Re: PR35503 - warn for restrict pointer

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

On 30/08/16 16:54, Tom de Vries 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.

So, what I had in mind is something like this:
...
Warn when an argument passed to a restrict-qualified parameter pointing 
to a non-const type aliases with another argument.

The warning will trigger for the following illegal use of restrict:

  void foo (int *restrict a, int *b) { *a = 1; *b = 2; }
  void foo2 (void) { int o; foo (&o, &o); }

The use of restrict is illegal because in the lifetime of the scope of 
the restrict pointer 'a' both:
- the object 'o' that the restrict pointer 'a' points to is modified,
  and
- the object 'o' is accessed through pointer 'b', which is not based on
  restrict pointer 'a'.

However, the warning will also trigger for the following legal use of 
restrict:

  int foo (int *restrict a, int *b) { return *a + *b; }
  int foo2 (void) { int o = 1; return foo (&o, &o); }

The use of restrict is legal, because the object 'o' that the restrict 
pointer 'a' points to is not modified  in the lifetime of the scope of 
the restrict pointer 'a'. The warning can be silenced by changing the 
type of 'a' to 'const int *restrict'.
...

Thanks,
- Tom