> On Nov 2, 2023, at 8:09 AM, Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Thu, Nov 02, 2023 at 12:52:50PM +0100, Richard Biener wrote:
>>> What I meant is to emit
>>> tmp_4 = .ACCESS_WITH_SIZE (&s.b[0], &s.a, (typeof (&s.a)) 0);
>>> p_5 = &tmp_4[2];
>>> i.e. don't associate the pointer with a value of the size, but with
>>> an address where to find the size (plus how large it is), basically escape
>>> pointer to the size at that point. And __builtin_dynamic_object_size is
>>> pure,
>>> so supposedly it can depend on what the escaped pointer points to.
>>
>> Well, yeah - that would work but depend on .ACCESS_WITH_SIZE being an
>> escape point (quite bad IMHO)
>
> That is why I've said we need to decide what cost we want to suffer because
> of that.
>
>> and __builtin_dynamic_object_size being
>> non-const (that's probably not too bad).
>
> It is already pure,leaf,nothrow (unlike __builtin_object_size which is
> obviously
> const,leaf,nothrow). Because under the hood, it can read memory when
> expanded.
>
>>> We'd see that a particular pointer is size associated with &s.a address
>>> and would use that address cast to the type of the third argument (to
>>> preserve the exact pointer type on INTEGER_CST, though not sure, wouldn't
>>> VN CSE it anyway if one has say
>>> union U { struct S { int a; char b __attribute__((counted_by (a))) []; } s;
>>> struct T { char c, d, e, f; char g __attribute__((counted_by (c)))
>>> []; } t; };
>>> and
>>> .ACCESS_WITH_SIZE (&v.s.b[0], &v.s.a, (int *) 0);
>>> ...
>>> .ACCESS_WITH_SIZE (&v.t.g[0], &v.t.c, (int *) 0);
>>> ?
>>
>> We'd probably CSE that - the usual issue of address-with-same-value.
>>
>>> It would mean though that counted_by wouldn't be allowed to be a
>>> bit-field...
>>
>> Yup. We could also pass a pointer to the container though, that's good
>> enough
>> for the escape, and pass the size by value in addition to that.
>
> I was wondering about stuff like _BitInt. But sure, counted_by is just an
> extension, we can just refuse counting by _BitInt in addition to counting by
> floating point, pointers, aggregates, bit-fields, or we could somehow encode
> all the needed type's properties numerically into an integral constant.
> Similarly for alias set (unless it uses 0 for reads).
counted_by currently is limited to INTEGER type. This should resolve this
issue, right?
Qing
>
> Jakub
>
