> On Nov 3, 2023, at 2:22 AM, Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Fri, Nov 03, 2023 at 07:07:36AM +0100, Martin Uecker wrote:
>> Am Donnerstag, dem 02.11.2023 um 17:28 -0700 schrieb Bill Wendling:
>>> On Thu, Nov 2, 2023 at 1:36 PM Qing Zhao <qing.z...@oracle.com> wrote:
>>>>
>>>> Thanks a lot for raising these issues.
>>>>
>>>> If I understand correctly, the major question we need to answer is:
>>>>
>>>> For the following example: (Jakub mentioned this in an early message)
>>>>
>>>> 1 struct S { int a; char b __attribute__((counted_by (a))) []; };
>>>> 2 struct S s;
>>>> 3 s.a = 5;
>>>> 4 char *p = &s.b[2];
>>>> 5 int i1 = __builtin_dynamic_object_size (p, 0);
>>>> 6 s.a = 3;
>>>> 7 int i2 = __builtin_dynamic_object_size (p, 0);
>>>>
>>>> Should the 2nd __bdos call (line 7) get
>>>> A. the latest value of s.a (line 6) for it’s size?
>>>> Or B. the value when the s.b was referenced (line 3, line 4)?
>>>>
>>> I personally think it should be (A). The user is specifically
>>> indicating that the size has somehow changed, and the compiler should
>>> behave accordingly.
>>
>>
>> One potential problem for A apart from the potential impact on
>> optimization is that the information may get lost more
>> easily. Consider:
>>
>> char *p = &s.b[2];
>> f(&s);
>> int i = __bdos(p, 0);
>>
>> If the compiler can not see into 'f', the information is lost
>> because f may have changed the size.
>
> Why? It doesn't really matter. The options are
> A. p is at &s.b[2] associated with &s.a and int type (or size of int
> or whatever); .ACCESS_WITH_SIZE can't be pure,
.ACCESS_WITH_SIZE will only load the size from its address, no any write to
memory.
It still can be PURE, right? (It will not be CONST anymore).
> but sure, for aliasing
> POV we can describe it with more detail that it doesn't modify anything
> in the pointed structure, just escapes the pointer;
If we need to do this, where in the gcc code we need to add these details?
> __bdos can stay
> leaf I believe;
That’s good! (I thought now _bdos will call .ACCESS_WITH_SIZE?)
Qing
> and when expanding __bdos later on, it would just
> dereference the associated pointer at that point (note, __bdos is
> pure, so it has vuse but not vdef and can load from memory); if
> f changes s.a, no problem, __bdos will load the changed value in there
> B. if .ACCESS_WITH_SIZE associates the pointer with the s.a value from that
> point, .ACCESS_WITH_SIZE can be const, but obviously if f changes s.a,
> __bdos later will use s.a value from the &s.b[2] spot
>
> Jakub
>
