xref: /freebsd/contrib/llvm-project/clang/lib/CodeGen/CodeGenTBAA.cpp (revision 47ef2a131091508e049ab10cad7f91a3c1342cd9)
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 "ABIInfoImpl.h"
19 #include "CGCXXABI.h"
20 #include "CGRecordLayout.h"
21 #include "CodeGenTypes.h"
22 #include "clang/AST/ASTContext.h"
23 #include "clang/AST/Attr.h"
24 #include "clang/AST/Mangle.h"
25 #include "clang/AST/RecordLayout.h"
26 #include "clang/Basic/CodeGenOptions.h"
27 #include "clang/Basic/TargetInfo.h"
28 #include "llvm/ADT/SmallSet.h"
29 #include "llvm/IR/Constants.h"
30 #include "llvm/IR/LLVMContext.h"
31 #include "llvm/IR/Metadata.h"
32 #include "llvm/IR/Module.h"
33 #include "llvm/IR/Type.h"
34 #include "llvm/Support/Debug.h"
35 using namespace clang;
36 using namespace CodeGen;
37 
38 CodeGenTBAA::CodeGenTBAA(ASTContext &Ctx, CodeGenTypes &CGTypes,
39                          llvm::Module &M, const CodeGenOptions &CGO,
40                          const LangOptions &Features)
41     : Context(Ctx), CGTypes(CGTypes), Module(M), CodeGenOpts(CGO),
42       Features(Features), MDHelper(M.getContext()), Root(nullptr),
43       Char(nullptr) {}
44 
45 CodeGenTBAA::~CodeGenTBAA() {
46 }
47 
48 llvm::MDNode *CodeGenTBAA::getRoot() {
49   // Define the root of the tree. This identifies the tree, so that
50   // if our LLVM IR is linked with LLVM IR from a different front-end
51   // (or a different version of this front-end), their TBAA trees will
52   // remain distinct, and the optimizer will treat them conservatively.
53   if (!Root) {
54     if (Features.CPlusPlus)
55       Root = MDHelper.createTBAARoot("Simple C++ TBAA");
56     else
57       Root = MDHelper.createTBAARoot("Simple C/C++ TBAA");
58   }
59 
60   return Root;
61 }
62 
63 llvm::MDNode *CodeGenTBAA::createScalarTypeNode(StringRef Name,
64                                                 llvm::MDNode *Parent,
65                                                 uint64_t Size) {
66   if (CodeGenOpts.NewStructPathTBAA) {
67     llvm::Metadata *Id = MDHelper.createString(Name);
68     return MDHelper.createTBAATypeNode(Parent, Size, Id);
69   }
70   return MDHelper.createTBAAScalarTypeNode(Name, Parent);
71 }
72 
73 llvm::MDNode *CodeGenTBAA::getChar() {
74   // Define the root of the tree for user-accessible memory. C and C++
75   // give special powers to char and certain similar types. However,
76   // these special powers only cover user-accessible memory, and doesn't
77   // include things like vtables.
78   if (!Char)
79     Char = createScalarTypeNode("omnipotent char", getRoot(), /* Size= */ 1);
80 
81   return Char;
82 }
83 
84 static bool TypeHasMayAlias(QualType QTy) {
85   // Tagged types have declarations, and therefore may have attributes.
86   if (auto *TD = QTy->getAsTagDecl())
87     if (TD->hasAttr<MayAliasAttr>())
88       return true;
89 
90   // Also look for may_alias as a declaration attribute on a typedef.
91   // FIXME: We should follow GCC and model may_alias as a type attribute
92   // rather than as a declaration attribute.
93   while (auto *TT = QTy->getAs<TypedefType>()) {
94     if (TT->getDecl()->hasAttr<MayAliasAttr>())
95       return true;
96     QTy = TT->desugar();
97   }
98   return false;
99 }
100 
101 /// Check if the given type is a valid base type to be used in access tags.
102 static bool isValidBaseType(QualType QTy) {
103   if (const RecordType *TTy = QTy->getAs<RecordType>()) {
104     const RecordDecl *RD = TTy->getDecl()->getDefinition();
105     // Incomplete types are not valid base access types.
106     if (!RD)
107       return false;
108     if (RD->hasFlexibleArrayMember())
109       return false;
110     // RD can be struct, union, class, interface or enum.
111     // For now, we only handle struct and class.
