Re: PR35503 - warn for restrict pointer

[Tom de Vries <Tom_deVries at mentor dot com>]

On 30/08/16 17:34, Prathamesh Kulkarni wrote:
> On 30 August 2016 at 20:24, Tom de Vries <Tom_deVries@mentor.com> wrote:
> > On 26/08/16 13:39, Prathamesh Kulkarni wrote:
> > > Hi,
> > > The following patch adds option -Wrestrict that warns when an argument
> > > is passed to a restrict qualified parameter and it aliases with
> > > another argument.
> > >
> > > eg:
> > > int foo (const char *__restrict buf, const char *__restrict fmt, ...);
> > >
> > > void f(void)
> > > {
> > >   char buf[100] = "hello";
> > >   foo (buf, "%s-%s", buf, "world");
> > > }
> >
> >
> > Does -Wrestrict generate a warning for this example?
> >
> > ...
> > void h(int n, int * restrict p, int * restrict q, int * restrict r)
> > {
> >   int i;
> >   for (i = 0; i < n; i++)
> >     p[i] = q[i] + r[i];
> > }
> >
> > h (100, a, b, b)
> > ...
> >
> > [ Note that this is valid C, and does not violate the restrict definition. ]
> >
> > If -Wrestrict indeed generates a warning, then we should be explicit in the
> > documentation that while the warning triggers on this type of example, the
> > code is correct.
> I am afraid it would warn for the above case. The patch just checks if
> the parameter is qualified
> with restrict, and if the corresponding argument has aliases in the
> call (by calling operand_equal_p).

> Is such code common in practice ?

[ The example is from the definition of restrict in the c99 standard. ]

According to the definition of restrict, only the restrict on p is 
required to know that p doesn't alias with q and that p doesn't alias 
with r.

AFAIK the current implementation of gcc already generates optimal code 
if restrict is only on p. But earlier versions (and possibly other 
compilers as well?) need the restrict on q and r as well.

So I expect this code to occur.

> If it is, I wonder if we should keep
> the warning in -Wall ?

Hmm, I wonder if we can use the const keyword to silence the warning.

So if we generate a warning for:
...
void h(int n, int * restrict p, int * restrict q, int * restrict r)
{
  int i;
  for (i = 0; i < n; i++)
    p[i] = q[i] + r[i];
}
h (100, a, b, b)
...

but not for:
...
void h(int n, int * restrict p, const int * restrict q, const int * 
restrict r)
{
  int i;
  for (i = 0; i < n; i++)
    p[i] = q[i] + r[i];
}
h (100, a, b, b)
...

Then there's an easy way to rewrite the example such that the warning 
doesn't trigger, without having to remove the restrict.

Thanks,
- Tom