> On Jul 28, 2025, at 12:48, Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Wed, Jul 23, 2025 at 05:59:22PM +0000, Qing Zhao wrote:
>> struct S {
>> int n;
>> int *p __attribute__((counted_by(n)));
>> } *f;
>> Int *g;
>> void setup (int **ptr, int count)
>> {
>> *ptr = __builtin_malloc (sizeof (int) * count);
>> g = *ptr;
>> };
>> int main ()
>> {
>> f = __builtin_malloc (sizeof (struct S));
>> setup (&f->p, 10);
>
> This is neither read nor write, it is taking an address of f->p.
> The above case is definitely questionable because nothing really initializes
> f->n, so any later uses of f->p would be invalid unless it is initialized
> first.
> Anyway, the choices are not mark with .ACCESS_WITH_SIZE taking address of
> such pointers, or mark it with another mode and handle it differently later.
> At least for the start, I'd strongly suggest the former.
> With the above setup mess, it will always be just best effort, if it is
> inline, bos pass can see what it has been initialized to and associated
> with, if not, then it will simply not know it has an counted_by attribute.
>
>> C FE has no such capability to determine whether the f->p is a read or a
>> write. Is this right?
>
> C certainly can determine that, otherwise e.g. the -Wunused-but-set-*
> warnings wouldn't work.
> If there is an lvalue to rvalue conversion, it was read, so you can attach
> .ACCESS_WITH_SIZE to that if it is COMPONENT_REF with pointer type with
> counted_by attribute.
> If there is not an lvalue to rvalue conversion, it is write or something
> else.
> So, one possibility is e.g. to look for mark_exp_read calls.
> Another is try default_function_array_read_conversion and a few other spots.
I tried to look for “mark_exp_read” and identified the following 3 routines:
convert_lvalue_to_rvalue
default_function_array_read_conversion
default_conversion
And in the above 3 routines, check for COMPONENT_REF for pointer field, generate
Call to .ACCESS_WITH_SIZE for them.
And this works quite well. All my testing cases passed without any issue.
(Apparently, only “convert_lvalue_to_rvalue” is not enough..)
Let me know if you see any obvious issue with this.
Otherwise, I will prepare for the 9th version of the patch.
Thanks a lot for your help.
Qing
>
> Or another option might be don't mark even the loads with .ACCESS_WITH_SIZE
> when pointer type, tweak the content of the counted_by attribute (its
> argument) instead on the FIELD_DECLs such that the middle-end could figure
> it out and just handle it on the bos pass side. Though if counted_by
> argument is not just an identifier of a field in the same structure but
> complex expression, trying to reintroduce it into the IL might be too
> challenging at that point.
>
> Jakub
>
