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

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

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.

Thanks,
Sid