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

[Jonathan Wakely]

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).