112     if (RD->isStruct() || RD->isClass())
113       return true;
114   }
115   return false;
116 }
117 
118 llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
119   uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
120 
121   // Handle builtin types.
122   if (const BuiltinType *BTy = dyn_cast<BuiltinType>(Ty)) {
123     switch (BTy->getKind()) {
124     // Character types are special and can alias anything.
125     // In C++, this technically only includes "char" and "unsigned char",
126     // and not "signed char". In C, it includes all three. For now,
127     // the risk of exploiting this detail in C++ seems likely to outweigh
128     // the benefit.
129     case BuiltinType::Char_U:
130     case BuiltinType::Char_S:
131     case BuiltinType::UChar:
132     case BuiltinType::SChar:
133       return getChar();
134 
135     // Unsigned types can alias their corresponding signed types.
136     case BuiltinType::UShort:
137       return getTypeInfo(Context.ShortTy);
138     case BuiltinType::UInt:
139       return getTypeInfo(Context.IntTy);
140     case BuiltinType::ULong:
141       return getTypeInfo(Context.LongTy);
142     case BuiltinType::ULongLong:
143       return getTypeInfo(Context.LongLongTy);
144     case BuiltinType::UInt128:
145       return getTypeInfo(Context.Int128Ty);
146 
147     case BuiltinType::UShortFract:
148       return getTypeInfo(Context.ShortFractTy);
149     case BuiltinType::UFract:
150       return getTypeInfo(Context.FractTy);
151     case BuiltinType::ULongFract:
152       return getTypeInfo(Context.LongFractTy);
153 
154     case BuiltinType::SatUShortFract:
155       return getTypeInfo(Context.SatShortFractTy);
156     case BuiltinType::SatUFract:
157       return getTypeInfo(Context.SatFractTy);
158     case BuiltinType::SatULongFract:
159       return getTypeInfo(Context.SatLongFractTy);
160 
161     case BuiltinType::UShortAccum:
162       return getTypeInfo(Context.ShortAccumTy);
163     case BuiltinType::UAccum:
164       return getTypeInfo(Context.AccumTy);
165     case BuiltinType::ULongAccum:
166       return getTypeInfo(Context.LongAccumTy);
167 
168     case BuiltinType::SatUShortAccum:
169       return getTypeInfo(Context.SatShortAccumTy);
170     case BuiltinType::SatUAccum:
171       return getTypeInfo(Context.SatAccumTy);
172     case BuiltinType::SatULongAccum:
173       return getTypeInfo(Context.SatLongAccumTy);
174 
175     // Treat all other builtin types as distinct types. This includes
176     // treating wchar_t, char16_t, and char32_t as distinct from their
177     // "underlying types".
178     default:
179       return createScalarTypeNode(BTy->getName(Features), getChar(), Size);
180     }
181   }
182 
183   // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
184   // an object through a glvalue of other than one of the following types the
185   // behavior is undefined: [...] a char, unsigned char, or std::byte type."
186   if (Ty->isStdByteType())
187     return getChar();
188 
189   // Handle pointers and references.
190   //
191   // C has a very strict rule for pointer aliasing. C23 6.7.6.1p2:
192   //     For two pointer types to be compatible, both shall be identically
193   //     qualified and both shall be pointers to compatible types.
194   //
195   // This rule is impractically strict; we want to at least ignore CVR
196   // qualifiers. Distinguishing by CVR qualifiers would make it UB to
197   // e.g. cast a `char **` to `const char * const *` and dereference it,
198   // which is too common and useful to invalidate. C++'s similar types
199   // rule permits qualifier differences in these nested positions; in fact,
200   // C++ even allows that cast as an implicit conversion.
201   //
202   // Other qualifiers could theoretically be distinguished, especially if
203   // they involve a significant representation difference.  We don't
204   // currently do so, however.
205   //
206   // Computing the pointee type string recursively is implicitly more
207   // forgiving than the standards require.  Effectively, we are turning
208   // the question "are these types compatible/similar" into "are
209   // accesses to these types allowed to alias".  In both C and C++,
210   // the latter question has special carve-outs for signedness
211   // mismatches that only apply at the top level.  As a result, we are
212   // allowing e.g. `int *` l-values to access `unsigned *` objects.
213   if (Ty->isPointerType() || Ty->isReferenceType()) {
214     llvm::MDNode *AnyPtr = createScalarTypeNode("any pointer", getChar(), Size);
215     if (!CodeGenOpts.PointerTBAA)
216       return AnyPtr;
217     // Compute the depth of the pointer and generate a tag of the form "p<depth>
218     // <base type tag>".
