Re: [PATCH] c++/58796 Make nullptr match exception handlers of pointer type

[Jonathan Wakely <jwakely at redhat dot com>]

On 16/07/16 11:11 +0200, Jakub Jelinek wrote:
> On Fri, Jul 15, 2016 at 11:14:03PM +0100, Jonathan Wakely wrote:
> > On 15/07/16 22:53 +0100, Jonathan Wakely wrote:
> > >On 15/07/16 23:36 +0200, Jakub Jelinek wrote:
> > >>On Fri, Jul 15, 2016 at 10:07:03PM +0100, Jonathan Wakely wrote:
> > >>>>+      if (typeid (*this) == typeid(__pointer_type_info))
> > >>>>+        {
> > >>>>+          *thr_obj = nullptr;
> > >>>>+          return true;
> > >>
> > >>But you have the above store too.
> > >
> > >That doesn't write to the exception object, it only does a single
> > >dereference (compared to the double dereference of the racy write), so
> > >it writes to the local variable in the PERSONALITY_FUNCTION in
> > >eh_personality.cc
> > >
> > >So that shouldn't race with other threads. I think.
> > >
> >
> > TSan agrees. When I compile my test and yours (and include
> > libsupc++/pbase_type_info.cc in the executable, so the writes are also
> > instrumented by tsan) then I see races for the *(ptrdiff_t*)*thr_obj
> > writes but not the *thr_obj ones.
> >
> > It's only the ptr-to-data-member case that scribbles in the actual
> > exception object.
>
> In:
>
> struct A { int a; };
> int a;
>
> int
> main ()
> {
>  try {
>    throw nullptr;
>  } catch (int * const &p) {
>    __builtin_printf ("%p %p\n", p, &p);
>  }
>  try {
>    throw nullptr;
>  } catch (int A::* const &p) {
>    __builtin_printf ("%p\n", &p);
>    asm volatile ("" : : "r" (&p));
>  }
>  try {
>    throw &a;
>  } catch (int * const &p) {
>    __builtin_printf ("%p %p\n", p, &p);
>  }
> }
>
> I see &p being the address passed to __cxa_throw only in the second case,
> where does the reference point to in the other two cases?  Some temporary in
> main?

I'm not sure what the difference is there. There's some difference in
how pointers and non-pointers are handled.

> Does that mean if you rethrow the exception multiple times in
> different threads you get references to the same object for PMF and to
> different objects for pointers?

No, for the throw &a case you'll get the same exception object in
every thread.



say anything about the identity of the caught objects when nullptr is
thrown).