[PATCH] D51340: Add /Zc:DllexportInlines option to clang-cl

[Takuto Ikuta via Phabricator via cfe-commits]

takuto.ikuta added inline comments.


================
Comment at: clang/lib/AST/ASTContext.cpp:9552
+    // overwrite linkage of explicit template instantiation
+    // definition/declaration.
+    return GVA_DiscardableODR;
----------------
hans wrote:
> takuto.ikuta wrote:
> > takuto.ikuta wrote:
> > > hans wrote:
> > > > Can you give an example for why this is needed?
> > > Sorry, this change does not need. Removed.
> > Sorry, this change is necessary still.
> > 
> > Without this, definition of inline function in explicit template instantiation declaration is not be emitted, due to GVA_AvailableExternally linkage.
> > But we stop exporting definition of inline function in explicit template instantiation definition too.
> > 
> > So without this, definition of dllimported inline function of explicit template instantiation declaration won't be available.
> > 
> Can you provide a code example of why this is needed?
If we don't change function linkage, following code will be compiled like below with -fno-dllexport-inlines.

```
template<typename>
class M{
public:
  void foo() {}
};

template class __declspec(dllexport) M<int>;

extern template class __declspec(dllimport) M<short>;

void f (){
  M<int> mi;
  mi.foo();

  M<short> ms;
  ms.foo();
}
```

```
$"?foo@?$M at H@@QEAAXXZ" = comdat any

; Function Attrs: noinline nounwind optnone
define weak_odr dso_local void @"?foo@?$M at H@@QEAAXXZ"(%class.M* %this) #0 comdat align 2 {
entry:
  %this.addr = alloca %class.M*, align 8
  store %class.M* %this, %class.M** %this.addr, align 8
  %this1 = load %class.M*, %class.M** %this.addr, align 8
  ret void
}

; Function Attrs: noinline nounwind optnone
define dso_local void @"?f@@YAXXZ"() #0 {
entry:
  %mi = alloca %class.M, align 1
  %ms = alloca %class.M.0, align 1
  call void @"?foo@?$M at H@@QEAAXXZ"(%class.M* %mi)
  call void @"?foo@?$M at F@@QEAAXXZ"(%class.M.0* %ms)
  ret void
}

declare dso_local void @"?foo@?$M at F@@QEAAXXZ"(%class.M.0*) #1
```

M<short>::foo() is declared, but inline function is not dllexported (See M<int>::foo() is not dllexported). So this code cannot be linked because definition of M<short>::foo does not exist. If the function is properly inlined, we don't need to have this code. But I'm not sure why the function is not inlined.


https://reviews.llvm.org/D51340