219     unsigned PtrDepth = 0;
220     do {
221       PtrDepth++;
222       Ty = Ty->getPointeeType().getTypePtr();
223     } while (Ty->isPointerType());
224     // TODO: Implement C++'s type "similarity" and consider dis-"similar"
225     // pointers distinct for non-builtin types.
226     if (isa<BuiltinType>(Ty)) {
227       llvm::MDNode *ScalarMD = getTypeInfoHelper(Ty);
228       StringRef Name =
229           cast<llvm::MDString>(
230               ScalarMD->getOperand(CodeGenOpts.NewStructPathTBAA ? 2 : 0))
231               ->getString();
232       SmallString<256> OutName("p");
233       OutName += std::to_string(PtrDepth);
234       OutName += " ";
235       OutName += Name;
236       return createScalarTypeNode(OutName, AnyPtr, Size);
237     }
238     return AnyPtr;
239   }
240 
241   // Accesses to arrays are accesses to objects of their element types.
242   if (CodeGenOpts.NewStructPathTBAA && Ty->isArrayType())
243     return getTypeInfo(cast<ArrayType>(Ty)->getElementType());
244 
245   // Enum types are distinct types. In C++ they have "underlying types",
246   // however they aren't related for TBAA.
247   if (const EnumType *ETy = dyn_cast<EnumType>(Ty)) {
248     if (!Features.CPlusPlus)
249       return getTypeInfo(ETy->getDecl()->getIntegerType());
250 
251     // In C++ mode, types have linkage, so we can rely on the ODR and
252     // on their mangled names, if they're external.
253     // TODO: Is there a way to get a program-wide unique name for a
254     // decl with local linkage or no linkage?
255     if (!ETy->getDecl()->isExternallyVisible())
256       return getChar();
257 
258     SmallString<256> OutName;
259     llvm::raw_svector_ostream Out(OutName);
260     CGTypes.getCXXABI().getMangleContext().mangleCanonicalTypeName(
261         QualType(ETy, 0), Out);
262     return createScalarTypeNode(OutName, getChar(), Size);
263   }
264 
265   if (const auto *EIT = dyn_cast<BitIntType>(Ty)) {
266     SmallString<256> OutName;
267     llvm::raw_svector_ostream Out(OutName);
268     // Don't specify signed/unsigned since integer types can alias despite sign
269     // differences.
270     Out << "_BitInt(" << EIT->getNumBits() << ')';
271     return createScalarTypeNode(OutName, getChar(), Size);
272   }
273 
274   // For now, handle any other kind of type conservatively.
275   return getChar();
276 }
277 
278 llvm::MDNode *CodeGenTBAA::getTypeInfo(QualType QTy) {
279   // At -O0 or relaxed aliasing, TBAA is not emitted for regular types.
280   if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
281     return nullptr;
282 
283   // If the type has the may_alias attribute (even on a typedef), it is
284   // effectively in the general char alias class.
285   if (TypeHasMayAlias(QTy))
286     return getChar();
287 
288   // We need this function to not fall back to returning the "omnipotent char"
289   // type node for aggregate and union types. Otherwise, any dereference of an
290   // aggregate will result into the may-alias access descriptor, meaning all
291   // subsequent accesses to direct and indirect members of that aggregate will
292   // be considered may-alias too.
293   // TODO: Combine getTypeInfo() and getValidBaseTypeInfo() into a single
294   // function.
295   if (isValidBaseType(QTy))
296     return getValidBaseTypeInfo(QTy);
297 
298   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
299   if (llvm::MDNode *N = MetadataCache[Ty])
300     return N;
301 
302   // Note that the following helper call is allowed to add new nodes to the
303   // cache, which invalidates all its previously obtained iterators. So we
304   // first generate the node for the type and then add that node to the cache.
305   llvm::MDNode *TypeNode = getTypeInfoHelper(Ty);
306   return MetadataCache[Ty] = TypeNode;
307 }
308 
309 TBAAAccessInfo CodeGenTBAA::getAccessInfo(QualType AccessType) {
310   // Pointee values may have incomplete types, but they shall never be
311   // dereferenced.
