https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107952
--- Comment #13 from Qing Zhao <qing.zhao at oracle dot com> ---
> On Jan 25, 2023, at 2:32 AM, rguenther at suse dot de
> <gcc-bugzi...@gcc.gnu.org> wrote:
>>
>> A little confused here:
>> when the structure with a trailing flexible-array member is a middle
>> field of
>> an outer structure, GCC extension will treat it as a flexible-array
>> too? But the
>> maximum size of this flexible-array will be bounded by the size of the
>> padding
>> of that field?
>> Is the above understanding correct?
>
> That's correct - at least when using the get_ref_base_and_extent
> API. I see that when using array_ref_flexible_size_p it doesn't
> return true (but it's technically not _flexible_, we just treat it with
> a bound size that doesn't match the declared bound).
For the current array_ref_flexible_size_p, we include the following 3 cases:
A. a ref to a flexible array member at the end of a structure;
B. a ref to an array with a different type against the original decl;
for example:
short a[16] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
(*((char(*)[16])&a[0]))[i+8]
C. a ref to an array that was passed as a parameter;
for example:
int test (uint8_t *p, uint32_t t[1][1], int n) {
for (int i = 0; i < 4; i++, p++)
t[i][0] = …;
It basically mean: return true if REF is to an array whose actual size might be
larger than its upper bound implies.
I feel that the case "when the structure with a trailing flexible-array member
is a middle field of an outer structure” still fit here, but not very sure,
need more thinking...
>
> The first is handled by the function just fine,
No, even the first case is not recognized by the current
“array_ref_flexible_size_p”, it’s not been identified as a flexible array right
now.
Shall we include this case into “array_ref_flexible_size_p”? (It’s a GCC
extension).
> it's the second with the bound size that's not and that isn't a good fit
> there,
> though we do handle padding to the end of a declaration where
> we return true.
>
>> Handle them separately instead?
>
> I'm not sure how important it is to hande the middle array
> extending to padding, ISTR there was some real world code
> "miscompiled" when treating the array domain as written.
We can leave the 2nd case in a later time to resolve -:)
>
