I just updated PR121000: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121000.
Yes, the root cause is exactly what you mentioned in the other email:
“
The IL must clearly use the value (the size of the element), otherwise
DCE or other passes will happily optimize it away, they don't keep some
expression computation around just because it is referenced in
TYPE_SIZE_UNIT of some type somewhere.
“
Thanks.
Qing
> On Jul 8, 2025, at 10:30, Qing Zhao <qing.z...@oracle.com> wrote:
>
> Hi,
>
>> On Jul 8, 2025, at 01:18, Jakub Jelinek <ja...@redhat.com> wrote:
>>>
>>>> 5th argument ACCESS_MODE:
>>>> -1: Unknown access semantics
>>>> 0: none
>>>> 1: read_only
>>>> 2: write_only
>>>> 3: read_write
>>>> 6th argument: A constant 0 with the pointer TYPE to the original flexible
>>>> array type.
>>>
>>> Likewise, wouldn't this always be TREE_TYPE(TREE_TYPE(REF_TO_OBJ))? For a
>>> FAM, the frontend does array_to_pointer, so with the INDIRECT_REF at the end
>>> of build_access_with_size_for_counted_by gone, I reckon you should be able
>>> to get the type of the array element. Likewise if it was a pointer and not
>>> a FAM.
>>>
>>> TYPE_SIZE_UNIT may not work for them like you said, but there ought to be a
>>> usable expression that we can reach from the type, no?
>>
>> No. The IL must clearly use the value (the size of the element), otherwise
>> DCE or other passes will happily optimize it away, they don't keep some
>> expression computation around just because it is referenced in
>> TYPE_SIZE_UNIT of some type somewhere.
>>
>> Consider e.g.
>> void bar (int *);
>>
>> void
>> foo (int n, int m)
>> {
>> typedef int A[m];
>> struct S { int n, m; A a[2]; A b[] __attribute__((counted_by (n))); } *p;
>> p = __builtin_malloc (sizeof (struct S) + sizeof (A) * n);
>> p->n = n;
>> p->m = m;
>> int *q = &p->b[1][0];
>> bar (q);
>> q = &p->b[0][1];
>> bar (q);
>> }
>> There is a reason why e.g. COMPONENT_REF has 3 arguments rather than 2,
>> the last one used solely for the variable length structures (primarily Ada,
>> or GNU C extension like above).
>
> Just filed PR121000: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121000
> __builtin_dynamic_object_size should work for FAM with VLA element when
> annotated with counted_by
>
> Looks like that the current implementation of counted_by for FAM has a bug
> here to handle this case.
> Will study a little bit along with your comments in the other emails. And
> then respond in another email.
>
> Thanks a lot for the comments and testing case.
>
>> Jakub
