On Tue, 4 Oct 2011, Jakub Jelinek 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;
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.
Richard.
