[clang] [llvm] [PAC] Fix address discrimination for type info vtable pointers (PR #102199)

[Eli Friedman via llvm-commits]

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@@ -1056,12 +1056,18 @@ class ConstantPtrAuth final : public Constant {
     return !getAddrDiscriminator()->isNullValue();
   }
 
-  /// A constant value for the address discriminator which has special
-  /// significance to ctors/dtors lowering. Regular address discrimination can't
-  /// be applied for them since uses of llvm.global_{c|d}tors are disallowed
-  /// (see Verifier::visitGlobalVariable) and we can't emit getelementptr
-  /// expressions referencing these special arrays.
-  enum { AddrDiscriminator_CtorsDtors = 1 };
+  /// Constant values for the address discriminator which have special
+  /// significance to lowering in some contexts.
+  /// - For ctors/dtors, regular address discrimination can't
+  ///   be applied for them since uses of llvm.global_{c|d}tors are disallowed
+  ///   (see Verifier::visitGlobalVariable) and we can't emit getelementptr
+  ///   expressions referencing these special arrays.
+  /// - For vtable pointers of std::type_info and classes derived from it,
+  ///   we do not know the storage address when emitting ptrauth constant.
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efriedma-quic wrote:

The only caller of BuildVTablePointer is BuildTypeInfo... and BuildTypeInfo itself creates the global variable in question.  So it shouldn't be that hard.  Just move the call to BuildVTablePointer after we construct the global.

Actually, that might be slightly trickier than I'm making it sound because we don't compute the type before we create the ConstantStruct, but it should be possible to separate computing the type from constructing the ConstantStruct.

https://github.com/llvm/llvm-project/pull/102199