312   if (AccessType->isIncompleteType())
313     return TBAAAccessInfo::getIncompleteInfo();
314 
315   if (TypeHasMayAlias(AccessType))
316     return TBAAAccessInfo::getMayAliasInfo();
317 
318   uint64_t Size = Context.getTypeSizeInChars(AccessType).getQuantity();
319   return TBAAAccessInfo(getTypeInfo(AccessType), Size);
320 }
321 
322 TBAAAccessInfo CodeGenTBAA::getVTablePtrAccessInfo(llvm::Type *VTablePtrType) {
323   llvm::DataLayout DL(&Module);
324   unsigned Size = DL.getPointerTypeSize(VTablePtrType);
325   return TBAAAccessInfo(createScalarTypeNode("vtable pointer", getRoot(), Size),
326                         Size);
327 }
328 
329 bool
330 CodeGenTBAA::CollectFields(uint64_t BaseOffset,
331                            QualType QTy,
332                            SmallVectorImpl<llvm::MDBuilder::TBAAStructField> &
333                              Fields,
334                            bool MayAlias) {
335   /* Things not handled yet include: C++ base classes, bitfields, */
336 
337   if (const RecordType *TTy = QTy->getAs<RecordType>()) {
338     if (TTy->isUnionType()) {
339       uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
340       llvm::MDNode *TBAAType = getChar();
341       llvm::MDNode *TBAATag = getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
342       Fields.push_back(
343           llvm::MDBuilder::TBAAStructField(BaseOffset, Size, TBAATag));
344       return true;
345     }
346     const RecordDecl *RD = TTy->getDecl()->getDefinition();
347     if (RD->hasFlexibleArrayMember())
348       return false;
349 
350     // TODO: Handle C++ base classes.
351     if (const CXXRecordDecl *Decl = dyn_cast<CXXRecordDecl>(RD))
352       if (Decl->bases_begin() != Decl->bases_end())
353         return false;
354 
355     const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
356     const CGRecordLayout &CGRL = CGTypes.getCGRecordLayout(RD);
357 
358     unsigned idx = 0;
359     for (RecordDecl::field_iterator i = RD->field_begin(), e = RD->field_end();
360          i != e; ++i, ++idx) {
361       if (isEmptyFieldForLayout(Context, *i))
362         continue;
363 
364       uint64_t Offset =
365           BaseOffset + Layout.getFieldOffset(idx) / Context.getCharWidth();
366 
367       // Create a single field for consecutive named bitfields using char as
368       // base type.
369       if ((*i)->isBitField()) {
370         const CGBitFieldInfo &Info = CGRL.getBitFieldInfo(*i);
371         // For big endian targets the first bitfield in the consecutive run is
372         // at the most-significant end; see CGRecordLowering::setBitFieldInfo
373         // for more information.
374         bool IsBE = Context.getTargetInfo().isBigEndian();
375         bool IsFirst = IsBE ? Info.StorageSize - (Info.Offset + Info.Size) == 0
376                             : Info.Offset == 0;
377         if (!IsFirst)
378           continue;
379         unsigned CurrentBitFieldSize = Info.StorageSize;
380         uint64_t Size =
381             llvm::divideCeil(CurrentBitFieldSize, Context.getCharWidth());
382         llvm::MDNode *TBAAType = getChar();
383         llvm::MDNode *TBAATag =
384             getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
385         Fields.push_back(
386             llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
387         continue;
388       }
389 
390       QualType FieldQTy = i->getType();
391       if (!CollectFields(Offset, FieldQTy, Fields,
392                          MayAlias || TypeHasMayAlias(FieldQTy)))
393         return false;
394     }
395     return true;
396   }
397 
398   /* Otherwise, treat whatever it is as a field. */
399   uint64_t Offset = BaseOffset;
400   uint64_t Size = Context.getTypeSizeInChars(QTy).getQuantity();
401   llvm::MDNode *TBAAType = MayAlias ? getChar() : getTypeInfo(QTy);
402   llvm::MDNode *TBAATag = getAccessTagInfo(TBAAAccessInfo(TBAAType, Size));
403   Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size, TBAATag));
404   return true;
405 }
406 
407 llvm::MDNode *
408 CodeGenTBAA::getTBAAStructInfo(QualType QTy) {
409   if (CodeGenOpts.OptimizationLevel == 0 || CodeGenOpts.RelaxedAliasing)
410     return nullptr;
411 
412   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
413 
414   if (llvm::MDNode *N = StructMetadataCache[Ty])
415     return N;
416 
417   SmallVector<llvm::MDBuilder::TBAAStructField, 4> Fields;
418   if (CollectFields(0, QTy, Fields, TypeHasMayAlias(QTy)))
419     return MDHelper.createTBAAStructNode(Fields);
420 
421   // For now, handle any other kind of type conservatively.
