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

[Hans Wennborg via cfe-commits]

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@@ -19108,8 +19118,23 @@ bool Sema::DefineUsedVTables() {
         }
       }
 
-      if (IsExplicitInstantiationDeclaration)
+      if (IsExplicitInstantiationDeclaration) {
         DefineVTable = false;
+
+        // Ensure the instance of a virtual member function which is declared
+        // with `__attribute__((exclude_from_explicit_instantiation))` is
+        // accessible from the VTable.
+        for (Decl *decl : Class->decls()) {
+          auto *Method = dyn_cast<CXXMethodDecl>(decl);
+          if (!Method || !Method->isVirtual())
+            continue;
+
+          if (Method->hasAttr<ExcludeFromExplicitInstantiationAttr>()) {
+            MarkFunctionReferenced(Loc, Method);
+            DefinedAnything = true;
+          }
+        }
----------------
zmodem wrote:

Hmm. I guess I'm confused by the contradiction of setting "DefineVTable = false" and then "ensure ... is accessible from the vtable".

The problem seems to be that some parts of the code tries to suppress vtable emission, while a different part emits it.

Would it work if we don't do the `DefineVTable = false` part, i.e. just changed the the `if (IsExplicitInstantiationDeclaration)` to something like `if (IsExplicitInstantiationDeclaration && !HasExcludeFromExplicitInstantiationAttr)`?


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