On Thu, Nov 2, 2023 at 11:40 AM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Thu, Nov 02, 2023 at 11:18:09AM +0100, Richard Biener wrote:
> > > Or, if we want to pay further price, .ACCESS_WITH_SIZE could take as one
> > > of
> > > the arguments not the size value, but its address. Then at __bdos time
> > > we would dereference that pointer to get the size.
> > > So,
> > > struct S { int a; char b __attribute__((counted_by (a))) []; };
> > > struct S s;
> > > s.a = 5;
> > > char *p = &s.b[2];
> > > int i1 = __builtin_dynamic_object_size (p, 0);
> > > s.a = 3;
> > > int i2 = __builtin_dynamic_object_size (p, 0);
> > > would then yield 3 and 1 rather than 3 and 3.
> >
> > I fail to see how we can get the __builtin_dynamic_object_size call
> > data dependent on s.a, thus avoid re-ordering or even DSE of the
> > store.
>
> If &s.b[2] is lowered as
> sz_1 = s.a;
> tmp_2 = .ACCESS_WITH_SIZE (&s.b[0], sz_1);
> p_3 = &tmp_2[2];
> then sure, there is no way, you get the size from that point.
> tree-object-size.cc tracking then determines that in a particular
> case the pointer is size associated with sz_1 and use that value
> as the size (with the usual adjustments for pointer arithmetics and the
> like).
>
> 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) and __builtin_dynamic_object_size being
non-const (that's probably not too bad).
> 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.
> Jakub
>