422   return StructMetadataCache[Ty] = nullptr;
423 }
424 
425 llvm::MDNode *CodeGenTBAA::getBaseTypeInfoHelper(const Type *Ty) {
426   if (auto *TTy = dyn_cast<RecordType>(Ty)) {
427     const RecordDecl *RD = TTy->getDecl()->getDefinition();
428     const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
429     using TBAAStructField = llvm::MDBuilder::TBAAStructField;
430     SmallVector<TBAAStructField, 4> Fields;
431     if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
432       // Handle C++ base classes. Non-virtual bases can treated a kind of
433       // field. Virtual bases are more complex and omitted, but avoid an
434       // incomplete view for NewStructPathTBAA.
435       if (CodeGenOpts.NewStructPathTBAA && CXXRD->getNumVBases() != 0)
436         return nullptr;
437       for (const CXXBaseSpecifier &B : CXXRD->bases()) {
438         if (B.isVirtual())
439           continue;
440         QualType BaseQTy = B.getType();
441         const CXXRecordDecl *BaseRD = BaseQTy->getAsCXXRecordDecl();
442         if (BaseRD->isEmpty())
443           continue;
444         llvm::MDNode *TypeNode = isValidBaseType(BaseQTy)
445                                      ? getValidBaseTypeInfo(BaseQTy)
446                                      : getTypeInfo(BaseQTy);
447         if (!TypeNode)
448           return nullptr;
449         uint64_t Offset = Layout.getBaseClassOffset(BaseRD).getQuantity();
450         uint64_t Size =
451             Context.getASTRecordLayout(BaseRD).getDataSize().getQuantity();
452         Fields.push_back(
453             llvm::MDBuilder::TBAAStructField(Offset, Size, TypeNode));
454       }
455       // The order in which base class subobjects are allocated is unspecified,
456       // so may differ from declaration order. In particular, Itanium ABI will
457       // allocate a primary base first.
458       // Since we exclude empty subobjects, the objects are not overlapping and
459       // their offsets are unique.
460       llvm::sort(Fields,
461                  [](const TBAAStructField &A, const TBAAStructField &B) {
462                    return A.Offset < B.Offset;
463                  });
464     }
465     for (FieldDecl *Field : RD->fields()) {
466       if (Field->isZeroSize(Context) || Field->isUnnamedBitField())
467         continue;
468       QualType FieldQTy = Field->getType();
469       llvm::MDNode *TypeNode = isValidBaseType(FieldQTy)
470                                    ? getValidBaseTypeInfo(FieldQTy)
471                                    : getTypeInfo(FieldQTy);
472       if (!TypeNode)
473         return nullptr;
474 
475       uint64_t BitOffset = Layout.getFieldOffset(Field->getFieldIndex());
476       uint64_t Offset = Context.toCharUnitsFromBits(BitOffset).getQuantity();
477       uint64_t Size = Context.getTypeSizeInChars(FieldQTy).getQuantity();
478       Fields.push_back(llvm::MDBuilder::TBAAStructField(Offset, Size,
479                                                         TypeNode));
480     }
481 
482     SmallString<256> OutName;
483     if (Features.CPlusPlus) {
484       // Don't use the mangler for C code.
485       llvm::raw_svector_ostream Out(OutName);
486       CGTypes.getCXXABI().getMangleContext().mangleCanonicalTypeName(
487           QualType(Ty, 0), Out);
488     } else {
489       OutName = RD->getName();
490     }
491 
492     if (CodeGenOpts.NewStructPathTBAA) {
493       llvm::MDNode *Parent = getChar();
494       uint64_t Size = Context.getTypeSizeInChars(Ty).getQuantity();
495       llvm::Metadata *Id = MDHelper.createString(OutName);
496       return MDHelper.createTBAATypeNode(Parent, Size, Id, Fields);
497     }
498 
499     // Create the struct type node with a vector of pairs (offset, type).
