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

[Hans Wennborg via cfe-commits]

================
@@ -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>();
+          }))
----------------
zmodem wrote:

> Are there any cases to emit vtables, other than excluded constructors?

I'm not sure. I think they're emitted when used, which is typically by the constructor, but maybe there could be other cases.

> In other words, if control reaches here (`if (IsExplicitInstantiationDeclaration`) block), can we assume that some of constructors are excluded?

I don't think so. Why would we be able to assume that?

>From what I understand, all that we know at this point is that 1) there's a use of some class's vtable, 2) that class is an explicit instantiation decl.

The previous code assumes the explicit instantiation decl means *no* methods are emitted here, but will be emitted elsewhere (with the explicit instantiation definition).

With the addition of ExcludeFromExplicitInstantiationAttr, that assumption no longer holds true.

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