So, the basic question is:
Given the following:
struct fix {
int others;
int array[10];
}
extern struct fix * alloc_buf ();
int main ()
{
struct fix *p = alloc_buf ();
__builtin_object_size(p->array,0) == ?
}
Given p->array, can the compiler determine that p points to an object that has
TYPE struct fix?
If the answer is YES, then the current__builtin_object_size algorithm can be
improved to determine __builtin_object_size(p->array, 0) with the TYPE of the
struct fix.
Qing
> On Aug 3, 2023, at 1:34 PM, Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>
> One thing I need to point out first is, currently, even for regular fixed
> size array in the structure,
> We have this same issue, for example:
>
> #define LENGTH 10
>
> struct fix {
> size_t foo;
> int array[LENGTH];
> };
>
> …
> int main ()
> {
> struct fix *p;
> p = alloc_buf_more ();
>
> expect(__builtin_object_size(p->array, 1), LENGTH * sizeof(int));
> expect(__builtin_object_size(p->array, 0), -1);
> }
>
> Currently, for __builtin_object_size(p->array, 0), GCC return UNKNOWN for it.
> This is not a special issue for flexible array member.
>
> Qing
>
>
> On Aug 3, 2023, at 1:19 PM, Siddhesh Poyarekar <siddh...@gotplt.org> wrote:
>>
>> On 2023-08-03 12:43, Qing Zhao wrote:
>>>> Surely we could emit that for __bdos(q->array, 0) though, couldn't we?
>>> For __bdos(q->array, 0), we only have the access info for the sub-object
>>> q->array, we can surely decide the size of the sub-object q->array, but we
>>> still cannot
>>> decide the whole object that is pointed by q (the same reason as above),
>>> right?
>>
>> It's tricky, I mean we could assume p to be a valid object due to the
>> dereference and hence assume that q->foo is also valid and that there's at
>> least sizeof(*q) + q->foo * sizeof (q->array) bytes available. The question
>> then is whether q could be pointing to an element of an array of `struct
>> annotated`. Could we ever have a valid array of such structs that have a
>> flex array at the end? Wouldn't it always be a single object?
>>
>> In fact for all pointers to such structs with a flex array at the end, could
>> we always assume that it is a single object and never part of an array, and
>> hence return sizeof()?
>>
>> Thanks,
>> Sid
>
