> Am 25.10.2023 um 12:47 schrieb Martin Uecker <uec...@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 <uec...@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.
But if the access is through a pointer without the attribute visible even the
Frontend cannot recover? We’d need to force type correctness and give up on
indirecting through an int * when it can refer to two diffenent container
types. The best we can do I think is mark allocation sites and hope for some
basic code hygiene (not clobbering size or array pointer through pointers
without the appropriately attributed type)
> 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
>
>
>>
>
