Re: HELP: Will the reordering happen? Re: [V3][PATCH 0/3] New attribute "counted_by" to annotate bounds for C99 FAM(PR108896)

[Martin Uecker <uecker@tugraz.at>]

Am Mittwoch, dem 25.10.2023 um 06:25 -0400 schrieb Siddhesh Poyarekar:
> On 2023-10-25 04:16, Martin Uecker wrote:
> > Am Mittwoch, dem 25.10.2023 um 08:43 +0200 schrieb Richard Biener:
> > > 
> > > > Am 24.10.2023 um 22:38 schrieb Martin Uecker <uecker@tugraz.at>:
> > > > 
> > > > Am Dienstag, dem 24.10.2023 um 20:30 +0000 schrieb Qing Zhao:
> > > > > Hi, Sid,
> > > > > 
> > > > > Really appreciate for your example and detailed explanation. Very helpful.
> > > > > I think that this example is an excellent example to show (almost) all the issues we need to consider.
> > > > > 
> > > > > I slightly modified this example to make it to be compilable and run-able, as following:
> > > > > (but I still cannot make the incorrect reordering or DSE happening, anyway, the potential reordering possibility is there…)
> > > > > 
> > > > >   1 #include <malloc.h>
> > > > >   2 struct A
> > > > >   3 {
> > > > >   4  size_t size;
> > > > >   5  char buf[] __attribute__((counted_by(size)));
> > > > >   6 };
> > > > >   7
> > > > >   8 static size_t
> > > > >   9 get_size_from (void *ptr)
> > > > > 10 {
> > > > > 11  return __builtin_dynamic_object_size (ptr, 1);
> > > > > 12 }
> > > > > 13
> > > > > 14 void
> > > > > 15 foo (size_t sz)
> > > > > 16 {
> > > > > 17  struct A *obj = __builtin_malloc (sizeof(struct A) + sz * sizeof(char));
> > > > > 18  obj->size = sz;
> > > > > 19  obj->buf[0] = 2;
> > > > > 20  __builtin_printf (“%d\n", get_size_from (obj->buf));
> > > > > 21  return;
> > > > > 22 }
> > > > > 23
> > > > > 24 int main ()
> > > > > 25 {
> > > > > 26  foo (20);
> > > > > 27  return 0;
> > > > > 28 }
> > > > > 
> 
> <snip>
> 
> > > When it’s set I suppose.  Turn
> > > 
> > > X.l = n;
> > > 
> > > Into
> > > 
> > > X.l = __builtin_with_size (x.buf, n);
> > 
> > It would turn
> > 
> > some_variable = (&) x.buf
> > 
> > into
> > 
> > some_variable = __builtin_with_size ( (&) x.buf. x.len)
> > 
> > 
> > So the later access to x.buf and not the initialization
> > of a member of the struct (which is too early).
> > 
> 
> Hmm, so with Qing's example above, are you suggesting the transformation 
> be to foo like so:
> 
> 14 void
> 15 foo (size_t sz)
> 16 {
> 16.5  void * _1;
> 17  struct A *obj = __builtin_malloc (sizeof(struct A) + sz * sizeof(char));
> 18  obj->size = sz;
> 19  obj->buf[0] = 2;
> 19.5  _1 = __builtin_with_size (obj->buf, obj->size);
> 20  __builtin_printf (“%d\n", get_size_from (_1));
> 21  return;
> 22 }
> 
> If yes then this could indeed work.  I think I got thrown off by the 
> reference to __bdos.

Yes. I think it is important not to evaluate the size at the
access to buf and not the allocation, because the point is to 
recover it from the size member even when the compiler can't 
see the original allocation.

Evaluating at this point requires that the size is correctly set
before the access to the FAM and the user has to make sure 
this is the case. But to me this requirement would make sense.

Semantically, it could aöso make sense to evaluate the size at a
later time.  But then the reordering becomes problematic again.

Also I think this would make this feature generally more useful.
For example, it could work also for others pointers in the struct
and not just for FAMs.  In this case, the struct may already be
freed when  BDOS is called, so it might also not possible to
access the size member at a later time.

Martin


>