[TBAA] Don't emit pointer tbaa for unnamed structs or unions. (llvm#116596)

For unnamed structs or unions, C's compatible types rule applies. Two
compatible types in different compilation units can have different
mangled names, meaning the metadata emitted below would incorrectly mark
them as no-alias. Use AnyPtr for such types in both C and C++, as C and
C++ types may be visible when doing LTO.

PR: llvm#116596
(cherry picked from commit 41a0c66)

Author: fhahn

clang/lib/CodeGen/CodeGenTBAA.cpp

@@ -248,7 +248,22 @@ llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
       // Be conservative if the type isn't a RecordType. We are specifically
       // required to do this for member pointers until we implement the
       // similar-types rule.
-      if (!Ty->isRecordType())
+      const auto *RT = Ty->getAs<RecordType>();
+      if (!RT)
+        return AnyPtr;
+
+      // For unnamed structs or unions C's compatible types rule applies. Two
+      // compatible types in different compilation units can have different
+      // mangled names, meaning the metadata emitted below would incorrectly
+      // mark them as no-alias. Use AnyPtr for such types in both C and C++, as
+      // C and C++ types may be visible when doing LTO.
+      //
+      // Note that using AnyPtr is overly conservative. We could summarize the
+      // members of the type, as per the C compatibility rule in the future.
+      // This also covers anonymous structs and unions, which have a different
+      // compatibility rule, but it doesn't matter because you can never have a
+      // pointer to an anonymous struct or union.
+      if (!RT->getDecl()->getDeclName())
         return AnyPtr;
 
       // For non-builtin types use the mangled name of the canonical type.

clang/test/CodeGen/tbaa-pointers.c

@@ -190,8 +190,6 @@ typedef struct {
   int i1;
 } TypedefS;
 
-// FIXME: The !tbaa tag for unnamed structs doesn't account for compatible
-// types in C.
 void unamed_struct_typedef(TypedefS *ptr) {
 // COMMON-LABEL: define void @unamed_struct_typedef(
 // COMMON-SAME: ptr noundef %ptr)
@@ -239,5 +237,4 @@ void unamed_struct_typedef(TypedefS *ptr) {
 // DEFAULT: [[S2_TY]]  = !{!"S2", [[ANY_POINTER]], i64 0}
 // COMMON:  [[INT_TAG]] = !{[[INT_TY:!.+]], [[INT_TY]], i64 0}
 // COMMON:  [[INT_TY]] = !{!"int", [[CHAR]], i64 0}
-// ENABLED: [[P1TYPEDEF]] = !{[[P1TYPEDEF_TY:!.+]],  [[P1TYPEDEF_TY]], i64 0}
-// ENABLED: [[P1TYPEDEF_TY]] = !{!"p1 _ZTS8TypedefS", [[ANY_POINTER]], i64 0}
+// ENABLED: [[P1TYPEDEF]] = !{[[ANY_POINTER]],  [[ANY_POINTER]], i64 0}