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

[Jakub Jelinek <jakub@redhat.com>]

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, 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; __bdos can stay
   leaf I believe; 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