Re: RFC: the proposal to resolve the missing dependency issue for counted_by attribute

[Qing Zhao <qing.zhao@oracle.com>]

> On Nov 3, 2023, at 2:22 AM, Jakub Jelinek <jakub@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.zhao@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
>