500     SmallVector<std::pair<llvm::MDNode*, uint64_t>, 4> OffsetsAndTypes;
501     for (const auto &Field : Fields)
502         OffsetsAndTypes.push_back(std::make_pair(Field.Type, Field.Offset));
503     return MDHelper.createTBAAStructTypeNode(OutName, OffsetsAndTypes);
504   }
505 
506   return nullptr;
507 }
508 
509 llvm::MDNode *CodeGenTBAA::getValidBaseTypeInfo(QualType QTy) {
510   assert(isValidBaseType(QTy) && "Must be a valid base type");
511 
512   const Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
513 
514   // nullptr is a valid value in the cache, so use find rather than []
515   auto I = BaseTypeMetadataCache.find(Ty);
516   if (I != BaseTypeMetadataCache.end())
517     return I->second;
518 
519   // First calculate the metadata, before recomputing the insertion point, as
520   // the helper can recursively call us.
521   llvm::MDNode *TypeNode = getBaseTypeInfoHelper(Ty);
522   LLVM_ATTRIBUTE_UNUSED auto inserted =
523       BaseTypeMetadataCache.insert({Ty, TypeNode});
524   assert(inserted.second && "BaseType metadata was already inserted");
525 
526   return TypeNode;
527 }
528 
529 llvm::MDNode *CodeGenTBAA::getBaseTypeInfo(QualType QTy) {
530   return isValidBaseType(QTy) ? getValidBaseTypeInfo(QTy) : nullptr;
531 }
532 
533 llvm::MDNode *CodeGenTBAA::getAccessTagInfo(TBAAAccessInfo Info) {
534   assert(!Info.isIncomplete() && "Access to an object of an incomplete type!");
535 
536   if (Info.isMayAlias())
537     Info = TBAAAccessInfo(getChar(), Info.Size);
538 
539   if (!Info.AccessType)
540     return nullptr;
541 
542   if (!CodeGenOpts.StructPathTBAA)
543     Info = TBAAAccessInfo(Info.AccessType, Info.Size);
544 
545   llvm::MDNode *&N = AccessTagMetadataCache[Info];
546   if (N)
547     return N;
548 
549   if (!Info.BaseType) {
550     Info.BaseType = Info.AccessType;
551     assert(!Info.Offset && "Nonzero offset for an access with no base type!");
552   }
553   if (CodeGenOpts.NewStructPathTBAA) {
554     return N = MDHelper.createTBAAAccessTag(Info.BaseType, Info.AccessType,
555                                             Info.Offset, Info.Size);
556   }
557   return N = MDHelper.createTBAAStructTagNode(Info.BaseType, Info.AccessType,
558                                               Info.Offset);
559 }
560 
561 TBAAAccessInfo CodeGenTBAA::mergeTBAAInfoForCast(TBAAAccessInfo SourceInfo,
562                                                  TBAAAccessInfo TargetInfo) {
563   if (SourceInfo.isMayAlias() || TargetInfo.isMayAlias())
564     return TBAAAccessInfo::getMayAliasInfo();
565   return TargetInfo;
566 }
567 
568 TBAAAccessInfo
569 CodeGenTBAA::mergeTBAAInfoForConditionalOperator(TBAAAccessInfo InfoA,
570                                                  TBAAAccessInfo InfoB) {
571   if (InfoA == InfoB)
572     return InfoA;
573 
574   if (!InfoA || !InfoB)
575     return TBAAAccessInfo();
576 
577   if (InfoA.isMayAlias() || InfoB.isMayAlias())
578     return TBAAAccessInfo::getMayAliasInfo();
579 
580   // TODO: Implement the rest of the logic here. For example, two accesses
581   // with same final access types result in an access to an object of that final
582   // access type regardless of their base types.
583   return TBAAAccessInfo::getMayAliasInfo();
584 }
585 
586 TBAAAccessInfo
587 CodeGenTBAA::mergeTBAAInfoForMemoryTransfer(TBAAAccessInfo DestInfo,
588                                             TBAAAccessInfo SrcInfo) {
589   if (DestInfo == SrcInfo)
590     return DestInfo;
591 
592   if (!DestInfo || !SrcInfo)
593     return TBAAAccessInfo();
594 
595   if (DestInfo.isMayAlias() || SrcInfo.isMayAlias())
596     return TBAAAccessInfo::getMayAliasInfo();
597 
598   // TODO: Implement the rest of the logic here. For example, two accesses
599   // with same final access types result in an access to an object of that final
600   // access type regardless of their base types.
601   return TBAAAccessInfo::getMayAliasInfo();
602 }
603