypeNode)
346 return BaseTypeMetadataCache[Ty] = nullptr;
347
348 uint64_t BitOffset = Layout.getFieldOffset(Field->getFieldIndex());
349 uint64_t Offset = Context.toCharUnitsFromBits(BitOffset).getQuantity();
350 uint64_t Size = Context.getTypeSizeInChars(FieldQTy).getQuantity();
351 Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size,
352 TypeNode));
353 }
354
355 SmallString<256> OutName;
356 if (Features.CPlusPlus) {
357 // Don't use the mangler for C code.
358 llvm::raw_svector_ostream Out(OutName);
359 MContext.mangleTypeName(QualType(Ty, 0), Out);
360 } else {
361 OutName = RD->getName();
362 }
363
364 if (CodeGenOpts.NewStructPathTBAA) {
365 llvm::MDNode *Parent = getChar();
366 uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
367 llvm::Metadata *Id = MDHelper.createString(OutName);
368 return MDHelper.createTBAATypeNode(Parent, Size, Id, Fields);
369 }
370
371 // Create the struct type node with a vector of pairs (offset, type).
372 SmallVector<std::pair<llvm::MDNode*, uint64_t>, 4> OffsetsAndTypes;
373 for (const auto &Field : Fields)
374 OffsetsAndTypes.push_back(std::make_pair(Field.Type, Field.Offset));
375 return MDHelper.createTBAAStructTypeNode(OutName, OffsetsAndTypes);
376 }
377
378 return nullptr;
379}
380
381llvm::MDNode *CodeGenTBAA::getBaseTypeInfo(QualType QTy) {
382 if (!isValidBaseType(QTy))
383 return nullptr;
384
385 const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
386 if (llvm::MDNode *N = BaseTypeMetadataCache[Ty])
387 return N;
388
389 // Note that the following helper call is allowed to add new nodes to the
390 // cache, which invalidates all its previously obtained iterators. So we
391 // first generate the node for the type and then add that node to the cache.
392 llvm::MDNode *TypeNode = getBaseTypeInfoHelper(Ty);
393 return BaseTypeMetadataCache[Ty] = TypeNode;
394}
395
396llvm::MDNode *CodeGenTBAA::getAccessTagInfo(TBAAAccessInfo Info) {
397 assert(!Info.isIncomplete() && "Access to an object of an incomplete type!");
398
399 if (Info.isMayAlias())
400 Info = TBAAAccessInfo(getChar(), Info.Size);
401
402 if (!Info.AccessType)
403 return nullptr;
404
405 if (!CodeGenOpts.StructPathTBAA)
406 Info = TBAAAccessInfo(Info.AccessType, Info.Size);
407
408 llvm::MDNode *&N = AccessTagMetadataCache[Info];
409 if (N)
410 return N;
411
412 if (!Info.BaseType) {
413 Info.BaseType = Info.AccessType;
414 assert(!Info.Offset && "Nonzero offset for an access with no base type!");
415 }
416 if (CodeGenOpts.NewStructPathTBAA) {
417 return N = MDHelper.createTBAAAccessTag(Info.BaseType, Info.AccessType,
418 Info.Offset, Info.Size);
419 }
420 return N = MDHelper.createTBAAStructTagNode(Info.BaseType, Info.AccessType,
421 Info.Offset);
422}
423
424TBAAAccessInfo CodeGenTBAA::mergeTBAAInfoForCast(TBAAAccessInfo SourceInfo,
425 TBAAAccessInfo TargetInfo) {
426 if (SourceInfo.isMayAlias() || TargetInfo.isMayAlias())
427 return TBAAAccessInfo::getMayAliasInfo();
428 return TargetInfo;
429}
430
431TBAAAccessInfo
432CodeGenTBAA::mergeTBAAInfoForConditionalOperator(TBAAAccessInfo InfoA,
433 TBAAAccessInfo InfoB) {
434 if (InfoA == InfoB)
435 return InfoA;
436
437 if (!InfoA || !InfoB)
438 return TBAAAccessInfo();
439
440 if (InfoA.isMayAlias() || InfoB.isMayAlias())
441 return TBAAAccessInfo::getMayAliasInfo();
442
443 // TODO: Implement the rest of the logic here. For example, two accesses
444 // with same final access types result in an access to an object of that final
445 // access type regardless of their base types.
446 return TBAAAccessInfo::getMayAliasInfo();
447}
448
449TBAAAccessInfo
450CodeGenTBAA::mergeTBAAInfoForMemoryTransfer(TBAAAccessInfo DestInfo,
451 TBAAAccessInfo SrcInfo) {
452 if (DestInfo == SrcInfo)
453 return DestInfo;
454
455 if (!DestInfo || !SrcInfo)
456 return TBAAAccessInfo();
457
458 if (DestInfo.isMayAlias() || SrcInfo.isMayAlias())
459 return TBAAAccessInfo::getMayAliasInfo();
460
461 // TODO: Implement the rest of the logic here. For example, two accesses
462 // with same final access types result in an access to an object of that final
463 // access type regardless of their base types.
464 return TBAAAccessInfo::getMayAliasInfo();
465}
466