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.
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);
?
It would mean though that counted_by wouldn't be allowed to be a
bit-field...
Jakub
