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

[John McCall via cfe-commits]

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@@ -249,6 +249,21 @@ llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
       if (!Ty->isRecordType())
         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.
+      const auto *RT = Ty->getAs<RecordType>();
+      if (RT && !RT->getDecl()->getDeclName())
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rjmccall wrote:

This `getAs<>` is now redundant with the `isRecordType()` above; please do the `getAs<>` above and test `RT` there, and then you can test this here.

https://github.com/llvm/llvm-project/pull/116596