[Bug c++/14639] New: Incorrect emission of unused compiler-generated destructor at -O2

[Mark Mitchell]

> So when the function itself gets marked as necesary we automatically
> emit the thunk when the mark_used_thunk has been called, right?

Yes.  Whenever a function is emittted, so are all of its associated thunks.

> I guess we need two thinks - we need to manage that once we discover
> thunk to be necesary we need to mark the function it is refering to and
> we need to emit thunks iff they are really reachable (not just used,
> right)?

Yes.

> It would be ideal if the thunk was behaving like ordinary function, but
> for some reason it is not.  Is there way to recognize thunk in C++
> independent way?

No.  But, you could always change things so that was possible, I 
suppose.  Thunks cannot be represented as ordinary functions until we 
have multiple entry points, and we don't yet have that.

> I guess we will need kind of hooks - tell cgraph code that given decl is
> thunk and thunk is like function but not quite as it has no body and
> reffers to other function and it is not necesay to assemble it as it
> gets assembled magically with the function body.

Yes.

> Hmm, perhaps I am getting overzelaous here (hope so).  I am not quite
> sure how emit_p works. When the emit_p is set?  If it is at function
> expansion time, it is too late - we need to be finished this in advance.

emit_p is set from emit_associated_thunks, which is called for a 
function's thunks right before the function itself is sent to 
tree_rest_of_compilation.

-- 
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com