[MinGW] Emit typeinfo locally for dllimported classes without key functions

This fixes building Qt as shared libraries with clang in MinGW
mode; previously subclasses of the QObjectData class (in other
DLLs than the base DLL) failed to find the typeinfo symbols
(that neither were emitted in the base DLL nor in the DLL
containing the subclass).

If the virtual destructor in the newly added testcase wouldn't
be pure (or if there'd be another non-pure virtual method),
it'd be a key function and things would work out even before this
change. Make sure to locally emit the typeinfo for these classes
as well.

This matches what GCC does in this specific testcase.

This fixes the root issue that spawned PR35146. (The difference
to GCC that is initially described in that bug still is present
though.)

Differential Revision: https://reviews.llvm.org/D42641

llvm-svn: 324059

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -2761,6 +2761,11 @@ static bool ShouldUseExternalRTTIDescriptor(CodeGenModule &CGM,
     // N.B. We must always emit the RTTI data ourselves if there exists a key
     // function.
     bool IsDLLImport = RD->hasAttr<DLLImportAttr>();
+
+    // Don't import the RTTI but emit it locally.
+    if (CGM.getTriple().isWindowsGNUEnvironment() && IsDLLImport)
+      return false;
+
     if (CGM.getVTables().isVTableExternal(RD))
       return IsDLLImport && !CGM.getTriple().isWindowsItaniumEnvironment()
                  ? false

clang/test/CodeGenCXX/dllimport-missing-key.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -std=c++1y -O0 -o - %s -w | FileCheck --check-prefix=GNU %s
+
+class __declspec(dllimport) QObjectData {
+public:
+    virtual ~QObjectData() = 0;
+    void *ptr;
+
+    int method() const;
+};
+
+class LocalClass : public QObjectData {
+};
+
+void call() {
+    (new LocalClass())->method();
+}
+
+// GNU-DAG: @_ZTV11QObjectData = available_externally dllimport
+// GNU-DAG: @_ZTS11QObjectData = linkonce_odr
+// GNU-DAG: @_ZTI11QObjectData = linkonce_odr

clang/test/CodeGenCXX/dllimport-rtti.cpp

@@ -12,7 +12,7 @@ struct __declspec(dllimport) S {
 // MSVC-DAG: @"\01??_R3S@@8" = linkonce_odr
 
 // GNU-DAG: @_ZTV1S = available_externally dllimport
-// GNU-DAG: @_ZTI1S = external dllimport
+// GNU-DAG: @_ZTI1S = linkonce_odr
 
 struct U : S {
 } u;