On Tue, Oct 04, 2011 at 11:55:17AM +0200, Richard Guenther wrote:
> > On Tue, Oct 04, 2011 at 11:01:27AM +0200, Richard Guenther wrote:
> > > > > > void foo (int *p)
> > > > > > {
> > > > > > int * __restrict p1 = p;
> > > > > > int * __restrict p2 = p + 32;
> > > > > > int *q;
> > > > > > int i;
> > > > > > for (i = 0; i < 32; ++i)
> > > > > > p1[i] = p2[i];
> > > > > > p = p1;
> > > > > > q = p2 - 31;
> > > > > > for (i = 0; i < 32; ++i)
> > > > > > p[i] = q[i];
> > > > > > }
> > > > > >
> >
> > > In the above first loop the restrict pointers p1 and p2 access
> > > distinct object pieces. The second loop uses non-restrict qualified
> > > pointers p and q (that are based on the restrict variants p1 and p2
> > > though) to access overlapping pieces. Is the second loop invalid
> > > because p and q are based on p1 and p2 even though they are not
> > > restrict qualified?
> >
> > IMHO yes. The standard doesn't seem to talk about the accesses being done
> > through the restricted pointer, but about accesses that are based on
> > the restricted pointer, and as soon as you access in the associated block
> > (here the foo function) some object through an lvalue whose address is
> > based on some restricted pointer and the value is modified by any means,
> > then all accesses to that object need to be done through something
> > based on that restricted pointer.
>
> So when I change the above to
>
> /*p = p;*/
> q = (p + 32) - 31;
void foo (int *p)
{
int * __restrict p1 = p;
int * __restrict p2 = p + 32;
int *q;
int i;
for (i = 0; i < 32; ++i)
p1[i] = p2[i];
q = (p + 32) - 31;
for (i = 0; i < 32; ++i)
p[i] = q[i];
}
> then the code will be valid? When I obfuscate that enough I
> can get GCC CSEing p + 32 and thus effectively q will look
> like it is based on p2.
The above is still invalid. p[0] through p[31] is modified and
is accessed both through lvalue whose address is based on p1 (p1[i])
and through lvalues whose address is not based on p1 (p[i] and
q[i] (the latter only for p[0] through p[30])). If you take
the first loop out, it would be valid though.
Jakub
