On Thu, Dec 5, 2024 at 2:28 PM Bill Wendling <isanb...@gmail.com> wrote:
> On Thu, Dec 5, 2024 at 1:09 PM Qing Zhao <qing.z...@oracle.com> wrote:
> > > On Dec 3, 2024, at 10:29, Qing Zhao <qing.z...@oracle.com> wrote:
> > >> On Dec 3, 2024, at 10:07, Martin Uecker <uec...@tugraz.at> wrote:
> > >> The language extension does not exist yet, so there is no problem.
> > > Yeah, I should mention this as “corresponding future language extension”
> > > -:)
> > >>
> > >> But I hope we will get it and then specify it so that this works
> > >> correctly without this footgun.
> > >>
> > >> Of course, if GCC gets the "counted_by" attribute wrong, there will
> > >> be arguments later in WG14 why the feature is then different to it.
> > >
> > > I think that we need to resolve this issue first in the design of
> > > “counted_by” for pointer fields.
> > > I guess that we might need to come up with some additional limitations
> > > for using the “counted_by”
> > > attribute for pointer fields at the source code level in order to avoid
> > > such potential error. But not
> > > sure what exactly the additional limitation should be at this moment.
> > >
> > > Need some study here.
> >
> > Actually, I found out that this is really not a problem with the current
> > design, for the following new testing case I added for my current
> > implementation of the counted_by for pointer field:
> >
> > [ gcc.dg]$ cat pointer-counted-by-7.c
> > /* Test the attribute counted_by for pointer field and its usage in
> > * __builtin_dynamic_object_size. */
> > /* { dg-do run } */
> > /* { dg-options "-O2" } */
> >
> > #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
> > = (struct annotated *) malloc (sizeof (struct 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 ();
> > }
> >
> > This test case passed without any issue.
> >
> > Our previous introduction of the new internal function .ACCESS_WITH_SIZE
> > already resolved this issue.
> >
> > So, I think that as long as the whole structure is set at the same time,
> > should be fine.
> >
> > Let me know if you have more comment here.
> >
> Nice! I think this is going to be an issue with Clang's
> implementation. I'll need to create our version of .ACCESS_WITH_SIZE.
> It might end up simplifying some of the code. :-)
So my assumption that our GetElementPointerInst instruction would
"just work" in this case was wrong. By assigning x->c to p it loses
pretty much all information, e.g. the __counted_by attribute. It looks
like we'll have to go with a similar intrinsic to .ACCESS_WITH_SIZE.
-bw
