[MinGW] Produce a vtable and RTTI for dllexported classes without a key function

[mstorsjo]

Summary

This matches what GCC does in these situations.

This fixes compiling Qt in debug mode. In release mode, references to the vtable of this particular class ends up optimized away, but in debug mode, the compiler creates references to the vtable, which is expected to be dllexported from a different DLL. Make sure the dllexported version actually ends up emitted.

[mstorsjo]

Fixed wrapping of the RUN line in the test.

[rnk]

lib/Sema/SemaDeclCXX.cpp

5754–5756

This may be too early, you can get into situations like this if you start emitting the vtable (which will emit inline methods) before we get to the end of the outermost class. See this bug for example:
https://bugs.llvm.org/show_bug.cgi?id=40006

Maybe if you have a dllexported nested class with a virtual method that references the constructor of the outer class which has a late-parsed member initializer... you can get things to go wrong as in the bug above.

I think the fix will be to touch the vtable when we process delayed dllexported classes from the list just above this line.

test/CodeGenCXX/dllexport-missing-key.cpp

20

OK, good, I was going to ask if it became weak_odr, but looks like it all works.

[mstorsjo]

lib/Sema/SemaDeclCXX.cpp

5754–5756

Ok, will upload a new version of the patch.

[mstorsjo]

Moved the code to work on DelayedDllExportClasses instead, as suggested, which still works for the testcase. (I've yet to test that approach on a larger codebase though.)

[rnk]

lgtm

lib/Sema/SemaDeclCXX.cpp

           FD->addAttr(NewAttr);
         }
       }
     }
   }
 
   if (ClassExported)
     DelayedDllExportClasses.push_back(Class);
+
+  if (ClassExported &&
+      Context.getTargetInfo().getTriple().isWindowsGNUEnvironment())
+    MarkVTableUsed(Class->getLocation(), Class, true);
 }
 
 /// Perform propagation of DLL attributes from a derived class to a
 /// templated base class for MS compatibility.
 void Sema::propagateDLLAttrToBaseClassTemplate(
     CXXRecordDecl *Class, Attr *ClassAttr,
     ClassTemplateSpecializationDecl *BaseTemplateSpec, SourceLocation BaseLoc) {
   if (getDLLAttr(

test/CodeGenCXX/dllexport-missing-key.cpp

+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -std=c++11 -o - %s | FileCheck --check-prefix=GNU %s
+
+class __declspec(dllexport) QAbstractLayoutStyleInfo {
+public:
+  QAbstractLayoutStyleInfo() : m_isWindow(false) {}
+  virtual ~QAbstractLayoutStyleInfo() {}
+
+  virtual bool hasChangedCore() const { return false; }
+
+  virtual void invalidate() {}
+
+  virtual double windowMargin(bool orientation) const = 0;
+
+  bool isWindow() const { return m_isWindow; }
+
+protected:
+  bool m_isWindow;
+};
+
+// GNU-DAG: @_ZTV24QAbstractLayoutStyleInfo = weak_odr dso_local dllexport
+// GNU-DAG: @_ZTS24QAbstractLayoutStyleInfo = linkonce_odr
+// GNU-DAG: @_ZTI24QAbstractLayoutStyleInfo = linkonce_odr