Am Freitag, dem 06.12.2024 um 16:13 +0000 schrieb Qing Zhao:
>
> > On Dec 6, 2024, at 10:56, Martin Uecker <uec...@tugraz.at> wrote:
> >
> > Am Freitag, dem 06.12.2024 um 14:16 +0000 schrieb Qing Zhao:
> > >
...
> > > >
> > > > I think the relevant scenario is where you assign the struct and
> > > > not a pointer to the struct, i.e. something like the following:
> > > >
> > > > #include "builtin-object-size-common.h"
> > > >
> > > > struct annotated {
> > > > int b;
> > > > int *c __attribute__ ((counted_by (b)));
> > > > };
> > > >
> > > > struct annotated __attribute__((__noinline__)) setup (int attr_count)
> > > > {
> > > > struct annotated p_array_annotated;
> > > > p_array_annotated.c = (int *) malloc (sizeof (int) * attr_count);
> > > > p_array_annotated.b = attr_count;
> > > >
> > > > return p_array_annotated;
> > > > }
> > > >
> > > >
> > > > int main(int argc, char *argv[])
> > > > {
> > > > struct annotated x = setup (10);
> > > > int *p = x.c;
> > > > x = setup (20);
> > > > EXPECT(__builtin_dynamic_object_size (p, 1), 10 * sizeof (int));
> > > > EXPECT(__builtin_dynamic_object_size (x.c, 1), 20 * sizeof (int));
> > > > DONE ();
> > > > }
> > > >
> > >
> > > With the above testing case, my current implementation based on
> > > .ACCESS_WITH_SIZE succeed without any issue. -:)
> > > The design of .ACCESS_WITH_SIZE already resolved this issue.
> >
> > Ok, thanks! But I am a bit confused, because it seems it behaves
> > this way also for FAMs
> >
> > https://godbolt.org/z/64a6z4cna
>
> The behavior of the above testing case is exactly the additional feature we
> provided for counted_by attribute for FAM:
>
> =====
> One important feature of the attribute is, a reference to the
> flexible array member field uses the latest value assigned to the
> field that represents the number of the elements before that
> reference. For example,
>
> p->count = val1;
> p->array[20] = 0; // ref1 to p->array
> p->count = val2;
> p->array[30] = 0; // ref2 to p->array
>
> in the above, 'ref1' uses 'val1' as the number of the elements in
> 'p->array', and 'ref2' uses 'val2' as the number of elements in
> 'p->array’.
> =====
>
> So, it’s the correct behavior for the counted_by attribute for FAM based on
> our previous discussion and agreement.
If it is indeed that the value of p->count last stored before p->array is
*referenced* which counts, then everything is well.
Somehow I thought for FAMs it is the value p->count last stored before
p->array is *accessed* (possibly indirectly via another pointer). Probably
it was just me being confused.
>
> However, as you pointed out, when the “counted_by” attribute is extended to
> the pointer field, this feature will be problematic.
> And we need to add the following additional new requirement for the
> “counted_by” attribute of pointer field:
>
> p->count and p->array can only be changed by changing the whole structure
> at the same time.
Actually, I am then not even sure we need this requirement. My point was only
that
setting the whole structure at the time should work correctly, i.e. without
changing
the bounds for old pointers which were stored in the struct previously. With
the
semantics above it seems this case also works automatically.
Martin
>
> So for a structure that includes a pointer field with a counted_by attribute,
> the p->count and p->array must be updated at the same time by changing the
> whole structure, therefore the testing case will not be valid for pointer
> field, we MUST change it as the following to make it valid for pointer field
> with counted_by attribute:
>
> int main(int argc, char *argv[])
> {
> struct annotated *x = setup(10);
> int *p = x->c;
> x = setup(5);
> printf("%d\n", (int)__builtin_dynamic_object_size (p, 1));
> printf("%d\n", (int)__builtin_dynamic_object_size (x->c, 1));
> exit(0);
> }
>
>
> Hope this is clear this time -:).
>
> Qing
> >
> > and I thought this was changed, i.e. so that the bounds
> > track the current value of b for this case. But apparently
> > I was mistaken. I am fine with the behavior as shown here.
> >
> > Martin
> >
> > >
> > > Qing
> > > > Martin
> > > >
> > > >
> > > >
> > >
> >
> > --
> > Univ.-Prof. Dr. rer. nat. Martin Uecker
> > Graz University of Technology
> > Institute of Biomedical Imaging
>
>
