[patch] Trying to fix pr13124

[Rafael Espíndola]

ccing cfe-commits

On 26 August 2013 21:44, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> I finally got back to pr13124.
>
> Last time it was discussed
> (http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-June/022606.html)
> John was in favor of emitting a copy of the function when emitting the
> available_externally vtable. The folks working on the equivalent gcc
> bug (gcc.gnu.org/pr53808) agree, so I started trying to implement this
> in clang.
>
> The first issue I hit is that if codegen is going to output a
> function, sema has to mark it used. Given llvm.org/pr9114, it looks
> like sema cannot be more aggressive at marking functions used because
> of vtables.
>
> This leaves us with a few unpleasant options:
>
> * Marking functions in vtables used if possible. This sounds a bit
> sloppy, so I would like to avoid it.
> * Producing available_externally vtables only when all the functions
> in it are already used or weak_odr. This would cover cases like
>
> --------------------
> struct foo {
>   virtual ~foo();
> };
> struct bar : public foo {
>   virtual void zed();
> };
> void f() {
>   foo *x(new bar);
>   delete x;
> }
> void g(bar *x) {
>   x->~bar(); // force the destructor to be used
> }
> --------------------------
>
> and
>
> ----------------------------------
> template<typename T>
> struct bar {
>   virtual ~bar();
> };
> template<typename T>
> bar<T>::~bar() {
> }
> extern template class bar<int>; // make the destructor weak_odr
> instead of linkonce_odr
> void f() {
>   bar<int> *x(new bar<int>);
>   delete x;
> }
> ----------------------------
>
> These look like corner cases, so it is unclear if it is worth it.
>
> * And finally: Just nuke this optimization. That is what this patch implements.
>
> Cheers,
> Rafael