[PATCH] Fix libstdc++/C++ fallout from RTL alias export

[Richard Guenther]

On Thu, 21 Jan 2010, Paolo Carlini wrote:

> Hi,
> > Like
> >   D.12950_92 = (struct _Simple_type_wrapper *) 
> > &D.12951.D.8815._M_functor._M_pod_data[0];
> >   *D.12950_92 = D.12949;
> >   __old_functor = D.12951.D.8815._M_functor;
> >
> > where it stores a struct _Simple_type_wrapper into _M_pod_data and
> > then reads from it via a union _Any_data type.
> >
> > Generated from tr1::function::swap.  Looks innocent but in reality
> > isn't:
> >
> >       void swap(function& __x)
> >       {
> >         _Any_data __old_functor = _M_functor;
> >         _M_functor = __x._M_functor;
> >
> > because you are effectively doing
> >
> >   int a, b;
> >   *(float *)&a = 0.0;
> >   b = a;
> >
> > and expect the read from a to conflict with the earlier store.
> > BUT - the type of the lvalue 'a' is int and you are not allowed
> > to access an object with effective dynamic type float via it.
> >   
> I see. That _Simple_type_wrapper looks like an hack to me, I think we
> should replace it altogether.
>
> > In the struct case it gets of course less obvious.  In functional you
> > do
> >
> >  struct Data {
> >    char mem[...];
> >  } a, b;
> >
> >  *(T *)&a.mem[0] = ...;
> >  b = a;
> >
> > but with the same argument this fails.  An aggregate copy
> > is _not_ like memcpy as far as type based aliasing goes!
> >
> > So a fixed swap() would be using memcpy.  Or a fixed _Any_data
> > would return ref-all pointers.
> >   
> As I said already, I'm envisaging two completely different approaches
> for C++0x and TR1: for the latter, any dirt trick is fine, I would even
> say, if you don't get feedback from me decently quickly, just add
> attributes at your will and be done with it. For C++0x I think we should
> do re-design things in a much better way, I can't believe we can't do
> anything else but use memcpy...

I have to see how to fix PR42834 first, but yes - I'll try to come up
with a sensible solution (which I believe will be using memcpy in swap()).

Richard.