clang-cl: Only mark dllexported constexpr functions once

[• ehsan]

Summary

This fixes PR21718.

[hans]

Nice!

I think the patch description could be a little more explanatory, maybe something like "don't mark constexpr members of dllexport classes referenced, since they considered are referenced already".

lib/Sema/SemaDeclCXX.cpp

4745

The comment is a little vague (and should end with a period). Maybe something like "constexpr functions are already marked referenced."

Also, David pointed out that this does not only apply to user-defined functions, so it should be moved back to where you first suggested in the PR. Sorry for my misleading suggestion here before.

test/CodeGenCXX/dllexport.cpp

621

Nit: the template is declared as a struct, so it should be referenced as a struct when instantiated too.

Please also add a test where the constructor is explicitly defaulted, i.e.

template <typename> struct P { constexpr P() = default; };

[• ehsan]

lib/Sema/SemaDeclCXX.cpp

4745

What about http://llvm.org/bugs/show_bug.cgi?id=21718#c6?

[hans]

Sorry, it seems I'm reading email in the wrong order today.

lib/Sema/SemaDeclCXX.cpp

4745

Maybe what we should do is something like

if (MD->isReferenced())

continue;

Would that pass all the old tests and your new one?

[• ehsan]

lib/Sema/SemaDeclCXX.cpp

4745

That doesn't fix the original issue.

[• ehsan]

Per http://llvm.org/bugs/show_bug.cgi?id=21718#c9.

lib/Sema/SemaDeclCXX.cpp

if (!getDLLAttr(Member)) {
       auto *NewAttr =
           cast<InheritableAttr>(ClassAttr->clone(S.getASTContext()));
       NewAttr->setInherited(true);
       Member->addAttr(NewAttr);
     }
 
     if (MD && ClassExported) {
       if (MD->isUserProvided()) {
+        // Don't mark constexpr functions again
+        if (MD->isConstexpr())
+          continue;
+
         // Instantiate non-default methods..
 
         // .. except for certain kinds of template specializations.
         if (TSK == TSK_ExplicitInstantiationDeclaration)
           continue;
         if (TSK == TSK_ImplicitInstantiation && !ClassAttr->isInherited())
           continue;
 

test/CodeGenCXX/dllexport.cpp

 struct NonExportedBaseClass {
   virtual ~NonExportedBaseClass();
 };
 NonExportedBaseClass::~NonExportedBaseClass() {}
 
 struct __declspec(dllexport) ExportedDerivedClass : NonExportedBaseClass {};
 // M32-DAG: weak_odr dllexport x86_thiscallcc void @"\01??1ExportedDerivedClass@@UAE@XZ"
 
+// Exported constexpr function is only codegened once.
+template <class> struct Q { constexpr Q() {} };
+template class __declspec(dllexport) Q<void>;
+// M32-DAG: define weak_odr dllexport x86_thiscallcc %struct.Q* @"\01??0?$Q@X@@QAE@XZ"(%struct.Q* returned %this)
 
 //===----------------------------------------------------------------------===//
 // Classes with template base classes
 //===----------------------------------------------------------------------===//
 
 template <typename T> struct ClassTemplate { void func() {} };
 template <typename T> struct __declspec(dllexport) ExportedClassTemplate { void func() {} };
 template <typename T> struct __declspec(dllimport) ImportedClassTemplate { void func(); };