[clang] [Clang] Apply exclude_from_explicit_instantiation to dllimport/dllexport (PR #168171)

[Tomohiro Kashiwada via cfe-commits]

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@@ -19109,7 +19119,17 @@ bool Sema::DefineUsedVTables() {
         }
       }
 
-      if (IsExplicitInstantiationDeclaration)
+      if (IsExplicitInstantiationDeclaration &&
+          llvm::none_of(Class->decls(), [](Decl *decl) {
+            // If the class has a virtual member function declared with
+            // `__attribute__((exclude_from_explicit_instantiation))`, the
+            // explicit instantiation declaration shouldn't suppress emitting
+            // the vtable to ensure that the excluded member function is
+            // accessible through the vtable.
+            auto *Method = dyn_cast<CXXMethodDecl>(decl);
+            return Method && Method->isVirtual() &&
+                   Method->hasAttr<ExcludeFromExplicitInstantiationAttr>();
+          }))
----------------
kikairoya wrote:

> "vtable will live with the explicit instantiation" doesn't necessarily hold if some methods are excluded from explicit instantiation.

I'm sorry, but I still don't understand. Why are non-virtual functions allowed to affect vtables? Is it just an optimization?

In the testcase, the implicitly instantiation of the excluded constructor triggers emitting the vtable. If there is no other way to run this block, the whole `if (!KeyFunc)` block could be omitted. Otherwise, that means this block may run even if no constructors are excluded, so I think that any non-virtual members that are not instantiated shouldn't affect code generation.

https://github.com/llvm/llvm-project/pull/168171