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

[Richard Biener <richard.guenther@gmail.com>]

> Am 25.10.2023 um 12:47 schrieb 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.

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
> 
> 
>> 
>